ERDA-76/154

/JBS112-T6- iiu

I)

JOINT U.S.-U.S.S.R. TEST

OF U.S. MHD ELECTRODE SYSTEMS

IN U.S.S.R. U-02 MHD

FACILITY

(PHASE I)

FINAL REPORT

ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION
' ^ Hosier, W. R., Editor, Joint U.S.-U.S.S.R. » test of U.S. MHD electrode systems in . U.S.S.R. U-02 MHD faciliUy (phase l).

Final Report Energy Research and Development

Administration, ERDA-76/15^, 262 pages = (Available from the National Technical

Information Services, Springfield, VA | 1976). !

313

5 iR 7^V///

j

Available from:

National Technical Information Service (NTIS)

U.S. Department of Commerce

5285 Port Royal Road

Springfield, Virginia 22161

Price; Printed Copy:

Microfiche

$9.00

$3.00
ERDA-76/154

U90G

Joint U.S.-U.S.S.R.

Test of U.S. MHD Electrode Systems

in U.S.S.R.

U-02 MHD Facility

(Phase I)

FINAL REPORT

Edited by

William R. Hosier

National Bureau of Standards

Washington, D.C. 20234

Prepared for

Energy Research and Development Administration

by

Battelle Pacific Northwest Laboratory

National Bureau of Standards

Westinghouse Research and Development Laboratory

U.S.S.R. Institute of High Temperatures
1
PREFACE

This report was jointly prepared for the Energy Research

and Development Administration hy the Battelle Pacific

Northwest Laboratory, the National Bureau of Standards, the

Westinghouse Research and Development Laboratory and the

U.S.S.R. Institute of High Temperatures. The report presents

the results of the first joint U.S. -U.S.S.R. test of U.S.

MHD electrode materials in the U.S.S.R. U-02 MHD Facility.

The electrode walls were designed and built by the Nestinghouse Research and Development Laboratory.

The joint test was conducted in implementation of the

understanding regarding open-cycle MHD cooperation reached

between representatives of the U.S. and U.S.S.R. Joint

Working Group for Energy held in Moscow, October 6, 1972,

and endorsed as a cooperative program by the U . S . -U . S . S . R

Joint Committee for Scientific and Technical Cooperation.

at its meeting in Washington, D.C., March 21, 1973. Work

on the Program was affirmed by the U . S . -U . S . S . R . Joint

Committee on Energy at its October 3-4> 1974 meeting in

Washington, D.C.

Members of the U.S.-U.S.S.R. Ad-Hoc Working Group

for MHD Materials

U.S.

G. Rudins - Energy Research and Development Administration

S. Schneider - National Bureau of Standards

L. Bates - Battel le Pacific Northwest Laboratories

K. Bowen - Massachusetts Institute of Technology

L. Crawford - University of Tennessee Space Institute

H. Frederikse - National Bureau of Standards

B. Rossing - Westinghouse Research and Development Laboratory

D. D. Marchant - Argonne National Laboratory

U.S.S.R.

G. P. Telegin - Institute of High Temperatures

D. K. Burenkov - Institute of High Temperatures

A. I. Romanov - Institute of High Temperatures

V. V. Kirillov - Institute of High Temperatures

Organizations that have actively participated in work described in this report are

1. Battelle Pacific Northwest Laboratory, Richland, Washington.

2. National Bureau of Standards, Washington, D.C.

3. Westinghouse Research and Development Laboratory, Pittsburgh, Pa.

4. Institute of High Temperatures of the Academy of Sciences, Moscow, U.S.S.R

Personnel that have contributed to the contents of this report or actively

participated in the joint test are:

U.S. U.S.S.R.

G. Rudins

R. J. Wright

B. R. Rossing

W. E. Young

W. R. Hosier

J. L. Bates

T. Negas

C. L. McDaniel

E. N. Farabaugh

A. J. Armstrong

A. Perloff

S. J. Schneider

H. P. R. Frederikse

J. L. Daniel

D. K. Burenkov

G. P. Telegin

V. V. Kirillov

V. I. Zalkind

0. B. Zamyslov

Yu. L. Dol inski

A. I. Romanov

Yu. L. Krasulin

T. I. Borodina

A. S. Golobkova

V. L. Vinogradov

D. A. Yysotskii

F. A. Akopov

A. B. Ivanov

V. D. Liubimov

V. G. Gordon

TABLE OF CONTENTS PAGE

ABSTRACT 1

INTRODUCTION 3

CHAPTER I

DESCRIPTION OF THE TEST FACILITY AND OPERATING CONDITIONS DURING THE

TEST 4

A. Description of the Test Facility 4

B. Operating Modes for the Electrode Wall 11

B.l Characteristic Thermal Operating Mode for the Electrode Wall .... 11

B.2 Characteristic Electrical Parameters for the Life Test 22

B.3 Electrical Characteristics of the Channel Before and

After the Life Test 25

CHAPTER II

MODULE DESCRIPTION, ANALYTICAL PROCEDURES AND RESULTS OF ANALYSIS 42

A. Module Design and Preparation of Materials 42

A.l Overview of Design Considerations 42

A. 2 Preparation of Materials 43

A. 3 Electrode Wall Design 45

A. 4 Instrumentation 49

B. Pre-Test Materials Characterization and Analysis ^ 65

B. l Thermal Diffusivity 65

B.1.1 MgO Insulator 66

B.l.

Electrodes 67

B.2 Thermal Expansion 67

B.3 Chemical Composition 73

B.4 Microstructure 73

B.4.1 MgO Insulator 73

B.4.

Electrodes 83

B.5 Electrical Conductivity 92
B.6 Phase Composition 92

B. 7 Porosity and Pore Size Distribution 92

C. Post-Test Materials Characterization and Analysis 103

C. l Module Dissassembly and Materials Distribution 103

C.2 Phase Composition and Chemical Reactions 109

C.2.1 Cathode Materials 113

C.2. 1.1 Introduction and Methods 113

C.2. 1.2 Results, Phase Analysis and Discussion 113

C.2. 1.2.1 CeO^ Containing Materials 113

C.2. 1.2.

^2^3 Stabilized ZrO^ (cathode) and

MgO Insulator 118

C.2. 1.2.

Special Considerations 119

C.2.Anode Materials 119

C.3 Electrical Conductivity ^ 145

C.4 Metallography and Electron Microprobe Analysis . 145

C. 4.1 Specific Observations Concerning Various Materials of

the Tested Channel 164

C. 5 Radiograph and Current Lead-out Analysis 169

D. USSR Results on Pre- and Post-Test Materials Characterization and

Analysis 185

D. l Magnesium Ceramics in the Channel Insulating Walls 185

D.2 Phase Composition and Structure of Materials Tested in the U-02 . . 195

D. 2.1 Objects and Methods of the Studies 195

D.2.Results of X-Ray Phase Analysis 196

D.2. 3. Discussion of Results 199

D.3 Electrochemical Investigations of Electrodes with Compositions

I (88 ZrO^ - 12 Y^O^) and II (82 Zr02 - 18 CeO^) 214

D.3.1 Hot Ceramic Electrodes with Compositions Consisting of

88 ZrO^ - 12 • 214
D.3.2 American Specimens with a Composition 18 CeO^. .221

D.4 Qualitative Analysis of the Stability of Electrode Materials,

using the High-Temperature Treatment Method 240

D.4.1 Investigations of the ZrO„ + (8.7 mol%) carried

out in the U.S.S.R. 247

CHAPTER IV

CONCLUSIONS (U.S. and U.S.S.R.) 256

A. Conclusions from Phase Composition, Chemical Reactions,

Metallography and Microprobe Analysis (U.S.) 256

A.l Conclusions from Phase Composition and Chemical Reactions 256

A. 2 Conclusions from Metallography and Microprobe Analysis 257

1

ABSTRACT

The first (Phase I) joint U.S.-U.S.S.R. test of U.S. electrode materials was carried out in Moscow between September 25 and October 8 , 1975 in the

Soviet U-02 MHD facility. The test procedure followed closely a predetermined work plan designed to test five different zirconia based materials and

the cathode and anode electrode wall modules under MHD operating conditions.

The materials which were selected by the Ad Hoc Materials Working Group were

88Zr 02- 12Y203 , 82Zr02- 18Ce02 , 50Zr02-50Ce02 , 25Zr02~75Ce02 and 20Zr02-78Ce02-

2Ta205 . The electrode modules were constructed by Westinghouse Research and

Development Laboratory. Each of the five electrode materials had four different current densities established between the anode and cathode during the

experiment which lasted a total of 127 hours; 100 hours under MHD power

operational conditions.

There were four main phases in the test schedule:

1st phase - Start-up of the channel over a specific heating period. No seed

(K2 CO 3 ) introduction - 18 hours.

2nd phase - Electrical tests at operating temperature to investigate electrophysical characteristics of the channel and electrodes - 6 hours.

3rd phase - Operating life test - 94 hours.

4th phase - Shut-down of the channel over a specific cool do\^m pei?iod -

9 hours.

There were also extensive tests made on the modules before their shipment

to the U.S.S.R. and after their installation into the U-02 channel but before the

start of the 1st phase. U.S. and U.S.S.R. personnel were in attendance

at all times during the continuous test which was extended from the

originally planned total time of 100 hours to 127 hours.

All except six electrode pairs performed satisfactorily during the entire

test. These were the pairs which were designated to carry maximum or near maximum current density. Five pairs failed early in the life test

and the sixth pair failed in the last several hours. Failure was not

due to the electrode materials, however, but due to lead-out melting

caused by joule heating in the platinum wires.

The U-02 facility has been fully described and the operational parameters

are given for each phase of the test. The electrode and insulating walls have

been described .and the appropriate parameters that are used to predict the performance of the module are given.
2

Extensive pre- and post-test materials characterizations were made to

determine the effect of the MHD environment on the electrodes and insulators.

Pre-test measurements were made to determine: thermal diffusivity, thermal

expansion, chemical composition, microstructure, electrical conductivity,

phase composition, closed and open porosity and pore size distribution. After

the test, the electrode walls were carefully packed and shipped back to the

National Bureau of Standards where they were disassembled in such a manner as

to allow extensive post-test materials characterization. The analysis work

was done by NBS , Battelle NW Laboratories, Westinghouse Research Laboratories

and U.S.S.R. Institute for High Temperature Research. These analyses include phase

composition and chemical reactions, electrical conductivity, metallography, and

electron microprobe analysis. In addition, radiograph and current lead-out

analyses were made on the electrode wall sub-modules. There was extensive

attack by the seed on the cathode wall resulting in chemical reactions and

phase changes of the electrode materials. There was also seed penetration

through cracks and pores in both insulator and electrode.

A comparative analysis has been made between the results of this test and

the results of previous tests made on materials in the U.S.S.R. U-02 facility.

Significant conclusions were made from the test results which will be of

great benefit to future tests in the joint US-USSR cooperative testing program

and to the overall program of MHD development for electrical power generation.

A final and important result of this first cooperative test between the U.S.

and U.S.S.R. is to show that a project of this size can be arranged, carried out

and assessed in the spirit of cooperation and mutual respect that has continued

throughout the entire cooperative program.
3

INTRODUCTION

Tliis report is the final document concerning the entire Phase I U-02

test carried out in Moscow between September 25 and October 8, 1975 according

to the work program agreed to by the U.S. and the U.S.S.R. sides. The work

is in accordance with the existing intergovernmental agreement signed on May 24, 1972, which describes the Program of Scientific and Technical Cooperation

between the U,S. and the U.S.S.R. in the field of developing and building

an open cycle MHD power station of commercial scale. One of the important

directions in the U.S. -U.S.S.R. cooperation in this area is the study of materials

The report includes pre- and post-test information on the materials and on the

electrode modules (anode and cathode) as well as data relating to the experimental

conditions during the test period.

The National Bureau of Standards, Westinghouse Research and Development

Laboratory, Battelle Northwest Laboratories and U.S.S.R. Institute for High Temper

atures participated in preparing various sections of this report. Individual

credits are given in text.

The report is divided generally into four chapters as designated in Appendix

75, Record of the Fourth Meeting (4-lM) of the U. S.-U. S. S.R. Ad Hoc Working Group

MHD Materials Development.

Chapter I - Description of the Test Facility and Operating Conditions During

the Test (U.S.S.R.).

Chapter II A. Module Design and Materials Preparation (U.S.).

B. Pre-test Materials Characterization and Analysis (U.S. results).*

1. Thermal Diffusivity

2. Thermal Expansion

3. Chemical Composition

4. Microstructure

5. Electrical Conductivity

6. Phase Composition

7. Porosity and Pore Size Distribution

C. Post-test Materials Characterization and Analysis (U.S. results).

1. Module Disassembly and Materials Distribution

2. Phase Composition and Chemical Reactions

3. Electrical Conductivity

4. Metallography and Electron Microprobe Anaysis

5. Radiograph and Current Lead-out Analysis

D. U.S.S.R. Results on Pre- and Post-test Materials Characterization

and Analysis

Chapter III - Comparative Analysis (U.S.S.R.)

Chapter IV - Conclusions (U.S. and U.S.S.R.)
4

CHAPTER I

Description of the Test Facility and Operating

Conditions During the Test

A. Description of the Test Facility (D. K. Burenkov, B. P. Telegin,

V. V. Kirillov, V. I. Zalkind, 0. B. Zamyslov, Yu. L. Dolinskii)

The experimental channel of the MHD generator consisted of the

nozzle section, working section and the diffuser section. At the entrance,

before the operating section and at the exit were located water-cooled

sections with a gas wall troweled with a coating of zirconia. The

working section of the MHD generator channel with a cross section of

64 X 260 mm^ and 490 mm long consisted of electrode and insulating

walls

.

The electrode walls consisted of 20 electrodes with platinum current

leads mounted in magnesia blocks (see Fig. A.l). The electrode wall was made of five units with four electrodes of the same material mounted in

each one. Each unit had its own copper water-cooled base. During the

test, the following groups of the electrode material were included,

listed here in the order in which they are located along the channel.

Group I - 88% Zr02 + 12% Y 2 O 3 Group II - 82% Zr02 + 18% Ce02

Group III - 50% Zr02 + 50% Ce02

Group IV - 25% Zr02 + 75% Ce02

Group V - 20% Zr02 + 78% Ce02 + 2% Ta205 .

The insulating walls were made of hexagonal ceramic modules made

with a MgO base. Each module was soldered or brazed to its copper base

which served to remove the heat from the ceramic and transmit it to the

cooling water, and the base, in turn, was attached to the metal wall

structure (Fig. A. 2).

The channel design provided for the possibility of measuring during

the experiment thermal fluxes into the components of the electrode and

insulating walls by means of calormetric measurements of the cooling

water. During the experiment, the temperature of the surface of the

fourth anode was measured by means of an optical pyrometer. The temperature of the various electrode components (Fig. A. 4.1, Chapter II) and of

the upper insulating walls was measured by means of thermocouples imbedded

in them. The plasma potential was measured by means of platinum probes

which protruded into the plasma stream and which were located on the
5

upper insulating wall (see Fig. A. 3). The potential of separate components

of the channel was also measured. The temperature of the combustion

products at the entrance into the channel was measured during the experiment by means of sodium spectral line reversal.

The program of joint experiment consisted of the following components:

- Preheating of the loop and the selection of the thermal operating

mode (1st phase)

Study of electrophysical characteristics of the channel (2nd

phase)

Duration of life test (3rd phase).

- Cooling off of the loop (4th phase)

The preheat of the structure was made according to the requirements

of the temperature increase rate (on the average no greater than 2.5 °C

per minute) and the schedule for preheating the structure during the 1st

phase was developed accordingly (see Fig. A. 4). Prior to the start-up

of the combustion chamber, the structure was preheated with air which

was electrically heated to 250 °C. The combustion chamber was then

started up to operate with the maximum oxidizer excess (a)which would

allow stable operation of the combustion chamber. Then, after a 30-

minute wait, the gradual preheat of the structure was continued by

bringing the combustion chamber to the level where a = 1 and with a subsequent increase of the degree of oxygen enrichment.

The operating mode of the loop for the duration tests of the electrode

walls was selected in order to achieve the temperature on the surface of

the fourth anode (measured by an optical pyrometer) in the range of 1700

to 1775 °C; the plasma temperature at the entrance of the channel should

at this point have been 2600 K.

The operating mode for the loop was established accordingly and

during the 2nd and 3rd phases of the experiment was maintained with

precision ±5%. The following basic parameters were maintained:

Combustion Product Temperature

Total Mass Flow Rate

Static Pressure

Potassium Content in the Plasma

Temperature of the Electrode Surface

2573 K

0.75 kg/sec

0.83 atm

1% (weight)

1710 °C optical pyrometer

The cooling down of the structure during the 4th phase of the

experiment corresponded to the reverse schedule for the preheat.
6

INSULATION

Figure A.l. Schematic Diagram of a Submodule Unit.
7

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10

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Schematic diagram of a submodule unit.

Schematic diagram of the insulating wall.

Location of thermocouples and probes in the uppt

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Graph of the unit heating.
11

B. Operating Modes for the Electrode Walls

B.l Characteristic Thermal Operating Mode for the Electrode Walls

As was noted earlier, the permissible rate of preheat of' the electrode

wall structure components was set on the average of 2.5 °C/min while the

maximal preheat rate for a short period during the start-up of the

combustion chamber would be between 50 and 80 °C/min. With this preheat

rate, it was supposed that the thermal stresses in the elements of the

structure would not produce destruction or cracking. The analysis of

the preheat rate was conducted precisely with this in mind.

The surface temperature of the electrode walls, after the operating

mode for duration tests was established, should have been between 1700

and 1775 °C which, along with the gradual preheat of the structure, was

the second purpose of establishing this operating mode. Figures B.1.1

to B.l. 6 show thermocouple readings taken during all four phases of the

test (the thermocouples were located in various electrodes). On all

these figures the duration of the experiment was counted from the moment

when the air preheater was turned on. Figure B.l. 7 shows the changes in

temperature of the electrode surface 2104, as measured by the optical

pyrometer. After the air preheat temperature has been raised by means

of an electrical heater, the combustion chamber was started up with the

maximum permissible rate of oxidizer excess. The average rate of temperature increase according to the readings from the thermocouples located

in the electrode modules was about 50 °C/min. While the facility was approaching a stable operating mode, a = 1, the rate of temperature

increase on the electrode modules was as follows: on the anode 2104

where the thermocouple #51 showed the highest temperature, the rate of

increase was on the average 1 °C/min; while on the anode 2517, where the

thermocouple #54 showed the lowest temperature, the rate of increase was

on the average 0.5 °C/min. Further increase of temperature occurred

during the gradual enrichment of air with oxygen, when the temperature

increase rate, measured with the thermocouples described above was about

3 °C/min. During the eleventh hour of the experiment, a fairly rapid

change in the parameters of the loop was noted, due to the interruption

in the operation of the oxygen tanks while they were being switched.

The maximum rates of temperature change registered by the thermocouples

were 110 °C/min; i.e., they were roughly twice as great as those noted

during the start-up of the combustion chamber. However, these changes

occurred while the temperature of the ceramics was higher; this appears

to lessen the hazard of thermal shock. Subsequently, during the actual

duration or life test, two more relatively abrupt temperature changes

occurred in the channel elements (20 '^C/min) due to temporary complications

in the operation of the facility loop (Figs. B.l. 3 and B.l. 5).

When the planned thermal parameters for the duration tests were reached and the electrodes were loaded with the appropriate currents, as per Figs. B.l. 2, B.l. 3, and B.l. 4,. the electrode temperatures as read
12

from thermocouples 3 millimeters from the electrode surface were recorded

by means of the Kaye 8000 data logging system.

The temperature on the anode 2101 (thermocouple 50-51) was 1180 to

1250 °C, on anode 2104 (thermocouples 52 and 53) it was 1100 to 1200 °C, on anode 2517 (thermocouples 54 and 55) it was 950 to 1200 °C. Thus, a certain small decrease in temperature of the electrodes was observed

along the channel (100 to 130 °C) . The temperature readings from the

thermocouples located in the hot area of the MgO insulators on the

cathode wall was between 980 and 880 °C.

As can be seen in Figures B.1.8 and B.1.9, the thermal fluxes on

the electrode wall increased gradually in the course of preheating and

then, with a few exceptions, were sufficiently stable or, at any rate,

changed within a fairly narrow range during the life tests. It would be

appropriate, however, to note the gradual reduction of thermal fluxes by

20 to 25% during the period between the 39th hour and the 111th hour of

the experiment. The average thermal fluxes during the period indicated

above on all the anode and cathode units are shown in Table B.1.1.

Figure B.1.7 shows changes in time of the surface temperature on

the anode 2104, measured by means of an optical micropyrometer OMP-43.

As can be seen in Fig. B.1.7, the temperature was 1710 °C, after

the steady operating mode was reached. During the 34th hour of the

experiments, the measurements ceased since the optical porthole was, by

this time, clogged with ionizing seed. Evidently, the last measurements

of the temperature also should not be taken into account, since they

were already distorted by the relatively cold accretions of K2 CO 3 within

the optical porthole. However, the accuracy- of measurements taken prior

to the formation of K2 CO 3 crust is not in doubt.

As can be seen in Table B.1.1, the thermal fluxes are not uniform

along the electrode walls; the smaller thermal fluxes occur where the

thermocouples and the probes are located. Evidently, part of the heat

is lost along the thermocouples and the probes. Also, it is possible

that the thermal fluxes are reduced due to the loss of air seal and the

leakage of cold air into the system.

Table B.1.1

2 2 Thermal Fluxes to the Electrode Units (10 kW/m )

21 22 23 24 25

Average Thermal

Flux to Anode Wall

1. 7-1.8 2. 1-2.3 1.45-1.55 2.15-2.25 1.34-1.4 1.75-1.86

11 12 13 14 15

Average Thermal Flux

to the Cathode Wall

2.3-2. 5 1.9-1.95 2. 2-2. 4 1.8-1.95 1. 8-2.0 2.16-2.0
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tfl

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cno

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zsoooe

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•

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19

CJ

o

Figure

B.1.7.

Changes

of

the

Surface

Temperature

of

the

Anode

2104

with

Time

as

Measured

with

an

Optical

Pyrometer.
watts/m

20

Figure

B.1.8.

Thermal

Flux

on

the

Anode

Wall

Submodule

Unit
13

SUBMODULE

Zi
22

The analysis of experimental and calculated thermal fluxes, as well

as the readings from the thermocouples placed inside the electrodes and

in the MgO units, made it possible to determine surface temperatures on

the rest of the electrode units. During this analysis, an evaluation

was made of the difference between the experimental and the calculated

thermal fluxes, due to heat losses on insulating walls and along the

thermocouples, probes and current lead-outs. These estimates made it

possible to establish that the temperature of electrode units of the

first cathode module exceeded the temperature of the first anode module

by 50 to 100 °C; thus, it was in the range between 1750 and 1800 °C.

Taking into consideration the reduction of thermal fluxes along the

channel, which agrees sufficiently well with the calculated data provided

by the Westinghouse Corporation, the temperature on the electrode units

was evenly reduced and on the fifth cathode unit was between 1650 and

1700 °C. Also of interest is the change of temperature on the surface

of the electrode when the current passes through the electrode.

As can be seen from Fig. B.1.2 during the 17th hour of the experiment,

when the electrodes were for the first tim.e under a load, the temperature

of electrode 2101 (at the depth of 3 millimeters) increased by 40 °C

with a current of 5.4 amperes, and at the beginning of the duration run

the temperature increased by 80 °C at 7.8 amperes; the Joule heating of

the composition V (anode 2517, thermocouple //54) was not very great: it was between 20 and 30 °C at I = 7.8 amp.

During the first 34 hours of the duration test, the temperatures on

the electrodes remained constant and the range of readings from the same thermocouples did not vary greatly in time, (as a rule no more than ± 20 °C,

if one does not count two or three deviations up to 50 °C) . However,

beginning with the 34th hour of the duration test (57th hour of the

experiment), a temperature increase of 20 to 40 °C in the course of one hour was shown on all thermocouple readings and, subsequently, a slow

increase in temperature of the anode wall elements by some 20 to 40 °C

during the next 12 hours, upon which the electrode temperatures had once again stabilized.

B.2 Characteristic Electrical Parameters During the Life Test

The duration tests of the electrodes were conducted in accordance

with the working program of tests which provided for the operation of

electrodes of each material group at three levels of current density on

the electrodes: 0.24-0.33 A/cm~, 0.5-0. 7 A/cm^, and 1.2 A/cm^. The

operation of electrodes under current was attained by loading them with

active resistance with an electrical load coefficient K = 0.2 to 0.5;

K = 0 (short circuit operating mode) and K = -0.2 to -0.3 (stagnation

operating mode with additional current to electrode pairs from separate

external voltage sources) . In. addition, each material group had one pair operating at K = 1 (open circuit)

.
23

Table B.2.1 shows the values of average current and deviations from

average currents for different pairs of electrodes during the life test.

As can be seen from Table B.2.1, the electrode couples numbered 9

and 17 (50% Zr02 and 50% Ce02 and 18% Zr02 plus 80% Ce02 plus 2% Ta205 , i.e. , compositions 3 and 5) had currents at 8.5A and 8.3A (1.20 and 1.18

A/ cm“) , respectively ; the electrode pairs numbers 2, 10, 14 and 18

(compositions 1, 3, 4, and 5) had currents from 3A to 4.8A (0.43 to 0.68

A/cm^); electrode couples numbers 3, 6, 11, and 19 (1, 2, 3, and 4

compositions) had currents of 1.6A to 2.8A (0.23 A/cm^ to 0.31 A/cm^). The

deviations from the average values did not exceed 15 to 18%.

The fourth electrode in each electrode group was tested in the

absence of current, i.e., while operating in an idling mode. Figures

B.2.1, B.2.2, and B.2.3 show the time dependence of anode-cathode potential

difference in electrode pairs tested with various loading regimes. A

significant change of the anode-cathode voltage for electrode pairs

operating in a stagnation mode was due to the necessity of maintaining

current values dictated by the test program schedule. Figure B.2.2

shows the change of the total acting voltage (VgQurce + d x B*Y; i.e.,

the sum of voltage from the sources and induced voltage) . As can be seen from Fig. B.2.2, the size of the operating voltage fluctuated within ±15%.

The applied voltages ranged from 50 to 10 volts; i.e., they were no more

than 20% of the effective -voltage. The indicated current fluctuations

and effective voltages in the range ±15% could quite easily be explained

by random changes in the thermal parameters of the loop, which, as was provided by the operating schedule, were maintained within ±5% range.

However, in addition to the random fluctuations mentioned above, a

gradual, if not very large, reduction of the applied voltage necessary

for maintaining the currents at the assigned level (from 30 to 60 volts

down to 10 to 20 volts) was observed during the entire test. Such a change could have little connection with the subsequently described

anode-cathode resistance drop (current leaks could have accounted for

about 0.5A by the end of the duration test); to a great extent this

could have been caused by the drop of the effective internal resistance

of the channel, due to the increase during the second half of the test

of the surface temperature on the anode wall (and judging by the decrease

of thermal fluxes, a possible increase in the cathode wall temperature

as well) , which led to a reduction of electrical resistivity of the near electrode boundary layers of the plasma.

Figure B.2.3 shows the changes in time of the voltage values for 4,

5, 12, 13, and 20 pair of electrodes, operating in an open circuit mode.

As was to be expected, these values were smaller than those theoretically

calculated. Such difference is due to the fact that next to the indicated

pairs there were pairs operating in other modes (for instance, stagnation

mode) . The abrupt decrease in the open circuit voltage of the fourth

and fifth pairs of electrodes after 10 hours of duration tests can,

evidently, be explained by the fact that the insulating characteristics

of the nozzle section have decreased. This section was troweled with

cement and was located upstream from the working section.
Average

Current

Values

and

Deviation

of

Current

on

the

Electrode

Pairs

24

cn01

E-tco

CO C

&0

•H

(5

00

ro 0) T)OU

«JO0) O

C*-;

cn

COO

CN<3-

u-1 CO

CO

CO

'cr0) ao

COo

CMo

CO

•<r

COO

MO

CM

00

CM

r-' CD

CM

un

CO

00o

<vao

CO

i-i
25

Figures B.2.4 and B.2.5 show the changes in the currents on the

electrode pairs of various material groups in the course of duration

tests. The duration test analysis shows that the eleptrical parameters

were basically stable and were within the set limits, while the small

deviations observed had a definite qualitative explanation. Thus, the

electrical and thermal characteristics of the tested electrodes showed

them capable of adequate performance on all the electrode pairs, with

the exception of those which suffered destruction of the platinum current

lead-outs

B.3 Electrical Characteristics (before and after the Life Test)

The study of the electrical characteristics of the MHD generator

channel was conducted both before the life tests and after them. The

main purpose of the test was a comparison of the electrical characteristics

of the electrodes made of different materials, as well as the study of

the characteristics of the channel as a whole, which made it possible to

judge the state of the channel insulation.

Tables B.3.1 and B.3. 2 show the short circuit current values and

the voltage of the open circuit operating mode before and after the

test

As can be seen from the Tables B.3.1 and B.3. 2, the voltage during

the open circuit operation before the life test was 155 to 160 volts,

the short circuit current from a pair of electrodes from 3 to 4A, while

Hall voltage along the operating section was on the order of 90 to 100

volts (B = 1.7 T)

The main purpose of studying the channel wall characteristics was

to determine the ratio of the possible current leaks along the insulation

of the electrode and insulating walls. The study included measurements

of the currents through electrode pairs in different operating modes

during simultaneous measurements of the potentials of various elements

of the design, various subassemblies of the structure (the potentials of

the nozzle section, diffuser, electrodes and potential probes located in

various parts of the upper insulation wall) in order to determine the

plasma potential. The measured potential distributions made it possible

to determine the values of the voltages in the electric field of the

plasma core and along the electrode walls. The comparison of the results

in order to evaluate the insulation characteristics of the working

section are best done utilizing the indicated values of the voltage

where

E

X

X experimental

E

X theoretical
+

3rd

ELECTRODE

PAIR o

6th

ELECTRODE

PAIR

26

CT3

CO

CD

LO

CD

oo

CO

CO

CO

CO

LD

LO

CD

CO

CO

CO

OJ

ooo

Figure

B.2.1.

Changes

in

the

Anode-Cathode

Potential

Difference

with

Time

for

Electrode

Pairs

Tested

with

an

Electrical

Load

Coefficient
27

I—o

Figure

B.2.2.

Changes

in

the

Anode-Cathode

Potential

Difference

with

Time

for

Electrode

Pairs

Tested

with

an

Electrical

Load

Coefficient
28

CO

CD

LD

C73

CO

on

I

CO

CO

LO

LSD

CT5 CO

CO

CO

CSI

CO

o

Figure

B.2.3.

Changes

in

the

Anode-Cathode

Potential

Difference

with

Time

for

Electrode

Pairs

Tested

in

Open

Circuit

Mode.
3rd

ELECTRODE

PAIR

29

ro O

(N

><I

(N OW

tsi

CX) 'Xp

urp

•rJ t,o

4J

U-1o

1-1 •H

03 cx

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X

a c -P

03

a

X-

(Na

(Ud

cn

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(82

ZrO18

CeO )

Material

Groups

During

the
9th

ELECTRODE

PAIR

30

A

A

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Voltage

for

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on

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Pairs

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f-H

m <r «n > m pH O

00

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pH

r>. pH r>» 1 1 > lo r>*. I 1 pH

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O 1 1 m CO 1 1 pH

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pH •H

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pH

CN

•H

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m o > vO 00 m 00

pH pH

iH o -y CM in > vO GO CM P*^ pH tH OpH 00 ^ CO CM > m 00 pH P«n pH pH

c^ 00 r-i CM ON > m 00 pH vO

pH pH

00 in cvj 1 1 > m 00 1 1 pH

CN CN 1 1 > m GO 1 1 ^Ho o pH > m 00 Os m

pHm

O ST > >4* 00 m

pH

<r vO cn > m in pH

CM pH On cm > m r>. m <r

^H

CN

O

m 00 nO > CM sC >T m

pH

cn vO \0 vO > o m CM pH

pH

00 > m lO ON

03 00 CM

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03 in in

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m 00

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d pH o

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m P-* 'J’
32

where experimental definite experiment value of the Hall field

voltage and where theoretical is th® value of the Hall field

which would exist at the given average density of the transverse current

in a case where the Hall current equals 0; i.e., in the absence of Hall

current leaks along the insulation. The results shown in Table B.3.3

indicate that the average value of the indicated Hall field before and

after duration tests differs insignificantly (roughly by 10-15%) which

shows that the insulation preserves its resistance on the electrode

walls (in the axial direction) in fairly good condition for the entire

duration of the life tests. In the experiments with a special grounded

combustion chamber after the life tests, we have noted a relatively larg(

decrease of the Hall current in the operating portion of the channel.

Evidently, this is connected with certain deterioration of the insulatior

level between the working section of the channel and the nozzle and

diffuser sections. The measurements of the insulation resistance betweer

the given elements testify to this fact. Figures B.3.1 and B.3.2 show

the distribution of the potential along the channel in an open circuit

mode for several magnetic fields at the beginning and at the end of a run. The average current densities of transverse current in the channel

are

29

J

1 , av

where is the current through electrode pair where S = h x L (h is the

height of the channel and L is the length) and these average currents

are accordingly equal to 0.22 A/cm^ and 0.17 A/cm^. The uniformity of

the Hall field along the channel indicates the absence of local short

circuits between the electrodes and the ground and between the electrodes.

The program to study the resistance of insulation included:

1. The determination of the effective resistance of insulation

between the cathode and the anode of different electrode pairs

(on the basis of experience with short circuit and open circuit

operation)

2. The measurements of electrode resistance to the adjacent

elements, insulation of interelectrode spaces, insulation

between electrodes and the water-cooled units and insulation

of the electrodes in relationship to ground.
33

O

Figure

B.3.1.

Distribution

of

Potentials

Along

the

Channel
34

O

LO O LT3

LU

C_D

<

h—

Si2

a

c

Figure

B.3.2.

Distribution

of

Potentials

Across

the

Channel

in

a

Short

Circuit

Operating

Mode.
35

Table B.3.3

Relative Values of Hall Voltages at the Channel in

Different Operating Modes

Time B (tesla) hi

T7 X (core ) (anode)

E x(cathode) Notes

hrs

.

min

.

19 50 1.65 57 0.95 1.0 1.0

27 0.9 1.0 0.9

14 1.0 1.0 1.0

7 0.85 0.85 0.85

0 0.87 0.77 0.77

20 50 0.8 0 1.0 0.93

21 10 1.65 0 0.67 0.67 \.c. To ground I-llA

21 15 1.3 0 0.69 0.69 \.c. To ground I=6A

21 20 1.35 0 1.0 0.9 0.9

115 45 1.55 0 0.9 0.75 0.75

115 50 0.84 0 0.73 0.73 0.73

116 10 1.55 0 0.47 0.3 hc.c.

To ground I=15A

116 20 0.84 0.53 0.3 “•3

"l.c.

To ground 1=4. 2A
36

On the basis of the measured voltages of the open circuit mode and

the currents of the short circuit, the anode-cathode resistances of all

electrode pairs have been determined (Table B.3.4). On the first day of

the run, operating with ionizing seed (2nd phase) prior to the beginning

of the life tests, the resistance of insulation between anode and cathode

fluctuated within the range of 100 to 500 ohms. On the first three and

last two electrode pairs, it varied between 60 and 180 ohms, on pairs 4 and 5 it was between 175 and 280 ohms, and on the 6 through 18 pairs, it was between 300 and 500 ohms.

After the duration tests had been conducted, the second set of

experiments was conducted with the same operating mode in order to

determine the cathode-anode resistance of insulation. These experiments

have shown that the overall level of insulation of all the electrode

pairs has decreased. The electrode pairs from:

1 to 5 pair - down 16 to 80 ohms;

9 to 14 pair - down 100 to 150 ohms; and

18 to 20 pair - down 60 to 100 ohms.

Thus, the reduction of insulation resistance was in the range of 30

to 40%. The electrical characteristics can also help to judge the

effective resistance of the channel in the transverse direction. The

study of electrode operation in the open circuit and the short circuit

mode prior to the duration tests has shown that the open circuit mode

voltage differs from the theoretical value by 10% at the 6th to 17th

electrode pairs and by 30% at the electrode pairs 1 to 5 and 18 to 20.

During the run, the open circuit mode voltage had changed on the

9th to 14th electrode pairs by 30%, on the first six pairs it diminished

two-fold (at the 2 to 6 pairs)

The reduction of the open circuit mode voltage on the electrode

pairs at the end of the channel is, evidently, due to the reduction in

the insulation of the nozzle and diffuser sections, troweled with concrete

and adjacent to the operating section.

Direct measurements of the insulation level between the elements

along the channel have shown that the resistance of the interelectrode

space on the first day of the run, when operating without ionizing seed

with Gq = 200 g/s, along the cathode side 400-600 ohms and along the

2 anode side 600-1000 ohms.
Effective

Anode -

Cathode

Resistance

37 o CN vO CO CM CO rs. pH I oe: •H 1

CO lO >3“ O

00 ^ r««. rs. cc yO

CO vO o 00

CO CO 00

cd CN CN

p>. o o ON CN 1 1 Qi CO CO 1 1

yO CN pH 1 1 CH CO 1 1

u-|

O \ 1 i 1 yO 1 1 un

'J* CO lA UO pH

CN o UO

Cri >3* pH CN

cn uo

O UO vj* xO O UO r*^ oc ^ xO pH CN

CM CO o Qy CN

CO o xO <•

oc

m r>» pH CN

fH

fH uO

O P«-. 1—

O O UO O'

Od lO xo pH pH pH o o o CN ^

>TO CN CO

p:: <r tn pH pH »H

^3*

<T> o m « cc UO pH \0 ^ CN vO

00 <r o

a: 00 o 1 1 vy 1—

i 1 pH o o

CEi

O yO 1 1 CN 1 1 pH

yO CN <r COO

0^ xO

O P'S. <j\ CO y£>

m CN uO -HO

00 CO •<T UO

CN <r

CO pH CM P^

0^ yO 00 ^

pH CN CN

UO cn UO o P"» ON

0^ CN • pH pH CN

CO CM CO • fiC O pH xD pH ^ pH pH CN

sO CN • • Qd CM <r i 1 xO sO i i

fC

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S_/ ^ CD pH

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pa . . U3 CO cf U

C

u JZ "p^ (U j= a a a UO

ON

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O

—^ UO ^ UO
38

The measurements of insulation in the middle of the run gave the

following results; along the cathode side the resistance was 20 to 60

ohms while along the anode side it was 80 to 150 ohms. After the duration

tests had been conducted, the resistance of insulation of the interelectrode space on the cathode wall was between 100 and 300 ohms (with

the exception of the spaces between the electrodes 1207-1208, 1208-1309,

1312-1413, and 1413-1414, where the resistance was 10 to 30 ohms); which

along the anode side the resistance of the insulation of the interelectrode

spaces was 50 to 100 ohms. Direct measurements of resistivity of the

insulation between the electrodes and the water-cooled units showed that

the insulation level on the anode wall was fairly high during the entire

run and was on the order of several kilo-ohms and up, and on the cathode

wall at the beginning of the run, it was about 5 to 10 kilo-ohms, but by

the end of the run it was reduced to several tens of ohms (see Table

B.3.5). This was possibly due to the penetration of potassium compounds

into the walls during tests under current.

The estimate of possible current leaks to the insulation shows that

when the voltage level from external sources is '^50 volts and the anodecathode resistance is on the order of 100 ohms, the current losses in

the transverse direction by the end of the run could reach 0.5A. With a current level at most pairs from 3 to 8A; thus, the losses through

insulation leaks were from 5 to 15%. The currents of the leaks in the

axial (Hall) direction before and after the test did not exceed 0.1 to

0.2A. Relatively large current leaks of about lA could develop along

the cathode wall through the copper base within each block, however,

with the Hall current at 10 to 13A they were relatively small.

Thus, the entire set of tests of the electrical characteristics of

the channel and of the direct insulation measurement has shown that

during the experiments, the resistivity of insulation of both the insulating and electrode walls was sufficiently high and there were no conspicuous

current leaks between the segments of the whole structure.

The measured values of the near-electrode voltage drops, when the

current in the electrode pair is 3A was 40 to 50 volts with anode and 10

to 20 volts on the cathode. The near electrode drops were determined on

the basis of the difference between the potential at the current leadouts and the potential voltage probe, located 2.5 cm from the gas surface

of the electrode. This difference of the potential included the voltage

drop in the electrode body itself, the voltage drop in the area of the

space charge above the electrode and the voltage drop in the gasodynamic

boundary layer.

The drop of voltage at the electrodes measured as a difference of

the potential between probes located in the electrode and the platinum

lead-out was not very great (0.7 to IV) with a current of 4 to 8A and

represented a small share of the total voltage drop. This testifies to

the relatively small possible Joule heat of the electrodes. The voltage
Table

B.3.5

Electrode

Resistance

on

Water-Cooled

Electrode

Units

in

Ohms

m

CNI . CN CN

<r

CNl 8 8

m

CN

r*) <r <r

CN

CN o

m 00

• • CNJ

1

1

j

1

]

1 1-H

1 rH 1 1 • • O') o

rH rH X

CO 1

1 1 • CN 00 o

CTs <d" x>

COO

rH CN 1 00 X

• o X

oc rH CN 1—! rHo

CO o

<d* X

<r 1 IT) 1

1 o CO o

a^ CO X

m

LO

•

1 r--

1 rH 1 X

LO rH 00 •<r o

CN o

CO m rH

1 X

1

1 1 o

1 VsO

, 00 vO 1—

CN rH r—i

O O uO X

i-H CO LO

^£1 X! sD X
40

drop within the electrode

moment when the platinum

electrical resistance of

''^e.s stable and ac; a currant- 1 a

’ ^he urrent lead-out was destroyed, showed the electrode body

very

a stable
List of Figures

Figure B.l

Figure B.l

Figure B.l

Figure B.l

Figure B.l

Figure B.l

Figure B.l

Figure B . 1

Figure B.l

Figure B .

Figure B.2

Figure B .

Figure B.2

Figure B .

Figure B.3

Figure B .

1. Changes of electrode temperature with time (thermocouple

readings)

2. Changes of electrode temperature with time (thermocouple

readings)

3. Changes of electrode temperature with time (thermocouple

readings)

4. Changes of electrode temperature with time (thermocouple

readings)

5. Changes of electrode temperature with time (thermocouple

readings)

6 . Changes of electrode temperature with time (thermocouple

readings)

7. Changes of the surface temperature of the anode 2104 with

time as measured with an optical pyrometer.

8 . Thermal flux on the anode wall submodule units.

9. Thermal flux on the cathode wall submodule units.

1. Changes in the anode-cathode potential difference with

time for electrode pairs tested with the electrical load

coefficient of 0 . 2- 0 . 5.

2. Changes in the anode-cathode potential difference with time

for electrode pairs tested with an electrical load coefficient

of -0.2 to -0.3.

3. Changes in the anode-cathode potential difference with time

for electrode pairs tested in open circuit mode.

4. Current changes on electrode pairs of the first ( 88Zr02 - 12Y 203 ) and second ( 82 Zr 02 - 18Ce02 ) material groups during the life

test

5. Current changes on electrode pairs of the third (50Zr02-50Ce02)

,

fourth (25Zr02-75Ce02) and Fifth ( 20 Zr 02 - 78Ce02 - 2 Ta 205 ) material

groups during the life test.

1. Distribution of potentials along the channel.

2. Distribution of potentials across the channel in a short

circuit operating mode.
42

CHAPTER II

Module Description, Analytical Procedures,

and Results of Analysis

A. Module Design and Preparation of Materials

A.l Overview of design considerations (B. Wright, Westinghouse

Research and Development Laboratory)

The known mechanical constraints on the design are ones of

physical size and mechanical capability with the existing components of

the Soviet U-02 MHD facility. The overall dimensions of a single electrode

wall are length, 491 mm; height, 100 mm; width (at hot wall surface),

63-64 mm; width (at channel exterior) , 112 mm. The electrode wall design

has no limitations as to the precise electrode configuration. The short

channel length implies a constant cross-sectional flow area. Thus, a segmented electrode Faraday design is the most practical, although not

the most desirable, from the viewpoint of power generation from an MHD

generator. To design the electrode wall, the U-02 MHD plant operating

characteristics must be known. These specifications have been transmitted

via reference [ 1 ].

The concept "cold” or "hot" as it applies to MHD channel design

is rather conventional. The term cold wall refers to one whose gas side

surface is made of strongly cooled metal parts or modules, fastened in one way or another to a supporting plate or beam. The term hot, wall refers

to one whose gas side surface is made of ceramic parts fastened to watercooled metal parts or modules, which in turn, are fastened to a supporting

beam or plate. The objectives of the U.S.-U. S.S.R. cooperative materials

program necessitate a hot wall design.

Plasma property values needed for the design of the generator

come from typical working conditions of the Soviet U-02 MHD facility.

The fuel is natural gas containing methane 97% by volume, ethane 2% by

volume, and Inerts 1% by volume. Combustion occurs with an oxidant

which is moist air with 1 % moisture by weight and enriched with oxygen

to the extent of 40% mole concentration in dry air. Seeding is accomplished

by a K2 CO 3 solution having 50% by weight K 2 CO 3 and 50% by weight water. At

the entrance to the channel the desired plasma conditions are:

Temperature

Static pressure

Potassium by weight

Mass flow

2600 K

0.80 atm

1 %

0.75 kg/

The internal dimensional envelope of the channel as given by

the Soviets specifies a height of 0.260 m and a width of 0.064 m yielding

a constant cross-sectional flow area of 0.0166 m^ . For practical purposes

the velocity in the channel can be considered a constant. With stoichiometric

combustion, the plasma has an average density of 0.1 kg/m. Thus, from
43

continuity of mass, the average channel velocity is 450 m/s. The

magnetic field has a strength of 1.5 T which is constant the length of

the channel. The average plasma conductivity and mobility are 10 mhos/m

and 0.7 m^/Wb, respectively. The Hall parameter has a numerical value

near unity.

A. 2 Preparation of materials (B. Rossing, Westinghouse Research

and Development Laboratory)

The requirements of good resistance to both thermal stress damage

and seed attack demand, in turn, careful control of microstructure

and microchemistry for both electrodes and insulators. Thermal stress

damage resistance can be increased by the presence of such crack arresting

microstructural features as pores, microcracks, and second phases. In

single phase materials, this can be accomplished by use of a wide variation

in grain structure and a moderate level of porosity. Seed resistance

is achieved by low and/or closed porosity and low levels of impurities

(absence of secondary phases at grain boundaries) . Both electrodes and

insulators are thus fabricated by making dense high porosity grain, followed

by sizing, pressing and refiring. The fabrication details for both

electrodes and insulators are detailed in the following discussion.

The processing scheme for producing the ceramic electrodes is

shown in Figure A. 2.1. High purity, reactive powders are used as the

starting materials. These materials are purchased from the following

commercial sources: Zr 02 from Engine Kuhlmann, Paris France; Ce02 and

Y 2 O 3 from Research Chemicals, Phoenix, Arizona; and Ta205 , Kaweki Beryelco,

Bayertown, Pennsylvania. All powders are claimed to be greater than

99.9% purity with less than 100 ppm of either Fe 203 or Si 02 . The purity

of the three major powders has been checked by emission spectroscopic

analysis; these results are shown in Table A. 2.1. These results are accurate within a factor of 3. The particle size distribution has also

been determined by the centrifugal instrumentation method. The particle

size distribution of the four starting powders is shown in Figure A. 2. 2.

The average particle size (by weight) of the Zr02 , Y 2 O 3 , Ce02 , and Ta205

powders are 1.5, 4.5, 6 , and 11 micrometers, respectively.

A slurry is prepared consisting of distilled water, 1200 gm

of oxide powder and organic binder (a mixture of polyvinyl alcohol and

polyethylene glycol) . This slurry is milled in a rubber lined ball mill

filled with yttria-stabilized zirconia balls. Finally, this slurry is

introduced into a spray drier to form a free flowing, homogeneous powder.

These powders are, in turn, pressed at 30,000 psi into 300 to 400 gm

bars. These bars were subsequently fired for 14 hours at 1500°C.

Pycnometer densities using -30 +70 mesh particles gave the calculated

percent theoretical densities shown in Table A. 2.2.

These sintered bars are crushed into fine particles using a laboratory

crusher. Any iron contamination is removed by magnetic separation and
44

Table A. 2.1

Optical Emission Spectrographic Analysis of Starting Powders (ppmw)

Research Chemical Engine Kuhlmann Research Chemical

Element 99. 9 Ce02 'ver' grade Zr 02 99 .9 Y

2 O

3

A1 30 100 10

Bi * *

Ca 10 *

Cd * •k

Co * *

Cr * * *

Cu * * *

Fe * 1 1

Mg 10 10 * Mn * * * Na 100 * * Ni 20 * *

Pb * *

Sb * * *

Si 100 20 10

Sn * * * Ti * JU *

Zn * * *

*Not detected , detection. limits for Na, 100 ppm, for Cd , Sb , Sn, Tl, and Zn,

30 ppm, for Bi, Ca, Co, Ni, Pb , 3 ppm, and for all others 1 ppm .

Composition

12 m/o - 88 m/o

18 m/o Ce02 - 82 m/o

50 m/o Ce02 - 50 m/o

75 m/o Ce02 - 25 m/o

78 m/o Ce02 - 20 m/o

Table A. 2.

Zr02

Zr02

Zr02

Zr02

Zr02 -

% of Theoretical Density

91.7

97.9

94.8

93.4

2 m/o Ta 96.7 20 ^
45

leaching with acid solutions. A check of iron content by emission spectroscopy at this point indicates iron levels of 30 ppm or less. The particles

are subsequently sized by screening into three size fractions, a coarse

(-30 +70 mesh) fraction, a medium (-70 +325 mesh) fraction, and a fine

(-325 mesh) fraction. Additional fine (-325 mesh) material is produced by jet

milling coarse grain material. The average particle sizes for the crushed

-325 mesh and the jet milled powders are 15 and 6 micrometers, respectively.

The three size fractions are mixed together in the ratio of 52 coarse to

15 medium to 33 fine. Polyethylene glycol is used as an organic binder

in pressing this granular mixture. Samples 14 mm x 35 mm x 8 mm are pressed at 15,000 psi on a double action press. These samples are fired

at 1600° C in air. Linear shrinkages during this heat treatment are less

than 2.0%. The porosity of the five electrode compositions varies from

14.6 to 19.9%. The pore size distribution for these materials is sho\m

in Figure A. 2. 3. Finally, the electrode is machined to the desired

dimensions using diamond grinding tools.

The electrodes are metallized using a commercial (Engelhard

# 6082) platinum paste. Four individual thin coats are brushed on and

fired. During the application and firing of the last three coats, a platinum screen is also attached to the metallized surface. Finally, a platinum (0.020") wire is welded to the platinum screen.

The magnesia insulators are purchased from the Norton Company

of Worchester, Massachusetts. A high purity fused grain of magnesia is

sized and hot pressed to produce a body containing approximately 15%

porosity. The processing parameters in producing this material are regarded as proprietary by the manufacturer. The hot-pressed blocks

which were 152 mm x 152 mm x 36.5 mm are heated in air to remove any

graphite pickup and then cut into blocks 1-2 mm larger than the desired

insulator dimensions. Finally, these blocks were machined into the

desired insulator dimensions and shapes.

A. 3 Electrode wall design (B. Wright, Westinghouse Research and

Development Laboratory)

The MHD channel is the Faraday type. In each of the electrode

walls, there are twenty separate electrodes with the pitch between

electrodes being 24 mm. The electrodes proper are 12 mm long and 62 mm

wide. Each electrode is individually housed in an insulator made of

magnesium oxide. Channel insulating walls which separate the two electrode

walls are designed and constructed by the Institute of High Temperature of

the U.S.S.R. Academy of Sciences. The insulating wall width determines the

distance between electrode walls. Figures A. 3.1 through A. 3.5 illustrate

the construction of the channel.

One effect of finite segmenting in the generator is to reduce

the value of the scalar conductivity and Hall parameter. The conductivity

and Hall parameter are reduced to apparent values of 9.6 mhos/m and 0.95,

respectively. Each pair of electrodes is connected to a separate load.
46

Under open circuit conditions (K=l) Ey' =0, Ey = uB = 675 V/m, and jy = 0,

while for the axial fields E^^' = E^^ = -BEy' = 0 and = 0. For short

circuit conditions (K=0) Ey ' = uB = 675 V/m, Ey = 0 and jy = oEy' = 6413 A/m^

,

and the axial fields are E^' = E^ = -BEy' = -641 V/m. Thus, the current

per electrode under short circuit conditions is 4.77 A, while the onen circuit voltage is 175 V. For a Hall field of 641 V/m, the voltage

between adjacent electrodes would be 15 V.

Previous studies [3,4] offer a theoretical explanation and model

of interelectrode breakdown in MHD generators. It has been shown that

interelectrode voltages in excess of a certain critical value cannot be

sustained by the plasma over the insulator. Instead, an electrical breakdown occurs in which the local Hall voltage across the insulator segments

decreases suddenly and a large axial current flows over the insulator. In

combustion drive MHD generators operating at pressures near atmospheric

with electrode segments of 1 cm length scale, the critical breakdown

voltage is of the order of 40-100 V [5]. Electrode segmentation should

be sufficient to prevent the critical breakdown voltage from being exceeded.

Arcing processes may cause significant short-circuiting of the channel, or

they can result in unacceptable damage to the channel walls. In the present

design with electrode segments of 1.2 cm, 15 volts between adjacent

electrodes is more than adequate for the prevention of arcing.

To isolate the electrodes from the channel support structure,

a NEMA G-lOh phenolic is used for the mounting block material. In

addition, the heat sink of each electrode wall is subdivided into five

individual segments. Each heat sink is a water-cooled copper block

which has been coated with an electrical insulating paint called

LIMITRAK. All exterior surfaces of the heat sink are painted except where

the magnesium oxide joins to the heat sink. Each heat sink attaches to

the single piece mounting block by two bolts.

A constant nuisance to a majority of designs is coolant water

leaks. Such leaks normally occur at joints or seals in the cooling system.

To minimize potential water leaks, the number of joints or seals should

be held to an absolute minimum. Thus, the leak-tightness of the cooling

system is an important construction feature. Each heat sink has only

one water inlet and outlet. Internally, the cooling water channels and

manifolds are drilled, access holes plugged with copper plugs and then

brazed in place in a vacuum furnace. After this operation, the integrity

of the joint is checked by a helium leak test using a Model 925 NRC

Mass Spectrometer Leak Detector. A calibration source establishes a sensitivity to helium leaks of 2 x 10~^^ cc/s. At this sensitivity, no detectable leaks were found in any of the heat sinks.

The cooling water passages are designed for laminar flow heat

transfer from the copper blocks to the cooling water. The design

equation used is
47

Q

= ttDL - mC 4T

P

( 1 )

where the water passage wall temperature is assumed to be 373 K and the

effective water passage length is 60 mm. Figure A. 3. 6 for total cooling

water flow into each heat sink illustrates the small water temperature

rise expected.

The thickness of the electrode is a function of the thermal

conductivity of the material, the heat flux and the desired face and

back surface temperatures of the electrode. Each of the five electrode

materials has a different thermal conductivity. All the materials have

thermal conductivities which are a function of temperature. An electrode

face temperature of 2073 K is used for design. Temperatures higher than

2073 K result in more rapid vaporization of the stabilizing materials

in the electrodes. Temperatures lower than 2073 K result in a more

resistive electrode. Because all 20 electrodes have the same thickness,

the thickness per se is determined by the least conductive of the electrode

materials. This material, 82 m/o Zr02 and 18 m/o Ce02, could not operate

as an electrode when any part of it has a temperature below 1472 K,

that is, at a conductivity less than 10~^ mho/cm. Depending on the method

used for electrical contact to the ceramic electrodes, a maximum back

face temperature may become a design consideration. For example, a ceramic- to-metal contact should not be operated at a temperature that

enhances oxidation of the metal. The electrical contact in the present

design is made with pla'tinum which has extremely good oxidation resistance

and a melting point of 2046 K. Thus, there are two primary design

constraints which are: (1) an electrode must be of low overall resistance,

on the order of one ohm or less; and (2) the mechanism for current take-off

from the electrode must function under a changing, severe environment and

still retain a long lifetime.

Energy transfer to the electrode wall is by convection and

radiation heat transfer from the plasma. Internally, there is joule

heating in the electrode. Heat transfer from the plasma to the electrode

is mostly by turbulent convection. The correlation for the Nusselt

number of flow along a flat plate is

Nu

X

0.0296 Re Pr^^^

X

( 2 )

while the Stanton number is

St = Nu /Re Pr . X XX ( 3 )
48

Convective heat flux through the electrode is given by

(q/A)^ St pu(h - h ) . X aw w

(4)

Boundary layer development begins 0.8 m upstream of the entrance to the

channel. The range of numerical values for the local Stanton number from

the entrance to the exit of the channel is 0.0025 to 0.0020. Convective

heat flux varies from 22 W/cm^ at the channel entrance to 18 W/cm^ at

the channel exit. Radiation heat transfer from the plasma to the electrode

is on the order of 2 W/cm^. Joule heating is difficult to calculate

because it requires knowing the three-dimensional temperature distribution

and current paths within the electrode. An estimate of the joule heating

is made by assuming current flows normal to and uniformly through the

electrode. Dividing the electrode thickness into equal segments, the

one-dimensional, centerline, midpoint temperature of each segment is

calculated. Knowing the one dimensional temperature distribution, the

resistance of each segment is determined. The electrode resistance becomes

the sum of the resistances of the individual segments. For example, using

an electrode thickness of 6 mm, the most resistive material, 82 m/o Zr02

and 18 m/o CeOo, has an electrode resistance on the order of 2 ohms when

calculated by the above procedure. Under short circuit conditions,

joule heating would contribute 6 w/cm^ to the heat flux. The value of

6 W/cm^ for joule heating is a severe case because it is a maximum' value

Thus, the largest expected value of heat flux is 30 W/cm^ and occurs at

the channel entrance. The smallest expected value of heat flux is 18 W/cm^

and occurs at the channel exit, assuming negligible radiation heat

transfer from the plasma and no joule heating.

Near the channel entrance, the electrode face temperature is

2073 K, the electrode back surface temperature is 1500 K and the temperature

at the interface between the Zr02 phosphate cement and MgO insulator

housing is 1200 K. These temperatures are based on a one-dimensional

analysis with a heat flux of 22.5 W/cm^, an electrode thickness of 6 mm,

and an electrode material of pure Zr02. Deviations from these temperatures

will occur due to electrode position with respect to downstream distance,

electrode material, current density in the electrode, and two-dimensional

effects. To minimize deviations from these temperatures, electrode materials

are positioned longitudinally in the channel beginning with the material

of lowest thermal conductivity and following in order of increasing thermal

conductivity. This order of materials is also one of decreasing electrical

resistance and thermal shock resistance. Near the channel exit, the electrode

face temperatures are 1900 K, the back surface temperatures, 1400 K, and

the interface temperatures between the MgO and Zr02 phosphate cement, 1130 K.

See Figure A. 3 . 7
49

Each copper water-cooled heat sink has attached to it four

electrode- insulator assemblies. A low thermal resistance attachment

between the copper heat sinks and MgO insulators is made by brazing the

insulator to the heat sink with a high temperature Ag-Cu-In alloy.

There inevitably appear locally very large electric fields at the

electrode-insulator boundary to maintain the Hall field in the flow,

which are apt to cause arc discharges due to large Hall effects causing

various undesirable phenomena referred to above. Therefore, in order to

realize more uniform potential distributions on the electrode walls

similar to that in the main flow, the power take off is made at the

extreme edge of the resistive segmented electrode opposite the edge

where the current usually concentrates. This should generate counter

electric fields which will cancel induced Hall fields in the vicinity of

the electrode surface. To prevent non-uniform current distribution on the hot surface of the electrode, platinum is placed on the side of the

electrode which is opposite the electrode edge normally experiencing

current concentrations. Electrical shorting between electrodes by the

condensation of seed in the pore structure of the insulator and in the

joints between insulators was avoided by two design features. First, a dense alumina plate was inserted into the insulator joints, and second,

the wire leadout was enclosed in a dense alumina tube. These features

are shown in Figures A. 3.1 and A. 3. 5. Figures A. 3. 8 and A. 3. 9 give

drawings of the Westinghouse module design.

A. 4 Instrumentation (B. Wright, Westinghouse Research and Development

Laboratory)

Thermocouples and voltage probes are located at various positions

within the two electrode walls. Three electrodes on the anode wall each

have two platinum 13% rhodium-platinum thermocouples buried approximately

3 mm deep at the electrode centerline. As referenced from the channel

entrance, these are the first, fourth and seventeenth electrodes. Five

electrodes have chrome 1-alumel thermocouples positioned at the interface

between the Zr02 phosphate cement and MgO insulator. On the anode wall,

these thermocouples are the third, tenth and eighteenth electrode positions.

On the cathode wall, these thermocouples are in the fifth and fifteenth

electrode positions. In addition, there are sixteen chrome 1-alumel

thermocouples positioned in the colder regions of both the anode and

cathode walls. A single voltage probe is located at the center of

gravity of one-half of an electrode at the first, ninth, and seventeenth

electrode positions on the anode wall, and at the eighth and sixteenth

electrode positions on the cathode wall. These voltage probes will

determine the resistance of the lower half of an electrode.

On the cathode wall, the fourth electrode position has an access hole for line of sight measurement of electrode surface temperature by

an optical technique. This arrangement allows the surface temperature of

the fourth electrode on the anode wall to be measured directly. The

Institute of High Temperatures (IVTAN) is responsible for the actual
50

temperature measurement. Just prior to entering the channel, the plasma

temperature is measured by the sodium line reversal method, again by

IVTAN. IVTAN also has voltage probes located laterally and longitudinally

on the top insulating wall to measure Hall and Faraday potentials. Thermocouples are buried at various depths in the MgO of the top insulating wall

near the entrance, midpoint and exit of the channel. All of the data is

to be recorded in the test protocols for analysis after the test.

Figure A. 4.1 is an open schematic of cathode and anode showing materials,

identification digits, and location of instrumentation.

The U.S. side would like to acknowledge the experience of the

U.S.S.R. MHD specialists in designing hot wall electrode modules [7]. This

information was helpful in designing the module described here.

References

[1] Kirillin, V. A., Sheindlin, E. A., et al.. Study of the U-02 MHD

facility during extented operations, Teplofizika Vysokikh Temperature

(High Temperature), IX [5] (1971).

[2] Rossing, B. R. , Cadoff, L. H. , and Gupta, T. K. , Fabrication and

properties of electrodes based on zirconium oxide, in "Proc. 6th

Int. Conf . in MHD Electrical Power Generation,” Washington (1975).

[3] Burenkov, D. S., Dolinskii, Yu. L., Zalkind, V. I., Zelikson, Yu. M.,

Kirillov, V. V., Sokolov, Yu. N. , and Uspenskaya, G. I., Study of

interelectrode breakdown in an MHD channel on the U-02 installation,

in "Second Joint US-USSR Colloquium on the Problems of MHD Conversion

of Energy," Washington, DC (1975).

[4] Oliver, D. A., Time dependent phenomena in magnetohydrodynamic

generators, in "Proc. First Joint Soviet-American Colloquium on the

Problems of MHD Conversion of Energy," Moscow, (February 1974).

[5] Louis, J. F., Lothrop, T., and Brogan, T. R. , Studies of fluid

using a large combustion driven MHD generator, Phys . Fluids, (7, 362-374 (1964).

[6] Zhimerin, D., Bashilov, V., and Motulevich, V., Results of electrophysical research on module section of magnetohydrodynamic generator

ENIN-2, in "11th Symposium on Engineering Aspects of MHD," California

Institute of Technology, Pasadena, CA (1970).

"Experimental Research on Model MHD Generator U-02 (Stage II)," Scientific

and Technical Report, Institute of High Temperatures, U.S.S.R. Academy

of Sciences (1971)

.

[ 7 ]
51

A.

2.

1“

Flow

sheet

for

preparation

of

electrodes
52

azis spiped papoipui ueqi jajeaJO iqBjaM Aq %

PARTICLE

SIZE

ANALYSIS

OF

STARTING

POWDERS
53

O

C\J

UPi

ro

oo

ir\

o

J8J3U1B1Q 8J0d pajB3!PUI UBgi J8}B8J9 3LUn|0A Aq %

I

CSJ <c

(D

LU

PORE

DIAMETER

(‘MICROMETERS)

PORE

SIZE

DISTRIBUTION

OF

CERAMIC

ELECTRODE

MATERIALS
54

-J|88Zr02/12Y20^

Optical Site Port

Copper Water Cooled Heat Sink

"|82 Zr02/18 Ce02

ALO, Platelet

qSOZrO^/SOCeO

Cathode

MgO Insulator Housing

Electrode

3ZrOj75CeO

24 mm

12 mm

18ZrO„/80 Ce 0«/Ta. 0.

2 2 2 5

Fig. A.3.1“Plan view of cathode wall
55

^ AI^O^ Platelet

Eledr^e

IVIgO Insulator Hous^

'9

Al^O^Tube

Hose Connector

Electrical Lead Out

Silicone Rubber

Fig. A. 3. 2 -Side view of anode wall
56

Fig. A. 3. 3- View showing relationship between electrode and insulating walls
57

Fig. A.3.4-Electrode insulator assembly
58

MgO Insulator

Housing

30 mm

30 mm

Electrode mm

Plasma Sprayed

Copper Layer

Platinum

Screen

Platinum

Coating

AI2O3 Tube

Platinum Lead

Copper Lead

Fig. A.3.5-Exploded view of electrode insulator assembly
59

Oo 'asiy aJH^Ejadiuai J9 }b/v\ 6u!|ooo

A.

3.

6

-Heat

sink

cooling

water

passage

design

equation
60

Plasma

2600 K

Fig A. 3. 7-Calculated temperatures for the

electrode insulator assembly


Current

Densities A/cm^

63

Anode

( Right Side)

Flow

Cathode

{ Left Side)

0 Optical Sight Port

f—Three Thermocouples ( K)

Hot MgO, Cold MgO and Heat Sink

?77Z^ Two Thermocouples ( K)Cold MgO and Heat Sink

• Voltage Probe

1 Thermocouple ( R): Electrode Center Line

Fig. A. 4. 1-Open schematic of cathode and anode showing materials, identification

digits and location of instrumentation
64

Figure A. 2.1

F igure A. 2 .

Figure A. 2.

Figure A. 3.

Figure A, 3.

Figure A. 3.

Figure A. 3.4

Figure A. 3.

Figure A. 3.

Figure A. 3 .

Figure A. 3 .

Figure A. 3.

Figure A. 4.1

List of Figures

Flow sheet for preparation of electrodes.

Particle size analysis of starting powders.

Pore size distribution of ceramic electrode materials.

Plan view of cathode wall.

Side view of anode wall.

View showing relationship between electrode and insulating

walls

Electrode insulator assembly.

Exploded view of electrode insulator assembly.

Heat sink cooling water passage design equation.

Calculated temperatures for the electrode insulator

assembly

Working drawing of electrode module.

Working drawing of sub-module.

Open schematic of cathode and anode showing materials,

identification digits and location of instrumentation.
65

B . Pre-Test Materials Characterization and Analysis (U.S. Results)

A number of samples of each electrode material was supplied by

Westinghouse Electric Research Laboratory to various laboratories for

pre-test analysis. In some cases, measurements were made on several

samples of the same material and these different samples are designated

by the appropriate suffix to the chemical composition. The samples were

hot pressed, high density MgO insulators and Zr02 based electrode materials.

The materials were received in the insulator or electrode geometry to be

used in the U-02 Phase I test and were subsequently cut into shapes

required for property measurements.

For the measurements in Sections B.l, B.2, B.3, and B.4, the

samples were sectioned into smaller pieces for specific characterization

and examination. Figure B.1.1. The large MgO reference insulator, 2.35

X 3.06 X 6.00 cm was cut in the following manner, Figure B.1.1. The

bars, 0.95 x 0.95 x 2.54 cm (M-1) were cut from one corner for thermal

expansion measurements. The ends of this bar (B-1 and T-1) were used

for metallography. A wafer, 0.15 cm thick, was cut from near the center,

normal to sample length (C-1) . Two discs, 1.07 cm in diameter, were cut

from this section for thermal diffusivity measurements. The remainder

was used for ceramography and chemical analysis.

The two electrodes of each composition were cut differently. One was cut in two equal sections with the cut parallel to the electrode.

Figure B.1.1. These sections were used for thermal expansion measurements.

The other was cut normal to the electrode length with wafers for metallography

and chemical analysis cut from the center and from one end. Two discs,

1.07 cm in diameter, were cut from one section, with the sample thickness normal

to the expected heat flow in the electrode during testing.

B.l Thermal Diffusivity (J. L. Bates, Battelle NW Labs)

The thermal dif f usivities of the MgO insulator and electrode

materials were measured using a laser pulse technique from. near room temperature to 1750°C. Briefly, this method consists of heating one surface of a thin sample disc with a short heat pulse from a ruby laser.

The heat pulse passes through the sample and the temperature transient

on the back surface of the sample is measured as a function of time.

This temperature transient is recorded and the thermal diffusivity

determined from the shape of the temperature-versus-time curve and the

measured sample thickness.

The sample was heated to the measurement temperature in a tungsten resistance furnace in a purified argon atmosphere at a slightly

positive atmospheric pressure. Temperatures were measured with W-5%Re

versus W-26%Re thermocouples. The samples, 1.07 cm in diameter and 0.07

to 0.14 cm thick, were mounted in the furnace in an a-Al 203 holder.
66

The apparatus was calibrated using Armco iron and pyroceram. The

absolute thermal diffusivity values of these materials are known. Thermal

diffusivity (a) can be calculated if the heat capacity (Cp) and density (p)

values are known as a function of temperature

A = a*C *p . (1)

P

B.1.1 MgO Insulator

The thermal diffusivity of MgO is shown in Figure B.1.2.

Measurements were made on two sample thicknesses, 0.101 and 0.139 cm. The

thermal dif f usivities were the same for both sample thicknesses. The reciprocal of the thermal diffusivity was plotted against temperature generating the

relationship from least squares fitting, (Figure B.1.3).

A+BT 6.35+0.0493T ' '

The thermal conductivity was calculated using available heat capacity

data (260-2000°C)

Cp = 0.288 + 7.71 X 10“^T - 4.03 x 10"^T“^cal/g-°C (3)

the expression for a, equation 2; and the recommended linear thermal expansion data:

11.6 X lO'^/'^C (21-540°C)

15.8 X 10~^ (540-980°C)

17.2 X 10"^ (980-2000°C)

The density variation is related to the thermal expansion as %

P "

(1 + AL/Lq)3

where p = 3.01 g/cm^ (measured room temperature density).

Thus, thermal conductivity for MgO in the direction normal

to the hot pressing direction can be expressed as:

( 4 )
X 1 1.49 + 0.0142T

wa't t s / cm- ° C

67

(5)

This equation is valid between 100 and 1500°C.

B.1.2 Electrodes

The thermal dif f usivities of the electrode materials are shown

in Figures B.1.4-B.1.8. Measurements were made to as high a temperature as possible, with the limitation being the potential reaction that the electrode

has with the a-Al 203 holder. Some samples, during measurements, eventually

sintered to the holder and altered its structure. This temperature varied

from 1465°C for the 0. 02Ta205* 0. 78Ce02* 0. 20Zr02 sample to near 1700°C for

the 0 . 5Ce02“0 . 5Zr02 electrode material.

The thermal dif f usivities for the electrode materials are summarized

in Figure B.1.9. There are substantial differences in the thermal diffusivity

values at lower temperatures. However, the diffusivity of all the electrodes

decrease with increasing temperature, which is expected for oxide materials where

lattice conduction is dominant. Near 1100-1200°C, the dif f usivities tend to

approach a common value, but tend to diverge at the higher temperatures as the

1.18 Ce02 • 0 . 82Zr02 begins to increase with increasing temperature, while the 50

and 75 mole percent Ce02 electrodes continue to decrease.

The data did not follow a usual linear 1/a versus temperature

relationship expected for lattice conductivity. All electrode materials ex- hibited inflections in the curves, some at temperatures as low as 900°C. These

inflections may be related to possible changes in phase or stoichiometry. Since

the thermal diffusivity measurements were made in an inert gas in the presence

of hot refractory metals, such as tungsten and molybdenum, some reduction could

occur. These conditions are substantially different from the air environment

during fabrication. In addition, destabilization of the Zr02 could also occur.

Calculations of thermal conductivity were not made, since heat

ca- acity and density changes with temperature are not known.

B.2 Thermal Expansion (J. L. Bates, Battelle NW Labs.)

The thermal expansions of the MgO insulator and reference electrodes

were measured from approximately 60 to 1500°C using an aluminum oxide push rod

dilatometer. Measurements were made in both argon and air at atmospheric pressure. The MgO samples were approximately 2.5 cm long and electrodes were

approximately 1.9 cm long. Heating and cooling rates were linear with temperature changes of 4°C per minute.

The expansion was measured with a transducer with a resolution of ±2.5

X 10 ^ cm. For MgO, which expands on the order of 2% at 1400°C, the precision

of the measurement is of the order of 0.05%. With a similar precision for the

measurement of the calibration standard, the overall precision is 0.1%.
63

ELECTRODES

THERMAL DIFFUSIVITY

Figure B.1.1 Cutting Procedures for Precharacteri zati on , U-02 Electrodes

and Insulators.
ThtRMAL

DIFfUSIVITY,

cm

/sec

69

Figure B.1.2. Thermal Diffusivity (a) of MgO Insulator Showing Thermal

Conductivity (A) Equation Calculated from Heat Capacity

and Density Data.

Figure B.1.3. Thermal Resistivity (1/a) of MgO Insulator.
THERMAL

DIFFUSIVITY,

cm^/sec

Figure B.1.4. Thermal Diffusivity of 0.12 Y^O^- 0. SSZrO^ (Y-13)

Figure B.1.5. Thermal Diffusivity of 0. 18Ce02*0.82Zr02 (C2-15)
71

Figure B.1.7. Thermal Diffusivity of 0. 75Ce02* 0. 25Zr02 (C4-19)
72

Fi.gure B

,

Figure B.

1.8 Thermal Diffusivity of 0. TSCeO^* 0. 20Zr02* 0. OZTa^O^ (C5-9).

.9. Summary of Thermal Diffusivi ties for U-02 MKD Electrode

Materials.
73

The accuracy of the measurements was determined using a tungsten

standard (SRM 737) supplied by the U.S. National Bureau of Standards,

which is accurate to ±5%. Temperatures were measured with a platinum

versus platinum- 10% rhodium thermocouple, which has been calibrated

against a standard platinum resistance thermometer. The accuracy of the

temperature measurement in the system is within 5°C. The overall accuracy

of the thermal expansion data is estimated to be better than 5%.

The thermal expansion data are illustrated in Figures B.2.1 - B.2.6.

Differences were observed in the thermal expansions (a) measured in argon and

air, with the values of a larger in argon than in air. The magnitude of these

differences varied between materials being smallest for the Y 203 ~Zr 02 electrode

and MgO insulator, and greatest for the Ce02*Zr02 electrodes. The samples which

showed different expansion in air and argon also experienced changes in sample

length during the measurements to 1400-1500°C (Table B.2.1). In general, those

samples which had larger a values in argon also showed the largest increases

in length. In addition, the AL/L values for the as-received electrodes were

consistently higher during the cooling cycle.

These changes in a during heating in argon suggest possible stoichiometry or phase changes resulting from the lower oxygen pressures. These changes

appear to be reversible as seen in the results for MgO, Figure B.2.1, and for

the 0.75 Ce02'0.25 Zr02 (C4-16) , Figure B.2.5, in which the thermal expansion re- turned to the original air values after "being first heated in argon and then in

air

The average thermal expansion coefficients from 100 to 1400°C

calculated from AL/L (Table B.2.1) data are summarized in Table B.’2.2.

The a values for measurements made in argon are values obtained only on cooling since the values on heating can represent possible changes in

sample composition or stoichiometry.

It is interesting to note that for the insulator and all

electrodes, the thermal expansion values, a(100-1400) , are larger in

argon by a factor of approximately 1.1 than in air. In addition the a values for Ce02 -Zr 02 electrodes increase with increasing Ce02 concentration.

B.3 Chemical Composition (J. L. Bates, Battelle NW)

The composition of the MgO was determined by emission spectrochemical

analysis to a precision ± factor of 2. The results indicate a high purity oxide

containing 20 ppm Al, 50 ppm Ca, 50 ppm Si, and 50 ppm V. All other elements

were less than detectable concentrations.

The compositions of the electrode materials were determined by x-ray

fluorescence. Table B.3.1. These results are considered accurate to ±5%.

B.4 Microstructure (J. L. Bates, Battelle NW Labs.)

B.4.1 MgO Insulator

The electrode and insulator samples for metallography were vacuum

impregnated with resin and polished for microstructural examination. The microstructures of the MgO insulator were substantially different depending on the

orientation relative to the hot pressing directions. Figures B.4.1-B.4.3.
AL/L(x

10^)

74

Figure B.2.1. Thermal expansion of MgO (M-1).
AL/Ux

10)

75

Figure B.2.2. Thermal Expansion of 0.12 Y^O^’O.SB Zr02 (Y-11).
AL/L(X

10^)

76

Figure B.2.3. Thermal expansion of 0.18 Ce02-0.82 Zr02 (C-2-13).
77

Figure B.2,4. Thermal expansion of 0.5 CeO^'O.S ZrO^ (C-3-14).
AL/L

(X

10)

78

Figure B.2.5. Thermal expansion of 0.75 CeO2-0.25 ZrO^ (C-4-16)
79

Figure B.2.6. Thermal expansion of 0.78 CeO -0.20 ZrO -0.02 Ta O . (C-5-11)

2 2 5
80

TABLE B.2.1

DIMENSIONAL

OF

CHANGES DUE TO THERMAL

INSULATOR AND ELECTRODES

EXPANSION

Length (cm)

Sample Atmosphere Before After Change

MgO (M-1) Argon^ 2.5334 2.5362 +0.0028

Argon^ 2.5362 2.5357 -0.0005

Air2 2.5359 2.5362 +0.0003

Y-11 Argon 1.8160 1.8763 +0.0003

Air 1.8628 1.8628 0.0000

C-2-13 Argon 1.9136 1.9131 -0.0005

Air 1.9167 1.9157 -0.0010

C-3-14 Argon 1.9126 1.9162 +0.0036

Air 1.9215 1.9210 -0.0005

C-4-16 Argon^ 1.9000 1.9075 +0.0075

Air 2 1.9075 1.9022 -0.0053

Air 1.9055 1.9055 0.0000

C-5-11 Argon 1.9126 1.9182 +0.0056

Air 1.9053 1.9058 +0.0005

^Measurements accurate to 0.0005 cm. ^All measurements made on same MgO sample in order of their listing.

^Both measurements made on same sample, first in argon followed by measurement

in air.
81

TABLE B.2.2

THERMAL EXPANSION OF MgO AND ELECTRODES

-i/°C(10~^) (100-1400°C)

Argon Air

MgO (M-1) 15.6 14.0

15.5

0.12 ¥ 203 - 0.88 Zr02 (Y^ll) 12.3 10.9

0.18 Ce02*0.82 Zr02 (C-2-13) 13.1 11.5

0.5 Ce02*0.5 Zr02 (C-3-14) 13.6 12.2

0.75 Ce02*0.25 Zr 02 (C-4-16) 14.5 13.2

0.78 Ce02 * 0.20 Zr 02 * 0.02 Ta 20 s (C-5-11) 14.9 13.6
82

TABLE B.3.1

Composition and Designation of Electrode Materials

Electrode Reference Mole Percent

Zr02 Y2O3 CaO^ Ta^Oj

0 .,12 Y 2 O 3-• 0 . 00 00 Zr02

1

—

1

I—

1

1 f-13* 88 .,8 11.2

0 .,18 Ce 02 -> 0 ,,82 Zr 02 C2-13, C2-15* 77.,8 22 ..2

0 .,50 Ce 02 '• 0 .,50 Zr 02 C3-14, C3-22* 46.,2 53.,8

0 .,75 Ce 02 •• 0 .,25 Zr 02 C4-16, C4-19* 19.,6 80,.4

0 .,78 Ce 02 0 .,20 Zr 02 C5-9*, C5-11 16..5 82,.2

• 0.02 Ta20s

•k Analytical samples
83

Normal to the hot pressing direction (B-1 'and T-1) in Figures B.4.1 and B.4.2,

the pores were irregular in shape and uniformly distributed around the MgO particles with no apparent orientation. In contrast, in the direction parallel to

the hot pressing direction, the pores appear smaller and are longer in one direction than the other. The smaller width is parallel to the hot pressing direction.

The grains were very dense but show evidence of deformation. The average grain

size is 10 ym with some grains as large as 0.02 cm. The pore size varied up to

5 ym across

.

B.4.2 Electrodes

[ 0 .

I

2Y2 O 3 *0

.

88 Zr 02 (Y-13-E,M)] The microstructure of the yttria

stabilized electrode is sho\«m in Figure B.4.4. The sections from the center (Y-13-M)

and edge (Y-13-E) were essentially the same; however, they differed in color.

The Y-13-M was off-white, while the Y-13-E was mottled with the large grains and

edges in particular being a light gray. The larger grains were round or oval in

shape and contained a large number of very small pores uniformly distributed throughout. The size of the grains varied up to 65 ym. The edges of the large grains were

irregular, indicating sintering between the oxide fines. Sintering was also extensive in the fines between the larger grains.

[ 0 . 18Ce02-0 . 82Zr02 (C2-15 E,M)] The microstructure from the center

and end of the electrode was essentially the same. Figure B.4.5. The electrode was mottled off-white and light gray. The lighter areas were near the edges. A second

phase (light gray) was observed in some selected areas. The size of the large grains

varied up to 0.065 cm. These large grains contained substantial small pores uniformly distributed throughout the grains. Some evidence of sintering between the

larger grains and small filler material can be seen.

[ 0 . 5Ce02 • 0 . 5Zr02 (C3-22E,M)] The electrode was mottled with a light

yellow background and light gray areas, Figure B.4.6. The middle (C3-22 M) was substantially darker than the end. The darker areas do not appear to be associated with

any particular area. A possible second phase may also be present. The electrode

contained a number of microcracks. The size of the large grains was as large as 0.065 cm. The fines filled the spaces between the grains and some sintering is evident. The larger grains contained a uniform distribution of small pores.

[0.75 Ce02 *0 . 25Zr02 (C4-19 E,M)] This high ceria-containing electrode

was multi-colored, with large areas of yellow, tinted with green, which were interlaced with gray. Figure B.4.7. These areas of different color were related to specific

microstructures, the lighter yellow to areas free of the large grains and the darker

gray to areas containing a large number of large grains. Both sections contained a substantial number of cracks which ran parallel to the length of the sample. The

large grains were approximately 0.065 cm or less in diameter, and were uniformly

dispersed with small pores, <0.001 cm in diameter.

[ 0 . 02Ta205 • 0 . 78Ce02 *0 . 2Zr02 (C5-9 E,M)] This electrode material was uniformly mottled with yellow green background and very dark green spots. Figure

B.4.8. These dark green areas were directly related to the large crystallites of

the material, while smaller particles are light green. These effects are easily seen

in polarized light. The regular surfaces of the porosity in the areas which bind the

larger particles together suggest the presence of a liquid phase during sintering.

Some large and small cracks lace the electrodes, especially in the center section.

The large grains were 0.065 cm or less in diameter. The pores in the larger grains
84

12X

lOOX Center

Figure B.4.1. Representative Mi cros t-ructure of MgO Insulator, B-1 with

Hot Pressing Direction Normal to the MgO Surface.
85

f

; h U ••. . • ... -Jf,.

' 4>v> -.' -- -Vir.- i-v • ^-r-- .• ’•'i'V:' -V

• ',< Vi,'. .•>., ..sV.--'

••

. •_ '.'• /'JCI' V* • v .^v W '

• ' •, .» V.'5^ .'> ^ .-.u 4 ^ -V .•

’

’•' r.

•«.'' '- < V -l'- ' 'T -'. ';

' •• v4'- .•*-- se‘ K^., > /

• ;.•

..'..V- .'y v:

'

- '*'• ’ v,‘.

‘ * \

'. ,’v. 12X

Hot Pressing Direction

Figure B.4.2. Representative Microstructures of MgO Insulator T 1, with

Hot Pressing Direction Normal to the MgO Surface (As-Pol i shed)
86

loox

4-

Center

Figure B.4.3. Representative Microstructures of MgO Insulator C-1 with

Hot Pressing Direction Parallel to MgO Surface (As -Pol i shed )
87

lOOX Center

Edge

lOOX

Figure B.4.4. Representative Microstructure

Zr02 (Y-13) MHD Electrode (As

of 12 Mole% Y 2O 3-88 Mole%

-Polished)
88

lOOX lOOX

Edge

f r, "

.

^ V • , 2*

* :* •* • :V:^v^'vnv*‘

** '

. ‘•<'l* . • < y

* - .*

* ^ • tL. *. •- • • ^

t • ’v ' • iT*. ^

V

*. • 'i- .. »V •*. •<•

• • * • ^ .ftf. '..». . , J Center

Representative Mi crostructures of 18 Mole% Ce0?-82 Mole% Zr02 (C2-15) MHD Electrode (As-Polished).

Figure B.4.5
89

rdge

Figure B.

lOOX Center lOOX

4.6. Representative Microstructure of 50 mole% Ce02~50 mole% ZrO^

(C-3-22) MHD Electrode (As-Polished).
90

Figure B.4.7. Representative Mi crostructure of 75 Mole% Ce02-25 Mole%

Zr02 (C4-19) MHD Electrode (As-Polished).
91

Center 100X

Figure B.4.8. Representative Mi crostructure of 78 Mole% Ce02~20 Mole%

Zr02-2 Mole% 13285 (C5-9) MHD Electrode (As-Polished).
92

were generally spherical in shape and varied widely in size up to 0.0006 cm

in diameter.

B.5 Electrical Conductivity (W. R. Hosier, NBS)

The electrical resistivity of the five test electrode materials

is shown in Figures B.5.1-B.5.3. The data is given as a function of temperature and at several oxygen pressures. The data points represent equilibrium

values. Since, in all samples (except the 88 Zr 02 - 12Y 203 ) , the conductivity

of the material depends on the ratio of the Ce^’^/Ce"^'*', the amount of reduction

is important and this is determined by the oxygen pressure. Equilibrium times

of several hours were sometimes necessary to obtain equilibrium at the low

temperatures

All data were obtained using a d.c. four probe method, thus minimizing

any measurement difficulties due to contact emf’s or polarization effects. Resistance measurements were made as a function of current density, and current

dependent effects were observed below a power density value of '^^1.25 watts/cm^.

Generally, below 1200°C, current densities of 1 amp/cm^ would cause joule heating.

The effect, of course, is dependent on the resistivity of each sample at 1200°C.

The sample containing 78 Ce02 - 20Zr 02- 2Ta 205 has the highest conductivity and

therefore showed the smallest joule heating at 1 amp/cm^ at 1200°C. The effect

of the 5 valent tantalum is to force some Ce^"*" into Ce^'*' independent of the

oxygen pressure and to maintain a conducting material through electron exchange

between the Ce^"^ and Ce^'^.

No long term current dependent effects were made on the ionically con- ducting 88Zr 02 ~ 12Y 203 where one might expect some reduction of the Zr 02 at the

cathode. Current densities and measurement times were too low to expect to

observe a change in these samples.

Figure B.5. A shows the conductivity of high purity MgO ceramic material

as a function of temperature at several oxygen pressures. This data is taken

from C. M. Osburn and R. W. Vest, J. Am. Ceram. Soc. , 5^, No. 9 p. A28-435

(1971). The material was approximately 10% porous and the data has not been

corrected for this effect. (Multiply given values by 1.09 to correct for

porosity.

)

B .6 Phase Composition (A. Perloff)

Tables B.6.1 and B. 6 . 2 list pertinent x-ray diffraction data for pretest, U-02 Phase I materials, listed in terms of their nominal compositions.

Materials for anode and cathode use are identical. Phases present in these

nominal compositions are characteristic of reaction products formed at elevated

temperatures under relatively oxidizing conditions, in accordance with established

thermochemical data. These untested phase and phase assemblages are discussed

further with reference to post-test results (Chapter II - Section C.2).

B.7 Porosity and Pore Size Distribution (B. Rossing, Westinghouse Res. and

Dev. Lab)

The porosity and pore size distribution were determined for each

electrode composition and for the magnesia insulators by conventional techniques.
CONDUCTIVITY(ohm-cm)

93

T(K)

Figure B.5.1. Electrical Conductivity of Test Materials in Air.
CONDUCTIVITY

(ohm-cmr'

94

10^

T(K)

Electrical Conductivity of Test Materials at 10

Oxygen in Nitrogen.

Fig'ore B. 5 . 2 . atm
95

a(K)

" 6 Figure B.5.3. Electrical Conductivity of Test Materials at 6 x 10 atm

Oxygen in Nitrogen.
CONDUCTIVITY(ohm-cm)'

96

Figure B.5.4. Electrical Conductivity of MgO Ceramic
Sample

Major

Compos! t

ion

~n^ol %

Designation

Crystallinity

Phase

97 o

•H

XI3OO 0 cn 0) 0 0) 0) (U u c tn c c ra •H (t3 0 0 s x: z z El

04 o < LTJ rrH LO m m • CN • . in • • in m

in cn

It

II II

II II

03

03u

CO <TJ

*• - - 6 u u • • • 0) < u 4J 0 c o • 0) • 0 • 4-1 4J <4-1 z <4-1 0 < o < 0 < 0 r—

CN cn

m 0 < O < m in m 0 < 0 < CN CN CN <n O O II II II 0 0 o o • • u 0 U • • +1 +1 r—l 1—

(—1 +1 +1 CN 1—1 ttj - (TJ - CTS *. CN CN

r-

in c m

C <x

m

CN f—

0 1—

o CN 0 CN cn

m • • • • tj< • 0^ • CP • « • . . 0 rr o IT) n3 Ln «3 m cO m 0 m 0 m * • 0 i-4 • . u II O II 4-1 II 4-1 II 4J II 0 II 0 II • • 0) CU CD • • 4-1 <T3 4-1 fO +J <T3 4-1 cQ 4J cd 4-1 <d f a

dJ 0)

cn cn

(T3

m

x: x:

04 a, u o o o n C

TJ T! 0 0 u TJ td -i-i 0 0 •r-J *H •H 0 e e 0 0 03 <d 03 0 U 0 Cm 04 t4 U 1 1

T3

O

OO 't3 C3 li.

vD

m cn p' 0 0 CN CN 4—

1-^ 1—< CN CN f—

I-H

4—11

i-HI 1 CN 1 CN 1 cn 1 cn 1 1 1 m 1 m > >4 U U U U U 0 U U

cn CM CN CN CM CM CM CN CMm CN 0 0 0 0 0 0 0 0 0 0 0

CN

V4 CD tH CD kl (D tH <D CM iH >4 N U N U CN CD N U 03 Ch

CN 0 oP aP aP aP aP aP aP aP aP aP CP CN CD CD CN 0 0 <n m CO <yP 0 s 1—4 CD i-H CD m m CN CN CN
TABLE B.6.2

98

Pretest Samples

Sample X-ray Results

MgO Pure, well-crystallized f.c.c. MgO pattern,

a = 4.2121.002 A

12 % Y2 O 3 Y-12, Y-14Both samples well-crystallized, single

phase f.c.c., a = 5.1511.002 A

18% Ce02 Both samples almost all tetragonal ^ C2-14, C2-16 zirconia phase with a = 5.15, c = 5.24 A.

Trace of cubic phase (?) indicated by the

presence of one very small peak. Crystallinity

only fair relative to C4 and C5 samples.

50% Ce02 Crystallinity only fair for both.

C3-15 Primarily tetragonal zirconia phase with

a = 5.26, c = 5.30 A plus a very small amount

of a cubic (?) phase with a = 5.15 A.

C3-23 Major component - tetragonal zirconia phase

0 a = 5.27, c = 5.31 A. Also contains a small

amount of a tetragonal phase with a = 5.15

and c = 5.24 A plus a very small amount „ of what may be a cubic phase with a = 5.34 A.

75% Ce02

C4-17, C4-20

Both samples well crystallized single phase

f.c.c. with a = 5.342±.002 A.

73% Ce02 , 2% Ta 205 Both samples contain two f.c.c. phases. The

C5-10, C5-12 major component^is well-crystallized with

a = 5.352±.005 A. The minor component is ^ not as well crystallized with a = 5.371.01 A.
99

The determination of the total porosity was carried out by grinding the specimen

finely enough to open all pores and measuring the true density by the pycnometer

method. The total porosity was calculated from;

d -cL P = -y ^ X 100

d

t where d^ is the true density, d^ the bulk density, and P the percentage of total

pores in relation to the bulk volume. The bulk volume was taken from measurements

of a solid rectangular specimen. The open porosity was measured by three weighings

substituted into the following equation:

P

W-D

W-A

X 100

where P is the percent of open pore volume in relation to the bulk volume, W is

the weight of a water saturated sample in air, D is the weight of a dry sample

in air, and A is the weight of a water saturated sample submerged in water.

Saturation with water was accomplished under vacuum. The closed porosity is then

calculated as the difference between total and open porosity. The pore size

distribution was accomplished by using a commercial (Micromeritics Model //903-1)

mercury penetration porosimeter. Sample volumes of approximately 20 cm^ subjected

to mercury penetration at various pressures. The volume of mercury penetrating

at varying pressures gave the (open) pore size distribution. The porosities and

pore size distributions for all materials are shown in Table B.7.1.
100

TABLE B.7.1

POROSITIES AND PORE SIZE DISTRIBUTION OF

INSULATORS AND ELECTRODES

78% Ce02

MgO

12 % Y 2 O 3 88% Zr 02

18% CeO?

82% Zr 02

50% Ce02

50% Zr02

75% Ce02

25% Zr02

20% ZrO;

2 % Ta20

Total

Porosity (%) 16.9 17.7 15.8 19.9 17.6 14.6

Closed

Porosity (%) <0. 5 <0. 5 2.1 5.5 3.3 1.1

Pore Size

Distribution

%less than: 20 pm 100 100 100 100 98 99

15 pm 99 100 98 98 95 98

10 pm 98 100 56 60 5 24

5 pm 95 95 39 41 1 1

3 pm 89 84 20 20 <1 <1

2 pm 44 48 5 5 - -

1 pm 11 11 <1 2 — —

0.5 pm 3 2 - <1

0.2 pm 1 <1
101

List of Figures

Figure B . 1 . 1. Cutting procedures for precharacterization, U-02 electrodes and

insulators

Figure B . 1 . 2

Thermal diffusivity (a) of MgO insulator showing thermal conduc-

(A) equation calculated from heat capacity and density data.

Figure B.1.3. Thermal resistivity (1/a) of MgO insulator.

Figure B. 1. 4. Thermal diffusivity of 0.12 Y^O^’O.SS ZrO^ (Y-13).

Figure B.1.5. Thermal diffusivity of 0.18 Ce02*0.82 Zr02 (C2-15).

Figure B.1.6. Thermal diffusivity of 0.5 CeO^'O.S ZrO^ (C3-22).

Figure B.1.7. Thermal diffusivity of 0.75 Ce02*0.25 ZrO^ (C4-19)

^ -gure B.1.8. Thermal diffusivity of 0.78 CeO^'O.ZO ZrO^'O.OZ Ta^O^ (C5-9)

Figure B.1.9. Summary of thermal dif fusivities for U-02 MHD electrode materials.

> Figure B.2.1. Thermal expansion of MgO (M-1)

Figure B.2.2. Thermal expansion of 0.12 Y^O^'O.dd ZrO^ (Y-11).

Figure B.2.3. Thermal expansion of 0.18 Ce02*0.82 ZrO^ (C-2-13).

Figure B.2.4. Thermal expansion of 0.5 Ce02*0.5 ZrO^ (C-3-14).

Figure B.2.5. Thermal expansion of 0.75 Ce02*0.25 Zr02 (C-4-16)

Fi ,ure B. 2. 6. Thermal expansion of 0.78 Ce02*0.20 Zr02*0.02 Ta20^ (C-5-11)

Figure B.4.1. Representative microstructure of MgO insulator, B-1 with hot

pressing direction normal to the MgO surface.

Figure B.4.2. Representative microstructures of MgO insulator T-1 with hot pressing

direction normal to the MgO surface (As-Polished)

Figure B.4.3. Representative microstructures of MgO insulator C-1 with hot pressing

direction parallel to MgO surface (As-Polished)

Figure B.4.4. Representative microstructure of 12 mole% Y_0_-88 mole% Zr0_

(Y-13) MHD electrode (As-Polished)

Figure B.4.5. Representative microstructures of 18 mole% Ce0„-82 mole% Zr0„

(C2-15) MHD electrode (As-Polished)

Figure B.4.6. Representative microstructure of 50 mole% CeO^-50 mole% Zr02 (C-3-22)

MHD electrode (As-Polished).
102

Figure B.4.7.

Figure B.4.8.

Figure B.5.1.

Figure B.5.2.

Figure B.5.3.

Figure B.5.4,

Representative microstructure of 75 mole% Ce02-25 mole% Zr02 (C4-19)

MHD electrode (As-Polished)

Representative microstructure of 78 mole% Ce0„-20 mole% Zr0_-2

mole% Ta^O^ (C5-9) MHD electrode (As-Polished).

Electrical conductivity of test materials in air.

-3

Electrical conductivity of test materials at 10 atm oxygen in

nitrogen.

—6

Electrical conductivity of test materials at 6 x 10 atm oxygen

in nitrogen.

Electrical conductivity of MgO ceramic.
103

C . Post-Test Materials Characterization And Analysis (U.S.)

C.l Module Disassembly and Materials Distribution (W. R. Hosier and T. Negas,

NBS)

After completion of the test in Moscow on October 1, 1975, at 6:20 a.m.,

the electrode section was removed from the U-02 channel. Extensive photographs

were taken at this time of the electrode section before disassembly and after the

electrode sections were removed from the insulating walls.

A physical description of each electrode and insulator surface area was made by Dr. L. Bates, Battelle N.W. Labs. , just after the test.

Anode

2101 - off-white color with localized areas of erosion, no cracking

on upstream side. MgO shrunken away from electrode side near

2102.

2102 - similar to 2101 .

2103 - similar to 2101 and 2102.

2104 - similar to 2101 except some cracking or spalling on bottom

and top of electrode.

2205

- some erosion on upstream side, no cracking with electrode

sections sintered together. Very light tan^color.

MgO appears to have eroded on upstream side between 2205

and 2206.

2206 - same as 2205.

2207 - same as 2205 with MgO eroded between 2106 and 2107. Some

possible reaction with MgO.

2208 - same as 2205 with some possible reaction with MgO.

Some bloating of bottom of 2208.

2309 - some bloating and spalling with surface appearing friable.

Reaction with MgO limited. Light tan in color. Possible

reaction with insulating strip (AI 2 O 3 ). MgO appears intact

but cracked.
104

2310 - same as 2309.

2311 - same as 2309.

2312 - same as 2309.

2413 - Light tan with some brown color. Top electrode in best

condition of the series (2413-2416). Bottom extensively

cracked and spalled. Large section missing. Some possible

reaction'' with areas between insulating walls and electrode MgO

fractured and lost on downstream side.

2414 - Same color as 2413 but with light green tint.

Best of this composition on anode. Cracked in a criss-cross

pattern. Some evidence of spalling.

2415 - Light tan yellow cast and green tint. Eroded on up.-tream side.

Extensively bloated and presence of a hole under suila.e on upstream side.

2416 - some erosion. Light tan with some yellow and green. Evidence

of reaction at interface of electrode and MgO. Some cracking.

2517 - Li^ht green in color, but darker than 2518 and 2510.

Bloating significantly less than cathode wall. Surface

more dense. Electrode section did not appear to sinter

together. MgO eroded between 2517 and 2518 and is light tan

in color. Slight fracture.

2518 - Green in color and darker than 2519 and 2520 - similar to

2517. Some spalling of the surface.

2519 - Light green, but lighter than 2517 and 2518. Some erosion

on upstream side. Some cracking as in 2517.

2520 - Same as 2519 and cracked. Some spalling.
105

Cathode

1101 - Off white color. Bloating worst in center with erosion

upstream. Erosion lines parallel to plasma flow. Bloated

area in bottom electrode section has upstream hole approximately

1 cm long and 0.3 cm wide. Some possible reaction with MgO.

1102 - Loss near center with extensive bloating. Light tan

surface, but off-white in area lost. Some erosion on upstream side. No sintering between electrodes.

1103 - Similar, to 1102. Platinum showing on sides. Possible

seed penetration in MgO between 1102 and 1103. MgO

spalled between 1102 and 1103.

1104 - Light tan in color. No cracking, some evidence of bloating.

Best in series. Some erosion on upstream side. Some

possible reaction with AI 2 O 3 spacers. This electrode has

pyrometer sight port. Seed clogging is visible.

1205 - Very light tan. Extensively swelled «nd bloated. Very

little evidence of corrosion. Electrode sections sintered

together. Some possible reaction with MgO insulator and

insulation wall. AI 2 O 3 spacer between MgO insulator appears

to extrude from crack. MgO light tan to brown and recessed

on the edges

1206 - similar to 1205.

1207 - similar to 1205.

1208 - similar to 1205.

1309 - light yellow tint in electrode center, but brown near MgO

insulator walls. Electrode sections joined together.

Extensively swelled and spalled. Some possible reaction

with MgO.

1310 - similar to 1309.

1311 - similar to 1309.

1312 - similar to 1309, but cracked vertically.

1413 - Light tan in color. Electrode section bonded together. Entire

surface appears to have been removed - clean in appearance,

crystalline and rough.
106

1414 - same as 1413.

1415 - same as 1413.

1416 - similar to 1413 except spalled extensively on bottom, top

bloated, porous in appearance. MgO spalled upstream of

1416. Reaction with MgO possible.

1517 - yellow with green tint. Bloated and swelled. Porous.

MgO light grey. Electrode sections joined together. Some

portions of electrode removed and cracked.

1518 - same as 1517. Top and bottom spalled. Possible reaction

with MgO.

1519 - yellow to light brown. Bottom spalled Swelling extensive.

General Comments on Both Modules

The MgO insulator blocks were cracked down the center, parallel

to the module length on both electrode walls.

The AI 2 O 3 spacers in most areas either swelled or react--u

with seed. This effect was more pronounced on the ca'!i'<de.

The MgO insulator walls eroded more deeply on the left side

of the channel (cathode side looking downstream)

The cathode was more severely swelled and spalled than the

anode

Within four hours after the section was dismantled, the electrode

walls were placed in especially designed metal containers along with an appropriate amount of dessicant. These containers were then sealed in plastic bags

after alternate flushing with inert dry gas and pumping. They were placed in

the original shipment boxes for return to the U.S.

The shipment container was opened at NBS on October 29, 19/5

Dr. U. Sokolov from U.S.S.R. was present and assisted in all stages of the module

disassembly. After the electrode modules were removed from the metal containers,

extensive photographs were made again to determine any damage that might have

occurred during shipment. Figures C.1.1 and C.1.2 show the anode and cathode

after shipment. No damage was detected.

In order to best analyze the results of the test, one half of

each electrode was removed from each module (Figure C.1.3). These samples were

used for phase and chemical composition, electron microprobe analysis and electrical conductivity measurements, where the material removed was of sufficient size

to obtain usable samples. Looking downstream, the upper half of the anode

electrodes (anode on right) were removed and the lower half of the cathode electrodes (cathode on right) were removed. These samples were carefully labeled

and divided. One part from each sample was supplied to the U.S.S.R. (through Dr.

Sokolov) for post- test analysis. The remaining part was retained at NBS for phase
107

Figure C.1.1 Anode
108

FIGURE

C.1,3

CATHODE

OR

ANODE

WALL

SCHEMATIC
109

and chemical composition work. Only samples 2102, 2205, 2206, 2201 , 2208

were large enough to permit electrical conductivity measurements (see Section

C. 3 , this chapter)

In order to assess the effects of the temperature gradient,

current concentration, and seed penetration into the electrodes and into insulator blocks, it is necessary to look at a cross section of the electrode wall

perpendicular to the electrode surface. In order to obtain slices of the anode

and cathode modules, the entire module was potted in epoxy resin. This was done in a specially designed vacuum chamber where the resin could penetrate the

electrode and insulator blocks, but the samples removed would retain their original shape and relative positions. After potting, each electrode wall was separated

into sub-module sections by sawing through the acrylic holding block just below

each water-cooling block sub-module division. This, then, left a total of ten

sub-modules, 5 anode and 5 cathode. Each anode sub-module and each cathode

sub-module held a set of 4 materials of one kind.

Each sub-module was sliced (using a diamond saw and ethylene glycol

as coolant) perpendicular to the electrode surface and lengthwise, parallel to the

plasma flow (Figure C.1.3). Three slices, each approximately 5 mm thick, were

removed from each sub-module and carefully labeled. Figure C.1.4 (anode) and

Figure C.1.5 (cathode) show the five outside slices from each sub-module. The

inside slices from each sub-module was delivered to U.S.S.R. through Dr. Sokolov for

post-test analysis in the U.S.S.R. The center slices from each sub-module were

delivered to Dr. L. Bates for electron microprobe analysis at Battelle NW. The

outside slices were retained by NBS and used for correlation purposes associated

with the phase and chemical composition determinations.

Extensive color and black and white photographs were taken at hvery

stage of disassembly. These 35 mm slides are cataloged, but reproduction

of the whole disassembly process would be impractical.

Figure C.1.6 shows a schematic diagram of the cathode and anode

materials and the identification digits. See Chapter II, Section A, Figure A. 4.1,

for instrumentation and load factors for the various electrode pairs. In Figure

C.1.6, the cross-hatched electrode is the side of electrode pair that went open

circuit during the test. Table C.1.1 gives the approximate time after the beginning of the life test (3rd phase) when the electrode pair failed. During the

2nd phase, different currents and voltages were applied to the electrode pairs

to establish the working conditions of the module for use in the life test, so that

the times recorded in Table C.1.1 are not the total times of operation. Additional

information on the electrode lead failure is given in Chapter II, C.5.

C.2 Phase Composition and Chemical Reactions (T. Negas , C. L. McDaniel,

A. Perloff)

To obtain a fundamental understanding of the chemical behavior of Zr02/Ce02 materials, subsolidus phase relations in the system Zr02 “Ce02 _x were determined up to

1600°C in air. Additionally, the vaporization of cerium oxides from several com- positions in the system was measured up to 1900°C. This work, together with a preand post-test phase analysis of Battelle proof-test materials and the voluminous

literature on Zr02-cerium oxide reactions and properties constitutes a data base for

the untested, "virgin” materials. These data were summarized previously by NBS in
outside

110

^i4e

slice

*

<
Ill

CATHODE ANODE

88 Zr 02

12 Y

2

O

3

2101

1102 • 2102

1103 2103

llOi) 2104

1295

82 ZRO

2

18 CEO

2

2205

1206 2206

1207

1208 <^2208;:<::^

1309

50 ZRO

2

50 CEO

2

2309

1310 2310

1311 2311

1312 2312

1413

25 Zr02

75 CEO

2

2413

1414 2414

1415 ^2415^:^

2416

1517

20 ZrOo

78 CEO2

2 TA 2

O

5

1513 2518

1519 2519

1520 2520

OPEN SCHEMATIC OF CATHODE AND ANODE SHOWING MATERIALS.

IDENTIFICATION DIGITS AND ELECTRODE LEAD"OUT FAILURE.

SHADED AREAS INDICATE WHICH LEAD FAILED (aNODE OR CATHODE).

FIGURE C.l. 6

GAS

FLOW

DIRECTION
112

TABLE C.1.1

Electrode Pair

Approximate Time To Lead-Out

Failure From Start of Life

Test (3rd phase)

1207 - 2207 7 min

1208 - 2208 7 min.

1101 - 2101 1 hour 20 min.

1415 - 2415 1 hour 45 min.

1416 2416 2 hours 20 min.

1517 - 2517 92 hours 40 min.
113

/ Quarterly Reports to ERDA. By comparing post-test characterization data

with the pre-test data base, it is possible to decipher and understand the

chemical and/or phase changes which occurred within the electrode during

testing. For cathode materials, it was deemed necessary to reproduce certain

chemical reactions in the laboratory and to correlate these data with the x-ray

diffraction results from tested materials.

C.2.1 Cathode Materials (T. Negas and C. McDaniel)

C.2.1.1 Introduction and Methods

When the U-02, Phase I, Westinghouse module was received, dismantled, and dissected CChapter II, C.l) at NBS, a visual observation of the

electrodes immediately suggested significant alteration of the Ce 02-containing

Zr02 materials at the cathode wall. These materials were severely bloated,

fragmented, and banded, generally into three zones distinguished by white (top - 1-3 mm in thickness) , yellow (1-3 mm below surface), and yellow to olive-green

(bottom), colorations. These zones are not of constant thickness within

an individual electrode, nor similar in thickness in electrodes of

identical starting composition, but with varying current density.

Generally, the discolored bands increase in thickness toward the outer

MgO spacers, provided by the Soviet side, to fill gaps in the module.

That the zones of discoloration are indicative of phase changes of original materials and of reaction with potassium seed was considered

obvious. Several x-ray diffraction powder patterns of selected specimens

confirmed these initial assumptions and more extensive work followed.

The bloating or swelling of cathode materials probably occurred

during and after cool-down. Almost the entire cathode wall is saturated with

seed, mostly as condensed K2 CO 3

. This includes the underlying MgO blocks which

were cracked extensively. When exposed to moisture, K2 CO 3 (as is well known)

expands (and ultimately dissolves) by hydrating. This expansion, abetted by the

mismatch in the cohesion among zones (see above) of dissimilar phases and alteration products, and hence, dissimilar mechanical properties, most likely caused

the swelling. Indeed, it is unlikely that the electrodes experienced bloating

during operation as distended material would have been swept by the gas stream.

Numerous specimens were hand-picked carefully from within each

zone of discoloration of every electrode. The 88 Zr02 :

2 Y 2 O 3 electrodes are not

zoned, but samples representative of the top, middle, and bottom portions were

taken. All specimens were investigated by x-ray diffraction powder analysis using

filtered Cu-radiation.. X-ray dif f ractograms were made within the 90° - 5° 29

range (1/4° 20 per minute) . The magnetic susceptibility of selected specimens

was measured at room temperature. In contrast to most anode materials, the

fragmented cathode materials could not be removed intact so that x-ray diffraction

analysis directly of every surface was rendered impossible.

C.2.1.2 Results, Phase Analysis and Discussion

C.2.1. 2.1 Ce02-Containing Materials

X-ray diffraction analysis results from cathode materials

are summarized in Table C.2.1. The outstanding characteristics of all Ce02 - containing materials are; a) alteration through phase transitions, and/or
114

b) reaction with potassium seed. The identification of post-test phases in

electrodes is relatively simple, provided care is taken in accounting for not

only every strong diffraction line, but' every weak maximum in dif f ractograms

.

This point is crucial, particularly for materials taken from the yellowish

interior zones of most Zr02/Ce02 electrodes. For example, the phase termed

(Cl) in Table C.2.1 is not a true cubic solid-solution based on the fluorite

structure, but rather a cubic material based on a similar structure-type . This

identification can be made only on the presence of certain weak diffraction

maxima

.

It is a particularly difficult task to reconstruct a selfconsistent scenario which encompasses electrochemical/environmental factors

during testing , and, therefore, which accounts for the observed post-test facts

in the form of a series of established phases and phase assemblages. Additionally, the effects of the cool-down process during which reoxidation of materials

and, possibly, back-diffusion of seed can occur must be factored into the

scenario. Clearly then, attempts to reconstruct such a scenario is similar to

piecing together a complex jigsaw puzzle which lacks important parts. A detailed account of phase transitions and chemical reactions in Zr 02 /Ce 02 materials

is beyond the scope of this report. These data will be detailed by T. Negas (NBS)

elsewhere. Several typical examples, displaying the alteration of Ce02/Zr02 cathode

materials, however, are illustrated below.

Figure C.2.1a is a portion of the x-ray diffraction powder

pattern of the untested 82 (mol %) Zr02~18Ce02 composition. It consists of

single-phase, tetragonal zirconia solid solution (T) . Figures C.2.1b, c, d, are equivalent x-ray tracings for this material (electrode 1205, open circuit)

after the U-02 test. Figure C.2.1b is the pattern for the upper (white),

"hot" zone of the electrode. Notice the crystallographic changes within

this zone (compare with Figure C.2.1a). These changes are unrelated to

reactions with potassium, but are typical for materials subjected to elevated

temperatures followed by reoxidation on cooling. Single-phase, stabilized

tetragonal zirconia at the 82-18 composition when heated above 1700-1750° under

relatively oxidizing conditions (? 02 ^ 10“^ atm), decomposes to a Zr02-rich

tetragonal solid-solution, plus a cubic fluorite solid solution richer in cerium

oxide. If the temperature is high enough, a single-phase cubic fluorite solid

solution develops. If reducing conditions, whether provided by a gaseous en- vironment or made effective by electrochemical means, are prevalent, this stabilized tetragonal zirconia solid-solution decomposes, even at relatively low

temperatures, to a zirconia-rich solid solution plus a cubic phase richer in

reduced cerium oxide but not a true fluorite. Concurrent with these phase transitions is the vaporization of cerium oxide. Loss of cerium oxide, particularly

at the "hot" surfaces, drives the bulk composition (82-18) toward compositions

richer in Zr02. When cooled to room temperature, and if not enough time is

permitted to completely re-equilibrate the bulk composition to the thermodynamically stable phase(s), a multi-phased, reoxidized assemblage results. Zr02~r'ich

solid solutions formed at the elevated temperatures via decomposition reactions,

or through vaporization localized at the surface, no longer are stabilized but

transform to monoclinic zirconia when cooled through the tetragonal-monoclinic

transition. These phenomena are not peculiar to the 82-18 composition, but are reflected by most Ce02-containing electrodes at the cathode and anode (hot

surfaces)

.

Figure C.2.1c and C.2.1d are pertinent x-ray tracings of the

yellow, and yellow to olive-green zones, respectively, of the electrode (see
115

Figures C.2.1a and C.2.1b). These phases appear to be related to reductiondecomposition reactions and/or reactions with seed in situ , followed by reoxidation on cooling. The symbol M represents nearly pure, destabilized monoclinic

Zr02; (Cl), a cubic phase similar to, but not a true cubic fluorite;- and P, a new (K, Ce, Zr)-oxide. Figure C.2.2 is a portion of the x-ray pattern of

this new mixed oxide, prepared in this laboratory, coexisting with Ce 02 .

Figures C.2.3a, b, c are the pertinent x-ray tracings for

similar zones (white, yellow, and yellow to olive-green, respectively, for

electrode 1208, 1.25A/cm^. These also should be compared with the untested

82-18 material. Figure C.2.1a. These patterns indicate little difference

between electrodes 1205 (open circuit) and 1208 (1.25A/cm^).

Figure C.2.4a represents the 50Zr02~50Ce02 composition prepared under oxidizing conditions below 1500°C. It is obvious that this composition consists of two phases, a Zr02 -rich, tetragonal solid-solution plus a Ce 02“ rich, cubic fluorite solid-solution, as predicted by phase diagrams. I'Jhen heated

above 1550°C, this two-phase mixture transforms to a single phase, cubic fluorite

solid-solution. Figure C . 2 . 4b . U-02 test specimens for this composition (anode

and cathode) are in very poor condition, and an x-ray pattern directly of the electrode surface could be obtained only from 2312 (anode). Nevertheless, it is

evident from the anode data (see this report) and from the Battelle post-proof

test data (anode and cathode, short-term test), that the "ho.t” portion of the

50-50 electrodes shows direct evidence (materials of smaller cell dimensions) for

vaporization of cerium oxide.

Figures C.2.5a and C.2.5b represent the yellow, and yellow

to olive-green (bottom) zones, respectively of electrode 1310 (50Zr02-50Ce02)

,

0.8A/cm . Notice the significant structural-chemical change when compared with

Figures C.2.4a and C.2.4b. Indeed, it is possible to identify the (K, Ce, Zr)-

oxide (P) phase as well as nearly pure Ce02 (C 2 ) . The (Ci)-phase, mentioned

above, is also obvious. Electrode 1312, 50Zr02“50Ce02 , open circuit, retains

these features but phase (P) is far less evident. In general, open circuit

materials, especially within the 14 and 15 series (high Ce02~content) appear to

be far less altered by seed.

The purpose of comparing the x-ray dif f ractograms of

several pre- and post-test materials, is to demonstrate unequivocally that

chemical/structural alterations have resulted for Ce02-containing electrodes.

These alterations can be attributed to the combined effects of electrochemical/

environmentally imposed reduction, reactions with seed, and partial reoxidation during

cool-down. Let us now consider the x-ray evidence with respect to a pictorial

review of the information.

Figures C.2.7 and C.2.8; Figures C.2.9 and C.2.10; Figures C.2.11

and C.2.12; and Figures C.2.13 and C.2.14, compare the gross features of anode and

cathode cross-sections, respectively, of the 82Zr02-18Ce02 , 50 ZrO2-50CeO2,

25Zr02~75Ce02, and 78Ce02-20Zr02-2Ta205 materials, respectively. Reduction/

reaction of cathode materials, manifested by the discoloration and banding or zoning, alluded to above, is clearly evident. Compare, for example, the

colorless anodes in Figure C.2.9 with the cathode counterparts. Figure C.2.10.
116

(The Ta 205 -containing material is greenish in color to begin with and, hence,

distinctions are more subtle, see below.) Let us now continue and "zero in" on individual electrodes to indicate certain features which cannot be

ignored by those seriously attempting to characterize these materials.

Figure C.2.15 shows electrode 1205 (cathode, 82Zr02-18Ce02)

.

Here we observe clearly the various zones characterized by x-ray diffraction in

Table C.2.1. The lowermost, olive-green, band is a reaction zone which

consists predominately of monoclinic zirconia plus the K, Ce, Zr-oxide

(Phase P) compound. The yellowish interior zone consists predominately

of monoclinic zirconia plus the cubic pseudo-fluorite (Ci) phase. This

zone, in turn, grades into an upper white zone characterized by monoclinic

Zr 02 plus poorly crystalline cubic fluorite and tetragonal solid solution

variants. The (P)-phase and (Ci)-phase are discussed below. Figures

C.2.16 and C.2.17, cathode electrodes (82Zr02-18Ce02) 1206 and 1207,

respectively, reveal nearly identical zones differing only in thickness,

not phase assemblage. Figure C.2.18 is the anode electrode 2207 corresponding

with cathode 1207, Figure C.2.17. The bottom of 2207 consists of unaltered,

fully oxidized, tetragonal zirconia solid-solution. However, above this

zone, notice the greenish "bleb" (also evident in electrode 2208, see Figure C.2.7). This is a reaction zone, characterized by the (P-phase)

K, Ce, Zr-oxide plus monoclinic Zr 02 , but which now is located, not at the

electrode base, but within the interior. ^ Figures C.2.19, C.2.20, C.2.21, and C.2.22 show cathode

electrodes, 1309, 1310, 1311, and 1312, respectively, 50Zr02-50Ce02 . This is a particularly interesting series as these electrodes functioned throughout the

test period and, therefore, would be expected to best characterize reduction/

reaction phenomena. Again, notice the distinct coloration banding which, in

contrast, is absent for anode materials (see, for example. Figure C.2.23, anode

2312, 50Zr02“50Ce02) . Because the initial bulk composition is richer in Ce02 , phase assemblages, of course, are different from the previous ( 82 Zr 02 - 18Ce02 ) series. In Figures C.2.19 and C.2.20, there are three zones of discoloration

clearly evident. The greenish bottom zone is one involving reaction with

seed and consists of the (P)-phase plus the (Ci)-phase together with phase

(C 2 ) , (see Table C.2.1), a cubic fluorite having a cell parameter approaching

that of nearly pure Ce02 . The yellow zones are nearly pure (Ci)-phase.

These grade into the white zones, characterized by cubic fluorite solidsolution variants. The 0.5 A/cm^ and open circuit materials (Figures

C.2.21 and C.2.22) show the yellow (Ci)-phase band extending to the

electrode base. Extensive reaction with seed is not apparent although

x-ray data reveal small amounts of the (P)-phase in both materials (see

Table C.2.1) .

The 25Zr02-75Ce02 composition cathode series is illustrated

in Figures C.2.24 (1413, open circuit), C.2.25 (1414, 0.5 A/cm^), C.2.26

(1415, 0.8 A/cm^), C.2.27 (1416, 1.25 A/cm^ , interior cross-section) and

C.2.28 (1416, 125 A/cm^, outside cross-section). Again, because of differences

in initial bulk composition and electrochemical characteristics, phase

assemblages for this series differ somewhat from those of the previous two
117

series. Electrodes 1413 (Figure C.2.24, open circuit) and 1415 (Figure

C.2.26, 0.8 A/cm^ , but failed during the test) have similar features. Both

appear unaltered (i.e., no reaction with seed or phase transitions), although

reduction is evident by the yellowish (fine-grained) -blue (coarse-grained)

bottom zones. The extent of reduction during operation is unclear because,

on cooling, reduced Ce/Zr-oxide readily reoxidizes. It appears that the

finer-grained yellowish groundmass has reoxidized to a greater extent than

the blue coarse grains. Electrode 1414 (0.5 A/cm^) which operated

throughout the test and electrode 1416 (1.25 A/cm^) which failed during

the test reveal additional features. Notice the yellow-green zone

in Figure C.2.25 (1414) and the yellow-green outer edges of electrode 1416

in Figure C.2.28. These are reaction zones characterized by the (P) -phase

plus a cubic fluorite solid-solution with a cell parameter approaching

pure Ce02 (C2 -phase, Table C.2.1). Yellow to white zones consist of cubic

fluorite solid solutions similar to the starting material. Figure C.2.27,

an interior cross-section of 1416, shows only diffuse reaction areas, generally

limited to the lower-outer edges of the electrode. The blue-zone is reduced

cubic fluorite solid solution.

Figures C.2.29, C.2.30, C.2.31, C.2.32, C.2.33, and

C.2.34 are cathode and anode materials for the 78Ce02-20Zr02-2Ta205 composition.

Electrodes 1517, 1.25 A/cm^ , (Figure C.2.29) and 1518, 0.8 A/cm^ (Figure

C.2.30) show nearly identical features. The olive-green bottom zones are reaction areas and consist of the (P)-phase plus cubic fluorite solid

solutions with variable cell parameters (but predominately a fluorite

phase having a cell parameter approaching that of nearly pure Ce02) . Upper

yellow-blue portions consist of a mixture of several cubic fluorite solid

solutions similar to the original starting material. In contrast, electrodes

1519, 0.5 A/cm^, (Figure C.2.31) and 1520, open circuit, (Figure C.2.32) are not altered by seed, but were subjected to some reduction. Compare, for

example, electrode 1519 (Figure C.2.31) with its anode counterpart, 2519,

Figure C.2.33. The green coloration at the anode is more typical of the

original starting material. Anode electrode 2520, open circuit. Figure

C.2.34, also provides an interesting contrast with its cathode equivalent,

1520, Figure C.2.32. Notice the finer-grained, olive-green zone (lower

right. Figure C.2.34). Again, this is a reaction zone characterized by

the (P)-phase plus nearly pure Ce02. Notice also the thin, white, uppermost

zone in Figures C.2.31, C.2.32, C.2.33, and C.2.34. This zone reflects

loss of coloration due to loss of Ta205 , presumably via vaporization.

Figures C.2.35, C.2.36, and C.2.37 provide additional

views of electrodes 1205 and 1208. Figure C.2.35 is a longitudinal fracture

surface of 1205, while C.2.36 is the cross-section of an interior fracture

surface. Both clearly show the distinct banding previously discussed.

Figure C.2.35 also clearly shows a white, vitreous-like (recrystallized?)

uppermost zone which is typical of most materials, including the Zr 02 /Y 203

materials. Figure C.2.37 is a cross-section, fracture surface, of electrode

1208. Again, the zoning is obvious, but the lowermost (reaction) zone is

somewhat thicker than in electrode 1205.
118

Several lines of evidence have: a) aided in the

identification of phase and phase assemblages in tested electrodes, and

b) permitted the observation that considerable reduction of materials,

particularly, 82 Zr 02- 18Ce02 and 50Zr02-50Ce02 , must have taken place

during operation followed by reoxidation on cooling. The P-phase

,

associated with reactions with seed, is a cubic perovskite. Single crystals

of the material identical to that found in electrodes were grown in the

laboratory. EDX analysis revealed a composition near KCeZr 20 g. The phase

termed (Cj^), a cubic pseudo-fluorite with pseudo a, = 5.26 A is clearly a pyrochlore- type compound with true a. = 10.52 A. Within the yellow zones

of the 50Zr02/50Ce02 electrodes, it is found in nearly single phase form.

This is not unusual as the pure pyrochlore Ce 2

^'*’Zr 207 has the correct Ce/Zr

ratio existing in the electrode material. However, pure Ce 2^"*’Zr 207has a cell parameter, a. = 10.74 A, which is considerably larger than the material

found in the electrodes. With increased amounts of the smaller Ce^"^ cation

in a pyrochlore formulation such as 2^l+\ / lyi'f the cell parameter

should decrease (provided that transformation to a cubic fluorite does not

occur). Thus, to arrive at a small cell parameter of a = 10.52 A, a reduced pyrochlore must be at least partially oxidized. The experimental

proof is straightforward. First, pure Ce 2 Zr 207 was prepared at 1475°C, under vacuum,

in a thermogravimetric apparatus. The grey-green material has a. = 10.74 A.

This material was next reoxidized in air, but not permitted to transform

to the equilibrium phase assemblage in air. The result is a yellow product,

nearly fully oxidized, but yet a pyrochlore with ^

= 10.52 A, identical with

electrode alteration products. Additional experiments have revealed how

Zr02 /Ce02 materials react with alkali seed to yield the phase assemblages

(including monoclinic Zr02 ) observed in tested electrodes.

It is difficult to attribute reduction of materials at

the cathode totally to the MHD Pq environment, particularly within the lower

temperature regimes at the base oi lower portions of the electrodes. Electrochemically aided reduction at the cathode base must remain a strong

possibility. Such a process would yield relatively unaltered, oxidized , phase and phase assemblages at the base of anodes. Indeed, this is exactly

what is observed. Seed penetration may also contribute to electrochemical

processes

C. 2. 1.2.

Y 203 -Stabilized Zr 02 (cathode) and MgO Insulators

The 88 Zr 02 “ 12Y 203 cathode materials exhibited less

obvious alteration. The upper (<1 mm surface) of these electrodes (cathode)

apparently recrystallized, forming a surface with a glassy-like appearance.

This zone separated and buckled from the underlying electrode groundmass. Within

portions of the bottom of these electrodes, monoclinic (destabilized) zirconia,

as well as a cubic zirconia solid-solution, with a cubic cell parameter smaller

than the pure (82-18) material was identified. These phases may result from

reaction with seed or from electrochemical action but time has not permitted

definitive answers based on laboratory experiments.

MgO insulators (cathode and anode) were investigated by

x-ray diffraction for potential chemical/phase changes. Apart from the extensive

amount of seed (as K2 CO 3 and/or K2 CO 3 hydrate) found beneath the cathode electrodes within the MgO, insulators exposed to the plasma were altered chemically
1 L 9

at their surface. Surprisingly, x-ray lines (only a portion shown in Figure

C. 2 . 6 ) corresponding to a cubic, spinel-like material are present in the typical

MgO pattern. Virgin material does not contain this phase. If this material

is a spinel, the small, cubic cell parameter suggests that it is MgAl204. This

phase is also present in trace quantities at the surfaces of many of the anode

materials and probably of cathode materials. The latter could not be confirmed,

as cathode surfaces were too fragmented and irregular for direct x-ray analysis.

If MgAl204 is the phase present, then AI2O3 somehow must have been transported

via the vapor phase to react with available MgO. This may appear to be a trival

point, but it should be recognized that a chemical reaction leading to the formation

of a new chemical species could have a degrading effect on the original mechanical/

electrical properties of a virgin material.

C. 2 . 1 . 2.Special Considerations

Although this portion of this report is concerned only with

the documentation of phase/chemical characterization of electrode materials, several

related questions remain open to interpretation and further study. Given that

alteration of materials proceeded at the cathode at an accelerated rate with re- spect to the anode, how does seed reach, penetrate and react with materials? Is

the difference in reactivity of cathode and anode with seed due to better sealing

or higher working temperatures at the anode? If not, are alterations at the

cathode accelerated by electrochemical means? It appears unlikely that seed as K(g) is the dominating factor, otherwise anode and cathode materials should re- flect nearly identical chemical reaction phenomena. Possibly, attracted

from the plasma to and/or formed at the cathode surface (and attracted) according

to K(g) -> K"^g) + e is the controlling factor. K"*'(g) at the anode necessarily would

be repelled except, perhaps, for open circuit materials. Given the reality of

seed penetration and reaction, which should accelerate with time, can cathode

materials based on Ce0,2./Zr02 components, even with their additional, inherent

problems such as vaporization of cerium oxide and phase transitions particularly

at low effective Pq 2 environments, be "sealed" to prevent attack in future, longer

duration tests? Finally, to what extent are the phase transitions and attack by

alkali seed detrimental to electrode performance?

C. 2.2 Anode Materials (A. Perloff)

Anode materials show little evidence associated with alteration

by potassium seed. The distinct color zoning, typical of cathode, Ce02/Zr02

electrodes is not obvious although a faint yellowish discoloration is apparent

at the bottom of some electrodes. Direct evidence for reaction with seed was

found by T. Negas at the bottom of electrode 2520 significantly, an open circuit material as well as 2207 and 2208 (see above)

Generally, the hot surface material of anode electrodes tends

to be monoclinic, cubic and/or tetragonal phases with smaller unit cells than the

original bulk material. This suggests that the surface phases are richer in Zr02

and were formed via vaporization of cerium oxides. For example, the Zr02“Ce02

compositions initially rich in Zr02 ( 82 % Zr02, 18 % Ce02, 50 % Zr02, 50% Ce02) also

show the formation of some monoclinic Zr02 on the surface. The crystallinity of

surface materials is generally poorer (i.e., finer particles or more disorder) than

the original materials, which is an additional indication of decomposition. Many

of the samples have small amounts of a spinel phase (most probably MgAl204) on

the surface. Table C. 2.2 summarizes pertinent data.
120

28 29 30 31 32 33 34 35 36

20CuKa

Figure C.2.1a. Portion of X-ray diffraction powder pattern of the 82Zr02-18Ce02

,

1500°C material prepared under oxidizing conditions (untested). T = tetragonal

zirconia solid-solution. Compare with Figures C.2.1b, C.2.1c, C.2.1d and C.2.3a,

C.2.3b, C.2.3c.

28 29 30 31 32 33 34 35

26CuKa

Figure C.2.1b. Portion of X-ray diffraction powder pattern of cathode electrode

1205, open circuit, 82Zr02-18Ce02 (tested) . t-Thite upper surface, see Table

C.2.1. M = monoclinic zirconia; C, T = poorly crystalline cubic fluorite and/or

tetragonal solid-solutions.
121

28 29 30 31 32 33 34 35

20CuKa

Figure C.2.1c. Portion of X-ray diffraction powder pattern of cathode electrode

1205, open circuit, 82Zr02~18Ce02 (tested) . Yellow interior zone, see Table

C.2.1. M = monoclinic zirconia; (C.) = cubic pseudo-fluorite (cubic subcell

a=5.26A); P = (K, Ce, Zr-oxide) . ^

28 29 ' 30 3F" 32 3J“ 3T" 35

'

29CuKa

Figure C.2.1d. Portion of X-ray diffraction powder pattern of cathode electrode

1205, open circuit, 82Zr02-18Ce02 (tested). Yellow, olive-green bottom zone,

see Table C.2.1. M = monoclinic zirconia; (C-) = cubic, pseudo-fluorite (cubic

subcell a=5.26A); P = (K, Ce, Zr-oxide). ^
122

ZeCuKa

Figure C.2.2. Portion of X-ray diffraction powder pattern of synthetic products

resulting from reactions of ZrO^, and K

2

O. P = (K, Ce, Zr-oxide) . Compare synthetic phase P with P phase in Figures C.2.1c, C.2.1d and C.2.5b.

2« 29 ”30“^T“

29CuKa

Figure C.2.3a. Portion of X-ray diffraction powder pattern of cathode electrode

1208, 1.25 A/cm^, 82Zr02-18Ce02 (tested) . White zone, top, see Table C.2.1.

M = monoclinic zirconia; C = two or more cubic fluorite solid-solutions.
123

28 29 30 31 32 33 34 35

20CuKa

Figure C.2.3b. Portion of X-ray diffraction powder pattern of cathode electrode

1208, 1.25 A/cm , 82Zr02-18Ce02 (tested). Yellowish interior zone, see Table

C.2.1. M = monoclinic zirconia; (C^) = cubic, pseudo-fluorite (cubic subcell

a=5.26A.

20CuKa

Figure C.2.3c. Portion of X-ray diffraction powder pattern of cathode electrode

1208, 1.25 A/cviy (tested). Yellowish-green bottom zone, see

Table C.2.1. M = monoclinic zirconia; (C^) = cubic, pseudo-fluorite (cubic

subcell a=5.26A; P = (K, Ce, Zr-oxide)
124

n

lie

28 29 30 31 32

‘ ""34 " 35

zeCuKa

Figure C.2.4a. Portion of x-ray diffraction powder pattern of ttie 50Zr02“50Ce02

>

1500®C oxidizing conditions (untested) . T = tetragonal zirconia solid-solution;

C = cubic fluorite solid-solution. Compare with Figures C.2.4b, C.2.5a and

C . 2 . 5b .

26CuKa

Figure C.2.Ab. Portion of X-ray diffraction powder pattern of the 50Zr02~50CeO^

,

1600®C oxidizing conditions (untested) . C = cubic fluorite solid-solution.

Compare with Figures C.2.4a, C.2.5a and C.2.5b.
125

28 29 30 31 32 33 34 35

20CuKa

Figure C.2.5a._ Portion of X-ray diffraction powder pattern of cathode electrode

1310, 0.8 A/cm , 50ZrO--50Ce02 (tested). Yellow interior, see Table C.2.1.

(C^) = cubic, pseudo-fluorite (cubic subcell a=5.26A); P = (K, Ce, Zr-oxide)

Compare with Figures C.2.4a and C.2.4b.

(C2) iiP

I,

'

IC.)

1 il !'='

' i

28 29 30 31 32 33 34 35

20CuKa

Figure C.2.5b.- Portion of X-ray diffraction powder pattern of cathode electrode

1310, 0.8 A/cm^, SOZrO^-SOCeO^ (tested). Yellowish-green bottom zone, see

Table C.2.1. (C-.) ~ cubic, pseudo-fluorite (cubic subcell a=5.26A); P = (K, Ce,

Zr-oxide); (^

2

) = cubic fluorite solid-solution a^=5.38X.
126

20CuKa

Figure C.2.6. Portion of X-ray diffraction powder pattern of a typical MgOinsulator, "hot", surface. M = MgO, S = spinel, probably MgAl20^. S not

detected in untested MgO.
127
128

Figure C.2.10 Cathode
129
130

132

Figure

134
135

137

W 3 H- c n 3 n

O' rf n

•-I o

Q.n 0

K)1 H-* 00noo

00nno oO h-i 3 3

•3 H» O n

05

CO ^ 3 ft> D

00 CO n zr

rr

O

(b 3 O *3 3 M

(bC

3*I OO

(b

(b3

fb

(b 3

fD rr 3 rD O' H- o o

n- r-i fTO *3 S H-C

H.

€ 3 3* 3 M

0) 3 M *3O 3 CO 3 M. ^ O N 3

fO ^ 2 O O

0\ T3 > h-c

-3 3 3* O 3 3

CO

O o n

CO H*

O 3

ix (t>

|3 3 3

rr o o 3 3 O

h- 3oo

rr H- O N

.-n '1O

nro c

cr + O "33*

CO 3 O CO ^ /-s o •03 r>- on N H. I CO 3 O f-t O X O

w. 3 ^3 a. 00 3 *— o w • fsj

|3

II Ui +

Njn

3N

C > cr

“3 3 3 O 3 i-h O3

TJO

3"

O 3 h- W

rD 3 3 C

H- O. « O K H3* C

M.

O

mN 0 ^

h- o

KfO O "3 H- O c o o3

CO

n + 3 C >-l cr t-*

o. rD O 3 n o3

rr

O

(bn

CO + 3

(b fb ^ DO o Ln < • D

a.

»-*•

D

n 2

SUMMARY

OF

X-RAY

DIFFRACTION

PHASE

ANALYSIS

FOR

CATHODE

MATERIALS
//3

darker

yellow

bottom

At

least

two

cubic

fluorite

solid

solutions.

Major

phase,

^=5.3AA;

minor

phase

lias

somewhat

larger ^

138

00 U) > t-* o

o

5

ho

NJ Ud

Ui O

> VO

o

ft) o

LnO^3

On

n>o

UloN

i0

sJ

1 L/1 O

o

ft) o

oN

Ho

fto

ft (t 3“ =r

T3 3 M. Z

D =r ft ft (t

CA3*

O ft €

3 O n "Tj (t ^r(H* *3 O M

»-t c

CQ

fth- ft 0 h- c »-•

1 O

00 C

l-{ H-

(t W

ft 3" 3

H- cr 3 O rr rr (t rrO H- S O

ft

I 00

(tft3

O'o

COO' 3“

OC

r-{ m

o

3

O'o

> CO a > 0 O' /-N 3 > w Z 0 3 X y-V rt y-*\ X rr 0 3 3 z

h-* c 0 rr O c o 3 v-» c ft rn^ O ir 3 3 r{ o 3 3 rh O' 3 3 D

3 CO 3 I-* rit-* OQ 3 X 3 3 N-X 3 3 'w' ?) Cl 3

O o H- 3 'W >» 3 C o n 3 'w' ft 1 O w ft O 1 y—V O') 1 rt

cn ft 3 ft 1 CO 1 ft CO ft »—• O 1 ft 3 O 1 ft 3 o ft 3 3 M

rf 3 3 rr *3 f-* 3 rr rr •3 3 3 3 3 O' 3 3 O' NJ O' X

rr 3 CO 3* 3* M* 3“ H* rr 0 O' X X 3 O O* X 3 3 'w' y—

3 X 3 k«i. rr rr 3 CP 3 o 3 H- *3 n 3 rr X C 3 O X 1 X X 3 3

C O' r-{ CO * CO C cr C M X y—

ft CT X y—S 3 r 3 rr ft n C

»-{ M- -3 rr CU (t rr 3 cn H« -1 ft rr w- a 0 cr rt o rt

ft 1—

O' 3* <• M c (t 1—ft 3 Z ho X o • ro n < 3 O < 3 3

« cu n W CO H- < c 'w' rr H- O X 3 ft CL

rr w ft c *3 rr n —s. rr V n ft rnjo HU r{ O OQ rt 3 ft ft cr r—

H- *-f o ri X o rf n 1—O r- li »-* 3 X X n C; O V H- 0 3 O TJt— <• h-i X X c u> 3 C Cn L_> H* I® C-* o CO n O CO n 3 rr '—' rt cr f—* CQ C • o n X + X C” n ro 3 c CQ CQ 1 N) 1 O X O rt hO vO rr S rr H- 1 1— 3 O' •3 o ft *3 cn •3 rv> rr o + HU O' •9 CV3 rt X o 3

o rt h** c »-f + O' o r< rr > rr O rr 3 O 3- O n 3

|3 3 n 3 “3 O X y-V 3 3 o 3 3* 3 3 3 3 o 3 CO 3 H* X • X h-

3 o n < 3 hO CP 1—CP 3* X

I-* cn •3 ro o Cl rr ft ft C CP hO X ft (P 3 3 3

o* CL X h- •» X H-* 'w' 0 rt 3 •r C 3 X w

CO o s: CO h-

h** 13 o |3 CL 1 K- X ft rf 3 H- ft rr (0 o c rr 3 11 OVT /«-N. 1! 3 Hi* rr X 3 3 c 1® X X tx>* ft rr h-4 CO H* rr cn X r-i n cn X 3“ O. X H^ rr O X X U H- CQ CO 3' 1-^ o • OQ 3 fsO • o 3 1 rr O rt O c M* cn

n w D. rr 3 n; ho II O ho )—

X to r^ O ft 3 y-S 3 • o CO \ 3 Ui O' o ft 1 o\ c D 0 c X 3 O rr o H> ro 3 •3 3 CO 3 —

o > 9 •3 > rr ^ 3 c CQ N) tsj I-* Ov

ft M 1—

c 3 o n O ro 3 3* + C CP CO y-V 3 >

VD OJ 0 h- (t O « + O 3 O r-r o x-u. o I ri o « »-c c 3 X 3 M. 4- ft o H- o 3

c 't rr cr rr O ft c X n C 3 h-

>>- X H* D“ t7* X f-f M* 1— o rn n ft K- 3 ^ r* I cr 3 c

(t 0 'w' r> at) )— 3 |3 'w' 3 M* 1 3 1-^ X

o n |3 3 f X O H* pr H 1 cr 3 3" o I® o -t « "3 •3 CO ft 3 cn 3 1 H*- O' 3 rr c X 2T 3* }i X—». r* 3 H-i 3 X rh rr ri H- CO lit 3 3 o 3 n 3 ho 3 O' h" X ft uw 0 3 c rr bp CO CO C 'w' VO X 3 rr ft • c O

o 3 3 Cn ft n h-3 ^3 1 > 3 X X o y—

3

3 H- cn O rt 3 D • + 3 rt o X

H* CQ l-rf U) o O 3 O Hrr 3 U) C ho 3 3 < hO y—

\

rr rr 'w' O

ft ft 3 > H* X O. P cn o H* ft rr> rr ft rr 3 3 (t 3 0 N) o •

CO « •3 O' 1— 3 1® ?r O 'W' 3 S

o O Jt OQ 3 H • 1 1® o

H- C CO 3 3 Cn + 3 X H 3

CO ft a. • *—

3* H*- cn O

Q. 3 n rr hO 3 o 3 3 • o

1 o H* OV 1-^ O' X U) L-

CO n hO n rt > 3 X 3 OD O 3 3 3 O CP 3 > 5

L_< n n ri II + rt H* c ft p ts) H* O
#4

green

bottom

Nearly

single-phase

cubic

fluorite,

^=5.35A

139

O3 O n f— • HM* V,/! r1 N> > h- O O VO C O

H.

g

rr N3

O• H-* 00 Ln > ^ '— 00 na

hO

K>

M£«• > »-• 3 O00 H- >O5

|NJ I K) OO H n 0) <v NJ o

Oo

Ui I NJo

ro I «vj hO 00 H n

Q>

n

s) o

Oo ^ t hOoNO INO 00 H O

CD (t s) O

Oo

ji I NO I -vJ NO CO H n

to

n

so o

CKO

J* I NO

NO NO Ln Kjr NJ tsl

»-t 0 O

NO NO 1 I 'vj Ln Ln n O

to to o o

NO NO

NO

Ln

NJ0

NO1 Lnn

too

NO

=«s ^ =55i )-• CJ UO

too

•3No3

to

w

toto

cro

fT

rrO3

09

toto

croo3

NO

rr L- O to C M

rr

w

O 3* o

00

l-{ toto3No3

to

to0c1 OOn

toto3

a*o

rr

ftO3

to0t1 OO

toto3

croo3

toNo3

to

to

-3O3O3 O3 H

s:0

a.o3333Oc

c*

to TJrr3W

tow

uoON>

cn ^ cn !oo >

DO

O to rr 3 ctQ Ln

to

X to . h00 LJ to to II to CD to w + CO 0) O rr ^ Ln ^ CD =55: • rr NO NO N; LJ 3"

--I • to K- to Ln Ln to — »— 3 to VO VO a. c « cr Ln M. 3 • to to LO

00 Ln *3 3 > rr

to to rr to H« to to to

toC

to

toto 3 to

NO

CD>

to

NO

Ln o

to

to X

to •o

toto

toa. o

cP H'

Cl Z

10. °3

Ln to 3

to to

*3

rr

toto

to

to 33O

tooc

crn

to3

toto

-o uo

3* 0^

to >

to

to > M H- 3 >

rr

O

O P* ft CN OT3

h-i

X >- o hto OO CO to to

to It II to

to o I—* to to ft • • 3* ft

H-* LJ to rr Ov vO to ft 3* ’••to €

to to o

to to 3 3*

to to to o

i-w • OO to

C 3 3 3*

•3 to to ^ O p 3 n

=sfe to I— to (—•to O h- r-n

to

to3

toH

to3

OO

to^ 3* > —

i-ri -n O

hCnc

3*

to *33*

to

to

totoH3*

to

toC

toto

to *3

rr • t-u ro "3 rr CO rr

4-U > Q) — to M. g to rr to to X 00 • - 3

to N) rr 2! Ln •-*• to o o <-»• n o • ton c O to *3

<-nn 3*

to to

C -3 to 3 ft to

€

rr3*3

toto

toa9

torr

ton

p- Ln

u- LJ>

NO -

Lno O I *3

3*

to

to

to+3

too

r^nc

3*O

to

Ito >

rr Ln

LO to CD to > toft

to3 ft a. rrn

to to

(-•• to 3

r-»

O

P*C

to a* n r-u n o *3 3 to C to

rt to

to to n +

toO ^ NO rj

to

to

to

•3

rr

to

to

to

to

to I o 3 n

3*

to "3

to 3"

to to • to

to

O' 2

to u^O

r-l •3

to

CO

to3*

toCO

O'

ton 3 C

to cr

|to c

uoI Ln

cJLn>

T5

(—C

tooc

3^OmCon

toC

3*

00

OO z

totonXoco

Ito

II

uo

is>

*3rr

to

CQ

to

Ln

UO>

to rh r-9 p* o

CO P o o

*3 to *3 C C n p 3“

to to p*

to to

to o 3 to tf, p O P*

NO

C • O p to 3O *3 p rr

to •3 CO rr to

to CO

(0 • torr 2

to to

CO Uuo

CO P pP -3 00 rr

rr to P CO p to

CO II 3 Ln

to • p uo

c»

to > p •

|to >

IIp

Ln• p

u> to Ln to > CO • • p 3 p p Z 3 O Op toc

*3 O*

3* P

to to

CO

to

p

3-

C

to o

to to p

00 CD

to o p po3

CO

|to 3 O • to p

to p. p -3

PJ P 3* w v< to I CD *3 P to 3* 3*

to to

to
140

n

c

fT O• M

Ln H- > O

U) n

B

NJ

Ln o > S

QD

00N

00

QDM

OU)

hJK

K)o

I •o3

* (D

•o

COn

c

a.o

|n>HU1

ON>

*: H. O* D O

CO rr C rr M O

fi) 3

rr • On CO

Q>3

rsM

ro3

<t>

X

00o

H* a* D o

CO rr C rr H* O

to 3

3

(03

r<3

00o
141

TABLE C.2.2

Anode Electrode X-ray Results

1. Composition — 88% Zr02 , 12% Y 2 O 3

O Original - Very well-crystallized f.c.c., a=5.151A

O Post Proof Test - Well-crystallized f.c.c., a=5.136A.

Very slight suggestion of line splitting which could

indicate lower symmetry or multiple phases.

O Post Phase I Test Samples - bulk material f.c.c., a=5.15A

2101 Surface crystallinity good, f.c.c., a''^5.14A.

Intensity varies from original. Line

sharpness varies to suggest splitting in

some directions.

2102 Surface crystallinity good, f.c.c., a''^5.14A

Oriented intensitites . Some peaks show

distinct line splitting.

2103 Similar to 2101. Oriented intensities and

probable line splitting. Approximately

O f.c.c.

,

a=5 . 14A.

2104 Approximately f.c.c., a'^^5.14A. Oriented

intensities and some distinct line splitting

Small amount of spinel visible.

2. Composition — 82% ZrO 2, 18% CeO 2

O Original - Almost all tetragonal, a=5.15, c=5.24A

O Post Proof Test - Almost all f.c.c., a=5.11A. (Surface)

Post Phase I Test Samples - Crystallinity generally poor although some

of the poor resolution may be due to uneven surfaces.

2205-2208 All surface patterns show primarily a cubic (or very

O nearly cubic) phase with av5.1A. They also show monoclinic Zr02 and spinel (probably MgAl 204 )
142

3. Composition — 50% Zr02 , 50% Ce02

O Original - Primarily tetragonal (a=5.26, c=5. 30A) plus a small

O amount of a cubic phase (a=5. 15A) or tetragonal

O (a=5.15, c=5.24A) and a trace of a probably cubic

O phase (a=5 . 34A)

O Post Proof Test - Very poor pattern which is primarily f.c.c., a=5.12A.

Post Phase I Test Samples - very poor condition. Only the 2312 sample

could be used to obtain a surface pattern. All other patterns

were obtained by grinding samples from the top and bottom portions

of the electrodes.

2309 Top portion is mostly cubic (or nearly cubic) a=5.26A.

It also contains small amounts of approximately cubic

O O cells with cell edges of 5.21A and 5.16A plus a small

amount of monoclinic Zr02 .

Bottom portion has three phases present which are

O similar to the original material: cubic, a=5.33A;

O * O tetragonal a=5.27, c=5.3LA; tetragonal a=5.15, c=5.24A.

2310 Top portion is mostly nearly cubic (probably tetragonal

O a=5.26, c=5.29A) plus small amounts of material with

smaller cells (^5.21A, v5.13A).

O Bottom portion is mostly cubic (a=5. 34A) plus some

O tetragonal (a=5.15, c=5.24A).

O 2311 Top portion is almost all tetragonal (a=5.26, c=5.30A)

plus a trace of a smaller tetragonal phase (a=5.15,

c=5.24A)

O Bottom portion is mostly cubic (a=5.34A) plus some

O tetragonal (a=5.15, c=5.24A).
143

2312

/ 0 The surface is mostly two phases: - cubic (a=5.14A) and

0 tetragonal (a=5.18, c=5.26A). Small amounts of monoclinic Zr02 and spinel are also visible.

The bulk surface material is primarily cubic (or nearly

0 so) with a v 5.27A. It also contains smaller amounts

of phases with smaller cells with cell edges v5.21,

5.16, 5.14A.

4. Composition — 25% Zr0 2 , 75% CeO 2

O Original - Well crystallized f.c.c., a=5.342A

Post Proof Test - Very poorly crystallized mixture of at least three phases

of cubic or nearly cubic materials with cells a.5.33, 5.26,

O 5 . 19A (surface)

Post Phase I Test Samples were considerably fragmented. Surface patterns

could be obtained only on the 2413 and 2416 electrodes.

2413

0 The surface is mostly a cubic phase (a=5.14A). There

is also some spinel visible plus traces of monoclinic

0 Zr02 and a possible cubic (a=5.26A) phase.

2414 Could not get a useable surface pattern. The ground - upper portion is mostly the same as the original

0 material (cubic, a=5.34A). It also contains small

amounts of two other phases: tetragonal, a=5.23,

0 0 c=5.29A; probably cubic a%5.16A.

2415 No surface pattern. A loose piece which was mostly

surface material was ground for x-ray examination.

0 It was mostly cubic with av5.13A plus a small

0 0 amount of two probably cubic phases, a=5.22A, a -5.19A.

2416 Surface pattern is predominantly cubic (or nearly so)

0 with av5.2A. Small amounts of a minor cubic component

0 (av5.3A) and spinel are visible.
144

5. Composition — 20% Zr02, 78% Ce02, 2% Ta205

Original - Consists of a major component (well crystallized f.c.c.,

O a=5.35A) and a minor component (not as well crystallized

f.c.c., a=5 . 37A)

Post Proof Test - Pattern is nearly cubic, but the crystallinity is only

fair. Slight splitting of high angle lines suggest that

O it may be tetragonal with a=c-5.34A or two cubics with

a=5.34A and a'=5.35A.

Post Phase I Test Samples - All samples have a thin white coating

(presumably due to loss of Ta) and are green like the original

beneath this surface coating. The bulk green material is the well

O crystallized f.c.c. phase (a=5.35A).

The white coating, when chipped off and ground, is primarily the

O f.c.c. phase (a=5.35A) but, also, contains at least two other

’ O O phases with smaller cubic or nearly cubic cells (a''^5.2A, a"'^5.3A)

which are presumably richer in Zr02 .

The surface patterns of all these samples show primarily a poorly

O crystallized pattern of the 5.2A material plus smaller amounts of

O the 5.3A material. Additionally, the surfaces showed:

2517 Traces of spinel and an unidentified material.

2518 Traces of monoclinic Zr02 and an unidentified

material.

2519 No additional phases.

2520 Trace of spinel.
145

C.3 Electrical Conductivity (W. R. Hosier, and A. J. Armstrong, NBS)

Since most electrode sections were cracked and eroded to some extent,

it was impossible to remove each half electrode referred to in Section C-1 of

this chapter in original condition. Measurement of electrical conductivity re- quires a sample of sufficient size to permit attachment of electrodes. Only

those anode electrodes, 2102, 2205, 2206, 2207, 2208 could be used for posttest electrical data.

In addition, the significance of post-test electrical data is in

question. It is difficult to reproduce in the laboratory measurement apparatus,

the exact conditions that were present in the test facility plasma. These

conductivity measurements are bulk measurements taken with the entire sample at

the same temperature and in a particular atmosphere of oxygen partial pressure.

No alkali seed was present in the measuring apparatus. It would seem likely that

during the measurement, the condition of the sample could be altered considerably from that actually existing while it was exposed to the test plasma.

However, the measurements do indicate that no permanent damage had been

done to the anode electrodes measured since the pre- and post-data are very similar

See Figures C.3.1 and C.3. 2. This data should be compared with that given in

Figures B.5.1, B.5.2, and B.5.3 of section B.5, this chapter. While the series

of samples containing 82 Zr 02 - 18Ce02 have the lowest electrical conductivity of

any material tested, its mechanical resistance to fracture on the anode is the

greatest since all anode electrodes of this series were removed intact.

C.4 Metallography and Electron Microprobe Analysis (J. L. Bates, J. L. Daniel

Battelle Pacific NW Laboratories)

The cross sections of U-02 channel provided by the National Bureau of

Standards were vacuum impregnated in mounting resin and polished in a non-aqueous media. Special precautions were required to prevent the

smearing of potassium. The following electrode pairs were examined initially:

Electrode Pair

1103 - 2103 ( 1102 ) 1206 - 2206

1309 - 2309 (1310) (1312)

1414 - 2414 (1416)

1517 - 2517 (1518)

(The electrodes in parentheses received only cursory metallographic examination

at lower magnifications.)

Photographs of poMshed cross sections were taken at lower magnification

and a montage was prepared for this report.
CONDUCTIVITY(ohm-cmr'

146

T(K)

Post-test Electrical Conductivity of Electrode 2102

(88 ZrO^ - 12 Y^O^)

Figure C.3.1.
147

icf

T(K)

Post-test Electrical Conductivity of Electrode 2206

(82 ZrO^ - 18 CeO^)

e C.3.2.
148

The cross section was examined with a scanning electron microscope

and electron microprobe. These techniques, coupled with the optical

microscopy, were used to identify the location and composition of the phases

present in the MgO and electrodes. These compositions are expressed as approximate mass ratios of the elements.

The following is a detailed description of structural characterizations

of each electrode pair:

0 . 12Y202* 0 . SSZrO^ (1100-2100 Electrode Pairs)

1103 Cathode (Figure C.4.1): The electrode and MgO were extensively fractured.

The electrode has expanded to a thickness of 0.71 cm and raised 0.35 cm above the MgO surface. Break-up occurred mainly between the larger, dense

particles. There is also extensive reaction between the cement, potassium,

and the MgO, especially at interior corners.

The Pt separated from the electrode, with little evidence of the wire

mesh remaining. Some Pt has diffused into adjacent areas of the electrode

and cement

The electrode is single phase and contains no potassium except a trace

adjacent to the Pt. The black area below the Pt, originally the Zr02

cement, is very hxgh in potassium and in many areas is almost pure potassium.

The potassium extends down into the MgO and is located in the lower density areas which surround the dense MgO particles. The K concentration in the MgO decreases

as the distance from the electrode increases.

The MgO adjacent to the electrode and at the gas surface has recrystallized

and is single phase with large grains and few pores. About 0.1-0. 2 cm below this layer is another recrystallized structure with columnar-like grains.

This section also contains a second phase of Zr-Ca located in cracks and

pores (Zr/Ca = 17/1). There is also a dark band of pure zirconia containing

no calcium.

Below this, the MgO is free of Zr and Ca with some K, which increases toward

the cooler parts of the insulator.

2103 Anode (Figure C.4.1): There is little change in the overall structure

of the electrode and insulator. Some separation has occurred in the cement

causing the electrode to raise. The MgO has fractured with cracks initiating

from the internal, machined corners.

The electrode has recrystallized to a depth of 0.05 cm below the surface and

contains two second phases. The phase at the grain boundaries is composed

of Al, K, and Ca with ratios of Al/K » 6 and Al/Ca = 10. This phase extends

only to a depth of 0.02 cm from the surface. The lighter colored continuous

phase is composed of Zr and Y, and is free of potassium. The center of the
149

Figure C.4.1. Cross sections of anode and cathode for electrode

pair 2103 and 1103 (0.12 Y202*0.88 Zr02)-
150

electrode has a single phase Zr/Y structure, similar to the reference

material, but is somewhat fractured. However, in some isolated areas, the

lower density porous oxide areas contain varying amounts of K and Ca.

One area, for example, has Zr/K = 2.5 and Zr/Ca = 7.

The electrode immediately adjacent to the Ft and cement contain some K

and some second phase with a Zr/Ca of 1.3.

The large Zr02 particles in the cement have a Zr/Ca ratio of 10. The

smaller particulates surrounding these larger particles contain K with

a different Zr/Ca ratio of 3.4. The Zr/K ratio is ==7.3. The potassium is

present in the low density portions of the cement.

The MgO below the cement contains both K and Zr with ratios Mg/K =7.1 and Mg/Zr = 15.

This phase is concentrated in the pores and cracks. The larger MgO is free of

K. The potassium diminishes rapidly away from the cement and at 0.1 cm from

the cement only a trace of K is found. At 0.2 cm, no K can be detected.

The MgO at the plasma surface has recrystallized to a depth of 0.31 cm,

from a dense surface into columnar type grains and pores and eventually

into equiaxial grain growth.

Large second phase particles, up to 45 ym, are found near the surface and

are composed of Zr and Ca with Zr/Ca = 5. Some of this phase persists below

the surface into the columnar grains, but decreases as the distance

from the surface increases. The MgO has a trace of Fe, but is free of K.

However, the potassium begins to increase outside the columnar grain area

and at 0.3 cm, near the cement area, 80 ym particles of Zr, Ca, and K

phase are found with ratios of K/Zr =2.5 and K/Ca = 1.3. It is very prevalent

in the cracks.

0.18Ce02*0. 82Zr02 (1200-2200 Electrode Pairs)

1206 Cathode (Figure 4.2): The electrode has broken, segmented, swollen

extensively and has interacted with the cement and MgO. The top of the

0.67 cm thick electrode has recrystallized to a depth of 0.2 cm into a single

phase, dense body (Zr/Ce = 3) with large rounded pores near the surface.

There is, however, a trace of a second phase with Zr/Ca ratio of 15. The

electrode is free of this phase below 0.24 cm and is also free of K except

in the area immediately adjacent to the Ft. This area also contains a second phase with a general composition of Zr/Si =3.5, Zr/K = 2.4, and

K/Ce = 3.2.

The reacted area surrounding the electrode adjacent to the MgO, which may have

contained the Zr02 cement, is three phase and is composed of fine grains

and small pores. These phases are yet to be determined, but does have a high concentration of K.
151

The MgO surface exposed to the plasma has recrystallized to a depth of =0.33

cm and contains two phases containing Zr-Ca and Si-Ca (Si/Ca = 5).

These precipitates, 1 to 5 pm in diameter, are found only near the surface.

A 0.13 cm thick columnar grain formation is located immediately below the above area.

It also contains some second phase material at the pores of a composition

similar to that at the surface. The cracks are parallel to the surface.

The cement and MgO below the electrode have reacted extensively with the potassium.

The resultant phase has penetrated between the platinum and the electrode.

Some areas are pure potassium.

2206 Anode (Figure C.4.2): The electrode has recrystallized only on the

surface near the plasma. The remainder of the electrode has changed

relatively little, although there is some evidence of break-up in the lower

density areas surrounding the larger particles. The electrode surface,

which was exposed to the plasma, contains three phases, one of which can only be resolved above lOOOX. A major phase (gray) contains Mg, Al,

and Ca with Al/Mg 3 and Al/Ca 1. Inside this gray area, the very

fine phase (light) contains Al, Zr, and Ca with Al/Zr 16 and Al/Ca 3.5.

The lighter phase which surrounds these two phases is Zr/Ca v 10.4. This

surface layer contains little, if any Ce, but is high in Ca which decreases

rapidly away from the surface. At 0.3 pm from surface, the Ca is gone,

and Cephas returned to the original composition. This area is free of

potassium. The remainder of the anode is single phase. However, the microprobe

results indicate the presence of potassium > 0.24 cm below the surface.

The K occurs in several forms, one of which is a finger-like structure,

that often crosses some cracks, indicating it may have resulted during

polishing. Another form of the potassium appears as a particle, 3‘-10 pm

in size, sitting on the surface. In the areas between this debris, no potassium is detected. Thus, the presence of K in the lower portion of the

electrode will require some more resolution. However, since all' other areas

show the presence of relatively little K, the source, other than the anode

itself, cannot be defined.

The cement contains significant amounts of potassium, although the

concentrations are orders of magnitude less than at the corresponding

cathode. The K is located in the lower density areas (Zr/K 11) surrounding

the larger calcia stabilized, zirconia particles, Zr/Ca 6.

The MgO, immediately below the cement, contains Zr and K, Mg/Zr 27 and Mg/K 'v- 11.

These elements are found only within a 0.05 cm band in the MgO adjacent to the

cement

The MgO exposed to the plasma has recrystallized to a depth of 0.2 cm. A second phase composed of Zr and Ca is found in this area, Zr/Ca v 9. The

MgO surrounding this phase, contains only a trace of iron, Mg/Fe 30.

This area also contains no potassium.

Below 0.2-0.25 cm, the potassium concentration gradually increases,

expecially in the cracks. Below the large crack, near the bottom of the

electrode, there are areas of high potassium in the low density MgO.

Concentrations of K in the anode are small compared to corresponding areas in

the cathode.
152

Figure C.4

Anode

.2. Cross Sections of Anode dnd Cathode for Electrode

Pair 2206 and 1206 (0.18 Ce02 • 0.82 Zr02)

Cathode
153

0 . SCeO^ • 0 . SZrO^ (1300-2300 Electrode Pairs)

1309 Cathode (Figure C.4.3): This cathode was fractured more than any

other of the electrodes examined to date with total separation of electrode

from the insulator. Pieces had to be replaced in some locations and

may be mis-oriented

.

The microstructure of the electrode was completely changed. There is a parallel crack 0.2 cm from the surface and the material has recrystallized

extensively on both sides of this crack. Below this area, the electrode

is cracked and has a multiphase structure.

The structure of the MgO has changed to a depth of 0.38 cm. The surface

is dense to a depth^of 0.08 cm with a restructured columnar grain effect.

Pores run parallel to these grains and to the thermal gradient. To more

clearly describe these results, areas have been numbered (Figure C.4.3) and

each of these areas are described in detail.

° 1-2: This detached recrystallized area of the electrode is single

phase and only a trace, if any, K is present. The surface

appears depleted in Ce (Zr/Ce 2) and the area of recrystallized

columnar grains has a Zr/Ce ratio 3 indicating some loss or migration of Ce. The recrystallized top surface does not

contain any potassium.

° 3: In the center of the above area of the electrode, there

is a Mg-Al containing second phase of unknown origin.

° 4: About 0.1 cm from the surface, the potassium begins to appear

and continues to increase toward the bottom of the electrode.

° 5: About 0.1 cm below the surface, and extending from the

bottom of the K free area, the electrode is composed of

two phases, totally different from the original oxide. The

matrix phase is composed of about equal amounts of Zr and

Ce plus K. The second phase, 20% of total area, is

almost all Ce and contains no K with trace amounts of Zr.

° 8-9: Near the bottom of the electrode, just above the Pt, there

is an area which contains significant amounts of Ca in

addition to Zr and Ce. The Ce content is low. Some Hf

is also present.

° 10: Just below the Pt, a black area is very high in K with some very small amounts of Zr , Ce, and Zn.
154

11 :

12 :

13:

14-15:

16-17:

18-19:

20 :

In the MgO near the bottom of the electrode and adjacent

to the cement, the black area is nearly pure K with only

trace ainounts of Ce, Hf, and Mg. In the MgO, the K is

concentrated in the low density areas surrounding the large

MgO particles.

In a crack in the upper left corner of the electrode, there is a wide band of solidified K.

At the interface between the original MgO and electrode, a new phase has developed either from the cement or the electrode.

It is a porous structure composed mainly of Zr with substantial

Hf, K, and some Ca. In the MgO adjacent to this area, the MgO

contains a second phase of Zr and Hf, but is free of Ca or K.

The AI 2O 3 spacer between insulators has become potassium

3-alumina. The top section (14) contains K (Al/K ~ 30)

The K appears to be diffusing into the AI 2 O 3 from the outside

edges forming a distinct cone shaped area at the reaction boundary.

The K is concentrated at the pores and the reaction area is

also more porous. The AI 2 O 3 below (15) contains no K.

In the high density, recrystallized MgO surface, there is a bright

second phase composed of Zr with smaller amounts of Ca

(Zr/Ca = 24) and Ce (Zr/Ce = 6 ) and some Mg and Hf.

In the columnar grain region of the MgO, a second phase

is found at the pores. These pores are Ca stabilized Zr02

containing Hf and Ce.

In the MgO, which has only partially recrystallized, the

Zr containing second phase is not present and K is present

in the irregularly shaped pores. The K concentration increases

as the distance from the surface increases.

2309 Anode (Figure C.4.3): The electrode has fractured and top surface

('^ 0.2 cm) is missing. The exposed surface which remains has recrystallized

into a dense body with spherical pores. The electrode is also cracked

and has separated from the cement and platinum. The cement remains relatively

intact with some separation from the MgO.

At the surface, the MgO is recrystallized to the same depth as the electrode.

A second phase is found near the surface to a depth of 0.05 cm. The MgO

again is cracked, initiated at the machined internal corners which seat

the electrode.
155

Figure C.4.3 Cross Sections of Anode and Cathode for Electrode Pair

2309 and 1309 (0.5 Ce02 • 0.5 Zr02)
156

The remaining exposed surface of the electrode, which has recrystallized,

also has a gray second phase located principally at the grain boundaries.

This phase contains Ce, K, Zr, and A1 (Ce/K = 0.7, Ce/Zr = 4, Al/Ce = 6).

These ratios vary, but the A1 and K are always high. This second phase is

limited to a narrow band, 0.1 cm thick along the surface.

Near the center of the electrode, there appears to be a light gray phase

in the optical micrograph. However, this has the same composition as the

matrix and is thought to be shallow pores or cracks. Some of the large

cracks in the electrode interior contain K, Al, and Si with ratios

K/Al = 1 and K/Si = 2.

A dark gray zone between the platinum and the electrode contains Zr with

some Ca, apparently originating from the cement. It does not, however,

contain any K.

The larger particles in the cement immediately below the Pt are Zr and

Ca (Zr/Ca = 7). The low density areas surrounding these particles contain

about equal amounts of Zr and K plus the Ca. At 0.1 cm from the top

edge of the cement, the Zr/K ratio is =15 and at 0.15 cm, the ratio is = 17.

In the MgO below the cement, the large particles are pure MgO. The low

density MgO is high in K with some Zr. The K concentration decreases as the distance from the electrode increases.

In the recrystallized MgO near the electrode surface, there are two phases.

The light gray phase occurs only near the surface and contains Zr, Ca,

Ce, and Si. The ratios are Zr/Ce = 4, Zr/Ca = 3.4, and Zr/Si =4.4. The

bright phase contains Mg and Si with Mg/Si ratio =1.2/1.

A crack in the MgO contains a 100-150 ym band of almost pure Ca.

Near the Pt on the side of the electrode between the MgO, there is a phase

composed of nearly 100% Zr with a trace of K. There is no Ca present.

The material adjacent to the MgO in this same area contains Zr with high K.

0. IbCeO^* 0. 25Zr02 (1400-2400 Electrode Pairs)

1414 Cathode (Figure C.4.4): The changes in the upper portion of the electrode

were extensive with the electrode separating from the Pt and cement. The

MgO was cracked, but not extensively degraded. The top half of the electrode

had fragmented into smaller pieces, with only remnants of larger particles

remaining. The surface had recrystallized to a depth of only 0.03 cm.

The Pt paste and mesh had separated from the electrode in the center and

from the cement. The mesh was eroded slightly.
157

The recrystallized band in the electrode contained very small, 10-25/um

diameter, second phase particles composed of Al, Ca and Ce, with ratios of

Ce/Al = 7.5 and Ce/Ca =«12. This phase did not contain Zr and was found in

a band to a depth of 300 ym from the surface.

Next to the surface is a narrow band of almost pure K. Potassium is also

found in some cracks in this recrystallized area. Some pores in this area

also contain a phase composed of Zr, K, and Ce with ratios Ce/Zr =« 5 and

Ce/K =5. The continuous matrix contains no potassium. In the broken central

portion of the electrode, the oxide contains no K with Ce/Zr = 5.

The bottom of the electrode is high in K, especially in the low density

areas surrounding the large particles and in the cracks found in the larger

particles. Some 1-5 ym particles of pure K are scattered in the low density

area. There is a narrow band of pure K adjacent to the bottom of the electrode.

The platinum is partly eroded, presumably by the K, since the pores in the

Pt are also high in K.

The cement contains significant quantities of K located in the void spaces

around the large solid particles. These particles contain Zr but no Ca.

The Ca is located in the smaller granular phase which surrounds these particles.

The MgO immediately below the cement is high in K. However, the K diminishes

rapidly and 0.2 cm from the cement, the MgO is free of K.

In the recrystallized MgO near the electrode surface, there is a high concentration

of K in the pores and cracks. In addition, there is a light second phase

containing Zr, Ce, Ca, and Mg with ratios Zr/Ce ~ 10, Zr/Ca ~ 16, and

Zr/Mg « 20. This phase seems to be located only in the more dense band

0.1 to 0.6 cm next to the surface. Below this band, this phase is less

dense and is smaller in size and composition changes to Ce/Zr ratio of 5.0

with no Ca or Mg.

The potassium in the MgO increases as the distance from the surface increases,

0.6-0. 8 cm, then decreases as the distance exceeds 0,8 cm.

2414 Anode (Figure C.4.4): The electrode and insulator are relatively intact

although the electrode is badly cracked and separated from the Pt and cement.

The electrode separated horizontally and parallel to the surface about midway

between the top and the bottom. Recrystallization occurred to a depth between

0.06 and 0.1 cm from the surface. The lighter phase in the recrystallized

area does, however, contain extensive second phase containing Zr, Ca, and

Ce with ratios Ce/Zr ~ 4, Ce/Ca ~ 10. There is no K in this phase. A

light gray phase contains Al, Cr, K, Ca, and Ce with ratios Al/Ce ~ 3,

Al/Zr « 31, Al/K ~ 15, and Al/Ca ~ 5. These phases appear isolated near the

surface. The pores in the electrode near the Pt contains Zr, Ca, and

Ce inclusions with Zr/Ca 7 and Zr/Ce ~ 5, This portion of the electrode is

free of K. The low density portion of the cement contains K with Zr/K ratio

near 3. This ratio varies widely within the cement.
158

Cathode 4X

Figure c.4.4. Cross Sections of Anode and Cathode for Electrode Pair

2414 and 1414 (0.75 CeO^ • 0.25 Zr02)
159

The MgO, adjacent to the cement, contains a small amount of K. At 0.05 cm,

a trace of K is found, and at 0.1 cm from the cement, the MgO is free of

potassium.

The MgO at the surface adjacent to the electrode is crystallized containing

large spherical pores and a bright second phase at the surface. This phase

is composed of Ce, Zr, Ca, and a trace of Si. Ratios are Ce/Zr ~ 2,

Ce/Ca ~ 5, and Ce/Si « 8. The continuous phase in this area contains

Mg, Zr, K, Ca, Fe, Ni with ratios Mg/Zr ~ 33, Mg/K ~ 11,

Mg/Ca = 13, Mg/Fe ~ 22, and Mg/Ni== 33. This latter phase is concentrated

at the surface and at 0.1 cm from the surface, only Si and K remain. There is a large increase of K in the cracks of the MgO.

The copper bonding has separated from the back of the MgO. The Cu interface

is two phase, being composed of silver and copper.

0.78 Ce02*0.20 Zr02*0.02 Ta^O^ (1500-2500 Electrode Pairs)

1517 Cathode (Figure C.4.5): The electrode has experienced substantial fracture

with nearly complete restructuring. The MgO insulator and cement supporting

the electrode is extensively degraded with the Pt separated from the electrode.

The surface of the electrode exposed to the plasma has recrystallized to a depth of "'^^O.OA cm and is a two phase region also containing large pores.

Along the very outer edge ('^^0.01. cm width) there is a phase containing Zr,

Ce, and Ca with a Zr/Ce ratio of =4 and a Zr/Ca ratio near 7. There is

little or no Ta in this layer. This area of high Zr decreases very rapidly

and at about 100 ym below the surface the Ce/Zr=a^ and the Ta begins to

increase. This area is free of K.

Beyond 0.01 cm, there are three phases, a light gray phase containing Ce,

Ta, Ca, and Zr with Ce/Taf==4; Ce/Ca;®^4; and Ce/Zr=3. The darker phase

contains Ce, Ca, and Zr with ratios Ce/Ca=3.5 and Ce/Zr=6. The area is

also free of K. The original continuous phase has a Ce/Zr^l2 with a trace

of Ta. At approximately 0.1 cm beneath the surface, the Ce/Zr/Ta ratios

approach that of the reference electrode.

At 0.25 cm beneath the surface, K begins to appear with a Ce/K ratio 12.

At 0.35, 0.45 and 0.55 cm, the surface of the Ce/K ratios are approximately

8, 6, and 6 respectively.

In the center of the electrode a second phase comprising 1-2% of total area

appears. The continuous phase is Ce and Zr only with Ce/Zr^2. The second

phase contains Ce, K, Zr, and a trace of Ta with Ce/K~12, Ce/Zr~3. This

structure extends down to the bottom of the electrode.

Between the separated Pt and the electrode is a large porous structure

area containing long needle-like grains. This area is almost pure Zr and

is free of Ce, Ca, but does contain some K. The black area below the Pt

is very high in K. The remnants of the cement contain large Zr02 particles

with Zr/Ca?5i8. The area surrounding these particles, which orginally

were low density cement, is nearly pure K.
160

The MgO below the cement contains large quantities of K. The Mg/K ratios

at the ceraent-MgO interface and 0.1 cm below are 4 and 3 respectively.

The MgO at the plasma surface has also restructured. Immediately adjacent to

the surface is a single phase recrystallized area, some of which appears to

have spalled away. This qrea contains Ce, Ca, K, Zr, and Ta with ratios

Ce/Ta~12, Ce/Ca?s5, Ce/K~6, and Ce/Zrsi2.

Immediately below this begins a two phase region which at a depth of 0.2 cm contains pure Mg and a grain boundary, minor phase containing Ce, K, and Zr,

with ratios Ce/K 1.6 and Ce/Zr 1. This second phase decreases toward the

bottom of the electrode whereas the potassium begins to increase especially

at the grain boundaries\

2517 Anode (Figure C.4.5): The electrode has fractured considerably. However

this was an instrumented section and the central hole may have contributed

to the fragmentation. The surrounding MgO is also cracked with most

structural changes occurring near the plasma surface. The Pt has separated

from the electrode while the cement remains partially intact.

The surface of the electrode has recrystallized to a depth of 0.25-0.3 cm. Along the entire surface is a two phase area, a gray phase containing

A1 and K with Al/K«3.4 and a light phase containing Zr, Ca, and Ce, with

Ce/Ca~4 and Ce/Zr psI. This is much lower than normal. The concentration

of this phase deareases rapidly toward the interior of the electrode

and at 0.05 cm below the surface the Zr/Ce ratio has returned to the expected

value.

The remainder of the electrode has a normal composition, but small amounts

of K are found in the cracks.

The cement contains high concentrations of K located in the low density

porous areas of the cement.

The MgO immediately adjacent to the cement contains considerable K and

also some Zr, with the Mg/K ~ 7 and the Mg/Zr«14. Both the Zr and K are located in the lower density MgO and decrease rapidly in concentration

toward the bulk MgO. At 0.1 cm below the cement, the Mg/K«13 and at

0.4 cm the MgO is free of K.

The MgO surface has recrystallized to varying dgrees to depths up to 0.4 cm. This area is generally single phase MgO except at the surface where some two

phase particles are attached. The lighter phase contains Zr and Ca only

with a Zr/Ca ratio ofs:r8. The darker continous gray phase contains A1

,

Mg, and traces of Fe. The Al/Mg ratio is~2.8 and the Al/Fe ratio is~19.

This area is free of K. Some of these phases, in particular the Zr-Ca phase,

extend into the MgO to a depth of ~3.05 cm.
161

The MgO contains only small amounts of K which increases as the distance

from the surface increases. Only trace amounts of K are found near the

surface and at 0.18 and 0.35 cm below the surface, the Mg/K ratios are ~20

and ~4, respectively. Below the large crack which is found 1 cm below the

surface the K again begins to decrease.
162

Figure C.4.5. Cross Sections of Anode and Cathode for Electrode

Pair 2517 and 1517 (0.78 CeO^-0.20 ZrO *0.02 Ta 0 )

2 2 2 5
163

Figure c.4.6. Cross Section of Cathode 1102 (0.12 Y

2O

3

• 0.88 Zr02 )

Figure c.4.7. Cross Section of Cathode 1310 (0.5 Ce02 • 0.5 Zr02)
164

C.4.1 Specific Observations Concerning Various Materials of the Tested

Channel.

Potassium

The highest concentrations of potassium were associated with the cathode

side of the channel, concentrating in that portion of the materials which were

at operating temperature of approximately 600 to 1300 °C. In the MgO , cement,

and in some electrodes, the potassium is associated with the lower density,

porous areas which surround the large oxide particles. Potassium is also found

in cracks. It appears that the transport of potassium in the oxide is directly

related to the open porosity associated with the low density structure of the

materials

Little potassium is found on or near the plasma surface of the MgO or cathode. Some potassium was found on the anode surfaces.

MgO Insulator

The MgO exposed to the plasma had recrystallized to depths as much as 0.4 cm. Although not observed in all electrode pairs, the recrystallization of the MgO

generally consisted of three different structures.

(1) Dense band near the surface which varied in width from 0.02 to 0.10 cm. This layer was 3 to 5 times wider on the cathode side than on the anode. This

layer also contained a second phase composed principally of zirconia and calcia.

However, on the downstream electrodes, this phase also contained increasing

amounts of cerium. The calcium concentration was highest on the anode. The

anode also often contained silicon. No potassium was observed at the surface.

Other impurities, eg. Fe, Ni, A1 were also observed on specific electrodes, but

in small amounts.

(2) Columnar type grains extended to depths between 0.7-0.18 cm. (The depths

were approximately the same for the cathode and anode) . These long grains

were associated with a high porosity at the grain boundaries. On the cathode

side, a second phase was found at the grain boundaries. This phase is thought

to be a Ce02 stabilized ZrO„ since more and more Ce02 was found in the downstream

MgO. Little, if any, second phase was present in the columnar type grains of

the MgO near the anode. This area, except in selected cracks, was also free

of potassium.

(3) Equiaxed grain growth occurred to a depth from the surface of approximately

0.3 to 0.4 cm. This recrystallization appeared as a densif ication of the low

density fines which surrounded the larger particles, in a decrease in the number

of pores, and in an increase in pore size. This recrystallized area of the

cathode only sometimes contained a second phase and was also the area where

potassium was first observed in the MgO.

- The erosion, degradation and recrystallization of the MgO appeared to be more

severe on the MgO which was on the upstream side of the electrode. Perhaps

associated with this effect, is the apparent increased reaction between the MgO,

the cement and the upstream side of the electrode. This effect appeared to be

more pronounced on the anode than on the cathode.
165

-The cracking of the MgO appeared similar for all electrodes. Cracks

appeared to initiate from the corners of the machined recesses in the

insulator. In most insulators, these cracks were found '^0.60.7 cm from

the surface. Most of these cracks contained potassium.

-The potassium penetrated deep into the MgO below the cement and cathode.

Penetration in the MgO below the cement of the anodes was minor, with onlytrace amounts 0.05 cm below the cement-MgO interface. The potassium penetration

was greatest in the high current density cathodes.

Electrodes

The microstructure of each electrode pair was unique but some general

conclusions may be made.

- The surface of most electrodes had recrystallized into a dense layer often

with large spherical pores. This layer varied in thickness (0.02-0.1 cm) from

electrode to electrode. With the exception of the 1103 cathode, the surface

contained from one to three phases, with the concentration of the second

phases highest at the plasma surface. These second phases on the cathode

were principally zirconia containing varying amounts of calcium and

cerium. On the anode surface, more than one phase was found and in

addition to the zirconiacalciaceria phase, phases high in aluminum were

present. Potassium was also found in some phases. These results are summarized in Table C.4.1.
166

Table C.4.1 PHASES AbT) APPROXIMATE COMPOSITIONS IN ELECTRODES: NEAR

PLASMA SURFACE

MAJOR PHASE* MINOR PHASE*

Cathode

1103 Zr/Y '^2.6

1206 Zr/Ce '^^3 (1) Zr/Ca '^.15 1309 Zr/Ca

1414 (1) Ce/Al '^7.5

Ce/Ca %12

1517 (1) Ce/Zr '\.12 (Cont.) (1) Ce/K '^^12, Ce/Zr '^^3

Anode

2103 Zr/Y Al/Ca ^^10,

Al/K

2206 Zr/Ca 'vlO.4 (at surface) (1) Al/Mg 'V3,

Al/Ca ^11

Zr/Ce ^\.3 (>0.3 m) (2) Al/Zr '^^16, Al/Ca -^3.5

2309 Zr/Ce ^-0.3 (1) 'Ce/K ^.7,

Ce/Zr ^>^4, Al/Ce ^5.5

2414 Ce/Zr '^4.5 (1) Ce/Zr

Ce/Ca '^^10

(2) Al/Ce '^>3, Al/Zr '^^31, Al/K '^^15, Al/Ca ^\^5 2515 Ce/Zr ^1,

CeICa^(>0.025 cm)

Ce/Zr ^>7 (>0.025 cm)

(1) Al/K '^^3.4

*The compositions are expressed in mass ratios of the metals.
167

- The phase compositions at the center of the electrodes also varied, and often

contained more than one phase. These results are summarized in Table C.4.2.

Table C.4.2 PHASES AND APPPvOXIMATE COMPOSITIONS IN ELECTRODES: NEAR CENTER

MAJOR PHASE* MINOR PHASE*

Cathode

1103 Zr/Y v2.6

1206 Zr/Ce v2.6 (1) Zr/Ca '^^ll to no Ca

1309 Zr/Ce vl, Zr/K vO.5 (1) Ce (20%)

1414 Ce, Zr , K (1) K (in cracks and

pores)

1517 Ce/Ta v4 , Ce/Ca v4, (1) Ce/Ca v3 . 5

,

Ce/Zr v3

(2)

Ce/Zr v6

Zr/Ce ^4,

Zr/Ca v7

Anode

2103 Zr/Y '^^2.6 (1) Zr/K v2.5,

Zr/Ca v7 (in pores

and cracks)

2106 Zr/Ce v2.6 (1) K (high in low

density oxide)

2309 Zr/Ce vO.2 (1) K/Al vl,

K/Si '^2 (in cracks)

2414 Ce/Zr v4.5

2517 Ce/Zr v7 (1) K (trace in cracks)

*The compositions are expressed in mass ratios of the metals.
168

- Similarly, the electrode near the platinum current lead-out also was contaminated. The possible phases and compositions are summarized in Table C.A.3.

Table C.4.3 PHASES AND APPROPRIATE COMPOSITIONS IN ELECTRODE: ADJACENT PT LEADS

Cathode

MAJOR PHASES* MINOR PHASES*

1103 Zr/Y %2.6 (1) K

1206 Zr/Ce ^^3.2 (1) Zr/K V2.4

(2) Zr/Si ^3.5

1309 (Zr+Ce)/K vl (1) K (in pores)

1914 Ce, Zr, K (1) K

1517 —N.D. +N.D.

Anode

1103 Zr/Y -V2.6 (1) Z?-'K v9,

Zr/Ca ^1.3

1206 Zr/Ce 'v^3 (1) K

2309 Zr/Ce '^^0.2 (1) Pt

(2) Zt/Ca

2414 Ce/Zr ^4.5 (1) Zr/Ce v 5 , Zr/Ca '\il 2517 Ce/Zr v7 (1) K

*The compositions are expressed in mass ratios of the metals.

Cement

The cement acts as a major source of potassium concentration and possible

path of seed transport into the MgO and electrode. The highest concentration

of potassium in the cement was found in the cathodes and increased with current

density. Total degradation of the cem^ent can occur. Conversely, the cement

associated with the anode remained intact, but contained vlO% potassium associated

totally with the low density portion of the cement.

Aluminum Oxide

The Al^O^ spacers reacted with the potassium, but did not appear to react in

any other way with the other materials. The higher temperature top edge of the

sintered Al^O^ plate appeared to form B-Al^O^ with Al/K v30. The potassium diffuses

into the A1 0^ from the outside edges forming a distinct cone-shaped reaction

boundary. The potassium is concentrated at pores within the Al^O^. The 6-Al^O^

has a higher porosity than the original Al^O^.
169

C.5 Radiograph and Current Lead-out Analysis (E. Farabaugh, NBS)

In contact radiography, the image contrast is due entirely to

the variation in x-ray absorption from point-to-point in the specimen.

Distinguishing specific features in the radiograph depends on the relative

contrast, and thus, on the differential absorption of x-rays in materials

of differing density and atomic constitution. This method of characterization requires only an x-ray source and an emulsion to record the image.

The accompanying radiographs (Figures C.5.1-C.5.5) were taken using an uncollimated beam of x—rays from a tungsten rotating anode operating at

200 kv. This high voltage source allowed the viewing of the thick anode

and cathode electrodes in transmission. The images were recorded directly

on type 57 Polaroid film. The specimen to source and specimen to film

distances were 20 cm and 2 cm, respectively. In the radiographs, the exact

shape of the electrode is not always recorded. This is due to the geometry

of the experimental arrangement and the size of the x-ray source. However,

the important features, the breaks in the platinum leads, are always easily

detected

In the series of radiographs of the anode side, breaks are seen in A2-7,

A2-8, A4-15 and A5-17 . These electrodes were noted as opening up during

the test run. On the cathode side, Cl-1, Cl-4, C3-9, C4-15 and C4-16

are seen to be discontinuous. Of the cathode elements, Cl-1 was noted

to have opened up during the test. C3-9 was part of electrode //9 which

behaved erratically during the latter part of the test but never did open

up completely. The radiograph of C3-9 shows that the leads are not in

good condition, explaining this behavior. C4-15 and C4-16 were part of

electrodes #15 and #16 which also were recorded as opening up during the

test. Cl-4, which served as a position of an optical sight port, shows

no leads in the radiograph. Thus, electrode #1 failed on the cathode side

while #7 and #8 failed on the anode side. The poor performance of #9 was due to cathode lead problems, the radiograph showing the leads are very nearly broken. Electrode #15 appears to have opened up obviously, on the anode side and not quite so dramatically on the cathode side.

Electrodes #16 and #17 failed on the cathode and anode sides, respectively.

Four breaks, therefore, occurred in the anodes and 3 in the cathodes, so the discontinuities in the platinum leads showed no preference for anode

or cathode assembly.

It should be noted that there is one case where one of the double leads

failed, while the other remained intact allowing the electrode unit to

function for the complete test. The radiograph of Cl-2 shows only one

of the two leads to be broken, allowing the electrode to operate properly.

In all of the cases where leads opened up, it appears that the platinum

melted at a point near its connection to the electrode and then, forming

a ball, melted back along the lead. In fact, a ball of what is believed

to be platinum, is seen near the base on A2-7 , A5-15, A5-17 and Cl-1.

To address ourselves to the problem of lead melting, it was decided to

remove several of the leads that failed and examine them optically and

with the SEM. Anode electrodes A2-7 , A2-8 and A5-15 were selected for

this examination.
170

The leads of A2-7 were easily removed and an optical micrograph of the leads

is sho\^m in Figure C.5.6. The ball of platinum is seen on the end of one of

the leads. The other lead does not have such a strong indication of melting,

but it is believed that the round shaped dark im.age in the radiograph of A2-7

is melted platinum which dropped off the lead. (Note is taken here of the

material adhering to the melted end of the platinum lead. More will be said

about this later) A4-15 also was easily removed. An optical micrograph of

platinum leads of A4-15 is shown in Figure C.5.7. In this case, the leads have

melted together with material between them. Note the nearly identical images

in the radiograph of A4-15 and the optical micrograph. The leads of A2-8

were not removed as easily. In fact, in attempting to pull the leads out of

the ceramic tube, the leads parted, leaving some of the lead (including the

melted end) in the AI2 O 3 ceramic tube. (It should be pointed out here that

none of the platinum lead assemblies were securely held to the ceramic tube by

paint on the end of the tubes. In fact, it seems possible that some of the twists

in the leads, as indicated by the radiographs, could be attributed to movement

of the lead assembly when nuts were placed on or removed from the threaded ends

of the lead assembly) . In order to optically examine #A2-8 electrode assembly

(electrode and the ceramic tube cemented to the electrode) , it was removed from

the MgO insulator. Once removed, the AI2 O 3 tube containing the leads was cut so

as to reveal the lead inside the tube. Figure C.5.8 shows the assembly as removed

from the insulator with the ceramic tube cut to show the lead. Figure C.5.9 is

a higher magnification optical micrograph of the same electrode. In this micrograph, the melted end is clearly seen as is the necked down end where the lead

pulled apart in the removal attempt. It is observed that the melted bead has formed

on the lead in a region of tube where there was a void in the cement. This ex- plains why the lead could not be easily pulled out. The cement in the tube is a zirconia cement which was forced down the tube when the platinum le’ads were put

in place. In looking back over the 'radiographs , it is seen that all the breaks in

the leads could be seen, that is, they did not occur in the opaque areas as indicated in the radiographs . The opaque areas correspond to the presence of Zr or Zr compounds which have a high absorption for x-rays. Thus, the opaque areas

in the radiographs show the parts of the ceramic tube in which the platinum leads

are well surrounded by zirconia cement. One is brought to the conclusion that

breaks occurred in the leads at places where the leads were not embedded in the

cement. It is thought that the cement may have enhanced the performance of the

leads by conducting away heat from the lead and protecting them from damage due to

joule heating. The cement also provides support to the leads when connections are made to the lead assembly. Consideration, however, must be given to the possibility

of chemical attack and degradation of the lead, especially in light of Figure C.5.10,

which showed the melted end of the lead and the powdery material adhering to it.

Figure C.5.10 is a secondary electron SEN micrograph showing the melted end of the

lead of A2-8 , and the powdery material which was white in color. Some of the

powder was scraped off and an x-ray powder pattern was taken. The analysis of

the data from this film showed the material to be KCl. Comparison made on the two

strongest lines and three other weaker lines showed good agreement. The lead

as shown in Figure C.5.10 was also analyzed with EDX on the SEM. Figure C.5.11

shows the spectrum obtained by looking at the white material at 950 X with an accelerative voltage of 20 kV on the SEM. The two strongest peaks shown in the

spectrum are due to Cl (K2 at 2.62 kev) and K(K2 at 3.31 kev) . Thus, these two

techniques confirm that the material seen on the end of the lead is KCl. It is

felt that the KCl had nothing to do with the lead failure. It probably came down

on the lead after it melted in the form of a polycrystalline powder and a few

small crystals (some well developed crystallites can be seen at high magnification

under the SEM) . The K for the KCl has obviously come from the seeded
171

plasma in the channel. Other SEM work has indicated the K is present

almost everywhere in the insulator and electrode assemblies. The Cl

has probably come from a small weight percent of oxy-chloride frequently

used as a binder in the zirconia cement.

The conclusions of this examination are:

1. The leads melted and were not chemically attacked. It is

suggested that in assembly, a scratch or bend was put into the lead

in a region which was not surrounded by the cement. Joule heating

developed and without the cement to conduct away the heat, the lead

melted.

2. Thus leads should:

a. be double and each of sufficient size to carry the

current without excessive joule heating.

b. be embedded as well as possible in the cement.

c. the lead assembly should be more securely attached

to the ceramic tube at the rear of the electrode

insulator assembly in order to prevent possible

damage to leads by twisting when making electrical

connections
172

A2

C.5.1. Rad iogiraphs of anodo and cathod© submodule 1.
173

Figure C.5.2. Radiographs of anode and cathode submodule 2
174

Figure C.5.3. Radiographs of anode and cathode submodule 3.
175

Figure C.5.4. Radiographs of anode and cathode submodule 4.
176

C5

Figure C.5.5. Radiographs of anode and cathode submodule 5.
177

Figure C.5.6. Optical micrograph of melted lead of A2-7.

Figure C.5.7. Optical micrograph of leads melted together of A4-15.
178

Figure C.5.8. Optical micrograph of electrode and ceramic tube of

A2-8 cut to show the melted platinum lead.

Figure C.5.9. Higher magnification optical micrograph of A2-8.
179

Figure C.5.10. Secondary electron SEM micrograph of melted end of

the platinum lead of A2-7.

Figure C.5.11. EDX spectra taken from powdery material formed on

the melted lead. The strong peaks are due to Cl

and K.
180

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

List of Figures

1.1. Anode wall after shipment to NBS.

1.2. Cathode wall after shipment to NBS.

1.3. Cathode or anode wall schematic.

1.4. Inside surface of the outside slice from each anode submodule

(see Figure C.I.3.).

1.5. Inside surface of the outside slice from each cathode submodule

(see Figure C.I.3.).

1.6. Open schematic of cathode and anode showing materials, identification digits and electrode lead-out failure. Shaded areas indicate

which lead failed (anode or cathode)

.

2.1a. Portion of x-ray diffraction powder pattern of the 82 Zr02-18 CeO^,

1500 °C material prepared under oxidizing conditions (untested)

.

T = tetragonal zirconia solid-solution. Compare with Figures C.2.1b,

C.2.1c, C.2.1d and C.2.3a, C.2.3b, C.2.3c.

2.1b. Portion of x-ray diffraction powder pattern of cathode electrode

1205, open circuit, 82 Zr02-18 CeO^ (tested). White upper surface

see Table C.2.1. M = monoclinic zirconia; C, T = poorly crystalline

cubic fluorite and/or tetragonal solid-solutions.
181

Figure C.

Figure C.

Figure C.

Figure C.

Figure C

Figure C

Figure C

Figure C

2.1c. Portion of x-ray diffraction powder pattern of cathode electrode

1205, open circuit, 82 ZrO^-lSCeO^ (tested). Yellow interior zone, see Table C.2.1. M = monoclinic zirconia; (C]) = cubic pseudofluorite (cubic subcell _a = 5.26 A); P = K, Ce, Zr-oxide) .

2 . Id . Portion of x-ray diffraction powder pattern of cathode electrode

1205, open circuit, 82ZrO -IdCeO^ (tested). Yellow, olive-green

bottom zone, see Table C.2.1. II = monoclinic zirconia; (Ci) = cubic pseudo-fluorite (cubic subcell a = 5.26 A) ; P = (K, Ce, Zroxide) .

2.2. Portion of x-ray diffraction powder pattern of synthetic products

resulting from reactions of CeO^ , ZrO^, and K^O. P = (K, Ce, Zroxide). Compare synthetic phase P with P phase in Figures C.2.1c,

C.2.1d and C.2.5b.

2.3a. Portion of x-ray diffraction powaer pattern of cathode electrode

1208, 1.25 A/cm^, 82Zr0^-18Ce02 (tested). White zone, top, see Table C.2.1. M = monoclinic zirconia; C = two or more cubic

fluorite solid-solutions.

2.3b. Portion of x-ray diffraction powder pattern of cathode electrode

1208, 1.25 A/cm^, 82Zr02-18Ce02 (tested). Yellowish interior zone, see Table C.2.1. M = monoclinic zirconia; (C^) = cubic pseudo-fluorite

(cubic subcell a = 5.26 X.

2.3c. Portion of x-ray diffraction powder pattern of cathode electrode 1208,

1.25 A/cm^, 82Zr02-18Ce02 (tested). Yellowish-green bottom zone, see Table C.2.1. M = monoclinic^zirconia; (C^) = cubic, pseudofluorite cubic subcell a = 5.26 A; P = (K, Ce, Zr-oxide).

2.4a. Portion of x-ray diffraction powder pattern of the fOZrO^-fOCeO^

,

1500 °C oxidizing conditions (untested) . T = tetragonal zirconia

solid-solution; C = cubic fluorite solid-solution. Compare with

Figures C . 2 . 4b , C.2.5a and C.2.5b.

2.4b. Portion of x-ray diffraction powder pattern of the fOZrO^-bOCeO

1600 °C oxidizing conditions (untested) . C = cubic fluorite so solution. Compare with Figures C.2.4a, C.2.5a and C.2.5b.
182

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

Figure C

2.5a. Portion of x-ray diffraction powder pattern of cathode electrode

1310, 0.8 A/cm^ , 50Zr0 -50Ce0 (tested). Yellow interior, see Table C.2.1. (Ci) = cubic, pseudo-fluorite (cubic subcell a = 5.26 A); P = (K, Ce, Zr-oxide) . Compare with Figures C.2.4a and

C.2.4b.

2.5b. Portion of x-ray diffraction powder pattern of cathode electrode

1310, 0.8 A/cm^, 50 Zr02-50Ce02 (tested). Yellowish-green bottom

zone, see^Table C.2.1. (C^) = cubic, pseudo-fluorite (cubic subcell ^

= 5.26 A) ; P = (K, Ce, Zr-oxide); (C 2 ) = cubic fluorite solidsolution a = 5.38 A)

2.6. Portion of x-ray diffraction powder pattern of a typical MgO-insulator

,

"hot”, surface. M = MgO, S = spinel, probably MgAl^O, . S not detected

in untested MgO.

2.7. Anode series 2205 through 2208, 82 Zr02-18Ce02.

2.8. Cathode series 1205 through 1208, 82 ZrO^-lSCeO^.

2.9. Anode series 2309 through 2312, 50 Zr02-50Ce02*

2.10. Cathode series 1309 through 1312, 50Zr02~50Ce02

2.11. Anode series 2413 through 2416, 25Zr02~75Ce02

2.12. Cathode series 1413 through 1416, 25Zr02-75Ce02

2.13. Anode series 2517 through 2520, 78Ce02-20Zr02~2Ta20^

2.14. Cathode series 1517 through 1520, 78Ce02-20Zr02-2Ta20^

2.15. Cathode electrode 1205, open circuit, 82Zr02-18Ce02

2.16. Cathode electrode 1206, 0.5 A/cm^ , 82Zr02-18Ce02

2.17. Cathode electrode 1207 , 0.8 A/cm^ , 82Zr02~18Ce02

Anode electrode 2207, 0.8 A/cm^, 82ZrO^-18CeO , compare with

Figure C.2.17.

Figure C.2.18.
183

Figure C.2.19. Cathode electrode 1309, 1.25 A/cm^, SOZrO^-SOCeO^

.

Figure C.2.20. Cathode electrode 1310, 0.8 A/cm^, SOZrO^-SOCeO^

.

Figure C.2.21.

2 Cathode electrode 1311, 0.5 A/cm , 50Zr02~50CeO^

.

Figure C.2.22. Cathode electrode 1312, open circuit, 50Zr02~50Ce02

.

Figure C.2.23. Anode electrode, 2312, open circuit, 50ZrO^-50Ce02

.

Figure C.2.24. Cathode electrode 1413, open circuit, 25ZrO„-75CeO_

.

Figure C.2.25. Cathode electrode 1414, 0.5 A/cm^, 25Zr02-75Ce02

.

Figure C.2.26. Cathode electrode 1415, 0.8 A/cm^, 25Zr02-75Ce02

.

Figure C.2.27.

2 Cathode electrode 1416, 1.25 A/cm , 25Zr02~75Ce02 , interior

cross section.

Figure C.2.28. Cathode electrode 1416, 1.25 A/cm^, 25Zr02“75Ce02

,

outside cross section.

Figure C.2.29. Cathode electrode 1517, 1.25 A/cm^ , 78Ce02~20Zr02-2Ta20^

.

Figure C.2.30. Cathode electrode 1518, 0.8 A/cm^, 78Ce02-20Zr02~2Ta20^

.

Figure C.2.31. Cathode electrode 1519, 0.5 A/cm^ , 78Ce02-20Zr02“2Ta20^

.

Figure C.2.32. Cathode electrode 1520, open circuit, 78Ce02-20Zr02~2Ta20^

.

Figure C.2.33. Anode electrode 2519, 0.5 A/cm^ , 78Ce02-20Zr02~2Ta20^

,

compare with Figure C.2.31.

Figure C.2.34. Anode electrode 2520, open circuit, 78Ce02-20Zr02-2Ta20^

,

compare with Figure C.2.32.

Figure C.2.35. Cathode electrode 1205, open circuit, 82Zr02“18Ce02

,

longitudinal section, fracture surface.

Figure C.2.36. Cathode electrode 1205, open circuit, 82Zr02~18Ce02

,

cross section, fracture surface.

Figure C.2.37.

2 Cathode electrode 1208, 1.25 A/ cm , 82Zr02~18Ce02

,

cross section, fracture surface.

Figure C.3.1. Post-test electrical conductivity of electrode 2102

(88 Zr02-12 Y

2

O

2

)

.

Figure C.3.2. Post-test electrical conductivity of electrode 2206

(82Zr02-18Ce02)

.
184

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

Figure C.

4.1. Cross sections of anode and cathode for electrode pair

2103 and 1103 (0.12 Y^O^.O.SS ZrO^)

4.2. Cross sections of anode and cathode for electrode pair

2206 and 1206 (0.18 CeO2.0.82 ZrO^)

4.3. Cross sections of anode and cathode for electrode pair

2309 and 1309 (0.5 CeO^.O.S ZrO^)

4.4. Cross sections of anode and cathode for electrode pair

2414 and 1414 (0.75 CeO2.0.25 ZrO^)

4.5. Cross sections of anode and cathode for electrode pair

2517 and 1517 (0.75 Ce02.0.20 Zr02-0.02 Ta^O^)

4.6. Cross section of cathode 1102 (0.12 Y^O^.O.dd ZrO^)

4.7. Cross section of cathode 1310 (0.5 CeO^.0.5 ZrO^)

5.1. Radiographs of anode and cathode submodule 1.

5.2. Radiographs of anode and cathode submodule 2.

5.3. Radiographs of anode and cathode submodule 3.

5.4. Radiographs of anode and cathode submodule 4.

5.5. Radiographs of anode and cathode submodule 5.

5.6. Optical Micrograph of melted lead of A2-7.

5.7. Optical micrograph of leads melted together of A4-15.

5.8. Optical micrograph of electrode and ceramic tube of

A2-8 cut to show the melted platinum lead.

5.9. Higher magnification optical micrograph of A2-8.

5.10. Secondary electron SEM micrographs of melted end of the

platinum lead of A2-7

5.11. EDX spectra taken from powdery material formed on the

melted lead. The strong peaks are due to Cl and K.
185

D . USSR Results on Pre- and Post-Test Materials Characterization

and Analysis (G. P. Telegin, A. I. Romanov, Y. P. Gokhshtein,

Yu. L. Krasulin, T. I. Borodina, A. S. Golubkova, V. L. Vinogradov,

and D. A. Vysotskii, L. G. Smirnova, A. B. Ivanov)

D. 1. Magnesia Ceramics in Channel Insulating Walls

Magnesia ceramics have been used as ceramic material for the

insulating walls of magnetohydrodynamic generator channel walls.

Electrosmelted magnesia served as a raw material in producing the

magnesia ceramics. The granular magnesia powder (grain size < 1 mm) was mixed with the finely dispersed component and charged with a magnesium chloride solution. Magnesium chloride acts as a plasticizing

agent during extrusion and facilitates the formation of magnesium oxychloride

and magnesium hydroxide in the reaction with finely dispersed magnesium

oxide. With the decomposition of the oxychloride and hydroxide, highly

active magnesium oxide forms in the firing process, which facilitates

caking of the electrosmelted magnesium oxide. The material was produced

by pressing with a specific pressure of 3000 kg/cm^. Firing was carried

out at 1600°C. The concentration of magnesium oxide in the magnesia

ceramic was about 97%. The apparent porosity of the product was 13-14%.

Figure D.1.1 shows a microphotograph of a transparent section of

the magnesia ceramic. The filler (the light portion) consists of

magnesia granules with diameters of up to 800 microns; they form

idomorphic crystals with clearly differentiated three-dimensional

cleavage fissuring. The connecting mass consists of small MgO crystals.

In the granular filler, magnesia crystals deformed along the

cleavage plane are observed. Thin streaks of silicate are seen around

the magnesia grains. These streaks are composed of calcium silicate

("rankinite" , a-Ca2 Si 04 ) , and magnesiof errite

The distribution of pores of the ceramic specimens (in regard

to size) was determined by the mercury porosity determination method.

98.6% of all the pores had radii between 100,000 and 31,620 A.

The magnesia ceramic offers adequately high electrical resistance

under high-temperature conditions (1 to 1.5*10^ ohm cm. at 1500°C)

The change in conductivity of the magnesia ceramic with an increase in

temperature is shown in Figure D.1.2.

The heat conductivity of the magnesia ceramic in the temperature

range from 200-1600°C (Figure D.1.3) was determined by the "pipe” method;

this method utilizes radial thermal flow from an internal heat source

located along the axis of the cylindrical samples.

The coefficient of thermal expansion, determined in a vacuum for

the temperature range of 20-1800°C, equals 16.0 x 10 ^/degree. Maximum

expansion at 1800°C is 2.93%.
186

A study of the thermomechanical properties of the magnesia

ceramic was carried out under isothermic conditions in a temperature

range of 20-1600°C, with single-axis compression or expansion in an oxidizing environment. The testing was carried out using automatic testing equipment at the Moscow State University Institute

of Mechanics. Testing and calibration of this equipment provides a constant temperature with an accuracy of zO.5%, measurement of the

load with an accuracy of ±1%, and of the characteristics of deformation

with an accuracy of ±2%.

In Figures D.1.4 and D.1.5, the deformation curves B versus e of the

magnesia ceramic samples at compression and under tension are shown for

various temperatures. At a temperature of 1200°C, the magnesia ceramic

becomes plastic and the character of the curve becomes nonlinear.

The "creep" testing was conducted under conditions of static load

and exposure to constant compression load, smaller in magnitude than the

limit of proportionality. "Creep" curves £ versus time (t) (Figure D.1.6)

show the continuous growth in deformation of the test samples with time.

The rate at which "creeping" stabilizes increases with an increase in

temperature or with an increase in load.

The rate of vaporization was determined in "bi-parabolic" optical

ovens using test pieces 8 mm in diameter and 30 mm long. The rate of

vaporization in relation to temperature increases according to a nearly

eicponential law. Up to 2200 K the amount of evaporation is not significant;

primarily the impurities contained in the MgO evaporate. In the

temperature range from 2300-2500°C, the rate of vaporization increases sharply.

The data is shown' in Figure D.1.7.

During the tests, the insulating walls were in the form of panels

of small modules, in which the ceramic material and the reinforcing

foundation were divided into sections, in order to insulate the modules

from one another. This construction of the insulating panels is shown

in Figure A. 2, Chapter I. The basic ceramic element is a hexagonal module

of magnesia ceramic (1), attached to a copper base (2) by brazing through

the copper under-layer. The copper base serves to conduct heat away from

the ceramic to the cooling water, and also for attachment to the carrier plate (3)

The carrier plate of the channel is made of stainless steel.

The working portion of the channel has a cross section 64 x 220 mm^

and a length of 490 mm.

After testing for 125 hours, the surface of the ceramic modules had not

experienced any change. Neither cracking nor chips were observed (D.1.8).

Because of the high surface temperatures of the plate at the time of the

test, there were no seed deposits on the modules. Analysis of the seed

amount absorbed by the ceramic in the course of testing indicated

that the ceramic had accumulated an insignificant quantity of seed at

the zone of its maximum concentration (3-4 mm from the surface); this was not more than 1%.

During testing, basic attention was directed to the determination

of the changes in insulating characteristics of the wall during extended

operation. Results of measurement of the resistance of the channel

elements showed a high level of insulation of the channel wall sections.
187

Figure D.1.1. Microstructure of test piece of magnesium

ceramic. Transparent section, 46X.
(ohms/cni)

188

Dependence of the conductivity or magnesium ceramic on

temperature

Figure D. 1.2.
189

CD

C\J

Figure D . 1 . 3

Dependence of the thernial conductivity of magnesium

ceramic on temperature.

1000

1200

1A00

1600
190

Figure D.1.4. Deformation curve "B-E" of test pieces of

magnesium ceramic under compression.

OQ
191

CM

e

u

O'

CQ Figure D . 1 - 5 Deformation curve "B-E" of test pieces of magnesi

ceramic under tension.

um

0,02

0.03
192

PO

j::

V.

Figure D.1.6. "Creeping" curve £ vs t of magnesium ceramic.
193

Figure D.1.7. Dependence of the speed of vaporization of

magnesium ceramic on temperature.

195

Analysis of the test modules of magnesia ceramic showed that the

upper part of the module, in contact with the gas flow, acquired a lighter coloration and showed a more prominent grainy texture as a result

of the migration of iron oxide from the insulator to the working space.

While the structure of the ceramic in the low temperature zones did not

differ from the initial structure, recrystallization of the magnesia was observed in the working part of the insulator.

D.2 Phase Composition and Structure of Materials Tested in the U—02

D.2.1 Objects and Methods of the Studies

A study of the changes occurring in the composition of

electrode materials as a result of electrode tests in the channel of the

U-02 facility was made by x-ray phase and x-ray spectrum analysis. We

studied the composition of samples consisting of sections from the side

parts of electrodes. In tests in the channel these electrodes were

subjected to lower temperatures than the central sections that were

studied by the U.S. side.

Since the U.S, side made a detailed analysis of the

phase composition of all the electrodes tested, the U.S.S.R. side made

similar studies only for certain electrodes. One of the aims of the

phase study was to determine possible differences in the composition

of the materials of the central and the side segments of the electrodes.

For the studies, two pairs of electrodes for each of the

compositions tested were selected, one of each of the two pairs of

electrodes was run At open circuit during the test. We studied the

composition of materials for the following electrode pairs: 2 and 4

(without current) (initial composition 88 Zr02 - 12 Y2 O 3 ) ; 5 (without

current) and 7 (j = 0.8 A/cm^) (initial composition 82 Zr 02 - 18 Ce02 )

10 (j =0.8 A/cm"^) and 12 (without current) (initial composition

50 Zr 02 - 50 Ce02 ); 13 (without current) and 15 (j = 0.8 A/cm^) (initial

composition 25 Zr02 - 75 Ce02 , 17 (j = 1.25 A/cm^) initial composition

20 Zr02 - 78 Ce02 - 2 Ta205).

For each of these samples laminar dif f ractograms were made

of the successive surfaces of the sections in the area between the

working surface and the platinum. The width of the zone studied in each

shot was 1 mm. For samples with the initial composition 88 Zr 02 - 12 Y2 O 3 a phase analysis of the cement material and the cold parts of the insulator

was also made. The dif f ractograms were made on filtered Cu-radiation in

the range of angles of 29 of 3-100° with a sample rotation rate of 2 degrees/min.

Moreover, in order to make the phase composition more

accurate, additional dif f ractograms have been made in a number of cases

in the range 26: 27-35°; 46-50° and 55-60° with a sample rotation speed

of 0.5 degrees/min.

For all the samples listed above, curves of the intensity

of distribution of the -radiation of potassium and zirconium were U 1

^ obtained by means of x-ray spectral analysis. In the samples containing

CeOo the distribution curves for the intensitv of L -radiation and

ai
196

in the samples containing Y 2 O 3 - Pt were recorded. Recordings of

the intensity of radiation from K, Zr and Ce in the microvolumes of each

electrode were performed along lines perpendicular to the working surface

of the electrode. For comparison of the distribution of the intensity

radiation K in various parts of the electrode, curves along lines

parallel to each other were recorded at points equidistant from the

"hot” layer of the electrode, the topography of the surface was the same

or differed little.

D.2.2 Results of X-Ray Phase Analysis

The results of x-ray phase analysis of the electrode

materials are shown in Table D.2.2.1 and D.2.2. 2. In the description of

the recorded phases, in order to simplify comparison of the data for the

central and side segments of the electrodes, the designations introduced

by the U.S. investigators were used: phase with a pyrochlor structure

and the period a = 10.52 A (or with the "pseudofluorite" lattice, a = 5.26 A); C 2 phase with fluorite structure and period close to the period

of the lattice of pure Ce02 : a = 5.38 to 5.39 A; P phase with a cubic

O perovskite structure and lattice period a = 4.09 A. Additional studies

of the composition of the and P phases were not performed by the U.S.

side to establish the composition of these phases. Thus, under phase

we include the phase Ce 203 * 2 Zr02 , in which part of the Ce^"^ cations are replaced by Ce*^'*’. The P phase is apparently a product of the reaction

of the seed with Zr 02 and Ce02 .

The data of phase analysis of the samples before their

tests differ slightly from the corresponding U.S. data. On the

dif fractograms of the samples of all the initial electrode materials and

MgO there are weak maximums not pertaining to the basic phases and

indicating the presence of admixture phases in the initial materials.

Identification of these phases was not successful, and in Table D.2.2.1

they are given under the arbitrary designations A, B, D, E. In the

sample with composition of 50 Zr02 - 50 Ce02 , in addition to the other

secondary phases, there are probably and C 2 phases.

The phase compositions of samples taken from central and

side segments of the tested electrodes have noticeable differences in a number of cases.

Materials with the initial composition 88 Zr 02 - 12 Y 2 O 3 electrode

pairs 2 and 4 (without current)

Change of the electrode materials containing Y 2 O 3 appears mainly in

the appearance of monoclinic Zr 02 , which is a product of partial

destabilization of the fluorite solid solution with a Zr02 base. Monoclinic

Zr02 appeared in all the electrodes studied with the exception of anode

2104. The content of monoclinic Zr 02 increases in areas adjacent to

the platinum. The greatest amount of monoclinic Zr 02 was recorded in

cathode 1102. For this cathode in the regions around the platinum we observed a decrease in the period of the fluorite solid solution lattice in
197

comparison with the period of the initial material lattice. On all the

electrode sections studied we recorded traces of the presence of KHCO 3

.

The composition of the materials of the central and side sections of

the electrodes is the same.

Materials with the initial composition 82 Zr02-18 Ce02 electrode

pairs: 5 (without current and 7 (j =0.8 A/cm^)

For the electrodes of the 5th and 7th pairs the presence of a significant amount of monoclinic Zr 02 is common for the entire cross section of the electrode from the working surface to the platinum and a significant amount of KHCO 3 , the content of which increases as the

platinum is approached.

The main difference between cathodes 1205 and 1207 is the fact that

in 1205 the reaction of the seed with Zr 02 /Ce02 occurred across the

entire section of the sample, and the P-phase is dominant, while in 1207

there is only a small amount of P-phase, the zone of its presence not

being adjacent to platinum. On the other hand, the C^-phase, indicating

reduction of Ce^'*', is dominant across almost the entire section of 1207, while

in 1205, it is present only in the - 2 mm zone, beginning from the "hot”

surface of the electrode. In 1205 the dominating phase (P) is accompanied

by the cubic fluorite solid solution enriched with Ce02 , while in 1207, in

addition to the dominating Ci-phase there is a tetragonal solid solution

similar to the initial solid solution.

In the central segments of these cathodes there is a different

distribution of and P phases across the cross section of the electrodes.

The zone of reaction of the seed with the electrode material is

adjacent to platinum, and the phase is present in the middle regions

of the sections. In addition, unlike the lateral segments, in the

central areas of 1207 across the entire section there is no tetragonal

S' lid solution similar to the initial solution.

The material of anode 2205 contains a significant amount of tetragonal

solid solution unchanged in comparison with the original. In the material

of anode 2207 there was breakup of the initial solid solution into two

cubic fluorite solid solutions enriched with Zr 02 « As noted above, the

materials of both these anodes, as well as the cathodes, contain significant

amounts of monoclinic Zr 02 , which indicates the intense destabilization

processes occurring in these electrodes. But the degree of destabilization

in the material of the side segments of the electrodes is less than in

the central segments, thus in the side segments there is an unchanged

(in comparison with the initial) tetragonal solid structure, usually

absent in the central segments. In addition, the sections of anodes

studied by us do not contain the P-phase in distinction from the central

part of anode 2207.

Materials with the initial composition 50 Zr 02 - 50 Ce02 electrode

pairs 10 (j = 0.8 A/cm^) and 12 (without current)

In these cathode sections the dominant phase is the Cj-phase,

indicating reduction of Ce^'*’. The difference between cathodes 1310 and
198

1312 is mainly the fact that in 1310 there occurred the reaction

Zr 02 /Ce 02 with the seed (the reaction zone coincides with the zone of

yellow color) , while in cathode 1312 no traces of this reaction were

detected. In addition, in 1312 next to the working surface a certain

amount of tetragonal solid solution was retained, similar to the initial.

In 1310 there is no tetragonal solid solution.

In the upper parts of anodes 2310 and 2312 there was formation of

C^-phase, while 2310 also retained tetragonal solid solution, the main

phase for the initial composition 50 Zr02 - 50 Ce02

In the lower parts of the anodes the fluoride solid solution is

O dominant with the lattice period a = 5.29 A and tetragonal solid solution

(a = 5.15, c = 5.24 A) characteristic of compositions with a low constant

of Ce02, for example, for the composition 82 Zr02 - 18 Ce02. In addition,

in areas adjacent to platinum cubic fluorite phases are present, enriched

with Ce02 . These regions are the zones of the reaction of Zr02/Ce02

with the seed.

The composition of the materials of the central and side areas of

the electrodes studied (with the exception of 2312) is generally similar,

although the sections from the side areas of the cathodes did not have a green color zone, in distinction from the central areas. It should be

noted, however, that in the central area of anode 2310 the P-phase noted

in the side areas of this anode are absent.

Materials with the initial composition 25 Zr02 - 75 Ce02 electrode

pairs: 13 (without current) and 15 (j = 0.8 A/cm^)

The main phase for the materials of electrode pairs 13 and 15 is

fluorite solid solutions similar to the initial solution (a = 5.34 A). At

the same time, all the samples studied have cubic solid solutions at

fluorite with smaller lattice periods. On the sections of cathodes 1413

and 1415 we recorded a significant amount of KHCO 2 , but no P-phase was

revealed. In distinction from the cathodes, the reaction of the seed with

Zr02/Ce02 occurred in both anodes; in 2415 we observed only traces of Pphase, while in 2413 there is a strong P-phase in the zone not adjacent

to platinum and coinciding with the region of gray-brown color.

In cathode 1413 and in both anodes, there is monoclinic Zr02 near the

platinum. Anode 2413 is distinguished by the presence of almost pure

Ce02 (C 2 ~phase) in it.

The composition of the cathodes in the central and side sections is

rather similar. The composition of the anodes differs significantly, at

least in the surface layers.

In the central segments at these anodes, the cubic phase with an

a = 5.14 period predominates. This indicates a significant enrichment

at the solid solution with zirconia; this is substantially less pronounced

in the lateral segments at the electrodes.
199

Materials with the initial composition 20 Zr 09 - 78 Ce02 - 2 Ta205

electrode pairs 17 (j = 1.25 A/cm^) and 20 (without current)

The main phase of the upper parts of cathodes 1517 and 1520, as well as anode 2520, is fluorite solid solution (A = 5.35), similar to

the initial. In anode 2517 a phase richer in Ce02 dominates. Reaction

with the seed occurred in all electrodes. The reaction zones fit

the olive-green-yellow and olive-green areas and consist of areas 2-3 mm

wide, adjacent to platinum. In all the electrodes studied we observed

an assembly of cubic phases with varying lattice periods. The difference

in cathode 1517 is the presence of the C^-phase in the upper part of the

electrode.

On the whole, for electrodes 17 and 20 we observed agreement of the

composition in the central and side segments.

Cement

In the cement material (both on the cathode and on the anode walls)

there was partial destabilization of the cubic solid solution on a Zr 02

base. The amount of monoclinic Zr 02 forming as a result of the breakup

of the cubic solid solution increases from platinum to the cold part of

the cement. The cement is a concentrator of potassium carbonate hydration

products. In general, KHCO 3 is present in the cement. X-ray spectrum

analysis shows that the amount of potassium increases with approach to

the cold end of the cement, the K content being higher in the cement on

the cathode wall than in the cement of the anode wall.

In the gray areas of the cement, in addition to KHCO 3 , we also

recorded both K2 CO 3 - 1-1/2 H2 O and K2 C 03*(2 KHC 02 )*( 1.5 H2 O) . In these same sections we noted the presence of magnesium oxide.

In the cement layers adjacent to the platinum lattice we observed a certain amount of Pt. The width of the platinum diffusion zone varies

from about 0.2 to 0.3 mm for the different electrodes, and in some cases

(for example, anode 2104) it spreads over all the section of the cement.

The Mg02 insulator

In the cold parts of the cathode wall insulators we recorded a significant amount of potassium hydration products: KHCO 3 K2 CO 3 - 1-1/2 H2 O, K2 C03*(2 KHCO 3

) • 1. 5 (H 2 O)

On similar parts of the cathode

wall insulators, K is absent.

D.2.3 Discussion of the Results

As follows from the results of x-ray phase analysis, significant

changes occurred in the electrode materials due to the breakup on the

initial solid solutions and reactions with the seed. Here the materials

with the initial composition 82 Zr 02 - 18 Ce 02 and 50 Zr 02 - 50 CeO"

underwent the greatest changes. It is for these electrodes that the

breakup of the solid solutions is most clearly expressed, as well as the

reaction of the electrode material with the seed.

For the materials of all the compositions tested breakup

of the initial solid solutions is observed both on the cathodes and on
200

the anodes, it is also clearly expressed on electrodes without current.

The breakup products are monoclinic Zr 02 and an assembly of cubic phases

with varying content of Ce02 ; it is possible to separate solid solutions

enriched or depleted of Ce 02 compared with the initial composition.

The materials of these electrodes are to a greater or lesser degree premeated with potassium, hydration of which forms the

compound KHCO 3

. As is shown by x-ray spectrum studies, K is present in

all the samples studied both directly on the working surface and in

deeper layers. As a rule, the intensity of radiation K increases at

a depth of 2 to 3 mm from the working surface, and has a significantly

higher value in this zone than in the hot layers of the electrode.

In the majority of electrode materials containing Ce02 , there was a reaction of Zr 02 /Ce 02 with the seed. The reaction occurred

(both in the cathodes and in the anodes) mainly in zones 1 to 3 mm wide,

either adjacent to platinum or approximately 1 mm from it.

Analysis of the results obtained for cathode and anode

materials containing Ce02 shox^/s that at this stage of the studies there

is not a sufficient basis to unequivocally equate the coloring of each

of the color zones observed on the electrodes with the presence of any

single phase. It is possible that the yellow color of the material is

to a certain degree connected with reduction «f Ce^'*’.

The composition of the electrode materials of the central

and side areas generally agree in some cases, and differ significantly

in other cases. The differences are apparently connected with the

temperature differences on these electrode segments. It should be noted

that, as study has shown, the phases present in the materials are irregularly

distributed through the volume of the electrodes. For example, the

phase composition of the materials of two neighboring sections of the same sample, at a distance of approximately 1 mm from each other, in the

majority of cases differ at least in the ratio of the amount of main

phases and also in the amount of KHCO 3

. Thus, in addition to the change

in the phase composition from the hot to the cold layers of the electrode,

there are also variations of the composition in layers parallel to the

working surface. Probably, the nonuniformity of the phase composition

at various parts of the electrode is connected with the irregularity of

distribution of the seed penetrating into the electrode. These electrodes

are characterized by an extremely irregular distribution of K. In the

electrode layers parallel to the working surface, the intensity of the K(^j,

for K undergoes severe changes. For example, the zone of high

intensity on the working surface changes in width from 0 to 0.7 mm. Similar zones in the cold parts of the electrode also change their size

and intensity. Figures D.2.3.1a,b and D.2.3.2a,b show the distribution

curves of the intensity of K along lines perpendicular to the working

surface for the 1310 cathode and 2520 anode, illustrating this change in

the width and intensity of the zones of K in the hot and cold parts

of the electrode. Similar situations are also observed on electrodes of

other compositions.

In addition, experiments have shown that the amount of K

recorded on sections exposed to air over a long period proves to be

greater than on corresponding fresh sections. The K distribution patterns

for these sections are similar.

This result is apparently connected with the hydration of

the seed penetrating into the depth of the electrodes.
201

All the electrode material composition changes recorded

and discussed above occur as a result of the simultaneous effect of

three main factors: high temperatures (and subsequent cooling), seed

and electrochemical processes. It is their simultaneous action that

leads to the complex phase composition situation in these electrodes.

It is apparent that in order to explain the phase conversion mechanism

occurring in the electrodes it is necessary to have a clear idea of the

role of each of these factors. But at the present stage of study we can apparently speak only of their cumulative effect, since to determine

the degree of influence of each of the factors requires additional

experiments and studies.

Table D.2.2.1. Sample Composition before Tests.

O O Sample (mol.%) Main phase (a,c - A) Additional phases (a,c A)

88 ZrO^ -

12 Yj03

82 ZrO^ -

18 Ce02

50 ZrO^ -

50 CeO^

25 ZrO^ -

75 CeO^

20 ZrO^ -

78 Ce02 -

2 Ta^O^

MgO

f . c . c

a = 5.152

Tetragonal solid sol.

a = 5.15; c = 5.24

Tetragonal solid sol.

a = 5.26; c = 5.33

f . c . c

a = 5.34

f .c.c. a = 5.35

Admixture phase A

Admixture phase A + B

C^-phase, a = 5.26 +

tetragon, a = 5.15

c = 5.24 + f .c.c. , a = 5.34 + C^-phase,

a = 5.38 + admixture

phase B

f.c.c., a = 5.23 +

admixture phase D

f.c.c., a = 5.15 +

admixture phases D + B

f.c.c.

a = 4.210

admixture phase E
1

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• N» Ln 9 ro Ln o. Pt» n Ou M T3 + r> Q. 9 • CD • It X ft o + H- O t} + h-» CO 9 R 9 • Pi R 9 X U Ln H- Ln o Ln 9 O • • 9 X O

Pi rt •m 9 Ln rr « H- • n Ln >- O o Ln H-* O Ln Pi • • Pi 9 • • 9 9 9 o • 9 9 • 9 o Ps> 9 a • L*^ ro • 9 LJ L^ • 00 LJ n Ln O o -c^ it O 9 11 < 9 II /—

+ c 0 9 9 + CD < X + Ln pt> + Ln Pi Ln 9 2

• • O • • o • Pi 0 ro

Pt> NJ p^ N> o *9 O X

o o X 9 o U) • • U)

*9

9*

Q}

CD

fD9

+ 09O

rf 9 B 9 • 9 Ln O ^3

ro i-l H* Q o a^

NJ « o^ ^ n

9

f9O

o

c

cr

2

o

LJ

o

9

OQ

UiU>+

82

Zr0_

-

5

to

1

cream-white

cream-white

cub.

solid

solution

tetr.

solid

solution
204

n

moto

GDN>N

Oto

I Ln

00

li(0

re9

reH*H*?+

O.»

7^

U>

reh-§

re •9 •9 C

re re re re 9* 00 M M

3 re re rr

re O re 9 c re re rr H

V re re re re re 3 g g

reH* +

h*

•9

re 9

re •9

reHre

re

fi

re§

•9

re •9reM

re

re

re

re3

•9 re 3 n Ln re 3 n rr rr n r> re 3 n re rr •9 n re + n O 3 n re rr

re 1 o O H-* 1 • o o h- •-t to o 0 1—

ti X o h-* rn o re rr 9 M 9 1 *9 ro H* 9 1 re re 1 II 1—

9 1 It re • II H-» 3 1 9 1 H rr

ri 9* • •9 =r 9 • •9 re re •9 9 • •9 o H* O •9 • •9 fi O re a 9* re • 9- re re X Ln • X Ln re Ln Om 9 X X Ln • 9 re N re re + CO re re re re • CO re • re z • • re O CO re • 00 re re re re re re re to /»*s n re to re to re re O »i re re o O re rr o re o o re Z O re to o re o CO re LJ o re •o 0 M to + i-l a to Hn re to i-n 7T M n to H- < 9 re a re + re re + 9 O re re Hre * + t: re Ln + 7? re 7T + n • to + O O.

re R X re • II X 1 rr a c re re a 1 7T re re n re rr o •9 It n rr X c re + rr 1 re c Ln o Ln re o X re re re Ln • 0 X Ln rf Ln o

1 9* • 0 « fT • u> re Ln o rr • • 0 rr • re • Ln M* + • to »-h n to re • re a »-t to •9 ui re to re to • 9

o^ re • 9N re u> re « X X • Ln rr X re X to • s

• rt» VO Ln • u> H» • rl a o* s H* M + 1 re + + 0 • re + • • LJ C M • +

n U1 C o + i-n H* 0 o Ln o O H- Ln h- + ri « + ri • • • •

Ul re 3 re re re 3 re re re re 9 3 re re o re 3 re re re re ft re M% re re

re • re O o c re € O o c 9 X 0 o c c O 0 c M M» o c

rr to re 9 H* n 9* rr re 9 H-* II 9“ O. 9 j— H X X 9 H-* Q X it re B c H- X

»i ro o • H* • rt re • r* • • • H- • re o 9 • O 9 a Ln O ?r Q. Ln re € o. Ln O. Ln Ln re Ln • 9 re Q. N • 9 'w' CO i-n H* CO • My •9 CO • • re • • 00 9 re H* OO re H-* h- It n re H- H-* re t- H* O to re HO. re O o c s-/ re o o •O C X o o C • O o C CD o H- re c

c to h- c to h- O Ln to H- o • « to 1—

o My o B o

a 9* 9 + n a 9 + ri • •9 9 + 9 + + + • + • • + • • O re + • • re + • • re H* Ln H* u re Ln re CD n n re c 3 O. • Q.

•9 rr re c re • re re c re re c re It fT re c re X X o i-» re Ln 9* n 9* 0 N) c 9* o + 0 X o n X o n 9 re • re re . II • — ON 9* t) • h- o. re It • M Ln re n

• H- o re • o re O re re H* h*» 2 re o H* • re H* LJ M + 0

00 re re Ln • CU + Ln »-n a. re Ln My &. M re LA My O. •9 9 H-* re u • — *o • 1—• o re H- to re • X • re + •• U) C re 2

IT ro C re o re to re to C re • O c D. o to o 0 re VO 0 0 LJ o U1 o o + o >-• o o •• to X

re h-» n re n H- It rn n H* s + • 9 o re + • 9 -f • 9 + • 9 + n n U) Ln n •3m o • c w X O w • u>
205
phase

a

=

5.17

+

5.23 +

cub.

fluor.

solid

206

Ln

I

n t

C H-n

^ 3- n o

3 c

rr rt

O

N

O

3 ^

D» A M h- m h-* O

o C

II +

T3

Q) n

00

3*

oo

3“

003*

OO3*

n

v*s 00

'Im

3

s

n

o

0)

II

N3O'+

o

3

00

CD

It

COo

3

00

n

c

o*

3 3 o 3 a C UN rr 3 UN n 0 Ul rr n N *3 + rr> UN n

S 0 h- 3* I-* • rl • h- • 3 1—

rt h- • H*

C-5 3 II 1 •

II 1 M 3 II M 1 ts> rr i O 12 rr • ro 1 o • •3 •3 OJ O ON •3 o ON 3 •3 N3 rt ON •3 U) Ln 3“ r-s UN 3“ 3 UN 3* c w • 3* UN 3 3 3“ 3 • 3 + 3 + 3 a* 3 + • n O + 3

ro CO 3 ro 3 ho 3 O 3 3 LiJ 3 h- 3 o ON 3 ON 3 rr O O' rr 3 0 3 7i UN 3 • 3

ho 1^ rr. « 3 •3 1 f-i X 1 + ««•/ + ^ 3 rr 3- • 3 O + O 3 •3 /—

s

+ < < O n o ri < • UN o rh 0 zr 3

rr n 3 n 3 3 c • 3 • 3 fl rr H-* 3 rr rr n ri C rl 3 a* II rt 3 w O r^ O 3 3 rt

rl 0) II CT'^ • 3 • O I— UN y«—

3 C • 3 •

fi> o * 0 Ul O 12 3 • c O O* ''w' n 00 UN «• rn *3 • M 3 + • to 3 3 • 3 +

m CO rr • m rr 3* • rr Ul O' 3 3 3

CO n H- n • o rt « rr 3 *3 U n

0 o C 0 X 3 • h- rn + B O’ c n 0 rr, 3 O 3 n 3 + O •w 'U 3 11 o. 3 UN cr

i-h 00 3 OO o I )-• O 3 3 •

• 1) 3 3 3 • 3 h-

• a m

0 CO ui CO n 3 + UN n UN rr Ul n UN UN n UN •3 3 3 3 n

i-r» 0 • o H-

N5 < • H-* • rl • h- • • I—

< • 3“ O c

H-* N3 h- 1 1 II fT 3 ND 1 U) 3 3 N) 1 ISJ 3 NJ 1 3 M fl h- II O' 3 3 U> •3 •3 3 ON 3 UN O 3 O' "3 O' 3 O' -3 3 3 • 0 • CL 3* 3* UN rr ?r 3* 3 7T zr K 3* ?r 3 UN • Ul 3 + 3 3 * 3 'w' + 3 3 + 3 + 3 w • • rr. CO 3 OT K3 3 3 3 12 12 NJ NJ H* o O m fO 3 rr 3 O 3 rr 3 3 rr 3 + *«sj 3 \o • N II c h- CO 3 3 3 UN Ul rr O' 3 3 s o II rr /-V H rr r—

s

S rr y-^ • + rt + 3 O Ul • € n H-* 1^ w n rt < < X b UJ O 0

N) • 11 m U» • rr UN « 3 UN • 3 o CO O' rr 3 rr h-

U) Q> 3 11 o. rl o* • rl • rt o « 00 3 • + 1—

UN 7T 1—•* 3 0 • 1—

3 h- 3 • UJ + 3 'w' rr

II UN 3 UN O 3 UN 0 UN O 12 n 3

rr + O h-* 00 *3 h- 3 H- 3 *N3 3 3 O

1^ CO UN o 3 hJ H* w 3* • rr • rr 1 UN 3

Q) rr 0 3 n CL • n rt 3 *3 •

II 3 3 O M h- H- + II 3 3 3 3* u> n 0 • 0) H* + II 3 3 II O 1 0 3 c UN KCO n a « rr UN .—V O II h- >-• 3 « O' • • n C UN H- 12 UN 3 3 3 3 3 • H-* o CO n rr w O 3 • • 12 NJ sO 3 N> N5 U Q r
207

phase

a

==

5.3^

+

5.29 +

cub.

fluor.

solid
A-5

blue-green

Cub.

fl.

solid

208
•209

H

01
210

j

-«r>

Distribution of the intensity of radiation K for the

1310 cathode: Along an arbitrary line perpendicular to

the working layer of the electrode (central part of the

section)

Fi gure D. 2 . 3.1a

WM
211

. Distribution of the intensity of K<;xi radiation K for the

1310 cathode: Along a line parallel to line a) and 500 mcm

from it.

Figure D.2.3.1b

MM
Figure D. .3.2a. Distribution of the radiation intensity K for the

2520 anode: Along an arbitrary line perpendicular to

the working layer of the electrode (central part of the

section)

.
Figure D.2.3.2b. Distribution of the radiation intensity K for the

2520 anode: Along a line parallel to line a) and 800 mcm

from it.

nri
214

D.3 Electrochemical Investigations of Electrodes with

Compositions I (88 ZrO_, 12 Y„0„) and II (82 ZrO„,

18 CeO^).

D.3.1 Hot ceramic electrodes with composiitons consisting

of 88 mol% Zr02 12 mol%

o The fluorite structure has a lattice constant a = 5.151 ± 0.002 A. The open porosity is 17.7%; the closed porosity amounts to

less than 0.5%. The ceramics specimen is a solid electrolyte with

anionic conductivity. In accordance with the data from the "VDN Limited”

and Magnesium Electron Limited, the following admixtures predominate in

specimens having the above-mentioned composition: 0.2% TiO^; 0.2% Si02 ; 0.02% Fe^O^ and so forth. The admixture composition in Soviet specimens

differ somewhat from the admixture composition contained in American

specimens. Table D.3. 1.1 presents data concerning the electrical conductivity

of ceramics as a function of the temperature.

Table D.3. 1.1. The specific electroconductivity of the binary system 88

mol% Zr 02 - 12 mol% '^ 2^2 ^ function of the temperature.

a ohm cm Temperature

°C American data Data from laboratories

using electrochemical

methods in their investi

gation

Reference

data

900 2.5 • 0

1

3.0 • lO"^ 1.8 • 10

1000 6.5 • 10-2 6.0 • 10“^ 2.4 • 10

1100 9.6 • 10"^ 1.0 • 10”^ 7.7 • 10

1200 1.3 • 10”^ 2.0 • 10"^ 1.1 • 10

1300 2.4 • lo"^ 3.0 • 10"^ 2.0 • 10

1400 3.8 •

1

—

1

rH 0 4.2 •

1

—

1

0

1

—3.5 • 10

The conductivity values given show that the experimental

IVTAN data practically coincide with the data of American investigators.

The a values (IVTAN) refer to the Zr02 ~Y 203 specimens, which were produced

in laboratories that use electrochemical methods for the investigation

of materials. A good correlation between the data attests to the fact

that the admixture conductivity does not contribute anything at all to

the general conductivity at this temperature interval.

The microstructure of the cathode interphase layer exerts

a significant influence on the critical current (the limiting current of

oxygen diffusion). Its thickness depends on the method by which the platinum

current outlet is attached to the ceramics. If the platinum is pressed
215

into the ceramic, then the thickness of the unbroken layer of the solid

electrolyte, which is adjacent to the current outlet, attains approximately 10 or 10 cm. However, in the case where the surface of the ceramic is coated

with platinum, which then is brazed, the continuous surface becomes thicker

and more vacuum tight. For this reason, it is desirable to choose the

method in which the platinum is coated on the surface of the ceramics.

We assume that U.S. investigators used the method described below.

It is well known that the critical current, the area current,

is connected with a certain diffusion mechanism. Two mechanisms are possible

for the delivery of the O 2 to the platinum cathode:

1 ) O 2 diffusion in the pores down to the triphase boundary:

cathode-solid electrolyte-gas;

2 ) O 2 diffusion in the pores with subsequent diffusion in the

thin layer of the solid electrolyte to the boundary: electrolytecathode. The O 2 is dissolved in the ceramic with the formation of the

0 ^” anion-vacancy pairs, which diffuse with the diffusion coefficient

D = 1 to 2 • 10"^ cm2/sec at 1300-1400°C.

Independent of the concrete geometry of the cathodic space,

the conclusion can be drawn from the experimental data that the oxygen passes

the unbroken thin layer in the solid electrolyte after diffusion in the

pores of the remainder of the path to the cathode. The second stage is

significant in that it:

1 ) makes possible the participation of oxygen in the cathode

proces^s, which is guided by the electric field at the cathode-electrolyte

boundary; and

2 ) limits the value of the critical current.

The current goes through the current leadout-anode because

of the discharge of the oxygen anions. The anions are supplied to the anode

through migration in the electric field, but the discharge of O 2 from the

anode does not regulate the current but it is regulated by the supply

of O 2 to the cathode. For this reason, with the increase of the voltage

in the specimen the dependency of the current on the anode potential does

not disclose the limiting current plateau. It is precisely the cathode

which is responsible for the critical current density. [See Ya. P.

Gokhstein, R. A. Khaikin, Teplofixika vysokikh temperatur 7, No. 5, 1031

(1969); Ya. P. Gokhstein, DAN, 190, No. 1, 140 (1970); Ya. P. Gokhstein,

A. A. Safonov, Teplofixika vysokikh tem^peratur 8 , No. 2, 398 (1970);

Ya. P. Gokhstein, A. A. Safonov, DAN, 195, No. 1, 109 (1970)].

A U.S. specimen with a composition of 88 mol% Zr 02 - 12 mol% Y 2 O 3 was tested under laboratory conditions, in air, at a temperature of 1400°C and in the absence of thermal-shock (detailed account

will be presented later)

.

The faces of two specimens were coated with platinum and

using arc welding, a platinum grid was attached to this platinum. A

platinum wire with a 0.5 mm diameter was soldered on the grid. The

specimens were provided with platinum probes at distances of v 0.1 and

0.16 cm from the cathode and anode, respectively.

The tests were conducted in a Silit furnace and the

temperature of the furnace was determined with the aid of two platinum-
216

rhodium thermocouples. At a temperature of 1400°C ± 10°C the furnace

worked in a stable fashion. Together with the main specimen, a completely

analogous control specimen was introduced into the furnace; a current

did not pass through this specimen.

During the experiments of this study, the potential of the

electrode-platinum current leadout was measured at a preset current in

comparison to the probe. The volt-ampere curves "current density-potential"

were recorded by means of PDS-021m (double coordinate recording potentiometer)

both in the negative and the positive areas of the potential. On the "current

density-potential" curve in the negative area of the potential, where the

current leadout providing the measurements serves as a cathode, the

extreme oxygen diffusion current (critical current density) is found at

'^0.1 a/cm^ across the sample. For Zr02 -Y 203 ceramics with approximately

the same composition, an open porosity of 25% (IVTAN specimens), and

platinum disks pressed into the specimen, the limiting current of the

oxygen diffusion appeared at 0.62 a/cm^ in the beginning and 0.8 a/cm^

at the end (in the cross section of the specimen). (Figure D.3.1.1).

It is well known that the limiting current of the oxygen

diffusion jm in the case of unidimensional diffusion is determined by

the following equation

j m

nF

C o

D D, o 1

1

o h1

where n - the flow rate of the electrons in the reaction (n = 2);

F - the Faraday number;

Cq - the O2 concentration outside the specimen; the diffusion

layers is composed of two layers a porous layer (0) and an unbroken

one (1) with thickness 1 and 1. and diffusion coefficients D and D,

o 1 o 1

The diffusion of oxygen through the unbroken layer (the

second stage of the diffusion mechanism) is the basic, essential stage,

limiting the value of the critical current. When the platinum is coated

on tb^ ceramic specimen, an unbroken, vacuum-tight layer, l^, is formed

thicker than the unbroken layer 1^ of the sample with a platinum current

leadout pressed into the ceramic. The oxygen must be diffused through a thicker and denser ceramic layer. For this reason the diffusion coefficient

Dj is less than the value in the case where the platinum is pressed

into the ceramic and, as a consequence of this, the critical current

becomes significantly smaller than jjjj which was obtained from a specimen

with a pressed platinum leadout.

The gaseous oxygen (diffusion coefficient at the given

temperature, 14 cm^/sec), which comes from the air to the surface of the

platinum (not continguous with the ceramics), does not have an effect

on the value of the limiting current of the oxygen diffusion. The

investigation established the oxygen diffusion mechanism at the

phase boundary: ceramic-platinum (coated on the surface of the

ceramics^' at a temperature close to the temperature of the current
217

fN

0) -H e 4J 3 -H Ifl <U C c

•H >0

4J

U

(0ouo 0 •H J2 IT3 TD > 0 P 0) (U C 0) Qu X X m (U x: • tn p p

j=p (N •H

(N

p T3 c c c TJ ‘H • •H • T3 c •H C (U c c C *H itJ P <N CN 0)

(0 T3 (T3 E E E E

(0 0) CNO •H o •H . 0) C U) \ o \ o

(A •H 0) • (0 0) <0 (U to fO • > (/) a a.

TJ 0 p

w P •Hw vO

w

C f-H XJ •H 3 X O' o 0 0) O (TJ o 0) p (U o > • X • X

•H . p T3 fl o p o p E (p 0 C W

w x:p (tJ

W II II 0) 0 T3 p •H > C 0 P u p p IT3 o 3 w - E C4 f 3 0 0 X •n u U.4 p <D X

CDn

(V XI0

CU

<u T3oc T

Sh

d(> oe

CN

(N

(N o

N1

dPrHoE

00

00c

OJe

•Ho

0)cu

U1c

nJu

•H

0)

0) x:

+Jo

<ww

OJ>3oco

*Hp

(0N

*Hp

(T3

u

M-io

0)p3

+J

IT3 P

<Ui0) 4-)

O (0 a -p

0) (T3 O P C -H

(0 (tJ 'O c C -H

TJ

(U 0) X} c O -H

x: (0 -p -p

fO XI o o

rod

0)P3

O'
218

leadout under the operating conditions of the electrode in the U-02 channel.

The control specimen did not change; the coated platinum on the two

faces was not damaged.

Following is a description of electrodes with a composition

of 88 mol% Zr 02 - 12 mol% Y 2 O 3 on the cathode and anode walls of the

U—02 channel (a cross section of the cathode and anode walls is investigated)

A sketch of the construction of the U.S. electrode shows

that besides the presence of the platinum coating on the surface of the

ceramics, the platinum coated face with the platinum grid is fixed

in the insulator with cement basically containing Zr02 , stabilized by

calcium oxide. The passage of direct current through the specimen must

induce electrochemical reactions both in the interphase layer, the

"platinum-ceramics” "flayer , and on the boundary of the platinum with the

cement. This somewhat complicates the method for determining the critical

current density since one has to investigate separately the processes on both sides of the coated platinum. Computations on the basis of readings

from thermocouples which were located in the MgO at a distance of 8.3 mm

from the hot surface of the ceramics gave the following figures: on

the boundary of the platinum-ceramics division the temperature fluctuated

within the range of 1300-1350°C, but on the surface of the platinum

which was immersed in cement, the temperature attained 1250°C.

Let us first of all look at electrode pair 1104 and 2104;

the current did not pass through this electrode. On the cathode the

electrode remained practically intact. The height of the ceramics was approximately 6.2 mm and its breadth, approximately 13.2 mm. Between

the cement and the ceramics one can see the platinum layer. However, on

the right side of the ceramics one can observe some corrosion in the

platinum, which reacted with the ceramics at 1300°C, as a consequence of

the penetration of the KOH fusion into the ceramic pores. As a result

of the reaction two layers of gray color are formed on the boundary with

the platinum and a bit lower in the cement area. The cement remained

unaffected and exhibited a rose tint. There was no microfissures on the

cathode. On the cathodic wall of the insulator there were two small

fissures

On the anode the electrode also remained practically

intact. The height of the ceramics was approximately 6.1 mm and the

breadth, approximately 12.8 mm. The platinum between the ceramics and

the cement was not affected since the seed practically did not

penetrate the anode. Neither in structure nor in color did the cement

change in comparison with the cement on the cathode. The MgO did not

bear any fissures.

Thus, one may ascertain that in the absence of a current

the cathode and anode of this electrode pair were not corroded.

A current of 8.5 A passed through electrode 1101-2101;

this amounted to 1.09 A/cm^. After supplying current to the specimen

for a period of 15-60 minutes, the electric circuit was disrupted because

the platinum wire on the cathode melted. The melting of the platinum on

the cathode was corroborated by an x-ray photograph.
219

It is interesting to note that under laboratory conditions

it was established that for the U.S. specimen with the same composition,

flaking of the brazed platinum in the ceramics had occurred from the

composition face on the cathode after a current with a density of 1

A/cm^ had been transmitted during a period of 17 hours. The ceramic

specimen with its two faces brazed with platinum was hung in the Silit

furnace with the aid of a platinum wire. The electrical circuit was not

disrupted at 1400°C. Electrical measurements attested to this fact.

Because of the low value of the limiting current of the

oxygen diffusion, the potential of the platinum cathode, beyond very

short time intervals, attains values at which the reduction of Zr^"^ from

the solid electrolyte with a composition of 88 mol% Zr02 - 12 mol%

Y 2 O 3 begins in the cathodic area. The reduced zirconium dioxide

(Zr203 or ZrO) is oxidized by the oxygen of the air into Zr 02 . The reverse

conversion of the electrochemical reaction products into zirconium

dioxide does not at all denote the reduction of its initial structure.

In the general case, the atoms are already rearranged during the formation

of the blackened zone and in the reverse process, the zirconium dioxide

is obtained in the form of a tetragonal modification, i.e. the original

ceramics specimen is destabilized. During the cooling of the specimen

within the range of 1000-1100°C in the thin layer of the face adjacent

to the platinum current outlet, the transition from a tetragonal modification

into a monoclinic system takes place. Since the transition from one modification to another is accompanied by a change in the volume of the

zirconium dioxide, the deformation of the surface of the face on which

the platinum was brazed led to a flaking of the platinum from the surface

of the ceramics. Consequently, under laboratory conditions in atmbsphere

air at 1400°C, electrochemical processes took place in the cathodic

region and led to the destabilization of the ceramics with a composition

of 88 mol% Zr 02 - 12 Y2 O 3

. With a gradual cooling of the specimen

within the range of 1000-1100°C, the brazed platinum separated from the

face on the cathodic side of the ceramics because of the transition of

the tetragonal modification of the zirconium dioxide to the monoclinic.

However, the U.S. specimens, which were placed in the

U—02 channel, contained cement in which the back side of the brazed

platinum with the platinum grid soldered to it was located. In the

process of starting, this mass was thermoprocessed and sintered at

1300°. The sintered mass retained the platinum in a stable manner, not

allowing it to be separated from the surface of the ceramics during the

lengthy transmission of the current through the electrode. In such a manner, the conditions were created in the U—02 channel for the electrochemical

processes both in the interpahse layer, the "ceramics-platinum" layer,

and in the back side of the platinum immersed in cement.

A current which significantly exceeded the limiting

current of the oxygen diffusion was supplied in electrode pair 1101- 2101. This excess led to electrochemical processes (according to the

scheme stated above) occurring at a significant rate in the interphase

layer, the "platinum-ceramics" layer. During the imposition of the

electric field, the penetration of the KOH fusion (admixture) into the
220

cathode of electrode 1101 contributed to the corrosion of the platinum

and the interreaction of the layer with the reduction of the zirconium

dioxide. As a result of such an interreaction, the blackened zone is

formed. Electrochemical processes also take place from the back part of

the platinum, which is immersed in cement, to the boundary of the phases,

leading to the penetration of the blackened zone and the barrier layer.

(We assume that the cement basically contains Zr 09 - CaO) . During the

passage of the current through the current outlet the platinum wire

heated up intensely and melted at some distance from the soldered joint

as a result of the formation of the blackened zone. (See Chapter II,

Section Q.5.)

The height of the electrode is 7 mm; its breadth is 13.8

mm. The area of the cathode of electrode 1101 increased in comparison

to the area of the cathode of electrode 1104 through which the current

did not pass. The area of the blackened zone in the ceramics in the

immediate vicinity of the platinum current outlet is 5 x 1.5 mm but the

area of the blackened zone in the cement region is 9 x 3 mm. From the

right side on the cathode of electrode 1101 there are fissures. From

the right and left sides on the MgO insulator one can see fissures. The

height of the anode of electrode 2101 is approximately 6.1 mm; its

breadth is 12.6 mm, i.e., its area practically did not change in comparison

with the area of the anode of electrode 2104 through which the current

did not pass. In this way the anode of electrode 2101 was not damaged

in spite of the fact that a significant amount of current passed through

it.

Electrode pair 1102-2102 was subjected to an electric

current of 6.0 A, which amounts to approximately 0.77 A/cm^. For 18

hours after applying an electric field to it, the current dropped to 4.0

A and the power of the current was maintained practically constant for a period of 110 hours. The mean current density in the specimen cross section amounted to 0.53 A/cm^. The formation of blackened zones on both sides of the platinum current outlet attests to the fact that the

basic electrochemical reaction took place—the formation of the reduced

zirconium dioxide in the cathodic regions and the destabilization of the

ceramics. This fact indicates that in the given case the current was supplied continuously to the electrode, exceeding the critical current

density.

The area of the cathode of electrode 1102 increased in

comparison with the area of the cathode of electrode 1104 through which

the current did not pass. The height of the cathode of electrode 1102

is 8.2 mm; its breadth is 13.1 mm. The zone of the destabilized ceramics

formed close to the platinum current outlet in the cathodic area. In

the given case, the melted KOH penetrated the ceramics of the cathode

and corroded the platinum. It interreacted with the product of the

electrochemical reaction. Such an interreaction caused the formation of

the blackened zone in the cathodic layer of the ceramics with a 3 mm

thickness. Beyond this zone the fissure, follows. In the cement region

bordering on the platinum there are also electrochemical reactions,

which lead to the formation of a blackened zone with a 5 mm thickness.

The expansion of the zone in the cement region caused the formation of

deep fissures in the insulator.
221

The anode of electrode 2102 was not damaged. Its area,

in comparison with the area of the anode of electrode 2104, did not

change. The height of the anode of electrode 2102 is 6.1 mm; its breadth

is 12.8 mm.

A current of 2.5 A passed through electrode pair 1103-

2103. It passed continuously for a period of 110 hours. However, after

18 hours the current dropped to 1.8 A; after 53.4 hours, to 1.2 A; after

77 hours, to 1.4 A; and then the power of the current again fell to 1.2

A. After some time the current rose to 1.58 A. On the average one can assume the current density on a cross section of the specimen was approximately

0.2 A/cm^. The formation of the blackened zone on both sides of the

platinum current outlet is also proof of the reduction of Zr^"^ on the

platinum cathode and of the destabilization of Zr 02 - Y 2 O 3 in the cathodic

region of the ceramics. The platinum also interreacted with the electrochemical

reaction products because of the penetration of the admixture into the

cathode. This fact suggests that the value 0.2 A/cm^ in the cross section of the specimen also exceeds the critical current density. The

area of the cathode of electrode 1103 increased in comparison with the

area of the cathode of electrode 1104, through which the current did not

pass. The height of the cathode of electrode 1102 is 8.8 mm; its breadth

is 13 mm. Close to the platinum the blackened zone developed with a thickness of 2 mm. A fissure resulted between this zone and the remaining

portion of the ceramics. The thickness of the blackened zone in tne

immediate vicinity of the platinum in the ce^nt region is 2.5 mm. A

fissure resulted between this zone and the remaining portion of the

ceramics. The thickness of the blackened zone in the immediate vicinity

of the platinum in the cement region is 2.5 mm. A zone which is not as dark followed it and had a thickness of 3 mm. Fissures were rormed in

the MgO insulator. The anode of electrode 2103 did not change in comparison

with the anode of electrode 2104.

<» D.3.2 American specimens with a composition of 18 mol%

Ce02 - 82 mol% Zr02

A ceramic sample with a low content of ceric oxide can be

represented b^ a tetragonal system with the lattice parameters a = 5.15

and c = 5.24 A; the cubic phase is practically absent. The structure of

the ceramic with the composition given attests to the incomplete stabilization

of the zirconium dioxide since in the process of sintering, the composition

does not crystallize according to the fluorite type. Owing to the

presence of Ce02 in the system the transition of the monoclinic phase of

the Zr 02 to the tetragonal one is facilitated. According to the data of

U.S. investigators the open porosity amounts to 17.7% and the closed,

to less than 0.5%.

In the process of sintering in atmosphere air (at 1P50°C)

,

the following electrochemical reaction takes place

2CeJ + ^ 1/2 0- + 2Ce: + (1) Zr 0 2 Zr o X

In this equation the symbols denote respectively: Ce 2 i- -

Ce^"*" in the cationic sublattice: Oq - oxygen anion (0^“) in the anionic

sublattice; Ce~^ - center in the binary system which is the characteristic
222

transition of Ce^”^ or Ce^"*" according to the equation Ce^j. + e Cezr +

the twice ionized vacancy of the oxygen, which is formed as a result of

the Vq giving off two electrons.

The binary system, 18 mol% Ce02 - 82 mol% Zr02 , possesses the

lowest electrical conductivity in comparison with other types of ceramic

compositions which contain a significantly larger amount of Ce02. Table

D. 3.2.1 gives data on the conductivity of American and Soviet specimens with

a composition of 18 mol% Ce02 - 82 mol% Zr02. The Soviet specimens (IVTAN)

were investigated in 1970 in laboratories using electrochemical methods for

the investigation of fireproof materials.

Table D.3.2.1. The specific electroconductivity of the system 18 mol%

Ce 02 - 82 mol% Zr 02 as a function of the temperature.

Temperature, °C

a ohm ^ -1 cm

American specimens IVTAN specimens

780 2.9 • 10“^ 3.6 • 10~J

838 5.0 • 10 2 5.6 • 903 8.2 • 10 8.0 • 977 0.95 • 10 :: 1.04 • loi

1060 3.07 • 10 :: 3.15 • loi

1156 5.36 • 10 ^ 6.00 • 10-3

1265 8.00 • 10 ^ 8.88 • 10-3

1393 2.21 • 10

^ 2.88 • 10-3

Experimental data on conductivity, which were obtained

by U.S. and U.S.S.R. investigators, correlate well with each other.

A juxtaposition of the conductivity results proves that admixtures at

the given temperature interval make a negligibly small contribution

to the values of the specific electroconductivity.

American specimens with a composition of 18 mol% Ce02 + 82 mol%

Zr02 were investigated under laboratory conditions in air, at 1400°C and

in the absence of thermal shock.

The specimens had the form of a rectangular parallelepiped;

the platinum was brazed on two of its faces. A platinum mesh was arc- welded to the platinum coated surface. The presence of two probes at

different points along the specimen permitted a determination of the

change in the resistance of the solid solution during the passage of

direct current through it. In the initial state there were no cracks

in the two specimens. The geometrical dimensions of the specimens

are shown in Figure D.3.2.1.

Tests were carried out in the Silit furnace; the temperature

of the furnace was determined with the aid of two platinum-platinum

rhodium thermocouples. At the temperature of 1400 ± 10°C the furnace

worked in a stable manner. Together with the basic specimen, an identical

specimen through which the current did not pass was introduced into the

furnace.
CM

B

IT) ^in ^ ro • ^ • • O rH O rH

223

CO ro ro ro

cn <C ^ m

T5 0)

-PW0) 4J

(D

.C

-P

4-1o

U)co

•H

cnc0) g

•H

ndO • •H C P 0) ^>H g O O Q)

cu

a

O U)

CMro• QGP

Cn

•H
224

The conductivity of the U.S. specimen was measured

under laboratory conditions duirng the passage of direct current by

using polarity to exclude the influence of the thermoelectromotive

force. The data obtained correlated with the conductivity values determined by the IVTAN in 1970. The values of the specific conductance

are shown in Table D.3.2.2.

Table D.3.2.2. The specific electrical conductivity as a function of the

temperature for two compositions of the binary system Zr 02 - Ce 02 .

Composition

of the

American

specimens

Temperature °C

\

u -1 -1

a ohm cm Composition

of the

American

specimens

Temperature °Ca ohm 1 -1 cm

18 mol% CeO^ 1020 1.17 • 10~^ 15-16 mol% - -

82 mol% . 1070 1.54 • 10”^ CeO^ - 85-84 1060 1.52 • 10~^

1120 1.93 • 10"^ mol% ZrO^ - -

1150 2.42 • 10"^ 1156 3.60 • 10~^

1200 3.47 • lO"^ - -

1240 4.33 • lO"^ 1265 5.4 • 10"^

1290 6.9 • 10"^ - -

1340 9.75 • lO”^ - -

1390 , 1.53 .• 10"^ 1393 1.45 • lO"^

1400 1.66 • 10"^ - -

The a values, which are given in Table D.3.2.2, show that

the samples with a composition 18 mol% Ce02 - 92 mol% Zr02, sent

later to the IVTAN from the USA, differ somewhat in the ceric

oxide content from the American specimens tested in the U—02 channel.

Evidently, instead of 18 mol% Ce02, they contain 15-16 mol% Ce02.

However, a small difference in the ceric oxide content in the binary

system does not influence the mechanism of the electrochemical reactions

which take place in the interphase layers.

The ion transfer number was measured using a concentration

galvanic cell without the transfer of the tested mass as a function of

the temperature for the two compositions of the binary system Zr 02 - Ce02

.

Table D.3.2.3 gives the data on the number of ion transfers.
225

As can be seen from the data in Table D.3.2.3, the electronic

part of the conductivity in the ceramics increases with an increase in

temperature which confirms that a reaction has taken place in the

oxidizing medium:

2 CeO^ ^ Ce^O^ + 1/2 0^

* *

• (2)

the oxygen is released into the medium.

Table D.3.2.3. The number of ion transfers for the binary system

Zr 02 - Ce 02 as a function of the temperature (P 02 ) cathode = 1 atm;

(P 02 ) anode = 10~^ atm.

Composition

Ion transfer number Electron transfer

(t.), % number (t ^), %

1 el

t° 112>°C 1327°C 1527°C 1127°C 1327°C 1527°C

15 mol% CeO^ -

oc -,0V 66.5 50.0 44.0 33.5 50.0 56.0

18 mol% Ce0„ -

- 82 mol% 65.5 45.4 39.4 34.5 54.6 60.6

The sesquioxide (Ce 203 ) content increases in the binary

system Zr02 - Ce02 with an increase in temperature. Ceramics with the com- positions indicated above are mixed semiconductors. It would be desirable

to determine the ceric sesquioxide content in the ceramics with the aid

of x-ray diffraction analysis.

At a set current level, the potential of the electrodeplatinum current leadout was measured with respect to the probe. Figure

D.3.2.2a and D.3.2.2b give the "current density-potential" curves in the

positive and negative regions of the potentials, where the platinum

current outlet (on which the measurements are carried out) serve as a cathode or anode. These data are obtained when the direct current

initially passed through the specimen. Curves 1 and 2 in Figure D.3.2.2a

were plotted on the double coordinate, self-recording potentiometer PDS021M without regard for the ohmic losses in the volume of the specimen

as the potential changes toward negative values (1) and back (2)

Curves 3 and 4 in Figure D.3.2.2b are also obtained on the PDS without

regard for the ohmic losses in the volume of the specimen as the potential

losses toward positive values (3) and back (4)

The time for plotting

the curve in one direction was 30 minutes.

In the cathode and anode volt-ampere characteristics

between the direct and reverse course of the curves there arose a hysterisis

(lagging) , which was proof of the occurrence of electrochemical reactions

in the interphase layers. The limiting current of oxygen diffusion

practically did not appear. On the cathode branch of the curve, a bend

is barely visible at -1.8 v with reference to the platinum probe.
obtained

in

the

226

C3- o O

cnH

COou

4-» u

0)

OJ JZ

4J

U-lo

<N

OJ t:C (T3

(T3 4-1 to

to to

OJ *H > 4-) ^ -H 3 C U -H

Figure

D.3.2.2a,b.

Cathode

and

anode

curves

for

the

specimen -

18

mol%

CeO

-82

mol%

ZrO

obtained

in

air

ata

temperature

of

1400

°C. ^
227

For clarity. Table D.3.2.4 gives values for the voltage

(v) between the anode and cathode, the anode and the probe, the probes,

the probe and the cathode in the initial stages of current transmission

through the specimen; these values were obtained while measuring the

volt-ampere characteristics.

Using the data in Table D.3.2.4, we computed the values

of specific resistance p between the anode and the probe, the cathode

and the probe as a function of the current density. The results of the

computation are tabulated in Table D.3.2.5.

Table D.3.2.4. Volt- ampere characteristics obtained for the specimen

with a composition of 18 mol% Ce02 - 92 mol% Zr02 in the initial stage

of current transmission in air at 1400°C in the furnace (direct course

of the curve). (Measurement time, 30 minutes.) Polarity + -.

V

Anodecathode

Voltage

V

anode

probe

Voltage

V

probe probe

Voltage

V

probe

cathode

Voltage

current I A

Current

density in

the cross section of

the specimen

11.62 0.55 10.30 0.78 0.05 0.1094

16.15 0.80 14.30 1.05 0.07 0.1532

22.95 1.20 20.30 1.45 0.10 0.2188

32.05 1.75 28.20 2.10 0.125 0.2735

44.55 2.60 39.10 2.85 0.225 0.4923

48.60 2.95 42.50 3.15 0.30 0.6564

49.46 3.05 43.10 3.30 0.33 0.7220

The p values in the electrode layer during the initial stage

of current transmission (Table D.3.2.5) show to what extent the basic

lattice defects of the tested ceramic in the specimen volume were

changed in the cathode and anode regions; this had a direct influence

on the conductivity of the system. As can be seen from the data in

Table D.3.2.5, the p values between the anode and the probe, and the

cathode and the probe differ significantly; the deviation between the

p values increases with an increase in the current density in the cross section of the specimen. Undoubtedly the Joule effect exerts an influence on the change of the specific resistance in all portions of the

specimen. However, from the data of Table D.3.2.5 one can see that this

effect is not decisive since the p changes in the electrode regions and

between the probes differ substantially according to the value. The

Joule effect would have to lead to a monotonous drop in specific resistance

in all portions of the ceramics when the current density is increased. However,

this is not observed in the anode region of the specimen (see the data

in Table D.3.2.5). With all the values of j, the specific resistance of

the ceramic between the anode and the probe is much higher than p between the cathode and the probe. Furthermore, the values of the

specific resistance of the ceramics between the anode and probe increase

considerably within the range of 0.11 - 0.27 A/cm^ ; then with an increase

in j , 0 (anode-probe) gradually declines and according to the value it

is 58-61% higher than the values of p between the cathode and the probe.
228

Table D.3.2.5. p values between the anode and the probe and the cathode

and the probe as a function of the current density at 1400°C.

anode

cathode

V

anode

probe

,

ohm

R

anode

probe

,

ohm* cm

P

cathode

probe

,

ohm

R

cathode

probe

ohm* cm

P

A/ cm^

% deviation between

values

Note

11.62 11.00 67.02 15.60 54.84 0.1094 +22.5 values in

16.15 11.43 69.64 15.00 52.73 0.1532 +31.8 the cathodic

22.95 12.00 73.12 14.50 50.97 0.2188 +43.4 region taken

32.05 14.00 85.30 16.80 59.05 0.2735 +44.4 to be beyond

44.55 11.55 70.37 12.66 44.60 0.4923 +57.7 100%

48.60 9.83 59.89 10.50 36.91 0.6564 +62.2

49.46 9.30 56.67 10.00 35.15 0.7220 +61.2

It is necessary to clarify the reason for the lack of

agreement between the p values in the near electrode layers and the specific

resistance of the specimen (p between the probes) . For this purpose, the values

of the voltage (v) between the probes (as given in Table D.3.2.4) were

used. Let us compute the specific resistance of the ceramic in the

volutne of the specimen and compare it with the p values, obtained in

Table D.3.2.5. The results of the computation are given in Table D.3.2.6.

From the data in Table D.3.2.5 and D.3.2.6 one can see

that the specific resistance of the ceramics in the volume of the specimen

is lower than the p values between the anode and probe but it is greater

than the p values in the volume of the specimen in the cathode region.

This fact indicates the change in the composition of the ceramics near

the platinum cathode. Evidently, as a result of the electrochemical

reaction in the cathode a new phase is formed; the conductivity of this

phase grows with an increase in current density. In the anode region

one observes another picture, the formation of a double layer, of an electrochemical barrier layer, the resistance of this increases with an increase in current density.

The conductivity of the ceramics between the probes

within the range of 0.1 - 0.22 A/cm^ practically coincides with the a values, which are given in Table D.3.2.2 for 1390-1400°C. A drop in the

conductivity of the ceramics is observed only at 0.27 A/cm^ ; then, with

an increase in the current density, the a values gradually increase,

evidently because of the Joule effect. Let us clarify the effect of the

heating-up of the specimen on its conductivity, due to the passage of

current through the specimen.

The surface, with an emissivity e, radiates energy

E = coT^ where a = 5.67 • degree^. For a system of 82 mol% Zr02 + 18 mol% Ce02, e = 0.42 at 1700 K (1427°C) [See Handbook of thermophysical

properties of solid materials, Pergamon Press, 1961]

In the differential form;

AE = 4ecT^AT. .

(3)
229

where AE is the increment of radiative energy due to the generation of

heat within the specimen and AT the corresponding temperature increment

of the surface.

If heat with an intensity A is generated within an infinite

cylinder with radius 0, with respect to the unit volume, then the difference

between the temperatures on the axis of the cylinder and on its surface amounts

to

6T = A0^/4K. .

. (4)

where K is the coefficient of the heat conductivity [G. Karslou, D.

Eger, The heat conductivity of solid bodies, "Nauka,” Moscow, 1964].

Let us evaluate the maximum temperature drop between the

center and the surface of the rectangular specimen. Such an estimate is

known for the cylindrical specimen (see the above-mentioned reference)

Since the temperature drop increases with an increase in the diameter of

the cylinder, this analysis can be applied to a parallelepiped under the

condition that the maximum transverse dimension (the cross section of

the specimen) be used as a diameter.

Let us assume that the American specimen in the form of a rectangular parallelepiped is inscribed into a circle and that the

length of the edge of the base is a. Then the length of the diagonal of

the transverse section of the specimen d = a/2 and 0 = a//2 . If instead

of 0 we substituted a//2 into (4), then we obtain,

2

6T = A |- / 4 K . . . (5)

Thus, if the temperature at all points of the parallelepiped

was equal to T before heating, then with a stationary heat generation it

rises in the center of the specimen to Tq = T + AT + 6T. The Tq found

in this way can be used as the maximum value of the temperature for a parallelepiped of finite length.

Table D.3.2.6. Values of the specific resistance of the specimen between

the probes; comparison of p (probe-probe) and p (anode-probe) as a function of the current density. Medium air, 1400°C in the furnace

(initial stage of current transmission through the specimen)

probeprobe

V

probeprobe

ohm R

probeprobe

ohm* cm

P

specimen

(ohm*cm) ^ a

Difference

between

values p (anode-probe)

and p (probeprobe) ohm* cm

j a/cm

10.30 206.0 65.4 1.53 10“^ 1.62 0.1094

14.30 204.3 64.8 1.54 10

~ 4.84 0.1532

20.30 203.0 63.3 1.58 10 ^ 9.82 0.2188

28.20 225.0 71.4 1.40 10 ^ 13.90 0.2735

39.10 170.4 54.1 1.85 10 ^ 16.27 0.4923

42.50 141.6 44.9 2.22 10 ^ 14.99 0.6564

43.10 130.6 41.4 2.42 10

^ 15.27 0.7220
230

The temperature equalization rate in different places

of the body is determined by the heat conductivity coefficient K,

which is divided into the heat capacity of the unit volume of the body,

i. e. , the value

a p-c.

cmsec

cal cm sec degree

_g_ . cal

_ cm g'degree

cal* cm *g*degree cm

cal • g • cm* sec • degree sec

( 6 )

Here p - density;

C - the heat capacity of the unit mass of the body under constant

pressBre;

a - temperature conductivity.

In agreement with the data of American investigators for

the composition of 18 mol% Ce02 - 92 mol% Zr02j at 1673 K (1400°C) a = 0.0045 cm^/sec; p = 5.3 g/cm^.

C = 0.288 + 7.71 * 10

P

-6

1673 - 4.03*10

-3

^^^ 0.301

1673'

cal

g* degree

K = 0.0045 * 5.3 * 0.301 = 7.18 * 10

-3

= 0.288 + 0.013 =

cal

cm* sec * degree

The process of heat transfer takes place in a quiescent

medium. This means that in the medium there are no drops in pressure

which would lead to the creation of motion in it.

Let us carry out the computation using the data

of one of the tests given in Table D.3.2.4. The length of the specimen

is 2.01 cm; the length of the edge of the base a = 0.676 cm; the

current = 0133 a (which corresponds to 0.72 A/cm^); and the ohmic

drop in the specimen is 49.46 v. The lateral surface of the specimen

S = 5.435 cm^ ; the volume V = 0.9185 cm^.

The power of the heat generation W = 0.33 a * 49.46

V = 16.32 watts (W) . Moreover,

AE w

S 16.32

5.435

3.003 w

2 cm

5

w ^ 16.32

V 0,9185 17.76 W

3 cm
3.003

231

AT —j-z = 67.32 degree

4x0,42x5.67x10 xl673'^

6T

17.76 X 0.2285

4x7.18 X 10"^

141.3 degree

T = 1673 + 67 + 141 = 1881 K = 1608°C.

o The maximum drop in temperature between the center and

surface of the specimen amounts to 208°.

The results of the computation for other values of the

current are tabulated in Table D.3.2.7.

From the data in Tables D.3.2.6 and D.3.2.7 it follows

that within the range 0.1 - 0.22 A/cm^ the heating of the specimen

due to the passage of current through it is negligibly small since the

values of the specific electrical conductivity of the ceramics (p between

probes) practically coincided with the conductivity of the ceramics,

measured with the four-probe method during the transmission

of a direct current of 5 ma through it (see the data in Table D.3.2.2).

Within this current density range the p values between the probes are lower than the p values between the anode and the probe, but much

higher than the specific resistance in the volume of the ceramics in

the near-cathode region. This experimental fact indicates that already in

the initial stage of current transmission through the specimen (even

with small current densities in the platinum cathode) , an electrochemical

reduction of Zr^"*" occurs; there may also be a reduction of the Ce*^"^,

leading to a change in the composition of the ceramics in the cathodic

region. Simultaneously in the anode region, a double electric layer,

electrochemical barrier layer, begins to be formed.

Table D.3.2.7. Temperature drop between the center and the surface of the

parallelepiped-shaped sample. Temperature T = 1673°C (1400°C)

Current

I a

j (A/ cm^) w

watts

AE=^

w/ cm^

AT

degree

A= ^

V

w/ cm^

6T,

degree

T K

o degree

T°C Maximum

temperature drop,

degrees

0.05 0.1094 0.58 0.1067 2.39 0.6314 5.02 1680 1407 7 0.07 0.1532 1.13 0.2079 4 . 66 1.23 9.78 1687 1414 14

0.10 0.2188 2.29 0.4213 9.45 2.493 19.84 1702 1429 29

0.125 0.2735 4.00 0.7359 16.50 4.355 34.65 1724 1451 51

0.225 0.4923 10.01 1.842 41.30 10.90 86.70 1781 1508 108

0.30 0.6564 14.58 2.682 60.13 15.87 126.2 1859 1586 186

0.33 0.7220 16.32 3.003 67.32 17.76 141.3 1881 1608 208

With 0.27 A/cm^ one observes a drop in the temperature between

the center and the surface of the specimen, amounting to 51 degrees. However,

such a maximum possible temperature drop in practice has little effect on the
232

heating up of the specimen since in all portions of the specimen there is an increase in the specific resistance of the ceramics in comparison to the p value for 0.22 A/cm^ (see the data in Tables D.3.2.5 and D.3.2.6). Evidently,

with such a current density there is a strong interaction between the characteristic defects of the lattice, leading to a drop in the conductivity of the

ceramics. However, in this case the p value between the probes is lower than

the p value between the cathode and the probe. Beginning with 0.49 A/cm^ and

higher current densities, one observes a heating up of the specimen due

to the transmission of current through it. A comparison of the conductivity

values and the data a, which we and U.S. investigators found for ceramics

having two compositions: 15-16 mol% Ce02 - 85-84 mol% Zr02 and 18 mol%

Ce02 - 82 mol% Zr02, shows that the values 1.85 • 10 2.22 • 10~^

and 2.42 • 10“^ 1/ohm* cm correspond approximately to the following

temperature levels: 1450°, 1490°, and 1500°C.

As a result of the heat flow from hotter to cooler places,

which continues until the temperature is equalized in the whole body, an equilibrium temperature is established after a certain period of time.

In this manner, the heating of the specimen by means of current passage,

causes a rise in the temperature of the specimen body as a function of

the current density within the range of 0.49 - 0.72 A/cm^; this temperature

rise does not exceed 50-100°. However, even the heating of the specimen

by the passage of current did not change the picture. In all cases p between the probes was lower than p between the anode and the probe and

significantly higher than the values of p between the cathode and the

probe. The heating up of the specimen due to the transmission of the

current did not stop the electrochemical processes in the near-electrode

regions; on the contrary, it increased the rate of the electrochemical

reactions and enhanced the interreaction of the characteristic lattice

defects

.

Table D.3 .2.8 presents a comparison of the conductivity

of the ceramics and the conductivity of the near-cathode layer zone

.

Table D .3.2.8. Comparison of the specif ic electrical conductivity of the

ceramics and the conductivity of the cathode layer zone as a function

of the current density. 1500°C in the furnace.

• / 2

j a/cm a ceramics a cathode Percentage

(ohm* cm) layer zone deviation be Note:

(ohm* cm) tween the

values

-2 -2

0.1094 1.53*10

^ 1.83*10^ +19.6 Values of

0.1532 1.54*10

^ 1.90*10^ +23.0 the ceramics

0.2188

0.2735

1.58*10

^ 1.40*10

^

1.96*10 ^ 1.69*10"^

+24.0

+20.7

(between the

probes) assumed

0.4923 1.85*10

^ 2.24*10 ^ +21.0 to be 100%

0.6564 2.22*10

^ 2.71*10

^ +22.0

0.7220 2.42*10 ^ 2.85*10 ^ +17.70
233

On the average the a values of the zone in the cathode

region exceed by 21% the value of the specific conductivity of the

ceramics, which attests to the fact of a change in composition

of the ceramics close to the platinum cathode as a result of the

electrochemical reduction of the Zr^"*" in the cathode. Let us determine the potential difference b at the boundary of the platinum

anode with the ceramics. To do this we use the formula

= V anode-probe

V • £ z-z z-z

I z-z

(7),

where V , , anode-probe

probe, and between

and V are the voltages between the anode and the z-z

the other probes, respectively.

The results of the computation are given in Table D.3.2.9.

The data in Table D.3.2.9 illustrate the formation of a double electric layer at the boundary of the platinum anode with the

ceramics. The potential difference increases with an increase in the

current density in the specimen cross section.

Table D.3.2.9. Potential difference (}) in the interphase layer anode probe

as a function of the current density in the initial stage of the current

transmission through the specimen.

j A/ cm^

V anode-cathode

mV

V anode-probe

mV

V -1 z-z z

pA/ z-z mV

mV

0.1094 550 10300 536.4 13.6

0.1532 800 14300 744.8 55.2

0.2188 1200 20300 1057 143.0

0.2735 1750 28200 1468 282.0

0.4923 2600 39100 2036 564.0

0.6564 2950 42500 2214 736.0

0.7220 3050 43100 2244 806.0

Let us examine the influence of the direct current on

the ceramics duirng its continuous passage through the specimen. To

do this a current of 0.1 a (j = 0.2188 A/cm^) was passed through the

U.S. specimen for a period of 16 hours. After a certain period of time

(as indicated), the volt-ampere characteristics were measured. The results

of the measurements are tabulated in Table D.3.2.10.

After the current had been transmitted through the

specimen for a period of 16 hours, it did not change the appearance of the

sample; there were no cracks; the brazed platinum on both faces of the parallelepiped and the platinum wire welded to it were in good condition.
23A

Table D.3.2.10. Volt-ampere characteristics obtained for a specimen

with the composition of 18 mol% Ce02 - 82 mol% Zr 02 after 16 hours of

transmitting a current of 0.1 a in air at 1400°C. Direct course of the curve measured after 30 minutes. Polarity +

anodeV cathode

V

anodeV probe

V

probeV probe

probeV cathode

current

a

j A/cm^

4.78 0.53 3.65 0.60 0.02 0.044

9.58 1.08 7.30 1.20 0.04 0.087

14.40 1.60 11.00 1.8 0.06 0.1313

19.90 2.10 14.50 2.40 0.08 0.1750

23.40 2.60 17.90 2.90 0.10 0.2188

27.10 3.00 20.70 3.40 0.12 0.2625

30.80 3.50 23.50 3.80 0.14 0.3063

33.30 3.70 25.5 4.10 0.16 0.3501

36.30 4.10 27.70 4.50 0.18 0.3936

39.70 4.50 30.20 5.00 0.20 0.4376

42.50 5.00 32.00 5.50 0.23 0.5032

44.40 5.20 33.40 5.80 0.25 0.5470

Let us determine the potential difference on the boundary

of the platinum anode with the ceramics after subjecting it to a current

of 0.1 a (0.2188 A/cm^) during a period of 16 hours. Formula (7) was used

for the calculation. The results of the computation are tabulated in

Table D. 3. 2.11.

The potential drop (}> in the anode region includes the

potential difference in the double layer and the ohmic decrease in

the surface of the ceramics region which is located beyond the limits

of the double layer. The values (p show a significant change in the

electrochemical parameters of the tested specimen as a result of the

continuous transmission of the current through it. This change in cf) was caused by the interaction of the basic lattice defects in the

electrode region. The cj)^ values point to the formation of an electrochemical

barrier layer in the anode region; this layer impedes the transmission

of the current. The potential drop in the anode region significantly

exceeds the (p values obtained for the same specimen investigated in the

initial stage of the current transmission (see Table D.3.2.9). Thus,

for example, for 0.2188 A/cm^ the <})^ value exceeds by 11 times the value

of (}), and for 0.5 A/cm^ the value is almost 6 times larger than the

value of the potential difference in the double layer.
235

Table D.3.2.11. Potential drop in the anode region as a function of

the current density after 16 hours of processing the specimen with a current of 0.1 a (0.2188 A/cm^).

j A/cm^ ^ anode-probe.

^ probe-probe. V • i z-z z mv mv ;) /b Z-Z

mv

mv

0.0440 530 3650 190.1 340

0.0870 1080 7300 380 700

0.1313 1600 11000 572 1028

0.1750 2100 14500 755 1345

0.2188 2600 17900 932 1668

0.2625 3000 20700 1078 1922

0.3063 3500 23500 1224 2276

0.3501 3700 25500 1328 2372

0.3938 4100 27700 1442 2658

0.4376 4500 30200 1573 2927

0.5032 5000 32000 1666 3334

0.5470 5200 33400 1739 3461

It is also necessary to clarify to what extent the electrochemical properties of the ceramics are changed as a result of the con- tinuous transmission of current through the specimen. To <fo this the

data in Table D.3.2.10 are used.

First, let us examine the change in the conductivity of

the ceramics as a result of the continuous transmission of a current

of 0.1 a through the specimen (j = 0.2188 A/cm^) . Table D.3.2.12

presents a comparison of the a values and the specific conductivity

of the specimen in the initial stage of the electrolysis. The specific

electrical conductivity of the ceramics was determined between two probes

located in different cross sections of the specimen. The value of the

current density, which was selected in the laboratory, is close to the

value 0.25 A/cm^, which is the value used in conducting the continuous

experiment in the U—02 channel.

From the data in Table D.3.2.12 it follows that after

subjecting the specimen to a current of 0.1 A, the conductivity of the

ceramics increased in comparison with its conductivity in the initial

stage of the current transmission. Within the range of 0.04 - 0.22

A/cm^, a (= 1.74 • 10 ^ ohm~^ • cm~^) was 12% larger than the previous

value of a (= 1.55 • 10“^ ohm“ " • cm~^). With the indicated current

densities the heating up of the specimen due to the transmission of the

current was negligibly small. At the end of the continuous current

transmission through the specimen, the conductivity of the ceramics was measured by the four-probe method and good agreement was found between

the value of the specific electrical conductivity of the ceramics and

the a values, which were obtained for the specimen after long time

current passage. With j » 0.5 A/cm- the deviation between the a values

amounted to 20%, which attests to the fact of the increase in the con- ductivity of the ceramics after subjecting it to the current.
236

Table D.3.2.12. Comparison of the values of the specific electrical

conductivity of the specimen, obtained in the initial stage of the current

transmission, and the a values, measured after a lengthy current transmission (0.2188 A/cm^) for 16 hours. 1400°C. Medium Air.

j

a/ cm^

In the initial stage of

current transmission

(0.1 A) through the

specimen j

A/ cm^

After a lengthy

of a current of

the specimen

transmission of

0.1 A through

R z-z,

ohm

£ z-z

,

ohm* cm (ohm cm)

^ R z-z

,

ohm

£ z-z, 0

^ ohm- cm (ohm cm)

a by fourprobe method

-2 -2 -2

0.1094 206 65.4 1.53-10

^ 0.044 182.5 57.9 1.73-10_: 1.75 10

0.1532 204.3 64.8 1.54-10

^ 0.087 182.5 57.9 1.73-10_: - 0.2188 203.0 63.3 1.58-10

^ 0.1313 183.3 58.17 1.72-10^ - 0.2735 225.0 71.4 1.40-10 ^ 0.1750 181.2 57.50 1.74-10

^ - 0.4923 170.4 54.1 1.85-10

^ 0.2188 179.0 56.80 1.76-10"^ - 0.6564 141.6 44.9 2.22-10

^ 0.2625 172.5 54.74 1.83-10 ^ - 0.7220 130.6 41.4 2.42-10 0.3069 168.0 53.30 1.88-10

^ - 0.3501 160.0 50.78 1.97-10_: - 0.3938 154.0 48.87 2.05-10_f - 0.4376 151.0 U7.92 2.09-10_: - 0.5032 140.0 44.43 2.25-10_: - 0.5470 133.6 42.40 2.36-10 ^ -

Evidently, a continuous current transmission through the

specimen contributes to an increase in the electronic part of the conductivity

in comparison with the electron transfer number obtained for the same ceramics in the absence of an electric field. The effect of increasing

the conductivity of the ceramics was caused by the reaction (1) tending

to form the centers Ce„ . Zr

However, a different pattern is observed in the near-electrode

layers as a result of the current. Table D.3.2.13 gives the p values

for the cathodic region of the ceramics, which were obtained in the

initial stage of the current transmission through the specimen and after

it had been subjected to the continuous transmission of the current (0.1

A).

As can be seen from the data in Table D.3.2.13, the

specific resistance of the ceramics in the cathode region increased

significantly in comparison with the p values between the cathode and

the probe, which were measured in the initial stage of processing the

specimen with the current. Such an increase in the resistance of the

ceramics in the cathode region can clarify the reduction of Zr^"^ in the

platinum cathode and the subsequent oxidation of the reaction products

to a tetragonal and monoclinic modification of ZrO-, i.e., the initial

system of 18 mol% Ce 02 - 82 mol% Zr 02 in the cathode region is destabilized.

The transition from one phase to another is accompanied by a change in

the specific volume of the binary systerm. The deformation can cause a flaking of the brazed platinum on the cathode side of the specimen with

a current density exceeding 0.4 A/cm'^
237

Table D.3.2.13. Comparison of p values "between the cathode and probe, obtained

in the initial stage of current transmission, and the p values probe cathode, com- puted after a long time current transmission (0.2188 A/cm^) during a period of 16

hours. T = 1400°C.

3 2 A/ cm

In the initial stage of

current transmission

through specimen

^ 2 A/ cm

After a lengthy

of a current of

specimen

transmission

0.1 a through

R cathodeprobe ohm

p cathodeprobe ohm* cm

R cathodeprobe ohm

p cathode-probe

ohm • cm

0.1094 15.60 54.84 0.044 30 105.4

0.1532 15.0 52.73 0.087 30 105.4

0.2188 14.50 50.97 0.1313 30 105.4

0.2735 16.80 59.05 0.1750 30 105.4

0.4923 12.66 44.60 0.2188 29 101.9

0.6564 10.50 36.91 0.2625 28.3 99.49

0.7220 10.00 35.15 0.3063 27.1 92.26

0.3501 25.6 90.00

0.3938 25.0 87.88

0.4376 25.0 87.88

0.5032 23.91 84.05

0.5470 23.20 81.55

Table D.3.2.14 gives p values for the anode region of the

ceramics, obtained in the initial stage of subjecting the specimen

to the current, and after a lengthy current transmission.

In the anode region of the ceramics—already indicated

above—an electrochemical barrier layer is formed as a result of the

interaction of the defects of Ce^j- with a twice ionized vacancy of

oxygen. Moreover, an associate is formed; the composition of this

associate has been studied in supplementary investigations (the influence

of the double layer on the barrier layer has been explored with highfrequency methods). Work, in the study of these ceramics is being

continued

Following is a description of U.S. specimens with a composition

consisting of 18 mol% Ce 02 - 82 mol% Zr 02 after having undergone lengthy tests

in the U—02 channel.

1. First let us look at electrode pair 1205-2205, through

which no current was passed. The cathode of electrode 1205 has a height

of 6.8 mm and a length 13.6 mm. The platinum is visible. Beyond the platinum,

cement follows. On the cathode close to the platinum, a green zone 2.8 mm

high is visible; beyond it, there is a zone with a light yellow color. One

can see a black zone in the cement region. The area is 5 x 3 mm. This zone wedges into the MgO insulator, forming a fissure in it. The cathode of

electrode 1205 is practically intact.
238

The anode of electrode 2205 looks good. Its height is

6.8 mm; its length is 13.8 mm. The platinum is visible. Beyond it, cement

of a light yellowish color follows. The ceramics are light yellow.

2. A current of 2.8 A passed through electrode pair

1206-2206 but after an hour a current of 1.6 A was transmitted through

it. Eighteen hours after beginning the test the current rose to 1.8 A.

Such a current strength was maintained for 60 hours; then the current rose

to 2.2 A. Electrode pair 1206-2206 worked for more than 110 hours. One

can state that on the average a current of 2 A passed through the electrode;

this amounts to 0.25 A/cm^.

Table D.3.2.14. Comparison of p values between the anode and the probe,

obtained in the initial stage of current transmission through the specimen

and after a lengthy current transmission for a period of 16 hours. 1400°C.

Medium air.

^ 2

In the initial stage of

current transmission j 2

After a lengthy current

transmission through the

A/ cm through the specimen A/ cm specimen

R anode- p anodeprobe probe

ohm ohm* cm

R anode- p anode-probe

probe ohm* cm ohm

0.1094 11.0 67.02 0.044 26.5 161.4

0.1532 11.43 69.64 0.087 27.0 164.5

0.2188 12.00 73.12 0.1313 26.7 162.7

0.2735 14.00 85.30 0.1750 26.25 159.9

0.4923 11.55 70.37 0.2188 26.0 158.4

0.6564 9.83 59.89 0.2625 25.0 152.3

0.7220 9.30 56.67 0.3063 25.0 152.3

0.3501 23.0 140.7

0.3938 22.8 138.9

0.4376 22.5 137.1

0.5032 21.7 132.4

0.5470 20.8 126.7

The cathode of electrode 1206 became deformed; its height is

12.6 mm; its length, 13.8 mm. The platinum is visible. The cement follows

the platinum. Its area increased. The cement wedged into the MgO, causing

deep cracks in the insulator. On the boundary of the cement with the

MgO there were small black spots. Close to the platinum the ceramics has

a yellow-green zone, 2.5 mm high. A yellow-green zone follows this zone in the

ceramics. The ceramics specimen is intact.

The anode of electrode 2206 is in good condition. The color of

the ceramics is light yellow; the cement practically did not change in shade.

The insulator is intact. The height of the ceramics is 6.8 mm; the length is

13 mm. The platinum is visible. The anode of electrode 2206 is in good con- dition.
239

3. A current of 8.5 A was 'passed through electrode 7, which

amounts to 1 A/cm^. However, after 15-60 minutes the platinum wire which was welded to the platinum grid melted. The melting of the platinum in the

anode caused the formation of a barrier layer in the anode region (see

the results of the investigation of a specimen with a composition consisting

of 18 mol% Ce02 - 82 mol% Zr02 in the electrochemical laboratory IVTAN)

The platinum wire was heated to the melting temperature by the current

because of the presence of a barrier layer in the anode layer.

The cathode of electrode 1207 was not destroyed. The platinum

was greatly deformed but it was visible. Close to the platinum the ceramic

has a zone of a green color with small dark spots; a light green zone follows

this zone. The dimensions of the ceramics are the following: height 7.1

mm; length, 13.9 mm. The cement follows the platinum; its area increased. The

cement wedged into the MgO, forming a fissure in the insulator. On the boundary

with the MgO there is a black zone close to the cement.

The anode of electrode 2207 is in good condition. Substantial

changes are not visible in the ceramics of the electrode.

4. A current of 8.5 A was passed through electrode pair 1208-2208,

which amounts to 1 A/cm^. After 15-60 minutes the platinum wire also melted in

the anode. The reason for the melting of the platinum wire is the same as that

for electrode pair 1207-2207.

The cathode of electrode 1208 has approximately the same zones

as in the ceramics of cathode 1207. The insulator was destroyed to a great

extent

The anode of electrode 2208 is practically intact. The cement

is also intact.
240

D.4. Qualitative Analysis of the Stability of Electrode Materials,

using the High-Temperature Treatment Method

Recrystallization, evaporation and erosion are among the processes

which cause the destruction of materials in the MHD-generator channel.

All these phenomena were detected after tests on American electrode

walls in the U—02 unit.

In order to have a supplementary qualitative comparison of the

stability of the candidate electrode materials in a high temperature

range, a thermal processing of these materials was undertaken in a high

temperature gas furnace. Samples of compositions 1-5 (Table D.4.1),

distributed in crucibles made of zirconium dioxide, were subjected to

heating using the products of methane combustion of oxygen up to 2200°C

at a rate of 300°C per hour. This thermal processing at 2200°C lasted for

5 hours.

Table D.4.1 gives data on the weight changes in the samples tested.

As can be deduced from the table, zirconium dioxide stabilized by

yttrium oxide has the maximum stability.

The structure of the materials before and after the tests was examined using the scanning electron microscope. The corresponding

photographs are presented in Figures D.4.1. to D.4.6.

Table D.4.1. Weight change in electrode material samples after thermal

processing at 220°C.

Composition

No.

1 2

ZrO^ - 88 ZrO^ - Composition

mol%

'^2°3 - 12 Ce02 -

Weight of

original

sample

0.5431 1.1126

Weight of

sample after

testing, g

0.5423 0.9589

Weight

losses, g

0.0008 0.1537

Weight

losses, %

0.15 14

3

ZrO^ - 50

CeO^ - 50

0.8022

0.4753

0.3269

4 5

ZrO^ - 25 ZrO^ - 20

CeO^ - 75 Ce02 - 78

Ta^Oj - 2

sample sample

melted evaporated

completely

40
241

Figure D.4.1. Sample structure I before testing (x 1200).
242

Figure D.4.2. Sample structure I after testing (x 1200).
243

Figure D . 4 .Sample structure II before testing (x 600) .
244

Figure D.4.4. Sample structure II after testing (x 600).
245

Figure D.4.5. Sample structure III before testing (x 1500)

.
246

Figure D.4.6. Sample structure HI after testing (x 1500).

*
247

The observed structure changes in the samples after thermal processing

indicate the compression of the materials and the growth of the grains and

pores. These phenomena develop at a higher rate in materials containing

cerium oxide.

D.4.1. Investigations of the ZrO„ + (8.7 mol%) carried

out in the U.S.S.R. ^ ^

Electrode walls must operate in a hot MHD-generator channel for a period of several hundred hours at a surface temperature ^ 1800°C.

Stabilized zirconium dioxide and, in particular, zirconium

dioxide stabilized by yttrium oxide are among the more promising

electrode materials.

The temperature at which the material must operate as an electrode is found in the range of intensive recrystallization.

Besides recrystallization the material experiences the

influence of alkaline seed. These two factors lead to a substantial change in the initial mechanical properties and, as a result,

to the destruction of the electrode modules.

From 1966 to 1970 comprehensive investigations of the properties of

materials made of zirconium dioxide stabilized by yttrium oxide, which

were obtained through traditional technological methods, were carried

out at the Institute of High Temperature. In addition, electrodes were

tested in the U—02 unit [1].

Table D.4.1.1 gives data concerning the evaporation rate of

zirconium dioxide stabilized by 15 weight % yttrium oxide as a function

of the temperature and the composition of the medium in which the

tests were carried out. The samples had an open porosity of approximately

25%.

From the data presented one can see that the evaporation rate

is determined both by the temperature of the hot surface as well as by

the environment. The observed drop of the evaporation rate in argon is

evidently caused by a lower rate of diffusion of the evaporation products into the gas, which is under excess pressure [2].

During the operation of the electrodes in contact with a high

temperature flow of ionized plasma, the mass may be carried away not

only as a result of the evaporation, but also as a result of the dynamic

effect of the current and chemical interaction of the low temperature

plasma with the material of the electrode. The tests showed that a surface temperature of 2300 K and a plasma flow velocity of 500 m/sec,

the rate at which the mass is carried off amounted to 0.02 g/m“ sec. With the introduction of potassium seed into the plasma, the carrying

off rate is increased by no more than 15%. It is characteristic that as time passes a relatively constant erosion rate is established: this

explains the carrying away of weakly bound material grains, the selective

evaporation of admixtures, and the additional sintering in the surface

layer of the material. Comparative investigations of erosion resistance
248

of zirconium dioxide stabilized by potassium and yttrium oxides as well

as studies of the evaporation process permit us to come to a conclusion

as to the increased stability of zirconium dioxide stabilized by yttrium

oxide

Table D.4.1.1. Temperature dependency of evaporation rate in various

environments

T°K

Evaporation rate g/m'^ sec • M O

to

3000 2800 2600 2500 2450 2400 2300 2200 2100

Vacuum

1,10~" 2.72 2.26 1.33

mmHg

Air 4.42 1.76 1.47 — 1.40 1.00 0.88 —

Argon

0,2 atn 1.35 1.29 1.06 0.76 0.59

A comparison of the data on the evaporation rate (Table D.4.1.1)

and the erosion removal rate with the results from lengthy (up to 80

hours) erosion resistance tests [3] shows that this material is practically

not affected by evaporation and erosion at a temperature up to 2100 K.

Thermal conductivity . The dependence of the coefficient of

thermal conductivity on temperature has been investigated in the range

500-2100 K. At a temperature of 2100 K the value of thermal conductivity

amounts to 1.35 watts/m degree. The deviation from this value in the

total investigated temperature range does not exceed ± 15%.

Spectral emittance . In the thermal computations of the MHDgenerator channel it is necessary to take into account the radiant heat

exchange. Table D.4.1.2 gives the temperature dependence of the spectral

degree of blackness for zirconium dioxide stabilized by yttrium oxide.

Table D.4.1.2. Temperature dependency of spectral emittance of Zr02(Y) [1].

T°K 2000 2150 2250 2300 2350 2400

£ X 0.47 0.64 0.71 0.78 0.81 0.81

in the

The value of the

200-2300 K range and

spectral

changes

emittance

slightly

increases

in the 2300

almost linearly

-2400 K interval

Thermal expansion . The value of the coefficient of thermal ex- pansion in the 900-2000°C interval remains practically constant (1.3 - 1.35 • 10 degree~0 for samples fired both at 1750°C and at 2000°C.
249

Compression strength . Table D,. 4.1.3 presents the values of the

elastic modulus, E, the yield strength, Y, and the deformation, Ai, before

destruction

Table D.4.1.3. Stability properties Zr 02 + ^

2^3

Properties

T°C 20°C 2000°C

E. 10^ kg/ cm^ 2.0 0.026

Y. kg/ cm^ 180 5

Ai . % 0.1 0.22

Time of experiments, hours 1 2 3 4 5

Creep rate, % 0.7 0.36 0.33 0.30 0.27

The creep rate = 0.5 kg/cm, T = 2000°C) decreases during

the experiment and this decline is particularly sharp in the first hour

of the experiment.

Electrical conductivity . Table D.4.1.4 presents data on the

electrical conductivity of the material consisting of zirconium dioxide

stabilized by 8.7 mol% (15 weight %) and having a porosity of 25%. The

temperature dependence of the electrical conductivity is investigated by

the four-probe method in air.

Interaction with potassium and its compounds . Complex chemical,

x-ray, and microscopic investigations show [4] that upon the migration

of potassium compounds into the body of the electrode, a chemical interaction

of the stabilized zirconium dioxide and the potassium salts takes place

in the region of the ceramic electrodes located between two isotherms

having temperatures corresponding to 1250 and 950°C.

Table D.4.1.4. Electroconductivity dependency of temperature.

T°K 1430 1570 1650 1770 1820

(ohm* cm)

^

1 1 1— 0

1

—

2.10’^ 2 . 5 . 10"^

3 . 4 . 10

"^

1

1

—

1

0

LO

J. Dubois [5] determined absolute values for the chemical

corrosion rate of zirconium dioxide stabilized by potassium oxide and

oxides of certain rare earth elements after exposure to potassium sulfate

and potassium carbonate salts at 1200°C.

It was established that potassium sulfate interreacts with

the stabilized zirconium dioxide in a more active manner than potassium

carbonate. With a chemical corrosion rate of 0.1 mg/cm* hour the change
250

in the linear dimensions of the specimens of stabilized zirconium under

conditions of surface interaction with these salts amounts to less than

0.2 mm after 1000 hours of operation. In the presence of admixtures

(Si 02 , AI2 O 3 , Fe203 ) in the stabilized zirconium dioxide, which form

low temperature phases along the grain boundaries, the chemical interreaction

of K 2 SO 4 and K2 CO 3 with Zr 02 basically occurs along the boundaries of

the grains where the glass phase is distributed and is accompanied by

crazing of the samples and partial destabilization. Under the effect of

a direct -current the corrosion process is intensified due to electrochemical

processes on the boundaries of the solid electrolyte-molten salt division.

Investigations of the corrosion resistance of stabilized

zirconium dioxide to potassium compounds at high temperatures (1800-

2000°C) indicate that under these conditions there is no detectable interreaction of the seed with the electrode material. After testing electrodes

made of stabilized zirconium dioxide in the experimental MHD-generator

channel with a wall temperature of 1900°C, no reaction products of Zr02

with potassium were detected [ 6 ]. Such reactions are improbable on the basis

of the following thermodynamic data;

AZ°

298 Kcal/g - atmosphere oxygen; K^O 81.2

ZrO^ 123.8

U°2 140.5

Potassium zirconates are subjected to dissociation when

heated [7] (see Table D.4.1.5).

Table D.4.1.5. Thermal decomposition of potassium zirconates..

Temperature of the Temperature range of Phases

initial decomposition thermal decomposition

K^ZrO^ 570° T

a - K^Zr^O^ 720° 570

6 - K^ZrO^ 720° 720

K„Zr. 0 „ 720° 850

2 3 2

< 570° K

2

Zr03

720° K„Zr0_^a-K„Zr _0^

2 3 2 2 5

850° K-Zr0 „^a-K_Zr„ 0 ;- 2 3 2 2 5

< T K2Zr0^-B-K2Zr20^TZr02

In conformity with the data [ 8 ] more than 80% of the potassium

in the flow of combustion products is found in the form of a hydroxide

at 1950°C; 15-20% is in atomic form. At a temperature of 1950°C almost

all the potassium is converted into KOH; as the temperature drops, the

potassium condenses and due to the absorption of CO 2 from the combustion

products is converted into K2 CO 3 and KHCO 2 —compounds which are distinguished

by being highly hygroscopic. Therefore, the destruction of the stabilized

Zr 02 the temperature range between the isotherms 1250-950°C can be
251

explained by the interreaction between trhe condensed potassium salts and the

admixtures in the refractory material (mainly with Si02 ) and by the

secondary hydration of potassium carbonate and bicarbonate which is

accompanied by significant volume changes.

Experiments carried out in the U—02 confirmed the data of

French investigators [5,6]. The maximum content of potassium compounds

(up to 7 weight %) was detected in zones of the electrodes which are located between the isotherms 1250 and 850°C. Hence, the low temperature

limit for the use of porous ceramic electrodes should be set at 1300-

1400°C. At the same time, it should be noted that in order to avoid the

condensation of potassium compounds on the ceramic walls while the

channel is operating in the transitional mode, the start of the seed

injection into the plasma and the termination of the seed supply should

be carried out only when the channel is thoroughly heated.

Recrystallization . After testing electrode materials in the

U—02 unit for a period of 1 hour at a surface temperature of 2000°C, it was established that over their total cross section the electrodes are composed of a monophase material of different densities; the difference

in the structure is determined by the operating temperature of the electrode

wall

Structural changes appear more clearly in a narrow band at a distance of 1 mm from the operating surface. In this region there is an abrupt growth of grains and pores, and shrinkage is being observed.

When the experiment was prolonged to 100 hours under similar

conditions [3] the directional character of the recrystallization was

noticed. This was accompanied by a tenfold increase of the grain size

(from 10 y in the original material to 100-120 p after the test).

Grains and large pores acquire an elongated form and are oriented in a direction perpendicular to the hot surface. Channel-shaped pores break

up the material into discrete microblocks, which leads to a sharp decline

in the strength of the material [9].

References

[1] A. B. Ivanov, et. al.

,

Oxide materials for the hot channel of the

open cycle MHD-generator , in Material for the MUD-generator channel,

pp. 52-62, Nauka, 1969.

[2] V. V. Yanulis and I. S. Mayauskas, The use of solar and arc

furnaces for the determination of the evaporation rate of

refractory materials, Geliotekhnika, No. 6, 1967.

[3] I. S. Mayauskas, R. I. Abraitis and G. A. Grinyute, The erosion

effect of the flow of combustion products containing potassium

on zirconium dioxide. Report SM-107/83 at a symposium for obtaining electroenergy with the aid of MHD-generators , MAGATE,

Warsaw, 1968.
252

[4] J. Millet, Les mechanismes de corrosion des electrodes ceramiques

MHD, SM-107/67, Electricity from MHD , 1968, JAEA, Vienna, 1968.

[5] J. Dubois, Corrosion chimique de materiaux les composes alcalines

de la simence, EDF, 1966.

[ 6 ] A. B. Ivanov, Zirconia and zirconia compounds. Electricity from

MHD, 1968, Vol. )a, pp. 3835-3845, IAEA, Vienna, 1968.

[7] M. Tournoux, Rev. Haut. Temper, et Refract., 1, N4, 343, 1964.

[ 8 ] M. A. Styrikovich, A. E. Sheindlin and I. L. Mostinskii, Ionizing

seed in an open-cycle MHD-generator operating on gaseous

fuel, in The MHD-method for obtaining eleotrioat power, Energiya,

p. 40, 1968.

[9] A. T. Andrianov, A study of the thermal aging of ceramics made of

pure oxides AI 2 O 3 , MgO, Zr 02 , MgO • AI 2 O 3 , Avtoref. kand . dis.

Mosk, khimikotekhnolog . in-ta, 1971.
253

List of Figures

Figure D.,1.1. Microstructure of test piece of magnesium ceramic.

Transparent section, 46X.

Figure D.,1.2. Dependence of the conductivity of magnesium ceramic on temperature.

Figure D.,1.3. Dependence of the thermal conductivity of magnesium

ceramic on temperature.

Figure D.,1.4. Deformation curve "B-E” of test pieces of magnesium

ceramic under compression.

Figure D.,1.5. Deformation curve "B-E" of test pieces of magnesium

ceramic under tension.

Figure D.,1.6. "Creeping” curve e vs t of magnesium ceramic.

Figure D.,1.7. Dependence of the speed of vaporization of magnesium

ceramic on temperature.

Figure D..1.8. External view of the insulating wall after testing in

U—02 channel.

Figure D.,2.3.1a. Distribution of the intensity of K i radiation K for

Ct the 1310 cathode: along an arbitrary line perpendicular to the working layer of the electrode (central

part of the section)

Figure D,,2.3.1b. Distribution of the intensity of K i radiation K for

the 1310 cathode: along a line parallel to line a)

and 500 mcm from it.

Figure D.,2.3.2a. Distribution of the K i radiation intensity K for the

2520 anode: along an arbitrary line perpendicular to

the working layer of the electrode (central part of

the section)

Figure D.,2.3.2b. Distribution of the K i radiation intensity K for the

2520 anode: along a ^ine parallel to line a) and 800

mcm from it.

Figure D.,3.1.1. Cathode and anode polarization curves for the American

specimen 88 mol% Zr 02 - 12 mol% Y 2 O 3 obtained in air at

a temperature of 1400°C.

Figure D.,3.2.1. Geometric dimensions of the tested specimen.

Figure D,, 3.2.2a,b. Cathode and anode curves for the specimen - 18 mol%

Ce02 - 92 mol% Zr 02 - obtained in air at a temperature

of 1400°C.

Figure D.4.1. Sample structure I before testing (x 1200)
Figure D.4.2. Sample structure I after testing (x 1200).

Figure D.4.3. Sample structure

Figure D.4.4. Sample structure

Figure D.4.5. Sample structure

Figure D.4.6. Sample structure

II before testing (x 600).

II after testing (x 600).

III before testing (x 1500)

Ill after testing (x 1500).
255

CHAPTER IV

Conclusions - U.S. and U.S.S.R.

A. Conclusions from Phase Composition, Chemical Reactions, Metallography,

and Microprobe Analysis (U.S.)

A.l Conclusions from phase composition and chemical reactions

1.

The effects caused by seed penetration are more pronounced at the cathode

wall

a. All Ce02/Zr02~based cathode materials within lower, cooler, zones show

alteration products typical of reactions with seed. Reaction intensity is more

evident for current-carrying materials.

b. Within the ZrO„-cements and MgO insulators below cathode materials, seed,

condensed as K^CO^ and/or hydrate, is ubiquitous.

c. Reaction with seed at the anode electrodes is less pronounced and,

generally occurs within localized interior zones.

2.

Except for Y^O^-stabilized ZrO„ materials, and contrary to anode materials,

CeO-ZZrO^ cathode materials appear to have been subjected to reduction in situ

foliow;ed by complete to nearly complete reoxidation during shut-down.

a. Phase transitions due to reduction are manifested particularly by the

presence of monoclinic Zr02 , together with certain additional phases.

3. Surface x-ray analysis, particularly of Ce02/Zr02~anode materials, show

depletion of Ce02- The Ta-containing electrodes also show Ta-depletion

4. Phase transitions leading to alteration of the microstructure of virgin

cathode materials may have enhanced seed penetration. Cements also provide good

avenues for seed penetration.

5. MgO insulators (anode and cathode), especially where exposed to the plasma,

show compounds with Al 20 _ and cubic zirconia from x-ray diffraction data.

These may contribute to the alteration and the degradation of the erosion

resistance of virgin materials.

6. Bloating of cathode materials apparently was caused by hydration and expansion

of condensed seed during and after the cool-down period.

7 . Lead-out failure in electrode pairs was not caused by chemical attack, but

rather by joule heating and eventual melting. Evidence for presence of KCl

within lead-out tubes indicated K-penetration within this area.

8. There appears to be no evidence for damage of electrode materials solely

by current transport.

9. Zr02/Y202 materials at the cathode show evidence for partial destabilization

of cubic zirconia. Presently, it is unclear whether this process is related to

seed reactions or electrochemical phenomena.

10. The information from Phase I U-02 test is extensive, but complete analysis

of all elements would require years of study.
256

A. 2 Conclusions from Metallography and Microprobe Analysis

Based on SEM and microprobe analysis carried out at Battelle Pacific

Northwest Laboratories, a number of conclusions can be drax^7n. These

conclusions were made from the metallographic and electron microscope evaluations

of eight to twelve of the electrodes. Since the examination and evaluations

are not complete, these conclusions are considered preliminary and may be

modified after the structural characterization is completed.

1. The cathode degradation was substantially greater than the anode, presumably

due to electrochemical effects.

2. Cement, which acts as a source of potassium concentration, is a major design

problem.

3. Potassium transport and concentration occurs principally through and in

the open porosity of the low density phases of the MgO, cement and electrode.

4. The mechanical swelling of the cement and MgO appears to result from

hydration of the K

2 CO

2 which is accompanied by a 40% increase in volume.

5. Microstructural and electron microscopy results agree in general with the

x-ray diffraction phases studies.

6. Contamination of the insulator and electrode surfaces are apparently due to

oxide contaminants carried by the plasma.

7. There is a preferential loss of Ce and/or Ta from the surfaces of the high

Ce containing electrodes.

8. Potassium not only influences the mechanical breakup of the MgO, cement

and electrodes, but also appears to influence or enhance recrystallization,

transport of other elements and formation of new phases in the electrodes and

insulators

* U.S. GOVERNMENT PRINTING OFFICE 1977 0-240-980/533