oes i'{t qb i/o ( Cfz)

E- (49-1) -3300-4

District Category UC-30g

DEVELOPMENT, TESTING AND EVALUATION CF

MKD-MATERI ALS

Quarterly i\eport

for the osriod Ami - June 1975

ti

1r$

2

M’t

•J4

iI

H. P. R. Frederikse, T. Negas and S. J. Schneider

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards ' Washington, D. C. 20234

Date Published : June 30, 1976

PREPARED FOR THE UNITED STATES

ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

Under Contract No. E (49-1) -3600

|

I

j

*

4

ABSTRACT

In February, ERDA requested NBS to assist in the analysis and

evaluation of channel wall materials after a 95 hour test of the AVCO

Mark VI, C-channel. During this quarter, therefore, activities were focused on. this task. All work was completed, a report was presented

at the 15th MHD Symposium, Philadelphia, PA and a summary report will

be transmitted to ERDA.

The Final Report for the Phase 1, U-02 test will be completed upon

receipt of translated Soviet contributions.

The viscosity of slag from "standard coals" (Illinois #6 and Rosebud) was measured and electrical conductivity measurements are in progress.

Based on encouraging electrical and chemical data, an iron-containing,

MgAl2C>4 spinel solid-solution served as the basis for the development of

several electrode module designs. This material will be tested during

U-02, Phase II and in the ANL, U-25 materials screening program.

More recent data suggest that measured potassium pressures over silicate

solutions depend on Knudsen effusion hole size and, hence, previous

estimates of K^O solubility in a slag are probably too high.

ZrO^-based materials containing rare earth oxides from La to Tb readily

react with I^O. ZrC^/Y^C^ materials are more resistant to attack by seed.

A scanning electron microscope-energy dispersive x-ray analyzer

facility was installed and is operational for future analysis of materials.

Diffusion couple experiments were conducted to determine optimum methods

for data collection and theoretical work is in progress to elucidate diffusion

processes in MHD electrode systems.

As part of a continuing program, additional thermophysical/chemical

data for alloys were compiled. Based on these properties, together with

environmentally imposed MHD requirements and cost estimates, the selection

of metals for steam plant components can be facilitated.

\

1

*

Objectives and Scope of Work *

The overall objectives of this program are to obtain chemical and physical

definition of high temperature materials which have shown promise for use

in coal-fired open-cycle MHD power systems. Major problem areas in which

investigations will be concentrated are:

1. Characterization of coal slag and its effects on system components

and performance at prototype temperatures.

2 . Development of electrode materials which provide adequate performance

over extended periods of time.

•

• 3 . Insulating materials which limit thermal losses and are resistant

to prolonged thermal and erosion effects.'

4 . Preheater materials which can withstand the operating modes of

separately and directly fired operation.

5 . Seed recovery methods from slag which are technically and economically

feasible.

« 6 . Phase equilibria and diffusion rates of seed in slag and the corrosive

action of combination on system components and materials.

7. Durability of prototype MHD sub-systems.

The program is designed to contribute to the solution of these problems bv

providing much needed data on candidate mar.erials and by evaluating test sam- ples and structures that have been subjected to real or simulated MHD conditicns,

The activities are grouped -under six tasks

:

G. Program Management Coordination (Assisting EPDA in coordination , planning

and review of the various MHD-Materiais Development Programs)

.

H. U-02 Materials Testing and Characterization (Coordination of U-02 Test

Activities, Phase I)

.

I. Operational Design Properties (viscosity, electrical conductivity,

vaporization)

.

J. Corrosion by Seed and Slag (phase equilibria, diffusion)

.

K. Materials Testing and Characterization (test coordination, ore- and

post-test analysis)

.

L. Assessment of Steam Plant Components (corrosion resistance of metals

and alloys)

.

Summary

Task G. Program Management Coordination

- Conducted and participated in ERDA program review meetings and sessions

devoted to Phases I, II and III of the U-02 and U-25 Bypass Materials

Testing Programs.

Task H. U-02 Materials Testing and Characterization

- All work for this task is complete. A Final Report will be issued

after contributions from the Soviet side are translated, received, and

meshed with the U. S. input.

Task I. Operational Design Properties

a . Viscosity of Coal Slag

- Analysis of slag from "standard coals" was initied. Viscosity of a synthetic

slag simulating Illinois #6 coal slag was measured.

- Viscosity of a bottom-ash from a power plant using "Rosebud" coal was measured and compared with the synthetic variant. The real slag has a viscosity one order of magnitude higher than the synthetic suggesting a considerable difference in bulk composition. This will be resolved by

chemical analysis.

b. Electrical Conductivity

- A detailed investigation of the electrical conductivity of the candidate

electrode material, SMgAl^O^ :Fe_0 1 (spinel solid-solution) was completed.

This material has a melting ‘point^near 1860 °C and could operate within the

500-1700 °C range.

c . Vaporization '

- Mass spectrometric-Knudsen effusion measurements of the potassium pressure

over simple and complex silicate solutions were performed. Potassium

pressures over K^O/SiO^ solutions were found to depend on effusion hole

size suggesting £hat equilibrium concentration of K 0 in slag will be lower

than previously estimated.

Task J. Corrosion, Processes, Seed-Slag Interaction, Diffusion

- A scanning electron microscope-energy dispersive x-ray analyzer system

was installed and is to be utilized for work pertinent to this task and,

particularly, to Task K and related materials testing programs.

- Data collection and measurement techniques necessary to determine diffusion

coefficients from diffusion couples were developed and will be refined.

- Matrix and diffusion of probability techniques are being used to calculate

diffusion . parameters for MHD materials. Analysis of perturbation effects

from temperature and electric field gradients permits results to be related

to diffusion processes in MIID electrode systems.

- Completed a study of subsolidus phase relations in the K^O-Al^O^-SiO^

system and initiated a study of the K^O-iron oxide-SiO^ system.

Task K. Materials Testing and Coordination

- An electrode system utilizing an iron-containing, MgAl^O spinel solid

solution, flame sprayed onto a wire alloy mesh was designed. After fabrication this module will be tested during the ANL U-25 electrode screening

program. This spinel system but with different design, fabricated by

Battelle-Northwest and Westinghouse , also will be tested during Phase II, U-02

- Completed a comprehensive analysis of AVCO Mark VI, C-channel electrodes

operated for 95 hours.

- Analyzed by x-ray diffraction several preheater ceramics tested at

FluiDyne Corporation.

Task L. Assessment of Steam Plant Components

- Environment conditions (and hence performance requirements) for components

"below" the preheater (and seed condensor) are constantly updated and modified

- Reports are being analyzed dealing with the potassium (or sodium) corrosion

and oxidation resistance of various alloys (stainless steel, Inconel,

Hastelloys, etc) in the range 370 - 1000 °C.

4

NBS Papers and Reports on MHD, April - June 1976

Inorganic Materials Division

Title Expected Date of Report or Publication

1(a) Final Report U-02 Test (Phase I)

W. R. Hosier, editor with contributions

from Battelle NW, Westinghouse , and NBS.

Completed draft to Steering Committee,

June 1976 (USSR contributions

received for translation)

.

(b) Joint Test of the Electrode System in

USSR U-02 Facility, W. D. Jackson, S. J.

Schneider, W. E. Young, A. E. Sheindlin,

G. P. Telegin, D. K. Burenkov*

15th MHD Symposium, Philadelphia, Pa.

May 28, 1976.

(c) Investigations of the Thermophysical

Properties of High Temperature Materials

for MHD Generator Channel, G. P. Telegin,

S. J. Schneider, et al.(et al. not given).

5-th European Conf. on the Thermophysical Properties of Materials at

High Temperature, Moscow, 5/18/76

2. Chemical Aspects of Materials

Containing Cerium Oxide, T. Negas,

R. S. Roth, H. S. Parker, C. L. McDaniel

and C. D. Olson (preprint available)

.

Moscow Conference (see above),

5/18/76

3. Spinels for MHD Electrodes,

H. P. R. Frederikse, W. R. Hosier,

A. J. Armstrong, T. Negas

15th MHD Symposium, Philadelphia, Pa.

May 24, 1976.

4. Crystallization and Vaporization

Studies of Synthetic Coal Slag

Compositions, L. P. Cook, E. R. Plante,

T. Negas, R. S. Roth, C. D. Olson

15th MHD Symposium, Philadelphia, Pa.

May 24, 1976.

5. Influence of K^O on the Cerium OxideZrO^ system, T. Negas, R. S. Roth, C. L.

McDaniel, C. D. Olson (preprint available)

.

12th Rare Earth Research Conf. , Vail, Colorado, 7/18/76

6. Oxidation-Reduction Reactions of

CeM04+x (M=Ta or Nb) Phases, T. Negas,

R. S. Roth, H. S. Parker, C. L. McDaniel,

C. D. Olson (preprint available) .

12th Rare Earth Research Conference,

Vail, Colo., 7/18/76.

7. Analysis of AVCO Mark VI C Channel

Electrodes After Long Duration Test,

W. R. Hosier, T. Negas, S. W. Petty

15th MHD Symposium, Philadelphia, PA

May 24, 1976.

8. Materials for Open Cycle MHD, Div. 11

of the US-USSR Status Report on MHD Power

Generation, S. J. Schneider, H.P.R.

Frederikse, T. Negas (ed. M. Petrick,

Argonne National Lab.).

1st draft submitted to Steering Comm.

June 1976. USSR contributions

received for translation.

! . 9. The System K^O-Al^-SiO . Part 1.

Crystal Chemistry of Phases on the

KAlSiO -KA10 0 Join, L. P. Cook, R. S.

Roth, H. S. Parker, T. Negas (preprint

available) .

Submitted to American Mineralogist,

May 1976.

Milestones - April - June 1976

Listed below are recent accomplishments based on the projected

milestones of the ERDA-NBS MHD contract. It should be recognized that

our work scheduling is strongly influenced by the needs of ERDA and

associated contractors. For ‘example, programs such as U-02 Phase I and AVCO Mark VI, C-channel, as well as the forthcoming ANL, U-25

Screening Test and U-02, Phase II, require a continuing shift of the

duties of our limited staff. Hence, some milestones will change

while others will be subordinated to more urgent work.

4(a). Measured the viscosity of "standard" slag from a midwestern

and western coal.

11a. Completed an investigation of a portion of the K^O-Al^O^-SiO^

system.

b. Extended the study of this system by adding CaO.

c. Measured potassium pressures over multicomponent synthetic sJags

containing the oxides above.

19. Measured the electrical conductivity of promising (new) electrode

materials (spinels)

.

28. Assembled and put into operation a scanning electron microscopeenergy dispersive x-ray analyzer system for measuring diffusion and

penetration of seed/slag into MHD- components

.

39,40. Completed an analysis of AVCO

a) Mark VI C-channel electrodes.

b) U-02 Materials Testing and Characterization, Phase I, is complete.

c) Completed U-02, Phase I Report except for Soviet contributions to

be translated.

41. Compiled and analyzed (from data and experience of MHD facility operators)

the range of envircnmental conditions under which steam plant components

operate. This analysis will be continually updated as new information

becomes available.

42. Assessment of the corrosive attack by potassium compounds in the flue

gas and general guidelines for the initial selection of promising alloys

resistant to this type of attack has been carried out by making use of

Na2 S 04 data from marine gas turbine research as a best approximation to

supplemert the relatively little data available on K^SO^ and K2 CO3 attack.

43a. A compilation of data on promising steam superheater tube alloys is

well underway.

45a.' Reasons have been given for recommending that hot corrosion experimental

data be obtained on certain of these alloys under conditions more closely

approximating the actual superheater tube environment.

6
f
Technical Progress

Task G. Program Management Coordination

U.S. ~ U.S.S.R. Materials Program Activities

S. J. Schneider, (NBS-) ERDA Materials Coordinator participated in several

(AVCO, MIT, FluiDyne) program review meetings as well as meetings related

to the US-USSR cooperative program on Materials Testing and the ANL, U-25

Materials Screening Test.

On April 14 a meeting of the Materials Working Group, including representatives

from ERDA, MBS, Westinghouse , UTSI and Battelle Northwest was held at NBS

.

Discussions involving materials selection, fabrication and overall design

of the module for the Phase II, U-02 test were conducted.

The US-USSR MHD Steering Committee met in Washington, June 7-8. S. J. Schneider

participated in discussions leading to tentative plans for the U-02, Phase II

and U-25 Bypass testing of materials. Present scheduling suggests that the

Westinghouse U-02 Phase II module will be delivered to Moscow, August 10

followed by a test sometime between Sept. 15-25. Phase III testing will be

conducted using a U-25-Bypass-dropout section sometime during early 1978.

USSR contributions to the Division 11 Materials Status Report and to the

U-02 Phase I Report were received during this meeting.

T. Negas, with Dr. B. Rossing (Westinghouse) and Dr. D.P.R. Hasselman

(Montana Energy and MHD Res. and Dev. Inst.) attended meetings in

Moscow, May 18-22, for discussions concerned with U-02 Phase II

testing, U-02 Phase I Final Report, Materials Status Report, and

Soviet testing in US facilities.

7

Task H. U-02 Materials Testing and Characterization (Phase I) (W. R. Hosier)

Pre- and post-test materials analysis and characterization for the U-02

Phase I test have been completed. The Final Report awaits translation of

Soviet contributions which, finally, were received June 4. Translation is

in progress and the Final Report should be completed by late July 1976.

Reproducible copies will be transmitted to ERDA for approval and distribution. A paper was presented at the 15th MHD Symposium, Philadelphia, May

24-25, 1976. This paper, "Joint Test of a U.S. Electrode System in the USSR

U-02 Facility", is included in Appendix I of this report. We are grateful

for the cooperation, effort, and technical contributions made by personnel

from Westinghouse Corn, and Battelle NW for the Final Report, U-02, Phase I test.

This task is considered complete and will be terminated as of June 30, 1976.

«

8
I
Task I. OPERATIONAL DESIGN PROPERTIES

a. VISCOSITY OF COAL SLAGS (W. Capps and D. A. Kauffman, Inorganic Glass Sec.)

Introduction

It has been requested that NBS characterize the slags from the several

coals adopted as "standard" coals for forthcoming MKD tests. These coals

include Montana Rosebud and Illinois #6. The coals to

be used in the joint US-Soviet tests will be American coals that resemble

those of special interest to the Soviets. The Soviet coals are Krushnitz

and a Siberian coal. The Montana Rosebud coal apparently resembles the

Siberian coal closely enough to be used as their "standard". The Krushnitz

coal counterpart is not yet known. In comparing the ash analysis for the

Krushnitz (according to Dr. Crawford of UTSI) with those in Selvig and

Gibson, it appears that there are similar coals from Pennsylvania, Kentucky,

Alabama, Illinois, and New Mexico. The proposed characterization consists

of 1) the estimation of expected slag compositions in the several parts

of an MHD-system based on expected conditions of temperature, etc., 2) the

determination of viscosity-temperature relationships of real slags and of

synthetic slags of expected compositions, and 3) the determination of the

electrical conductivity of these slags.

Slag Preparation and Measurement

Progress to date consists of the following:

1) A bottom-ash sample from a Montana power plant from Montana Rosebud

coal was obtained and its viscosity was determined. A piece of this remelted

slag v;as annealed and is being used for conductivity measurements.

2) A synthetic slag was prepared having a composition similar to that

of Montana Rosebud given in ECAS Specifications from a NASA study.

The viscosity of the melt from the coal ash was an order of

magnitude higher than the synthetic slag. The reported analysis of that

coal was probably determined under conditions that retained the sulfur.

The analysis showed extremely high sulfur content (26.2%). It is possible

that the bottom ash did not have such a high sulfur content and

that much of the sulfur went up the power plant stack as gases or as flyash.

Both the real and the synthetic slags will be analyzed for sulfur.

3) To test this theory, a second synthetic slag was prepared identical

to the first but sulfur was omitted. The viscosity was also very lew and not too different from the sulfur-bearing counterpart.

Sulfur retention may be a critical factor and must be examined by analysis

of specimens after viscosity determination.

9
,

"
4) A synthetic slag representing Illinois #6 coal, as given in the

ECAS Specs., was prepared and its viscosity was determined. Mo real

111 inois #6 slag has been obtained but is expected from PERC.

Viscosity-temperature curves will be given in the next report when

chemical analyses of the samples have been completed.

PLANS

1. Prepare synthetic Krushnitz coal slag 'and determine viscosity

furnish specimen for conductivity measurement.

2. Have chemical analyses made to determine if real and synthetic

slags resemble the analyses in the ECAS Specifications.

3. Obtain and measure viscosity of Illinois #6 slag and compare with

synthetic model slag.

X

10
'i
b. ELECTRICAL CONDUCTIVITY (W. R. Hosier, A. J. Armstrong, H.P.R. Frederikse)

During this reporting period, a detailed study of the electrical

conductivity of a sintered, high density spinel of composition 75 mole

percent MgAl^O plus 25 mole percent Fe^O^ was conducted; the electrical

conductivity or this material was measured from room temperature to

2001 K under three oxygen partial pressures. The electrical conductivity

was found to be 3.24 x 10“^ ohm- ^cm- -*- at room temperature and 2.80 x 10 - 3 ohm~lcm_ l at 186 °C (the maximum temperature for use of copper in air)

.

The electrical conductivity at several C>2 pressures from 769 K to 2001 K

is shown in Figure 1. As the iron component in this spinel may undergo

oxidation at temperatures near 1200 °C (and below) in an oxidizing atmosphere (air) , we conducted a long term experiment to explore this possibility.

A sample was heated at 1200 °C in air for 117 hours while the electrical

conductivity was monitored continuously. Only a small decrease (1%) in

the electrical conductivity was observed during this time. Subsequent

examination indicated damage to the platinum electrodes but no physical

damage to the sample. Under these conditions it appears that the dense

ceramic spinel retains adequate electrical conductivity for use as an electrode material.

A paper, "Spinels for MHD Electrodes", was presented at the 15th

Symposium on Engineering Aspects of MHD (see Appendix I)

.

Samples of two slags from western and midwestern "standard" coals have

been obtained and are being prepared for electrical conductivity measurements.

Samples of 3MgAl 0 ./iFe 0 . spinel and a graded ceramic based on ZrO„-

ceria compositions, to be used in the U-02 Phase II test, will be investigated using the laser thermal gradient apparatus.

N

11
'
oxygen

partial

pressures.

12
1 V

•1
c Vaporization Studies (E. R. Plante)

Vaporization studies during the last two quarters have involved

measurements of the K(g) pressures produced by the reaction;

K20(£, dissolved in slag or SiC>2) = 2 K(g) + l/2C>2(g).

Pertinent data, results, and conclusions from studies of two synthetic

slag samples were given in a paper by Cook, Plante, Negas, Roth and

Olson at the 15 th MHD Symposium in Philadelphia (May 24- 25 , 1976 )^.

A copy of this paper is appended to the present report so that only a summary of these studies is needed here.

Briefly, mass spectrcmetric-Knudsen effusion measurements were made

using Knudsen cells having nominal .5 ram diameter orifices. Use of

small effusion holes has the effect of decreasing the sensitivity of the mass spectrometer so that measurements in the liquid range of the slags

could be made. The conclusions were that the K(g) pressures over the

slags at a fixed wt % K9O and temperature were significantly higher over

the synthetic slags than over solutions containing just Si02 ana K2O

obtained from previous N3S dataz .

This conclusion would be consistent if S.d.CL ition of other slag

components such as AI-.O3, CaO, MgO, and Fe^O^ had a tendency to destabil

the K20-Si02 solutions. However, this result would also be possible if

the measured pressures depended on the orifice size (1 mm diam.) of the

Knudsen cell in the l^O-SiC^ solution studies.

To check on this possibility, mass spectrometric Knudsen effusion

studies were made on a sample of ^O-SiC^ (£) contained in a Knudsen cell

having a .5 mm diameter orifice. The results show that the K(g) pressur

are significantly higher than those previously measured using a 1 mm diameter orifice. As an additional check on the influence of the size

of the effusion hole on the apparent K(g) pressure, measurements of a K20-Si02 solution are being made using a 1 mm diameter effusion hole.

Initial results confirm that there is an important hole size effect.

In addition, the mass spectrometer results show satisfactory agreement

at 40 wt % I<20 with previous NBS results.

Two important conclusions can be drawn from these results. First,

the thermodynamic esrinate for the equilibrium concentration of K2O in

slag is roughly a factor of two too high. Secondly, the failure to

achieve equilibrium under relatively ideal Knudsen cell conditions with

a sample area/escape area ratio of about 60 can probably be taken as fairly strong evidence that the equilibrium concentration of K2O in slag

will not be attained except possibly for slag which has a long life on the channel walls.

Future Work

Vaporization measurements on selected slags will be continued to

determine the effects of various components on the activity of J^O.

Analysis of current data for use in engineering calculations will be

attempted.

References

1. L. P. Cook, E. R. Plante, T. Negas, R. S. Roth and C. D. Olson,

Crystallization and Vaporization Studies on Synthetic Coal Slag Compositions, 15th MHD Symposium, Philadelphia, Pa., May, 1976.

2. E. R. Plante, C. D. Olson and T. Negas, Sixth International Conference

on Magnetohydrodynamic Electrical Power Generation II, 211.

14

Task J. CORROSION PROCESSES, SEED-SLAG INTERACTION, DIFFUSION

a. Seed-Slag Interaction (L. P. Cook)

An investigation of subsolidus phase relations for a portion of the

K20-Al203~Si02 system was completed and a paper, "The System K^O-Al _CLSiO^. Part I. Crystal Chemistry of Phases on the KAlSiO -KA10 o Join", "was

submitted to the American Mineralogist. Due to the length of tnis paper

only an abstract is included in Appendix I. However, preprints are available

upon request. This paper presents a wealth of information for materials

specialists involved in slag-seed characterization studies or in thermodynamic calculations of slag/seed interactions. The K^O-Al^O, -SiC^ system

is far more complex than that portrayed by equilibrium diagrams presently

available.

In another paper, "Crystallization and Vaporization Studies of Synthetic

Coal Slag Compositions" (Appendix I) , it is demonstrated that high CaO contents

in slag can lead to higher K^O activity and, hence, less absorption of

seed by slag.

Future work will concentrate on the effect of iron oxide on the

K^O-Al^O^-SiO^ system.

b. Seed-Refractory Oxide Reactions (C. L. McDaniel)

Post-test analysis of U-02 Phase I, ZrO^/cerium oxide electrode materials

revealed substantial alteration via reaction with potassium, particularly

at the cathode where cerium oxide was reduced during operation. The

reactivity of other ZrCK/rare earth oxide materials with K2 O, therefore, was investigated. All materials containing rare earth oxides from La to Tb

readily react with K2O to form identical corrosion products. ZrO /Y „0

and, probably, HfO^/Y^O^ materials are less susceptible to corrosion"

by K

20.

15

c Diffusion (E. N. Farabaugh, J. R. Manning)

1. Experimental Results

t Further work on the diffusion of potassium into MHD materials

has been accomplished. A major part of this effort has been in the

area of data collection from diffused specimens. Primarily this work

was done on an AI 2 O 3 ceramic specimen diffused at 1400°C for 96 hours.

It has been evident from work done that the simple technique of running

a single line scan from either an electron probe or SEM across a ceramic specimen to determine a concentration profile does not give

satisfactory results. The profiles obtained by this method are very

irregular and do not yield a curve from which a diffusion coefficient

can be determined. As the electron beam transverses the specimen in

this method, it goes across flat areas and voids. When the electron

beam is incident on a void, the proper signal will not get back to

the detector. As the beam goes across the diffused area, many of

these voids are transversed and the resulting profile is very irregular.

In an attempt to produce a smoother curve a diffused specimen surface

was polished with a succession of diamond grits down to 3 micron

diamond. Again a line scan was run on a polished diffused surface

but the resulting curve was still too irregular to allow any calculation of a diffusion coefficient. Next, it was decided to integrate

for potassium over the area of a strip of the specimen parallel to

the edge from which the potassium diffused. This was done by using

an electron probe to raster along a strip approximately 10 microns

by 2 mm. (The long dimension of the scrip is parallel to the edge

of the specimen.) The raster was then moved 100 microns in a direction normal to the edge and another strip was integrated over

for potassium counts. This was done completely across the specimen

from edge to edge. This data collection resulted in a much better " concentration profile, from which one could make an estimate of a diffusion coefficient of the order of 10” ° to 10” ^ cm^/sec. Since

there are not enough data points in the present profile, data will

need to be collected in perhaps 20 micron steps rather than 100

micron steps in order to give improved accuracy in the profile shape

and obtain a more accurate value of the diffusion coefficient.

In addition to the measurements described above, which were done

with an electron probe, our new SEM also was used with its EDX

accessory utilizing a rectangular 10x 300 micron partial field to

measure concentration profiles. A similar technique of stepping

across the specimen was used and collection of data representative

of a strip of diffused material parallel to an edge was taken at

intervals of 100 microns. This data was nearly the same as that

obtained by the electron probe on the same material, AI 2 O 3 ceramic

diffused 96 hours at 1400°C. Then to get a better defined curve,

a second traverse was made with data being collected every 10 microns.

However, this time instead of a strip, a 10 micron square area was integrated over in several places in the 10 micronx 300 micron strip.

The areas selected for data collection were those areas in the strip

which were either free from voids, or at least had a very small part

1 A

of its area occupied by voids. This did not yield a greatly improved

profile shape. Irregularities still occurred. Thus, one is led to

bel ieve that both voids and non-uniform diffusion of potassium in the

ceramic contribute to the sometimes irregularly shaped profile. It

appears that the technique of integrating over as long a strip as possible parallel to an edge must be the approach to use if we are

to obtain usable concentration profiles. These data collection techniques are currently being worked on, and it is felt that most of

the diffusion data can be collected using the new SEM facility with

only a minimal reliance on electron probe analysis.

Other diffusion couples have been made. One of the couples was made up of single crystal AI2O3 embedded in B-AI2O3 powder. The

single crystal specimen_was rectangular in shape with the edges

parallel to [OOOlJ , [ 1010 ] and [ 1120 ]

. Thus proper sectioning of

the diffused specimen has yielded samples for which diffusion profiles

in the three crystallographic directions can be determined. This

couple was heated at 1400 °C for 96 hours. This was done so we can compare the results of the single crystal diffusion with diffusion

into AI2O3 ceramic for which we have our best data and profiles.

Although it is recognized that single crystal AI2O3 plays no active

role in MHO applications, diffusion data on it will give us a base

line to work from. These single crystal samples are currently being

examined, and we have no data on them yet.

An MgO ceramic diffusion couple, heated at 1400 °C for 24 hours was analyzed with the SEM. Just as was the case with the AI2O3

ceramic run at 1400 °C for 24 hours, only small concentrations of

potassium could be found in the specimen and no profile could be

determined. The question arises if during the run the potassium

in the 3-AI2O3 pellet was depleted. As a check on this problem,

powder from the 8-AI2O3 pellets on this and subsequent couples which

was next to the host material was removed from the pellet. Care was taken that only material which was in contact with some part of the

specimen was removed. An x-ray powder pattern was taken of the

material showed only the 8-AI2O3 lines, no AI2O3 lines at all. This

indicates no significant depletion of the potassium in the 3-AI2O3.

In the next quarter we plan to further refine our data collection

technique, study diffusion in more dense AI2O3 ceramic, obtain data

on single crystal AI2O3, and measure a concentration profile from

which a reliable diffusion coefficient can be determined.

2 . Theory

Diffusion in MHD channel wall materials, particularly in the

electrodes, will be affected by the electric fields and temperature

gradients present. It has been shown in previous work at the National

Bureau of Standards that an important kinetic force which influences

diffusion in such fields and gradients is the vacancy wind force,

which arises whenever there is a net vacancy flux. One quantity which

must be known to calculate the magnitude of the vacancy wind force is

the effective return frequency H of vacancies to the various migrating

atoms. Values of this quantity are being calculated for MHD materials.

17

t The frequency H is related to the correlation factor f for

diffu sion in the absence of driving forces; and the equation

f = H/(2w2+H), where w2 is the jump frequency for exchange of the

particular migrating acorn, with a neighboring vacancy, can be used

to help determine H. Here f is a measure of deviations from

random v?alk diffusion behavior. The kinetic calculation of f in the

absence of electric fields and temperature gradients is much simpler

than a calculation with one of these driving forces present. Also

studies at NB5 have shown that f itself is unaffected by driving

forces, even though the various components of H are subject to change

from these forces. Thus, in the present calculations, it is planned

first to calculate H in the absence of driving forces and then to

determine the linear perturbations to this result introduced by the

electric fields and temperature gradients.

With this approach in mind, various methods of calculating f

and hence H in situations which may arise in MHD electrodes have

been investigated. In particular, it was concluded that the

comparison made between correlation factor calculations for metal

alloys and various defects in metals [j. R. Manning, Comparison of

Matrix Inversion, Monte Carlo, and Diffusion of Probability Techniques

for Calculation of Diffusion Correlation Factors, Proceedings of

Conference on Computer Simulation for Materials Applications,

Gaithersburg, Md. , April 1976, pp. 109-120, Nuclear Metallurgy,

Vol. 20] should apply equally well to most systems proposed for use

as MHD electrodes, including spinel structure materials (hercynite,

magnetite, MgAl2Ou) and fluorite structure materials (Ce02 , stabilized

zirconia) . It appears that matrix techniques and diffusion of

probability techniques can be used with confidence for kinetic

calculations of f and H in these materials. Consequently, these

techniques are being used for the present calculations. An analysis

of perturbation effects from temperature gradients and electric

fields then will allow these results to be related to diffusion

processes in MHD electrodes.

The diffusion flux J in the x-direction in both electric fields

and temperature gradients can be represented by the equation

J = -D* (3c/3x) + cFD*/kT

where D* is the tracer diffusion coefficient of the diffusing species,

c its concentration, F the driving force from electric fields and

temperature gradients, k Boltzmann's constant and T the absolute

temperature. In the linear perturbation region, the forces qE from

an electric field E and Q* 3lnT/3x from a temperature gradient add

linearly to the vacancy wind force to yield the net driving force F

.

Here the effective charge q and the effective heat of transport Q* are parallel quantities, both being properties of the diffusing ions

and the materials they diffuse in, whereas E and 31nT/3x are properties

of the MHD environment. Thus, q and Q* are directly dependent on the

particular electrode materials chosen, as also is D*.;. and these are

the key kinetic quantities that will differ from one electrode material

to another. Kinetic expressions are being developed for these

quantities, starting with calculations of f and H as mentioned above.

18

L

Even in originally homogeneous electrodes the cF term creates a diffusion flux which can cause significant changes in the oxygen

and metal ion distributions and lead to degradation and failure

of electrodes. In graded electrodes where an original concentration

gradient is introduced to provide uniform conductivity in a temperature

gradient, diffusion effects may make the concentration gradient

difficult to maintain. Both the 3c/Sx and cF terms can contribute

greatly to the diffusion flux in graded electrodes.

Similarities and differences between the diffusion effects

from electric fields and temperature gradients as described above

were discussed in a talk on "Diffusion in Temperature Gradients and

Electric Fields" presented by J. R. Manning as part of the Symposium

on Control and Utilization of Defects in Non-Metallic Solids at the

Spring 1976 Meeting of the Electrochemical' Society. Significant

differences can arise both from the non-uniform ion fluxes created

by diffusion in a temperature gradient and from the possibility

that vacancy concentrations may not be maintained at local equilibrium

values during diffusion in a temperature gradient. Nevertheless,

because of the similarities, the equations for the diffusion flux J as written above allow electric fields and temperature gradients

to be treated in a nearly parallel fashion. Present indications

are that these equations provide a good framework for the description

of kinetic diffusion processes in MHD electrodes.

19

Task K. MATERIALS TESTING AND CHARACTERIZATION

a. Phase II, U-02

Plans previously formulated for Phase II, U-02 are being implemented.

.The test schedule was delayed (by mutual agreement, US/USSR), however, until

September 15, 1976. The test also has been scheduled for 100 hours maximum

instead of 100 hours minimum duration. Design considerations have not been

changed and sample and module preparation by Westinghouse Research and Development Lab. are on schedule. Shipment of the module will be delayed until mid

August 1976.

b. Phase III, U-02 Plans

According to the latest plans of the US-USSR-MHD Steering Committee,

the Phase III, U-02 materials test will be shifted to a U-25 bypass dropout section. A cooperative US/USSR effort is contemplated for the design

of the electrode svstem(s). The test will be of 100 hour duration with

maintenance (replenishment) and is scheduled, tentatively, for early 1978.

At this time, absolutely nothing is known about pertinent engineering

parameters and, hence, planning activities cannot be initiated or implemented.

c. Post-test Analysis of AVCO MHD electrodes (W. R. Hosier, T. Negas)

The analysis of the AVCO Mark VI, C-channel electrodes was completed

according to the general plan established in the last Quarterly Report.

The analysis was based primarily on results from scanning electron microscope involving microprobe and EDA.X analysis and elemental x-ray mapping

of specific areas as well as x-ray diffraction measurements to determine

crystalline phases present. Chemical analyses of pre- and post-test

materials also were made as needed. A paper, "Analysis of AVCO Mark VIC-Channel Electrodes After Long Duration Test" , was presented at the 15th

Symposium on Engineering Aspects of MHD in Philadelphia, PA (May 24, 1976)

and is included in this report in Appendix I. Results will also be

published in the near future in the Bulletin of the American Ceramic Society.

The numerous photographs in this paper make reproduction ('t 60 copies) difficult

and costly. Original reprints, however, are available on request.

d. ANL U-25 Materials Screening Tests (W. R. Hosier)

The electrode material to be used in fabrication of the US built U-25

channel has not been selected. Argonne National Laboratory is conducting

at the Reynolds Facility a screening test directed toward the

selection of a suitable material. Electrode systems submitted by a number

of organizations will be tested according to a predetermined test schedule

and conditions.

It is the responsibility, as designated by ERDA, for NBS to perform

post-test analysis on these electrode systems. Results of the analyses

are to be reported within 2 to 3 weeks after the samples are received at

NBS. As this is a stringent time requirement for a thorough analysis,

a special NBS team of specialists has been designated to perform this

task as their primary function when the systems are received. Electrode

systems will be analyzed using x-ray diffraction, SEM, optical microscopy,

and chemical analysis when appropriate. Each system will be handled on an individual basis depending on its condition upon arrival at NBS. After

thorough discussions by the team members, procedures to obtain maximum

results will be instigated. In addition, it is desirable for the most
.

'

.
meaningful analysis to have information about the electrode system construction

and layout in the test chamber together with appropriate test infromation

such as temperature at various points, times of different test conditions,

unplanned excursions in test conditions or parameters etc. For these

considerations, it is desirable to have a NBS team representative at the

test to collect the information necessary for the analysis.

Testing was scheduled to begin about June 1. Unforseeable and

unavoidable delays have caused a change in the schedule and NBS must be

notified when the first test is to begin.

e. NBS Magnesia-Alumina-Iron Spinel Electrode Material (W. R. Hosier,

T. Negas)

Spinel solid-solutions of the MgAl^O^-Fe^O^ system appear to be very

attractive electrode module constituents. Tne ‘electrical conductivity

of a typical solid solution consisting of 3MgAl

9 0^/Fe^0 , (see Task I) is

excellent as the carve remains fairly flat with“ temperature and is nearly

independent of oxygen pressure (see Figure 1) . This particular spinel

has adequate conductivity down to 500 °C and has a melting point near

1860 °C (reducing conditions) as reported by J. L. Bates, BNW Laboratory.

Higher melting temperatures can be achieved by simply decreasing the iron

oxide content of the spinel subjected to the highest temperatures (i.e.,

grading the material) . Better electrical conductivity at lower temperatures

can also be accomplished by grading the material toward higher iron oxide

content. Indeed, one advantage of this material, is that grading laterally

to a pure MgAl^O^ (or MgO plus MgAl^O^) spinel insulator is possible.

NBS does not have the facilities to fabricate complete electrode

modules utilizing these spinels. However, much effort was devoted toward

the development of methods and techniques to produce spinel powders which

can be fabricated into dense, sintered ceramics or which can be utilized

for flame or plasma spraying. This was accomplished through close cooperation and consultation with a firm independent of NBS.

By taking material properties into consideration (i.e., electrical

and thermal conductivity, thermal expansion, adaptability for sintering

or spraying, etc.) is is possible to suggest an electrode module design.

Major problems, however, are not related directly to the material but to

problems imposed by environmental parameters. For example, the large heat

fluxes for the ANL U-25 Screening Test dictate electrodes having particularly

small dimensions (thickness) . Large temperature gradients and, therefore,

large stresses over small dimensions, therefore, are unavoidable. Joining

the thin, conducting ceramic to an effective leadout then becomes a formidable

problem. One possible design based on an assumed wall heat flux of 100 W/cm > is illustrated in Figure 2. For the leadout, a mesh composed of sintered

metal fibers is suggested for two reasons. Flame or plasma sprayed spinel

forms a tenacious bond with the mesh which, in turn, helps relieve mechanical

stresses created by large thermal gradients and the mismatch of the thermal

expansion of the oxide and most metals.

This design (Figure 2) as well as others have been .discussed with Argonne

National Laboratory, AVCO MHD Lab., Westinghouse , and others. The spinel

material will be tested in the U-02, Phase II program scheduled in Moscow

21

Figure 2. Schematic Design of MHD Electrode.

22

for September 1976. Fabrication of an electrode system based on Figure 2 is in progress and this module will be tested in the ANL U-25 Screening

Tests. As indicated above, appropriate adjustments in composition can lead

to higher (and lower) working, temperatures and better insulation characteristics. These refinements must await future work.

f . X-ray Examination of Corhart X-317 Ceramics (magnesia spinels) Tested

at Fluidyne Eng. Corp . (A. Perloff)

Preliminary SEM work by E. N. Farabaugh on the magnesia spinel

(MgO-MgAl £> 4) sample FN-27 done last quarter on the old unit and recon- firmed this quarter on the new SEM unit has shown the elemental composition of the surface material to be primarily Mg, Al, Si, K and Ca plus a small amount of Fe. The interior composition is primarily Mg

and Al plus a small amount of Ca.

The X-ray examination of three similar specimens (FN-12, FN-17, FN30) is in agreement with these findings inasmuch as all identifiable

surface phases have been silicates of K, Al, Mg and Ca. The dark coloration of the surface is probably due primarily to the presence of Fe. No

specific iron compound could be identified so it is likely that the iron

exists in substitution for Ca or Mg (Fe 2) and Al(Fe 5

) .

The patterns of the internal material of all samples show essentially

only MgO and MgAl 2C>4 (spinel) as would be expected.

Sample FN-30 (1600°K, 30 hrs.)

The powdered surface material yields a pattern containing moderate

amounts of MgO and spinel. It probably, also, contains CagMg(Si0 4

) 2 (merwinite) plus a number of other silicates. The most likely candidates

include ct-K 2MgS i

30 8 , K2MgSi0 4 and KAlSi0 4

.

Sample FN-17 (1700°K, 30 hrs.)

The powdered surface material pattern shows large amounts of MgO

and spinel. The spinel probably exists in two compositions. Slight

splitting of high angle lines suggests two spingls, • in approximately

equal amounts, whose cell sizes differ by vO.OlA.

Many weaker lines are present in the pattern. Most of these lines can be approximately satisfied by assuming a mixture of KAlSi0 4

, CaMgSiO^

(monticellite) , and Al 6Si 20 13 (mullite) . This identification must be

considered only tentative in such a complex pattern.

Sample FN-12 (1800°K, 5 hrs.)

The powdered surface material pattern contains large amoynts of MgO

and two spinels. The spinel cell sizes differ by about Oi0.02A and there

appear to be about equal amounts of each spinel.

The remaining weaker lines can be partially identified as probably

containing CaMgSi0 4 (monticellite) but there still remain a number of

weak lines for which no satisfactory identification has been found.

23

Two slag samples (FN-23 and FN-29) have, also, been examined by x- ray diffraction.

Sample FN-23 (Accumulation from slag tap)

This pattern contains MgO and spinel along with several other

materials for which a completely satisfactory identification has not

been made to date. Among the possible candidates are compounds resembling

NaAl}iC>i 7 or NaAl^On or KAl^O^, CajA^Og and assorted potassium aluminum

silicates.

Sample FN-29 (Accumulated seed and slag)

This pattern contains a large amount of K2 SO 4 , but no MgO or spinel.

There are significant amounts of other phases present, but identification

has been unsuccessful to date.

24

g. SEM Installation and Testing (L.P. Cook and E.N. Farabaugh)

A scanning electron microscope [SEM] equipped with energy dispersive

X-ray analyzer was installed during this quarter. The instrument has been

tested, the -majority of specifications have been validated, and it will be

accepted pending correction of problem areas by the manufacturer. This

will constitute completion of a major milestone, it is hoped, before the

end of the quarter. The instrument has been used for about three weeks

during which time users have become familiar with its operation, capabilities

and limitations. It is thought necessary to discuss briefly the SEM vacuum

system, since an expenditure above the price of a conventional system was involved. The ion-pumped system has proved beneficial so far primarily

in increased filament life and reduced aperture contamination at high beam

currents. Use of the airlock provided has been found to be essential for

routine work. This introduces some, but not serious, restrictions on the

size of samples which can be exchanged routinely. With the airlock, sample

exchange and pump down times at 10_o torr operation are comparable to those

for conventional oil-pumped systems, provided exceptionally gassy samples

are not used. We have had no difficulty thus far in working with one-inch

diameter epoxy-potted electrode specimens. It is expected that the principal c value in the ion-pumped system, which has ultra-high vacuum capability of 10

torr, lies in the extension of the instrument to include certain techniques

of surface microchemical examination, such as Auger analysis, windowless

energy dispersive x-ray analysis, and secondary ion mass analysis employing

a quadrupole mass spectrometer. These and other techniques have immediate

application in the more detailed aspects of the MHD materials characterization

program. Pending installation of any such accessory equipment as can be

purchased it is hoped and expected that the instrument will fullfill all

the functions of a conventional high-performance SEM, with the added

advantages of an oil-free vacuum. The accompanying illustrations give

examples of work done on the instrument during the "shake-down" period.

Figures 3a,b,c,d, and e shows SEM and x-ray map photographs of the

border of an inconel cathode (rail-type electrode) run under slagging

MHD conditions for ^100 hr in the AVCO facility (1) . Segregations of the

elements are apparent, and an interesting correlation is shown between Ca

and Cr, both of which seem to be concentrated in a zone near the unaltered

inconel. In detail, however, there is not a one-to-one correspondence,

suggesting the existence of two Cr-rich phases, one without substantial Ca

.

Similarly an interesting gross correlation between K and Al within voids

in the inconel exists - to what extent this has been influenced by the

cutting and polishing process is uncertain. Fe and Ni appear to remain

concentrated in the inconel despite Cr-depletion

.

The new SEM facility was also used in collecting data for determination

of potassium concentration profiles in various ceramic materials. Utilizing

the EDX accessory and the partial field mode of operation x-ray analysis were

made of various regions in the diffused specimen. In this investigation and

25
' 5)

'
also in some limited investigations of a Cohart refractory, the x-ray analysis

accessory has proven easy to use and yields reliable and reproducible results.

The ability of moving the x-ray detector closer to the specimen by means of a telescoping adjustment provides the capability of obtaining purer spectra

and higher counting rates.

Reference

1. W.R. Hosier, T. Negas, and S.W. Petty, 1976 , Analysis of AVCO Mark VI C Channel Electrodes After Long Duration Test, Proceedings of the 15th

Symposium on Engineering Aspects of Magnetohydrodynamics, Philadelphia.

26

Figure 3a. SEM photograph of a portion Figure 3b. X-ray map of area in Fig. 3a

of an inconel cathode electrode, rail- for Ca.

type, from AVCO test (see text). Lower

part is inconel grading through altered

.inconel toward the upper part near the

slag/metal interface.

2 ?

Figure 3c. X-ray map of area in Fig. 3a.

for Cr.

Figure 3d. X-ray map of area in Fig. 3a

for Fe.

Figure 3e. X-ray map of area in Fig. 3a

for Ni.

u

Task L. ASSESSMENT OF STEAM PLANT COMPONENTS (J. R. Cuthill)

This is the fourth quarterly report since the initiation of Task L as a three year program. The objective of Task L is the selection of

commercially available alloys for critical components of the down stream

steam plant, notably the boiler tubes and valve seats, after making a judgment as to the anticipated service as more detailed data are made available

from computer model system studies, while simultaneously reviewing alloy

properties

.

In connection with the boiler tubes a principal concern has been the

unknown severity of the hot corrosion problem (attack by the K SO,, K„CO , and KCl in the flue gas). With this phenomenon foremost in mind, property

data have been collected on alloys that would be expected to exhibit good

hot corrosion resistance. This list has again been updated from the previous

quarterly report by the addition of Inconel 718 and which is expected to

exhibit good hot corrosion resistance. See Tables 1, 2, and 3 which are compositions, mechanical and thermal properties, respectively. (Table la

is the original table of alloy compositions of which Table 1 is a continuation, and is reproduced here from the second quarterly progress report

for convenient reference) . It should be noted that only wrought alloys

have been tabulated thus far because, although there are many cast alloys

on the market with excellent resistance to hot corrosion attack as a result

of marine gas turbine research, there seems to be no application for cast

components in boiler and hear exchangers subjec ted to the flue gas. (Cast

alloys, IN 713C and W152 were included in the original alloy composition

table because reference was made to them in a discussion of hot corrosion

behavior in the second quarterly progress report) . Some of the INCO alloys

in that list are comparatively new alloys and Table 4 in this quarterly

report gives seme price information on them, obtained for us by James R. Hunt, INCO Washington District Manager. Although these alloys appear relatively expensive, superheater tube sections have been installed in commercial

power plants, generally as a replacement for stainless steel tubing.

However, it may be that hot corrosion attack will not be the major

problem - clogging of the boiler with huge deposits of K^SO^ and ^CO^

,

and serious reduction in heat transfer efficiency may be of primary importance. The anticipated amount of these deposits based on STD Corporation's

state point calculations will be discussed below.

It appears quite obvious from the litter data on simulation of boiler

tubes in downstream MHD steam plants that has been obtained so far, such as the Bruceton tests-*- and the British tests 2 that the low alloy steels commonly

used for boiler tubes used in the past, will not hold up. On the other hand

the service conditions are complex and the ultimate al-loys in respect to

hot corrosion resistance may not be needed, because hot corrosion attack is

the; most severe where the salt is molten on the surface of the metal and

this does not appear to be the condition that will exist on the surface of

the boiler tubes. Rather, it appears that the surface of the boiler tube

will act as a chill surface which will condense out as solid mixtures of

K

2 SO , K

2 c°3

, and KCl in varying proportions on the tube service. The deposit

would continue to build up and the thermal conductivity through the deposit - covered tube wall will continue to drop until the temperature on the surface of

the deposit increases to the melting point of the salts, at which point further

29

material condensing out as a liquid will sluff off and no further build

up will occur. This anticipated behavior is based on calculated state

point data from the STD Corp. For a 2000 MW plant the STD data shows

768.6 kg/sec.' of flue gas entering the high pressure steam boiler at a temperature of 1445°K (see Fig. 4 )*. If the steam temperature inside the

boiler tube is in the 1000 to 1050°F range, the outside surface of the

metal wall of the boiler tube will be at about 900°K. From the distribution

of molecular species at 900°K in the flue gas predicated by the STD Corp.

calculation it appears that virtually all of the potassium will precipitate

out in the form of K

? SO_,, K o C0_ and KC1 on the surface of the boiler tubes,

with the initial rates of precipitation on the bare boiler tube as follows:

ppt. during 1st hr.

5 Kg/sec. of K^SO^ 18 metric tons

9.5 Kg/sec. of K^CCL 34 metric tons

0.2 Kg/sec. of KC1 0.8 metric tons

If in actual fact the only potassium salts in contact with the metal

are well below their melting points the hot corrosion might not be as severe as it might otherwise be. It has even been reported that the oxidation

of Cobalt and Co-Cr alloys is slower with a Na^SO, coating than without . Therefore it is imperative that simulated boiler cube testing be carried out

on these new alloys where the flue gas composition, velocity, and temperature

is controlled, such as the type of test carried out at Bruceton

0

, to permit

a determination of the lowest price alloy that will do the job. It is

realized that these tests themselves are costly so therefore it would be

prudent to do some preliminary screening by conducting "crucible" tests.

Some additional guide lines have been drawn from the recent hot

corrosion research in the marine gas turbine area, involving Na^SO^. These

guidelines are as follows:

1. Alloys that form only Al^O- films for oxidation protection

are highly susceptible to hot corrosion attack whereas alloys

that form predominately a Ci^O-^ film, show generally good

hot corrosion resistance.

These STD Corp. calculations are based on Montana Rosebud coal. Results

of similar comDuter model calculations have been received from PERC (Bruceton,

Pa) but they were carried out on the basis of upper Freeport Seam coal.

Also, some preliminary calculations have been received from Argonne

.

3

Recently, a more detailed Baseline system diagram has been presented

showing flue gas temperatures at corresponding points in the down stream

steam plant that run somewhat higher than those in Figure 4, although the

temperature in the combustor is virtually the same. However, with the high

pressure steam still remaining in the 1000-1050 °F range the seed condensation problem would be expected to remain similar.
.

.
2. Alloys high in Mo that precipitate Mo carbides to impart

strength are susceptible to hot corrosion pitting.

Tungsten has a similar but less pronounced effect. (See

previous quarterly report.)

3. Alloy additions of the rare earths, Y, and La, increase

the stability of the protective film resisting hot

corrosion attack.

There are other locations and components in the system that are not subject

to hot corrosion, but simply to high temperature oxidation in air, such as air

ducts in the air preheating system. As a guide to the selection of alloys for

such applications. Table 5 is included which lists some representative con- struction alloys together with the temperature to which they are stated to be

resistant to oxidation. In some cases there have been specific rulings under

the ASME Boiler and Pressure Vessel Code on the maximum temperature at which

the alloy can be used in such applications, and where available, these are given.

Future Plans

Continue compiling data on wrought alloys for heat exchange tubes, hot air

ducts, etc. Begin compiling data on hard facing alloys that are hot corrosion

resistant, for valve seats, etc., and continue to obtain data from the various

MUD contractors to update the assessment of service conditions.

References

1. D. Bienstock, R. J. Demski, and R. C. Corey, Corrosion of Heat-Exchange

Tubes in a Simulated Coal-Fired MHD System, J. Engineering for Power, £5

Series A, #2, 1971, pp. 249-256.

2. J. B. Heywood and G. J. Womack, eds.. Open Cycle M.H.D. Power Generation,

Pergamon Press, 1969, Chapter 10.

3. W. D. Jackson, R. V. Shanklin III, R. L. Lawit , R. A. Stoudt, M. G. Klett,

S. T. Demetriades, J. C. Cutting, C. D. Maxwell, and R. F. Schnorr,

Development of a Baseline Reference Design for an Open Cycle MHD Power

Plant for Commercial Service, Proceedings of the 15th Symposium, Engineering

Aspects of Magnetohydrodynamics , Philadelphia, Pa., May 24-26, 1976.

4. D. M. Johnson, D. P. Whittle and J. Stringer, Oxidation of NagSO^ -Coat ed

Cobalt Base Alloys, Corrosion Science, 1975, Vol. 15, pp. 649 to 661.

31

Wrought

Alloy

Compositions

(cont.

from

previous

Quarterly

Report)

bv& 1 w

00oo3u

oon

a

ITl o

9S

csi• 01

CMo 3Om

CQsDO

CDU

0)Eu0

w-(

0) *o

x>p

CO CJ

.05 .05

r-i1* \oo

.35 .07 .05-15

m

r—i1 mo

00o

Si max i .50 1 i i 1.0 .35 m K I m CN o in c e 1 1 m X E • r—i •

vO

r—i i 1 1 i i 1 1 l cn CN cXH

X>

CN

CO •oo

CLE0

4J1 •H

•C>

CD aj

CD «H

CDO

0> ua

"5.

r—i3

CDC

0) 4J

r—ioE

CD 4J

CD •H

CDa« a

CL>H

>soawa

CD ou

L- u I I f o C. » r—i1 a 00

m 1 I I ! I I I I I I I I |N I I I I I I m

CN CC X)

C5 XI

CN

r~> CNm

^3* m <

CNo

CN

CD

O

r—i r—i CN CN

O

t—<

CN CN

CN

CNI O

CN

CNik

CO X)

cO

CNI mS3 mmi m

CN

Ti 1

.35 .38 .38 i i i i o

00

—i

00 00 o i m C~> 1 ^ 1 i CN O , O o o o O CN r-i u. 1 vD vX • 1 vX m r>%

C3 X

<r

mmi o

O'I

Base

Alloy

Type I

Carbide

formers

»H

«J

KI mrHo1 CQ •c I I in m ao

<N o o o o o u 0.1 .25 max CC X O C3 • E X

CN

C • £

.08.2 .15

X

eg£ mo K O O O in

pH « • • 1 i to t pH pH rH rH o X N o o m m c 1 • • {SI 1 1 1 0L* £ pH CN CN rH •

\© o pH

C* 1 1 1 1 1 1 •H

<•

pH

CN c2 1 1 1 1 *? > i i 00n

rH c 1 1 * eg H . AJ0

CO -

JO2 1 1 1 1 CN1 1 .75 CO

pH <

>111 = i i i i i i i i

in

pH

uN 1 1 1 1 in 1 | 1 c

Csi o • CO

T1 , 1 l 1 i , 1 n • • CN m Csl so CO < 1 1 1 1 m in - 1 1 in CN , , pH m x O X y eg rg cj • eg • eg 1 1 cn E

Xi *“

X CM c . O 0 rH CN u eg 1 1 1 1 1 eg eg 1 ^ JO CO X X -<r pH

ph CN • , , pH t—i

CN i-H pH pH

O CN z ! 1 1 1 a a l I in co & a> o X X

pH pH

_ so CO

— o c u CN CN pH CN pH CN

O o CJ 1 | 1 § | 1 • m cn

Os •a sO CO CN

m CN CN pH CN pH r~' c

pH

Oo

pH

m . , . o §

vO 03

r CO CO CO *0 o ^ X i • • • u co 0) fl pH

CO CO CO CO pp

C ai

CJ o >j CN

O eg c • p o sO sC o X m o r- u o o u pH p-i CO <—* o C f- u

CC eg com

X u. 3 »-H PH c

u-t PH PH 2*

0* z o O o CO o o o o o

CO o sO sD in 2 pH pH •sj 3 ro co <r o O

CO CO CO 2 z * * o 3 U 3 3 3 3 c a

*H c

Vh o Uh

0) •H o > AJ 0 E CC 3 13 y C/ S’ eg c

aj •H c eg c m c o ^* eg "

«h

m ccx c

AJ eg

eg

y

O pH E a x eg H H 60 c CC Uh 13 o c

pH

U *H «p»

C • eg AJ A^ o m 13 eg eg 13 flj C Z PH •H AJ CC <y eg AJ cj y

4J cn

x

y 0 co AJ C

pH CC

u

pH 13 c CC

<5a

H c ^

4J AJ *H o

Uh *h c 13 0 C o 3 C

cn pH eg E o u a

a> o jC E

AJ

X Ch *H

CO 4J a> eg y CO

60

C cj o

•H c •H 13

CD

X

a CJ *rH 3 AJ AJ c >

CO pH 4-) o 0 c > GO Va» E cg c.

co

m E c x o AJ 03 O 0 u C E U pH CJ o

pH eg E AJ

QC

>s c m

J- 03 13 X a co *H C > « 13 o cc

«J cc c

pH pH P -rH • pH (S a> 13 J-i nh aj AJ c a E O m eg x . y E pH E

AJ c

eo 13 rH

m z

>H o E w

co

y -a

C 13 ' 3 eg u a

GC 13 3 -H C 0 60 '13 'pH -H O pH p a y c U 13

H

X >H

C

H E H ca >> V eg 3H O X C 2 (C *0 H H o U 41 H —<

•o -H D. eg *J o v- o • c

pH CJ pH 13

W c E CJ C 13

fc > (5

U 4) o cj cm u u h -c u

eg c y cn aj eg > O U H « > 0 m -o c

0)

—< f
' *

« )
M**c!innlcnl

Prooertlon !iil

Our O y *j

»-< o y 4J

n ~ O 3 O u »-< cd o *-» a

«o

O « y

u >—< c *+4 o - —< y O *J CL 3 > .3

ccxcri-.Tr

4) uo

y vi

ya w_

C

C CD CD

O

VO *0

C 3 e a > -o >-4 c 5^' 0 u. o o *4-. u y 4) -x o o

-c - u o »« a cl — o = c~

00 Dn f— t»3

x

y

®

x c u. 3 3 E t u- y y o

c. o E m

y

2:

O

— o c

V- CM .0 c y

lt « ^ o <m y c > — t H w o — *-> sl a a y ac

*. 3 ~o o uu l.

w

y c cdry o C L- C C- 2: y o >> xcmtw- y >>•»-* v- c u x u «-> 3 y rj ct 3 E w <5 C <9

c.a ie t

c- o E 3 l- > O y ^ £1 a- «

34

Table 3

Thermal Properties

Alloy Thermal Conductivity Mean Coefficient of

“ watt . cm/

cm^ °C

BTU. in .

/

ft^.hr. °F

Thermal Expansion

x10 6 /°F

Haynes 25 . 204 (600° C) 142 (1100°F) / 8. 2 (70-1200°F)

\9 . 4 (70-1800° F) •

% 304 S.S. 149 (68-932° F)

' 11.0(68-1600°F)

310 S.S. (130(68-932°F)

\ 99 (212°F)

10. 9 (68-1832°F)

316 S.S. 113(212°F) 10.8 (68-1832°F)

446 S.S. 171 (68-932° F) 6 . 5 (68-1472°F)

U500 135 (1100° F

)* 8 . 2 (70-1200° F)

INCO CLAD 671/800H

Inconel 671 109(70°F) 7 . 99 (78-1200° F)

Incoloy 800H 146 (1100° F) 9 . 6(70-1200° F

)

Inconel 617 • 155 (1100° F) 8.0(78-1200°F)

Incoloy 800 146 (1100° F) 9 . 6 (70-1200° F)

Haynes 188 . 210 (593° C) 146(1100° F) 8 . 6 (70-1200° F)

Hastelloy X . 206(600 °C) 144 (1100° F) 8 . 6 (79-1200° F)

Inconel X-750 138 (1100° F)* 8. 5 (70-1200° F)

Incoloy 802 142 (1100° F) 9 . 5 (70-1200° F)

*Estimated from graph

35

COLD

DRAW

TUBING.

FKL.

ANN.

RDM.

10,000

If

QUANTITIES

a Tt o < r - • «M CO VO O b U cm *—* CM O u. • • • • O co vO O' r» CO • • o C7> o NO <o

V*

g ri O < - r • VO CO CO »—c o o u <o cm o iO O wn u. • • • • O fO • • </> O c cm

€-1 oo < r r • n o CO O vo AJ cm O <T o f'W u • • • * O r~* CO H IO <V • • </> co cm H OO t5 o < r r • CO CO o o iO u o co r““i O U. • • « • 10 rn co CTN • • </> r^- co VO r>j NO ^ o < r - • •—<

CO o o o b aj CN cw o m U- • • • • tn <nj vo CO CD crv • • </> m vD co 7—< N • a ^ o < - - • CTn O O o O vo aj On MT o O oj u. • • • • VO r—i o • • </> <o CO CM H

CMa r? c < - - • *-M r-M o O to AJ CO <T o O u • • • • O co \ CTN On CM r—< • • o vo VO <T n r? • o <r • r*^ r—J lO co b o aj co o <r o O 10 u • • • * O N CM o ' o VO • • </> m JO r~4 D ts O < - r • VO fO CM CO O b aj CM o lO H O CM u • • • • O ^ kO o co • • </> cm CM r-4 oo

CO

r^» CM o C o M O o

r-* NO CO CO H< 9 >- >*

•JO

PJ S O

-J o o o o V <~> o

/- -J1 —1 •“* -

Table 5

Some Representative Construction Alloys and their Reported Resistance

' + to Oxidation In Air

Resistant to Oxidation 4. 4. Alloy Major Constituents up to, °F

Plain carbon steels 800 (1)

Low alloy steel 875 (1)

Type 406 S.S. 13 Cr, Fe-base 1400 (2)

" 446 " 26 Cr, Fe-base 1950 (3)

" 304 " 18 Cr, 8 Ni, Fe-base 1550

(A)

" 316 "*

17 Cr, 12 Ni, Fe-base 1500 (4)

" 310 .. 25 Cr, 20 Ni, Fe-base 1900 (5)

Haynes 25 20 Cr, 10 Ni, 15W, Co-base 1900

j 2000

Hastelloy X 22 Cr, 19 Fe, Ni-base

| 1650 (6)

Haynes 188 22 Cr, 22 Ni, Co-base

/ 2100

Haynes 556*** 22 Cr, 20 Ni, 20 Co, Fe-base 2000

Inconel 600 15.5 Cr, 8 Fe, Ni-base 2150

Inconel 601** 23 Cr, 14 Fe, Ni-base 2300

Inconel X750* 16 Cr, 7 Fe, Ni-base 2000

(formerly designated:

Inconel X) -

Inconel 718* 48 Cr, 18 Fe, Ni-base 1800

Incoloy 800A 21 Cr, 32.5 Ni, Fe-base 1150 (?)

Incoloy 617 22 Cr, 12.5 Co, Ni-base 2300

(?)

Incoloy 802 21 Cr, 32.5 Ni, Fe-base 1500

(?)

Incoloy 800 21 Cr, 32.5 Ni, Fe-base 1600 (?)

37

Notes pertaining to Table 5

t The temperatures listed , unless otherwise noted are generally

those stated as such by the producer. However, these values

should only be taken as a guide. The detailed oxidation

behavior can vary significantly.

ft Other constituents sometimes total 10% or more and play an important role in oxidation resistance and strength.

1. Generally recommended maximum in steam boiler application.

Specific temperature varies with the type steel.

2 . Scaling temperature in continuous service; 50°F higher in

intermittent service.

3 . Scaling temperature in continuous service; 100°F higher in

intermittent service.

A. Scaling temperature in intermittant service; 100°F higher in

continuous service.

5 . Scaling temperature in intermittant service; 150°F higher in

continuous service.

6. Allowed by the ASME Boiler and Pressure Vessel Code.

for

7 . Recommended by producer/boiler tube type applications.

* Highly resistant to chloride ion-induced stress corrosion.

** Good resistance to sulfidizing atmospheres.

*** Developmental alloy.

38

WATER

FLOW 0) 4=4-* M4) 4-1

<4-1 u

41

toC

ai "Vcoo

X)

O'4) (0 JC

4J •H

CO

<y

i-i 3

4J

toM4) a.£4) 4Joa0) 4-> to

4-1 TO(X

y-1 ou

43OP

to4) TO4) PS

60

Pn 2O . cQh

tf)

tn

i-«DHIT)

39
<
SPINELS FOR Ml ID-ELECTRODES

H. P. R. Frederiksc, W. R. Hosier, A. J. Armstrong and T. Negas

National Bureau Of Standards

Washington, D. C. 20234

Abstract

Solid solutions of Kg-chromite and Mg-aluminate

spinels and nagretite appear to be promising materials for MHD electrodes. This paper presents high

temperature electrical conductivity data and discusses some structural and chemical properties of

these compounds. The Mg-Al-spinels show good resistance to seed corrosion, while the Mg-Cr-spmels

appear to stand-up well in a slagging environment.

A third spinel, composed of and Co-,0_, , is a very good electrical conductor and has potential as

a lead-out material.

I. Introduction

Recent tests of several promising preheater

materials conducted by the FluiDyne Engineering

Corporation have indicated that Xg-Cr-and Mg-Alspinels can withstand the rigors of an MUD environment remarkably well . ^ In these tests the materials were subjected to a slag and seed environment.

Two runs of about 100 hours were made during which

the sample temperature was cycled periodically

reaching a maximum of 1300 K. These results have

further encouraged us to explore the possible use

of spinels as the basic materials for MHD-electrodes. The question is: can oxide additives be

found which change the usually high resistance Mgspinels into reasonably good conducting semiconductors? Fortunately, this can be achieved rather

easily by mixing the insulating spinels with good

conducting magnetite (Fe 2 04

, which also has the

spinel structure) to form single phase solid solutions.

Spinels are cubic solids with 24 cation sites,-

8 tetrahedrally surrounded by oxycen ions, and 16 octahedrally coordinated. Hence, the general chemical formula is When all the trivalent

ions are on the B-site, the spinel is called ''nor- mal”; on the other hand when equal numbers of tnand divalent ions occupv the B position one speaks

of an "inverse" spinel.

In this paper we will focus our attention on

KgCr20

4 and MgAl-,0 , spinels, containing up to 30 mole percent Fsj04 in solid solution. The distribution of the cations over the tetrahedral (A) sites and octahedral (B) sites was first discussed

by Verwey and his coworkers . 3 < 4 Mq (Cr ) -,0, and

Hg(Al)-»D4 are normal spinels due to the rather

strong preference of Al 3 + and specifically Cr J+ for

the octahedral B sites. On the other hand, MgFe->04 and Fe^04 are inverse spinels, because Fe 3 + favors

the tetrahedral site more than most other ions.

Hence, the chemical formulas for the latter two compounds can be written as follows:

Fe 3+ (Mg 2+ Fe 3

'r )04 and F

e

3 ’’ (Fe 2+ Fe 3

‘,')0

4

. The large

conductivity of magnetite is easily explained; it stems from the electronic exchange between ferrous

and ferri-ions on equivalent (octahedral) lattice

sites. Using the same reasoning one can understand

why pure MgCr-,04 is an insulator. Verwey et al . have shown that the conductivity of the mixed

Crystals increases rapidly with increasing iron Content. 3 However, iron decreases the melting

point of these compounds: MgCr,0 melts at 2400 °C,

a 4

MqAl

20

4 at 2110 °C , while magnetite (Fe^0

4

) has its

melting point at 1591 °C, hercynite (FeAl 204 ) at 1730 °C and HgFe

20

4 at 1717 °C. 6 ' 7 The phase dia- grams of the systems Mq0 -Cr 203 -Fe 203 and Mgo-Al^03 - Fe 2 03 (Figures 1 and 2) indicate that the spmei

fields are quite extensive in both cases. 3 However,

the melting temDeratures are low ar.d hence ccmixssi- tions rich in FeOx should be avoided. Iron also

tends to raise the volatility and the chemical reactivity. All these chemical facts are reasons for using just enough magnetite to produce solid solutions with sufficient electrical ccnduction, and at the same time to retain most of the stability of the higher melting spinels.

Figure 1. The system XgO-Cr C -Fe O at 130C °C in

air. (Ref. 4)

.

Of course, it is by no means obvious that relatively small amounts of iron oxide will lead to a reasonable degree of conductivity. Let us take as an example a mixed spinel consisting of 1/4 magnetite

and 3/4 magnesium chromite. (In terms of hematite

the ratio is 1 Fe 20g to 2 >lgCr-,04

) • The electrical

conductivity and cation distribution of this mixture

has been discussed by Verwey et a_I . - Because of the

strong preference of Fc JT for the tetrahedral position no trivalent iron will be found on the B-sicas

in this concentration range. On the other hand, the Fe^-ions can be located either on the A or the

B-site leading to the following two extreme distributions :

I. (Fe 2+ Mg

2+

) (Fe

2 + Xg

2+ Cr2+ )0

2_

1 1/2 9 l/2 1 1/4 -1/4 3/24

3+ 2+ 2-t- 2+ 3+ 2- H. Fe

1/4

Mg

1/4

) (Mg 1/2

(- r

3 /2

)cV

Only spinel II and di- and trivalent iron on equivalent lattice positions (A-site) and hence would show

appreciable electronic conductions; spinel X will be an insulator. However, the difference in energy

between, those two site arrangements is small and an intermediate distribution is far more likely, espe- cially at high temperatures. Jueh a "half -way"

arrangement may have the composition:

II. 2.1

3+ 2+ 2+ 2+ 2+ 3+ 20

<FC l/2Fc l/8 M<J 3/8

) (Fc

1/8

M<3 3/8 Cr

3/2

)0 ‘

This spinel is expected to be a (semi) conductor

because both Fe J+ and Fe zf are present on cquiva- ’ lent A-sites (although not in equal amounts)

The above reasoning assumes that the spinel is

stoichiometric and fully oxidired. A reducing at- mosphere will give rise to a relatively larger Fe“ + content, which certainly will enhance the electrical conductivity.

MgO

Figure 2. The system MgO-AljOj-Fe^O^ at 14C0 °C. S = broad spinel band, S. = .MgFe20lj ss,

S-> = MgAlpOq ss, H = hematite ss, C = corundum ss, M = MgO, FA = FeAlOj ss. [After W. Kwestroo, J. Inorg. Nucl. Chem.

9, 67 (1959)].

A similar argument car. be used to explain the

electrical conductivity of spinels formed from

magnetite and magnesium, aluminatc. Although the

octahedral site preference energy for Al^+ is con- siderably smaller than that of Cr^ T

, the other ions

Kg2+

, Fe2+ and Fe^+

, strongly favor the tetrahedral

site.^ Hence, one expects that the physical behavior of Mg-Fe-aluminate and Mg-Fe-chromite will be

quite similar, including their respective conductivities. The somewhat higher conductivity

observed in the chromites may indicate that not only the Fe-icns but also the Cr-ions are present

in more than one valence state.

TiK) Figure 3. Electrical conductivities of 5 commercial materials containing MgO, Cr^O , Al^O^ and Fe203 as major constituents

[see Table I. Data on sample B-3226 D (not shown) are nearly identical to those obtained on sample 8-3210 D]

Figure 4. Electrical conductivities of 5 laboratory prepared spinels (see Table I)

II . Conductivity Measurcnents and Results

El ectrical conductivity measurements were made

on a number of Mg-Cr-Fe- and Mg-Al-Fe-spinels in the temperature range between 1000 and 1900 K. The

experimental method has been described

previously.^'1 The oxygen partial pressure was varied between 0.2 and 6 x 10“® atm. Some 10 samples of chromite and aluminate spinels have

been investigated of which 6 were obtained from a commercial source and 4 were prepared in our laboratories. Details concerning composition and

density are presented in Table I. Beside the

chromites and aluminates, the conductivity of one

sample of iron-cobalt spinel was also measured.

Results of these measurements are presented in

Figures 3 and 4.

Of the commercial materials only the sample

labeled ISO-C has an., acceptable level of conductivity (c i 10 -z ohn*ic.n*M and a sufficiently small

temperature dependence. However, a large excess of

in this material is a considerable drawback

because of the high V3pcr pressure of this oxide

and its strong tendency to react with potassium.

Hence, one expects that the ISO-C materials will

deteriorate rather rapidly in a seeded atmosphere

at high temperatures.

The single-phase Mg-Fe-chromite (#6) shows

excellent conductivity .over the entire range between

1000 and 1900 K. This is not too surprising because

the iron oxide content is higher tnan in any of the

other Al- or Cr-spmels tested. It should be noted

II. 2.
>
fable I. Comj<>si t ion of Spinel Samples Studied

MgO Cr 2°3 A1 2°3 F0 2°3 , (Fc 0 ) Co O. SiG_ CaO porev, i ty ( l ) 3 4 3 4 2 \sO-B70 (w/o) 70 11.7 12.0 5.3 12.3

sp. (m/o) 50+ 16.7 26.0 7.3

ft ISO-B50 (w/o) 50 20.1 20.0 8.8 6.3

Sp. (m/o) 50+ 17.7 25.6 7.2

^1SO-B30 (w/o) 30 28.3 28.0 12.7 10.4

sp. (m/o) 50+ 17.2 25.2 7.3

AIF0-C (v/o) 5.8 79.3 3.14 4.8 2-3

sp. (m/o) 50 19.8+ 28.8 10.4 V3201D (w/o) 27.0 38.7 10.4 22.3 0.82 0.27

sp. (m/o) 50+ 25.5 10.

2

5 14.2

B-3226D (w/o) 38 31.7 10.7 18.3 0.96 0.31

6p. (m/o) 50 24.0 12.5 11.5

•Chromite #2- (n/o) 42.1 42,1 15.8

(44.4 44.4 11.1)

•Chromite #6 (m/o) 36.4 36.4 27.2

(40.0 40.0 20.0)

•Aluninate #3 (m/o) 42.1 42.1. 15.8

(44.4 44.

4

D 11.1)

•Aluminate *5 (m/o) 40.0 40.0 20.0

(42.

S’* 42.

8

b 14.3)

‘Fe-Co spinel (m/o) 70.8 29.2

These samples were prepared by the A. P. Green Refractories Company. The chemical composition in weight

percent is given on the 1st line of each sample. These specimens were not single c'-ase. but a mixture

of spinel and excess (cubic ) MgO or excess Cr^0, (samole ISO-C) . The second line gives the composition

of spinels (so.) in nolo pc rcer.t ( + indicates large amc.;:.t3 of excess MgG or Cr_C ) . - These samples were prepared in the Solid State Chenistr'y Section of MBS. All samples were single phase

spinel. The fac t that the :nole percentage of MgO is smaller than 50 indicates that these spinels are partially "inversed"

.

that only above 1500 K. the conductivity is independent of the partial oxygen pressure. Below 1500 K

a core oxidizing atmosphere - expected to prevail

on the anode wall of an MHO channel - will depress

the electrical conductivity.

Conductivities of acceptable magnitude are

observed also in the Mg-Fe-alumir.ate spinels. At

about 1100 K the conductivity drops below 10“-

ohm'^cm-

!. Hence, it is desirable to combine this

spinel with a better conducting material for the lower temperature range.

A prime candidate to r-erferm this function (of

"lead-out”) is a Fe-Co spinel with the composition

70 m/o Fe-jO_,-30 m/'o CojO^ (see Table I). This

particular composition melts aoove 1600 °C and has

the spinel structure over a large temperature interval as shown in Figure 5. The electrical conductivity is high and independent of the oxygen

pressure down to 000 K. At this temperature the material can be brazed to a met3i-alloy lead-out

(e.g. , stainless steel) without much danger of

corrosive decay through oxidation.

Ill . Conclusions

A number of high temperature spinels appear to hold promise as building blocks for {tID-electrodcs.

Above 1200 K (and in some cases even '.'00 K) the electrical conduction in iron-doped Mg-chromite and

Mg-aluminate spinels is sufficiently high to carry

the current generated in MHD-cnanr.e i s . Fe-Cotpinel seems to have all the properties desired for

a good lead-out material. Mg-Al-Fe spinel is recommended for use with clean fuels, because its

reactivity with potassium apparently is very small.

On the other iiund the chromite spinel has shown

good corrosion resistance in a slagging environment.

A slac layer on top of a chromite electrode would

absorb much of the seed, hence lower the potassium

activity and thus diminish the possibility of strong

K^O-CrjO^ reactions.

Fiqure 5. Phase diagram of the system Fe,0 -Co, 0. in air. (Kef. 11) 3^34

II. 2.

3

References

•Supported by Energy Research and Development

Administration.

1. Quarterly Progress Report (to ERDA) , Fluidyne

Eng. Corp. , August 1975.

2 . "Introduction to Ceramics", by W. D. Kingery,

Publ. John Wiley, New York, (1967), p. 115 and

also p. 750.

3 .

4 .

5 .

E. J. K. Verwey and E. L. Heilmann, J. Chem.

Phys. 15, 174 (1947).

G. C. Olmer and W. J. Smothers, J. Am. Ceram.

Soc. , 51, 315 (1963)

.

E. J. W. Verwey, P. W. Kaayman and F. C. Romeyn,

J. Chem. Phys. 15, 1S1 (1947).

6. L. H. Atlas and W. K. Sumida,

41, 150 (1958).

J. Am. Ceram. Soc

7. J. C. Willshee and J. White,

Ceram. Soc. 6C, 541 (1967) . Trans. Brit.

8. "Phase Diagrams for Ceramists

Levin, C. R. Robbins and K. F by Am. Ceram. Soc., Columbus,

", Ed. by E. M.

. McMurdie, (Publ.

Ohio, 1964).

9. Arthur Miller, J. Appl. Phys.

(1959)

.

30 (Sup.) 24S

10. H. P. R. Frederikse and W. R. Ceram. Soc. 56, 418 (1973).

11. Arnulf Muan , in "High Tempera

Vol . 5, Part I, Ed. A. M. Alp

Press, New York, 1970, p. 334

ture Oxides,

er, Publ. Academic

\

II. 2.

4

ANALYSIS OF AVCO MARK VI C CHANNEL

ELECTRODES AFTER LONG DURATION TEST*

W. R. Hosier and T. Negas

Nati onai Bureau of Standards

Washington, D. C. 20234

and

S. W. Petty

Avco Ever ett R esearch Laboratory

Everett, Mass. 02149

L Introduction and Procedures

•

*

~ The recent AVCO 100 hour test* marked a milestone in the U- S- MKD effort to produce energy through MHD. Although a distinct success, problems developed in both the anode and cathode

electrode walls as the test continued. Most important are:

1 . the degradation of the electrical insulation

between the cathode electrode segments

together with the erosion/ corrosion of the associated copper fin on the downstream

side of the insulator gap. Figure la. and,

2 . the considerable erosion/ corrosion of the anode electrode walL

To investigate possible causes for these main

problem areas, sections were removed from, the anode, cathode and sidewall anode and cathode in the upstream portion of the channel where the construction was of the Inconel button tvpe. Figure lb. Corresponding sections also were removed from

the channel rrudsection consisting of rail-type electrodes. Figure la. These sections, containing

two or more adjacent segments, were examined in detail using optical and scanning electron microscopy (SEM) in conjunction with chemical and Xray diffraction analyses. After appropriate samples were taken for cherrucai and X-ray diffraction

analyses, the electrode sections were potted in epoxy resin and subsequently sawed to yield cross

sections which expose significant areas for optical

and SEM examination. Figure 2 shows the dis- sected electrode sections. Unfortunately, while

the slag on the walls remained nearly intact after cool down of the channel (especially on the cathode),

during the section removal process much of the slag cracked loose from the metal surtace (primarily on the anode). An effort was made to pre- serve the slag in the same general area above the metal since this interface is a critical area for analysis. Table I lists semiquantitative analyses

for several slag specimens from various portions

of the MHD system together with the dominant

components of the original fly-ash.

II. Results and Discussion

A. Cathode Wall

The problem concerning the degradation of the electrical insulation at the cathode wall is ex- tremely important because the result is a snort in

the axial. Hall voltage. In turn, this causes electrical stress (overloads) on adjacent electrodes

*This research was supported by ERDA.

which then are subjected to accelerated degradation

leading to premature failure of the generator.

Figure 3' shows a cathode rail wall cross-section and the particular area where insulation degraded. The copper cooling fin is brazed onto the downstream electrode from the insulator gao. Initially, this fin had a rectangular cross section so

the copper has eroded/corroded during the test. We believe that the shorting of the insulating gao

is caused by one or a combination of several factors. Originally, the area just upstream and

downstream from the rail cf each electrode was

filled with castable zirconia (CaO stabilized) and

alumina castable cements, respectively. A strik- ing feature of every section taken is that the alu- mina cement has been altered and nearly completely replaced by slag saturated with seed. This

phenomena is associated with the zone wherein

shorting occurs. The narrower, adjacent (upstream) gap filled with zirconia castable remains

Drastically unaffected and is relatively free of seed

deposits.

As cold coal slag of the type used at AVCO is a relatively good insulator, the shorting process

must be due to other factors. Figure 4, a schematic drawing of the cathode electrode rail assembly, illustrates the area where insulation has degraded. The entire areas, a. b, and c contain

potassium as evident from X-ray diffraction and

EDX data. Area c, however, has a significantly

higher potassium content. Indeed, when exposed

to ambient conditions (after cool down), hydration

in this area and in area b commences and a solution oozes from the surface. This observation as well as x-ray diffraction data, indicate that zone c and, at least, the lower portion of zone b, are sat- urated with condensed K7 CO3

. (Fig. 5). X-ray

diffraction patterns of "slag" specimens from

zones within areas in Figure 4 further demonstrate

that there is a potassium concentration gradient

(differential) within the slag at the cathode in contrast to slag at the anode. Zone (a) is glassy and

has no crystalline phases containing potassium,

although some potassium, presumably in solution,

is present. Intermediate area (b) contains crystalline, orthorhombic KAlSiO^ as the predominant

phase. However, toward the base of area (b), tetragonal and cubic KiO-aiuminosiiicates . crystalline phases richer in K-,0 than KAlSiO,,

clearly are present. These materials have been

characterized previously by Cook e_t a_l . *• at NBS.

Area (c) represents the zone saturated with po- tassium so indicated above. The differentiation

of potassium at the cathode conceivably could be

attributed to temperature gradients. However,

1

Table 1. Semi-quantitative spcctrochemical analysis of bulk slap materials

from several locations in the generator system. Only elements

significant to the experiment are listed (wt %).

Burner

Upstream

Cathode

Upstream

Anode

Cathode

Exit

Anode

Exit Diffuser

Original

Fly- Ash

A1 >13 >13 >13 >13 >13 >13 13. 5*

Si >13 >13 >13 >13 >13 >13 16. 7* K >13 >13 > 13 >13 >13

Ca 1. 5 2 1 1. 5 1 1 4. 08

Cr . 05 A O <. 05 0. 15 0. 15 . 05

Ni . 05 0.

2

0. 2 0. 5 0. 2 0. 2 Fe >13 >13 5-10 5-10 5-10 >13 1 3 Cu . 15 3 1. 5 0. 05 1. 5 . 05

*Wet Chemical Analysis

anode slags, subjected to similar temperature

gradients, are completely different. Crystalline

KAlSi04 is distributed throughout the anode slag

but phases richer in K^O and areas comparable to Figure 4 c, do not exist. These data point to one

of the possible causes for the shorting at the cathode. As area (c), and probably part of (b). Figure

4, are saturated with potassium, it is possible that ionic conductors consisting of condensed t^CO-^/

KOH solutions could short-out adjacent electrodes. These solutions can exist at temperatures below

1300° to about 370°C. It should be recognized that I^O-rich solutions can be stable and coexist with

crystalline slag components saturated with K2O.

X-ray diffraction and magnetic susceptibility

measurements of "slag" extracted from various areas in Figure 4 . reveal that iron also differentiates at the cathode. Area (a) contains magnetite,

Fe^O^, and bulk magnetic susceptibility data indi- cate ~15% FejOq. It is unclear, however, whether

Fe 3 °4 is primary or is a product which crystallized during the cool-down process. The region

between areas (a) and (b) contain crystalline

KAlSiC>4 (see above) and, as susceptibilitv data in- dicate, ~ 33 % FejO^p Anode slag yields magnetic

data corresponding to ~ 3 % FejC^ while glassy

slag from the combustor yields ~ 1 % FejC^. These

percentages represent lowe

r

limits for the iron

content calculated by comparing magnetic data for pure Fe3C>4 with that from the slag specimen. Iron

not contributing to the magnetism is excluded. The

relative percentage differences for iron are significant and indicate that on the cathode highly conductive. crystalline Fe3<D4 concentrates between

(a) and (b). Figure 4 . This differentiation easily

could contribute to interelec trode shorting. Concentration of iron may result from crystallization

and phase separation in a steep temperature gradient. These concentration gradients also may be influenced by electrochemical reactions involving

either transport of or oxidation/ reduction of iron

cations.

Another manifestation of the differentiation of iron is suggested by the composition of the

• lag at anode and cathode surfaces. Directly above

cathode Inconel buttons. Inconel rails and copper

fins, the slag surface consists of a thin (< 0. 25 mm), red-brown crust X- ray diffrac tion data reveal that, hematite, Fe^C^, is the dominant

crystalline phase associated with this zone which

presumably was subjected to the highest temperatures. Below this zone, and in areas between

metal components, reduced iron oxide, Fe,0^

dominates (e. g. , area (a). Figure 4 ). It might be supposed that the upper surface of the slap, at the cathode oxidized during cool-down. This hypothesis is negated by the absence of the Fe20i-ricn

crust at the surface of the slag between raiis and,

indeed, its absence at the surface of most anode

slag (see below). Thus, a contradiction is apparent. Given a fixed oxygen activity (e. g. , lO'^atm)

and a temperature gradient, reduced Fe 3O4 and

oxidized Fe2C>3 (primary or secondary in origin)

should exist within the high and low temperature

regions of a slag, respectively. We observe precisely the opposite type of differentiation. We

speculate that electrochemical reactions and current transport processes w'ithin the molten slag

may control the oxidation state of iron. Anode sla on the other hand contains Fe2C>3 in regions well

below the surface. However, the slag at the sidewall, rail anode, which has not degraded totally

(see below) does contain a thick crust (> 0 . 2 5 mm)

containing Fe2C>3. This crust occurs above and

between metallic components.

Just below the plasma/slag interface and be- tween the copper fin of the downstream electrode

and the rail of the upstream electrode, region )a), Figure 4 , the molten (glassy) slag layer thickens

and lies in a trough along the entire rail. This

periodic contour is obvious in the full cathode wall

photographs taken of the disassembled channel. We

believe that these areas contribute to the observed

shorts for several reasons. Mo Ite

n

slag containing iron is a good conductor. Generally, this is a projected problem in any coal-fired MHD channel

because the conductivity provides a path for current leakage along the wall. However, if the molten slag layer remains thin above the insulator

the effective axial resistance is high. Thickening

of the molten slag layer at this region (a). Figure

4 is precisely where it is not desired.

As evident from Figure 3 , the cathode copper

fin is eroded on the upstream side. This fin is at approximately 20 volts positive potential with re- spect to the upstream adjacent electrode. X-ray

diffraction and EDX data peveal concentrations of copper upstream from the fin possibly resulting

2

from corrosion of the copper via electrolytic processes. In effect, the area in Figure 4 may represent a miniature electrochemical cell containing

dissimilar electrodes, copper and Inconel. The

copper at positive potential could degrade ticcordto, Cu -*Cu| + e . At negative potential,

processes including, K + e — K. Fe + e —

Fe^ +

, and Ca 1' + + 2e -* Ca could balance the reaction. We note, further, that copper degrades precisely within the area where inter eiec trode shorting

occurs. A second possibility is that the Faraday current in the form of arc discharges is concentrated

on the upstream edge of the cathodes by the JxB

force on the arcs. This hypothesis is corroborated

by: a) the unmarked condition of the inconel buttons in the cathode design of Figure lb indicating an absence of current activity and b) the unmarked

condition of the downstream side of all cathodes

which one would expect should show arc tracks or copper deposition if interelectrode arcs or electrochemical activity were occurring.

An interesting observation that may be of significance to very long term operation of a generator

and which may relate to the discussion above was made from an analysis of the cathode rails by

SEM. Figure 6 is a SEM micrograph montage of a typical cathode rail electrode. Although this wail

withstood the test very well compared to the anode, some corrosion is evident both at the slag-metal

interface and the undercut rail surfaces. The In- conel shows trans-granular cracks which are

penetrated by calcium and, more deeply, by no - tassium. Chromium from the Inconel is associated

with these penetrations. The SEM micrograph,

Figure 7 a, of a typical zone and the pertinent X-ray

maps for Ca, Figure 7b, and for Cr, Figure 7c. demonstrate this phenomenon. Nickel is not present within areas of alteration and, indeed, at the underside of the rails, alteration has proceeded

under the original nickel plate. The chemistry of the Inconel alloy at the slag interface is altered by

potassium and calcium which preferentially react with chromium. These reactions alter the Inconel

from a non-magnetic alloy to a ferromaenetic material. Potassium and calcium as metailic species

in the alloy are unlikely and probably exist as K2CO3/KOH and CaO/ Ca(OH)? phases or are associated with chromium oxide compounds such as I^CrO^ and CaCrO^. Significantly, corresponding Ni-containing compounds do not exist.

Although tne cathode rail wall degrades via trans - granula r cracking and penetration of calci- um and potassium, we cannot define, unequivocally,

the processes or mechanisms involved. The fact

that the rail degradation occurred rather uniformly

on the underside as well as the top surface would seem to rule out a significant correlation with the main Faraday current which appeared to favor the upstream copper as it did in the button desiiin. The significant difference in the inconel operating

temperatures (rail =<2000o F vs button ^1000°F)

may account for the fact that t.de rails sustained

potassium/ calcium attack while the buttons did

not. It is suspected, however, that this attack is electrochemical in nature and could, therefore,

have some tie-in with the main current transport

mechanism Reactions could also be enhanced by

localized heating from micro-arc discharge

through a thin, cold slag layer near the metal.

The top and downstream side of the copper fin are

not eroded/ corroded suggesting that substantial current was not present in this area for the rail wail design.

Figure 8 shows a cathode button wall cross

section. Here, again, conditions similar to the cathode rail wall prevail and result in interelectrode degradation. In this case, however, it is clearly evident that most of the Faraday current was carried by the copper. SEM micrographs

show little damage to the Inconel buttons in contrast to the rails but the copper is highly eroded

both on the top and the upstream edge where the current erosion is similar to that observed in the rail electrode. Figure 8 demonstrates particularly well, the thickening of the molten slag layer

in the critical area above the insulator. The rectangular dark area is the Inconel button. The

other dark-area to the left is the molten (glassy

when cooled quickly) slag area. The erosion/ corrosion of the Inconel rails on

the cathode wall is not extensive for this length of

test. Current transfer through the thin, cold, coal

slag layer just above the Inconel is assumed to be primarily by arc column transport with possibly

smaii components of electronic conductivity associated with Fe^ T/Fe + ratios, and ionic conductivity associated with cations and oxygen. As an

arc column is propagated through the dielectric layer and continued through the slag/ metal boundary, material around the arc column may melt and

vaporize filling the column with ionized gases. The

arc root at the cathode metal (Inconei) could melt

a local area. Since the electrode is negative with

respect to the plasma stream, metal 10 ns in the vapor would be driven toward the cathode thus minimizing erosion through vaporization. At the same time, if the oxygen activity is low at the cathode metal/ slag interface rapid oxidation of the molten metal components would be minimized.

Any oxygen ion transport through the slag layer is away from the metal cathode. This arc or spark

erosion process parallels closely that observed in the well-known process of elec tro -discharge machining. On the cathode, the polarities are such

that the arc is not favorable for metal removal.

However, surface cracking has been observed-5 in experimental electro -discharge machining and

this might be the source of the transgranular at- tack shown in Figure 7a. The suggested current

transport processes through the slag layer are

speculative. Arc "tracks" through the cathode

slag could not be detected.

B. Anode Wall

Erosion/corrosion of the anode metals is very

severe. Figure 9 shows a cross section of a rail anode wail which was eroded almost entirely.

Similar to the cathode, the anode is completely

covered with slag during operation of the generator and it may be assumed that the current transfer through the cold slag layer near the metal

surface is. again, primarily via arc columns.

The arcs continue to the slag/ metal boundary and

may provide the source for localized, intensive

heating which contributes to melting and vaporization of the metal surface. In contrast to the cath- ode, ionized metal vapor would be repelled from

the anode (positive with respect to the plasma) and

transported by the arc column. The activity of oxygen may be high at the anode as a result of

3

• everal factors. Oxygen ions may be transported

through the molten and solidified but porous slag.

The concentration of oxygen or hydroxyl ions also

could be enhanced at the anode by the electric field. Oxygen gas would be evolved if these species were

discharged at the anode metai/slag interface. Metals, therefore, could be subjected to destructive oxidation accelerated by localized heating and

melting due to micro-arcing.

Another mechanism for anode corrosion is in- dicated by the SEM micrographs shown in Figures

10 a and b. On the rail wall, the Inconel is eroded

away (Figure 9) so that erosion mechanisms cannot be assessed. On the anode rail sidewall, however, rails remain and it is evident that their shape is similar to the original but reduced in size. Erosion appears to occur not only on the rail top but around the rounded edges and toward the neck. The SEM micrographs indicate that there are essentially three zones near the slag interface. The

innermost zone is InconeL The intermediate zone

is vessicular and consists of Inconel ridges (lighter areas. Figure 11a) and irregular cavities rich in chromium and iron. This suggests that the nickel

is oxidizing preferentially. The outer zone (Figure

lib) has a granular microstructure and consists of nickel, chromium and iron. We suggest that this area represents an oxidized scale which ultimately

spalls and dissolves in the slag. The mechanism

for this degradation must be related to active oxidation of the metal by chemical reaction with oxy- gen and/or to electrochemical corrosion reactions.

It is unclear, however, whether the corrosion observed for the sidewall anode rails is the dominant

process leading to destruction of the anode wall,

in general. If destructive oxidation dominates,

one would expect to find metal oxide comnonents of Inconel in the slag adiacent to the anode electrodes. We searched for these components in the anode

slag but could not find significant amounts. However, if the slag is exchanged frequently, metal

components would not be present in the newly

formed slag. As the totai mass ratio of metal re- moved to coal slag fed to the channel during the 100 hour run is small, a dilution effect is operative. Previous analysis of AVCO cathode and

anode slag under different conditions did show 1 to 5 wt% of nickel in localized spots in the anode

slag.

Our interpretation of the erosion processes at the anode are based to a large extent on speculation. It is difficult to elucidate possible mechanisms without further investigation of the anode

electrodes while an experiment is in progress using instrumentation aimed directiy at the problem.

There is considerable evidence, however,

from operation with previous Mark VI channels

that the recession rate of anode metal is very temperature dependent and. in fact, slag covered in- conel anodes operating at less than 100CFF metal

temperature showed negligible erosion alter 110 cumulative hours of operation in the VI B channel.

11L Conclusions

Several conclusions are inferred from the results of this investigation. It may be beneficial

to design an electrode system for a slagging generator in a configuration such that the axial temperature along the wall is nearly constant. Single

metal alloy construction would be the simplest

and least susceptible to electrochemical effects in the interelectrode gap. High density ceramic in- sulation, having good thermal conductivity should

be used to prevent seed penetration, erosion/

corrosion by plasma and slag flow, and to avoid

large temperature gradients in the axial direction. Chrorruum-contaimng metal alloys for the cathode

may not survive extensive attack by potassium and

calcium during long term generation of power. At

the anode, the selection of a metal depends on what

degr adationai processes dominate. Erosion via micro-arcing combined with destructive oxidation

require the use of a metal with a high melting

point, good thermal conductivity and high resist- ance to oxidation. For extended anode durations,

it may be necessary to utilize a conducting oxide

which is compatible with coal slag. It seems evi- dent that optimal designs for anodes and cathodes

wili be considerably different and the engineering

and materials selection process must recognize

this fact _

In this paper, we illustrate several erosion/

corrosion processes and demonstrate some re- markable differences in the chemistry of slag at anode and cathode wails. Mechanisms to explain

these observations are complex and multiple in nature and need to be "unraveled" by thorough

research. ExDerimentation using te st- rigs , high

temperature electrochemical ceils, etc. could

provide invaluable data concerning the nature of the current and mass transport processes occurring at every interface within a cathode and anode

waiL Without these data, selection of materials

and wail design is subjective and wed to the trial- and-error process.

References

1. Petty, S. , Soibes, A. , Enos, G. and Duntcn,

A. , "Progress on the Mark VI Long- Duration

MHD Generator. " 15th Symposium on Engineering Aspects of MHD, Philadelphia, PA,

May 1976.

2. Cook, L. , Roth, R. S. , Parker, H. S. and

Negas, T. , to be published in American

Mineraiogis t.

3. Williams, R. O. , "Elements of ElectroDischarge Machining, Technical Activities

Seminar, Sept. 30-Oct. 10, 1957, Cincinnati, Ohio.

4
'
Fig. .3 Cathode rail wall cross section.

Fig. la Ra il wall design. Plasma flow from left

to right for cathode, right to left for anode.

.a - w. .-or. RUBBER BOOT

Fig. 4 Schematic diagram of cathode area between rail wall and copper fin.

a. molten slag, b. solidified siag,

c. high concentrations of K and Ca.

Fig. lb Button wall design. Plasma flow from

left to right for cathode, right to left for anode.

FigFig 2 Sliced electrode sections.

5

5 SEM map of;K in area c of Fig. 4. (30X).

Fig. 6 SEM montage of cathode rail. (30X).

Fig. 7a SEM micrograph of damage to cathode

Inconel rail. (150X).

Fig. 7b SEM map for Ca of area in Fig. 7a. The white dots indicate Ca concentrations.

Fig. SEM map for Cr of area in Fig. 7a. The white dots indicate Cr concentrations.

Fig. 8 Cathode button wall cross section. Rectangular dark area is Inconel

button. Dark area to left of Inconel

is glassy slag.

Fig. 9 Anode rail wall.cross section. Inconel rail nearly eroded away.

6

'i *

^ ' ' <k v,y , /3

*w \

p^rr

**&*$ 3*“

->. ^ >

TV »T^*.”’'''%

< t, ^ *'„£

./ t

*u .', .?

' fc. '£ "*

< •£*

±i*m

Fig. 10a SEM micrograph of anode sidewall

rail. Slice taken near channel

corner. (30X).

Fig. lla_ SEM micrograph of intermediate zone of Fig. 10b. (3G00X).

Fig. 10b SEM micrograph of metal-slag Fig. 11b SEM micrograph of outer zone of boundary of Fig. 10a showing Fig. 10b. (3000X).

three general areas. (750X)

\

7

THE SYSTEM I^O-Al^-SiC^ PART 1. CRYSTAL CHEMISTRY OF PHASES ON THE

.

KA1S10.-KA10. JOIN

4 2 L. P. Cook, R. S. Roth, II. S. Parker, and T. Negas

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

Orthorhombic KAlSiO^ synthesized at 950°C (a=9 . 057 (2)A, b=15 . 642 (2)A

,

>C=8.582(2)A transforms above 1485°C to another orthorhombic phase having

a larger unit cell (a=18 .110(3)A, b=15 .600(3)A, c_=8.560(2)A. Nonstoichiometry

of the high temperature orthorhombic phase is suggested by analysis of

x-ray powder diffraction data. Evidence is presented for the reversibility

of this sluggish transition. An excess of KAIO^ appears to increase the

rate of the reverse reaction to produce orthorhombic KAlSiO^ with falling

temperature

.

A body-centered J tetragonal tridymite-like J phase K, , Al., , Si., 0. with

1+x 1+x 1-x 4 x < 0.1 and unit cell dimensions a=8 . 943 (1)A , c=5.221(l)A has been found

to be stable at the expense of orthorhombic KAlSiO^ and cubic KAlO^-silica

solid solution in the range 1400-1600°C.

Experiments suggest that the compound previously reported to be

K„Al_SiO, is really a f.c.c. solid solution Al, Si 0„ with x = 0.25.

2 2 6 1-x 1-x x 2 This compound is the limiting end-member of the cristcbalite-like solid

solution series at 1600°C.

I

CRYSTALLIZATION AND VAPORIZATION STUDIES ON SYNTHETIC COAL SLAG COMPOSITIONS

{

I L. P. Cook, E.R. Plante, T. Negas , R.S. Roth, and C.D. Olson

National Bureau of Standards

Washington, D.C. 20234

Abstract

Insight as to the chemical behavior of coal

Islags accumulating on the wails of MHD systems can be gained by examining the results of high temperature experiments on K 2 0-Ca0-Al 2 C> 3 -Si0 2 mixtures.

;An increase in CaO content may increase a _

q

appreciably for certain compositions. Potassium

aluminate - silica solid solutions, some of which

are relatively water soluble, occur over a significant range of silica-poor and Ca-rich compositions

at 1400°C

.

Two synthetic slags, with compositions falling

close to an observed distribution of coal ash

analyses, have been studied with the mass spectrometer using the Knudsen effusion technique. One

slag having initially 15 wtc

s CaO yielded a higher

K pressure at 10 wt°s K 2 0 than a K 2 0-Si0 2 solution

containing 30 wtT K 2 0. A second slag with relatively high Si0 2 and AI 2 O 3 and 15 wt% K 2 0 gave a

IK pressure above that of a K 2 0-Si0 2 solution with

;20 wt% k2 o.

i

I

I I. Introduction

i ! The interaction of seed, as K 2 0, with slag,

: leads to a number of important problem areas in 'the development of open cycle, coal fired MHD

: electric power generation. These include corrosion

by the slag phase in contact with the electrode-

! insulator structure and seed recovery.

I '

» Significant effort has been made in recent

years in the development of thermodynamic models,

whose function is to predict equilibrium concentrations of K 2 0 in the slag phase as a function of

temperature. Apparently, the consensus is that

such models overestimate the amount of seed cap- tured by the slag . 1

’ 2

I Previous efforts at NSS have involved measurements of the K(g) pressure over K 2 0-Si0 2 solutions and phases in the K 2 0 -A 1 2 03 system.

These data have been used in modeling slag 1 with

the result that the amount of seed absorption pre- dicted by the model is less than that predicted by

an ideal solution model. Thermodynamic data for

two component systems provide useful insights into

the seed-slag interaction problem but its use in

realistic models confronts the thermodynamicist

with a severe challenge. Obviously, the most

serious problem with thermodynamic models for seed,

coal-slag interaction results from our almost

total ignorance of what the effect of each of the

four or five slag components is on the activity

of the K 2 0 component.

II. Chemical Variation in Coal Slag

Inspection of analyses listed in U.S. Bureau Of _Mines_Bulletin_S67'* shows extensive variation

jin coal ash compositions; however, most of this

‘variability is due to fluctuation in Fe and Ca content. Atomic Si/Al ratios are less variable,

as would be expected if these elements originate

largely in the clay fraction of the mineral matter

- for example, Si/Al in illites 5 typically varies

from 1.2 to 1.4. Twenty-eight recalculated coal

ash analyses originally reported by Selvig and

Seaman* and discussed by Selvig and Gibson 4 are shown 'in Figure 1. These analyses, from different

horizons within a bed of Pennsylvania coal, illustrate the sort of variability which can occur. Figure 1 shows a marked concentration at Si/(A1 + Fe

+0

) = 1.5, while exh ibiting a wide

variation in content of CaO + MgO + FeO.

Figure 1. Recalculated analyses of synthetic slags

K512 and K531 (circles with dots) and coal ash from

Pennsylvania coal (dots), shown in relation to compounds in the system Ca0-Ai 2 0 3 -Si0 2 (open

! circles). Dashed line shows Si/ (A1 + Fe'*’

-1 ) = 1.5.

It is desirable to discuss briefly the manner

of treating comoositionally complex coal slags,

for this leads to three or four component models

which serve as a starting point for certain experimental studies. To recalculate the analyses as shown in Figure 1, it has been necessary to make

several assumptions. Si, Al, Fe, Ca and Mg are the

dominant cations; alkalies 3 re present in subordinate amounts of at most a few weight percent and

have been ignored, along with Ti0 2 , P 2 0s, and SO3

(not appreciably soluble in most silicate phases) . All iron in the "natural" slags was reported in the original analyses as Fe 2 03 . Although this

, depends to a large extent on bulk chemistry, a imore realistic approximation to Fe

+3

/Fe"’* would be

‘0.20 at a temperature of 1400-1600°C and an oxygen

partial pressure of 10

~ J atm in a silicate melt.' An assumption of this sort does not involve an appreciable error in representation of bulk

V

chemistry for slags in which Si0 2 , AI 2 O 3 and CaO

•are major constituents. The Fe ^ 3 recalculated in this way was combined with Al 2 C> 3 f assuming ideality

of substitution in the silicate phases present.

MgO and FeO change the important qualitative aspects

of the liquidus and subsolidus relationships little

if substitution for CaO occurs in amounts up to 15 mole % FeO 8 and 5 mole % MgO 3

. For this reason, mole percent CaO + MgO + FeO have been combined

in Figure 1 providing, it is felt, a more realistic

yet simplified representation of slag bulk chemistry

than would the neglect of MgO and FeO. Compositions of synthetic slags (Table 2) used in vaporization studies described below are also plotted

in Figure 1, and it is apparent that they fall

nearly within the distribution of the "natural"

slags.

, Compositions within the system Ca0-Al 2 0s -Si02

provide the most useful starting point as a model

system in the experimental study of equilibrium

relations - to which of course K 2 0 must be added

for MHD simulation. As Figure 1 and most of the

'other analyses given by Seivig and Gibson 4 indicate,

K2 0-free compositions may be approximated as being

made up of some combination of Si0 2 , Ca2Al 2 Si07 , CaAl 2 Si 2 0a, CaSiG 3 and CaaSiO^, as well as, for more CaO-rich compositions, CaO-rich calcium

aluminates.

III. The MHD Environment

The open cycle coal-fired MHD systems cur- jrently under development 15 ' 1 contain several postcombustion components where seed/slag interaction

may occur: generator channel, diffuser, radiant

boiler, and preheater. Within each, different

conditions prevail, which are dependent upon the

specific operation and design parameters of a given installation; the temperatures which are discussed in this section are therefore to be re- garded as a first order approximation. Plasma

passing through the generator, including entrained

slag droplets, will approach the combustion temperature of 2400-2600°C. V.'all temperatures within

! the generator at the plasma/slag interface are

l much lower, near 1300-140C°C . * * The temperature

i of plasma entering a radiant boiler section could

, approach 2100°C, while that leaving might typically

be 1700°C. Preheater entrance and exit temperatures would thus be approximately 1700 and 1500°C

respectively, while plasma entering a seed con- denser at 1500°C would be cooled to 1000°C or below at exit. 1

1

Superimposed upon these MHD temperature

regimes is the partial pressure of potassium, P^,

whichdepends upon the percentage of seed added,

as well as the temperature

.

s Pq

2 similarly shows

a strong dependence upon temperature for a given

equivalence ratio, but it may be expected to be

within the 10’ 2 - !0~ 5 atm range at generator wall

surface temperatures,* 3 subject to modification by

transverse MHD effects. j

I Principal seed/slag interaction will occur upstream of the condenser section. In combustion

systems containing appreciable amounts of sulfur,

some of the seed condenses as kjSOi. and can be recovered by washing the slag and fly ash with

water. The condensation temperature of K 2 SQh in

& combustion system free of slag seeded with

sufficient K, as k 2 SO<., to provide a potential

atomic concentration which is 1 wt% of the combustion products, is estimated to be about 132S°C. 1U At temperatures above 1325°C slag can still dissolve

appreciable amounts of potassium as K 2 0. Condensation of K 2 SOi, in the MHD system should therefore

be viewed as a mechanism of recovering seed which

has not yet reacted with slag.

From the standpoint of significant seed/slag

interaction, two temperature regimes must be con- sidered for each section - that of the plasma,

and that of the wall accumulations. Interactions

within the plasma of the generator channel will

have the dominant effect on material passing

directly through the system due to higher temperatures - given short residence times, it is doubtful if substantial reequilibration will occur further downstream, except possibly in the pre- heater. The wall accumulations, on the other hand,

are characterized by longer residence times, of

length sufficient for appreciable equilibration.

Alkali penetration, especially, is expected to reach equilibrium values in the hotter portions

of this temperature regime (estimated at 1300- 1400 J C) . Material is eventually removed from

walls by run-off or spall-off.**’** Results

from one test facility 1 * suggest that wall accu- mulations may be separated from plasma-entrained

droplets in the seed/slag separator deposits on the basis of particle size. These results indicate

that roughly 50% of the coal ash + seed accumu- lated on walls downstream of the combustor, and

50% passed through the system into the separator

without first being deposited on the wails. Obviously, less K 2 0 will be absorbed by slag carried

directly along with the plasma while the material

deposited on the walls at 1400°C and below has

potential for significant K 2 0 absorption.’ The

quench experiments described below and their discussion have application primarily tc wall slag

accumulations . The potassia vaporization studies

have application to higher temperature plasma/slag

interaction in addition.

IV. Quench Experiments on K 2 0 -Ca0 -Al 2 03 -Si 0 ; Mixtures

Quench experiments were conducted in sealed

platinum capsules at 1300-1400^0 as summarized

in Table 1 celcw. Starting materials were crystalline single-phase compositions weighed out in a dry box, calcined at the appropriate temperature,

and characterized by x-ray diffraction. Products

were maintained at temperature in a Ft-wound

quench furnace, as monitored with an XBS-calibrated

optical pyrometer or a Pt/Pt 1C% Rh thermocouple

calibrated against the melting points of palladium

and gold. Runs were quenched in water, opened.

Tftftl* 1. QuMrfc [ipfrUnii In tfe* Snm l.O-C»b-ai .Oj-SIOj

.

UMpetttlon 'on frmcrian) *c* tartt t am

fcx X-rrr InaPultan

rt UmIU

(UISlOj)

s IWSilO 12 |

IftJI ) • *-*l,Oj ItfftC* | (UUSiOJ jff-CASiOj)

J lmuo 12 1 1 U 1 (4oAlM*t) • into pfcoeeic XAlSiCj

(KAlSi0

4

] jlCftjAljStOj)

j m«tio U art •crNaso l c UI3lO

( • C*jM,SlO,

out 10 » IAlSl-0^ (lauunll • •rtftorfco«elc MlSl04

• |Ul»

(*.*» ,°4 ) l (CftJU

rS10?

I t u crOcrrftcMic (A1S10, > ft^-C.,510^

lUlOjI jiCJU.Sl.Oj)

s imtio 12 srtkn-kaMlC IAI3104

• fUsi

iuuOj) jU-c^siOj)

J JMiH « ,1-1**, 1-«*1»,0

I

• ?-c«^io

4y

(CUOjJ ,(0^1^10.) , i« U S-* lA, i-«

,l

» )0

i • C4 ^

, : si0

t fuiOj) yv-CftjUO^

t \»U19 12 S-« fA, l-«

,,

i

,°2* 1 * 01 *

^ m peerlv rm 1III 1 1 1*4 M

tl*«.

41 f fW« «- r*r |PO-4CT HlltfU, u!U4 IftMlI (|aU« if IM
r

1
'examined for evidences of melting, and analyzed by

iX-ray diffraction. |

I

1

) Examination of results leads to the conclusion

•that KAlSiOi. and Ca 2 SiOi, coexist at 1400°C. Results

further show that Ki_x(Al i-xSix )°2 solid solutions

coexist with Kj +xAli +x Sii_x Oi. and Ca 2 SiOi, and that

KAlSiOi, and KAlSi 2 0s coexist with liquids ir. the

plane KA10 2 -Si0 2 -Ca0.’ Taken together

,

t these

iobservations strongly suggest that the plane KA10 2 - SiOj-CaO is a true ternary system over most of its extent. This contention is supported bv isolated

observations of other workers, as well. 5 For MHD

•applications this is helpful, for it means that

the effect of adding CaO to compositions rich in

KAlSiOi,, a compound shown to be a constituent of MHD fly ash and thought to be responsible for tying

up a large fraction of the unrecoverable k 2 0,

0 can be demonstrated within the model system K 2 0-Ca0-

MIID applications. The importance of slag bulk

ichemistry can be shown in determining: a) melting

properties, b) absorption of K 2 0, c) tendency to form solid compounds which are insoluble (KAlSi0 4 ) or relatively soluble in water (Kj.xCAli.xSixjOi)

as the case may be.

The addition of other components may alter

these effects. FeO should exnand the liquid

region at any given temperature as well as affecting other slag properties such as viscosity

and electrical conductivity.* 7 It is expected

that for a given FeO content, addition of CaO will

still raise ag 2 Q for most bulk compositions. This

must be demonstrated more conclusively than can be

done with the two synthetic slags studied in this paper, however.

V. Valorization of Potassia From Svnthetic Slazs

! Al20s-Si02. Referring to Figure 2, if CaO is progressively added to compositions near KAlSiOi,

,

•three-phase fields A, 3 and C are successively

encountered. If a slight excess of AI2O3 is present, ay

2 o

fi xec* at progressively higher values

within each three phase field. Within the two- phase field Ki_ x (Ali_ xSix )02 - Ca 2 Si 0 u, ay.Q

increases contir.ously from the value of B to that

of the three phase assemblage C.

i

Figure 2. Effect of adding CaO to compositions

rich in KAlSiOi. and near the ternary plane KA10 2 - Si02 -Ca0.

Other experiments in Table 1 demonstrate

the relative stability of Ki

_

x (A1 i_ x Si x )0 2 solid solutions with Ca 2 Al 2 Sii7 and the existence

of a [relatively] low melting region of Ca-rich

liquids between KAlSiOi. and CaSiOi and between

KAlSiOi, and CaA^SiiQ*. These experiments

indicate a strong partitioning of K between

K 2 0-Al 203 -Si 02 solids and CaO-Al : 0 3 -Si0 2 solids

and liquids. Again, this may be significant for

NBS has an active program on characterization

of the viscosity of slags and the effects of

various components therecn. To carry out these

measurements, large sarnies of comlex, homoge- neous, synthetic slags have been prepared.* 3 These

slags are well characterized with respect to composition except for the content of dissolved

gases which they can absorb since the preparation

and measurement is carried out in contact with the

atmospheric environment. Also, the oxidation state

of the iron oxide dissolved in the slag is unknown.

The object of the vaporization measurements

is to measure the activity of K 2 0 in the liquid

slag solution. The primary vaporization process

is ‘

;

1 K^O (dissolved in slag) = 2K(g) + 1/2 0 ^ (g)

The activity of K2 O in the slag phase is defined as

fp

2 1/2 2 1 / 2 ,

1 K 0

2

J slag/L K

l 0

2

j K

2 0(2, pure)

! These data can then serve as a measure of the

activity of K 2 0 in the liquid slag solution. If

' data is available over a range of temperatures

;and compositions, calculation of the K 2 0 concen- tration in the slag phase in equilibrium with

specified K(g) and 0 2 (g) pressures in the plasma

phase is possible. In the present paper, however,

i we will compare only the measured K(g) pressures

i measured over two different synthetic slags with

I the K(g) pressures nreviously measured over \ 2 0 - Si02 solutions at N'BS.

3

j

j Ideally, measurements should be made under

conditions where the slags are entirely liquid

and the amount of :< 2 0 loss should be kept to a minimum so that the sample composition remains

I essentially constant. In practice, seme of the

I Beasurements extended into the region where

| crystallization of some of the slag components

: is possible. Since the slag contains some dissolved gases, sample sizes were kept small (ICO

mg) to avoid having the sample ejected from the

crucible by evolution of gas when the slag becomes

fluid. At higher temperatures, fractional loss of

• K 2 0 from the sample is appreciable and concentration

changes are important. The specific data points

' are therefore primarily of qualitative significance.

1 . . . [^Further details will be submitted for publication in American Mineralogist^

V /

V

Measurements were made using a nodulated beam, mass spectrometer. The slag sample was contained

tin a platinum Knudsen cell having a 0.5 mm diameter

.orifice. Temperatures were read in the orifice' of

the effusion cell with an optical pyrometer through

'a prism, window, and the modulation disk. Temperaitures were corrected for the reflection of light

1 ®t the prism and window and for the 50a transmisision of the modulation disk. Ion currents due to

!nass peaks of interest (primarily K* and 0 2 ) were Measured on a lock-in amplifier. Ions were generated by electron bombardment of neutrals at 20 volts. The modulated beam system measures only

that signal in phase with the reference signal

which is generated by a light and photodiode on either side of the modulation disk.

I

i The pressure of K(g) was determined from the

relationship

P = k • I + r »T K

K K ‘where ky is an instrument constant for K of mass

j39, I y is the ion current and T is the absolute

temperature. The value of k y was evaluated from

the vaporization of a known amount of k 2 0 from

the slag solution at known temperature and periods

of time. The assumptions required in order that

ithis calibration procedure be valid are that the

1 only significant vaporization products from the

'slag phase are k(g) and 02(g) having a formal

'stoichiometry of k 2 0 and that this accounts for

;all of the observed weight loss. The ratio of Wpo2 over the slag solutions was compared with

that obtained from similar mass spectrcmetric

‘measurements performed in this laboratory on

• KAlSiO^s) and ;<~C-Si0 2 solutions. In contrast,

the ratio for the slag solutions was less than

expected below 120O°C and greater than expected

above 1200°C. During most of the measurements the

; 0a pressure was about twice the value expected for

a solution in which the vaporization of K2O control-

. led the O2 pressure. It is presumed that "FesCU"

in the slag is responsible for this behavior. No attempt was made to account for the excessive O2

i pressure in making the calibrations. The potassium

pressure is dependent on the oxygen pressure to

| only the 1/4 power so doubling the O2 pressure

j would decrease the K pressure only by 20s . VaporI ization of species from the slag such as Fe(g) and

SiO(g) were negligible throughout the temperature

range of these studies. i

I

I Data have been obtained on two synthetic slags

having the initial compositions shown in Table 2. The choice of slag K512 was made in part to test the idea that slags high in calcia would have a relatively high k : 0 activity while slag k531 was selected in order to determine the effect of

relatively high Si02 ana AI2O3 content.

I

I Table 2. Initial Compositions of Synthetic Slags

(wt%) • i

Component K512 JC531 Si02 30.00 48.00

A1 2 0 j 11.25 20.00

Fe j0«, 11.25 8.00

CaO 15.00 2.40

MgO 7.50 1.60

K:0 25.00 20.00 TooToo 100.00

l J

Potassium pressures for the KS12 and KS31

slags are shown in Figures 3 and 4. In each ; figure, the measured K(g) pressures are plotted

along with curves representing the K(g) pressure

over solutions of K 2 0 in silica with indicated

weight percentages of K 2 0 as previously determined

at NBS. Each figure shows the concentration

range of the slag for an experimental sequence

during which temperatures were generally increased

from one constant temperature to another. An exception to this procedure occurred for the data

ranging from 15.9-10.5 wt’s K 2 0 in Figure 3 for

which the temperatures were decreased between measurements after a maximum temperature of about

1500°C was obtained. The different concentration

ranges were obtained during evaporation of the same sample. The concentration ranges indicated

were obtained from the mass calibrations and

integrated ion intensities. Data points con- nected by a vertical line were obtained at a fixed temperature. Increase or decrease in presures is indicated by an arrow on the vertical line.

Both slag samples show significantly higher K pressures than would be expected for slags having

the same K2O concentration but containing onlv

K2O and silica. The K pressures over slag K512

are higher than those over a 30 wtl K20-S102

solution until the K 2 0 composition has decreased

to only 10 wt% of the sample. The K pressures

over slag KS31 while not as high as those ever

the K512 slag are higher than those over a 20 wta

K20-Si02 solution when the k'53i slag contains

only 13 wt% K2O. The implication of these results

TEMPERATURE °C

Figure 3. Potassium pressures as a function of

10 /T and varying composition over K512 slag.

4

Figure 4. Potassium pressures as a function of

: if Vt and varying composition over K531 slag.

, is that simple seed-slag calculations in which one uses data for pure K 2 G-Si0 2 solutions to model the

interaction will predict a concentration of K 2 0 . in the slag solution which is 2 to 3 times greater

than that which would actually be obtained. The

, idea that high calcia content in a slag will

| increase the K 2 0 activity by reducing the Si0 2 activity appears to be correct.

Comparison of K pressures over slags K512 and

K531 with each other are probably quite valid but

comparison of these pressures with pressures over

K2 0-Si0 2 solutions must be considered more tenuous because of the possibility of systematic errors between the two different experimental techniques.

These latter comparisons must be considered tentative until tested experimentally. i

: The data presented in Figures 3 and 4 show

some unexpected effects. Samples placed in the

effusion cell were coarsely ground. The pressures

measured initially over these samples are signif-

_

icantly higher than the pressures measured later

after the slags had become sufficiently fluid to flow into a molten pool. Thus in Figure 3, the

initial series of points show relatively high

pressures until they go through a maximum at about

1250 e C. In this pressure range the rate of loss

of K 2 0 would not be expected to be sufficiently

high to cause the large decrease in pressure be- cause of a concentration change. Likewise, the

data in Figure 4 show a noticeable decrease in

pressure at about 1350°C. It is noteworthy that

the K531 slag is about 250 times as viscous as the KS12 slag. 10 The temperatures at which the

pressures decrease is believed to be the softening

point of the slags. i

Two possibilities exist to explain this

behavior. First, if some crystallization of the

slag had taken place, the glassy phase might have

a much higher K 2 0 concentration than the bulk

composition. When the slag became sufficiently

molten, the crystallized phase might redissolve

in the melt leading to a decrease in the K 2 0 concentration. The second possibility is that

the decrease in pressure is a consequence of

reducing the sample area. If this is the case then the pressures measured over the liquid melts

do not represent equilibrium saturation pressures

and the rate of transport of k and 0 2 from the

slag to the vapor phase above the slag is kinetically controlled even under the relatively ideal

conditions of a Knudsen cell. This would probably

mean that present thermodynamic estimates of seedslag interaction are too pessimistic. This effect

needs to be examined carefully to determine which

or V7hat combination of these possibilities is responsible for this behavior. ,

An additional factor bearing on whether or not equilibrium is attained was noted. During

vaporization of the K531 slag it was observed

that the K

+ and 0 2 ion currents were not con- stant but increased for short periods by as much as

a factor of two from their nominal values at a temperature of about 1450°C. This behavior had

been observed previously in this laboratory during measurements on the vaporization of K 2 0-Si0 2 melts.

: At that time it was assumed that it was due to sample inhomogeneity. The K531 sample however had

undergone a great deal of stirring and homogenization during its preparation and use in viscosity

measurements so an inhomogeneous sample would net

be expected. A common property of the K 2 0-Si0 2 melts and the KS31 slag is their relatively high

viscosities. Since both of these samples had been

melted in an air environment they would be expected

to be saturated. IVhen heated in vacuum, air can

be released. An attempt to observe release of

dissolved gases was made by monitoring mass peaks

such as H 2 0

+

, N

t 2 , K

+

, 0 2 , and C0 2 by making

1 simultaneous measurements of two of the peaks on the A.C. (chopped) channel and D.C. (unchopped)

channel of the spectrometer. The result was that

simultaneous increases in the K

+ and 0 2 signals

could be observed regardless of which channels

they were monitored on while increases of N 2 , C0 2 or H 2 0

+ were not observed when increases

in 0 2 or K

+ took place. However, in D.C. spectroI metry there are relatively large background

; signals at these mass positions so that increases

might be difficult to observe. In A.C. spectrometry, the H2 0

+

, N 2 , and C0 2 mass positions

have very small signals and any pressure burst

would be averaged out by the relatively large

time constant of the amplifier. A rough calculation

shows that over 991 of the gas released by a bubble of sufficient size to initially raise the

partial pressure of the trapped gas to 10'' atm

in the Knudsen cell would have effused from the

. cell within .3 sec.

Presumably, if a bubble could form on the

surface of the slag it would disturb the slag

layer when it burst. The ensuing disturbance

would bring a fresh layer of slag to the surface

which would have a higher surface concentration

than the undisturbed surface layers. Evapo rati on 1

| of the fresh K 2 0 surface layer would give rise to

(the observed increases in the K and 0 2 pressures.

iThis again would be evidence that equilibrium

7. E. M. Levin, C. R. Robbins and H. F. McMurdie,

Phase Diagrams for Ceramists, (1964) 60.

[pressure measurements are not being obtained. 8.

i Ibid., 204.

Verification of the observations thus far

'Bade is important. If equilibrium measurements

9. Ibid., 210.

| have not been obtained on the K312 ar,d K531 slags,

it is likely that previous measurements of the

j activity of K 2 0 in K 2 C-Si0 2 solutions are too

‘low because of ur.saturation effects. Experimental

10. H.P.R. Frederikse, T. Negas and S. J. Schneider, Development, Testing and Evaluation

of MHD-Materials , Quart. Rept. Oct-Dec 1975.

investigations of these possibilities will be

attempted by decreasing the site of the effusion

orifice to see if pressure increases at fixed

: temperatures and K 2 0 concentration can be observed.

11. University of Tennessee Space Institute,

Development Program for MHD Direct Coal-Fired

Power Generation Test Facility, Quart. Frog.

Rept. Nos 1,2; Month. Tech. Prog. Rept. July,

[ i August, September 1975. VI. Conclusions i i

j Experiments on K 2 0 -Ca0 -Al 2 03 -Si 0 2 mixtures

demonstrate partitioning of K 2 0 between K 2 0-

Al 2 03 -Si02 solids and Ca0 -Al 2 03 -SiG 2 solids or CaO-rich liquids. Addition of CaO to KAlSiGs.-

! 12. D. B. Stickler and R. DeSaro, Sixth Inter- national Conference on Magnetohydrcdyr.amic

Electrical Power Generation, II, June 1975,

31.

!J rich compositions causes a corresponding increase

‘in a^

2 o* Potassium pressures ever two synthetic

' slags were compared to X pressures over K 2 0-Si0 2 solutions having similar K-.O concentrations and were found to be significantly higher. One slag

13. E. M. Levin, S. J. Schneider, and E. R. Plante, 13th Symposium on Engineering

Aspects of Magnetohydrcdynamics, March 1973, - IV. 5.1.

with a relatively high, calcia content had a X pressure higher than K 2 0-Si0 2 solution with 50 wt%

KjO when the K 2 0 component had decreased to 10 wt?

s of the sample. The second slag with a relatively

14. C. L. McDaniel and E. R. Plante, 14th

Symposium on Engineering Aspects of Magnetohydrodynamics, April 1974, IV. 6.1.

high Si02 and Al 2 03 content showed a K pressure

above that of X 2 C-Si0 2 solution with 20 wt°i K 2 0

when its K 2 0 content was about 13 wtt X 2 0. Vaporization phenomena which might be ascribed to

15. T. I. Avdeeva and A. A. Novolodskava, I;v. Sibirsk. Otd. Akad. Nauk SSSR. Ser. Khim. Nauk

2 , 82 (1964).

non -equilibrium behavior were observed. Further

studies to investigate these effects are planned.

16. C. Karr, P. Waldstein, J. J. Kovach, J. Ir.st. Fuel 47, 177 (1974)

.

VII. Acknowledgements 17. Theodor Myslivec, Sb. Ved. Pr. Vvs. Sk. Ban.ske

Ke would like to exnress our aonreciation to Ostrave. Rada Hutn. 20, 39 (1974).

Dr. K. Haller of the Inorganic Glass Section for

an enlightening discussion on the vaporization

behavior of glasses ar.c to Webster Capps who pro- vided the slag samples. This work was sponsored

IS. S. J. Schneider, K. Capps, H.P.R. Frederikse,

W. R. Hosier, D. A. Kauffman, E. M. Levin,

C. L. McDaniel, T. Negas and E. R. Plante,

NBSIR 74-543 (1974).

! in part by ERDA, Office of Fossil Fuels.

VIII. References

1. F. E. Spencer, J. C. Her.drie, Jr., and D. Bienstock, Sixth International Conference on Magnetohydrodyr.amics Electrical Power Generation, Washington, D.C., June 1975, II, 181 .

I 2. L. K. Crawford, K. E. Tempelmeyer, J. Martin,

and J. Muehlhauser, Ibid II, 51.

3. E. R. Plante, C. D. Olson, and T. Negas, Ibid

II. 211.

j

i 4. K. A. Selvig and F. H. Gibson, Bureau of

Mines, Bull. 567 (1956).

5. V. A. Deer, R. A. Howit and J. Zussman, RockForning Minerals, Volute 3 fSheet Silicatesj

(1962). !

I 6. V. A. Selvig and Henry Seaman, Carnegie Inst. Techno 1 . Mining and Metallurgical Advisory

Boards, Min. and Met. Investigations, Coop.

Bull. 43 (1929),

JOINT TEST OF AN U. S. ELECTRODE SYSTEM IN' THE U.S.S.R. U-02 FACILITY

W. D. Jackson

Energy Research and Development Administration, (U.S.A.)

S. J. Schneider

National Bureau of Standards, (L'.S.A.) W. E, Young

Vestinghouse Research Laboratory, (U.S.A.)

A. E. Sheindiin , G. P. Telegin and D. K. Burenkov

Institute for High Temperatures, (U.S.S.R.)

Abstract I. Introduction

The first (Phase I) joint U.S .-U.S .S .R. test

of U.S. electrode materials vas carried out in Moscow between September 25 and October 1, 1975 in

the U.S.S.R. U-02 MHD facility. The test procedure

followed closely a predetermined work plan designed

to test five different zirconia based materials in cathode and anode electrode wall modules under MID

operating conditions. The electrode materials were:

(in Mol %) 83Zr02 :12Y 2 0 3> 82Zr02

: 18Ce0 2 , 50Zr02

: 50Ce02 , 25Zr02 :75Ce02 and 20ZrC2

: 7SCe0 2

: 2Ta 2 0 5

. 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 MKD power

operational conditions.

Fourteen electrode pairs performed satisfactorily during the entire test. Five pairs failed

early in the life test and the sixth pair failed in

the 1 ast several hours. Failure was not due to the

electrode materials, however, but due to lead-out

melting caused by Joule heating in the platinum

vires.

Extensive pre- and post-test materials characterizations were made to determine the effect of

the MKD environment on the electrodes and insulators. Measurements included: thermal diffusivity,

thermal expansion, chemical composition, microstructure, electrical conductivity, phase composition,

closed and open porosity, pore size distribution,

and radiography. Results indicate that there was

extensive attack by the seed on the cathode wall

causing chemical reactions and phase changes of the electrode materials. There was also seed penetration through cracks and pores in both insulator and

electrode.

\

* The following persons also participated

cent and/or aided in the preparation of

From the U.S.:

G. Rudins (ERDA)

R. J. Wright (Vestinghouse)

B. R. Mossing (Vestinghouse)

J. L. Bates (Battelle-Northwest)

V. R. Hosier (NBS)

E. Farabaugh (NBS)

A. Period f (N3S)

T. Negas (NBS)

As part of the U.S .-U . S .S .R. Cooperative Pro- gram on MKD, a series of joint tests are planned

in the facilities of both countries to aid in the

development of high temperature electrode systems

and materials. Between September 25 and October 1, 1975 the first of these experiments was conducted

in the U.S.S.R. U-02 facility on a section of a MHD generator consisting of U.S. designed anode

and cathode modules and U.S.S.R. supplied insulatin

walls. The main objective of the test was to evaluate the behavior and performance of the electrode system and to establish the characteristics

of the various materials of constructions (particularly electrodes and insulators) during long term operation under real MHD conditions.

The entire test, from conception to completion

was carried out on a joint basis with several

organizations* in both countries participating in the various activities necessary to mount, conduct

and analyze the experiment. The Materials Ad Hoc Working Group, with approval of the Standing

Steering Committee on MHD, planned the overall

experiment, formulated test objectives and criteria and generally provided guidance for the entire

activity. The Energy Research and Development

Administration (ERDA) sponsored U.S. activities

with Vestinghouse Research Laboratory having the responsibility for module design and construction

and the National Bureau of Standards and Battelle

Northwest Laboratory performing pre- and post-test

materials characterization. The Institute for High Temperatures in the U.S.S.R. provided the U-02

facility for the test and in addition performed

complementary materials characterization studies.

The actual test carried out on the U-02 followed a predetermined work plan and \.as monitored by a joint working group consisting of six U.S. and six

U.S.S.R. specialists, some of whom were always

present at the facility.

the work, the conduct of the actual experl-

. From the U.S.S.R.:

V. V. Kirillov IVTAN

0. B. Zamyslov IVTAN

V. I. Zalkind IVTAN

A. B. Ivanov IVTAN

V. G. Gordon IVTAN

D. A. Vysotskii IVTAN

Yu . L . Dolinskii IVTAN

A. I. Romanov IVTAN

in the planning of

this report:

1 . 1.1

A complete report describing the entire test results and conclusions will be published separately. The purpose of this paper is tc present a synopsis of the experiment including significant , details on the electrode system design and construction, test conditions, materials performance and preliminary conclusions.

II. Electrode System

Module Design

Plasma property values needed for the design

of the electrode system were derived from typical

working conditions of the Soviet U-02 MED facility.

The fuel is natural gas containing by volume 97% methane, 2% ethane, and 1% ir.erts. 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 50%K2C02 and 50% water mixture (total 1% by weight). At the entrance to the channel the design plasma conditions were: temperature - 2600K, static pressure - 0.S0 atm

and mass flow - 0.75 kg/s. Cesign heat fluxes at the channel entrance and exit were 30 V.'/cm/ and 18 V.'/cn^ respectively for the electrode walls.

The internal dimensional envelope of the channel as given by the facility operator specifies a height of 0.260 m and a width of 0.C64 m yielding

a constant crcss-sec ciena 1 flew area of 0.0166 n

z

. For practical purposes the velocity in the channel

was considered constant. Kith stoichiometric com- bustion the plasma has an average density of

C.l kg/ra. Thus from continuity of mass the average

channel velocity is 450 m/s. The magnetic field

has a strength of 1.5 T which is constant over the length of the channel. The average plasma conductivity and mobility are 10 mhos/m and 0.7 m

2 /'7b

,

respectively. The Pall parameter has a numerical

value near unity ('v .95).

The 13® channel is the Faraday type. Figure 1 illustrates the pre-test cathode wall of the

electrode system. In each of the electrode walls

there are twenty separate electrodes with the pitch

between electrodes being 24 ran. The electrodes

proper were 12 mm long, 6 mm deep and 62 mm wide

and have platinum lead-outs. Each electrode was individually housed in an insulator made of magnesium oxide. Electrode pairs were connected tc separate loads; voltage between electrodes was estimated to be 15V. Channel insulating walls

which separate the two electrode walls were

designed and constructed by the Institute fer High

Temperatures of the U.S.S.F.. Academy of Sciences.

They consisted of hexagonal llgO blocks braised to

a copper base.

The electrode modules were isolated from, the

channel support structure by a phenolic mounting

material. In addition the heat sink of each electrode wall was sub-divided into five individual

segments. Each heat sink consisted of a watercooled (one inlet and cne outlet) ccpper block

which had been coated with an electrical insulating paint. Ail exterior surfaces of the heat sink were painted except where the magnesium cxide

joined to the heat sink.

Preparation cf Materials

The two electrode (anode and cathode sections)

walls were made up of five separate segments each

having four electrodes of the same material. The

groups of electrode materials were (see Figure 2 for identifying number code) :

I group = 83%'ZrO;, :12'

V

2 0 3 (mol %)

II group = 82%Zr0 2 :18%Ce02

111

group = 50%Zr0 2 : 507.Ce0 2 IV group = 25%Zr02 :75%C.e0 2 V group = 20%Zr02

: 78%Ce0 2 :2%Ta 2 C5

The requirements of good resistance to both

thermal stress damage and seed attack demand careful control of microstructure and nicrcchem.istrv

for both electrodes and insulators. Thermal stress damage resistance can be increased by the mreser.ee of such crack arresting micros tructurai features

as pores, nicrocracks, and second phases. In single phase materials this can be acccr.pl is:. e.d by use of a wide variation in grain structure and a moderate level of porcsity. Seed resistance is achieved by low and/or closed porosity and lev

levels of impurities (absence of secondary phases

at grain boundaries). Seth electrodes and insulators are thus fabricated by raking dense hign

porosity grain, followed by sizing, pressing and

refirir.g. The fabrication details for both

electrodes and insulators are detailed in the

following discussion.

High purity, reactive powders were used as starting materials for the electrodes. These

materials, Zr0 2 , Ce0 2 , Y 2 O 3 and TapOc, were pur- chased from commercial sources and are claimed tc

be greater than 99.5% pure with less than ICO ppm

of either FepC^ or Si02

. The particle size

(average) of the ZrCp, V2 O 3 , Ce0 2 and Ta 2 C; powders were 1.5, 4.5, 6 and 11 microns respectively. A

slurry mas prepared consisting cf distilled water,

1200 g of oxide powder and organic binder (a mixture of polyvinyl alcohol and polythylene glycol).

This slurry was milled in a rubber lined ball mill

filled with yttria stabilized zirccnia balls and

introduced into a spray drier to form a free

flowing, homogeneous powder. These powders were,

in turn, pressed at 30,000 psi into 3C0 to 4C0 g bars and subsequently fired for 14 hours at 150u°C.

The sintered tars were then crushed into fine

particles using a laboratory crusher. Any iron

contamination was removed by magnetic separation

and leaching with acid solutions (to less than

30 ppm). The particles were subsequently sized by

screening into three size fractions, a course

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

fraction, and a fine (-325 mesh) fraction. Additional fine. (-325 mesh) material was produced by

jet milling course grain material. The average

particle sizes for the crushed -325 mesh and the jet milled powders were 15 and 6 microns, respectively. The three size fractions were mixed

together in the ratio of 52 coarse to 15 -edium to

33 fine. Polythylene glycol was used as an organic

binder in pressing this granular mixture. Samples

14 mm x 35 mm, x 8 mm were pressed at 15,000 psi on

a double action press and fired at 1 600° C in air.

Linear shrinkages durirc this heat treatment was

less than 2.0%. The final porosity of the five

electrode compositions varied from 14.6 to 19.9' . The electrode was thqr. machined tc the desired

dimensions using diamond grinding tools.

The electrodes were metallized using a commercial platinum paste. Four individual thin coats

1 . 1.2
'

'
were brushed on and fired. During the application

and firing of the last three coats a platinum

screen was also attached to the metallized surface.

Finally a plat inum (0.020”) wire lead-out was welded to the platinum screen.

The magnesia insulators were purchased from a commercial source. _ A high purity fused gr^in of

magnesia was 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 het-pressed blocks which were 152 mm >: 152 mm

x 36.5 mm were heated in air to remove any graphite

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

the desired insulator dimensions. These blocks were machined into the desired insulator dimensions

and shapes.

Finally open spaces between the electrode and

insulators in the module assembly were filled with

a commercial zirconia phosphate cement. A calciazircor.ia cement also was used to fill voids in other parts of the channel not related to the electrode system.

Instrumentation

Thermocouples and voltage probes were located

at various positions within the two electrode walls.

Figure 2 is an open schematic of cathode and anode

showing materials, idencif ication digits ar.d location of instrumentation. Three electrodes on the anode wall each had two platinum 13% rhodiumplatinum thermocouples buried along the electrode

centerliue. As referenced from the channel

entrance these were the first, fourth and seventeenth electrodes. Five electrodes had chrcmelalumel thermocouples positioned at the interface

between the Zr 02 phosphate cement and MgO insulator. On the anode wall these thermocouples were

the second, tenth and eighteenth electrode positions. On the cathode wall these thermocouples

were in the sixth and fifteenth electrode positions.

In addition there were sixteen chromei-alumel

thermocouples positioned in the colder regions of both the anc-de and cathode walls. A single voltage

probe was located at the center of the 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 were used to determine the resistance cf the lower half of an electrode.

On the cathode wall the fourth electrode

position had an access hole for line of sight measurement of electrode surface temperature by an optical technique. This arrangement allowed the

surface temperature of the fourth electrode on the anode wall to be measured directly. Just prior to entering the channel the plasma temperature was measured by the sodium line reversal method.

Voltage probes also were located laterally ar.d longitudinally on the top insulating wall to measure Hall ar.d Faraday potentials, thermocouples

were buried at various depths in the MgO of the

top insulating wall near the entrance, midpoint

and exit of the channel.

III. Test Procedures, Operating Conditions

and Electrode System Performance

The testing of the electrode nodules in the

li-02 facility followed a predetermined work plan.

This plan or test procedure was divided into four

prinary phases:

Phase I (18 hours duration)

- pre-start tests of the electrode modules;

- heating of the system and selection of

thermal conditions (no seed);

Phase II (6 hours duration)

- study of the electrical characteristics

of the nodules at the full working conditions ; Phase III (94 hours duration)

- operating life test at the full working

conditions

;

Phase IV (9 hours duration)

- cooling down the system (no seed) ; - post-run tests and dismantling the channel.

The pre-start tests showed that the hydraulic ar.d electrical characteristics cf the section corresponded to the design specifications. In particular, the average insulation level between adjacent

elements of the working section was equal to 50 kohms at an applied test voltage of 500 V.

Before igniting the combustor the system was

heated by air electrically preheaced to 250’C. Next the combustor was operated at the maximum

possible turndown ratio which would support stable

operation of the combustion chamber. The average

temperature rise as read by the thermocouples

mounted in the electrode modules was 50°C/min.

After a 30-mir.ute hold period the combustion

chamber was then gradually brought up to operating

conditions (= = 1). Oxygen enrichment then allowed

the channel to be brought to life test conditions.

The operating conditions of the lccp for conducting the life tests of the electrode modules were

established at values to achieve temperatures on

the 4th anode between 170G°C ar.d 1775°C. These

were maintained during Phase II ar.d Phase III of

the experiment with an accuracy of ±5%. The

following describe the working conditions:

Plasma temperature, K 2573

Total mass flow rate, kg/s 0.75

Static pressure, atm 0.83

Content of potassium in plasma 1.0 wt. X Magnetic field 1.7 T

The hot surface temperature of Croup

as measured by an optical pyrometer

The temperature of the electrode as thermocouple at a depth of 3 cim frcQ

face was 1180°C.

I electrodes

was 17 10°C

.

measured by a the hot surDuring Phases II and; III three short duration

excursions in the plasma temperature took place.

The temperature change as measured by the in-depch

electrode thermocouples was 360°C during 2 1/2

minutes in the first case and approximately 425 c C during 5 minutes in the second case and 140°C in

T 1
r
2 minutes in the last case.

The loop was cooled during Phase IV according

to the schedule in the reverse order of the warmup period (Phase I).

Analysis of the thermal data revealed that the

thermal flux on the electrode walls increased

gradually in the course of preheating and then,

with a f ev exceptions, was sufficiently stable or,

at any rate changed within a fairly narrow range during the life tests (Phase III). There was however a gradual reduction of thermal flux by 20 to 25% during the period between the 39th hour and

the 111th hour of the experiment. The average

thermal flux during this period in the cathode

module was 1.96 x 10^ kcal/n‘- hour (23 V.'/cm 2

) and

in the anode module 1.94 x 10° kcal/m 2 -hour (22.9

K/cm2

). Furthermore the thermal flux was not uniform along the electrode walls; the smaller thermal

flux occurs where the thermocouples and the probes

are located. Evidently, part of the heat is lest

along the thermocouples and the probes. Also, it

is possible that the thermal flux was reduced due

to the loss of air seal and the leakage of cold air

into the system..

Thermal flux data as well as the readings

from the thermocouples placed inside the electrodes

and in the MgO units, made it possible to estimate

surface temperatures on the rest of the electrode

units. These estimates established that the temp- erature of electrode units of the first cathode

nodule exceeded the temperature of the first anode

module by 5C to 1G0°C. Thus, surface temperatures were in the range between 1,730 and 1,S00°C. Taking into consideration the reduction of thermal

flux along the channel (which agrees sufficiently

well with the calculated data) the surface temperature of the electrode units was evenly reduced in

the downstream direction. The fifth cathode module

had a gas side temperature between 1,650 and

1 ,700°C.

Also cf interest is the change of temperature

on the surface of the electrode when current passed

through the electrode. During the 17th hour of the

experiment, when the electrodes were for the first

tine under a load, the temperature of electrode

2101 (at the depth of 3 millimeters) increased by

40°C under a current of 5A (.73 A/cn2

) and at the

beginning cf the duration run by 80°C with 7 . 8A (1.12 A/cm2 ). The Joule heating of composition V

(anode 2517) was not very great; between 20 and 30°C at J=7 . SA.

During the first 34 hours of the duration test

the temperatures of the electrodes remained con- stant and the change with time of readings from

the same thermocouples did net vary greatly, (as a rule no more than + 2C°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 one hour was indicated by all thermocouples

readings. Subsequently, a slow increase occurred

in temperature of the anode wall elements by some

20 to 40°C during the next 12" hours, upon which

the electrode temperatures once again stabilized.

During the life test (Phase III) the electrodes of each material group were operated at four levels of current density: OA/cm2

, 0.24 - 0.33 A/cm2

, 0.5 - 0.7 A /era2 and 1.2 A/cm 2

. The

operation of electrodes under current was attained

by loading them with controlled resistance with an electrical load coefficient at K * 1 (open circuit),

K = 0.2, to 0.5; K c 0 (short circuit) and K = (-0.2 to -0.3) (stagnation operating mode with

additional current to electrode pairs from separate

external voltage source).

The electrode couples numbers 9 and 17 (5C% ZrC2

: 50%Ce0 2 and 20%Zr0 2 : 767Ce02 :2%Ta 2 0s i.e.,

compositions 3 and 5) had currents at 8.5A and

3.3A (1.18 to 1.2 A/cm2 ); the electrode pairs

numbers 2, 10, 14, 18 (compositions 1, 3, 4, anc 5) had currents from 3A to 4.8A (0.43 to 0.68 A/cm-);

electrode couples numbers 3, 6, 11, and 13 (1, 2,

3, and 4 compositions) had currents of 1.6A to 2.18A (0.23 A/cr.2 - 0.31 A/cn~). The deviations

from the average values did not exceed 15 to IS' . These fluctuations could be explained on the basis

of random, changes in the thermal parameters of the

loop, which were maintained to within ±52. In addition to the random fluctuations mentioned

above, a gradual reduction was observed in the applied voltage necessary to maintain the currents

at the assigned level (from 30 to 60 volts ccvn to

10 to 20 volts) . Such a change could have little

connection with anode-cathode resistance drops

(current leaks could have accounted for about 0.3A

by the end of the duration test). To a great extent this could have been caused by the crop of

the effective internal resistance cf the channel,

due to the increase during the second half cf the

life test of the surface temperature cn the anode

wall (and judging by the decrease of thermal flux,

a possible increase in possibly the cathcde wall

temperature as well). This could have led to a reduction of electrical resistivity of the near electrode boundary layers of the plasma.

During Phase III (life tests) discontinuities

in electrical contacts were observed in six loaded

electrodes (identified as due to platinum lead-cut

failures). The failing electrode pair and time to failure were;

Composition 6 Electrode Pair

Failure Time From Start

(3rd Phase)

Current Density A/c

(nominal)

Mol % 82Zr0 2 :lSCe0 2 12C7-2207

7 cin

.

.65

82Zr02: 18Ce0 2 1208-2208

7 min. 1.10

88Zr0 2 :12Y 2 0 3 1101-21011 hr. 20 min. 1.10

25Zr02 :75Ce0 2 1415-2415

1 hr. 45 min. .65

25Zr0 2 :75Ce0 2 1416-2416

2 hr. 20 r.in. 1.10

20Zr02 :73Ce0 2

: 2Ta ; 0 5 1517-2517

92 hr. 40 min. 1.10

The life test data basically indicates that

the electrical parameters were stable and were within the set limits. The small deviations

observed had a definite qualitative explanation.

Overall, the electrical and thermal characteristics

of the electrodes tesxed shoved them capable of adequate performance on all the electrode pairs,

with the exception of those which suffered distraction of the platinum current lead-outs.

1 . 1.4

IV. Materials Characterization* *

Extensive pre- end post-test materials

characterizations were made to determine the effect

of the KHD environment on the electrode system as

a vhole as well as on individual electrodes and insulators. Pre-test measurements were used to obtain design information and to establish a base

for comparison of post-test results. Pre-test

examinations included measurements on: thermal

diffusivity, thermal conductivity, chemical composition, microstructure , electrical conductivity,

phase composition, porosity and pore size distribution.

Following the test and immediate examination

visually, the modules were packed and shipped back

to the United States where they were disassembled

in such a manner to allow for extensive post-test

examination. These analyses involved phase composition and chemical reactions, electrical conductivity, metallography, electron nicroprobe studies

and radiography of current lead-outs.

The following describes some (not all) of the core significant results of the pre- and post-test

analysis

.

Pre-Test Analyses

Thermal Diffusivity : The thermal diffusivities of the KgO insulator and electrode materials

were measured using a laser pulse technique from

room temperature to 1750°C. The results for

electrode materials are indicated in Figure 3.

Thermal Expansion : The thermal expansion of

MgO insulator and electrode materials were measured from about 60°C to 15G0 c C using an alumina push

rod dilatoceter. Table 1 lists the coefficient of

expansion for the various materials.

Chemical Composition : The compositions of the NgC and electrodes were determined by emission

spectrochenical analyses (precision + 5%). The KgO

contained less than 170 ppm total impurities. The

compositions of the electrodes are given in Table

2 .

Microstructure : The electrode and insulator

samples selected for metallography were vacuum

impregnated with resin and polished for microstructural details. A brief description is as follows

:

(KgO) Normal to the hot pressing direction the pores were irregular in shape and uniformly distributed around the KgO particles with nc apparent

orientation. In contrast, in the direction paraljel 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 urn with some grains as large as 0.02 cm. The pore size varied up to 5 urn across

.

(887,2 rO? : 122VoO ->) The microstructure of the vttria

stabilized electrode sections from the center and

edge were essentially the same. However, they

differed in color (off white to 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 um. 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.

( 827ZrO? :182CeO:>) The microstructure from the center and end of the electrode was essentially the same. A second phase was observed in some selected areas. The size of the large grains varied up

to 0.065 cm.

( 50%Zr0? :50%CeQ->) The electrode was mottled with

a light yellow background and light gray areas. The middle 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.

( 25%ZrO? :75%CeO->) This high ceria-containing

electrode was multi-colored, with large areas of yellow, tinted with green, which were interlaced

with gray. These areas of different color were related to specific microstructures. 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

.

( 2Q7ZrO? :7S%CeO? :2Tap0

Q

This electrode material

was uniformly mottled with yellow-green background

and very dark green spots. These dark green areas were directly related to the large crystallites of the material, while smaller particles ere light

green. The regular surfaces of the porosity in the areas which bind the larger particles together

suggest the presence of a liquid phase curing

sintering. Seme large and small cracks lace the

electrodes, especially in the center section. The

large grains were 0.065 cm or less in diameter.

Phase Composition : Table 3 lists pertinent

x-ray diffraction data for the insulator ar.d electrode materials. Phases present are characteristic of reaction products formed at elevated

temperatures under relatively oxidizing conditions.

Anode and cathode materials are identical.

Electrical Conductivity : The electrical con- ductivities of the electrode materials were determined using a d.c. four probe method in various

oxygen partial pressures. Figure 4 illustrates

typical values for 10~ 3 atm FO 2

.

Post-Test Analyses

Physical Description : After completion of the

test in Moscow, the electrode section was removed

from the U-02 channel. Extensive photographs were

** Characterization activities are being carried measurements are not fully completed the pre- and

L'.S.S.R. results are expected to conform to that

in degree since they depend upon interpretations

out by both U.S. and U.S.S.R. sides. Because thes

post-analysis are based on primarily U.S. results

reported here, although some' firm conclusions may

of difficult experiments.

e

differ

1 . 1.5
»1
taken at this tine of the electrode section before

disassembly and after the electrode sections were

removed from the insulating walls.

i Immediately after the test visual inspection

of the modules revealed:

(Condition of Joints) The joints between the nodule

and the nozzle and diffuser did not leak during the

test. However, seed penetrated into these joints

to a depth of 10 to 15 cm. The AlrOj spacers in most areas either swelled or reacted with seed.

This eff ect was more evident on the cathode w^all. Heavy seed concentration v?as noted in all joint

areas.

(Electrode Condition) The electrodes showed traces

of erosion, corrosion and cracking. Erosion and

corrosion was greater on the cathode than on the anode. The electrodes exhibited swelling and spalling which was substantially greater on the cathode. The ZrC>2:Y203 compositions appeared to be

less degraded. Electrodes under current exhibited

greatest swelling.

(Inter-electrode Insulator Condition ) The MgO insulators, in general, were eroded at the joints between the insulators to a depth between 1-3 mm with

the cathode side being core affected. The insulators were cracked longitudinally near the centers

of the insulator. This condition was present in

both anode and cathode walls. The greatest

destruction occurred near electrodes working at maximum current.

(Insulation Malls Condition ) The general appearance of the insulating walls remained unchanged

for the cost part. Differences in color were

noted.

Microstructure

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. Three slices, each approximately 5 cm thick, were removed from each submodule for various studies. Figures 5 and 6 shew

a composite of anode and cathode slices.

The highest concentrations of potassium were

associated with the cathode side of the channel,

especially in that portion of the materials which

were at operating temperature of approximately 700

to 13C0°C. In the MgO, cement, and in seme electrodes, the potassium, is associated with the lower

density, porous areas which surround the large

cxide 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.

The MgO insulator 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.

(2) Colunnar type grains extended to depths between 0.7-0.18 cm. (The depths were approximately

the same for the cathodes and anodes.) 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 CaO stabilized ZrO; which

contained more Ce02 in the MgO downstream.

(3) Equiaxed grain growth occurred to a depth

from the surface of approximately 0.3 to 0.4 cm. This recrystallizaticn appears as a den.sif icaticn

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.

The erosion, degradation and recrystallizaticn

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.

The cracking of the MgO appeared similar for

all electrode assemblies. Cracks fanned cut from

the corners initiating at the machined recesses in

the insulator. In most insulators, these crac'-'s were found 'tO.6-0.7 cm from the surface. Most cf

these cracks contained potassium.

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

at least one and up to three phases, with the

concentration cf these 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 zirccnia-calcia-ceria phase,

phases high in alumina were present. Potassium

was also found in some phases.

The phase compositions at the center of the

electrodes also varied, and often contained more

than one phase. Similarly, the electrode near the

platinum current lead-out also was contaminated.

The cement between the electrode and insulation acted as a major sink of potassium concentration and possible path of seed transport into tne MgO and electrode. The highest concentration cf

potassium in the cement was found in the cathodes

and increased with current density. Total degradation of the cement occurred. Conversely, the cement associated with the anode remair.ee intact,

but contained 'VICE potassium associated totally

with the low density portion of the cement.

The AI2O3 spacers reacted with the potassium,

but did not appear to react in any other way with

the other materials. The higher temperature top

edge cf the sintered AI2Q3 plate appeared to

1 . 1.6

form porous S-AI 2 O 3

.

Radiography of the lead-outs revealed that

several failures occurred through the melting of < the platinum wires. These failures do not appear

to be due to chemical attack. It is suggested

that in assembly, a scratch or bend was put into

the lead wire in a region which was not surroundc-d by ZrC>2 cement. Joule heating developed and without the cement- to carry away the heat, the platinum lead melted.

Electrical Conductivity : Since most electrode sections were cracked and eroded to some

extent, it was impossible to remove each half

electrode in its original condition. Measurement

of electrical conductivity requires a sample of

sufficient size to permit attachment of electrodes.

Only those anode electrodes, 21Q2, 2205, 2206,

2207, and 220S could be used for post-test electrical data.

It is difficult to reproduce in the laboratory measurement apparatus, the exact conditions

that were present in the test facility plasma.

The 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 con- dition of the sample could be considerably different from that actually existing while it was

exposed to the test plasma.

However, the conductivity results do indicate that no permanent damage had been done to the anode electrodes measured since the pre- and

post-test data are very similar. While the series of samples containing S 2 Zr02 rlSCeCb 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.

Phase Composition and Chemical Reactions : When the nodule was received, dismantled,

and dissected, a visual observation of the electrodes immediately suggested significant alteration of the Ce02 containing ZrO? 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. That the zones of discoloration are indicative of phase changes of

original materials and of reaction with potassium

seed was considered obvious. The bloating or swelling of cathode materials probably occurred

during and after cool-down. Almost the entire

cathode wall is saturated with seed, costly as condensed K2 CO 3

. This includes the underlying

KgO blocks which were cracked extensively. When

exposed to moisture, K2 CO 3 (as is well known)

expands 40-60'' (and ultimately dissolves) by

hydrating. This expansion, abetted by the mismatch in the cohesion among zones of dissimilar

phases and alteration products, and hence, dis;irnilar mechanical properties, most likely caused the

swelling. Indeed, it is uni iVe 1

y

that the electrodes experienced bloating d uri ne on era tier, as protruding material would have been swept away by

the gas stream.

Numerous specimens were selected carefully

iron within each zone of discoloration of every

electrode. The 8 SZr02 :

I

2 Y 2 O 3 electrodes are not zoned, but samples representative of the top, middle

and bottom portions were also taken. All specimens

were investigated by x-ray diffraction powder analysis using filtered Cu-rad iation.

The outstanding characteristics of all cathode

CeC> 2 -ccntaining materials are their; a) alteration

through phase transitions, and/or b) reaction with

potassium seed. A detailed account of phase transitions and chemical reactions in Zr 02 :CeC 2 materials is beyond the scope of this report.

Several typical examples, displaying the alteration

of the- cathode materials, however, are given in

Table 4.

Anode materials shew little evidence associated with alteration by potassium seed. The distinct

color zoning, typical of cathode electrodes, is not

obvious although a faint yellowish discoloration is apparent at the bottom of some electrodes.

Generally, the hot surface material of anode

electrodes tends to be monoclinic, cubic and/cr

tetragonal phases with smaller unit cells than the

original bulk material. This suggests that the

surface phases are richer in Zr02 ana were formed

via vaporization of cerium oxides. For example,

the Zr 02 :Ce02 compositions initially rich in ZrOs

(32%Zr02 : 18%Ce02 , 50%Zr02 : 5 G;"XeC> 2 ) also show the

formation of some m.onoclinic Zr 02 on the surface.

The crystallinity of surface materials is generally

poorer (i.e., finer particles or core 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

M2AI 2 O 4 ) on the surface.

MgO insulators (cathode and anode) were also

investigated for potential chemical/phase changes

by x-ray diffraction. 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 at their surface. Surprisingly, x-ray

lines 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 MgA^Oi,. This phase is

also present in trace quantities at the surfaces of

many of the anode materials ana probably of cathode

materials. If KgAljOu is the phase present, then

AI 2 O 3 somehow must have been transported via the

vapor phase to react with available MgO. This may

be important since it should be recognized that a chemical reaction leading to the formation of a new

chemical species could have a degrading ertect on

the original r.echanical/electrical properties of a virgin material.

Although this portion of this report is concerned only with the documentation of phase/chenical characterization of electrode materials, several

related questions remain open to interpretation

1 . 1.7

and further study. Given that alteration of materials proceeded at the cathode at an accelerated

rate with respect to the anode, how does seed reach,

penetrate and react with materials? Is the difference in reactivity of cathode ar.d 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 cathece materials

should reflect nearly identical chemical reaction

phenomena. Possibly, K^Cg) attracted from the plasma to and/or formed at the cathode surface

(and attracted) according to K(g) -* iC'fg) + e is the controlling factor. iC(g) at the anode necessarily would be repelled except, perhaps, for open

circuit materials.

V. General Ccr.clusions

Some general conclusions can be drawn even though the behavior of materials subjected to l'-02 conditions is complex and analyses of results are difficult

:

1. The effects caused by seed penetration are more pronounced at the cathode wall . In general,

cathode degradation is substantially greater than

at the anode.

a. All Ce02 : Zr02~based cathode materials within

cooler zones shew new phases typical of reactions

with seed. Reaction intensity is more evident for

current-carrying materials.

b. Within the Zr02~cements and MgO insulators

below cathode materials, large qusntitites of seed,

condensed at K2CO3 and/or hydrate, are present.

C. Reaction with seed at the anode electrodes

is significantly less pronounced and, generally

occurs within localized interior zones.

2. Bloating of cathode materials apparently was caused by hydration and expansion of condensed seed

during and after the cool-covn period.

3. Contrary to the YjOj-stabiiized ZrOj materials,

and the anode materials, Ce02:Zr02 cathode materials appear to have been subjected to reduction

in situ followed by complete to nearly complete

reoxidation during shut-down. The phase transitions due to reduction are manifested particularly

by the presence of moncclinic ZrC>2 , together with

certain additional phases.

4. Surface x-ray analysis, particularly of Ce02:

ZrC>2-anode materials, show depletion of CeC>2. The

Ta-containing electrodes also show Ta-depleticn.

5. 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.

6. UgO insulators (anode and cathode) , especially

where exposed to the plasma, show compounds with

A1 2 0 3 and cubic zircor.ia from x-ray diffraction

data. These may contribute to the alteration and

the degradation of the erosion resistance of virgin

oaterials.

7 . Lead-out failure in electrode pairs was not caused by chemical attack, but rather by Joule

heating and eventual melting. Evidence for prescr.c

of KC1 within lead-out tubes indicated E-pcnetraticn within this area.

8. Electro-chemical effects probably enhanced

degradation of the electrode materials. However

other factors also were influential ar.d the overall

damage to the materials probably was caused by a combination of events.

9 . ZrC>2:Y203 materials at the cathode show evidenc

for partial destabilization of cubic zirconia.

Presently, it is unclear whether this process is related to seed reactions or electro-chemical

phenomena.

10 . Even though extensive degradation cf materials

occurred, the electrode system generally performed

satisfactorily. It may be possible through improved microstructures to control the degradation and

thereby permit the operation of these systems for longer times.

Copper V.

Figure 1. Plan View of Cathode Wall

1 . 1.8

e Optical S15M Port CH3 Three Therrococpies I r.) . Hot M$0, Ce'<J V^O anO Heal Sink r77A Too Thermocouples ( K) : Cck! .V5C and Heat Sink

• Voila^e Probe

I Tnermcco.p'-e f P>: Electrode Center lire

Figure 2. Open schematic of cathode and anode

showing naterials identification digits and

olocation of instrumentation

T(K)

Figure 4. Materials

Electrical Conductivities

at 10 -3 atm P 02

of Electrode

*c

Figure 3. Thermal Dif fusivities of Electrode

Materials (Fre-Test)

Figure 5. Anode Sections After Test

1.1.9

Figure 6. Cathode Sections After Test

TABLE 1

THERMAL EXPANSION OF MgO AND ELECTRODES

Material u/° C(10- 6 )*

Argon Air

MgO 15.6

15.5

14.0

12%Y20 3 :88%ZrO2 12.3 10.9

18%Ce02

: S2"Zr02 13.1 11.5

50ZCe02 :50ZZr02 13.6 12.2

75ZCe02 :25LZr0 2 14.5 13.2

78%Ce02:20%Zr02 :2%Ta2 05 14.9 13.6

* a e — where L = length

Differences in values probably due to stoichiometric changes caused by different oxygen pres- sures

•TABLE 2

COMPOSITION OF ELECTRODE MATERIALS

Measured * Nominal Zr02 Y2O3 Ce0 2 Ta^Os

* Mol 7. 12%Y2 03 :8SZZr0 2 88.8 11.2

18ZCc0 2 :82%Zr02 78.8 22.2

50ZCeO2 :50ZZrO2 46.2 53.8

75ZCe02

: 2 5%Zr02 19.6 80.4

78ZCe02 :20/:Zr0 2 : 16.5 82.2

2ZTa 2 0 5

*Determined by x-ray fluorescence techniques

(results accurate to ±5%)

TABLE 3

PRE-TEST PHASE COMPOSITION (X-RAY ANALYSIS)

ANODE AND CATHODE HATERIALS*

Composition Major Phases Minor Phases

MgO cubic o None

a = 4.212A

12%Y2 0 3 f.C.C. o None

88%Zr02 a = 5.151A

18%Ce0 2 tet 0 Trace of cubic (?)

82%Zr0 2 a = 5.15A

c = 5 . 24A

50%Ce0 2 tet 0 f.c.c., a = 5 . 15 a

50%Zr02 a = 5.26a

c *= 5.3GA

75%Ce0 2 f.C.C. None

25%ZrC2 a = 5.342A

78%Ce0 2 f.C.C. f.c.c. , a = 5.37A

2%Taa0 5 20%ZrO2

a *= 5.352a

*f.c.c. = face centered cubic (Fluorite)

tet c tetragonal; all compositions are white in color with exception of that containing 2%Ta205

(green)

1 . 1.10

TABLE 4

U-02 , Phase I, Post-Test

I POST-TEST PHASE COMPOSITION (X-RAY ANALYSIS) FOR CATHODE MATERIALS

Initial Bulk

Co-position (nol %)

Relative Position

of Material Analyzed

Results

Phases Present and Corrnentsi2ZrO?:lSCe0 2 .-1205' Open Circuit

S2Zr02 :lECe02 #1203

1.25A/cn2

50Zr02 :50Ce02 #1312

Open Circuit

50Zr02 :50CeO 2 #1307

1.25A/cn2

25Zr02 :75Ce0 2 #1414

•5A/cm2

25Zr02

: 75Ce02 #1416

1.25A/cn2

upper 1-2 mm white material

white surface

yellow interior zone

yellow, olive-green bottom

white zone, top

yellowish interior zone plus

snail portion of white zone yellowish interior zone plus

yellow-green botton zone yellow-green botton zone

yellow’ interior

yellow’-green bottom

top white zone + yellow zone

Nearly pure cor.o Zr02 + f.c.c. solid-solution +

cubic and/or tetragonal solid-solution

Mono Zr02 + poorly crystalline f.c.c. and/or tet sol id- so lut ions Mono Zr02 (strong) + (C;) , (strong) , + a trace phase (P) Mono Zr0 2 + (P)-Phase (very strong)

Mono Zr02 + two or more f.c.c. solid-solution phases Mono Zr0 2 + strong (Ci)-phase, a_=5.26A

Mono Zr02 + (C 2 )-phase + (P)-phase

(P)-phase strong + Mono Zr0 2 + (Cj)

Almost pure (Cj) -phase a=5.26A + trace mono Zr02 (Ci)-phase strong + additional cubic phases with

similar but slightly larger a_ + trace (P)-phase

(Ci)-phase, + cubic solid-solution, + trace of a third cubic phase

yellowish interior

yellowish-green botton

Nearly pure (Ci) -phase, (a=5.26A) + traces of mono

Zr02 end a second cubic phase

(P)-phase (very strong) + (C 2

N a=5.35A phase

(strong) t small amount (Ci)-phase

white (top) zone

yellow-brovm bottom

dark yellow botton

white (top) zone white + yellowish zones yellow-green bottom

Two f.c.c. solid-solutions. Major phase, a=5.35A;

minor phase has larger a_ Two f.c.c. phases. Major phase a=5.38A, nearly

that for pure Ce02 (a_=5.42) (P)-phase also present

At least three cubic phases. Major phase a=5.34A

f.c.c. a'wS.BA-S.lSA + f.c.c. with slightly smaller £

At least three cubic phases

At least three cubic phases + (P)-phase. Major

phase has a^5.38A (similar to pure Ce0 2 )

(Cx)-phase refers to a pyrochlore type phase ;; (P)-phase is (K, Ce,

similar to pure Ce02 ; f.c.c. « face center cubic (Fluorite); Tet = Zr-oxide) perovskite; (C 2 ) phase

Tetragonal ; Mono = Monoclinic.

NOTE: Compare results (phase assemblages and cell dimensions) with that given in Table 3 to assess change in starting materials. Color changes are indicative of chemical changes.

1.1.11