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NBSIR 78-1472

Materials For Fuel Cells

L. H. Bennett, M. I. Cohen, A. L. Dragoo, A. D. Franklin, A. J. McAlister and K. F. Young

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

Annual Report — January 1977 to December 1977

Issued July 1 978

Prepared for

> Power Systems Division

Department of Energy

Washington, D.C. 20545

100

. U56

78-1472

''2$0i»al Oureaa of Standards APR i ( 1879

I

NBSIR 78-1472

MATERIALS FOR FUEL CELLS

L. H. Bennett, M. I. Cohen, A. L. Dragoo, A. D. Franklin, A. J. McAlister and K. F. Young

National Measurement Laboratory

National Bureau of Standards

Washington, D C. 20234

Annual Report — January 1977 to December 1977

Issued July 1 978

Prepared for Power Systems Division

Department of Energy

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman. Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

AJ6S

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Table of Contents

Abstract

1. Introduction

2. Subtask Reports

2.1 Electrocatalysis

Introduction

2.1.1 Electrocatalysis on Non-Noble Metals

and Alloys

Introduction

Acid Stability Tests

Tungsten Carbide

Rhenium

W

0.5

Tl0.5 C

Mo, W C

1—x x Phosphides and Sulfides

2.1.2 Microprocessor - Controlled Potentiostat

Electrochemical Measurements

Introduction

The Potentiostat

Basic Experimental Set-up and Design Criteria

The Microcomputer

Board-by-Board Details

Programming

System Use

Cost of Implementation

Acknowledgements

Page

1

2

3

3

3

4

4

4

5

7

7

8

14

16

16

16

17

19

22

38

40

46

46

i
Table of Contents (Cont'd) Page

2.2 High Temperature Solid Electrolytes 49

Introduction 49

2.2.1 Electrolyte Materials 51

Introduction 51

Homogeneous Precipitation of the Carbonates 54

Preparation of Mixed Oxide Powders via

Organic Resins 58

Preparation of Mixed Oxide Powders via

Extraction of an Organic Complex 62

Summary 63

Acknowledgement 65

2.2.2 Impedance Measurements 65

Introduction 65

Data Handling 65

Experimental Results - The Bulk Arc 67

Experimental Results - The Electrode Arc 79

Summary 90

2.3 Battery Materials 94

Introduction 94

2.3.1 Beta-Alumina 94

Introduction 94

Experimental 99

Results 102

Analysis 105

Discussion 109

Conclusions 117

References 118

ii
TABLES

Page

1. Elec trocataly tic Activity of Tungsten Carbide Specimens. 6

2. Data Clock Rate Selection 28

3. Circuit Board Parts Cost and Commercial Availability 47

4. Resistance and Capacitance Values from ArcFits in Fig. 28.

5. Parameters for Electrode Arc for Hot-Pressed Ce„ - Y_ 0., „ r . 0.9 0.1 1.9b

6. Comparison of Internal Friction Relaxation Energies.

iii
Figures Page

1. Rhenium metal, in 1 M H^PO^ at 22°C. Sweep rate is 0.002 Hz.

Reference electrode is calomel.

2. c Ti_ ,-C, in 1 M H o P0. at 22°C. Sweep rate is 0.002 Hz. Reference 0.50.5 34

electrode is calomel.

3. M_ 0 W_ _ C, once leached in hot concentrated H„P0. . Voltammetric

0 . o 0 . 2 3 4

sweeps in 1M H^PO^ at 22°C. Sweep rate is 0.002 Hz. Reference

electrode is calomel.

4. M_ 0 W_ _ C, once leached in hot concentrated H„P0, . Electrode

0 . o U . Z 3 4

is pretreated by Cathodic Polarization in presence of "Au".

Voltammetric sweeps in 1M H^PO^ at 22°C. Sweep rate is 0.002 Hz.

Reference electrode is calomel.

5. Block diagram of potentiostat and cell.

6. Block diagram of complete system.

7. Schematic of CPU Board as modified. Also shown are components

mounted on front panel.

8. Memory map of system.

9. Schematic of 8 kilobyte RAM Board. Also shown Power-on Reset

circuit which is included on one RAM Board only.

10. Schematic of Variable speed serial interface.

11. Schematic of Timer-Converter Board: Address decoding system.

12. Timer-Converter Board: Timer section.

13. Timer-Converter Board: DAC section.

14. Timer-Converter Board: modification to DAC section proposed for

Interrupt-Triggered return to zero output.

15. Timer-Converter Board: ADC section.

16. Schematic of Signal Distribution Board.

9

10

11

12

18

23

25

26

27

29

30

31

33

34

36

39

IV
Page

17. a) Cross-assembled Data Acquisition Program 41

b) Data Acquisition Program (continued) 42

c) Data Acquisition Program (continued) 43

18. a) Sample of System Output 44

b) System Output (continued) 45

19. Aggregation and porosity.

19a. Aggregate particle from a precipitation of Ce-Y hydroxides by

the addition of ammonium hydroxide to a nitrate solution. 53

19b. Fracture surface of a sintered specimen which was fabricated

from a powder containing aggregates similar to that shown in

Fig. 19a. 53

20. Schematic of the carbonate procedure. 55

21. Ce-Y carbonate platelets. 60

22. Ce-Y oxide nodules prepared by homogeneous precipitation and

dispersal in a succinic acid resin. 60

23. Ce-Y oxide, coarse aggregates after grinding; powder prepared

by nonhomogeneous precipitation of the hydroxides and dispersal

in a tartaric acid resin. 61

24. Ce-Y oxide aggregate and fines; powder prepared by nonhomogeneous

precipitation of the hydroxides and dispersal in a tartarto-ethylene

glycol ester resin. 61

25. Ce-Y oxide aggregate; powder prepared by nonhomogeneous precipitation of the salicylate and extraction into isoamyl alcohol. 64

26. Equivalent Circuit for Y-doped CeO., Ceramic with Sputtered Pt

Electrodes. 66

27. Complex Impedance Plot for Bulk Arc for Ce^

9 1

*^1 95 ^*2 ^

^

and N 9 (x) at 350°C. 69

v
Page

28. Complex impedance plots for Ce Y CL QC. at temperatures near

u.y u.x x* yD

250°C. The curves are fitted to the data points from 31.6 kHz

(indicated by the arrow) to 12 MHz (extreme left). They represent:

1, 0

2 at 253°C ; 2, N

2 at 252°C; 3, 0

2 at 248°C; 4, N

? at 248°C;

5, N

2 at 248°C. The data were taken in the order indicated, with

an excursion to 804°C in 0

2 between curves 1 and 2, and to 804°C in

both 0

? and N

2 between curves 2 and 3.

29. Complex impedance plot for sintered Ce Yn

CL Q(- at 250°C in 0„.

Calculated values are from the equivalent circuit of Fig. 26 (high

frequency side), fitted to the data. The arc represents Z* , . gb

30. Complex impedance plot for hot-pressed Ce^

9 Y

o 1 ^1 95

at ^50°C in

0 2< Calculated values are from the equivalent circuit of Fig. 26,

fitted to the data. The arc represents Z* . : gb

31. Complex impedance plot for hot-pressed Ce^

9 ^0 1 ^1 95 an at

several temperatures : D , 252°C; o, 350°C; x, 470°C; *, 580°C.

32. Arrhenius plot for bulk resistance, R^, for hot-pressed Ce^

9 ^0 1 ^1 95

in N

?

.

33. Complex impedance plot at low frequencies for hot-pressed

Cen Y 0 in 0 at 702°C. D , experimental points; x, calu.y u*j_ x.yj z culated using eqn (9) with fitted values for R. and x, and with

d

the exponent 1/2 replaced by n, whose value is selected by fitting.

34. Schematic of Porous Electrode-Electrolyte Interface. R^ denotes

the resistive path for the oxygen discharge reaction, and W the

diffusive path.

35. Comparison of experimental values (Ref. 63) for effective electrode

capacitance per unit area ^) to values calculated with eqn. 16

(see text). The solid line is illustrative of the expected behavior.

70

72

73

75

76

80

81

84

vi
Page

36. Complex impedance plots at low frequencies for hot-pressed

Ce^

9

Yq

i Of 99

in 0^ at 478°C (x) and N

2 at 470°C ( )

37. Complex impedance plot at low frequencies for hot-pressed

Ce

Q 9 Yq 1 0

1 99

in 0

? at 593°C (+) and in N

2 at 582°C ( ).

38. Complex impedance plot at low frequencies for hot-pressed

Ce

0 9 Y

0 1 °1 95

ln °? at 702 ° C ( +) and in n

2 at 6 9A°C ( ).

39. Complex impedance plot at low frequencies for hot-pressed

Ce

Q 9 Y

q p 95

at 804°C in 0

2 (') and in N

2

( ).

40. Temperature dependence of TAZ for Ce~ _ Y_ , 0, oc at 804°C

el 0.90.11.95

in Tank 0 9 (') and Tank N 9 ( ).

41. Temperature dependence r of C for Ce„ „ Y_ 0, in tank 0 o z 0.90. 11. 95 2 and tank N

2

«

42. Arrangement of ions in the conducting plane of beta-alumina. Dotted

lines enclose "cell"; two such "cells", on adjacent conducting planes,

are included in the crystallographic unit cell. Na+

ion sites are:

1, BR site; 2, aBR site; and 3, MO site. Mirror planes described

in text are indicated by dashed lines.

43. Flexural strain produced in specimen bar for two orientations.

44. Experimental arrangement for internal friction experiment.

45. Internal friction for the sodium beta-alumina specimen with the

width horizontal. Falling temperature, x. Rinsing temperature 0.

46. Internal friction for the sodium beta-alumina specimen with the

width vertical. Falling temperature x. Rising temperature 0.

47. Internal friction for the sodium beta alumina specimen with the

width vertical, details of the temperature variation. Falling

temperature x, rising temperature D , estimated equilibrium data +.

86

87

88

89

91

92

96

100

101

103

104

106

vxi
Page

\

48. Maximum systematic temperature error in internal friction data.

49. Arrhenius plot for internal friction in sodium beta-alumina. Data

of Barmatz and Farrow (Ref. 74) o, our data with maximum systematic

error added x, and subtracted . The error bars are reported

random error standard deviations.

50. Arrangement and jumps of split interstitials from one "cell" to

another in sodium beta-alumina. aBR sites lie at intersections

of trigonal network; MO sites are indicated by small filled circles.

Lines connecting MO sites indicate possible split interstitials

and arrows the paths from one "cell" to the next.

51. Approach of the split interstitial model for sodium beta-alumina

to the activated state for both transport and reorientation jumps.

The large circle is the lattice oxygen ion, the shaded circles

are sodium ions on MO sites, and the small circle is the unoccupied

MO site.

108

110

112

114

viii
ABSTRACT

Research is described on hydrogen oxidation electrocatalysis on a number

of compounds related to tungsten carbide (WC). Compounds in the molybdenumtungsten carbide series, Mo, W C with x vO . 2 , were found to be active,

x —x x although some uncertainty exists about the purity of the samples. Some activity

was also noted in

^ Ti^

^ C and rhenium metal'. Experiments on the preparation of molybdenum phosphide, MoP , and iron phosphide, FeP.

?

, are described.

The microprocessor-controlled potentiostat for electrochemical measurements was completed and is described in detail.

Experiments on several techniques for the preparation of very finely divided

Y-doped Ce0

9 powder are described, the powders to serve as raw materials for

hot-pressing pure, dense, homogeneous ceramics. Straight sintering of some

of these specimens yield densities of 95-97% theoretical.

The equivalent circuit previously suggested appears to fit very well

the frequency-dependence of the impedance of sintered specimens of yttrium-doped

cerium dioxide, Ce Y , 0, , with sputtered or evaporated platinum (Pt)

\J • kJ v7«-L electrodes. The bulk part of the data is independent of oxygen partial pressure

and varies with porosity as expected. An activation energy for the crystal

conduction was obtained in agreement with literature values. High temperature

instability in the Pt electrodes was observed. The electrode polarization

impedance is fitted rather well by a Cole-Cole expression. It cannot be attributted to a model proposed earlier involving oxygen atom diffusion in the Pt electrode.

Internal friction experiments on a single crystal of sodium beta alumina,

Na B-Al 0„, are interpreted in terms of the sodium ion motion. They are compatible with the split-interstitial model on mid-oxygen sites for the excess

sodium ions, but not with the simple interstitial occupying anti-Beevers-Ross

sites

1
1. Introduction

The National Bureau of Standards has undertaken a program of

research on materials for fuel cells and batteries. This program includes

studies of electrocatalysis in liquid-electrolyte cells and of solid

electrolytes, in high-temperature cells and in batteries. The overall

objectives of the program are:

1. to provide data allowing selection of optimum materials

for critical aspects of fuel cell operations;

2. to improve the measurement capability and standards

for assessment of performance (e.g., efficiency of electrode

processes and lifetime of components) of materials in fuel

cells and batteries.

This report will describe the results for the second year of the program.

The Materials for Fuel Cells Program at NBS consists of four major

elements, which taken together reinforce each other in techniques and

concepts; these elements embrace:

i. electrocatalysis, especially oxygen reduction, on

non-metallic surfaces;

ii. electrocatalysis, especially hydrogen oxidation, on non-noble

metals and alloys;

iii. degradation mechanisms involving solid oxygen- transporting

electrolytes; and

iv. mechanisms controlling transport of sodium ions in sodium

beta-alumina

2
Items ii and iii were partially supported by DOE during this past

year, and item i was essentially dormant but work will be resumed in the

coming year. This report will cover progress in items ii, iii, and iv

2. Subtask Reports

2.1 Electrocatalysis

Introduction

In most fuel cell applications a major contribution to the overvoltage,

and therefore to a reduction in useful voltage and to efficiency, comes

from electrode polarization. For use in hot H^PO^ no satisfactory

substitutes for Pt have been found, but Pt is not completely satisfactory.

In catalyst loadings sufficiently large to ensure adequate efficiency

and lifetime it makes a significant contribution to the cost. At the

anode its sensitivity to CO poisoning forces the use of higher temperatures

and catalytic shift conversion in the fuel processing train. At the

cathode a mixed potential situation seems to exist and the open-circuit

voltage lies below what should be possible. Thus a continued study of

electrocatalysis and electrocatalysts in H^PO^ is worthwhile.

In our program we have elected to study refractory metal-metalloid

compounds (e.q. WC) as potential anode catalysts. Work on oxide catalysts

for use at the cathode was suspended for lack of funds, but some effort

has gone into automating the electrochemical measurements used in studying

electrocatalysis of either kind. These two lines of work are reported

here

3
2 . 1 . 1 . Electrocatalysis on Non-Noble Metals and Alloys

Introduction

A limiting factor in full scale implementation of the low

temperature acid fuel cell for electric power production is the scarcity

and expense of Pt, the current material of choice for fuel and oxygen

electrocatalysts. In the search for adequate non-precious substitutes for

Pt, the "refractory hard metals" - alloys of transition metals with

carbon, boron, nitrogen, etc. - offer a promising area of investigation,

for they are platinum-like in a number of respects: they have d-band

vacancies, have refractory properties, and display considerable chemical

activity. Several such materials, notably WC and CoP^, have been shown to

have useful oxidation activity and acid stability over practical ranges

of voltage. This project entails a general search for other acid stable,

catalytically active materials, and includes detailed characterization of

known active materials as a guide to achieving this end.

Acid Stability Tests

Four samples of transition metal compounds having the NiAs structure

were obtained commercially and examined. The NiAs structure is closely

related to the WC structure; WC has metal atoms at simple hexagonal sites,

and C repetitively occupies but one of the two trigonal sites in the

primitive cell; in the NiAS structure, the unit cell is doubled, with the

metalloid occupying the trigonal sites alternately. Samples of FeSe, VS,

CoS, and NiS were examined. All reacted visibly with hot concentrated

H

3 P0

4 (85%, 82°C),the NiS more slowly than the others. Studies of the NiS

by cyclic voltammetry showed no activity for H

9 oxidation in H^PO^.

4
A number of commercially obtained phosphides were also examined.

FeP, Fe^P, Co^P, anc^ FIi_,P a ^ reacted rapidly with the hot, concentrated

H^PO^. A sample of "MoP" proved to be only unreacted elements.

Fe^^Ni^^P^^Bg ("Metglas") reacted very slowly, but showed no II^ oxidation

activity.

Samples 'of the superalloy component Ni Al^ ^Ti are attacked very

slowly by hot H._,PO^. Their stability and possible activity will be

studied voltammetrical ly

Tungsten Carbide

Since the earliest work on WC , it has been realized that oxidative

1 treatment tends to enhance its catalytic activity. We predicted [1] on

the basis of H 0 uptake and Auger surface analysis measurements on

variously prepared WC samples by Ross and Stonehart [2] that alloying

substitution of oxygen for carbon in WC was responsible for this

enchancement. We therefore carried out hydrogen oxidation measurements on

a series of WC samples prepared in different ways, some intended to

include oxygen as an alloying addition in the bulk lattice. Two samples

were carburized tungsten foils. The rest were powders of mesh size

ranging from -200 to +400. Idenification of the product was by x-ray

diffraction. Powder electrodes were prepared by hot pressing Teflon 30

bonded powders onto Au disks coated with a thin layer of teflon. Cyclic

voltammetric measurements were carried out on flooded electrodes, in

stirred lM H P0^ at 22°C, with either or N 9 bubbled as appropriate.

Results are summarized in Table 1.

Figures in brackets indicate the literature references given at the end

of this report.

5
Table 1

Electrocataly tic Activity of Tungsten Carbide Specimens

H

2 Oxidation Current Per Unit

Sample No. Form Preparation Capacitance at +0,3 Volts (RHE)

1 foil W + CH

4

5.7 X 10- 3 A/F

2 ' powder W + C 10.1 X 10~ 3 A/F

3 powder W0^+ c 17.1 X 10- 3 A/F

Preparation and Support:

1) Sheet, formed by exposing 0.5 mil W sheet to a 5% CH^-95% mixture

for 4 hours at 1500°C. Supported on epoxy.

2) Commercially obtained powder, formed by direct reaction of W with

carbon. Supported on Ta sheet.

3) Powder, formed by heating WO^ and carbon powder for 2 hours at 1800°C

in H

2

- Supported on Ta sheet, and on Au with teflon bonding.

Current is normalized to capacitance, evaluated from the sweep rate and

current at +0.3 volts (RHE).

Clearly, those samples likeliest to contain alloyed oxygen in the

bulk show the greatest specific activity. This result has been confirmed

in much greater detail in a recent paper by Ross and Stonehart [4] who

report activity and characterization (x-ray diffraction. Auger, and ESCA)

measurements on variously prepared samples, and definitely correlate

higher activity with the presence of carbon substituted oxygen.

The mechanism of this oxygen-alloying effect remains unclear.

Perhaps it is simply a local electronic structure effect, but there are

indications that crystallite morphology plays an important role here. We

are preparing WC samples of varying oxygen content and activity by the

method of Kulcickyj and Petrdlck (CO carburization of tungstic acids

variously precipitated from ammonium metatungstate solution)

6
Microstructure and crystallite morphology will be studied by x-ray

diffraction and electron microscopy and correlated with sample hydrogen

oxidation activity.

Hydrogen oxidation activity and a useful range of acid stability

were observed in several materials this year. Brief discussions of each

material follow.

Rhenium

In our published studies of the electronic structure of WC [3] we

noted similarities between the experimental state distribution of WC and

the calculated density of states of the hep metal Re (filled to an

electron per atom ratio of 6, as a model for hexagonally coordinated W)

It was of interest, therefore, to examine Re metal for stability and

oxidation activity in H^po^ . Fig. 1 shows voltammetric sweeps on a Re

electrode fabricated by pressing Teflon 30 bonded Re powder (99.99% stated

purity) onto a teflon coated Au disc. Activity is observed, with

corrosion onset at ^ +0.7 volts (RHE)

W

5 Ti

,5

C

WC is catalytically active while W metal is not. We have shown [3]

that this change in chemical activity accompanies a change in the local

distribution in energy of electronic states at W sites from W to WC. Two

factors play a role here: an electronic factor arising from interaction

of the metal with the metalloid; and a geometric factor associated with

the difference in lattice symmetry. This latter factor makes it of

interest to look for activity by W in other than hexagonal refractory hard

metals. Voltammetric sweeps of the alloy W Ti -C (cubic NaCl structure)

are shown in Fig. 2. The material is active. If one assumes its specific

7
capacitance to be approximately equal to that of WC, then its specific

activity scales about as the concentration of W, relative to a WC sample

of low or no bulk oxygen content.

Mo, W C 1-x x We have continued our study of the Mo^ ^w^C system, which condenses

in the WC structure. We have examined additional electrodes made (a) from

our original sample of Mo W n C, and (b) from a second sample of

• O • Mo w -i Q C. All of these electrodes show some activity for H„ oxidation,

but none as high overall or so good at low overpotential as that upon

which we commented in earlier reports.

This compound is difficult to make, and up to 2% by weight of Ni

and/or Co is added to the reactants to catalyze the solid state reaction.

Neither Ni or Co is believed to dissolve in the bulk, but some small

amount may. In addition, Fe metal was found to contaminate the first

sample as received; it was probably introduced during the grinding to

powder of the porous lumps produced during compound formation. These

contaminants react slowly, with visible discoloration, with hot concentrated phosphoric acid. The present measurements were made on

electrodes fabricated from powders (a) cleaned by leaching in hot

concentrated H^po^ for several days, then washing, or (b) by repetition of

step (a) until no further discoloration of the acid could be observed.

The active electrode of Mo w c described in our last report was

• O • ^ made from once- leached powder, as in procedure (a)

bound with diluted

Teflon 30, and hot pressed onto an Au disc coated with teflon. It was

discovered that this support "leaked"; that is, the Au was exposed to the

acid, and its voltammetric characteristics could be seen at high

8
300 x

10

Amp

o

9

Reference

electrode

is

calomel.
60*

I0

-D

Amp

10

Sweep

rate

is

0.002

Hz.

Reference
60x|0' b

Amp <3- di

11

C,

once

leached

in

hot

concentrated H3

PO4.

Vol

tammetric

sweeps

in

at

22°C.

Sweep

rate

is

0.002

Hz.

Reference

electrode

is

calomel.
60x

I0" D

Amp

-O

a)

ra

aj s:

s- r— >—

-)-> O)

ai c e

s- •>- o

CL i—

i/> ro co Cl O

•r- <u

a> co

CD 2 r- -o co o OI

12
potential; moreover, the Au stock from which the discs were fabricated was

slightly active, due to contamination, although not as active as the given

electrode. Two new samples on Au discs were fabricated from the Mo w C

• 8 .2

lot, one from the once-leached powder, and one from repetitively leached

powder of the same lot. Neither sample "leaked".

The once-leached material is active (Fig. 3), though not nearly as

active as our earlier "Au-leached" sample. The activity of the

once-leached sample can be enhanced somewhat by a pretreatment in which

the electrode is placed in contact with a strip of (contaminated) Au stock

and repetitively swept up to the evolution potential. (Fig, 4)

A recheck of our earlier data on WC, Re, and W Ti r C, indicated

.5 .5

that none were affected by Au "leakage", but currently, all electrodes are

pressed onto teflon alone, with electrical contact being made with Ta

metal which, as we have shown in earlier work, is passive over a wide

voltage range in H^po^.

Evidently, the compound Mo

-l_ xwx

c is active for the oxidation of

although not as active as our first experiment indicated. It is difficult

to prepare this material in pure form; we have not yet succeeded in doing

so ourselves. Studies on the Mo^

xW

x

C system continue. Eight new

2 production lot samples from WaChang-Teledyne have been leached in hot

H^po^ to remove Fe contamination, and teflon mounted electrodes are being

prepared for activity testing.

^Certain commercial equipment, instruments, or materials are identified in

this report in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the material or equipment

identified is necessarily the best available for the purpose.

13
Phosphides and Sulfides

A number of phosphorous rich transition metal phosphides and layered

transition metal sulfides have been reported to possess some range of

stability and hydrogen oxidation activity in sulfuric acid. We are

working on the extension of these results to phosphoric acid, and on

determining the electrochemical properties of related compounds which can

be prepared in similar ways, particularly intercalated compounds of the

layered species. We are currently engaged in efforts to fabricate such

compounds, most of which are not commercially available (studies of some

commercially available phosphides and sulfides which were found unstable

or inactive have been described in earlier reports of this series.) So

far we have concentrated on phosphides, particularly MoP and FeP^. Our

first approach was to isolate the metal and red phosphorous in

stoichiometric proportion in evacuated quartz ampoules. The sealed

ampoules were heated for 2 hours at 550°C. X-ray diffraction studies of

the products showed that the Mo and P were unreacted, and the Fe and P

reacted to form an unidentified product of complex diffraction pattern

which reacted slowly with H^po^ and proved inactive for H

2 oxidation. In

the second attempt, we tried to produce the compound MoP^ by reaction of

the metal with Ca^P^. The reactants were mixed, placed in a Ta crucible,

and induction heated in vacuum for 2 hours at 600°C. The product was

washed in 0.1N HC1 to remove Ca, with careful venting and burning of

gaseous products to remove any poisonous phosphine produced. The product

appeared to be acid stable, but was inactive for H oxidation. It had a

complex x-ray diffraction pattern, not containing those of Mo or MoP^. We

are currently setting up a third preparative method, in which metal and

14
metalloid are independently heated, the metal to a suitable reactive

temperature and metalloid to vaporization. Both are placed in a steam of

inert carrier gas, the metalloid upstream of the metal.

15
2.1.2 Microprocessor-controlled Potentiostat

Introduction

The advent of the microprocessor (yP) [5] has opened up a relatively low

cost approach to digital techniques for control and data handling in the

laboratory. Although somewhat limited in throughput and "number crunching"

ability (as compared to minicomputers) , these devices are more than adequate

for many applications in instrument control.

For our purposes, the device is considered as a Central Processing

Unit (CPU) on a single chip capable (at maximum speed) of handling

250,000 instructions per second (estimated by utilizing an average of

2 bytes per instruction with 2 cycles per byte at a clock rate of 1 MHz)

With the addition of suitable input-output interfaces and peripherals and

of memory [both Read-Only-Memory (ROM) and Random-Access-memory (RAM) ] , a

microcomputer is constructed. By the use of appropriate Digital to Analog

Converters (DAC) and Analog to Digital Converters (ADC) we can interface this

microcomputer with analog instruments for operational control and data

acquisition

The primary goal of this work was to design and implement a relatively

low cost control package to be utilized with a standard laboratory potentiostat.

The bulk of this paper is concerned with the implementation of this concept

for the operational control of and data acquisition from a potentiostat used

in electrochemical measurements. However, the System Use Section will be

concerned with some planned uses of the equipment.

The Potentiostat

A potentiostat is an electronic device used by an electrochemist to

control the potential of a cell in an accurate predetermined manner. The

primary quantity that the electrochemist measures is the current as a function

of the potentiostatically controlled cell potential and the rate of change of

that potential. The cell potential, of course, varies with the input voltage

which may be either AC or DC. The waveshape and frequency response are governed

by the characteristic impedances of the overall system.

16
It is useful at this point to review the characteristics of a

Potentiostat [6] for the purpose of defining the factors that must be

controlled by the microcomputer. The instrument can be modeled (Fig. 5)

as two amplifiers, one an error amplifier and the other a power amplifier.

The electrochemical cell with which the potentiostat is used can be

represented by the impedances Z^ and (for simplicity we assume that

the cell contains only resistive components) . Z is the series impedance in

s relationship to the counter electrode, the cell electrolyte and the current leads,

while Z is the impedance between the reference electrode and the working

c electrode (sample) across which the potential is developed that is to be controlled

by the potentiostat. The operation of the system can be outlined as the

comparison of the potential across Z with some input potential, amplific cation of the resulting difference signal and application of this amplified

potential to the cell. The resulting current changes the potential across

Z and the process continues until the difference signal is essentially zero,

c Basic Experimental Set-up and Design Criteria

A very common experimental arrangement consists of a motorized potentiometer used to generate an input potential which varies linearly with

time (ramp or triangular waveshape) as input to the potentiostat. The

output of the instrument is the Cell current which is plotted against cell

potential on an X-Y recorder. Going beyond this experimental minimum, we

find varying arrangements for input and output. The source potential is

generated by oscillators, stepping-motor-driven sources, pulse generators,

etc. These circuits allow the use of DC, ramps, stepped sweeps, waves, (sine,

square and triangle), and pulse inputs. Compensation for changes in

potentials due to IR drop across the cell's internal impedance is often

added. Output data is often acquired by 2 pen recorders, logarithmic

recorders, digital voltmeters, etc.

17
5. Block diagram of potentiostat and cell.

18
From this, we see that ideally, a digital control system for potentiostatic measurements should be capable of generating DC and AC potentials

whose waveform must be easily controlled. Data acquisition should be high

speed and the data should be of high resolution. The output of the

acquired data must be flexible in format. To be useful, the instrument

must be able to make at least simple computations using the acquired data

so that results will be in the most desirable form.

Additional operational requirements can be imposed. The equipment

should operate unattended (except for start-up) , and must be able to

terminate the experiment according to criteria established by the operator.

If the device cannot perform the required data analysis by itself, it must

have provision for communication with a larger machine. Finally, operation

and programming of the instrument must be relatively simple and fast.

All of these functions can be performed by a system consisting of

appropriate DAC's and ADC's combined with a digital computer. A minicomputer

dedicated to this purpose would be under-utilized and was therefore not

considered. The use of a communication line to a central computer with

dedicated hardware at the instrument was considered but not used since

cost estimates indicated it would be essentially no cheaper than the use

of a microcomputer. A desk calculator could be used but is more expensive.

The Microcomputer [ 7

The system designer must first evaluate the trade-offs necessary to

determine which microprocessor (NP) matches the application best. Many factors

enter into this decision and they must be carefully evaluated before

proceeding. We have stressed the following:

1. Word size - The word length of a microprocessor at the present

time can be 4, 8, 12, 16 bits, etc. In general, the power of the unit

increases with increasing word length, but so does the overall

system cost.

19
2. Speed - The speed of a uP is not only a function of clock frequency,

but also depends upon other factors such as the instruction cycle and

the instruction set.

3. Instruction set - In general, the larger the instruction set, the

more powerful the device. We must note, however, that in some

cases, a device may require special instructions for a function

(e.g., input-output instructions) that are not needed by other

devices

4. Chip family - This factor is of great importance to the researcher.

A well integrated and compatible set of circuits that reduce the

time and effort necessary for design and construction of a system

can more than offset other factors.

5. Available software - If we examine the overall costs of a system,

programming is by far the most expensive element. If a body of

software already exists, then the overall expense of the system

is effectively reduced.

6. Available hardware - Obviously, the less time and effort taken from

the researcher for design and construction, the better. New devices

and much new hardware are appearing all the time and the designer

should be alert.

Details necessary for evaluation of many of these factors are available

from the yP manufacturer's data sheets. The relative importance of each factor

is dependent upon the particular project, the potentiostat used and the

individual designer.

Our choice was the 6800 Microprocessor. [8] It is an 8 bit device, with a

clock frequency of up to 1 MHz and a minimum instruction time of 2 cycles.

It has a reasonably extensive instruction set with several addressing modes

giving great flexibility. Peripherals are treated as memory locations, thus

requiring no special instructions. (This is probably the most important

20
reason for selecting the 6800. This feature, desirable for the potentiostat

is not, in general, found in other microprocessors at similar cost). The

accessory chips are appropriate allowing relatively simple and easy access

to peripherals with a minum of design. Although the available software is

not as extensive as that available for the 8080 microprocessor (Intel Corp.)

it is adequate for our purposes. There is a considerable variety of peripheral

hardware available for microcomputer systems based on the 6800 yP, enabling the

designer to add on functions to his system with minmal problems of design

and construction.

The 6800 based microcomputer we have constructed is based on the

manufacturer’s evaluation board, [9] which has been somewhat modified. This

board provides the microprocessor with its support circuits (clock, gating

circuits, etc.). It also provides interface adaptors for 3 peripherals

[2 parallel using Peripheral Interface Adaptors (PIA) and one serial

using an Asynchronous Communications Interface Adaptor (ACIA)]. One of

the parallel ports is dedicated to communication with a teletype or low

speed RS-232-C device. (A detailed discussion of all the circuit boards

will follow later on in this paper). In our system, communication with a

central computer, if desired, is provided through a terminal with a modem

and an acoustic coupler. However, if we wish, data can be recorded on

cassette tape at the terminal (at 30 to 120 characters per second) for

later processing by the central computer.

The rest of the microcomputer consists of 5 circuit boards. There are

two 8196 byte (one byte equals an 8 bit word) RAM boards, one converter (DAC's,

ADC) and timer board, one board for signal distribution, and one variable speed

serial interface board. The two memory boards feature data buffers and fully

decoded address circuitry and include switching operations to place their base

address at any 4K boundary in the address field. The variable speed, serial

interface provides interfacing for RS-232-C communication levels at 8 speeds

ranging from 10 characters per second to 960 characters per second.

21
The converter and timer board contains three PIA's with interfacing,

one of which is dedicated to a programmable timer. This has a range of

microseconds to 40 minutes per timing point and generates an interrupt

signal at the conclusion of the period. A second PIA is dedicated to

two 10 bit digital to analog converters which have an output range of

-5.12 to +5.11 volts. The third PIA interfaces with a 10 bit tracking ADC.

The signal distribution board provides connection points for input and

output to the potentiostat or other device. Of the two DAC outputs one is

normally connected to the input potential point on the potentiostat. The other

-5 6 output is connected to a voltage controlled oscillator (3 x 10 to 3 x 10 Hz).

There are three ADC connectors. One provides an output of the analog voltage

generated by a DAC which output is subsequently compared with the input (identified

in Fig. 16 as ECHO) , while the other two connectors are inputs which are connected

thru a switch to a precision voltage clipper (discussed below) . The output of

the clipper provides the input to the ADC. These two inputs have a common return.

(One of these inputs carries the full signal, while the other input carries half

of this potential)

Communication between operator and equipment occurs via a Teletype or

RS-232-C interface to a terminal. There is a monitor program in the ROM which

obviates the need for panel switch programming. In fact, the only switches needed

(other than power on) are a reset and communication speed set switch. Power

requirements are +5V @ 6A, +15 V @ 50 mA, and +12 V @ 100 mA. A small cooling

fan is necessary if the microcomputer is enclosed in a cabinet. Figure 6

shows the overall block diagram of the system.

Board-by-Board Details

The CPU of the microcomputer is a somewhat modified version of the

manufacturer's evaluation board #1. The original board was designed to be

a stand-alone unit with limited memory. Addressing is redundant and expansion of the memory is possible only by removing the redundancy. This is

easily accomplished by removing all memory from the evaluation board except

22
6. Block diagram of complete system.

23
for the monitor ROM (address E000 hex) and its associated scratch-pad RAM

(address A000 hex). An additional redundancy exists for the peripheral addresses.

Since it is desirable for the peripheral addressed to be separated from main

memory, we added an address line to the scratch pad enabling several blocks of

addresses in the memory region that is identified by Axxx hex. • An additional line

was run from the PIA used for a low speed terminal. This line allows switch

selection of 10 or 30 character per second operation. A schematic of

the resulting board, including only operational circuits, is shown in

Fig. 7. A map of available memory is shown in Fig. 8.

The monitor ROM (MIKBUG*) , which is included in the evaluation kit, allows

for loading from and generating of tapes, displaying and changing the contents

of memory locations, displaying the contents of the primary registers and

entering a user written program. Since most of the commercially available

software is written using MIKBUG subroutines for control of input and output, we

have retained it. Although limited in scope, it is adequate for our purposes.

The 2 phase multivibrator clock oscillates at a frequency of 1 MHz for

operation of the microcomputer. Three buses (data, address and control) are

brought out through a dual 43 pin edge connector. As previously mentioned, there

are 3 on-board peripheral adaptors. One of these adaptors (a PIA) is reserved

to interface with a planned addition of a floppy disc unit.

The 8K memory boards (Fig. 9) are based on 1024 x 1 bit static memory

Integrated Circuits (IC) . These memories have access times of 500 nanoseconds (nsec)

or better. Address and data lines are buffered and base address selection is

made by means of a hexadecimal coded switch. Connection to the bus is via a

dual 43 pin edge connector. Only one board contains the Poweron Reset [10]

circuit shown in Fig. 9.

*Registered Trademark Motorola (Firmware ROM program)

24
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8. Memory map of system.

26
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GND

A 0

A 1

A 2

A 3

A 4

A 5

A 6

A 7

A 8

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A 1 0 A 1 1 A 1 2

A1 3

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Z-4 74LS27 TRIPLE 3-INPUT NOR GATE

Z-5 74LS138 3 LINE TO 8 LINE DECODER

Z-6 555 TIMER

Z-7.8 74LS04 HEX INVERTER Z-9 74LS266 QUAD 2 INPUT EXCLUSIVE NOR (OPEN COLL.) Z-10->73 21L02-1 RAM (1024 x 1) S-1 AMP 53137-1 HEX SWITCH

9 . Schematic of 8 kilobyte RAM Board. Also shown Power-on Reset

circuit which is included on one RAM Board only.

27
Figure 10 is a schematic of the variable speed interface together with

the low speed terminal connections. RS-232-C interface connections are shown for

mating with an Automatic Send Receiver (ASR) terminal that has the capability for

120 characters per second operating speed when recording or playing back with its

built-in casette tape drives. If desired, data rates other than those shown can be

obtained through' the use of the MC 14411 bit rate generator. [9,12] These data

rates can be achieved by wiring pins 22 and 23 (Rate Select B and Rate Select A)

according to Table 2 or by connecting them to a peripheral interface and programming them. There are also 8 other speeds directly available from the chip at

any rate select configuration. (Note that the output frequency for each of

the positions is found by multiplying the characters/sec by 160)

Table 2. Data Clock Rate Selection

Rate Multiplier Rate Select A Connection Rate Select B Connection

XI 0 0

X8 1 0

X16 -----------o----------- 1 (present connections)

X64 1 1

where : 0 = GND

1 = +5V

The timer-converter board schematics are shown in Fig. 11 through Fig. 15.

The interval timer [11] section (Fig. 12) is software programmable and runs off

the system clock (1 MHz). Thus, its accuracy is dependent upon the stability and

frequency of that signal. Since we are not using a crystal controlled oscillator

the stated set times must be considered to be nominal values. At the present time,

they are sufficiently precise for our application. Installation of a two phase

crystal controlled clock is not difficult and can be done when necessary.

The setting of the time interval is done in two steps. First a code,

defining which output from the 7490-74452 decade divider chain is to be

fed to the presettable counter (74455), is loaded, via the PIA A side,

into the 9312 multiplexer. Then, the 74455 is preset with the number of

counts desired (0-256) from the B side of the PIA, and counting (down count)

28
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11. Schematic of Timer-Converter Board: Address decoding system.

30
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(AFF4-

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31

Timer-Converter

Board:

Timer

section
is started. When the desired number of timing pulses has been counted, the

74455 puts out an interrupt pulse to the CPU. The minimum interval time for

the timer is 1 microsecond (nsec) . The maximum interval is 42.6 minutes.

However, the maximum repetition rate is dependent on the interrupt handling

routine in the microprocessor. We have not directly determined this elapsed

time for the shortest program but estimate it. to be on the order of 30 nsec.

The program we are using takes on the order of 250 ysec. As will be shown, this

places a lower limit on the effective pulse time period for the input to the

potentiostat

There are two identical DAC ' s (Fig. 13) on this board. [13] Under program

control, each one can be independently loaded from the PIA utilizing the 7475

latches. As presently formatted, the input to the DAC remains the same until

changed by the CPU. Thus, the output of the DAC, which is fed to the potentiostat,

remains at a constant level until the next value is placed on the latches. For

our purposes this is satisfactory. A simple addition of circuitry (shown as

Fig. 14) will remove this feature and allow the input pulse width to the

potentiostat to closely follow the timer period.

As a check on performance, a program for generating a triangle wave (in steps

of 10 millivolts (mV)) was written. An effort was made to minimize program steps.

When run, this program generates a DAC output triangular wave of 25 Hz. The

time per point works out to be 20 ysec. From this we can estimate that a waveform generated from a lookup table would take about 50 ys per step (based on a

minimum number of instructions) . It is obvious that were we to generate individual

points of a waveform and use them individually, as generated by the program,

we would have much longer times per point. Pulses of variable duty cycles would

be limited in repetition rate by the interrupt handler (see timer discussion) to

about 30 ysec, although "on" time could be adjusted by the technique discussed

before (Fig. 14 and text of previous paragraph)

32
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33

Timer-Converter

Board:

DAC

section
SEO (PIN 1 5 OF Z14)

o +5V

14. Timer-Converter Board: modification to DAC section proposed for

Interrupt-Triggered return to zero output.

34

wwn
Repetitive waveforms of normal complexity (sine, square, triangle, ramp)

for input to the potentiostat can be generated under program control in an

alternative manner. The output of one of the DAC's can act as input for a

Voltage Controlled Oscillator (VCO) (the VCO we use has a frequency deviation

range of 30 yHz to 3 MHz) allowing program selection of output frequency.

The inverted CB^ output of the timer PIA is used as a trigger signal for the

VCO. This oscillator puts out one cycle of the selected frequency when triggered.

Since peak-to-peak voltage, offset voltage, apd starting phase are all

controllable, we have considerable flexibility for the generation of waveforms.

One final comment is in order. The DAC section of the board contains

logic for limited multiplexing (although not fully implemented) of the input

and output signals. With the installation of some reed relays we can choose

between two distinct analog sources and simultaneously between four outputs.

Some minor circuitry changes would allow all of these lines to be dedicated

to some other arrangement of signals or to extend the multiplexing range.

The ADC section (Fig. 15) of the board reflects some major compromises

that were made after careful trade-off considerations of system speed requirements

and costs-. Ideally, the analog to digital converter would have a resolution of

one millivolt or better and would acquire the data in one ysec or less. Such a

device is not feasible at this time. Examination of the experimental needs allows

relaxation of these requirements particularly in the area of high-speed operation.

The experiment as planned calls for the input to the potentiostat to be stepped

through a cycle in which the input potential is changed by a fixed value, and then

sustained for a period of time. At this time the current is measured and after

this reading the cycle is continued. As indicated in the timer discussion the

fastest rate at which the input can be changed is 30 ysec (or 33.33 kHz). This

establishes an upper limit on the required operating speed. Resolution requirements

can be minimized by amplification of the input signal (preferably under program

control. In our case, automatic amplification is not performed, our choice being a

manual change in output amplification on the potentiostat) . In addition to the

factors already noted, cost and circuit complexity are vital considerations.

35
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We should also note here that the output current from the potentiostat

(our analog signal source) will often be slowly falling with time due to

the charging of the reactive components of the cell. It is desirable for

the ADC to follow this fall closely. (When this exponential fall-off has

reached a quasi-constant level, a "steady state" condition has been

reached in the cell, and this is a region of interest).

After weighing all these points, we decided to use a 10 bit tracking

ADC (also known as counter-ramp ADC). [14,15] This type of ADC is normally

thought of as a "slow" converter, since the time to acquire the signal is

(

n

— 1 ) times the step rate of the counter for full scale changes (n is the

number of bits in the DAC component of the converter). In our case, this

is about three milliseconds.. However, this number is valid only for full

scale excursions. For less than full scale the time is reduced by a factor

proportional to the change in the number of counts needed. Indeed for a change

in input potential of 100 mV (10 counts) the time is 30 psec. This is estimated

for a step change. For a slow change the speed would be even better. The

overall limitation on ADC conversion (for small analog change) rate is

the conversion speed of the component DAC.

The tracking ADC converter follows the input potential continuously,

and the digital output is always available. It is a relatively simple

circuit [14] consisting of an up/down counter, a DAC, an amplifier, a comparator

and a gate. We are using a 10 bit DAC that has a conversion rate of

'MO kHz. Our clock source for the counter is the CPU clock of 1 MHz.

The ADC could be run at a maximum frequency of 2.5 MHz if desired. This

would allow an ADC conversion rate of 800 Hz for full scale excursions

and proportionally reduced times for small changes. Overranging is a

problem with this converter. If the input potential exceeds the full

scale potential, the counter will continue to count indefinitely and the

digital data will be in error. To avoid this, a precision voltage limiter
is employed on the input to the ADC. [16] This is physically located on the

signal distribution board (Fig. 16) in close proximity to the potentiostat

The full scale input to the ADC ranges from -5 to +5 volts.

Programming [17,18]

To no one's surprise, initial programming is a major fraction of the

time needed to develop an operating system. . Fortunately, the commercial

availability of a higher level language (BASIC) has reduced the size of

this effort considerably. There are compromises needed, however, which

must be carefully weighed.

Assembly language programming yields a more concise and faster running

code than the use of a higher level language, especially when that language

is implemented as an interpreter. Since relatively high speed data acquisition is an integral part of our experimental concept, alternatives to BASIC

must be considered. Two approaches are immediately obvious. First, we

can implement in hardware a Direct Memory Access (DMA) system. This would

have the advantage of providing the fastest possible storage of data.

However, flexibility would be reduced. The second approach (the one we

adopted) would involve the calling of a machine language program as a

subroutine while the overall program would be in BASIC. Under this concept,

we would maintain the ease and flexibility of programming in a higher level

language while maintaining reasonable data acquisition speeds. This requires

that the BASIC program (written or purchased) include a command to call a

machine language program, a means to return from that program and the ability

to handle the data in the form generated by the data acquisition program.

There are several commercial programs available (one of which we have

acquired) [19] that satisfy these requirements.

For generation of machine language programs the use of an assembler is

suggested. This can either be "resident" in the microcomputer or can be

38
26

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Schematic

of

Signal

Distribution

Board.
available via time-sharing on a large computer (cross-assembler) . An example

of a cross assembled program is shown in Figure 17. This particular program

is of interest since it is the data acquisition program we currently are using.

Some comments are in order. First, this program utilizes only one of the two

DAC's. Second, initial clearing of the data storage area is accomplished by

the BASIC control program (automatically on RUN) . Third, this is probably not

the fastest possible program (as indicated earlier it uses about 250 ysec per

point) . Fourth, proper organization of the BASIC program will enable this

program to be utilized for signal averaging. For this purpose, the data storage

alloted per point, limits the maximum number of sweeps to 63. Addition of one

more byte of data storage per point would allow more data accumulation by

allotting memory space for the addition of 256 more sweeps.

The required memory for the BASIC interpreter and the machine language

program is approximately 8K byte. The BASIC control program and its required

buffer memory takes an additional 4K byte leaving 4K byte for data storage.

Since 4 bytes are required per point, only 1000 points can be stored without

installing additional RAM. In the future, we expect to place these programs

into ROM thereby freeing the 16K bytes of RAM for additional data storage.

At present, however, they are recorded on casette tape and are loaded into

RAM when needed.

System Use

We have described the implementation of a microprocessor control and

fast data acquisition system for use with a standard laboratory potentiostat. Figure 18 shows a result from one of the experiments we are

conducting using this instrument. Using this as an example we can

illustrate several features. First, we can accumulate digital data at

selected rates. If we sweep at fast rates, the data can be plotted in a

manner yielding the equivalent of a potentiodynamic scan. If we use

long delay times per point - long enough for steady state to be reached -

40
I ME RDDR El B2 E3 E DRTR ACQUISITION PAGE

1 ARM DRTR ACQUISITION

3 THIS PROGRAM OUTPUTS R POTENTIAL

4 BETWEEN -5 AND +5 VOLTS FOP R

cr FIXED TINE PERIOD* AT THE

if. END OF WHICH, R POTENTIAL IS READ.

“7 THE UPPER AND LOWER LIMITS CRN EE

8 SET EXTERNALLY, AS CRN BE THE

9 STEP SIZE AND CLOCK PERIOD.

1 1 ORG 81 EEF

1 iZ 1 EEF IE RF CLKCT ECU 81 ERF H I = T I ME.- COUNT : LO=“COUNTS

1 3 1 EEF IE B3 STEF'L ECU 81 EES

14 1 EEF IE EE' STEF'H ECU 81EE2

1

cr

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1 Cj 1 EEF IE B9 TREG ECU 8 1 EE 9 POINTER TO TOP DF DRTR STAC

1 9 1 EE F IE EE F'PEG ECU 8 IEEE POINTER TO BOTTOM OF STACK

c 0 1 EEF RF F4 TIMER ECU 8RFF4

C i 1 EEF HF F8 DRC ECU SflFFS

-I 3 1EEF AF FC ADC ECU 8RFFC

c 1 EEF RO 0 0 IOINT ECU SA0 00 INTERRUPT VECTOR

•I 4 1 EEF HO 14 TEN PR ECU SR 01

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c if. 1 EEF R Ci 16 I NP E G E C IJ 8 R 0 1 6

c r" 1 EEF R 0 18 SREG ECU 3 A 0 1

c. 1 EEF IE El SWEEP ECU 3 1 EE 1 SAVE “OF SWEEPS

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-

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2 q 1 EC F FF IE ED STX TF'EGW

4 0 1 EE £ EE IE E4 LDS HILIM

41 1 EE 5 HF 0 0 STS 00, X LOAD LIMIT INTO T REGISTER

4 iZ 1ED7 CE RF F4 SETUP LDX -TIMER SET UP PERIPHERALS

4 1EDA 4F CL PA

44 IEEE R7 02 STAR 02,

4

cr 1 EDD R7 03 STAR 03,

46 1EDF R7 06 STAR 06,

4 1 EE 1 R7 07 STAR 07,

4 1 EE 43 COMA

17. a) Cross-assembled Data Acquisition Program

41
LINE hit DR El E8 B3 E DATA ACQUISITION

49 1 EE4 A7 0 0 star oo*:-: 5 0 1 EE6 R7 01 STAR 01 *X

51 1 EE8 R7 04 STAR 04 * X

CT 1 EEA R7 05 STAR 05* X

crJO 1 EEC 86 04 LDAA “804

54 IEEE R7 08 STAR 08* X

cr cr 1EF0 R7 OR STAR 0AH*X

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Cjp

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6 0 1 EFA R7 OE STAR OBHjX

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6c' 1 EFE R7 07 STAR 07?

63 1 F 0 0 0E DATAT CLI

64 1 F 0

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65 IF 04 FF RO 0 0 STX I DINT

66 IF 07 FE IE ED LDX T REGLI 67* IF OR HE 0 0 LBS 00?

6 3 1F0C EF RF F8 STS DAC OUTPUT-ANAL DS VOLTAGE

69 IF OF FE IE AF LDX CLKCT

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71 1 F 1EE RO 18 LDS SREG

72 1 F 13E LIAI WRIT FOP INTERRUPT

7 1 F 101 MOP

74 1F1A 8 0 OF ERA CONT

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76 1F1E E7 RF F4 STAR TIMER STOP TIMER

i*’ 1F81 B6 RF F5 LDAA TIMER+1 READ OUT TIMER

“7;Z; 1 F84 FE RF FC LDX ADC READ ADC

79 1 F87 FF RO 14 STX TEMF'R

8 0 1 F8R 3E RTI

81 1F8E FE IE ED CONT LDX TEEGW

C* c 1 F8E R6 03 LDAA 03* X RECALL PREVIOUS VALUE

oo 1F3 0 E6 08 LDAE 08*

84 1 F38 EE RO 15 ADDA TEMPR+1 ADD NEW VALUE

r

1 F35 F9 RO 14 ADCD TEMF'R 86 IF 88 R7 03 STAR 03* X STORE SUM

;Z; 1? 1 F3A E7 08 STAB 08 *X

88 1F3C R6 01 LDAA 01 *X

09 1 F3E E6 0 0 LDAE 00* X

9 U 1 F 4 0 7D IE E8 TST D I REG UP OR DOWN?

91 1 F 4 3 C. l‘ 19 EEC DOWN

98 1 F45 B0 IE E3 UP SUER STEAL UP! THEN DECREMENT

93 1F48 F8 IE E8 SBCE STEF'H

94 1 F4B R7 05 STAR 05*

95 1 F4D E7 04 STAB 04 *X STORE NEW D-A DATA IN

96 NEXT LOCATION

97 1 F4F FF RO 14 STX TEMF'R SAVE INDEX

9 3 IF 58 EE 04 LDX 04 *X

99 1 F54 EC IE E4 CPX HILIM COMPARE NEW X TO UPPER

1 0 0 1 F57 C l"' ED EEC END GREATER? THEN GD TO END

1 0

1 F59 FE RO 14 LDX TEMF'R ELSE? RESTORE INDEX

1 08 1 F5C £0 17 ERA NEXT

PAGE

17. b) Data Acquisition Program (continued)

42
L I ME A DDR B

E2 B3 E DATA ACQUIS IT ION PAGE

1 03 1 F5E EE IE B3 DU UN ADDA S TEF'L DO UN! THEN INCREMENT

1 04 1F61 F'r IE B£ ADCE STEF'H

1 05 1 F64 A7 05 STAR 05.- X

1 06 1 F66 E7 04 STAB 04-

STORE NEU X

1 07 1 F6S FF A0 14 STX TEMPR SAVE INDEX

1 03 1F6E EE 04 LDX 04 - X

1 09 1 F 6 D EC IE B6 CPX LOLIM COMPARE NEU X TO LO LIMIT

1 1 0. 1 F 7 0 C. i oc EEC REVER LESS? THEN TURN AROUND

1 1 1 1 F73 FE AO 14 LDX TEMPR ELSE- RESTORE INDEX

1 12 1 F75 03 NEXT I NX

1 1

IF 76 03 I NX

114 1 F77 03 I NX

115 1 F73 03 I NX

116 1 F79 FF IE ED STX TREGU SAVE NEU POINTER

117 1 F7C 3C S3 ERA DATAT NEXT POINT

118 1 F7E 7C IE ES REVER INC DIREG CHANGE DIRECTION

119 1 F 3

FE AO 14 LDX TEMPR

1 3 0 1 F34 3 EF ERA NEXT NEXT POINT

131 1 FS6 FE IE ED END LDX TREGU SAVE POINTER TO LAST DATA

1 33 POINT

133 1 FS9 03 I NX

134 1 F3A 03 I NX

135 1 FSB 03 I NX

136 1 F3C 03 I NX

137 1F3D FF IE EE STX RREG

133 1 F90 FE IE E9 LDX TREG RESTORE ALL REGISTERS

139 IF 93 FF IE ED STX TREGU RESTORE UORKING T REGISTER

1 3 0 1 F96 EE AO IS LBS SR EG

131 1 F99 FE AO 16 LDX INREG

133 1 F9C S' c. PUL A 133 1 F9D fit TAP

134 1 F9E I* -1 PUL E 135 1 F9F 'I PUL A 136 1 F A 0 39 RTS

137 1 FA 1 END

TOTAL assembler errors = 0

BATA ACQUISITION PAGE

SYMBOL TABLE

1 ADC AFFC CLKCT 1 EAF CDNT 1 FSB DAC AFFS

DATAT 1 F 0 0 DIREG 1 EES DOUN 1 F5E END 1FS6

H I L I

1 EE4 INREG A 0 1

1 0 1 NT A 0 0 0 LOLIM 1 EES

NEXT 1 F75 REVER 1 F7E RREG IEEE SERV 1 FI C

SETUP 1 ED7 SREG A 0 1

STEF'H 1 EES STEF'L 1 EES

SWEEP 1 EE 1 TEMPR AO 14 TIMER AFF4 TREG 1 EE9

TREGU 1EED UP 1 F45

17. c) Data Acquisition Program (continued)
REA IfV

it RIJM ELECTROCHEM I CAL DATA TAKER VERSION £

DATE

Y A 1 9 7" 7

SAMPLE

? TItTA

FULL SCALE CURRENT-MA

? 1 ENTER TIME •COUNT

CODE

0

1

4

? ij ENTE R it OF CO U N T S : 0 -255

TIME IN SEC

1 0E1

1 CEO

1 OE-1

1 0E-£

1 OE-3

1 OE-4

1 OE-5

1 ME -6

it OF SWEEPS: 0-355

? 1 S TEF-SIZE M L V T

S - S TEPS OF 1 0

? £50

HILIM-MLVT

? 1 0 0 0 LOLIM-MLVT

? -£000

TO START-ENTER 1 - TO CHANGE PARAMETERS-ENTER 0 ? 1

STARTED!

1 9s 77 T I : TA 1 MA FULL S

POTENTIAL <V> CURRENT CNR) LOG CURRENT

1 -0. 014 -11. 1764532

0. 75 - 0 . 014 -11. 1764532

0 . 5 -0. 014 -11. 1764533

0. £5 -0. 01 -11. 5 1 29255

0 -0. 014 -11. 1 764532

-0. £5 -0. 014 -11. 1764533

-0.5 - 0 . 0 1

-11. 1764533

|'| “7CT -0. 013 -10.9351 383

-1 -0. 034 -1 0. £391493

- 1 . £5 -0. 054 -9. 82652653

-1 .

-0. 03-9. 40379133

-1.75 0. 933 — 6 • 97176 ij 6* £

C. '•ill UJ 1 co -1£. 0237511

-1 . 75 -0. 906 -7. 00647137

-1 .

-0. £9 -3. 14563934

-1 . £5 -0. 03 -1 0. 414313

-1 - 0 . 014 -11.1 764532

-0. 75 £. E- 03 -13. 1333634

- 0 .

6 . E- 03 -13. 0337511

-0. £5 6. E-03 -13. 033751

0 £ . E- 03 -13. 1333634

0. £5 -£. E- 03 -13. 1333634

0. 5 -£. E-03 -13. 1333634

0. 75 -0. 01 -11. 5139355

18. a) Sample of System Output

44
9/ 19/77 T I : Th 1 f'lfi FULL SCALE

F'LDT OF CURRENT VERSUS POTENTIAL

CURRENT

- o +++++++++ 0+++++++++ 0+++++++++ n

9/ 19/77 TI:TA 1 MA FULL SCALE

POTENTIAL VS LOG (CURRENT.)

LOG OF CURRENT +++++++++ 0+++++++++0+++++++++ 0+++++++++0++++++++ + M+++++

0 +

“ OF SWEEPS LEFT=0

RUN COMPLETE! WANT ANOTHER 7 YES=1>NO=0

. b) System Output (continued)

4 5

18
we can generate Tafel plots [20]. (Alternatively the machine language program

can be changed to one that will check the raw data for steady state

conditions after which the data is plotted)

Second, we can perform most of the necessary computations without

an operator. This means that the instrument will not only give us the data

as acquired, but will perform the necessary mathematical steps to generate a

more usable form of information (e.g., calculate the log of the current and

then plot those values) . The BASIC program in the yP, although slow compared

to a big machine, is capable of most of the operations (except Matrix

functions in particular) that BASIC in a larger computer could perform.

Computational speed is quite adequate, and for those occasions when it

is not, communication with a, larger machine is provided.

Third, once programmed and started, the instrument can run unattended.

The length of time a run takes does not have to be fixed. The microcomputer

can run an experiment for as long as it needs to run. It can be programmed,

if necessary, to take emergency measures when conditions exceed limits.

This instrument has, as indicated before. Alternating Current (AC)

capabilities. For discussion purposes, we will separate pulse techniques from

other methods. In general, in pulse techniques we observe the effects of an

impulse input on the cell by observing the decay of its effect. Pulses can be

derived either from the timer (minimum width 1 ysec) or from the voltage controlled

oscillator (minimum width 200 nsec) . Other waveforms can be provided by the

DAC or the VCO (which depend upon the frequency desired-see DAC discussion)

If needed, one DAC can provide a DC level which can be summed with the VCO

output (controlled by DAC #2) and directed to the potentiostat input.

Direct measurement of AC potentials is possible using the existing ADC

for a limited frequency range. To obtain all the information in an AC signal,

the sampling rate must be at least twice the highest signal frequency. [21]

Our sampling rate for full scale excursions is 300 Hz and thus our maximum
Table 3. Circuit Board Parts Cost and Commercial Availability

PC Board Prototype

Parts Cost

Similar Board Available

Commercially*

Commercial

Pricing

Microcomputer $200 Yes Same or higher

8K Byte RAM 180 Yes Higher

8K Byte RAM 180 Yes Higher

Variable Speed

Serial Interface 25 Yes Higher

Converter & Timer

Board (including

Signal Distribution) 250 No —

Power Supplies

& Motherboard 150 Yes Same or higher

TOTAL $985

*Partial listing of possible sources for directly compatible boards:

Motorola, Creative Micro Systems, Electronic Product Associates, and

many others.
frequency would be 150 Hz. In line with our previous discussion, this ' established maximum would increase as the AC peak to peak voltage decreases.

As a practical consideration, however, the increase in bandwidth is not wide

enough in general, and another technique should be considered. This can

either take the form of a different type of ADC with wider bandwidth or

some form of AC to DC converter whose output, can be measured by the microcomputer ADC. At present, the response is adequate for our purpose but we are

actively pursuing the resolution of this problem for the future.

Cost of Implementation

The system described here was constructed for a parts cost of

slightly less than $1000 (exclusive of terminal) . Table 3 shows the

breakdown of costs per board,. Depending upon desired parameters, systems

similar to this can be constructed that range in parts cost from about $800

on up. The lowest system price is based on the use of minimum memory hardware

(no BASIC). Decisions as to memory size, other peripherals, and/or purchase

of assembled boards from commercial sources will have a significant effect

on overall expense. There is, at present, no commercial source for a tracking

type analog to digital converter (similar to the one used here) available, that

mates with this bus structure. Successive approximation type converters are

available (usually with multiplexed inputs) but they are more expensive, are

relatively limited in throughput, and do not contain the necessary timer circuit.

Conclusions

In conclusion, an instrument has been constructed that offers improvement in

measurement techniques over manual methods. This device incorporates one of the

recent advances in instrumentation (the microprocessor) which allows rapid,

concise and consistent measurements not before realized by manual methods.

Acknowledgements

The authors wish to thank H. P. R. Frederikse, A. D. Franklin, and

L. H. Bennett for useful discussions. A very special thanks to U. Bertocci

without whose advice and encouragement this work would not have been completed.

48
2.2 High Temperature Solid Electrolytes

Introduction

Oxide solid solutions having a defective fluorite structure and

based on ZrO^, CeO

^ and ThO ?

, doped with CaO, or the trivalent rare

earth oxides, have high oxygen ion mobilities relative to the pure

constituent oxides. The high oxygen ion mobilities make these oxide

solid solutions potentially useful solid electrolytes for high temperature

fuel cells. The high oxygen ion mobility results from the large concentration of oxygen vacancies introduced into the oxygen sublattice of the

host oxide, about 5 to 15 mole % of dopant may be added depending upon

the oxide system.

ZrO^-based solid solutions have been used as electrolytes for hightemperature electromotive force measurements since the mid-1930's [22,23],

and ThO ? -based electrolytes, since 1957 [24]. ZrC^-based electrolytes

have been studied for fuel cell applications principally at Westinghouse

and G.E. in the United States, and elsewhere in France, Germany and the

Soviet Union. Recent results [25,26] on CeC^Iu^O^, where Ln represents

3+

a variety of lanthanide ion or Y , have revealed oxygen-ion conductivities

essentially equivalent to that of ZrO^CaO but at temperatures about

200°C lower. Thus, fuel cells using a CeC^-based electrolyte might

operate at about 800°C as compared to one with ZrO ? -based electrolyte

which would require 1000°C . However, CeO ? -based electrolytes begin to

show appreciable electronic conduction at higher oxygen partial pressures

than do the ZrO ? -elec trolytes ; for example, one-half of the electrical

-14

conductivity is electronic in (CeO„) n (Y„CL) at P = 10 atm

U t y 3 J U e W J (J 2 (^10 ^ Pa) for t = 800°C, whereas for Zr02:Ca0, P^ = 2.5x10 ^ atm

2 . 5xl0~ 36Pa) for t = 1000°C [27].

4 9
At the high oxygen vacancy concentrations present in these electrolytes, the oxygen vacancies are probably nearly all associated with the

dopant cations [28]. If these electrolytes undergo prolonged anneals at

moderate temperatures (<1000°C) , the oxygen vacancies, and perhaps to a

lesser extent, the dopant cations, may order into a structure coherent

with the fluorite structure of the host oxide. Formally, in terms of a

coherent structure, the vacancies no longer exist. In actual consequence, the ionic conductivity of the electrolyte may decrease (resistance

may increase) because the vacancies, relative to the fluorite structure,

must then move cooperatively.

The existence of coherent structures is well documented for Zr0 o -

based solid solutions [29-36]. Coherent structures in CeO ? -based solid

solutions have not been identified.

Carter and Roth [33] found that the increase in resistance of

calcia stabilized zirconia (CSZ) when annealed at <1000°C was associated

with the formation of coherent structures, observed as superstructure in

neutron and x-ray diffraction. This "aging" of CSZ could be reversed

("deaged") by annealing at 1400°C. In a high temperature fuel cell

aging of the electrolyte would reduce the efficiency of the cell.

A second aging process which may alter the long-term performance of

a high temperature fuel cell electrolyte is the redistribution of the

dopant cations along the grain boundaries. Relative to the lattice, the

dopant cations appear as negative charges. Thus, they can be expected

to migrate under the influence of an applied field. Although the mobility

of these cations within the interior of the grain is negligibly small

for practical purposes, their mobility in the grain boundaries can be

significant. For example, from the tracer diffusion measurements for

50
ions to cross a 20 ym

2+ 2+

Ca ions in CSZ [37], the time required for Ca

thick electrolyte film with a potential difference of 0.8 V is on the

order of hundreds of years. On the other hand, the same tracer diffusion

2+

study showed that migration of Ca ions in the grain boundaries is

perhaps 1000 times larger at 1800°C than the bulk diffusion rate, a

ratio that would be expected to be even larger at lower temperatures.

Hence, migration of the dopant ions along grain boundary paths over

significant distances during times of the order of a year or less can be

expected in all of the fluorite-structure oxides of current interest as

solid electrolytes.

The migration of dopant ions along grain boundaries can be expected

to result in at least three deleterious effects: (1) a build-up of a

resistive layer at either electrolyte boundary, (2) a build-up of a

space charge at the electrodes resulting effectively in an overvoltage,

and (3) a possible interference with the electrode reaction due to a

change in electrolyte composition at the electrolyte-electrode boundaries.

The NBS program of studies on high-temperature electrolytes currently

contains two elements: a study of degradation mechanisms in CeO^-based

oxygen-ion electrolytes, and the development of the impedance measurement as a method of analyzing fuel cell performance with the particular

aim of developing a lifetime-prediction test.

2.2.1 Electrolyte Materials

Introduction

In an effort to produce Ce0 9 :Y20^ specimens of high density and

uniform composition, chemical preparation of sinterable powders with the

desired compositions was undertaken this year. This work is summarized

51
here, and in particular, the details of one procedure, "the carbonate

procedure", are given. The carbonate procedure is now in use to prepare

600 to 700 g batches of powder for hot-pressing. Two other procedures

which show promise are briefly described. These two procedures utilize

complexes of Ce(III) and Y(III) with organic reagents. Cerium-yttrium

nitrate stock solutions, whose preparation yzas described in the previous

annual report [54], were used as the starting point for all of the chemical

procedures described here.

Chemical preparation of ceria-yttria powders generally requires one

or more coprecipitations. Insufficient control of the precipitation

process can result in aggregation of the precipitate particles. The

pores incorporated into the. aggregates can persist to the final sintered

specimens unless further chemical processing, extensive grinding or

isostatic pressing of the green-bodies to high pressures (300 to 700

MPa) [38] are used to break up the aggregates.

An example of the persistence of pores is shown by the SEM micrographs in Fig. 19. Figure 19a shows an aggregate particle from a powder

obtained by coprecipitating the ceria-y ttrium hydroxide from a nearly

concentrated nitrate stock solution by the addition of ammonium hydroxide;

followed by rinsing successively with water, ethanol, acetone and toluene;

and then by air-drying. Upon examination at higher magnifications than

that of Fig. 19a, the particle shows rather even surfaces punctuated

with pores, such as that in the lower part of the picture, and surfaces

consisting of billowy masses of particles, such as that on the upper

portion of the aggregate particle. Fig. 19b shows a fracture surface of

a specimen fabricated from such material and sintered for 4 days at

1525°C. Although there appears to be regions where the grains are

52
19a. Aggregate particle from a precipitation of Ce-Y hydroxides by

the addition of ammonium hydroxide to a nitrate solution.

19b. Fracture surface of a sintered specimen which was fabricated

from a powder containing aggregates similar to that shown in

Fig. 19a.

53
thoroughly sintered together, the specimen also appears to be interlaced

with rather large pores. The bulk density of specimens isostatically

3 pressed and sintered from the aggregated material was 5.51 to 5.65 g/cm

(79 to 81% of theoretical)

Aggregation can be reduced significantly by using the homogeneous

release within the solution of the precipitating agent. The science and

techniques of homogeneous precipitation were discussed by Gordon [39,40]

and by Gordon et . al . [41]. The hydrolysis of trichloroacetic acid to

precipitate the carbonates and of the decomposition of urea to precipitate the hydroxides were examined in our work as possible techniques for

preparing sinterable ceria-yttria powders.

Homogeneous Precipitation of the Carbonates

The preparation of rare earth carbonates by the hydrolysis of

trichloroacetic acid solutions of the rare earth salts was first reported

by Salutsky and Quill [42]They prepared the carbonates of lanthanum,

neodymium and samarium. This method was subsequently extended to the

other rare earths [43-46]. Charles [47] carried out an investigation of

the compositions of the rare earth carbonates prepared by this method.

Head and Holley [48] investigated the hydrolysis reaction under CC^

atmospheres of moderate pressure and studied the thermal decomposition

of the carbonates.

Figure 20 is a diagram of the procedure we have developed based on

the hydrolysis of trichloroacetic acid to obtain the rare earth carbonates.

Ce and Y hydroxides are prepared by diluting 1 part of nitrate stock

solution with 2 parts water and then neutralizing the solution by the

dropwise addition of ammonium hydroxide diluted by the same proportion.

54
Carbonate Procedure

20. Schematic of the carbonate procedure.
The size of our apparatus limits us to about 500 ml of stock solution at

a time, the equivalent of about 50 g of oxide, so that blending is

required at later stages of the operation. The progress of the neutralization can be followed by the color changes of the solution: the deep

red of the ceric solution at high acidity fades as the solution is

neutralized; the color is yellow-orange to yellow at a pH of 1 to 2; at

a pH of 2 the color of the solution begins to shift from yellow to green

and the pH begins to decrease again; the pH decreases to 1, where the

solution has a green tint, the yellow having completely faded; the pH

then rises rapidly and precipitation starts at pH = 2.8. If the final

solution is made slightly alkaline, the precipitate takes on a muddy

violet-gray color which suggests the presence of some cerosoceric

hydroxide [49] . After the precipitate has been rinsed several times

with water containing a small amount of ammonium hydroxide to promote

settling, a pale yellow to white precipitate is obtained. With each

rinsing the precipitate is warmed and stirred to ripen the precipitate.

Following each rinse, the precipitate is allowed to settle before the

supernatant liquid is pumped off through a fritted disc.

The carbonate is prepared from the hydroxide by treating the precipitate with 1000 ml of nominally 25 w/ o trichloroacetic acid, pH

about -0.1 to -0.2, and 25 ml of 30% hydrogen peroxide, added while the

solution and precipitate are stirred together. The hydrogen peroxide is

added in about a 1:1 mole ratio to the amount of Ce present to promote

the dissolution of the precipitate. The mixture of precipitate and

solution becomes brown immediately upon the addition of the hydrogen

peroxide, and a thick head of foam develops; after about 30 minutes the

solution becomes clear and the foaming ceases.

56
Hydrogen peroxide has long been known as an aid to dissolving ceric

oxide in hydrochloric acid [50], chlorine being evolved as the oxide is

dissolved and reduced. However, in the present case the hydrogen peroxide

appears to attack primarily the Ce(IV) ions either in the precipitate or

in the solution, reducing them to Ce(III) and apparently forming a

colored complex with the Ce(III) as an intermediate in the reaction

[49,51]

After the solution clarifies, it is diluted to give a total volume

of 3000 ml and warmed to 80-100°C to hydrolyze the trichloroacetic acid.

When the hydrolysis is completed, about 30 g of ammonium bicarbonate,

NH^HCO^, are added to neutralize the solution and to bring down all of

the precipitate. The solution is warmed for an additional 24 hours to

ripen the precipitate and to reduce the volume of the supernatant liquid.

Several of the precipitates may be blended at this point. The supernatant liquid is pumped off through a fritted disc, and the precipitate

is rinsed several times with water containing a small amount of NH^HCO^,

then with ethanol and finally with acetone. The precipitate is dried

first in air and then in a vacuum oven at 60°C.

An SEM examination of the Ce-Y carbonate prepared by the above

procedure showed many small platelets, see Fig. 21, plus larger masses

having a layered structure. No structure lines were observed with x-ray

diffraction.

After calcination of the material at 700°C for 6 hours, the fluorite

structure oxide was obtained. A portion of the initial oxide powder

prepared by the carbonate procedure was isostatically pressed into two

specimens and calcined at 1500°C for 3 days to yield sintered specimens

3 with bulk densities of 6.32 and 6.53 g/cm (95 and 97 % of theoretical).

57
Preparation of Mixed Oxide Powders via Organic Resins

In an attempt to produce finer, unaggregated powders the coprecipitated Ce-Y hydroxide was dispersed in, or complexed by, several organic

resins. The following organic media and processes were investigated:

1. homogeneous precipitation of the hydroxide in excess succinic

acid;

2. dispersion of the hydroxide in tartaric acid;

3. dispersion of the hydroxide in citric acid;

4. dispersion of the hydroxide in the tartarto-ethlyene glycol

ester

5. dispersion of the hydroxide in the citrato-ethylene glycol

ester.

For process "1." the hydroxide was formed by the decomposition of

urea in the presences of succinic acid —6 moles of acid to 1 mole of

oxide. The solution was evaporated to dryness with stirring to disperse

the hydroxide in the organic matrix. For processes "2." to "5." the

hydroxide was coprecipitated from the nitrate solution by the addition

of ammonium hydroxide, then rinsed and dispersed in the organic matrix.

As in the case of "1." the solution was evaporated to dryness with

stirring. The incorporation of the hydroxide into an ester of a polycarboxylic acid and ethylene glycol was suggested by Pechini [52] . The

resin then was burned off to yield the oxide powder. For process "1."

the burn-off was carried out in a Pt crucible heated to 700°C for 41/2

hours in a furnace. Since complete removal of the resin from the beaker

in which it was made was difficult, the resins obtained by the other processes

were burned off jin_ situ by heating the beaker on a hot-plate and directing

a stream of oxygen on to the surface of the resin.

58
SEM micrographs of the oxide powders from the succinato-resin , the

tartrato-resin and the tartrato-ethylene glycol ester resin are compared

in Fig. 22, 23 and 24. Results for the citrate-based resins were similar

to those for tartrate-based resins.

The oxide particles from the succinato-resin. Fig. 22, are somewhat

spherical nodules with a diameter of 32+8 ym* . The nodules appear to be

hollow, fragile structures as evidenced by the broken one to the upper

left of center in the micrograph. These small, uniformly sized nodules

probably resulted from the homogeneous precipitation. The subsequent burningoff of the resin appears to leave these structures more-or-less intact.

In contrast to Fig. 22, the oxide particles from the tartratoresin, Fig. 23, are massive aggregates similar to those obtained when

the hydroxides are coprecipitated under nonhomogeneous conditions, see

Fig. 19a. The large particles, diameter = 0.23+0.03 mm*, with similar

shapes, in Fig. 23 are from the coarse fraction which was obtained after

grinding and sieving to remove the fines for surface area and sintering

studies. Although the size of the coarse particles is not indicative of

the size of the initial particles in the precipitate, the micrograph

still suggest that considerable aggregation occurred during precipitation

and that the cohesion within an aggregate particle may be fairly strong.

The oxide particles from the tartrato-ethylene glycol ester resin,

Fig. 24, show some massive aggregates among many fine particles. This

sample also was obtained from the coarse fraction after grinding and

sieving to remove the fines for further study. The mixture of coarse

* The uncertainty expresses the 95% confidence limits.

59
21. Ce-Y carbonate platelets.

22. Ce-Y oxide nodules prepared by homogeneous precipitation and

dispersal in a succinic acid resin.

60
23. Ce-Y oxide, coarse aggregates after grinding; powder prepared

by nonhomogeneous precipitation of the hydroxides and dispersal

in a tartaric acid resin.

24. Ce-Y oxide aggregate and fines; powder prepared by nonhomogeneous

precipitation of the hydroxides and dispersal in a tartarto-ethylene

glycol ester resin.

61
aggregates and fines in the SEM sample suggests that there is less

cohesion within these aggregate particles than for the particles from

the tartrato-resin

Although these results are preliminary, they suggest that homogeneous precipitation of the hydroxide may be required if large, and

possibly hard, aggregates are to be avoided in using any of the resin

processes

Preparation of Mixed Oxide Powders via Extraction of an

Organic Complex

This method was suggested by the work of Sudarikov et . al . [53] on

the extraction of scandium, yttrium, cerium, lanthanum, uranium and

thorium salicylates into isoamyl alcohol (3-methyl - 1-butanol) . They

found that both the Ce-complex and the Y-complex could be extracted

quantitatively at a pH >5. In our preliminary work we obtained yields of

70 to 90% for the oxide powder depending upon the amount of salicylate

used

The pH of the nitrate stock solution was adjusted to about 1 by the

addition of ammonium hydroxide. The solution then was diluted and

treated with hydrogen peroxide to reduce the Ce(IV) to Ce(III) . Salicylic

acid was added to 25 ml aliquots of the solution in the molar ratio of

about 3:1 for three samples and of about 6:1 for three samples. The

samples then were titrated with an ammonium hydroxide solution, about

0.25 M, to a pH of about 5.5 - 6.0. A slimy brown precipitate was

obtained which was extracted quantitatively into isoamyl alcohol. The

isoamyl alcohol fraction was transferred to a distillation apparatus

where most of the alcohol was recovered. Once most of the alcohol had

62
been removed from the precipitate, the distilling flask was removed from

the apparatus and heated to decompose the precipitate to an oxide powder

having the fluorite structure.

Figure 25 is an SEM micrograph of one of the oxide particles.

Although the particles generally appeared to be small, they showed

considerable aggregation, as illustrated by the particle in the micrograph. Adjustment of the pH homogeneously by the decomposition of urea

may reduce the amount of aggregation.

Summary

Although homogeneous precipitation of the carbonate is being used

at present to prepare batches of the oxide powders, the resin techniques

and the extraction technique show enough promise to encourage further

efforts to perfect them. There are three observations which hold for

all of the techniques.

First, a preliminary coprecipitation of the hydroxides from the

nitrate solution is desirable so that the fluoride and the excessive

nitrate present in the stock solution can be removed. This precipitation can be carried out rapidly by neutralization with ammonium hydroxide

rather than with one of the slower homogeneous processes. The hydroxides

can be dissolved and the Ce(IV) can be reduced to Ce(III) in a moderately

acid solution.

Second, a subsequent coprecipitation is best performed from a

homogeneous solution to minimize aggregation of the precipitate.

Third, the supernatant liquid can be removed more rapidly by pumping it off

through a fritted disc than by collecting the precipitate on a filtering

funnel

63
25. Ce-Y oxide aggregate; powder prepared by nonhomogeneous precipitation of the salicylate and extraction into isoamyl alcohol.

64
The carbonate procedure requires several rinses of the carbonate

precipitate to remove any chloride ions produced by side-reactions during the

hydrolysis of the trichloroacetic acid. The resin techniques and the

extraction technique avoid the slow rinsing operations, but these

techniques entail either the evaporation of the aqueous solution or the

distillation of the alcohol, both of which may be time (and energy)

consuming. The extraction technique seems to offer the possibility for

further purification of the oxides since Na-ions, a major contaminant,

may remain mostly in the aqueous phase when the Ce-Y salicylates are

extracted into the isoamyl alcohol.

Ac knowledgement

We wish to thank Gail Cooper for performing many of the chemical procedures reported here.

2.2.2 Impedance Measurements

Introduction

As described in our last Annual Report, for calendar year 1976

[54]

we are studying the ac impedance of specimens of Y-doped Ce0 7 fitted with symmetrical sputtered Pt electrodes over the range of

frequency from 10 Hz to 13 MHz and of temperature from room temperature

to about 850°C. Tank 0

? and tank N 0 are being used at present to vary

the oxygen partial pressure, but equipment is being installed to control

and measure this pressure quantitatively.

These data are being handled in the framework of equivalent circuits

of the kind shown in Fig. 26. Z ^ is a contribution to the impedance

that has a broad distribution of time constants and is thought to arise

65
CM

66

Electrodes
from the presence of inhomogeneities in the interior of the specimen.

We find it can be represented by a Cole-Cole [55] equation:

AZ

( 1 ) 'gb ,,,. sl-a 1+(jwt)

The electrode impedance, Z,, can also be represented by an equation of

this sort. In eqn (1), x is the most probable time constant and a is a

measure of the width of the time constant distribution. The circular

frequency is w, and AZ

^ is the contribution of this mechanism to the dc

resistance.

The impedance data as a function of frequency are fitted to the

equations for the equivalent circuit by a steepest-descent least-squares

fitting procedure that gives as output the "best" values for the parameters of the equivalent circuit and their standard deviations. The fit

is achieved by minimizing the sum of the squares of the distances in the

complex plane between the experimental and calculated points. A test

[56] of the entire measurement and data handling procedure was run on a

dummy specimen composed of two resistors and two capacitors whose values

were measured independently to better than 1%. The impedance measurements

were fitted using the actual circuit as the equivalent circuit and determining the resistances and capacitances as "best" values by the fitting

technique. The results gave an excellent account of the frequency dependence of the data, and the "true" values for the resistances and capacitances

were reproduced to within +10%.

Experimental Results-The Bulk Arc

According to the equivalent circuit of Fig 26, at high frequencies

the impedance data are determined essentially by the part of the circuit

67
on the left hand side of the dotted line, the so-called bulk effect.

The circuit elements here are the geometric capacitance, C , the resistance R of the interior of the conducting grains, and an inhomogeneity contribution indicated by Z*^ as given by eqn. (1).

All of these elements should be independent of the oxygen partial

pressure, and indeed the indications are that they are. Figure 27 shows

measurements made in 0

9 at 350°C, and then after replacing the 0

9 with

N . This figure represents the complex plane for the impedance, with Z'

the real and -Z" the imaginary part. Very little change is seen. On

the other hand, the sputtered Pt electrodes under study appear to age

persistently. Figure 28 shows the data taken at 250°C for several runs

on another specimen. The very low frequency data, on the right hand side,

represents the inhomogeneity effect. The curves, which are numbered in

the order in which the data were taken, represent a parallel R-C network

fitted to the highest frequency data, starting at the point indicated by

the arrow in Fig. 28. Curves 1 and 3 refer to data taken in 0

9

, while

the data in curves 2, 4, and 5 were taken in N Q

. Between curves 1 and

2 the specimen was heated to 804°C in N

9 and 0 9

.

Sintering of the electrode could reduce the contact between the

solid electrolyte and the metal electrode, and thus increase the specimen

resistance. To a first approximation, there should be little or no

effect on the capacitance, since the ratios of the spacing between the

electroylte and non-contacting electrode particles to the electrolyte

thickness should be much smaller than the inverse dielectric constant of

the electrolyte. The parameters drawn from the arc fits are shown in

Table 4.

68
rn

LO

pj

S3

Lrl m

pj

a:mmsm

to

LjJ

5WHD ' . . Z uoo

in

tn

-P

03+

69 Z1/

DHM5

27.

Complex

Impedance

Plot

for

Bulk

Arc

for

CeY0

in

0,

(O)

and0. 90• -L

1*95 ^

Z'

is

the

real

and

Z"

the

imaginary

parts

of

the

impedance.
HPB

2HB

N2

37

m

rm

m

to

Ld

U.

<19 5 h E 2

M

5WHD 7

I I Z

70

HM5
Table 4. Resistance and Capacitance Values from Arc.. Fits in Fig. 28.

Curve Atmos

T°C V C >pF

R

RC , rad/

1 °2 253 1.9xl0 5 13.2 3.9 x 10

5

2 N

2 252 4.7xl0 5 6.9 3.1 x 10

5

3 °2 248 5.0xl0

5 7.1 2.8 x 105

4 N

2 248 5.4xl0 5 6.9 2.7 x 10

5

5 n

2 248 5.5xl0 5 6.9 2.6 x 10

5

After the first curve the capacitance does stay essentially fixed

while the resistance rises almost 20%, finally stabilizing at a value

not much influenced by the atmosphere. Continued sintering and loss of

contact seems a likely explanation for the instability.

Because of this lack of stability in the Pt electrodes, comparisons

of absolute magnitudes from one specimen to another must be done cautiously.

However, some meaningful comparisons can be. made. Figures 29 and 30

compare the data for sintered and hot-pressed specimens of Ce Y

u . y u • _l

0, n , the latter being considerably denser (^96% of theoretical) than

j- * y j

the former (^85%) . The data in each case were fitted with the highfrequency part of the equivalent circuit of Fig. 26. The arc shown in

* each figure represents Z , alone. The span of the arc is AZ , , and the

gb r gb

left-hand intercept is lb, (refer to Fig. 26 and eqn. 1). In Figs. 29

-D and 30 it is clear that, relative to the bulk resistance, R , the more

porous sintered specimen exhibits a larger value of AZ^ than does the

denser hot-pressed specimen. This observation is consistent with our

interpretation of Z as arising from inhomogeneities in the specimen,

gb

in this case the pores.

The appearance of the right-hand part of the high-frequency arc

depends strongly on temperature. Figure 31 shows data on hot-pressed

71
ID

lil

EC

U

E

Ci

u

q:

u

h

Z

m

k

CN O 40

too c •H •H 40

• CJ 40

o vO M O Csl •X

u~i N CM • oo CO •u •H 4J

03 (0

a) LTU-I CO ON o 0)

•

S-4 HW a O •H 0) 3 u

rH CJ •H CJ OH H

>•> O CO

ON4-I a) •10 40 05) H

QJ i—

U CO> • T) H cO

0) 0) 4J H cr cO

0) 0) TO

4-) C 0) 0) •H 40 40

CO J 4-1 H e o

O o 4-1 M-l

>4-1 TO

4-1 0) O <D 4-1

i—

H 4J ex cO •H

M-l a) CO u CD 3 03 —

CO rH 0) TO CO T3

0) > •H

CX CO s TO •H 0) >N

•U a

X CO (0 CD i—

0)

1—

3 3

CX CJ a1 6 rH a) o CO u u CJ M-l

j 1

z i

a

I f-Ba4i3

5 3 0

I

on

CNI

smu'ai

72
(VJ

5WHQ ' i .Z

d

•H *- X O 04 oO • LO 00 04 -H

!d

4-4 d <4-4 O

LO

CTV4J •H H3 O CJd i—4H •O O

>4uc • COO) Xi

60 003 •K

01 > N

cj •Hd CO T3 cr 4-1 CU 0) d

CO 0)

CO 01 CO

cu -d CU d 4-1 d

CL CL 1 B CU 4J o d 0 d X M-i CJd d 01 d o d

4-t 03 OJ X U CO H O OJ i—

d

CL i—

• d d

0) > 4-4 o d d X X

o3 0) X 4-J OJ

01 03

X

CL

T 1 4-1

B

d

•H CJ o i—

4-4

X

d

0) CJ

X i—

04 CL 4-1 B . •d o OsM

CJ

O 4-1 o

CO

73
Ce^

9 Yq

i Oi at four temperatures from 252 to 580°C. These data

have been scaled so that the left-hand portions roughly coincide, in

order to reveal the changes taking place in the right-hand side as the

temperature increases. At the lowest temperature (252°C) there almost

appear to be two arcs, and as the temperature rises these move together,

until at the highest temperature the tangent at the real axis to the

curve defined by the data is almost vertical, indicating behavior like

that of a simple R-C parallel circuit with only a single time constant.

What appears to be happening is that the magnitude of the inhomogeneity

impedance is decreasing relative to the bulk resistance as the temperature rises. This is probably the result of the increase in bulk conductivity with increasing temperature, which would tend to produce an

increase in screening of the inhomogeneities and a consequent reduction

in their influence.

An estimate of R , the resistance of the interior of the ceramic

D grains, can be obtained from the fits of the equivalent circuit to these

data. Figure 32 shows these estimates plotted in the form of log(T/R ) D

against T \ where T is the absolute temperature. The solid line is a

least-squares fit to all the points except that at the lowest temperature, where the data are more uncertain. From the slope of this line we

calculate an activation energy of 74.3 kJ/mol , in excellent agreement with

the value 73.3 kJ/mol obtained by Tuller and Nowick [25] from four-probe

dc measurements on somewhat more porous specimens of the same composition. Our value is also in rough conformity with the high temperature

value found by Kudo and Obayashi [26] for the same dopant (Y) and composition. This latter value is estimated as about 62.7 kJ/mol from the

74
EB

ZN

Z5Z

BdH m

r-m

L/1

LU 9 S h E Z I

cO

Cs)Q_> Z oo e oo

•H LTl LO * avx

rH • •* o ao r-O cxr

>4c^X

lti

z:xsN c

QJ o o 0o

TO UO

ai CO

CO

CO r>

<L> •

S-i fX • rs 1 CJ

4-1 o o CN X LO

CN1

!-i o

CH

4-4 0 o • • 1—1 CO cx cu

S-i aj d a 4-1 c cO

CO S-i TO QJ

QJ CX

cx e B QJ •H 4-1 X i—1 QJ cO i—1 S-i CX 0) 6 > o 0) u CO

5WHD 7 i i Z

75
T/R.

(K/Q)

T. °C

800 700 600 500 400 300 250

32. Arrhenius plot for bulk resistance, Rg, for hot-pressed Ce^

g ^o.l *^1.95

in N

2

-

76
curves given in their paper, but which must be raised to about 70.4 kJ/mol

when corrected for the fact that they plot In (resistivity) rather than

In (resistivity/temperature) - see the discussion below.

Both Tuller and Nowick and Kudo and Obayashi made their measurements at temperatures from about 400°C to about 1000°C. Kudo and Obayashi

observed a break in their plots of In (resistivity) vs T

^ at about 500

to 600°C, a break which does not appear in the data of Tuller and Nowick

and also probably not in ours either. The increase in value found by

Kudo and Obayashi may arise from a combination of their method of

measurement and their specimen preparation. They prepared dense ceramics

(>96% of theoretical) by incorporation "when needed" of 3 mole % MgO,

which by analogy to results in zirconia [57,58] can be expected to

produce partially blocking grain boundaries. They measured the resistance using either an ac bridge at a frequency of 1 kHz or a dc 4-probe

method. One kHz is a sufficiently low frequency that at 400°C and up we

find all of the inhomogeneity effect to be included in the real part of

the resistance. Thus they measured not just the bulk resistance but the

bulk resistance plus whatever contribution the inhomogeneities made. We

observe in our specimens that the inhomogeneity contribution has a

greater temperature dependence than does the bulk, and begins to be a

significant part of the total at temperatures below about 500°C. Thus

we suggest that Kudo and Obayashi's low- temperature activation energy

reflects inhomogeneity effects; it is their high-temperature value that

corresponds to the true conduction process in the interior of the grains.

Tuller and Nowick [25], on the other hand, worked with unadulterated but

porous (>80% theoretical density) material and a dc measurement. They

must have included an inhomogeneity contribution in their resistance

77
measurements but its temperature dependence would have been identical to

that of the true bulk resistance and therefore its inclusion would not

have affected the measured activation energy. The specimens we have

worked with to date, while of very low porosity (>95% theoretical density)

and containing no sintering additives, have not been fully reacted and

therefore contain small amounts of phases other than the major fluorite

phase. These can account for our observation of an inhomogeneity contribution

to the resistance with a temperature dependence different from that of

the bulk. Thus an appropriate comparison may be made, as above, among

the activation energies drawn from the data of Tuller and Nowick [25],

the high-temperature data of Kudo and Obayashi [26], and our data over

the whole temperature range where the inhomogeneity effect has been

removed. Among these activation energies there is reasonable agreement,

as noted above.

Kudo and Obayashi [26] plotted In (resistivity) against T

^

However, for ionic conduction one expects (see for instance eqn. 3.63,

Ref 59 and the discussion following it, the following equation to be

valid for a single mobile defect:

p = AT exp (Q/kt) , (2)

where p is the resistivity, A a constant, T the absolute temperature, Q

the activation energy, and k the Boltzman constant. It follows that the

slope of a plot of lnp against T

^ is given by

Q’ = Q - kT. (3)

That is, the apparent activation energy Q* will be smaller than the real

value by a term kT , which ranges from 5.8 to 10.6 kJ/mol in the temperature

78
range from 400 to 1000°C. This is the source of the7.7 kj/mol we added to

the Kudo and Obayashi observed value for the apparent activation energy

to correct for this effect.

Experimental Results - The Electrode Arc

At high temperatures and low frequencies the data, when plotted in

either the impedance or the admittance complex plane } produce a flattened

arc. This behavior is reproduced by the equivalent circuit of Fig. 26,

k

using an equation similar to eqn. 1 for Z ^

1 + (jwT

e ^)(l-a)

Figure 33 shows representative data compared to values calculated using

eqn (4) with fitted parameters. The data are quite well reproduced.

One possible model for this equivalent circuit is a diffusioncontrolled Warburg impedance in parallel with a simple resistance, .

The parameter a would have the fixed value 0.5 for this model.

Physically this model suggests flow of electrochemically reacting

oxygen through two parallel paths (Fig. 34). In one, giving rise to ,

there is rapid interchange between the oxygen in the atmosphere and the

oxygen ions in the electrolyte. This path involves the three-phase

electrolyte-electrode-gas contact. According to the experiments of

Schouler et al [60] on Ag-electroded stabilized zirconia, R^ depends on

temperature and oxygen partial pressure according to

(4)

-1 /?

R

d = BP

q exp (QR

/kT)

and they suggest the rate-controlling step is the reaction between 0

atoms and 0

? molecules adsorbed on the electrode-gas interface;

20 , -* 0 o ,

. ads Zads

B is a constant depending on the length of the 3-phase line interface.

(5)

( 6 )

79
JRN

21/

1

970

HPB

702

02

BG

M

5WHD ' i i

80

/

DHM5
ELECTRODE

METAL C/) <

CD

c\l i 81

34.

Schematic

of

Porous

Electrode-Electrolyte

Interface. Rd

denotes

the

resistive

path

for

the

oxygen

discharge

reaction,

and W

the

diffusive

path.
The other path involves interchange between oxygen ions in the

electrolyte and oxygen atoms in the electrode material. The rate

controlling step is diffusion of the oxygen atoms in the electrode

material, and this gives the Warburg-type impedance

* _ a -1/2

Zw

~ T

E

where A is the area of electrode-electrolyte contact. Grahame [61] has

£j given an expression for o for the case of smooth homogeneous electrodes

with a uniform current density where some reactant is transported by

diffusion through the electrolyte to the interface. The situation is

essentially unchanged whether the diffusion path lies in the electrode

or the electrolyte, and so we adopt Grahame' s expression for a:

( 7 )

a = RT

2/2“ F

2 cD

1/2 ’

in which F is the Faraday, R the gas constant, c the molar volume concentration of diffusing species (0 atoms) and D its diffusion coefficient.

&

The combination of R. and Z produces an expression for the total

d w impedance of the electrode equivalent circuit [54],

, R

J el

1 + ( jiot ) 1/2

( 8 )

(9)

in which t is given by

T - f (W •

a The plot of Z in the complex plane is an arc whose span on the real

axis is R^ and for which the angle between the real axis and the line

joining the left intercept with the center of the arc is tt/ 4. Equation

(7) can be valid only at higher frequencies; at low frequencies the

Warburg impedance must approach a finite limit.

( 10 )

82
This electrode arc can be characterized by two parameters. Schouler

et al [60] expressed their data in terms of (R^/2) and

C = ( t

/

2R ) z d

= A^R

d

/(2a)

2

,

-2s Given measured values of R it should be possible to calculate C (A ) using

d z hi data in the literature on the solubility and diffusion coefficient of

atomic oxygen in the metallic electrodes.

We have made this comparison for the impedance data of Schouler et

al [60] for Ag electrodes on Ca-stabilized Zr0 9

. The data on diffusion

and solubility of atomic 0 in Ag were drawn from the work of Ramanarayanan

and Rapp [62].

.-3

D = 4.9x10 exp

„ „„-3

5840 -

i 2 "I

exp [-

T

| cm s

exp [-

5790 -

i d 1/?

T

1 V

-3

Schouler et al [60] give for the experimental values of the parameters

in eqn (5) at 900K:

Rj = 1.19xl0

_1 ° exp (23789/T)P„

_l/2

Q.

d U

^ Combining eqns (8), (12), (13), (14), and (15),

C A

-2

= (144. 2/T

2 )exp(6371/T)P 1/2 F/cm4

. Z Hi U

2

In a later paper, Schouler and Kleitz [63] give a plot of C

z /A vs

P„ at 900K, where A is the geometric electrode area. Figure 35 compares

2 their experimental data with values for C /A estimated with eqn. (16).

Z H

Whereas we expect on the basis of the model that the data should lie

along a line of unit slope such as the one drawn on Fig. 35, in fact

there is very little variation of the observed values for C A

^ with

z oxygen partial pressure.

(ID

( 12 )

(13)

(14)

(15)

(16)

83
1N3 IAII U3dX3 ' z

UJ0 /J rl '( t _Vz 3) DO!

S-i (D « O X '—I *H O > 3 • 3

jj 3 jc o cr 3

a) 3 43 i—

3 ,C "O

4-1 0) 3 <H 4-1 > £ a

•h a)

4-i xi a. u 3 X 3 4-1 3

4-1 ccj 4-1 rH <U

0) 3 r3 a 4-1 3 i—

o 3 44 44 a o

CO 3

cn 3 >

vO 3 •H rH 4-1 • 3 3

4-4 > 3 3 4-1 pi O 3 ^— 4-1 3

r-l co r"N r—

cm •H 31

i-H < co 3> CJ 3

rH 3 3 3 •rl 41 0) rH c 3 3 cd X e •H

•H 4J t—

3 H O 3 3 CO a 3 x 3

0) 3 -3

CU

H

44

a

O

CU • 3 CJ s o 3 4-1 CO 3 X

•H 4-1 3 3 •H 4-1 3 a

0- 3 3 e Cu 3 o 3 CO u CJ s

*

Lnm o

84
A less detailed comparison can be made with our own data for Pt

electrodes. The measurement temperatures range from 478 to 804°C, in

tank oxygen and in tank nitrogen. Figures 36 through 39 illustrate the

effect of changing from an oxygen to a nitrogen atmosphere. We believe

the placement of these curves along the real axis has been affected by

the instability of the Pt electrodes discussed above, and expect that

with stable electrodes, the left-hand intercepts will be the same for

the oxygen and nitrogen atmosphere. The arcs represent the fits of eqn

(1) to the data. At the lower temperatures in nitrogen the fitting

involves a long extrapolation and the results are obviously somewhat

uncertain

From the fits we derive the values for the parameters of eqn (1)

given in Table 5. The parameter t was converted to Cz using eqn (10).

Table 5 . Parameters for Electrode Arc for Hot-Pressed Ce^

9

Y

0.

°1 . 95

T°C

AZ . el.

S2 Cz , yF a T°C

AZ , ft el c

z ,yF

a

478 2280 69.3 .56 470 2250 60.9 .36

593 496 244 .53 582 1961 116 .27

702 43.7 169 .39 694 534 154 .32

804 7.2 116 .38 804 51.0 208 .38

804 6.8 112 .40

Figure 40 and 41 show the variation of AZ , and

el

C

z> respectively. with

temperature for both atmospheres. At the higher temperatures, where the

parameters are best known, AZ

^ appears to exhibit an exponential dependence

on T \ with an activation energy of 164 to 328 kj/mol. This value is a bit

more than twice the activation energy, 73.3 kJ/mol, given by Tuller and

85
FEB

10/

I37B

HPB

H7B

D2

BH

5WHD ' « ,

86

HMS
rvi m

PsJ

pjm

Lri

CD

a_mmm

CD

LU

LornN 5 W H /

87

nhd
38.

Complex

impedance

plot

at

low

frequencies

for

hot-pressed

Ce^9

Iqi

Of

in0^

at

702°C

(+)

and

in^

at

694°C

(S3)

IS rmZ

88 /

DHM5
hE

IU

h0B

BdH m

r~m

CS3

ta

LtJ

mx

Ki

co

5WHD ' i i 

89

Complex

impedance

plot

at

low

frequencies

for

hot-pressed

Ce09Yqi

Oq95

at

804°C

in0^

(.)

and

inN2

(O)
Nowick [25] for oxygen transport in the same composition. For comparison.

Schouler et al [60] found the activation energy for AZ

^ to be 198 kJ/mol

for ZTq Yq ^ 0^ with Ag electrodes whereas that for oxygen transport

in stabilized Zr0„ is about 125 kJ/mol [64]. It seems likely that AZ is z el

not controlled by transport of oxygen in the electrolyte, but reflects

some process on the interfaces.

On the other hand, does not appear to depend on whether the

atmosphere is oxygen or nitrogen, in agreement with the results of

Schouler and Kleitz [63] reflected in Fig. 35. Furthermore, a does not

appear to equal 1/2. This behavior is so at variance with the diffusion

model discussed above and expressed in eqn (16) that we feel compelled

to reject that model for the electrode arc and to continue the search

for an explanation for the behavior.

The temperature dependence of is expressed by

C = C exp (Q /kT)

z zo z

with the following values for C and Q : zo z

Atmosphere: 0

^ ^

C , pF 4.53 3.30xl0 3 zo

Q , eV 0.30 -.26

z

This reversal in sign of the activation energy may be related to the

observation of Schouler et al [60] that Pt electrodes apparently have a

layer of oxide in high oxygen pressures but not at low, in this temperature range.

Summary

The high frequency impedance arc appears to contain information

about both the bulk resistance in the interior of the grains, R , and a

* contribution, AZ , from inhomogeneities. The inhomogeneities may be

gb

(17)

90
TAZ

e)

, KQ

40. Temperature dependence of TAZ for Ce Y„ 0, __ at 804°C

in Tank 0^ (•) and Tank ^ (OfT

91
n

CM

Jri ZQ

CN

92

41.

Temperature

dependence

ofC

for

Ce

_ Y

0

in

tank

0

and

tank

N_
pores, second phase inclusions, or grain boundaries. The impedance data

can be quite well fitted with the equivalent circuit of Fig. 26, using a

desk- top computer with a least-squares technique. This part of the data

appears to be independent of the atmosphere. More porous specimens show

relatively more important inhomogeneity effects, as expected. For hotpressed Ce^ Yq

^ 0^ gj-, in which inhomogeneities exist due to incomplete

reaction, the variation with temperature of the inhomogeneous contribution.

AZ , was greater than that of the bulk resistance, R . The effect of

gb B

the former had essentially disappeared for temperatures higher than

about 500°C. The bulk resistance (R ) values give an activation energy

D in agreement with data in the literature [40, 41] for the same composition.

The sputtered Pt electrodes were found to be unstable, and to

approach a steady-state condition only after high-temperature (800°C)

anneal, and even then only very slowly. This aging results in an apparent

increase in bulk resistance, as if it might involve a decrease in effective

contact between the solid electrolyte and the electrode metal. On the

other hand, it appears to have less influence on the measured geometric

capacitance, .

The low-frequency arc reflects reactions at the electrode-gas phase

electrolyte interface. The impedance is well fitted by a Cole-Cole

expression. An unsuccessful attempt was made to reproduce the experimental

behavior by a model involving two parallel paths for the flow of oxygen,

one in which the impedance is controlled by diffusion of oxygen atoms in

the metal, giving rise to a Warburg impedance, and the other in which

the impedance is a pure resistance dependent on the inverse square root

l

! of the oxygen pressure, as observed.

93
2.3 Battery Materials

Introduction

The advent of fast ion conductors, and particularly the discovery

of the very rapid sodium ion transport in sodium beta-alumina, has

opened up the possibility of relatively cheap, high power density, high

energy density secondary batteries for automotive use, and also utility

load-levelling. Solid-electrolyte batteries operating with liquid or

solid electrodes obviate the problem of controlling dendritic growth on

the electrodes, and shorting during the charge cycle.

Presently the leading candidates for automotive power batteries

include sodium-sulfur, lithium-sulfur, and zinc-chlorine. All suffer

serious drawbacks, including the fact that none has as high power- and

energy-density as desirable. There is thus considerable interest in

finding systems capable of operating with lithium or hydrogen under nottoo-extreme conditions of temperature. To this end, a more complete

knowledge of the basic mechanisms controlling the desirable properties

in candidate materials is required. In the work on this program we are

currently contributing to the understanding of the sodium ion transport

mechanism in sodium beta-alumina.

2.3.1 Beta-Alumina

Introduction

One of the leading candidate systems for high energy density and

high power density batteries for transport use, and perhaps also utility

load-following use, is the sodium/sulfur battery. The electrolyte is

the ceramic, sodium beta-alumina, in which sodium ion conduction is very

high with a transference number of essentially unity. Because of the

94
potential usefulness of these batteries there is great current interest

in the properties of beta alumina, and particularly in the details of

the mechanism by which the very rapid transport occurs.

The structure of beta-alumina is well known [65]. The unit cell

3+ 2-

consists of blocks of spinel structure formed by A1 and 0 ions stacked

in the c-axis direction and interspersed with’ special planes, parallel + 2 - to the crystallographic basal planes, in which all the Na and some 0

ions reside, and in which the rapid Na

+ transport occurs.- The ionic

conduction mechanism can be viewed in a sense as two-dimensional transport in these planes.

The stoichiometric beta-alumina composition is Na.

? 0‘ll Al-^O^.

However, this does not actually occur, and real specimens contain an

excess of 20-35% Na20 [66]. There appears to be proportionality between

the ionic conductivity and this excess Na+ concentration [67], and it is

now thought that these excess ions occupy interstitial sites among the

regular Na+lattice sites and are responsible for the rapid Na+

transport

[68]

Models of this transport in terms of liquid-like motion of the

Na+

ions have been proposed [69]; however, we will discuss our work

using the point defect description.

Two defect models have been proposed [68]

In the description that

follows, "cell" will refer to the translation repeat unit of one of the

two conducting planes in the crystallographic unit cell. Figure 42

illustrates this "cell" in schematic fashion. In the simple interstitial

model [70] the excess Na+

ions constitute the interstitials and occupy

the anti Beevers-Ross (aBR) sites. Transport involves an interstitialcy

motion in which the interstitial moves from the aBR site to an adjacent

regular Na+

site (Beevers-Ross , or BR site) while the occupant moves to

an aBR site in an adjoining "cell" and becomes the interstitial.

95
OXYGEN l)

SODIUM

/\ ) \

CO

X 3

a) 3 •• JJ (J 0) oQ P XI

cu as a) X

&0 CO P 3 <u p P P CJ

4-> P CO a co ai 3 xi 3o CO H 3 33

•H

i—i o 3 cj I + 3 4-i cd P 3 Z

0) 3 X o p 3 • O

4-1 •i—) P P o X P 3 3 •H 3 o 2 3 3 3 O P i—i P • 3. - 3 3 = 3 P

toO CO P a P CJ CO •H p p P 3 ,3 o

CJ

O P.S 3 r cti X3 X too ro O CJ O a 3 P X

CO 3 3 3 3 X 2 u P S CO • r> p 3 3 P P

•H o •H Z CO

CO

r

—1 3 3 P X 03 o 3 P PQ •H CJ 3 = 3

cp •H * O a> CN

CO

X P o a) . r. c P x 3

ai

O 3 P S 3 P P

Cl) a>

a CO

toO 3 3 CO P Pi

cc! CD

X P 3 3 P p p •* <) H J) i—i

CNI

96

text

are

indicated

by

dashed

lines.
In a second defect model [71], an interstitial (excess) Na +

ion

and a regular Na+

ion share a "cell", both occupying mid-oxygen (MO)

sites, leaving the BR and aBR sites both vacant. Transport occurs when

either ion of this split interstitial "dumb-bell" moves through the

adjacent aBR site to an adjoining "cell", where it and the regular Na+

of the new "cell" form a new split interstitial.

In order to help to identify which of these processes is more likely

to be responsible for Na+

ion transport, we have performed an internal

friction experiment on a single crystal of sodium beta alumina. In this

experiment an alternating tensile-compressive stress of approximately

fixed frequency is applied along the <110> crystallographic direction

and the energy loss per cycle (expressed as the inverse of the quality

factor, Q \ of the resonating crystal) observed as a function of

temperature. A prominent peak in the TQ ^ vs T plot signals a process

in which mobile ions are moving under the stimulus of the applied stress.

The first defect model described above, in which the Na+ interstitial

occupies an aBR site and displaces a regular Na+

ion which in turn moves

to another aBR site (interstitialcy mechanism), would not produce internal

friction. The result of the unit transport step would be the disappearance

of an interstitial from one aBR site and the appearance of an interstitial

at an adjacent one. Since the aBR sites are crystallographically

equivalent by virtue of a unit translation, an equivalency that is not

removed by a homogeneous stress, no motion from one such site to another

would be induced by the stress, and therefore no internal friction would

be observed

97
On the other hand, the second mechanism, involving the split

interstitial on MO sites, could produce an internal friction peak.

There are three crystallographically equivalent orientations for the

split interstitial, related by a pair of mirror planes parallel to the

c-axis. (cf. Fig. 42) Application of a <110> compressive stress removes

one mirror plane and reduces the set of three equivalent orientations to

two sets, one composed of a pair of orientations still equivalent by

virtue of the remaining mirror plane and symmetrical disposed about the

<110> axis along which the stress has been applied. When the extra Na+

ion moves from one cell to another in the transport process, the effect

as described above is to move the split interstitial from one cell to

the next. Some of the jumps of this kind involve a reorientation of

the new split interstitial relative to the old. Such jumps would be

stimulated by the stress and could therefore contribute to the internal

friction as well as to the transport. In addition, reorientation of the

split interstitial within the "cell" could also contribute to the internal

friction, but, in this case not to the transport. The activated state

for this non- transport reorientation is probably not very different from

that for the transport process. Hence the split interstitial model

could result in very similar, but not necessarily identical, activation

energies and pre-exponential factors for the transport and internal

friction experiments. The observation of internal friction in the

experiment reported here thus tends to eliminate the simple interstitialcy

mechanism and to support the split interstital model as most likely to

be important to the Na+

transport process.

98
Experimental

Internal friction data in the temperature range from 90K to above

room temperature were collected on a sample of single crystal Na 8-Al ? 0^.

The sample was cut from a boule grown from the melt by the Crystal

Products Department of the Union Carbide Corporation. Such crystals

typically contain 15-35% excess Na+

. The sample was a bar of dimensions

7.82 x 1.17 x 0.82 cm, with the long dimension within 1/2 degree of the

<110> crystallographic (and also crystal growth) direction. The c-axis

was parallel to the 1.17 cm dimension. Cutting was performed with a

diamond saw under pump oil, and the crystal was stored and handled in an

argon atmosphere to protect it from atmospheric water. The bar was

resonated in the fundamental flexure mode. Because of the rectangular

cross-section, the frequency exhibited by this mode when the displacement was parallel to one side of the bar was different from that when

displacement was parallel to the other. Figure 43 shows schematically

the flexural strain generated in the bar, and the orientation of the

conducting planes relative to this displacement, for the two orientations. In both cases a tensile-compressive stress is generated oriented

along the <110> crystallographic direction. However, there is additionally

a shear tending to produce slip on planes which are parallel to the

conducting planes for the 16 kHz resonance but which are perpendicular

to the conducting planes for the 12 kHz resonance.

To achieve this flexural vibration, the sample was suspended in a

temperature-controlled, evacuable chamber (Figure 44) suspended (by fine

fibers) near one end from a phonograph cutting head and near the other

from a phonograph pickup with a Rochelle salt transducer. The suspension

fibers were of glass (150 pm diameter) coated with a thin layer of

plastisol. They were located on the crystal near the nodes of the

fundamental flexural vibration. The temperature was measured with a

99
12 KHz VIBRATION

16 KHz VIBRATION

43. Flexural strain produced in specimen bar for two orientations.

100
44. Experimental arrangement for internal friction experiment

101
platinum resistance thermometer calibrated at the National Bureau of

Standards, held near to but not touching the specimen. The thermometer

was encased in plastic to simulate the thermal transport properties of

the specimen. Temperatures below room temperature were varied by passing

cold nitrogen gas and liquid nitrogen through coils surrounding the

specimen chamber. Cooling and warming rates- of 1/2 to IK per minute

were used during the measurements.

The internal friction data were obtained by a variation of Forster's

method, using a new instrument developed at the National Bureau of

Standards by Parks and Linzer [72]. Briefly, a voltage-controlled

oscillator is inserted into a feed-back loop designed to locate the frequency

of maximum signal output from the pickup, thus allowing a continuous

measurement of the resonance frequency as the temperature was varied. A

phase-lock detector yielded a direct measure of the internal friction

(Q by measuring the frequencies at which some predetermined shift in

phase relative to the phase at resonance occurred. In practice phase

shifts of 45, 22.5, and 11.25 degrees were used to check the consistency

of the results. All values reported here are for a 22.5 degree phase

shift and represent the mean of 10 readings.

Results

The two expected flexural resonances were observed. At room temperature the fundamental resonance frequencies were 12.276 kHz, for the

specimen lying flat with the larger cross-sectional dimension (width)

horizontal (12 kHz orientation), and 16.257 kHz with the width vertical

(16 kHz orientation) . Figures 45 and 46 display the internal friction

data (TQ ^) over the temperature range from about 90K to room temperature.

There is a < + 6K discrepancy in temperature between the data taken with

rising and falling temperatures, such that the thermometer leads the

102
LjJ

€C

Z>

5

o

LjJ

CL

LjJ

o

LO

<}

(fr-OI) NOIlOiyj 1VNy3J.NI

103

Internal

friction

for

the

sodium

beta-alumina

specimen

with

the

width

horizontal.

Falling

temperature,

x.

Rising

temperature
104

100

150

200

250

300

TEMPERATURE

(K)
specimen. This is due to the fact that the thermometer could not directly

contact the specimen. This discrepancy was largest in the middle of the

temperature range where the rate of rise or fall was largest.

For the 12 kHz orientation, with the width horizontal (Fig. 45),

there is a single strong internal friction peak near 100K. With the

width vertical (16 kHz orientation) an additional very strong peak

occurs near 235K (Fig. 46). The orientation suggests that this peak

arises from the effect of the shear component of the stress, probably

interacting with small cracks parallel to the conducting planes observed

in the large faces of the crystal. Annealing the crystal in vacuum for

3 hours at 125°C to reduce the H

2 O content [73] had no influence on the

peak near 100K in either orientation, but reduced the peak at 235K with

the 16 kHz orientation to less than 50% of its former amplitude. This

higher temperature peak will not be discussed further.

Analysis

Because of the large temperature uncertainty in the middle of the

range, a detailed analysis of the internal friction peaks would not be

worthwhile. However, an analysis of the variation of the peak temperature, when combined with the similar data of Barmatz and Farrow [74],

would help in comparing the processes producing internal friction with

those producing Na+ion transport.

The following scheme was adopted to minimize the impact of the

temperature hysteresis exhibited in Figs. 45 and 46 on the estimate of

the peak temperature. It was observed that the data taken with rising

temperatures could be fit very well (Fig. 47) with a hyperbolic secant:

TQ

-1

= A Sech [B(T

_1

- T

_1

) ]

m

Using this as an interpolation formula for the rising- temperature data,

an estimate for the true temperature corresponding to equilibrium was

(18)

105
s

m

/ I

106

Internal

friction

for

the

sodium

beta

alumina

specimen

with

the

width

vertical,

details

of

the

temperature

variation.

Falling

temperature

x,

rising

temperature©,

estimated

equilibrium

data
obtained as the mean of the temperature of a data point taken with

falling temperature and the temperature of the point on the rising

temperature curve at the same TQ ^ value. Figure 47 shows these estimated equilibrium data points as pluses.

Equation 18 was found to produce an excellent fit to the estimated

equilibrium data points as well, and it was therefore fitted to these

data by least squares, minimizing the sum of the squares of the deviations in TQ ^ from the fitted curve. The estimated standard deviation

in the peak temperature, Tm, due to random error is about 0.2K. In

addition, however, there is a systematic error because the equilibrium

data may not be correctly represented by the mean of the rising and

falling data. This systematic error in choosing the correct temperature

for each equilibrium data point is bounded by the experimental values

for the rising and falling temperatures. Figure 48 displays the maximum

systematic error (one half the distance on the temperature scale of Fig.

^7 from the rising to the falling temperature curve) as a function of

the temperature for each equilibrium data point. The error decreases as

the temperature goes down because the rate of rising or falling of the

experimental temperature decreases near the end of the temperature range

and the rising and falling temperatures of the experiments performed

with temperature converge on the equilibrium values. Figure 48 shows

that at the peak temperature (100 - 103K) the systematic error must be

less than +1K, and in reality must be considerably less. We adopt

values of about +1K at 102K and + 0.6K at 101K as the maximum values

for the systematic temperature error.

107
ISn

108

48.

Maximum

systematic

temperature

error

in

internal

friction

data.
The extreme values for T including this maximum value for the m

systematic error are:

Frequency, kHz 12.34 16.39

Tm, Upper limit, K 101.6 102.9

Tm, Lower limit, K 100.4 100.9

The relaxation times obtained from these data combined with the

very similar data of Barmatz and Farrow [74] obey the Arrhenius equation

quite well (Fig. 49). Least-squares fits were made to the two sets of

four experimental points (the two values of Barmatz and Farrow together

with either our upper or our lower limiting values) , weighting them

according to the stated random errors in the peak temperatures and

assuming no errors in the frequencies. These fits gave two limiting

pairs of values for the activation energy and pre-exponential factor,

__-j^ g ^ g namely 20 kJ/mol and 5.6x10 s, and 23 kJ/mol and 7.4x10 s.

Discussion

These activation energies are quite close to those reported [68]

for various Na+

transport processes 16.4 kJ/mol, and if seems very probable

that the internal friction reflects the same processes. However, the

small discrepancy of 6 kJ/mol persists and may be real. Furthermore,

the pre-exponential factors appear to be unrealistically small, and may

be hard to reconcile with any realistic model.

The simple interstitialcy model discussed above cannot give rise to

internal friction, and can be ruled out in our experiment. The split

interstitial is consistent with our results. A detailed comparison can

be made between the behavior of this model and the various experimental

results. We use here the formalism of the relaxation mode analysis [75]

to calculate the ionic conductivity and the relaxation frequency for

internal friction. Work is still continuing on calculating the Na+

tracer diffusion coefficient.

109
( oas ‘ ±

901

110

Arrhenius

plot

for

internal

friction

in

sodium

beta-alumina.

Data

of

Barmatz

and

Farrow

(Ref.

74)

our

data

with

maximum

systematic

error

added

x,

and

subtracted

OThe

error

bars

are

reported
Figure 50 displays a block of "cells" in which only the MO and aBR

sites are shown. In the central cell the 3 orientations of the split

interstitial are shown, and the directions of possible interstitial

jumps indicated by arrows. In the surrounding cells the orientations of

the new split interstitials reached by the jumps are shown by heavy

lines. According to this figure, the rate equations governing the

occupation probabilities P_^ of the various orientations for the split

interstitials, neglecting defect interactions except for site availability,

are

3 3 3

P

1

- -

2

FP

1 +4

rP

2 +4

rP 3'

P

2 * !

PP

1 'I rP2 + t

rP 3’ (19)

3 3 3

p = —TP + — rp - — rp

3 4 1 4 2 4 3’

where T = v(l-p) + (4/3)y, (20)

with v the probability of one member of the split interstitial jumping

into an empty adjacent "cell" and y the probability of its jumping into

the vacant MO site in its own "cell". The fraction of excess Na+

ions

is p , so that (1-p) is the probability that a given adjacent cell does

not contain an interstitial.

If we define a 3-dimensional vector space in terms of the 3 orientations so that the coordinates represent the occupation probabilities of

the orientations, then a general solution to eqns (19) in the undriven

case can be written

P=-{- (1,1,1) +— (1,1, -2) +— (1,-1, 0) } (21)

/3 /3 /6 /2

where the three terms represent three relaxation modes, and

f . = f . exp ( — X . t) , (22)

l 10 r 1

111
I

112

50.

Arrangement

and

jumps

of

split

interstitials

from

one

"cell"

to

another

in

sodium

beta-alumina.

aBR

sites

lie

at

intersections

of

trigonal

network;

MO

sites

are

indicated

by

small

filled

circles.

Lines

connecting

MO

sites

indicate

possible

split

interstitials

and

arrows

the

paths

from

one

"cell"

to

the

next.
in which X. is the relaxation time for the ith mode and f. is determined

1 10

by the applied stress and the response of the material to it.

In particular, a <110> stress will excite the second mode but not

the third (The first is the equilibrium mode). For this second mode the

relaxation time is given by

T _1

= X = 9r/4

= v(l-p) + 3y

In this equation we will assume that v and y are given by the usual

Arrhenius expressions for activated processes:

v = V

Q exp (-Q^/kt)

y = y exp(-Q / kt)

o y

To calculate an expression for the Na+

ionic conductivity, we again

return to the relaxation modes, but this time in the presence of an

electric field sufficiently small such that only linear terms in the

changes induced by the field in the occupation probabilities need be retained

To proceed we adopt a more detailed idea of the movement of the split

interstitial toward the activated state. Following Wang et al [71], we

assume a coupled motion in which one Na

+

ion moves into a nearby aBR

site while the other moves to the BR site in the same cell (Fig. 51).

This is assumed to be the activated state for both transport and reorientation without transport. Transport occurs when the Na +

ion at the aBR

site moves on into another cell, reorientation without transport when it

moves back again to the MO site from which it came, the other Na+

ion

then moving away from the BR site toward either remaining MO site, the

one it came from or the one originally empty. In our model here we have

taken the activated state to be identical for the two processes, transport

(23)

(24)

(25)

113
Td

Q)

114

51

.

Approach

of

the

split

interstitial

model

for

sodium

beta-alumina

to

the

activated

state

for

both

transport

and

reorientation

jumps

The

large

circle

is

the

lattice

oxygen

ion,

the

shaded

circles

are

sodium

ions

on

MO

sites,

and

the

small

circle

is

the

unoccupi

MO

site.
and reorientation without transport, but they need not be exactly the

same, and in particular the activation energies could be somewhat

different

In the presence of a field. Equations (24) and (25) are modified by

/Jea including with the activation energies Q and Q a term +a = + ———E,

. y v — — 4kt

where E is the electric field strength, assumed to lie in the <110>

direction, e is the electronic charge, k is the Boltzmann constant, T

the temperature in kelvins, and a the lattice parameter. The plus or

minus sign is used when the movement from ground to the activated state

involves the motion of the Na+

ions in a direction opposite to or the

same as that of the applied field. For the case in which the original

split interstitial is oriented perpendicular to the field, there is no

net motion of the charge as the defect approaches the activated state

and therefore no field-dependent term added to Q and Q . y v With eqns (24) and (25) modified in this way, the equilibrium mode

replacing the first term in eqn (21) is

P

1 (P 11’ P 12 ’ P 13'>

p = r _!— —A —I—)

1

l 2+y ’ 2+y ’ 2+y

J ’

with y

= (3/2) v(l-q) + 2y(l+3a) , (3/2)v + 2y

where v and y are given by eqns (24) and (25) as written. The ionic

current is given by

given J = E J

i

where the subscript i denotes the orientation of the split interstitial

before jumping to a new position, and

J

± = (Npe) p ic

E Tij bij

j

(26)

(27)

(28)

(29)

115
with N the number of regular Na sites per cm , p (as before) the fraction

of interstitial sites occupied, e the unit electronic charge (positive),

p^c the probability of occupation of the ith orientation in the equilibrium

mode in the presence of the field (see eqn. 26), Tij the probability per

second of a j-type transport jump from an i orientation, and bij (which

can be positive, negative, or zero) the sum of the distances moved by

the two ions of the split interstitial in making the entire jump.

Correcting the activation energies for the transport jumps for the

effect of the field and retaining only the linear terms as described

above., and using the values for p_^ implied in eqns (26) and (27) and

taking into account the availability of nearby sites into which to jump,

the ionic condcutivity becomes'

3Np(l-p)e^a^ 2v+3y v

0

6k 3v+4y T

Note that v and y in this equation are given by eqns (24) and (25) , in

which the activation energies in the absence of the field are to be

used. If and are equal, then eqns (23) and (30) predict that the

activation energies for the internal friction and conductivity experiments will be the same. Otherwise, they can differ.

The conductivity data of Wittingham and Huggins [76] have been used

in eqn. (30) to calculate V

q

for their crystal (p = 0.16), assuming

v>>y, and this in turn used in eqn. (23) to calculate f^, the frequency

for which 100K would be the temperature of the internal friction peak

for a crystal such as ours (p = 0.3), and the result compared in Table 6

to the value interpolated from our experimental data. We also list the

attempt frequency and activation energy obtained by Walstedt et al [77]

23 from Na NMR nuclear-relaxation time T^ measurements on crystals similar

to ours, and the corresponding value of f .

(30)

116
Table 6 - Comparison of Internal Friction Relaxation Energies

Source Ref . v ,Hz

o Q, kJ/mole

£ el’

Hz

Conductivity 76

12

2.8x10 15.9 3.4xl0 3

NMR 77 inll ^10 ^ 14 ^6xl0 3

This Work _ _ _ 8.4xl0 3

Conclusions

The agreement among these frequencies lends support to the proposition

that the various experiments are all measuring essentially the same

process, the motion of the excess Na+

ions. The difficulty in representing

the temperature variation of the frequencies in terms of activation

energies and pre-exponential factors in a way that is consistent among

the experiments may result from unresolved errors in the measurements,

from curvature in the Arrhenius plot combined with the fact that the

various experiments do pertain to somewhat different temperature regimes,

or may indicate that each experiment reflects a somewhat different

averaging over a distribution of processes. The internal friction

result is inconsistent with the simple interstitial model in which the

excess Na+

ions occupy the aBR sites. It is consistent with and strongly

supports the split interstitial model in which the excess Na+

ion and

the regular Na~*~ ion in the same cell occupy two out of the three available

MO sites. The close correspondence with the transport process suggests

also that transport involves the split interstitial motion.

If resources permit, it is intended to test the possibility of

curvature of the Arrhenius plot for the internal friction by extending

the accessible frequency-temperature regime using ultrasonic attenuation

measurements

117
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39. L. Gordon, Anal. Chem., 2^7, 1704 (1955).

40. ibid., Record Chem. Progr . , 17 , 125 (1956).

41. L. Gordon, M. L. Salutsky and H. H. Williard, Precipitation from

Homogeneous Solution , John Wiley, New York (1959) .

42. M. L. Salutsky and L. L. Quill, J. Am. Chem. Soc., 72_, 3306 (1952).

43. T. Moeller and E. P. Horwitz, J. Inorg. Nucl. Chem., 12 , 49 (1959).

44. W. W. Wendlandt and T. D. George, Texas J. Sci., 13_, 316 (1961).

45. C. B. Bishop, Ph.D. Thesis, Michigan State University (1959).

46. J. M. Axelrod, Ph.D. Thesis, University of Maryland (1959).

47. R. G. Charles, J. Inorg. Nucl. Chem., 27_, 1489 (1965).

48. E. L. Head and C. E. Holley, Jr., "The Preparation and Thermal Decomposition of Some Rare Earth Carbonates',' Rare Earth Research II , ed . by

K. S. Vorres, Gordon and Breach, New York (1963).

49. R. C. Vickery, Chemistry of the Lanthanons , Academic Press, New York,

and Butterworths , London (1953), pp . 214-216.

50. ibid. , Analytical Chemistry of the Rare Earths , Pergamon Press,

New York, (1961), p. 13.

51. G. Czapski, H. Levanon and A. Samuni, Israel J. Chem. ]_, 375 (1969).

52. M. P. Pechini, U.S. Patent No. 3,330,697; July 11, 1967.

53. B. N. Sudarikov, V. A. Zaitsev and Yu. G. Puchkov, Nauch. Doklady

Vysshei Shkoly, Khim. i Khim. Tekhnol. 1959 , No. 1, 80-3; Chem. Abstr.,

53, 13738h (1959)

120
54. L. H. Bennett, et. al., "Materials for Fuel Cells", NBSIR 77-1270,

May 1977.

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121
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Rodriquez

122
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U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

78-1472

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Materials for Fuel Cells

5. Publication Date July 1978

6. Performing Organization Code

7. AUTHOR(S) L. H. Bennett, M. I. Cohen, A. L. Dragoo , A. D. Franklin, A. J. McAlister and K. F. Young

8 . Performing Organ. Report Nc

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 5610494

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Power Systems Division

Department of Energy

Washington, D. C. 20545

13. Type of Report & Period Covered

Annual Report

Jan. 1977 to Dec. 1977

14. Sponsoring Agency Code

15.

SUPPLEMENTARY NOTES

16.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) . , c , i Research is described on hydrogen oxidation electrocatalysis on a number of compounds

related to WC. Compounds in the series of Mo Wx C, with x MSIO . 2 , were found to be

active, although some uncertainty exists as to ?he purity of the samples. Some

activity was also noted in W

Q

Ti

Q 5 C and rhenium metal. Experiments on the preparation of MoP and FeP„ are described.

The microprocessor—controlled potentiostat for electrochemical measurements was completed and is described in detail.

Experiments on several techniques for the preparation of very finely-divided Y-doped

CeO powder are described. The powders to serve as raw materials for hot-pressing pure,

dense, homogeneous ceramics. Straight sintering of some of these specimens yield

densities of 95-97% theoretical.

The equivalent circuit previously suggested appears to fit very well the frequencydependence of the impedance of sintered specimens of Ce

0 g °]_95 with sputtered or evaporated Pt electrodes. The bulk part of the data is independent of P°2 and varies

with porosity as expected. An activation energy for the crystal conduction were obtained in agreement with literature values. High temperature instability in the Pt

electrodes was observed. The electrode polarization impedance is well-fitted by a Cole—Cole expression. It cannot be attributed to a model proposed earlier involving

oxygen atom diffusion in the Pt electrode. „ . . . n . ^ . , . Internal friction experiments on a single crystal of Na 8—A^O^ are interpreted m

terms of the Na+

ion motion.

17.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Electrocatalysis; electrochemical measurements; fuel cells; phosphoric acid;

polarization; refractory hard metals; cerium dioxide; sodium beta alumina; solid

electrolytes

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