NATL INST OF STAND & TECH

A11107 3^0275

NBS

PUBLICATIONS

NBSIR 83-2747

Materials for Fuel Cells

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Washington, DC 20234

Final Report--January 1979 to December 1979

Issued January 1984

Prepared for

r p

*"svision of Fossil Fuel Utilization

l'L * - Apartment of Energy

10 J ashington, DC 20545

,U56

33-2747,!

1934

C.2

NATIONAL

OF STAi UBRi

NBSIR 83-2747

MATERIALS FOR FUEL CELLS

c

/

C. K. Chiang, M. I. Cohen, A. L. Dragoo, and A. D. Franklin

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Washington, DC 20234

Final Report--January 1979 to December 1979

Issued January 1984

Prepared for Division of Fossil Fuel Utilization

Department of Energy

Washington, DC 20545

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
,

I
1 -

Table of Contents

Abstract 7

1 . Introduction 9

2. Subtask Reports 10

2.1

High Temperature Solid Electrolytes 10

Introduction 10

2.1.1 Preparation and Characterization of High-Density

Ceria-Yttria Ceramics 13

Introduction ........ 13

Coprecipitation. 14

Experimental Details .... 17

Results and Discussion .............. 22

Summary 32

Acknowledgments .33

2.1.2 Electrical Proprties of Y-doped CeC

2 Ceramics

Comparison of ac and dc Measurements 34

Introduction 34

Experimental Details 38

Results 40

Discussion 47

Summary 50

2.1.3 Slow Transients in Y-doped CeO^ 51

2.1.4 Precipitation Phenomena in Mg-doped Zr0

2 53
- 2 -

2.2 Interconnector Materials 54

2.2.1 Oxygen Sensitive Electrode Impedance in Sr-doped

LaCrO^ 54

Introduction 54

Results 55

Summary 58

2.3 Development of Instruments 59

2.3.1 Low Frequency and Pulse Operation of MicroprocessorControlled Potentiostat 59

Introduction 59

Hardware 59

Software: Interfacing to a Commercially-Available

Operating System 63

References 66
- 3

-

Tables

1. Flame emission/atomic absorption analyses for selected impurities,

ppm.

O 2. X-ray diffraction results (CuKa, X = 1.54178 A)

3. Effective crystallite dimensions,

4. Determinations of the lattice constant of Ce0

2 :Y0i 5

5. Equivalent circuit parameters for Y-doped CeO^ ceramic specimens at

230-250 °C.

6. Comparison of total ac with 4-probe dc conductivities at 500 °C for

"chemi cal ly-prepared" 8.5 cation per cent Y-doped CeC^ ceramics.

7. Activation energies for ac and dc conductivities in "chemicallyprepared" 8.5 cation percent Y-doped Ce0

2 ceramic.

8. Addressing requirements, disk dri ver-converter section.
- 4 -

Figures

la: Agglomerated particle from a precipitation of Ce-Y hydroxide by the

addition of ammonium hydroxide to a nitrate solution.

lb: Fracture surface of a sintered specimen which was fabricated from a

powder containing particles such as the one shown in Figure la.

2: Schematic of the precipitation procedure.

3: SEM Micrographs of the precipitate and oxide powder: (a) the

initial precipitate; (b) the aged precipitate; (c) the oxide powder

obtained by calcining the precipitate at 620 °C.

4: Micrographs of HP YDC: (a) fracture surface formed during hotpressing, chemically etched to reveal grain boundaries; (b) mechanically fractured surface, chemically etched; (c) bright field

STEM image of ion-beam thinned specimen [28].

5: Equivalent circuit used to represent the impedance data for Y-doped

Ce02 ceramics. The circuit elements and parameters are explained

in the text.

6: Impedance data in the complex plane for "mixed oxide" specimen B3

at 241 °C (a), and "chemically-prepared" specimen B7 at 251 °C (b).

The observed data are indicated by squares, and the values calculated for the same frequencies from the equivalent circuit, as

described in the text, are represented by pluses. The drawn arcs

represent the behavior of in parallel with C

Q

(left-hand arc)

and Z.j in series with (right-hand arc).

7: Temperature dependence of the conductivity of "chemical ly-prepared"

Y-doped Ce02 (8.5 cation percent) ceramic, measured by the 4-probe

dc method.
- 5

-

8: Impedance data in the complex plane for a "chemi cal ly-prepared"

ceramic specimen of 8.5 cation percent Y-doped CeO^ at 713 °C. The

electrodes are sputtered Au/Pd alloy. The observed data are indicated by squares and the calculated by pluses. See text for

details.

9: Comparison of temperature dependence of 4-probe and ac conductivities of "chemically-prepared" Y-doped CeC^ (8.5 cation percent)

ceramic. The symbols, +, A, and 0 refer to different ac specimens;

x indicates the 4-probe dc data on a fourth specimen.

10: Dependence of R^/R^ at ^ 240 °C upon porosity for "mixed oxide"

specimens of 10-20 cation percent Y-doped CeO^ ceramics. The

dotted line is the least-squares line through the origin. The

solid lines are solutions to Eqn. [3] for e = 30, x

= 0 And the

values of a/b shown.

11: Effect on complex impedance of Zr

Q

g-jgMdg Qg-|0] g-jg at 446 °C in

air of precipitation of tetragonal phase. The solution-annealed

specimen is indicated by x, the precipitation-annealed by 0. The

insert shows the vector difference.

12: Phase diagram of MgO-rich portion of the MgO-ZK^ binary, after

Grain, ref. (76).

13: Typical impedance spectrum (5 Hz to 13 MHz) of Lag gg2^ r

o . 008

Cr0

3 at room temperature, equilibrated with tank at 52 °C. The high

frequency region contains a temperature-dependent fine structure.

Inset shows this fine structure revealed by making measurements

at -130 °C.
6

14: Time dependence of the electrode resistance of Au electrodes on

La

0 992

Sr

0 008^3 at 52 °C* ^ pen c

'irc ^ es represent data taken

when the atmosphere was switched from tank ^to tank CL. Filled

circles are for the reverse.
- 7

-

Abstract

The preparation of very dense Y-doped CeO

? ceramic is described, by

hot-pressing powders obtained by the homogeneous precipitation of mixed

cerium and yttrium carbonates from trichloracetate solutions. The

resulting ceramic was very homogeneous, judging by the sharpness of the

x-ray diffraction peaks, had a density of 99.4 ± 0.06 percent of the

theoretical fully-dense value for the composition, and exhibits equiaxed

grains with diameters in the range from one to four urn.

Diffusion equations have been written in finite-difference form for

the flow of neutral atomic oxygen into or out of an oxygen-ion-conducting

solid electrolyte specimen through which a constant oxygen-ion flux is

maintained. The neutral oxygen diffuses to or from the electrode/

electrolyte interface simultaneously through the electrode and the

electrolyte. The slow establishment of a steady-state gradient in

oxygen activity produces a slowly-changing voltage difference between

two potential probes placed along the specimen axis. This potential

difference exhibits a number of the features we have seen in the slow

transients observed when measuring the dc resistance of Y-doped CeO^

ceramics by the 4- probe method.

Measurements of the impedance of Y-doped CeC^ ceramics containing

about ten or 20 cation percent Y indicate that there is a sizable contribution from inhomogeneities, perhaps grain boundaries, to the total

impedance. Separating the intrinsic or lattice impedance from the

inhomogeneity contribution appears to be possible using the frequency

response of the impedance. The intrinsic conductivity is somewhat, but

not heavily, dependent upon the specimen preparation. Values obtained

here for the activation energy and pre-exponential factor for this
- 8 -

intrinsic conductivity (aT plot) are in reasonable agreement with values

from the recent literature.

The inhomogeneity contribution to the impedance, and particularly

the equivalent parallel capacitance, appears to be strongly dependent on

the specimen preparation. For "mixed oxide" materials, the ratio of the

inhomogeneity resistance to the intrinsic resistance scales approximately linearly with porosity, but the connection appears to be indirect.

The existence of regions, perhaps at the grain boundaries, of variable

dopant concentration is a possible explanation, with the incidence of

such regions correlating with porosity. For the "chemically-prepared"

materials, a much larger equivalent inhomogeneity capacitance is observed than with "mixed oxide" materials. The differences cannot be

accounted for on the basis of grain boundary geometry, but may reflect

again a chemical effect at grain boundaries, such as an increase in the

concentration of electronic charge carriers.

With polycrystal 1 i ne specimens of materials of this kind, the use

of impedance spectroscopy, the analysis of the frequency dependence of

the impedance over a wide frequency range, appears to be essential to a

full understanding of the mass and charge transport processes.

The impedance of partially-stabilized zirconia (Ma-doped Zr09

)

annealed at temperatures at which particles of tetragonal ZrC^ solid

solution are expected to precipitate out, when compared to material

annealed wholly in a single-phase region, exhibits an additional relaxation. We attribute this to the presence of the precipitate particles,

with resistivity and/or dielectric constant somewhat different from that

of the matrix.

Barrier layers are formed on Sr-doped LaCrO^ that give rise to an

appreciable electrode impedance with electrodes of Au or Pt. The
- 9

-

corresponding electrode resistance is quite sensitive to the oxygen

activity in the atmosphere, following atmospheric changes at temperatures near room temperature with time constants of the order of minutes

or days. The electrical properties of these barrier layers at room

temperature do not conform to the usual models of Schottky layers on

semiconductors, but exhibit changes in current and differential capacitance much more symmetrical in the applied bias.

Work is continuing on adapting the microprocessor-controlled

potentiostat to low-frequency and pulse operation, to allow, especially,

solid-electrolyte impedance measurements in the low-frequency range.

This work has mainly been involved in redesigning a new converter board

and in developing software to allow interfacing with a commercial lyavai Table Operating System.

1. Introduction

The program of research on materials for fuel cells and batteries

at the National Bureau of Standards includes work on both liquidelectrolyte and solid-electrolyte systems. Formerly, progress in both

topics was reported in the same report, but starting this Fiscal Year

will be covered in separate reports. This report deals with work on

solid-electrolyte systems.

The overall objectives of the research are:

1. To provide data allowing selection of optimum materials for critical aspects of fuel cell operations, and

2. To improve the measurement capability and standards for assessment

of performance (e.g. , efficiency of electrode processes, lifetime

of components, etc.), of materials in fuel cells and batteries.

The work on materials for solid-electrolyte during this year involved research on materials for several components of the overall

solfd-electrolyte fuel cell. These include:
- 10 -

1. High Temperature Solid Electrolytes,

2. Interconnector materials, and

3. Development of instruments.

2. Subtask Reports

2.1 High Temperature Solid Electrolytes

Introducti on

Oxide solid solutions having a defective fluorite structure and

based on Zrt^, Ce02, and Tl^, doped with CaO, Y^O^ or the tri valent

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 percent of dopant may be added

depending upon the oxide system.

ZrC^-based solid solutions have been used as electrolytes for hightemperature electromotive force measurements since the mid-1930's [1,2],

and ThC^-based electrolytes, since 1957 [3], 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 [4,5] on CeC^tLn^, where In represents

a variety of lanthanide ions or Y^

+

, have revealed oxygen-ion conductivities essentially equivalent to that of ZrC^CaO but at temperatures

about 200 °C lower. Thus, fuel cells using a Ce02 ~based electrolyte

might operate at about 800 °C as compared to 1000 °C for ZK^-based

electrolytes. However, Ce-based electrolytes begin to show appreciable

electronic conduction at higher oxygen partial pressures than do the

Zr0

2 -electrolytes ; for example, one-half of the electrical conductivity
11

is electronic in (Ce0

2 )g 95^2^0 05

at P 0

= atm 10~ 19

Pa)

for T = 800 °C, whereas for ZrC^’.CaO, Pq = 2.5 x 10~ 3

^ atm (2.5 x

10" 36

Pa) for -> 1000 °C [6].

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

the dopant cations [7]. 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 ionic conductivity of the electrolyte

may decrease (resistance may increase) because the vacancies, relative

to the fluorite structure, must then move cooperati vely.

The existence of coherent structures is well documented for ZrO^-

based solid solutions [8-15]. Coherent structures in CeC^-based solid

solutions have not been identified.

Carter and Roth [12] found that the increase in resistance of

calcia-stabi 1 ized 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
12 -

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

be significant. For example, from the tracer diffusion measurements for

Ca ions in CSZ [16], the time required for Ca ions to cross a 20 \im

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 diffu2 + sion 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

sol id 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 hi gh- temperature electrolytes currently contains two elements: a study of degradation mechanisms in

CeC^-based oxygen-ion electrolytes and other materials, 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.
13 -

2.1.1 Preparation and Characterization of High-Density

Ceria-Yttria Ceramics

Introduction

Yttrium-doped cerium dioxide (YDC), like the perhaps better known

calcia- and yttria-stabi 1 ized zirconia, is a fl uori te-structure oxide

into which a great many oxygen vacancies are introduced by substituting

trivalent yttrium for quadrivalent cerium. When about 9.5 atom percent

Y is substituted for Ce, the ionic conductivity of YDC at 1000 °C is

about 0.15 (ohm*cm)~^ [4]. At these high concentrations of yttri um— for

example, 2.4 percent of the oxygen sites are vacant in 9.5 percent YDC--

oxygen vacancies most probably interact and may form ordered structures,

given sufficient time to do so.

To obtain research quality specimens for a study of long-term ac

impedance behavior of YDC, we investigated means for preparing ceramic

specimens which had nearly full density, uniform microstructure, homogeneous composition and high purity. Nearly complete densi fication was

required in order to avoid changes in the electrical properties arising

from rearrangement or elimination of pores during the long-term anneals.

Uniform microstructure and homogeneous composition were needed so that

the material would have a simple and interpretable ac response both for

the intrinsic impedance of the grains and for the grain-boundary impedance.

Our early attempts to prepare specimens by calcining, grinding,

compacting and thermally sintering (TS) or hot-pressing (HP) the mixed

oxides resulted in specimens of low density. Bulk densities ranged from

70 to 90 percent of theoretical density (TD) in the case of the TS

specimens and from 93 to 94 percent in the case of specimens cut from a

hot-pressed billet. The highest densities were obtained if the mixed
14 -

oxides were not calcined prior to sintering. However, the HP ceramic in

which an uncalcined oxide blend was used showed the presence of Ce0

2

-

related fluorite-structure phase and a Y^-related c-structure phase.

The c-structure phase disappeared when the material was annealed at

1550 °C for 40 hours. Since extensive coironinution of calcined oxides

was precluded because it was potential source of impurities, a chemical

procedure for preparing well -mixed and reactive ceria-yttria powders was

sought.

We have examined several chemical procedures employing coprecipitation from homogeneous solution to prepare Ce-Y compositions. Of these

procedures, we have used the coprecipitation of carbonates through the

hydrolysis of trichloroacetic acid to prepare a large batch of oxide

powder. This powder was used to prepare specimens by hot-pressing and

by thermal sintering.

Coprecipi tation

Coprecipitation of components can yield a thoroughly blended powder; however, the precipitation must be designed to avoid the entrapment

of large numbers of pores in hard, agglomerated precipitate particles.

Such particles result if Ce and Y are precipitated inhomogeneously--for

example, by neutralizing a Ce-Y nitrate solution with arrmonium hydroxide. Figure la is a SEM picture of an agglomerated Ce-Y hydroxide

particle obtained by precipitating the hydroxide inhomogeneously from a

nitrate solution. Grinding the calcined precipitate with an alumina

mortar and pestle did not destroy the pores so that the sintered specimens contained large pores which were reminiscent of the precipitate

particles. An example of the pores in one of these specimens is shown

in Fig. lb.
la: Agglomerated particle from a precipitation of Ce-Y hydroxide by the

addition of ammonium hydroxide to a nitrate solution.

lb: Fracture surface of a sintered specimen which was fabricated from a

powder containing particles such as the one shown in Figure la.

- 15 -

Willard and Tang [17] were the first to demonstrate the usefulness

of precipitation from a homogeneous solution for rapid preparation of a

dense precipitate. Subsequent investigators developed a variety of

methods for generating the precipitating agent--oxalate, sulfate, iodate, carbonate, etc. --homogeneously throughout the solution and for

precipitating a variety of metallic ions. The science and techniques of

precipitation from homogeneous solutions have been reviewed by Gordon [18,

19] and by Gordon, et al . [20].

The precipitation of rare-earth carbonates from homogeneous solution was described first by Salutsky and Quill [21], who precipitated

the carbonates of La, Nd and Sm by hydrolysis of strong trichloroacetic

acid solutions of these rare earths. The reaction proceeds by the

decomposition of the trichloroacetate ion [22], and the overall reaction

can be written as,

2Ln(Cl

3

CC00)

3 + 3H0H = 3C0

2 + 6CHC1

3 + Ln2 (C0

3

) 3

, (1)

where Ln designates a trivalent lanthanide ion.

Quill and Salutsky [23] observed that a yellow, gelatinous Ce (IV)

trichloroacetate complex formed in hot ceric solutions. Head and Holley

[24] prepared a cerous trichloroacetate solution by treating a ceric

sulfate solution with hydrogen peroxide, precipitating the cerous

hydroxide with urea and dissolving the cerous hydroxide in a slight

excess of trichloroacetic acid solution. They investigated the preparation of Ce carbonate at ambient C0

2 pressure and at CO^ pressures of

200-300 and 1000 psig. They identified their product as a hydrated

normal carbonate. In contrast, Charles [25] found that a hydrated basic

carbonate was obtained if a trichloroacetic acid solution of cerium

chloride was used for the hydrolysis reaction. The basic carbonate
16 -

precipitate was observed to be nongel ati nous and nearly white.

Wendlandt and George [26] prepared a hydrated normal yttrium carbonate

using the method of Salutsky and Quill.

Salutsky and Quill [21] noted that precipitation of the carbonates

did not recover all of the La, Nd or Sm, oxalate having to be added to

bring down the remainder of the rare earths from solution. Charles [25]

used trichloroacetic acid slightly in excess of the stoichiometric

amount for Ce ( III) trichloroacetate and obtained a 42 percent yield.

These yields of the carbonates appear to be surprisingly low in

view of the low solubility of Ce and Y carbonates. Trofimovich and

Sedel'nikov [27] determined the solubility of Ce carbonate in water at

25 °C to be 1.6-2. 4 mg/L. Trombe, et al . [28] found a comparably low

value for the solubility of Y carbonate. They also observed that the

solubility of Ce carbonate was not significantly influenced by CO^

pressure up to one atm.; whereas, the solubility of Y carbonate increased to nearly 40 mg/L as the C0

2 pressures was increased. Quill and

Salutsky [23] ascribed the failure to obtain a quantitative yield to the

formation of some rare earth chloride as a result of the secondary

oxidation of the chloroform in the hot solution. In addition to the

interference by Cl~-ions, the solubility of Ce and Y carbonate may be

increased by alkali and ammonium ions [29].

The results of previous work indicated that the hydrolysis of

trichloroacetic acid could be used to precipitate Ce and Y carbonates

but that yield and fractionation would be influenced by temperature and

concentration of carbonate as well as Cl , NH^, Na and K . In this

work we attempted to increase the recovery of Ce and Y by using excess

trichloroacetic acid, by adjusting the pH at the completion of the

hydrolysis reaction to slightly alkaline conditions with ammonium bi-
- 17 -

carbonate to bring down additional Ce and Y either as carbonate or as

hydroxide, and by ripening the precipitate to increase the particle

size, thereby improving the settling out of the precipitate. We obtained a yield of 68 to 70 percent as determined from the oxide recovered from the process.

Experimental Details

a. Materials

Cerium dioxide and yttrium oxide ( Y 2^3 ) Powders 99.99 percent

purity were used as the starting materials. The manufacturer's spectrographic lot analysis indicated the presence of Mg, Si, Ca and Fe at

less than 50 ppm and of other rare earth at less than several hundred

ppm. Alkalis were not reported. The first and second rows of Table 1

present flame emission/atomic absorption (FE/AA) analyses of blended

Ce0

2 -Y

2 03 powder and of a specimen hot-pressed with that powder. The

Ce0

2 had a weight loss on ignition (LOI) of 0.865 percent when heated at

1000 °C for several hours; the Y

2 0

3 had a 2.3 percent LOI.

Ce0

2 was dissolved in concentrated nitric acid containing a small

amount of hydrofluoric acid [30]*, and the appropriate amount of Y

2 03

,

dissolved in nitric acid, was added to give stock solutions with cation

concentrations in the range of 0.3 to 0.5 mol/1, and with an average CeY composition, corrected for LOI, of 90.35 atom percent Ce to 9.65 atom

percent Y. Since there was some bottle- to-bottle variation in the

compositions, the solutions were blended into one another as the contents of the bottles were removed.

*Subsequent work has shown that Ce0

2 can be dissolved much more effaciously in hot concentrated nitric acid to which 30 percent hydrogen

peroxide is slowly added than with the nitric acid-hydrofluoric acid

used in this work.
18 -

Table 1. Flame Emission/Atomic Absorption Analyses of Selected

Impurities, ppm.

Material Na M£ Al_ Si K Ca Fe RE

CeOp-YpO^ 30 3.7 31 <5 7.7 19.4 27 NR

Mech. mixed powder

Ce0 9 -Y 9 0. 28 4.3 108 <5 4.9 22.6 19 NR

Mech. mixed,

hot-pressed ceramic

Ce0

2 -Y2 03 111-124 NR 37-60 NR 8.2-8. 5 NR NR NR

Carbonate process,

hot-pressed ceramic 3

NR--not reported

aTwo samples
- 19 -

b. Preparation and Characterization of the Precipitate

The procedure for preparing the Ce-Y precipitate is diagrammed in

Fig. 2. About 500 mL of a stock solution was used for each precipitation. To reduce the nitrate concentration, Ce-Y hydroxides were precipitated by the dropwise addition of an ammonium hydroxide solution (1:2)

to the nitrate solution, also diluted 1:2. After the ripened precipitate had settled, the supernatant liquid was siphoned off through a tube

closed with a fritted disk to prevent loss of the precipitate. The

precipitate was rinsed with distilled water and the supernatant liquid

removed as before.

The hydroxide precipitate was dissolved in about three times the

stoichiometric amount of a nominally 25 percent trichloroacetic acid

solution. To achieve dissolution of the ceric hydroxide, it was necessary to add 30 percent hydrogen peroxide in an amount slightly in

excess of a 1:1 ratio to the Ce content. Upon addition of the hydrogen

peroxide, the slurry became brown and a thick layer of foam developed.

After about 30 minutes the foaming subsided and a clear solution was

obtained.

The solution was diluted to about 3 1 and was warmed to 80-100 °C

to decompose the trichloroacetic acid. When the reaction was completed,

about 20-30 g of NH

4

HC0

3 were added to neutralize the solution and to

precipitate any Ce or Y remaining in the solution. The solution was

tested by adding a small amount of NH^OH. The solution was warmed for

about 24 hours to ripen the precipitate and to reduce the volume of the

solution. Two or three batches of precipitate were often combined at

this point. After the precipitate had settled, the supernatant liquid

was pumped off as before, and the precipitate was rinsed three times

with water, then with ethanol and finally with acetone. The first
PREPARATION SCHEME: "CARBONATE PROCESS"

2: Schematic of the precipitation procedure.
20 -

aqueous rinse showed cloudiness when tested with silver nitrate, but the

third rinse remained clear. The precipitate was dried first in air and

then in a vacuum oven at 60 °C. About 880 g of material was obtained.

Samples of the initial and the aged precipitates were examined by

SEM and by x-ray diffraction.

c. Preparation of the Oxide Powder

A thermal decomposition study of Ce2(C0

3

) 3 *8H

2 0 by Head and Holley

indicated that the water of hydration was driven off below 200 °C and

that the anhydrous carbonate decomposed below 600 °C. X-ray diffraction

of a sample calcined at 620 °C showed that only broad fl uorite-structure

peaks were present so that initial calcination of the powder was carried

out at this temperature. The yield of oxide powder, when corrected for

an L0I of 2.65 percent, was 655 g, which amounted to a recovery of 68-70

percent of the oxides.

Samples of the oxide powder were calcined further for one hour at

700, 800 and 900 °C and were examined by x-ray diffraction and by SEM.

Some TS specimens were prepared from these powders.

d. Preparation and Characteri zation of Ceramic Specimens

About 350 g of the calcined powder was formed into a billet without

binder, isostatically pressed and then hot-pressed in an alumina die for

30 minutes at 1350 °C with an applied load of 28 MPa. The billet exhibited excessive shrinkage during hot-pressing and broke into several

large pieces.

Specimens were cut from the billet for electrical studies, to be

reported on elsewhere [31], and for examination by optical microscopy,

SEM and x-ray diffraction. Densities of the polished specimens were

obtained from mass and size measurements. An unetched specimen was

examined by electron micro-probe [32] for possible chloride contamina-
>
- 21 -

tion at the grain boundaries. Fracture surfaces from several smaller

pieces were examined by SEM and by STEM [33], both with analytical x-ray

capabil ities.

X-ray fluorescence [34] was used to measure the Ce and Y content of

the HP material. Samples of the crushed and ground material were fused

in lithium tetraborate to form borate-glass disks. Standard samples

containing known amounts of Ce and Y were prepared in a similar manner.

Estimates of the impurity concentrations were obtained by semiquantitative spectrochemical analysis.

A small amount of the billet was ground to a fine powder and mixed

with a small portion of tungsten powder as a reference for a precise

determination of the lattice constant [35]. A vertical, computerautomated x-ray diffractometer, with computer analysis of the count

rate, was used for these measurements. Ni-filtered CuKa radiation was

used, with the x-ray tube operated at 40 KV and 30 mA. Since the Kc^

and Kc^ diffraction peaks were well-resolved for 2e > 75°, only the

midchord positions of the Ka-j (x = 1.540498 A) peaks from 75° to 150°

were determined. The observed midchord positions were corrected using

the simultaneously-determined midchord values for tungsten; the lattice

constant of tungsten is 0.316524 nm, 0.000004 nm standard deviation [36].

The computed lattice constants were corrected for absorption by the

Nelson-Riley method [37,38].

Some specimens were prepared by forming them with a pellet press,

i sostatical ly pressing to 69 MPa to produce a uniform and larger green

density and thermally sintering at about 1520 °C for one day. Bulk

densities were measured by an Archimedes technique with mercury as the

buoyant medium.
22 -

Results and Discussion

a. Composition and Crystal Structure of the Precipitate

If the precipitate is assumed to have the general composition

Ln(C0

3

) a (0H)

6 .nH

2 0 , (2)

where Ln = Ce (III) , Y and

2a + 3 = 3 (electroneutral ity) , (3)

the ranges of a, 3 and n can be estimated from the weights of the precipitate, 880 g, and of the oxide, 655 q, obtained by calcining the

precipitate. In setting up the material balance equation which constrains a, 3 and n, we further assumed: (1) that the amounts of Ce and

Y are the same for the precipitate and for the resulting oxide, (2) that

calcination of the precipitate converts all of the Ce to Ce0

2 and all of

the Y to Y

2 0

3

, and (3) that C0

2 and H2 0 are the gaseous products evolved

during the calcination. The Ce-Y composition of the oxide is reported

in Section e. below. The electroneutral ity condition and material

balance equation can be solved to find a and 3 in terms of n; thus,

a = 2.02 - 0.69n

3 = -1.03 + 1 . 39n ; (4)

a and s have physically meaningful values simultaneously only when

0.74 < n < 2.91 . (5)

The x-ray diffraction results for the precipitates in Table 2 show

that, apart from some improvements in intensities and resolution and the

appearance of a few additional lines, the initial precipitate and the

aged precipitate had essentially the same phases. Differences in the dvalues reflect difficulties in locating the centroid positions of rather
- 23 -

broad diffraction maxima. Only a few of the d-values can be assigned to

a previously identified material, hydroxyl-bastnaesi te [39]. Although

this assignment supports our estimated composition, it is inconclusive

due to the remaining large number of unidentified lines, some of which

are stronger than the lines tentatively assigned to hydroxyl-bastnaesi te.

b. Grain Size and Particle Size in the Oxide Powder

Calcination of the precipitate at temperatures in excess of 600 °C

replaced the crystalline structure of the precipitate with the fluorite

structure of CeO-?, as shown by the results given in Table 2 for a calcination at 620 °C. The diffraction maxima were generally weak and very

broad, and only the maxima at low angles were observed.

Estimates of crystallite size were made from the broadened diffraction maxima for powders calcined between 620 and 900 °C. The measured

peak widths, at half-maximum intensity for the (111), (200), (220)

and (311) reflections were corrected to peak widths, due to

crystallite size broadening, first for instrumental broadening by means

of the curve for "Low-angle reflections" in Fig. 9-9 of Ref. [40]. and

then for Ka-jc^ doublet broadening using curve E in Fig. 9-8 of the same

text. A powder sample obtained from the HP YDC, see Section f., which

had sharp diffraction maxima was used as a reference in making these two

corrections

.

The Scherrer [26] equation in the form

L hkl = 0.89x/B 1/2 (hkl)cose ? (6)

see eq. (9-106) of [40], was used to obtain the effective crystallite

dimension which are given in Table 3. As Stokes and Wilson [42]

showed, L hk -j can be considered to be a ratio,

L hkl

= <T

2 >/<T> (7)
24 -

Table 2. X-Ray Diffraction Results (CuKa, g

= 1.54178 A).

Precipitate

Initial Aged Oxide , 620 °C

0 0 0 d, A l d, A l d, A ]_ hkl

1.51 7 1.36 VW 400

1.59 ? 1 .56 VW 222

1.68 ? 1.63 W-M 311

1.73 ? 1.825 VW

1.85b VW

1.854 vw 1 .855 W

1.90b 7 1.90b VW

1.91b ? 1.912 M 220

1.92b 7 1.92b VW*

1.955 vw 1 .951 w 2.005 vw 2.008 W-M

2.011 w 2.02 ? 2.03b VW

2.05b vw*

2.07 vw 2.070 w 2.073 W-M 2.073 W-M*

2.09 7 2.11 7

2.12b VW

2.14b VW 2.14b vw 2.16b vw 2.16b vw 2.348 M 2.348 M

2.431 W 2.429 W

2.47 7 2.506 W 2.501 w*

2.61 ? 2.65 b w 2.703 M 200

2.78 ? 2.88b VW

2.90b W 2.90b vw 2.914 W

2.957 M 2.952 M*
- 25 -

Precipitate

Initial Aged Oxide, 620 °C

o O O d, A 1 d, A l d, A I hkl

3.120 S 111

3.37b M 3.36b M

3.55b VW

3.58b W 3.58b VW*

3.601 W 3.601 W

3.695b W-M 3.695b M

3.720 M 3.717 M-S

4.03 ? 4.28b W-M 4.28b M

4.324 S 4.316 S 4.89b VW

4.912 W 4.917 W

4.969 w 4.950 W*

5.560 W-M 5.553 M

Abbreviations : b = broad peak; ? = peak uncertain; VW = v

weak; W-M = weak to medium; M = medium; M-S = medium to st

strong. The indicates that the peak corresponds to a analog of hydroxyl-bastnaesi te.
26 -

of volume averages of the crystallite dimension T normal to the reflecting planes. These estimates assume the absence of strain broadening

and, therefore, may be considered to underestimate the crystallite size.

From 620 to 800 °C the crystallites appear to be somewhat larger in

the (hOO) than in other directions (see Table 3 below). The somewhat

smaller values of l_ hklin the other directions suggest that the particles may have had irregular prismatic shapes. Calcining the 620 °C

powder at 700 °C for one hour had little additional affect on the

crystallite size. Calcining the 620 C C powder at 800 °C for one hour,

however, appears to have induced a doubling of the crystallite dimensions. Calcining at 900 °C appears to have produced no further coarsening in the [hOO] directions but to have produced growth in the other

directions so that the crystallites appeared more nearly rounded.

Chord-sizing of a sample of the powder calcined at 620 °C with an

image analyzer yielded a particle size distribution character!' zed by the

following statistical quantities:

mean diameter 1.2 ym

standard deviation 1.2 ym

coef. of skewness 0.7

kurtosis 5.

Since the smallest size fraction contained a preponderance of the particles, and since there appeared to be particles just below the detection

threshold of the instrument, 0.25 ym, which could not be distinguished

for the purpose of counting and sizing, the measured particle size

distribution very likely was biased toward the larger size particles.

Comparing the particle size and crystallite size, it appears that an

5 average particle may have contained on the order of 10 crystallites.
- 27 -

Table 3. Effective Crystal

Calcination on)

Temp., °C nm

620 17

700 23

800 34

900 37

ite Dimensions,

(200) (220) (311)

nm nm nm

24 18.5 18.5

25 23 18.5

51 30 34

51 52 64
- 28 -

c. Microstructure of the Precipitate and Oxide Powder

The initial precipitate exhibited a lamellar structure. Fig. 3a,

with many platelets several micrometers across. Aging the precipitate

disrupted this microstructure and produced many irregular shapes ranging

in size from a few micrometers to several tens of micrometers across.

Figure 3b is a micrograph of a portion of a large agglomerated particle

from the aged precipitate.

Calcining the precipitate produced a powder with particles which

did not differ significantly in appearance, see Fig. 3c, from those of

the aged precipitate. This powder was very friable.

d. Density and Some General Features of the HP Ceramics

3 The bulk density of specimens cut from the HP YDC was 6.96 kg/m ,

3 with a 95 percent confidence limit of ± 0.04 kg/m . From the composition, Section e., and the lattice constant. Section f., the theoretical

3 density was calculated to be 7.003 ± 0.008 kg/m . Thus, the bulk density was 99.4 ± 0.6 percent of TD. Thermal sintering resulted in specimens with densities in the range of 92 to 98 percent of TD.

The overall appearance of the HP material was that of a beigecolored ceramic, with a glassy luster and conchoidal fracture. Pieces

about a millimeter thick were translucent; sections 40-50 um thick were

transparent. The material was resistant to etching with strong acid and

very resistant to erosion by an ion beam [43]--very long thinning times

were required to prepare a HP sample for STEM analysis.

e. Composition of the HP Ceramic

The HP YDC was found to consist of 91.4 mol percent Ce0

2 and

8.6 mol percent YO-j with a 95 percent confidence limit of ± 0.3 mol

percent.

'

-

I
- 29 -

Semiquanti tative spectrochemical analysis of the HP YDC material

indicated that the A1 impurity had increased, but that the other impurities either had not changed significantly or had been reduced. A

sample was analyzed by FE/AA analysis to obtain a more sensitive and

precise determination of the A1 , Na and K impurities in the HP ceramic.

The FE/AA analysis, third row of Table 1, indicated that Al-content

had increased by up to 100 percent, the Na-content by 300 percent and

the K-content had not increased. A larger increase in A1 content was

noted for a HP sample prepared from mechanically blended Ce0

2 -Y

2 03 powder, second row of Table 1. The alumina die used for the hotpressings is suspected to have been the source of the A1 contamination

in these two HP ceramics. A thin brown reaction zone was present at the

surface of the billets after hot-pressing, and some brown flecks were

found beneath the surface of the ceramics suggesting that grains of the

"lubricating" alumina were pressed into the billet. The Na contamination probably resulted from the chemical processing.

Although tests of the supernatant liquids from the rinsing of the

precipitate did not show the presence of Cl” by the third rinse, we

still considered it possible that Cl "-ions were adsorbed by the precipitate.

Scans of HP YDC samples by electron microprobe and analytical STEM

did not detect Cl . However, neither technique apparently was capable

of examining solely the narrow grain boundary regions where chlorides or

oxychlorides may have segregated.

f . Crystal Structure and Lattice Constant of the HP Ceramic

X-ray diffraction showed that the HP ceramic was a single phase

material having the fluorite structure and with a lattice constant of

-6

0.541088 nm, 7 x 10 nm standard deviation. Electron diffraction [43]
30 -

from individual grains, using smallest available selected area aperture,

gave patterns corresponding to a fl uori te- type (FCC) lattice. No diffuse or extra reflections were observed.

Our determinations of the lattice constant is compared in Table 4

with the results of other investigators [44-49], all of whom used photographic methods, in most cases the Debye-Scherrer method. With the

exception of the earliest measurement by McCullough and Britton [44-45],

all of the previous measurements gave values which were lower than our

diffractometer result. Only the Debye-Scherrer measurements by Bevan,

et al. [47] and by Anderson and Wuensch [48] approach the precision of

our diffractometer measurement.

With the exception of results of McCullough and Britton the cited

values of the lattice constant in Table 4 were interpolated to a composition of 8.6 mol percent Y0-| ^ from the lattice constant vs. composition data reported by the respective investigators. McCullough and

Britton obtained the same value for the lattice constants of pure ceria

and ceria doped with 9.99 mol percent Y0-| ^

. To indicate the precision

of the respective measurements, the uncertainties cited in the table are

those given by the respective authors, with the exception of Adham [49]

where the uncertainty is the standard deviation of the fit; thus, these

uncertainties do not contain any additional error due to the interpolation.

There are important differences between our determination of the

lattice constant and the precise Debye-Scherrer determinations by

Bevan, et al . and by Anderson and Wuensch with respect to x-ray technique, material preparation and material characterization, as well as

possible errors resulting from the interpolation of the previous results

to our composition. Each of these differences may serve wholly, or in
Table 4. Determinations of the Lattice Constant of CeO^^O-j

^

Investi qators

Preparative Method

and X-ray Diffraction Method

Compos i tion

,

mol % YO-j 5

Lattice

Constant,

nm

McCullough & Britton

Copptn. of hydroxides;

ignition at 1100-

1200 °C. Powder diffraction photography.

9.99 0.5411

± .0002

Brauer & Gradinger

Copptn. of oxalates;

ignition at 1400 °C.

Debye-Scherrer.

8.6 0.5409 a,b

± .0001

Bevan, Barker,

Martin & Parks

Both copptn. and mech.

mixed powders; pressed

pellets sintered at

1600 °C for four or more days. DebyeScherrer .

8.6 0.54091 c

± .00001

Anderson & Wuensch

Mech. mixed powders;

pressed samples sintered for 100 hr. at

1750 °C, equilibrated

for 50 days at 800 °C.

Debye-Scherrer

.

8.6 0. 54098

b

± .00003

Adham Copptn. of oxalates;

calcined at 1000 °C;

pressed specimens sintered at 1600 °C.

Debye-Scherrer.

8.6 0. 5410

d

± .0001

This work Copptn. of carbonates;

calcined at 620 °C;

oxide billet hot- pressed

Automated powder diffractometer.

8.6 0.541088

± .000007

a. Lattice constant adjusted to x(CuKa-|) = 1.54051 A.

b. Lattice constant interpolated to 8.6 mol percent Y0-| ^ by fitting

a quadratic function to lattice constant-concentration data for

the fluorite structure region.

c. Lattice constant calculated from eq. for 0-10 Dercent Y0-. r (Ref. [4], p. 459).

d. Lattice constant interpolated to 8.6 mol percent Y0-| ^ by least

squares fit of lattice constant-composition data.
32 -

part, to account for the discrepancy between these values for the lattice constant. A satisfactory explanation of the discrepancy will

require measurements on the same material by the two techniques,

g. Microstructure of the HP Ceramic

Micrographs of fracture surfaces of the HP material are shown in

Figs. 4a and b. Figure 4a shows a fracture surface, which formed

during hot-pressing; Fig. 4b shows one formed by mechanically fracturing

the material. Both surfaces were chemically etched with a solution of

nitric acid-hydrofluoric acid (nitric acid: hydrofluoric acid: water

90:10:100) for about 20 minutes with ultrasonic stirring. Figure 4c is

a bright field electron micrograph [43] of an ion-beam thinned specimen.

A very small amount of porosity is evident in these three micrographs. Figures 4a and c reveal that most of the porosity is along the

grain boundaries and not within the grains. The grains shown in Fig. 4a

appear to be about 2-4 urn in size, whereas those shown in 4c are on the

order of one urn, or perhaps less. It is not clear whether this difference is due to more than variation resulting from sampling. In both

cases the grains appear to be equiaxed.

The surface formed during hot-pressing shows well -etched grain

boundaries, whereas the mechanically fractured surface shows very little

evidence of etching. This difference may have resulted either from

thermal etching of the former surface or from exsolution of a more

soluble phase to the fracture surface during hot-pressing.

Summary

Homogeneous coprecipitation of Ce and Y by the decomposition of the

trichloroacetate can be used to oroduce a fine precipitate as a precursor for a sinterable oxide powder. The preciDitate obtained in this

work apparently was a mixture of hydrated carbonate, hydroxy-carbonate
criiimeany

fractured

surface,

chemically

etched;

(c)

bright

field

SUM

image

of

ion

beam

thinned

specimen

f?8|.

33 -

and hydroxide rather than a simple carbonate. The process suffers from

the disadvantage that it is not quantitative and that there was some

loss of Y relative to the Ce. However, the sharpness of the x-ray

diffraction peaks indicate that the resulting ceramic was very homogeneous so that significant fractionation of the Ce and Y during the

process does not appeared to have occurred.

The oxide powder was sintered by hot-pressing under relatively mild

conditions--applied load of 28 MPa and 1350 °C for 30 minutes--to yield

a material with a density of 99.4 ± 0.6 percent of TD. The ceramic

appeared to consist of rather equiaxed grains, with dimensions in the

range 1-4 ym, with the porosity mainly along the grain boundaries.

The billet cracked during hot-pressing, possibly as a result of

excessive shrinkage; however, this difficulty may be avoided by calcining the oxide powder at a higher temperature prior to the sintering

stage. Also, there was some evidence of contamination of the YDC by A1

,

which most likely came from the A1

2 03

die used in the hot-pressing, and

by Na, which probably originated from the chemical processing.

Acknowledgments

The characterization of our ceria-yttria ceramic was greatly

assisted by contributions of a number of individuals: J. Drennan,

June M. Epp, C. R. Hubbard, D. E. Newbury and P. A. Pella. Their

specific contributions are referenced in the paper. Finally,

L. P. Domingues of Trans Tech, Inc. was an active collaborator and will

be a coauthor on publications on this work.

We also gratefully acknowledge the financial participation of the

Fuel Cell Program of the Department of Energy in support of this work.
34 -

2.1.2 Electrical Properties of Y-Doped Ce0

2

:

Comparison of ac and dc Measurements

Introduction

The study of electrical transport processes in real, polycrystalline solids can be complicated by the presence of grain boundaries,

porosity, second phases, and other forms of inhomogeneity within the

bulk of the specimen. Where the conductivity is anisotropic, a distribution in orientation of the crystal grains will also act as a source of

inhomogeneities. In general, electrical inhomogeneities give rise to

depolarizing fields at a local level within the material and as a result

the observed dc conductivities, measured with the classical 4-probe

technique, do not correspond to the intrinsic conductivity of the

crystalline material. On the other hand, it is the latter, as measured

with a single crystal devoid of such inhomogeneities, that is the proper

subject of theoretical treatments of lattice conduction.

The frequency dependence of the electrical transport properties

contains information on the separate contributions made by intrinsic

transport and by the various inhomogeneities. (For a recent review, see

Bottelberghs [50]). In this paper we present impedance data for Y-doped

Ce0

2 ceramics prepared by several techniques, and examine the influence

of the preparation technique upon, especially, the inhomogeneity impedance. We also point out the inadequacy of the standard 4-probe dc

technique for measuring conductivity where a significant inhomogeneity

contribution exists, and the need for measurements over a sufficient

range of frequencies rather than at a single "high" frequency, as is

often reported,

a. Equivalent Circuit

Bauerle [51] first suggested that grain boundaries and other inhomogeneities gave rise to contributions to the impedance of an elec-
- 35 -

trical conductor, and proposed to represent these elements in an equivalent circuit as a parallel resistance-capacitance network, in series

with a resistance representing the intrinsic resistance of the grain

interiors. In our work we have found that the inhomogeneity impedance,

Z. , often appears better represented not as a single RC network, but

rather as a series ensemble with a distribution of time constants.

Following Schouler, et al . [52] we find we can represent the experimental data rather well if we use a Cole-Cole [53] distribution. The

equivalent circuit we therefore use is shown in Fig. 5. The circuit

elements and parameters are as follows: C

q is the geometric capacitance; R^ is the intrinsic or lattice resistance contributed by the

grain interiors; is the impedance arising from the inhomogeneities,

such as grain boundaries, second phase particles, or pores; and Z

-j is

the electrode impedance. Our data were taken at sufficiently high

frequencies that Z -| did not contribute to the impedance in any important way, and Z

-j will be considered below only incidentally. Z. is

represented by

Z, =

R

i

1 + (Jut)

( 8 )

where R. is the inhomogeneity resistance, n is a parameter between 0 and

1, expressing the width of the Cole-Cole distribution of time constants

(for a single R-C network n = 1), x is the most probable time constant

in the distribution, and u> is the circular frequency. An equivalent

capacitance, C. , can be defined by

* * R

i ci (9)
Equivalent

circuit

used

to

represent

the

impedance

data

for

Y-doped

Ce0

o

ceramics.

The

circuit

elements

and

parameters

are

explained
- 36 -

This same equivalent circuit, often without invoking a distribution

of time constants, has been used in a number of studies. Powers and

Mitoff [54] analyzed impedance data for polycrystal 1 ine sodium beta

alumina and found an inhomogeneity contribution that they attributed to

grain boundaries and that appeared to call for a distribution of time

constants. Hooper [55] compared impedance data for single and polycrystalline sodium beta alumina and found the inhomogeneity contribution

only with the polycrystall ine material. Again, although not considered

in Hooper's study, the data suggest a distribution of time constants for

the inhomogeneity contribution.

Raistrick et al . [56], on the other hand, represented very similar

data on lithium silicates and aluminosilicates by a different equivalent

circuit. In their work the grain boundary effects were represented by a

circuit element placed parallel to the intrinsic resistance and geometric capacitance. The admittance of this grain boundary element was

taken as frequency dependent according to Y* = Aco a

+ iB(j a

. We have

shown previously [57] that the electrical behavior of the equivalent

circuit of Raistrick et al . [56] can be essentially the same as that of

the circuit in Fig. 5, so that the choice is not necessarily dictated by

the data.

b. Source of Inhomogeneity Impedance

The appearance of such an inhomogeneity contribution to the impedance in polycrystall ine specimens is often attributed to grain boundaries, and indeed, as in the work of Hooper [55] above there is good

reason to suppose that grain boundaries are a likely cause. However,

Armstrong et al . [58] pointed out that in the case of an anisotropic

material like sodium beta alumina a distribution in grain orientations

will also give rise to such an effect. Also, the appearance of apparent

- 37 -

inhomogeneity contributions in impedance data on single crystals suggest

other important causes.

For instance, Levitskii et al. [59,60] found what appear to be

inhomogeneity contributions to the impedance of single crystals of CaF^

and BaFg. This contribution was sensitive to the oxygen activity in the

atmosphere and probably arises from highly conducting surface layers

produced by incorporation of oxygen ions on anion sites plus anion

vacancies. The effects are similar to those noted and attributed to

surface layers on oxidized fluorite crystals by Franklin et al . [61] and

Johnson et al . [62] some years ago and more recently by Wapenaar and

Schoonman [63]. Van Alpen and Bell [64] also found an inhomogeneity

contribution in the impedance of single crystals of LigN, and suggested

that dislocations and subgrain boundaries might be responsible.

Beekmans and Heyne [65] observed an inhomogeneity contribution in

the impedance of Cag Zr^

gg 0-j gg

ceramics which correlated with the

presence of a second phase, rich in impurity SiOg, on the grain boundaries. Chu and Seitz [66] also observed a strong inhomogeneity contribution in CaO-doped ZrOg, however without being able to detect the

presence of second phases. They attributed the inhomogeneity impedance

to grain boundaries. On the other hand, De Jonghe [67] has shown that

the observed inhomogeneity contribution may reflect a mix of intrinsic

and grain boundary properties, so that it may not be possible to interpret the experimental data simply in terms of separate intrinsic and

grain boundary contributions.

Heterogeneous dielectrics, composed of discrete particles of a

minority phase with conductivity and dielectric constant differing from

those of the matrix in which the particles are embedded, have long been

known to exhibit relaxations in the dielectric properties (Maxwell

Wagner effect). The existence of such a relaxation in the dielectric
- 38 -

constant implies that a relaxation will also appear in the impedance.

Van Beek [68] has treated this problem, for dilute heterogeneous systems, in a rather general way. His equations, when cast into the impedance representation, describe exactly the behavior of the equivalent

circuit in Fig. 5. In particular, it can be shown that they lead to

R- A Ux-U 2 (l-p)p

_L -

a (10)

R

L [(l-p){A

a (x-l ) + 1} + xP]C A

a 0-P)0-e) + e]

2

where e = /e^ and X

= X

2 /X 1

> and e

i

> e 2 and X

1

’ x

2 are the dielectric

constant and conductivity of the matrix, dispersed phase; p is the

volume fraction of the latter, and A depends on the axial ratio a/b of

a the spheroidal particles, which are assumed identical and non-interacting. A a is given by:

a

prolate a/b > 1

A

a =

-1 + (a/b)

In (a/b) + /(a/b)

2

- 1 (11)

[(a/b)

2

- 1] (a/b)

2

- 1

sphere a/b = 1

A

a * 1/3 (12)

d

oblate a/b < 1

[1 - (a/b) ]

Cos"

1 (a/b)

A - (a/b)

2

(13)

Experimental Details

a. Specimen Preparation

Ceramic specimens of Ce-j x ^ 2

» with x near 0-"' or 0-2 5 were

sintered from Y-doped CeCU powder prepared by several methods. "Mixed
- 39

Oxide" specimens started with high-purity Y

2 03 and Ce02 powders, which

were mechanically mixed and calcined in air at temperatures around 1000-

1200 °C, isostatically pressed and fired in air at -v 1500 °C for about

16 hours. Densities measured by flotation in a dense but fluid fluorocarbon liquid ranged from 72 to 94 percent of theoretical. Bulk densities calculated from specimen masses and geometries were often lower

than flotation densities, indicating considerable open porosity in some

of the specimens. One specimen (HP8) was hot-pressed at 1350 °C and

28 MPa pressure for 45 minutes, with a resulting flotation density of

94 percent theoretical. However, both x-ray diffraction and microscopic

examination showed incomplete reaction, with two phases present ( Y 2°3

and fluorite, presumably a solid solution of Y

3

0

3 in Ce0

2

).

"Chemically prepared" specimens started with Y-doped Ce0

2 powder

prepared by homogenous precipitation of the mixed carbonate from trichloracetic acid solution, followed by calcination at 600-800 °C [69].

This powder was isostatically pressed and then hot-pressed at 1350 °C

and 28 MPa in air for 30 minutes. Densities (both flotation and bulk)

exceeded 95 percent of theoretical. The x-ray diffraction spectrum exhibited only the fluorite structure, and the diffraction peaks were very

sharp. The material was light brown in color and somewhat translucent.

The grains were essentially equi-axed, with a rather uniform distribution in sizes in the 2-4 pm range. Details of this preparation and

characterization of the resulting material will be given in a subsequent

publ i cation.

b. Electrical Measurements

The impedance was measured on either disks or flat square plates of

about 2 mm thickness and 8-10 mm diameter or width. Electrodes were Pt

or Au, sputtered onto the two large surfaces after polishing with 1/4 pm
- 40 -

diamond powder followed by a brief final polish with Linde B.* The

electrode thickness required to maintain stability up to about 1000 °C

was about 200 nm. An HP-3570A network analyzer system was used to

measure sample impedance from 50 Hz to 13 MHz.

Meaningful data could be obtained at temperatures from about 200 °C

to about 800 °C. Oxygen partial pressures were controlled in the range

from about ten Pa to 0.02 MPa (10~^ to one atmosphere) by bleeding O2

into an argon stream, and were measured with an oxygen gauge downstream

of the specimen.

The 4-probe dc measurements were performed on a bar-shaped specimen

with dimensions of 1.7 mm x 1.7 mm x 13 mm. The electrodes were Pt

paste (fired at 1350 °C) with Pt leads. A high ac current was passed

briefly through all electrodes in an attempt to render them porous. All

contacts were ohmic for the currents used in the experiments (three to

2 300 uA/cm depending on sample resistance). A high impedance electrometer was used to measure the voltage drop across the potential probes.

The temperature was measured with a pair of Pt/Pt-1 0% Rh thermocouples

placed next to the specimen. Oxygen partial pressures were controlled

and monitored as in the impedance experiments.

Resul ts

The impedance is conveniently displayed in the complex plane, with

the frequency as a parameter. Figures 6a and 6b show impedance plots

for two contrasting specimens, all in the neighborhood of 250 °C. In

Fig. 6a the specimen (B3) is a "mixed oxide" specimen, sintered in air

*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.
6: Impedance data in the complex plane for "mixed oxide" specimen B3

at 241 °C (a), and "chemically-prepared" specimen B7 at 251 °C (b)

The observed data are indicated by squares, and the values calculated for the same frequencies from the equivalent circuit, as described in the text, are represented by pluses. The drawn arcs

i| represent the behavior of R^ in parallel with C

a (left-hand arc)

and Z. in series with R, (right-hand arc).

I

- 41 -

at 1420 °C for 16 hours. In Fig. 6b we show data for a "chemicallyprepared" specimen (B7), sintered in air at 1525 °C for 72 hours.

In both cases, the data points at the very highest frequencies

approach the real axis essentially vertically, indicating that at these

highest frequencies the data are controlled by the combination of R

L (the intrinsic or lattice resistance) and (the geometric capacitance). With this as a clue, the equivalent circuit was fitted to the

data as follows. Using the highest frequency data, the imaginary part

(Z") of the complex impedance was plotted against the inverse circular

frequency, m, and the slope taken as C

Q

~^

. The impedance data were then

inverted to admittance and ojC^ subtracted from the imaginary part, Y".

The result was reinverted to impedance and the resulting values fitted

with the Cole-Cole form [Eqn. (8)] of the impedance plus a constant,

R^, by a least-squares technique.

This technique works as follows: first the radius and center of

the arc in the complex plane describing the data are found by minimizing

the sum of the squares of the differences between the radius and the

distances from the experimental points to the arc center. This establishes the values of R^ and 1 . , and gives an estimate of n, but no

information about t. The latter is estimated from the equation:

/(R. + Z. - V ) 2 + Z"

2 log(v/u) = log * ? = n logUx). (14)

/(Z 1 - r

l t + l"

Log (v/u) is plotted against log w, using observed values of Z

1 and Z",

and fitted near log (v/u) = 0 by least squares to a straight line,

assuming no error in log w. The estimate for t is then

-1 o

T = 00 (15)
- 42

where ou is the value of w for which the fitted line crossed the line

o log (v/u) = 0. The slope of the fitted line gives a second estimate for

n. Where this differs appreciably from the first estimate, the mean of

the two is taken.

To refine these values for the parameters in the equivalent circuit, a steepest-descents least-squares 5-parameter fitting program is

used, minimizing the sum of the squares of the distances, in the complex

plane, of the data points from values calculated for the same frequencies. In practice it is usually found that this refinement changes the

parameters very little.

The calculated values are displayed in Figs. 6a and 6b by pluses.

The figure also displays, in the form of solid arcs, the impedance plot

for the parallel R. C network (left-hand arc) and for the series R, -Z.

L g Li

network (right-hand arc). The fit of the calculated to the observed

data suggests that the equivalent circuit of Fig. 5, with use of the

Cole-Cole distribution of time constants for the inhomogeneity impedance, is a reasonable representation of the behavior of these materials,

although it is not possible in every case to produce really good fits.

To assess these uncertainties, a set of eight measurements was made

on one specimen (B3) in the temperature range from 220 to 340 °C. It

was found that the temperature dependences of (R^/T), (R^/T), and x all

could be described by Arrhenius equations. A measure of precision for

each was obtained by calculating the root mean square deviation (in

percent) of the experimental value from the value calculated from the

least square line in the Arrhenius plot, assuming no error in the

temperature. These estimates of precision were all about 20-30 percent;

a high value reflecting the imprecision attending computer fitting of

only partially resolved relaxations. A measure of precision of C p was
- 43 -

obtained from five independent measurements for each of two specimens

with a total temperature span no more than 10 °C. The standard deviation in one case was four percent, and in the other it was one percent

An uncertainty of not less than about five percent in C

q must be recog

nized where computer fitting is needed because the intrinsic and inhomogeneity relaxations overlap. The uncertainty in n appears to be

about ± 0.05, based on the two independent estimates obtained as described above when the equivalent circuit is fitted to the data.

The equivalent circuit and other parameters for several specimens

at temperatures near 230-250 °C are displayed in Table 5.

Table 5 Equivalent Circuit Parameters for Y-doped Ce0£

Ceramic Specimens at 230-250 °C

L

Specimen Cation

%Y

Flotation

Density

%Theory

p , fl • cm e L V\ C

i

, pF n

"Mixed Oxide"

B3 10 83 1.5 x 10

5 26 6.67 15 .5

A10 20 90 4.0 x 10

5 32 3.50 16 .4

A12 20 94 1.0 x 10

6

16 2.33 17 .5

HP8 10 94 3.1 x 10

5 33 1 .43 225 .4

II Chemically Prepared"

B7 8.5 84 2.6 x 10

4

45 14.6 1 .7 x 10

4

.9

B16 8.5 94 2.4 x 10

4

45 109 1.3 x 10

4

.9

CHP-2 8.5 >99.5 1.5 x 10

5 35 67.4 3.4 x 103

.9

Legend

p l = Resistivity calculated from R

l and the dimensions

= Dielectric Constant calculated from C

q and the

dimensions
- 44

The variation of the parameters from one group of specimens to the

other appears to be outside limits of the error in their determination

and to reflect real differences in the groups. A clue is seen in the

data for specimen A12, which was found to have an obvious internal crack

that provides an explanation for the rather low apparent dielectric

constant and might also give rise to an abnormally large apparent

resistivity. Such effects may account for some of the variation. The

ratio R -j /Rj_ » on the other hand, might be expected to be less sensitive

to internal cracks than the individual resistance values themselves, so

that its variation is probably real. This is probably also true of the

generally lower values of p^ and especially of the much higher values of

C.j and n observed for the "chemically prepared" as compared to the

"mixed oxide" specimens.

As indicated in Fig. 6a and Table 5, "chemically prepared" specimens mostly exhibited a considerable difference in time constant for the

impedance relaxation due to the intrinsic properties from that attributable to inhomogeneity effects. We have therefore used several of these

specimens to study the temperature dependence of both the intrinsic and

inhomegenei ty conductivities and also the total as measured by the 4-

probe method. An Arrhenius plot for this latter dc conductivity is

shown in Fig. 7. There is a distinct bend in the plot at about 900 °C,

and two activation energies can be defined, as already observed by Kudo

and Obayashi [5] and in a somewhat less definite fashion by Dirstine et

al. [70]. Tuller and Nowick [4], on the other hand, report 4-probe dc

conductivity data on 10 cation percent Y-doped Ce0

2 "mixed oxide" specimens that exhibit the characteristics of intrinsic conductivity, without

an appreciable inhomogeneity contribution to the impedance, although

data reported in a later paper, Nowick et al . [71], for a two cation
1400 1000 600 200

T (C)

400

7: Temperature dependence of the conductivity of "chemically-prepared"

Y-doped Ce0 9 (8.5 cation percent) ceramic, measured by the 4-probe

dc method.

i

- 45 -

percent Y-doped "chemically prepared" specimen do appear to exhibit a

large and important inhomogeneity effect. The conductivity displayed in

Fig. 7 was independent of oxygen partial pressures from 0.02 MPa down to

-4

about 10 Pa (0.2 atm down to about 10 atm), over the range of measurement temperatures from below 200 °C to above 1000 °C.

As the temperature increases the critical frequencies of various

processes in the specimen increase, and the frequency range sampled by

the electrical measurements moves to lower values relative to these

critical frequencies. As shown by Bauerle [51] and Schouler et al

.

[52], the result in the complex plane representation for the impedance

(or admittance) is the successive appearance or development of lowerfrequency arcs as the temperature is increased. In our studies here we

observe three such arcs, intrinsic and inhomogeneity arcs already described and expected on the basis of the equivalent circuit, and at the

highest temperatures at least the high-frequency side of an arc ascribed

to electrode processes because of its sensitivity to the oxygen partial

pressure in the atmosphere.

Figure 8 illustrates the appearance of such data at higher temperatures. The experimental data points (squares) are compared to those

calculated for the same frequencies from the equivalent circuit determined by an 8-parameter fit to the data. For this purpose, the electrode impedance was represented by a second Cole-Cole distribution. The

frequencies lie too low for the data to exhibit much influence from the

intrinsic relaxation. The main arc derives from the inhomogeneity

relaxation, and in this case exhibits a broad distribution of time

constants (n = 0.7) although the specimen was "chemically prepared".

3

The equivalent capacitance, 6.4 x 10 pF, is consistent with the "chemically prepared" material. Notable in this figure is the way in which

the presence of the electrolyte relaxation effects the data in the
(siuijOjOl-) „Z

0)s.

A3 co as

4->

4-> A3

A3 to U

-a

ID <u • >

CO i0) u- l/l o JOo c

OJ Si E

•r— 1— u

o>CL • m >> • o </l u r—

•r* r— = as 03 Li

s- TO 01 OJ C. TO o 3 s_ r < o

-o <4- OJ •a

S- ai •u

rt3 i.

X

Q. a» 0) a D D

s_ _> a. 3 OJ i Q. OJ >» IS) 00

r~- r— OJ

<T3 s_ • O A3 VI r— OJ E 00 SI OJ a; 3

-C -a r— u o o. z $-

4-> >)

rO

u

J

<u

i- r— TO o a OJ <+- -i-J a A3 a r~ ,— d H Z! A3 <J r“ • A3 o U X 0

OJ OJ r— CO Cl f— > =O TO u u c

A3 A3 OJ JC CM 00 Li o a

0) s- c o <3 — 3__

A3 0) S)

4-> A3 o >* -a TO -=

0) >- TO u 3 c Li -*->

IX3 A3 u U

QJ

u

—

C- s_ T3 E 0) <= CL #—

CO
- 46 -

region of the inhomogeneity relaxation. Were the data to the right of

the dotted line missing, a simple compass fit of an arc to the data

might easily overestimate the inhomogeneity resistance by five percent

and also the depression of the arc. This effect of the overlap of the

relaxations becomes worse the broader the distribution of time constants

and the closer the relaxations are in most probable time constant.

In Table 6, we compare the total ac conductivity at 500 °C, derived

from + r and the specimen geometry, with the 4-probe dc conductivity

at the same temperature, for "chemically prepared" specimens of 8.5

cation percent Y-doped Ce0

2

.

Table 6 Comparison of Total ac with 4-probe dc conductivities at

500 °C for "Chemical ly-Prepared" 8.5 cation percent Y-doped

CeO^ Ceramic.

Method El ectrode a. S/cm

ac Au, sputtered 0.44 x 10"4

ac Au, sputtered 2.52 x 10‘ 4

ac Pt, sputtered 1.16 x 10" 4

dc Pt, paste 1.85 x 10"4

Once again, there is variation from specimen to specimen beyond

expected experimental error, but within this variation the ac and dc

values are compatible, and no effect of the nature of the electrode is

seen.

In Fig. 9 the temperature dependence of the 4-probe dc conductivity

is compared to the independent ac measurements, for both total and

intrinsic conductivity, for each of the three ac measurements listed in

Table 6 above. The ac data have all been scaled to bring the total
a

T

(Kfr

1

cm"

0.8 1.0 1.2 1.4 1.6 1.8 2.0

10 3

1 1T (K' 1

)

9: Comparison of temperature dependence of 4-prooe and ac conductivities of "chemically-prepared" Y-doped Ce0 9 (3.5 cation percent

ceramic. The symbols, +, A, and • refer to different ac specimens

x indicates the 4-probe dc data on a fourth specimen.
- 47

conductivity into agreement with the 4-probe dc value at 250 °C. Thus

scaled, the data for both conductivities for the three specimens agree

well among themselves and the total ac conductivity agrees with the 4-

probe dc conductivity. Combining the ac measurements for the three

specimens, we obtain the activation energies in Table 7.

Table 7 Activation Energies for ac and dc Conductivities in

"Chemically-Prepared" 8.5 Cation Percent Y-doped CeO^

Ceramic

Method E-j , ev E

2

, ev

ac 1.08 .76

dc 1.10 .82

where E^ , represents the total ac or low-temperature dc conductivity,

and E

2 the intrinsic ac or high temperature dc conductivity.

Pi scussion

The variation of the ratio R./R^ with closed porosity, as measured

by flotation density, for "mixed oxide" specimens shown in Table 5

suggests that for these materials at least the inhomogeneity impedance

is related to the presence of internal pores in the material. Figure 10

shows that /Rj_ is roughly linearly dependent upon the volume fraction

porosity, taken as one minus the density expressed as a fraction of the

theoretical density, with a slope of about 37. It does not appear

possible, however, to reconcile this behavior with the simple model for

a dilute suspension of pores in a conducting matrix represented by

Eqn. (10), and the correlation of Fig. 10 disappears if bulk density is

used, thereby including open porosity as well. For spherical pores

(e^ = 1, x

2 = 0) i n a conducting matrix the approximately linear behavior is suggested by Eqn. (10), but the slope should not exceed 0.5,
CO N. CO LO CO 04 r-

© o

CM

LOo© c -a

O) -r- O l—

5-

O

O) (/) CL 0) c .c o P C• mmo

*-* O

(0

u. LLa) EO>

COoo

CL

TJ0

(OOo

(CuoCMt c

*CT

s-o0) jc

to .c c cr. xl <u 3 s E O «

•r— s- O £

Ol 4-> c c.

l/l aj

V c 0 s •*— Ol r- •o iC •r— to > X <u o s_ CJ (O £ o 3 *. a> CT X to t

•?— 1 : E 4-> « z to

ITJ • s_ 0) C o r—

<+- l a) >> -C X

-UJ 4-> •r— • to to c o •r— to s_o 1) u Q. c

*r- t> c r— o 5- c. “3 3 ai 4-

<_) *->

n 0 o CO T3 O 3 • CM C t— c:

<? UC

*-> . C

<T3 to •w CJ _i >r* to CSC e

.-a 0

•r— s- •f- CC 3 +J U

41- f— o CM o CO a) 3

CJOa c s- a; T3 «

-a 3 c CL to 0J O 3

Q.

a c

CJ 1 •r Q >- o

'a/ia
- 48 -

and should be less for a matrix dielectric constant greater than unity.

On the other hand, oblate pores produce a larger inhomogeneity resistance than do spherical pores, but the behavior with respect to porosity

is not simply linear. Figure 10 includes curves calculated using

Eqn. (10) with a matrix dielectric constant of 30 and for the axial

ratios shown. Furthermore, such a model would require that the temperature dependence of the inhomogeneity resistance be essentially the same

as that of the intrinsic resistance, and this is not the case experimentally. It therefore does not appear possible to account for the

observed data on the grounds of effects from isolated pores.

If, instead of pores, the distributed phase were to consist of

second-phase particles, then the dielectric constant ratio in Eqn (10)

would be smaller than for pores, resulting in larger values for R./R^.

In the event that the dielectric constants of the distributed phase and

matrix were the same, Eqn. (10) would become, for non-conducting particles.

R

i Aa r

L

‘ (16)

For oblate spheroids, A approaches unity as the axial ratio apa proaches zero, and thus rather large inhomogeneity resistances can occur

when there are flattened regions present with conductivity lower than

that of the matrix, such as might occur at grain boundaries. Why the

extent of such regions should correlate with the closed porosity in

these materials is not clear, however. Microscopic examination has not

revealed any discrete grain boundary phases, although specimen HP8 is

known from both x-ray diffraction and microscopic examination to be

incompletely reacted and to contain discrete grains of fluorite and

Y^O^-c structures.

- 49 -

The equivalent circuit parameters for "mixed oxide" and "chemically

prepared" specimens in Table 5 suggest several comparisons. The intrinsic lattice resistivity of the "chemical ly-prepared" material appears to

be somewhat lower than that of the "mixed oxide" material, and may

reflect the more homogeneous distribution of dopant in the former, as

also suggested by the observation of somewhat sharper x-ray diffraction

peaks for the "chemically-prepared" material. If the inhomogeneity

impedance arises from variations in dopant concentration near the surface of the grains, it would also account for the broader distribution

of time constants, reflected in the lower value of n in Table 5, for the

"mixed oxide" materials.

The outstanding difference between the two materials, however, lies

in the values for the equivalent capacitance, C. , of the inhomogeneity

impedance. This difference, amounting to several orders of magnitude,

does not arise from differences in grain morphology. Grain shapes and

sizes are essentially the same for the two cases, and the differences

are reflected in the time constant t [Eqn. (9)], which should be independent of morphology. For this same reason, the large value of

for the "chemically-prepared" specimens probably does not simply indicate thinner regions of inhomogeneity. One mechanism from which large

values of could result is partial compensation for the Y-doping at

the grain surfaces either by incorporation of other impurities or by

segregation of the dopant itself into the grain interior. As a result

of this partial compensation, the concentration and therefore electronic

conductivity of the surface region would be larger than that in the

grain interior. For such an electronically conducting region in an

otherwise el ectronical ly poorly conducting matrix, the dielectric constant would be enhanced by a factor (1 + sn/n) where n is the electron

concentration in the matrix and 5n the increase in the better conducting
- 50 -

region. In our experiments the electronic conduction, and therefore

electron concentration n, is very small, so that the dielectric enhancement in the surface regions could be large.

As shown by the data in Tables 6 and 7, the resistivity measured by

the usual 4-probe method includes both the intrinsic and inhomogeneity

contributions, and to obtain and interpret the intrinsic lattice conduction properties, the inhomogeneity contribution must be determined by ac

measurements. As already recognized by Hooper [55] and Nowick, et al

.

[71], a break in the Arrhenius plot for conductivity, as seen in the

data of Kudo and Obayashi [5], as well as in Fig. 7, can be due to this

combination of inhomogeneity and intrinsic impedances and need not

reflect changes in the intrinsic transport mechanisms themselves. The

activation energies displayed in Table 7 are in reasonable agreement

with the values already given by Kudo and Obayashi [5] and by Tuller and

Nowick [4] (intrinsic transport only) and Nowick, et al . [71], for about

the same concentration of Y in Ce02 (10 cation percent). Kudo and

Obayashi [5] derived their activation energies from plots of ln(p) vs. J~\ rather than 1 n (p/T) , and their activation energies must be increased by a few hundredths of an eV to be compatible with the others,

based on In(aT) plots. The pre-expontential factor for the lattice

5 conductivity from Fig. 5 is 5 x 10 SK/cm, in reasonable agreement with

Tuller and Nowick's [4] value of 2 x 10^, and about an order of magnitude above a value appropriate to the data of Kudo and Obayashi [5].

Summary

Measurements of the impedance of Y-doped Ce02 ceramics containing

about 10 or 20 cation percent Y indicate that there is a sizeable contribution from inhomogeneities, perhaps grain boundaries, to the total

impedance. Separating the intrinsic or lattice impedance from the
- 51 -

inhomogeneity contribution appears to be possible using the frequency

response of the impedance. The intrinsic conductivity is somewhat, but

not heavily, dependent upon the specimen preparation. Values obtained

here for the activation energy and pre-exponential factor for this

intrinsic conductivity (aT plot) are in reasonable agreement with values

from the recent literature.

The inhomogeneity contribution to the impedance, and particularly

the equivalent parallel capacitance, appears to be strongly dependent on

the specimen preparation. For "mixed oxide" materials, the ratio of the

inhomogeneity resistance to the intrinsic resistance scales approximately linearly with porosity, but the connection appears to be indirect. The existence of regions, perhaps at the grain boundaries, of

variable dopant concentration is a possible explanation, with the incidence of such regions correlating with porosity. For the "chemically

prepared" materials, a much larger equivalent inhomogeneity capacitance

is observed than with "mixed oxide" materials. The differences cannot

be accounted for on the basis of grain boundary geometry, but may reflect again a chemical effect at grain boundaries, such as an increase

in the concentration of electronic charge carriers.

With polycrystal 1 ine specimens of materials of this kind the use of

impedance spectroscopy, the analysis of the frequency dependence of the

impedance over a wide frequency range, appears to be essential to a full

understanding of the mass and charge transport processes.

2.1.3 Slow Transients in Y-Doped CeO^

In our last Annual Report [72] and in Quarterly Reports for 1 April

to 30 June, 1979 and 1 July to 30 September, 1979, we discuss the observation of a slow transient in the voltage measured across the potential

probes of a 4-probe dc conductivity measurement [73]. At temperatures
- 52

of the order of 900 °C in air, the time constants of these transients

were of the order of tens of seconds. The time constants obeyed the

Arrhenius equation with an activation energy of 30.1 kj/mol (0.83 eV),

while the slope of the overvoltage vs (time) ' plot seemed to depend on

1 /2

Pq , where Pq is the oxygen partial pressure.

This phenomenon suggests one of two explanations. The overvoltage

reflects a change in relative oxygen activity at the potential probes

induced by the initation of a current; and, therefore, an oxygen flux,

through the specimen.. The oxygen activity at the cathode can be expected to fall, and to rise at the anode, when this flux is turned on.

Apparently these changes in oxygen activity are transmitted to the

potential probes by diffusion of neutral oxygen out of the specimen,

which appears possible either as oxygen atoms or molecules moving presumably in the grain boundaries, or else as coupled electrons and oxygen

vacancies in ambi polar diffusion in the lattice.

We are investigating these models by numerical solution of the

diffusion equation with appropriate boundary conditions. The diffusion

problem for volume diffusion of a neutral oxygen atom has been cast in

finite-difference form and solved for a variety of values of oxygen

partial pressure in the atmosphere and of the ratio of diffusion coefficient for neutral oxygen in the electrolyte to that in the electrode, for comparison to the experimental observations.

The results of this partial study shows that the neutral oxygen

diffusion model reproduces rather well the experimental data. However,

the study is not complete until a similar investigation of the ambipolar

diffusion model has been compiled, so that a comparison between the two

models can be made and one presumably eliminated. It is planned during

the next year to complete this study.
- 53 -

2.1.4 Precipitation Phenomena in Mg-Doped Zr0

2 Zirconia electrolytes possess the high-conductivity fluorite structure by virtue of the presence of aliovalent dopant cations. Without

these stabilizing cations, the fluorite structure transforms to tetragonal at temperatures below about 2400 °C or to monoclinic below about

1240 °C, and loses the very high oxygen-ion conductivity. If in fuel

cell operation a long-term drift of the dopant cations occurs under the

force applied by the potential drop across the fuel cell solid electrolyte, and a loss of the fluorite structure results, the overall effect

could be a degradation of the fuel cell output and efficiency.

The de-stabilization of the fluorite structure through loss of

dopant cation is expected to occur by precipitation of the tetragonal

phase in the form of very small particles [74]. These particles would

fulfill the condition, discussed in Section 2.1.2, that their dielectric

constant is not very different from that of the matrix; and, therefore,

they could produce an appreciable inhomogeneity impedance. Figure 11

shows a comparison of the impedances in specimens quenched from the

solid solution and two-phase regions [75]. The precipitation anneal

results in the development of an additional arc in the impedance plot.

The additional arc is quite depressed, the value of n in the Cole-Cole

equation (Eqn. [2. 1.2-1]) being 0.69. This suggests a rather broad

distribution of particle properties, which is to be expected on the

basis of the precipitation mechanism as described by Heuer [74], in

which the composition follows the solvus lines as the specimen temperature is changed during reaching and leaving of the precipitation anneal

temperature.

The ratio of the inhomogeneity resistance attributable to the

precipitates to that from the rest of the specimen is about 0.2.
11/2

JB

o-F85

X“F102

File*

85

Zr

II-l,Hlr,446
- 54 -

According to Eqn. (16) this ratio for spherical non-conducting particles

of the dielectric constant as the matrix would be

Ri/Rs = P/2 , (17)

where Ri is the inhomogeneity resistance introduced by the precipitates,

Rs is that from the rest of the specimen, and P is the volume fraction

of precipitate. Thus, for spheres, the data suggest P ^ 0.4. The phase

diagram given by Grain [76] and sketched in Fig. 12 suggests that this

value might just be reached if precipitation were to take place at the

eutectoid temperature of about 1400 °C, but that a smaller value is to

be expected. Somewhat oblate ellipsoidal, instead of spherical particles, could account better for the data.

2.2 Interconnector Materials

2.2.1 Oxygen Sensitive Electrode Impedance in Sr-Doped LaCrO^

Introduction

In order to connect cells together in a high-temperature fuel cell,

the interconnector material must be a good electronic conductor at

operating temperatures (^ 1000 °C) and in a wide range of oxygen partial

pressures (from air to the fuel mixture), must not allow passage of too

many oxygen ions or atoms, and must be compatible with the other materials present. These conditions limit the choice of material drastically.

One of the candidate materials for this application is Sr-doped

LaCrO-, although there is some evidence for oxygen transport [77]. It

was not clear whether this 0 transport was intrinsic to the crystal

lattice or occurred mainly in the grain boundaries. It, therefore,

appeared profitable to study this material using the impedance spectroscopy technique. This work was done in collaboration with
CVJ

Phase

diaqram

of

MqO-rich

portion

of

the

Mq0-Zr0

2

binary,

after

Grain,

ref.

(76).
- 55 -

Richard A. Snellgrove of the General Refractories Co., who supplied the

specimens and their characterization.

Resul ts

The major result of the work to date is the observation of an

oxygen-sensitive electrode impedance with noble-metal electrodes on Srdoped LaCrO^. Changes induced by changing the oxygen partial pressure

in the atmosphere occurred at temperatures as low as room temperature

and in times of the order of days or weeks. We discuss here experiments

demonstrating this sensitivity and bearing on the nature of this electrode impedance.

The ceramic samples used in this study were prepared by firing

mixtures of oxides at 1750 °C in air. The samples had the composition

La

0 992^ r 0 008^3 ancl were b ^ acl< in c o^ or - The density was 97 percent

of the theoretical value. The grain size was approximately 3-10 ym.

Sharp x-ray patterns were obtained, and the material appeared to be

single-phase. Impedance measurements, made with an RMS amplitude of

0.3 V at' frequencies from 56 Hz to 13 MHz, and two-probe and four-probe

dc measurements were used to examine the electrical properties from

52 °C to below -100 °C under atmospheres of air, tank nitrogen and tank

oxygen. For the impedance and the two probe dc measurements, square

plate samples with dimensions 0.8 x 0.8 x 0.2 cm were used. Sputtered

platinum and evaporated gold electrodes were applied to surfaces polished to 0.5 ym diamond abrasive powders and then cleaned. These electrodes were either symmetrical, with a diameter of .64 cm, or were

unsymmetrical with diameters of 0.15 and 0.64 ym, for a ratio or areas

of 0.057. Point contacts opposite large area electrodes were made by

pressing the tip of a wire on the unelectroded surface. Bar samples

with dimensions of 1 x 0.12 x 0.12 cm were used for four-probe dc measurements, using silver paste contacts.
- 56 -

Figure 13 shows the impedance in the complex plane for the sample,

at two temperatures in tank N^, with frequency as a parameter. The

arrow at the left on the real axis indicates the bulk resistance as

measured with the four-probe specimen, and demonstrates that the arc to

the right, at lower frequencies, represents an electrode effect.

Figure 14 demonstrates the sensitivity of this electrode impedance

to the oxygen activity of the atmosphere. For the data of Fig. 14, the

sample was first held in at atmosphere of tank at 52 °C until all

changes ceased; and then the atmosphere was rapidly changed to tank

and the sample held at constant temperature in this atmosphere. The

impedance was occasionally measured as a function of frequency, and the

equivalent parallel resistance and capacitance determined by fitting

the data to a Cole-Cole [53] function. There was some quantitative

variation among specimens depending on prior heat treatment, but the

main effects were independent of specimen. Over a period of several

weeks, the electrode resistance increased by a factor as large as 35.

The capacitance decreased at the same time, but by less than a factor of

two.

After replacement of the ^

by 0^, still at 52 °C, the resistance

decreased again, much more rapidly than the increase that had occurred

in N^, and the capacitance increased. This reduction and reoxidation

process is essentially reversible, the resistance and capacitance after

the second oxidation returning to close to their original values. The

process is strongly temperature dependent, and occurs slowly enough

below room temperature so that measurements up and down the temperature

scale can be made without hysteresis. At low temperatures, the impedance data allow a determination to be made of the bulk resistance that

agrees in detail with the four-probe dc measurements, and gives values

independent of the changes in oxidation state carried out at 52 °C. The
CO

E

o

VMM

N

I

13: Typical impedance spectrum (5 Hz to 13 MHz) of La

Q g^Srg

008

Cr^3

at room temperature, equilibrated with tank N

9 at 52 °C. The high

frequency region contains a temperature-dependent fine structure.

Inset shows this fine structure revealed by making measurements at

-130 °C.

i
1250 r©

6000 c

O)Q. oooCM

LO

•o

QJ

•M CM O

CO O

C i- «3 O -t-> 00o o O -t->CM

5-

</0CM

CTi c O «3 • +J O<T3 E

_l £ C *4- o a V) CJ <u o o o 4-> s- *r—

4-> 3 u VI a

V)

<y (O5 3< OJs_

<4- CJ O JZ

c.

0) VO a s c

ra

p

fO

(/> •r—

a

00 .c

<V -t-> i- c

CJ a; -c jz o 2

s_ 4-> c u ai a

i—

oj -t->

CJ <C M

+j <a •a

<4-o 4-> C

CJ CJ U 00

CJ 5 s_ "O c.

CJ a> s_ c.a 00 c 0)

r*—

0J o S s- u— u

a>

i/t iCJ>

at

a>

<o

VI

0)u—

(swi| 0 ) 33MV1SIS3H
- 57 -

bulk resistance and the electrode resistance yielded approximate linear

graphs when plotted in the form of log (R/T) vs. T~^ in the range from

-160 to 75 °C. The activation energies were 0.16 eV for the bulk process; for the electrode process the values were 0.21 eV when equilibrated with dried tank O

2 at 52 °C, and 0.33 eV when exposed for three

weeks to dried tank at 52 °C.

The existence of a contact resistance and capacitance suggests a

barrier layer at the electrodes. Experiments at room temperature in air

on the effect of bias on both the current and the differential capacitance indicate that the barrier does not behave like a simple Schottky

depletion layer [78]. The I-V curves are roughly symmetrical about zero

bias and exhibit very little rectification. The capacitance is not

strongly changed by bias, but the effects are also quite symmetrical,

the capacitance at first decreasing with increasing bias; and then, when

the bias exceeds about 20 volts, decreasing again.

When plotted on a log-log basis, the I-V curves exhibit an ohmic

region at low bias followed by one square-law region and a transition to

another when the bias exceeds about ten volts. Applying a steady bias

for several hours always resulted in a decrease in the ohmic resistance.

For instance 20 V for two hours decreased the ohmic resistance by two

orders of magnitude. This effect also was symmetrical and reversible.

A sensitivity of the electrical properties to oxidation or reduction is to be expected in this material, which appears to be a p-type

conductor with the Sr ions acting as acceptors [79]. In low-oxygen

atmospheres, some reduction will take place, producing oxygen vacancies

which, because they act as donors, will at least partially compensate

2 + the Sr ions. Hence, the number of uncompensated acceptors will change

with the oxidation state. That changes in oxidation state are reflected

in electrode properties at temperatures as low as room temperature is
- 58 -

consistent with the apparent thinness of the critical region under the

electrodes. The impedance measurements at -109 °C revealed enough of

both bulk and electrode arcs simultaneously to allow estimates to be

obtained for both the intrinsic and electrode capacitances. From the

measured thickness of the specimen and assuming the same dielectric

constant for bulk and surface regions, we estimate the barrier width to

_5

be about 1.6 x 10 cm. For the sake of comparison, if we consider a

Schottky barrier with a barrier height of one V, this suggests an uncom1 fi pensated acceptor concentration of about 4 x 10 cm in this surface

2 + region, very much less than the overall Sr concentration of

20 -3

1.3 x 10 cm but reasonable if, in the surface, reduction has occurred, introducing almost enough oxygen vacancies to compensate the

2 + Sr ions, so that the chemical formula is close to Sr

x

La-| x

^ r^

3

_ x / 2

’

where x = .008 in our specimens. Why the surface should behave so

differently from the interior is not clear.

Summary

Barrier layers are formed on Sr-doped LaCrO^ that give rise to an

appreciable electrode impedance with electrodes of Au or Pt. The

corresponding electrode resistance is quite sensitive to the oxygen

activity in the atmosphere, following atmospheric changes at temperatures near room temperature with time constants of the order of minutes

or days. The electrical properties of these barrier layers at room

temperature do not conform to the models of simple Schottky layers on

semiconductors, but exhibit changes in current and differential capacitance much more symmetrical in the applied bias.

Acknowledgments

The specimens and much information about them were furnished by

R. A. Snellgrove, of the General Refractories Co., who is coauthor of a

version of this report being submitted for publication.
- 59

2.3 Development of Instruments

2.3.1 Low-Frequency and Pulse Operation of Microprocessor-Control! ed

Potentiostat

Introduction

In previous work [72,80], we have described the development of a

microprocessor control and fast data acquisition system for use with a

standard laboratory potentiostat. As discussed in ref. [72], this

system is, in principle, capable of ac and pulse measurements, and

offers a low-frequency capability below the range of our present

impedance-measuring equipment 10 Hz). Work this past year has been

directed toward realizing this potentiality. As discussed previously [80], this has necessitated the design and construction of a new

converter board. The construction of the new converter board has been

plagued by component delivery problems. The unavailability of normallycommon components led to a total review of the design with the intent of

redesign around more available parts. The result of this reexamination

has been a modification of the original design. These modifications,

the most important of which is a change to the addressing scheme, will

be discussed in detail below.

Much of the year was occupied with software. The difficulties

encountered with interfacing the disk units (to be described later) led

us to examine the possibility of writing our own operating system. We

soon concluded that this would be even more difficult, and the idea was

dropped.

Hardware

The fundamental design of the converters has remained the same.

The majority of the changes made have been in the portions of the circuit

devoted to the decoding of the addresses in the circuit. Several

changes have been made to the disk driver circuit. (It is worth noting
- 60 -

here that many of the changes to this part of the circuit are for the

purposes of improving the efficiency of programming and are a direct

result of the generation of the Disk Driver Routines.) The final alterations to be noted are in the DAC converter Reference Amplifiers.

The changes in the Reference Amplifiers will be discussed first.

The originally specified REF-01 and REF-02 are among the devices not

received. (See Figure 34 in Annual Report, Jan. 1978 to Dec. 1978,

Ref. [80].) They have been replace by MC 1404-10 and LM-10, respectively. On the schematic, these changes are shown as label changes, but

mechanical modifications due to physical device configurations are

necessary. The electrical specifications of the REF-01 and REF-02 are

apparently more desirable, and a return to the original design would be

justified when the devices are received. (Note that there is an error

in the references figure. The register select line labelled RC' should

be relabelled as R5.

)

The rest of the changes to the system are summarized below. Reference

should be made to the previously published schematics (specifically,

Fig. 28 and Fig. 31 in the Jan. 1978 to Dec. 1978 Annual Report,

Ref. [80].) The addressing changes will be discussed first.

The addressing requirements of the segments of the disk driverconverter section of the system are shown in Table 8. (Note that the

base address of this section (BFC0 Hexadecimal) can be changed by appropriate inversions of the address line inputs to the 74S133 13 input

NAND date. Also, that the addresses in the available range up to 3FFF

not used by the Disk Driver converter section can be decoded by the use
- 61 -

of the BFDO hex and BFFO lines as Enables to decoders for the four

lowest address lines.) The primary chip Enables are generated by the

combination of the 74S133 NAND date and half of the 74S133 two line to

four line Dual decoder. The Enables (inverted where necessary) are used

in combination with the four lowest address lines to generate each

necessary unique address. Although it's not an address function, the

other half of the 74LS139 generates the acknowledge strobe to the appropriate DMA channel, which is a decode operation and, therefore,

included in this discussion.

Table 8

Driver-Converter Section Addressing Requirements, Disk

Needed # of Decoded Addresses Used

IC Addresses (decimal) Function (Hexadecimal

)

MC6844 23 DMA Controller

(four channel

)

BFCO - BFDG

MC6820 4 Disk Driver PI A BFDC - BFDF

MC6840 8 Timer BFFO - BFF7

74154 16 Converter Register

Select

BFEO - BFEF

5 Available but unused BFD7 - BFD8

8 Available but unused BFF8 - BFFF

The resulting address decode system is simpler to understand,

because of it's more straightforward layout, and results in somewhat

fewer IC's needed.

The final revisions to be discussed are those made to the Disk

control part of the PI A. As mentioned earlier, the primary reason for

these changes was the improvement of programming efficiency. It should
- 62

be noted in passing, that the original control concepts can still be

implemented in the revised version. The alterations have simply added

another mode which is more efficient.

The major changes made have been to the manner in which the

DTR, DTAK, BUSY and ERROR signals are handled. In the original version, DTR going low caused the P1A CB1 Interrupt flag to be set. If

the Interrupt was masked, this condition would be detected by reading

the PI A B control register. If the Interrupt was not masked, the system

would automatically transfer to an Interrupt handling routine. In

either use, the Interrupt flag would have to be cleared, data transferred,

DTAK set low, and then high. These operations would be distinct sets of

instructions for each function. By reassigning DTR to the P1ACA1 Interrupt, and CA2 to DTAK, we can improve the situation. In this case, we

can program the A side of the PI A for CA2 pulse mode. Now upon detection of DTR low, we transfer data and read the A data side of the P1A.

This read operation automatically clears the Interrupt flag and sends a

negative going pulse on DTAK. Thus, we do in one instruction an operation that involved a minimum of five instructions in the original

design. Those considerations that influenced the first version (e.g.,

control of the pulse length of DTAK) can still be accomodated in the

present design, simple by changing the operational mode cf the PI A

.

Similarly, the rearrangement of the BUSY and ERROR signals so that

either one activates the CB1 Interrupt flag allows the same kind of

data handling as above, upon completion of a data transfer or detection

of an error. Note that BUSY goes high when the transfer is completed,

while ERROR goes low when one occurs. Thus, the Interrupt signal has to

be the NAND of ERROR with BUSY inverted.
- 63

Software: Interfacing to a Commercially Available Operating System

An operating system for a microcomputer is a program that provides

control of all the system functions. These functions include input,

output, dynamic file management, floppy disk control, etc. Most of

these functional programs are not hardware independent. Thus, the

adaptation of an operating system written for one hardware configuration

to another configuration is a difficult procedure. This is even more

true when the source code for it is not available. However, the programming of an entire system is even more difficult and time consuming.

Our decision to adapt a commercial system took note of these factors. A

short discussion of the adaptation method follows.

The first step in this procedure is to write programs for the

necessary disk driver functions. (Driver in this case is the control

program to set up and start the disk for a particular command function,

and to manage the transfer of data to (from) the disk from (to) memory.)

When these are completed, they must be matched to the particular operating system (OS) selected. This is done by finding the format used to

pass parameters and error signals back and forth between the OS and the

drivers. If this information is not provided by the OS manual, the

existing driver programs in the object code for the OS must be found,

disassembled and translated into source code. When this is completed,

our drivers are modified to reflect the now known parameter format. The

next step is to try and find all program calls to the disk drivers and

to other input--output routines. Wherever these are found, they must be

replaced by calls to our disk drivers and I/O routines. The I/O routines,

of course, are essentially the ones already written for the hardware

configuration. Finally, the Interrupt handling routines in the operating system must be translated into source code, and modified so that

the occurrence of an Interrupt will not destroy the program.
- 64

We have two operating systems potentially available for use. T'ne

relevant driver sections of both have been disassembled and are being

translated into source code. The primary difficulty seems to rest in

the format and handling of the error signals from the disk unit. At

present, it appears that it will be necessary for us to write a routine

that will simulate the appropriate error conditions upon receipt of a

signal from the drive.

The second most important difficulty is that of Interrupt handling.

We are not, as yet, able to pinpoint the areas in the OS where these

routines are. (The programmers who wrote the OS have used self-modifying

code in some regions of the program. This leads to some confusion.) We

feel, however, that this problem is capable of resolution once appropriate drivers are available.

I/O and disk driver calls are resident throughout the OS. Finding

all of them is simply a matter of disassembling the full code.

The latest versions of our disk driver are completed and are

written around the new hardware design. These drivers are written to

mesh with one of the operating systems available to us. One program, in

addition to the disk driver routines, is included that has not been

discussed. This is the routine used to format new disks. Each OS has

its own data and directory format requirements for a disk. Thus, every

new disk must be preformatted before it can be utilized by the operating

system.

Of the remaining drivers, one has to be operating system- independent.

This routine is to reside in ROM and functions as a loader for the

operating system-independent drivers and the loader program resident in
65

the OS. This OS independence is qualified by the proviso that the size

of the sectors on disk must correspond to that required by the OS.

Thus, our BOOT program requests the identification of the OS to be

loaded

.
- 66 -

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2 Ceramics:

Comparison of ac and dc Measurements", to be published.
- 68 -

32. Electron microprobe analysis was performed by D. E. Newbury,

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. Part 2, Non-Stoichiometry in

Ternary Rare-Earth Oxide Systems. Part A. The System Ce02~Y20

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",

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, Ag/Ce02~Based Solid Electrolyte/

Ag, O

2 ", D. Engr. Thesis, Universite de Grenoble, 1978.

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W. van Gool , Eds., Academic Press, New York, 1978.

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- 70 -

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submi tted.

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73.

- 71 -

C. K. Chiang, A. D. Dragoo and A. D. Franklin, "Slow Transient

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75. We are indebted to Prof. A. H. Heuer of Case Western Reserve

University for providing these specimens in their heat-treated

and quenched condition.

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78. E. H. Rhoderick, Metal-Semiconductor Contacts , Clarendon Press,

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A. J. McAlister and K. F. Young, Annual Report on Materials for

Fuel Cells, NBSIR 78-1472, January to December 1977.
NBS-114A IREV. 2«6C

)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 83-2747

2. Performing Organ. Report NoJ 3.

4. TITLE AND SUBTITLE

Publication Date

January 1984

Materials for Fuel Cells

Annual Report--January 1979 to December 1979

5. AUTHOR(S)

C. K. Chianq, M. I. Cohen, A. L. Draqoo, and A. D. Franklin

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10. SUPPLEMENTARY NOTES

I

j Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report covers work on the preparation of very dense Y-doped Ce0 2 (YDC)

ceramics; ac impedance studies of YDC, partially stabilized zirconia (Mg-doped)

and Sr-doped LaCr0 3 ; a theoretical analysis of current-initiation induced

voltage transients in YDC four-probe specimens; and the adaptation of a microprocessor-controlled pofentiosfat to low-frequency solid electrolyte

measurements

.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Solid electrolytes; Yttrium-doped ceria; Magnesium-doped zirconia; Strontiumdoped lanthanum chromite; ac impedance spectroscopy; Microprocessor-controlled

potentiostat ; Interconnector materials.

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