Intrinsic Creep Resistance of Two

Solid Oxygen Fuel Cell

Electrolytes: Cubic Zirconia

Versus Doped Lanthanum

Gallate

ARL-TN-133 March 1999

Jeff Wolfenstine

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ARL-TN-133 March 1999

Army Research Laboratory

Adelphi, MD 20783-1197

Intrinsic Creep Resistance of

Two Solid Oxygen Fuel Cell

Electrolytes: Cubic Zirconia

Versus Doped Lanthanum

Gallate

Jeff Wolfenstine 

Sensors and Electron Devices Directorate

Approved for public release; distribution unlimited.
Knowledge of the creep parameters for cubic zirconia and doped

lanthanum gallate allowed for a determination of the intrinsic creep

resistance of each material in the temperature range from 800 to

1000 °C, where they are to be utilized as electrolytes in solid oxygen

fuel cells. The results revealed that the intrinsic creep resistance of

cubic zirconia is much higher than that of doped lanthanum

gallate. This is most likely a result of their different crystal

Abstract

structures.

ii
iii

Table

1. Creep parameters for two materials ................................................................................................... 3

Figure

1. Grain size and temperature-compensated steady-state creep rate versus applied stress for

La0.8Sr0.2Ga0.85Mg0.15O2.825 and cubic zirconia .............................................................................. 3

Contents

Introduction ............................................................................................................................................... 1

Determining Creep Resistance .............................................................................................................. 2

Conclusion ................................................................................................................................................. 4

Acknowledgments .................................................................................................................................... 4

References .................................................................................................................................................. 5

Distribution ............................................................................................................................................... 7

Report Documentation Page................................................................................................................... 9
1

Introduction

For a material to be used as an electrolyte in solid oxygen fuel cells

(SOFCs), there are many important primary requirements. These include

(i) high ionic conductivity, (ii) chemical stability with cell components,

and (iii) thermal expansion match with that of the cell components [1–4].

In addition, there are some secondary mechanical property requirements

such as good high-temperature creep resistance and room-temperature

fracture toughness. Investigation of the high-temperature creep properties

is important to gain information about the dimensional stability of the

material at temperature under load.

It was previously shown at 1300 °C that the creep resistance of cubic

zirconia was greater than that of doped lanthanum gallate (La0.8Sr0.2

Ga0.85Mg0.15O2.825 (LSGM-2015)) [5]. I wanted to determine if this comparison would hold in the temperature range from 800 to 1000 °C, where

these materials are to be used as electrolytes in SOFCs. Thus, this report

presents the first comparison of the intrinsic creep resistance of cubic

zirconia versus LSGM-2015 over this temperature range.

The creep behavior of a polycrystalline oxide in general can be described

by the following equation [6–8]:

ε = A σnL– p P O2 m exp –Qc/RT , (1)

where ε is the steady-state creep rate, σ is the applied stress, n is the stress

exponent, L is the grain size, p is the grain size exponent, P(O2) is the

oxygen partial pressure, m is the oxygen partial pressure exponent, Qc is

the activation energy for creep, A is a constant, T is the absolute temperature, and R is the gas constant.

What we mean by intrinsic creep resistance is that the creep rate is a

function only of the material’s intrinsic properties (i.e., modulus) and not

a function of external variables such as grain size, oxygen partial pressure, etc. For example, from equation (1) we see that for the same material

of different grain sizes the creep rate will be different if p ≠ 0. Thus, to

obtain the intrinsic creep resistance of the material, the parameter p must

be known for that material. Consequently, from equation (1) we see that

to compare the intrinsic creep resistance of different materials tested at

different stresses, grain sizes, and oxygen partial pressures at various

temperatures, we must know the creep parameters n, p, m, Qc

, and A for

each material.
2

Determining Creep Resistance

The creep parameters for cubic zirconia [9–13] from several sources are

summarized in table 1 along with those for La0.8Sr0.2Ga0.85 Mg0.15 O2.825

[5,14]. I would like to make several points from table 1. First, both materials exhibit a fine grain size (the linear intercept grain size is ≤

20 μm).

Second, the steady-state creep rate varies approximately inversely with

the grain size, and is approximately squared for both cubic zirconia and

LSGM-2015. Third, the stress exponent is close to unity for both materials.

These three points suggest that both materials deform by the same

mechanism: either diffusional creep or grain boundary sliding accommodated by diffusion [15–18]. Finally, the steady-state creep rate is

independent of oxygen partial pressure (m = 0) for both materials.

Using the information from table 1 with the two assumptions below, the

intrinsic creep resistance of cubic zirconia and LSGM-2015 in the temperature range from 800 to 1000 °C can be compared to determine which is the

most creep-resistant. The first assumption is that the constant A in equation (1) is the same for both materials. This is very reasonable since A for

either a diffusional creep or grain boundary sliding accommodated by

diffusion mechanism is primarily a function of grain shape and type of

loading (i.e., compression versus shear) [15–18]. This assumption is

justified since both materials exhibit equiaxed grains, deform by the same

mechanism, and all tests were conducted under compression. The second

assumption is that the same mechanism that controls deformation (i.e.,

creep parameters) in the temperature range shown in table 1 remains the

same in the temperature range from 800 to 1000 oC. Without experimental

data in this temperature range, this assumption cannot be confirmed. To

obtain creep data in this temperature range would take an exceptionally

long time and, thus, is not feasible within a normal laboratory time frame.

However, in a first approximation, it is reasonable to assume that the

same mechanism controls deformation in both temperature ranges since

the average temperature in the lower range (≈ 900 oC) is only 150 oC lower

than the lowest temperature in table 1.

Using the above two assumptions and the data from table 1, the intrinsic

creep resistance of cubic zirconia and LSGM-2015 are shown in figure 1.

The data in figure 1 are plotted as grain size and temperaturecompensated steady-state creep rate versus applied stress on a double

logarithmic scale. From figure 1 several points are observed. First, the

creep data after compensation for cubic zirconia from different sources

with different dopants are in excellent agreement with each other. Second

and most important, at a given value of applied stress, the temperaturecompensated steady-state creep rate for LSGM-2015 is between 1,000 to

4,000 times faster than that for cubic zirconia. This result reveals that the

intrinsic creep resistance of cubic zirconia is much higher than that for

LSGM-2015. One possible explanation for this is the difference in crystal
3

structures: fluorite [19] for cubic zirconia versus monoclinic [20] or some

other structure for LSGM-2015 (this might be orthorhombic [21] or cubic

[1,22], as several different crystal structures have been suggested for

single-phase LSGM-2015). More work is needed to understand this

difference.

Table 1. Creep parameters for two materials.

Grain size Temperature Qc

Material (μm) (oC) n pm (kJ/mol)

La0.8Sr0.2Ga0.85Mg0.15O2.825 [5,14] 8 1050–1300 1.3 1.8 0 521

Cubic zirconia [9–13] 1–20 1050–1600 ~1 ~2 0 460

Figure 1. Grain size and

temperature-compensated

steady-state creep rate

versus applied stress for

La0.8Sr0.2Ga0.85Mg0.15O2.825

[5,14] and cubic zirconia

[9,10]. The dopants for

ZrO2 are shown in the

figure.

1015

1014

1013

1012

1011

1010

109

σ (MPa)

4 6 8 10 30

Y2O3

Sc2O3

CaO

Dopants

Cubic ZrO2

La0.8Sr0.2Ga0.85Mg0.15O2.825

L2 exp(

Qc/RT) ε
4

Conclusion

Knowledge of the creep parameters n, p, m, and Qc for cubic zirconia and

LSGM-2015 allowed for a determination of the intrinsic creep resistance

of each material in the temperature range from 800 to 1000 oC, where they

are to be utilized as electrolytes in solid oxygen fuel cells. The results

reveal that the intrinsic creep resistance of the currently used cubic zirconia is much better than that for the recently discovered LSGM-2015 over

the entire temperature range. This is most likely a result of the difference

in their crystal structures.

Acknowledgments

I would like to acknowledge the support of the U.S. Army Research

Laboratory. I also wish to thank Drs. P. Huang and A. Petric of the Department of Materials Science and Engineering, McMaster University,

Hamilton, Ontario (Canada), for supplying the LSGM-2015 material.
5

References

1. K. Huang, M. Feng, and J. B. Goodenough, J. Amer. Ceram. Soc., 79 (1996),

1100.

2. T. Ishihara, H. Matsuda, and Y. Takita, J. Am. Chem. Soc., 116 (1994), 3801.

3. J. Drennan, V. Zelizko, D. Hay, F. T. Ciacchi, S. Rajendran, and S. P. S.

Badwal, J. Mater. Chem., 7 (1997), 79.

4. N. Q. Minh and T. Takahashi, Science and Technology of Ceramic Fuel Cells,

Elsevier, New York (1995), p. 69.

5. J. Wolfenstine, P. Huang, and A. Petric, Solid State Ionics, in press.

6. J. Wolfenstine and D. L. Kohlstedt, J. Mater. Sci., 23 (1988), 3550.

7. R. T. Pascoe and K. A. Hay, Phil. Mag., 27 (1973), 897.

8. A. Dominquez-Rodriquez, C. Monty, and J. Philibert, Phil. Mag. A, 46

(1982), 869.

9. P. E. Evans, J. Am. Ceram. Soc., 53 (1970), 365.

10. R. G. St-Jacques and R. Angers, J. Am. Ceram. Soc., 55 (1972), 571.

11. D. Dimos and D. L. Kohlstedt, J. Am. Ceram. Soc., 70 (1987), 531.

12. W. R. Cannon and T. G. Langdon, J. Mater. Sci., 18 (1983), 1.

13. M. S. Seltzer and P. K. Talty, J. Am. Ceram. Soc., 58 (1975), 124.

14. J. Wolfenstine, P. Huang, and A. Petric, Solid State Ionics, 118 (1999), p 257.

15. B. Burton, Diffusional Creep of Polycrystalline Materials, Trans. Tech.,

Aedermansdorf, Switzerland (1977).

16. R. W. Evans and B. W. Wilshire, Introduction to Creep, The Institute of

Materials, London (1993).

17. O. D. Sherby and J. Wadsworth, Progr. Mater. Sci., 33 (1989), 169.

18. J. L. Routbort, K. C. Goretta, A. R. de Arellano-Lopez, and J. Wolfenstine,

Scripta Mater., 38 (1998), 315.

19. M. Barsum, Fundamentals of Ceramics, McGraw-Hill, New York (1997),

p. 65.

20. P. R. Slater, J. T. S. Irvine, T. Ishihara, and Y. Takita, Solid State Ionics, 107

(1998), 319.

21. W. Stevenson, T. R. Armstrong, D. E. McReady, L. R. Pederson, and

W. J. Weber, J. Electrochem. Soc., 144 (1997), 3613.

22. P. Huang and A. Petric, J. Electrochem. Soc., 143 (1996), 1644.
7

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Intrinsic Creep Resistance of Two Solid Oxygen Fuel

Cell Electrolytes: Cubic Zirconia Versus Doped Lanthanum Gallate

March 1999 Interim, Dec 1998 to Jan 1999

Knowledge of the creep parameters for cubic zirconia and doped lanthanum gallate allowed for a

determination of the intrinsic creep resistance of each material in the temperature range from 800

to 1000 °C, where they are to be utilized as electrolytes in solid oxygen fuel cells. The results

revealed that the intrinsic creep resistance of cubic zirconia is much higher than that of doped

lanthanum gallate. This is most likely a result of their different crystal structures.

High-temperature, stress, crystal structures

Unclassified

ARL-TN-133

9NE6V1

622705-H94

AH94

62705A

ARL PR:

AMS code:

DA PR:

PE:

Approved for public release;

distribution unlimited.

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Jeff Wolfenstine

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