Creep of Lanthanum Gallate
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Timothy R. Armstrong Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 21 June 2001; accepted 10 December 2001)
Strontium- and magnesium-doped lanthanum gallate (LSGM) was deliberately prepared to give A-site deficient nonstoichiometry with compositions (La0.9Sr0.1)z(Ga0.8Mg0.2)O3−␦ (z ⳱ 1.0, 0.98, and 0.95). Creep tests in four-point bending for 950 °C < T < 1350 °C and 15 MPa < < 75 MPa in air demonstrated that all three compositions shared a common stress dependence, n ⳱ 1.49 ± 0.10, and a common apparent activation energy, Q ⳱ 426 ± 9 kJ/mol. Despite this agreement, the creep rates of the different compositions depended on grain size in different ways: p ⳱ 3.1 ± 0.2 for z ⳱ 0.98, and p ⳱ 1.9 ± 0.1 for z ⳱ 0.95. The measured apparent activation energy, Q, for creep is similar, though statistically significantly smaller, than that measured in another LSGM. Both are nearly twice as large as reported activation energies for cation impurity diffusion. The absolute magnitude of the creep rates, after correction for grain size, were 30 to 100 times slower than in another LSGM of similar composition.
I. INTRODUCTION
Magnesium- and strontium-doped lanthanum gallate, (La1−xSrx)(Ga1−yMgy)O3−␦, usually denoted LSGM, is a potential replacement for yttria-doped zirconia as the electrolyte for solid oxide fuel cells because of its high ionic conductivity.1 Although lanthanum gallate fuel cell electrolytes must survive thousands of hours at elevated temperature under mechanical load, with few exceptions,2 very little is known about their high-temperature deformation resistance. The creep behavior of polycrystalline ceramics,3,4 and perovskites in particular,5–13 is generally represented by . a phenomenological relation between creep rate, ⑀, and stress, , temperature, T, and grain size, d. Nearly every creep mechanism, whether Nabarro–Herring, 14,15 Coble,16 or grain boundary sliding (GBS), accommodated by intragranular dislocation motion17–19 or diffusion20–23 reduces to ⑀˙ =
冋 册
Q A1 −p n d exp − T RT
,
(1)
where n is the stress exponent, Q is the apparent activation energy for creep, p is the grain size exponent, and all the other constants are combined in the prefactor,
a)
Address all correspondence to this author. e-mail: [email protected]
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A1. It is sometimes possible to differentiate between creep mechanisms by examining the measured values of n, Q, and p. This manuscript reports results on the creep of LSGM that was processed in an attempt to intentionally produce a lanthanum site deficiency. Other perovskites with lanthanum excess tend to form La2O3 which can hydrate in humid environments24 and degrade mechanical properties. These creep data are interpreted in light of and contrasted with that in the literature,2 especially with regard to the creep mechanism. Existing theories of creep of doped lanthanum gallate cannot reconcile the differences between the three compositions of this study or those reported in the lit
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