Kinetic demixing in yttria-doped zirconia part II - theoretical analysis

Abstract

Abstract. This paper presents a formal analysis of the transport processes in yttria-doped zirconia under a temperature gradient. Due to the simultaneous diffusion and drift of the species when the material is exposed to a thermodynamical potential gradient, a kinetic demixing process appears on the cationic sublattice if the diffusion coefficients of the cations are different. Experimental results obtained with yttria-doped zirconia are discussed on the basis of this analysis. They confirm that kinetic demixing processes during cooling must be taken into account in the interpretation of the grain boundary conductivity of doped zirconia. IntroductionXPS analysis [1] shows a near surface enrichment of silicon and yttrium in yttria-doped zirconia lower in air quenched materials than in samples cooled at 25 K/h. These observations are consistent with those of Aoki et al. [2], for calcium and silicon in calcium-doped zirconia. However, while Aoki et al. suggest that these cation redistributions are likely confined to a layer near the interfaces close or lower than 5 nm, we have observed that the segregation layer thickness is higher than the depth of the space charge region and depends on the cooling rate of the sample after sintering, indicating that cation redistributions occur during cooling. The purpose of this paper is then to provide an analysis of the kinetic demixing phenomena in oxide solid electrolytes, in order to be able to control such effects in materials in which the minority defects are present on the cationic sublattice.

Kinetic demixing and grain boundary conductivity of yttria-doped zirconia part I - experimental observations

“…Furthermore, we have shown that the near surface composition depends on the cooling rate, indicating that segregation processes occur during cooling. In part II [27], wc have analyzed these cation redistributions in order to be able to minimize such effects in materials in which the prevailing defects are present on the anionic sublattice.…”

Operational Inhomogeneities in La0.9Sr0.1Ga0.8Mg0.2O3–δ Electrolytes and La0.8Sr0.2Cr0.82Ru0.18O3–δ–Ce0.9Gd0.1O2–δ Composite Anodes for Solid Oxide Fuel Cells

“…Although, cation diffusivities are normally several orders of magnitude lower than that of oxygen ions [21], at sufficiently long times, high temperatures [22], and electric fields [23][24][25], substantial cation migration may still occur. Note that the migration of the different cations are not identical; certain elements migrate faster, others slower [26].…”