Vaporization and Diffusion Studies on the Stability of Doped Lanthanum Gallates

Abstract

Vaporization and diffusion determine the stability of doped lanthanum gallates under SOFC operating conditions. Systematic vaporization studies of Ga and other elements were carried out using the vapor transpiration method. It was shown that the Ga vaporization is controlled by diffusion from the bulk to the surface. Diffusion coefficients DGa and vaporization coefficients αGa were determined by fitting the measured vaporization data to a vaporization model. Secondary phases formed as a result of the vaporization were detected. The influence of different doping levels of Sr, Mg and Fe on the Ga vaporization was elucidated. Moreover, cation self‐diffusion of 139La, 84Sr and 25Mg as well as cation impurity diffusion of 144Nd, 89Y and 56Fe in polycrystalline samples of doped lanthanum gallate were directly determined for the composition La0.9Sr0.1Ga0.9Mg0.1O2.9 as an example, from diffusion profiles determined by SIMS. It was found that diffusion occurs by means of bulk and grain boundaries. The bulk diffusion coefficients are similar for all cations with activation energies which are strongly dependent on temperature. The results are explained by a frozen‐in defect structure at low temperatures in the ABO3 perovskite lattice and by proposing a defect cluster containing cation vacancies in the A and B sublattices, as well as oxygen vacancies.