Measurements of Oxygen Diffusion Coefficients D and D and Checking of the Darken Theory in Oxides: Example of Rare-Earth Nickelates

摘要

Measurement and understanding of oxygen diffusivity in oxides is very important in the search of better materials for oxygen permeable dense ceramic membranes or SOFC cathodes with mixed (O~(2-)/e~-) conduction. Ionic conductivity is often estimated from diffusion coefficient using the Nernst-Einstein relation. Generally, in mixed conductor oxide materials, the ionic conductivity is low compared to the electronic one. That is the reason why direct ionic conduction measurements can not be performed and isotopic oxygen exchange techniques or permeation measurements are required. In this study, chemical oxygen diffusion coefficients D were measured by a simple conductivity relaxation experiment on both a nickel oxide ceramic La_2Cu_(0.5)Ni_(0.5)O_(4+δ) and a nickelate single crystal with composition La_2NiO_(4+δ) Thisquick and accurate method consists in abruptly changing the oxygen partial pressure in the surrounding atmosphere of the oxide. The resulting change in electrical conductivity as a function of time for a fixed temperature is recorded. The transient behaviour in the re-equilibration process is then analysed by fitting the relaxation data with the solution of Fick's second law. The variation of the chemical diffusion coefficient versus temperature shows a Arrhenius-type behaviour. The activation energy of D was therefore determined. With the same sample, we have also measured the oxygen self-diffusion coefficient D_i~* by isotopic exchange technique with SIMS analysis of the exchanged ceramic.