Kinetics of oxygen exchange in strontium titanate

Abstract

The kinetics of oxygen exchange are of primary importance for the application of titanates as fast resistive oxygen sensors. The sensor’s conductivity is correlated with the oxygen partial pressure pO2 of the surrounding atmosphere: Due to oxygen surface transfer and subsequent diffusion of oxygen vacancies V O ·· , a pO2 change gives rise to a conductivity change of the sample. While bulk diffusion usually occurs very fast, the surface transfer reaction becomes the rate determining step for thin samples and for low temperatures. We have shown that in the case of acceptor doped SrTiO3 the kinetics of the surface transfer reaction can be strongly influenced through stoichiometric changes brought about by thin coatings of alkaline earth metal oxides (e.g. SrO). In contrast to the commonly used jump method (conductivity response to a sudden pO2 change in the time domain), a model is presented which is based on the frequency-domain analysis of amplitude and phase shift of the response signal obtained from a pO2 modulation in a fast kinetic measurement set-up. This method allows not only for measuring response times in the sub-millisecond range but also for distinguishing between behaviour either controlled by volume diffusion or by surface transfer reaction.