Current limiting electrochemical pumping cells (amperometric sensors) based on zirconia are commonly used for engine control applications. Fast resistive-type sensors adapted from semiconducting metal oxides are a promising alternative for future exhaust gas monitoring systems. Therefore among the interesting characteristics of the materials system Sr(Ti0.65Fe0.35)O3, including high sensitivity and temperature independence at high oxygen partial pressures (pO2 > 10-4 bar), a short response time (t90 = 30 ms) is obviously the most salient.
The latter is determined by the kinetics of the oxygen surface transfer and subsequent diffusion of oxygen vacancies V‥
O. For thin samples and low temperatures the surface transfer is dominant, since bulk diffusion usually occurs very fast. The presented model 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 setup. This method allows both the measurement of response times in the sub-millisecond range as well as the distinction of the behaviour either controlled by volume diffusion or by surface transfer reaction in Sr(Ti0.65Fe0.35)O3 ceramics.