Chemical diffusion of oxygen in tin dioxide

Tin(IV) oxide is widely used in the Taguchi sensor for the detection of redox-active gases. The performance of the sensor critically depends on oxygen transport parameters since perceptible values of the chemical diffusion coefficient of oxygen cause severe drift effects. The present work investigates the kinetics of oxygen exchange by two independent methods, an EPR relaxation technique and the well-known conductivity relaxation technique. The experiments were performed on nominally undoped single crystals which contained traces of iron and aluminum. In the temperature range from 700 to 1000°C the chemical diffusion coefficient is obtained as Dδ=0.02 cm2 s−1 exp(−0.9 eV/kT). The results of the relaxation experiments were complicated by additional processes occurring on different time scales. A doping independence assumed, we conclude that the kinetics of drift processes are determined by the surface exchange reaction of oxygen on SnO2 rather than by the chemical diffusion of oxygen.