The kinetics of oxygen incorporation are of fundamental importance for an application of acceptor doped strontium titanate as a resistive-type oxygen sensor. The electrical conductance of the sample depends on the ambient oxygen partial pressure pO2 due to oxygen exchange between gas phase and solid state which leads to a flow of charge carriers. The kinetics of the incorporation process can be separated into two steps: the oxygen surface transfer and the subsequent bulk diffusion of oxygen vacancies. The rate of the slower step determines the kinetics of the overall incorporation process and thus the sensor's response behavior.
A method for the investigation of the kinetic behavior is presented which is based on a frequency-domain analysis. In the underlying model, a SrTiO3 single crystal is exposed to a harmonically modulated pO2. This leads to a modulation of the sample's electrical conductance. By way of calculation, it is shown that the shape of the frequency response clearly allows to distinguish whether the kinetics of oxygen incorporation are determined either by bulk diffusion or by surface transfer.
The experimental validation of this model is demonstrated by investigations performed on slightly acceptor doped SrTiO3 single crystals of various thicknesses in a kinetic measurement setup at different temperatures.