Kinetics of Oxygen Incorporation into SrTiO3 Investigated by Frequency-Domain Analysis

Abstract

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 pO 2 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 SrTiO 3 single crystal is exposed to a harmonically modulated pO 2 . 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 SrTiO 3 single crystals of various thicknesses in a kinetic measurement setup at different temperatures.

Oxygen transport in epitaxial SrTiO<sub>3</sub>/SrTi<sub>1 − <i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3</sub> multilayer stacks

“…STO is a perovskite structure with well-studied electrochemical properties (e.g., Balachandran and Eror, 1981;Chan et al, 1981;Wagner et al, 2004;De Souza, 2009;Ohly et al, 2006;Gömann et al, 2005Gömann et al, , 2004. The oxygen partial pressure dependence of its mixed ionic-electronic conductivity makes it a good candidate for high temperature gas sensor applications (Fergus, 2007;Hu et al, 2004).…”

“…The −1/4 and +1/4 slopes of n and p type electronic conductivities indicate that the oxygen vacancy concentration is independent of the oxygen partial pressure. Here, the experimental data are fitted using the formula given by, e.g., Yoo et al (2002):…”

Oxygen sensors based on SrTi0.65Fe0.35O3−δ thick film with MgO diffusion barrier for automotive emission control

“…The oxygen diffusion is kinetically fast enough only at high temperatures [52,53], therefore, the STF35 sensor should normally operate at temperatures >700 • C. Under this condition, the power for heating becomes a considerable issue.…”

Resistive Oxygen Gas Sensors for Harsh Environments

“…In other words, the PMM can be used as an in-situ method for “measuring” oxygen transport parameters in sensor materials, especially the diffusion coefficient, D , and the surface reaction parameter, k . Further details can be found in [111]. This useful method measures the amplitude of the sensor output, H ( f ), for sine wave modulations of the total pressure at constant oxygen percentages at different frequencies, f , i.e.…”