Kinetics of Oxygen Exchange in Sr(Ti 0.65 Fe 0.35 )O 3
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Kinetics of Oxygen Exchange in Sr(Ti0.65Fe0.35)O3 Th. Schneider, S. F. Wagner, W. Menesklou, E. Ivers-Tiffée Universität Karlsruhe (TH), Institut für Werkstoffe der Elektrotechnik, 76128 Karlsruhe, Germany, [email protected] ABSTRACT 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 VO.. . 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. INTRODUCTION The kinetics of oxygen exchange are of primary importance for the application of semiconducting metal oxides, e.g. strontium titanate, as fast resistive oxygen sensors. The sensor’s electrical conductivity reflects the equilibrium between the oxygen partial pressure pO2 of the surrounding atmosphere and the bulk stoichiometry at high temperatures (T > 700 °C). Due to oxygen surface transfer and subsequent diffusion of oxygen vacancies within the sample, a pO2 change gives rise to a change of the sample’s electrical conductivity σ :
σ∝e
(
− EA
kT
) pO m 2
(1)
The first r.h.s. factor describes the temperature dependence, EA denotes an activation energy. Oxygen partial pressure dependence is described by the second factor, including m as a constant which depends on the dominant type of bulk defect (|m| ≤ + ¼) and corresponds to the sensor’s sensitivity. Negative values of m signify n-type, positive values p-type conduction mechanism. The defect chemical basics which lead to the characteristic σ( pO2 ) curve of strontium titanate have been treated in several papers, e.g. [1,2], and shall therefore only be briefly summarized here. The reduction of an oxide can be described by the defect reaction OO ↔ VOii + 2e − + ½O 2
(2)
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in which oxygen leaves its regular lattice sites (OO) generating a doubly ionized oxygen vacancy VO.. thermally activated by the reduction enthalpy ∆H Red , and two electrons e– in the conduction band. The generation/recombination of electronic defects depends on the material’s band gap Eg: n⋅ p ∝ e
E − g kT
,
(3)
n and p bein
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