Knowledge of the exchange kinetics of O2 in SrTiO3 allows us to design appropriate strategies to separate the ionic and the electronic conductivity. In the low‐temperature range, where the overall surface reaction is very slow compared to bulk diffusion and measuring time, electrochemical cells of the type Pt|SrTiO3|Pt are self‐blocking and self‐sealing and a Wagner–Hebb‐type polarization succeeds without the necessity of using selectively blocking electrodes. In the present study the ionic conductivity data obtained for Feand Ni‐doped SrTiO3 in this way are compared to data obtained from the analysis of the oxygen partial pressure dependence of the total conductivity as well as to defect chemical calculations. In complete contrast to the low temperature situation, at high temperatures, where the surface reaction is fast, the emf technique is conveniently applicable. Results are presented for Pt, O2|SrTiO3|O2, Pt cells. The conductivity behavior of SrTi(Fe)O3 as a function of temperature (20°–1000°C) is complex, due to partially frozen‐in equilibria, but even details can be quantitatively understood in terms of a simple defect chemistry. The turnover of the diffusion‐controlled regime to the surface reaction‐controlled regime can be shifted to significantly lower temperatures by using YBa2Cu3O7–8 electrodes.
Interfaces (grain boundaries and surfaces) are studied in acceptor (Fe) doped SrTiO3 bicrystals, single crystals, and ceramics as a function of temperature (423 K T 1023 K), oxygen partial pressure (1 Pa Po io Pa), and Fe doping content (1.9 1018 cm3 c 9.5 1O' cm3) using electrochemical methods. In particular, impedance spectroscopy and dc polarization techniques have been applied. The electrochemical investigation of tilt and twist grain boundaries in bicrystals combined with structural and chemical grain boundary charactizationby transmission electron microscopy, electron diffraction x-ray analysis, and electron energy loss spectroscopy allowed us to clarify grain boundary effects in SrTiO3. The use of reversible (nonblocking) YBa2Cu3O6+ electrodes proves to be a convenient technique to measure conductivities without electrode effects, since blocking effects at the sample surface were minimized. These results have been compared with those obtained for grain boundaries in polycrystalline samples as well as with the interfaces between metallic electrodes and SrTiO3 single crystals. Besides individual features, all findings at the investigated boundaries and interfaces could be consistently explained by the appearance of pronounced Schottky barriers which were composed of depletion layers of mobile majority charge carriers (h, V).
Oxygen content and oxide kinetics determine response time Diffusion profiles have been recorded in situ and evaluated in bulk conductivity sensors, drift effects in surface conductivusing an optical technique. In this way, bulk diffusion coefity sensors, speed of oxygen permeation in chemical filters, ficients have been reliably and quantitatively measured. It performance of oxide electrodes, lifetime in some perovskite is shown that the values agree with calculations without capacitor materials stressed by applied voltage, annealing temusing adjustable parameters if the coupling of the diffusperature in high-temperature superconductors, 11,13 and other ing species to internal redox changes of the dopants is phenomena. accounted for. Measurements on single crystals and on bicrystals with and without crack formation provide worthwhile information on the influence of free relaxed surfaces, II. Experimental Procedure freshly produced (crack) surfaces, and grain boundaries onMeasurements were performed on single crystals and bicrysthe surface reaction rate and diffusional process.
The time evolution of oxygen stoichiometry profiles in Fe-doped SrTiO, single crystals has been detected and analyzed in-situ using an optical absorption technique. The profiles can be fitted according to Fick's second law by a constant chemical diffusion coefficient. The data obtained are in good agreement with earlier results from integral detection techniques and can be quantitatively understood by including the influence of internal coupling reactions to bare ambipolar transport via short range interaction.lntroduction ln this communication we report on an in-situ and contactless determination of oxygen diffusion profiles and on a precise determination of the chemical diffusion coefficient of oxygen (Do) in SrTi03. To our knowledge this is the first example of such a direct measurement of the space and time behaviour of diffusion kinetics of oxygen exchange. The method has been developed from earlier variants in which either the integral response had been measured in-situ at elevated temperatures or the profile had been frozen-in [l 1. Integral absorbance measurements have been reported in Ref.[I] (on SrTi03) and [2] (on NiO), and frozen-in profiles in reduced SrTi03 [3]. Closest to our approach is a report by Ben-Michael and Tannhauser [4] on the in-situ investigation of a colour front upon reduction of Zr02. There, however, profiles could not be evaluated.The knowledge of the chemical diffusion coefficients of oxygen in oxides is of great interest for both fundamental and technological reasons. This parameter determines or at least limits the overall rate of chemical reactions; it determines leakage rates in ion-conducting membranes, permeation rates in chemical filters, response times in bulk conductivity sensors, or drift effects in boundary conductivity sensors. Surprisingly, even the chemical diffusion of such intensively investigated materials as SrTi03 or stabilized Z r 0 2 is not fully understood yet. During our experimental and theoretical work it turned out that the coupling of internal defect chemical equilibria to the diffusion provides the clue for an adequate description [ 1, 51.Our decision to focus primarily on SrTi03 had a twofold origin and exhibits a basic and a technological aspect. On the one hand, the bulk equilibrium defect chemistry is wellunderstood, and even subtle details can be evaluated [5]. On the other hand, chemical diffusion in SrTi03 is crucial with respect to the performance of oxygen sensor and the durability of a perovskitic capacitor or actuator materialsThe basic oxygen incorporation reaction can be formulat-[6-81.ed as (1) 1 -02(g)+VG * 0 & + 2 h ' .2In Eq. (1) Vt;, 0 x 0 , and h' denote an effectively twofold positively charged oxygen vacancy, an effectively neutral regular oxygen ion, and an electron hole, respectively (Kroger-Vink notation). The mass action constant for Eq. (1) is taken as [5] The defect chemistry of nominally pure perovskite materials such as SrTi03 is greatly influenced by dopants, mostly transition metal ions. Therefore we have used deliberately ...
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