An oxygen sensor using a process of high-temperature oxidation of metal

An oxygen sensor is proposed which is represented by an electrochemical cel l: metal \ oxide scale \ sensing electrode, where the metal, its oxide scal e, and sensing electrode work as reference electrode, electrolyte, and samp le electrode respectively. Here the oxide scale is required to be an oxide- ion conductor, and the sensing electrode is not to be reactive with the oxy gen. It is expected that the electrolyte is self-restorative because it can be reformed by high-temperature oxidation, The electromotive force (EMF) m easurements were carried out at 873 K with cells using zirconium as the met al electrode and Pt as the sensing electrode. At p(O2) = 1-10(-4) atm, the EMF vs. log p(O2) plot lies on a straight line and its gradient is 2.303 RT /4F, suggesting unity of the oxide-ion transform. number a the surface of t he scale. The EMF steeply decreases with decreasing p(O2) at p(O2) <10(-4) atm. which cannot be explained by the increase in the electronic conductivi ty. The oxidation behaviors showed linear oxidation. Assuming repetition wh ich constituted of parabolic oxide film growth until a certain thickness an d its crack formation, the linear rate constants were described as a functi on of the oxygen partial pressure. It was considered that the steep decreas e in EMF is caused by the change of the rate-determining process to form th e scale, (C) 1999 The Electrochemical Society. S0013-4651(98)03-077-8. All rights reserved.