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

An oxygen sensor is proposed which is represented by an electrochemical cell: metal \ oxide scale \ sensing electrode, where the metal, its oxide scale, and sensing electrode work as reference electrode, electrolyte, and sample 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 oxygen. It is expected that the electrolyte is self-restorative because it can be reformed by high-temperature oxidation, The electromotive force (EMF) measurements were carried out at 873 K with cells using zirconium as the metal 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 the 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 conductivity. The oxidation behaviors showed linear oxidation. Assuming repetition which constituted of parabolic oxide film growth until a certain thickness and its crack formation, the linear rate constants were described as a function of the oxygen partial pressure. It was considered that the steep decrease in EMF is caused by the change of the rate-determining process to form the scale, (C) 1999 The Electrochemical Society. S0013-4651(98)03-077-8. All rights reserved.