IMPACT OF THE REFERENCE ELECTROD DESIGN ON THE METROLOGICAL CHARACTERISTICS OF SOLID ELECTROLYTE OXYGEN SENSORS IN GASES

Abstract

The article is devoted to the internal construction of solid-electrolyte oxygen sensors in gases developed at the SSC RF - IPPE, namely: a liquid-metal reference electrode and an electrical lead. The influence of their design on the metrological characteristics of the sensor is considered. The lack of use of molybdenum electrical lead is shown - oxidation of molybdenum in oxygen-saturated liquid bismuth with the formation of an oxide layer having mixed electron-ionic conductivity. As a result, an additional source of EMF appears on the surface of molybdenum electrical lead, contributing to the total sensor signal and causing instability in time. The degree of influence of molybdenum oxidation on the metrological characteristics of oxygen sensors in gases and in liquid metals differ greatly due to differences in operating conditions: - higher rate of temperature change in the gas (thermal shock); - higher oxidative potential of the external environment - air (high operating currents and leakage currents). These differences cause a more intensive formation of the oxide layer on the surface of the molybdenum electrical lead and the low mechanical strength of this layer in oxygen sensors in gases. Various ways of solving the problem of signal instability have been proposed, in particular, replacing the material of electrical lead from molybdenum to lead or bismuth oxides, as well as steel grades EI-852 (Russian nomenclature), AISI 321, electrical steel with silicon content 4.5 wt. % and others. The results of comparative tests of solid-electrolyte oxygen sensors in gases with molybdenum electrical leads, with electrical leads based on lead and bismuth oxides, and steel are presented.