Resistance degradation in Y(Cr,Mn)O₃-Y₂O₃ composite NTC ceramics in hostile environments

A series of composite, negative temperature coefficient (NTC) ceramics were carefully processed with compositions based on the Y(Cr,Mn)O₃+Y ₂O₃ system and these were investigated for resistance stability in hostile environments. This specific system is of interest for high-temperature automobile thermistors, however either through the processing or in use of these, materials can be exposed to reducing atmospheres at temperatures around 900°-1000°C. The thermochemical processes at intermediate temperatures and low <10^-10atm can influence the resistance of the given ceramics. Through an impedance analysis it is determined that the resistance increase is associated primarily with a grain boundary resistance increase. The grain and grain boundary elements are modeled through parallel constant phase element and resistance equivalent circuits connected in series. Possible origins of the defect chemistry being controlled through high-temperature processes at the sintering are partial Schottky reactions that are compensated through a superoxidation reaction on cooling and aging. The reduction process reversed the superoxidation reaction and transited the grain boundary surfaces to ionically compensated B-site vacancies with oxygen vacancies.