Influence of oxygen atmosphere annealing on the thermal stability of Mn1.2Co1.5Ni0.3O4±δ ceramic films fabricated by RF magnetron sputtering

This paper presents the optimal atmosphere annealing conditions for Mn1.2Co1.5Ni0.3O4±δ ceramic thin films fabricated by the RF magnetron sputtering method. The microstructure and oxygen distribution, together with electrical properties, are combined and applied for determining thermal stability. All of the Mn1.2Co1.5Ni0.3O4±δ films, which are annealed at various oxygen atmosphere from 1 × 10−3 to 1 × 105 Pa, exhibit a negative temperature coefficient characteristic and show a poly-crystalline spinel structure. The film which annealed at 10 Pa with the most uniform and most dense surface morphology has the minimum resistivity compared to the others. It is characterized by the highest Mn3+ and Mn4+ pair content, which gives the highest carrier concentration of ceramic films. Combined with the aging test at 125 °C for 500 h, the films annealed at 10 Pa have the minimum resistance drift (ΔR/R0 = 2.35%), which is mainly affected by the oxygen vacancy concentration. This demonstrates that the film thermistors annealed in a hypoxia state will never be stable. This is because there will be several oxidation reactions leading to a continuous generation of cationic vacancies during high temperature aging. The present results will open a way to design desired stable negative temperature coefficient thermistors by adjusting the annealing oxygen atmosphere of films.