Formation of nickel-manganese oxide thermistors studied by XRD, SEM and auger spectroscopy

Abstract

This paper describes the formation of nickel-manganeses oxide thermistor bodies at 1000–1340° C, employing analytical techniques of XRD, SEM/EPMA and AES. The micro-structural studies revealed that the main phase of nickel manganite coexists with a solid solution of NiO in Mn₃O₄ in the final product. The optimum firing conditions to achieve the necessary electrical properties as well as the development of the desired microstructure could be selected around 1200° C, for 4 hrs in an ambient atmosphere. Above this temperature, the density begins to decrease while the resistivity increases. These anomalous electrical resistivity variations could be partly attributed to the trapped oxygen which was evolved from the decomposition of the unreacted α-Mn₂O₃. At-lower temperatures, unreacted nickel oxide residuals as well as a high porosity of the samples would yield specimens with high resistivity.