Decomposition of NiMn2O4 spinel: an NTC thermistor material

Nickel manganate based NTC thermistors are widely used in industrial and domestic applications where cost efficient but reliable temperature sensing, fluid flow rate or pressure sensing devices are required. Single phase, ceramic samples of NiMn2O4 (2:1 Mn:Ni) were synthesised using a conventional mixed oxide route. Thermal analysis confirmed the 2:1 spinel structured material decomposes upon heating above 907°C. After heating at temperatures between 1050 and 1200°C and subsequently quenching to room temperature, regions containing florets of spinel structured material were found using TEM. The surrounding matrix was a NiO rich rock-salt structured material and the florets were manganese rich. A decomposition mechanism based on degree of inversion is proposed to explain the appearance of the florets. Slowly cooling samples from 1250°C led to a microstructure principally composed of cubic spinel but also with regions of much smaller spinel florets in a rock-salt structured phase. The slow-cooled samples showed a larger drift in resistance over time than single phase samples when held at 400°C. XRD measurements carried out before and after electrical characterisation showed a reduction in the amount of rock-salt structured material present in the slow-cooled samples.