Decomposition of NiMn2O4 spinel: an NTC thermistor material

Nickel manganate based NTC thermistors are widely used in industrial and do mestic applications where cost efficient but reliable temperature sensing, fluid flow rate or pressure sensing devices are required. Single phase, cer amic samples of NiMn2O4 (2:1 Mn:Ni) were synthesised using a conventional m ixed oxide route. Thermal analysis confirmed the 2:1 spinel structured mate rial decomposes upon heating above 907 degreesC. After heating at temperatu res between 1050 and 1200 degreesC and subsequently quenching to room tempe rature, regions containing florets of spinel structured material were found using TEM. The surrounding matrix was a NiO rich rock-salt structured mate rial and the florets were manganese rich. A decomposition mechanism based o n degree of inversion is proposed to explain the appearance of the florets. Slowly cooling samples from 1250 degreesC led to a microstructure principa lly composed of cubic spinel but also with regions of much smaller spinel f lorets in a rock-salt structured phase. The slow-cooled samples showed a la rger drift in resistance over time than single phase samples when held at 4 00 degreesC. XRD measurements carried out before and after electrical chara cterisation showed a reduction in the amount of rock-salt structured materi al present in the slow-cooled samples. (C) 2001 Elsevier Science Ltd. All r ights reserved.