Preparation and characterization of spinel-type Mn Ni Co O negative temperature coefficient ceramic thermistors

Powders of ternary Mn-Ni-Co oxide negative temperature coefficient thermistors were synthesized by a low-temperature alternative route. The procedure allowed straightforward preparation without the addition of any binder, of highly densified Mn-Ni-Co-O semiconducting ceramics as cubic single-phase spinels, at relatively moderate temperature (approximate to 1000 degrees C). A tentative cation distribution for the Mn1.5Ni0.6Co0.9O4 spinel oxide has been proposed, and its variation with temperature has also been considered. The dilatometric and X-ray powder diffraction studies carried out for Mn1.5Ni0.6Co0.9O4 showed that sintering takes place in a single stage between 900 and 1000 degrees C, and yields highly densified ceramic with an apparant density larger than 96% of the calculated X-ray density. Scanning electron microscopy showed different microstructures for the Mn(1.5)Ni(0.6)Co(0.9)O(4)pinel oxide, depending on sintering conditions. The value of the sensitivity index, beta = 3068 K, indicates a good technological thermistor performance for this material. (C) 1998 Chapman & Hall.