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 the rmistors 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 (approx imate to 1000 degrees C). A tentative cation distribution for the Mn1. 5Ni0.6Co0.9O4 spinel oxide has been proposed, and its variation with t emperature has also been considered. The dilatometric and X-ray powder diffraction studies carried out for Mn1.5Ni0.6Co0.9O4 showed that sin tering takes place in a single stage between 900 and 1000 degrees C, a nd yields highly densified ceramic with an apparant density larger tha n 96% of the calculated X-ray density. Scanning electron microscopy sh owed 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 perfo rmance for this material. (C) 1998 Chapman & Hall.