Preparation and characterization of thermistors with negative temperature coefficient, Ni x Mn 3–x O 4 (1⩾x⩾0.56), from formate precursors

Single-phase nickel manganite spinets, NixMn3–xO4 with 1x0.56, have been prepared by mild heating of solid solutions of nickel–manganese formate obtained by coprecipitation. A dilatometric and X-ray diffraction study was conducted at different temperatures, showing the different stages (788, 1033 and 1083 K) of the sintering process of highly densified nickel manganite ceramics (density, 96–98%). A granulometric and scanning electron microscopy (SEM) study showed that the oxide particles formed agglomerates of 2 µm, with a narrow size distribution. The oxide particles obtained in air were larger than those obtained in a controlled atmosphere.The resistivity showed a minimum of 1580 Ω cm and the sensitivity index was at ca. 4000 K. This value corresponds to x= 0.72; this implies that the nickel is almost entirely in octahedral holes. This minimum is independent of the oxide synthesis process.