Mn-Fe-Co-Ni系酸化物サーミスターの熱処理に伴う結晶構造と電気抵抗の変化

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This investigation was carried out to understand the remarkable change in electric resistance of thermistor material before and after the glass-coating process adopted in making the commercial thermistor. The thermistor material was made from nitrates of Mn, Fe, Co and Ni with the molar ratio of 6.1 : 2.9 : 1.3 : 1.0 by firing at 1400℃ in air atmosphere. Electric resistance of the thermistor material was measured continuously in the temperature range from room temperature to 700℃, with a rate of 1℃/min, in air and nitrogen. In air, a remarkable change in electric resistance was observed between 300° and 500℃, and this change was reversible in both heating-up and cooling-down processes. In nitrogen, a similar change in electric resistance appeared in the heating-up process. However, no detectable change was observed in the cooling-down process in nitrogen. X-ray diffraction analysis of heating-up process in air revealed that the crystal change, in which mixed crystal of cubic and tetragonal spinel transfered to cubic spinel, occurred between 420° and 480℃. This crystal change in air was reversible and was observed in both heating-up and cooling-down processes. On the other hand, in the case of nitrogen, similar crystal change was observed only in heating-up process, and no crystal change occurred in cooling-down process. In heating-up process in nitrogen, the starting mixed crystal of cubic and tetragonal spinel changed to cubic spinel above 480℃, and this cubic spinel remained unchanged even though heat-treated up to 1000℃. In cooling-down process in nitrogen, cubic spinel structure remained unchanged to room temperature. These results propose that the effect of glass-coating on electric resistance of thermistor material is attributed to crystal change before and after the glass- coating.
社団法人日本セラミックス協会の論文
1988-12-01

Mn-Fe-Co-Ni系酸化物サーミスターの熱処理に伴う結晶構造と電気抵抗の変化

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