Spinel forming ceramics of the system FexNiyMn3–x–yO4 for high temperature NTC thermistor applications

The thermal stability and aging of spinels of the system Fe2O3–NiO–Mn-oxide are studied by measuring the electrical properties of ceramic samples obtained by sintering up to 1350°C. Based on the formula FeIIIxNiIIyMnz3−x−yO3+x2+δ, chemical analyses of ceramic samples with defined values of x and y make it possible to determine the average oxidation number z of manganese with z=2δ3−x−y+2 thus leading to the share of oxygen 2−x−2δ4 O2 which is liberated during sintering. X-ray diffraction measurements of samples with higher NiO and lower Fe2O3 content indicate NiO separation. Hence, the analytical results are suitable to elucidate the phase constitution of the ceramics consisting of a mixture of NiO and of a spinel with modified composition. The formation of the spinel ceramics FeNi0.5Mn1.5O4 (x=1,y=0.5) and FeNi0.7Mn1.3O4 (x=1,y=0.7) in a single-phase state, i.e. without oxygen loss on sintering at 1350 or 1300°C in air, is due to the lower content of NiO at a sufficiently high fraction of Fe2O3 in the composition. The compounds show ferrimagnetic behavior. FeNi0.5Mn1.5O4 has a Curie temperature of 245 5°C. At increasing temperature, commonly above 150°C or 200°C up to 400°C the specific electrical resistivity ρ25°C and the value of the B25/100°C constant of both of the two compounds depend on time and on the thermal pre-treatment of the samples. Aging is due to the frozen-in state of the equilibrium of distribution of cations between the tetrahedral and octahedral sites of the spinel structure. This tends to shift towards equilibrium when the temperature is increased step by step during aging; it can be followed by measuring the electrical properties. The changes are observed to arise approaching the Curie temperature. Soaking of the samples at 650°C after aging for 72 and 144 h up to 500°C shows that the variations of the ρ25°C and B25/100°C values can be repeated in a following aging cycle, provided that the same cooling rate is applied. On the other hand, above 400 °C the ralaxation effects fail. Changes in this range of higher temperature involve fast rates of cation re-distribution thus leading to short waiting times until a constant value of the electrical resistivity is achieved. Hence, above 400°C, FeNi0.5Mn1.5O4 ceramics appear completely stable against aging within common measuring times. Therefore, they should be suitable for high temperature NTC thermistor applications. The comparatively high value B25/100°C=7470 K makes it possible to measure the temperature via the electrical resistivity with satisfactory sensitivity, e.g. α=1ρdρdT=0.8% is achieved in the range around 750°C.