Microstructural and electrical properties of Y0.2Al0.1Mn0.27−xFe0.16Ni0.27−x(Cr2x)oy for NTC thermistors

Mixtures of refractory oxides and transition metal oxides for yttrium–aluminum–manganese–iron–nickel–chromium–oxygen (Y–Al–Mn–Fe–Ni–Cr–O) systems were prepared using normal ceramic processes, for use as negative temperature coefficient (NTC) thermistors to measure a wide temperature range. Platinum–rhodium (Pt–Rh) alloy electrodes were inserted into the body during the formation process to increase the reliability at high temperatures and decrease the contact resistance. The properties of the thermistors were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and resistance measurements. The shrinkage rate, grain size, and density of the specimens doped with Cr2O3 were higher than those of non-doped specimens. The crystallinity of the grains decreased with the addition of 0.006 mol of Cr2O3. The grain size increased with Cr2O3. The resistance behaviors were similar for the two specimens, but the values were higher with the Cr2O3. The B constant value (the coefficient of temperature sensitivity) was 3636 K for the non-doped specimen and 3876 K for the specimen doped with 0.006 mol of Cr2O3 across a temperature range of 0–600 °C. The specimens exhibited a linear relationship between their electrical resistance and reciprocal temperature over a wide temperature range, indicating NTC thermistor characteristics. The mixtures of refractory oxides and transition metal oxides extended the measurement range from 0 °C to 600 °C, and the addition of Cr2O3 increased the resistance and the B constant in the doped sample.