Electrical and microstructural characterization of metal oxide doped negative temperature coefficient thermistors

Manufacturing sensors that detect environmental temperature changes is of great value in materials science, as they are crucial for many industries. Temperature sensors are materials called thermistors, whose electrical resistance changes with temperature. For commercial thermistors, which are obtained by mixing various metal oxides, the material constant values are expected to be in the range of 2000–5000 K. Electrical properties vary depending on the selected composition, the type and amount of additives, and the production conditions. In the first phase of the study, the effects of chromium oxide, titanium oxide, and iron oxide additives on the electrical and microstructural properties of Ni-Co-Mn-O-based compositions were examined. Boron oxide was added to these samples, which contained various metal oxide additives, to investigate the liquid phase effect. The samples with boron oxide were not calcined, whereas those without it were subjected to calcination at 900°C for 2 hours. The samples in this phase were sintered at 1100°C for 5 hours. In the second phase, the changes in the electrical and microstructural properties of Ni-Co-Mn-O-based NTC thermistors were investigated by increasing the amount of chromium oxide additive and the sintering temperature. Vanadium oxide was added to the samples in the second phase to examine the liquid phase effect, and these samples were not calcined. The sintering temperatures were set at 900, 1000, and 1100°C. Samples without vanadium oxide were calcined at 900°C for 2 hours, and their sintering temperatures were set at 1100, 1200, and 1300°C. Phase formation in all samples was determined by X-ray diffraction (XRD) analysis. Bulk density measurements were performed using the Archimedes method. The microstructure of the samples was examined using a scanning electron microscope. X-ray photoelectron spectroscopy analysis was performed to obtain information on the surface chemical properties of the samples. The electrical resistance of the samples was measured in the range of 25–85°C to calculate their resistivity, material constant, sensitivity coefficient, and activation energies. According to the XRD analyses, cubic spinel phase formation occurred in all samples produced in the first and second phases. The material constant values of the samples produced in the first phase were found to be in the range of 3669–4481 K. It was determined that the samples produced in the second phase had values of 3799–5063 K. These values are within the desired range for industrial NTC thermistor applications. It was also found that boron oxide is an important additive for NTC thermistor production in terms of energy consumption.