Microstructure and Electrical Properties of Microwave-Sintered PTC Thermistors

Abstract

BaTiO3—based positive temperature coefficient (PTC) thermistors have been prepared by microwave sintering. The sintering process was carried out in a 2.45 GHz multimode microwave tube furnace without any susceptor. The heating behavior of the PTC material, influence of processing conditions on microstructure, and electrical properties of the sintered samples were investigated. The sintering temperature range varied from 1275 °C to 1350 °C. The optimum density and microstructure were achieved between 1300 °C to 1325 °C. Samples were also microwave sintered at 1300 °C for various soaking times from 10 min to 90 min. It was found that soaking between 30 min to 60 min generated a uniform microstructure. Cooling rate varied from 7.5 °C/min to 2 °C/min. The study showed that the cooling rate has an influence on PTC effect but not on microstructure. Slower cooling rate is essential for achieving better PTC effect. Compared to conventional method, the microwave-processed samples exhibited higher density and better PTC effect. The relative density of microwave sample was 93.5%, compared to 90.2% of the conventional sample. The R25, αp ß, Tc, and Rmax of the microwave sintered samples are 17.6 Ω, 15.8, 6.7 × 105, 88.0 °C, and 11.5 MΩ, respectively, compared to 12.1 Ω, 14.6, 1.1 × 105, 88.1 °C, and 1.3 MΩ, respectively, for the conventional sample.