Abstract
Semiconducting (Ba0.9Sr0.1)TiO3 ceramics with 0.1mol% Sb2O3 and 0.05mol% Ta2O5 were prepared by changing sintering temperature and heating rate. Microstructure, specific resistivity at room temperature, magnitude of PTCR effect and breakdown voltage of the obtained ceramics were investigated. Abnormal grain growth was observed from 1300°C and became remarkable at the temperature regime between 1320°C and 1360°C. The specific resistivity at room temperature decreased and the magnitude of PTCR effect increased with increasing the degree of abnormal grain growth. When the heating rate is increased from 180°C/h to 500°C/h at the temperature regime between 1300°C and 1360°C, the grain size of the samples decreased from 44μm to 18μm. The specific resistivity at room temperature increased from 5Ωcm to 7.5Ωcm by decreasing average grain size. However, both the magnitude of PTCR effect and the resistivity in the grain evaluated by the complex impedance measurement were independent of the grain size. The breakdown voltage increased from 31V/mm to 60V/mm with decreasing the grain size from 44μm to 18μm. There was the linear relationship between the breakdown voltage and the reverse of the grain size, and the breakdown voltage of a grain boundary was estimated 0.78V from the slope of this relationship. In this study, it was revealed that the heating rate is one of the critical factors for properties of (Ba, Sr)TiO3 ceramics. Rapid heating at a proper temperature regime is a preferable condition for the ceramics with fine microstructure, which results in the improvement of their breakdown voltage without significant increase of the specific resistivity at room temperature.
