Electrical property of semiconducting (Ba,Sr)TiO3 ceramics with the microstructure controlled by heating rate

Semiconducting (Ba0.9Sr0.1) TiO3 ceramics with 0.1mol% Sb2O3 and 0.05 mol% 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 degrees C and became remarkable at the temperature regime between 1320 degrees C and 1360 degrees 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 degrees C/h to 500 degrees C/h at the temperature regime between 1300 degrees C and 1360 degrees C, the grain size of the samples decreased from 44 mu m to 18 mu m. The specific resistivity at room temperature increased from 5 Omega cm to 7.5 Omega cm 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 31 V/mm to 60 V/mm with decreasing the grain size from 44 mu m to 18 mu m, 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 0.78 V 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.