Resistivity and microstructure of alumina ceramics added with TiO₂ fired in reducing atmosphere

Alumina ceramics containing various amounts of TiO₂ were fabricated by sintering in humid reducing atmospheres and also in air. The sintered samples were treated at various temperatures in reducing atmospheres. The electrical resistivity measurements and microstructural observation were conducted on the obtained samples to clarify the relationship between the ceramic microstructure and the resistivity. The resistivity decreased with increasing TiO₂ content and the resistivity change with addition of TiO₂ behaved as the percolation curve, when the samples were sintered in humid reducing atmosphere. However, the percolation curve disappeared and the resistivity decreased gradually with increasing TiO₂ content after the heat treatment. In order to clarify the relationship between electrical resistivity and microstructure of sintered body, EPMA analysis and X-ray diffraction were conducted. Rutile, Mg-Al-Ti solid solution and Ti in Si rich grain boundary phase were detected in the fabricated samples and Ti solid solution was also confirmed in the alumina grains. It is considered that the great decrease in the electrical resistivity, which was observed in the samples containing more than 2.0wt% TiO₂, is caused by the formation of the connections among low resistivity Ti-compounds. This microstructural change is considered to cause the percolation curve. After the heat treatment, the low resistivity Ti-compounds disappeared into microstructure It is considered that the disconnections of low Ti-compounds increase the resistivity of sintered body.