Dielectric breakdown characteristics of alumina

The AC 50 Hz sine dielectric breakdown of alumina ceramics versus thickness (0.127 to 2.54 mm) and purity (92%, 96% and 99.5%) was investigated. The results have shown that the dielectric strength decreases with the thickness and increases with the purity. Furthermore, the investigation of the failure morphology using the scanning electron microscopy (SEM) show for each specimens submitted to breakdown the presence of a canal terminated by a crater at the surface. This crater is resulting from the extraction and ejection of the matter during the breakdown process caused by the intensification of the local electric stress. An evolution of the crater size versus the sample thickness have also been observed. The analysis of the experimental results and the SEM observations confirm that the built up of the crater is a crucial step in dielectric breakdown process and the mechanism best suited to describe the dielectric breakdown in these materials is based on an electromechanical one.