| The dielectric breakdown strength of alumina was investigated with spherical electrodes of different diameters and two dielectric liquids (castor oil and DialaRTM oil). The dielectric breakdown strength, expressed in terms of the maximum electric field in the ceramic at the electrode contact point, decreased with increasing electrode radius. The breakdown strengths were higher for the DialaRTM oil as compared to the castor oil. These effects of the electrode geometry and of the media on the breakdown strength of the alumina suggested a size effect expected for weakest-link failure. However, an analysis of the breakdown strength distributions using the two-parameter Weibull distribution and finite element analysis (FEA) of the axi-symmetric electric field distribution on the surfaces of disk specimens indicated that the measured variations in the breakdown strength with electrode size and dielectric media were much greater than what would be expected for the Weibull distributions and their characteristic shape parameters. On the other hand, it was noted that the Joule heating rate (J/s) in the volume of the puncture attained a constant critical value at the breakdown voltage independent of the electrodes and the dielectric media. At the breakdown voltage, the Joule heating rate is adequate to raise the temperature of the ceramic to its melting point, thus initiating the thermal breakdown. |
