Degradation Due to Wet Hydrogen Firing on the High-Voltage Performance of Alumina Insulators in Vacuum Applications

The HV characteristics of plain and doped alumina ceramics fired under air and air plus wet hydrogen conditions were tested in a uniform field vacuum gap using dc excitation. Wet hydrogen firing of doped alumina ceramics considerably degraded voltage hold-off as compared to air firing. Baking hydrogen-fired samples in vacuum did not improve their performance and the V-I curves remained unaltered. Gap currents measured for plain and doped air-fired samples were essentially the same (~1 to 10 nA), however, for wet hydrogen-fired samples the currents were much higher for doped than for undoped alumina samples. In general, the non-resistive current component obtained from the V-I curves was found to follow the Fowler-Nordheim equation only in the high-field region (> 60 kV/cm) for air-fired alumina samples. Processing the wet-hydrogen-fired samples by subsequent firing in an atmosphere of dry hydrogen revived the voltage hold-off of the doped alumina ceramics; the currents being comparable to that of wet-hydrogen-fired samples in the low voltage range (< 50 kV).