Effect of annealing and polishing on flashover characteristics of ceramic in vacuum

Experiments were carried out to study the effects of surface polishing and annealing of ceramic specimens on the dielectric breakdown characteristics of their surfaces. It was found that when the samples were annealed at 1000°C, allowing the residual stresses in their surfaces to decrease, the breakdown voltage rose, whereas with the polished samples, reductions of residual stresses resulted in a slight rise of the breakdown voltage. Prior to dielectric breakdown, a surface glow was observed, due to luminescence from electron bombardment and insulator surface defects. The starting voltage of luminescence rose when residual stresses were reduced. It is suggested that the mechanical strain due to surface defects contributes to dielectric breakdowns on ceramic surfaces