A Study of Incomplete Breakdown in Dielectrics and Glass

Abstract

Works aimed at studying electrical breakdown of solid dielectrics placed in sharply nonuniform fields have revealed a number of interesting phenomena for partial and sequential breakdown cases. The article analyzes the properties pertinent to partial breakdown of dielectrics. Partial breakdown depends on the applied voltage level, time, thickness, and crack length growth rate. When a dielectric is placed in electric field, charges appear on the surface of its air inclusions. Once the field strength caused by these surface charges reaches the breakdown voltage of air inclusions, air ionization processes are initiated. The ionization will take place until air ions have neutralized the surface charges. After that, the cycle is repeated. The ratio between the electromagnetic field attraction and repulsion forces that arise when the atoms approach each other determines the stable existence of a solid body and its mechanical properties, adhesion, and surface tension. Formulas for calculating the adhesion energy are obtained, and the calculation results are presented. The theoretical values of these quantities can be used in performing a comparative analysis of adhesion energy variations for different solid materials. The electrical breakdown of glass coated with an electrically conducting film was investigated. A 2...3 mm wide part of the film was removed by means of a laser to form insulating cut-offs. Such cut-offs on the surface of glass coated with a conducting film and placed in electric field give rise to nonuniformities in electric field distribution. These nonuniformities concentrate over the perimeter of insulation cut-offs and at the corners. To investigate the electrical breakdown of glass, its samples were prepared with various types of cutoff shapes: linear cut-offs and cut-offs with the perimeter of a square or rectangle. The glass plate was laid with its non-coated side facing the breakdown installation’s lower electrode, whereas the upper electrode was placed on the film surrounded by cut-offs or insulated strips without film. The electrical breakdown phenomena were investigated by applying AC voltage. The breakdown occurred over the film periphery at the boundary with cut-offs. It has been found from the experiment that numerous discharges are observed over the boundary between the film and insulating strip during the breakdown. It has also been found that glass melting occurs on the glass surface at the air ionization spots. Experimental data on partial breakdown of glass having different configurations, with and without cut-offs, for glass samples with film coated on one side are presented. It is shown that partial breakdowns in glass occur at field strengths less than those causing the real breakdown, which is characteristic of the gas medium in which the breakdown occurs.