The deterioration of mica under the action of electrical discharges

This paper describes an investigation into the discharge resistance of high grades of Muscovite and Phlogopite micas. Internal discharge experiments have been performed in air at NTP with electric stress up to 8 kV mm^-1with a frequency acceleration factor of 24. The samples were not tested to breakdown but examined at selected times up to an equivalent period of ten years at power frequency. Both types of mica experienced a similar pattern of deterioration starting with a surface crazing and the development of minute nodules. With continued discharge attack the crazed surface began to peel exposing newly cleaved surfaces which subsequently, repeated the peeling effect, resulting in puncture of the flake at the centre of these areas. During this development initially curled flakes and surface nodules develop dendritic formations modifying the electric stress and enhancing the discharge attack. Micro-probe analyses of the surfaces and surface products at various stages of degradation indicated that depletion and removal of the metallic cations had occurred in the micas resulting in a collapse of their crystalline stability and the formation of silica. The lost cations were found present as metallic salts in solution and solidified residues on the mica surfaces.