S.E.M. study of aging phenomena in dielectrics at cryotemperatures

This paper presents studies and investigations into the mechanism of deterioration and complete failure of solid polymers at cryogenic temperatures. The studies incorporate a scanning electron microscope (SEM) which enabled individual discharge sites to be identified and the deposits and erosion of polymers to be clearly observed. Tracks of damage were examined to determine their growth rate and behaviour at different stages of failure. The effects of different times of voltage application at different stresses on the degradation processes was determined. Polyethylene samples were tested at temperatures ranging from room temperature (293°K) down to that of LHe (5°K). It was concluded that the energy damaging the dielectric is predominatly carried by the moving electrons. These impinging electrons are the prime factor of degradation. The damage density is dependent upon the frequency and magnitude of the applied stress. Generally, the behaviour of discharges was seriously reduced, the rate and density of damage were greatly lowered and the lifetime of dielectic was significantly increased specially under d-c voltages at LN2 temperatures.