The relation between space charges in polymer dielectrics at cryogenic temperature and characteristics of polymer materials

The distribution of space charges in a polymeric dielectric was measured by electro-acoustic pulse techniques. Tests show that the amount of space charge goes up apparently when the environmental temperature decreases from room temperature to cryogenic temperature. Results were examined by thermal stimulated current measurements on polymer samples polarized at cryogenic temperature and were related to the shallow energy level of localized states in polymers. The localized states with shallow energy levels may trap carriers at the surface close to the electrode and have a large effect on the electrical properties of polymers at low temperatures. At low temperatures, the polymer will most likely be broken down at the moment of short circuit or at the reverse of the voltage polarity. It is concluded that polymers used for cryogenics as insulating materials should properly be selected according to the conditions at operation to adjust the densities of shallow traps and to decrease the effect caused by thermal stress