Space charge characterization

The same poling conditions have been currently used for making “electrets” and for investigating aging and breakdown phenomena. These conditions consist to pole during a long time under elevated temperature and then to short circuit and to ground the electrodes before space charge measurements. These conditions are may be adequate for forming and orienting dipoles but are a priori inadequate for trapping charges. That is because the trapping probability depends on the temperature, on the electric field and other stress as mechanical stress. Moreover, a trapped state is a metastable state that can exist only in a temperature domain. Therefore, it is first necessary to determine the existence domain of this state for optimizing the poling conditions. Because the poling and measuring conditions having been chosen without pre determining the trapping domain it is understandable why a low charge density is currently found (C/m3). Injected charges flow through the sample but let behind them a memory that is sample representative and even if the poling conditions are inadequate for predicting the material properties, they allow to differentiate different materials. The challenge is now to predict which one material is the best for electrical insulation and to verify the conclusion by doing high voltage tests