Effect of Surface Layer on the Space Charge Accumulation in Epoxy Samples under DC Stress

Space charge measurements according to the pulsed electroacoustic (PEA) method are performed in disc samples of epoxy at electric fields from about 2 to 20 kV/mm and at temperatures of 25 and 40 degrees C. The accumulation of homo-charges and hetero-charges in the epoxy samples is found under DC stress. Both homo- and hetero-charges form stable space charge zones. The accumulation of space charges is influenced by a surface layer of the samples, probably caused by casting and oxidation. If the surface layer is removed, no hetero-charge accumulation is observed. Further, the injected homo-charges do not form a stable space charge region in front of the electrode, but travel further into the material, forming an initial wave-front-like charge distribution. A charge transport model is developed taking into account charge injection, charge accumulation and charge transport. The charge transport model is validated by the space charge measurements.