Role of thermal and electrical relaxations for the long-term conduction current in polyethylene

The characterization of the electrical conduction behavior of polymeric insulating material is a challenging task. Typical reasons are the underlying complexity of the physical processes, the smallness of the current, and the long relaxation times. Moreover, even if the external conditions like applied voltage and temperature are time-constant, often a steady-state direct current (DC) is not established during the measurement time, which may last up from hours to months. This work reports on DC current measurements on 1 mm thick press molded LDPE and XLPE samples. The results show that the conductance is slowly decaying sub-linearly with time, I~t
^p
, with 0 <; -p <; 1, p often around -0.5. In certain cases an intermediate current maximum is observed. There is strong indication that the decay of the effective conductivity is due a thermal process rather than to an electrical process.