Transient current in a semi-crystalline PET controlled by the injection and transport of space charges

Injection and transport of space charges have been observed in a semi-crystalline PET under 6.25 kV/mm dc at 60°C. Measurements were made 30 s after volt-off along a 74 h poling using the Laser-Induced Pressure Pulse (LIPP) method. They revealed only negative charges that spread throughout the inter-electrode gap and accumulated near the anode. The space charge distribution stabilized after about 30 hours. Once stabilized, the distribution of charges led to an increase of the field at the anode of 30% and a decrease of the field at the cathode of 20%. The measured density of charges held at the anode was in good agreement with the computed densities of charges induced by the space charges. The measured transient current is similar to the current derived from the charges induced at the anode. This suggests that in these conditions the transient current is a kind of displacement current entirely due to the transport of the space charges. A criterion taking into account the fields at the anode and at the cathode is proposed to explain the end of the transient régime.