Current Reduction Caused by Strong Local Field in Low-Density Polyethylene under Various Temperatures

This paper presents experimental studies on J-E characteristics and space charge distributions for a low-density polyethylene (LDPE) sheet between semi-conduction-anode and Al-cathode under high dc field. In normal cases, the steady current density J in a polymer sheet increases with increasing the mean applied field strength E. We measured the value of J under each applied field E from 0.1 to 1.5 MV/cm for the LDPE sheet under various temperatures between 30 and 60°C. The relationships between J and E for the LDPE can be expressed as J α E
²
until E exceeds a threshold value E
₀
. The J-E characteristics for E <; E
₀
can be explained with the well-known space charge limited current (SCLC) model. However, J is reduced with increasing E when E > E
₀
. The threshold value E
₀
increased with increasing temperature. We confirmed that a large number of positive space charges were injected into the bulk of LDPE when E > E
₀
. They were accumulated in the LDPE bulk, so that the local field strength near the cathode became much larger than the applied field. These results imply that the charge mobility of the space charge in the local high field area must be lower than that in the low field area. The positive charge density in the LDPE bulk was reduced with increasing temperature. The field distortion caused by the space charge accumulation in the LDPE bulk cannot be observed at 60°C. The current reduction phenomenon was not also observed at 60 °C. On the basis of these results, the reason why the charge mobility was reduced by the field concentration into the local area will be discussed.