Electric charge transport mechanism at medium fields in low-density polyethylene

The transient conductivity in low-density polyethylene is studied. Isochronal current-voltage measurements for 4 h and 4 days time intervals are carried out under a dry N₂ atmosphere. The activation energy is around 0.70 eV. It changes if the measurements are not carried out close to the steady state condition. At a constant heating rate of 0.12 K/min and low field the apparent activation energy for temperatures lower than about 318 K is ≅ 1.2 eV and for temperatures higher than 318 K it is ≅ 0.9 eV. When there is a small quantity of trapped charge in the sample the isochronal current-voltage characteristic reveals an ohmic behavior. When the next field increase is applied without sample discharging, the current-voltage characteristic is super-quadratic. We explain this increase of the current assuming that a fraction of the previous injected charge is detrapped by the field and it contributes to current increase. This is mainly a bulk effect explaining way the bulk mechanisms provide better explanations. Consequently the current-voltage characteristic is strongly dependent on the time lag between two successive rises in the field. Neither the Poole-Frenkel mechanism nor the Richardson-Schottky mechanism can by used to explain the experimental results. The isothermal charging and discharging currents are explained assuming the movement of injected charge in the resultant local field. The values obtained for the adjustable parameters of the model are in good agreement with the values in the literature.