Conduction and breakdown in polymers

High field conduction in polymer films and single crystal of long chain hydrocarbon molecule was measured with short pulsed electron beam. The induced conduction in polyethylene consists of the fast and the slow components, which are attributed to the carrier transport in the crystalline and in the amorphous region respectively. The mobility of the former component seems to be very high. Logarithmic plot (Scher-Montroll plot) of the latter component vs. time gives a knee at the time T_r, which is considered to be the transit time of the slow carrier front between electrodes. The apparent slow carrier mobilities at 343 K are estimated from T_r to be 5.6 × 10^-7 cm² /V·sec and 3.2 × 10^-7 cm² /V·sec for the electron and the hole respectively at the field strength of 1.2 MV/cm and the sample thickness of 12 μm. These apparent slow carrier mobilities thus obtained depend on both sample thickness and field. The activation energy of the mobility is in good agreement with the trap depth obtained from the simultaneous TSC and TL measurements. The dependences of breakdown voltage in polyethylene on the temperature, crystallinity, waveform of the applied voltage etc. are well explained in terms of the conduction mechanism mentioned above and the space charge effect of the trapped carrier. The quantum yield of the electron beam induced current is much larger in aromatic systems than in linear polymers. At low fields, even in the polymer with it electron system, clear knee was not observed in the induced charge signal. In such a case, the mobility can be also estimated from the Scher -Montroll plot (7.0 × 10^-5 cm² /V·sec for electron in PS), which is in good coincidence with that estimated from knee point at higher fields in our previous reports.