The Study Of The Nonlinear Conduction Behavior Of High-density Polyethylene/Graphite Nanosheets Conducting Composites Under Electric Field

Study on the nonlinear conduction behavior of conducting composites under electric field in particular electrical breakdown behavior is important , not only for the better understanding of mechanism conduction of conducting composites, but also for the application of conducting composites by providing a reasonable and reliable basis. In this thesis, study on the nonlinear conduction behaviors of high-density polyethylene/ graphite nanosheets (HDPE/GN) conducting composites is presented.The main contents are as following:1) The percolative behavior of HDPE/GN conducting compositesHDPE/GN conducting composites were prepared by melt blending of HDPE and GN. The percolative behavior of prepared HDPE/GN composites had been investigated. It was found that the conduction of conducting composites possessed a typical percolative phenomenon. The transport behaviors could be well described by percolation equation. The lower percolation threshold in HDPE/GN conducting composites illuminated that GN with higher aspect ratio possessed advantages in forming the conducting network via channel in the polymer matrix. However, the critical exponent was different from the universal value predicted in classical percolation theory, which indicated that the transport behavior in HDPE/GN conducting composites was non-universal transport behavior.2) The reversible nonlinear conduction behavior of HDPE/GN conducting compositesThe investigation of the nonlinear conduction in HDPE/GN conducting composites indicated that the nonlinear crossover current density scaled with the linear conductance of sample. The value of exponent x in the scale equation was in the range of theoretic value predicted by using dynamic random resistor network model (DRRN), which illuminated that the macroscopic nonlinear conduction of HDPE/GN conducting composites might originate from tunneling effect in conduction network. Investigation on the degree of nonlinear conduction deviating from linear conduction in HDPE/GN conducting composites indicated that the nonlinear conduction behavior enhanced when the GN content was close to the percolation threshold.3) The critical electrical breakdown behavior of HDPE/GN conducting compositesAll above, the nonlinear responses of HDPE/GN conducting composites under applied currents were studied. Under applied currents, an electrical-thermal equilibrium state in composites was ultimately reached, associating with the resistance and temperature both reached steady values. The nonlinear conduction behavior in electric-thermal equilibrium state for HDPE/GN conducting composites under applied currents was investigated. The result showed that nonlinear conduction behavior in electric-thermal equilibrium state was induced by tunneling effect and Joule heating effect in the conducting network. The thermal relaxation behavior of resistance in composites was studied. In the process of thermal relaxation, the thermal relaxation time became longer with increasing current. It was also found that the thermal relaxation diverged at the critical electrical breakdown current. Investigation on the critical electrical breakdown field of HDPE/GN conducting composites showed that the critical electrical breakdown field scaled with the linear resistance of sample. The effects of GN content, the size of sample, breakdown cycle on the ratio of the critical electrical breakdown resistance to the linear resistance ( Rb / R0) were also investigated. It was found that the ratio of the critical electrical breakdown resistance to the linear resistance in HDPE/GN conducting composites assumed a fixed value of about 1.30. The investigation also found that the Rb / R0 value in other HDPE conducting composites filled with different graphite indicated that the value Rb / R0 depended on the property of conducting fillers.4) The dynamic electrical breakdown behavior of HDPE/GN conducting compositesThe model of resistance and time was established by studying the changes of resistance over time in HDPE/GN conducting composites under applied breakdown current. Furthermore, the electrical breakdown time of HDPE/GN conducting composites under different currents was measured and the scale equation of electrical breakdown time and current was established. The changes in temperature of HDPE/GN conducting composites in dynamic electrical breakdown process indicated that phase transition of HDPE appeared when the electrical breakdown occurred. The positive temperature coefficient of resistivity (PTCR) behavior under temperature field was also discussed. The mechanism of electrical breakdown in HDPE/GN conducting composites under applied currents was investigated, associating with the PTCR effect of HDPE/GN conducting composites under temperature field.5) The self-heating behavior of HDPE/GN conducting composites under electric fieldThe nonlinear responses in HDPE/GN conducting composites under applied field were investigated. Under sufficient electric field, the resistance and temperature increased initially due to the Joule heating effect and eventually reached a steady value. The electrical switching properties under self-heating effect in HDPE/GN conducting composites were studied. The results showed that the switching ratio increased and the switching time became shorter with the increasing applied field. The threshold voltage value of thermal switching was studied and the scale equation of threshold voltage value vs. resistance was established.