【作者】 周丽春；

【导师】 陈国华；

【作者基本信息】 华侨大学 ， 材料学， 2009， 博士

【摘要】 研究导电复合材料在电场下的非线性导电行为特别是电击穿行为,不仅能更好地了解导电复合材料的导电性能和机理,还可以为导电复合材料的应用提供合理、可靠的依据。本论文主要研究了高密度聚乙烯/纳米石墨微片(HDPE/GN)导电复合材料在电场作用下非线性导电行为,着重研究了在电流作用下的电击穿行为。本论文主要研究工作如下:(1)HDPE/GN导电复合材料渗滤导电行为研究本文以GN与HDPE熔融复合制备的HDPE/GN导电复合材料作为研究对象,研究了HDPE/GN导电复合材料的渗滤导电行为。GN的含量与HDPE/GN导电复合材料电阻率之间的关系符合渗滤导电方程。HDPE/GN导电复合材料的渗滤阈值较低,说明具有较大径厚比的GN容易在高分子基体中形成导电通路。渗滤临界指数值与理论渗滤网络普适值差异较大,说明HDPE/GN导电复合材料渗滤体系的传输行为是非普适性传输行为。(2)HDPE/GN导电复合材料可逆非线性导电行为研究本文研究了HDPE/GN导电复合材料在电流下的I ? U曲线,得到非线性转变电流密度与线性电导率之间的标度模型。标度模型指数x值的范围在DRRN网络模型预测的理论指数值范围内,说明引起HDPE/GN导电复合材料可逆非线性导电行为的主要机理是导电网络的隧道效应。分析了HDPE/GN导电复合材料非线性导电行为偏离线性导电行为的程度,结果表明当GN的含量接近渗滤阈值时,HDPE/GN导电复合材料非线性导电行为得到增强。(3)HDPE/GN导电复合材料临界电击穿行为研究本文首先研究了电流( I≤Ib)作用下HDPE/GN导电复合材料的非线性响应。在电流作用下,导电复合材料内最终达到电热平衡状态,材料的电阻和温度在电热平衡状态下达到稳定值。研究了HDPE/GN导电复合材料电热平衡态的非线性导电行为,电热平衡态的非线性导电行为是导电网络内隧道效应和焦耳热效应共同作用的结果。研究了电流作用下HDPE/GN导电复合材料的电阻热松弛行为,热松弛时间τh随着电流的增加而变长,热松弛时间τh在临界击穿点发散。研究了HDPE/GN导电复合材料临界击穿电场,建立了临界击穿电场与线性电阻的标度模型。研究了GN的含量、测试样品的大小、电击穿的测试次数对HDPE/GN导电复合材料临界击穿电阻与线性电阻比值的影响,表明HDPE/GN导电复合材料的临界击穿电阻与线性电阻的比值Rb / R0为定值。研究了不同石墨填料/HDPE导电复合材料Rb / R0值,表明Rb / R0值与导电填料的性质有关。(4)HDPE/GN导电复合材料动态电击穿行为研究本文研究了在击穿电流作用下HDPE/GN导电复合材料的电阻随时间的变化,建立电阻与时间的标度模型。研究了导电复合材料在电流作用下的电击穿时间,建立击穿电流与击穿时间的标度模型。研究了HDPE/GN导电复合材料在动态电击穿过程中温度的变化,结果表明当HDPE/GN导电材料发生电击穿时,HDPE的相态发生了变化。研究了在电流作用下动态电击穿过程中HDPE/GN导电复合材料的电阻随温度的变化,结合温度场下HDPE/GN导电复合材料的PTCR效应,探讨了在电流作用下HDPE/GN导电复合材料的电击穿的机理。(5)电压作用下HDPE/GN导电复合材料自加热行为研究本文研究了在自加热效应作用下HDPE/GN复合材料的电阻和温度对时间的依赖性。研究了电压作用下HDPE/GN复合材料电热平衡态的非线性导电行为,探讨了电热平衡态非线性导电行为的机理。研究了HDPE/GN导电复合材料在自加热效应下的电开关特性,材料的电开关值随电压的升高而增大,电开关时间随电压的升高而变短。研究HDPE/GN导电复合材料的电开关临界电压值,建立临界电压值与样品的线性电阻的标度模型。更多还原

【Abstract】 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.更多还原