电场和温度对聚合物空间电荷陷阱性能的影响

模拟分子的结构和行为有助于更深刻地分析空间电荷陷阱性能变化的微观机理.利用Materials studio软件建立聚乙烯模型,通过分子链段运动产生的能量和自由体积变化对微观结构和电荷陷阱进行分析.结果表明:温度由298 K逐渐升高至363 K的过程中,聚合物分子热运动加剧导致的滑移扩散现象,使自由体积和陷阱能级在363 K处分别出现1542.07 ~3和0.66 eV的最大值和最小值.然而在Z轴方向施加0.0007 Hartree/Bohr(1 Hartree/Bohr=5.2×10~(11)V/m)电场作用时,由于电致伸缩产生Maxwell应力,使分子链段出现局部有序排列,增大范德瓦耳斯能至-360.18 kcal/mol(1 kcal/mol=4.18 kJ/mol),而自由体积降低了279.77 ~3,导致陷阱能级减小0.45 eV.当363 K和0.0007 Hartree/Bohr联合作用时,聚乙烯的陷阱能级相比同温无电场作用降低0.17 eV.分子模拟结果与实测结果相符.利用分子热运动和电致伸缩效应,初步探讨了材料自由体积和范德瓦耳斯相互作用能变化的微观机理,证实分子链段运动改变了微观结构,从而影响电荷陷阱特性.并且与温度相比,电场作用会使材料产生更低能级的空间电荷陷阱.

Simulations of the effects of electric field and temperature on space charge traps in polymer

The simulations of the structure and behavior of the molecule in the simulation software are an effective way to analyze the microscopic mechanism associated with performance change of space charge trap in the polymer. To achieve this, in this paper we first present the polyethylene molecular model which is developed by using the simulation software Materials Studio (MS). Then, the microstructure and property of space charge trap are analyzed by the changes with the energy and the free volume in the polyethylene due to the chain segment motion under the universal force field (UFF), respectively. Some important findings are extracted from simulation results. First, in the process of the temperature gradually increasing from 298 K to 363 K, the phenomena of slippage and diffusion of the molecule due to the enhanced thermal motion of molecules are observed. These phenomena lead to the free volume increasing and the space charge trap energy level decreasing gradually, whose maximum value is 1542.07ų and the minimum value is 0.66 eV when the temperature is 363 K. Second, when an electrostatic field of 0.0007 Hartree/Bohr is applied to the polymer, molecular chain segments are oriented by the Maxwell stress that is generated by the electric effect. Molecular chain segment orientations induce the van der Waals interaction energy to increase to -360.18 kcal/mol (1 kcal/mol = 4.18 kJ/mol), the free volume to decrease by 279.77 ų, and the space charge trap energy level to decrease by 0.45 eV. Third, by comparing the cases of applying the temperature field and the electric field to the polyethylene, it is found that the electric field has stronger effect on charge trap. Specifically, the space charge trap energy level of the polyethylene associated with 0.0007 Hartree/Bohr electric field is reduced by 0.17 eV compared with that associated with the temperature of 363 K. Moreover, simulation results and measured results are compared with each other and they are well consistent. Finally, it is concluded that using electric effect and molecular thermodynamic movement is an very effective way to analyze the microscopic mechanism of changes with free volume and van der Waals interaction energy. This analysis confirms that molecular motion changes the microstructure of the polyethylene and generates charge traps. In addition, it confirms that the influence of the electric field on the polyethylene generates the lower level of space charge trap than the effect of the temperature field.