Chen Xi, Guo Guangzhi, Hao Yifan, Li Jiaming, Li Wendong, Deng Junbo, Zhang Guanjun, Zhai Mingming
State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an, 710049, P. R. China.
Phys Chem Chem Phys. 2020 Oct 15;22(39):22207-22216. doi: 10.1039/d0cp01992e.
Polymers, especially polyethylene (PE), are widely employed as insulating materials in electrical power transmission systems. However, the insulation still faces the problem of space charge, which distorts the electric field distribution and accelerates electrical aging. Experimental results show that after the fluorination process, less charge injection occurs compared with pure PE. To clarify the mechanism, classical molecular dynamics was employed to build a PE/fluorinated layer interfacial model and first principles calculation was utilized to get the band offset at the interface. The results calculated by both the bulk plus band lineup method and the layer-decomposed density of states method show that the energy band of the fluorinated PE layer is overall lower than that of the PE side, and the band offsets are around 2 eV. Charge transport results based on Marcus theory and kinetic Monte Carlo simulations also show that charge can easily accumulate at the interfacial area under an electric field and the band offset can suppress charge injection. The conduction band offset acts as an energy barrier for the excess electrons at the fluorinated layer side to cross the interface, while the valence band offset has the same effect on hole transport because of the energy barrier caused by the inverted region. Our findings provide a fundamental and theoretical basis for material modification and space charge inhibition.
聚合物，尤其是聚乙烯（PE），在输电系统中被广泛用作绝缘材料。然而，这种绝缘材料仍然面临空间电荷问题，该问题会使电场分布发生畸变并加速电气老化。实验结果表明，氟化处理后，与纯PE相比，电荷注入减少。为了阐明其机理，采用经典分子动力学构建了PE/氟化层界面模型，并利用第一性原理计算得到界面处的能带偏移。通过体加能带排列方法和层分解态密度方法计算得到的结果均表明，氟化PE层的能带总体低于PE一侧，能带偏移约为2 eV。基于马库斯理论和动力学蒙特卡罗模拟的电荷输运结果还表明，在电场作用下，电荷很容易在界面区域积累，而能带偏移可以抑制电荷注入。导带偏移对氟化层一侧的多余电子穿越界面起到能量势垒的作用，而价带偏移由于反向区域引起的能量势垒，对空穴输运也有同样的作用。我们的研究结果为材料改性和空间电荷抑制提供了基础理论依据。
