Characterisation Of Interfacial States In The SiO₂/LDPE Nanocomposite From First Principles Computations

Due to its excellent performance, polymer insulating material has been widely used in the insulation materials of power equipment. However, with the fast development of modern electric power industry, higher requirements of the performance and reliability of the insulating material are put forward. The nano dielectric materials can be prepared by adding nano-scale dimension inorganic fillers into the polymer and making the inorganic fillers evenly dispersed in the polymer. The outstanding performance of nano-dielectrics has been paid more and more attention by people. As long as doping a small amount of nanoparticles, the performance of the polymer can be improved significantly. It has been an accepted fact that interface plays an important role in controlling the process of charge transport. Many excellent performance of nano-dielectric are considered related to the interface structure and behavior. The formation of traps caused by interface action has important influence on the internal charge transfer and space charge properties in the polymers. Some research indicates when the ability of nanocomposite space charge is improved, the depth and density of trap changes at the same time. Therefore, in the process that nanoparticles improve the polymer dielectric properties, we need to insight into these characteristic changes of the interface trap distribution after filling the nanoparticles. Introducing the nanoparticles into composite materials or changing the properties of nanoparticles will produce chemical defects at the interfaces. So it will have an experimental and theoretical reference value for us to study the trap's depth and density of states introduced by different defects.In this paper, by using the method of first principle computation, we calculated the characteristic of trap introduced by different defects at the interface formed by silica nanoparticles and polyethylene matrix. In addition, we studied the degree of depth of trap states and the size of density of states when the interface respectively has these five kinds of structure defects: carbonyl, carbon carbon double bond, conjugated double bond, vinyl and dienone. The results prove that: structure defects is one of the reasons why the trap exists; when the structure of defects is different, the characteristic of trap is also different; we found these defects all introduce the deep electron trap. For example, the depth of deep trap introduced by carbonyl and dienone is between 3.0 to 3.7eV. Although the depth of the deep trap produced by dienone is deeper than that produced by carbonyl, the size of density of states of deep trap introduced by carbonyl is bigger. The depth of the deep trap,between 2.5 to 2.9eV,produced by conjugated double bond is a little shallower, and the density of states is slightly smaller. In comparison, the depth of the deep trap, about 1.5eV, brought by ethylene and carbon carbon double bond is shallowest, but there is not much difference in the density of states when compared with carbon-carbon double bonds. Dienone and conjugated double bond introduce the obviously shallow trap states near the conduction band. Carbonyl, conjugated double bond and ethylene introduce the shallow trap states near the valence band. The shallow trap not only has the electron trap above the Fermi level but also has the hole traps under the Fermi level.