Dielectric Properties of SiO2/MMT/LDPE Micro-nano Composites

Low-density polyethylene is one of the most comprehensive used as insulation materials in the power equipment. How to improve dielectric properties by doping inorganic particles in LDPE has always been concerned by many researchers. In this paper, SiO₂ particles and MMT particles were added to LDPE, the order of adding particles was changed, and different micro-nano composites was made. The crystallization characteristics of composites were analyzed, and the εr and tanδ changes of each material were investigated with frequency and temperature. The results of crystallization show that the crystalline properties and crystallinity of the composites are higher than the matrix LDPE, the addition of SiO₂ particles increases the composites' crystallinity significantly, the intercellular spacing of micro-nano composites is the smallest among all materials, and adding order in first small size particles and then large size particles is beneficial to the combination of each particle in matrix, composites' crystal structure become more tight and crystallinity of composites raise. Experimental data of dielectric frequency spectra show that the dielectric constants of micro-nano composites and composites added MMT particles are lower than the matrix, the addition of MMT particles can raise the composites' dielectric loss, however the dielectric loss of SiO₂-MMT/LDPE composites prepared by adding small size particles first and large size particles later is the lowest among all composites. The experimental data of dielectric temperature spectra show that the permittivity of SiO₂-MMT/LDPE is still at a low level under temperature conditions. The results show that the crystallization and dielectric properties of SiO₂-MMT/LDPE micro-nano composites can be improved by the method of adding small size particles first and large size particles later, and the adding method can limit molecular chain movement and establish the polarization difficultly, however it also can destroy the original interface area and reduce the ability to bind chains to the thermal motion of molecules.

I. Introduction

Low density polyethylene (LDPE) is widely used in the production of high voltage cables because of its excellent electrical insulation, good mechanical performance and technological properties^1–2. In order to meet the higher requirements of scientific and technological progress and social development on polymer insulation, researchers try to improve some of its physical and chemical properties by adding inorganic nanoparticles to the polymer. A large number of studies have shown that the introduction of nanometer filler into the polymer insulation material can obviously improve the breakdown field strength of the composite³, improve its conductivity⁴, electrical tree resistance and partial discharge performance⁵, as well as inhibit space charge accumulation^6-7. However, due to the agglomeration of nanoparticles in the polymer, it is difficult to achieve good dispersion. Therefore, the research conclusions obtained by different researchers are quite different. So far, the mechanism of the effect of inorganic nanoparticles on the structure and properties of matrix polymer is not clear. In order to improve the dispersion of nanoparticles in polymer, people try to add micro and nano particles into the matrix polymer at the same time, hoping to obtain composite materials with excellent performance through the interaction of micro and nano particles. At present, existing studies have shown that adding micron particles together with nanoparticles into the matrix can not only avoid the agglomeration of nanoparticles, but also increase the performance of the matrix^8–10. At a certain proportion, the introduction of micron particles will highlight the unique properties of nanoparticles to a certain extent. It makes some properties of micro-nano-composite materials are similar to nano-composite materials and even better than nanocomposite materials.