Molecular Dynamics Simulation Of Thermal Aging Characteristics And Mechanism Of Nano-SiO₂/MVQ

Composite insulators have low intrinsic thermal conductivity,and may gradually age or even crack during network operation due to high temperature,resulting in the degradation of electrical insulation performance,which is not conducive to the safe and stable operation of power systems.Therefore,the insulating properties of composite insulators under high temperature conditions are of great interest to scholars.Nanometer SiO2 is the additive of choice for silicone rubber materials(MVQ)for composite insulators because of its excellent insulation and thermal conductivity.The nano-SiO2/MVQ composites are therefore widely used in power systems.Considering that the electric field strength of composite insulators during operation is related to both dielectric properties and thermal conductivity.In this paper,we simulate the dielectric properties and thermal conductivity of nano-SiO2/MVQ before and after thermal aging based on the molecular dynamics method.High temperature aging simulations of nano-SiO2/MVQ to investigate the microscopic effects of SiO2-filled nanoparticles on the thermal aging of silicone rubber composite insulation materials.To analyze the high temperature cracking mechanism of nano-SiO2/MVQ and to analyze the effect of filled nano-SiO2 on the thermal aging condition of the material at the microscopic level.This study is expected to provide insight into the improvement of the dielectric properties and thermal conductivity of silicone rubber composite insulators after high temperature thermal aging by nano-SiO2 particle filling,and the inhibition of the high temperature thermal aging process of the material.The results of the study provide a theoretical basis for reducing the aging loss of composite insulators and developing new highly thermally conductive insulating materials.The main study contents and related findings are as follows:(1)A model of composite insulator substrate MVQ and nano-SiO2/MVQ system with 5%,15%and 25%added mass fraction was constructed using Materials Studio software.Structural optimization and energy minimization of the constructed model to achieve a stable conformation.The reliability of the proposed model is verified by the corresponding temperature,system energy,and system density variation curves with simulation time obtained through molecular dynamics simulation calculations.(2)Thermal aging simulations were conducted for four systems at different temperatures,and the changes in relative permittivity and thermal conductivity of each system before and after thermal aging were compared and analyzed.The effects of nano-SiO2 content and thermal aging temperature on nano-SiO2/MVQ composites were investigated.The thermal conductivity of the 25%nano-SiO2/MVQ system at room temperature was increased by 107.2%compared to that of the neat MVQ system,while the material dielectric constant was increased by 50.26%.The relative permittivity and thermal conductivity of each system model showed a slow decrease with the increase of temperature from 300K to 500K.The lower filling amount makes the effective thermal conductivity pathway between molecules not formed,and the increase of temperature can improve the phonon transport inside the material,which makes the thermal conductivity of the two systems models of neat MVQ and 5%nanoSiO2/MVQ increase slowly with temperature.Therefore,the filling of nano-SiO2 particles significantly enhances the thermal conductivity of the material,improves the dielectric properties of the material,suppresses the increase of dielectric constant with temperature,and alleviates the problems of charge aggregation and surface overheating of the material.(3)Based on the molecular dynamics method of ReaxFF reaction to simulate the thermal cracking process of composite insulators,the inhibitory effect of filled nano-SiO2 on the thermal cracking of nano-SiO2/MVQ composites was investigated.When the system temperature is 2000K～4000K,with the increase of temperature,the motion and twist of molecular main chain of neat MVQ and 25%SiO2/MVQ increase,and the aging cracking of the two systems becomes more and more serious.At 200 ps,the fragmentation index of 25%SiO2/MVQ at 2000 K is 29.6%less than that of neat MVQ fragmentation index,while less small molecule products such as CH4,C2H2 and H2 are generated at each temperature condition.It indicates that the presence of nano-SiO2 increases the energy required for the cleavage of material molecules to occur and effectively inhibits the degree of thermal cleavage of MVQ.Model cleavage of the main small molecule products at each temperature,the rate and amount of product generation gradually increases with temperature,but methane,acetylene and other products will improve early decomposition at high temperatures.A large number of methyl and hydrogen radicals are shed from the MVQ side chain at the beginning of the MVQ thermal cleavage reaction,and various small molecule alkane,olefin and alkyne products are produced.It can be seen that under the action of high temperature,the main chain is continuously cleaved,a large number of chain siloxanes are cross-linked,the cross-link density increases and the material becomes brittle.Small molecules of gas gather and diffuse,further breaking the MVQ molecules and eventually leading to material insulation failure.