| The epoxy resin is widely used in the field of electrical equipment insulation casting.However,the thermal conductivity of traditional epoxy resin is only about 0.2 W/(m·K),which is difficult to meet the requirement of timely heat dissipation of equipment.It is of great significance to develop epoxy resin casting materials with high thermal conductivity and high filling to solve the structural heat dissipation problem of electrical equipment.In this dissertation,epoxy resin matrix composites were prepared by the combination of designing low viscosity filler system to achieve high filling and improving thermal conductivity of resin matrix,using ordinary epoxy resin(EP)and liquid crystal epoxy resin(LCEP)as matrix respectively,and alumina(Al2O3)as filler.The effects of matrix type,filler content,coupling agent treatment and other factors on the thermal conductivity and electrical insulation properties of composites were studied,and the microscopic mechanism of thermal conductivity and electrical properties was revealed.It was found that the spherical-like Al2O3 with different particle sizes treated by coupling agent and particle size ratio of 8 to 15 and mass ratio of 3:2 can significantly reduce the viscosity of the composite and achieve high filling.The infrared spectra indicate that chemical bonding occurred between the coupling agent and the surface of micro-nanoparticles.The liquid crystal phase was observed by polarizing microscope during the cooling process of LCEP.After adding the compound anhydride curing agent,the lower limit of temperature range of liquid crystal phase decreased by 30? and the average size of the liquid crystal domain decreased by 1/3.The obvious liquid crystal domain appeared in the cured product of LCEP.Compared with anhydride curing agent,the thermal conductivity of LCEP/curing agent system cured with DDM curing agent is higher.The thermal conductivity of the system decreases when the initial curing temperature is too high.The results indicate that the thermal conductivity of the composites can be improved by filling with larger particle sizes or mixing fillers with different particle sizes.The thermal conductivity of composites increases with the increase of filler content,and increases sharply when the filler content is higher than 60 wt%.The thermal resistance of the interface between micro-nanoparticles and resin matrix was reduced by coupling agent treatment,and the thermal conductivity of the composite was improved obviously by coupling agent treatment at high filling content.EP-A and LCEP-B composites were prepared by filling EP and LCEP with Al2O3 treated by coupling agent respectively.When the filler content is higher than 60 wt%,the thermal conductivity of LCEP-B is obviously higher than that of EP-A.When the filler content is 80 wt%,the thermal conductivity of EP-A is 1.935 W/(m·K),which is 10.75 times that of pure EP,the thermal conductivity of LCEP-B is 2.471 W/(m-K),which is 7.02 times that of pure LCEP.The test results of electrical insulation indicate that the AC breakdown field strength of resin matrix decreases with the doping of micro-nano Al2O3,and decreases more seriously with EP as matrix than with LCEP.When the filler content is 80 wt%,the breakdown field strength of EP-A and LCEP-B decreases by 55.31%and 31.57%respectively compared with the corresponding pure resin matrix.The doping of micro-nano Al2O3 can inhibit the high temperature ablation of insulating materials during the breakdown process.The volume conductivity of resin matrix increases with the doping of micro-nano Al2O3,especially when the filler content is higher than 60 wt%.The volume conductivity of pure LCEP and LCEP-B increases with the increase of temperature,and increases sharply when the temperature is higher than 100?.The dielectric constant and the dielectric loss at high temperature of the resin matrix were significantly increased by micro-nano doping.The dielectric constant of epoxy resin and its composites increases with the increase of temperature.When the temperature is higher than 120?,the dielectric loss of various materials increases sharply with the increase of temperature.Coupling agent treatment enhances the interfacial interaction between micro-nanoparticles and resin matrix,improves the AC breakdown field strength of composites,and reduces the volume conductivity,dielectric constant and dielectric loss of composites. |
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