Study On The Performance Of High Thermal Conductivity And Insulation Epoxy Resin Composites

Epoxy resin composites are widely used in the fields of UHV power transmission and electronic devices because of their excellent insulation and stable chemical properties.In this paper,a series of epoxy resin composites were prepared by doping Al₂O₃ particles of various sizes and morphologies with epoxy resin（EP）as the matrix.The electrical insulation properties,breakdown properties,dielectric properties and thermal conductivity of EP composites were investigated in the paper.Firstly,nano-spherical Al₂O₃ was sintered at high temperature to obtain Al₂O₃ with dendritic structure（b-Al₂O₃ ）.The b-Al₂O₃ /EP composites were prepared by casting method with EP as the matrix.Then,the Al₂O₃ nanosheets with uniform particle size were synthesized by the molten salt method with sulfate as molten salt,and flake Al₂O₃ /EP composites were prepared by casting.Finally,micron-sized spherical Al₂O₃ was doped to prepare micron Al₂O₃ /EP composites.1.The b-Al₂O₃ with dendritic structure was prepared by high temperature sintering of nano-spherical Al₂O₃ .B-Al₂O₃ /EP composites were prepared by doping b-Al₂O₃ with different mass fractions into the EP matrix.After sintering,the nano-spherical Al₂O₃ is bonded to form b-Al₂O₃ with a dendritic structure,which can keep the structure unchanged after doping into the EP matrix.It forms continuous overlapping network structure inside the composite,which provides an effective pathway for heat transfer.The results show that the thermal conductivity of b-Al₂O₃ /EP composite is 1.13 W·m^-1·K^-1when the doping amount of b-Al₂O₃ is 70wt%,and the thermal conductivity is 7 times higher than that of pure EP.The main reason is that the prepared b-Al₂O₃ has a multi-branched structure,and the branches overlap each other to form a continuous network structure after doping into the matrix,which forms a thermal conductivity channel and effectively reduces the interfacial thermal resistance,resulting in a significant increase in the thermal conductivity of the material.The volume resistivity of EP composites is increased due to the continuous structure that limits the carrier migration process.Compared to pure EP(2.08×10¹⁴Ω·m),the b-Al₂O₃ /EP resistivity increases to 9.64×10¹⁴Ω·m when the mass fraction of b-Al₂O₃ reaches 70%.The maximum breakdown field strength of the composites was increased by 6.62%compared with pure EP at 30 wt%of b-Al₂O₃ addition,and the breakdown field strength of the material was reduced when the content was too high.The dielectric properties were tested and found that the dielectric constant of b-Al₂O₃ /EP composites increased with the increase of b-Al₂O₃ addition and the addition of b-Al₂O₃ all reduced the dielectric loss of the composites to some extent.2.The nanosized Al₂O₃ flakes were prepared by the molten salt method,and were observed by scanning electron microscopy,which revealed that the Al₂O₃ nanosheets had a uniform particle size distribution and a distinct flake structure.The thickness of nanosheets are between 200 and 400 nm.The XRD characterization was performed to show that it was well crystallized.Composites with different Al₂O₃ nanosheet doping amounts were prepared by casting with epoxy resin,and the effects of the addition of flake Al₂O₃ on the thermal conductivity,insulation breakdown and dielectric properties of EP composites were explored.It was shown that the doping of flake Al₂O₃ could significantly improve the thermal conductivity,volume resistivity and breakdown properties of EP composites.When the doping amount of Al₂O₃ nanosheets was 70wt%,the thermal conductivity of EP composites increased by 4.76 times compared with undoped.The breakdown field strength increased from 48.8 k V/mm of pure epoxy resin to 58.5 k V/mm,which improved by 19.88%.Meanwhile,the dielectric constants of the composites and their resistivities both improved significantly with the increase of alumina content.3.EP composites with different micron size spherical Al₂O₃ doping were prepared by casting method using micron size spherical Al₂O₃ as filler and epoxy resin as matrix.The effects of micron spherical Al₂O₃ doping on the thermal conductivity and insulation breakdown properties of EP composites were explored.The results show that the doping of micron spherical Al₂O₃ can effectively improve the thermal conductivity of the composites and the enhancement effect is more significant than the doping of equal amount of nano spherical Al₂O₃ .The thermal conductivity of the material is 1.15W·m^-1·K^-1when the Al₂O₃ is 70wt%.The volume resistivity of the material also increases with the increase of alumina content,when the micron Al₂O₃ is 70wt%,the volume resistivity is 8.34×10¹³Ω·m,which is 2.1 times higher than that of pure epoxy resin.With the increase of micron spherical Al₂O₃ addition,the breakdown field strength of the composites gradually decreases.At 70wt%of micron spherical Al₂O₃ ,the breakdown field strength of the composite is only 35.3 k V/mm,which is significantly lower than that of pure EP（48.9 k V/mm）,which is mainly related to the large number of defects between the micron particles and the epoxy resin matrix.