| Adding high filling ratio(≥70 wt%)Al2O3 to epoxy resin is one of the most direct and effective methods to obtain high thermal conductivity epoxy composite insulation materials.There are several problems that need to be addressed in the casting preparation of high filling ratio Al2O3/epoxy composite insulation materials.Firstly,the density of Al2O3 is much larger than that of the epoxy resin matrix,causing a significant sedimentation phenomenon of Al2O3 fillers during the casting and forming process of the epoxy composite material,it causes uneven distribution of the thermal,electrical and other properties of the epoxy composite material.Secondly,under high filling ratios,the research on the relationship between the particle size of Al2O3 and the properties of Al2O3/epoxy composite materials is relatively scarce.Moreover,at high filling ratios,the viscosity of Al2O3/epoxy composite materials is too high,leading to poor processability.Finally,the compatibility between Al2O3 and epoxy resin is poor.How to improve the compatibility between Al2O3 and matrices at high filling ratios is also a significant concern in the research of Al2O3/epoxy composite materials.In response to the above issues,this article mainly conducted the following three aspects of research.(1)In the preparation of Al2O3/epoxy composite insulation materials,the focus of this study is to investigate how to suppress the settling of Al2O3 fillers and obtain a high thermally conductive Al2O3/epoxy composite insulation material with uniform filler distribution.By adjusting and optimizing the pre-crosslinking and curing processes,the viscosity of the epoxy composite is increased to effectively inhibit the settling of Al2O3 fillers in the epoxy resin.The impact mechanism of the pre-crosslinking and curing processes on the settling of Al2O3 fillers is studied in depth by testing the rheological properties of the composite system.The Al2O3 settling rate of the composite material prepared by the optimal process is only 0.4%,and the thermal conductivity of the composite material is 1.31 W·m-1·K-1.It also has excellent electrical properties,with a dielectric constant of 4.94,volume resistivity of 1.64×1017 Ω·cm,and breakdown strength of 33.4 kV/mm.(2)The effects of factors such as the Al2O3 filling ratio,particle size,and different particle size Al2O3 composite on the viscosity and properties of Al2O3/epoxy composite insulation materials are investigated in this study.It is found that the thermal conductivity of the Al2O3/epoxy composite material increases with the increase of the Al2O3 filling ratio,and the thermal conductivity of the epoxy composite material increases rapidly when the filling ratio exceeds 50 wt%.The breakdown strength and mechanical properties of the Al2O3/epoxy composite material decrease with the increase of the Al2O3 filling ratio.Under the same filling ratio,the thermal conductivity of the epoxy composite material increases with the increase of the Al2O3 particle size,while the insulation performance decreases.In addition,the blending of Al2O3 with different particle sizes can significantly reduce the viscosity of epoxy composites,effectively improving the processability of the composite material,with the lowest viscosity of the composite prepared according to the densest packing formula.A uniform 79 wt%filled Al2O3/epoxy composite material was successfully prepared using the densest packing compound,with a thermal conductivity of 1.46 W·m-1·K-1,a dielectric constant of 6.40,a breakdown strength of 26.5 kV/mm,a volume resistivity of 7.18 ×1015 Ω·cm,and good insulation properties.(3)This study investigates the impact of modifying Al2O3 particles with KH560 silane coupling agent and polydopamine on the properties of Al2O3/epoxy composite insulation materials with a 70 wt%filling ratio.The results show that KH560-modified Al2O3 exhibited improved compatibility with epoxy resin compared to unmodified Al2O3.When the coupling agent dosage was 2 wt%of Al2O3 mass,the modified Al2O3/epoxy composite material demonstrated an increased thermal conductivity of approximately 0.10 W·m-1·K1,a higher breakdown strength of about 1.4 kV/mm,and a decreased dielectric constant of 0.2.Similarly,polydopamine-modified Al2O3 also displayed enhanced compatibility with epoxy resin compared to unmodified Al2O3,resulting in a higher thermal conductivity of around 0.32 W·m-1·K-1,a greater breakdown strength of about 1.1 kV/mm,and a reduced dielectric constant of 0.23,when the self-polymerization coating modification time was one hour.Furthermore,the study revealed that the thermal conductivity of the prepared epoxy composite material was significantly improved.However,prolonged coating modification time resulted in a gradual decrease in the thermal conductivity of the epoxy composite material. |
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