Study On The Structure-design And Properties Of Electrical Insulating Silicone Rubber Composites With High Thermal Conductivity

With the rapid development of electronic,electric and energy industry,the power density of the electronic integrated circuits,electronic devices and electronic equipment increased rapidly,resulting in the more and more serious heat dissipation problems.The heat accumulation will raise the operating temperature of the devices and machines,leading to the performance degradation and lifetime reduction.Due to the technology limitation,the present thermal conductive insulating materials cannot meet the requirement of the development of the electronic and electric industry for the thermal conductivity,flexible and insulation at the same time.Besides,the present methods for preparing these materials are complex,which are not suitable for large scale.Therefore,it is of great importance to prepare the flexible insulating polymer composite materials with high thermal conductivity.In this thesis,the silicone rubber composites with high thermal conductivity and excellent insulating properties were prepared via constructing the three-dimensional(3D)filler network,assembling the aligned filler structure,or fabricating the layered composite structure.In the meantime,the flexibility and other properties of those composites were studied.Moreover,the industrialized application of the silicone rubber composites in the composite insulator was also explored.The contents of the research were as follows:(1)We constructed hybrid three-dimensional(3D)filler networks by simply incorporating a relatively low content of one-dimensional carbon nanotubes(CNTs)and a certain content of two-dimensional boron nitride(BN)in silicone rubber(SR)matrix.Compared to the SR/BN composite with the same BN content,the SR/BN/CNT composites exhibited improved thermal conductivity,slightly increased volume resistivity,and comparable breakdown strength without a largely decreased flexibility.When 0.25 vol.%CNTs were incorporated,the SR/BN/CNT composite exhibited 75%and 25%higher thermal conductivities relative to the pure SR and SR/BN composite with 30 phr BN(B30),respectively.The volume resistivity of the SR/BN/CNT composites was increased by the addition of very small amount of CNTs.and the breakdown strength of SR/BN/CNT composites was?23 kV mm-1,which was comparable to B30.Moreover,as compared to the SR/BN composite,the SR/BN/CNT composites exhibited the higher tensile strength and better flexibility,and the increment in elastic modulus was much lower than that in thermal conductivity.Furthermore,silane coupling agent A-171 was added to strengthen the interfacial interaction between filler and rubber.The thermal conductivity of the composites was increased by 10%at least after modifying with A-171.The composite modified with A-171 also exhibited the higher breakdown strength and lower dielectric loss.(2)The SR/vertically aligned BN composites were prepared by mechanical shearing followed by a two-step vulcanization process,and the through-plane thermal conductivity of the composites was systematically investigated.The through-plane thermal conductivity of SR/ABN150 reached 5.4 W m-1 K-1,which was?6.3 and?33 times higher than those of SR/BN150 and pure SR,respectively.Analyzing by EMA model,the interface heat resistance RBd3 of SR/ABN composites was found to be 2.5×10-8 m2 K W-1,an order of magnitude lower than RBd1,and also an order of magnitude lower than that of SR/BN composites with the same BN loading.In addition,the surface thermal infrared analysis also indicates that the vertically aligned SR/ABN composites possess excellent heat transfer capacity during the heating and cooling processes.Moreover,the SR/ABN composites showed the lower linear thermal expansion coefficient than SR/BN composites,indicating the better resistance to heat deformation.At the meantime,the SR/BN composites showed the excellent electric insulating performance,with the volume resistivity all higher than 1016?·cm.(3)The SR/aligned BN-SR/aligned expanded graphite(EG)composites(SR/ABN-SR/AEG)were prepared by mechanical shearing,respectively.The laminated alternating structure constructed by the thermal conductive SR/AEG layer and the electrical insulating SR/ABN layer was designed.The thermal conductivity in the in-plane direction of the obtained laminated SR/ABN-SR/AEG composites reached 23 W m-1 K-1 since the BN and EG fillers were highly oriented along the in-plane direction.Besides,the in-plane thermal conductivity of SR/ABN-SR/AEG composites increased with the layer increasing.Compared to random SR/BN/EG(SBE)composite and pure SR,the maximum increment of the thermal conductivity of SR/ABN-SR/AEG was?180%and?2 order of magnitudes,respectively.Moreover,the volume resistance of the SR/ABN-SR/AEG composites was?1014 ?·cm,which was?5 order of magnitudes higher than that of SBE composite.(4)The SR composites filled with ATH,irregular silica(IS)and sphere silica(SS)were prepared by mechanical blending,respectively,and the effects of filler type and filler shape on mechanical,electrical and thermal properties of SR/ATH,SR/IS,SR/SS composites were systematically investigated.It showed that the tensile strength of SR/IS and SR/SS composites were up to 6.5 MPa and 6.6 MPa,respectively,which were both nearly 2-folded compared to that of SR/ATH composite.Besides,the breakdown strength of SR/ATH composite was only 18.9 kV mm-1,while those of SR/IS and SR/SS composites were 24.9 and 24.8 kV mm-1,respectively.The SS with spherical shape showed excellent compatibility and processability in the SR matrix.Among the three SR composites,the SR/SS composite showed the highest elongation at break,the lowest elasticity modulus,the best performance on dielectric properties and thermal stability,moreover,the highest thermal conductivity at high temperature.Furthermore,the SR/SS composite exhibited the best arc aging resistance due to the good thermal stability and thermal conductivity at high temperature.