| As semiconductors/electronic products are developing towards minimization, lightweight and more efficient, it is going to exacerbate the thermal trouble, so it is urgentto take measures to solve the problem of heat dissipation. Thermal interface materials(TIMs) can fill the space between heat source and heat sink in order to expel the airand quicken heat dissipation, which are applied in LED lighting,electromagnetic/electronic information, communications equipment, aviation,automobile, home appliances etc. This paper chosed elastic silicone rubbers as matrix,inorganic nonmetallic oxide, carbon materials and metal as fillers, aimed to product aseries of high thermal conductivity and insulated TIMs with low cost. The thermalconductivity, dielectric properties, mechanical properties, coefficient of thermalexpansion, thermostability and microstructure were measured by laserflash thermalanalyzer, megger, mechanical tests, thermal mechanical analyser, thermogravimetricanalyzer, SEM, FT-IR, etc. Then the effects of fillers, content, morphology, surfacemodification, combined distribution on these properties were discussed detailedly.(1)Low cost platelet alumina and spherical alumina were used to prepared TIMs andexplored the influencing factors at different stages of thermal conductive network. Theexperiment found that thermal conductivity was only with intrinsic thermalconductivity when fillers’ loading below20vol.%. Then the growth rate K becamefaster during the loading from20vol.%to45vol.%, and the sequence was as follows:K(200nm Al2O3)> K(40μm p-Al2O3)> K(40μm s-Al2O3)> K(5μm p-Al2O3)> K(5μms-Al2O3), so we can conclude that platelet Al2O3is easier to form thermal conductivitythan spherical Al2O3. When the loading greater than45vol.%, which be regarded asthreshold according to percolation theory, the thermal conductivity rose sharply.Besides, the maximum loading of spherical Al2O3can reach1000-1200phr.(2) Different sizes of spherical Al2O3were added into rubber in order to find out thebest ratio of composites.The results shown that only the loading greater than the45vol.%, compounding synergistic effect can appear. At fixed65vol.%, the highest thermal conductivity reached2.197W/m·K when40μm and5μm spherical Al2O3at7:3mass mixing ratio. Then65vol.%multi-scale Al2O3were added into silicone rubber toproduce better composites, which can reach3.136W/m·K and5.827W/m·Krespectively without pressure and with120psi pressure, and with electrical insulation,low thermal expansion coefficient and good mechanical properties.(3) Based on above works, nano-Al2O3and BN substituted20vol.%micro-Al2O3inthe best binary size formula, then the thermal conductivity of new composites were2.723W/m·K and3.416W/m·K, respectively. This illustrated that small amounts ofnano-Al2O3or BN could further increase thermal conductivity by decreasing thermalboundary resistance.(4) High thermal conductive and electrical insulating silicone rubber interfacematerials could be produced by adding small amounts of carbon materials into binarysize fillers system. The results showed that the measured volume resistivity ofcomposites filled with3g carbon fiber or graphene or carbon nanotubes(CNTs)reached3.9×1014,7.86×1014,5.43×1014Ω·cm respectively. And the thermalconductivity is2.634,3.118,4.619W/m·K respectively, of which the highest is CNTs.CNTs/Al2O3/SR composites reached higher thermal conductivity (5.032W/m·K) under120psi pressure. In addition, the thermal conductivity and mechanical property bothincreased after surface treatment by Si-69coupling agent.(5) Thermal conductive networks were enhanced by using low melting metals，creatively. Metals with70℃and220℃melting points were used to substitute10vol.%micro-Al2O3in the best binary size formula. The thermal conductivity increasedfrom2.179W/m·K to2.91W/m·K and3.41W/m·K, while the volume resistivitydecreased from9.8×1014Ω·cm to4.2×1014Ω·cm and3.41×1013Ω·cm, respectively. Thismet the criteria of electrical insulator, but had a negative impact on mechanicalproperties. |
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