Structure Design Of Thermal Conductive Fillers And Their Epoxy-based Composites

To meet the demand of high-performance electronic equipment,semiconductor integrated circuit is on the way of super-large scale,high density,high power and ultra-high speed.How to quickly conduct the heat generated by the operation of devices becomes a difficult problem,and the key is to choose appropriate materials to achieve effective thermal management.There has been a long-standing interest in polymers,which possess lightweight,electric insulation and corrosion resistance,to meet the lightweight requirement of electronic devices.However,the low intrinsic thermal conductivity of polymers has limited their applications.To enhance the thermal conductivity of the polymer,the thermal conductive fillers have been incorporated into polymer,and this approach is still an important research direction for the preparation of high thermal conductive polymer materials.In this dissertation,different thermal conductive filler systems consisting of fillers with different sizes and dimensions were designed and prepared to compound with epoxy matrix,optimizing the processing properties and improving the thermal conductivity of epoxy-based composites.Meanwhile,the influences of the structure of filler systems on the morphology and comprehensive properties were investigated.The following issues were studied:1.Binary micron spherical alumina（S-Al₂O₃）with particle diameter ratio of 6 were used as the thermal conductive fillers in the epoxy resin composites.Theoretical maximum packing fraction was achieved by adjusting the volume fraction of small-particle（Xs）in the binary filler system,thereby improving the processing fluidity of the uncured epoxy/binary S-Al₂O₃ mixtures at high loading.At a constant loading of 50 vol%and Xs of 0.2,the lowest viscosity of 21.8 Pa s of the mixture was obtained and the thermal conductivity of the resultant composite was 518%higher than epoxy resin.This result indicated that the contradiction between the processing performance and thermal conductivity of highly filled composites were solved.Furthermore,the epoxy/binary S-Al₂O₃ composites exhibited reduced thermal expansion coefficient,and acceptable adhesion properties,mechanical properties and dielectric properties.2.The simultaneously exfoliated and amino-functionalized boron nitride nanosheets（BNNS-NH₂）were synthesized by urea-assisted ball milling of hexagonal boron nitride（h-BN）.Then,the in-situ reduced silver nanoparticles（AgNPs）with 5-10 nm diameter were deposited on the flat surface of BNNS-NH₂ to obtain AgNPs decorated BNNS.The thermal stability and mechanical properties of epoxy/BNNS@AgNPs composites were significantly improved due to the enhanced dispersibility and interfacial strength of BNNS in/with epoxy resin matrix.Taking advantage of the sintering of AgNPs on BNNS after a thermal annealing process for the composites at 300 ^oC,which could make BNNS connect each other to reduce the contact thermal resistance of fillers,the thermal conductivity of resultant composites was further improved to 1.13 W/mK by increasing the filler loading to 20 wt%.3.A facile and controllable graphene-templated approach was used to synthesize silica coated graphene oxide（GO）（denoted as GO@SiO₂）via a Sol-Gel method of silane in a"water-in-oil（water/benzyl alcohol）"microemulsions taking advantage of the amphiphilic characteristics of GO.This strategy solved the problems of poor controllability,uniform coating and complex separation procedures of traditional methods in ethanol/water cosolvent.The thermal reduced GO@SiO₂（denoted as TRGO@SiO₂）fillers were compounded with epoxy to prepare epoxy/TRGO@SiO₂ composites.The results showed that the SiO₂ layer improved the dispersion of the filler in matrix and made the processing and heat resistance of the composite significantly better than these of the epoxy/TRGO composite with same loading.According to the principle of modulus matching,SiO₂ played the role of modulus transition layer between stiff TRGO and soft epoxy to reduce the interfacial phonon scattering.Thermal conductivity enhancement per 1 wt%TRGO of epoxy/TRGO@SiO₂ composite with5 wt%loading was 7.3-fold higher than that of epoxy/TRGO composites.4.Mesoporous silica coated thermal reduced GO（denoted as TRGO@MSiO₂）was synthesized via the hydrolysis-condensation reaction of tetraethyl orthosilicate（TEOS）on GO surface using cationic surfactant as template and a subsequent thermal reduction treatment.High-density grafting of epoxy-terminated silane coupling agent onto the surface of TRGO@MSiO₂ was carried out to prepare silane coupling agent grated TRGO@MSiO₂（denoted as TRGO@GMSiO₂）,which was used to improve the thermal conductivity of epoxy resin.Owing to the interface interaction and dispersion of TRGO@GMSiO₂ in epoxy resin,which was promoted by the surface modification of TRGO@MSiO₂ using silane coupling agent,the uncured epoxy/TRGO@GMSiO₂ mixture showed good processing performance.The dynamic mechanical properties and thermal stability of epoxy/TRGO@GMSiO₂composites were improved because the interfacial strength between the filler and matrix was enhanced.More importantly,the enhancement of interfacial interaction and SiO₂ modulus transition layer helped to decreasing the interfacial thermal resistance in the composites.Hence,the thermal conductivity of epoxy/TRGO@GMSiO₂ composites was 56.5%higher than that of epoxy resin.