Silica-Coated Graphene And Thermally Conductive Epoxy Nanocomposites With Electrically Insulating

With the continuing miniaturization of electronic devices and the increasing power output of electrical equipments, efficient thermal management is imperative for electronic packaging materials. Polymer materials have attracted considerable attention due to their good processability, electrically insulating and low cost. Because of the lower thermal conductivity of polymers, many fillers with high thermal conductivity have been used to improve the thermal conductivity of polymers. However, the improvement effects are usually not as satisfactory as expected due to the interfacial thermal resistance between fillers and matrix, especially for nano-scaled fillers. Therefore, adjusting the interface structure is important for the improvement of thermal conductivity of polymer nanocomposites. Graphene exhibits a superior thermal conductivity, but it is also electrically conductive. In this thesis, an electrically insulating coating was used to reduce the electrical conductivity of graphene sheets and improve the interfacial compatibility between graphene and polymer matrix to benefit the interfacial thermal transfer. Such thermally conductive and electrically insulating nanocomposites are key materials in developing integrated multifunctional structural/electronic systems and now being used in thermal-control and electronic-packaging areas.The main work of this thesis includes:1. Graphene oxide (GO) was reduced and functionalized simultaneously by reacting with3-aminopropyltriethoxysilane (APTES) without the use of conventional reducing agents. Results showed that the functionalization and reduction of GO with APTES was realized via the nucleophilic substitution reaction between the epoxide groups of GO and the amine groups of APTES. By preparing and studying the APTES functionalized graphene (A-graphene) with different reaction temperature and time, we drew that GO could be reduced and functionalized completely by APTES for reacting at75℃for12h.2. SiO2and SiOx were subsequently formed in situ on A-graphene sheets by a sol-gel approach using tetraethyl orthosilicate (TEOS) and APTES as the precursors, respectively. We studied the morphology of the products by the sol-gel process using the organosiloxane with different structures and analyzed the effect of the alkyl chain in APTES. The coating of silica nanoparticles on A-graphene surface by the hydrolysis of TEOS was confirmed by SEM and TEM observations. The A-graphene sheets were closely packed with silica nanoparticles with sizes of10～20nm. However, Si-O-Si network cage structure was obtained by the sol-gel process using APTES in the presence of the alkyl chain.3. A-graphene, silica-coated A-graphene (A-graphene-SiO2) and Si-O-Si wrapped A-graphene (A-graphene-SiOx) were compounded with the epoxy resins (EP). We investigated the influence of silica layer in A-graphene-SiO2on the thermal conductivity and electrically insulating properties. The thermal conductivity of the epoxy nanocomposite with8wt%A-graphene-SiO2was improved by72%in comparison with that of neat epoxy, which was much higher than the thermal conductivities of other nanocomposites with the same content of fillers, while the electrically insulating feature of the nanocomposite was retained. The improved interfacial interaction of A-graphene-SiO2and EP were well reflected in the dynamic mechanical properties of the epoxy nanocomposites. So we concluded that the silica layer not only improved interfacial interaction between the filler and the epoxy matrix, but also reduced the modulus mismatch between nanofillers and polymer matrix.It was a positive factor on interfacial thermal conductance enhancement.