| Graphene-based composites have been extensively applied in many fields, due to thier outstanding mechanical strength, superhigh conductivity, excellent light transmittance. However，there are main three problems needed to be resolved: first, how to be large-scale preparation of high-performance graphene is still a critical technical problem; second, the interfacial compatibility between the graphene and matrix is still needed to improve; the third one is that the structural changes of grahene during composite moulding would cause the degradation of the mechanical and physical properties, such as interfacial fractures, aggregates. Especially for polymer matrix, the graphene and graphene oxide with the chemical modification could effectively obtain the good compatibility, however the modification would reduce the conductivity and optical properties of graphene/polymer composites. Therefore, how to solve the above metioned three problems and improve the properties of graphene-based composites is still a hard task.In this subsjuct, the graphene oxide (GO) are prepared by a modified Hummers method. The GO dispersion in N, N-dimethylacetamide solvent are performed by ultrasonic treatment. Then, the reduced graphene oxide (RGO) are prepared by a low temperature thermal reduction at200℃for2hours(h) and solvothermal reduction at180℃for24h. Their structures and morphologies are characterized by XRD, TEM, AFM and Raman spectrum. The sizes and thickness of the graphene oxide could be controlled by the reacted temperature and times.The results demonstrate that solvothermal reduced graphene oxide (ST-RGO) possesses the integrated structure and good sizes distribution, which is adaptable to apply in the polyimide matrix composite as a high performance reinfroced material.Uniform and thickness controllable RGO films with large area were prepared by evaporation-induced self-assembly at a liquid/air interface on glass substrates in combination with low temperature thermal reduction at200℃. In this process, the technical parameters are determined by semi-quantitative analysis. This process has the advantage of good compatibility with flexible and non-flexible substrates. The films are of centimeter scale and their thickness can be controlled. The structural evolution was characterized. The obtained thermal RGO films exhibit excellent optical properties in the visible and near infrared regions, which remain close to70%. Nanoindentation anslysis shows the excellent mechanical properties, e.g. the elastic modulus and film hardness are increased by8.7%and67.6%, which finally possess a high elastic modulus of76.18GPa, and a hardness of6.89GPa.The high-purity stable dispersions of RGO are obtained by solvothermal method. Base on these results, the RGO/polyimide are synthesized by in situ polycondensation of BTDA and ODA. The thermal-resistant, mechanical, conductivity/dielectric and optical absorption properties are studied by TG/DTA, tensile measurements, four point probe method, impedance analysis and transmittacne spectrum. The results demonstrate that the heat-resistant temperature have an increase of40℃, the glass temperature have an increase of10℃, the tensile strength has an enhancement of56%, and the elongation has been increased by19%.The influence of graphene on the carbonization and graphitization of polyimide have been studied. First, the enhancement mechanism of graphene on carbonization process of polyimide films are measured by TG-DSC analysis technique. Furthermore, the structural transformations of RGO/PI composite films with different RGO content are investigated by XRD and FTIR experiments. The results display that the carbonization rate are increased, and carbonized temperature are decreased with the addition of GO content. Due to the2dimensional structure of graphene, the in-plane induced graphitization process have been researched by XRD and Raman spectra. The toughening effect and graphitized ehancement of graphene have been investigated by TEM and SEM micrographs in cross section. These results display that the graphitized degree of polyimide composite with3%RGO content have a great deal of improvement, and grain sizes also have improve with the increase of RGO content. Moreover, since the cost of RGO, the low-cost silicon carbide (SiC) nanoparticles have been used to research the graphitization process. The obvious ehancement of graphitization degree and growth of grain sizes are also observed by XRD, Raman spectrum and TEM. Finally, conductivity of these graphitized films have been measured by four point probes method, which display an obvious increase for PI/SiC composite films with3%SiC content. |
