Preparation And Structure Properties Of Graphene/Polyimide Composite Carbon Fiber

With the development of aerospace technology, electronic equipment of spacecraft thermal control system tends to be compact, multi-functional and operation efficiently, which makes the feature of high power density of the electronic devices more and more obvious. These components can generate large amount of heat which would seriously affect the stability and security of the electronic equipment. In order to meet the needs of heat dissipation of high-power devices, materials with high thermal conductivity, lightweight and good mechanical properties are attracted more and more attention. Polyimide (PI) molecule contains a large amount of planar aromatic heterocyclic structure, and PI graphite films with high thermal conductivity of 1800W·m-1K-1 were obtained after graphitization, which is much higher than the copper with 398W·m-1K-1. So theoretically, PI fibers, the same molecular structure with PI films are also possible to obtain high thermal conductivity of PI based graphite fibers. However, the preparation of PI fibers is very difficult, and mechanical property is far below the theoretical level. This is mainly due to the lower preferred orientation along the fiber axis of PI molecular chains. This is closely related to its spinning solution and heat treatment process During spinning process, the role of drawing in the axial direction induces PAA molecule to preferred orientation, but in the process of heat imidization and carbonization, PI molecule is easy to produce a de-oriented due to thermal motion of molecules. So in this paper, we make a research on this problem in order to obtain high thermal conductivity of PI based graphite fibers.Graphene as new carbon material, is excellent performance enhancing material, due to its high mechanical strength, thermal and electrical conductivity. Moreover, graphene oxide (GO) as its precursor contains a lot of oxygen-containing functional groups on layers and edges, which makes it more easily to disperse in organic solvent. In this paper, GO was doped into the PAA, and during spinning process, lamellae GO promotes the orientation degree of PAA molecular along the axial direction, and inhibits the disorientation of the fibers during imidization and carbonization process. Moreover, GO can promote the process of carbonization and graphitizationas as nuclei crystallites, and thus the target material is obtained.First PAA solution was prepared by polycondensation. The pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA) at a molar ratio of 1.02:1 was added in the N, N1-dimethylacetamide(DMAc) solvent, and the polycondensation was carried out at low temperature and under an inert atmosphere protection, thus the PAA spinning solution with 12% solid content was obtained. GO/PAA spinning solution was prepared by in-situ polymerization process:GO was prepared using the modified Hummers method, and then GO was dispersed in DMAc by sonication. Then ODA and PMDA were added into DMAc in order, and in results, polycondensation was reacted to obtain GO/PAA spinning solution with different contents of GO.PI fibers were prepared by dry-jet wet spinning process, and the processing conditions of how to get high-quality fibers were discussed in this paper. The results shows that the spinning conditions of PAA fibers is that the best spinning temperature is between 25-30 ?, water coagulation bath ratio is Vwater:Vethanol= 9:1, the best solidification time is 3min and drawing rate is 32m/min. Then PAA fibers were thermally imidized treated by 100?,200?, 300? were treated 1h,350? treated 30min, and thus PI fibers with a tensile strength of 1.2GPa and dense structure were obtained.GO/PI composite fibers were by in-situ polymerization method. The effects of the contents of GO on the structure and properties of composite fibers were investigated. By XRD analysis, PI fibers shows semi-crystalline structure, but the composite fibers have a tendency to transform crystalline. By TG and DSC analysis in both oxygen and an inert atmosphere, the results shows that when GO content is 1.0wt%, the thermal characteristics of the composite fibers Td5 and Td10 (respectively the temperature of 5% and 10% mass loss) both increase nearly 40? respectively compared to the pure PI fibers. This illustrates the heat resistance of GO/PI composite fibers. Decomposition temperature of the composite fibers with 1.0wt% GO improved 76? compared to the pure PI fibers. This illustrates the improved thermal stability of the composite fibers. By the analysis of SEM observation and mechanical properties, the tensile strength the composite fiber with 0.3wt% GO increases 8.3%, and the structure of composite fibers is more compact and smooth.G/PI composite carbon fibers were prepared through carbonization and graphitizition. The effects of contents of GO on the structure and properties of composite carbon fibers were tested by XRD, Raman, mechanical properties, conductivity and other analysis. The results show that with the increase of the content of GO and the increase of carbonization temperature, graphitization degrees of the fibers improves and the grain sizes increase. When GO content is 2.0wt%, La and Lc are 13.47nm and 29.19nm respectively, which indicates that graphite structure is more perfect. When the GO content is 0.3wt%, the mechanical properties of composite carbon fibers when carbonized at 1000? increase by 67.7%. The thermal conductivity of composite graphite fibers with 2.0wt% GO is as high as 435.57 W·m-1·K-1, increased by 40% compared to the pure PI based carbon fibers, which presents excellent thermal potential. The orientation degree of GO/PI composite graphite fibers improved 4.6° compared with PI-based graphite fibers. This illustrates that graphene can induce orientation and promote carbonization and graphitization process as graphite nuclei microcrystalline.