Study On Fabrication Process And Properties Of C/C Composites By Catalytic CVD

Carbon/carbon (C/C) composites are possessed with excellent high temperature property, and widely used in thermal field. Chemical vapor deposition (CVD) is the most common preparation process to fabricate C/C composites, but this process has several disadvantages including long preparation cycle, high cost, energy wasting and so on. With these disadvantages of CVD, this paper has made use of transition metal catalysts and applied improved catalytic chemical vapor deposition technique for fabricating C/C composites. The microstructure and phase composition of catalysts and C/C composites were characterized by XRD、PLM、SEM and TEM analyses, and the bending property and thermal conductivity were investigated. The main contents and conclusion are listed as follows:The surface morphology of carbon fibers and catalysts uniformity of different pretreatment were investigated. The results show that specific surface area and activity are improved after heat treatment and acid treatment. Meanwhile, the grooves and oxygen containing groups are increase. Catalysts are uniformly distributed on carbon fibers which have been treated. On the contrary, the original carbon fibers are smooth, which leads to solution migration and catalysts misdistribution.Catalysts were loaded on the carbon fibers by spraying method, dipping method and homogeneous precipitation method. The catalysts uniformity was stuied. The results show that catalysts loaded by sprsy method, are misdistribution on the carbon fibers. The homogeneous precipitation method is a more ideal way because of uniformity and fine particle for catalysts.C/C composites were deposited from natural gas by catalytic CVD without any carrier gas or diluents gas. The density and pyro-carbon structure of catalysts were discussed. The effect of deposition temperature, gas flow and residence time on density and pyro-carbon microstructure were researched. Experimental results indicate that catalysts can accelerate the C/C composites deposition, change the pyro-carbon microstructure, and conduce to formation of rough laminar (RL). The temperature and gas flow are interactions in the density and composition of C/C composites. The smooth laminar (SL) and low density C/C composites are deposited when temperature and gas flow are low or high at the same time. On the other hand, RL could be got under medium temperature and gas flow. The residence time results shows that medium residence time is good for RL’s growth..By analyzing the process of catalytic CVD, the mechanism is preliminary studied. The catalytic deposition mechanism may consist of two processes. First of all, the CNFs/CNTs are in situ grown by Ni catalytic, and then, the different pyro-carbon is formed based on these CNFs/CNTs which are regard as crystal nucleus.The bending property and thermal conductivity of C/C composites by catalytic CVD were investigated. In the meantime, the influences of density and microstructure on the properties were also discussed. The results show that the higher the density, pore number and size of the smaller of C/C composites, the better bending property and thermal conductivity. The RL C/C composites have stronger bending property and thermal conductivity than SL samples, band and composite structure are in center. The thermal properties of all samples are decrease along with increasing test temperature. Furthermore, the parallel thermal properties of samples are better than perpendicular ones.