Effect Of Insitu Carbon Nanotube On The Microstructure And Thermophysical Properties Of C/C Composites

C/C composites have been widely used in many fields for their excellent high-temperature properties. But in traditional C/C composites, the microstructure of pyrocarbon can't be easily controlled, and the tiny concentric cracks often appear between pyrocarbon and carbon fibers, so these drawbacks will cut down the properties of C/C composites. For strengthening the adhesion between pyrocarbon and carbon fibers, improving the microstructure of pyrocarbon and enhancing the thermophysical properties of C/C composites, in situ catalytic grown carbon nanotubes (CNT) were explored to change the surfacial properties of carbon fibers, and acted as a secondly reinforcement in C/C composites. In the present research, differert loading methods of catalyst on to carbon fibers were firstly analysed and discussed. Then CNTs were in situ grown on catalysts, which were loaded by the best loading methods of catalyst, using methane as carbon source. Finally in situ CNT/C/C composites were prepared by traditional chemical vapor infiltration, using natual gas as carbon source. The effects of different CVI procedures,weight percentage of CNT in the carbon felt and initial graphitization degree of CNT on interfacial structure,texture of pyrocarbon and thermophysical properties of C/C composites were investigated in order to get the relationships between them. And some of reinforcement mechanisms that govern the microstructure and thermaophysical properties of CNT were also discussed. The results show that:1) Catalysts prepared by in situ co-precipitation method and homogeneous precipitation method were uniformly distributed on the carbon fibers. And these catalysts with narrow particle size distribution showed high activity in catalytic decomposing of methane. Then three dimensional netlike CNTs with bamboo-like structure were in situ grown on carbon fibers. At the same time, sol-gel method and impregnation were not suitable for catalytic decomposing of methane, because of them showed some defects in the process of preparation or low activity as catalyst.2) Under the CVI condition which mainly formed SL structure, CNT conduced to formation of banded structure mainly consist of RL structure and better interface between the fibers and pyrocarbons. The thermal conductivity of in situ CNT/C/C composites is general 1.8 times that of C/C composites. When under the CVI condition which mainly formed RL structure, both kinds of composites formed RL typed pyrocarbon. However, the thermal conductivity of in situ CNT/C/C composites is 13.0% higher than C/C composites with nearly the same density and mean graphitization degree. But both thermal conductivity and mean graphitization degree of in situ CNT/C/C composites and C/C composites are higher than that of in situ C/C composites prepared under the CVI condition which mainly formed SL structure.3) Catalysts, introduced in C/C composites by CNT, conduced to growth of hemispherical pyrocarbon. Then much more of tiny granular structure was formed on the surface of carbon fibers, which resulted in mean graphitization degree of composites decreasing sharply, so do thermal conductivity.4) There is a interfacial structure changing (RL to ISO) area between carbon fibers and pyrocarbons, which resulted from three dimensional netlike CNTs on the surface of carbon fibers. The interfacial area becoming thicker, mean graphitization degree of in situ CNT/C/C composites increasing slightly, crystallite size and thermal conductivity of in situ CNT/C/C composites decreasing sharply when the weight percentage of CNT in the carbon felt increasing.5) mean graphitization degree of in situ CNT/C/C composites decreasing slightly, crystallite size and thermal conductivity of in situ CNT/C/C composites increasing slightly when the initial graphitization degree of CNT decreasing.6) Thermal diffusivity and thermal conductivity of C/C composites could be stabilized by CNT in the wide range of temperature, which decreased more slower than traditional C/C composites as the temperature rised. And thermal expansion of C/C composites was also stabilized in a small range by CNT from 200℃to 1000℃.