Study On Insitu Growing Carbon Nanofibers/Nanotubes On The Carbon Fibers By CVD And Related Application In C/C Composites

Carbon fiber reinforced carbon matrix(C/C) composites have been widely used in many fields in the past 30 years for their excellent properties. But in traditional C/C composites, the microstructure of pyrocarbon can't be easily controlled, and the tiny cracks often appear between pyrocarbon and carbon fibers, which will cut down the properties of C/C composites. Because carbon fibers are main part of C/C composites, so their structure and properties directly affect the properties of C/C composites. In order to strengthen the adhesion of pyrocarbon and carbon fibers, change the microstrature of pyrocarbon and improve the thermal conductivity of C/C composites, in situ catalytic grown carbon nanofibers/nanotubes (CNFs/CNTs) were explored to change the properties of carbon fibers, and act as a bridge to secondly reinforce C/C composites.In this work, first nano-Ni catalyst particles were coated on the surfaces of carbon fibers by KOH-impregnation-reduction method. Then CNFs/CNTs were in situ grown by catalytic pyrogenation. Finally carbon nanofibers(nanotubes)/carbon/carbon composites(CNFs(CNTs)/C/C) were fabricated by CVI technology. The effects of different catalysts,concentration of Ni(NO₃)₂ solution,reduction temperature and time,reaction temperature and time in CVTK carbon source,CO/H₂ ratio,sulfur content were investigated and discussed. The growth mechanism of CNFs/CNTs was also researched. Then the effects of CNFs/CNTs on the microstructure, thermal conductivity, graphitization degree, apparent dentisy and prosity of composites were primarily studied. And the main results have been obtained:(1) After treated by HNO₃ and KOH, the surface morphologies of carbon fibers were changed. The activeness and wettability of carbon fibers had been improved.(2) Among the catalysts of Fe,Co,Ni,Mo and their mixtures, their catalytic mechanisms were different. It was also found that Ni catalyst particles were the best, Co catalyst particles were the better, and the last were bad. (3) In impregnation-reduction process, the concentration of Ni(NO₃)₂ solution,reduction temperature and time had a great influence on the morphology and yield of Ni particles and CNFs/CNTs. The experimental results showed that the suitable Ni particles (40 run) could be produced with Ni(NO₃)₂ solution concentration(0.1 mol/L), reduction temperature (400～450°C)andtime(40⁶0min).(4) In CVD process for growing CNFs/CNTs, the different CNFs/CNTs were produced with the changing of the CO/H₂ ratio,reaction temperature and time. The optimized condition for CNFs/CNTs were achieved: reaction temperature at 700～750°C, reaction time at 30～40 min, the CO/H₂ ratio at about 1: 4. Sulfur added to the starting materials was acrucial factor for growing CNFs/CNTs by CVD method. This work showed that only the optimal amount of sulfur additive (thiophene) could produce filamentous carbon product.(5) The pyrogenation process of CO was simpler than that of CH4, so it was easily controlled, and the grown CNFs/CNTs were better than that of CH₄.(6) The growth mechanism of in situ grown CNFs/CNTs by CVD was top-model. The growth process accorded with dissolution-diffusion -deposition. The growth driving force of CNFs/CNTs was from the grads of temperature and carbon concentration between the front and back-end of catalysts particles.(7) The in situ grown CNFs/CNTs could strengthen the adhesion of pyrocarbon and carbon fibers, reduce the appearance of crack, and conduce to formation of high-textured rough laminar(RL). So the apparent density and graphitication degree of CNFs(CNTs)/C/C compostites increased, but the porosity reduced, compared with traditional C/C compostites under the same experimental conditions. Especially the longitudinal thermal conductivity of CNFs(CNTs)/C/C compostites was generally 2～5 times that of traditional C/C composites.