Preparation, Modification And Phenolic Impregnation Of Fiberform

The rigid carbon fiber preform (Fiberform), a kind of low density carbon/carbon composite material, has been widely used in the aerospace and thermal insulation industry due to its unique properties including low density, low thermal conductivity, low thermal expansion coeffient and high porosity, etc. More typically, Fiberform has been employed as the structure-enhanced matrix for the phenolic impregnate carbon ablator (PICA), which is a new generation of ablative materials developed by NASA.In this thesis, Fiberform with fine microstructure and extremely low density has been prepared via vacuum molding using chopped carbon fiber as raw material, boron phenolic resin powder as the crosslinking agent and polyacrylamide as solution dispersant, followed by drying, curing and carbonization. Fiberform was further modified by graphitization or silicone coating to improve its compressive stress and oxidation resistance. Furthermore, Fiberform and modified Fiberform ware impregnated with thermoplastic phenolic resin solution to prepare different types of PICA. The major conclusions are summarized as follows:(1) Fiberform was prepared via vacuum molding using chopped carbon fiber as raw material, boron phenolic resin powder as the crosslinking agent. The morphology and compressive strength can be optimized and controlled by changing different types of chopped carbon fiber, the concentration of polyacrylamide used, the weight ratio of carbon fiber (CF) and phenolic resin (PF), curing temperature and carbon temperature. The resultant Fiberform has very similar physical properties to that of reported by NASA. The optimum conditions for preparation of Fiberform using pitch carbon fiber P800 were determined as follows:the polyacrylamide solution concentration of 0.2%, the weight ration of CF and PF of 1:1, curing temperature of 140?, carbonization temperature of 900?. The resulting Fiberform has an interconnected carbon fiber network with typical anisotropy structure. When in 10% deformation, Fiberform with the bulk density of 0.178 g/cm3 has longitudinal and transverse compressive stress of 0.339 MPa and 0.794 MPa, respectively. The longitudinal and transverse thermal conductivity were 0.0240 W/(m·K) and 0.1517 W/(m·K), respectively.(2) Fiberform was further modified by graphitization treatment and silicone impregnation. The results show that graphitization could slightly increase the density of Fiberform, but significantly improve the compressive stress up to 10% and oxidation resistance from 600? to 850?. The thermal conductivity of Fiberform is almost no change before and after the graphitization. Silicone modification could slightly increase the density of Fiberform, but significantly improve the compressive stress up to 25% and oxidation resistance (16.2% residue was left after oxidation in 900?). The thermal conductivity of Fiberform is increased by four times after silicone modification.(3) Fiberform and modified Fiberform ware impregnated with different contents of thermal-plastic phenolic resin solution to prepare PICA for thermal protecting applications. The results show that PICA has a fiber-enhanced aerogel-like composite structure consisting of phenolic nanoparticles and carbon fiber framework. The density of PICA could be adjusted by changing the concentration of phenolic resin solution. After the carbonization, aerogel-like composite structure can be well maintained, but at a cost of slightly decreased compressive stress. The silicone modified PICA had a significantly improved compressive property and oxidant resistance, but its thermal conductivity is increased. When in 10% deformation, the PICA with the density of 0.277 g/cm3 had a longitudinal and transverse compressive stress of 0.426 MPa and 1.172 MPa, respectively. Its longitudinal and transverse thermal conductivity were 0.0458 W/(m·K) and 0.0486 W/(m·K), respectively. The as-prepared PICA has very similar mechanical and thermal properties with PICA developed by NASA. When in 10% deformation, the CPDMS-PICA with the density of 0.246 g/cm3 had a longitudinal and transverse compressive stress of 0.893 MPa and 1.255 MPa, respectively. Its longitudinal and transverse thermal conductivity were 0.0605 W/(m·K) and 0.0509 W/(m·K), respectively.