Research On Mechanical And Thermal Properties Of Carbon Bonded Carbon Fiber Composite

Carbon-bonded carbon fiber composite material(CBCF) is a low density carbon/carbon composite of chopped carbon fibers, which has high porosity, light weight, excellent high temperature resistance and structure stability. It has been widely used as thermal insulation materials of atmosphere vacuum pyrolysis furnace and fuel capsule of heat sources. Phenolic impregnated carbon fiber composite is a novel kind of lightweight ablative thermal protection and thermal-insulated integrated material, which has made CBCF has a wide application in the field of aerospace aircraft.Rayon-based carbon fibers have low thermal conductivity. Here we have prepared the fibrous chopped carbon fiber skeleton by a series of technological processes, including a pressure filtration technique with water-based slurry, curing at low temperature and carbonizing under high temperature. It is a high porous carbon/carbon composite consisted of short carbon fibers which are bonded by pyrolysis carbon. The prepared composite has a homogeneous microstructure without agglomerations and layering and the carbon fibers maintain its original microstructure even after high temperature treatment. Due to the pressure filtration technique, carbon fibers are oriented preferentially in x-y plane, making it shows anisotropic in two directions. Fibers in the vertical direction of pressure form layered structure and distribute randomly in parallel to pressure direction. This distributing characteristic determines the anisotropy of mechanical and thermo physical properties of the CBCF composite.The compression performance of CBCF presents an anisotropic feature. With compression load parallel to the pressing direction, the compression test shows a pseudoplastic deformation, while the transverse compression test shows similar characteristics with plastic deformation in the other direction. The parallel compressive strength S and elastic modulus E decrease with the increase of fiber length, while increase with the rising of material density. And the higher the density is, the more sensitive of these two parameters to the density changes. The yield strength and energy density of the material are not affected by the fiber length, while increase with the rise of material density in transverse compression process.Heat conduction analysis of the composite material is examined and simulated by the 2D and 3D model to show its anisotropy characteristics. Two dimensional models are established of CBCF composites with different fiber orientations and densities through a random generation-growth method. Thermal conductivity has been calculated by lattice Boltzmann method. When the fiber orientation angle is 90°, all the fibers distribute randomly which makes the thermal conductivity of materials maintain isotropic characteristics. With the limit value of the fiber orientation angle decreased, the thermal conductivity of the material increases along the temperature gradient direction and decreases perpendicular to the temperature gradient direction, showing an increasing anisotropy characteristics. Furthermore, the thermal conductivity of material has an approximate linear growth trend with the fiber volume fraction. According to the fiber orientation tensor, the Geo Dict software was adopted to calculate the thermal conductivity of different 3D conduction model, the result shows thermal conductivity is anisotropic and the more closer the material tend to layered distribution, the more significant anisotropy characteristic showes in calculations.The thermal conductivity of the material was measured by laser flash method at room temperature, and the result shows that the CBCF composite has obviously anisotropy characteristics in different directions, which was consistent with the calculation results of 2D and 3D model. However, the thermal conductivity in z direction is about 80% the number of it in x-y direction, while this value is about 54% or even smaller in calculations. Carbonaceous binders promote the heat transfer in x-y direction and inhibit the heat transfer in z direction leading to the anisotropy characteristic not as obvious as it in calculations.