Preparation And Performance Study Of Mesophase Pitch-based Carbon/carbon Composites

This paper is based on the high thermal conductivity of carbon/carbon(C/C)composites in space applications.Mesophase pitch is used as the matrix precursor,and the whole pre-oxidized felt is used as the reinforcement.The chemical vapor deposition(CVD)and the liquid phase impregnation-carbonization are combined to form a dense body.The mechanical,thermal properties and microstructures of C/C composites were analyzed.The influence of different mesophase pitch precursors on the structure and thermal physical properties of C/C composites was studied in order to prepare the comparative high heat-conducting C/C composites.Based on the reason that the influence of the reinforcement is weakened as much as possible,the integral felt with low fiber content is used as a preform.The main results of the study are as follows:1)The type,structure and content of matrix carbon have a great influence on the mechanical and thermal properties of C/C composites.By selecting matrix precursor and densification process,the physicochemical properties of composites can be controlled.The CVD green body with a density of 0.90 g/cm~3 was pitch-impregnated-carbonized and densified.After 5 cycles of atmospheric impregnation-carbonization and 3 cycles of high-pressure impregnation-carbonization,the high temperature coal pitch-based C/C composite density was obtained.It reached 1.70 g/cm~3,flexural strength was 40.3 MPa,and radial thermal conductivity at room temperature was 99 W/(m·K).To reach the same density,the densification cycles of the two mesophase pitch-based C/C composites are reduced by one atmospheric-impregnation carbonization and one high-pressure impregnation-carbonization,in which the mesophase pitch-based C/C material is bent.The strength is 19.6 MPa,the radial thermal conductivity at room temperature is 102 W/(m·K),the bending strength of the naphthalene-based mesophase pitch-based C/C material is 33.2MPa,and the radial thermal conductivity at normal temperature is 147 W/(m·K).Compared with high temperature coal bitumen,under the same condition,the mechanical properties of the naphthalene mesophase pitch based C/C composites are quite good,and can also effectively reduce the densification cycle and greatly increase the thermal conductivity of the material.2)The naphthalene-based mesophase pitch was used as matrix precursor to densify different density CVD green bodies,and C/C composites with different mesophase pitch carbon contents were prepared.With the increase of asphalt carbon content,the bending strength of C/C composites gradually decreased,while the thermal conductivity showed a nonlinear change.The thermal conductivity of C/C composite with density of 0.70 g/cm~3CVD is the highest,and the radial thermal conductivity at normal temperature reaches 166W/(m·K).3)As can be seen from the analysis of the microstructure of C/C composites,the pyrolytic carbon produced by the CVD process is tightly bound to the carbon fibers,and is coated around the carbon fibers to form a“cylindrical”structure;Filled in the periphery of pyrolytic carbon,it is"inlaid"or"streamlined"structure.The analysis shows that bituminous carbon is difficult to enter into the pores inside the fiber bundles,resulting in an increase in the internal interface thermal resistance.Therefore,as the content of bituminous carbon increases,the thermal conductivity of C/C composites does not increase linearly.Pure pitch carbon samples The thermal conductivity is the lowest,while the pyrolytic carbon can fill the pores inside the fiber bundles,but the CVD process is difficult to prepare high density C/C composites.Therefore,in order to obtain a C/C composite material with higher thermal conductivity,a CVD process may be first selected to generate a pyrolytic carbon with a good combination of fibers,and then mesophase pitch carbon may be introduced to fill the pores in the material,thereby regulating the heat.The content ratio of charcoal to bituminous carbon is decomposed to obtain the highest possible thermal conductivity,which gives full play to the superiority of the heat transfer performance of mesophase pitch charcoal and provides a theoretical basis for the space application of mesophase pitch based C/C composites.