Preparation And Thermal Conductive Performance Of Graphitic Foam

Graphitic foam derived from mesophase pitch is a newly developed, functional porous material which exhibits low density, high porosity and high thermal conductivity. Since its network of interconnected graphite ligaments is high thermal conductive and its open-cell structure is permeable to fluid, it can used as lightweight heat radiator material in aerospace and aviation industry and microelectronics for which the weight should be much concerned. Preparation of mesophase pitch based graphitic foam and analysis of its bulk thermal conductivity have been in this work which include the optimization of process parameters, numerical analysis of effective thermal conductivity of graphitic foam and the depiction of the randomness in graphitic foam's structure and its impact on graphitic foam's thermal properties.Apart form the properties of solid phase, the thermal properties of cellular material is dependent on its micro-structure, such as the size of the cellular structure and its distribution to a large certain extent can not be neglected. As for the graphitic foam produced and analyzed in this work, its micro-structure is largely decided by the process parameters (pressure, temperature and so on) during the foaming and carbonization process. Due to its'complexity and hard to visual observation, pore uniformity is proposed as a parameter to characterize the micro-structure of carbon foam, its micro-structure is analyzed by using the SEM image after the carbonization process. The influence of the parameter pressure, temperature and heating power on the micro-structure of carbon foam is obtained.Although the connection between the thermal properties of porous material and its micro-structure does exist, it is hard to be described clearly. Due to the difficulty and high cost and the lack of generality of experimental outcome in practice for the experimental method and the inaccuracy of the simplified models for the theoretical and semi-empirical methods, numerical method is widely used in the investigation of porous material with the development of computer technologies. Effective thermal conductivity is one of the most important properties of porous material. Semi-empirical analysis has been conducted for the effective thermal conductivity of porous material, but it is usually fit for low porosity material. Therefore numerical method is used by most researchers. In this work, face centered and body centered cubic models have been set up for high porosity material. Based on the models, FEM method was carried out and efficient thermal conductivity is obtained. Compared with semi-empirical results, it is found that shape factor from relevant investigation of others can be used in face centered cubic model when porosity is between 0.75 and 0.90. But the larger deviation exists when it is used in body center cubic model.The properties of homogeneous porous material such as the conductivity are generally expressed as the function of the conductivity of its solid phase and its porosity based on the homogeneous assumption. But except for a few man-made porous materials, the properties of most porous materials deviate far from the homogeneous assumption in production process, especially for graphitic foam through blow method with high temperature and high pressure. The impurity of the raw material, the non-homogeneity of the raw material granule, and the disturbance of process parameter can led to random diversification of its pore structure and result in complex nonlinear properties. So, apart form the conductivity of solid phase and its porosity, the geometrical structure of pores is also one of the influence factors of the thermal conductivity of porous material. For the expression and investigation of the relationship between thermal conductivity and geometrical structure of pores, randomness factor is used to modify the location of pores and the diameter of pores. Models with random geometrical structure were set up by using programming and CAD technologies to simulate the pore structure of non-homogeneous porous material. The influence of pore location randomness and the randomness of pore size on conductivity of porous material was investigated. Analysis was conducted regarding pore uniformity with combination of these two randomness. It is point that the effective thermal conductivity of porous material can be expressed as function of the conductivity of solid phase, macro-porosity and pore uniformity. This can be used as in manufacturing of graphitic foam and the research for non-homogeneous porous material.