Theory Study On The Thermal Conductivity For Carbon/Carbon Composites With Pitch-based Matrix

Carbon/carbon composites are a kind of high thermal conductive materials, which have been successfully used in various aerospace applications such as aircraft disc brakes, rocket re-entry nose tips, and rocket nozzles. Carbon/carbon composites are composed of carbon fibers, matrix, porosities and cracks. Due to the complex structure and many factors affected to thermal conductivity, the measurement of thermal conductivity is limited. It is necessary to develop a model to predict the thermal conductivities according the real structure of carbon/carbon composites. The carbon/carbon composites with pitch-based matrix have been widely applied and have the similar structure. Therefore, this paper develops a model to predict the thermal conductivities of carbon/carbon composites with pitch-based matrix.In this paper, a nano-scale sub-model is proposed to describe the thermal conductivity of the matrix based on its microscopic structure. This model describes the relationship of the thermal conductivity with the sizes and inclination angle of graphite crystallite. Then the sub-model is incorporated into a micro-scale model to predict the equivalent thermal conductivity of the composites by the so-called equivalent circuit approach. Finally, the model is used to predict the thermal conductivities parallel and transverse to the fiber axis for five carbon/carbon composites, and the predictions are compared with experimental data. In addition to non-uniform graphitization degree in the matrix, this model considers the effects of the thermal conductivities of fiber, the volume fraction and diameter of the fiber, the fiber/matrix interface, and pores/cracks. However, some of the parameters cannot be got the precise measurements, so we can only fit the parameters in the given range which reduces the operability of the model.In order to reduce the parameters of fitting, this paper develop a numerical model to predict the carbon/carbon composites with pitch-based matrix. This model not only considers the affecting of the fiber, interface and cracks, especially for the non-uniform graphitization degree of matrix gives a more detailed study. Moreover, the model provides a new method for studying (he thermal conductivities of the two-dimensional and three-dimensional carbon/carbon composites. The results predicted by the present model agree well with those from the experimental measurements. Besides, this paper gives the discussion of the affecting factors of thermal conductivities of carbon/carbon composites. The results show that, in addition to the thermal conductivity of the matrix graphite which nearest the fiber, the change of the non-uniform graphitization degree of matrix is the important factor which affecting the thermal conductivities of carbon/carbon comprises. And the linear relationship is same between the thermal conductivity of the matrix graphite which nearest the fiber and the axis thermal conductivity of the carbon/carbon composites until the change of the model structure. The finding will provide a basis for measure the axis thermal conductivity of the carbon/carbon composites. And the present model is expected to provide a theoretical guideline for structure design of carbon/carbon composites.