Theoretical Prediction of the Anisotropic Effective Thermal Conductivity of Composite Materials

The composite is made of a matrix and a fiber-reinforced material to form a non-homogeneous anisotropic material. Thermal behaviour of composite materials is very important in many applications as heat shields and heat guides. The present paper investigates theoretically a composite material made of a silica matrix and a fiber reinforcement made of steel. The steady state effective thermal conductivity in the main directions are calculated theoretically for two extreme thermal assumptions, i.e. parallel isothermal lines and parallel heat flux lines. The effective thermal conductivity of the composite is evaluated for a variable thickness of the reinforcement, i.e. for a variable volume fraction. The anisotropy degree, defined as the ratio between the thermal conductivities along the two main directions, increases with the ratio between the thermal conductivities of the reinforcement material and the matrix. The composite material, made of two homogeneous and isotropic materials, is thermally anisotropic and can be used to drive heat towards colder regions. This phenomenon is very useful when a device, such as a spacecraft, must be thermally protected.