Characterization on axial thermal conductivity of carbon fiber and its influence factors

Two measurement methods for carbon fiber's axial thermal conductivity are compared, which use fiber bundle and unidirectional fiber reinforced composite samples, respectively. The influence of fiber volume fraction and sample thickness on the measurement was discussed. The applicability of the method based on fiber bundle was investigated by testing polyacrylonitrile-based high-strength and high-modulus carbon fibers, and mesophase pitch-based carbon fiber. The relationship between thermal conductivity and microstructure was further discussed. The results indicate that unidirectional composite shows lower axial thermal diffusivity than fiber bundle sample due to the thermal conduction between fiber and polymer matrix in composite. Accurate thermal conductivity can be obtained by using fiber bundle sample, while unidirectional composite sample yields bigger thermal conductivity. For the measurement with unidirectional composite, the calculated thermal conductivity increases with the increase of fiber volume fraction and sample thickness. On the contrary, fiber bundle samples result in stable thermal conductivity which is not affected by fiber volume fraction. Moreover, the thermal conductivity increases in the order of mesophase pitch-based carbon fiber, high-modulus and high-strength carbon fiber. Lower lattice spacing and larger crystallite size leads to bigger thermal conductivity. These results may contribute to the accurate characterization on axial thermal conductivity of carbon fiber and the structural design of highly thermal conductive fiber composite.