Anisotropic thermal and electrical conductivities of individual polyacrylonitrile-based carbon fibers

Understanding the anisotropy of thermal and electrical properties of carbon fibers is important for many applications, but transverse transport properties are difficult to measure. We apply time-domain thermoreflectance (TDTR) mapping to determine spatial variations in the transverse thermal conductivity for different radial positions for two polyacrylonitrile (PAN)-based carbon fibers, the IM7 and AS4 fibers. Single-point TDTR measurements determine the longitudinal and transverse thermal conductivities of the carbon fibers. The transverse and longitudinal electrical resistivities of the AS4 fibers are determined by the van der Pauw method and four-point method, respectively. The transverse thermal conductivity of IM7 fibers is approximately uniform (2.0 W/m-K) at different radial positions. In contrast, the transverse thermal conductivity of the core (3.0 W/m-K) in AS4 fibers is higher than the shell (2.4 W/m-K). The longitudinal thermal conductivity of IM7 fibers is 7.5 W/m-K and the value of AS4 fibers is 6.9 W/m-K. The transverse electrical resistivity of AS4 fibers is 6.3 × 10⁻⁵ Ωm and the longitudinal electrical resistivity is 1.5 × 10⁻⁵ Ωm; the electrical resistivities of IM7 fibers were reported previously. Both the thermal and electrical measurements indicate a larger anisotropy of the IM7 fiber than the AS4 fiber.