TRANSPORT PROPERTIES OF CARBON FIBERS DERIVED FROM PETROLEUM- BASED MESOPHASE PITCH WITH MODIFIED TRANSVERSE MICROSTRUCTURES FOR ENHANCED TENSILE STRENGTH

摘要

Carbon fibers are extensively used in aerospace, defense, and automobile industry due to their lightweight and superior strength. Some of these applications demand high electrical and thermal conductivity, which glass fibers cannot provide because glass is an insulating material (in contrast to conducting carbon). For instance, in spacecraft electronic housing, it is important that carbon fiber reinforced composite be able to dissipate heat. Mesophase pitch-based carbon fibers possess excellent electrical/thermal conductivity due to their high graphitic crystallinity. Also, they can be produced from petroleum-based mesophase pitch, potentially a low-cost precursor. In this study, transport properties of such carbon fibers are reported. Precursor fibers possessing a wide range of micro-texture (radial to circular alignment) of discotic liquid crystalline molecules were produced using novel spinneret design. The design is well-suited for high-volume production of pitch fibers using continuous extruders. After thermo-oxidative stabilization in an air environment at 220°C and atmospheric pressure, fibers were graphitized at 2100°C. The tensile strength of carbon fibers with circular (non-radial) micro-texture was measured at 2.9 ± 0.4 GPa whereas that for radial texture was 2.4 土 0.2 GPa. The higher strength for non-radial texture is a consequence of deflection of crack in the hoop direction along circular layers. Often, such an improvement in strength is accompanied by a reduction in other properties. However, the current results indicate that the longitudinal electrical resistivity of carbon fibers was measured to be about 5 μΩ·m for both types of micro-textures. As estimated from the Issi-Lavin correlation, this corresponds to a thermal conductivity of 300 W/m·K, which is about 1500% greater than that of the best PAN-derived carbon fibers. Thus, micro-textural modification that led to enhanced tensile properties did not deteriorate transport properties. Further, X-ray diffraction revealed that the graphitic doo2 layer spacing was 0.34 nm for both micro-textures, indicating about 20% graphitic crystallinity. Interestingly, the circular (non-radial) micro-texture that helped to enhance tensile strength did not result in reduced graphitic content, so conductivity did not deteriorate. It is hypothesized that the longitudinal texture of the graphitic crystallite remains unchanged even as the transverse texture changes from radial to circular, a phenomenon that will be investigated in future studies.