| The research presented in this dissertation represents the first attempt to determine the influence of process parameters on the texture and microstructure and, ultimately the heat transfer capability of mesophase pitch-based carbon fibers. The textures and microstructures of three different experimental mesophase pitch-based ribbon-shaped carbon fibers were studied using the complementary techniques of scanning electron microscopy (SEM), high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM) and wide angle x-ray diffraction (WAXD).; To increase the applicability of the qualitative structural information obtained using the electron microscopy techniques and more accurately relate it to the fibers' heat transfer capabilities and processing parameters, imaging techniques were developed and utilized to quantify the fibers' textures and microstructures.; Using these complementary structural analysis techniques, the effect of mesophase pitch precursor and capillary entry design on the structure and heat transfer capabilities of the ribbon-shaped fibers was investigated. To determine the effect of precursor choice on fiber structure, and in turn, the fiber's heat transfer capabilities, two sets of ribbon fibers melt spun from a rectangular, flat-entry capillary, using a heat-soak petroleum-based mesophase and a synthetic naphthalene-based mesophase, respectively, were compared. Subsequently, to determine the effect of capillary entry design on fiber structure and heat transfer capability, a third set of ribbon fibers, melt spun from a naphthalene-based mesophase using a profiled-entry capillary, was compared to the fibers spun from the same mesophase but using a flat-entry capillary.; It was found that ribbon fibers melt spun from the naphthalene-based mesophase had higher degrees of preferred orientation and graphitization, larger crystallite sizes and lower electrical resistivities, and thus, structures more conducive to higher thermal conductivities than those of fibers melt spun from the heat-soaked mesophase. However, capillary entry design proved to be critical to the development of a more homogeneous and linear transverse fiber structure. This structure allowed for the development of higher degrees of graphitization and larger crystallite sizes within these fibers compared to those of the fibers spun from the flat-entry capillary. Thus, the profiled-entry fibers had structures highly conducive to high thermal conductivity. This fact was confirmed by their low electrical resistivities. |
