| This study concerns the chemical and structural changes that occur during thermal treatment of mesophase pitch-based carbon fibers. This task was part of a larger project sponsored by the Manufacturing Technology Program of the Department of the Navy aimed at producing high thermal conductivity carbon fibers from a thermally polymerized petroleum residue. The pitch was melt spun into circular- and ribbon cross section fibers, having radial (point) and line-origin textures, respectively. Then, the stabilization and carbonization processes were studied using a combination of dynamic and post-treatment techniques.; As a first step, some fundamental relationships were developed between the fiber crystal structure and lattice-dependent physical properties, such as modulus or thermal conductivity. They were based upon the theory of specific heat capacity and phonon conduction in solids. These relationships provided a useful way of characterizing the quality of graphitic structure developed during thermal treatment and allowed the prediction of lattice-dependent physical properties from a minimum of structural data.; Next, the stabilization and carbonization processes were examined by a series of analytical techniques. Such experiments were designed to clarify the chemical and structural changes that occur, and to use this information to develop thermal treatment processes for the production of high quality carbon fibers. Several significant discoveries were made, but the most important concerned the role of an impurity upon graphite structural development. Large quantities (several mass percent) of sulfur are present in the petroleum-based heat-soaked pitch and cannot be removed from the fiber until temperatures in excess of 1600{dollar}spcirc{dollar}C. There, sulfur removal can result in puffing, misorientation of the graphite basal planes with respect to the fiber axis, and cracking. Proper design of the carbonization schedule, including rapid heating to the maximum temperature, allowed the formation of elongated cracks that aided in sulfur removal, but prevented basal plane misorientation. Improved lattice-dependent physical properties resulted, at the expense of tensile strength. Increasing the time at maximum temperature, however, promoted crack closure and improved tensile strength. |
