Structural development in mesophase pitch-based carbon fibers.

摘要

The development of a high thermal conductivity carbon fiber is dependent on developing a highly ordered graphitic structure within the fiber. This structure includes a high degree of preferred orientation along the fiber axis, large crystallite dimensions, and an interlayer spacing approaching that of single crystal graphite. Without this type of structure, the physical properties (i.e. lattice dependent properties) of the fiber will not approach the theoretical values possible with graphite.; Earlier work has shown that chemical impurities, namely sulfur, in petroleum-derived precursor materials can reverse the graphitization process during high temperature thermal treatment. As sulfur bearing gases are evolved at elevated temperatures, the diffusing molecules distort the two-dimensional crystallites, increasing the basal plane miorientation. Also, the large lattice strains which develop as the gases diffuse through the fiber tend to "crack" the crystallites and increase the interlayer spacings of the crystallites to a more turbostratic structure. As a result, lattice dependent properties such as tensile modulus, electrical resistivity, and thermal conductivity are adversely effected. To overcome these problems, costly thermal treatment processes, must be employed to minimize or reduce the structural damage.; The present research utilizes a chemically pure precursor, synthetically-derived from naphthalene, in an attempt to eliminate the damage caused by sulfur evolution from the petroleum-derived precursors. Gas evolution also occurs during low temperature heat treatment, where substantial quantities of various gas species are evolved from the fiber. These gases, which include CO, CO{dollar}sb2{dollar}, CH{dollar}sb4{dollar}, and H{dollar}sb2{dollar}O, are a result of carbon fiber production and cannot be eliminated. As was the case with high temperature thermal treatment, thermolysis tends to damage the fibers crystal structure. Although the structure is damaged during pyrolysis, the AR fibers crystal structure is able to quickly heal itself during the subsequent high temperature heat treatment. Thus, this research attempts to study the structural changes that occur during thermal treatment and correlates those changes to physical properties. In addition, various structural properties in AR-derived carbon fibers are optimized through modifications in high temperature thermal treatment. Finally, relationships between various structural parameters are introduced, and their influence on lattice dependent properties are also discussed.