Carbon Fiber: Tensile Properties, Alternative Precursors, and Crystalline Structures

Carbon fiber (CF) has played a significant role in advancing the field of fiber composites, with CF reinforced-composites (CFRC) yielding superior strength to weight ratios over most other structural materials. The market for CF has grown steadily since its commercialization in the 1970s. In recent years, demand has increased for high performance, low-cost CF, particularly for wind energy and transportation applications. The current iteration of CF has several issues that need to be overcome prior to broader adoption of this material. Primarily, these issues are its high cost and lack of sustainability. CF is commercially produced from two major precursor materials, both of which are exclusively derived from fossil fuels. Furthermore, the production route involves an energy-intensive and emissions-heavy pyrolysis process. An improvement in current CF properties, or at least maintaining them, is required while addressing these issues. Understanding the unique processing-structure-property relationships that constitute CF is essential for making further progress with this material.

As a synthetic material, CF is unique as the range of bonding configurations and allotropes that carbon can take on enable a broad range of material properties tailored for specific applications. This dissertation encompasses four novel explorations into the processing, structure, and properties of CF. In Chapter 1, an overview of carbon fiber manufacturing processes, tensile properties, and structure at various length scales is presented. In Chapter 2, the role of carbon fiber misalignment in single-fiber tensile testing is elucidated and discussed. Single-fiber tensile testing is a popular method for CF property determination, and it is of particular importance in the development of novel CF varieties as it requires only small amounts of material. As a result, new best practices for single-fiber tensile testing of carbon fiber are presented. In Chapter 3, PA6 was explored as a novel, low-cost precursor for CF. Microstructural and analytical characterization techniques were employed to gain further insight into fiber morphology, crystallinity, chemical composition, and thermal stability at various stages of processing. As a result, the findings in this chapter provide recommendations for further development of CF derived from PA6. In Chapter 4, the relationships between crystalline parameters and mechanical properties of commercial PAN-based CF were determined and analyzed using wide-angle x-ray diffraction (WAXD). The results presented in this chapter suggest that PAN-based CF contains small turbostratic crystallites, with smaller crystallites having increased misorientation with the fiber axis. An imperfect basal plane structure is implied by these results, indicating the presence of curvature and/or wrinkling of the layers. In Chapter 5, WAXD patterns acquired from a set of heat-treated mesophase pitch-based CF were used to ascertain relationships between crystalline parameters and fiber orientation. Using a combined simulation and experimental approach, an in-depth interpretation of WAXD patterns of CF is presented. In Chapter 6, a summary of the contained work is presented and an outlook for the future of CF is discussed.