Rational Design of Carbon Fibers via Mechanistic and Structural Studies

Abstract

Carbon fibers are promising materials for a sustainable society and have been widely used across various application fields. In the case of high mechanical performance group, carbon fibers are utilized as structural components in automotive, aerospace, and sporting goods industries. While high-mechanical-performance carbon fibers dominate over 90% of the market, highly porous activated carbon fibers also exist and are applied in separation, purification, and electrochemistry. In the carbon fiber market, polyacrylonitrile (PAN)-based carbon fibers represent almost 90% of the market size due to their high mechanical properties, scalability, and low processing costs. Nevertheless, there remains room for improving the performance of carbon fibers. To address this challenge, understanding the structure-process- property relationships is essential.

In this dissertation, the fundamental microstructural evolution/development during heat treatments of PAN-based fibers was unveiled in Chapters 2–4. Through various heat treatments, different types of carbon fibers (carbonized fibers, graphitized carbon fibers, and activated carbon fibers) were fabricated.

In Chapter 2, PAN fibers were carbonized in a continuous carbonization oven, and microstructural changes were observed using advanced X-ray analysis (voids and crystallites) and radial heterogeneity analysis. Microstructural analysis revealed correlations carbon crystallites, voids, and tensile modulus. Moreover, radial structure variations showed trends closely correlating with the tensile strength of carbon fibers. As a result, optimal continuous carbonization conditions were identified.

In Chapter 3, carbon fibers were graphitized in a continuous graphitization oven. Structural changes during graphitization were observed through void analysis, crystallite analysis, and mechanical property analysis. At a certain graphitization temperature, tensile strength decreased dramatically while tensile modulus increased linearly with temperature. This trade- off phenomenon was elucidated through microstructural analysis. During graphitization, the high thermal conductivity along the in-plane direction, characteristics of carbon fibers, promoted structural reorganization and merging of crystallites near the fiber surface. This resulted in an accumulation of grain boundaries at the surface layer, which contributed to the observed trade-off reaction characteristics.

In Chapter 4, PAN fibers were stabilized and activated to manufacture activated carbon fibers that resolve the bottlenecks of current sorbents. The engineered ACFs possess an extraordinary micro/mesoporous structure with a surface area exceeding 2,900 m2 g-1 while maintaining mechanical and thermal stability. The resulting fibers demonstrate a superior iodine capture capacity of 2.89 g g-1 and a capture rate of 2.56 g g-1 h-1. Furthermore, a novel oxygen-doping strategy was developed to enhance iodine capture performance beyond conventional methods. Strategic oxygen doping dramatically improves performance, achieving 66% higher capacity (4.41 g g-1) and 91% faster rate (4.89 g g-1 h-1).

The key objectives of this dissertation are to provide a fundamental understanding of structural changes in PAN fibers during various heat treatments and to demonstrate the relationships between structure and properties of PAN-based carbon fibers.