Chemistry Evolution of Carbon Fiber from Mesophase Pitch During Thermal Treatment

Abstract

Carbon fibers are widely used in various industries due to their unique properties, including low mass density, high thermal and electrical conductivity, and excellent mechanical properties. Carbon fibers are mainly made by the controlled pyrolysis of polyacrylonitrile (PAN) precursor. During this process, the precursor material undergoes a series of thermal treatments and leaves behind a carbon-rich fiber. Due to the high cost and low carbon yield of PAN precursor, the development of carbon fibers using mesophase pitch (MP) precursors has emerged. However, because MP carbon fiber's lower strength than PAN precursors, developing MP carbon fiber with improved mechanical performance is crucial for industries with high-strength durability. Therefore, understanding chemical mechanisms is crucial for producing high-performance carbon fibers because it allows precise control over the synthesis process. Therefore, this work aims to the optimal thermal treatment conditions of the MP fiber and analyzes the chemical characteristics of the MP carbon fiber during stabilization, carbonization, and graphitization processes. The thermal behavior of MP was examined by Thermogravimetric analysis (TGA). TGA obtained information on the decomposition point and maximum oxygen uptake temperature to set the spinning and stabilization temperature for actual carbon fiber synthesis. In addition, chemical composition and microstructural change during thermal treatment was studied through X-ray photoelectron spectroscopy (XPS), Electron Probe Micro Analyzer (EPMA), and Raman spectroscopy. Several factors could affect oxygen uptakes, such as the fiber diameter, heating rate, and residence time. If the fiber diameter is reduced to tens of microns, the stabilization temperature and residence time can be reduced to achieve fully cross-linked fibers. Analysis of the D and G peaks suggested that stabilization does not create new graphitic domains but forms bonds between existing domains. Carbonization did not significantly change the defect density or domain size but improved the bonding quality. Laser-induced graphitization (LIG) was conducted to improve the graphitization domains of the fibers further due to the limitations of the tube furnace. The results showed much better improved 2D peak intensity and larger crystallite size and defect-to-defect length after graphitization, indicating the success of LIG.