Microwave-Assisted Single-Stage Carbonization of PAN-Based Carbon Fiber

Hung-Chun Hsu^1*, Sheng-Chin Juang¹, Hsien-Wen Chao¹, Tsun-Hsu Chang¹

¹Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Hung-Chun Hsu, email:z11452@yahoo.com.tw

Conventional carbonization of polyacrylonitrile (PAN)-based carbon fibers typically employs multi-stage heating to prevent pore formation—first performing a low-temperature step (~700 °C) to release volatiles, followed by a higher-temperature treatment (up to ~1500 °C) to develop ordered carbon structures. Although effective, this approach increases energy consumption and process complexity.

This study investigates microwave-assisted single-stage carbonization as an alternative that maintains fiber quality while simplifying the process. A penetrating microwave reactor capable of operation up to 1300 °C under nitrogen atmosphere was developed. All samples were processed through a single-step, isothermal heating procedure, without staged temperature transitions. Stabilized PAN fibers supplied by Formosa Plastics were carbonized at fixed temperatures ranging from 600 °C to 1300 °C, with holding times of 5 minutes and 3 minutes for comparison. Elemental analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), and density measurements were performed to examine the evolution of chemical composition, crystallinity, and morphology.

Microwave heating differs fundamentally from conventional thermal conduction: it provides volumetric and selective heating, enabling uniform temperature distribution throughout the fiber bundle. This homogeneous internal heating promotes smoother gas evolution and prevents internal pressure buildup, thus avoiding pore formation even in a single-stage process. Moreover, localized electromagnetic fields accelerate dehydrogenation and denitrogenation reactions, facilitating the formation of graphite-like structures at relatively lower temperatures.

The fiber sample carbonized at 900 °C for 5 minutes achieved carbon content, density, and crystallite size (Lc) comparable to conventionally two-stage carbonized fibers, without visible pore formation. These results demonstrate that microwave-assisted single-stage carbonization can achieve efficient and uniform chemical reactions through a simplified, energy-efficient process, offering a promising route toward sustainable carbon fiber manufacturing.

Keywords: Microwave, Carbonization, Polyacrylonitrile, Single-stage process, Volumetric heating