Jieying Liang

Effects of deformation-induced orientation on cyclization and oxidation of polyacrylonitrile fibers during stabilization process

Orientation of copolymer polyacrylonitrile (PAN) chains during their deformation prior to stabilization and the further effect on the stabilization were investigated in detail. Results reveal that the orientation of PAN chains presents a saturation point of 69.51% when the deformation ratio reaches approximately 1.07, meanwhile the cyclization rather than the oxidation has a stronger dependence on the orientation of PAN chains during stabilization. The cyclization is facilitated that the cyclization degree is increasing while the activation energy is decreasing obviously as a consequence of the developing orientation of PAN fibers before the saturation point; however, it is restrained during the further deformation of PAN fibers after the point. The resulting carbon fibers obtained from the PAN fibers prepared at the saturation point possess the highest mechanical properties of 4.07 GPa in tensile strength and 249.0 GPa in tensile modulus.

The demonstration of a dense and stable circumferential layer in the subsurface of polyacrylonitrile fibers for carbon fibers

The radial structure of polyacrylonitrile (PAN) copolymer fibers was investigated quantitatively by etching layer by layer in an improved permanganic etchant; meanwhile the effect of the etchant on the fiber surface was taken into consideration. The aggregated structure (crystal size, crystallinity, orientation and density) and thermal stability of each circumferential layer of PAN fibers were determined in detail according to a model proposed in the study. A denser layer with a thickness of about 1 µm was observed in the subsurface (2 µm from the PAN fiber surface), possessing a greater crystal size and crystallinity as well as a relatively higher thermal stability than other layers. This layer was considered to be a barrier for the diffusion of oxygen into PAN fibers during the stabilization and accelerated the formation of a core-shell structure in the resulting carbon fibers.

Effects of γ-Ray Irradiation on the Radial Structure Heterogeneity in Polyacrylonitrile Fibers during Thermal Stabilization

The radial structural heterogeneity of thermally-stabilized polyacrylonitrile (PAN) fiber is considered to be a limiting factor affecting the mechanical properties of the resulting carbon fibers. In this study, we demonstrate that a low-dose (60 kGy) γ-ray irradiation pretreatment can effectively mitigate the radial structural heterogeneity of PAN fibers after thermal stabilization. The characterization results indicate that low-dose γ-ray irradiation only affects the physical structure of PAN through decreasing its crystalline size and crystallinity, rather than inducing chemical cross-linking and/or intramolecular cyclization. It is proposed that an increased amorphous region in PAN fibers prompted by low-dose γ-ray irradiation can facilitate oxygen diffusion from skin to core during stabilization, which results in the increased structural homogeneity of stabilized PAN fibers.

Rapid and Continuous Preparation of Polyacrylonitrile-Based Carbon Fibers with Electron-Beam Irradiation Pretreatment

Thermal stabilization is a critical, yet time- and energy-consuming process during the preparation of PAN-based carbon fibers. In this work, automobile-grade carbon fibers with a 2.85 GPa tensile strength and a 203 GPa modulus are continuously produced with electron-beam (e-beam) irradiation pretreatment and 24 min thermal stabilization. Thermal and structural analyses reveal that e-beam irradiation can lower the onset temperature of the cyclization reaction and mitigate the heat release. Meanwhile, during the process of stabilization, e-beam irradiation can facilitate the evolution of both the chemical structure and the crystalline structure of polyacrylonitrile (PAN) fibers. Comparing to the industrial production of carbon fiber with a 40 min stabilization time, e-beam irradiated PAN fibers can achieve the same degree of stabilization with a 40% time savings.