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Syntactic foams with high strength and light weight are essential for aerospace and deep sea explorations. In this study, high-performance carbon fiber/hollow glass microspheres/epoxy syntactic foams were prepared, and the reinforcing mechanism of carbon fiber is discussed. The syntactic foam with 3 wt% short carbon fiber achieves a uniaxial compressive strength of 141.7 MPa, which is among the highest values reported in the literature. The thermal conductivity of syntactic foam increased by 66.7% with the addition of 3 wt% short carbon fibers, which helped to control the cracking issue caused by the inner temperature surge during the curing process. Further stress characterizations using fiber Bragg gratings (FBGs) confirmed that carbon fiber can relieve the evolution of thermal stress and reduce the residual thermal stress. [ABSTRACT FROM AUTHOR]

Electron-beam irradiation has demonstrated the promising results for tuning the exothermic behavior of polyacrylonitrile (PAN) fibers, and the stabilization time can be shortened accordingly. However, the mechanical properties, especially the tensile strength, of carbon fibers based on irradiated PAN fibers are usually lower than those of standard carbon fibers. In this study, PAN fibers irradiated with an electron beam to 1200 kGy are continuously stabilized and carbonized to produce carbon fibers (i-CFs). The mechanical properties of the i-CFs are optimized by controlling the density of the stabilized fibers, and the best performing i-CF achieves a tensile strength of 2.27 GPa and a modulus of 174 GPa. The structure and properties of the carbon fibers and the precursor fibers are investigated to determine the origin of the low tensile properties of the i-CFs. The results show that the graphite crystalline structures are comparable between the CFs and the i-CFs. However, two distinct peaks at 2020 and 1668 cm−1 emerge in the FT-IR spectra of PAN fibers after irradiation that correspond to the C=C=N (ketenimine) and C=N–N=C conjugation groups. These cross-linking structures may disrupt the cyclization reaction and hinder the formation of the aromatic structure of PAN during stabilization, which bring about defects in the resultant carbon fibers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]