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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]

Carbon nanofibrous nonwoven mats (CNFMs) prepared via electrospinning offer excellent electrical and structural properties and have been used as frameworks for electrodes in electrochemical energy storage devices. However, lack of mechanical strength hinders the broad applications of CNFMs in flexible electronics or industry use. In this work, a rapid stabilization method is developed to prepare high-strength and flexible CNFMs. Studies of the effects of stabilization time on the structures of the stabilized polyacrylonitrile (PAN) nanofibers and the subsequent carbon nanofibers reveal that there is an optimal stabilization time for making high-strength CNFMs. Long stabilization time results in excessive oxidation of the stabilized PAN nanofibers and unwanted defects in the carbon nanofibers. Short stabilization time results in carbon nanofibers with less crystalline structures due to insufficient formation of the thermally stable ladder-like structure. Robust and flexible CNFM with the highest tensile strength of 192.7 MPa is obtained using an optimized total stabilization time of 40 min. To demonstrate the application of the flexible CNFMs, they are fabricated as an electrode framework to load TiO2 nanoparticles without use of organic binders. Lithium ion half-cell based on this electrode demonstrates superior rate cycling performance owning to the porous structure and highly conductive fibrous carbon network of CNFM. [ABSTRACT FROM AUTHOR]