Papers by Author: Liang Hua Xu

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Authors: Ai Jun Gao, Zuo Guang Zhang, Liang Hua Xu
Abstract: The mechanism for the change in the size of the geometry of graphite crystallites in polyacrylonitrile-based carbon fibers heat treated at 1500–2500 °C is proposed. The study indicated that the growth transition temperature of graphite crystallites in carbon fibers is 2000 °C. Above 2000 °C, the interlayer space (d002) decreases slowly and an ordered graphitic structure forms. Below 2000 °C, the variation in the size of the crystallites along the directions of the axis (La) and the radius (La) of the carbon fibers that were heat treated at various temperatures was almost the same and the crystallite form factor, La/La, which is nearly 1. Above 2000 °C, La/La increases rapidly because the anisotropy of the arrangements of the crystallites in the carbon fibers and the geometry of the crystallites change from being square to becoming rectangular. The crystallite thickness (Lc) also increases with an increasing temperature and below 2000 °C the crystallite form factor (La/Lc) increases rapidly. Above 2000 °C, the La/Lc stabilized at 3.1. The different growth models in the different directions are the intrinsic root for the variations in the crystallite geometries that underwent high-temperature heat treatments.
Authors: Yuan Jian Tong, Liang Hua Xu
Abstract: Non-woven hemp fiber mat has been used to reinforce unsaturated polyester to make natural fiber composites. Thermal properties of the hemp fiber mat were investigated to discover the range of heat treatment temperatures suitable for the hemp fiber mat. Loss of weight during heat treatment and absorption of moisture from the environment during storage of the hemp fiber mat were also studied. Both hand lay-up technique and compression molding were used to make hemp mat composites. Due to the low fiber fraction, no significant reinforcing effect was found in the composite made by the hand lay-up technique. The effects of heat treatment of fibers, water content in the fibers, fiber fraction, and manufacture methods on tensile properties of the resulted composites were investigated. Hemp mat composites with tensile strength and modulus comparable to those of [±45°]4 glass fiber reinforced polyester were achieved by compression molding at a molding pressure of 2MPa.
Authors: Na An, Qiang Xu, Liang Hua Xu, Si Zhu Wu
Abstract: The microstructure of the polyacrylonitrile (PAN) precursors can strongly affect the structure of the carbon fibers which determine the final mechanical properties of carbon fibers. The orientation factors ( n f  ) of PAN precursors under different draw ratios were characterized by the birefringence ( n f ), which was obtained through polarized microscopy. The results show that the increasing of the draw ratio may enhance the degree of orientation, which can correspondingly improve the mechanical properties. However the increasing speed of the orientation factor slows down as the draw ratio reached to 8.
Authors: Ai Jun Gao, Zuo Guang Zhang, Liang Hua Xu
Abstract: Pyrolysis mechanism of nitrogen in carbon fibers, heat-treated at various temperatures, was investigated, in relation to the nitrogen chemical bonding states. Nitrogen content shows exponential decreasing with increased temperature and extended time. The pyrolysis of nitrogen exhibited time-temperature equivalence. Nitrogen atoms containing hydrogen in the amorphous region and at the edge of microcrystalline, and the double-bonded nitrogen in pyridine rings are easily pyrolyzed, while nitrogen atoms in the six-member ring of the graphite sheets are difficult to be pyrolyzed.
Authors: Jie Yang, Li Xin Mao, Liang Hua Xu
Abstract: The rheological properties and viscoelasticity of PAN/DMSO solutions were studied by means of revolving rheometer under the scan modes of steady shear rate and dynamic frequency. The effects of shear rate and temperature of PAN/DMSO solution on its Non-Newtonian viscous index, zero shear viscosity, activation energy of viscous flow and modulus of the solutions were studied. It was found that PAN/DMSO solution is a kind of pseudoplastic fluid. And the more temperature of solution, the less Non-Newtonian viscous index at steady experiment curve, the less the loss modulus and storage modulus of solution at dynamics experiment curve. In addition, the liquidity and spinnability of the solution also increased when the temperature went up.
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