Yanxiang Wang

One step convenient synthesis of crystalline β-Si3N4

Well-crystallized β-Si3N4 was directly prepared at an initial reaction temperature of 150 °C through the reaction of SiCl4 and NaN3 in the presence of a small amount of CCl4. Characterization by X-ray diffraction, high-resolution electron microscopy, X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy indicates that the product synthesized is crystalline β-Si3N4. The yield of β-Si3N4 is about 86% based on the amount of precursor SiCl4 used at the initial reaction temperature of 150 °C, and is more than 90% at the reaction temperature of 200 °C with the product β-Si3N4 in high crystallinity. The dominant morphology of the product is short rods with the growth axis along the [001] direction. The formation mechanism of crystalline β-Si3N4 was discussed briefly, and the role of CCl4 in the formation process of β-Si3N4 was analyzed.

Controllable growth of uniform carbon nanotubes/carbon nanofibers on the surface of carbon fibers

An efficient method to obtain a good and uniform catalyst coating on the surface of carbon fibers was developed by modifying carbon fibers with electrochemical anodic oxidation (EAO), the homogeneous growth of carbon nanotubes (CNTs)/carbon nanofibers (CNFs) was then achieved on the surface of carbon fibers via chemical vapor deposition (CVD). According to the study on the effect of both the catalyst type and concentration on CNT/CNF growth, it was found that when the concentration of catalyst precursor is higher than a critical value, catalytic efficiency decreases apparently with the increase of catalyst concentration regardless of the catalyst type employed. The influence of CVD temperature on tensile strength of CNT/CNF-grafted carbon fibers was also investigated. At low temperatures, such as 500 °C and 550 °C, growing CNTs/CNFs without any degradation of mechanical properties of carbon fibers was successfully achieved, which demonstrated the feasibility of growing CNTs/CNFs directly on carbon fibers. A mathematical model for CNT growth was established to explain the experimental results successfully, which can be used to accurately control the morphology and yield of CNTs/CNFs grown on the surface of carbon fibers. Hence it provides theoretical guidance for the large-scale synthesis processes.

Spinnability of Polyacrylonitrile Gel Dope in the Mixed Solvent of Dimethyl Sulfoxide/Dimethylacetamide and Characterization of the Nascent Fibers

In this work, acrylonitrile copolymers were prepared via precipitation polymerization. The copolymer solutions prepared at various ratio of dimethyl sulfoxide and dimethylacetamide were tested to prepare the nascent fibers by one-step wet-spun method. The effect of temperature, solvent ratio, molecular weight and the solid content on the rheological properties of polyacrylonitrile gel solution in different mixed solvent were studied. It was shown that the viscosity decreased with the increase of the temperature and fluctuated with the different solvent ratio reaching the minimal value at the ratio of dimethyl sulfoxide to dimethylacetamide equal to 1.25. The crystallinity of copolymers and the structure of the nascent fiber surface also depended from the solvent ratio in polymerization. The optimum conditions for spinnability of copolymers were determined. The high-quality polyacrylonitrile precursor was achieved with the controllable range of 0.5–0.8 dtex and the toughness of polyacrylonitrile precursor was greater than 6.0 cN/dtex after the wet spinning process, while the tensile strength of carbon fiber is up to 6.25 GPa after their pre-oxidation and carbonization process.

Synthesis and growth mechanism of carbon nanotubes growing on carbon fiber surfaces with improved tensile strength

An effective approach has been developed for the catalytic decomposition of acetylene (C2H2) by chemical vapor deposition (CVD), to achieve homogeneous growth of carbon nanotubes (CNTs) on the surfaces of carbon fibers. The morphology of CNTs grown on carbon fiber surfaces was observed by a scanning electron microscope and high-resolution transmission electron microscope, which revealed the uniform coverage of CNTs on the carbon fiber surfaces. The single fiber tensile test demonstrated that the tensile strength of carbon fibers could be increased by more than 12% with the catalytic growth of CNTs on their surface. The reparation of the damage caused during the formation of catalyst nanoparticles, and the cross-link of neighboring graphite crystallites induced by CNTs all occurred during the CVD process, which were considered to be the main reasons for improvement. The growth mechanism model of CNTs formation was established based on the thermodynamics principle and the interface microstructure of CNT-grown carbon fiber, illuminating the detailed mechanism for the growth of CNTs and the change of the shape of catalyst particles.

Research on the multi-scale microstructure of polyacrylonitrile precursors prepared by a dry-jet wet spinning process:

An understanding of the properties of polyacrylonitrile (PAN) precursors is an essential precondition for manufacturing high-performance carbon fibres, and the structure of the precursors has a direct and profound effect on the performance of carbon fibres. In this study, PAN precursors, formed in a multistage coagulation bath, were spun by a dry-jet wet spinning process, and the multi-scale microstructure and morphology of the precursors were investigated by separating the fibrils from the precursors. Scanning electron microscopy and high-resolution transmission electron microscopy were employed to examine the surface morphology, cross-sectional morphology and microstructure of the precursors. X-ray diffraction was used to characterize the crystal structure. The micropore sizes of the precursors were determined with nitrogen adsorption experiments; the adsorption increased after ultrasonic etching and decreased with an increase in the treated concentration. All the results demonstrated that the PAN precursors had a multi-scale microstructure, the precursors consisted of fibrils with diameters of 80–200 nm and the fibrils consisted of some microfibrils with diameters of 20–40 nm, including the periodic tissues with thicknesses of 16–30 nm perpendicular to the fibre axis.

Preparation and characterization of t-BN coatings on the surface of carbon fibers

ABSTRACT Turbostratic boron nitride (t-BN) coatings were successfully synthesized on the surface of carbon fibers by dip-coating method. Boric acid and urea were used as boron source and nitrogen source separately and the nitridation process was carried out at 850°C for 4h under the protection of N2. Dipping times were considered as variables to find out optimum amounts. The structure, chemical composition, phase and properties of BN coated carbon fibers were characterized. Results showed that continuous and uniform coatings were prepared after three times dipping and adhered well with carbon fibers. The coatings were mainly composed of the phase of t-BN. The oxidation property and high temperature resistant performance of the BN modified carbon fibers were greatly improved.

Preparing of high-quality carbon film from polyimide films by gradient heat treatment process

A series of carbon-graphite films were prepared via the gradient heating based on polyimide films under nitrogen atmosphere. FTIR spectroscopy, X-ray diffraction, Raman spectrum were used to systemically investigate the evolutions of chemical structure, crystal structure, thermal stability, morphological microstructure and properties of the carbon-graphite films. The study shows that the segment fracture and thermal cross-linking reaction occur first during the low temperature pyrolysis process of PI films, and the thermal cross-linking reaction changes the structure of polyimide. Radicals start restructuring and the thermal condensation becomes sharp with the increase of heat treatment temperature, and the amorphous structure of film is transformed into the ordered graphite structure, at the same time the crystallinity and orientation of the films are improved. When it reaches a certain temperature, the crystal lattice is further improved, and the structure of film can be transformed into polycrystalline graphite structure. With the increase of the temperature, the carbon content of film is gradually increasing, and the surface resistivity is smaller and smaller. It is obvious that once the continuous conductive network of carbon formed, the conductivity of composite sheet films hardly changed.