Zhijun Dong

Influence of infiltration temperature on the microstructure and oxidation behavior of SiC–ZrC ceramic coating on C/C composites prepared by reactive melt infiltration

SiC–ZrC ceramic coating on C/C composites was prepared by reactive melt infiltration (RMI) using a powder mixture composed of Zr, Si and C as the infiltrator. The phase composition and microstructure of the ceramic coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The oxidation resistance of the as-prepared composites was tested at 1550 °C in static air. The results indicate that the infiltration temperature has remarkable effects on the phase composition and microstructure of the ceramic coating, as well as on the oxidation resistance of the composites. The SiC–ZrC coated C/C composites prepared at 2000 °C exhibit an excellent oxidation resistance. They gain weight about 5.9 wt% after oxidation at 1550 °C in static air for 5 h, whereas the SiC–ZrC coated C/C composites prepared at 1800 °C lose weight about 3.2 wt%. As a comparison, SiC coated C/C composites prepared at 2000 °C by RMI show an inferior oxidation resistance. After 5 h oxidation, SiC coated C/C composites are severely damaged and their weight loss reaches up to 44.3 wt%. The outstanding oxidation resistance of the SiC–ZrC coated C/C composites prepared at 2000 °C can be attributed to the rapid formation of a continuous glass-like layer composed of ZrO2, ZrSiO4 and SiO2, which covers the surface of the composites and retards the oxygen diffusion and the attack on the underlying C/C substrate. For SiC coated C/C composites, the large SiC particles formed on the surface of the composites are difficult to oxidize rapidly and so a continuous and dense SiO2 layer cannot be formed in time to significantly hinder fast oxygen diffusion leading to the consequent severe oxidation of the C/C substrate.

Synthesis of tantalum carbide from multiwall carbon nanotubes in a molten salt medium

Abstract Tantalum carbide (TaC) nanofibers and coatings were synthesized using multiwall carbon nanotubes (MWCNTs) with different structures as templates and the carbon source in a KCl-LiCl molten salt mixture (41.2/58.8 mol/mol). The TaC and MWCNTs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and selected area electron diffraction. Results indicate that the microstructure of the MWCNTs has a distinct influence on the formation of a TaC coating on the MWCNTs. MWCNTs heat-treated at 2 900 °C have a higher crystallinity and are harder to react with Ta to form TaC than those without the heat-treatment. The formation of TaC nanofibers or TaC coatings on MWCNTs is dependent on the molar ratio of tantalum to carbon nanotubes. The morphology of the polycrystalline cubic TaC nanofibers and the TaC coating is similar to that of MWCNTs. The reaction time and temperature have a great influence on the conversion of carbon to TaC and its crystallite size.

Silicon Carbide-Derived Carbon Coated Graphitized MesocarbonMicrobead Composites for Anode of Lithium-Ion Battery

Unique silicon carbide-derived carbon (SiC-CDC) and graphitized mesocarbon microbead (GMCMB) composites (GMCMB@SiC-CDC) were prepared by solid reaction of GMCMB and silicon and following etching reaction with chlorine. The microstructure of SiC-CDC was characterized as mainly amorphous carbon combined with some short and curved sheets of graphite. N2 adsorption/desorption isotherms and pore size distributions proved the composites presented both an enormous amount micropores centered at around 0.5~0.6 nm and a few mesopores (2~50 nm). The GMCMB@SiC-CDC composites showed higher specific surface area and pore volume, especially the microporous surface area and volume. The composites as the anode materials manifested much higher charge specific capacities of 877.6 mAh·g-1 at the first cycle compared with pure GMCMB of 359.6 mAh·g-1, which was probable that Li ions could insert into not only carbon layers but also micropores. The GMCMB@SiC-CDC composites presented better discharge specific capacities at higher charge/discharge rates. The component and electrochemical performances of the composites can be optimized through changing the molar ratio of GMCMB/Si to control the corresponding proportion of SiC-CDC and GMCMB.