Xuanke Li

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.

The synthesis of single-walled carbon nanotubes over an Al2O3Fe2O3 binary aerogel catalyst

An Al2O3Fe2O3 binary aerogel was used as a catalyst for the synthesis of singlewalled carbon nanotubes. The carbon products were synthesized by catalytic decomposition of methane or a CH4/H2 mixture at 820-960°C for 30 min. The influence of preparation atmosphere on the growth of single-walled carbon nanotubes was investigated. The morphology and the structure of carbon products were investigated by TEM, HRTEM and Raman spectroscopy analyses. The carbon products prepared under a methane atmosphere are mainly amorphous. High-purity individual single walled carbon nanotubes (SWCNTs) and bundles were synthesized under the mixed atmosphere of CH4/H2. The results show that the atmospheres exhibit a close relationship to the yield and structure of carbon products formed.

Supported Catalytic Growth of SWCNTs using the CVD Method

The growth of carbon nanotubes (CNTs) from supported metal catalysts using the CVD method with CH4 as the carbon feedstock was investigated using SEM and TEM. Studies include the influence of the substrate structure, the metal catalyst content and other experimental parameters on the nature of the CNTs produced using calcined aluminium nitrate and delta-alumina nanoparticles (~13nm). The iron catalyst precursors are ferric sulphate and also iron oxide nanoparticles. Using an aberration corrected STEM and a FEGTEM BF imaging has been used to identify symmetries of tubes produced, as well as a TEM-STM tip to measure I-V curves of SWCNTs. It appears the optimum iron precursor and catalyst support for production of SWCNTs is either ferric sulphate or iron oxide nanoparticles supported on deltaalumina nanoparticles.

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.

Structure of ultrafine carbon powders prepared using a carbonaceous gel

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Growth of Carbon Nanotubes from Supported Metal Catalysts

The growth of carbon nanotubes (CNTs) from supported metal catalysts is under investigation using the chemical vapour deposition (CVD) method with CH4 as the carbon feedstock. Studies include the effects of the structure of the support media, metal catalyst content and other experimental parameters on the CNTs produced. The effects of the surface structure on the catalyst particles and the CNTs produced are being investigated using various alumina-based supported iron catalysts. Supported catalysts have been prepared from ferric sulphate and either aluminium nitrate or deltaalumina nanoparticles in order to produce different substrate morphologies. Preliminary TEM studies indicate that under the same experimental conditions Fe supported on alumina nanoparticles produces mostly bundles of DWCNTs, whereas Fe supported on alumina derived from aluminium nitrate produces predominantly SWNCT bundles with some MWCNTs. The effects of catalyst content on the CNT production is also being investigated with Fe content varying between 5% and 30%. Preliminary TEM results show the presence of bundles of SWCNTs for all the Fe contents except 5% Fe; no CNTs have been observed in this sample.

The effects of Al and Si additions on the microstructures of precipitates in low carbon steels containing Sn

The microstructures of low carbon steels with Sn additions were investigated using scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and energy dispersive X-ray spectroscopy. Four steels based on Fe-0.9Nb-0.3Sn-0.05C (wt%) with different levels of Al and Si additions were prepared by arc melting under an argon atmosphere. The effects of heat treatment and the level of alloying elements Al and Si on the precipitation of Sn-rich phases were studied. After ageing at 1150°C and 850°C, NbC precipitates were found in all samples, as well as AlN in the higher Al content steels. The concentration of Al in steel was also found to affect the formation of Sn-rich compounds after heat treatment at 850°C for 96 hours. In the lower Al or Al-free steels, a η-Fe2Nb3 phase, which dissolves a significant amount of Si, was observed. In the higher Al steels, a Fe2Nb-based Laves phase, which dissolves both Si and Sn was detected. A mechanism based on both size factors and thermodynamic considerations is described, which accounts for the experimental observations.