Wenming Guo

Progress and applications of recrystallized SiC and its composites

Recrystallized silicon carbide (RSiC), a high-purity material sintered by using evaporation-condensation without additives, is one of the most important materials in high-temperature structural applications. However, its low density and porous structure, caused by the sintering mechanism in the absence of shrinkage, restrict the engineering applications of RSiC. This paper reviews research and related technologies on the preparation of high-density RSiC, porous RSiC, and RSiC composites. The fluidity of SiC slurries can be improved by electrostatic stabilization or steric stabilization, which has been achieved by modifying the surfaces of fine SiC raw powders with grafted polymers based on silane coupling agents; the resultant slurry had high solid content, and when sintered produced RSiC with a relatively high density (2.75 g/cm3). Using the interconnected pores of RSiC as a modification path, its density has been further increased (2.99 g/cm3) by combining polymer impregnation-pyrolysis of polycarbosilane (PIP, 1400℃) and recrystallization (2400℃). SiC has excellent chemical stability, thermal stability, and mechanical properties, and its sintering mechanism does not produce shrinkage. Because of these advantages, various porous RSiC elements have been manufactured with relative ease. These elements can be gradually substituted for those made from conventional materials, such as Al2O3, cordierite, reaction-bonded SiC, and mullite, in ceramic membranes used for concentrating hot corrosive solutions, filtering diesel particulate, and filtering fine particles from hot gases, as well as in volumetric honeycombs used to collect and convert solar energy. Because SiC is compatible with many metallic elements, RSiC-(inter)metallic composites have been prepared by infiltrating molten liquid through the interconnected pore structure of RSiC. Dense RSiC-MoSi2 composites directly infiltrated at 2050~2100℃ with molten MoSi2 exhibited excellent oxidation resistance, relatively low resistivity (~10-1 W cm) and a ~40% improvement in flexural strength compared to RSiC. The interface between SiC and MoSi2 has been modified by PIP pre-treatment of polycarbosilanes (PCS) (or phenolic resin), which either directly forms a SiC layer (with PCS) or reacts with Si while infiltrating the MoSi2-Si-X (X=Ti, Cr, Al) alloy, further improving strength (171.4 MPa) while decreasing the resistivity (10-3 W cm) and infiltration temperature (1800℃ for MoSi2-Si-X). This modified material can be used as structural material and as a high-temperature electrothermal material, substituting for MoSi2. Using molten infiltration assisted by gas pressure, a RSiC-Al composite with very high thermal conductivity (250 W/(m K)) and low thermal expansion coefficient has also been fabricated, a material which could meet the requirements of electronic packaging materials for high-power electronic components.

Synthesis and growth mechanism of NbC-SiC micro/nanowire by carbothermal reduction

Abstract NbC–SiC micro/nanowires (MNWs) with NbC content varying from 5 to 20 mol.-% were synthesised at 1600–1800°C via carbothermal reduction utilising silica sol, niobium pentoxide powder and carbon black as starting materials. The synthesis process and growth mechanism of NbC–SiC system were investigated. Results show that the morphology of the synthesised products mainly appears as curve shaped microwires or nanowires. The crystalline consists of both SiC and NbC phases which doped with each other by substitution and interstitial reactions in solid solution. NbC–SiC MNWs were developed by vapour–liquid–solid mechanism according to the existence of liquid droplet phase in the tip at reaction temperature. β-SiC twin crystal growing along [112] direction was formed in the stem, and NbC polycrystal was dissociated from Nb–Si liquid phase. The varied concentration of Nb and Si in the Nb–Si liquid phase could be a significant reason for the curved growth of NbC–SiC MNWs.

Synthesis of TiC–TiB2 composite powders via carbothermal reduction and its reaction mechanism

ABSTRACT Titanium carbide–titanium diboride (TiC–TiB2) composite powders were synthesised through a carbothermal reduction method by using titanium dioxide, boric acid, and different carbon sources (namely, carbon black, sucrose, and glucose) as starting materials. The thermal decomposition behaviour of the precursors was studied by thermogravimetry–differential thermal analyser. Phase compositions and morphologies of the synthesised products were characterised by X-ray diffractometer and scanning electron microscope. When n(Ti):n(B):n(C) = 1.0:2.5:5.0, the blended stock mainly formed TiB2 with sucrose or glucose as a carbon source, whereas the stock produced TiC when carbon black was the source. At an optimum reaction temperature, the particles of the powders synthesised from carbon black as a carbon source were the smallest at approximately 100 nm. With increasing amount of boric acid in the precursor, the morphologies of the samples changed into less spherical particles, and more flaky grains and small particles with irregular structures were observed.

Mechanical and electrical properties of MoSi2–RSiC composites via a combination of phenolic resin infiltration–pyrolysis and MoSi2–Si–Ti alloy-activated melting infiltration composite processes

A three-dimensional interpenetrated network structure composite was designed and prepared via a combination of phenolic resin infiltration-pyrolysis and MoSi2–Si–Ti alloy-activated melting infiltration processes to effectively merge the desirable properties of MoSi2 and RSiC. Influence of infiltration temperature on the microstructure, mechanical and electrical properties of the composites was examined. Almost dense MoSi2–RSiC composites with the designed structure were obtained at 1900°C. Formation of the gradient interface modified the interface combination and enhanced the mechanical and electrical properties of the composite. Flexural strength of the composites reached approximately 114.262 MPa (room temperature) and 128.392 MPa (1400°C), respectively, indicating corresponding increases of 37.08 and 35.69% compared with the RSiC matrix. Volume resistivity decreased to 57.63 mΩ cm, nearly five orders of magnitude lower than that of RSiC. Influence of the interpenetrated network structure and interface combination on the electrical conductivity behaviour of the composites was discussed via a modified mixture rule.