Key Engineering Materials Vols. 368-372

Abstract: SiC powder was synthesized by carbothermal reduction method using silica sol and amylum as reactants in closed and open graphite crucibles, respectively. The phase composition and morphology of the powders were characterized. Formation of phase-pure silicon carbide can be achieved at 1700°C for 1h. Increasing reaction temperature and extending holding time can greatly promote the production of SiC. The products synthesized in open graphite crucibles are mainly composed of equiaxial β-SiC particles while those synthesized in closed graphite crucibles are β-SiC particles and nanorods.

Abstract: A new method for producing silicon carbide platelets with low cost and high yield was introduced. The silicon carbide platelets were synthesized by double-heating technique with carbon black and SiO2 powder as raw materials without using any catalysts. The starting mixtures were heated at a temperature in the range of 1800-2000°C for the duration of about 2-4h to produce substantially completely unagglomerated silicon carbide platelets with the thickness of 5-15μm and the average diameter of 50-150μm. Compared to the conventional heating, double-heating technique has different heating mechanism and has advantages of less investment for equipment, easy to manufacture and convenient to operate. Furthermore, it is very suitable for realizing the scaled production because of the lower synthesizing temperature, shorter reaction time and greater output.

Abstract: SiC fibers containing Y and Al were prepared by sintering the amorphous KD-Y fibers at different temperatures from 1200 to 1800 °C. The tensile strength of fibers increases when the sintering temperature increases from 1200 to 1400 °C due to the dispersion strengthening of β-SiC microcrystal and decreases to the lowest point when being sintered at 1600 °C, then rose again with the further increase of temperature to 1800 °C. The tensile strength reached to 2.15 GPa. The oxygen content of KD-Y fibers determined the properties of the sintered SiC fibers and sintering process. Through chemical vapor curing (CVC), the oxygen content was controlled below 7.0 wt% and the tensile strength of KD-Y fibers reached to 3.08 GPa. Further-more, the sintering process of KD-Y fibers was discussed and the grain size of β-SiC grew up with the heat treatment temperature.

Abstract: A new heating apparatus for synthesizing SiC whiskers was introduced. SiC whiskers were synthesized in electric field furnace with carbon black and SiO2 powders as raw materials and some influencing factors were discussed. SiC whiskers with diameter of 0.1-0.4μm, length of 5-70μm and average purity of 99.6% were acquired at lower temperatures of 1200-1400°C for a shorter holding time of 2-4h. The results showed that the high voltage field has great catalysis action on synthetic of SiC whiskers by reducing activation energy and enhancing reaction speed.

Abstract: In this paper, green bodies of RBSC were prepared through the gel-casting process. The effects of monomer concentration and ratio of MBAM to AM on the strength and microstructure of the green bodies were studied. When the monomer content increased from 10wt% to 20wt %( relative to water in the slurry), the flexural strength increases efficiency. However, the flexural strength decreases when the monomer content was above 20wt%. Similarly, when the ratio of MBAM to AM increased from 1/19 to 1/9, it had the same effects on the flexural strength of green bodies. It was found that silicon carbide particles were packaged by the carbon particles with gel. When the monomer content was 20wt% and the ratio of MBAM to AM was 1/9, the green body showed the best homogeneous microstructures and its flexural strength achieved 8MPa.

Abstract: The application and the transpirational behavior of oriented porous SiC as transpiration cooling materials in a simulating test were studied. The test sample was shaped by freeze-casting process and reaction sintered. The testing device was composed by an electric arc combustor and a hydrogen coolant providing system. It was found that the permeability, the amount of coolant required and the uniformity of coolant infiltration were all effected by the microstructure of porous SiC, which furthermore influced the cooling efficiency of the testing system. In this test, the inner face temperature of the combustion chamber was maintained less than 400K throughout the thermal testing period contrasting with the fire temperature up to 3600K by the utilization of oriented porous SiC transpiration cooling materials.

Abstract: Using the coat mix process, porous SiC ceramics are fabricated using commercially available silicon powders and phenolic resin as the starting materials. The phase composition, morphology, pore size and pore size distribution of the obtained products are characterized by X-ray diffraction, scanning electron microscopy and mercury intrusion porosimeter. The results show that high porosity SiC ceramics with a narrow pore size distribution can be fabricated at 1500°C in vacuum by the coat mix process. The open pore porosity can reach up to 60%. The pore size varies in the range of 1-6 'm.

Abstract: Polyacrylonitrile (PAN) based carbon fiber felt which contains abundant various SiC microthreads and some other microstructures was prepared through sintering the pretreated felt at high temperature at low nitrogen pressure. XRD, SEM, TEM, HRTEM analyses for the sintered felt were carried out to study its components and microstructures. There are SiC nanothreads, SiC submicron threads, SiC micron threads and a few SiO2 two- or three-dimensional microstructures (possibly intermix with the similar form of SiC) existed within the inner hollow spaces of the felt. The complex permittivity, complex permeability of the sample in the X-band frequency range were obtained.

Abstract: The effects of deposition temperature on the growth characteristics of CVD SiC coatings were investigated. CVD SiC coatings were made by pyrolysis of methyltrichlorosilane (MTS) in hydrogen at a low pressure of 5kPa. The ratio of MTS to hydrogen was 1/12. The deposition temperatures were varied from 1373K to 1503K. Optical microscope and SEM were used to observe the surface morphology and microstructure of the coatings. XRD was used for characterization of the phase composition. Results indicated that the deposition rate and the surface roughness varied with deposition temperature. At 1373K, the deposited grains were mainly equiaxed with the crystallite size of 22 nm. However, when the deposition temperature was 1503K, the SiC grains were mainly showed faceted columnar structure with the crystallite size of 32 nm. Grain size increased with the increase of deposition temperature.

Abstract: 2D-C/SiC composites with high performance were prepared at temperatures as low as 900 °C. The flexural strength of the composites reached 329.61MPa, the same level as the composites prepared at 1200°C, and shear strength and fracture toughness were 32.14MPa and 14.65MPa·m1/2, respectively. The microstructure and mechanical properties of the composites after heat-treatment at 1600°C were also investigated to determine the potential applications at high temperature.