Microstructural Evaluation of Sintered Nanoscale Sic Powders Prepared by various Processing Routes
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contain equiaxed grains below 500 nm with a fairly uniform grain size distribution. The demonstrated ability of both combustion synthesis methods"1 and preceramic polymers12 to produce nanocrystalline SiC ceramics was expected to enhance the potential for obtaining dense 13-SiC using applied high pressures, EXPERIMENTAL METHODS Powder Preparation
Polycrystalline SiC powders were obtained by self-propagating, high temperature synthesis (SHS) techniques described previously' 3 . Materials obtained using this method were 5 found to be microcrystalline 13-type; cubic structure 3C with strong one dimensional disorder14" Combustion synthesized nanophase silicon carbide powders were prepared in a near-spherical high pressure stainless steel vessel. A central spark was used to ignite accurately prepared gas mixtures of silane and acetylene. A resulting combustion wave propagates throughout the gas mixture, producing H2 and nanophase SiC powder as major products, with acetylene, methane, free C and free Si as possible minor by-products. Previous work showed that pure silicon carbide (uncontaminated with free C or Si) can be prepared over an extremely narrow range of the initial reactant gas pressure ratio or mole fraction". That work experimentally confirmed that the predicted thermodynamic equilibrium lies at the C/Si ratio in the reactant mixture slightly greater than one for initial reactant pressures equal to about 100 kPa. Loss of the excess C occurred via gasification as methane. The SiC powders produced were agglomerates of crystalline particles ranging in size from about 10- 100 nm with bulk BET specific surface areas (N2 adsorption) in the range of 25-75 m2/g. Powders were predominantly crystalline SiC by XRD and chemical analysis (ANSI Method B74.15-1992) with free C and Si contents as low as 0.1 wt%. Powders used in this work were produced from silane/acetylene mixtures near the optimum ratio" and at a range of initial total reactant pressures. Powder exhibiting 8 nm grain sizes were produced using subatmospheric initial pressure (77 m2/g), while the 30 nm grain size powder (34 m2/g) was produced at 100 kPa. It is significant that although precise silane/acetylene chemical stoichiometries were prepared for constant volume combustion, the pressure increased as the combustion wave propagated through the reaction mixture. Hence, the collected ceramic powder products were produced over a range of pressures - which may have influenced the homogeneity of crystallinity, agglomeration or size properties. Samples used in this work were well mixed to be representative of the bulk powder produced. Preceramic polymers were prepared and/or handled in a N 2-filled glovebox or with typical Schlenk techniques. Poly(methylvinylsilane), (PMVS; [(CH 2=CH)Si(CH 3)],), a precursor to Crich SiC, was prepared from suitable chlorosilanes as described previously' 6 . Blends of PMVS with perhydridopoly(silazane) (Tonen's PHPS precursor to Si-rich Si3N 4; [H2 SiNH]n) were used to prepare powders containing nanocrystalline p_-SiC16,1 7. Po
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