Formation of Nanocrystalline SiC Powder from Chlorine-Containing Polycarbosilane Precursors
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surface chemistry, and dispersion uniformity of the SiC particulate all play significant roles in ultimate property determination. Little work regarding SiC particulates with nanometer-sized grains formed from chlorinecontaining polysilane/polycarbosilanes (PS/PCS) has been reported. In previous work,"' the polysilane to polycarbosilane conversion, the conversion of organosilicon compounds into a random SiC network, and crosslinking of molecules has been investigated. SiC derived from polymeric precursors shows inhomogeneities in the nanometer range.8' 9 Stoichiometric silicon carbide can be obtained as a consequence of the chlorine content in poly-(methylchlorosilane) which allows an adjustment of the carbon content in the precursor molecules in contrast to silicon polymers with alkyl or aryl groups."1" 2 In this work, we elaborate upon the structural changes that take place during heat treatment and subsequent transformation from chlorine containing PS/PCS to cubic (P3-) polycrystalline SiC with nanometer-sized grains. EXPERIMENT Chlorine-containing PS/PCS was synthesized by catalytic redistribution of bis(chloromethylsilane)s to yield monosilanes and poly(chloromethylsilane) oligomers. 9 The oligomers were converted to poly(chloromethylsilane) by thermally-induced crosslinking reactions, increasing the molecular weight, accompanied by simultaneous cleavage of trichloromethylsilane 205 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
(monosilane). Styrene was added to the reaction mixture at the redistribution reaction, yielding a copolymer with improved spinning properties. Pre-pyrolysis was performed with ground, solid poly(chloromethylsilane-co-styrene) of about 2-10 g by heating at 2C /min to 700'C and holding for 1 hour under argon. The pre-pyrolyzed sample was an inorganic amorphous network with Si-Si-, Si-C-, Si-Cl-, and Si-CH 2 bonds as identified by XRD, IR, and NMR. The resulting chlorinecontaining PS/PCS sample was fractionated into 200-315 jtm diameter batches which were used as the precursor of nanocrystalline SiC powder. Pyrolysis and crystallization of the precursor were performed on a TA Instruments DTA 2920, at heating rates of 5, 15, 20, 30, and 40'C/min to 1600'C in argon. The transformation processing of nanocrystalline SiC from the precursor was analyzed on a TA Instruments SDT 2960; gas evolution from the SDT was monitored by a FISONS quadrupole mass spectrometer (QMS) and a Bruker FS55 Fourier Transform Infrared (FTIR) Spectrometer. XRD analyses were performed on an Scintag XDS 2000 X-ray diffractometer, with CuKci radiation. Mean grain sizes were determined using the Scherrer and Wilson' 3 equations from the breadth of the O3-SiC [220] and [311 ] diffraction lines, unless specified otherwise. The morphology and chemical characterization of the crystallized SiC sample were carried out on a conventional and analytical Philips CM-200 TEM equipped with EDS. RESULTS Differential Thermal 10 Analysis (Figure 1) shows two exothermic peaks (except at -4 15°C/min where there ar
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