Preparation and Characterization of Sic From Alkoxide-Derived Polymeric Precursors
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PREPARATI[ON AND CHARACTERIZATION OF SIC FROM ALKOXIDE-DERIVED POLYMERIC PRECURSORS Zhiping Jiang and Wendell E. Rhine
Ceramics Processing Research Laboratory Massachusetts Institute of Technology, Cambridge, MA 02139
ABSTRACT Crystalline SiC was prepared by carbothermic reduction of alkoxide-derived polymeric precursors under a flow of argon at 1400-1600°C. The pyrolysis products from the precursors with a molecular source of carbon, i.e., (R1)1 (R2)ySiO 1 .5 (Rl=phenyl; R2=propyl, vinyl), showed different morphologies from those of the precursors derived from TEOS and a polymeric source of carbon (polyfurfuryl alcohol). Furthermore, the pyrolysis chemistry of theses precursors and the microstructure of their corresponding pyrolysis products were studied by means of FTIR, TGA, XRD, SEM, BET, elemental analyses, etc. INTRODUCTION SiC can be prepared by the carbothermic reduction of alkoxide-derived polymers. These polymers are usually prepared by hydrolysis of silicon alkoxides or their derivatives. The carbon source for the reduction is either an external polymeric material, such as polyfurfuryl alcohol [1], polyacrylonitrile [2] and phenolic resin [3-4], or an internal organic group attached to Si, such as alkyl, vinyl, allyl and phenyl groups [5-9]. Here, we report the pyrolysis study of alkoxidederived precursors with various carbon sources such as furfuryl alcohol (I), polyphenylpropylsilsesquioxane (II), or polyphenylvinylsilsesquioxane (III). The purpose of this study is to compare the pyrolysis chemistry of these polymers and the effect of carbon source on the morphology of the pyrolysis product. EXPERIMENTAL PROCEDURE Polyphenylpropylsilsesquioxane (phenyl:propyl=7:3) (II) and polyphenylvinylsilsesquioxane (phenyl:vinyl=9:1) (III) were used as-received (Huls Petrarch Systems). Si(OC 2H5 )4 and furfuryl alcohol (FuOH) were purified by distillation. Pyrolyses were carried out in a carbon furnace under argon with a heating rate of 200°C/h. Oxidation or ammonolysis of the ceramic products was performed in either a box furnace under air or a tube furnace under a flow of ammonia. Precursor (I) was prepared by mixing Si(OC 2H5 )4, H20 and furfuryl alcohol in a molar ratio of 1:1.2:0.75 using H2SO4 as the acid catalyst in THF solvent. The resulting solution was refluxed under nitrogen for 2 h, leaving a deep brown solution. The product, after removal of solvent under vacuum at 120'C, was a dark solid which was slightly soluble in hydrocarbon solvents. Based on a similar procedure, precursors with a Si:FuOH ratio of 1:0.85, 1:1 and 1:1.25 were prepared.
Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
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RESULTS AND DISCUSSION Pyrolvses of Precursor (I) It is well known that both TEOS/H 2 0 [10] and FuOH [11] will undergo polymerization under acid catalysis. Furthermore, FTIR studies of the precursor (I) indicated the existence of OC.H 5, SiO-H, and C=C groups in the structure. Therefore, the as-synthesized polymer can be represented as a mixture of SiO,(OC 2H5),(OH)z and polyfurfuryl alc
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