Synthesis of silicon carbide powders from fumed silica powder and phenolic resin
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Silicon carbide powders were synthesized by application of sol-gel processing. Fumed silica powder was used as the starting material for the silicon source, and phenolic resin was used for the carbon source. The effects of mixing ratio and difference between hydrophilic and hydrophobic types of fumed silica powders were investigated. The stirred mixtures of fumed silica powders and phenolic resin became apparent sol states owing to homogeneous distribution. SiC powders were formed derived from both the types of fumed silica powders after the gelation and pyrolysis up to 1800 °C. The hydrophilic silica powder was effective to synthesize more homogeneous SiC powders than the hydrophobic type. Single-phase SiC was obtained in the C/Si (molar ratio after pyrolysis at 1000 °C) range between 1.0 and 2.5, while free carbon was contained in the obtained powders of C/Si > 2.5. In contrast, formation of SiC was insufficient, and SiO2 was retained at ratios of C/Si < 1.0.
I. INTRODUCTION
Silicon carbide (SiC) ceramics are attractive engineering materials for high-temperature application as well as hard materials because of their high mechanical properties and chemical stability at elevated temperatures. Recently SiC ceramics have become more important in semiconductor industries, and hence high-purity SiC is demanded for such valuable applications. SiC ceramics can be synthesized by some kinds of process routes. The carbothermic reduction process of silica by carbon has been most widely used to industrial mass production. However, some additional treatments to remove unreacted silica and carbon are usually required in this process because the carbothermic reduction is a heterogeneous reaction between solid powders. The difficulties caused by the heterogeneous reactions can be mitigated by using fine powders or liquid organic compounds as starting materials. Wei et al. have synthesized SiC powders derived from various kinds of silicon sources (colloidal silica and methyltrimethoxysilane) and carbon sources (sucrose, petroleum pitch and phenolic resin)1. Tanaka and Kurachi have proposed a method to prepare a homogeneous precursor derived from ethylsilicate and phenolic resin.2 Such carbothermic reduction processes of hybrid precursors prepared with sol-gel techniques
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0138 J. Mater. Res., Vol. 21, No. 5, May 2006
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from liquid silicate compounds have been widely investigated,3–6 as well as some kinds of another silicon and carbon sources.7–16 SiC powders that do not contain any residual SiO2 and free carbon can be synthesized by adjusting the blending of starting materials to appropriate ratios in this method. An additional superior quality of such precursor-derived processing is that high-purity silicon carbide ceramics can be obtained by using liquid organic compounds containing much less metallic impurity. On the other hand, fumed silica powders are synthesized by a
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