Growth of SiC whiskers by VLS process
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j3 - SiC whiskers were synthesized by the vapor-liquid-solid (VLS) process using Fe catalyst. Whiskers show smooth surfaces and no ramifications. They have uniform diameter (0.5-1 /xm) and lengths between 50 and 300 ^ m . A catalyst droplet was observed on the tip of almost all the whiskers. The transport of iron from the substrate surface to the SiO generators, where growth took place, occurs fundamentally via vapor phase. Fe was deposited over surfaces containing C, and whisker growth was produced where there were Fe droplets of appropriate size and SiO available in great quantity. The need for reaching a threshold size ( 2 - 3 /y-m) of the catalyst droplet before whisker growth is proposed as a possible explanation for the formation of whiskers with uniform size in zones with a high partial pressure of SiO.
I. INTRODUCTION Whiskers are high-purity, single-crystal fibers with strength values approaching theoretical ones. They are widely used as a reinforcing material for composites. In particular, SiC whiskers are useful in enhancing the fracture toughness of ceramic matrix composites. Raw materials for the solid-state formation of SiC are silica and carbon. Most of the commercial whiskers are produced from rice hulls, a waste product of rice milling. In rice hull ashes, silica is present in a high percentage (15 to 20%) and carbon is originated by thermal decomposition of cellulose, also present in rice hulls.1 The vapor-liquid-solid (VLS) process with iron as a catalyst was studied by Bootsma et al.2 These authors studied the system S i O 2 - C - H 2 , and found that the rate of whisker growth was controlled by the rate of supply of gaseous silicon and carbon compounds. They also observed different SiC phases, depending on the substrate used to grow the single crystals. Moreover, they obtained whiskers with fluctuations in their diameters; this was attributed to variations in Si/C ratio in the F e - S i - C alloy droplets, and could be prevented by equilibration of the alloy with solid carbon. Milewski et al? studied the VLS process in order to establish a relationship between the conditions of growth and the morphology of the SiC whiskers formed. They used a graphite substrate coated with catalyst particles (stainless steel), a gaseous mixture of CO, CH 4 , N 2 , and H 2 , and silicon monoxide generators. SiO is formed by the following reaction: SiO 2 (s) + C(s) —
SiO(g) + CO(g)
(1)
Since CO is a product of reaction (1), it is added to the reactor feed gases in order to moderate and control the rate of release of SiO. N-? is added as a diluent J. Mater. Res., Vol. 9, No. 11, Nov 1994 http://journals.cambridge.org
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controlling supersaturation levels and also as stimulant to the VLS reaction. The growth morphologies were related to the degree of supersaturation and stoichiometry of the reactive gases at the temperature studied (1400 °C). A mechanism in which SiO and CH4 react with the catalyst and the crystalline SiC precipitates on the growth substrate, reaction (2), when the liquid
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