Elsevier

Carbon

Volume 39, Issue 2, February 2001, Pages 159-165
Carbon

Synthesis of nanometre silicon carbide whiskers from binary carbonaceous silica aerogels

https://doi.org/10.1016/S0008-6223(00)00020-8Get rights and content

Abstract

A novel synthesis process, based on a carbothermal reduction reaction of binary carbonaceous silica aerogels, has been developed for producing nanometre silicon carbide whiskers. The process uses binary carbonaceous silica aerogels as precursors. The precursors, which are derived from carbonaceous sol and silica sol, are prepared by sol–gel supercritical fluid drying and provide a high contact area of reactants. The homogeneous distribution of silica within carbon and their loose structure result in the formation of purer nanometre silicon carbide whiskers. The precursors and their carbothermal reduction products are investigated by TEM, EDX and XRD analyses. The effect of heat-treatment temperature on the production of SiC whiskers is discussed.

Introduction

Silicon carbide (SiC) whiskers are ceramic materials with high mechanical strength, good thermal and chemical properties. They are used as a reinforcement material in high-strength lightweight metal–matrix composites and damage-tolerant refractory ceramic–matrix composites. β-SiC whiskers are commonly synthesized by chemical vapor deposition (CVD) and carbothermal reduction reaction using various starting materials [1], [2], [3].

The preparation of a high-quality SiC whisker requires a set of optimum synthesis conditions. Firstly, because the carbothermal reduction of SiO2 is a heterogeneous solid-state reaction, the mixing condition of two reactants greatly influences the properties of SiC whiskers. Preiss et al. [4] reported a binary sol mixing method by which the carbonaceous sols and silica sols, which act as the sources of SiO2 and carbon, could be homogeneously stirred. The silicon carbides and the Zr(C,O,N) solid solutions [5] have been produced using binary carbonaceous–silica hydrogels and binary carbonaceous–zirconia gels, respectively. Furthermore, the particle sizes and agglomeration of reactants also strongly influence the reaction conditions of the carbothermal reduction and the properties of products. The sol–gel supercritical fluid drying method has been widely applied to the preparation of ceramic materials and catalysts of inorganic nanoparticles [6], [7]. The supercritical fluid drying of binary carbonaceous–silica sols provides the possibility of high contact area and loose structure between carbon and SiO2.

In the present work, nanosized silicon carbide whiskers have been synthesized by a new precursor preparing process. The first step consists of preparing binary aerogel precursors from carbonaceous sols and silica sols by supercritical drying. This offers intimate contact between reactants and an even homogeneous distribution of silica nanoparticles among carbon nanoparticles. The second step involves the formation of SiC whiskers by carbothermal reduction reaction under argon in the range of 1000–1500°C. The binary aerogel precursors and the SiC whiskers are characterized by TEM, EDX and XRD analyses.

Section snippets

Preparation of binary carbonaceous silica aerogel

Tetraethoxysilane (TEOS) and ethanol were AR grade. TEOS (111 ml) mixed with ethanol in a 500-ml ground long-necked flat-bottomed flask served as stock solution (TEOS=1.0 M), and was sealed to avoid hydrolysis. One hundred millilitres of stock solution were added to a conical flask in which 10 ml of water were added previously. The whole liquor was adjusted to pH=8 using ammonia liquor and stirred with a magnetic stirrer at room temperature for 2.5 h to form silica sol. The silica sol was

The carbothermal chemistry of binary aerogel

SiC whiskers are synthesized by the carbothermal reduction reaction of SiO2 according to the overall reactionSiO2+3C→SiC+CO

Reaction (1) seems to involve a multiple-step process. The first step begins with the reduction of SiO2 by carbon to form SiO gas according toSiO2+C→SiO+CO

Once carbon monoxide (CO) is formed, SiO can also be produced according to reaction (3)SiO2+CO→SiO +CO2

Any CO2 produced will be consumed immediately by the surrounding carbon particles to form CO gasCO2+C→2CO

The gaseous

Conclusions

The XRD profiles of binary aerogel and its heat-treated products show that carbothermal reduction reaction begins at about 1200°C. The binary carbonaceous–silica aerogel precursor prepared by supercritical fluid drying is composed of spheric binary silica–carbon particles over a range of 10 to 60 nm. The silicon carbide whiskers with nanosized diameter and larger L/D ratio can be synthesized by carbothermal reduction reaction of binary aerogel under flowing argon. The XRD profile of produced

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