Synthesis and Characterization of Nanosized Silicon Carbide
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ABSTRACT Nanosized silicon carbide powders are synthesized by the Chemical Vapor Synthesis (CVS). Grain and particle sizes are measured by XRD and BET. Agglomeration, crystallinity and the chemical composition of the SiC samples are investigated as a function of storage conditions and the CVS process parameters: precursor material, synthesis temperature and reactor length. Grain and particle sizes below 10 nm are observed in all samples. INTRODUCTION Silicon carbide (SIC) is an advanced material with a wide spectrum of technical applications [13]. Nanosized SiC exhibits higher hardness and sinterability at lower temperatures than conventional, micrometer sized powders [4]. However, nanosized SiC powders are also extremely oxidation sensitive[5] due to their large surface to volume ratio. Currently, this limits the use of these powders in ceramic processing and applications. Therefore, it is important to develop methods for scalable synthesis of nanosized, less agglomerated powders with a high purity, and a suitable way of handling and storage. Naturally, gas flow processes are continuous and deliver high quality powders in large quantities and high yields. Nanocrystalline 3-SiC powders can be produced in gas phase reactions by laser-enhanced, plasma-enhanced, and thermal decomposition of volatile compounds containing silicon and carbon. In this paper the influence of synthesis parameters and storage conditions on the powder characteristics are investigated. EXPERIMENTAL The organosilicon precursors hexamethyldisilazane (HMDSZ, (CH 3 ) 3 Si-NH-Si(CH 3 )3 ), hexamethyldisilane (HMDS, (CH 3 )3 Si-Si(CH 3 )3 ) and tetramethylsilane (TMS, Si(CH 3) 4) are thermally decomposed in a hot wall tube reactor (figure 1). The HMDSZ precursor is delivered into the reactor by bubbling helium gas through the liquid precursor held at 423 K. The TMS, HMDS precursors and helium carrier gas are delivered by thermal MKS Instruments mass flow controllers (MFC). The powders are collected on a rotating cylinder cooled with liquid nitrogen from which the powder is scraped off and collected continuously. Crystalline phases of the silicon carbide powders are identified by X-ray powder diffraction with a Siemens D 5000 diffractometer using a CuKcx source. The grain size is estimated from the XRD line broadening of the (220) peak using the Scherrer equation. The specific surface area (S) is measured by nitrogen adsorption according to the BET method using a Quantachrom Autosorb-3B instrument. The particle size is calculated from the specific surface area assuming spherical particles and using the bulk density of SiC. The relative errors of the size measured by XRD and BET are estimated to be 5 % and 1 %, respectively. The value of the total carbon and oxygen content is determined in a LECO CS 344 system by a combustion method for carbon and by hot extraction in a helium gas flow for oxygen, followed by infrared (IR)- spectroscopic detection of the carbon dioxide gas. The relative error of the chemical analysis is 0.3 - 0.6 % for both elements.
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