In-situ Analysis of the Chemical Vapor Synthesis of Nanocrystalline Silicon Carbide by Aerosol Mass Spectrometry

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In-situ Analysis of the Chemical Vapor Synthesis of Nanocrystalline Silicon Carbide by Aerosol Mass Spectrometry In-Kyum Lee, Markus Winterer, Horst Hahn, Christian Janzen1, Dirk Lindackers1, Paul Roth1 Darmstadt University of Technology, Institute of Materials Science, Petersenstr. 23, 64287 Darmstadt, Germany 1 Institut of Combustion and Gasdynamics, Gerhard-Mercator University, 47048 Duisburg, Germany Abstract An Aerosol Mass Spectrometer (AMS) is a combination of a Quadrupol- (QMS) and a Particle Mass Spectrometer (PMS) and enables the in-situ analysis of gas phase processes for the generation of nanoparticles. Size distributions of ultrafine silicon carbide particles in the range of 104 atomic mass units (amu) to 107 amu are measured in the PMS. Simultaneously, molecular species up to 300 amu can be detected in the QMS. Aerosols containing nanocrystalline silicon carbide are produced from tetramethylsilane (TMS) by thermal decomposition. In situ process analysis with the AMS as a function of process parameters was performed to elucidate the formation and growth mechanism of SiC nanoparticles. 1. Introduction Powders of small grain size, narrow size distribution, low agglomeration and high purity are required for the production of solid nanocrystalline materials and the exploitation of size effects in applications (Winterer 2001 and multiple references therein). The synthesis of particles from the gas phase is one important technique to produce ultrafine particles with a narrow size distribution and high purity. Chemical Vapor Synthesis (CVS) is a method to generate particles in the size range below 10 nm, narrow size distribution, geometric standard deviation smaller than 1.2 and low degree of agglomeration. Silicon Carbide is an advanced ceramic material for high performance applications. Sintering of SiC to high density is very difficult because of low diffusion coefficient even at high temperatures due to strong covalent bonds. Ultrafine particles of SiC are promising for the production of dense bulk solids due to the small grain size of such powders. Up to now, CVS powders have been analysed only by ex situ methods (Klein 1998). In situ analysis methods of CVS processes have to be compatible with high temperatures, low pressures, high particle number concentration and very small particle size of the order of a few nanometers. One instrument which meets those requirements is called AMS, which has been used for particle size measurements in premixed low pressure flames (Roth 1993, Homann 1987). 2. Methodology Chemical Vapor Synthesis In the Chemical Vapor Synthesis (CVS) process a precursor vapor with carrier gas is delivered into a hot wall reactor. The precursor is thermally decomposed and particles form by coagulation and sintering. Process parameters like temperature, pressure, mass flow, precursor concentration, precursor material and reactor geometry influence the time-temperature history of the particles and determine the particle size and chemical purity of the powder. For the synthesis of SiC the pre