10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Synthesis of Silicon Nanoparticles with Controlled Morphology from Silane Pyrolysis in a Helium Atmosphere and Its Relative Nucleation and Condensation Rates
MIGUEL VAZQUEZ PUFLEAU, Martin Yamane, Pratim Biswas, Elijah Thimsen, Washington University in St. Louis
Abstract Number: 419 Working Group: Aerosol Chemistry
Abstract Chemical nucleation is the first step by which gas phase molecules produce solid nanoparticles via chemical reactions. This phenomenon has tremendous technological and environmental relevance, yet, it is still not well understood. Numerous industrial processes including silicon chemical vapor deposition (SiCVD) and silane fluidized bed reactor (SiH4-FBR) are energy intensive processes that could profit significantly from better understanding the initial stages of silicon nanoparticle formation and quantifying the growth rates. However, many difficulties exist to extract experimental kinetic rates in the initial stages of aerosol formation. One of the main challenges is to temporally resolve the kinetics of nucleation and condensation as they occur in the sub millisecond regime. Nevertheless, since the temporal scale of condensation and nucleation are expected to be comparable, the ratio of the speed of both processes can be used to obtain kinetic information.
In this work, a silane pyrolysis flow reactor was used to obtain insights in the nucleation phenomenon. Primary particle size distribution was obtained from transmission electron microscopy (TEM) and supported on X-Ray diffraction (XRD) spectroscopy. These results were contrasted with aerosol online characterizations by scanning mobility particle sizer (SMPS). Interestingly the agglomerate size given by SMPS was not correlated to the reactor temperature but primary particle size distribution was a strong function of it. After finding a suitable temperature where sintering was negligible, primary particle analysis was used to determine the relative condensation and nucleation rates of silane pyrolysis. The results show a relative rate of silicon condensation over nucleation with a dependence to the order of 1.5 with respect to SiH4 precursor. Similarly, the analysis reveals that the total number of formed nuclei is correlated to the initial silane concentration to the power of 2/3. The methodology described here is proposed as a tool to help elucidate the kinetics and morphology of rapidly evolving aerosols made from other materials in a regime where primary particles can be identified and are independent of residence time in the reactor. Such understanding provides experimental information on the nature of the dominant chemical reaction mechanism and can be applied to help engineer silicon nanoparticles for a whole range of applications that require sharp control of primary particle size such as catalysis, battery anodes and biomaterials.