(235c) Influence of the Quenching Process on Thermal Plasma Synthesis of Engineered Nanoparticles
AIChE Spring Meeting and Global Congress on Process Safety
Wednesday, April 26, 2006 - 5:10pm to 5:30pm
The present work describes the synthesis of nanoparticles using an inductively coupled plasma (ICP). The advantages of the ICP process over other synthesis routes for non - oxide ceramic and metalic nanoparticles are mainly: a high temperature and energy density of the plasma allowing the evaporation and condensation of high refractory nanoparticles even from solid precursors, the absence of electrodes and solvents results in high purity products, and the possibility to work under a controlled atmosphere, which can be either inert or reactive. The ICP process can use solid, liquid, or gaseous precursors which are vaporised and atomised due to the high plasma temperatures of around 8,000 - 10,000 K. The ICP process is based on an evaporation ? condensation mechanism: cooling of the hot vapour phase and the subsequent condensation results in the formation of nanoparticles. The final particle size distributions, chemical and crystallographic phase compositions of the synthesised nanoparticles, are heavily influenced by the thermal history of the formed particles. The thermal history can be influenced and even controlled by the use of rapid cooling, also called quenching. As a consequence the properties like size and agglomeration, chemical and crystallographic phase composition can be controlled as well. From the several methods to quench a hot gas phase, like there are, rapid expansion, gas mixing, and contact with cold surfaces, the gas mixing has been selected for its flexibility. Metallic nanoparticles, e.g. silicon, and carbides, like tungsten- and silicon carbide, have been synthesised as a function of relevant plasma parameters (gas flow rates, working pressure, plate power, etc.) and quenching parameters (representing gas flow rates and compositions, quench positions and quench jet geometries). The conducted experiments have been supported with modelling, in-situ diagnostics and ex-situ particle characterisation. In this work computational fluid dynamics (CFD) has been used to determine the optimal quench flow rate and position. The effect of the plasma and quenching parameters on the plasma characteristics (enthalpy and velocity) has been investigated by performing enthalpy probe measurements. The synthesis chamber has been equipped with a multitude of view ports allowing in-situ diagnostics and in-line particle sampling. Therefore Optical Emission Spectroscopy (OES), and Fourier Transformed Infrared (FTIR) spectroscopy could be used to obtain information on the species present in the gas phase. Also an in-line sampling device has been used to track on particle growth during plasma processing at different axial and radial positions. The synthesised products were characterised afterwards by X-ray diffraction (XRD), gas adsorption measurements (BET), thermal gravimetrical analysis and differential thermal analysis (TGA-DTA), and high resolution Scanning Electron Microscopy (SEM). Furthermore, a particle growth model describing the particle growth, by coagulation and coalescence, has been used to predict the particle size in case of the experiments using silicon. This growth model is based on the model developed by Kruis et al., AIChE, 19, 1993. The most important results obtained from these experiments are: a certain threshold amount of quenching gas is necessary to penetrate the plasma and to cause a cooling effect. This value is around 20 slpm argon (Ar) in case of the currently used process conditions and was verified by enthalpy probe measurements. The BET value of ICP synthesised silicon nanoparticles could be increased over 100% by improving the quenching operation, to a value of 145 m2/g, resulting in an average BET diameter of 18 nm. Although the growth model does not result into a 1 to 1 reproduction of the obtained results, it is able to predict the observed trends very well. The in-line sampling device is a powerful tool to track particle growth at different positions in the reaction chamber during plasma processing. The effect of quenching is clearly present in the particle size distribution, chemistry, and crystallographic phase composition and therefore allowing the control of these properties. The main conclusions from this research are: the combination of experiments, modelling and diagnostics is a powerful tool to optimise the synthesis of nanoparticles. Properties like size distribution and chemistry can be controlled by proper quenching. A reactive plasma atmosphere allows changing of the final chemical composition and coating of the nanoparticles.
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