Aspects of heat transfer to particles in thermal plasma processing

Abstract

Thermal plasma technology is potentially useful for a range of materials processing applications, such as the synthesis of sub-micron, ultra-pure ceramic powders. Thermal plasma reactors are characterised by short residence times (between 10 and 100 ms). Consequently, for chemical reactions to proceed to completion, reactants must be in the gas phase. Reaction rates of solids and liquids are too slow to proceed to any great degree in a thermal plasma, and unvaporised particles can contaminate product material. However, many useful reagents for plasma synthesis are available in particulate form, and thus particles must be completely vaporised if they are to be effective. In this thesis, vaporisation of particles in thermal plasmas was investigated both numerically and experimentally. A numerical model of particle vaporisation in a thermal plasma was developed, which considers the effects of particle vapour on thermodynamic and transport properties of the plasma. This was compared with a simpler model which neglects vapour contamination effects on the plasma. Results showed that the simpler model greatly over-estimated vaporisation times of copper, aluminium, and tungsten particles in argon plasmas at temperatures less than 11000 K, but reasonable accuracy was obtained at higher temperatures. It was found that heat and mass fluxes, and vaporisation time could be expressed in a reduced form which is independent of initial particle diameter. Heat and mass fluxes during vaporisation were found to be linear functions of the inverse of particle radius. Gas-vapour property data are generally difficult to obtain, and guidelines are recommended for using pure argon properties to estimate vaporisation time. The two major types of thermal plasma are the DC (direct current) arc, and the RF (radio-frequency), or induction, plasma. The RF plasma has several advantages over other techniques for the synthesis of powders. Reactions occur in primarily in the gas phase, resulting in good mixing between reactants. Rapid quenching of the tail flame can be used to promote homogeneous nucleation and fine particle size. There is no source of external contamination, because the RF plasma torch lacks electrodes, and a wide variety of reactants can be used, including corrosive and oxidising reagents. The plasma has a relatively low velocity and large diameter, and axial feeding of particles results in better vaporisation of particulate reagents than other thermal plasma torches. In the experimental programme, two RF plasma torches were designed and constructed using the same 13.5 MHz, 15 kW power supply. Fluidised bed feeders and a vibratory feeder were constructed to feed low flow rates (less than 0.2 g/min) of powders, and other apparatus were designed for collecting product particles and quenching the plasma tail flame. The final torch design was used to study heat transfer to particles of a range of materials and particle sizes in the plasma. The materials studied covered a range of boiling points and heats of vaporisation, so that the effects of these properties could be investigated. Particles of alumina, titanium carbide and magnesium oxide smaller than 38 μm diameter were found to vaporise completely. Condensation of vapour produced particles approximately 100 nm diameter which were probably agglomerates of smaller particles formed by homogeneous nucleation. Inspection of morphologies of unvaporised particles showed that the treatment of particles in the plasma is not always uniform, as particles follow a wide range of trajectories and experience various temperature histories. From a semi-empirical analysis of partial vaporization of a range of particle sizes it was estimated that the mean residence time of particles was 18 ms and the mean plasma temperature was 9400 K A heat transfer coefficient of 8000 W/m2K was estimated for partially vaporising particles, which was similar to heat transfer coefficients obtained by numerical modelling. These three parameters may be used to predict the degree of vaporization of particles in an RF plasma torch. Thermodynamic analyses of plasma synthesis of titanium carbide and nitride were performed, indicating the feasibility of the synthesis of these materials in thermal plasma reactors and possible reactant combinations which may be used.