Thermal plasmas, source of very high enthalpy, high chemical reactivity offer rapid evaporation rate, steep temperature gradients, wide area and high deposition/growth rate, and an attractive and chemically nonspecific route for the synthesis of fine powders down to the nanometer size range. Among various types of thermal plasma reactors, radio frequency induction thermal plasma (ITP) reactors offer several advantages such as high purity due to absence of electrode, large volume of plasma for processing, low plasma velocity, simple power supply unit and wide pressure range. ITP reactors, which basically convert the electrical energy into heat energy, have been widely used as a clean heat source for synthesis of nanoparticles. The high-temperature of ITP leads to short evaporation time which translates into relatively small torch with high throughput. Powder synthesis needs sharp temperature gradient for rapid condensation in the reaction chamber. ITP offers the distinct advantage of providing essentially one-step processes, avoiding the multitude of steps required in the creation of particles with conventional methods. The feasibility of producing nanoparticles of various intermetallic compounds, alloys, oxides, and nitrides by vapor-phase synthesis in reactive thermal plasma systems will be demonstrated in this chapter. To investigate the complicated phenomena in the synthesis processes of nanoparticles by ITP, theoretical and numerical studies are powerful approaches. The precursory powders of the raw materials are injected into the plasma and vaporized due to the heat transfer from the plasma. The vapor is transported downstream with the plasma flow. Since the saturation pressure drastically decreases with the rapid temperature drop at the tail of the plasma, the vapor becomes supersaturated. As a result, nuclei are generated by homogeneous nucleation, and the supersaturated vapor easily condenses on the nuclei by heterogeneous condensation. This process is the fundamental formation mechanism of nanoparticles by ITP. Simultaneously nanoparticles collide and coagulate among themselves. Coagulation also plays an important role for the nanoparticle growth. The mathematical models are introduced to simulate the processes. The fields in the plasma are expressed on the basis of the electromagnetic fluid dynamics. The trajectory and temperature history of the precursory powders are examined by Lagrangian approach taking into account the rarefied gas effects. The nanoparticle formation can be modeled by the aerosol dynamics, basically with Eulerian approach, taking into account not only nucleation, condensation, and coagulation but also convection, diffusion, and thermophoresis. Through the computation with these models, the particle size distributions or compositions of the produced nanoparticles are obtained and the formation mechanisms are clarified.
|Title of host publication||Nanoparticles|
|Subtitle of host publication||Properties, Classification, Characterization, and Fabrication|
|Publisher||Nova Science Publishers, Inc.|
|Number of pages||64|
|Publication status||Published - 2010 Feb|
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