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Model Formulation and Numerical Investigation of Thermal- Plasma-Aided Nanopowder Production

Lecture by Visiting Professor Masaya Shigeta

Mar 03, 2010 from 06:00 PM to 07:00 PM

Where Residenza di Studi Superiori, via Fantin, 15

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Thermal plasma is anticipated as a potent tool for effective production of nanopowders because thermal plasma provides several distinctive advantages: high enthalpy, high chemical reactivity, and a steep temperature gradient. The production process starts from vaporization of raw materials, which is easily achieved by the remarkably high enthalpy of a plasma, even if the materials have high melting or boiling points. Subsequently, the vapor is transported to the plasma tail at high cooling rates, and the vapor becomes highly supersaturated. As a result, nanopowders are rapidly generated through nucleation, condensation, and coagulation.

However, in experiments, it is extremely difficult to directly observe or measure the detailed processes of nanopowder formation. Only the characteristics of the products have been evaluated. Therefore, it is strongly expected that theoretical or numerical-modeling approaches will extract significant information that is unobtainable from experiments, even though the system includes complicated phenomena.

The objective of the present study is to formulate mathematical models to simulate the comprehensive system of nanopowder formation in thermal plasma processing and to clarify the substantial formation mechanisms. First the thermal plasma flow is expressed based on the electromagnetic fluid dynamics. The trajectory and temperature history of the raw material particles are examined using a Lagrangian approach, taking into account the rarefied gas effect. The nanopowder formation is modeled by the aerosol dynamics taking into account not only nucleation, condensation, and coagulation but also convection, diffusion, and thermophoresis.

In the lecture, the model formulation and numerical demonstrations are presented. Also, a newly-developed model to numerically analyze the nanopowder formation in binary systems will be introduced. In particular, the latest result of metal-silicide nanopowders will be shown and their formation mechanisms will be discussed.