Production of Nanopowders in Inductively Coupled Plasma, Multiphysics Modeling
The production of nanopowders, whose properties differ from the bulk material due to their size close to molecular dimensions, is a nanotechnology that has already had an important industrial impact. Often these materials show completely new properties. Nanometallic powders, nanoceramics or nanocomposites have mechanical, chemical, optical, magnetic and electronic properties that are enhanced compared to micrometric powders with the same chemical composition. For example, nanometric iron oxide acquires super-paramagnetic capabilities at room temperature. Energetic materials are nanopowders whose properties change compared to the micrometric material, nano-aluminum powders have a spontaneous ignition depending strongly on their size. Among the technologies available to produce nanopowders at industrial scale, the Inductively Coupled Plasma (ICP) presents numerous advantages: high temperatures to vaporize precursors of any type, relatively large size and low velocities with a controlled chemical environment. In an ICP reactor, the vapor cloud from the precursor vaporization condenses and forms nanoparticles that subsequently grow through coagulation. As the flow, temperature and concentration fields in such reactors are complex, the nanopowders produced in such reactors depend heavily on these fields and a complete characterization is of outmost importance for better design. This has lead to a number of experimental and modeling studies of the complex relationships between reactor geometry, quench and powder size and morphology. The development and validation of an inductively coupled thermal plasma model for the production of nanopowders is a truly multi-physics problem dealing with the coupling between the electromagnetic fields and the fluid flow, with the introduction of precursors in the plasma flow, the nucleation, growth, aggregation and transport of the nanoparticles. The model presented here describes the evaporation of the micron-sized precursor particles in the plasma ﬂow and the subsequent formation of the nanoparticles in the quenching zone of the reactor. The plasma ﬂow is described by a coupled system of the ﬂuid mechanics equations of continuity, momentum, and energy with the vector potential formulation of Maxwell equations. In the case presented in this work, since the precursors injected in the plasma flow are in the form of micron-sized particles, a two-phase flow model is used where the solid particles precursors are treated following a Lagrangian approach, evaporating and creating a vapor ﬁeld in the plasma ﬂow. A population balance model (PBM) is used to describe the formation of nanoparticles by simultaneous nucleation and growth by condensation and coagulation.