This paper presents a short overview of the high flexibility and potential of inductively coupled thermal plasmas for the synthesis of nanoparticles and especially high refractory carbide nanopowders. The high energy and temperature of theses plasmas enable the evaporation of all materials allowing the use of solid precursors that are cheap and commercially available. By controlling the temperature history of each particle, the growth mechanisms can be stopped at the requested particle size. Therefore in‐situ monitoring and process modelling are proved to be powerful tools.

Silicon nanopowders are investigated intensively for application in microelectronics and energy among them. [1][2][3] However, for the moment the control of the product high quality and the processing costs are limiting their breakthrough. RF-thermal plasmas could respond to both criteria as it is a continuous process achieving high production volume while ensuring a good synthesis control from the gaseous phase. The absence of electrodes for igniting the plasma and the controlled process atmosphere of RF-plasmas are beneficial for producing high purity materials. Typically a supersaturated gaseous phase containing the precursor is condensed rapidly after nucleation took place. The nanoparticles grow then subsequently by coagulation and coalescence. [4] This rapid condensation is achieved either by natural thermal gradients in the plasma or by quenching using a cold source (gas or cold surface) or by using an expansion. [5][6][7][8] This paper addresses the development of a specific quenching nozzle design combining rapid cooling with an expansion for controlling the size of silicon nanopowders processed by an inductively coupled plasma (ICP). The design of the quenching device has been supported by computational fluid dynamic (CFD) calculations aiming at modelling the plasma properties like among them the temperature and the velocity of the powderfree plasma. The modelling has been validated by in-situ plasma characterization using an enthalpy-probe coupled to a mass spectrometer. The produced nanopowders were collected either on a filter membrane, or directly in the gas phase using an on-line and in-situ sampling system made of a TEM-grid fixed on a moveable support and then ex-situ characterized by XRD, Raman, microscopy and surface specific area measurement using the BET technique. Experimental Set-up and Plasma CharacterizationThe ICP apparatus devoted to the synthesis of nanoparticles has been already described in details elsewhere. [9] Its dimensions and its geometry have been introduced in the CFD modelling code. Plasma gases typically used for processing are argon and hydrogen. This reducing atmosphere is generally used to ensure oxide-free particles. The plasma torch (PL-35, Tekna) has been also modelled with the different gas inlets; carrier gas (Ar), central gas (Ar) and the sheath gas that is a mixture of Ar and H 2 . The gas composition is referred in the following as carrier gas-central gas-sheath gas (Ar/H 2 ) in slpm. An enthalpy-probe (ENT-476, Tekna) has been used to measure in-situ the enthalpy and the velocity of the plasma in powder-free conditions at different radial positions and heights in the reactor (z=0 corresponds to the plasma torch exhaust). The plasma properties have been calculated outside the electromagnetic field, meaning below the torch exhaust where the quenching should be performed. Therefore, in a first approximation, the plasma process has been considered as a steady state process with the plasma being modelled as a heat source with a homogeneous power distribution located ...

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Among other methods, inductively coupled plasma (ICP) torches can be used for the synthesis of nanoparticles. In this process, the precursor material is vaporized in the first step in the plasma core. In the second step, nucleation and condensation occur in the synthesis chamber where the plasma gets colder and high-purity nanoparticles are synthesized, the growth of which is stopped by gas quenching. From their low velocity and high temperature, induction plasmas are particularly adapted for this application. Numerical modeling is a good way to achieve a better knowledge and understanding of the process since non-intrusive diagnostics are fairly difficult to implement. In the present article, a twodimensional model of an ICP torch was developed and validated on the basis of comparisons with data obtained by some other authors. Finally, the current frequency (13.56 MHz), pressure level (0.04 MPa), and gas flow rates were adjusted for the specific conditions of nanoparticles synthesis.