Synthesis of ultra fine particles by plasma transferred arc: Influence of anode material on particle properties

There is a great deal of interest today in the special properties of nanoparticles and their potential applications. Gas-phase process, although having some drawbacks, has the largest control possibilities and is therefore the method chosen here. The size of the primary particles depends on the temperature/time history and material properties. Further growth of the particles strongly depends on the properties of the flow into which they are imbedded. In the current work, a transferred arc is used to produce nanometric particles from the condensation of metallic vapours obtained by controlled evaporation of the anode material, which becomes the solid precursor of the synthesis. As all types of anode materials would be used, depending of the nature of the desired particles, this technique requires a good control of the heat transfer and its application time to a given anode location, as well as the separation of evaporation/nucleation-growth steps. That is why a new and original experimental set-up was built in order to control vapour production and its thermal history. Experiments showed that the heat transfer at the anode precursor strongly depends on the cold boundary layer (CBL) properties close to the anode. For adequate parameters, it becomes possible to generate either diffuse or constricted stable arc root, and so to control vapour production. Orientation and so dissociation of evaporation and nucleation events is also achieved by tilting the angle between the jet issued from the cathode and the anode. The vapours produced are then naturally entrained towards a temperature controlled zone, where they are collected onto a water cooled substrate. It thus becomes possible to control the residence time and the thermal vapour history of the particles. Whereas aluminium oxide particles synthesized are clearly nanoparticle chain aggregates, iron oxides particles are spherical, in the micrometric range and no aggregates or agglomerates are visible. These experiments show that particle morphology, size and shape, for given working parameters, strongly depend on the properties of the material to be vaporized. (c) 2006 Elsevier Ltd. All rights reserved.