Special session on induction plasma spraying: Diagnostics on a thermal RF plasma process used for the evaporation of zirconia powders

摘要

The radio frequency generated thermal plasmas offer a 10000 K hot region free from electrode contamination. The long residence time - compared to DC plasma jets - for particles injected into this kind of plasma jet makes it suitable for the production of ultrafine particles and coatings. In addition to plasma spraying, where melting of injected precursor materials is sufficient, the plasma flash evaporation technique [1] requires complete evaporation of the precursor to produce high-performance coatings and nanosized powders. The method consists of an evaporation phase, where the injected solid precursor materials are vaporized. Rapid quenching causes a limited growth of nucleating particles or condensation of a coating on an appropriate substrate, respectively. Solids as starting materials are available at low costs for a wide class of materials, and, since the pure substances can be used, no unwelcome reaction products are formed. Boiling point and heat of evaporation are often much higher for the solid starting materials compared to other precursors, so special care has to be taken for the complete evaporation of the particles. Zirconia (ZrO_2) is a material with extreme thermal properties (melting temperature: 2950 K, evaporation temperature: 4548 K, [2]). The high thermal stability of ZrO_2 is used in technical applications such as high temperature thermal insulation and solid oxide fuel cells [3]. On the other hand, this stability makes it difficult to achieve a complete evaporation of zirconia in a thermal plasma, so up to now, not much work has been reported on the successful evaporation of solid ZrO_2 [4]. Numerical modeling of the plasma zone and the behaviour of the injected particles supports the prediction of the interaction between plasma properties (temperature and velocity field, gas flow and gas composition), particle properties, probe position and geometry [5-10]. Optical emission spectroscopy is an independent approach to check the numerically predicted particle evaporation experimentally [10,11]. The velocity of the injected particles and thus the residence time of the particles in the plasma plays an important role for the evaporation. The velocity is measured by the LDA technique.