Thermal Plasma Processing Of Materials Research Articles

Thermal Plasma Technology: Where Do We Stand and Where Are We Going?

In this overview, an attempt is made to assess the present and future research and development in thermal plasma processing of materials restricted to (1) thermal plasma coating technologies, (2) thermal plasma synthesis of fine powders, (3) thermal plasma waste destruction, and (4) thermal plasma spheroidization and densification. Since thermal plasma processing is, in general, governed by a large number of parameters, implementation of controls becomes mandatory. The lack of sufficient controls combined with economic drawbacks in some cases has been the main obstacle for the growth of thermal plasma technology. Present R&D efforts, however, address these problems.

Materials Processing With Thermal Plasmas

Thermal plasmas in which the plasma state approaches local thermodynamic equilibrium may be produced by high-intensity arcs (current I > 100 A) or by radio-frequency (rf) discharges at high-power densities and at pressures exceeding 10 kPa (0.1 atm). In such plasmas, electron and heavy-particle temperatures are identical, and particle densities are only a function of the temperature (chemical equilibrium).Thermal-plasma technology has passed through a drastic transition stage from primarily space-related activities in the 1960s to a more and more materials-oriented focus in the 1980s and 1990s, and thermal-plasma processing of materials is now attracting increasing interest for industrial applications.Thermal-plasma processing of materials however must be viewed in the context of much broader technology trends, some of which are visible today. There is no question that materials and materials processing will be one of the most important technical issues as we approach the turn of the century. This trend will not be restricted to the development of new materials but will also include the refining of materials, the conservation of materials (by hard facing, coating, etc.), and the development of new processing routes that are more energy-efficient, more productive, and less damaging to our environment. Thermal-plasma processing is already playing an important role in these developments. Its potential for developing new materials-related technologies is increasingly recognized, and many research laboratories all over the world are engaged in advancing the frontiers of our knowledge in this exciting field. As a result of these efforts, thermal-plasma processing has emerged as a powerful technology that is finding numerous industrial applications.

Thermal plasma processing of materials: A review

The use of thermal plasma in materials processing industries is becoming an increasingly active and attractive field for the development of new technology. The potential applications of thermal plasma processing technology cover a wide range of activities, such as: the extraction of metals, the refining/alloying of metals/alloys, the synthesis of fine ceramic powders, spray coatings, and the consolidation and destruction of hazardous wastes. A review of thermal plasma applications in materials processing is presented.

Thermal plasma materials processing?Applications and opportunities

Thermal plasma processing of materials is a rapidly growing area of research. The commercialization of these processes, however, has been limited by the lack of fundamental understanding of how the various processes work. Research has historically focused on developing models of fluid flow and heat transfer to particles injected into either DC arc or RF plasma jets. These models in the past have simplified boundary conditions to meet computational limitations. Recent advances in models have now been made, allowing evaluations of more of the plasma process variables. Supersonic flow modeling in a DC jet and modeling of the effects of particle loading (particulate feed rate) have been accomplished and are reviewed here. Materials processing using thermal plasmas has been separated into the categories of synthesis, melting, and deposition, and is discussed in view of the processing effects on the resultant material structures. Process modeling leading to process understanding is reviewed with an emphasis on process control and optimization. Commercialization of plasma processes requires controls and process transducers which result from experimentation and process models. Approaches to develop process controls from the current technical base are presented.

Particle behavior in thermal plasmas

In this overview, effects exerted on the motion and on heat and mass transfer of particulates injected into a thermal plasma are discussed, including an assessment of their relative importance in the context of thermal plasma processing of materials. Results of computer experiments are shown for particle sizes ranging from 5–50 μm, and for alumina and tungsten as sample materials. The results indicate that (i) the correction terms required for the viscous drag and the convective heat transfer due to strongly varying properties are the most important factors; (ii) noncontinuum effects are important for particle sizes <10 μm at atmospheric pressure, and these effects will be enhanced for smaller particles and/or reduced pressures; (iii) the Basset history term is negligible, unless relatively large and light particles are considered over long processing distances; (iv) thermophoresis is not crucial for the injection of particles into thermal plasmas; (v) turbulent dispersion becomes important for particle <10 μm in diameter; and (vi) vaporization describes a different particle heating history than that of the evaporation process which, however, is not a critical control mechanism for interphase mass transfer of particles injected into thermal plasmas.

Particle dynamics and particle heat and mass transfer in thermal plasmas. Part I. The motion of a single particle without thermal effects

A particle injected into a thermal plasma will experience a number of effects which are not present in an ordinary gas. In this paper effects exerted on the motion of a particle will be reviewed and analyzed in the context of thermal plasma processing of materials. The primary purpose of this paper is an assessment of the relative importance of various effects on particle motion. Computer experiments are described, simulating motion of a spherical particle in a laminar, confined plasma jet or in a turbulent, free plasma jet. Particle sizes range from 5 to 50 µm, and as sample materials alumina and tungsten are considered. The results indicate that (i) the correction term required for the viscous drag coefficient due to strongly varying properties is the most important factor; (ii) non-continuum effects are important for particle sizes <10 µm at atmospheric pressure and these effects will be enhanced for smaller particles and/or reduced pressures; (iii) the Basset history term is negligible, unless relatively large and light particles are considered over long processing distances; (iv) thermophoresis is not crucial for the injection of particles into thermal plasmas; (v) turbulent dispersion becomes important for particle <10 µm in diameter.

Heat and momentum transfer to particles in thermal plasma flows

Abstract

Effects of metallic vapor on the properties of an argon arc plasma

During thermal plasma processing of materials, vapor generated from injected particulate matter will enter the plasma. Even traces of metallic vapors may have a strong effect on the properties and the behavior of the plasma and on the associated heat flux to the injected particles. In this paper a model system is considered in which an argon plasma at atmospheric pressure is “contaminated” by small amounts of copper vapor. By using the Chapman-Enskog approximation for a multicomponent gas mixture the transport properties are calculated for such a contaminated argon plasma. The results show that there is a drastic effect on the electrical properties. For temperatures below 104 K, the electrical conductivity, for example, increases by more than an order of magnitude if metallic vapor is present. The presence of metallic contaminants is also somewhat felt by the reactional thermal conductivity. In contrast, there is no effect on the heavy-particle properties as long as the percentage of the contaminants remains small.