Plasma Chemistry and Plasma Processing (v.35, #3)

Professor J. V. R. Heberlein, 1939–2014 by Maher Boulos; Pierre Fauchais; Emil Pfender (417-420).

Thermal Plasma Sources: How Well are They Adopted to Process Needs? by Javad Mostaghimi; Maher I. Boulos (421-436).
Thermal plasma devices have evolved over the years from an aerospace R&D tool for the simulation of re-entry of space vehicles into powerful sources for a wide range of applications including materials processing, nano-powder synthesis and deposition of functional coatings, waste treatment and biomass gasification. With the development of full scale industrial applications, it is increasingly apparent that the plasma source is an integral part of the process, and that the success of a technology depends to a large extent on the way the plasma source satisfies the process needs. In this paper, a short review is presented of a selected number of commercially available plasma sources that are using DC and/or RF inductively coupled plasma torch technologies.
Keywords: Thermal plasma processing; DC plasma torches (non-transferred); Radio frequency inductively coupled plasma torches

New Methods to Look at an Old Technology: Innovations to Diagnose Thermal Plasmas by J. Schein; K. Hartz-Behrend; S. Kirner; M. Kühn-Kauffeldt; B. Bachmann; E. Siewert (437-453).
The use and development of diagnostics for thermal plasmas is motivated by the industrial importance of thermal plasma applications like welding, cutting or thermal spraying. While the physical fundamentals of plasma diagnostics were introduced decades ago new technologies allow to perform a more detailed analysis of the mentioned applications with increased spatial and temporal resolution, enabling the investigation of complex processes thereby moving the focus from pure plasma to plasma-material diagnostics. An attempt is made to demonstrate current and future possibilities provided by technical progress using “old physics” with the help of few examples. The examples introduced here focus mostly on welding applications and include the use of high-speed cameras for the spectrally resolved analysis of plasma radiation and two-color pyrometry. In addition the use of Thomson scattering in gas metal arc welding is proposed as well as the use of magnetic field measurements for non-intrusive current density measurements.
Keywords: Plasma diagnostics; Welding; Thermal spraying; Thermal plasmas

Perspectives on Thermal Plasma Modelling by Alain Gleizes (455-469).
As in many other fields of research, modelling is an important tool to study thermal plasmas, to interpret their behaviours in different applications and to try to predict some of their global properties. Even though each kind of application tends itself to privilege a specific model, the approach adopted in this paper is to present perspectives of the modelling relative to scientific problems and not to specific applications. Of course for each kind of scientific problem we give some links to the corresponding applications.
Keywords: Thermal plasmas; Modelling; Electric arcs; Plasma properties

The state-of-the-art on arc welding research is considered, with unresolved questions and future directions highlighted. Both diagnostics and modelling are discussed. The focus is on the arc plasma, and its interactions with the electrode and workpiece, in tungsten–inert-gas and metal–inert-gas welding. Areas in which the need for further work is identified include development of techniques to measure current density distributions, calculation of the distribution of different gasses in the arc plasma (for example vapours of different metallic elements when welding alloys), computational methods for modelling metal transfer, and treatments of the sheath regions. It is shown that a thorough understanding of the arc is important in welding research and development. For example, reliable calculation of the heat flux to the workpiece requires the interactions between the arc and electrodes to be considered. Computational models of welding that take into account these interactions can already predict the shape and depth of the weld pool. Extensions of these methods would enable the determination of important properties of the welded metal, such as microstructure, residual stress and distortion, raising the possibility of the development of a “virtual manufacturing” capability.
Keywords: Arc welding; Thermal plasmas; Computational modelling; Diagnostics; Virtual manufacturing

A Perspective on Plasma Spray Technology by Armelle Vardelle; Christian Moreau; Nickolas J. Themelis; Christophe Chazelas (491-509).
Plasma spraying is often assumed to be a mature technology in which all the important phenomena have been observed and described adequately. However, the intricate interactions between the electrically conducting fluid and electromagnetic, thermal and acoustics phenomena that affect the operation of the plasma torch are not fully understood as yet. Also, variants of the plasma spray process are emerging and raise new scientific questions. These technologies include the spraying of liquid feedstock in the form of submicrometric particles or chemical precursors in a solvent and, coatings formed by vapor condensation onto the substrate. These relatively novel techniques make possible the production of thinner coatings than in air plasma spraying with a fine and even nanostructured microstructure. This paper attempts to define some of the current important issues and research priorities in the plasma spray field.
Keywords: Surface engineering; Plasma spraying; Plasma spray torch; Splat formation; Suspension and solution plasma spraying; Very low pressure plasma spraying; Sustainable manufacturing

Key Challenges and Opportunities in Suspension and Solution Plasma Spraying by P. Fauchais; M. Vardelle; S. Goutier; A. Vardelle (511-525).
Over the last 15 years many research papers have presented the potential of suspension and solution plasma spraying to achieve coatings, essentially ceramic coating, with a nanometric or sub-micrometric microstructures. Such coatings exhibit improved properties (e.g. lower thermal diffusivity, higher hardness and toughness) in comparison with coatings with a micrometric microstructure. This paper presents first the techniques that are now used to characterize these plasma spray processes and the resulting coatings; it, then, examines, the main plasma torches and their limits, the preparation of suspensions and solutions, the interactions between the liquid feedstock and plasma jet, the processing of droplets and particles and the coating formation. Finally, the key issues for the transfer of these technologies from the laboratories to industry will be discussed.
Keywords: Suspensions; Solutions; Plasma spraying

The research on high voltage circuit breakers using SF6 is addressed. The current state of the art in this field is shortly reviewed and future research directions are discussed. These are mainly the radiative energy transport, the understanding and description of ablation processes at the nozzle and contact surfaces, the influence of such vapors on pressure build up and interruption capability, electric breakdown processes, departures from equilibrium, turbulence and the importance of magnetic fields and 3D modelling.
Keywords: Arc; Circuit breaker; High voltage; SF6

Accurate numerical modeling is prerequisite of predicting plasma behavior and understanding the complex physical and chemical processes in a plasma system. The evolution of nonequilibrium modeling of arcjet to its current state of development is traced, and some uncertainties in the way of further progress are discussed. It is demonstrated that the accuracy of two-temperature plasma transport coefficients can be improved by adopting more reasonable interatomic potentials. A comparison of the chemical kinetic rate coefficients for the same kinetic process shows some discrepancies, which further indicates that there exist some uncertainties on the inelastic cross sections obtained from experimental measurement or theoretic calculation. Application and extension of three-level atomic model of argon in nonequilibrium modeling are briefly reviewed, and the criteria required in the choice of chemical kinetic processes are discussed. The elementary processes involved in high velocity plasma flow can be investigated by collisional radiative model (CR model). The method of applying CR model on the analysis of nonequilibrium plasma processes in arcjet is presented as an example in some detail.
Keywords: Thermal plasma flow; Nonequilibrium modeling; Arcjet