Anthony B. Murphy

The 2012 Plasma Roadmap

Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap.

Transport coefficients of argon, nitrogen, oxygen, argon-nitrogen, and argon-oxygen plasmas

Calculated values of the viscosity, thermal conductivity, and electrical conductivity of argon, nitrogen, and oxygen plasmas, and mixtures of argon anti nitrogen and of argon anti oxygen, are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. These three combined diffusion coefficients fully describe di fusion of the two gases, irrespective of their degree of dissociation or ionizati on. The calculations, which assume local thermodynamic equilibrium, are performed! for atmospheric-pressure plasmas in the temperature range /torn 300 to 30,000 K. A number of the collision integrals used in calculating the transport coefficients are significantly more accurate than values used in previous theoretical studies, resulting in more reliable values of the transport coefficients. The results are compared with those of published theoretical and experimental studies.

Transport coefficients of air, argon-air, nitrogen-air, and oxygen-air plasmas

Calculated values of the viscosity, thermal conductivity and electrical conductivity of air and mixtures of air and argon, air and nitrogen, and air and oxygen at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. The calculations, which assione local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30,000 K. The results for air plasmas are compared with those of published theoretical and experimental studies. Significant discrepancies are found with the other theoretical studies; these are attributed to differences in the collision integrals used in calculating the transport coefficients. A number of the collision integrals used here are significantly more accurate than values used previously, resulting in more reliable values of the transport coefficients.

Thermal plasma waste treatment

Plasma waste treatment has over the past decade become a more prominent technology because of the increasing problems with waste disposal and because of the realization of opportunities to generate valuable co-products. Plasma vitrification of hazardous slags has been a commercial technology for several years, and volume reduction of hazardous wastes using plasma processes is increasingly being used. Plasma gasification of wastes with low negative values has attracted interest as a source of energy and spawned process developments for treatment of even municipal solid wastes. Numerous technologies and approaches exist for plasma treatment of wastes. This review summarizes the approaches that have been developed, presents some of the basic physical principles, provides details of some specific processes and considers the advantages and disadvantages of thermal plasmas in waste treatment applications.

The effects of metal vapour in arc welding

Metal vapour is formed in arc welding processes by the evaporation of molten metal in the weld pool, and in the case of gas–metal arc welding, in the wire electrode and droplets. The presence of metal vapour can have a major influence on the properties of the arc and the size and shape of the weld pool. Previous experimental and computational works on the production and transport of metal vapour in welding arcs, in particular those relevant to gas–metal arc welding and gas–tungsten arc welding, are reviewed. The influence of metal vapour on the thermodynamic, transport and radiative properties of plasmas is discussed. The effect of metal vapour on the distributions of temperature, current density and heat flux in arcs is examined in terms of these thermophysical properties. Different approaches to treating diffusion of metal vapour in plasmas, and the production of vapour from molten metal, are compared. The production of welding fume by the nucleation and subsequent condensation of metal vapour is considered. Recommendations are presented about subjects requiring further investigation, and the requirements for accurate computational modelling of welding arcs.

Thermal plasmas in gas mixtures

The calculation and measurement of the properties of thermal plasmas in mixtures of different gases are reviewed. The calculation of composition, thermodynamic properties and transport coefficients is described. Particular attention is given to the calculation of diffusion coefficients, which is a significant problem in mixed-gas plasmas. The combined diffusion coefficient formulation is shown to be a useful method for the treatment of diffusion. Computational fluid dynamic modelling of thermal plasmas in gas mixtures is considered, using the examples of demixing in welding arcs, the turbulent mixing of atmospheric air into a plasma jet and a plasma waste destruction process. Diagnostic techniques for mixed-gas plasmas, in particular emission spectroscopy, laser scattering and laser-induced fluorescence, are discussed.

Transport Coefficients of Hydrogen and Argon–Hydrogen Plasmas

Calculated values of the viscosity, thermal conductivity, and electrical conductivity of hydrogen and mixtures of argon and hydrogen at high temperatures are presented. Combined ordinary, pressure, temperature, and electric field diffusion coefficients are also given for the mixtures. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30,000 K. The results are compared with those of previously published studies. Generally, the agreement is reasonable; those discrepancies that exist are attributed to the improved values of some of the collision integrals used here in calculating the transport coefficients.

Thermal plasmas for nanofabrication

In this paper, we review the recent progress in nanofabrication by thermal plasmas, and attempt to define some of the most important issues in the field. For synthesis of nanoparticles, the experimental studies in the past five years are briefly introduced; the theoretical and numerical modelling works of the past 20 years are reviewed with some detailed explanations. Also, the use of thermal plasmas to produce nanostructured films and coatings is described. A wide range of technologies have been developed, ranging from chemical vapour deposition processes to new plasma spraying processes. We present an overview of the different techniques and the important physical phenomena, as well as the requirements for future progress.

Atmospheric pressure plasmas: Infection control and bacterial responses

Cold atmospheric pressure plasma (APP) is a recent, cutting-edge antimicrobial treatment. It has the potential to be used as an alternative to traditional treatments such as antibiotics and as a promoter of wound healing, making it a promising tool in a range of biomedical applications with particular importance for combating infections. A number of studies show very promising results for APP-mediated killing of bacteria, including removal of biofilms of pathogenic bacteria such as Pseudomonas aeruginosa. However, the mode of action of APP and the resulting bacterial response are not fully understood. Use of a variety of different plasma-generating devices, different types of plasma gases and different treatment modes makes it challenging to show reproducibility and transferability of results. This review considers some important studies in which APP was used as an antibacterial agent, and specifically those that elucidate its mode of action, with the aim of identifying common bacterial responses to APP exposure. The review has a particular emphasis on mechanisms of interactions of bacterial biofilms with APP.

Treatment of non-equilibrium phenomena in thermal plasma flows

Thermal plasma flows provide a uniquely high specific enthalpy source that is well suited to transformation of matter, often via phase changes. As a consequence, numerous thermal-plasma-based processes have been developed to, for example, destroy pollutants, modify surfaces (e.g. cutting and welding), synthesize nanostructures and deposit functionalized nanostructured coatings. In many cases, departures from equilibrium (both thermal and chemical) occur in regions of such plasmas; for example, in electrode erosion phenomena or in the injection of a liquid into a plasma jet. This paper reviews the treatment of non-equilibrium phenomena in thermal plasma flows, in particular the methods of calculation of the composition and transport coefficients of non-equilibrium plasmas, which are required for modelling the above processes. The focus is on two-temperature plasmas, in which electrons and heavy species are at different temperatures. Methods of calculation of the composition of plasmas both in local chemical equilibrium (LCE) and out of LCE are presented. A comparison of the different methods shows large discrepancies, even assuming LCE. Two-temperature transport coefficients obtained from simplified expressions, from the modified Chapman–Enskog method and from the Stefan–Maxwell relations are presented, as well as examples focusing on the influence of plasma composition. Different methods of calculation of the collision integrals required in determining the transport coefficients are also reviewed. Particular attention is paid to diffusion, in particular to the combined diffusion coefficient method, which simplifies treatment of plasmas in LCE. The method of calculation of the reactive thermal conductivity and the influence of excited states on transport coefficients are also addressed in some detail.