Plasma Chemistry and Gas Conversion

Low-temperature non-equilibrium gaseous discharges represent nearly ideal media for boosting plasma-based chemical reactions. In these discharges the energy of plasma electrons, after being received from the electromagnetic field, is transferred to the other degrees of freedom differently, ideally with only a small part going to the translational motion of heavy gas particles. This unique property enables the important application of non-equilibrium plasmas for greenhouse gas conversion. While the degree of discharge non-equilibrium often defines the energetic efficiency of conversion, other factors are also of a great importance, such as type of discharge, presence of plasma catalysis, etc. This book is focused on the recent achievements in optimization and understanding of non-equilibrium plasma for gas conversion via plasma modeling and experimental work

Non-Thermal Atmospheric-Pressure Plasma for Sterilization of Surfaces and Biofilms

Bacterial resistance to antimicrobial methods is a critical issue in many fields of medicine. This work describes the studies performed to characterize and optimize the bacterial inactivation effects of a non-thermal atmospheric-pressure plasma brush and plasma jet on a laminate surface inoculated with Acinetobacter baumannii and Staphylococcus aureus, and a cultivated Enterococcus faecalis biofilm, respectively. These treatments are pilot studies for eventual application to surface sterilization in hospitals and root canal disinfection. To evaluate bacterial inactivation, after treatment and recovery, the bacterial colony forming units (CFUs) are counted. Several different methods are used to optimize the antimicrobial effect. For the plasma jet, the optimal fraction of oxygen in the helium feed gas is determined, resulting in a 1.5 log (97%) bacterial inactivation. For the plasma brush treatment of a dry laminate environment, the addition of water to the feed gas and surface is investigated. These variations greatly alter the plasma chemistry, which is characterized for the plasma brush by the use of optical emission spectroscopy to detect the presence of excited reactive species. These and other species generated in the plasma plume play a large role in the deleterious effect of plasma on bacterial cells. In a humid discharge, bacterial inactivation is maximized at 0.6 log (75%), whereas adding water to the laminate surface before treatment yields a 4 log (99.99%) reduction in bacterial growth. Thus, while the optimized oxygen or water concentration in a noble feed gas improves inactivation effect of the plasma, the most significant criterion for maximal bacterial inactivation in these studies is the presence of water at the treatment surface

Wall fluxes of reactive oxygen species of an rf atmospheric-pressure plasma and their dependence on sheath dynamics

A radio-frequency (rf) atmospheric-pressure discharge in He–O2 mixture is studied using a fluid model for its wall fluxes and their dependence on electron and chemical kinetics in the sheath region. It is shown that ground-state O, O+2 and O− are the dominant wall fluxes of neutral species, cations and anions, respectively. Detailed analysis of particle transport shows that wall fluxes are supplied from a boundary layer of 3–300μm immediately next to an electrode, a fraction of the thickness of the sheath region. The width of the boundary layer mirrors the effective excursion distance during lifetime of plasma species, and is a result of much reduced length scale of particle transport at elevated gas pressures. As a result, plasma species supplying their wall fluxes are produced locally within the boundary layer and the chemical composition of the overall wall flux depends critically on spatio-temporal characteristics of electron temperature and density within the sheath. Wall fluxes of cations and ions are found to consist of a train of nanosecond pulses, whereas wall fluxes of neutral species are largely time-invariant

Streamers, sprites, leaders, lightning: from micro- to macroscales

"Streamers, sprites, leaders, lightning: from micro- to macroscales" was the theme of a workshop in October 2007 in Leiden, The Netherlands; it brought researchers from plasma physics, electrical engineering and industry, geophysics and space physics, computational science and nonlinear dynamics together around the common topic of generation, structure and products of streamer-like electric breakdown. The present cluster issue collects relevant articles within this area; most of them were presented during the workshop. We here briefly discuss the research questions and very shortly review the papers in the cluster issue, and we also refer to a few recent papers in other journals.Comment: Editorial introduction for the cluster issue on "Streamers, sprites and lightning" in J. Phys. D, 13 pages, 74 reference

Conversion of Greenhouse Gases to Value Added Products Assisted By Catalytic Nonthermal Plasma

The negative impact the greenhouse gases, especially carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) on the environment is well established and development of suitable technologies is warranted in order to regulate their increasing concentrations in the atmosphere. Co-processing of these gases has the advantage of waste minimization, energy production, resource utilization and pollution control. However, as the activation of these gases is highly endothermic, conventional thermocatalytic techniques may not be effective. In this context, nonthermal plasma (NTP) generated by electrical discharges was proposed as an alternative to the conventional methods. NTP created by electrical discharges generate the energetic electrons that colloid with the target gas molecules to decompose them without increasing the temperature of the back ground gas. However, as NTP is non–selective, catalytic NTP was proposed, in order to improve the selectivity to the desired product. The objectives of this research work were to explore NTP for the conversion of the selected greenhouse gases to value added products and to arrive at the suitable catalyst combination to obtain the best selectivity to the desired products like syngas and methanol. This presentation will focus o

Absolute O3 and OH densities measurement by two-beam UV-LED absorption spectroscopy in atmospheric pressure plasmas

Low temperature atmospheric pressure plasmas (APPs) create rich environment of reactive particle species and chemical-physical interactions at close-to-room temperature and ambient pressure which calls for a wide range of fundamental and application studies. APPs for biomedical applications is one of the emerging interdisciplinary researches. Its fundamental mechanisms have been studied using different numerical models and various diagnostic techniques. With hundreds of particle species and complex reactions, each species requires unique measurement techniques. In a typical APP, ozone (O3), one of the key species in living-cells inactivation, is produced from the complex reaction chain of short-lived oxygen atoms and excited molecules. Measurement and theoretical predictions of O3 densities can have high uncertainties. The measurements of O3 densities inside the small plasma volume are challenging due to the sensitivity to non-plasma parameters. In this work, two-beam UV-LED absorption spectroscopy has been developed by using a Mach-Zehnder configuration for O3 density measurements on the core of a homogeneous, He-O2 capacitively coupled, 13.56 MHz RF-driven APP. The improved technique allows for high-sensitivity measurement in the order of 10−3 absorption signal with 10−4 of uncertainty. The anticorrelation between O3 density and gas temperature was observed and described based on the plasma chemistry models. For controlling-parameter effect, the duty-cycle in frequency modulations showed a significant influence on the spatial profile of O3 density in the plasma channel. From an application perspective, the developed technique was able to provide 2D O3 density distribution in the effluent region of a co-axial DBD kHz-driven APPJ when applied to biological samples. The correlation between radial O3 density profiles and bacterial inactivation areas was investigated. In the relatively realistic condition with higher H2O vapour admixture, hydroxyl (OH) density, which is one of important radical species, can be measured using the UV absorption technique. Thus, the setup has been adjusted in order to measure both species. Furthermore, O3 density in the CO2-CO conversion 40.68 MHz RF-driven APP, an important process in chemical research, was observed. The O3 density as a function of plasma power and CO2 concentration provided a significant contribution to the main production and destruction channels of the conversion processes

Plasma–liquid interactions: a review and roadmap

Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas

Plasma-liquid interactions: a review and roadmap

Plasma-liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas