Vacuum ultraviolet emission from argon wave driven microwave (2.45GHz) plasmas operating at low pressures (0.2 – 1 mbar) has been investigated. The emitted spectra show the presence of argon atomic and ionic lines in the range 80 – 110 nm. The relative emission intensities of excited Ar atoms (at 104.8 nm and 106.6 nm) and ions (at 92.0 nm and 93.2 nm) were investigated as a function of the microwave power and pressure of the discharge. Both atomic and ionic emission lines increased linearly in intensity as the power was raised. Concerning the dependence on pressure, experimental results show that atomic and ionic lines decreased linearly in intensity as the pressure was increased. The plasma electron density was estimated to be in the order of 10 cm.

The spatial structure of a microwave plasma torch driven by an azimuthally symmetric surface wave operating in a N2–Ar mixture at atmospheric pressure is investigated. A two-dimensional (2D) self-consistent theoretical model is developed to investigate the entire spatial structure of the source, including the discharge zone, sustained by the field of the surface TM00 mode, and the postdischarge plasma. Maxwell’s equations, the rate balance equations for the most important excited species—vibrationally and electronically excited states, ions and nitrogen atoms N(S4)—and the Boltzmann equation for electrons are consistently solved. Model calculations of the 2D spatial distributions of species of interest such as charged particles (electrons and positive ions), N2(Χ Σ1g+,v) vibrationally excited molecules, N2(A Σ3u+) metastable molecules, and N(S4) ground state atoms are presented and discussed.

The production of cellulosic ethanol from sugarcane biomass is an attractive alternative to the use of fossil fuels. Pretreatment is needed to separate the cellulosic material, which is packed with hemicellulose and lignin in cell wall of sugarcane biomass. A microwave 'tornado'-type air-water plasma source operating at 2.45 GHz and atmospheric pressure has been applied for this purpose. Samples of dry and wet biomass (similar to 2 g) have been exposed to the late afterglow plasma stream. The experiments demonstrate that the air-water highly reactive plasma environment provides a number of long-lived active species able to destroy the cellulosic wrapping. Scanning electron microscopy has been applied to analyse the morphological changes occurring due to plasma treatment. The effluent gas streams have been analysed by Fourier-transform infrared spectroscopy (FT-IR). Optical emission spectroscopy and FT-IR have been applied to determine the gas temperature in the discharge and late afterglow plasma zones, respectively. The optimal range of the operational parameters is discussed along with the main active species involved in the treatment process. Synergistic effects can result from the action of singlet O-2(alpha (1) Delta g) oxygen, NO2, nitrous acid HNO2 and OH hydroxyl radical.

Self-standing graphene sheets were synthesized using microwave plasmas driven by surface waves at 2.45 GHz stimulating frequency and atmospheric pressure. The method is based on injecting ethanol molecules through a microwave argon plasma environment, where decomposition of ethanol molecules takes place. The evolution of the ethanol decomposition was studied in situ by plasma emission spectroscopy. Free gas-phase carbon atoms created in the plasma diffuse into colder zones, both in radial and axial directions, and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the hot region of the plasma in the outlet plasma stream where nanostructures assemble and grow. Externally forced heating in the assembly zone of the plasma reactor has been applied to engineer the structural qualities of the assembled nanostructures. The synthesized graphene sheets have been analysed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. The presence of sp3 carbons is reduced by increasing the gas temperature in the assembly zone of the plasma reactor. As a general trend, the number of mono-layers decreases when the wall temperature increases from 60 to 100 °C. The synthesized graphene sheets are stable and highly ordered.

Microwave plasma steam reforming of ethanol under vortex gas flow and atmospheric pressure conditions has been investigated. The main gas products of the steam reforming are H2 and CO as detected by mass spectrometry and Fourier transform infrared spectroscopy. A “black” carbon deposit on the wall has been observed. A previously developed theoretical model for ethanol decomposition accounting for the gas thermal balance and the chemical kinetics has been further extended to account for the addition of steam to the argon/ethanol feeding background gas. The mechanisms of ethanol and water decomposition depend on the ethanol/steam ratio, and several hydrogen production regimes have been identified and discussed. An integral reaction scheme for ethanol/water decomposition is suggested.

Generation of energetic hydrogen atoms, with energy in the range 4-8 eV, was detected throughout the volume of a surface wave generated (500 MHz) plasma column in H-2 at pressure p = 0.01 mbar. The H-beta, H-gamma, H-delta, and H-epsilon, line profiles were found to be bi-Gaussian towards the plasma column end. The kinetic temperatures corresponding to the Doppler broadening of the H-beta, H-gamma, H-delta, lines are higher than the rotational temperature of the hydrogen molecular Fulcher-alpha band and the wall temperature. At pressure p = 0.2 mbar, the kinetic temperature of excited H (n = 4-7) atoms, as determined from the fitting of the spectral lines with a single-Gaussian profile, increases with upper level principal quantum number. The experimental results have been analyzed in the framework of a global self-consistent kinetic model describing this surface wave sustained plasma column.

It is well recognized at present that the unique, high energy density plasma environment provides suitable conditions to dissociate/atomize molecules in remediation systems, to convert waste and biomass into sustainable energy sources, to purify water, to assemble nanostructures, etc. The remarkable plasma potential is based on its ability to supply simultaneously high fluxes of charged particles, chemically active molecules, radicals (e.g. O, H, OH), heat, highly energetic photons (UV and extreme UV radiation), and strong electric fields in intrinsic sheath domains. Due to this complexity, low-temperature plasma science and engineering is a huge, highly interdisciplinary field that spans many research disciplines and applications across many areas of our daily life and industrial activities. For this reason, this review deals only with some selected aspects of low-temperature plasma applications for a clean and sustainable environment. It is not intended to be a comprehensive survey, but just to highlight some important works and achievements in specific areas. The selected issues demonstrate the diversity of plasma-based applications associated with clean and sustainable ambiance and also show the unity of the underlying science. Fundamental plasma phenomena/processes/features are the common fibers that pass across all these areas and unify all these applications. Browsing through different topics, we try to emphasize these phenomena/processes/features and their uniqueness in an attempt to build a general overview. The presented survey of recently published works demonstrates that plasma processes show a significant potential as a solution for waste/biomass-to-energy recovery problems. The reforming technologies based on non-thermal plasma treatment of hydrocarbons show promising prospects for the production of hydrogen as a future clean energy carrier. It is also shown that plasmas can provide numerous agents that influence biological activity. The simultaneous generation in water discharges of intense UV radiation, shock waves and active radicals (OH, O, H2O2, etc), which are all effective agents against many biological pathogens and harmful chemicals, make these discharges suitable for decontamination, sterilization and purification processes. Moreover, plasmas appear as invaluable tools for the synthesis and engineering of new nanomaterials and in particular 2D materials. A brief overview on plasma-synthesized carbon nanostructures shows the high potential of such materials for energy conversion and storage applications.

Vacuum ultraviolet emission from Ar-H2 wave driven microwave (2.45 GHz) plasmas operating at low pressures (0.1–1 mbar) has been investigated. The emitted spectra show the presence of the Ar resonance lines at 104.8 and 106.7 nm and of the Lyman-α,β atomic lines at 121.6 nm and 102.6 nm, respectively. The increase of the hydrogen amount in the mixture results in an abrupt increase of the Werner and Lyman molecular bands intensity. The Lyman-β intensity shows little changes in the range of 5%–30% of hydrogen in the mixture while the Lyman-α intensity tends to decrease as the percentage of hydrogen increases.

Microwave atmospheric pressure plasmas driven by surface waves were used to synthesize graphene sheets from vaporized ethanol molecules carried through argon plasma. In the plasma, ethanol decomposes creating carbon atoms that form nanostructures in the outlet plasma stream, where external cooling/heating was applied. It was found that the outlet gas stream temperature plays an important role in the nucleation processes and the structural quality of the produced nanostructures. The synthesis of few layers (from one to five) graphene has been confirmed by high-resolution transmission electron microscopy. Raman spectral studies were conducted to determine the ratio of the 2D to G peaks (>2). Disorder D-peak to G-peak intensity ratio decreases when outlet gas stream temperature decreases. (C) 2013 AIP Publishing LLC.

This paper, the third in a series of three, describes work carried in the context of Plasma Data Exchange Project of the Gaseous Electronics Conference (PDEP-GEC) to compare electron collision cross-sections sets from ground-state, noble gases atoms and to check their consistency with measured swarm parameters. Such consistency is a minimum requirement if the cross-section data are to be used for modelling low-temperature plasmas. In this paper, we present intercomparisons of the independently compiled sets of electron cross-sections from ground-state, neutral Kr and Xe atoms presently available on the LXCat open-access website (www.lxcat.laplace.univ-tlse.fr). Swarm parameters (reduced mobility, characteristic energy, reduced longitudinal diffusion coefficient, reduced ionization coefficient) calculated in a Boltzmann solver or Monte Carlo simulation using these cross-sections sets are compared with experimental data, also available online on the LXCat site.

A detailed collisional-radiative model for Ar at intermediate and high pressures is developed. The model is coupled with the electron Boltzmann equation and includes several highly excited Ar states and charged particles. The densities of all neutral and charged species considered are calculated self-consistently. Detailed information concerning the electronic data used is presented. The model will be further used for analysing the contraction phenomena in noble gas discharges and eventually for theoretical description of the kinetic properties of high pressure Ar excimer laser.

In this work, an experimental investigation of microwave plasma-assisted reforming of different alcohols is presented. A microwave (2.45 GHz) 'tornado'-type plasma with a high-speed tangential gas injection (swirl) at atmospheric pressure is applied to decompose alcohol molecules, namely methanol, ethanol and propanol, and to produce hydrogen-rich gas. The reforming efficiency is investigated both in Ar and Ar+ water vapor plasma environments. The hydrogen yield dependence on the partial alcohol flux is analyzed. Mass spectrometry and Fourier transform infrared spectroscopy are used to detect the outlet gas products from the decomposition process. Hydrogen, carbon monoxide, carbon dioxide and solid carbon are the main decomposition by-products. A significant increase in the hydrogen production rate is observed with the addition of a small amount of water. Furthermore, optical emission spectroscopy is applied to detect the radiation emitted by the plasma and to estimate the gas temperature and electron density.

The overall performance of a surface wave driven air-water plasma source operating at atmospheric pressure and 2.45 GHz has been analyzed. A 1D model previously developed has been improved in order to describe in detail the creation and loss processes of active species of interest. This model provides a complete characterization of the axial structure of the source, including the discharge and the afterglow zones. The main electron creation channel was found to be the associative ionization process N + O → NO+ + e. The NO(X) relative density in the afterglow plasma jet ranges from 1.2% to 1.6% depending on power and water percentage, according to the model predictions and the measurements. Other types of species such as NO2 and nitrous acid HNO2 have also been detected by mass and Fourier Transform Infrared spectroscopy. The relative population density of O(3P) ground state atoms increases from 8% to 10% in the discharge zone when the input microwave power increases from 200 to 400 W and the water percentage from 1% to 10%. Furthermore, high densities of O2(a1Δg) singlet delta oxygen molecules and OH radicals (1% and 5%, respectively) can be achieved in the discharge zone. In the late afterglow the O2(a1Δg) density is about 0.1% of the total density. This plasma source has a flexible operation and potential for channeling the energy in ways that maximize the density of active species of interest.

Investigations on plasma interaction with living matter are presently at the frontiers of plasma research and development. Plasmas contain numerous agents that influence biological activity. For example, plasmas can provide essentially two types of biocidal species: reactive species, such as oxygen atoms that lead to lethality of micro-organisms through erosion, and UV radiation that can damage the DNA strands [1]. An important aspect to be addressed is whether the biological objects are exposed directly to the discharge plasma or to its flowing afterglow. Combined, reactive species and UV photons can lead to significant synergistic effects. Furthermore, non-equilibrium plasmas are able to initiate, promote, control and catalyze complex behaviors and responses in biological systems, in a variety of ways different from the thermal effects [2]. Significant advancement in the plasma surgery, wound healing and tissue regeneration has been achieved with the development of the so-called “Plazon” system (a dc arc), based on a jet of hot air plasma which is rapidly cooled and provides relatively high NO concentrations with significant therapeutic effect [3]. Today, it is well established that NO serves a multitude of essential biological functions in the human organism – it regulates blood vessel tone and blood coagulation, the immune system and early apoptosis, neural communication and memory, etc.

Spatially resolved emission spectroscopy techniques have been used to determine the gas temperature, the electron, and N2+ ion densities and the relative emission intensities of radiative species in a microwave (2.45 GHz) plasma torch driven by a surface wave. The experimental results have been analyzed in terms of a two-dimensional theoretical model based on a self-consistent treatment of particles kinetics, gas dynamics, and wave electrodynamics. The measured spatial variations in the various quantities agree well with the model predictions. The radially averaged gas temperature is around 3000 K and varies only slowly along the discharge zone of the source but it drops sharply down to about 400 K in the postdischarge. The experimental wave dispersion characteristics nearly follow the theoretical ones, thus confirming that this plasma source is driven by a surface wave.

Doppler broadened Hγ emission was detected in high frequency (350 and 500 MHz) hydrogen surface wave sustained discharges, revealing the presence of fast excited H atoms with kinetic energies in the range 4–9 eV. Spatially resolved measurements of the Doppler-broadened emission indicate that these fast atoms are predominantly formed near the wall, which suggests that their generation may result from acceleration of H+ ions in the radial dc space charge field followed by recombination at the wall and the return of the neutral atom to the gas phase.