Volume 78, Issue 6

The 17th International Symposium on Plasma Chemistry (ISPC 17) was held in Toronto, Ontario, Canada from 7-12 August 2005. ISPC 17 is sponsored by the International Union of Pure and Applied Chemistry (IUPAC) and the International Plasma Chemistry Society (IPCS). This is the second time that the ISPC was held in Canada (ISPC 6, Montreal, Quebec, 1983). Two short courses with 60 participants in low- and high-pressure plasma processing were offered prior to the start of the symposium (4-6 August 2005). A one-day industrial workshop with 63 participants was also held on 7 August 2005. ISPC is the main symposium on partially ionized gases and plasma processing science. During ISPC 17, 560 researchers, including 183 students, from 48 countries presented their latest discoveries in this field. 540 contributed papers, 5 plenary and 18 invited abstracts covering 15 topics in plasma processing science were presented during the symposium. All submitted abstracts were reviewed by the International Organizing Committee. The complete text of the papers was provided on a searchable CD. The high number of contributed papers and the high level of registration in both the industrial workshop as well as the short courses reflect the continued support and interest of researchers in plasma processing science community in this bi-annual event. The invited plenary and topical lectures, selected by the International Organizing Committee, are representative of the great progress in the different areas of plasma chemistry. This issue of Pure and Applied Chemistry offers an overview on the recent advances in high- and low-pressure plasmas as well as on more specific developments in plasma processing science. These topics were presented at the ISPC 17 by plenary and topical invited speakers. Javad Mostaghimi Conference Chair

The recent global trend of materials R&D is shifting from "monolithic" to "materials system" and from "bulk" to "thin film and coating". It is thus a natural consequence that plasma spray processing, as an affordable and effective thin film and coating technology, is attracting global attention in materials engineering. Unfortunately, however, the current plasma spray technology for thin film and coating seems to not yet reach the sufficient technological level to meet the requirements from the fields where this technique was not applicable in the past. In this context, this paper will point out the disregarded but important issues involved in conventional plasma powder spraying to fit for high technologies and infer the future potential of novel plasma spray processing with the use of extended feedstock like gases, liquids, and various-sized powders. In particular, special attention will be given to recent challenges aiming at a development of a co-deposition process of droplets and vapors, namely, comprehensive plasma spraying.

Nanoparticles can form via chemical nucleation from gas-phase species during plasma processing of silicon films. Nanoparticle-plasma interactions are studied by simulating finite-rate charging and transport of particle nuclei in a low-pressure processing plasma. Results show little change in electron temperature and concentration during the early stages of particle nucleation. However, the ion density profile changes drastically as ions accumulate near the reactor center. Increased ion concentration corresponds to the growing concentration of negatively charged particles, which are shown to have the highest production rate in the reactor center where they are trapped. A significant number of neutral particles are deposited on reactor walls or onto a deposition substrate by diffusion. Positively charged particles impact the substrate at low concentrations but with high energies, which may affect film morphology during plasma-enhanced chemical vapor deposition (PECVD).

This paper presents the growth of vertically aligned carbon nanotubes by plasma-enhanced chemical vapor deposition (PECVD) using Ni catalyst and C 2 H 2 /NH 3 feedstock. The role of plasma in aligning the carbon nanotubes during growth is investigated both experimentally and computationally, confirming that the field in the plasma sheath causes the nanotubes to be aligned. Experiments using a plasma analyzer show that C 2 H 2 is the dominant precursor for carbon nanotube growth. The role of NH 3 in the plasma chemistry is also investigated, and experimental results show how the interaction between NH 3 and the C 2 H 2 carbon feedstock in the gas phase explains the structural variation in deposited nanotubes for differing gas ratios. The effects of varying the plasma power during deposition on nanotube growth rate is also explored. Finally, the role of endothermic ion-molecule reactions in the plasma sheath is investigated by comparing measured data with simulation results.

Homogenous mechanism of soot formation in moderate-pressure Ar/CH 4 /H 2 microwave discharges was analyzed with the help of kinetics modeling of the thermally nonequilibrium plasmas. Two main reaction mechanisms based on either neutral molecular growth and condensation reaction nucleation process were considered. These mechanisms were incorporated in a numerical model that solves for the plasma species and energy equations under a quasi-uniform plasma assumption. This enabled us to estimate the plasma species density and temperature along with the nucleation rate at different discharge conditions. The results showed that soot particles might form at significant density values by both neutral and ionic mechanisms. Their formation mainly takes place at the discharge edges where the temperature level favors the development of large molecular edifices. Simulations showed that the formation of soot is unlikely to happen in the bulk of the discharge where the gas temperature is high and the large molecular hydrocarbon (HC) cannot form at significant concentrations.

The operating parameters of a miniature atmospheric pressure glow discharge torch (APGD- t ) are optimized for the production of excited atomic oxygen, and the effect of the plasma jet on endothelial cells grown in Petri dishes is studied. We first demonstrate the importance of accounting for the effect of the voltage probe used to measure the electrical parameters of the torch on its ignition and operation characteristics. When operated with a main plasma gas flow rate of 1 SLM He and a power level of ~1 W, the torch shows an optimum in the production of excited atomic oxygen for a O 2 flow of ~3.5 SCCM injected downstream from the plasma-forming region through a capillary electrode (i.e., 0.35 v/v % O 2 /He). It is shown that endothelial cells are detached from the Petri dishes surface under the action of the optimized plasma jet and that this effect does not originate from heating and fluid shearing effects. It is postulated that the cell detachment is caused solely by plasma-induced biochemical processes taking place at the cell-substrate interface.

Applications of atmospheric pressure nonequilibrium plasmas, because of their special advantages of forming reactive plasmas in a simple reactor, are spreading into various engineering fields, not only of materials processing, but also into energy and environment areas. Our group has explored new applications of both filamentary and diffuse barrier discharges, including the establishment of appropriate modeling, which enables better optimization of given plasma processes. More recently, microplasmas produced in submillimeter to micrometer reactors are also highlighted in association with atmospheric pressure nonequilibrium plasma because such small-scale plasmas frequently require high-density media to produce. This paper overviews our recent projects: (1) steam reforming of methane using filamentary barrier discharge; (2) deposition of carbon nanotubes in atmospheric pressure radio frequency discharge (APRFD); and (3) synthesis of silicon nanoparticles using microplasma.

The abatement of perfluorinated compound (PFC) gases is investigated using atmospheric pressure microwave-surface-wave plasmas. These PFCs are diluted in nitrogen gas with concentration ranging from 5000 to 10 000 ppmv. The abatement mechanisms of SF 6 and CF 4 are examined, and conversion schemes are presented in the case where oxygen is added to the gas mixture. The PFC fragments are oxidized, forming acid-like by-products that are finally trapped irreversibly using a humidified soda lime scrubber. Gas-phase analysis was performed using Fourier transform infrared (FTIR) spectroscopy. The abatement efficiency is found to increase with increasing absorbed microwave power and gas residence time. The energy efficiency of the abatement process is shown to increase, with PFC concentrations in the gas mixture up to 10 000 ppmv. A complete conversion of SF 6 is achieved for energy densities ranging from 700 to 1200 J/cm 3 for concentrations ranging from 5000 to 10 000 ppmv. Lowering the microwave excitation frequency and using swirling flow are shown to reduce the energy cost per abated molecule.

Our imaging of radicals interacting with surfaces (IRIS) method was used to investigate radical-surface reactions during low-temperature plasma-enhanced chemical vapor deposition (PECVD) processes. Special emphasis was placed on the analysis of surface reactivities for CH, SiH, CN, NH, NH 2 , CF 2 , and SiCl 2 radicals during film growth. The effects of plasma parameters, such as radio frequency (rf) power and gas composition, substrate temperature, and substrate bias on radical-surface reactivity were analyzed. Different radicals exhibit different behavior at the surface of a depositing film. Specifically, CH, SiH, and CN are "sticky", with high surface reactivities. In contrast, other species such as NH, CF 2 , and SiCl 2 do not stick to the surface of growing films and, in some cases are actually generated at the surface of the depositing film. Different plasma systems and parameters can have an effect on the stickiness of some of these species. Our IRIS measurements indicate a molecule's surface sticking probability may also be related to the molecule's electronic configuration and stability, with the most reactive species being molecules with a doublet electron configuration. In contrast, the singlet species examined here tend to be generated at the surface during film deposition. Our results also indicate that when a molecule scatters with greater than 100 % probability, it is likely to be strongly affected by energetic ion bombardment of the film surface.

Numerous experiments have shown that low-energy H ions and neutrals can erode amorphous carbon at ion energies of 1-10 eV, where physical sputtering is impossible, but at erosion rates which are clearly higher than those caused by thermal ions. In this paper, we will first review our computer simulation work providing an atom-level mechanism for how this erosion occurs, and then present some new results for H and He bombardment of tungsten carbide and amorphous hydrogenated silicon (a-Si:H), which indicate the mechanism can be of importance in a wide range of covalently bonded materials. We also discuss how the presented mechanism relates to previously described abstraction and etching mechanisms.

Streamer discharges in atmospheric gases are currently receiving increased attention in connection with environmental issues or new material treatment technologies. The chemical reactivity of streamer-produced plasma, created by various active atomic and molecular species, is a critical parameter for most application areas. Recently, advanced diagnostics based on optical and energy transfer methods have been applied to study dynamics and spatial distribution of several important streamer-produced species. This paper presents an overview of the most recently reported experimental achievements to monitor and detect reactive species, such as, e.g., NO, OH, and O 3 radicals, nitrogen and oxygen atoms, or N 2 (A 3 Σ) metastables.

The present status of catalyst preparation using nonthermal plasma treatment has been summarized in this paper. Improved dispersion, better low-temperature activity, enhanced stability, and better anti-carbon deposition performance can be achieved with nonthermal plasma-treated catalysts. The improvement in catalyst preparation with nonthermal plasma treatment can reduce or avoid the use of hazardous chemicals. Nonthermal plasma catalyst treatment has especially induced a new development of nonthermal plasma for catalyst reduction. The reduction using hydrogen at high temperatures or using hazardous liquid chemicals can be replaced by the developed plasma reduction process. The mechanism for nonthermal plasma treatment has been presented. An analog between the man-made gas discharge plasmas and the environment inside the zeolite pores and around catalyst surface defects is also proposed.

Metal halide lamps are very efficient light sources based on a Hg plasma arc with metal halide salt additions. In spite of their high efficiency, the lamps suffer from several problems, such as color separation and instabilities, which currently hinder large-scale use. These phenomena are caused by a complex interaction of convection and diffusion flows in the plasma. In order to unravel the various contributions, experiments under microgravity have been performed where convection is absent. The experiments confirm the previously held qualitative views, but also provide absolute data on densities and temperatures that will be used to validate numerical models of these lamps.

Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.

This paper presents an overview of experimental and numerical investigations of the nonequilibrium cold plasma generated under high overvoltage and further usage of this plasma for plasma-assisted combustion. Here, two different types of the discharge are considered: a streamer under high pressure and the so-called fast ionization wave (FIW) at low pressure. The comprehensive experimental investigation of the processes of alkane slow oxidation in mixtures with oxygen and air under nanosecond uniform discharge has been performed. The kinetics of alkane oxidation has been measured from methane to decane in stoichiometric and lean mixtures with oxygen and air at room temperature under the action of high-voltage nanosecond uniform discharge. The efficiency of nanosecond discharges as active particles generator for plasma-assisted combustion and ignition has been investigated. The study of nanosecond barrier discharge influence on a flame propagation and flame blow-off velocity has been carried out. With energy input negligible in comparison with the burner's chemical power, a double flame blow-off velocity increase has been obtained. A signicant shift of the ignition delay time in comparison with the autoignition has been registered for all mixtures. Detonation initiating by high-voltage gas discharge has been demonstrated. The energy deposition in the discharge ranged from 70 mJ to 12 J. The ignition delay time, the velocity of the flame front propagation, and the electrical characteristics of the discharge have been measured during the experiments. Under the conditions of the experiment, three modes of the flame front propagation have been observed, i.e., deflagration, transient detonation, and Chapman-Jouguet detonation. The efficiency of the pulsed nanosecond discharge to deflagration-to-detonation transition (DDT) control has been shown to be very high.