Volume 82, Issue 6

The 19th International Symposium on Plasma Chemistry (ISPC-19) was held at Ruhr-University Bochum, Germany, from 26 to 31 July 2009 under the sponsorship of the International Union of Pure and Applied Chemistry (IUPAC) and the International Plasma Chemistry Society (IPCS), and under the patronage of the Ministry of Innovation, Science, Research and Technology of North-Rhine Westphalia, Germany. Bochum also served as the host venue for ISPC-10 in 1991, and this return occasion of ISPC-19 was co-sponsored by the German Science Foundation, the Ruhr-University Bochum, and the town of Bochum. ISPC is a bi-annual international conference with topics encompassing the complete areas of plasma chemistry and plasma processing science. Almost 650 participants from over 40 countries presented 682 papers. Six plenary speakers and 22 topical invited speakers covered a broad range of topics. These keynote lectures were complemented by 105 oral contributions and about 530 poster presentations in 16 categorized scientific topics. The large number of papers reflects that the current field of plasma chemistry is lively and evolving, and crosses the borders between many disciplines ranging from natural to engineering sciences. This issue of Pure and Applied Chemistry contains 13 papers based on plenary lectures and topical invited talks presented at ISPC-19. This compilation illustrates very well that the traditional distinction between low-pressure and thermal high-pressure plasmas almost disappears; the current research topics on microplasmas and microplasma jets exhibit many similarities if compared to research on conventional arcs used for thermal spraying. We sincerely hope that this issue provides readers an overview of current and future scientific developments. Achim von Keudell Conference Chair Jörg Winter Conference Co-Chair

Unlike the generation of uniform plasmas in a large volume, structured plasma comprising microplasmas has attracted increasing scientific and technological interest in recent years. Similarly, the concept of plasma generation in artificial media with gas–gas, gas–liquid, and gas–solid interfaces under intentionally organized and controlled conditions has the potential to open a new field in plasma science and technology. Amongst the subtopics of plasma generation in artificial media, we discuss here those dealing with micro-plasma jets in flowing gas channels, microdischarges within microbubbles generated in aqueous electrolyte solutions, and discharges in bubbled water and misted air. Our discussion is supported by preliminary results and aims to stimulate future systematic and quantitative investigations.

Our recent work on optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) applied to dielectric barrier discharge (DBD) at atmospheric pressure (ATP) is reported. The interplay of discharge kinetics and diagnostic techniques features is evidenced, with emphasis on collision processes that, with their variety of state-to-state characteristics and rate constants, determine the observation of a peculiar collision-dominated non-equilibrium system.

Diagnostic-based modeling (DBM) actively combines complementary advantages of numerical plasma simulations and relatively simple optical emission spectroscopy (OES). DBM is applied to determine spatial absolute atomic oxygen ground-state density profiles in a micro atmospheric-pressure plasma jet operated in He–O 2 . A 1D fluid model with semi-kinetic treatment of the electrons yields detailed information on the electron dynamics and the corresponding spatio-temporal electron energy distribution function. Benchmarking this time- and space-resolved simulation with phase-resolved OES (PROES) allows subsequent derivation of effective excitation rates as the basis for DBM. The population dynamics of the upper O(3 p 3 P ) oxygen state ( λ = 844 nm) is governed by direct electron impact excitation, dissociative excitation, radiation losses, and collisional induced quenching. Absolute values for atomic oxygen densities are obtained through tracer comparison with the upper Ar(2 p 1 ) state ( λ = 750.4 nm). The resulting spatial profile for the absolute atomic oxygen density shows an excellent quantitative agreement to a density profile obtained by two-photon absorption laser-induced fluorescence spectroscopy.

Plasma-based treatment of chronic wounds or skin diseases as well as tissue engineering or tumor treatment is an extremely promising field. First practical studies are promising, and plasma medicine as an independent medical field is emerging worldwide. While during the last years the basics of sterilizing effects of plasmas were well studied, concepts of tailor-made plasma sources which meet the technical requirements of medical instrumentation are still less developed. Indeed, studies on the verification of selective antiseptic effects of plasmas are required, but the development of advanced plasma sources for biomedical applications and a profound knowledge of their physics, chemistry, and parameters must be contributed by physical research. Considering atmospheric-pressure plasma sources, the determination of discharge development and plasma parameters is a great challenge, due to the high complexity and limited diagnostic approaches. This contribution gives an overview on plasma sources for therapeutic applications in plasma medicine. Selected specific plasma sources that are used for the investigation of various biological effects are presented and discussed. Furthermore, the needs, prospects, and approaches for its characterization from the fundamental plasma physical point of view will be discussed.

Plasma technology offers many interesting possibilities for the production of high-value-added textiles. Nevertheless, textiles can have a considerable structural and chemical complexity, and their properties must be taken into account for the implementation of plasma processes. The influences of some of these properties are highlighted through several examples of recent interesting applications, such as the metallization of polyester yarns, the enhancement of fabric moisture wicking and the surface functionalization with plasma polymerization.

High-power impulse magnetron sputtering (HIPIMS) was introduced in the late 1990s as a unique physical vapor deposition method. The technology utilizes magnetron sputtering cathodes and high peak power density of up to 3 kW cm –2 on the target. The plasma produces a metal flux with high degree of ionization. HIPIMS has been successfully used as a substrate pretreatment method to enhance coating adhesion by promoting local epitaxial growth. As a deposition technology, HIPIMS produces high-density microstructure films. It has been industrialized and has successful applications in hard, electronic, and optical coatings.

There are many different techniques for the synthesis of carbon nanotubes (CNTs), and plasma technologies experience a significant competitor in thermal chemical vapor deposition (CVD) processes. A particular process is, therefore, selected according to the specific requirements of an application, which clearly differ for the development of composites as compared to nanoelectronics, field emission, displays, sensors, and the like. This paper discusses the method for the synthesis of CNTs using an atmospheric-pressure microwave (MW) torch. It was successfully applied in the fast deposition of multiwalled nanotubes (MWNTs) on a substrate without the necessity of any vacuum or heating equipment. Dense straight-standing nanotubes were prepared on Si substrates with and also without barrier SiO x layer. Therefore, it was possible to produce CNTs directly on conductive Si and to use them as an electron-emitting electrode of the gas pressure sensor. The CNTs grown in MW torch were also used to create a gas sensor based on the changes of electrical resistance measured between two planar electrodes connected by the CNTs.

Formation of dust particles is a common mechanism in low-pressure plasmas. These big particles (in comparison with other plasma species) are sometimes the desired final products of the process, but they may also constitute a severe drawback in certain contexts. In either situation, it is necessary to understand growth mechanisms well, in order to control or avoid dust particle formation. One of the problems that has to be overcome is that dust particle growth is usually a continuous mechanism: once started, it can enter into a cyclic regime where new generations of dust particles are succeeding one after the other. This cyclic phenomenon often induces a side effect consisting of instabilities of a few tens of Hz. This paper discusses the main characteristics of dust successive generations, and particularly the importance of dust-free spaces (void) involved in this process. Finally, some aspects related to deposition when several generations coexist will be presented.

In this paper, an overview is given of modeling activities going on in our research group, for describing the plasma chemistry and plasma–surface interactions in reactive plasmas. The plasma chemistry is calculated by a fluid approach or by hybrid Monte Carlo (MC)–fluid modeling. An example of both is illustrated in the first part of the paper. The example of fluid modeling is given for a dielectric barrier discharge (DBD) in CH 4 /O 2 , to describe the partial oxidation of CH 4 into value-added chemicals. The example of hybrid MC–fluid modeling concerns an inductively coupled plasma (ICP) etch reactor in Ar/Cl 2 /O 2 , including also the description of the etch process. The second part of the paper deals with the treatment of plasma–surface interactions on the atomic level, with molecular dynamics (MD) simulations or a combination of MD and MC simulations.

In low-pressure plasmas commonly used in materials processing, plasma–wall inter-actions play a crucial role in the evolution of the plasma properties both over time and across large-area wafers. We have recently studied the heterogeneous recombination of O and Cl atoms on reactor walls in O 2 and Cl 2 plasmas through both experiments and modeling. The Langmuir–Hinshelwood (i.e., delayed) recombination was investigated using a “spinning-wall” technique in which a portion of the substrate surface is periodically exposed to an inductively coupled plasma and to a differentially pumped chamber where either Auger electron spectroscopy (AES) or line-of-sight mass spectrometry (MS) is used to detect surface and desorbing species. In this paper, a review of the various effects driving the O and Cl atoms recombination dynamics on anodized aluminum (AA) and stainless steel (SS) surfaces is presented. It is shown that recombination probabilities, γ , can vary following plasma exposure due to surface conditioning. In Cl 2 plasmas, γ was also found to depend on the Cl-to-Cl 2 number density ratio, a mechanism ascribed to a competition for adsorption sites between Cl and Cl 2 . We have also determined the recombination rates of Cl atoms in Cl 2 high-density plasmas sustained by electromagnetic surface waves by comparing the measured degrees of dissociation of Cl 2 to those predicted by an isothermal fluid model. For a reactor with large SS and quartz surfaces exposed to the plasma, γ values and their dependence on the Cl-to-Cl 2 number density ratio were consistent with those obtained from the rotating substrate technique. Similar values were obtained for plasmas sustained in a quartz discharge tube. It is expected that for plasmas sustained in or adjacent to a silica tube or plate, the Cl atoms recombination coefficient becomes independent of chamber wall material due to reactor seasoning, producing a silicon-oxychloride layer.

The irradiation of an intense laser pulse onto a solid target immersed in liquid produces dense plasma. The plasma produced by liquid-phase laser ablation has unique features at high pressure and temperature, which are never realized by liquid-phase discharges. Another unique characteristic of liquid-phase laser ablation is the formation of a cavitation bubble. This article reports the fundamental aspects of liquid-phase laser-ablation plasmas, cavitation bubbles, and the formation processes of nanoparticles, together with some applications of liquid-phase laser ablation.

The combination of a nonthermal, atmospheric plasma with a catalyst is investigated as a means of destroying pollutants in waste gas streams. Using the examples of dichloromethane (DCM) and toluene in air streams, it is shown that the destruction of the pollutant can be increased whilst lowering the operating temperature, giving increasing energy efficiency. Unwanted by-products can also be reduced selectively by appropriate choice of catalyst and of the plasma–catalyst configuration. By studying the temperature dependence of plasma catalysis, some ideas can be obtained about the nature of the interaction between plasma and catalyst in the processing.

A stable 12-phase AC arc was generated by transformers at a commercial electric power system, and the arc behavior was characterized by image analysis. For the unique advantages, the multiphase AC arc was developed to apply to in-flight glass melting for the purpose of energy-saving and emission reduction. The effects of electrode configuration and sheath gas flow rate on the arc and melting behavior of granulated glass raw material were investigated. Results show that the discharge behavior and the high-temperature region can be controlled by the electrode configuration. The luminance area of the high-temperature region and its fluctuation reflect the discharge behavior. The vitrification degree of glass raw material is mostly dependent on the center temperature of arc. As sheath gas flow rate increases, the ratio of luminance area decreases and the center temperature of arc increases.