Plasma Chemistry and Plasma Processing (v.32, #3)

ISPC-20 by Gregory Fridman (409-409).

Advances in Plasma Arc Cutting Technology: The Experimental Part of an Integrated Approach by V. Colombo; A. Concetti; E. Ghedini; F. Rotundo; P. Sanibondi; M. Boselli; S. Dallavalle; M. Gherardi; V. Nemchinsky; M. Vancini (411-426).
The experimental part of an integrated approach to design and optimization of plasma arc cutting devices will be presented; in particular results obtained through diagnostics based on high speed imaging and Schlieren photography and some evidences obtained through experimental procedures. High speed imaging enabled to investigate start-up transition phenomena in both pilot arc and transferred arc mode, anode attachment behaviour during piercing and cutting phases, cathode attachment behaviour during start-up transient in PAC torches with both retract and high frequency pulse pilot arc ignition. Schlieren photography has been used to better understand the interaction between the plasma discharge and the kerf front. The behaviour of hafnium cathodes at high current levels at the beginning of their service life was experimentally investigated, with the final aim of characterizing phenomena that take place during those initial piercing and cutting phases and optimizing the initial shape of the surface of the emissive insert.
Keywords: Plasma arc cutting torches; Diagnostics; Electrode erosion; High speed imaging; Schlieren imaging

Thermodynamics, Transport and Kinetics of Equilibrium and Non-Equilibrium Plasmas: A State-to-State Approach by M. Capitelli; I. Armenise; E. Bisceglie; D. Bruno; R. Celiberto; G. Colonna; G. D’Ammando; O. De Pascale; F. Esposito; C. Gorse; V. Laporta; A. Laricchiuta (427-450).
Thermal non-equilibrium plasmas have been deeply investigated theoretically by means of the state-to-state approach, offering the unique opportunity of a detailed information about internal distributions affecting thermodynamics, transport coefficients and kinetics, properly accounting for the presence of excited states. The efforts made in the construction of knowledge on the dynamics of elementary processes occurring in the plasma with resolution on internal degrees of freedom, required by the method, are discussed. Boltzmann equation is solved for electrons self-consistently coupled to the chemical species collisional dynamics, reproducing very interesting features of strongly non-equilibrium internal distributions, characterizing plasmas.
Keywords: Thermodynamics; Transport; Kinetic modeling; State-resolved cross sections

Field Reversal and Particle Growth in DC Discharge by A. Michau; G. Lombardi; L. Colina Delacqua; M. Redolfi; C. Arnas; P. Jestin; X. Bonnin; K. Hassouni (451-470).
A modeling study of carbon clusters and dust particles formation through carbon graphite sputtering in argon DC discharges is presented. The model combines the description of plasma discharge kinetics, molecular growth and transport of carbon clusters and aerosol dynamics for dust particles. Results show that field reversal is a key effect that ensures trapping and growth of negatively charged molecular carbon clusters, which are the precursors for dust particles. The model enables prediction of the space–time distributions of carbon clusters density as well as dust particle density, average charge and average diameter. Results especially show that dust particles and carbon clusters exhibit a pronounced density maximum in the vicinity of the field reversal position. A parametric study is presented in order to analyze the model sensitivity to some key parameters used in the physical model.
Keywords: DC; Dust; Carbon; Cluster; Model

Development of Fast Ionization Wave Discharges at High Pulse Repetition Rates by Keisuke Takashima; Igor V. Adamovich; Uwe Czarnetzki; Dirk Luggenhölscher (471-493).
Positive and negative polarity fast ionization wave discharges in nitrogen and dry air are studied in a rectangular geometry channel over a wide range of time delays between the discharge pulses, τ = 10 ms to 25 μs (pulse repetition rates of ν = 100 Hz–40 kHz). The discharge is generated by alternating polarity high-voltage pulses (peak voltage 18–23 kV, pulse duration 50–60 ns). Ionization wave speed and electric field distributions in the discharge are measured by a calibrated capacitive probe. Plasma images show that for long time delays between the pulses, above τ ~ 1 ms (pulse repetition rate up to ν ~ 1 kHz), the positive polarity wave propagates along the channel walls and the negative polarity wave tends to propagate along the centerline. For time delays below τ ~ 0.5–1.0 ms (above ν ~ 1–2 kHz), both positive and negative polarity waves become diffuse, while wave speed and peak electric field in the wave front become nearly independent of the polarity. Wave speed remains nearly constant for τ ~ 1–10 ms (up to ν ~ 1 kHz), 0.8–0.9 cm/ns, and decreases for shorter time delays, by 30–40 % at τ = 25 μs (ν = 40 kHz). Peak electric field in the positive polarity wave decreases significantly as the pulse repetition rate is increased, from 2.7–3.0 kV/cm to 1.2–1.5 kV/cm. Comparison of experimental results in nitrogen with kinetic modeling calculations at high pulse repetition rates, when the discharge is diffuse, shows good agreement. Calculation results show that peak electric field reduction at high pulse repetition rates is mainly due to higher electron density in the decaying plasma generated by the previous discharge pulse. Reduction of wave speed and coupled energy at high repetition rates is primarily due to slower voltage rise and lower pulse peak voltage, limited by the pulse generator. The model shows that at high pulse repetition rates most of the discharge power is coupled to the plasma behind the wave, at E/N ~ 200–300 Td. Discharge energy loading at ν = 1–40 kHz is 1–2 meV/molecule/pulse.
Keywords: Fast ionization wave; Nanosecond pulse; Electric field measurements; Kinetic modeling

The present contribution is continuation of Part 1: Equilibrium composition and thermodynamic properties. This paper is devoted to the calculation of transport properties of mixtures of water and carbon at high temperature. The transport properties, including electron diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity are obtained by using the Chapman–Enskog method expanded to the third-order approximation (second-order for viscosity), taking only elastic processes into account. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 400 to 30,000 K for pressures of 0. 10, 1.0, 3.0, 5.0 and 10.0 atm. with the results obtained are compared to those of previously published studies, and the reasons for discrepancies are analyzed. The results provide reliable reference data for simulation of plasmas in mixtures of carbon and water.
Keywords: Carbon; Water; Thermal plasmas; Chapman–Enskog method; Transport properties; Viscosity; Thermal conductivity; Electrical conductivity; Diffusion coefficient

Thermal Plasma Synthesis of Superparamagnetic Iron Oxide Nanoparticles by Pingyan Lei; Adam M. Boies; Steven Calder; Steven L. Girshick (519-531).
Superparamagnetic iron oxide nanoparticles were synthesized by injecting ferrocene vapor and oxygen into an argon/helium DC thermal plasma. Size distributions of particles in the reactor exhaust were measured online using an aerosol extraction probe interfaced to a scanning mobility particle sizer, and particles were collected on transmission electron microscopy (TEM) grids and glass fiber filters for off-line characterization. The morphology, chemical and phase composition of the nanoparticles were characterized using TEM and X-ray diffraction, and the magnetic properties of the particles were analyzed with a vibrating sample magnetometer and a magnetic property measurement system. Aerosol at the reactor exhaust consisted of both single nanocrystals and small agglomerates, with a modal mobility diameter of 8–9 nm. Powder synthesized with optimum oxygen flow rate consisted primarily of magnetite (Fe3O4), and had a room-temperature saturation magnetization of 40.15 emu/g, with a coercivity and remanence of 26 Oe and 1.5 emu/g, respectively.
Keywords: Iron oxide; Nanoparticles; DC thermal plasma; Magnetic properties

Silicon oxide films are deposited in atmospheric-pressure (AP) He/O2/HMDSO plasma excited by a 150 MHz VHF power using a cylindrical rotary electrode. The atomic bonding configurations and deposition rate are studied by controlling the O2 concentration (O2/HMDSO source ratio) and VHF power density, the other parameters being maintained constant. Under the addition of 0.03 % O2 to the process gas mixture (O2/HMDSO ≈ 0.09), AP-VHF plasma greatly enhances the fragmentation and oxidation of HMDSO, so that an almost inorganic film is obtained at a very high deposition rate of 33 nm s−1. A silicon oxide coating on a polycarbonate pane is demonstrated with no significant thermal deformation of the pane, showing that AP-VHF plasma would be an efficient coating tool for polymer substrates.
Keywords: Atmospheric-pressure plasma; Chemical vaper deposition; Hexamethyldisiloxane (HMDSO); Silicon oxide; Very high-frequency plasma

Numerical Investigation of a Parallel-Plate Atmospheric-Pressure Nitrogen/Ammonia Dielectric Barrier Discharge by F.-L. Li; K.-M. Lin; Y.-W. Yang; C.-T. Hung; J.-S. Wu; J.-P. Yu (547-564).
In this paper, a planar atmospheric-pressure dielectric barrier discharge (AP-DBD) of nitrogen mixed with ammonia (0–2 %) is simulated using one-dimensional self-consistent fluid modeling with cell-centered finite-volume method. This AP-DBD is driven by a 30 kHz power source with distorted sinusoidal voltages. The simulated discharge current densities are found to be in good agreement with the experiment data in both phase and magnitude. The simulated results show that the discharges of N2 mixed with NH3 (0–2 %) are all typical Townsend-like discharges because the ions always outnumber the electrons very much which leads to no quasi-neutral region in the gap throughout the cycle. N2 + and N4 + are found to be the most abundant charged species during and after the breakdown process, respectively, like a pure nitrogen DBD. NH4 + increases rapidly initially with increasing addition of NH3 and levels off eventually. In addition, N is the most dominant neutral species, except the background species, N2 and NH3, and NH2 and H are the second dominant species, which increase with increasing added NH3. The existence of abundant NH2 plays an important role in those applications which require functional group incorporation.
Keywords: Fluid modeling; Finite-volume method; Atmospheric-pressure dielectric barrier discharge; Nitrogen; Ammonia; Townsend-like discharge

Catalytic Conversion of Simulated Biogas Mixtures to Synthesis Gas in a Fluidized Bed Reactor Supported by a DBD by Thorsten Kroker; Torsten Kolb; Andreas Schenk; Krzysztof Krawczyk; Michal Młotek; Karl-Heinz Gericke (565-582).
The catalytic conversion of methane and carbon dioxide was studied in a fluidized bed reactor supported by a 13.56 Hz driven coaxial DBD-reactor. Palladium or cupper catalyst which are covered on Al2O3 particles were used. The goal was to test whether biogas can be used for the production of synthesis gas. The influences of discharge power, catalysts and temperature of the catalyst bed on the product yield were studied. The starting material and product stream was analyzed by quadrupole mass spectrometry and infrared spectroscopy. H2/CO ratios can be adjusted in a range between 0.65 (without a catalyst) and 1.75 (using a copper catalyst). The process is highly selective for hydrogen production (up to 83%, using a Palladium catalyst). A copper catalyst increases the H2/CO ratio can from 1.04 to 1.16 and the palladium catalyst from 1.11 to 1.43 by heating the catalyst to a temperature of 250°C.
Keywords: Biogas; Cold plasma; Fluidized bed reactor; Synthesis gas; Online monitoring

The aim of this research work was to evaluate the possibility of upgrading the simulated biogas (70 % CH4 and 30 % CO2) for hydrogen-rich syngas production using a multi-stage AC gliding arc system. The results showed that increasing stage number of plasma reactors, applied voltage and electrode gap distance enhanced both CH4 and CO2 conversions, in contrast with the increases in feed flow rate and input frequency. The gaseous products were mainly H2 and CO, with small amounts of C2H2, C2H4 and C2H6. The optimum conditions for hydrogen-rich syngas production using the four-stage AC gliding arc system were a feed flow rate of 150 cm3/min, an input frequency of 300 Hz, an applied voltage of 17 kV and an electrode gap distance of 6 mm. At the minimum power consumption (3.3 × 10−18 W s/molecule of biogas converted and 2.8 × 10−18 W s/molecule of syngas produced), CH4 and CO2 conversions were 21.5 and 5.7 %, respectively, H2 and CO selectivities were 57.1 and 14.9 %, respectively, and H2/CO (hydrogen-rich syngas) was 6.9. The combination of the plasma reforming and partial oxidation provided remarkable improvements to the overall process performance, especially in terms of reducing both the power consumption and the carbon formation on the electrode surface but the produced syngas had a much lower H2/CO ratio, depending on the oxygen/methane feed molar ratio. The best feed molar ratio of O2-to-CH4 ratio was found to be 0.3/1, providing the CH4 conversion of 81.4 %, CO2 conversion of 49.3 %, O2 conversion of 92.4 %, H2 selectivity of 49.5 %, CO selectivity of 49.96 %, and H2/CO of 1.6.
Keywords: Syngas; Biogas reforming; Partial oxidation; Gliding arc discharge; Plasma

Gliding arc discharge process was used for the pretreatment of four azo dye solutions, which are classified to acidic, reactive and chemical indicator, as well as printing and dyeing wastewater with the objective of improving their overall biodegradability. The percentage color removal of all samples were found to be over 92% after 40 min treatment, and the color disappearance of four azo dyes followed the first-order kinetics completely. The biochemical oxygen demand (BOD5)/chemical oxygen demand (COD) ratio increased from 0.02 to 0.46 for acid orange II (AO7), 0.173 to 0.55 for methyl orange, 0.019 to 0.4 for direct fast black, 0 to 0.65 for reactive red K-2BP. The decolorization of printing and dyeing wastewater achieved 97.5% and the COD removal efficiency was 76.6%. The BOD5/COD ratio increased to 0.55 after 20 min treatment. The experimental results indicates it is possible to combine gliding arc discharge with conventional biological treatment for the remedy of wastewater containing generally non-biodegradable dye.
Keywords: Gliding arc; Degradation efficiency; Biodegradability; Azo dyes

Non faradaic yields of anodic contact glow discharge electrolysis (CGDE) originate through H· and OH· radical generated during the process. Scavenging effects of Fe(CN) 6 4− on OH· radicals, in alkaline media have been investigated. A kinetic analysis of the competing reactions of O with different species in the system leads to an yield of 9.8 mol mol electron−1 of OH· and H· radicals each in the liquid phase reaction zone of anodic CGDE in good agreement with the yield reported from a study involving H· radical scavengers.
Keywords: Contact glow discharge electrolysis; OH· scavenger; Alkaline ferrocyanide; Non-faradaic yields; Primary radical yield

Combining a functional plasma polymer matrix with antibacterially active silver (Ag) within a nanocomposite structure allows secure production and applications in various fields, especially in the medical sector. Therefore, nitrogen or oxygen containing hydrocarbon plasma polymers and Ag nanoparticles were simultaneously deposited. Functional groups such as amino or carboxylic groups as well as an adjusted amount of Ag can be incorporated into the growing films by controlling the plasma deposition properties. For this purpose, macroscopic kinetics were used to characterise the deposition behaviour also as a base for possible industrial up-scaling. XPS and ICP-OES were used to analyse the chemical composition of the polymer–Ag nanocomposites and the Ag content which could be incorporated depending on the plasma process conditions. Finally, the Ag release was determined in bi-distilled water for classification and comparison with the antibacterial properties. The antibacterial effect of the polymer–Ag nanocomposites was proofed with the gram− strain Pseudomonas aeruginosa PAO1 and the gram+ strain Staphylococcus aureus (ST12 Group) showing a clear efficacy dependence on the amount of released Ag and the possibility for tailor-made antibacterial active plasma films.
Keywords: Plasma polymerisation; Sputtering; Nanocomposite; Macroscopic kinetics; Antibacterial

Degummed Bombyx mori (B. mori) silk fabrics modified by cold oxygen plasma (COP) and/or titania sols (TSs) were investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction, field emission scan electronic microscopy (FE-SEM), thermo-gravimetric and differential thermal analysis, and ultraviolet (UV) transmittance methods in this study. FT-IR analysis demonstrated that titania particles were associated with B. mori silk fibers by forming organic–inorganic hybrid blends. Processing sequences of COP and TSs, and curing conditions showed significant impacts on the crystalline, thermal, micro-morphological, and UV resistant characteristics of silk fabrics. Crystallinity index by both area and height methods, and crystallite sizes of silk fabrics were calculated as well. Results showed that crystallinity index of finished samples approximate to that of degummed silk fabric could be obtained by applying TSs and curing at 160 °C for 2 min prior to COP treatment, or vice versa with lower temperature of 140 °C for 3 min, whereas the crystallite sizes of treated samples increased slightly. The initial decomposition temperatures of finished samples were elevated by 23–35 °C with increased char residues at 600 °C, while the transmittance of UVA and UVB of finished samples decreased by 11.7, 17.7%, respectively. FE-SEM analysis revealed that titania particles were associated on the fiber surfaces with different smoothness.
Keywords: Cold plasma; Dyes/pigments; Fibers; Gels; Proteins

Here we demonstrate that the emission spectra of the ablation-plasma produced by nanosecond laser pulses on metallic Al targets may be directly connected to the ablation rates and the dimensions of the ablated craters. We show that the variation of the individual spectral-lines intensities with pulse number gives direct, real-time information on the crater depth, whereas the relative intensities of the lines and their widths enable us to study the variation of the electron temperature and density with pulse number and laser fluence in direct connection to the ablation rates. To interpret these results we use a simple model in which the plasma-plume is treated as an ideal gas expanding away from the target with a velocity given by the electron-temperature, and exerting a recoil pressure determined by the electron temperature and density. The model correlates the plume hydrodynamic-length to the crater dimensions and succeeds in predicting the rims heights.
Keywords: Laser ablation; Laser-plasma interactions; Ablation plasma spectroscopy