Plasma Chemistry and Plasma Processing (v.36, #5)

Effect of TiO2 on Various Regions of Active Electrode on Surface Dielectric Barrier Discharge in Air by S. Pekárek; J. Mikeš; I. Beshajová Pelikánová; F. Krčma; P. Dzik (1187-1200).
For surface dielectric barrier discharge in air, we examined the effect of titanium dioxide on various regions of the active electrode on the electrical parameters of discharge, on its emission spectra, and for demonstration of the obtained results also on the concentration of ozone produced by the discharge. We used the active electrode in the form of nine interconnected parallel strips and a square counter electrode. The TiO2 layer covered either only the strips, the region between the strips, or all active electrode. As reference discharge we used the discharge without any layer of TiO2. We found that direct application potential has a version when the strips of the active electrode are covered with a layer of TiO2, because the concentration of ozone produced by the discharge is the highest in all investigated cases. This finding could therefore be used for construction of more efficient ozone generators.
Keywords: Surface dielectric barrier discharge; Photocatalysis; Titanium dioxide; Ozone

Atmospheric Pressure Ammonia Synthesis Using Non-thermal Plasma Assisted Catalysis by Peng Peng; Yun Li; Yanling Cheng; Shaobo Deng; Paul Chen; Roger Ruan (1201-1210).
This paper described a novel and green approach on catalytic ammonia synthesis using non-thermal plasma (NTP). The process studied in this paper involves the synthesis and absorption of ammonia under atmospheric pressure and low temperature. The effects of operational parameters including applied voltage, frequency, gas component and flow rate on ammonia synthesis under NTP conditions are studied in this paper. In addition, different selected catalysts and absorbents were investigated under different conditions of NTP treatment and the ammonia efficiency was reported and analyzed. Ru catalyst with carbon nanotube support, along with Cs promoter and micro porous absorbents including Molecular Sieve 13X and Amberlyst 15 yield the highest ammonia efficiency in this process. Results further indicated that frequency and applied voltage of 10,000 Hz and 6000 V, with N2:H2 feed ratio of 3:1 provided the optimized efficiency of ammonia synthesis of 2.3 gNH3/kWh.
Keywords: Non-thermal plasma; Catalytic ammonia synthesis; Atmospheric pressure; Low temperature

A novel plasma-catalyst converter (NPCC) was engineered in applying the carbon capture utilization technology for the destruction of carbon dioxide (CO2), which is a cause of global warming and is generated from the combustion of fossil fuels. The NPCC has an orifice-type baffle to improve an amount of gas feed with the higher CO2 destruction for a stationary point sources application . To examine its ability for the CO2 destruction, the performance analysis was conducted on the effects of methane additive, nozzle injection velocity, total gas feed, and catalyst type. The product gas from the NPCC was combustible components like CO, H2, CH4, THCs. The CO2 destruction and the CH4 conversion at a 1.29 CH4/CO2 ratio were 37 and 47 %, respectively, and the energy decomposition efficiency was 0.0036 L/min W. The nickel oxide catalyst among other catalysts showed the most effectiveness for the CO2 destruction and CH4 conversion at a lower temperature. The carbon-black produced without the catalytic bed has carbon nanoparticles with diverse shapes, such as spherical carbon particles and carbon nanotubes; and its high conductivity and specific surface area were suitable for special electronic materials, fuel cells, and nanocomponents.
Keywords: Global warming gas; Plasma-catalyst converter; Carbon dioxide; Methane reforming; Carbon-black

The Effect of pH on OH Radical Generation in Aqueous Solutions by Atmospheric Pressure Glow Discharge by Anna Khlyustova; Natalija Khomyakova; Nikolaj Sirotkin; Yuriy Marfin (1229-1238).
Formation of hydroxyl radicals in aqueous solutions by atmospheric pressure DC glow discharge was investigated. The ferrocyanide was used as an OH scavenger at different pH. Experimental results indicated that pH effect on hydroxyl radical formation rate could be explained by particularities of [Fe(CN)6]4− and reactive species interaction in different media.
Keywords: OH radicals; Atmospheric pressure glow discharge; Formation rate; Yield

Easy Synthesis of Ageing-Resistant Coatings with Tunable Wettability by Atmospheric Pressure Plasma by Bernard Nisol; Jonathan Ghesquière; François Reniers (1239-1252).
This study presents a simple and dry approach to synthesize stable, thin organic coatings with tunable wettability by injecting appropriate quantities of propargyl methacrylate (propaMA) and acrylic acid (AA) into a dielectric barrier discharge operating at atmospheric pressure. Deposition rates of up to 11 nm s−1 can be achieved, thanks to the high reactivity of the propaMA monomer in the discharge. The AA monomer exhibits a weaker reactivity but, as evidenced by IRRAS and XPS analyses, allows introducing polar groups into the coating, thereby modifying the surface wettability. The surface is thus shown to be tunable from highly hydrophobic (WCA = 140°, pure propaMA coatings) to highly hydrophilic (WCA = 15°, pure AA coatings) by adjusting the monomer ratio in the discharge. These coatings have been deposited on polypropylene (PP) substrates, and the resulting WCA is shown to remain constant for at least 64 days, ageing that is remarkably slow compared with plasma functionalization that usually leads to rapid hydrophobic recovery.
Keywords: Surface modification; Polymeric materials; Thin films; Ageing; Dielectric barrier discharge

Kinetics and Mechanism of Cr(VI) Reduction in a Water Cathode Induced by Atmospheric Pressure DC Discharge in Air by Dmitriy A. Shutov; Aleksandra V. Sungurova; Andrei Choukourov; Vladimir V. Rybkin (1253-1269).
The process of reduction of Cr6+ ions (solution of potassium dichromate, K2Cr2O7) in a water cathode was studied during a DC discharge in air. The concentration range of Cr6+ was (5.7–19) ×10−5 mol/l and discharge current range was 20–80 mA. Cr6+ ions were shown to be reversibly reduced under a discharge action. The equilibrium degree of reduction increased with increasing initial concentration of the solution at fixed discharge current. At fixed initial concentration the reduction degree increased with increasing discharge current. The reduction degrees so obtained were 0.34–0.84. A kinetic scheme of the processes taking place in a solution was proposed. The calculated data obtained as a result of application of this scheme described well the experimental results on Cr6+ kinetics. The main processes of Cr6+ reduction and Cr3+ oxidation were revealed. HO 2 · radicals and hydrogen peroxide were shown to be responsible for Cr6+ reduction whereas ·OH radicals and O2 molecules provide the reverse process of Cr3+ oxidation to Cr6+. The mechanism of action of phenol additives improving the process efficiency is discussed. The efficiency of phenol action as a radical scavenger was shown to be determined with its mass-transfer to the reaction area rather than chemical reaction rate.
Keywords: DC air discharge; Water cathode; Modeling; Chromium ions; Reduction; Oxidation

Surface Modification of Low-Carbon Steels by Plasma Electrolytic Nitrocarburising by S. A. Kusmanov; I. G. Dyakov; Yu. V. Kusmanova; P. N. Belkin (1271-1286).
This study investigates the effect of the plasma electrolytic nitrocarburising conditions and electrolyte composition (NH4Cl, NH4NO3 and glycerol) on steel structure and properties. The cross-sectional microstructure, composition and phase constituents of a modified layer are characterized. The thickness of each layer is determined by carbon and nitrogen diffusion, anode dissolution, and oxidation that occur simultaneously. These processes are affected by the concentration of electrolyte components. The aqueous solution containing NH4NO3, NH4Cl and glycerol enables to obtain the modified layer with the thickness of 0.20 mm and hardness of 930 HV, the decrease in the surface roughness from 1.01 to 0.15 µm, the corrosion rate by a factor of 4.4 and the weight loss after lubricate wear testing by a factor of 14.
Keywords: Plasma electrolytic nitrocarburising; Microhardness; Surface roughness; Wear behavior; Corrosion resistance

We have used two calibration free laser induced breakdown spectroscopy (CF-LIBS) methods for the quantitative analysis of Pb–Sn alloys with different compositions. The first method is based on Boltzmann plots where the elemental concentration is determined from the intercept along the y-axis whereas the second method is the electron density conservation method, in which elemental compositions are determined by comparing the experimentally measured number density with the theoretical results obtained by CF-LIBS. The neutral spectral lines of lead and tin have been used for the estimation of plasma temperature by the Boltzmann plot method whereas the Saha–Boltzmann equation has been used to calculate the electron number density. By comparing the results of both CF-LIBS methods, it is concluded that the CF-LIBS (electron number density conservation) method is more appropriate than the CF-LIBS (Boltzmann plot method) for the quantitative elemental analysis.
Keywords: LIBS; Self-calibration method; CF-LIBS; Eelectron density; Pb–Sn Alloys; Quantitative analysis

Determination of the Dominant Species and Reactions in Non-equilibrium CO2 Thermal Plasmas with a Two-Temperature Chemical Kinetic Model by Qingqing Gao; Aijun Yang; Xiaohua Wang; Anthony B. Murphy; Yunjia Li; Chaojun Zhang; Yanhui Lu; Li Huan; Zhaofang Zhu; Mingzhe Rong (1301-1323).
It has become increasingly clear that deviations from local thermodynamic equilibrium occur in thermal plasmas. This paper is devoted to investigating the non-equilibrium characteristics of CO2 thermal plasmas, which have wide application in industry. A two-temperature chemical kinetic model with a comprehensive chemical system is developed to calculate the non-equilibrium characteristics of CO2 thermal plasmas for a wide temperature range, from 12,000 to 500 K, at atmospheric pressure. The non-equilibrium results are compared to the equilibrium composition obtained by Gibbs free energy minimization, and significant deviations are found at lower temperatures. Based on the dependence of molar fractions on temperature, the dominant species are determined in three temperature ranges. The dominant reactions are then obtained by considering their contribution to the generation and loss of the dominant species. Using the dominant species and reactions, the full model is simplified into three simpler models and the accuracy of the simplified models is evaluated. It is shown that this approach greatly reduces the number of species and reactions considered, while showing good agreement with the full model, with a root-mean-square error of no more than 4 %. Thus, the complicated physicochemical processes in non-equilibrium CO2 thermal plasmas can be characterized by relatively few species and reactions. It is suggested that the two-temperature chemical kinetic model developed in this paper can be applied to the full range of pressures that occur in arc welding, arc quenching and other industrial applications. In addition, the simplified methods can be applied in multi-dimensional models to reduce the chemical complexity and computing time while capturing the main physicochemical processes in non-equilibrium CO2 thermal plasmas.
Keywords: CO2 thermal plasmas; Departures from thermal equilibrium; Dominant species and reactions; Simplified models

Synthesis of Nano-catalysts by Induction Suspension Plasma Technology (SPS) for Fischer–Tropsch Reaction by James Aluha; Kossi Bere; Nicolas Abatzoglou; François Gitzhofer (1325-1348).
Nanometric catalysts were synthesized through induction suspension plasma technology (SPS) for application in the Fischer–Tropsch synthesis (FTS). Carbon-supported single metal catalysts (Co/C, Fe/C), bimetallic formulations (Co–Fe/C), and ternary (Co–Fe–Mo and Co–Fe–Ni) systems have been considered in this work. SPS has been selected because it simultaneously allows for: (1) atomizing and generating metallic nanoparticles; (2) creating particularly Fe carbides, which are important in Fe-based FTS reaction mechanism; (3) in situ production of the nanometric graphitic-carbon matrix; and (4) saving time in catalyst synthesis, limiting sample preparation steps and eliminating post synthesis treatment before use. Porosity measurements by the Brunauer–Emmett–Teller method indicate that the samples are essentially non-porous. The synthesized catalysts characterized by X-ray Diffraction analysis show the presence of both metallic and carbidic species. The graphitic-carbon matrix has substantial structural defects that make it partly amorphous. Scanning Electron Microscopy analysis coupled with Energy Dispersive X-ray Spectroscopy mapping shows uniform dispersion of the metal moieties in the carbon support. Analysis by Transmission Electron Microscopy imaging displays metal nanoparticles with mean particle size within the 9–15 nm range enveloped in the carbon matrix.
Keywords: Induction plasma; Suspension plasma; Cobalt–iron–carbon catalyst; Fischer–Tropsch

Nano-Architecture of Facetted NiFe2O4/(Ni,Fe)O Particles Produced by Induction Plasma by Samuel Bastien; Christian Ricolleau; Nadi Braidy (1349-1362).
Facetted nickel ferrite (NiFe2O4) and bunsenite [(Ni,Fe)O] nanocrystals were grown from the decomposition of iron and nickel nitrate precursors using an inductively coupled plasma reactor. The full range of the two-phase region of the Fe2O3–NiO pseudo-equilibrium phase diagram was investigated by producing nanopowders with bulk Ni/(Ni + Fe) ratios of 0.33, 0.4, 0.5, 0.75 and 1.0. A Ni-poor [Ni/(Ni + Fe) ≤ 0.5] precursor solution produced truncated octahedron nanocrystals, whereas nanocubes were obtained at higher ratios [Ni/(Ni + Fe) ≈ 1]. In both cases, it is shown that the nanocrystals adopt a morphology close to the Wulff shape of the crystalline system (spinel and NaCl, respectively). As the bulk Ni/(Ni + Fe) ratio increases from 0.33 (the stoechiometric composition of nickel ferrite), bunsenite is epitaxially segregated on the {110} and {111} facets of nickel ferrite, while leaving the NiFe2O4 {100} facets exposed. A precursor solution at a Ni/(Ni + Fe) ratio of 0.75 gave an (Ni,Fe)O-rich nanopowder with a random and irregular interconnected morphology. The structure of these nanocrystals can be understood in terms of their thermal history in the plasma reactor. These results highlights the possibility of producing organized multi-phased nanomaterials of binary systems having two phases stable at high temperatures, using a method known to be easily scalable.
Keywords: Induction plasma; Multi-phased; Nanoparticles; TEM

In this work, a fluid model has been applied to study HBr/Ar capacitively coupled plasma discharges that are being used for anisotropic etching process. Based on time average reaction rates, the model identify the most dominant species in HBr/Ar plasma. Our simulation results show that the neutral species like H and Br, which are the key precursors in chemical etching, have bell shape distribution while ions like HBr+, Br+ and Ar+ which plays a dominant role in the physical etching, have double humped distribution and shows peaks near electrodes. The effect of HBr/Ar mixing ratios on densities of dominant species are analyzed. The addition of Ar to HBr plasma decreases H, Br and HBr+ densities slightly while increases Br+ and Ar+ densities. It was found that the dilution of HBr by Ar results in an increase in electron density and electron temperature, which results in more ionization and dissociation. The densities and hence the fluxes of the neutrals and positive ions for etching and subsequently chemical etching versus physical etching in HBr/Ar plasmas discharge can be controlled by tuning Ar concentration in the discharge and the desire etching can be achieved.
Keywords: HBr plasma; Chemical and physical etching; Plasma simulation; Sietching

In this study, three hydrophilic monomers; 2-acrylamido-2-methyl-1-propanesulfonic acid, 2-hydroxyethyl methacrylate, diallyldimethylammonium chloride (DADMAC) were selected and their performance as an antistatic finish on nylon 6,6 fabrics was investigated. A non-thermal, high density atmospheric pressure plasma was used to graft polymerize the monomers on nylon 6,6 fabrics. Fabrics were first treated with solutions of monomer in water, air dried and then treated with helium plasma to graft polymerize the monomer on the fiber surface. Surface resistivity values were measured before and after soxhlet extraction with water. Results showed that the DADMAC monomer provided better antistatic properties to fabrics. Further studies with DADMAC monomer were made; effects of plasma post exposure time, plasma pre-exposure time, plasma power, concentration of the monomer and existence of a crosslinker were investigated. Higher plasma power, higher concentration of the monomer and longer post exposure times all gave better antistatic properties to the nylon 6,6 fabrics. Acid dye staining, UV–Vis and FT-IR measurements were conducted and results confirmed a grafted poly-DADMAC layer on the fabric surface.
Keywords: Plasma; Antistatic; Textile; Surface modification