Plasma Chemistry and Plasma Processing (v.33, #5)

Gliding Arc Plasma Synthesis of Crystalline TiO2 Nanopowders with High Photocatalytic Activity by Shi-Xin Liu; Xiao-Song Li; Xiaobing Zhu; Tian-Liang Zhao; Jing-Lin Liu; Ai-Min Zhu (827-838).
A warm plasma generated by gliding arc discharge, combining the advantages of both thermal and cold plasmas, is explored to synthesize TiO2 nanopowders for the first time. Air is used as the discharge gas and titanium tetraisopropoxide aerosol is carried by N2 into the plasma. X-ray diffraction and X-ray photoelectron spectroscopy characterizations confirm that the as-synthesized nanopowders are fully crystalline TiO2. The weight fraction of anatase (f A) is higher than that of rutile and increases from 68.8 to 96.8 % by increasing specific energy input (SEI) from 46 to 76 kJ/mol. The effect of SEI on specific surface area (S BET) and BET-equivalent diameter (d BET) of the nanopowders is investigated. The typically spheric morphology of the particles is observed by transmission electron microscopy (TEM) and the most probable d TEM approaches the d BET. All the as-synthesized nanopowders show a high photocatalytic activity comparable to that of Degussa P25.
Keywords: Plasma synthesis; TiO2 nanopowder; Gliding arc discharge; Photocatalyst

Silicon Dioxide Coating of Titanium Dioxide Nanoparticles from Dielectric Barrier Discharge in a Gaseous Mixture of Silane and Nitrogen by Sebastian Dahle; Lienhard Wegewitz; Fei Qi; Alfred P. Weber; Wolfgang Maus-Friedrichs (839-853).
The coating of titanium dioxide nanoparticles with silicon dioxide has been carried out by dielectric barrier discharge (DBD) plasma treatments to enhance the thermostability of Titania for applications at high temperature processes. During the first coating processing step, a closed film of silicon nitride was produced via plasma treatment in a gaseous mixture of silane and nitrogen, while atmospheric surface contaminations got mainly removed. In the second processing step, the DBD plasma treatment in oxygen or air was used to convert the silicon nitride mainly into silicon dioxide. Remaining carbon impurities at the interfaces between titanium dioxide and silicon nitride after the nitrogen/silane plasma treatment were subsequently removed simultaneously. Atomic force microscopy and X-ray photoelectron spectroscopy were employed to study the DBD plasma treatments of the TiO2 nanoparticles.
Keywords: X-ray photoelectron spectroscopy; Atomic force microscopy; Core–shell-structure

Non-Thermal Plasma Assisted Regeneration of Acetone Adsorbed TiO2 Surface by L. Sivachandiran; F. Thevenet; A. Rousseau (855-871).
Improvement of indoor air quality regarding volatile organic compounds (VOCs) requires the development of innovative oxidation processes. This paper investigates the coupling of a metal oxide sorbent with non-thermal plasma (NTP) in an especially designed reactor. TiO2 was selected as model sorbent and acetone was used as model VOC. The analyses of gas phase species at the reactor downstream have been performed using FTIR spectroscopy. In a first step, acetone adsorption on TiO2 surface under dry air was characterized in terms of total amount adsorbed, as well as reversibly and irreversibly adsorbed fractions. Obtained results were compared and discussed with literature in terms of acetone reactive adsorption on TiO2 surface. Mesityloxide was proposed as the major compound in the irreversibly adsorbed fraction. In a second time, acetone saturated TiO2 surface was exposed to NTP surface discharge. Irrespectively of the injected power, <30 % of the initially adsorbed acetone has been recovered as CO, CO2 and desorbed acetone. Finally, thermal desorptions have been performed. They evidenced that (1) NTP treatment modifies the nature of the adsorbed organic species, (2) mineralization rate is considerably improved. Based on desorbed species temporal profile analysis, carboxylates and more especially formates are suggested as major adsorbed species after NTP treatment (Pinj > 0.2 W). This hypothesis has been evaluated and confirmed. This paper finally evidenced that NTP can be used as an efficient pretreatment technique to promote the mineralization of adsorbed acetone for further thermal treatment.
Keywords: Adsorption; Non-thermal plasma; Acetone; Surface regeneration

Pulsed plasmachemical deposited poly(maleic anhydride) nanolayers display high efficiency for the capture of toxic cadmium ions from aqueous solution (down to the low parts per billion range). Subsequent release of the isolated heavy metal species and host media regeneration is accomplished by rinsing in weak acid solution.
Keywords: Plasma polymer; Maleic anhydride; Capture and release; Water purification; Heavy metal ion

Plasma Treatment of Glass Surfaces Using Diffuse Coplanar Surface Barrier Discharge in Ambient Air by Tomáš Homola; Jindřich Matoušek; Martin Kormunda; Linda Y. L. Wu; Mirko Černák (881-894).
We report a study on the treatment of flat glass surfaces by ambient air atmospheric pressure plasma, generated by a dielectric barrier discharge of coplanar arrangement of the electrode system—the diffuse coplanar surface barrier discharge (DCSBD). The plasma treatment of glass was performed in both static and dynamic modes. With respect to wettability of the glass surface, treatment in static mode resulted in non-uniform surface properties, whereas dynamic mode provided a fully uniform treatment. A water contact angle measurement was used to determine the efficiency of plasma treatments in dynamic mode and also to investigate a hydrophobic recovery of plasma treated glass surfaces. The X-ray photoelectron spectroscopy measurements showed a decrease of overall carbon concentrations after plasma treatment. A deconvolution of C1s peak, showed that a short plasma treatment led to decrease of C–C bonds concentration and increases of C–O and O–C=O bond concentrations. An enhancing influence of the glass surface itself on DCSBD diffuse plasma was observed and explained by different discharge onsets and changes in the electric field distribution.
Keywords: Atmospheric pressure air plasma; DCSBD; Diffuse plasma; Glass surface; XPS

Atmospheric Pressure Plasma Treatment of Fused Silica, Related Surface and Near-Surface Effects and Applications by Christoph Gerhard; Tobias Weihs; Daniel Tasche; Stephan Brückner; Stephan Wieneke; Wolfgang Viöl (895-905).
We report on an atmospheric pressure plasma (APP) treatment of fused silica and its related surface and near-surface effects. Such treatment was performed in order to improve laser micro-structuring of fused silica by a plasma-induced modification of the glass boundary layer. In this context, an APP jet applying a hydrogenous process gas was used. By the plasma treatment, the transmission of the investigated glass samples was significantly decreased. Further, a decrease in the superficial index of refraction of approx. 3.66 % at a wavelength of 636.7 nm was detected ellipsometrically. By surface energy measurements, a decrease of the surface polarity of 30.23 % was identified. These determined modifications confirm a reduction of silicon dioxide to UV-absorbing silicon suboxide as already reported in previous work. Further, a change in reflexion by maximum 0.26 % was detected which is explained by the superposition of constructive and destructive interferences due to a surface wrinkling. With the aid of atomic force microscopy, an increase of the surface root mean squared roughness by a factor of approx. 19 was determined. It was found that both the surface energy and the strength of the fused silica surface were reduced by the plasma treatment. Even though such treatment led to a clustering of carbonaceous contaminants, a surface-cleaning effect was confirmed by secondary ion mass spectroscopy and energy-dispersive X-ray spectroscopy. The increase in UV-absorption allows enhanced laser ablation results as shown in previous work.
Keywords: Atmospheric pressure plasma; Glass; Optical properties; Surface modification

Decomposition of Dissolved Methylene Blue in Water Using a Submerged Arc Between Titanium Electrodes by Naum Parkansky; Evelina Faktorovich Simon; Boris A. Alterkop; Raymond L. Boxman; O. Berkh (907-919).
Low voltage, low energy submerged pulsed arcs between Ti electrodes with a pulse repetition rate of 100 Hz, energies of 2.6–192 mJ and durations of 10–40 μs, followed by aging in the dark, were used to decompose 10 mg/l methylene blue (MB) contamination in 40 ml aqueous solutions, with and without the addition of 0.5 % H2O2. The impact of the arc treatment on the MB removal ratio (C0–Cta)/C0 was considered as a function of aging time ta, where C0 and Cta are the MB concentrations initially and after ta (the time needed to complete removal of MB after cessation of exposure of the arc). Particles eroded from the electrodes during the discharge enabled MB decomposition during aging. The particles were studied by XRD, XPS and Raman analysis, and titanium oxides and peroxides were found. MB decomposition during aging is explained by the formation of a surface layer of titanium peroxide that forms by the interaction of titanium dioxide with H2O2, which produce radicals which oxidize the MB. The 99.6 % MB removal yield (G99.6 = 90 g/kWhr) of the submerged pulsed arc process with Ti electrodes and addition of 0.5 % H2O2 was more than 60 times larger than obtained at 50 % removal with other plasma methods.
Keywords: Submerged pulsed arc; Electro-hydraulic discharge; Water treatment; Plasma; Decontamination

Ultra-high molecular weight polyethylene membranes were modified and subsequently polymer coated using the underwater plasma produced by glow discharge electrolysis. This plasma pretreatment generated various O-functional groups among them OH groups have dominated. This modified inner (pore) surface of membranes showed complete wetting and strong adhesion to a hydrogel copolymerized by glow discharge electrolysis also. The deposited hydrogel consists of plasma polymerized acrylic acid crosslinked by copolymerization with the bifunctional N,N′-methylenebis(acrylamide). Tuning the hydrogel hydrophilicity and bio-compatibility poly(ethylene glycol) was chemically inserted into the copolymer. Such saturated polymer could only be inserted on a non-classic way by (partial) fragmentation and recombination thus demonstrating the exotic properties of the underwater plasma. The modification of membrane was achieved by squeezing the reactive plasma solution into the pores by plasma-induced shock waves and supported by intense stirring. The deposited copolymer hydrogel has filled all pores also in the inner of membrane as shown by scanning electron microscopy of cross-sections. The copolymer shows the characteristic units of acrylic acid and ethylene glycol as demonstrated by infrared spectroscopy. A minimum loss in carboxylic groups of acrylic acid during the plasma polymerization process was confirmed by X-ray photoelectron spectroscopy. Additional cell adhesion tests on copolymer coated polyethylene using IEC-6 cells demonstrated the bio-compatibility of the plasma-deposited hydrogel.
Keywords: Glow discharge electrolysis (GDE); UHMWPE membrane; Pore modification; Adhesion; Copolymer hydrogel

Activation of PET Using an RF Atmospheric Plasma System by Mick Donegan; Vladimir Milosavljević; Denis P. Dowling (941-957).
The plasma treatment of polymer surfaces is routinely used to enhance surface properties prior to adhesive bonding or biomolecule interaction. This study investigates the influence of plasma treatment conditions on the surface activation of polyethylene terephthalate (PET) using the SurFx Atomflo™ 400L plasma source. In this study the effect of applied plasma power, processing speed, gas composition and plasma applicator nozzle to substrate distance were examined. The level of polymer surface activation was evaluated based on changes to the water contact angle (WCA) of PET samples after plasma treatment. PET surface properties were also monitored using surface energy and X-ray photoelectron spectroscopy (XPS) analysis. The heating effect of the plasma was monitored using thermal imaging and optical emission spectroscopy (OES) techniques. OES was also used as a diagnostic tool to monitor the change in atomic and molecular species intensity with changes in experimental conditions in both time and space. XPS analysis of the PET samples treated at different plasma powers indicated that increased oxygen content on samples surfaces accounted for the decreases observed in WCAs. For the first time a direct correlation was obtained between polymer WCA changes and the OES measurement of the atomic hydrogen Balmer Hα and molecular OH line emission intensities.
Keywords: Atmospheric plasma jet; Surface activation; Polyethylene-terephthalate; XPS; Optical emission spectroscopy

In most of the numerical approaches proposed for modeling high-intensity plasma-arcs, the effects of turbulence on the arc structure are often excluded because of the intricate physics originating from the interaction of turbulent scales, high-temperature gas dynamics, magnetohydrodynamics (MHD) and chemical kinetics. The goal of this study is threefold: to develop a generic turbulent MHD model to simulate free-burning arc discharges, to validate the code with available experimental data, and to investigate the effect of an external field and turbulent cross flow on the free-burning arc configuration. The governing equations are solved in conservative form using a hybrid scheme that combines a high-order monotonic upwind scheme with a second-order central scheme. The fluid and MHD turbulence are resolved using a large eddy simulation (LES) approach with a recently developed sub-grid closure model. An implicit scheme is used to compute the magnetic diffusion term appearing in the magnetic induction equation to alleviate the severe time-step constraint. The comparison of the model prediction with experimental data for Argon arcs at different current intensities shows generally good agreement. When an external field is applied, the overall shape of the free-burning arc drastically changes. The straightening of the arc indicates the potential for stabilization of a free-burning arc by magnetic forces. Even though the turbulence is significantly attenuated as a result of the thermal expansion near the cathode, it adds an unsteady characteristic to the arc and, in general, has a negative impact on the stabilization of the electrical discharge.
Keywords: MHD; LES; Free-burning arc; Turbulence

Plasma transferred arc cladding process was used to fabricate CoNiCuFeCr multi-element alloys coatings. The experimental results show that the coating forms a face-centered-cubic solid solution phase. The microstructure of the coating is mainly composed of dendrite and discontinuous interdendritic segregation. The average hardness of the coating reaches 194.8 HV100. The nano-indentation testing indicates that the micro-hardness and elastic modulus of the coating are 3.64 GPa and 211 GPa, respectively. The CoNiCuFeCr high-entropy alloy coating has excellent wear and corrosion resistance. The wear resistance of the coating is about 1.7 times higher than that of Q235 steel substrate under the same wet sand rubber wheel abrasion testing conditions. In 1N hydrochloric acid solution, the coating presents lower i corr values in polarization curves and higher fitted R f values in EIS plots than that of as-cast 304 stainless steel.
Keywords: Corrosion; Tribological properties; Coatings

A robust single hidden layer feed forward neural network (SLFN) is used in this study to model the in-flight particle characteristics of the atmospheric plasma spray (APS) process with regard to the input processing parameters. The in-flight particle characteristics influence the structure and properties of the APS coating and, thus, are considered important parameters to comprehend the manufacturing process. The training times of traditional back propagation algorithms, mostly used to model such processes, are far slower than desired for implementation of an on-line control system. Use of slow gradient based learning methods and iterative tuning of all network parameters during the learning process are the two major causes for such slower learning speed. An extreme learning machine (ELM) algorithm, which randomly selects the input weights and biases and analytically determines the output weights, is used in this work to train the SLFNs. Performance comparisons of the networks trained with ELM algorithm and standard error back propagation algorithms are presented. It is found that networks trained with ELM have good generalization performance, much shorter training times and stable performance with regard to the changes in number of hidden layer neurons. The trends represent robustness of the trained networks and enhance reliability of the application of the artificial neural network in modelling APS processes.
Keywords: Artificial neural network; Extreme learning machine; Atmospheric plasma spray; In-flight particle characteristics; Error back propagation; Single layer feed-forward network

Plasma Spray Process Operating Parameters Optimization Based on Artificial Intelligence by T. Liu; M. P. Planche; A. F. Kanta; S. Deng; G. Montavon; K. Deng; Z. M. Ren (1025-1041).
During plasma spray process, many intrinsic operating parameters allow tailoring in-flight particle characteristics (temperature and velocity) by controlling the plasma jet properties, thus affecting the final coating characteristics. Among them, plasma flow mass enthalpy, flow thermal conductivity, momentum density, etc. result from the selection of extrinsic operating parameters such as the plasma torch nozzle geometry, the composition and flow rate of plasma forming gases, the arc current intensity, beside the coupled relationships between those operating parameters make difficult in a full prediction of their effects on coating properties. Moreover, temporal fluctuations (anode wear for example) require “real time” corrections to maintain particle characteristic to targeted values. An expert system is built to optimize and control some of the main extrinsic operating parameters. This expert system includes two parts: (1) an artificial neural network (ANN) which predicts an extrinsic operating window and (2) a fuzzy logic controller (FLC) to control it. The paper details the general architecture of the system, discusses its limits and the typical characteristic times. The result shows that ANN can predict the characteristics of particles in-flight from coating porosity within maximal error 3 and 2 % in temperature and velocity respectively. And ANN also can predict the operating parameters from in-flight particle characteristics with maximal error 2.34, 4.80 and 8.66 % in current intensity, argon flow rate, and hydrogen flow rate respectively.
Keywords: Plasma spraying; ANN; FLC; In-flight particle characteristics; Prediction