Plasma Chemistry and Plasma Processing (v.35, #6)

Plasma technology has been widely applied in the ozone production, material modification, gas/water cleaning, etc. Various nanomaterials were produced by thermal plasma technology. However, the high temperature process and low uniformity products limit their application for the high value added chemicals synthesis, for example the functional materials or the temperature sensitive materials. Microplasma has attracted significant attentions from various fields owing to its unique characteristics, like the high-pressure operation, non-equilibrium chemistry, continuous-flow, microscale geometry and self-organization phenomenon. Its application on the functional nanomaterial synthesis was elaborately discussed in this review paper. Firstly, the main physical parameters were reviewed, which include the electron temperature, electron energy distribution function, electron density and the gas temperature. Then four representative microplasma configurations were categorized, and the proper selection of configuration was summarized in light of different conditions. Finally the synthesis, mechanism and application of some typical nanomaterials were introduced.
Keywords: Microplasma; Microdischarge; Nanomaterial synthesis; Functional material; Plasma technology

Deposition of Ni Coatings by Electrolytic Plasma Processing by A. Smith; R. Kelton; E. I. Meletis (963-978).
Electrolytic Plasma Processing (EPP) is an emerging cleaning and coating technology capable of high deposition rates while being relatively simple and environmentally friendly. In the present study, Ni coatings were deposited via EPP on low carbon steel substrates. Initially, experiments were carried out at different processing conditions to determine their effect on the coating characteristics and select the most appropriate conditions for Ni deposition. The coated samples were characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction, and Transmission Electron Microscopy. EPP Ni depositions were conducted at the selected processing conditions for 10, 20 and 30 s. EPP produced high purity Ni coatings with an approximate grain size of 20 nm at a deposition rate of 166 nm/s. The corrosion properties were assessed in 3.5 % NaCl aqueous solution using open circuit potential measurements, potentiodynamic polarization testing and Electrochemical Impedance Spectroscopy (EIS). The microstructure and coating characteristics, both before and after corrosion testing, were studied using SEM. In spite of the short deposition time, EPP-coated Ni was shown to increase the corrosion resistance of the steel substrate by increasing the corrosion potential and significantly decreasing the corrosion rate. The EIS studies revealed that a similar corrosion mechanism prevails between pure Ni and EPP-coated Ni. Best corrosion behavior was obtained with a coating deposition time of 30 s, showing a positive shift in the corrosion potential by ~200 mV and more than an order of magnitude reduction in the corrosion rate of the low carbon steel.
Keywords: Electrolytic plasma processing; Nickel; Coating; Corrosion

Nitrogen Atmospheric-Pressure-Plasma-Jet Induced Oxidation of SnOx Thin Films by Guan-Wei Lin; Yu-Hao Jiang; Peng-Kai Kao; I-Chung Chiu; Yu-Han Wu; Cheng-Che Hsu; I-Chun Cheng; Jian-Zhang Chen (979-991).
SnOx thin films that were rf-sputter-deposited under various gas flow ratios ([O2]/([O2] + [Ar]) OFR = 3.0, 3.6, 4.2 and 4.8 %) were rapidly annealed using atmospheric pressure plasma jets (APPJs) in temperature range of ~350–386 °C for up to 5 min. The original electron probe micro-analysis [O] contents in the as-deposited films were ~25, ~30, ~35 and ~40 % for films deposited at ([O2]/([O2] + [Ar]) gas flow ratios OFR = 3.0, 3.6, 4.2 and 4.8 %, respectively. APPJ annealing increased the [O] content to ~35 % for films deposited at OFR = 3.0 and 3.6 %, where the [O] content remained in similar levels for films deposited at OFR = 4.2 and 4.8 %. Crystalline metallic Sn was identified in films as-deposited at OFR = 3.0 and 3.6 %; on the other hand, an X-ray amorphous SnOx phase was identified in films as-deposited at OFR = 4.2 and 4.8 %. Crystallization and oxidation by APPJ annealing improved the transmittance and blue-shifted the absorption band edge to ~420 nm. All APPJ-annealed films exhibit n-type conductivity that may be contributed by the mixed phases of SnO, SnO2 and a small amount of Sn.
Keywords: Atmospheric pressure plasma jet; Oxidation; SnOx ; Sn; SnO; SnO2

Spectroscopic Characterization of Argon–Nitrogen Surface-Wave Discharges in Dielectric Tubes at Atmospheric Pressure by J. A. Bravo; R. Rincón; J. Muñoz; A. Sánchez; M. D. Calzada (993-1014).
The influence of nitrogen addition (up to 10 %) to an argon surface-wave atmospheric-pressure discharge has been investigated by spectroscopic means. Nitrogen introduction results in a significant increase of gas temperature, from 2000 to 4000 K, and a drastic reduction of electron density, from 1014 to 1013 cm−3. These variations come along with the disappearance of the discharge contraction and the apparition of a diffuse, less bright zone (postdischarge) in the downstream zone. Modifications of the argon discharge kinetics leading to a significant decrease of the density of argon excited states due to the presence of nitrogen have been discussed to the light of the spectra emitted by the discharge and a simplified kinetic model, showing the relevance of argon species in Penning excitation and charge transfer reactions.
Keywords: Argon; Nitrogen; Atmospheric pressure; Spectroscopy; Dielectric tubes

The super-hydrophobic nano-films were synthesized by atmospheric pressure plasma jet using hexamethyldisilazane. In this paper, the atmospheric pressure plasma jet reacting with air was used to determine the formation of plasma polymerized nano-film. The atmospheric pressure plasma polymerized nano-film surface properties were determined by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and atomic forced microscopy. Specifically, it has been observed that atmospheric pressure plasma polymerization with the appropriate monomer gas flow rate cause the formation of the super-hydrophobic film. The surface properties of atmospheric pressure plasma polymerized nano-films were determined as the Cassie–Baxter state. It was examined that super-hydrophobic nano-film surface exhibits the organosilicon sphere stacking structure. Such sphere stacking structure does not only cause the hydrophobicity, it also stabilizes the Cassie regime, and thus favors the water repellency.
Keywords: Atmospheric pressure plasma jet; Super-hydrophobic film; Hexamethyldisilazane; Plasma polymerized films; Cassie–Baxter state

Experimental Study of the Transient Response of Bunsen Flame to Nanosecond Pulsed Discharges by Shaohua Zhang; Xilong Yu; Heng Xiong; Hui Zeng; Fei Li (1029-1042).
The transient processes associated with the interaction of a Bunsen flame and nanosecond pulsed discharges (NPD) are explored experimentally with two optical methods. A nanosecond-gated schlieren system is employed to image the shockwave propagation and the hydrodynamic response of the flame to NPD while the time-resolved optical emission spectroscopy measurements are carried out to determine active species and temperature in the plasma region created by the discharges. Therefore, the unsteady process of the interaction of the flame with the discharges is recorded in real-time by the combined measurements. Numbers of experimental evidences for understanding the dynamics of non-equilibrium plasma produced by NPD and performing further numerical simulation are offered.
Keywords: Nanosecond pulsed discharges (NPD); Time-resolved optical emission spectroscopy; Schlieren

Generation of In-Package Cold Plasma and Efficacy Assessment Using Methylene Blue by N. N. Misra; K. M. Keener; P. Bourke; P. J. Cullen (1043-1056).
In-package cold plasma processing is highly desirable in the food and biomedical industries as it allows for efficient sterilisation, and prevents against post-packaging contamination. The sensitivity of methylene blue dye to the reactive species generated from cold plasma is tested in this work for possible use as a marker of process efficacy. A large gap dielectric barrier discharge (DBD) operating in air was employed to generate the plasma discharge within the sealed package. The discolouration of methylene blue dye placed inside the package was studied as a function of DBD operation time, applied voltage and spatial position. Ozone concentrations were measured immediately after treatment as an indicator of one of the key meta-stables produced by the approach with values of up to 1800 ppm recorded. Visible absorption spectra and pH changes of the dye were measured. A decrease in peak absorbance of the dyes and pH was observed as a function of treatment time and ozone concentration. Optical emission spectroscopy of the discharge revealed the generation of excited nitrogen and reactive oxygen species. The results of kinetic modelling revealed that the dye discolouration can be used as a suitable marker reaction for treatment times within the order of 30 s.
Keywords: Ozone; AOP; Non-thermal plasma; Dielectric barrier discharge; Methylene blue

The present work provides a detailed kinetic analysis of the time-resolved dynamics of the gas heating during the arc reattachment in nitrogen gas in order to understand the main processes leading to such a fast reattachment. The model includes gas heating due to the relaxation of the energy stored in the vibrational as well as the electronic modes of the molecules. The results show that the anode arc reattachment is essentiality a threshold process, corresponding to a reduced electric field value of E/N ~ 40 Td for the plasma discharge conditions considered in this work. The arc reattachment is triggered by a vibrational instability whose development requires a time of the order of 100 µs. For E/N < 80–100 Td, most of the electron energy is transferred to gas heating through the mechanism of vibrational–translational relaxation. For larger values of E/N the electronic–translational energy relaxation mechanism produces a further intensification of the gas heating. The sharp increase of the gas heating rate during the last few µs of the vibrational instability give rises to a sudden transition from a diffuse (glow-like) discharge to a constricted arc with a high current density (~107 A/m2). This sudden increase in the current density gives rise to a new anode attachment closer to the cathode (where the voltage drop between the original arc and the anode is the largest) thus causing the decay of the old arc spot.
Keywords: Anode arc reattachment; Vibrational instability; Fast gas heating; Plasma torches

Properties of Argon–Nitrogen Atmospheric Pressure DC Arc Plasma by Dragan Rankovic; Miroslav Kuzmanovic; Mirjana S. Pavlovic; Milovan Stoiljkovic; Jelena Savovic (1071-1095).
The effect of nitrogen addition (1–20 %) on atmospheric argon DC arc plasma parameters was investigated. Nitrogen was added to three initial plasma gas compositions: pure argon gas, argon gas with water aerosol, and argon gas with water aerosol containing 0.5 % KCl. Admixtures of nitrogen from 1 up to 10 % (in the case of Ar–N2–water aerosol plasma) or up to 5 % (in the presence of 0.5 % KCl) produced a more contracted discharge, with larger gradients of electron number density and temperature, as compared to plasma without molecular gas addition. The opposite effect was observed in gas mixtures with 20 % of nitrogen. In that case, the arc core was expanded, and the radial profile of electron density and temperature became flatter as compared to the case of pure argon plasma. The changes in the radial structure of the discharge could be explained by increased thermal conductivity of mixed gas plasma and the presence of molecular ions. Increasing nitrogen content brings plasma closer to thermal equilibrium. To evaluate the effect of nitrogen upon analytic performance of the arc source, limits of detection (LODs) were measured for Zn, Cu, Li, and Rb. In plasma without KCl, the addition of nitrogen resulted in lowering of LODs for Cu, Li and Rb, while the opposite was observed for Zn. In the presence of KCl the addition of nitrogen had little or no effect on LODs for Cu, Li and Rb, while detection limits for Zn were increased.
Keywords: Atmospheric pressure DC arc discharge; Argon/nitrogen plasma; Excitation temperature; Electron density; Thermal equilibrium

On the Selection of Integration Intervals for the Calculation of Mean Absorption Coefficients by P. Kloc; V. Aubrecht; M. Bartlova; O. Coufal; Ch. Rümpler (1097-1110).
In radiation modeling of thermal plasmas non-grey models are applied where the radiative transport is described in several frequency bands (spectral intervals). Hereby mean absorption coefficients have to be calculated by a spectral integration procedure, providing a constant mean absorption coefficient for each band. Depending on the number of bands, one or more integration boundaries have to be selected in order to do the integration. In this paper we evaluate the influence of the selection of these integration boundaries on the mean absorption coefficient and the also radiation transfer by applying the mean absorption coefficients in a radiation transport model. Using a simplified two-band model we demonstrate that the selection of the integration boundary has a large impact on the total model accuracy. We show that in some cases selecting a band boundary right at the frequency where the continuum absorption shows a jump can introduce a significant error into the radiation calculation. The process of the integration interval selection thus demands a global optimization procedure to properly evaluate the boundaries of each frequency band.
Keywords: Radiative transfer; Arc; Air; Spectral absorption coefficient; Planck mean absorption; Discrete ordinates method

Hydrogen Reduction of Germanium Tetrafluoride in RF-Discharge by R. A. Kornev; P. G. Sennikov (1111-1118).
A germanium tetrafluoride conversion into germanium and polyfluorogermanes in hydrogen inductively coupled RF-discharge plasma (13.56 MHz) has been experimentally studied. The product ratio is dependent on specific energy contribution into discharge. Conditions that ensure practically complete conversion of GeF4 and the yield of germanium equal to 86 % have been found. The analysis of exhaust gases, of chemically active plasma and of condensed phase by emission- and IR-spectroscopy as well as by mass spectrometry made it possible to propose mechanism accounting for the formation of observed intermediate species and final products.
Keywords: RF inductively coupled plasma; Germanium tetrafluoride; Hydrogen reduction

Pressure-Dependent Etching Mechanism and Induced Dielectric Properties Variation of BZN Thin Films in SF6/Ar Plasma by Liping Dai; Wenping Song; Shuya Wang; Zhiqin Zhong; Guojun Zhang (1119-1127).
Reactive ion etching was used to study bismuth zinc niobate (BZN) thin films etching in SF6/Ar plasma as a function of pressure. The etch rate increases as the pressure increases from 1 to 20 mTorr and decreases when the pressure exceeds 20 mTorr. The film surfaces were analyzed to determine the etching mechanism using X-ray photoelectron spectroscopy. Pressure is found to have an effect on etch reaction and non-volatile etch by-products removal through different ion density and energy, thus resulting in varying compositions, element chemical binding states on the film surface. Dielectric properties of the film, modified by SF6/Ar plasma, show an obvious variation with pressure. The permittivity shows a slight decrease in the range of 1–20 mTorr but a sharp decrease at 30 mTorr. Instead, the dielectric loss variation exhibits an opposite trend.
Keywords: Surface; Thin film; Etching; X-ray photoelectron spectroscopy (XPS); Dielectric property

A Model-Based Comparative Study of HCl and HBr Plasma Chemistries for Dry Etching Purposes by Alexander Efremov; Joon Hyub Kim; Kwang-Ho Kwon (1129-1142).
The comparative study of HCl and HBr plasma chemistries was carried out using plasma modeling. It was found that both gas systems are characterized by similar reaction schemes and exhibit only the quantitative differences in kinetics of both neutral and charged species. For one and the same ranges of electron temperature and electron density, the important features of the HBr plasma are: (1) higher dissociation degree and Br atom density; (2) higher electronegativity; and (3) higher efficiency of the physical etching pathway.
Keywords: HCl; HBr; Plasma; Dissociation; Ionization; Rate coefficient; Reaction rate