Plasma Chemistry and Plasma Processing (v.36, #3)

Nitric oxide (NO) is a vasodilator and antihypertensive agent as well as a universal anti-microbial factor killing bacteria, fungi and parasites without killing human cells provided that an appropriate dose level and treatment time are applied. Exogenous NO is often employed in inhalation therapies for treating pulmonary hypertension in children and adults. NO generation from air in high voltage electrical discharges is being developed for medical uses because it is technologically simple, economical and portable. The related literature is reviewed here. The plasma can be a thermal plasma, where the temperature is of the order of 10,000 K, or it can be a non-thermal plasma, where the electron temperature is very high but the average gas temperature can vary over a wide range from close to room temperature to thousands of degrees above room temperature. The plasma temperature has significant effects on the chemical composition of the treated gas. These effects are explained based on the chemical reaction mechanism. Further, NO generated by electrical discharges is usually contaminated with nitrogen dioxide and sometimes with ozone and particulate matter. The techniques that have been successfully hybridized with the electrical discharge devices or that can potentially be hybridized for the purification of NO are also reviewed. Recent successful testing of electrical discharge-based NO generators for inhalation therapy on animal models in the US and routine use of them in Russia and east Europe for wound decontamination and fast heeling suggests that the technique has a great potential for applications in future.
Keywords: Nitric oxide; Thermal plasma; Nonthermal plasma; High voltage electrical discharge; Pulmonary hypertension; Inhalation therapy; Wound decontamination; Wound healing; Plasma medicine

Generation of Antimicrobial NOx by Atmospheric Air Transient Spark Discharge by M. Janda; V. Martišovitš; K. Hensel; Z. Machala (767-781).
Atmospheric pressure air plasma discharges generate potential antimicrobial agents, such as nitrogen oxides and ozone. Generation of nitrogen oxides was studied in a DC-driven self-pulsing (1–10 kHz) transient spark (TS) discharge. The precursors of NOx production and the TS characteristics were studied by nanosecond time-resolved optical diagnostics: a photomultiplier module and a spectrometer coupled with fast intensified camera. Thanks to the short (~10–100 ns) high current (>1 A) spark current pulses, highly reactive non-equilibrium plasma is generated. Ozone was not detectable in the TS, probably due to higher gas temperature after the short spark current pulses, but the NOx production rate of ~7 × 1016 molecules/J was achieved. The NO2/NO ratio decreased with increasing TS repetition frequency, which is related to the complex frequency-dependent discharge properties and thus changing NO2/NO generating mechanisms. Further optimization of NO2 and NO production to improve the biomedical and antimicrobial effects is possible by modifying the electric circuit generating the TS discharge.
Keywords: Non-equilibrium air plasma; Transient spark; Nitrogen oxides; Antimicrobial agents; Time-resolved optical diagnostic

Low Temperature Diesel Particulate Filter Regeneration by Atmospheric Air Non-thermal Plasma Injection System by Yunxi Shi; Yixi Cai; Xiaohua Li; Hui Xu; Weijun Li; Xiaoyu Pu (783-797).
An experimental study of the regeneration of diesel particulate filter (DPF) was conducted through the use of a self-designed Non-thermal plasma (NTP) injection system with an experimental temperature of 20–300 °C, with atmospheric air being used as the gas source. The results revealed that the PM could be broken down into CO and CO2 by NTP, through a discharge reaction of the NTP reactor. As the temperature increases, the mass of C1 (mass of C in CO) showed an overall declining trend. Interestingly, the mass of C2 (mass of C in CO2) and C12 (the sum of C1 and C2) both showed an initial increase, followed by a decrease. The peak mass of C12 appears at 150 °C, and both axial and radial temperature gradients are less than the limit of DPF temperature gradient at this temperature. In conclusion, DPF can be regenerated by the NTP technology at a lower temperature, which can aid in the avoidance of thermal damage of DPF. The technology boasts a great advantage in adopting atmospheric air as its gas source, which can not only reduce costs, but also is convenient.
Keywords: Diesel engine; Diesel particulate filter; Regeneration; Non-thermal plasma; Air; Temperature

Synthesis, Characterization and Photocatalytic Application of TiO2/SnO2 Nanocomposite Obtained Under Non-thermal Plasma Condition at Atmospheric Pressure by Elie Acayanka; Duclair S. kuete; Georges Y. Kamgang; Serge Nzali; Samuel Laminsi; Peter T. Ndifon (799-811).
A plasma-assisted synthesis of TiO2/SnO2 nanocomposite is described. In this approach, a precursor containing a mixture of [TiCl3 and SnCl2] exposed to electric discharge was oxidized by plasma-generated reactive species (HO·/H2O = 2.85 eV/SHE). SnO2 microstructures with a diameter of 10–40 µm were coated by thin layers TiO2 nanorods with mean diameter of 6–8 nm. The obtained TiO2/SnO2 nanocomposite was characterized by transmission and scanning electron microscopy, X-ray diffraction and Fourier transform infrared. TiO2/SnO2 nanocomposite was found to be a promising new material for the photocatalytic discoloration of aqueous Remazol Brilliant Blue-R dye under daylight and UVA light sources, due to the combined effects of large specific surface area and heterojunction which efficiently separates the electron–hole pairs delaying the charge recombination. The leaching test indicated that the nanocomposite is stable easily reusable.
Keywords: Non-thermal plasma; TiO2/SnO2 ; Nanocomposite; Photocatalysis; Remazol Brilliant Blue R dye

Rotating Gliding Arc Assisted Water Splitting in Atmospheric Nitrogen by Hao Zhang; Fengsen Zhu; Xiaodong Li; Kefa Cen; Changming Du; Xin Tu (813-834).
In this study, hydrogen production from water splitting in N2 using an atmospheric pressure rotating gliding arc plasma was investigated. The effect of input H2O concentration and total flow rate on the performance of the plasma water splitting process (e.g., H2 and O2 yield, H2 production rate, and energy yield of H2) was investigated. N2 showed a pronouncedly facilitating effect on the H2O splitting and H2 production process due to the reactions of the excited N2 species [e.g., electronically excited metastable N2(A)] with the H2O molecules. The maximum H2 production rate reached up to 41.3 μmols−1, which is much higher than that of other typical non-thermal plasmas (e.g., ~0.2 μmols−1 for a dielectric barrier discharge). Optical emission diagnostics has shown that in addition to the NO, N2, and N2 + that were observed in the pure N2 spectra, strong OH and NH emission lines also appeared in the H2O/N2 spectra. OH radical is considered as a key intermediate species that could contribute to the formation of H2, O2, and H2O2. The increase of the H2O concentration could lead to a continuous enhancement of the OH intensity. The rotational temperature of N2 + dropped drastically from 2875 ± 125 to 1725 ± 25 K with the addition of 1 % (mol/mol) H2O into the N2 plasma.
Keywords: Rotating gliding arc; Water splitting; Hydrogen production; Optical emission spectroscopy (OES); Reaction mechanisms

Functionalization of Polyurethane/Urea Copolymers with Amide Groups by Polymer Treatment with Ammonia Plasma by Alenka Vesel; Rok Zaplotnik; Gregor Primc; Xiangyu Liu; Kaitian Xu; Kevin C. Chen; Chiju Wei; Miran Mozetic (835-848).
Samples of porous foam from polyurethane/urea copolymers based on polyethylene glycol (PURPEG) were prepared in the form of 1-mm-thick discs of diameter 10 cm and exposed to ammonia plasma created by inductively coupled radiofrequency discharge in either low density (E mode) or high density (H mode). The evolution of surface composition and structure upon plasma treatment was characterized by X-ray photoelectron spectroscopy. Treatment in the H mode caused depletion of oxygen even after 2 s of treatment, whereas treatment in the E mode caused gentle functionalization with amide groups. The concentration of functional groups depended on the discharge power, and the best results were obtained at moderately high power just before the transition from E to H modes.
Keywords: Polyurethanes; Polyurea copolymer; Biodegradable; Ammonia plasma treatment; Surface modification; XPS

Preparation of Silicon Thin Films of Different Phase Composition from Monochlorosilane as a Precursor by RF Capacitive Plasma Discharge by L. A. Mochalov; R. A. Kornev; A. V. Nezhdanov; A. I. Mashin; A. S. Lobanov; A. V. Kostrov; V. M. Vorotyntsev; A. V. Vorotyntsev (849-856).
Monochlorosilane/argon/hydrogen (SiH3Cl-Ar-H2) mixture of different ratios was investigated from the point of PECVD application. RF capacitive plasma discharge of 40.68 MHz frequency was used. The process of deposition was studied by optical emission spectroscopy. The silicon thin films of different phase composition were obtained. The thin films were characterized by Raman-spectroscopy, atomic force microscopy, and secondary ion mass spectrometry. The exhaust gas mixture was analyzed by IR-spectroscopy in outlet of the reactor during PECVD process. The chemical mechanism for the deposition process was also proposed.
Keywords: PECVD; Monochlorosilane; Raman spectroscopy; Silicon thin films; IR-spectroscopy

Br-based plasmas potentially provide selective etching of Si. The characteristics of homogenous discharge in mixed gases of HBr and He are investigated numerically based on a self-consistent 2D fluid model. The model takes into account the primary processes like excitation and ionization. The reactions of radicals with radicals, neutrals with neutrals and radicals and neutrals are taken into account in HBr/He discharge and therefore can adequately represent discharge plasma. Based on simulation results of the self-consistent 2D fluid model, the dominant species for Si etching in HBr/He plasma discharge are Br, Br+, H and HBr+. The impact of frequency, voltage, electrode gap, and gas mixture ratio on the densities of these important species in HBr/He has been explored. Simulation results indicate that elevating high frequency electrode’s frequency and voltage, enhances etching species densities. Increasing the electrode gap, the densities of all plasma species decrease and vice versa. The addition of He to HBr plasma decreases Br and HBr+ densities while increases Br+ density. Densities of active species for Si etching and subsequently chemical etching versus physical sputtering in HBr/He plasma can be controlled by tuning input parameters and the desired etching can be achieved.
Keywords: HBr Plasma; Simulation; Etching

Quenching Experiment Study on Thermal Plasma Pyrolysis Process of Coal Tar by Xuan Li; Changning Wu; Jiantao Han (869-880).
Quenching is a key approach to obtain high acetylene yield in the process of coal tar pyrolysis to produce acetylene in a thermal plasma reactor due to the thermodynamic characteristics of acetylene. Experiments of coal tar pyrolysis were carried out in a lab-scale H2/Ar plasma reactor under various quenching conditions. Meanwhile, thermodynamic analysis was performed to assist the optimization of quenching temperature and the maximization of acetylene yield. As quenching media in the experiments, hydrogen, argon, methane, and water were used separately to study the influence of quenching process on acetylene yield and specific energy requirement. The experimental results indicate that the acetylene concentration in quenched product gas was significantly affected by quenching operation, and the acetylene yield was significantly affected by quenching medium flow rate. The acetylene yields of 24.6, 17.8, 44.9 and 23.6 wt% can be reached by using hydrogen, argon, methane, and water as quenching media, respectively. The specific energy requirement analysis indicates that process energy efficiency can be improved by a suitable quench operation.
Keywords: Quench; Acetylene; Thermal plasma; Coal tar; Pyrolysis

The non-transferred direct current (DC) plasma torch has been widely used in various industrial applications due to its special jet characteristics. The jet characteristics are determined by different factors, including the working parameters, the torch construction, the gas injection angle (GIA) etc. As there is little study on the influence of the GIA on the jet characteristics, experimental study on the GIA’s effects on the jet characteristics has been carried out on a specially designed non-transferred DC plasma torch, whose GIA can be changed by replacing a gas injection component. The arc voltages and thermal efficiencies of the plasma torch, the specific enthalpies and jet lengths of the plasma jets at different working conditions were obtained and analyzed. It has been found that the GIA greatly affects the arc voltage, the thermal efficiency, the specific enthalpy and the jet length. Based on these findings, plasma torch with appropriate GIA could be used to help generating the plasma jet with desired characteristics.
Keywords: Non-transferred DC plasma torch; Gas injection angle; Jet characteristics; Specific enthalpy; Jet length

Arc-Enhanced Plasma Machining Technology for High Efficiency Machining of Silicon Carbide by Baolu Shi; Yifan Dai; Xuhui Xie; Shengyi Li; Lin Zhou (891-900).
Atmosphere plasma etching methods have been demonstrated efficient in the etching of fused silica or ULE. However, because of the high chemical stability of silicon carbide (SiC), the conventional plasma etching methods seem incapable of obtaining a high material removal rate (MRR). We have found that MRR will be significantly improved while the electric spark appears between the plasma and the SiC surface. As a result, a new plasma source is designed to generate stable arc at the surface. Due to the generation of arc, the MRR of 0.35 mm3/min is obtained, about 10 times as high as the conventional method. In this paper, the removal characteristics and the thermal effect of this method are presented. MRR and the surface temperature are investigated in dependence on plasma parameters: RF power, travel speed of plasma source, SF6 gas flow and O2 gas flow. Due to the negligible thermal effect, the surface figuring can be achieved using the conventional dwell time method. The shape error of a flat SiC surface is corrected, verifying the figuring capability and the effectiveness of this method.
Keywords: Atmosphere plasma etching; Inductively coupled plasma; Material removal rate; Silicon Carbide