Plasma Chemistry and Plasma Processing (v.34, #2)

On the LPCVD-Formed SiO2 Etching Mechanism in CF4/Ar/O2 Inductively Coupled Plasmas: Effects of Gas Mixing Ratios and Gas Pressure by Jinyoung Son; Alexander Efremov; Inwoo Chun; Geun Young Yeom; Kwang-Ho Kwon (239-257).
An investigation of etching mechanism of low-temperature SiO2 thin films in CF4/Ar/O2 inductively coupled plasmas at constant input power (900 W) and bias power (200 W) was carried out. It was found that that the variations of Ar/O2 mixing ratio (0–50 %) at constant 50 % CF4 fraction as well as the change in gas pressure (4–10 mTorr) resulted in non-monotonic SiO2 etching rates. The zero-dimensional plasma model with Langmuir probe diagnostics data provided the detailed information on formation-decay kinetics for plasma active species. The model-based analysis of etching kinetics showed that these effects were not connected with the non-monotonic change of fluorine atom density (as was found in several works for the binary CF4/O2 system), but resulted from the decrease in reaction probability and with the transition from neutral-flux to ion-flux-limited regimes of ion assisted chemical reaction.
Keywords: Low-temperature SiO2 ; CF4 plasma; Diagnostics; Modeling; Etching mechanism

Plasma Polymerization Inside Tubes in Hexamethyldisiloxanes and Ethyne Glow Discharges: Effects of Deposition Atmosphere on Wetting and Ageing in Solvents by Juergen M. Lackner; Martin Wiesinger; Reinhard Kaindl; Wolfgang Waldhauser; Daniel Heim; Paul Hartmann (259-269).
The coating deposition inside tubes becomes increasingly important for fluidic applications, in which inner surfaces are chemically and mechanically strained by the flowing liquid and by scratching of particles. The developed process for tube coating, presented in this work, is based on the discharge in the precursor gas atmosphere between two mesh electrodes at the ends of the tube. The gas mixture is introduced on one end and pumped through the electrode on the other end. Igniting plasma inside the tube, the tube walls are the barrier to the atmosphere. Especially pulsed DC discharges for plasma polymerization in this alignment lead to good coating results, which is shown in this work focusing on deposition in pure and mixed hexamethyldisiloxane, ethyne, and oxygen atmospheres. Chemical binding, wetting, and ageing are strongly influenced by the choice of the gas mixtures. Sufficient oxygen partial pressure in the deposition atmosphere leads to hydrophilic behavior of the SiO2-like polymer-like carbon coatings, all other applied atmospheres to generally hydrophobic behavior of pure and Si-doped plasma polymers, respectively.
Keywords: Plasma polymerization; Coating of tubes; Hexamethyldisiloxane; Ethyne

Characterization of Plasma Polymerized Hexamethyldisiloxane Films Prepared by Arc Discharge by Algirdas Lazauskas; Jonas Baltrusaitis; Viktoras Grigaliūnas; Dalius Jucius; Asta Guobienė; Igoris Prosyčevas; Pranas Narmontas (271-285).
Herein, we present a simple method for fabricating plasma polymerized hexamethyldisiloxane films (pp-HMDSO) possessing superhydrophobic characteristics via arc discharge. The pp-HMDSO films were deposited on a soda–lime–silica float glass using HMDSO monomer vapor as a precursor. A detailed surface characterization was performed using scanning electron microscopy and atomic force microscopy. The growth process of the pp-HMDSO films was investigated as a function of deposition time from 30 to 300 s. The non-wetting characteristics of the pp-HMDSO films were evaluated by means of contact angle (CA) measurements and correlated with the morphological characteristics, as obtained from microscopy measurements. The deposited films were found to be nano-structured and exhibited dual-scale roughness with the static CA values close to 170°. Fourier transform infrared spectroscopy analysis was carried out to investigate chemical and functional properties of these films. Methyl groups were identified spectroscopically to be present within the pp-HMDSO films and were proposed to result in the low surface energy of material. The synergy between the dual-scale roughness and low surface energy resulted in the superhydrophobic characteristics of the pp-HMDSO films. A possible mechanism for the pp-HMDSO film formation is proposed.
Keywords: Arc discharge; Plasma; Hexamethyldisiloxane; Morphology; Wetting

Non Thermal Plasma Functionalized 2D Carbon–Carbon Composites as Supports for Co Nanoparticles by Julien Souquet-Grumey; Philippe Ayrault; Olivier Heintz; Joël Barrault; Jean-Michel Tatibouët; Hervé Plaisantin; Jacques Thébault; Sabine Valange; Elodie Fourré (287-300).
Novel two-dimensional carbon–carbon composites made of carbon nanofibers (CNFs) supported on a carbon preform were functionalized by non thermal plasma treatment (room temperature, atmospheric pressure, humid air), before being used as supports for metallic cobalt nanoparticles. It was shown that the degree of functionalization of the carbon nanofibers depends on the plasma power input, the treatment time and the CNF loading. The size of the cobalt nanoparticles generated after subsequent reduction of the Co-containing plasma treated CNF/C composites under hydrogen flow seems to be independent of the amount of supported cobalt. Changes in surface characteristics were analyzed using thermogravimetric analyses coupled to a mass spectrometer, X-ray photoelectron spectroscopy analyses and Raman spectroscopy. Transmission electron microscopy was used to complementary characterize the final size, dispersion and location of the so generated Co nanoparticles.
Keywords: Carbon nanofibers; Carbon composites; Non thermal plasma; Cobalt nanoparticles; XPS

Atmospheric-pressure dielectric barrier discharge (DBD) cold plasma was employed to fabricate Pt/TiO2 photocatalyst using the mixture of Ar and H2 as working gas. X-ray photoelectron microscopy (XPS) and transmission electron microscopy (TEM) measurements were used to characterize the Pt/TiO2 photocatalyst. The results showed that H2PtCl6 was completely reduced to metallic Pt nanoparticles when the treating time was increased to 6 min. In addition, the Pt/TiO2 photocatalyst prepared by atmospheric-pressure DBD cold plasma (Pt/TiO2-P) showed high dispersion and smaller size of Pt particles, and enhanced metal-support interaction. The photocatalytic degradation of methylene blue (MB) was chosen as a model reaction to evaluate the activity of the Pt/TiO2 photocatalyst. The apparent rate constant of 0.5 wt% Pt/TiO2-P for the MB photodegradation was 1.4 times higher than that over 0.5 wt% Pt/TiO2-C sample (prepared by thermal reduction method). This may be attributed to the smaller size and high dispersion of Pt particles, and the enhanced metal-support interaction in Pt/TiO2-P sample, which was consistent with the results of XPS and TEM. The influence of Pt content on photocatalytic activity of Pt/TiO2-P was investigated, and the highest apparent rate constant was obtained at 0.5 wt% Pt content. This was much lower than that prepared by conventional methods. Optical emission spectra (OES) were observed during the reduction process of Pt ions by atmospheric-pressure DBD cold plasma, and the reduction mechanism was further discussed.
Keywords: Atmospheric-pressure cold plasma; Dielectric barrier discharge (DBD); Pt/TiO2 ; Photocatalyst; Optical emission spectra (OES)

Chemical Kinetics of Methane Pyrolysis in Microwave Plasma at Atmospheric Pressure by Mirosław Dors; Helena Nowakowska; Mariusz Jasiński; Jerzy Mizeraczyk (313-326).
Results of chemical kinetics modeling in methane subjected to the microwave plasma at atmospheric pressure are presented in this paper. The reaction mechanism is based on the methane oxidation model without reactions involving nitrogen and oxygen. For the numerical calculations 0D and 1D models were created. 0D model uses Calorimetric Bomb Reactor whereas 1D model is constructed either as Plug Flow Reactor or as a chain of Plug Flow Reactor and Calorimetric Bomb Reactor. Both models explain experimental results and show the most important reactions responsible for the methane conversion and production of H2, C2H2, C2H4 and C2H6 detected in the experiment. Main conclusion is that the chemical reactions in our experiment proceed by a thermal process and the products can be defined by considering thermodynamic equilibrium. Temperature characterizing the methane pyrolysis is 1,500–2,000 K, but plasma temperature is in the range of 4,000–5,700 K, which means that methane pyrolysis process is occurring outside the plasma region in the swirl gas flowing around the plasma.
Keywords: Pyrolysis; Modeling; Methane; Microwave plasma

This work presents the results obtained on the single-step route towards the synthesis of iron oxide nanoparticles in a microwave plasma torch. The torch is supplied by 660 sccm of Ar mixed with 1 sccm of Fe(CO)5 and a variable amount of O2. The influence of oxygen addition on the phase composition of the synthesized powder was studied. Magnetite and maghemite phases could not be distinguished using the standard X-ray diffraction (XRD) analysis. Therefore, a combined XRD and Raman spectra analysis had to be applied, which is based on fitting of selected diffraction peaks and spectral features. According to XRD and Raman spectroscopy, the powder synthesized from Ar/Fe(CO)5 consisted about 50 % of magnetite, Fe3O4, the rest being α-Fe and FeO. An increase in oxygen flow rate led to an increase in γ-Fe2O3 percentage, at the expense of α-Fe, FeO and Fe3O4. Almost pure γ-Fe2O3 was synthesized at oxygen flow rates 25–75× higher than the flow rate of Fe(CO)5. A further increase in the oxygen flow rate led to α-Fe2O3 and ε-Fe2O3 production. The distributions of nanoparticles’ (NPs) diameters were obtained using transmission electron microscopy (TEM) and dynamic light scattering (DLS). The mean diameter of the NPs measured by TEM was 13 nm while the DLS measurements led to the mean diameter of 12 nm. About 90 % of all particles had the diameter in the range of 5–21 nm but a few larger particles were observed in TEM micrographs.
Keywords: Iron oxides; Nanoparticles; Plasmachemical synthesis; Raman spectroscopy; X-ray powder diffraction; Microwave plasma

The objective of this paper is to analyse the oxidation of acetylene under nanosecond pulsed N2/O2 discharges generated in a complex multi-pin-to-plane (MPP) corona reactor in the frame of Yan’s generic chemical kinetic model. We made use of the results obtained from the detailed kinetic model published previously (Redolfi et al. in Plasma Chem Plasma Process 29(3):173–195, 2009) in order to propose a global reactor models based on Yan’s generic chemical model and taking into account the non-homogeneous and non-stationary character of the discharges. This enables us expressing the energy cost in terms of physical and kinetic parameters of the discharge. We checked the model validity by comparing predicted and measured energy cost-values for acetylene in MPP reactor. The methodology presented may be adapted to predict the energy cost in other complex corona reactor provided the model parameters are determined experimentally.
Keywords: Corona discharge; Kinetic model; Acetylene; Oxygen atom