Oxygen Glow Discharge Research Articles

Surface morphology and chemical composition of lamb’s lettuce ( Valerianella locusta) after exposure to a low-pressure oxygen plasma

The influence of a low-pressure oxygen glow discharge plasma on secondary metabolites of lamb’s lettuce was examined. After extraction and hydrolysis the compounds were analysed by means of RP-HPLC. Whereas pure compounds showed a time-dependent degradation (flavonoids) or remained unchanged (phenolic acids), the exposure of lamb’s lettuce to the oxygen plasma led to a significant increase of protocatechuic acid, luteolin, and disometin after 120 s treatment time, independent of the applied plasma driving voltage. Contact angle (CA) measurements show an increased wettability of adaxial leaf surfaces after plasma exposure. A successive degradation of epicuticular waxes and cutin of the plant’s epidermis has been indicated by means of FTIR (ATR) and scanning electron microscopy (SEM). These findings suggest that reactive species in the plasma play a crucial role for the observed changes upon exposure to non-thermal plasma.

Cyclohexane dehydrogenation on a copper-platinum catalyst

The reaction of the dehydrogenation of cyclohexane on a copper-platinum catalyst supported by silica gel (1 wt % Pt + 0.15 wt % Cu)/SiO2 was studied. The state of the catalyst surface was investigated using X-ray photoelectron spectroscopy. It was established that under both flow and static conditions, the activity of the copper-platinum catalyst is higher than the activity of a catalyst containing 1 wt % Pt/SiO2. The rise in activity as a result of the introduction of copper, due to a decrease in the activation energy, is explained by an increase in the fraction of carbon in the composition of active centers localized on particles of neutral (Ptm0) and positively charged (Ptn+δ) platinum, and by the formation of centers with increased activity as a result of the adsorption of Cu+δ on particles of Ptm0. It was demonstrated that treating the copper-platinum catalyst with the plasma of a glow discharge in argon and oxygen increases its activity, while treatment in high-frequency H2 plasma reduces it. The indicated changes in the activity are associated with the alteration of the activation energies and the number of active centers, revealed by X-ray photoelectron spectroscopy, that depend on changes in the catalyst surface composition.

Fuel removal from tile gaps with oxygen discharges: reactivity of neutrals

Carbon removal in the so-called remote areas, where no energetic species can reach the surface, is investigated with low-temperature glow discharges in pure oxygen. Plasma-deposited amorphous hydrogenated carbon thin films are used as a model system for redeposited films. Erosion measurements are performed on flat substrates as well as on two different three-dimensional (3D) test structures. One design consists of 19 mm deep gaps with widths ranging from 0.5 to 4 mm to simulate ITER tile gaps. A second design consists of a flat box-like structure where particles can enter only through a narrow slit. Measurements are performed for substrate temperatures ranging from 290 to 580 K. Erosion rates follow an Arrhenius-type dependence on substrate temperature with an effective activation energy of 0.25 eV. While at room temperature the surface loss probability of the dominant eroding species is 50% it becomes smaller at elevated temperatures. At elevated temperatures, the films are not only removed faster but simultaneously erosion penetrates deeper into the gaps.

Study of plasma–solid interaction in electronegative gas mixtures at higher pressures

Surface treatment at higher pressures has become an important issue of plasma science and technology. The analysis of processes in such plasmas is now often based on computer modelling; however, the main problem of simulation in medium and higher pressures is the increasing number of collisions among plasma particles and because of this, decreasing efficiency of simulations. A combination of experimental and computational approaches was used for the study of this problem in the present work. The measurements were performed in the positive column of a DC glow discharge in oxygen, both pure and in mixtures with rare gases, at medium pressures. These experiments were completed by computer simulations, where an attempt was made to extend the particle simulation technique to higher pressures. It was shown that the standard Particle-In-Cell/Monte Carlo simulation technique loses its applicability at higher pressures and must be replaced by more sophisticated computational methods. In the present work new algorithms for particle treatment in plasma at higher pressures were suggested and successfully tested.

Study of rotational temperature of oxygen DC glow discharge in Silica and Pyrex discharge tubes

Active DC glow discharges in oxygen have been studied in Silica and Pyrex discharge tubes for medium pressures up to 550 Pa and for discharge currents up to 40 mA. Electric field strength measured by a double-probe technique was found to increase with the pressure and to decrease with the discharge current, which is typical for DC glow discharges. We have focused on the emitted radiation. The rotational temperature ( T rot) of molecular oxygen was determined from the PP and PQ branches of the well-resolved atmospheric A-band of molecular oxygen at 760 nm. Good agreement between values of T rot obtained from particular branches was found. The increase of the rotational temperature with increasing pressure and discharge current has been observed in both discharge tubes, however, the values of rotational temperature were systematically higher in the tube made of Pyrex glass. This difference was explained by the particular thermal conductivity of the tube material.

Plasma sterilization using the RF glow discharge

In the present work, glow discharge oxygen plasma was used to sterilize the Pseudomonas aeruginosa on the polyethylene terephthalate (PET) sheets. In a self-designed plasma reaction equipment, active species (electron, ion, radical, UV light, etc.) were separated effectively, and the discharge area, afterglow area and remote area were plotted out in the plasma field. Before and after plasma treatment the cell morphology was studied by scanning electron microscopy (SEM). The results showed that after treatment of 30 s the germicidal effect is 4.26, 3. 84, 2.61, respectively in the three areas on the following conditions: discharge power was 40 W and gas flux was 20 cm 3/min. SEM results revealed the cell morphology before and after plasma treatment. The walls or cell membrane cracking was testified by determining the content of protein using coomassie light blue technique. The results from electron spin resonance spectroscopy (ESR) and double Langmuir electron probe showed that electron, ion and oxygen free radical played important roles in sterilization in the discharge area, but only oxygen radicals acted to sterilize the bacteria in the afterglow area and the remote area.

N–O mix optimisation in low energy dense DC glow surface Ti conditioning

Samples of pure titanium have been treated by means a plasma immersed ion implantation (PIII) process in a DC glow discharge in pure oxygen and in different nitrogen-oxygen mixtures. In contrast with conventional voltage supply based glow PIII, the present study has been conducted with a novel specifically designed high current supply which allows a high electron density to be kept constant, regardless of gas pressure variations, within the operational ranks. Thus, the acquired sample characteristics can be more clearly ascribed to the chemical composition of the mixture. One stratified TiO2 (rutile) and TiN0.26 layer was identified from XRD and Raman spectroscopy, both of these compounds reputedly being highly biocompatible. The superficial hardness of the samples was improved up to more than five times that of the untreated reference sample, namely, ∼1600 Vickers microhardness (10 g load) thanks to a 2–6 μm deep implanted layer. These optimal results have been obtained from an 80% nitrogen 20% oxygen mixture at 1×10-2 torr. Furthermore, with this gas proportion, the best roughness finishing of the sample set was accomplished, which can be relevant for biocompatible applications.

Carbon removal from tile gap structures with oxygen glow discharges

The removal of redeposited layers from tile gap structures with ITER-like geometries was investigated with various low-temperature glow discharges in oxygen. The test structure consists of 19 mm deep gaps with widths ranging from 0.5 to 4 mm. Erosion rates inside the gaps are reduced compared with a directly exposed witness surface. Smallest erosion is found for the lowest particle energies. Erosion at the bottom of the gap increases linearly with the gap width. Erosion on the side walls drops nearly exponentially with increasing distance from the top. Side wall erosion dominates the total eroded amount for all cases investigated. The total eroded amount integrated over the whole inner gap surface is larger than the amount that would be eroded on a flat surface with an area identical to the gap opening.

Optimization of non-oxidative carbon-removal techniques by nitrogen-containing plasmas

The continuous control of tritium inventory in ITER calls for the development of new conditioning techniques [G. Federici et al., Nucl. Fus. 41 (2001) 1967]. For carbon plasma-facing components, this implies the removal of the T-rich carbon co-deposits. In the presence of strong oxygen getters, such Be, the use of oxygen-based techniques will be discouraged. In addition, tritiated water generated by these techniques poses extra problems in terms of safety issues [G. Saji, Fus. Eng. Des. 69 (2003) 631; G. Bellanger, J.J. Rameau, Fus. Technol. 32 (1997) 196; T. Hayashi, et al., Fus. Eng. Des. 81 (2006) 1365]. In the present work, oxygen-free (nitrogen and ammonia) glow discharge plasmas for carbon film removal were investigated. The following gas mixtures were fed into a DC glow discharge running in a ∼200 nm carbon film coated chamber. Erosion rate was measured in situ by laser interferometry, RGA (Residual Gas Analysis) and CTAMS (Cryotrapping Assisted Mass Spectrometry) [J.A. Ferreira, F.L. Tabarés, J. Vac. Sci. Technol. A25(2) (2007) 246] were used for the characterization of the reaction products. Very high erosion rates (similar to those obtained in helium–oxygen glow discharge [J.A. Ferreira et al., J. Nucl. Mater. 363–365 (2007) 252]) were recorded for ammonia glow discharge.

Controlled carbon deposit removal by oxygen radicals

Amorphous hydrogenated carbon deposits are a present problem in fusion devices with carbon walls. They act as an unpredictable source of hydrogen retention and thus have to be regularly removed. In our experiment we studied removal of hard carbon deposits in an atmosphere of partially dissociated room temperature oxygen. The source of oxygen atoms was a radiofrequency glow discharge in oxygen at pressures in the vicinity of 100 Pa. The etching efficiency was observed as surface emissivity variations. Etchings rates are compared to those measured in the discharge region in presence of charged particles. Etching rates in the discharge region are of the order of 1 nm/s, etching rates in a neutral oxygen atmosphere were found to be strongly temperature dependent and reached the value of 10 nm/s at 300 °C.

Carbon removal in stainless steel wall and graphite sheet by using oxygen glow discharge

In ITER, oxygen glow discharge cleaning (He/O 2-GDC) is proposed as a method for removal of a-C:H co-deposited layer. In this study, graphite was irradiated by this method, and its erosion rate was quantitatively estimated from the CO and CO 2 desorption during the discharge. With He/O 2(7%)-GDC, the erosion rate was estimated to be 4.1 × 10 −6 g cm −2 h −1, which corresponds to 23 nm h −1. This erosion rate increased with O 2 mixture ratio in its discharging gas. However, the rate was not proportional to the oxygen mixture ratio because of an O 2 retention in graphite. From TDS, it is found that graphite exposed to He/O 2(7%)-GDC retained 3.1 × 10 16 O atoms/cm 2.

Selective Surface Modification of Poly(propylene) with OH and COOH Groups Using Liquid‐Plasma Systems

AbstractUnderwater plasma and glow discharge electrolysis are interesting new methods for polymer surface functionalization. The achievable content of O‐containing functional groups exceeds that of oxygen glow discharge gas plasmas by a factor of two (up to ca. 56 O/100 C). The percentage of OH groups among all O‐containing groups can reach 25 to 40%, whereas it is about 10% in the gas plasmas. Addition of hydrogen peroxide increases the fraction of OH groups to at most 70% (27 OH/100 C). The liquid plasma systems are also able to polymerize acrylic acid and deposit the polymer as very thin film on substrate surfaces or membranes, thereby retaining about 80% of all COOH functional groups (27 COOH/100 C).magnified image

Formation of ultrathin nanocomposite SiO 2:nc-Au structures by Pulsed Laser Deposition

A method for the formation of Au nanocrystal (nc) arrays embedded in an ultrathin SiO 2 layer in one vacuum cycle is proposed. The method is based on the co-deposition in vacuum of ∼1 nm thick uniform Si–Au amorphous layer at a specific composition ratio by Pulsed Laser Deposition on the pre-oxidized Si(1 0 0) substrate, followed by its oxidation in the glow discharge oxygen plasma at room temperature, resulting in the precipitation of Au ncs at the bottom interface and/or at the surface of the forming SiO 2 layer. The capping SiO 2 layer is formed by the glow discharge plasma oxidation of further deposited ultrathin Si layer. Au ncs 2–5 nm in size and with the separation of ∼3–20 nm from each other segregate during the oxidation of Au–Si mixture as evidenced by transmission electron microscopy (TEM). The evolution of Au and Si chemical state upon each step of the SiO 2:nc-Au nanocomposite structure formation is monitored in situ by X-ray photoelectron spectroscopy (XPS). The metrology of nanocomposite SiO 2:nc-Au structures describing the space distribution of Au ncs as a function of Au/Si ratio is presented.

On the stability of a dc-driven oxygen discharge in cylindrical geometry

Starting from the hydrodynamic equations describing the positive column of glow discharges in oxygen, the stability of the homogeneous state is investigated. The model contains electrons, positive and negative ions as well as the metastable molecule O2(a1Δg) as a detachment partner. It is shown that the transition from the H-mode to the T-mode of a dc-driven oxygen discharge is caused by an attachment-induced ionization instability. The stability boundaries are calculated for selected values of the plasma parameters. The comparison of experimental data and theoretical predictions shows satisfactory agreement.

Surface Modification of CrN Coating by Plasma Oxidation Process

In this study, the CrN coating was first deposited on tool steels by cathodic arc evaporation technique and then this coating was subjected to oxygen glow discharge to activate the oxidation process in order to form oxide layer on the surface of the CrN coating. The varied properties of CrN coatings with or without oxidation treatment were studied in terms of hardness, adhesion, microstructure and surface morphology by using XRD, AFM, Rockwell indentation, Nano-indentation tests. The result turned out that the effect of the plasma oxidation process on the hardness of CrN coating was improved due to the oxide layer on the surface.

The dehydrogenation of isopropanol on a nickel-manganese catalyst subjected to treatment in glow-discharge oxygen, argon, and hydrogen plasmas

The dependence of the activity of the (Ni 20 wt %-Mn 4 wt %)/SiO2 catalyst on the treatment of its surface in glow-discharge oxygen, argon, and hydrogen plasmas was studied. Catalytic experiments were performed in a flow reactor and under static conditions in a vacuum. The highest activity was observed after catalyst treatment in an argon plasma. Glow-discharge plasma treatment changed the structure and number of active centers, which resulted in a change in the reaction mechanism. Ab initio quantum-chemical calculations were performed using the Hartree-Fock (UHF) method for the Ni5 cluster. The results substantiated the suggested that the active center contained the hydrogen atom.

Oxygen glow discharge cleaning in nuclear fusion devices

Laboratory experiments were carried out as part of the preparations of an oxygen glow discharge cleaning experiment in ASDEX Upgrade. They aimed at evaluating the effect of mixing the oxygen with helium, the collateral damage caused by the glow discharge, as well as the influence of impurities in the films being eroded. Oxygen concentrations below 20% in helium are sufficient to achieve high erosion rates. The discharge can lead to the formation of oxide layers on surfaces which – as demonstrated for tungsten – can be rapidly reduced by post-treatment in a hydrogen discharge. For carbon, aluminum and iron the physical sputtering yield may become similar to the erosion yield of redeposited layers, but it is by more than one order of magnitude smaller for tungsten. Using a-B:C:H films with varying boron content, it was found that impurities can cause the erosion rate to drop by orders of magnitude.

Enhanced adhesion of plasma-sputtered copper films on polyimide substrates by oxygen glow discharge for microelectronics

Enhanced plasma-sputtered copper film adhesion onto polyimide substrates treated by oxygen glow discharge was investigated. The peel test demonstrates this improvement, with peel strengths of 0.7–1.2 g/mm for copper films prepared on un-modified polyimide substrates and 195.5–262.2 g/mm for copper films on oxygen plasma-modified polyimide substrates at certain plasma conditions. The enhanced adhesive strengths of plasma-sputtered copper films onto polyimide substrates by oxygen plasmas are due mainly to the increased surface energies of the polyimide substrates. Contact angle measurements indicate that the surface energies of polyimide substrates were greatly increased by oxygen plasmas. X-ray photoelectron spectroscopy analysis shows that the increased surface energies of polyimide substrates using oxygen plasmas occur because of the increased oxygen surface concentration and the increased C–O bond proportion.

Reduction of Pd/HZSM-5 Using Oxygen Glow Discharge Plasma for a Highly Durable Catalyst Preparation

Pd/HZSM-5 catalyst prepared via glow discharge plasma reduction followed by calcination thermally showed enhanced stability for methane combustion. In order to investigate the plasma catalyst preparation mechanism, oxygen glow discharge, initiated at room temperature, has been employed for reduction of Pd/HZSM-5 catalyst. The catalyst characterization using XRD and XPS analyses demonstrates that oxygen glow discharge plasma is also able to reduce Pd/HZSM-5 catalyst. This suggests that the electronic effect plays an important role in the treatment of catalyst using glow discharge. This also confirms that the thermal effect of the glow discharge reduction can be ignored. A highly durable catalyst for methane combustion has been obtained from calcination of the glow discharge reduced sample.

Measurements of the gas temperature in an oxygen plasma by spectroscopy of the O2(b 1Σ g + ) → O2(X 3Σ g − ) transition

A method for measuring the gas temperature in an oxygen plasma by spectroscopy of the electronic transition from the O2(b1Σg+, v = 0) metastable state of molecular oxygen into the O2(X3Σg−, v = 0) ground state is considered in detail. The method is verified experimentally for the plasma of dc glow discharge in pure oxygen. It is shown that the gas temperature can be determined by analyzing high-resolution spectra of the P branch of this transition, no matter whether its fine structure (PP and PQ branches) is resolved or masked, provided that the rotational structure of the spectrum is resolved. The feasibility of the method proposed in 1999 by P. Maco and P. Veis for determining the gas temperature from the ratio between the intensity maxima of the R and P branches of the O2(b1Σg+, v = 0) → O2(X3Σg−, v = 0) transition in a poorly resolved spectrum was studied experimentally. It is shown that, in order to use this method, it is necessary to know the spectrograph instrumental function. The effect of the spatial inhomogeneity of the temperature and concentration of O2(b1Σg+) molecules on the accuracy of integral (over the plasma volume) measurements of the gas temperature is investigated using spatially resolved spectroscopy of the O2(b1Σg+, v = 0) → O2(X3Σg−, v = 0) transition. It is shown that precise measurements of the temperature require that the optical measurement system be thoroughly adjusted in order for the temperature and concentration of the emitting particles to vary insignificantly over the optically selected volume.