Low-pressure Glow Discharge Research Articles

Surface modification of a polyurethane film by low pressure glow discharge oxygen plasma treatment

AbstractLow pressure oxygen plasma has been used to improve the surface wettability of a polyurethane film. The modifications induced by the plasma treatment in the material were analyzed using contact angle measurements. X‐ray photoelectron spectroscopy technique was used for surface characterization of the plasma‐treated films. Atomic force microscopy and scanning electron microscopy were used to analyze topography changes due to the plasma‐etching mechanism. The results show a much better surface wettability of the film even for short exposure times, with a considerable increase in the surface energy values. As expected, functionalization with oxygen plasma is mainly because of surface oxidation with species like (CO, CO, OH, etc). An aging process with regard to polar groups rearrangement has been observed, thus promoting a partial hydrophobic recovery. Besides functionalization, the surface wettability of the material improves as a consequence of a slight increase in surface roughness because of the etching effect of oxygen plasma. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Surface Modification of a Polyurethane Film by Low Pressure Nitrogen Plasma for Improved Adhesion to Polyethylene Foam for Automotive Industry Laminates

A low pressure glow discharge nitrogen plasma has been used to modify wettability of a polyurethane film and, consequently, improve adhesion properties to polyolefin foam in order to obtain technical laminates for uses in automotive industry. A significant increase in surface wettability is achieved after the plasma treatment as observed by contact angle measurements. Surface changes have been studied by X-ray photoelectron spectroscopy (insertion of polar groups) and atomic force microscopy (changes in surface roughness and topography). Laminates based on plasma-treated polyurethane film and polyolefin foam show a considerable increase in the T-peel strength of the adhesion joint; furthermore we can observe a slight ageing process under aggressive conditions (high temperature and relative humidity).

Broad beam gas ion source with hollow cathode discharge and four-grid accelerator system

A broad beam gas ion source based on low-pressure hollow cathode glow discharge is described. An axial magnetic filed produced by AlNiCo permanent magnets enhances the glow discharge in the ion source as a result of the magnetizing electrons between the hollow cathode and rod anode. The gas plasma is produced by magnetron hollow cathode glow discharge in the hollow cathode and a collimated broad ion beam is extracted by a four-grid accelerator system. A weak magnetic field of several millitesla is enough to ignite the magnetron glow discharge at pressure lower than 0.1Pa, thereby enabling stable and continuous high-current discharge to form the homogeneous plasma. A four-grid accelerator, which separates the extraction and acceleration of the ion beam, is used in this design to generate the high-energy ion beam from 10keV to 60keV at a working pressure of 10−4Torr. Although a higher gas pressure is necessary to maintain the low-pressure glow discharge when compared to hot filament discharge, the hollow cathode ion source is operational with reactive gases such as oxygen in the high-voltage continuous mode. A laterally uniform ion beam can be achieved by using the four-grid accelerator system. The effects of the rod anode length on the characteristics of the plasma discharge as well as ion beam extraction from the ion source are discussed.

Decomposition of Methanol in a Low-pressure DC Glow Discharge

Decomposition characteristics of methanol in a low-pressure DC glow discharge in nitrogen-oxygen mixture are investigated by mass spectrometry. We obtained CO, CO2, CH4, C2H2 and HCN as methanol fragments and by-products; C2H2 and HCN are minor intermediate products, and CO, CO2 and CH4 are end products in the discharge. It is found that the yield of CO2 increases with oxygen concentration, but that the yield of CO peaks around the oxygen concentration of 15%.

プラズマと分光分析 ＩＶ．直流放電プラズマとその発光分析への応用

This lecture describes the spectrochemical characteristics of direct-current discharge plasmas generated by arc, spark, and glow discharges, which are generally employed as an excitation source for direct analysis of solid samples in atomic emission spectrometry. Emission spectra excited from arc discharge plasmas under ambient atmosphere are much different from those from low-pressure glow discharge plasmas. Excitation phenomena in glow discharge plasmas are described in detail so that a particular excitation mechanism occurring in the reduced pressure plasma, called charge transfer collision, can be well understood.

Glow Discharge Formation over Water Surface at Saturated Water Vapor Pressure and Its Application to Wastewater Treatment

With the aim of wastewater treatment using the active reaction of OH radicals generated in gas discharges, DC, low frequency (LF, 100 kHz) and RF (13.56 MHz) glow discharges were generated over the water surface at saturated water vapor pressure. Low-pressure glow discharge over the water surface has an advantage of uniform OH radical production near the water surface. A very stable and uniform glow discharge was obtained with an RF power source, whereas the discharges obtained with the DC and LF power sources were sometimes localized. The effective OH radical production was confirmed from the strong optical emission of OH(A2Σ+–X2Π) near the water surface. The OH(A2Σ+–X2Π) emission intensity near the water surface increased almost linearly with an increase in discharge power. N,N-Dimethyl- p-nitrosoaniline (RNO) solution as a persistent test pollutant was treated by RF glow discharge over the water surface. We confirmed that the RNO solution was certainly decolorized by OH radicals generated in the RF glow discharge. It was found that the degradation of target compounds by OH radicals was concentrated near the water surface in the solution because very slow diffusion of target compounds limited the reaction rate.

Cumulative contributions of 3s–np (n⩾3) transitions in comparing O atom densities in low-pressure Ar–O2 and He–O2 glow discharges

Many-fold enhancements in the population densities of O atoms in low-pressure glow discharges in Ar–O2 and He–O2 compared with the discharge in pure O2 have been reported in recent years. In some reports, the 3sS3,5–3pP3,5 transitions of O atoms around 844.6 or 777.2nm are considered in estimating the density of atomic oxygen. The authors’ results demonstrate that the contributions of other possible transitions such as 3sS3,5–npP3,5 (n>3) directly feeding the lower 3sS3,5 levels are also significant and should be included for a better estimate of O atom densities.

Optical emission characteristics of glow discharges in Ar-O2 and He-O2 gas mixtures

We have investigated optical emissions from low-pressure DC glow discharges in Ar-O2 and He-O2 gas mixtures in the 300 to 850 nm spectral range. The ionic species present in these discharges reveal interesting energy transfer pathways between different constituents of the gas mixtures. In the case of Ar-O2 mixture, we observed the presence of highly excited Ar and O atoms as well as Ar+, Ar2 + and O2 + ions. Apparently, charge-exchange collisions between Ar2 + and O2 yield O2 + ions. The path ways of energy transfer are different for He-O2 gas mixture where O2 + ions are formed directly through collisions with excited He atoms. The O2 + ions subsequently capture free electrons and dissociate into excited O atoms.

Explosion dynamics of dusty plasma liquids induced by laser ablation on suspended dust particles

Dusty plasma liquids can be formed by suspending negatively charged micrometer sized dust particles in a low pressure glow discharge. We investigate the dynamical behaviors of the expanding plume generated by the ablation on one of the suspended particles using a focused nanosecond pulsed laser, through monitoring the white light emission intensity and dust particle trajectories. In the initial 10μs, the diameter of the expanding plume scales as D∝P−0.37t0.31, where P is the background pressure. The surrounding dust particles start to be driven away radially after tens of microseconds. The diameter of the expanding cavity (plasma bubble) with dust depletion scales as D∝t0.33.

Modelling and optimization of a low-pressure DC glow discharge in stable regime

A study, both theoretical and experimental, of the steady regime of a glow discharge is reported where a nitrogen plasma is produced by a DC power source operating at a constant current through a 200–500 mA range. Two different models of this plasma are developed. The first one was designed to reproduce the V–I characteristic curve where the transition from the background ionization regime to the saturation one takes place, and, from this, to the Townsend and breakdown regimes, respectively. The second model represents the plasma behaviour as that of a voltage operated current source which follows the power law depending on the gas pressure as i( υ) = Aυ n( p) . The experimental results confirm, to a high degree of coincidence, the predictions made by these models.

Surface modification of low density polyethylene (LDPE) film by low pressure O 2 plasma treatment

In this work, low pressure glow discharge O 2 plasma has been used to increase wettability in a LDPE film in order to improve adhesion properties and make it useful for technical applications. Surface energy values have been estimated using contact angle measurements for different exposure times and different test liquids. In addition, plasma-treated samples have been subjected to an aging process to determine the durability of the plasma treatment. Characterization of the surface changes due to the plasma treatment has been carried out by means of Fourier transformed infrared spectroscopy (FTIR) to determine the presence of polar species such as carbonyl, carboxyl and hydroxyl groups. In addition to this, atomic force microscopy (AFM) analysis has been used to evaluate changes in surface morphology and roughness. Furthermore, and considering the semicrystalline nature of the LDPE film, a calorimetric study using differential scanning calorimetry (DSC) has been carried out to determine changes in crystallinity and degradation temperatures induced by the plasma treatment. The results show that low pressure O 2 plasma improves wettability in LDPE films and no significant changes can be observed at longer exposure times. Nevertheless, we can observe that short exposure times to low pressure O 2 plasma promote the formation of some polar species on the exposed surface and longer exposure times cause slight abrasion on LDPE films as observed by the little increase in surface roughness.

On the reliability of low-pressure dc glow discharge modelling

Modelling approaches used for the description of the cathode region of dc glow discharges are reviewed, with the focus on hybrid models which combine the fluid description of positive ions and bulk electrons with the kinetic simulation of fast electrons. The reliability of the calculated discharge characteristics is analysed by testing the different assumptions of the models and the sensitivity of the calculated characteristics on the input data. The applicability of the particle-in-cell technique (complemented with Monte Carlo simulation of collision processes) for the simulation of dc glow discharges is also discussed.

Experimental and modelling investigations of a dielectric barrier discharge in low-pressure argon

The discharge behaviour of a dielectric barrier discharge (DBD) in low-pressure argon gas was investigated by experiments and modelling. The electrical characteristics and light emission dynamics of the discharge were measured and compared with the results of a two-dimensional fluid model. Our investigations showed that the discharge consisted of a single, diffuse discharge per voltage half-cycle. The breakdown phase of the low-pressure DBD (LPDBD) was investigated to be similar to the ignition phase of a low-pressure glow discharge without dielectrics, described by Townsend breakdown theory. The stable discharge phase of the LPDBD also showed a plasma structure with features similar to those of a classical glow discharge. The presence of the dielectric in the discharge gap led to the discharge quenching and thus the decay of the plasma. Additionally, the argon metastable density was monitored by measuring light emission from nitrogen impurities. A metastable density of about 5 × 1017 m−3 was present during the entire voltage cycle, with only a small (∼10%) increase during the discharge. Finally, a reduction of the applied voltage to the minimum required to sustain the discharge led to a further reduction of the role of the dielectric. The discharge was no longer quenched by the dielectrics only but also by a reduction of the applied voltage.

Repetitively pulsed CO2 laser driven by an electron accelerator with a gas-discharge plasma cathode

A repetitively pulsed electron-beam-controlled CO2 laser driven by an electron accelerator with a plasma electron emitter based on low-pressure glow discharge with hollow anode and cathode is presented. The application of the proposed emitter makes it possible to sharply increase the current and to control the electron-beam duration and, hence, the energy and time characteristics of the laser. It is demonstrated that the radiation pulse duration ranges from 300 to 1200 μs, whereas the radiation energy amounts to 200 J at an efficiency of 18%. In the course of lasing, the focal spot is not affected by the heterogeneous heating of the active medium. A significant heterogeneity in the gas flow upon an incomplete renewal of the gas in the zone filled with radiation leads to an increase in the focal spot owing to the wavefront distortions.

Pulsed excitation of low-pressure He–Xe glow discharges

Theoretical and experimental investigations of the temporal behaviour of the column plasma of square-wave-pulsed low-pressure glow discharges in mixtures of helium and xenon are presented. In the framework of the time-dependent, radially averaged model the equations for the external electrical circuit accounting for an approximate treatment of the electrode regions, rate equations for 13 xenon and 3 helium states and the time-dependent Boltzmann equations of the electrons are self-consistently solved. The experimental studies comprise time-resolved measurements of the electrical characteristics and of the axis densities of the lowest excited states of xenon. The main aspects of the self-consistent model and of the electrical and spectroscopic measurements are given and the results for the He–Xe mixtures containing 2% of xenon in a discharge tube with a diameter of 17.5 mm and an electrode distance of 20 cm at applied voltages between 140 and 250 V and gas pressures between 1 and 4 Torr are presented. The agreement between experimental and theoretical data is generally good during the entire pulse period. The analysis shows that the velocity distribution function of the electrons is subject to large structural alterations as a result of the periodic change between the on-phase of the discharge, driven by the electric field, and the off-phase when chemo-ionization process gets large importance. However, the chemo-ionization processes are not efficient enough to act as a reservoir of charged particles under the conditions considered. Concerning the excited xenon atoms it is found, in particular, that their temporal behaviour is significantly influenced by their mixing due to electron collision processes for optically forbidden transitions in the afterglow period.

Electric discharge fluid modelling with the contribution of convective and drift energy effects

A fluid model has been used in this work to analyze the electric and energetic behavior of a low-pressure DC glow discharge in Ar chosen as a gas test. The governing equations are the first three moments of the Boltzmann transport equations under their complete form without using the classical-drift-diffusion approximation for the momentum transfer equation while the energy conservation equation involves both thermal and drift energies. In the framework of the local energy approximation, the basic data needed more particularly in the collision source terms for both momentum transfer and energy equations are determined from a multi term solution of Boltzmann equation. Due to the strong coupling with electric field obtained from Poisson equation and the high sheath gradients, the transport equations are numerically solved using a powerful Galerkin finite elements method. This model, after a validation from comparison with literature results, is then used to analyze the convective and drift energy effects on the electric discharge characteristics. Present results show a large influence of the convective term in comparison to the drift-diffusion approximation, mainly on the electric field and charged density profiles due to the antagonist effect induced by this term on the electron and ion motion which reinforces the charge space. Present results show also the discharge characteristic changes mainly in the sheath due to the drift energy consideration.

Inactivation and Destruction of Bacillus Subtilis Using a Low Pressure Glow Discharge Plasma

Radicals like atomic oxygen, the hydroxyl OH and charged particles directly affect outer membranes of biological cells such as microorganisms. Low pressure glow discharges of oxygen and hydrogen peroxide (3% concentration H2O2) produced these radicals and charged particles, and their effect on the inactivation or destruction of Bacillus subtilis were investigated. Artificially contaminated 65 cm2 Petri dishes with a pure culture of the microorganism were used to test the efficiency of the plasma treatment. Cell density of the spore suspension of a 24-hour nutrient agar containing the culture is estimated at 1.5x109 cells/ml using standard plate count techniques. The effect of varied discharge conditions (450V and 500V), gas flow rate (20-60 sccm), sample temperature and exposure time (30-60 minutes) on the decimal reduction value (DRV or D-value), that is, the time required to inactivate or destroy 90% of the original population of microorganisms were determined. Results indicate that for the H2O2 plasma, at a discharge potential of 500V, the DRV is 11.83 minutes with a 99.6% destruction of microorganisms in 60 minutes. For oxygen plasma, the DRV is 10.57 minutes and 99.33% reduction in 60 minutes. The sample temperature in the H2O2 plasma exposure reached a maximum of 109 oC in an hour, while that in the oxygen plasma peaked at 46 oC in less than an hour. These DRV, cycle time and temperature values are much better when compared to common sterilization methods like autoclave, dry heat and gas like ethylene oxide.

Removal of volatile organic compounds in atmospheric pressure air by means of direct current glow discharges

A nonthermal plasma with an electron density on the order of 10/sup 12/ cm/sup -3/ and a gas temperature of 2000 K was generated in atmospheric pressure air, using a microhollow cathode discharge as plasma cathode. The plasma was sustained in a /spl sim/1 mm/sup 3/ micro reactor, by a voltage of 470 V between the plasma cathode and a planar anode, and at currents ranging from 12 to 22 mA. This direct current glow discharge has been used to study the remediation of methane and benzene, two of the most stable volatile organic compounds (VOCs). The removal fraction for 300-ppm methane in atmospheric pressure air, flowing through the 0.5-mm thick plasma layer, with a residence time of the gas in the plasma of less than 0.5 ms, was measured at 80% with an energy density of 4 kJ/L. For benzene, the remediation rate is as high as 90%, comparable to results obtained with low pressure glow discharges. The energy efficiency for benzene remediation is 0.9 g/kWh, higher than that obtained for benzene remediation in low pressure glow discharges in noble gases. However, the VOC fraction remaining was found to be limited to values of approximately 0.1 and 0.05 for methane and benzene, respectively. In addition to experimental studies, the VOC dissociation mechanism in a VOC/dry air mixture plasma was modeled using a zero-dimensional plasma chemistry code. The modeling results have shown that atomic oxygen impact reactions are the dominant dissociation reactions for VOC destruction in this kind of glow discharge. Diffusion of atomic oxygen to the dielectric walls of the reactor is assumed to cause the observed limitation in the VOC destruction rate and efficiency.

Rotational spectroscopy and equilibrium structures of S3 and S4

The sulfur molecules thiozone S3 and tetrasulfur S4 have been observed in a supersonic molecular beam in the centimeter-wave band by Fourier transform microwave spectroscopy, and in the millimeter- and submillimeter-wave bands in a low-pressure glow discharge. For S3 over 150 rotational transitions between 10 and 458 GHz were measured, and for S4 a comparable number between 6 and 271 GHz. The spectrum of S3 is reproduced to within the measurement uncertainties by an asymmetric top Hamiltonian with three rotational and 12 centrifugal distortion constants; ten distortion constants, but an additional term to account for very small level shifts caused by interchange tunneling, are required to reproduce to comparable accuracy the spectrum of S4. Empirical equilibrium (r(e)(emp)) structures of S3 and S4 were derived from experimental rotational constants of the normal and sulfur-34 species and vibrational corrections from coupled-cluster theory calculations. Quantum chemical calculations show that interchange tunneling occurs because S4 automerizes through a transition state with D2h symmetry which lies about 500 cm(-1) above the two equivalent C2upsilon minima on the potential energy surface.

Plasma based Ar+ beam assisted poly(dimethylsiloxane) surface modification

Plasma based Ar+ beam performed in RF (13.56MHz) low-pressure (200mTorr) glow discharge (at 100W, 1200W and 2500W) with a serial capacitance was employed for surface modification of poly(dimethylsiloxane) (PDMS) aimed at improvement of its interactions with living cells. The presence of a serial capacitance ensures arise of an ion-flow inside the plasma volume directed toward the treated sample and the vary of the discharge power ensures varied density of the ion-flow.XPS analysis was performed to study the changes in the surface chemical composition of the modified samples and the corresponding changes in the surface energy were monitored by contact angle measurements.We found that plasma based Ar+ beam transforms the initially hydrophobic PDMS surface into a hydrophilic one mainly due to a raising of the polar component of the surface tension, this effect being most probably due to an enrichment of the modified surface layer with permanent dipoles of a [SiOx]-based network and elimination of the original methyl groups. The initial adhesion of human fibroblast cells was studied on the described above plasma based Ar+ beam modified and acrylic acid (AA) grafted or not fibronectin (FN) pre-coated or bare surfaces.The cell response seems to be related with the peculiar structure and wettability of the modified PDMS surface layer after plasma based Ar+ beam treatment followed or not by AA grafting.