Pressure Radio Frequency Discharge Research Articles

Mass spectroscopy of oxygen plasmas with energetic ions

An experimental study of the plasma gas phase in low pressure radiofrequency discharges of oxygen is presented. The plasma phase has been studied by means of mass spectroscopy of the neutral and of the charged species, directly sampling the plasma gas phase. We also measured the ion energy distributions. We have studied the influence of the operating conditions on the plasma gas-phase composition. Ion density and energy have been measured as a function of discharge parameters. In particular, we have identified operating conditions in low pressure discharges allowing the extraction of ions from the plasma state with energies exceeding 50 eV. This makes plasma processing with energetic oxygen ions feasible in our device.

Patterned superhydrophobic surface fabrication by coupled atmospheric pressure RF and pulsed volume dielectric barrier discharges

AbstractSuperhydrophobic (SH) surfaces have great potential in numerous applications. Plasma polymerization is one of the most efficient technologies for engineering SH surfaces due to its unique feature of tailoring surface chemistry and surface topography simultaneously. Herein, a novel sandwich‐like plasma device that consists of contiguous two‐stage dielectric barrier discharges (DBDs) driven by the time‐modulated radiofrequency (RF) and pulsed power sources is proposed to polymerize hexamethyldisilazane (HMDSN) at atmospheric pressure for the purpose of SH surface engineering. The coordination of dual power sources shows effective performances in plasma operation and material surface treatment, compared to the case driven by any power source alone. Easy ignition and enhanced stability are achievable for the upper RF DBD with the assistance of the bottom pulsed DBD. Vice versa, a diffuser pulsed discharge is obtainable with the input of abundant active and energetic species and precursor fragments from the upper RF plasma. Diagnostic measurements by optical emission spectroscopy and Mie scattering demonstrate that HMDSN fragmentation and nanoparticle nucleation are initiated predominantly by the RF‐driven plasma. These species and nanoparticles are further fragmented and dispersed in the bottom pulsed discharge. Consequently, the desired SH surface is fabricated with a similar pattern to that of the pulsed DBD geometry. This study provides a new pathway based on the plasma‐assisted method to control surface hydrophobicity and provides insights on a new plasma deposition method suitable for atmospheric pressure material processing.

Investigation of atmospheric pressure RF discharge with coexisting α and γ-modes

An atmospheric pressure radio frequency discharge working at coexistence of α and γ-modes was investigated using electrical and optical measurements. The voltage and current characteristics showed a phase shift which is typical for α mode discharge. Time resolved discharge patterns were captured at a condition favorable for the coexistence of α and γ-modes to assist in analyzing the working mode. Spectral line profile of HeI line at 492.19 nm and its forbidden counterpart was recorded and analyzed to estimate the electric field distribution near the instantaneous cathode. The combined spectroscopic and imaging measurement showed the simultaneous presence of both modes in the plasma volume at certain conditions. The strong HeI line Stark broadening and consecutive analysis indicated a presence of a short plasma sheath with a strong electric field, characteristic for γ-mode, whereas a long extending low-field region was attributed to a presence of simultaneous α-mode. The electric field analysis method presented here can be used in a case where a very thin plasma sheath exists which is often detected in electrical discharges sustained at atmospheric pressure.

Experimental study on the transition between patterned and homogeneous glow in a pulse-modulated radio frequency atmospheric pressure dielectric barrier discharge in helium

In a pulse-modulated radio frequency atmospheric pressure dielectric barrier discharge in helium, the influence of the pulse width on the transition from a patterned glow discharge to a homogeneous glow discharge is experimentally investigated. The experimental results show that the discharge mode can transit from a patterned glow to a homogeneous glow at a moderate pulse width (7 μs) but only has a patterned glow at a short pulse width (4 μs) or a homogeneous glow at a long pulse width (18 μs). The characteristic regions of patterned and homogeneous glow discharges are also acquired. The results can be explained using the breakdown characteristics of the pulse-modulated radio frequency atmospheric pressure discharge.

Influence of cold plasma treatment parameters on the mechanical properties of polyamide homogeneous bonded joints

This study reports a systematic investigation of the effects of various cold plasma treatment parameters on adhesion characteristics of two polyamide substrates.Polyamide 6 and polyamide 6.6 specimens were treated with a low pressure radio-frequency discharge plasma using different treatment time, power inputs and working gas (air, argon and oxygen). Contact angle measurements with sessile drop technique were carried out for estimation of surface wettability, as well as surface roughness evaluation and X-ray Photoelectron Spectroscopy (XPS) analysis. Then, untreated samples and cold plasma treated samples were adhesively bonded together to form overlap joints. Single lap shear tensile testing of these adhesively bonded joints was performed to investigate the effect of different surface treatments on the joint strength.The experimental results show that the optimized plasma process may remarkably increase wettability properties of polyamide surface and shear strength of the bonded joints.

Effects of low pressure radio frequency discharge on the physical and mechanical characteristics and chemical composition of diffusion coating on a surface of complex configuration details

The work deals with the influence of low-pressure radio frequency (RF) discharge on the surface properties of metals and their alloys. As objects of research to study the interaction of the jet low pressure RF discharge into the surface of the material the following materials were chosen: tungsten cobalt alloy, high speed steel, structural steel. In the presence of the materials energy parameters of low pressure RF discharge flows in the discharge chamber and the electrode gap were studied. A quantitative assessment of the gas composition inside the chamber to determine the characteristics of the plasma flow, making the major contribution to the modification of the surface was carried out. The influence of the input parameters of the plasma unit on the discharge characteristics was held. Identification of the main processes responsible for the modification of the surface of metals and alloys with the metal sample in the plasma jet and the effect of samples of products complex configuration on its properties is determined.The results of studies of physical and mechanical characteristics and chemical composition of the surface layers of high-speed steels, alloys and steel before and after treatment by low pressure radio frequency discharges with the instrumental indentation methods and X-ray photo-electron spectroscopy. With the help of the quality control system of the inner surfaces tubular products were studied.

Experimental studies of the overshoot and undershoot in pulse-modulated radio-frequency atmospheric discharge

The overshoot and undershoot of the applied voltage on the electrodes, the discharge current, and radio frequency (RF) power were observed at the initial phase of pulse-modulated (PM) RF atmospheric pressure discharges, but factors influencing the overshoot and undershoot have not been fully elucidated. In this paper, the experimental studies were performed to seek the reasons for the overshoot and undershoot. The experimental results show that the overshoot and undershoot are associated with the pulse frequency, the rise time of pulse signal, and the series capacitor Cs in the inversely L-shaped matching network. In the case of a high RF power discharge, these overshoot and undershoot become serious when shortening the rise time of a pulse signal (5 ns) or operating at a moderate pulse frequency (500 Hz or 1 kHz).

Simulation of nanoparticle coagulation in radio-frequency C2H2/Ar microdischarges**Project supported by the Natural Science Foundation of Heilongjiang Province, China (Grant Nos. A2015011 and A2015010), the Postdoctoral Scientific Research Development Fund of Heilongjiang Province, China (Grant No. LBH-Q14159), the Program for Young Teachers Scientific Research in Qiqihar University (Grant No. 2014k-Z11), the National Natural Science Foundation of China (Grant No. 11404180), and

The nanoparticle coagulation is investigated by using a couple of fluid models and aerosol dynamics model in argon with a 5% molecular acetylene admixture rf microdischarges, with the total input gas flow rate of 400 sccm. It co-exists with a homogeneous, secondary electron-dominated low temperature γ-mode glow discharges. The heat transfer equation and flow equation for neutral gas are taken into account. We mainly focused on investigations of the nanoparticle properties in atmospheric pressure microdischarges, and discussed the influences of pressure, electrode spacing, and applied voltage on the plasma density and nanoparticle density profiles. The results show that the characteristics of microdischarges are quite different from those of low pressure radio-frequency discharges. First, the nanoparticle density in the bulk plasma in microdischarges is much larger than that of low pressure discharges. Second, the nanoparticle density of 10 nm experiences an exponential increase as soon as the applied voltage increases, especially in the presheath. Finally, as the electrode spacing increases, the nanoparticle density decreased instead of increasing.

Absolute and relative emission spectroscopy study of 3 cm wide planar radio frequency atmospheric pressure bio-plasma source

The dynamics of low power atmospheric pressure radio frequency discharge generated in Ar gas in long gap of 3 cm is investigated. This plasma source is characterized and analyzed for possible large scale biomedical applications where low gas temperature and potential-less effluent are required. The discharge forms a homogenous glow-like afterglow in ambient air at input power of 30 W with low gas temperature of 330 K, which is desirable in biomedical applications. With absolute calibrated spectroscopy of the discharge, electron density of 0.4 × 1018 m−3 and electron temperature of 1.5 eV are obtained from continuum Bremsstrahlung radiation of the source. Time and spatial resolved emission spectroscopy is used to analyze discharge generation mechanism and active species formation. It is found that discharge dynamics strongly correlates with the discharge current waveform. Strong Ar(2p) excited states emission is observed nearby the electrodes surface on a distance up to 200 μm in the plasma sheath region at 10 ns after the current peak, whereas OH(A) emission is uniform along of the interelectrode gap.

Particle modelling of magnetically confined oxygen plasma in low pressure radio frequency discharge

The main objective of this paper is the modelling and simulation of a radio frequency (RF) discharge in oxygen at low pressure and at room temperature, including the effect of crossed electric and magnetic fields for generation and confinement of oxygen plasma. The particle model takes into account one axial dimension along the electric field axis and three velocity components during the Monte Carlo treatment of the collisions between charged particles and background gas. The simulation by this developed code allows us not only to determine the electrodynamics characteristics of the RF discharge, but also to obtain kinetics and energetic description of reactive oxygen plasma at low pressure. These information are very important for the control of the deep reactive-ion etching technology of the silicon to manufacture capacitors with high density and for the deposition thick insulating films or thick metal to manufacture micro-coils. The simulation conditions are as follows: RF peak voltage of 200 V, frequency of 13.56 MHz, crossed magnetic field varying from 0 to 50 Gauss, and oxygen pressure of 13.8 Pa. In the presence of magnetic field, the results show an increase of the plasma density, a decrease of the electron mean energy, and also a reduction of the ratio between electron density and positive ion density. Finally in order to validate, the results are successfully compared with measurements already carried out in the literature. The conditions of comparison are from 100 to 300 V of the peak voltage at 13.56 MHz under a pressure of 13.8 Pa and a gap distance of 2.5 cm.

A Computational Study of Radio Frequency Atmospheric Pressure Discharge in Nitrogen and Oxygen Mixture Gases

The characteristics of radio frequency atmospheric pressure discharge in nitrogen and oxygen mixture gases are investigated by using a one-dimensional fluid model. The model consists of equations of particle continuity, Poisson's equation, and the electron energy equation. The effect of the concentrations of O2 in N2 on the discharge characteristics is analyzed. The results show that when the concentration of O2 in N2 is less than 12%, as the amount of O2 grows, the electron density and the N+4 ion density decrease; the main negative particles are electrons. When the concentration of O2 in N2 is greater than 12%, the electron density and the N+4 ion density increase with the increasing admixture of oxygen; the main negative particle is the O− ion. Moreover, the O− ion density, the O+2 ion density, the electron temperature and the mixture gases electronegativity increase with growth of O2 in the range from 4% to 20% and with density of O2 in N2.

Atmospheric-pressure plasma synthesis of carbon nanotubes

An atmospheric-pressure radio-frequency discharge (APRFD) has great advantages over vacuum-oriented plasma-enhanced chemical vapour deposition (PECVD) as well as other types of atmospheric-pressure plasma sources in terms of single-walled carbon nanotube (SWCNT) growth. We first provide an overview on the recent advances in PECVD synthesis of CNTs, ranging from low pressure to atmospheric pressure, and then we present our current work focusing on the analysis of reactive species generated in the cathodic plasma sheath for further understanding of the SWCNT growth mechanism in PECVD. It was found that the plasma-generated C2H2 is the main CNT growth precursor in PECVD. Approximately 30% of the CH4 (initial feedstock) was converted into C2H6, C2H4 and C2H2. A trace amount of C2H2 enabled the synthesis of SWCNTs in the thermal chemical vapour deposition (CVD) regime. H2 is necessary to grow SWCNTs using PECVD because H2 suppresses the formation of excess amount of C2H2; however, H2 does not eliminate amorphous carbon even at H2/C2H2 ratios of 300. PECVD using a binary mixture of C2H2 and isotope-modified 13CH4 demonstrated that CH4 does not contribute to CNT growth in C2H2-assisted thermal CVD. Atmospheric-pressure PECVD performed with a He/CH4/H2 system is equivalent to C2H2-assisted thermal CVD without an etching gas. APRFD appears to produce a hidden species, which influences the CNT growth process.

Fluorination of poly(dimethylsiloxane) surfaces by low pressure CF4 plasma – physicochemical and antifouling properties

Fluorinated surface groups were introduced into poly(dimethylsiloxane) (PDMS) coatings by plasma treatment using a low pressure radio frequency discharge operated with tetrafluoromethane. Substrates were placed in a remote position downstream the discharge. Discharge power and treatment time were tuned to alter the chemical composition of the plasma treated PDMS surface. The physicochemical properties and stability of the fluorine containing PDMS were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurements. Smooth PDMS coatings with a fluorine content up to 47% were attainable. The CF4 plasma treatment generated a harder, non-brittle layer at the top-most surface of the PDMS. No changes of surface morphology were observed upon one week incubation in aqueous media. Surprisingly, the PDMS surface was more hydrophilic after the introduction of fluorine. This may be explained by an increased exposure of oxygen containing moieties towards the surface upon re-orientation of fluorinated groups towards the bulk, and/or be a consequence of oxidation effects associated with the plasma treatment. Experiments with strains of marine bacteria with different surface energies, Cobetia marina and Marinobacter hydrocarbonoclasticus, showed a significant decrease of bacteria attachment upon fluorination of the PDMS surface. Altogether, the CF4 plasma treatments successfully introduced fluorinated groups into the PDMS, being a robust and versatile surface modification technology that may find application where a minimization of bacterial adhesion is required.

Inert Gas Admixtures in PECVD of Titanium Dioxide Thin Films on Polymers—Influence on the UV Absorptance of the Films

A low pressure radio frequency discharge (200 W for 5 min) was operated in oxygen, nitrogen/oxygen and argon/oxygen with small admixtures of titanium(IV)isopropoxide (TTIP). By means of the O2-flow, the O2-to-TTIP ratio q was varied in all the gas mixtures. Besides the growth rate, the absorptance of films deposited was determined in the range of 200–500 nm. Accordingly, their cut-off wavelength increased with q for mixtures containing N2 or Ar, while it was unaffected for O2/TTIP mixtures. At 310 nm, the absorption coefficient was a310 = 13 µm−1 for q ≈ 44 in Ar/O2/TTIP and marginally lower in N2/O2/TTIP as well as O2/TTIP. But, for decreasing q, a drastic decrease in a310 ≈ 8 µm−1 was witnessed for O2/TTIP mixtures. In contrast, a310 increased by 2 µm−1 for gas mixtures with Ar or N2.

Plasma polymerization of tetrafluoroethylene – towards CF2 dominated fluorocarbon films

AbstractFluorocarbon films with an exceptionally high CF2 content were obtained by plasma polymerization using a low pressure radio frequency discharge operated with a mixture of argon and tetrafluoroethylene. Substrates were placed in a remote position downstream the discharge. Gas pressure, discharge power, substrate position, gas composition and substrate temperature were changed to alter the chemical structure of the plasma polymers. The properties of the films were characterized by X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry and contact angle goniometry. A pronounced increase of the CF2 content was obtained for elevated substrate temperatures and increased amounts of tetrafluoroethylene in the process gas. Applied as a model surface in studies of interfacial phenomena on polytetrafluoroethylene (PTFE), transparent PTFE-like thin films enable the use of numerous optical techniques not applicable to common PTFE foils.

Fabrication of vertically aligned single-walled carbon nanotubes in atmospheric pressure non-thermal plasma CVD

A fabrication technique of high-purity vertically aligned single-walled carbon nanotubes (VA-SWCNTs) using atmospheric pressure plasma enhanced chemical vapor deposition is presented. Although densely mono-dispersed Fe–Co catalysts of a few nanometers is primarily responsible for VA-SWCNT growth, carbon precipitation was virtually absent in the thermal CVD regime at 700 °C. On the other hand, high-purity VA-SWCNTs without measurable defects were grown at 4 μm min −1 by applying atmospheric pressure radio-frequency discharge (APRFD) which has been previously developed for this purpose. The results proved that cathodic ion sheath adjacent to the substrates, where a large potential drop exists, also plays an essential role for the controlled growth of SWCNTs, while ion damage to the VA-SWCNTs is inherently avoided due to high collision frequency among molecules in atmospheric pressure. Operation regime of APRFD and tentative reaction mechanisms for VA-SWCNT growth are discussed along with optical emission spectroscopy of near substrate region.

UV Absorptance of Titanium Dioxide Thin Films by Plasma Enhanced Deposition from Mixtures of Oxygen and Titanium-Tetrakis-Isopropoxide

A low pressure radio frequency discharge was used to deposit films by mixtures of oxygen and titanium (IV) isopropoxide (TTIP) at powers of 200 W on films of polyethylene-terephthalat and samples of quartz glass. In the non-thermal plasma, films of rather pure TiO2 could be deposited as revealed by X-ray photo-electron spectroscopy. Besides the film growth rate and the chemical composition, the spectral behaviour of the spectral transmittance of visually transparent films was determined in the range from 200 to 500 nm. Furthermore, the absorptance of films has been derived at characteristic spectral positions of the transmission spectra of the films. Accordingly, cut-off wavelength was found to increase with deposition time from 5 to 10 min as well as with the concentration of TTIP in a range below 1.7%. At 310 nm, the spectral absorption coefficient (extinction coefficient × concentration) was 12 μm−1. While keeping other parameters constant, this coefficient decreased by 4 μm−1 due to an increase of the concentration of TTIP from 1.7% to 8%. Simultaneously, the surface roughness increased as revealed by profilometry. Thus, since the chemical structure of films was found to change only marginally, a decrease of the film density is likely to cause the observed dependence of the absorption coefficient with increasing precursor concentration.

Deposition of vertically oriented carbon nanofibers in atmospheric pressure radio frequency discharge

Deposition of vertically oriented carbon nanofibers (CNFs) has been studied in an atmospheric pressure radio frequency discharge without dielectric barrier covering the metallic electrodes. When the frequency is sufficiently high so that ions reside in the gap for more than one rf cycle (“trapped ions”), the operating voltage decreases remarkably and the transition from a uniform glow discharge to an arc discharge is suppressed even without dielectric barriers. More importantly, the trapped ions are able to build up a cathodic ion sheath. A large potential drop is created in the sheath between the bulk plasma and the electrode, which is essential for aligning growing CNFs. At the same time, the damage to CNFs due to ion bombardment can be minimized at atmospheric pressure. The primary interest of the present work is in identifying the cathodic ion sheath and investigating how it influences the alignment of growing CNFs in atmospheric pressure plasma-enhanced chemical-vapor deposition. Spectral emission profiles of He (706nm), Hα (656nm), and CH (432nm) clearly showed that a dark space is formed between the cathode layer and the heated bottom electrode. However, increasing the rf power induced the transition to a nonuniform γ-mode discharge which creates intense plasma spots in the dark space. Aligned CNFs can be grown at moderate input power during the initial stage of the deposition process. Catalyst particles were heavily contaminated by precipitated carbon in less than 5min. Alignment deteriorates as CNFs grow and deposition was virtually terminated by the deactivation of catalyst particles.

Materials processing at atmospheric pressure: Nonequilibrium effects on nanotechnology and mega-industries

Abstract Applications of atmospheric pressure nonequilibrium plasmas, because of their special advantages of forming reactive plasmas in a simple reactor, are spreading into various engineering fields, not only of materials processing, but also into energy and environment areas. Our group has explored new applications of both filamentary and diffuse barrier discharges, including the establishment of appropriate modeling, which enables better optimization of given plasma processes. More recently, microplasmas produced in submillimeter to micrometer reactors are also highlighted in association with atmospheric pressure nonequilibrium plasma because such small-scale plasmas frequently require high-density media to produce. This paper overviews our recent projects: (1) steam reforming of methane using filamentary barrier discharge; (2) deposition of carbon nanotubes in atmospheric pressure radio frequency discharge (APRFD); and (3) synthesis of silicon nanoparticles using microplasma.

Modeling of magnetically enhanced capacitively coupled plasma sources: Ar discharges

Magnetically enhanced capacitively coupled plasma sources use transverse static magnetic fields to modify the performance of low pressure radio frequency discharges. Magnetically enhanced reactive ion etching (MERIE) sources typically use magnetic fields of tens to hundreds of Gauss parallel to the substrate to increase the plasma density at a given pressure or to lower the operating pressure. In this article results from a two-dimensional hybrid-fluid computational investigation of MERIE reactors with plasmas sustained in argon are discussed for an industrially relevant geometry. The reduction in electron cross field mobility as the magnetic field increases produces a systematic decrease in the dc bias (becoming more positive). This decrease is accompanied by a decrease in the energy and increase in angular spread of the ion flux to the substrate. Similar trends are observed when decreasing pressure for a constant magnetic field. Although for constant power the magnitudes of ion fluxes to the substrate increase with moderate magnetic fields, the fluxes decreased at larger magnetic fields. These trends are due, in part, to a reduction in the contributions of more efficient multistep ionization.