Low Pressure Radio Frequency Plasma Research Articles

A combination of plasma diagnostics and Raman spectroscopy to examine plasma-graphene interactions in low-pressure argon radiofrequency plasmas

Graphene films were exposed to low-pressure capacitively coupled (E-mode) and inductively coupled (H-mode) argon radio frequency plasmas to investigate damage formation by very-low-energy ion irradiation. In the H-mode, plasma parameters were assessed by a Langmuir probe and plasma sampling mass spectrometry to determine the conditions of fixed ion fluence but with different average ion energies. The populations of argon metastable and resonant argon atoms were also measured by optical absorption spectroscopy to determine their contribution to the total energy flux during plasma treatment. In the H-mode, in which plasma-graphene interactions are dominated by ion irradiation effects, Raman spectroscopy reveals a significant rise in the D/G ratio and full width at half maximum of the G peak as well as the onset of graphene amorphization, even at very low ion energies (between 7 and 13 eV). In the E-mode characterized by comparable ion energy but much lower ion density, significant damage is also observed, a feature ascribed to the additional energy flux linked to the de-excitation of metastable argon species on the graphene surface.

Barrier and mechanical properties of carbon steel coated with SiOx/SiOxCyHz gradual films prepared by PECVD

Carbon steel is the most commonly used material for sheets, plates, bars and tubes in mechanical metallurgy and construction industry. However, the susceptibility of carbon steel to oxidation under real conditions of use turns preventive and corrective repairs mandatory. Many published studies show that plasma deposited silica and organosilicon layers can be used as corrosion barriers, being the first one more efficient due to its higher chemical density and inertness. However, the performance of SiOx layers may be affected by mechanical stress, and especially by defects generated in the deposition process or by the substrate finishing. To associate the good corrosion resistance of the silicon oxide films to the flexibility of organosilicon ones, some authors created multilayered systems, intercalating organosilicon and silica layers. However, the adhesion between the organosilicon and inorganic layers in these multilayered systems is still a problem, since there is a well-defined interface between them and both layers are just slightly interconnected. In this sense, the purpose of the present work is to create, from plasma polymerization of hexamethyldisiloxane, an organosilicon-silica gradual system and to investigate its barrier and mechanical properties. SiOx/SiOxCyHz gradual films were deposited by low pressure radiofrequency (13.56 MHz) plasmas using HMDSO, Ar and O2 mixtures. The change of an organosilicon to inorganic coating or vice versa was made only by adjusting the plasma conditions without interrupting the process. The influence of the carbon steel native oxide pre-deposition treatment in O2 plasma was also evaluated. Electrochemical Impedance Spectroscopy, EIS was used to evaluate the corrosion resistance provided by the gradual system to the carbon steel. Infrared spectroscopy, FTIR, was applied to analyze the chemical composition and molecular structure. The thickness of the films was measured by profilometry. Surface wettability was assessed from contact angle measurements using an automated goniometer. The morphology of the samples was inspected by secondary electrons micrographs acquired using a scanning electron microscope while the mechanical properties of the systems were examined by nanoindentation. The oxidation treatment performed on the substrate surface prior to deposition improve the corrosion resistance of the samples. The total corrosion resistance increases six orders of magnitude when carbon steel was coated with a quaternary gradual coating of only 2 μm.

Formation and behavior of negative ions in low pressure aniline-containing RF plasmas

This paper is focused on the formation mechanisms and the general behavior of negative ions in low pressure radio-frequency (RF) plasmas operated in a mixture of argon and aniline vapor. This type of plasma is mostly used for the synthesis of polyaniline, one of the most studied conductive polymers. Experiments based on mass spectroscopy measurements reveal the necessity to have a thin layer of plasma synthesized polyaniline on the electrodes to produce negative ions through complex surface reactions. In addition, thin-films deposited using this type of discharge are analyzed by means of Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The material analysis gives a first indication about the possible contribution of negative ions to polyaniline deposition.

The charge of micro-particles in a low pressure spatial plasma afterglow

In this letter, we present charge measurements of micro-particles in the spatial afterglow (remote plasma) of an inductively coupled low pressure radiofrequency plasma. The particle afterglow charge of , being deducted from their acceleration in an externally applied electric field, is about three orders of magnitude lower compared to the typical charge expected in the bulk of such plasmas. This difference is explained by a relatively simplistic analytical model applying orbital motion limited theory in the afterglow region. From an application perspective, our results enable further understanding and development of in situ plasma-based particle contamination control for ultra-clean low pressure environments.

Grafting of amine functions on cellulose acetate fibers by plasma processing

The aim of this study was to graft the amine functions on the surface of cellulose acetate (CA) fibers by two kind of plasma processing; namely, inductively coupled plasma (ICP) and radiofrequency plasma, using either NH3/He or mixture of N2/H2 gases. Optical emission spectroscopy was used to examine the various chemical species of the N2/H2 ICP system. Due to the entangled morphology of the CA fibers, only X-ray photoelectron spectroscopy (XPS) was used to characterize the surface. The effects of the different experimental conditions on the grafting efficiency have been studied. The results showed that in all cases NH2 groups were successfully grafted onto the surface of the CA fibers, and it seems that the ICP process is more effective for the amine grafting than low pressure radiofrequency plasma. Ageing tests carried out under laboratory conditions on the plasma-treated CA tow showed that the density of grafted amines was almost stable over a period of 60 days.

Oxygen content dependent etch rate of single polymer microparticles confined in the sheath region of a low pressure radiofrequency argon/oxygen plasma

To study the etching of polymer microparticles confined in low pressure radiofrequency plasmas, the size and refractive index of single 2 μm particles are experimentally obtained as a function of both time and oxygen content (0%–50%) added to the argon background gas. The etch rate was found to depend heavily on the oxygen (O2) content, especially for mixtures with low fractions of O2. As expected the etch rate was found to be close to zero in absence of O2 and increases to a value of 2 nm min−1 for 0.5% O2 and to roughly 3.5 nm min−1 for 5% O2. Above 5% O2 the etch rate saturates. It is shown that these results are consistent with a steady state etch model taking the effects of both atomic oxygen and positive ions into account.

Modification of the surface properties of glass-ceramic materials at low-pressure RF plasma stream

The surface roughness has a huge effect on the mechanical, optical, and electronic properties of materials. In modern optical systems, the specifications for the surface accuracy and smoothness of substrates are becoming even more stringent. Commercially available pre-polished glass-ceramic substrates were treated with the radio frequency (RF) inductively coupled (13.56 MHz) low-pressure plasma to clean the surface of the samples and decrease the roughness. Optical emission spectroscopy was used to investigate the plasma stream parameters and phase-shifted interferometry to investigate the surface of the specimen. In this work, the dependence of RF inductively coupled plasma on macroscopic parameters was investigated with the focus on improving the surfaces. The ion energy, sputtering rate, and homogeneity were investigated. The improvements of the glass-ceramic surfaces from 2.6 to 2.2 Å root mean square by removing the “waste” after the previous operations had been achieved.

Low-Pressure Plasma Synthesis of Ni/C Nanocatalysts from Solid Precursors: Influence of the Plasma Chemistry on the Morphology and Chemical State

Nanocatalyst materials based on metal nanoparticles (NPs) deposited on mesoporous carbon substrates are widely used in catalysis and energy storage; however, conventional wet-chemical deposition methods based on the reduction of metal salts are not always the best choice when looking for a process ensuring easy scalability and low environmental impact. Moreover, additional surface functionalization steps, such as the addition of nitrogen- or oxygen-containing groups, are more and more explored to increase the activity or the chemical stability of catalysts. In this work, we investigate a new methodology for the fabrication of nickel/carbon nanocatalysts relying on a low-pressure radio frequency plasma treatment of solid (powder) precursors. A mesoporous carbon xerogel is used as support for nickel NPs synthesized through the decomposition of an organometallic nickel precursor in a plasma discharge. Different plasma treatment conditions and chemical environments are applied by varying the plasma power and ...

Modification of starch using low pressure radio frequency air plasma

Cold plasma is one of the emerging nonthermal food processing technologies recently drawn interest as a chemical-free biopolymer modification method. In the present study, two starches (corn and tapioca) were exposed to low pressure radio frequency plasma at 40 W and 60 W for 10 and 20 min. The effect of air plasma treatment on the starch properties was determined by measuring amylose content, swelling power, powder flowability properties, gelatinization temperatures and pasting properties. The amylose content of corn and tapioca starches was observed to decreased and increased respectively. Increase in water binding capacity and swelling power of both starches. The results of pH showed partial oxidation of starches as evident from the decrease in pH. Rapid visco analyzer showed an increase in final viscosities after the plasma treatment. The increase in viscosities is the result of higher amounts of leach out of particles from the starch granules. Plasma treated corn starches exhibited lower peak temperatures and enthalpies whereas it increased in tapioca starches. X-ray diffractogram revealed an A-type pattern of both starches and plasma treatment did not show any significant effect on the crystallanity pattern. Cold plasma offers a unique route for starch modification without affecting its bulk properties.

Improvement of technical purpose materials performance characteristics with the radio frequency low pressure plasma

The main aim of the work is to solve the actual problem of increasing the competitiveness of tanning products by reducing the prime cost and improving the quality of finished products due to the increased durability of the working elements of tanneries. The impact of the low pressure radio frequency (RF) plasma in the processes of treating for modification of the materials for special purposes is considered in the article. The results of working elements of tanneries and the materials for special purposes sample processing by a RF low pressure plasma are described. As a result of leather materials nano structuring and nano modifying physical, mechanical and hygienic characteristics were increased. Processing of the technical purpose materials allows to increase operational performance of products and extend their lifespan.

Intensification of separation of oil-in-water emulsions using polysulfonamide membranes modified with low-pressure radiofrequency plasma

Effect of low-pressure radiofrequency capacitively-coupled plasma parameters on the separation of model “oil-in-water” emulsions using UPM-20 polysulfonamide membranes with a pore size of 0.01 μm has been studied. Atmospheric air has been used as a plasma gas. The plasma treatment time is 1, 5, 4, and 7 min at a plasma torch anode voltage of U a = 1.5–7.5 kV. The separated medium is a 3% emulsion based on the industrial oil I-20A; the stabilizer is the surfactant Kosintol-242. The highest performance is observed in the case of membranes treated with plasma for 7 min. It is found that plasma treatment leads to an increase in the emulsion separation efficiency from 90 to 99%. It has been shown that the plasma treatment contributes to the surface hydrophilization of the membranes owing to the formation of oxygen-containing functional groups. The membrane exhibits a contact angle of water of α = 59.6° before treatment and α = 19.5° after a 4-min plasma treatment at U a = 7.5 kV. Atomic force microscopy reveals that the plasma treatment leads to a change in the surface structure of the membranes, namely, to a decrease in their roughness. The internal structure of the membranes also undergoes changes which result in an increase in their crystallinity.

Enhancement of ZnO-Based Photocatalyst Activity by RF Discharge-Plasma Treatment*

The structure and photocatalytic properties of polydisperse zinc oxide (ZnO) powder treated with a low-pressure radio-frequency (RF) plasma discharge were studied. The photocatalytic properties of ZnO before and after the treatment were studied with respect to photodegradation of organic impurities using decomposition of methyl orange in aqueous solution by UV light as an example. It was found that the photodegradation kinetics were described well by first-order equations. The photodegradation mechanisms of methyl orange over untreated and plasma-treated ZnO samples were different. It was shown that the effectiveness of the photodegradation expressed in terms of reaction constants increased by 20% after RF discharge-plasma treatment of the photocatalyst.

A computational analysis of the vibrational levels of molecular oxygen in low-pressure stationary and transient radio-frequency oxygen plasma

Vibrational levels of molecular oxygen, O2(v < 42), are investigated in continuous and pulse-modulated low-pressure radio-frequency oxygen plasma with a global modelling approach. The model is benchmarked against a variety of pressure-, power- and time-resolved measurements of several inductive and asymmetric capacitive discharges available in the literature, and a good agreement is obtained. The sensitivity of the model with respect to the vibrational kinetics, the wall reactions and the spatial inhomogeneity of the charged particles are presented. The simulations without the vibrational levels are also shown for the sake of comparison.

Measurement of bi-directional ion acceleration along a convergent-divergent magnetic nozzle

Bi-directional plasma expansion resulting in the formation of ion beams travelling in opposite directions is respectively measured in the converging and diverging parts of a magnetic nozzle created using a low-pressure helicon radio-frequency plasma source. The axial profile of ion saturation current along the nozzle is closely correlated to that of the magnetic flux density, and the ion “swarm” has a zero convective velocity at the magnetic throat where plasma generation is localized, thereby balancing the bi-directional particle loss. The ion beam potentials measured on both sides of the magnetic nozzle show results consistent with the maximum plasma potential measured at the throat.

Experimental Investigation on Electromagnetic Attenuation by Low Pressure Radio-Frequency Plasma for Cavity Structure**supported by National Natural Science Foundation of China (No. 51107033) and the Fundamental Research Funds for the Central Universities of China (No. 2013B33614)

This paper reports on an experiment designed to test electromagnetic (EM) attenuation by radio-frequency (RF) plasma for cavity structures. A plasma reactor, in the shape of a hollow cylinder, filled with argon gas at low pressure, driven by a RF power source, was produced by wave-transmitting material. The detailed attenuations of EM waves were investigated under different conditions: the incident frequency is 1-4 GHz, the RF power supply is 13.56 MHz and 1.6-3 kW, and the argon pressure is 75-200 Pa. The experimental results indicate that 5-15 dB return loss can be obtained. From a first estimation, the electron density in the experiment is approximately (1.5-2.2) × 1016 m−3 and the collision frequency is about 11-30 GHz. The return loss of EM waves was calculated using a finite-difference time-domain (FDTD) method and it was found that it has a similar development with measurement. It can be confirmed that RF plasma is useful in the stealth of cavity structures such as jet-engine inlet.

Effect of Low-Pressure Radio Frequency Plasma on Ajwain Seed Germination

Effect of Low-Pressure Radio Frequency Plasma on Ajwain Seed Germination

Cyclic evolution of the electron temperature and density in dusty low-pressure radio frequency plasmas with pulsed injection of hexamethyldisiloxane

Optical emission spectroscopy was used to analyze the very-low-frequency cyclic evolution of the electron energy and density caused by repetitive formation and loss of dust nanoparticles in argon plasmas with pulsed injection of hexamethyldisiloxane (HMDSO, [CH3]6Si2O). After elaborating a Boltzmann diagram for Ar high-lying levels and a collisional-radiative model for Ar 2p (Paschen notation) states, temperatures characterizing the low- and high-energy parts of the electron population were calculated. Relative electron densities were also estimated from relative line emission intensities. Both temperatures increase when the dust occupation increases, and then decrease when dust is lost. The opposite trend was observed for the electron density. Such cyclic behaviors of the electron energy and electron density in the HMDSO-containing plasmas are in good agreement with the evolution processes in dusty plasmas, in which the formation of negative ions followed by an electron attachment on the surfaces of the nanoparticles is a critical phenomenon driving dust growth.

Surface roughness and hydrophilicity enhancement of polyolefin-based membranes by three kinds of plasma methods

The introduction of antibacterial property, conductivity, wettability and antithrombogenicity into polyolefin-based membranes has evoked much attention, which can be achieved by coating hydrophilic polymers. Therefore, it is necessary to modify the roughness and hydrophilicity of polyolefin-based membranes to enhance the coating ability. In this paper, three kinds of plasma methods, including inductively coupled (ICP) plasma, radio frequency low pressure (RFP) plasma and atmospheric dielectric barrier discharge (DBD) plasma, were used to modify the surface of the polyethylene (PE), polypropylene (PP) and polyester-polypropylene (PET–PP) membranes. The surface roughness of the plasma-modified PE, PP and PET–PP films was investigated by scanning electron microscopy (SEM) and atomic force microscope (AFM). The polar functional groups of films were observed by energy dispersive spectrometer (EDX) and X-ray photoelectron spectroscopy (XPS). Besides, the hydrophilicity of the plasma-modified PE, PP and PET–PP films was evaluated by water contact angle measurement. It was found that the surface roughness and hydrophilicity of plasma-modified PE, PP and PET–PP films increased with the generation of oxygen-containing functional groups (i.e. C―O, and CO). The PET–PP membranes were treated by RFP plasma at different processing powers and times. These results indicated that plasma is an effective way to modify films, and the treatment time and power of plasma had a certain accumulation effect on the membranes' hydrophilicity. As for the roughness and hydrophilicity, the DBD plasma modifies the PE film, which is the optimum way to get the ideal roughness and hydrophilicity. Copyright © 2015 John Wiley & Sons, Ltd.

A novel method for decoking of Pt–Sn/Al2O3 in the naphtha reforming process using RF and pin-to-plate DBD plasma systems

The effect of pin-to-plate dielectric barrier discharge (DBD) and radio frequency (RF) plasma on coked Pt–Sn/Al2O3 catalysts was investigated. Experimental results confirmed that both pin-to-plate atmospheric pressure DBD and low-pressure RF plasma were able to remove the carbon from the coked catalysts. A number of reactive species, such as O, O*, O2+ and Ar*, were present in the plasma medium, which contributed to the decoking reactions. The amount of carbon in the coked catalysts, as in the plasma-treated and thermal-regenerated catalysts, was determined by CHNS elemental analysis. Raman spectroscopy, TG/DTA and TPO analysis were used to confirm the removal of the coke from the Pt–Sn/Al2O3 catalysts. The plasma process was monitored by optical emission spectroscopy (OES). XRD confirmed the retention of crystallinity in the catalyst samples before and after plasma treatment. XRF analysis was used to measure the chlorine percent of fresh, coked and plasma regenerated catalysts. The BET analysis indicated that the surface area and pore volume of the plasma-treated catalysts were almost equal to those of the fresh catalyst sample. Metal surface area, metal dispersion ratio and average crystal size were determined by CO adsorption and these results indicated that during the plasma regeneration, the metal surface decreased 13.68% relative to the fresh catalyst. This decreasing is less than 14.53% for the thermal regenerated sample. Finally, the catalyst performance test indicated that the paraffin content in the feed was decreased from 39.3vol.% to 21.09 and 24.33vol.% over the fresh and DBD plasma regenerated catalyst. Also, the aromatic content was increased from 12.4vol.% in the feed to the 67.31 and 71.96vol.% over the fresh and regenerated catalyst.

Hydrophilic Stability of Plastic Surfaces Treated in Low- and Atmospheric-Pressure Radio-Frequency Plasmas

An atmospheric-pressure radio-frequency (RF) corona torch plasma and a low-pressure parallel-plate RF plasma are employed to increase hydrophilicity of polycarbonate and acrylonitrile butadiene styrene surfaces. The contact angle of water for the plastic surfaces is decreased by Ar plasma treatment in any condition, while the surface roughness is decreased, too, unexpectedly. The rate of decrease in contact angle versus treatment time in the atmospheric pressure plasma is about an order of magnitude higher than that in the low-pressure plasma. During postexposure of the plastic surfaces to ambient air, the contact angle is recovered gradually and saturated at a certain value depending on plasma treatment time. A longer plasma treatment time causes less recovery in contact angle. The results indicate that surface roughening as well as surface functionalization is responsible for improving the recovery characteristic and, hence, the hydrophilic stability.