Pressure Radio Frequency Plasma Research Articles

Concentration measurements of atomic nitrogen in an atmospheric-pressure RF plasma jet using a picosecond TALIF

The absolute concentration and spatial distribution of ground-state atomic nitrogen (N) in an atmospheric pressure plasma jet were measured using the two-photon absorption laser-induced fluorescence. The jet was ignited by radio frequency voltage in argon (or argon with nitrogen admixture) flowing through a silica tube. The spatially resolved measurements of atomic nitrogen concentration were realized in the effluent of the jet. In a pure argon plasma, the N concentration was increased with the distance from the silica tube and reached the maximum value ( 3.5⋅1014 cm−3) at the distance of 15 mm, and then sharply decreased at the end of the plume. On the contrary, plasma ignited in Ar with nitrogen admixture, the maximum N concentration was located directly at the end of the silica tube, where plasma starts to blow out into the ambient air. The highest N concentrations for 0.5% and 2% of N2 in the feed gas were 1.3⋅1015 cm−3 and 4⋅1015 cm−3, respectively.

Availability and reactivity of N2(v) for NH3 synthesis by plasma catalysis

Production of vibrationally excited N2 (N2(v)) in atmospheric pressure nonthermal plasma and loss of N2(v) by gas-phase reactions and reactions on catalytic surfaces are analyzed to examine the role of N2(v) in NH3 formation by plasma catalysis. Vibrational state-to-state kinetic models complemented with molecular beam mass spectrometry (MBMS) measurements demonstrate that N2(v> 0) is produced with densities 100× greater than the density of N radicals by a radiofrequency atmospheric pressure plasma jet. The experimentally measured loss of N2(v) corresponds with a state-to-state kinetic model that describes loss of N2(v) by surface-mediated vibrational relaxation without consideration of reactions that convert N2(v) to NH3 over the catalyst surface. Rate constants for vibrational relaxation of N2(v) on catalyst surfaces exceed upper bounds on proposed rate constants for NH3 formation reactions from N2(v) over Fe when v < 9, Ni when v < 18, and Ag when v < 39, which indicates that only higher vibrational levels can possibly contribute to catalytic NH3 formation faster than they undergo vibrational relaxation on the surface. Densities of N2(v> 8), vibrational levels that can possibly react over Fe to form NH3 faster than they undergo vibrational relaxation, are less than or similar to N densities at the inlet of the catalyst bed and measured NH3 formation for the investigated conditions in this work, while densities of N2(v> 17) and N2(v> 38) are orders of magnitude below the N density at the inlet of the catalyst bed and the measured NH3 formation. The loss of N2(v) by vibrational relaxation on the surface limits the ability of N2(v) to contribute to catalytic NH3 formation and explains why N2(v) does not produce NH3 in quantities that are comparable to NH3 formation from N even though N2(v > 0) is more abundantly produced by the plasma.

The effects of radio frequency atmospheric pressure plasma and thermal treatment on the hydrogenation of TiO2 thin film

The effects of radio frequency (RF) atmospheric pressure (AP) He/H2 plasma and thermal treatment on the hydrogenation of TiO2 thin films were investigated and compared in this work. The color of the original TiO2 film changes from white to black after being hydrogenated in He/H2 plasma at 160 W (gas temperature ∼381 °C) within 5 min, while the color of the thermally treated TiO2 film did not change significantly even in pure H2 or He/H2 atmosphere with higher temperature (470 °C) and longer time (30 min). This indicated that a more effective hydrogenation reaction happened through RF AP He/H2 plasma treatment than through pure H2 or He/H2 thermal treatment. The color change of TiO2 film was measured based on the Commission Internationale d’Eclairage L*a*b* color space system. Hydrogenated TiO2 film displayed improved visible light absorption with increased plasma power. The morphology of the cauliflower-like nanoparticles of the TiO2 film surface remained unchanged after plasma processing. X-ray photoelectron spectroscopy results showed that the contents of Ti3+ species and Ti–OH bonds in the plasma-hydrogenated black TiO2 increased compared with those in the thermally treated TiO2. X-ray diffraction (XRD) patterns and Raman spectra indicated that plasma would destroy the crystal structure of the TiO2 surface layer, while thermal annealing would increase the overall crystallinity. The different trends of XRD and Raman spectra results suggested that plasma modification on the TiO2 surface layer is more drastic than on its inner layer, which was also consistent with transmission electron microscopy results. Optical emission spectra results suggest that numerous active species were generated during RF AP He/H2 plasma processing, while there were no peaks detected from thermal processing. A possible mechanism for the TiO2 hydrogenation process by plasma has been proposed. Numerous active species were generated in the bulk plasma region, accelerated in the sheath region, and bumped toward the TiO2 film, which will react with the TiO2 surface to form OVs and disordered layers. This leads to the tailoring of the band gap of black TiO2 and causes its light absorption to extend into the visible region.

The impact of ion mobility coefficients on plasma discharge characteristics

In this paper, the high-accuracy ion mobility coefficient based on the Chapman–Enskog approximation to the solution of the Boltzmann equation for low pressure radio frequency plasma discharges is presented. We employ two-dimensional fluid simulations of the argon filled axisymmetric reactor, where the effect of new ion-kinetics-based fluid closure is compared to theoretical expressions and experimental data. The spatial profiles of plasma composition in the low pressure radio frequency capacitively coupled plasma are presented, which includes the metastable reactions in the simulation. Moreover, inelastic collision integrals terms, due to charge exchange inelastic collisions between ions and neutral species, have been also considered. A Monte Carlo simulation of kinetic ion energy distribution of impinging on the radio frequency powered electrode provides a measure of accuracy of the new transport model. From our simulation, the results that mirror the influence of ion mobility coefficient obtained by the Chapman–Enskog method on plasma physical quantities under different pressures, frequencies, and electrode gaps is in good agreement with experimental measurement results and theoretical expressions.

Experimental and modeling studies of the plasma chemistry in a humid Ar radiofrequency atmospheric pressure plasma jet

While humid atmospheric pressure plasmas are extensively modeled, reaction set validation for these conditions remains limited. We present a detailed comparison of a modelling and experimental study with a goal to elucidate the plasma chemistry in a humid Ar radiofrequency-driven atmospheric pressure plasma jet. A large group of species including radicals (H, OH, O, HO2) and long-lived species (H2, O2 and H2O2) in the jet effluent was experimentally quantified by molecular beam mass spectroscopy (MBMS). MBMS measurements of H2O2, OH and H were validated by direct comparison with a liquid phase colorimetric measurement, laser-induced fluorescence (LIF) and two-photon absorption LIF respectively. While an excellent agreement was found for OH and H2O2 by both techniques, a significant difference was found for H and shown to be due to boundary layer effects at the MBMS sampling substrate. The measured O, OH, HO2 and H2 are in good agreement with the plug model while H and O2 were underestimated and H2O2 was overestimated by the model. The accuracy of both the used reaction set and the diagnostics, as well as the observed discrepancies between the modeling and experimental results, are critically assessed. The results presented in this work enable us to identify further data needs for describing H2O vapor chemistry in low-temperature plasmas.

Development of a non-thermal atmospheric pressure plasma-assisted technology for the direct joining of metals with dissimilar materials

This work demonstrated the joining of SUS304 stainless steel to polycarbonate (PC) using an atmospheric pressure radio frequency (RF) plasma jet in conjunction with thermal press fitting. The tensile shear stress values of samples bonded after plasma treatment of both the SUS304 and PC were found to be higher than those of specimens in which only one or neither surface was exposed to the plasma. These results confirm that an atmospheric pressure RF plasma jet can be applied to the joining of different materials.

Effects of external circuit on current and voltage waveforms in atmospheric pressure RF symmetrical alpha-mode discharges

Evolutions of distortions in voltage and current waveforms in atmospheric pressure radio frequency (RF) symmetrical alpha-mode discharges were obtained by varying the driving frequency and the length of connection coaxial cable in the external circuit. Based on the fast Fourier transform analysis on waveforms, the pronounced current and voltage waveforms are, respectively, attributed to series and parallel resonances occurring between the atmospheric pressure RF plasma and the external circuit. Despite the weak nonlinearity due to the geometrical symmetry and high collision-frequency at atmospheric pressure, variations of the external circuit such varying the length of the connecting cable can still influence discharges at an equal power.

A self-consistent method for solving ion diffusion and mobility coefficients

A self-consistent method for computation and testing of transport coefficients in low pressure radio frequency plasma discharges is detailed by comparing the experimental fitting formula and previous results gained from the Langevin expression. In this paper, for the first time, we apply the Chapman–Enskog method for calculating transport coefficients of ions to a weakly ionized plasma which is laid in a thermodynamic non-equilibrium and chemical non-equilibrium state. The ion diffusion coefficient is obtained by using the Chapman–Enskog method expanded to the second order approximation. The corresponding ion mobility coefficient is deduced from Einstein's relation. A two-dimensional axisymmetric fluid model of capacitively coupled plasmas is employed to determine the plasma composition. Moreover, the comparison of plasma physical characteristics using ion transport coefficients calculated by the Chapman–Enskog method with those derived from theoretical and experimental methods is also investigated. The results show that the ion transport coefficients obtained by the Chapman–Enskog method and their effect on plasma physical quantities are in good agreement with the experimental measurement and theoretical expression. Furthermore, the results indicate that the set of exponential and Morse potentials is more reasonable for the ion–neutral particles' interaction.

Insights into the low‐temperature deposition of a dense anatase TiO2 film via an atmospheric pressure pulse‐modulated plasma

AbstractA pulse‐modulated atmospheric pressure radio frequency (RF) plasma is used to prepare a dense TiO2 thin film at a low temperature. With certain plasma on time (Ton = 50 ms), the discharge temperature decreases with plasma off time (Toff), and the lengths of the thin films increase. An anatase TiO2 thin film was successfully deposited on a temperature‐sensitive substrate (polyethylene terephthalate). The transmission electron microscope (TEM) results show that the film is composed from anatase TiO2 nanoparticles grown through plasma in 0.07 s. By using laser light scattering, the TiO2 particles are found to be trapped above the substrate at about 0.27 mm during plasma on, which is close to the simulated plasma sheath thickness (0.26 mm). The deposition mechanism of pulse‐modulated atmospheric pressure RF plasma has been investigated.

Investigation of an atmospheric pressure radio frequency helium planar plasma source in humid ambient air

Atmospheric pressure radio frequency (RF) helium plasma jets operating in open air are capable of producing abundant reactive species and are widely used in biomedical applications. A planar atmospheric pressure RF plasma is studied by diagnostics and numerical modelling in this work. A novel methodology with reduced computational time and complexity is developed by converting a two-dimensional plasma fluid model into a series of one dimensional models. The numerical model reproduced the electrical characteristics and the emission spectroscopy dynamics of the discharge in good agreement with experimental measurements. Furthermore, the air influence on the kinetics of short-lived species and long-lived species is investigated numerically. It is shown that electrons are the most abundant negative species in the discharge and their density decreases with air impurities. The increasing air impurities along the discharge channel can weaken the electric field resulting in lower ionization near the outlet, and even possibly destabilizing the discharge. Reactive oxygen species are mainly present as atomic oxygen. H2O2 is produced in the gas phase through the recombination of OH radicals and destructed by collisions with He m . It is revealed that the important for biomedical applications singlet oxygen O2(a) is primarily generated by direct excitation of O2 and destructed through recombination with O(1S). The presented model can be used to study the kinetics of reactive species in atmospheric pressure RF plasma jets working in humid ambient air typical of biomedical applications.

Radio Frequency Atmospheric Pressure Plasma Hydrogenated TiO2-x/Ni Foam as an Efficient Photocatalyst

Radio Frequency Atmospheric Pressure Plasma Hydrogenated TiO2-x/Ni Foam as an Efficient Photocatalyst

Cold atmospheric pressure He-plasma jet and plasma ball interactions with the Venus flytrap: Electrophysiology and side effects

Cold atmospheric pressure radio frequency plasma (CAPP) can play an important role in agriculture, medicine, biophysical and bioelectrochemical applications, disinfection and sterilization, synthesis of different compounds, nitrogen fixation, and treatment of surfaces. Here we found that reactive oxygen and nitrogen species, UV–Vis photons, and high-frequency strong electromagnetic fields with an amplitude of a few kV produced by a cold plasma jet can interact with bio-tissue and damage it if the plasma treatment is long enough. The electrophysiological effects of CAPP treatment of bio-tissue and electrical signals transmission were measured in the Venus flytrap. The plasma ball does not produce any visible side effects on the Venus flytrap, but induces electrical signals in bio-tissue with very high amplitude. Plasma (Kirlian) photography shows the existence of a blue aura around the plasma ball due to a corona discharge. Understanding the mechanisms of interactions between CAPP and bio-tissue and preventing side effects can contribute to the application of plasma technology in medicine and agriculture. The use of cold plasma in medicine and agriculture should be monitored for side effects from strong high-frequency electro-magnetic fields, UV photons, and reactive oxygen and nitrogen species to protect against undesirable consequences.

Absolute ion density measurements in the afterglow of a radiofrequency atmospheric pressure plasma jet

Atmospheric pressure plasma jets (APPJs) can enable large fluxes of reactive species impinging on treated substrates in many applications. The relative importance of ionic versus neutral reactive species fluxes produced by APPJs is to date mainly deduced from modeling due to the challenge of measuring absolute ion densities/fluxes. In this study, a calibration was performed to determine the absolute densities of cold ions in the afterglow region of an radiofrequency-driven APPJ in Ar + 1% O2 by molecular beam mass spectrometry. Positive ion densities were found to be about 3–4 orders of magnitudes lower than the densities of the dominant reactive neutral species (O, O2(a 1Δg), O3) in the afterglow region of the APPJ at a distance larger than 2 mm from the visible plasma plume. The results illustrate the ability of molecular beam mass spectrometry to enable a quantitative evaluation of both neutral and ionic species in many applications using APPJs.

Dielectric barrier discharge and radio-frequency plasma effect on structural properties of starches with different amylose content

The effect of the dielectric barrier discharge atmospheric (DBD) and radio-frequency (RF) cold plasma treatment on the morphology, helical order, and structural stability of starches with different amylose content (30, 50, & 70%) was investigated. The cavities formed after RF plasma treatment allowed the active species to modify the internal structure of the granule, probably reaching its center and expanding to the periphery, where the amorphous regions were damaged and gradually removed by the treatment, increasing the amylose helix order and thermal stability of starch molecule as suggested by the SEM, FTIR, DSC, and TGA analysis. In contrast, DBD treatment promoted a thicker coating deposition and HMDSO functional groups inclusion, which increased both the granular interaction and the decomposition temperature. Overall, active species of HMDSO plasma generated in RF and DBD reactors modified mainly the amylose chains in a distinctive way, favoring the ordering or stability of starch molecules, respectively. These findings could be useful when looking for potential applications in the food industry or in other fields like semiconductors. Industrial relevanceAtmospheric pressure dielectric barrier discharge (DBD) and radio-frequency (RF) plasma are sustainable and quick methods to modify the structural properties of corn starch. After short-time treatments (10 min) both reactors change the granule surface (HMDSO coating and holes), which resulted in structural changes at different extent, such as increase of helical order and high thermal stability.These results showed that the DBD and RF treatments are suitable methods to obtain starches with high resistance to gelatinization (≈91 °C), especially those with high amylose content; however, due to their distinctive properties, they could have different end uses. DBD treated starch with HMDSO chemical group incorporation could be used in the paper, packaging, and electronic industry.Although HMDSO plasma is not yet approved for food industry, the results of FTIR confirmed that RF plasma treatment does not incorporate new elements in the starch molecules but only promoted interactions between them. Therefore, HDMSO plasma treatment could be considered as a safe and chemical-residue free technique useful to modify starches, which can be explored in food matrices development. Furthermore, they could be employed in an industrial food process that involves the use of high temperature.

Influence of a Helium–Nitrogen RF Plasma Jet on Onion Seed Germination

This paper presents an experiment using a radio frequency atmospheric pressure plasma jet to generate cold plasma for pre-sowing stimulation of Wolska onion seeds. Impact of the He + N2 afterglow plasma on germination was investigated. Eight groups of seeds characterized by different exposure times (2, 5, 10, 15, 60, 120, 240, and 480 s) and distance from the electrodes (20 mm and 50 mm) were used. Pre-sowing plasma stimulation of the seeds improved the germination capacity and germination energy for all tested groups, relative to control. The impact of radio frequency plasma on the onion seed germination parameters was statistically significant. The highest germination parameters were obtained for seeds stimulated for 240 s at a distance of 50 mm. No significant differences in physical and morphological properties of onion seeds were found.

Effect of pressure and space between electrodes on the deposition of SiN x H y films in a capacitively coupled plasma reactor

The fluid model, also called the macroscopic model, is commonly used to simulate low temperature and low pressure radiofrequency plasma discharges. By varying the parameters of the model, numerical simulation allows us to study several cases, providing us the physico-chemical information that is often difficult to obtain experimentally. In this work, using the fluid model, we employ numerical simulation to show the effect of pressure and space between the reactor electrodes on the fundamental properties of silicon plasma diluted with ammonia and hydrogen. The results show the evolution of the fundamental characteristics of the plasma discharge as a function of the variation of the pressure and the distance between the electrodes. By examining the pressure-distance product in a range between 0.3 Torr 2.7 cm and 0.7 Torr 4 cm, we have determined the optimal pressure-distance product that allows better deposition of hydrogenated silicon nitride (SiN x H y ) films which is 0.7 Torr 2.7 cm.

Atmospheric-pressure plasma assisted engineering of polymer surfaces: From high hydrophobicity to superhydrophilicity

To engineer polymer surfaces with long-lasting wettability properties and with high coating stability in aqueous media, we investigated the surface wettability of polyethylene terephthalate (PET) films treated by radio frequency (RF) atmospheric-pressure plasma. By combining plasma activation and hexamethyldisiloxane (HMDSO)-based plasma polymerization, stable PET surface properties from highly hydrophobic to superhydrophilic were achieved. The results clearly showed that the wettability of PET surfaces could be tuned from stable high hydrophobicity (>140°) to stable superhydrophilicity (<10°) with a minimized aging effect by using plasma activation of plasma deposited coatings. Surface roughness increase and CH3 groups introduction are primarily responsible for the coating hydrophobization, while the introduction of more oxygen-containing polar functional groups is the main factor leading to superhydrophilization. Importantly, the coatings engineered by this unique combination showed high stability in water over 14 days. Overall, this work contributes to the easy-to-overlook links between plasma activation and plasma polymerization, demonstrates that atmospheric-pressure RF plasma can be a versatile tool to control surface properties of polymers in a wide range from high hydrophobicity to superhydrophilicity with high coating stability in aqueous media and a negligible aging effect, which is promising for emerging biomedical applications.

Excitation and dissociation of CO2 heavily diluted in noble gas atmospheric pressure plasma

The excitation and dissociation of CO2 admixed to argon and helium atmospheric pressure radio frequency plasmas is analyzed. The absorbed plasma power is determined by voltage and current probe measurements and the excitation and dissociation of CO2 and CO by transmission mode Fourier-transform infrared spectroscopy (FTIR). It is shown, that the vibrational temperatures of CO2 and CO are significantly higher in an argon compared to a helium plasma. The rotational temperatures remain in both cases close to room temperature. The conversion efficiency, expressed as a critical plasma power to reach almost complete depletion, is four times higher in the argon case. This is explained by the lower threshold for the generation of energetic particles (electrons or metastables) in argon as the main reactive collision partner, promoting excitation and dissociation of CO2, by the less efficient quenching of vibrational excited states of CO and CO2 by argon compared to helium and by a possible contribution of more energetic electrons in an argon plasma compared to helium.

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.