Non-equilibrium Atmospheric Plasma Research Articles

Simple Non-Equilibrium Atmospheric Plasma Post-Treatment Strategy for Surface Coating of Digital Light Processed 3D-Printed Vanillin-Based Schiff-Base Thermosets.

A simple non-equilibrium atmospheric plasma post-treatment strategy was developed for the surface coating of three-dimensional (3D) structures produced by digital light processing 3D printing. The influence of non-equilibrium atmospheric plasma on the chemical and physical properties of vanillin-derived Schiff-base thermosets and the dip-coating process was investigated and compared to the influence of traditional post-treatment with UV-light. As a comparison, thermosets without post-treatment were also subjected to the coating procedure. The results document that UV post-treatment can induce the completion of the curing of the printed thermosets if complete curing is not reached during printing. Conversely, the plasma post-treatment does not contribute to the curing of the thermoset but causes some opening of the imine bonds and the regeneration of aldehyde functions. As a consequence, no great differences are observed between the not post-treated and plasma post-treated samples in terms of mechanical, thermal, and solvent-resistant properties. In contrast to the UV post-treatment, the plasma post-treatment of the thermosets induces a noticeable increase of the thermoset hydrophilicity ascribed to the reformation of amines on the thermoset surface. The successful coating process and the greatest uniformity of the lignosulfonate coating on the surface of plasma post-treated samples are considered to be due to the presence of these amines and aldehydes. The investigation of the UV shielding properties and antioxidant activities documents the increase of both properties with the increasing amount and uniformity of the formed coating. Interestingly, evident antioxidant properties are also shown by the noncoated thermosets, which are deduced to their chemical structures.

(Invited) Visualization of Charge Density Distribution in Parallel-Plate Dielectric Barrier Discharge Under Atmospheric-Pressure Air

Dielectric barrier discharge (DBD) is one of the atmospheric nonequilibrium plasma and is typically driven by sinusoidal or pulse voltage with electrodes that dielectric is inserted. DBD has been used in some industries such as plasma medicine and material processing. However, the fundamental process of DBD has not been elucidated yet. When a parallel plate DBD is driven by sinusoidal voltage under atmospheric pressure, numerous filamentary discharges called microdischarges (MDs) are observed. The surface charge deposited on the dielectric after the discharge plays a major role in the behavior of these MDs. As the MD develops during the gap and reaches the dielectric surface, a surface discharge occurs and deposits the charge on the dielectric surface.We performed dielectric surface charge measurements using Bi4Ge3O12, a Pockels crystal, in a short-gap, parallel-plate DBD of 0.5 and 0.3 mm driven by a sinusoidal high voltage. The high-speed camera enabled measurement of the spatio-temporal distribution of surface charge and estimation of the net electric field and the characteristics of microdischarges (MDs).The net electric field strength in the gap was estimated from the measurement results. The axial reduced electric field strength generated by the surface charge was found to be equivalent to 50-60% of the peak value of the applied electric field with all the voltage amplitude and gap length in this study, and the gap electric field after discharge was suppressed to less than 100 Td. Such a strong field keeps the MDs in a nonequilibrium plasma and also prevents re-discharge in the same position at the half cycle. On the other hand, the lateral electric field strength generated by the surface charge is largely characterized by the charge distribution—i.e., the voltage amplitude, gap length, and charge polarity—as mentioned above. Higher amplitude of applied voltage leads to an increase in the area of high lateral electric field strength above 100 Td. Compared to the gap length of 0.5 mm, the lateral electric field weakened, while the axial electric field generated by the surface charge was stronger in the gap length of 0.3 mm. As for the relationship between the charge polarity and the lateral electric field, the lateral field at the anodic dielectric, which the negative charge is deposited on, is dominant in the region of less than 100 Td. Meanwhile, on the cathodic dielectric, which the positive charge is deposited on, the area of above 100 Td is larger. That is, the strong lateral electric field can be observed near the cathodic dielectric, rather than the anodic dielectric. Thus, different chemical reactions may occur in the vicinity of the anodic and cathodic dielectrics, respectively, because of the difference in the drifting charged particles and the lateral field strength.

Hydrogen Production from Methane in Atmospheric Non-Equilibrium Plasma

Hydrogen Production from Methane in Atmospheric Non-Equilibrium Plasma

Zero-dimensional numerical simulation of dry reforming of methane in atmospheric pressure non-equilibrium plasma

Recently, atmospheric non-equilibrium plasma has been proposed as a potential and novel type of “reaction carrier” for the activation and conversion of greenhouse gases (methane and carbon dioxide) into value-added chemicals, due to its unique non-equilibrium characteristics. In this paper, a zero-dimensional plasma chemical reaction kinetic model in CH<sub>4</sub>/CO<sub>2</sub> gas mixture is constructed, with an emphasis on reaction mechanism for plasma dry reforming of methane to syngas and oxygenates. Especially, the effect of the CH<sub>4</sub> molar fraction (5%–95%) on plasma dry reforming of methane is investigated. First, the time evolution of electron temperature and density with initial methane content is presented, and the results show that both the electron temperature and electron density vary periodically with the applied triangular power density pulse, and the higher initial methane content in gas mixture is favored for a larger electron temperature and density. Subsequently, the time evolution of number densities of free radicals, ions and molecules at different CH<sub>4</sub>/CO<sub>2</sub> molar fraction are given. The higher the initial methane content, the greater the number densities of H, H<sup>–</sup>, H<sub>2</sub>, and CH<sub>3</sub>, leading to insufficient oxygen atoms to participate in the reaction for oxygenates synthesis. The conversions of inlet gases, the selectivities of syngas and important oxygenates are also calculated. The conversion rate of carbon dioxide increases with the increasing methane content, but the conversion rate of methane is insensitive to the variation of methane content. As methane mole fraction is increased from 5% to 95%, the selectivities of important oxygenates (CH<sub>3</sub>OH and CH<sub>2</sub>O) are relatively low (<5%), and the selectivity of H<sub>2</sub> gradually increases from 13.0% to 24.6%, while the selectivity of CO significantly decreases from 58.9% to 9.7%. Moreover, the dominant reaction pathways governing production and destruction of H<sub>2</sub>, CO, CH<sub>2</sub>O and CH<sub>3</sub>OH are determined, and CH<sub>3</sub> and OH radicals are found to be the key intermediate for the production of valuable oxygenates. Finally, a schematic overview of the transformation relationship between dominant plasma species is summarized and shown to clearly reveal intrinsic reaction mechanism of dry reforming of methane in atmospheric non-equilibrium plasma.

Development and characterization of anti-biofilm coatings applied by Non-Equilibrium Atmospheric Plasma on stainless steel

Biofilm-mediated microbial persistence of pathogenic and spoilage bacteria is a serious problem in food industries. Due to the difficulty of removing mature biofilms, great efforts are being made to find new strategies to prevent bacterial adherence to surfaces, the first step for biofilm development. In this study, coatings of (3-aminopropyl)triethoxysilane (APTES), tetraethyl orthosilicate (TEOS) and acrylic acid (AA) were applied by Non-Equilibrium Atmospheric Plasma on stainless steel (SS) AISI 316, the SS most commonly used in food industry equipment. Their anti-biofilm activity was assessed against Listeria monocytogenes CECT911 and Escherichia coli CECT515 after incubation at 37 °C. The best results were obtained for L. monocytogenes, with coatings consisting of a base coating of APTES and a functional coating of TEOS (AP10 + TE6) or AA (AP10 + AA6) that reduced biofilm production by 45% and 74%, respectively, when compared with the uncoated SS. These coatings were further characterized, together with a variation of the best one that replaced the acrylic acid with succinic acid (AP10 + SA6). Their anti-biofilm activity was assessed under different incubation conditions, including two strains of L. monocytogenes isolated from processing environments of a meat industry. The coating AP10 + AA6 reduced the biofilm formation by 90% after incubation at 12 °C, a temperature more representative of those commonly found in food processing environments. The morphological and physico-chemical characterization of the selected coatings showed that the coating with the highest anti-biofilm activity (i.e., AP10 + AA6) had lower surface roughness and higher hydrophilicity. This suggests that the formation of a hydration layer prevents the adherence of L. monocytogenes, an effect that seems to be enhanced by low temperature conditions, when the wettability of the strains is increased.

Microwave absorption and optical emission spectrometry analyses of ambient air plasmas induced by pulsed electron beams

This work is devoted to the characterization of plasmas produced by an intense pulsed relativistic electron beam propagating in air at atmospheric pressure. A large range of a time integrated dose is investigated [0.75 to 7.4] kGy(air) inducing electron densities from 2 × 1012 up to 1.6 × 1014 cm−3. The air plasmas are analyzed by two complementary diagnostics: microwave absorption and optical emission spectroscopy, to deduce plasma parameters during the creation and relaxation phases, respectively. A reduced air kinetics model is used to calculate transmission measurements during the electron beam pulse and to infer electric conductivities, which are compared to the results of three-dimensional Maxwell calculations. Optical emission spectra are measured and compared to calculated ones, giving rotational and vibrational temperatures during the relaxation phase of the plasma. These plasma characterizations may be used to model non-equilibrium atmospheric air plasma chemistry in the framework of, for instance, streamer dynamics as well as corona or dielectric barrier discharges.

Mass spectrometry of neutrals and positive ions in He/CO2 non-equilibrium atmospheric plasma jet

Neutral species and positive ions are measured by means of mass spectrometry in the effluent of the non-equilibrium atmospheric plasma jet operated in He/0%–1%CO2 gas mixture. The capacitively coupled plasma source is the predecessor of the known COST reference jet with identical performance and it is operated in a diffuse mode with gas temperature close to the room temperature. Threshold ionisation molecular beam mass spectrometry is used to measure absolute densities of CO, O2, O and O3 species. The CO molecules are generated with densities up to 2 × 1015 cm−3 at 1 W absorbed power. The O2 density is ∼6 times lower than the CO density and even lower than the O density at level of 4.5 × 1014 cm−3. The O3 density is negligibly small at 4 × 1012 cm−3. The measured O2, O3 and O absolute densities add up to ∼57% of the expected oxygen amount. The high O density could indicate that the vibrational excitation of CO2 is ineffective in this plasma and gas mixture, because O would otherwise react quickly with vibrationally excited CO2(v) to form CO and O2, the low energy efficiency is in agreement with the possibly low vibrational excitation. The highest energy efficiency is just 5% with 1% conversion efficiency. Ion mass spectrometry have been used to measure positive and negative ions, where only positive ions have been detected. The ion with highest signal is the ion, probably due to its low ionisation energy. , , , but no CO+, and protonated clusters or clusters containing water molecules have been detected as well. Neutrals and positive ions have been measured as a function of CO2 admixture, applied power and distance to the jet nozzle. The observed trends can be used for validation of plasma-chemistry models.

The effect of hydrogen peroxide concentration on the partial oxidation of methane to methanol in an atmospheric dielectric barrier discharge

It was proved that atmospheric non-equilibrium plasma can be deemed as “reaction carrier”, and is an effective method of partial oxidation of methane to methanol and other higher hydrocarbons. In this paper, hydrogen peroxide vapor is selected as oxygen-containing oxidizer and used to activate and convert methane into methanol in an atmospheric dielectric barrier discharge. A detailed axisymmetric 2D fluid model in CH4/H2O/H2O2 gas mixture is developed, with an emphasis on gas-phase plasma chemistry for partial oxidation of methane and methanol formation. Especially, the effect of hydrogen peroxide concentration on the conversion of methane to methanol is studied. The spatial and temporal distributions of various plasma species are shown as a function of hydrogen peroxide concentration. In addition, the main plasma species and reaction pathways governing the production and loss of CH3OH and OH are determined. It is shown that the increasing hydrogen peroxide concentration results in increase of OH and CH3OH production. Hydroxyl appears to play a significant role during the process of methanol synthesis, which is primarily produced by electron-impact dissociation of H2O2 and H2O molecules.

Synthesis of Copper-Based Nanostructures in Liquid Environments by Means of a Non-equilibrium Atmospheric Pressure Nanopulsed Plasma Jet

The influence of the liquid composition on the chemical and morphological properties of copper-based nanostructures synthesized by a non-equilibrium atmospheric plasma treatment is investigated and discussed. The synthesis approach is simple and environmentally friendly, employs a non-equilibrium nanopulsed atmospheric pressure plasma jet as a contactless cathode and a Cu foil as immersed anode. The process was studied using four distinct electrolyte solutions composed of distilled water and either NaCl + NaOH, NaCl only or NaOH only at two different concentrations, without the addition of any copper salts. CuO crystalline structures with limited impurities (e.g. Cu and Cu(OH)2 phases) were produced from NaCl + NaOH containing solutions, mainly CuO and CuCl2 structures were synthesized in the electrolyte solution containing only NaCl and no synthesis occurred in solutions containing only NaOH. Both aggregated and dispersed nanostructures were produced in the NaCl + NaOH and NaCl containing solutions. Reaction pathways leading to the formation of the nanostructures are proposed and discussed.

Molecular dynamics simulation of S. cerevisiae glucan destruction by plasma ROS based on ReaxFF

Atmospheric non-equilibrium plasma sterilization technology has been applied in the food processing, medical and health fields because of advantages such as short application time, low-temperature operation, high efficiency and safety. Research has shown that the active particles in the plasma play a decisive role in sterilization. However, the micromechanisms underlying the interaction between the active particles and biological components remain unclear. In this paper, with the common deteriorative microorganism Saccharomycodes as the research object, we examined the interaction between reactive oxygen species (O, OH, HO2 and H2O2) and glucan in the cell wall using a reactive force field molecular dynamics (ReaxFF MD) simulation methodology. We found that these reactive oxygen species reacted with the glucan structure by hydrogen abstraction reactions to cleave chemical bonds (C–O and C–C), resulting in cell wall destruction. Of these species, the O and OH species attract H atoms from the structure; these atoms are highly active and can easily break C–C bonds and release monosaccharides from the branched glucan chains. The H atoms in HO2 and H2O2 are strongly attracted to the glucan structure. Next, the C–O bonds are easily broken, leading to destruction of the chain structure, mainly because of the cleavage of the glucoside bonds. This simulation study adds to the understanding of the micromechanism of the ROS-mediated destruction of the cell wall glucan of Saccharomyces cerevisiae and of plasma sterilization at the atomic level.

Free radical generation by non-equilibrium atmospheric pressure plasma in alcohol–water mixtures: an EPR-spin trapping study

Free radical species in aqueous solution—various alcohol–water reaction mixtures—by exposure to non-equilibrium cold atmospheric pressure Ar plasma (CAP), were monitored using electron paramagnetic resonance spin-trapping techniques with 3, 5-dibromo-4-nitrosobenzene sulfonate as a water soluble nitroso spin trap. The major radical species were formed by H-abstraction from alcohol molecules due to ·OH radicals. In the ethanol–water mixture ·CH2CH2OH produced by H abstraction from CH3 group of the ethanol and ·CH3 radicals were detected. The latter was due to the decomposition of unstable CH3·CHOH to form the ·CH3 radicals and the stable formaldehyde by C–C bond fission. These intermediates are similar to those observed by reaction with ·OH radicals generation in the H2O2–UV photolysis of the reaction mixtures. The evidence of ·CH3 radical formation in the pyrolytic decomposition of the reaction mixtures by exposure to ultrasound or in methane irradiated with microwave plasma have been reported previously. However, the pyrolytic ·CH3 radicals were not found in both plasma and H2O2–UV photolysis condition. These results suggests that free radicals produced by Ar-CAP are most likely due to the reaction between abundant ·OH radicals and alcohol molecules.

Influence of Gas Temperature in Atmospheric Non-Equilibrium Plasma on Bactericidal Effect.

In this study, the relationship between plasma gas temperature and the bactericidal effects on five of bacteria (Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Enterococcus faecalis and Bacillus cereus (spore)) in liquid was investigated using a temperature-controllable plasma source. We determined that the bactericidal ability improved as the plasma gas temperature increased. Specifically, the bactericidal ability on E. coli of 80-℃ plasma was enhanced by as much as 6.3 times compared to that of 10-℃ plasma. The relationship between plasma gas temperature and the amount of hydroxyl radical, singlet oxygen, hydrogen peroxide, and ozone introduced into the solution was investigated. Our results also showed that each reactive species production increased by 2.1, 9.0, 1.6, and 17 times, respectively, with 80-℃ compared to 10-℃ plasma. The relationship between the bactericidal ability and amount of reactive species indicated that singlet oxygen and ozone introduced to the solution mostly influenced the bactericidal ability as the plasma gas temperature increased. We conclude that the plasma gas temperature is the crucial parameter for plasma sterilization.

Antibody immobilization on poly(L-lactic acid) nanofibers advantageously carried out by means of a non-equilibrium atmospheric plasma process

In the present study, the comparison between a conventional wet-chemical method and a non-equilibrium atmospheric pressure plasma process for the conjugation of biomolecules on the surface of poly(L-lactic acid) (PLLA) electrospun fibers is reported. Physico-chemical and morphological characteristics of chemically and plasma functionalized mats are studied and compared with those of pristine mats. The efficiency in biomolecules immobilization is assessed by the covalent conjugation of an antibody (anti-CD10) on the functionalized PLLA fibers. The achieved results highlight that the proposed plasma process enables antibodies to be successfully immobilized on the surface of PLLA fibers, demonstrating that non-equilibrium atmospheric pressure plasma can be an effective, highly flexible and environmentally friendly alternative to the still widely employed wet-chemical methods for the conjugation of biomolecules onto biomaterials.

Effects of Continuous Volumetric Direct-Coupled Nonequilibrium Atmospheric Microwave Plasma Discharge on Swirl-Stabilized Premixed Flames

The effect of continuous volumetric direct-coupled nonequilibrium atmospheric microwave plasma discharge on swirl-stabilized premixed methane–air flames was investigated using quantitative OH planar laser-induced fluorescence and spectrally resolved emission. The plasma discharge was found to influence the dynamics of flame stabilization, i.e., plasma-assisted flames stabilized in the quiescent center body wake were relatively stable while swirl flames stabilized in the active inner shear layer were prone to local extinction due to aerodynamic shear. At coupled plasma powers corresponding to less than 3% of the thermal power output, in addition to the improved flame stability, significant improvement in the lean blow-out limit ( $\sim 43$ %) and OH number density ( $\sim 150$ %) was observed. The enhancements are shown to be nonequilibrium plasma effects and not predominantly ohmic heating as significant equivalence ratio dependence of OH number density in plasma-assisted flames was observed. Spectrographic measurements indicated nitrogen vibrational temperatures as high as 6100 K, suggesting both vibrational and electronic excitation of nitrogen molecules in the presence of a plasma discharge. The activation of highly reactive species through vibrational–vibrational relaxation and direct impact dissociation accelerates the combustion chemistry. It is demonstrated that microwave direct plasma coupling can drastically enhance the dynamic flame stability of swirl-stabilized flames, especially at very lean operating conditions.

Experimental approaches for studying non-equilibrium atmospheric plasma jets

This work reviews recent research efforts undertaken in the area non-equilibrium atmospheric plasma jets with special focus on experimental approaches. Physics of small non-equilibrium atmospheric plasma jets operating in kHz frequency range at powers around few Watts will be analyzed, including mechanism of breakdown, process of ionization front propagation, electrical coupling of the ionization front with the discharge electrodes, distributions of excited and ionized species, discharge current spreading, transient dynamics of various plasma parameters, etc. Experimental diagnostic approaches utilized in the field will be considered, including Rayleigh microwave scattering, Thomson laser scattering, electrostatic streamer scatterers, optical emission spectroscopy, fast photographing, etc.

Sterilization of bacteria suspensions and identification of radicals deposited during plasma treatment

Abstract In this paper we will present results for plasma sterilization of planktonic samples of two reference strains of bacteria, Pseudomonas aeruginosa ATCC 27853 and Enterococcus faecalis ATCC 29212. We have used a plasma needle as a source of non-equilibrium atmospheric plasma in all treatments. This device is already well characterized by OES, derivative probes and mass spectrometry. It was shown that power delivered to the plasma is bellow 2 W and that it produces the main radical oxygen and nitrogen species believed to be responsible for the sterilization process. Here we will only present results obtained by electron paramagnetic resonance which was used to detect the OH, H and NO species. Treatment time and power delivered to the plasma were found to have the strongest influence on sterilization. In all cases we have observed a reduction of several orders of magnitude in the concentration of bacteria and for the longest treatment time complete eradication. A more efficient sterilization was achieved in the case of gram negative bacteria.

Thomson scattering diagnostics of atmospheric plasmas in contact with ionic liquids

Laser Thomson scattering has been applied to a nonequilibrium atmospheric plasma in contact with an ionic liquid to measure both electron density (ne) and electron temperature (Te). The discharge plasma was produced between a needle and a plane ionic-liquid electrode separated by a 3 mm gap. In comparison with a conventional metal electrode, distinct differences were found in spatial profiles of ne and Te of the plasma produced with the ionic-liquid electrode.

Investigation of reactive species using various gas plasmas

In this study, atmospheric nonequilibrium plasmas were generated with six gas species using a multi-gas plasma jet.

Plasma-target surface interaction during non-equilibrium plasma irradiation at atmospheric pressure: Generation of dusty plasma

AbstractThe remaining challenges, developing the relativistic electron beam sources, stimulate the investigations of cathode materials. Carbon-fiber-aluminum composite is the most appropriate cathode materials to construct the robust relativistic electron beam sources. Carbon-fiber-aluminum composite is treated by a non-equilibrium atmospheric plasma torch with a copper electrode based on high-voltage gas discharge. The axial and radial distributions of the plasma torch temperature are measured to determine the optimal treatment temperature and location. Copper-oxide particles with diameters of less than 1 µm are deposited onto the surface of the carbon-fibers and a layer of copper-oxide covers the entire surface as the treatment time is increased. Raman spectroscopy suggests that although the locations of the D and G band are similar, the areas of the D and G bands increase after the plasma treatment due to the reduced graphite crystalline size in the carbon-fibers. Analysis of the copper electrode surface discloses materials ablation arising from the discharge which releases copper from the source. Our results reveal that the atmospheric plasma torch generated by high-voltage discharge is promising in the surface modification of the carbon-fiber-reinforced aluminum composite. Further, the plasma produced by atmospheric plasma torch is dusty plasma, due to the participation of liberated copper particles. The plasma torch was analyzed by fluid dynamics, in terms of plasma density, plasma expansion velocity, and internal pressure, and it was found that the plasma produced by atmospheric torch is supersonic flow.

Experimental and numerical study on the optimization of pulse-modulated radio-frequency discharges

In this paper we present an experimental and computational study on the optimization of pulse-modulated radio-frequency (rf) discharges at atmospheric pressure. Based on the measured and simulated data, to improve the electron density and electron temperature in a pulse-modulated rf discharge, a lower modulation frequency, smaller than 50 kHz, with a higher duty cycle, usually larger than 30%, should be used. But to reduce the power consumption and lower the gas heating, the duty cycle should be smaller than 60%. On the other hand, to achieve a quasi-continuous rf plasma in a whole modulation cycle, a modulation frequency larger than 100 kHz is preferred. Thus, by choosing an appropriate modulation frequency and duty cycle, the pulsed rf discharge can be effectively optimized to produce a high-density and non-equilibrium atmospheric plasma without the risk of gas heating but with a reduced power consumption.