Afterglow Plasma Research Articles

Development and fundamental investigation of He plasma ionization detector (HPID) for gas chromatography using DC glow discharge

In analytical chemistry, gas chromatography (GC) has been widely used because of the short measurement time and the low running costs. High-sensitivity detection is required for further improvement of the analytical performance. To date, several types of detectors have been developed. Thermal conductivity detectors (TCDs) are the most common GC detectors, and they are applicable for almost all samples. However, element-specific detection is impossible, and the limit of detection (LOD), which is on the order of 100 ppm, is not good. In this study, a new type of GC detector using atmospheric-pressure He plasma was developed. He has the highest ionization (24.58 eV) and metastable (19.82, 20.62 eV) energies among the elements. This means that He plasma can effectively ionize and excite all elements. In the helium plasma ionization detector (HPID), DC-powered He plasma and rod-like electrodes were utilized for ionization of the samples. For an ionization detector, the generation of very stable plasma is important. Therefore, we used DC He plasma. The sample was ionized when it was mixed into the He afterglow plasma. The concentration of each sample was then measured by the detection of the ion or electron current. When a 2 mL mixture of 20 ppm H2, N2, O2, CH4 and CO was introduced for GC, a LOD of 21–67 ppb was achieved. Thus, the detection ability of HPID was more than 1000 times better than that of TCD.

Emissive sheath measurements in the afterglow of a radio frequency plasma

The difference between the plasma potential and the floating potential of a highly emissive planar surface was measured in the afterglow of a radio frequency discharge. A Langmuir probe was used to measure the electron temperature and an emissive probe was used to measure the spatial distribution of the potential using the inflection point in the limit of zero emission technique. Time-resolved measurements were made using the slow-sweep method, a technique for measuring time-resolved current-voltage traces. This was the first time the inflection point in the limit of zero emission was used to make time-resolved measurements. Measurements of the potential profile of the presheath indicate that the potential penetrated approximately 50% farther into the plasma when a surface was emitting electrons. The experiments confirmed a recent kinetic theory of emissive sheaths, demonstrating that late in the afterglow as the plasma electron temperature approached the emitted electron temperature, the emissive sheath potential shrank to zero. However, the difference between the plasma potential and the floating potential of a highly emissive planar surface data appeared to be much less sensitive to the electron temperature ratio than the theory predicts.

Air–water ‘tornado’-type microwave plasmas applied for sugarcane biomass treatment

The production of cellulosic ethanol from sugarcane biomass is an attractive alternative to the use of fossil fuels. Pretreatment is needed to separate the cellulosic material, which is packed with hemicellulose and lignin in cell wall of sugarcane biomass. A microwave ‘tornado’-type air–water plasma source operating at 2.45 GHz and atmospheric pressure has been applied for this purpose. Samples of dry and wet biomass (∼2 g) have been exposed to the late afterglow plasma stream. The experiments demonstrate that the air–water highly reactive plasma environment provides a number of long-lived active species able to destroy the cellulosic wrapping. Scanning electron microscopy has been applied to analyse the morphological changes occurring due to plasma treatment. The effluent gas streams have been analysed by Fourier-transform infrared spectroscopy (FT-IR). Optical emission spectroscopy and FT-IR have been applied to determine the gas temperature in the discharge and late afterglow plasma zones, respectively. The optimal range of the operational parameters is discussed along with the main active species involved in the treatment process. Synergistic effects can result from the action of singlet O2(a 1Δg) oxygen, NO2, nitrous acid HNO2 and OH hydroxyl radical.

The effect of a direct current field on the microparticle charge in the plasma afterglow

Residual charges of individual microparticles forming dense clouds were measured in a RF discharge afterglow. Experiments were performed under microgravity conditions on board the International Space Station, which ensured particle levitation inside the gas volume after the plasma switch-off. The distribution of residual charges as well as the spatial distribution of charged particles across the cloud were analyzed by applying a low-frequency voltage to the electrodes and measuring amplitudes of the resulting particle oscillations. Upon “free decharging” conditions, the charge distribution had a sharp peak at zero and was rather symmetric (with charges concentrated between −10e and +10e), yet positively and negatively charged particles were homogeneously distributed over the cloud. However, when decharging evolved in the presence of an external DC field (applied shortly before the plasma switch-off) practically all residual charges were positive. In this case, the overall charge distribution had a sharp peak at about +15e and was highly asymmetric, while the spatial distribution exhibited a significant charge gradient along the direction of the applied DC field.

Recombination in Dense Ion Plasmas

AbstractA theory is developed to treat ion recombination in dense ion plasmas. Numerical results are obtained with the method of molecular dynamics. Exponential suppression of recombination is found for the discharge afterglow plasma in sulfur hexafluoride at strong Coulomb nonideality Γ. The reason for suppression is an increase of the recombination activation energy ΔE with increase of Γ. The range ΔE corresponds to the multiparticle fluctuations. Ion solvation is another suppression factor. Its importance is demonstrated for the discharge afterglow plasma in fluorine at weak Coulomb nonideality. The theory developed agrees with the experimental data available. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Towards a plasma treatment of corneal infections

Abstract Infectious keratitis of the eye is induced by proliferation and penetration of microorganisms in the cornea. The possibility of developing a novel treatment for this condition based on the action of a low temperature, atmospheric pressure plasma is presented. In-vitro results show that bacteria and fungi responsible of keratitis are inactivated by the exposure to the afterglow of a RF plasma produced in a helium flow mixed with ambient air. On the contrary, the viability of corneal cells is not significantly affected. Furthermore, no DNA damage has been detected in the treated cells, suggesting that no long time effects might be induced by this kind of treatment. An increase of intracellular Reactive Oxygen Species (ROS) has been measured following the treatment, suggesting that these chemicals can be involved in the plasma-induced effects. Overall, the presented results point to the feasibility of a plasma-based treatment for infectious keratitis.

Electron-ion dissociative recombination rate constants: Effects for benzene analogs with varying degrees of methylation

An in-depth study of the effects of varying degrees of methyl substitution on protonated benzene's electron-ion dissociative recombination (e-IDR) rate constants has been conducted. The e-IDR rate constants for protonated benzene, toluene, xylene, trimethylbenzene, tetramethylbenzene, pentamethylbenzene, hexamethylbenzene, and all of their corresponding isomers have been measured using a Flowing Afterglow equipped with an electrostatic Langmuir probe (FALP) at 300K. The addition of the first methyl substitution decreases the measured e-IDR rate constant by ∼50%. The addition of successive methyl substituents has little effect on the e-IDR rate constant. Isomeric forms also have no effect on the measured e-IDR rate constant. Implications of the observed trends to the chemistry of the Titan ionosphere are discussed.

Thin-film deposition modeling of hydrogenated amorphous silicon in the afterglow of argon plasma

This paper proposes an integrated numerical approach to model the thin-film deposition of amorphous silicon in the afterglow of argon plasma. The investigated plasma enhanced chemical vapor deposition (PECVD) process is governed by gas flow equations, chemical kinetics equations and surface reaction equations. Nineteen species, i.e. Ar, Ar*, Ar+, Si2H6, Si2H4, SiH4, SiH3, SiH2, SiH, H2, H, Si, Si+, SiH3+, SiH2+, SiH+, H2+, H+ and electron (e) are considered, where 52 chemical reaction equations are adopted to describe the reactions in space among silane, hydrogen and argon plasma. Continuity, momentum and energy equations are solved to describe the velocity, pressure and temperature field of gas flow in the process chamber, while the density of neutral species is calculated from the corresponding transport equation considering its convection and diffusion effect, as well as the production and destruction due to chemical reactions. The transport equations of charged species are modeled by a drift–diffusion approximation. The ambipolar diffusion is assumed for charged species in the afterglow of argon plasma due to the vanishing of external electrical field. The equation of energy balance for electron is applied to the estimation of the electron temperature in the process chamber. In this study, ten reaction mechanisms of thin film growth are taken into account. The species considered in the deposition process on substrates are silane, the free radicals of silane, hydrogen, hydrogen atom, silicon atom, the species with dangling bond on the substrate surface covered by active and the passive site. All governing equations are discretized by using a finite volume method and solved by an iterative approach. The variables, such as position, velocity and moment, are expressed by Cartesian coordinates, where the non-staggered variable arrangement is adopted to define field variables. The deferred correction approach is employed to approximate for both diffusion and source terms, where SIMPLE algorithm is used to decouple pressure and velocity. A parallelized numerical scheme is implemented via MPI library. Numerical simulations with different operation parameters are performed to show their influence on the deposition rate. The numerical results indicate the deposition rate exhibits a double-peak characteristic on the substrate and give an average value varying between 5Å/s and 10Å/s for the studied PECVD process in the afterglow of argon plasma.

Influence of gas and treatment time on the surface modification of EPDM rubber treated at afterglow microwave plasmas

The ethylene propylene diene monomer (EPDM) rubber possesses excellent physical/chemical bulk properties, is cost-effective, and has been used in the mechanical and aerospace industry. However, it has an inert surface and needs a surface treatment in order to improve its adhesion properties. Plasma modification is the most accepted technique for surface modification of polymers without affecting the properties of the bulk. In this study, an afterglow microwave plasma reactor was used to generate the plasma species responsible for the EPDM surface modification. The plasma modified surfaces were analyzed by means of contact angle measurement, adhesion tests, attenuated total reflection-infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. Two experimental variables were analyzed: type of the plasma gases and exposure time were considered. The predominant failure mode was adhesive, for long treatment times a mixture of adhesive and cohesive failure can be observed and the best conditions tested there was an increase of the rupture strength of about 27%, that can be associated mainly with the creation of oxygen containing functional groups on the rubber surface (CO, COC and CO) identified by spectroscopic methods. The predominant failure mode was adhesive, for long treatment times a mixture of adhesive and cohesive failure can be observed. In various conditions tested the contact angles easily decreased more than 500%. What can be concluded that high wettability is a necessary condition to obtain good adhesion, but this is not a sufficient condition.

Kinetic Theory of Plasma Sheaths Surrounding Electron-Emitting Surfaces

A one-dimensional kinetic theory of sheaths surrounding planar, electron-emitting surfaces is presented which accounts for plasma electrons lost to the surface and the temperature of the emitted electrons. It is shown that ratio of plasma electron temperature to emitted electron temperature significantly affects the sheath potential when the plasma electron temperature is within an order of magnitude of the emitted electron temperature. The sheath potential goes to zero as the plasma electron temperature equals the emitted electron temperature, which can occur in the afterglow of an rf plasma and some low-temperature plasma sources. These results were validated by particle in cell simulations. The theory was tested by making measurements of the sheath surrounding a thermionically emitting cathode in the afterglow of an rf plasma. The measured sheath potential shrunk to zero as the plasma electron temperature cooled to the emitted electron temperature, as predicted by the theory.

Discharging of dust particles in the afterglow of plasma with large dust density

The discharging of dust particles in the afterglow of plasma with large dust density is studied. We used measured electron and metastable dependencies to calculate the rate describing collection of electrons by dust particles by solving the electron balance equation. This rate is compared with the rate calculated using the orbital motion limited (OML) theory. It is found that the OML theory may not be applied for description of dust charging at large afterglow times, and the energetic electrons generated in metastable-metastable collisions significantly affect charging of dust particles. The time dependence for dust charge is calculated by two different approaches: first, the "standard" approach is used, which assumes that ion and electron fluxes to the dust particles are different in the afterglow. Second, the dust charge is calculated by assuming that desorption of electrons from dust particles is very fast. Both approaches gave similar results for dust charging. In addition, the effects of secondary emission due to ion-dust and metastable-dust collisions on dust discharging are investigated. The main source of dust charging in the late afterglow of plasma with large dust density are the energetic electrons generated in Ar(m) metastable-metastable collisions.

Enhancement of anatase functionalization and photocatalytic self-cleaning properties of keratins by microwave-generated plasma afterglow

A microwave-generated plasma afterglow (MWGPA) treatment was applied to keratin fibers to improve their adhesion to anatase nanocrystals by modifying their surface chemical and physical structure. The induced photocatalytic self-cleaning property has significantly been improved by 70% as evidenced by complete stain decomposition in 6h only. The effects of gas mixtures, gas flow, treatment distance, treatment duration and power flow of the MWGPA treatment on keratin's surface were investigated. The MWGPA-induced alteration of keratin's surface structure resulted in an increase in the uptake of anatase crystals and thus enhanced photocatalytic self-cleaning activities. The contact angle, absorption time, deposition behavior, adhesion and stability of anatase coating of keratin fibers are discussed. The anatase uptake and coating uniformity were characterized quantitatively using Time-of-Flight Secondary Ion Mass Spectrometry. This efficient and low energy plasma surface activation approach demonstrated potential toward the practical application of anatase in keratinous materials.

Physiological and transcriptional response of Bacillus cereus treated with low-temperature nitrogen gas plasma

This study was conducted to investigate the inactivation kinetics of Bacillus cereus vegetative cells upon exposure to low-temperature nitrogen gas plasma and to reveal the mode of inactivation by transcriptome profiling. Exponentially growing B.cereus cells were filtered and put on agar plates. The plates, carrying the filters with the vegetative cells, were placed into low-temperature nitrogen gas plasma at atmospheric pressure. After different exposure times, the cells were harvested for RNA extraction and enumeration. The RNA was used to perform whole-transcriptome profiling using DNA microarrays. The transcriptome profile showed a large overlap with profiles obtained from conditions generating reactive oxygen species in B.cereus. However, excess radicals such as peroxynitrite, hydroxyl and superoxide could not be detected using radical-specific fluorescence staining. Lack of UV-specific responses including factors involved in DNA damage repair is in line with the absence of UV-specific emission in the afterglow of the nitrogen gas plasma as analysed using optical emission spectroscopy (OES). Antibacterial activity of nitrogen gas plasma is not based on UV radiation. Exposure to nitrogen gas plasma leads to oxidative stress and inactivation of targeted cells. A secondary oxidative stress with the indicative formation of reactive oxygen species within cells could not be observed. This study represents the first investigation of differential gene expression on a genome-wide scale in B.cereus following nitrogen gas plasma exposure. This study may help to design economically feasible, safe and effective plasma decontamination devices.

Sterilization/disinfection of medical devices using plasma: the flowing afterglow of the reduced-pressure N2-O2discharge as the inactivating medium

Potential sterilization/disinfection of medical devices (MDs) is investigated using a specific plasma process developed at the Université de Montréal over the last decade. The inactivating medium of the microorganisms is the flowing afterglow of a reduced-pressure N2-O2 discharge, which provides, as the main biocidal agent, photons over a broad ultraviolet (UV) wavelength range. The flowing afterglow is considered less damaging to MDs than the discharge itself. Working at gas pressures in the 400—700 Pa range (a few torr) ensures, through species diffusion, the uniform filling of large volume chambers with the species outflowing from the discharge, possibly allowing batch processing within them. As a rule, bacterial endospores are used as bio-indicators (BI) to validate sterilization processes. Under the present operating conditions, Bacillus atrophaeus is found to be the most resistant one and is therefore utilized as BI. The current paper reviews the main experimental results concerning the operation and characterization of this sterilizer/disinfector, updating and completing some of our previously published papers. It uses modeling results as guidelines, which are particularly useful when the corresponding experimental data are not (yet) available, hopefully leading to more insight into this plasma afterglow system. The species flowing out of the N2-O2 discharge can be divided into two groups, depending on the time elapsed after they left the discharge zone as they move toward the chamber, namely the early afterglow and the late afterglow. The early flowing afterglow from a pure N2 discharge (also called pink afterglow) is known to be comprised of N2+ and N4+ ions. In the present N2-O2 mixture discharge, NO+ ions are additionally generated, with a lifetime that extends over a longer period than that of the nitrogen molecular ions. We shall suppose that the disappearance of the NO+ ions marks the end of the early afterglow regime, thereby stressing our intent to work in an ion-free process chamber to minimize damage to MDs. Therefore, operating conditions should be set such that the sterilizer/disinfector chamber is predominantly filled by N and O atoms, possibly together with long-lived metastable-state O2(1 Δg) (singlet-delta) molecules. Various aspects related to the observed survival curves are examined: the actual existence of two “phases” in the inactivation rate, the notion of UV irradiation dose (fluence) and its implications, the UV photon best wavelength range in terms of inactivation efficiency, the influence of substrate temperature and the reduction of UV intensity through surface recombination of N and O atoms on the object/packaging being processed. To preserve their on-shelf sterility, MDs are sealed/wrapped in packaging material. Porous packaging materials utilized in conventional sterilization systems (where MDs are packaged before being subjected to sterilization) were tested and found inadequate for the N2-O2 afterglow system in contrast to a (non-porous) polyolefin polymer. Because the latter is non-porous, its corresponding pouch must be kept unsealed until the end of the process. Even though it is unsealed, but because the opening is very small the O2(1Δg) metastable-state molecules are expected to be strongly quenched by the pouch material as they try to enter it and, as a result, only N and O atoms, together with UV photons, are significantly present within it. Therefore, by examining a given process under pouch and no-pouch conditions, it is possible to determine what are the inactivating agents operating: (i) when packaged, these are predominantly UV photons, (ii) when unpackaged, O2(1Δg) molecules together with UV photons can be acting, (iii) comparing the inactivation efficiency under both packaged and unpackaged conditions allows the determination of the relative contribution of UV photons (if any) and O2(1Δg) metastable-state molecules. Such a method is applied to pyrogenic molecules and to the enzymatic activity of lysozyme proteins once exposed to the N2-O2 flowing afterglow. Finally, the activity of the infectious prion protein is shown to be reduced when exposed to the present flowing afterglow, as demonstrated by both in vitro and in vivo experiments.

A non-invasive technique to determine ion fluxes and ion densities in reactive and non-reactive pulsed plasmas

A non-invasive diagnostic technique for measurements of ion fluxes and ion densities in low-pressure pulsed radio-frequency (RF) discharges is described. The ion flux towards an electrode is determined from the change in electrode dc-bias in the plasma afterglow. The measurements in various gas mixtures (Ar, N2, Ar/C2H2 and Ar with nanoparticles) show an increase in ion flux with the applied RF power. Ion densities are estimated from the ion fluxes and the plasma density at the plasma–sheath boundary, assuming the afterglow electron temperature to be known. Very good agreement between the estimated ion densities and independently measured electron densities is found in the bulk of dust-free plasmas assuming an afterglow electron temperature of 0.05 eV for argon and argon/acetylene plasmas and 0.2 eV for the nitrogen plasma. In dusty plasmas, the ion density at the plasma–sheath boundary can be determined from the measured ion flux and assuming an afterglow electron temperature of 0.05 eV. For the estimation of ion density in the dusty plasma bulk, the ion density distribution, influenced by the negatively charged dust particles, has to be taken into account.

Collisional-radiative recombination of Ar+ions with electrons in ambient helium at temperatures from 50 K to 100 K

Ternary electron-assisted collisional radiative recombination (E-CRR) and neutral-assisted radiative recombination (N-CRR) have very strong negative temperature dependencies and hence dominate electron recombination in some plasmas at very low temperatures. However, there are only few data for molecular ions and almost none for atomic ions at temperatures much below 300 K. In this experimental study we used a cryogenic afterglow plasma experiment (Cryo-FALP II) at temperatures from 50 K to 100 K to measure ternary recombination rate coefficients for ${\mathrm{Ar}}^{+}$ ions in ambient helium gas. The measured magnitudes and the temperature dependencies of the ternary recombination rate coefficients for electron-assisted and neutral-assisted collisional radiative recombination agree well with theoretical predictions (${K}_{\mathrm{E}\ensuremath{-}\mathrm{CRR}}\ensuremath{\sim}{T}^{\ensuremath{-}4.5}$ and ${K}_{\mathrm{He}\ensuremath{-}\mathrm{CRR}}\ensuremath{\sim}{T}^{\ensuremath{-}2.5}$ to ${T}^{\ensuremath{-}2.9}$) over a broad range of electron and He densities.

Atmospheric pressure, low temperature deposition of photocatalytic TiOx thin films with a blown arc discharge

Thin film deposition of TiOx is performed with an atmospheric pressure plasma afterglow from a nitrogen-fed blown arc discharge. Titanium tetraisopropoxide is used as titanium precursor molecule and injected in the afterglow by an ultrasonic nebuliser. The coatings are made of a homogenous thin film and agglomerates, whose mean size and number depend on the deposition time and on the reactivity of the afterglow. These agglomerates are created by aggregation of nanoparticles synthesised in the gas phase. The latter are crystalline and responsible for the photocatalytic activity of the coating. The presence of a crystalline phase with a high specific surface area induces a photodegradation of the stearic acid with kinetics close to that obtained with an anatase sample deposited by magnetron sputtering at 523K. The substrate temperature is always lower than 523K. Significant photocatalytic activity has even been achieved with a substrate temperature as low as 460K.

Bacterial Decontamination of the Inner Wall of Narrow Tubes by a Nitrogen Afterglow at Atmospheric Pressure and its Relation to Local Atomic Nitrogen Concentration

AbstractA quartz tube with inner wall contaminated by Escherichia coli bacteria is treated using an atmospheric pressure nitrogen afterglow flowing at 20 SLM. The biocidal efficacy is measured locally at both the input and at the output of a 57 cm tube, giving a reduction of survivors of around 5 and 4 log for a 40 min treatment time, respectively. A weak contribution of gas heating is observed and delineated from the effect uniquely due to the plasma afterglow. Based on previous work, a model allows the estimation of the variation of the axial concentration of N atoms, assumed to be the main active species, and its correlation to the biocidal effect of the afterglow. Extrapolation of the model opens the perspective of the decontamination of longer tubes with smaller diameters such as catheter and endoscope ducts.

A novel technique for measurement of thermal rate constants and temperature dependences of dissociative recombination: CO2+, CF3+, N2O+, C7H8+, C7H7+, C6H6+, C6H5+, C5H6+, C4H4+, and C3H3+

A novel technique using a flowing afterglow-Langmuir probe apparatus for measurement of temperature dependences of rate constants for dissociative recombination (DR) is presented. Low (~10(11) cm(-3)) concentrations of a neutral precursor are added to a noble gas∕electron afterglow plasma thermalized at 300-500 K. Charge exchange yields one or many cation species, each of which may undergo DR. Relative ion concentrations are monitored at a fixed reaction time while the initial plasma density is varied between 10(9) and 10(10) cm(-3). Modeling of the decrease in concentration of each cation relative to the non-recombining noble gas cation yields the rate constant for DR. The technique is applied to several species (O2(+), CO2(+), CF3(+), N2O(+)) with previously determined 300 K values, showing excellent agreement. The measurements of those species are extended to 500 K, with good agreement to literature values where they exist. Measurements are also made for a range of CnHm(+) (C7H7(+), C7H8(+), C5H6(+), C4H4(+), C6H5(+), C3H3(+), and C6H6(+)) derived from benzene and toluene neutral precursors. CnHm(+) DR rate constants vary from 8-12 × 10(-7) cm(3) s(-1) at 300 K with temperature dependences of approximately T(-0.7). Where prior measurements exist these results are in agreement, with the exception of C3H3(+) where the present results disagree with a previously reported flat temperature dependence.

Kinetic simulations of argon dusty plasma afterglow including metastable atom kinetics

The afterglow of a dusty plasma of rf discharge in argon is simulated by the particle-in-cell-Monte Carlo collision (PIC-MCC) method. The experimental observation that heavy dust contamination of plasma leads to an anomalous increase in the electron density at the beginning of afterglow is explained by release of electrons from the dust surface. Under the assumption that the floating potential of particles is in equilibrium with plasma conditions, the fast cooling of electrons in afterglow plasma due to a rapid escape of hot electrons from the volume leads to a decrease in the magnitude of the floating potential and hence to a loss of charge by dust. The intensive desorption of electrons from nanoparticles is the origin of anomalous behavior of the electron density. At the next stage of afterglow, when the electrons become cool, the plasma decay is defined by ambipolar diffusion. The effect of metastable argon atoms is also considered. Additional ionization due to metastable atom collisions affects the electron temperature but does not change the behavior of the electron density qualitatively.