Non-equilibrium Plasma Discharge Research Articles

Self-Consistent State-to-State Kinetic Modeling of CO2 Cold Plasmas: Insights on the Role of Electronically Excited States

This study focuses on the role of CO2 and CO electronically excited states in the kinetics of CO2 cold non-equilibrium plasma discharges by means of a state-to-state OD kinetic model based on the simultaneous and self-consistent solution of the electron Boltzmann equation and the master equations describing the vibrationally and electronically excited state kinetics and the plasma composition. A new CO2 dissociation model based on the use of the Biagi electron impact excitation cross sections, considered as fully dissociative, of several CO2 electronic excited states, in the energy range from 6.5 to 25 eV, is tested and compared with the results obtained by using the Phelps database in typical glow and microwave discharge conditions. Moreover, a refinement of the kinetics of the COa3Π\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm{CO}\\left({\\mathrm{a}}^{3}\\Pi \\right)$$\\end{document} excited state is proposed by including new production and loss terms and the effect of the change of its time evolution density on the eedf, the electron temperature, the CO2 and CO vibrational distribution functions, electron impact and vibrational induced dissociation rates is investigated. Finally, the contribution of the COa3Π\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm{CO}\\left({\\mathrm{a}}^{3}\\Pi \\right)$$\\end{document} state to CO2 dissociation is examined in terms of production and recombination (or back-reaction) processes both in microwave and glow discharge conditions.

(Invited) Plasma-Assisted Ammonia Combustion and Flare Gas Reforming to Enhance Reactivity and Control Emission

Increasing emphasis on the de-carbonization of energy production has led researchers to consider carbon-free, renewable, and green fuels such as hydrogen (H2) and ammonia (NH3). While H2 suffers from the major challenges of production difficulties, transportation, and storage, NH3 is plagued with challenges of low flame speeds causing unstable flames, high autoignition temperatures resulting in longer ignition delays, narrow flammability limits, and higher levels of NOx emission. Among the different solutions to overcome these challenges of NH3 combustion, non-equilibrium plasma-based igniters are significant owing to the promotion of localized volumetric ignition kernel development by both thermal and chemical assistance. Computational investigation of plasma-assisted combustion of ammonia-air mixtures in constant volume and constant pressure reactors are conducted, to determine the impact of operating conditions on ignition delays and NOx emissions. A mechanism has been assembled in this work using well-validated plasma reactions of NH3 with O2 and N2, alongside plasma kinetics of air from the literature. Subsequently, the newly developed mechanism was used to investigate the plasma-assisted oxidation of NH3. In particular, the impact of the reduced electric field (E/N), equivalence ratio, pressure, pulse frequency, and energy density on the ignition delays and NOx emission were investigated. A Global Pathway-based Analysis algorithm for plasma-assisted systems (PGPA) is used to analyze the nanosecond pulsed nonequilibrium plasma-assisted combustion of NH3/air mixtures. Firstly, a faster ignition and lower production of NOx are observed in the case of plasma discharges compared to thermal energy deposition, owing to the enhanced production of OH radicals and the early reforming of NH3 to produce N2 and H2 with plasma, respectively. At lower reduced electric fields (E/N), PGPA analyses elucidated the significance of gas heating due to vibrational-translational cycles of NH3 and N2 on the increased reactivity of NH3/air mixtures as compared to ignition at a higher E/N. The fuel-lean mixture is observed to exhibit higher production of NOx than stoichiometric and fuel-rich mixtures, resulting from plasma chemistry involving oxygen radical and electronic excited states of N2. Higher rates of collisional quenching at higher pressures during the inter-pulse gaps are found to result in a lesser amount of electronically excited states of N2 and O2, resulting in lower production of air-bound NOx during the pulses.Complementing combustion enhancements, the study also considers the role of plasma-assisted systems in gas reforming, thereby imparting specific desired characteristics lacking in the original mixture. For instance, plasma-assisted reforming can be utilized to control emissions by reforming specific emission precursors or by improving the gas reactivity to promote clean combustion. Natural gas associated with oil wells and natural gas fields is a significant source of greenhouse gas emissions and airborne pollutants. Flaring/burning of the associated gas removes greenhouse gases like methane (CH4) and other hydrocarbons. Our study explores the possibility of enhancing the flaring of associated gas mixtures (C1 – C4 alkane mixture) using nanosecond pulsed non-equilibrium plasma discharges. A well-studied conventional combustion chemistry for small alkanes is coupled with the plasma kinetics of CH4, C2H6, C3H8, and N2, including electron-impact excitations, dissociations, and ionization reactions. The newly developed plasma-based flare gas chemistry is then utilized to investigate repetitively pulsed nonequilibrium plasma-assisted reforming and subsequent combustion of the flare gas mixture diluted with N2 at different conditions. The results indicate an enhanced production of H2 and C2H4 in the reformed gas mixture, owing to the electron-impact dissociations of alkanes and subsequent H-abstractions and recombination reactions, thereby resulting in a mixture of CH4, H2, C2H4, C2H2, and other unsaturated C3. The reformed mixture exhibits significantly high reactivity as exhibited by their increased flame speeds and shorter ignition delays. The reformed mixture is also observed to promote increased CH4 destruction levels and complete flaring, thereby reducing the emissions of CH4 and other hydrocarbons.

Activation of vibrational-induced CO2 dissociation in cold non-equilibrium plasma

The activation of vibrational-induced dissociation of CO2 in cold non-equilibrium plasma discharges is investigated by means of a 0D self-consistent kinetic model, which, with a state-to-state approach, is able to calculate the CO2 vibrational distribution function (vdf) of the asymmetric mode levels, the electron energy distribution function and the corresponding vibrational-induced and electron impact CO2 dissociation rates. The conditions for the onset of such activation are linked to the achievement of a sufficiently high CO2 vibrational excitation characterized by the presence of a non-equilibrium plateau in the CO2 vdf, resulting from the combined effect of electron–vibrational and vibrational–vibrational collisions, which, by overpopulating the higher vibrational levels, enhances dissociation. Such non-equilibrium conditions are maximized at lower gas temperature, lower pressure and higher power density values. In particular, for the power density, an activation threshold value can be obtained from simulations and its dependence on the gas temperature and pressure can be investigated. The dependence of the maximum vibrational temperature reached at the end of the discharge as a function of the gas temperature and pressure is also analyzed. A satisfactory agreement from our simulation results with the Kotov’s criterion for vibrational activation has been found.

Computational Fluid Dynamics Modeling of Low Temperature Ignition Processes From a Nanosecond Pulsed Discharge at Quiescent Conditions

Abstract Recent interest in nonequilibrium plasma discharges as sources of ignition for the automotive industry has not yet been accompanied by the availability of dedicated models to perform this task in computational fluid dynamics (CFD) engine simulations. The need for a low-temperature plasma (LTP) ignition model has motivated much work in simulating these discharges from first principles. Most ignition models assume that an equilibrium plasma comprises the bulk of discharge kernels. LTP discharges, however, exhibit highly nonequilibrium behavior. In this work, a method to determine a consistent initialization of LTP discharge kernels for use in engine CFD codes like converge is proposed. The method utilizes first principles discharge simulations. Such an LTP kernel is introduced in a flammable mixture of air and fuel, and the subsequent plasma expansion and ignition simulation is carried out using a reacting flow solver with detailed chemistry. The proposed numerical approach is shown to produce results that agree with experimental observations regarding the ignitability of methane-air and ethylene-air mixtures by LTP discharges.

Effects of inter-pulse coupling on nanosecond pulsed high frequency discharge ignition in a flowing mixture

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H2/air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow.

Flames with plasmas

Non-equilibrium plasma discharges can create a favorable environment for combustion processes, at low energy cost. “How favorable?” and “How low?” are two open questions that can be answered only by gaining a deep understanding of the coupling between flames and plasmas. This paper reviews recent progress in plasma-assisted combustion with a special focus on flame dynamics. First, different non-equilibrium plasma discharges are presented and their effects on flame properties are discussed. Then, an additional level of complexity is introduced by considering the effect of such plasmas on the coupling between flames and acoustic waves. While initial results suggested that the best plasma actuation would combine thermal and chemical effects, a shift towards favoring a chemical impact is emerging. Reasons for this shift are explained and future research directions are finally proposed.

Thermoelectrical properties of ternary lead chalcogenide plumbum-selenium-tellurium thin films with excess of tellurium prepared by plasma-chemical vapor deposition

Ternary chalcogenide PbSeTe thin films with sufficient excess of tellurium over the stoichiometry Pb50Se50-xTex have been fabricated by Plasma-enhanced chemical vapor deposition technique. High-pure elemental lead, tellurium, and selenium were the starting materials, that were supplied by the flow of high-pure argon into the 40 MHz inductively-coupled non-equilibrium plasma discharge at low pressure 13 Pa. Argon also served as the plasma-feed gas. The structural properties and chemical composition of the films were characterized by scanning electron microscope and X-ray diffraction analysis, respectively. The dependence of the Seebeck coefficient, resistivity and thermoelectric power factor on structural properties and composition were investigated. The influence of film composition on thermoelectric characteristics was studied.

Lead-based chalcogenide thin films for mid-IR photoreceivers: plasma synthesis, semiconductor, and optical properties

Complex chalcogenide systems like PbS1-xSex seem to be promising semiconductors with a great potential for highly sensitive photodetectors of the mid-IR range and thermoelectric working at room temperature. The first group of problems that scientists face is how to synthesize materials with a homogeneous chemical and phase composition and a well-defined stoichiometry. The second is how to avoid contamination of such sensitive materials with residues of unreacted precursors and installation materials. In addition, the technological approach should allow the potential scale-up of the process for commercial applications of the above materials. In this work, we report the applicability of the plasma-enhanced chemical vapor deposition (PECVD) in preparation of PbS1-xSex complex inorganic chalcogenide materials of various stoichiometry and phase composition in function of plasma process conditions. Elemental high-pure lead, sulfur, and selenium were the initial substances. RF (40.68 MHz) non-equilibrium plasma discharge at low pressure (0.01 Torr) was used for the initiation of interactions between the starting materials. The PECVD process was studied by optical emission spectroscopy (OES). Various analytical methods were utilized to characterize the obtained materials.

A nanosecond pulsed discharge circuit model for engine applications

Non-equilibrium plasma discharges in spark gaps have been an increasingly studied method for alleviating cycle to cycle variation in lean and dilute combustion environments. However, ignition models that account for streamer propagation, cathode fall, and transmission line amplification over nanosecond time scales have so far not been developed. The present study develops such a model, with emphasis on the energy delivered from circuit to cylinder. Key pieces of the relevant physics and chemistry are summarized, simplified, and systematically coupled to one another. The set of parameters is limited to a handful of key observables and modeled using Modelica. Results show non-trivial behavior in the energy delivery characteristics of such discharges with important implications for ignition.

Effects of Non-Equilibrium Plasma and Spark Discharge on Low-Temperature Combustion in Lean Propane/Air Mixtures

Effects of Non-Equilibrium Plasma and Spark Discharge on Low-Temperature Combustion in Lean Propane/Air Mixtures

Heteroepitaxial Growth of Ga2O3 Thin Films of Various Phase Composition by Oxidation of Ga in Hydrogen-Oxygen Plasmas

We describe a novel plasma-chemical method for Ga2O3 thin films synthesis, in which high-purity elemental Ga is delivered by hydrogen flow into the plasma discharge zone to react with O2. Radio Frequency (40 MHz) non-equilibrium low temperature plasma discharge at low pressure (0.01 Torr) is applied for initiation of interaction between these precursors. Optical Emission Spectroscopy together with quantum-chemical calculations was used to determine the main reaction pathways and reacting species appearing in the plasma discharge. For a low growth temperature of 350 °C and high plasma power of 50 W, the Ga2O3 films deposited on basal plane sapphire are textured β-Ga2O3 with (−201) orientation. At low plasma power of 30 W and low growth temperature of 350 °C, the layers are dominated by the polycrystalline ε(k) phase. Increasing the growth temperature under these low plasma power conditions from 350 °C to 550 °C leads to the grains ripening and the conversion of the films into the polycrystalline β-polytype. Electrical measurements on the films grown under high power conditions on sapphire showed them to be semi-insulating, with the Fermi level pinned near Ec−0.7 eV. Films on Si substrates had low donor concentrations of ∼1015 cm−3, most likely due to Si autodoping from the Si substrate.

Fabrication and Characterization of Non-Equilibrium Plasma-Treated PVDF Nanofiber Membrane-Based Sensors.

The effect of a self-pulsing non-equilibrium plasma discharge on piezoelectric PVDF nanofiber membrane was investigated. The plasma discharge was generated in air with a DC power source, with a discharge current of 0.012 mA, a nominal interelectrode separation of 1 mm, and discharge voltage of ~970 V. In a continuous fabrication process, the electrospinning method was used to generate thin nanofiber membrane with a flow rate of 0.7–1 mL h−1 and 25–27 kV voltage to obtain the nanofiber with high sensitivity and a higher degree of alignment and uniformity over a larger area. Plasma treatment was applied on both single layer and multi-layer (three layers) nanomembranes. In addition, simultaneously, the nanofiber membranes were heat-treated at a glass transition temperature (80–120 °C) and then underwent plasma treatment. Fourier-transform infrared (FTIR) spectroscopy showed that the area under the curve at 840 and 1272 cm−1 (β phase) increased due to the application of plasma and differential scanning calorimeter (DSC) indicated an increase in the degree of crystallinity. Finally, PVDF sensors were fabricated from the nanofibers and their piezoelectric properties were characterized. The results suggested that compared to the pristine samples the piezoelectric properties in the plasma and plasma-heat-treated sensors were enhanced by 70% and 85% respectively.

Synthesis of gallium oxide via interaction of gallium with iodide pentoxide in plasma

The promising fields of gallium oxide application are the production of hybrid cars, electrical equipment of high-power, ultraviolet radiation sensors and uninterruptible power supplies. However, the main factor hindering its massive commercial use is the lack of synthesis technologies, that should be cheap, reproducible, and scalable. In this work we develop a novel plasma-chemical method of Ga2O3 synthesis. The high-purity elemental gallium was used as the precursor, which was delivered by argon flow to the reaction zone, where the interaction with iodide pentoxide took place. RF (40.68 MHz) non-equilibrium plasma discharge at low pressure (0.1 Torr) was employed for the initiation of interactions between precursors. Optical Emission Spectroscopy in tandem with quantum-chemical calculations allowed us to find out the reactive species formed in the plasma discharge. The properties of the solid phase obtained were studied as well.

Influence of dielectric coatings on pin-to-rod nanosecond-pulsed discharges in phosphate-buffered saline.

Plasma medicine is a rapidly expanding field that utilizes non-equilibrium plasma discharges at atmospheric conditions or in liquids for clinical applications. There is significant interest in the production of plasma in the liquid phase for wastewater treatment, agricultural applications, and medical purposes. However, little investigation has been done about the effects of dielectric coatings on submerged electrodes, which is of significant interest to limit electrical current flow in the liquid. This work investigates the effect of different dielectric coatings including aluminum oxide, parylene C, and bi-layer combinations, on plasma discharge characteristics in phosphate-buffered saline (σ = 18 mS/cm) from nanosecond high-voltage pulses. Observed results for aluminum oxide are consistent with past works, including micron-sized clusters of holes generated in the layer due to dielectric breakdown. A bi-layer combination of parylene C on top of aluminum oxide resulted in longer lifetime for electrodes, possibly due to the melting/solidification behavior of the polymer, which may have a "healing" effect. The use of a thick parylene C layer resulted in a different, "creeping", discharge regime, which is hypothesized to be similar to triple-gap discharge observed in space plasma physics and high-voltage insulators, in which the electric field is enhanced at the boundary of a conductor, dielectric, and a vacuum/fluid, resulting in discharge at this junction point. Temporally-resolved and high-spatial-resolution imaging are required for verification.

Numerical assessment of MILD combustion enhancement through plasma actuator

One promising solution to achieve moderate or intense low-oxygen dilution (MILD) combustion under lower temperatures may be using non-equilibrium plasma discharge. Plasma assisted combustion can extend the flammability limits by reducing the auto-ignition temperature of the reactants and can be a great approach to achieve steady MILD combustion under extreme conditions through decreasing ignition delay time. In particular, the influence of the outlet flows from a co-flowing cylindrical dielectric barrier discharge (DBD) on the CH4/N2 combustion under MILD combustion has been investigated numerically through some simplifications on plasma discharge. A three streams coaxial burner are modeled in a 2D-axisymmetric computational domain by a steady state approach. On the whole, different effects of simplified non-equilibrium plasma discharging, fuel dilution, and oxygen concentration of the preheated oxidizer on MILD combustion regimes are studied. It is found that applying the plasma discharge to the flame region, leads to an intensification of the reaction zone, increment of the rate of heat release by around a factor of 3, and 30% reduction of flame lift-off. Different aspects of plasma effect through chemical kinetics, mixing, and thermal heating on MILD combustion are investigated. Apart from all the benefits of plasma such as improving the mixing, it has a momentous impact on both the reaction zone length and its diameter via improving the ignition delay time.

Radar resonance-enhanced multiphoton ionization for measurement of atomic oxygen in non-equilibrium pulsed plasmas

In this work, coherent microwave Rayleigh scattering for the detection of resonance-enhanced multiphoton ionization (Radar REMPI) was used for the measurement of atomic oxygen in a nonequilibrium pulsed plasma discharge. Specifically, the detection of atomic oxygen is demonstrated in a pulsed discharge at 100 Torr in a molecular oxygen and helium gas mixture. REMPI was conducted using Nd:YAG pumped dye radiation at 618.9 nm mixed with the Nd:YAG 355 nm radiation, generating 225.6 nm laser light, of which two photons match the energy of the 2P33p(3P) electronically excited intermediate state of atomic oxygen. A third photon from the laser source then readily ionizes the atom in the 2 + 1 photon REMPI scheme. The Radar REMPI results are compared to two-photon absorption laser-induced fluorescence measurements from the literature, which shows reasonable agreement.

A novel method for synthesis of arsenic sulfide films employing conversion of arsenic monosulfide in a plasma discharge

A novel plasma-based method for synthesis of arsenic sulfide films with different structural units and stoichiometries is demonstrated. For the first time As-S films have been prepared via direct conversion of arsenic monosulfide (As4S4) as a single precursor in a low-temperature non-equilibrium RF plasma discharge at low pressure. The interplay between composition, structure and properties of the chalcogenide materials prepared has been studied. Quantum-chemical calculations have been performed to gain insight into the films structure and the mechanism of its formation in the plasma discharge. Raman spectroscopy proves that with the As-content increase in the As-S films the intensity of the line, corresponding to vibrations of homopolar As-As bonds in the realgar As4S4 units and in the S2As-AsS2 bridges increases. The last ones are responsible for the appearance of a photoluminescence phenomenon in chalcogenide glasses.

Purification of tellurium through thermal decomposition of plasma prepared tellurium hydride

Abstract For the first time tellurium has been purified by means of plasma-enhanced chemical transport reaction with intermediate formation of tellurium hydride (H2Te) and the following its thermal decomposition with formation of high-purity tellurium and hydrogen. Recently developed direct interaction of hydrogen with tellurium vapors at low pressure in inductively coupled non-equilibrium plasma discharge underlain the novel approach of purification. The metal and carbon-containing impurities behavior was also studied. The possible mechanism of their conversion during the process has been discussed. In order to demonstrate effectiveness of the suggested method, As2Te3 films have been prepared from the initial tellurium and tellurium after purification. They were studied by mass-spectrometry technique for comparison.

Behavior of Carbon-Containing Impurities in the Process of Plasma-Chemical Distillation of Sulfur

Sulfur was purified by plasma-chemical distillation at low pressure (0.1 Torr) under dynamic vacuum conditions. RF (40 MHz) non-equilibrium plasma discharge was used for initiation of chemical interactions. The carbon-containing impurities behavior was studied. The possible mechanism of their conversion has been discussed. The effectiveness of the suggested method has been compared with the ‘traditional’ distillation in terms of removal of heterophase impurities.

Numerical Investigation on Effects of Nonequilibrium Plasma on Laminar Burning Velocity of Ammonia Flame

Direct burning of ammonia is required to reduce the consumption of fossil fuels. However, ammonia has an exceedingly low laminar burning velocity. Therefore, it is necessary to improve the burning velocity of ammonia. In this study, the effect of nonequilibrium plasma on the problems of ammonia combustion is investigated. To reduce a calculation load from the difference of time scale, simulation is conducted in two stages: a section of nonequilibrium plasma discharge on a mixture and a section of combustion. The reactions of plasma discharge in a flow reactor are simulated as a perfect stirred reactor (PSR) using the SENKIN code in the CHEMKIN-II package. Second, the laminar flame located downstream of the flow reactor is simulated using the PREMIX code in the CHEMKIN-II package. From the simulation results, as the time of the application of the electric field increases, the amount of decomposed ammonia in the unburned gas increases and the laminar burning velocity becomes considerably fast. Moreover, it ...