Long-lived plasmoids generated by surface microwave discharges in chemically active gases

The generation of long-lived microplasmoids is observed during the irradiation of a metal-dielectric surface with a high-power microwave beam in a chemically active gas mixture (H₂ + O₂; CH₄ + O₂). The lifetime of these plasmoids substantially exceeds the characteristic recombination and cooling times of plasmoids arising at the target surface in a chemically inactive medium.     

Combustion of methane-oxygen and methane-oxygen-CFC mixtures initiated by a high-current slipping surface discharge

Results are presented from experimental studies of the destruction of chlorofluorocarbon (CF₂Cl₂) molecules in a methane-oxygen (air) gas mixture whose combustion is initiated by a high-current slipping surface discharge. It is found that a three-component CH₄+O₂(air)+CF₂Cl₂ gas mixture (even with a considerable amount of the third component) demonstrates properties of explosive combustion involving chain reactions that are typical of two-component CH₄+O₂ mixtures. Experiments show the high degree of destruction (almost complete decomposition) of chlorofluorocarbons contained in the mixture during one combustion event. The combustion dynamics is studied. It is shown that the combustion initiated by a slipping surface discharge has a number of characteristic features that make it impossible to identify the combustion dynamics with the formation of a combustion or detonation wave. The features of the effects observed can be related to intense UV radiation produced by a pulsed high-current surface discharge.     

Long-lived plasmoids generated by surface laser sparks in combustible gas mixtures

Results are presented from experimental studies of laser sparks generated at the surface of a metal target in a chemically active (combustible) gas mixture. It is shown that microplasmoids produced on the target surface penetrate into the ambient gas and initiate its combustion. The lifetime of microplasmoids is found to be anomalously long, up to 1–1.5 ms, which substantially exceeds their lifetime in a chemically inactive medium.     

Propagation of a surface microwave along the afterglow plasma column of a high-current pulsed discharge

It is demonstrated experimentally that the lifetime of the afterglow plasma of a high-current pulsed discharge in a dielectric tube filled with a mixture of argon with saturated mercury vapor is longer than 1 ms. Such a long lifetime, during which the electron density decreases from 10¹⁴ to 10¹² cm⁻³, is explained by the chemi-ionization of mercury vapor by long-lived metastable argon atoms. During this time, the afterglow plasma can serve as a microwave waveguide for a weakly damped low-noise E ₀-type axisymmetric surface mode, which allows one to use it for transmission of signals in the centimeter wavelength range.     

Energetics,kinetics and dynamics of self-interstitial clusters in bcc tungsten

The configuration, slipping and rotation of self-interstitial atoms cluster along <111> crystal orientation with different sizes in a tungsten are investigated systematically with molecular dynamics. It is found that (I) the SIA clusters with high symmetry are always favoured; (II) the SIA clusters can undergo one-dimensional fast migration along <111> direction, and their migration barriers are no more than 0.07?eV, which is expected due to the strong interaction in the SIA clusters; (III) the rotation energy barriers of the SIA clusters are rather high and they are basically positively correlated with the size of the cluster. For example, the reorientation barrier is 0.66?eV for 1 SIA, 1.2–1.8?eV for SIA_n (2?≤?n?≤?5) clusters and over 2.7?eV for SIA_n (6?≤?n?≤?7) clusters. Compared with slipping of SIA clusters, is an infrequent event, especially for larger SIAs cluster, the vast majority SIAs cluster would have already recombination with vacancies or annihilates at surface and grain boundary through slipping before rotation, which explained that there are very low density of SIAs cluster found in the experiment.     

Aneutronic Fusion in Collision of Oppositely Directed Plasmoids

Tri-Alpha and Helion energy companies have proposed an approach as the near future fusion reactor. The method used in this kind of reactor for attaining high fusion yield is based on the formation and throwing of two plasmoids toward each other. In this study, the optimized reaction rate for interpenetration of two head on colliding plasmoids is investigated. Calculations are performed by supposing the velocity of plasmoids ions as Maxwellian distribution function. Fusion output-to-input power ratio (Q factor) was computed by evaluation of the velocity-averaged cross sections and also ion?electron and ion?ion Coulomb power loss. A fluid model including a computational code has been used for the precise calculations of fusion power balance. The optimum interpenetration velocity and plasmoids parameters required to reach the ignition are studied for aneutronic fusion fuels, such as p–¹¹B and D–³He, as well as D?T and D?D fuels. The results of investigation show that the breakeven is attainable in specific collision velocity and plasma temperature for each fuel. Also, the plasma density has to be around 10²⁰ ions/cm³.     

A highly sensitive, flow through h(2) gas analyzer for use in nitrogen fixation studies

Studies of H₂ evolution by N₂ fixing systems are frequently limited by an inability to accurately measure H₂ gas concentrations of less than about 10 microliters per liter. In this study, a H₂ gas analyzer is described which is able to accurately and reproducibly detect up to 100 times lower H₂ concentrations than most thermal conductivity gas chromatographs or other conventional instruments used for the measurement of H₂ gas. This high level of sensitivity (maximum of about 0.02 microliter per liter H₂ per millivolt output) and the ability to continuously monitor H₂ concentration directly in a flowing gas stream, makes this instrument well suited for use in an open gas exchange system.

Since the sensor used in the instrument was also sensitive to other combustible gases, it was necessary to demonstrate that H₂ was the only combustible gas produced by the N₂ fixing system being studied. When an air stream was passed through a pot containing nodulated soybean (Glycine max L.) roots, gas chromatographic analysis of the effluent gas stream revealed that H₂ was the only combustible gas present. These results were supported by other studies in which no combustible gases were detected in the effluent gas stream from soybean roots nodulated with USDA 110, a Rhizobium strain which displays active uptake hydrogenase activity.

     

Ignition of a combustible gas mixture by a high-current electric discharge in a closed volume

Results are presented from experimental studies and numerical calculations of the ignition of a stoichiometric CH₄: O₂ gas mixture by a high-current gliding discharge. It is shown that this type of discharge generates an axially propagating thermal wave (precursor) that penetrates into the gas medium and leads to fast gas heating. This process is followed by an almost simultaneous ignition of the gas mixture over the entire reactor volume.     

Long‐Lived,Visible‐Light‐Excited Charge Carriers of TiO2/BiVO4 Nanocomposites and their Unexpected Photoactivity for Water Splitting

Different mole ratios of TiO₂/BiVO₄ nanocomposites with effective contacts have are fabricated by putting BiVO₄ nanoparticles into the TiO₂ sol, followed by thermal treatment at 450 °C. Based on the transient‐state surface photovoltage responses and the atmosphere‐controlled steady‐state surface photovoltage spectra, it is concluded that the photogenerated charge carriers in the TiO₂/BiVO₄ nanocomposite with a proper mole ratio (5%) display much longer lifetime and higher separation than those in the BiVO₄ alone. This is responsible for the unexpected activity for photoelectrochemical oxidation of water, for photocatalytic production of H₂, and for photocatalytic degradation of phenol as a model pollutant under visible irradiation. Moreover, it is suggested that the prolonged lifetime and increased separation of photogenerated charges in the fabricated TiO₂/BiVO₄ nanocomposite is attributed to the unusual spatial transfer of visible‐excited high‐energy electrons of BiVO₄ to TiO₂. This work will provide feasible routes to synthesize visible‐light responsive nanomaterials for efficient solar utilization.     

Combined Imaging of Bacteria and Oxygen in Biofilms

Transparent sensors for microscopic O₂ imaging were developed by spin coating an ultrathin (<1- to 2-μm) layer of a luminescent O₂ indicator onto coverslips. The sensors showed (i) an ideal Stern-Volmer quenching behavior of the luminescence lifetime towards O₂ levels, (ii) homogeneous measuring characteristics over the sensor surface, and (iii) a linear decline of luminescence lifetime with increasing temperature. When a batch of such coverslip sensors has been characterized, their use is thus essentially calibration free at a known temperature. The sensors are easy to use in flow chambers and other growth devices used in microbiology. We present the first application for combined imaging of O₂ and bacteria in a biofilm flow chamber mounted on a microscope equipped with a spinning-disk confocal unit and a luminescence lifetime camera system.     

A Novel Surface Treatment Method and New Insight into Discharge Voltage Deterioration for High‐Performance 0.4Li2MnO3–0.6LiNi1/3Co1/3Mn1/3O2 Cathode Materials

The Li‐rich cathode materials have been considered as one of the most promising cathodes for high energy Li‐ion batteries. However, realization of these materials for use in Li‐ion batteries is currently limited by their intrinsic problems. To overcome this barrier, a new surface treatment concept is proposed in which a hybrid surface layer composed of a reduced graphene oxide (rGO) coating and a chemically activated layer is created. A few layers of GO are first coated on the surface of the Li‐rich cathode material, followed by a hydrazine treatment to produce the reducing agent of GO and the chemical activator of the Li₂MnO₃ phase. Compared to previous studies, this surface treatment provides substantially improved electrochemical performance in terms of initial Coulombic effiency and retention of discharge voltage. As a result, the surface‐treated 0.4Li­₂MnO_3–0.6LiNi_1/3Co_1/3Mn_1/3O₂ exhibits a high capacity efficiency of 99.5% during the first cycle a the discharge capacity of 250 mAh g^?1 (2.0–4.6 V under 0.1C), 94.6% discharge voltage retention during 100 cycles (1C) and the superior capacity retention of 60% at 12C at 24 °C.     

Local‐field effect on the fluorescence relaxation of Tm3+:LaF3 nanocrystals immersed in liquid medium

Tm³⁺:LaF₃ nanocrystals were synthesized with hydrothermal technique. Local‐field effect on the radiative relaxation rate was studied in the system of Tm³⁺:LaF₃ nanocrystals immersed in several liquid media. The fluorescence lifetime was measured. It was found that the fluorescence decay presented the characteristics of second‐order exponential decay, for which the contribution from the ions inside the nanocrystal and ions at the interface of the nanocrystal were distinguished. Investigating the experimental results with proposed models, we found that the surface effect had to be eliminated. For rare earth doped LaF₃ nanocrystals, real‐cavity model well explains the influence of surrounding medium on the fluorescence relaxation rate. Copyright © 2009 John Wiley & Sons, Ltd.     

Plasma chemistry of the sealed-off slab CO laser active medium pumped by radio-frequency discharge with liquid-nitrogen-cooled electrodes

The long-term time behavior of the output power of a sealed-off cryogenic slab CO laser pumped by a repetitively pulsed RF discharge and operating on the overtone (λ = 2.6–3.5 μm) vibrational?rotational transitions of the CO molecule was studied experimentally. It is shown that adding of an anomalously large amount of oxygen (up to 50% with respect to the CO concentration) to the initial gas mixture CO : He = 1 : 10 leads to a manyfold (by several tens of times) increase in the duration of the laser operating cycle (until lasing failure due to the degradation of the active medium). In this case, the laser life-time without replacement of the active medium reaches 10⁵–10⁶ pulses. Using various diagnostics (including luminescence spectroscopy and IR and UV absorption spectroscopy), regularities in the time-behavior of the concentrations of the main component of the active medium (CO molecules) and the products of plasmachemical reactions (O₃, CO₂) generated in the discharge gap during the laser operating cycle are revealed. Time correlation between the characteristics of the active medium and the laser output power are analyzed. A phenomenological approach to describing the entirety of plasmachemical, purely chemical, gas-dynamic, and diffusion processes determining the behavior of the laser output characteristics throughout the laser operating cycle is offered.     

Dynamics of a high-temperature pinch in the presence of dust

Results are presented from experimental studies of the formation of a pinch in a plasma focus discharge in the presence of Al₂O₃ dust grains. Considerable attention is given to an analysis of the method for creating a dust target and determining its parameters. Phase transitions in a dust medium have been analyzed. It is found that the phase state of the dust component can be substantially affected by the radiation of the imploding plasma shell even in the preimplosion phase of the discharge. It is shown that a pinch produced in the presence of dust is more stable against MHD instabilities.     

Hydrodynamic model of a dielectric-barrier discharge in pure chlorine

A one-dimensional hydrodynamic model of a dielectric-barrier discharge (DBD) in pure chlorine is developed, and the properties of the discharge are modeled. The discharge is excited in an 8-mm-long discharge gap between 2-mm-thick dielectric quartz layers covering metal electrodes. The DBD spatiotemporal characteristics at gas pressures of 15–100 Torr are modeled for the case in which a 100-kHz harmonic voltage with an amplitude of 8 kV is applied to the electrodes. The average power density deposited in the discharge over one voltage period is 2.5–5.8 W/cm³. It is shown that ions and electrons absorb about 95 and 5% of the discharge power, respectively. In this case, from 67 to 97% of the power absorbed by electrons is spent on the dissociation and ionization of Cl₂ molecules. Two phases can be distinguished in the discharge dynamics: the active (multispike) phase, which follows the breakdown of the discharge gap, and the passive phase. The active phase is characterized by the presence of multiple current spikes, a relatively high current, small surface charge density on the dielectrics, and large voltage drop across the discharge gap. The passive phase (with no current spikes) is characterized by a low current, large surface charge density on the dielectrics, and small voltage drop across the discharge gap. The peak current density in the spikes at all pressures is about 4 mA/cm². In the multispike phase, there are distinct space charge sheaths with thicknesses of 1.5–1.8 mm and a mean electron energy of 4.3–7 eV and the central region of quasineutral plasma with a weak electric field and a mean electron energy of 0.8–3 eV. The degree of ionization of chlorine molecules in the discharge is ~0.02% at a pressure of 15 Torr and ~0.01% at 100 Torr. The DBD plasma is electronegative due to the fast attachment of electrons to chlorine atoms: e + Cl₂ → Cl + Cl^–. The most abundant charged particles are Cl ₂ ⁺ and Cl^? ions, and the degree of ionization during current spikes in the active phase is (4.1–5.5) × 10^–7. The mechanism of discharge sustainment is analyzed. The appearance of a series of current spikes in the active phase of the discharge is explained.     

Kinetics of the reactions involving CF2 and CF in a pure tetrafluoromethane plasma: II. Production and loss of CF2 and CF in the processes of fluorocarbon polymerization

Mechanisms for the production and loss of CF₂ and CF radicals in a glow discharge in pure CF₄ are investigated by the time-resolved laser-induced fluorescence method. The fluorocarbon polymerization processes are shown to contribute significantly to the production of radicals both in the plasma volume and on the surface of the discharge tube. The effective frequencies of both the volume and surface processes of radical production and loss are determined. An analysis of these frequencies allowed us to study the polymerization mechanism in a CF₄ plasma at a high relative concentration of F atoms and low ion energy. It is shown that, at elevated pressures, when the density of C_xF_y polymer particles in the plasma volume becomes comparable with the density of simple fluorocarbon radicals, the electron-impact dissociation of these particles is the main channel for the production of CF₂ and CF radicals. Another source of CF₂ and CF radicals is related to the reactions of C_nF_2m+1 unsaturated fluorocarbon particles both in the plasma volume and on the surface of a fluorocarbon film arising on the discharge tube wall. The C_xF_y fluorocarbon polymer particles form both in the discharge volume and on the fluorocarbon filmsurface also in the course of the film destruction. At lowered pressures, the main channel for the production of CF₂ and CF is the direct electron-impact dissociation of CF₄ molecules, whereas the loss of these radicals at the tube wall is the main loss channel. The probabilities of the heterogeneous losses of CF₂ and CF radicals on the heavily fluorinated surface of the fluorocarbon film at low ion energies are determined. Under these conditions, the surface recombination of the Fch chemisorbed fluorine atoms and CF _x ^ph physisorbed radicals with the production of an activated complex is shown to be the most probable mechanism for the heterogeneous losses of CF₂ and CF. The approximate activation energies for the production of F_ch · CF ₂ ^ph and F_ch · CF^ph surface complexes are found to be 750±70 K and 1030±100 K, respectively.     

Nitrogen dynamics of storm runoff in the riparian zone of a forested watershed

The influence of storm runoff processes on stream nitrogen dynamics was investigated in a headwater riparian swamp on the Oak Ridges moraine in southern Ontario. Hydrologic data were combined with analysis of an isotopic tracer (¹⁸0) and nitrogen (NH ₄ ⁺ , NO ₃ ^– ) concentrations in saturation overland flow and stream discharge. Storm runoff was separated into its event and pre-event components using¹⁸O in order to examine the effect of water source on nitrogen chemistry. Laboratory experiments were also used to study nitrogen transformation associated with storm runoff-surface substrate interactions in the swamp. In most storms N0₃-N and NH₄-N concentrations in the initial 3–4 mm throughfall increment were 10–20x and 20–100x higher respectively than stream base flow concentrations. Maximum stream N0₃-N concentrations were < 2x to 6x higher than base flow concentrations and preceded or coincided with peak stream discharge. Storm-to-storm variations in stream N0₃-N behaviour also occurred during the hydrograph recession phase. NH₄-N concentrations attained an initial peak on the rising hydrograph limb, or at peak stream discharge. A second NH₄-N increase occurred during the late recession phase 3–5 h after maximum stream discharge. Inorganic-N concentrations in surface runoff were similar to peak streamflow.The close agreement between observed N0₃-N concentrations and values predicted from a chemical mixing model indicate that stream N0₃-N variations were controlled mainly by the mixture of throughfall and groundwater in surface stormflow from the swamp. Laboratory experiments also indicated that N0₃-N in surface runoff behaved conservatively when mixed with swamp substrates. With the exception of the late hydrograph recession phase, observed stream NH₄-N concentrations were much lower than concentrations predicted by the chemical mixing model. The rapid loss of NH₄-N from mixtures of surface stormflow and swamp substrates in laboratory experiments and the absence of uptake in sterilized substrates indicated that NH₄-N retention in surface storm runoff was due to biotic processes.     

Optical characteristics and parameters of the plasma of a barrier discharge excited in a mixture of mercury dibromide vapor with nitrogen and helium

Results are presented from experimental and theoretical studies of the optical characteristics and parameters of the plasma of an atmospheric-pressure barrier discharge excited in a HgBr₂: N₂: He mixture, which was used as the working medium of a small-size (with a radiation area of 8 cm²) exciplex gas-discharge radiation source. The mean radiation power of 87 mW was achieved at the radiation wavelength λ_max = 502 nm. The electron energy distribution function, the transport characteristics, the specific energy lost in the processes involving electrons, the electron temperature and density, and the rate constants of elastic and inelastic electron scattering by the components of the working mixture were calculated as functions of the reduced field E/N. The plasma of a discharge excited in a HgBr₂: N₂: He mixture can be used as the working medium of a small-size blue-green radiation source. Such a source can find application in biotechnology, photonics, and medicine and can also be used to manufacture gas-discharge display panels.     

Surface Cleaning and Passivation Using (NH4)2S Treatment for Cu(In,Ga)Se2 Solar Cells: A Safe Alternative to KCN

With the aim of developing a safe alternative to the KCN etchant for the removal of Cu_xSe secondary phases at the surface of Cu(In,Ga)Se₂ (CIGSe) absorber, a method based on ammonium sulfide (AS) chemical treatment is proposed. Although lower etching rates are observed compared with the KCN reference solution, the AS solution is found to selectively etch Cu_xSe phases. In addition, it allows modifying the surface chemical state of the CIGSe absorber by incorporation of sulfur. As a consequence, the minority carrier lifetime located close to the surface of the absorber is found to be improved. Furthermore, it is demonstrated that optimizing the AS treatment time induces a remarkable enhancement in the electrical performances of the CIGSe‐based solar cells.     

Long-lived Ar-Hg plasma in the afterglow of a high-current pulsed discharge

High-density (n > 10¹² cm^?3) argon-mercury plasma produced by a short (t ～ 20 μs) high-power pulsed discharge in argon with an admixture of mercury vapor at a discharge current of ～50 A, an argon pressure of ～4 mm Hg, and a mercury vapor pressure of ～10^?3 mm Hg was studied using optical spectroscopy and radio physics methods. It is found that the lifetime of this plasma after the end of the discharge pulse is up to 10^?2 s. It is shown that such an abnormally long lifetime of such an afterglow plasma, as compared to the plasma of an argon discharge without an admixture of mercury vapor, is related to the long residence time of atoms and ions of both argon and mercury in highly excited states due to chemi-ionization processes involving long-lived metastable argon ions. It is suggested that dissociative recombination of highly excited molecular ions of argon play an important role in the transfer of excitation to argon atoms and ions that are close to autoionization states.