Negative Ions in a Glow Discharge of Tetrafluoromethane

The Langmuir — probe measurements in a CF₄ — glow discharge reveal a large number of negative ions (10¹⁰ cm^?3) in the plasma between the planar C-electrodes. Under special conditions the density of them exceeds the electron density at several orders of magnitude and sustains a quasi — electron free plasma. The ratio depends on the residence time of the molecular gas in the discharge reactor. It is due to dissociative electron attachment to highly molecular gas components arising from the plasmachemical conversion of CF₄. F^? and CF₃^? are the most important negative ions.     

Generation of H− ions in a low-pressure xenon-hydrogen discharge. I

A theory of a low-pressure discharge in a xenon-molecular hydrogen mixture is developed. It is shown that, in such a discharge, at an interelectrode distance of L = 1 cm and a total plasma pressure of p ₀ ～ 1 Torr, the density of negative hydrogen ions produced via the dissociative attachment of thermal electrons to vibrationally excited molecules H₂ can reach a value as high as N_H ^? ≥ 10¹² cm^?3. According to calculations, the electron temperature in discharge operating regimes under study attains T _e ≈ 1?2 eV, which corresponds to the maximum of the e-v exchange rate constant of H₂ molecules. This ensures a relatively high rate of vibrational pumping of H₂ molecules in the discharge.     

Vibrationally Excited Ozone Relaxation by CO

Time profiles of ozone concentration after pulsed UV laser photolysis in the O₂- O₃-Ar-CO mixture, measured using time-resolved absorption spectroscopy, are presented. The experimental results show the dominance of the stabilization channel over the reactive one for the interaction between the vibrationally excited ozone molecule O₃(υ) and carbon monoxide CO. The rate constant of the process O₃(υ)+CO→O₃ + CO, obtained by processing experimental data by the kinetic modeling method is (1.5 ± 0.2) · 10^?13 cm³/s.     

Anlagerungskoeffizienten und Driftgeschwindigkeiten von Elektronen in Luft

Electron drift velocities and attachment coefficients were measured in dry air (E/p=0.1–30 V/cm Torr) and in a 9∶1 nitrogen/oxygen mixture (E/p=0.2–3 V/cm Torr) in the pressure range from 50 to 200 Torr, using a modified spark chamber technique. The primary electrons were released by anα-particle. The temporal development of the electron density in the gap was determined from the amplitude of the current due to the avalanches, which were produced by applying high voltage pulses at different delay times. — It was found that in air the dissociative attachment sets in at higherE/p (～10–15 V/cm Torr) than in oxygen. At lowerE/p three body attachment is predominant. — When the high voltage pulses were applied after the transit time of the primary electrons, electron avalanches still appeared. It was concluded that they were started by electrons which were detached from negative ions. The estimated detachment rates indicate the formation of O ₂ ^? ions at lowE/p and of O^? ions at higherE/p.     

Ozone generation in oxygen-fed wire-to-cylinder ozonizer: effect of CH2Cl2, CHCl3 and CCl4 diluents

The inhibition effect of methylene chloride CH₂Cl₂, chloroform CHCl₃ and carbon tetrachloride CCl₄ on the ozone generation process from oxygen by negative corona discharge was experimentally investigated. The experiments were performed in a system of coaxial cylindrical electrodes at total gas pressure of 900 mbar and ambient temperature of gaseous mixtures. The rate of ozone generation as well as ozone concentration apparently decreases with a rising number of substituted chlorine atoms in the methane molecule at constant specific energy consumption dissipated in the discharge and at constant concentration of gaseous impurities in oxygen. In addition to experimental results, the paper presents theoretical considerations aimed at identifying the main process responsible for inhibition of ozone generation. The consumption of considerable fraction of oxygen atoms by CH _x Cl _y–1 radicals formed in discharge, is likely the most important mechanism responsible for the deleterious effect of such compounds on the efficiency of ozone production.     

On the dissociation of the ozone dication

A mechanism for the dissociation of the ozone dication formed by double ionisation of neutral O₃ is determined by ab initio calculations. The dication ground singlet state is found to have a linear equilibrium geometry that is stable with respect to dissociation to O⁺ and O₂⁺. However, at the Franck-Condon zone for formation of O₃²⁺ from the neutral molecule the singlet potential energy surface intersects with a dissociative triplet state. We propose that crossing to this dissociative triplet state can account for the absence of any long-lived O₃²⁺ ions in the electron-impact mass spectrum of ozone. Further calculations of the kinetic energy release for the fragmentation of O₃²⁺ to O⁺+O₂⁺ indicate that considerable vibrational excitation may be present in the O₂⁺ ion.     

Zur Radialstruktur der diffusionsbestimmten positiven Säule der Sauerstoff-Entladung bei niederen Gasdrucken

The radial distribution of the electrons, negative and positive ions in the cylindrical positive column of the direct current discharge in 0₂ has been calculated. The negative ions are formed by dissociative attachment (0₂ + e →^? + 0), the destruction of negative ions is effected by the reverse process (0^? + 0 0₂ + e). The pressure is assumed to be low enough that the radial variation of 0-concentration can be neglected. By use of the condition of the quasineutrality and the ambipolarity for the radial current from the balance equations of the electrons and the negative ions two differential equations of second order are derived, which has been solved numerically for different values of the ratio z_j/z_a and β[0]. Numerical results show that the radial distributions of the charge carriers are not-proportional. If z_j ≈? z_a the radial distribution of the electron density is enlarged, compared to a Bessel distribution. The radial distribution of the negative ions is also enlarged in the centre of the column, but near the wall of the tube a lack in the density exists. The effective diffusion coefficient of the negative ions is negative, i.e., they are moving from the wall of the tube to the centre of the discharge. The enlargement of the radial distributions of the charge carriers, which results in an improvement of the economy of the balance of electrons, increases if z_j/z_a and β/[0] are decreased.     

Experimental investigations and modelling studies of ozone producing corona discharges

The ozone generation by negative corona discharge in coaxial cylindrical system of electrodes have been studied experimentally in Ar+O₂ and N₂+O₂ mixtures. Both in argon and nitrogen mixtures with oxygen the monotonous decrease in ozone concentration [O₃] was observed at decreasing oxygen content in mixtures and the constant input energy density η. The rate coefficients for the ozone generation and ozone decomposition were obtained by fitting experimentally measured data [O₃]=f(η) with Vasiljev-Eremin formula. The calculated rate coefficient for ozone generation in N₂+O₂ mixtures at low content of oxygen (below 20%) was found considerably higher than that in Ar+O₂ mixtures. Increase in the rate coefficients for ozone generation and decomposition was observed with decreasing content of oxygen in both mixtures. The experimental results are in qualitative agreement with the simple model incorporating five main chemical processes in mechanism of ozone generation. The research was partially supported by Slovak Grant Agency under project 1/765920 The UK EPSRC (Grant GM/98944) and NATO Joint Project PST CLG 976544.     

Mechanisms of the Reactions of Radicals with Ozone

The available experimental data (rate constants and activation energies) for the reactions of the hydrogen atom and the R^?, RO^?, RO ₂ ^? , HO^?, and HO ₂ ^? radicals with ozone are analyzed using the method of intersecting parabolas (potential curves). The conclusion is drawn that the primary event in the reactions of H^?, R^?, HO^?, and RO^? with ozone is the addition of the radical to the ozone molecule with the subsequent fast decomposition of the labile polyoxide radical formed. The classical potential barrier for this addition reaction is close to that for the addition of radicals to molecules with multiple bonds. Peroxy radicals react with ozone by the associative decomposition mechanism, RO ₂ ^? + O₃ → RO^? + 2O₂. The ozone molecule reacts with the HO ₂ ^? radical by abstracting its hydrogen atom, O₃ + HO ₂ ^? → HO ₃ ^? + O₂. The experimental data were used to determine the parameters required to calculate the activation energies for the reactions under study.     

Reaktionskinetik der Ozonbildung in der Sauerstoffglimmentladung

Hg-light absorption at λ = 2537 ǎ by O₃ has been used to measure the concentration of O₃ in the positive column of an oxygen glow discharge under the nonstationary conditions of the Q-form (tube diameter 3:4 cm). For all gas pressures investigated (p = 7 — 12 torr) the O₃ density (n₃) as a function of the current reaches a remarkable maximum at i ≈ 15 mA. n₃ ^max was found to increase almost linearly with the gas pressure. At i = 80 mA, on the other hand, n₃ increases according to a quadratic relation. In order to understand the dependence of n₃ on the pressure and on the current density (or equivalently on the electron density n_e) a kinetic model previously developed has been used involving the following particles . For example, it can be shown that in the region of the maximum of n₃ one main production process of ozone is the associative detachment reaction . The calculated maximum of the ozone density is higher than the measured one and appears at higher electron density.     

Reduced Graphene Oxide decorated with Manganese Cobalt Oxide as Multifunctional Material for Mechanically Rechargeable and Hybrid Zinc–Air Batteries

Spinel MnCo₂O₄ nanoparticles on nitrogen‐doped reduced graphene oxide (MnCo₂O₄/NGr) are synthesized for advanced zinc–air batteries with remarkable cyclic efficiency and stability. The synthesized MnCo₂O₄/NGr exhibits good oxygen‐reduction reaction (ORR) activity with half‐wave potential E _1/2 of 0.85 V (vs reversible hydrogen electrode (RHE)), comparable to commercial Pt/C with E _1/2 of 0.88 V (vs RHE) along with superior oxygen electrode activity ΔE = 0.91 V for the ORR/OER (oxygen‐evolution reaction) in alkaline media. Durability tests confirm that MnCo₂O₄/NGr is more stable than Pt/C in alkaline environment. MnCo₂O₄/NGr functions with stable discharge profile of 1.2 V at 20 mA cm^?2, large discharge capacity of 707 mAh g^?1_Zn at 40 mA cm^?2 and a high energy density of 813 Wh kg^?1_Zn in a mechanically rechargeable zinc–air battery. The electrically rechargeable MnCo₂O₄/NGr zinc–air battery displays hybrid behavior with both Faradaic and oxygen redox charge–discharge characteristics, operating at higher voltage and providing higher power density and excellent cyclic efficiency of 86% for over 100 cycles compared to Pt/C with efficiency of around 60%. Moreover, hybrid zinc–air battery operates with a stable and energy efficient profile at different current densities.     

Co3O4 Hollow Nanoparticles and Co Organic Complexes Highly Dispersed on N‐Doped Graphene: An Efficient Cathode Catalyst for Li‐O2 Batteries

Rechargeable Li‐O₂ batteries are promising candidates for electric vehicles due to their high energy density. However, the current development of Li‐O₂ batteries demands highly efficient air cathode catalysts for high capacity, good rate capability, and long cycle life. In this work, a hydrothermal‐calcination method is presented to prepare a composite of Co₃O₄ hollow nanoparticles and Co organic complexes highly dispersed on N‐doped graphene (Co–NG), which acts as a bifunctional air cathode catalyst to optimize the electrochemical performances of Li‐O₂ batteries. Co–NG exhibits an outstanding initial discharge capacity up to 19 133 mAh g^?1 at a current density of 200 mA g^?1. In addition, the batteries could sustain 71 cycles at a cutoff capacity of 1000 mAh g^?1 with low overpotentials at the current density of 200 mA g^?1. Co–NG composites are attractive as air cathode catalysts for rechargeable Li‐O₂ batteries.     

O2掺杂对SiCOH低k薄膜结构与电学性能的影响

以十甲基环五硅氧烷(D5)和氧气(O₂)作为反应气体，采用电子回旋共振等离子体化学气相沉积(ECR-CVD)方法制备了k＝2.62的SiCOH薄膜.研究了O₂掺杂对薄膜结构与电学性能的影响.结果表明，采用O₂掺杂可以在保持较低介电常数的前提下极大地降低薄膜的漏电流，提高薄膜的绝缘性能，这与薄膜中Si-O立体鼠笼、Si-OH结构含量的提高有关. 关键词： SiCOH薄膜 2掺杂')" href="#">O₂掺杂 介电性能 键结构     

Negative Ions,Ozone, and Metastable Components in dc Oxygen Glow Discharge

In a dc glow discharge in oxygen, the concentrations of minor components of O₂(a¹Δ_g), O₂(b¹ Σ_g), O₃, O(¹D), as well as nagative ions and electrons have been measured. Balance equations have been derived which describe satisfactorily the stationary concentrations of these components as functions of gas pressure and discharge current. For the first time, the rate constants of important aeronomical reactions (a) O^? + O₂(a¹Δ_g) → O₃ + e, (b) O₂^? + O₂(a¹Δ_g) → 2O₂ + e and (c) e + O₃ → O₂^? +O have been measured as functions of gas temperature T and mean energies of ions E_i and electron E₆: K_a = (2.5 ± 0.5) · 10^?9 · (T/300)^{4 ± 0.4}· (E_i/0.04)^{?2.6 ± 0.4} cm³/s for T = 385?605 K and E_i = 0.10 ? 0.66 eV; K_b = (1.0 ± 0.3) · 10^?10 · (T/300)^{?2 ± 0.5} · (E_i/0.04)^{0.23 ± 0.05} cm³/s for T = 330?605 K and E_i = 0.09 + 1.5 eV; K_c for E_e = 0.8÷5 eV.     

CHCl3电子吸附速率常数的离子迁移谱

利用电晕放电离子迁移谱, 使用高纯氮气作为载气和迁移气体, 研究了电场强度在200～500 V/cm变化时CHCl₃的解离电子吸附速率常数, 得到样品所对应的电子吸附速率常数为1.26×10^-8～8.24×10^-9 cm³/(molecules s)．利用该装置测量了固定电场下,样品的电子吸附速率常数与样品浓度之间的关系．此外利用所获得的离子迁移谱图得到了不同电场强度下Cl^-与CHCl₃之间的离子分子反应速率常数.     

Untersuchungen über die Entwicklung des Kanaldurchschlags in Gasen

The temporal development of the current of a discharge leading to breakdown was investigated under static and homogeneous field conditions with wide band oscilloscopic techniques. The discharges were started by 10³ to 10⁵ electrons released within some 10^?9 sec along traces of single α-particles parallel to the electric field. Measurements have shown that streamer mechanism occurs in gases as CH₄ and CO₂ at static breakdown, if a sufficient density of space charge is produced by a great number of overlapping avalanches. The gas amplification of thesingle avalanche only has a value of about 10⁵. Also in O₂ and dry air at highpd-values breakdown develops with streamer mechanism. — In electronegative gases as freon 12 and O₂ a prolongation of the time necessary for streamer formation is caused by the reduction of the effective total space charge by the negative ions. The results of a detailed study of the time necessary for streamer development and the time constant of the increase of the current leading to breakdown confirm the model of streamer mechanism.     

Self consistent multi-fluid FDTD simulations of a nanosecond high power microwave discharge in air

FDTD simulations of the Maxwell equations are combined with the multi-fluid plasma equations to study the dynamics of a high power microwave discharge in air. The breakdown takes place in a short time of a few nanoseconds and the concentrations of electrons, ions, excited species, and the dissociation products are quickly enhanced. The breakdown time decreases with decreasing of the pressure and the pulse amplitude, while increases with increasing of the pulse width. N${{}_2}^{+}$ and O${}_2^{+}$ are the most important positive ions, whereas O^? is the most populated negative ion. For a single microwave pulse, the electron number density is large up to 1 μs, and the dissociation and excitation continue to increase the small radicals and excited species. Then the electron number density drops and the population of excited species declines. The ozone production becomes important after 1 μs when the three body association of O and O₂ dominates over the dissociation processes. The ozone number density continues to grow up to 5 ms, and then saturates at a value of 10²² m^?3. Quenching of electronically excited nitrogen molecules by O₂ molecules and the subsequent dissociation to atomic oxygen and generation of NO, are found to be important and can play a significant role in the ultrafast gas heating.     

The influences of gas and electron temperatures on electron attachment in gas electrical discharges

Various electron attachment processes are reviewed, emphasising the way in which the rates and products of some selected reactions vary with the attaching gas temperatureT _g, the temperature,T _e, and the energy of the attaching electrons. The examples illustrating the variety of reactions are the efficient dissociative attachment reaction to CCl₄, attachment to SF₆ which involves both dissociative and non-dissociative attachment, attachment to CHCl₃ which requires activation energy, and attachment to CCl₃Br which results in both Cl- and Br- product ions. A model has been presented which is able to quantitatively explain the difference influences ofT _g andT _e on the rates of some of these reactions. Also described are the unusually efficient attachment properties of the fullerene molecules C₆₀ and C₇₀ as revealed by our FALP experiments, noting that these molecules have potential importance as efficient suppressers of electrical breakdown through gases such as those used to insulate high voltage devices. We emphasise throughout this paper the importance of an understanding of the separate influences of gas and electron temperature on attachment reactions for the modelling of practical gas discharge media such as etchant plasmas. We dedicate this paper to Professor Jan Janča on the occasion of his sixtieth birthday in recognition of his major contributions to gas discharge physics.     

Elektronenverluste in einem abklingenden Wasserstoffplasma

The time dependence of the electron density in the afterglow period of an electrodeless discharge in hydrogen was measured by means of 4- and 8-mm microwave interferometry. An exponential decay was observed in the late afterglow permitting the evaluation of a time constant in the density range from 10¹²–10¹⁰ electrons per cm³. The decay time of the plasma was influenced by the discharge conditions. Electron losses could be explained by ambipolar diffusion and attachment to impurities. Two diffusion coefficients were found correlated to the discharge duration. The mobility value H₀=10.1±1.0 cm²/V · sec calculated from the diffusion coefficient found for short discharge pulses agrees with Saporoschenkos mobility value μ₀=10.2 cm²/V · sec for the H ₃ ⁺ -ion. A second mobility value μ₀=14.8±1.2cm²/V · sec found for longer discharge pulses might refer to the H⁺-ion.     

The reaction of •OH with O2, the decay of O and the pKa of HO – interrelated questions in aqueous free‐radical chemistry

In the reactions of ozone with organic compounds in aqueous solution, O is an abundant intermediate. A basic aspect of its conversion into ^?OH is addressed here. The reactions O^?? + O₂ ? O (1), H⁺ + O^?? ? ^?OH (8), ^?OH + O₂ ? HO (6), and H⁺ + O ? HO (5) are interconnected by a thermodynamic cycle. For equilibria (1) and (8) reliable equilibrium constants, and hence Gibbs energies are available (ΔG⁰(1) = ?32 kJ mol^?1, ΔG⁰(8) = 67 kJ mol^?1). For reaction (6), a Gibbs energy of ΔG⁰(6) = 47 kJ mol^?1 (K₆ = 10^?8.2 M) has now been calculated by G1. From the thermodynamic cycle one hence arrives at ΔG⁰(5) = ?12 kJ mol^?1. This relates to pK_a(HO) = ?2.1. Thus, the HO radical is a very strong acid. This value agrees with a value of ?2.0 obtained from the Bielski and Schwarz relationship for pK_a values of O_xH_y compounds. Reaction (6) must be very slow, 0.1 < k₆ < 10⁴ M^?1 s^?1. Copyright © 2010 John Wiley & Sons, Ltd.