Influence of Metallic Materials on SF<Subscript>6</Subscript> Decomposition Components under Positive DC Partial Discharge

Determination of the influence and mechanism of metallic materials on SF₆ decomposition under direct current (DC) partial discharge is one of the key aspects to improve SF₆ decomposition component analysis (DCA). In this study, three kinds of metallic materials, namely, aluminum, copper, and 18/8 stainless steel, were made into needle–plate electrons, and then used in the SF₆ positive DC partial discharge decomposition experiments. The influences of metallic materials on the five main decomposition components (i.e., CF₄, CO₂, SOF₂, SO₂F₂, and SO₂) were determined by gas chromatography–mass spectrometry. Results showed no significant correlation among the contents of CO₂ for the different kinds of metallic materials. However, the metallic materials considerably influenced the contents of the other four gases. The difference in SF₆ decomposition characteristics for the different metal electrodes was mainly due to the difference in anti-halogenation ability of metals and the passive film. Therefore, the impacts of different metallic materials should be considered when using SF₆ DCA for the condition monitoring and fault diagnosis of DC gas-insulated equipment.     

Identification of corona discharge-induced SF6 oxidation mechanisms using SF6/18O2/H2 16O and SF6/16O2/H2 18O gas mixtures

The absolute yields of gaseous oxyfluorides SOF₂, SO₂F₂, and SOF₄ from negative, point-plane corona discharges in pressurized gas mixtures of SF₆ with O₂ and H₂O enriched with¹⁸O₂ and H₂ ¹⁸O have been measured using a gas chromatograph-mass spectrometer. The predominant SF₆ oxidation mechanisms have been revealed from a determination of the relative¹⁸O and¹⁶O isotope content of the observed oxyfluoride by-product. The results are consistent with previously proposed production mechanisms and indicate that SOF₂ and SO₂F₂ derive oxygen predominantly from H₂O and O₂, respectively, in slow, gas-phase reactions involving SF₄, SF₃, and SF₂ that occur outside of the discharge region. The species SOF₄ derives oxygen from both H₂O and O₂ through fast reactions in the active discharge region involving free radicals or ions such as OH and O, with SF₅ and SF₄.     

Spectrokinetic study of SF5 and SF5O2 radicals and the reaction of SF5O2 with NO

UV spectra of SF₅ and SF₅O₂ radicals in the gas phase at 295 K have been quantified using a pulse radiolysis UV absorption technique. The absorption spectrum of SF₅ was quantified from 220 to 240 nm. The absorption cross section at 220 nm was (5.5 ± 1.7) × 10⁻¹⁹ cm². When SF₅ was produced in the presence of O₂ an equilibrium between SF₅, O₂, and SF₅O₂ was established. The rate constant for the reaction of SF₅ radicals with O₂ was (8 ± 2) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. The decomposition rate constant for SF₅O₂ was (1.0 ± 0.5) × 10⁵ s⁻¹, giving an equilibrium constant of K_eq = [SF₅O₂]/[SF₅][O₂] = (8.0 ± 4.5) × 10⁻¹⁸ cm³ molecule⁻¹. The SF₅ O₂ bond strength is (13.7 ± 2.0) kcal mol⁻¹. The SF₅O₂ spectrum was broad with no fine structure and similar to the UV spectra of alkyl peroxy radicals. The absorption cross section at 230 nm was found to (3.7 ± 0.9) × 10⁻¹⁸ cm². The rate constant of the reaction of SF₅O₂ with NO was measured to (1.1 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ by monitoring the kinetics of NO₂ formation at 400 nm. The rate constant for the reaction of F atoms with SF₄ was measured by two relative methods to be (1.3 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. © 1994 John Wiley & Sons, Inc.     

Nucleophilic Activation of Sulfur Hexafluoride: Metal‐Free,Selective Degradation by Phosphines

The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF₆) not only is of current environmental interest, but also offers new opportunities for applications of SF₆ as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF₆ by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF₆ into a bench‐stable, crystalline salt containing the SF₅^? anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (S_N2) for the initial fluorine abstraction from SF₆ by the phosphine. Furthermore, a scalable one‐pot procedure for the complete decomposition of SF₆ into solid, nonvolatile products is presented based on cheap and commercially available starting materials.     

Nucleophilic Activation of Sulfur Hexafluoride: Metal‐Free,Selective Degradation by Phosphines

The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF₆) not only is of current environmental interest, but also offers new opportunities for applications of SF₆ as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF₆ by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF₆ into a bench‐stable, crystalline salt containing the SF₅⁻ anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (S_N2) for the initial fluorine abstraction from SF₆ by the phosphine. Furthermore, a scalable one‐pot procedure for the complete decomposition of SF₆ into solid, nonvolatile products is presented based on cheap and commercially available starting materials.     

Mass-spectrometric investigation of the anions and cations in a hydrocarbon flame doped with sulphur/fluorine additives

A fuel-lean, premixed, CH₄O₂ flame at atmospheric pressure was doped with 0.2 mol% of SF₆ and 0.1 mol% of triflic anhydride (CF₃SO₂)₂O. Primarily the anions, but also the cations, occurring in the flame reaction zone and in the burnt-gas region downstream were observed by sampling the flame through a nozzle into a mass spectrometer. The main objective was to ascertain the ability of these sulphur/fluorine (S/F) additives and their combustion products to scavenge free electrons by forming negative ions in the flame gas. With either additive present, both total anions and cations increased in the reaction zone by a factor of between three and four. In the burnt gas, the total cations were essentially unchanged but the total anions showed a three-fold increase. With SF₆ additive only, major S/F cations were observed in the reaction zone (e.g.; SF⁺₃, SF₃O⁺, SF⁺₃, etc.) although H₃O⁺ with both additives was completely dominant downstream. Both additives produced major S/F anion signals in the flame reaction zone. Some of these were related to the individual additive structure (e.g. SF⁻₅ with SF₆; CF₃SO⁻₃ with (CF₃SO₂)₂O). Others were essentially unrelated but were observed with both additives (e.g. FSO⁻₃, HSO⁻₄, etc.) and persisted throughout the burnt gas. Some important features of the ion chemistry are discussed.     

The kinetics and the mechanism of the thermal decomposition of bis-pentafluorosulfurtrioxide (SF5OOOSF5)

The thermal decomposition of SF₅O₃SF₅ has been investigated between 5 and 25°C. In the presence of sufficient high pressures of O₂ the only products formed are SF₅O₂SF₅ and O₂: The reaction is homogeneous. Its rate is strictly first order with respect to the trioxide pressure and independent of the total pressure of the reaction products and of oxygen above a certain limiting pressure: The experimental results can be explained with the following mechanism: In the presence of O₂ > 100 Torr the concentration of SF₅ is insignificantly small. Therefore reactions (5) and (6) do not have to be considered any more, and steps (2) and (2′) will be of no importance. From reactions (1)–(4) it follows: The numerical value of the factor [1 + (k′₁²/2k₃k₄)^1/2] is small. It can be estimated that E₃ ? 2 ± 1 kcal; therefore, E – E₁ ≤ 1 kcal, and D = (26 – 1) ± 1.0 kcal.     

The ablation of quartz wall by SF6 under radiofrequency discharge

The behaviour of SF₆ in quartz and alumina tubes of a flow reactor capacitively coupled to a 35 MHz radiofrequency generator has been investigated at pressure of 20 torr, with power levels of 3.5÷5.5 cal cm^?3 sec^?1 and gas flow rates ranging between 0.1 and 2 1(STP) min^?1. A combination of gaschromatographic, mass spectrometric and infrared spectrophotometric techniques has shown the presence of SO₂F₂, SOF₄, SOF₂, SiF₄, F₂, O₂ together with unreacted SF₆ in the discharge products.A detailed quantitative investigation of the effluent products and of their concentration profiles versus space time and power is presented and a general mechanism for the ablation process of the quartz wall is suggested.     

Data on the formation and decomposition of bis(pentafluorosulfur) trioxide,bis(pentafluorosulfur) dioxide,bis(pentafluorosulfur) monoxide and of the radicals SF5O2 and SF5O. A review of earlier kinetic investigations

A method is described for the preparation of pure bis(pentafluorosulfur) trioxide. The mechanisms of formation and decomposition of SF₅O₃SF₅ and SF₅O₂SF₅ are described by reference to previous kinetic studies. The dissociation energies of these oxides and of the radicals SF₅O₂ and SF₅O are given and some reactions of SF₅·, SF₅O·, and SF₅O₂· are described.     

A model for the etching of silicon in SF6/O2 plasmas

A model has been developed to describe the chemistry which occurs in SF₆/O₂ plasmas and the etching of silicon in these plasmas. Emphasis is placed nn the gas-phase free radical reactions, and the predictions n( the model are compared with experimental results. Forty-seven reactions are included, although a subset of 18 reactions describes the chemistry equally well. Agreement between the calculated and measured concentrations of stable products downstream of the plasma is better than a factor of 2. The need for additional kinetic data and fàr well-characterized diagnostic studies of SF₆/O₂ plasmas is discussed.     

Kinetics of the thermal decomposition of bis-pentafluorosulfur trioxide(SF5O3SF5) in the presence of carbon monoxide

The thermal decomposition of SF₅O₃SF₅ in the presence of CO has been investigated between -9.8°C and + 9.9°C. Besides traces of S₂F₁₀, equimolecular amounts of SF₅O₂SF₅ and CO₂ are formed. The reaction is homogeneous. Its rate is proportional to the pressure of the trioxide and in dependent of the total pressure, the pressure of inert gases and of carbon monoxide: where k = k_1∞ = 10^16.32±0.40 exp(?25,300 ± 500 cal)/RT sec^?1. Consequently, In the presence of oxygen a sensitized CO₂ formation is observed. A mechanism is given which explains the experimental results.     

高压变电站事故预警监测探讨-新型氟化氢气体在线监测仪的研制

通过监测六氟化硫（SF₆）气体的变化发现高压变电站系统中设备的异常. 气体绝缘开关设备（GIS）中的高压开关、电闸及其它各种设备因接触不良、电阻值增加、发热、融解、拉弧、连续拉弧等原因会造成设备短路. 因保护这些电器的SF₆气体不可能非常纯净,如存在H₂O. 在上述这些故障发生时变成催化条件,使SF₆与H₂O作用后生成氟化氢气体. 通过监测氟化氢气体的突发变化,可以预警GIS系统设备可能出现的故障.     

Gas-phase reactions of SF5, SF2, and SOF with O(3 P): Their significance in plasma processing

Reactions of both SF₅ and SF₂ with O(³ P) and molecular oxygen have been studied at 295 K in a gas flow reactor sampled by a mass spectrometer. For reactions with O(³ P), rate coefficients of (2.0±0.5)×10^–11 cm³ s^–1 and (10.8±2.0)×10^–11 cm³ s^–1 were obtained for SF₅ and SF₂ respectively. The rate coefficients for reactions with O₂ are orders of magnitude lower, with an estimated upper limit of 5×10^–16 cm³ s^–1 for both SF₅ and SF₂. Reaction of SF₂ with O(³ P) leads to the production of SOF which then reacts with O(³ P) with a rate coefficient of (7.9±2.0)×10^–11 cm³ s^–1. Both SO and SO₂ are products in the reaction sequence initiated by reaction between SF₂ and O(³ P). Although considerable uncertainty exists for the heat of formation of SOF, it appears that SO arises only from reaction between SOF and O atoms which is also the source of SO₂. These results are discussed in terms of a reaction scheme proposed earlier to explain processes occurring during the plasma etching of Si in SF₆/O₂ plasmas. A comparison between the results obtained here and those reported earlier for reactions of both CF₃ and CF₂ with O and O₂ shows that there is a marked similarity in the free radical chemistry which occurs in SF₆/O₂ and CF₄/O₂ plasmas.     

Solvent isotope effects on the complexation and exchange kinetics of Tl(I), K and Na with 18-crown-6 by competitive NMR spectroscopy: Competition between homo- and heterobimolecular cations exchange

The thermodynamic properties of complexation and exchange kinetics of thallium by 18-crown-6 have been studied by thallium NMR spectroscopy. Effects of solvent isotope, counterion (ClO₄⁻ and NO₃⁻) and presence of competitive cations, such as Na⁺ and K⁺, on the exchange characteristics of the system have been considered. The obvious relationships between the effects of D₂O-H₂O solvent isotope on the thermodynamic properties and activation parameters of complexation have been investigated. In the absence of competitor cations, the mechanism of thallium exchange is unimolecular decomplexation and in the presence of competitor cations, homobimolecular cation exchange is the predominant mechanism at low concentrations of the ligand. At higher concentrations of the ligand, the measured rate constants show that the complexation/decomplexation process obeys a heterobimolecular cation interchange mechanism. The rate constants ratios (k_D2O/k_H2O < 1) for unimolecular mechanisms also show an inverse solvent isotope effect.     

Gas-phase reactions in plasmas of SF6 with O2 in He

Processes which occur in microwave discharges of dilute mixtures of SF₆ and O₂ in He have been examined using a flow reactor sampled by a mass spectrometer. Two classes of experiments were performed. In the first set of experiments, mixtures containing 6×10¹¹ cm^–3 SF₆, 6×10¹⁶ cm^–3 He, and O₂ in the range (0–3.6)×10¹³ cm^–3 were passed through a 20-W 2450-MHz microwave discharge. The gas mixtures arriving at a sample point downstream from the discharge were examined for SF₆, SF₄, SOF₂, SOF₄, SO₂F₂, SO₂, F, and O. In the second class of experiments, rate coefficients were measured for the reactions of SF₄ with O and O₂ and for the reaction of SF with O. The rate coefficient for the reaction of SF with O was found to be (4.2±1.5)×10^–11 cm^–3 s^–1. SF₄ was found to react so slowly with both oxygen atoms and oxygen molecules that only upper limits could be placed on the rate coefficients for these reactions. These values were 2×10^–14 cm³ s^–1 and 5×10^–15 cm³ s^–1 for reactions with O and O₂ respectively. The observed distribution of products from the discharged mixtures is discussed in terms of the measured rate coefficients.     

Effect of nano-transition metal oxides of Fe,Co and Ni and ferrites of Co and Ni on the multistage thermal decomposition of oxalates of Ce(III)

Semiconducting nano-metal oxides (Fe₃O₄, Co₃O₄, NiO, CoFe₂O₄ and NiFe₂O₄) were synthesized by thermal decomposition of their oxalate precursors. Using DSC technique, effect of nano-metal oxides [5 m m^?1 (%)] on the reaction pathway and mechanism of thermal decomposition of Ce₂ (C₂O₄)₃·10H₂O in flowing atmosphere of N₂ was investigated under linear non-isothermal condition. Performing the kinetic deconvolution method, physico-geometrical kinetic behavior of the two overlapping heat absorbing steps of both lower and higher temperature reactions was illustrated. Nano-Fe₃O₄ promoted the dehydration stages by lowering the Ea value to 35–36 kJ mol^?1. Nano-Co₃O₄, nano-CoFe₂O₄ and nano-NiFe₂O₄ promoted the dehydration as well as decomposition stages of cerium oxalate decahydrate by decreasing the Ea value. Nano-NiO has shown retarding effect on both dehydration and decomposition stages.

     

Studies on the thermal decomposition of the silver group of alkali metal nitritocobaltates(III)

The thermal decomposition of nitritocobaltate(III) of the silver group of general formula M₂Ag[Co(NO₂)₆] (where M = K⁺, NH⁺₄, Rb⁺ or Cs⁺) has been investigated. Based on the thermal curves of the investigated compounds and chemical and diffractometric analysis, the mechanism of thermal decomposition has been determined. The results obtained indicate that the decomposition proceeds in three stages. As a result of decomposition in the first stage (300°C), nitrates of alkali metals, metallic silver and Co₃O₄ are formed. In the second stage (500°C), a partial decomposition of nitrates to alkali metal oxides occurs, and in the third stage the products are alkali metal oxides, silver and Co₃O₄. This paper also presents the dependence of the decomposition temperature of nitritocobaltates(III) of the silver group on the ionic radius of the outer-sphere cation.     

Infrared multiphotonic excitation and the cell geometry problem: case of CH3Br and SF6

Gas samples of CH₃Br and SF₆ were irradiated by a focused CO₂ laser beam. Three cells with different sizes were used. With CH₃Br, fluorescence and decomposition were observed which depended upon the cell's dimensions. With SF₆ we always obtained decomposition and enrichment in ³⁴S by irradiation with the P(16) line.     

Absolute rate constants for F + CH3CHO and CH3CO + O2, relative rate study of CH3CO + NO,and the product distribution of the F + CH3CHO reaction

Using a pulse-radiolysis transient UV–VIS absorption system, rate constants for the reactions of F atoms with CH₃CHO (1) and CH₃CO radicals with O₂ (2) and NO (3) at 295 K and 1000 mbar total pressure of SF₆ was determined to be k₁=(1.4±0.2)×10⁻¹⁰, k₂=(4.4±0.7)×10⁻¹², and k₃=(2.4±0.7)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹. By monitoring the formation of CH₃C(O)O₂ radicals (λ>250nm) and NO₂ (λ=400.5nm) following radiolysis of SF₆/CH₃CHO/O₂ and SF₆/CH₃CHO/O₂/NO mixtures, respectively, it was deduced that reaction of F atoms with CH₃CHO gives (65±9)% CH₃CO and (35±9)% HC(O)CH₂ radicals. Finally, the data obtained here suggest that decomposition of HC(O)CH₂O radicals via C C bond scission occurs at a rate of <4.7×10⁵ s⁻¹. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 913–921, 1998