Inorganic volatile fluorides obtained from electrical decomposition of sulfur hexafluoride in a quartz tube

Recent investigations on sulfur hexafluoride decomposition have shown the need of a rapid and efficient method for the qualitative and quantitative analysis of the reaction products. An analytical method for characterizing the gas mixture obtained from the decomposition of sulfur hexafluoride in a quartz reactor submitted to an r.f. discharge, is presented. A combination of gas-chromatographic, mass spectrometric and infrared spectrophotometric techniques has shown the presence of SF₆, SO₂F₂, SOF₄, SOF₂, SiF₄ and F₂ in the gas mixtures examined. For quantitative purposes a gas-chromatographic method has been found to be most suitable.     

By-product formation in spark breakdown of SF6/O2 mixtures

The yields of SOF₄, SO₂F₂, SOF₂, and SO₂ have been measured as a function of O₂ content in SF₆/O₂ mixtures, following spark discharges. All experiments were made at a spark energy of 8.7 J/spark, a total pressure of 133 kPa, and for O₂ additions of 0, 1, 2, 5, 10, and 20% to SF₆. Even for the case of no added O₂, trace amounts of O₂ and H₂O result in the formation of the above by-products. However, addition of O₂ significantly increases the yields of SOF₄ and SO₂F₂, while SOF₂ is only slightly affected. The net yields for SOF₄ and SO₂F₂ formation range from 0.18×10^–9 and 0.64×10^–10 mol·J^–1, respectively, at 1% O₂ content to 10.45×10^–9 and 7.15×10^–10 mol·J^–1, respectively, at 20% O₂ content. The mechanism for SOF₄ production appears to involve SF₄, an important initial product of SF₆, as a precursor. Comparison of the SOF₄ and SO₂F₂ yield from spark discharges (arc and corona) shows that the yields from other discharges (arc and corona) shows that the yields can vary by at least three orders of magnitude, depending on the type of discharge and on other discharge parameters.     

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.     

Gas-phase reactions in plasmas of SF6 with O2: Reactions of F with SOF2 and SO2 and reactions of O with SOF2

The plasma chemistry of SF₆/O₂ mixtures is particularly complicated because of the large number of possible reactions. Over a wide range of conditions, products including SF₄, SOF₄, SOF₂, and SO₂F₂ can be formed but thre is considerable uncertainty about the major reactions which contribute to the formation of these species. In this work reactions of oxygen atoms with SOF₂ and fluorine atoms with SOF₂ and SO₂ have been studied in order to determine the principal sources of SO₂F₂ in these plasmas. Reactions were studied at 295 K in a gas flow reactor sampled by a mass spectrometer. No reaction could be detected between oxygen atoms and SOF₂, which for the conditions employed, means that the upper limit for the reaction rate coefficient is 1×10^–14 cm³ sec^–1. The reaction of fluorine atoms with SOF₂ was studied with the helium bath gas number density ranging from 3.1×10¹⁶ to 2.0×10¹⁷ cm^–3. Within this range the rate coefficient increased with increasing [He] from (4.1 to 10.8)×10^–14 cm³ sec^–1. SO₂ was found to react with fluorine atoms with a rate coefficient which appeared to be independent of the helium bath gas number density over the range given above. The possibility that this reaction occurred entirely on the walls of the reactor is discussed.     

Mass spectrometric study of SF6-N2 plasma during etching of silicon and tungsten

The reaction products in the SF₆-N₂ mixture rf plasma during reactive ion etching of Si and W have been measured by a mass spectrometric method. Two kinds of cathode materials were used in this work; they were stainless steel for the Si etching, and SiO₂ for the W etching. The main products detected in the etching experiments of Si and W included SF₄, SF₂, SO₂, SOF₂, SOF₄, SO₂F₂, NSF, NF₃, N₂F₄, N_xS_y, NO₂, and SiF₄. In the W etching with the SiO₂ cathode, additional S₂F₂, N₂O, and WF₆ molecules were also obtained. The formation reactions about the novel NSF compound and the sulfur oxyfuorides were discussed.     

Electron scattering and dissociative attachment by SF6 and its electrical-discharge by-products

Discrete electron-molecule processes relevant to SF₆ etching plasmas are examined. Absolute, total scattering cross sections for 0.2–12-eV electrons on SF₆, SO₂, SOF₂, SO₂F₂, SOF₄, and SF₄, as well as cross sections for negative-ion formation by attachment of electrons, have been measured. These are used to calculate dissociative-attachment rate coefficients as a function ofE/N for SF₆ by-products in SF₆.     

Negative ion formation in compounds relevant to SF6 decomposition in electrical discharges

Dissociative and nondissociative electron attachment in the electron impact energy range 0–14 eV are reported for SOF₂ SOF₄, SO₂F₂, SF₄, SO₂, and SiF₄ compounds which can be formed by electrical discharges in SF₆. The electron energy dependences of the mass-identified negative ions were determined in a time-of-flight mass spectrometer. The ions studied include F^– and SOF ₂ ^–* from SOF₂; SOF ₃ ^– and F^– from SOF₄; SO₂F ₂ ^–* , SO₂F^–, F ₂ ^– , and F^– from SO₂F₂; SF ₄ ^–* and F^– from SF₄; O^–, SO^–, and S^– from SO₂; and SiF ₃ ^– and F^– from SiF₄. Thermochemical data have been determined from the threshold energies of some of the fragment negative ions. Lifetimes of the anions SOF ₂ ^–* , SO₂F ₂ ^–* , and SF ₄ ^–* are also reported.     

Aspects of the chemistry of SF6/O2 plasmas

The production ofSOF ₄ initiated by the reaction of F atoms withSOF ₂ has been studied in a gas-flow reactor at 295 K for helium bath gas number densities in the range (3.0–27.0)×10¹⁶ cm^–3. The effect of O atoms on the formation ofSOF ₄ has been analyzed in terms of the competing reactionsSOF ₃+FSOF₄ andSOF ₃+OSO ₂ F ₂+F. This analysis leads to the conclusion that the rate coefficients for these two processes are equal within an uncertainty of about 50%. Furthermore, both experiment and calculations indicate that the rate coefficient for reactions between F atoms andSOF ₃ is close to its high-pressure limit under the conditions employed. The experiments set a lower limit on this rate coefficient of 5×10^–11 cm³ s^–1, while calculations based on unimolecular rate theory suggest that it may be greater than 1×10^–10 cm³ s^–1. These results make it clear that the two reactions shown above cannot explain the relative abundances ofSOF ₄ andSO ₂ F ₂ which are observed inSF ₆/O ₂ plasmas. This suggests thatSF ₂ is a major precursor in the sequence of reactions following the dissociation ofSF ₆.     

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.     

Real-time monitoring traces of SF6 in near-source ambient air by ion mobility spectrometry

The leakage of sulphur hexafluoride (SF₆) gas threats the global climate changes and personnel safety. Monitoring the concentration of SF₆ in its application places is an industry regulation. In this study, ion mobility spectrometry (IMS) was developed for fast monitoring traces of SF₆ in near-source ambient air. Due to the water is an important part of the natural air and affects most atmospheric measurements, the operating parameters of IMS monitoring SF₆ were optimised for quantitative analysis of SF₆ at different relative humidity (RH). It is discovered two main product ions SF₆^? and SOF₄^? by IMS at different RH. The calibration curves of SF₆ were investigated by its relationship with the peak intensity of SOF₄^– for real application. The time resolution of the measurement was obtained less than 1 s and the limit of detection (LOD) achieved 0.16–0.68 ppm with a data averaging of 30 times. At last, the simulated application of monitoring SF₆ leakage was tested in the fume hood of our lab. The results showed a great potential application prospect of IMS in monitoring SF₆ in the ambient air of its application places.     

Multidimensional SuFEx Click Chemistry: Sequential Sulfur(VI) Fluoride Exchange Connections of Diverse Modules Launched From An SOF4 Hub

Sulfur(VI) fluoride exchange (SuFEx) is a new family of click chemistry based transformations that enable the synthesis of covalently linked modules via S^VI hubs. Here we report thionyl tetrafluoride (SOF₄) as the first multidimensional SuFEx connector. SOF₄ sits between the commercially mass‐produced gases SF₆ and SO₂F₂, and like them, is readily synthesized on scale. Under SuFEx catalysis conditions, SOF₄ reliably seeks out primary amino groups [R‐ NH₂ ] and becomes permanently anchored via a tetrahedral iminosulfur(VI) link: R−N=(O=)S(F)₂. The pendant, prochiral difluoride groups R−N=(O=) SF₂ , in turn, offer two further SuFExable handles, which can be sequentially exchanged to create 3‐dimensional covalent departure vectors from the tetrahedral sulfur(VI) hub.     

Pentafluorethylschwefeltrifluorid: Synthese und Reaktionen

Pentafluorethyl Sulfurtrifluoride: Synthesis and Reactions By oxidation of (C₂F₅S?)₂ ( 1 ) with AgF₂ at 0°C a mixture of C₂F₅SF₃ ( 2 ) and C₂F₅SF₅ ( 3 ) besides C₂F₅S(O)F ( 4 ) is formed. With elemental fluorine only 3 is isolated, an intermediate in this reaction is (C₂F₅SF₄?)₂ ( 5 ). At ?40 to ?30°C the mixture of 2, 3 and 4 was reacted with TASF and AsF₅, to give TAS⁺ C₂F₅SF₄^? ( 6 ), TAS⁺ C₂F₅S(O)F₂^? ( 7 ) and C₂F₅SF₂⁺AsF₆^? ( 8 ), respectively. While 6 and 7 decompose rapidly in solution even at low temperatures, of thermally stable 8 the solid state structure was determined by x-ray diffraction.     

DC plasma etching of silicon by sulfur hexafluoride. Mass spectrometric study of the discharge products

Polycrystalline silicon wafers were etched in dc discharges of SF₆. SF_x species were extracted from the discharges and measured with a mass spectrometer. A systematic procedure was used to measure the SF _x ⁺ signals such that they are indicators of events in the discharge close to the sample undergoing etching. The picture that emerges is remarkably simple and shows the relative stability of several SF_x species including SF₆, SF₄, SF₂, and SF which are shown to be extracted from the discharge both in the presence and absence of the silicon sample. When silicon is being etched on the cathode of the discharge cell, the only significant additional products are SiF₄ and S₂F₂. A comparison of blank and sample data for opposite substrate polarities shows that there is only a small cation-assisted etching effect and suggests that ions do not play an important role in the etching of silicon by SF₆ discharges.     

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.     

The kinetics and thermochemistry of S2F10 pyrolysis

Data on the kinetics of S₂F₁₀ pyrolysis, which gives SF₄ + SF₆, have been reinterpreted to give a value for the equilibrium constant of S₂F₁₀ ? SF₄ + SF₆. This, together with statistical estimates of the entropy and heat capacity of S₂F₁₀, can be used to give for this reaction values of ΔH = 19.7 ± 1.0 kcal/mole and ΔS = 47.6 ± 2 gibbs/mole. ΔH(S₂F₁₀) = –494 kcal/mole. A compatible mechanism is shown to be S₂F₁₀ ? 2SF₅ (fast); 2SF₅ ? SF₆ + SF₄ (slow) with step 2 rate-determining. The overall, best first order rate constant is proposed as k_meas = 10^{17.42–43.0/θ} sec^?1 = K₁k₂, where θ = 2.303RT in kcal/mole. Independent measurements of δH and S° for the SF₅ radical, permits the evaluation of the equilibrium constant K₁ = 10^{8.92–(27.1 ± 6)/θ} l./mole-sec and yields k₂ = 10^{8.50–15.9/θ} l./mole-sec. The observed homogeneous catalysis by NO and CHCl ? CHCl can be explained in terms of a direct abstraction of F from S₂F₁₀ : C + S₂F₁₀ → CF + S₂F₉, followed by S₂F₉ → SF₅ + SF₄ and SF₅ + CF ? SF₆ + C (C ? NO or C₂H₂Cl₂).     

Untersuchung von Schwefel-Fluor-Verbindungen durch Codestillation und Cosublimation im Cady-Rohr

Zusammenfassung Das Trennverfahren der quantitativen Codestillation und Cosublimation ist zur Untersuchung von Schwefel-Fluor-Verbindungen eingesetzt worden, die vorwiegend Schwefel in niedriger Oxidationsstufe enthalten. Die neue Technik, die sich ideal für die Analyse luftempfindlicher und korrosiver Gasmischungen eignet, ermöglichte die Trennung kleiner Stoffmengen von SOF₂, SSF₂, FSSF und SF₃SF durch Destillation und die Charakterisierung dieser Stoffe durch ihre Dampfdrücke sowie die sich daraus ergebenden thennodynamischen Daten der Verdampfung bei sehr niedrigen Drücken. Mischungen aus SF₃SF und FSSF sowie FSSF und SSF₂ verhalten sich beim Sieden ideal, SSF₂ und SF₄ bilden ein azeotropes Gemisch mit SF₄ im Überschuß. SF₆ kann von den weniger flüchtigen Schwefelfluoriden durch Cosublimation leicht abgetrennt werden und als Standard für die Bestimmung von Stoffmengen und molaren Massen durch Verflüchtigung im Stickstoff- und Argonstrom dienen.Die Auswertung der Daten, die bei der Sublimation des Trifluorosulfoniumtetrafluoroborates, SF ₃ ⁺ BF ₄ ^– im Cady-Rohr erhalten wurden, bewiesen, daß die Flüchtigkeit dieser Verbindung auf ihre Dissoziation in SF₄ und BF₃ zurückzuführen ist.

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Investigation of sulfur fluorine compounds by codistillation and cosublimation in the Cady tube

Summary The separation concept of quantitative codistillation and cosublimation has been used to investigate sulfur fluorine compounds with sulfur in a lower oxidation state. The new technique, which is ideally suited to analyse air sensitive and corrosive gas mixtures, enabled the separation of small quantities of SOF₂, SF₄, SSF₂, FSSF and SF₃SF by distillation, and to characterize these compounds through their vapour pressures and thermodynamic data of vaporization at low pressures. Mixtures of FSSF and SF₃SF, and, FSSF and SSF₂ behave ideally on boiling, whereas SSF₂ and SF₄ form an azeotropic system with SF₄ in excess. SF₆ can easiliy be separated from the less volatile sulfur fluorides by cosublimation and may be used as a standard for the estimation of the amount of substances and molar masses through volatilization in nitrogen and argon. The evaluation of data obtained through the sublimation of trifluorsulfonium tetrafluoroborate in the Cady tube proved that the votalization process is caused by the dissociation of SF ₃ ⁺ BF ₄ ^– to yield SF₄ and BF₃.

     

The reaction of XeF+MF6− (M = As,Sb) with sulfuranes; preparation of CF3(O)F2+SbF6− and (CF3)2SOXeF+SbF6−

CF₃S(O)F, (CF₃)₂SO, CF₃SF₃, (CF₃)₂SF₂, and SF₄ react in different manner with XeF⁺MF₆^? (M?As, Sb). An oxidative fluorination is observed by CF₃S(O)F forming the persulfonium salt CF₃S(O)F₂⁺SbF₆^?, whereas by (CF₃)₂SO a simple addition product containing xenon can be isolated in form of the sulfonium salt (CF₃)₂SOXeF⁺SbF₆^?. On the contrary, the Lewis-acidic character of the XeF⁺-cation predominates against (CF₃)_nSF_4?n (n = 0 ? 2) leading to the corresponding fluorosulfonium salts (CF₃)_nSF_3?n ⁺MF₆^? (M?As, Sb) and XeF₂.     

Synthese von (C6F5)2SF+MF6− (M  As,Sb) und Kristallstruktur von (C6F5)2SF+SbF6− [1]

Preparation of (C₆F₅)₂SF⁺MF₆^? (M ? As, Sb) and Crystal Structure of (C₆F₅)₂SF⁺SbF₆^? XeF⁺MF₆^? (M ? As, Sb) reacts with (C₆F₅)₂S in HF to form (C₆F₅)₂SF⁺MF₆^?. The deeply violet sulfonium salts can be kept without decomposition up to 24 h at room temperature. The hexafluoroantimonate salt crystallizes in the monoclinic space group P2₁/n with a = 1056.4(7) pm, b = 1446.3(10) pm, c = 1102.9(8) pm, β = 91.29(6)° und Z = 4. The SF-bond distance with 158.4(3) pm is of unusual length. Cations and anions are connected via interionic fluorine contacts to an infinite chain, in which cations and anions form to ABAB sequence along the chain.     

Gas analysis by infrared spectroscopy as a tool for electrical fault diagnostics in SF6 insulated equipment

Infrared spectroscopy shows an enormous potential for the analysis of by-products generated from electrical discharges in sulfur-hexafluoride (SF₆) insulated equipment. Since by-product composition can be related to the fault genesis (arc, partial discharge or corona), the analysis of contaminated SF₆ provides a valuable diagnostic tool. The IR-spectrometric results from discharge experiments are presented, carried out with the application of SF₆ pressures around 300?kPa and an alternating voltage up to 30?kV. Under the discharge conditions used, the main by-products found are the sulfuroxyfluorides SOF₄ and SO₂F₂ with concentrations correlated to the discharge time. Due to its toxicity, special attention is also paid to S₂F₁₀. The experimental conditions and practical aspects for reliable quantitative analysis of reactive species are discussed.     

Multiphoton laser-induced chemistry of trifluorobromomethane

The multiphoton induced chemistry of trifluorobromomethane (CF₃Br), using a pulsed TEA CO₂ laser operating in the R branch of the 00°1–02°0 transition, and the effects of pulse number, pressure, and exciting wavelength has been investigated. The photolysis products are C₂F₆, CF₄, CF₂Br₂, and Br₂. A reaction scheme is proposed to account for the observed products and their dependence on experimental conditions. The product yield are found to increase with CF₃Br pressure but to decrease with increasing added bath gas pressure (N₂ or NO). Varying the exciting wavelength caused a variation in product yields resulting in a spectrum similar to that for single photon absorption but red shifted by ≈10 cm^?1. It was also observed that at the wavelength employed ¹²CF₃Br is preferentially dissociated at pressures below 2 Torr. Finally a comparison is made between the results of this work and those done on other freons and SF₆.