SF6 Content Research Articles

Tracking selenium in the Chalk aquifer of northern France: Sr isotope constraints

Groundwater at the southern and eastern edges of France’s Paris Basin has a selenium content that at times exceeds the European Framework Directive’s drinking-water limit value of 10μg/L. To better understand the dynamics of the Chalk groundwater being tapped to supply the city of Lille and the Se origins, we used a combination of geochemical and isotopic tools. Strontium isotopes, coupled with Ca/Sr, Mg/Sr and Se/Sr ratios, were used to identify the main groundwater bodies and their mixings, with the Mg/Sr and Se/Sr ratios constraining a ternary system. Groundwater in the agricultural aquifer-recharge zone represents a first end-member and displays the youngest water ages of the catchment along with the highest Sr isotopic signature (0.70842) and low Se contents. Anaerobic groundwater constitutes a second major end-member affected by water-rock interactions over a long residence time, with the lowest Sr isotopic signature (0.70789) and the lowest Se content, its low SF6 content confirming the contribution of old water. Se-rich groundwater containing up to 30μg/L of Se represents a third major end-member, with an intermediate Sr isotopic ratio (0.70826), and is mainly constrained by the clayey Se-rich formation overlying the Chalk aquifer. The spatial and temporal Se variability in the groundwater is clearly linked to the presence of this formation identified as Tertiary and also to the hydrological conditions; saturation of the Se-rich clays by oxygenated groundwater enhances Se mobility and also Sr adsorption onto the clays. This multi-tool study including Sr isotopes successfully identified the Se origins in the aquifer and has led to a better understanding of the regional mixing and processes affecting the Chalk groundwater.

Cathode Powder Surface Treatment Using Atmospheric Pressure Plasma

LiOH, Li2CO3 impurities of NCM (8:1:1) Li ion battery cathode powder were removed by using atmospheric pressure plasma. Specially designed plasma reactor was used for the study. The mixed gas of N2, H2, CH4, SF6 and NF3 was used for dielectric barrier discharge plasma generation. N2-SF6 and N2-NF3 mixed gas plasma showed the best removal performance and when the SF6 or NF3 content was increased, the removal efficiency was also increased with a plateau. Addition of a small amount of hydrogen containing gas such as H2 and CH4 into the plasma enhanced the impurity removal efficiency but the proportional relationship with the content increment with these gases was not shown. It is well known that the fluorine containing gas extinguishes plasma, however the hydrogen containing gas seems attenuated the extinguishing effect of the fluorine containing gas for this case. In the early stage of the study, the cathode powder was treated by direct contact with the plasma, however, it was also observed that the same removal effect was shown by just mixing the powder with the gas passed through the plasma generator. The plasma treatment of 3kW applied power could remove more than 50% of impurities from the NCM powder at 13kg/hr of the treatment speed. In the same impurity removal treatment condition, it was observed that fluorine passivation layer was formed on the surface of the cathode particle at the same time. Other than the impurity removal and fluorine passivation on the cathode active material, the atmospheric pressure plasma powder treatment can offer the solution for enhancement of dispersion property in making powder slurry and carbon adsorption on the active material surface etc.

Separation of sulfur hexafluoride (SF6) from ternary gas mixtures using commercial polysulfone (PSf) hollow fiber membranes

The separation of sulfur hexafluoride (SF6), a powerful anthropogenic greenhouse gas, was investigated using membrane technology. Permeation measurements in commercial polysulfone (PSf) hollow fiber membranes were performed using single gases (i.e. N2, O2 and SF6) to investigate the effect of temperature and pressure on separation performance. The permeation experiments of ternary gas mixtures (N2/O2/SF6) were also conducted under various operational conditions, including pressure, temperature, stage cut (permeation flow rate/feed flow rate) and gas compositions. The results showed that the SF6 treatment capacity increased with increase in temperature or pressure, but decreased with increasing stage cut and SF6 content in the feed gas mixture. At higher temperatures, the membrane exhibited higher performance for the separation, recovery and enrichment of SF6. A feed with a higher pressure or a lower stage cut resulted in lower SF6 separation and enrichment efficiency, but a higher recovery. The separation of SF6 from a gas mixture with higher contents of SF6 exhibited lower SF6 recovery and enrichment performance. Our current work demonstrated more realistic performance of the commercial PSf hollow fiber membrane for the separation, enrichment and recovery of SF6.

Investigation of atomic oxygen density in a capacitively coupled O2/SF6 discharge using two-photon absorption laser-induced fluorescence spectroscopy and a Langmuir probe

Two-photon absorption laser-induced fluorescence (TALIF) spectroscopy was used for detection of absolute atomic oxygen density in a low-pressure capacitively coupled plasma source. We investigated the variation of atomic oxygen density for various mixtures of O2/SF6 and report a significant five-fold increase of [O] when oxygen plasma was diluted with SF6 by only 5%. We attribute this increase in [O] to a combination of a change in surface conditions caused by constituents of SF6 plasma reacting with the reactor walls and also due to an increase in the electron temperature. Atomic oxygen production rates were determined using electron-energy distribution functions obtained with a cylindrical Langmuir probe. It was found that the effective electron temperature dramatically increased from approximately 1–8 eV as the SF6 content varied from 0% to 60% which consequently resulted in a three-fold increase in the atomic oxygen production rate. TALIF was also used to investigate the variation of [O] due to fluorination of the reactor walls and also after etching resist-coated wafers. It was found that [O] increased by over a factor of three after fluorinating the walls with SF6 plasma; on the other hand a coating formed on the reactor walls after a resist etch process resulted in a reduction of [O] by only 20%.

Study of Formation and Propagation of Streamers in SF6 and Its Gas Mixtures with Low Content of SF6 Using a One-Dimensional Fluid Model

Using a hybrid Monte Carlo Collision/Fluid model, the formation and propagation of streamers in SF6 and its gas mixtures are simulated. The simulation is based on an accurate numerical solution of Poisson's equation in conjunction with the continuity fluid equation for electrons, negative ions, and positive ions. The factors that influence the formation and propagation of streamers are investigated. The electron density, positive and negative ion density, and electric field in the discharge channel are also presented, which are very important in understanding the phenomena of streamers and in assessing the insulation strength of the gas mixture.

Investigation on creeping discharges propagating over epoxy resin and glass insulators in the presence of different gases and mixtures

This paper deals with the experimental characterization of discharges propagating over insulators of epoxy and glass, immersed in a gas or a gaseous mixture, under lightning impulse voltages (1.2/50 μs), using a point-plane electrode arrangement. The gases and mixtures we considered are SF6, N2, CO2, SF6-N2 and SF6-CO2. The morphology of creeping discharges and their final lengths are investigated versus the kind of insulator material, the amplitude and polarity of the voltage, the type of the gas (resp. mixture) and its pressure. It is shown that the shape of discharges and their final (stopping) lengths Lf depend significantly on the solid insulator and the type of gas. For given solid and gas, Lf increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges do not always present a radial structure as reported in the literature. For given voltage and pressure, Lf is longer when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one; and Lf is longer with glass than with epoxy. Lf is shorter in SF6 than in CO2 or N2. On the other hand, the increase of SF6 content in SF6-CO2 mixture leads to a significant decrease of Lf. Therefore, the addition of small concentration of SF6 in a given gas mixture improves the dielectric strength of insulating structure.

A simple method for pre-calibration storage of sulphur hexafluoride permeation tubes

Long term storage of sulphur hexafluoride (SF6) permeation tubes, used to determine enteric emissions from ruminants using a calibrated tracer (ERUCT) technique, is desirable to increase flexibility of post-manufacture tube handling. To be effective, such storage must decouple the date of tube manufacture from the date of subsequent expiration due to gas loss. We hypothesized that release of SF6 gas could be effectively inhibited by freezing tubes at −80°C without affecting their subsequent performance. Thirty permeation tubes with mean SF6 content of 3.081g were blocked by SF6 content and randomly allocated to one of two treatments being: immediate incubation at 39°C, or incubation at 39°C following 75 d storage at −80°C. The SF6 permeation rate at 39°C of all tubes was determined by repeated weighing over a 50 d period. Storage of permeation tubes at −80°C effectively inhibited release of SF6 but had no effect on subsequent permeation rate at 39°C. Deep frozen storage offers improved flexibility in handling of SF6 permeation tubes for the ERUCT technique.This article is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.

Back Diffusion of Electrons in Ar, $\hbox{SF}_{6}$, and $\hbox{Ar} + \hbox{SF}_{6}$ Binary Mixtures

The back diffusion processes in Ar, SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , and Ar + SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> binary mixtures are analyzed by a Monte Carlo simulation method employing realistic collision cross sections of the respective component gases in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> range from 50 to 500 Td. The escape factors in pure Ar are always higher than that of those in pure SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , and as the SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> content in the binary mixture increases, the escape factors decrease accordingly. Furthermore, the number of collisions observed for the backscattered electrons before returning to cathode in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> are smaller than that of those in Ar, and as the SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> content in the binary mixture increases, the mean number of collisions of the backscattered electrons decrease accordingly.

Effective ionization coefficients and transport parameters in binary and ultradilute SF6–Ar mixtures using Boltzmann equation analysis

The effective ionization coefficients and transport parameters such as electron mean energy drift velocity and transverse diffusion coefficient in binary and ultradilute SF6–Ar gas mixtures have been calculated for density reduced electric field strength E/N values from 10 to 400 Td. These calculations have been performed by using the two-term spherical harmonic expansion to obtain the numerical solution of the Boltzmann transport equation based on the finite element method under steady-state Townsend condition. In order to confirm the model and code developed in this study, the Reid ramp model has been used as a benchmark test and then effective ionization coefficients and transport parameters have been evaluated for SF6 contents of 1%, 10%, 25%, 50%, 70% and 100% in the binary mixture. Finally SF6 contents in the ultradilute mixtures of 0.1%, 0.3%, 0.5% and 0.7% are taken into account with the evaluated effective ionizations and transport parameters of electron mean energy, drift velocity and transverse diffusion coefficients.

Characteristics of Creeping Discharge Developed in Narrow Gap on a Filamentous Backside Electrodes

We describe about characteristics of creeping discharge developed in a narrow gap with a backside electrode, under μs pulse voltage application. As a result, in the gas with 3% SF6 mixture in N2, the flashover voltage in the pressures of 0.2 and 0.3MPa decreased by approximately 5% compared to that of 0.1MPa in both a filamentous straight backside electrode and a filamentous curve backside electrode. V-characteristics of which flashover voltage decreased were obtained by SF6 mixture in N2. We obtained V-characteristics on the flashover voltage at SF6 content in the range of D=3%-100% for a filamentous backside electrode. Flashover voltages at a filamentous straight backside electrode were higher than that of a filamentous curve backside electrode. From the corona observation using ultra-high speed camera, it was found that the flashover is generated along the backside electrode.

Effective Ionization Coefficients and Transport Parameters in Ultradilute &lt;formula formulatype="inline"&gt; &lt;tex&gt;$\hbox{SF}_{6} + \hbox{N}_{2}$&lt;/tex&gt; &lt;/formula&gt; Mixtures

The effective ionization coefficients in ultradilute SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> + N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas mixtures are calculated from a Monte Carlo technique over the range 100 ≤ E/N ≤ 270 Td together with electron drift velocities and pulsed Townsend mean energies. SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> contents in the mixtures of 0.1%, 0.3%, 0.5%, 0.7%, and 1% are considered. The effective ionization coefficients decrease with the increase of the SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> concentration in the mixtures at a given E/N, while the variation of the drift velocities and pulsed Townsend mean energies with respect to SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> concentrations is relatively constant in the E/N range of this paper.

Monte Carlo Simulation of Electron Swarm Parameters in the SF6/CF4 Gas Mixtures

We use a binary gas mixture Monte Carlo simulation model to calculate the electron transport parameters in SF6/CF4 mixtures in uniform electric fields. Electron collision cross section sets are assembled on the basis of the critical survey of Christophorou et al. [J. Phys. Chem. Ref. Data 25 (1996) 1341] for CF4 and Itoh et al. [J. Phys. D 21 (1988) 922] for SF6. The electron swarm parameters studied here are electron drift velocity, effective ionization coefficient and the ratio of longitudinal diffusion coefficient to mobility for the density-reduced electric field strength (E/N) in the range of 140≤E/N≤600 Td (1 Td = 10-17 V cm2); the SF6 contents in the gas mixtures are 0, 20, 50, and 100%. The differences between the present calculated and measured values of Urquijo et al. [J. Phys. D 36 (2003) 3132] do not exceed the overall measured uncertainties for the parameters. To our knowledge, only Urquijo et al. [J. Phys. D 36 (2003) 3132] have measured the electron swarm parameters in SF6/CF4 gas mixtures by a pulsed Townsend technique; however, simulation results have scarcely been reported.

Characteristics of N2/SF6 mixture gas in creeping discharge developing in narrow gap with backside electrode

AbstractThe flashover voltage with a backside electrode was found to be lower than that without the backside electrode. Under microsecond pulse voltage application, we describe the characteristics of a creeping discharge developed in the narrow gap with the backside electrode. Using a CCD camera and ultrahigh‐speed camera, we observed the corona extension processes. The lowest flashover voltage was found to be obtained at positive polarity with a SF6 content D = 3%. In the corona extension obtained using an ultrahigh‐speed camera, peculiar differences were observed in the corona extension process. The corona extension increased, and rapid flashover was observed at D = 3%. Using a CCD camera, small coronas were detected from the backside electrode. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(2): 31–38, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20431

Life cycle impact assessment of various treatment scenarios for sulfur hexafluoride (SF6) used as an insulating gas

AbstractA life cycle environmental impact assessment was conducted for sulfur hexafluoride (SF6) used as an insulating gas in electrical devices. Eight scenarios with a variety of SF6 treatment processes were considered. For each scenario, the life cycle power consumption was determined and an inventory analysis of pollutants and toxic materials was conducted. The scenarios were compared in terms of their life cycle environmental impacts, such as power consumption, global warming, acidification, eutrophication, generation of photochemical oxidants, and generation of substances toxic to human health. The power consumption is significantly reduced by the use of a gaseous mixture of SF6 and nitrogen in comparison to when pure SF6 is used, because the power consumption required for producing SF6, which is the major power consumption process, can be reduced according to the amount of SF6 replaced by nitrogen. On the other hand, larger amounts of the pollutants associated with the transportation process will be generated for the scenarios using gaseous mixtures with smaller SF6 contents in the insulating gas. The reuse of spent SF6 after purification can also reduce the power consumption. The calculated environmental impacts were unified with two types of environmental indexes using Eco‐Indicator 99 and LIME (Life‐cycle Impact assessment Method based on Endpoint modeling), and the scenarios were evaluated and compared. © 2006 American Institute of Chemical Engineers Environ Prog, 2006

Plasma etching of benzocyclobutene in CF4∕O2 and SF6∕O2 plasmas

Results for the reactive ion etching and patterning of benzocyclobutene (BCB) in CF4∕O2 and SF6∕O2 plasmas in a parallel-plate reactor with Langmuir probe and optical emission diagnostics are reported. The behavior of the O atom concentration [O] is similar in both plasmas, showing a strong maximum at a concentration of about 70% O2. The F atom concentration [F] has a maximum at 30% O2 in CF4∕O2, but the etch rate has a maximum at 70% O2. In SF6∕O2, by contrast, [F] and the etch rate increase continuously as the SF6 content is increased, and the highest etch rate is obtained with pure SF6. Also, BCB etched in pure SF6 gives a very smooth surface. Patterning is performed using two different masks: a conventional hard mask with Si oxide as the top layer and a different type of mask with BCB itself as the top layer. The anisotropy of the etching is evaluated in terms of the plasma conditions and type of masking. The aspect ratio dependence of the etch rate is investigated up to an aspect ratio of 2.5. The results are discussed in terms of possible etch mechanisms and their differences between SF6∕O2 and C4∕O2 plasmas. A simple model is used to show how the latter mask reduces the dependence of the etch rate on the aspect ratio.

Measurement of electron transport and effective ionization in SF6–air and SF6–O2 mixtures

The electron drift velocity ve, the density-normalized longitudinal diffusion coefficient NDL, the density-normalized effective ionization coefficient (α − η)/N and the limiting field strength E/Nlim have been measured for the binary mixtures SF6–air and SF6–O2 for SF6 concentrations ranging from 1% to 50%. The overall reduced electric field strength E/N was varied between 1.2 and 500 Td (1 Townsend = 10−17 V cm2). A pulsed Townsend technique was used to perform these measurements, which turned out to be sensitive enough to distinguish relatively small differences in the electron drift velocities and longitudinal diffusion coefficients over the mixture range 1 to 50% SF6 in SF6–air and SF6–O2. On the other hand, the effective ionization coefficients show a well-defined variation with the SF6 content in the mixture. The limiting value of E/N at which the effective ionization coefficient is zero, that is, the critical field strength, turned out to be smaller than that for the SF6–N2 mixture. However, the difference in behaviour between SF6–air and SF6–N2 is small. To the best of our knowledge, we are not aware of any previously published measurements of these parameters for these mixtures.

Groundwater dating with3H and SF6 in relation to mixing patterns, transport modelling and hydrochemistry

A comprehensive study of a sandy aquifer of deltaic origin in southern Poland included water chemistry, isotopes, dissolved trace gases and transport modelling. Tritium, sulphur hexafluoride (SF6) and freons (F-11, F-12 and F-113) showed the presence of modern waters in the recharge areas and shallow confined parts of the aquifer. The presence of older Holocene waters farther from the recharge areas was indicated by lack of 3H, SF6 contents ≤0·02 fmol l−1 and relatively low 14C values. The discharge from the system is by upward seepage in the valley of a major river. Pre-Holocene waters of a cooler climate, identified on the basis of δ18O, δ2H, 14C, Ne and Ar data, were found in some distant wells. Concentrations of N2, Ne and Ar determined by gas chromatography were used for calculating the noble gas temperatures, air excess needed for correction of SF6, and nitrogen content released by denitrification process. The time series of 3H content available for some wells supplied quantitative information on age distributions and the total mean ages of flow through the unsaturated and saturated zones. The derived 3H age distributions turned out to be very wide, with mean values in the range of about 30 to 160 years. For each well with determined 3H age, the SF6 data showed either a lower age range or the possibility of a lower age as expected due to shorter travel times of SF6 through the unsaturated zone, which most probably also resulted in different types of age distributions of these tracers. Freons appeared to be of little use for individual age determinations. A quantitative estimation of two-component mixing from SF63H relations is not possible unless the travel time of 3H through the unsaturated zone is comparable to that of SF6. The ratio of integrals of the response function over the age range with tracer and the whole response function yields the ratio of water with tracer to total flow of water. That ratio is a tracer-dependent function of time. Transport modelling of SF6 tracer done with MT3D code yielded initially large discrepancies between calculated and measured tracer concentrations. Some discrepancies remained even after calibration of the transport model with SF6. Simulation of tritium contents with a calibrated transport model yielded reasonable agreement with measured contents in some wells and indicated a need for further investigations, particularly in the eastern part of the aquifer. The existence of distinct hydrochemical zones is consistent with the tracer data; young waters with measurable 3H and SF6 contents are aerobic and of HCO3Ca or HCO3SO4Ca types. Slightly elevated Na and Cl contents, as well as the highest concentrations of SO4 and NO3 within this zone are due to anthropogenic influences. Anaerobic conditions prevail in the far field, under the confining cover, where pre-bomb era Holocene waters dominate. In that zone, dissolved oxygen, NO3 and U contents are reduced, and Fe, Mn and NH4 contents increase. In the third zone, early Holocene and glacial waters occur. They are of HCO3CaNa or HCO3Na types, with TDS values higher than 1 g l−1 and Na content higher than 200 mg l−1, due to either small admixtures of ascending or diffusing older water or freshening of marine sediments, a process that is probably occurring till the present time. Copyright © 2005 John Wiley & Sons, Ltd.

Characteristics of N2/SF6 Mixture Gas on Creeping Discharge Developed in Narrow Gap with Backside Electrode

The flashover voltage with the backside electrode was lower than one without the backside electrode. Under μs pulse voltage application, we describe about characteristics of creeping discharge developed in narrow gap with the backside electrode. Using CCD camera and ultra-high speed camera, we observed the corona extension processes. As a result, the lowest flashover voltage was obtained in positive polarity of SF6 content D=3%. In corona extension obtained using ultra-high speed camera, the peculiar differences were observed on the corona extension process. The corona extension increased, and rapid flashover were observed in D=3%. Using CCD camera, the small corona were detected from backside electrode.

Measurement of electron drift, diffusion, and effective ionization coefficients in the SF6–CHF3 and SF6–CF4 gas mixtures

The electron drift velocity, the longitudinal diffusion coefficient, the effective ionization coefficient and the limiting field strength for mixtures of SF6 with CHF3 and CF4 have been measured with a pulsed Townsend technique. The overall density-reduced electric field strength, E/N, could be varied between 60 and 520 Td (1 Td = 10−17 V cm2), while the SF6 content in the gas mixtures was varied over the range 1–50%. We have found that the electron drift velocity and the density-normalized longitudinal diffusion coefficient vary weakly with the amount of SF6 in the mixture. In contrast, the effective ionization coefficient shows a strong dependence with the SF6 content, becoming more electronegative as the amount of SF6 is increased. The measured limiting field strength for these two mixtures is lower than those for SF6–N2. To our knowledge, no previous ionization or electron transport data for these mixtures have been published.

Negative ion motion in the mixtures of SF6 with CF4 and CH4-Ar.

This paper deals with the measurement of the mobility of negative ions in the mixtures of SF6 with CF4 and the CH4-Ar (50:50) binary mixture with SF6 contents up to 50%. The pulsed Townsend technique was used to observe the integrated ionic avalanches over a range of the density-reduced electric field E/N for which ionization is either negligible or absent, and attachment processes are significant, leading to the formation of mostly SF-6. The E/N range of measurement was from 1 to 70 Td (1 Td=10(-17) V cm(2)), over which the measured mobilities were found to be almost constant. The mobility of the negative ions was also measured for trace amounts of SF6 in CH4 and Ar and 1% CF4, thereby providing a good value of the mobility of SF-6 in these pure gases, in order to test the measured mobilities with Blanc's law. We have found good agreement, within quoted experimental uncertainties, between calculated and measured values.