SF6 Gas Mixtures Research Articles

Gas gains over 104 and optimisation using 55Fe X-rays in low pressure SF6 with a novel Multi-Mesh ThGEM for directional dark matter searches

The Negative Ion Drift (NID) gas SF6 has favourable properties for track reconstruction in directional Dark Matter (DM) searches utilising low pressure gaseous Time Projection Chambers (TPCs). However, the electronegative nature of the gas means that it is more difficult to achieve significant gas gains with regular Thick Gaseous Electron Multipliers (ThGEMs). Typically, the maximum attainable gas gain in SF6 and other Negative Ion (NI) gas mixtures, previously achieved with an 55Fe X-ray source or electron beam, is on the order of 103 [1,2,3,4]; whereas electron drift gases like CF4 and similar mixtures are readily capable of reaching gas gains on the order of 104 or greater [5,9,7,8,6]. In this paper, a novel two stage Multi-Mesh ThGEM (MMThGEM) structure is presented. The MMThGEM was used to amplify charge liberated by an 55Fe X-ray source in 40 Torr of SF6. By expanding on previously demonstrated results [10], the device was pushed to its sparking limit and stable gas gains up to ˜50000 were observed. The device was further optimised by varying the field strengths of both the collection and transfer regions in isolation. Following this optimisation procedure, the device was able to produce a maximum stable gas gain of ˜90000. These results demonstrate an order of magnitude improvement in gain with the NID gas over previously reported values and ultimately benefits the sensitivity of a NITPC to low energy recoils in the context of a directional DM search.

Evaluation of the interruption capability of SF6 gas mixtures based on one-dimensional arc model

Evaluation of the interruption capability of SF6 gas mixtures based on one-dimensional arc model

Trend of Sulfur Hexafluoride (SF6) Gas Mixtures and Other Alternatives as Insulation Gas: an Overview

Research works are actively ongoing to seek a solution for reducing the usage of sulfur hexafluoride (SF6) as a gas insulator in high-voltage equipment since it was listed as one of the greenhouse gases under the Kyoto Protocol. However, manufacturers still in favor of using SF6 due to its excellent insulation and arc-quenching properties. One way to reduce SF6 usage is to mix it with other gases or replace it with alternative gas mixtures. The research status of SF6 gas mixtures and other potential substitutes is summarized in this paper. Electrical insulation properties, physical and chemical mechanisms, and environmental considerations, along with the scope and limitations of such mixtures were analyzed. The development prospects of SF6 gas mixtures were forecasted based on their best mixture ratio. Hybrid insulation gases consisting of three compounds (two electronegative gases with one buffer gas) have shown to be an ideal compromise in terms of cost, distinguished low environmental impact, and high dielectric strength. Trifluoroiodomethane (CF3I) offers great advantages among other dielectric gases, since it exerts minimal greenhouse effects, and has a considerable liquefaction temperature and higher dielectric strength. Therefore, the application of CF3I for reducing the usage of SF6 in gas mixtures was highlighted. The fundamental knowledge on the breakdown strength of the gas mixtures and their behavior under different electric fields need to be analyzed before any recommendations can be made.

Molybdenum nanopillar arrays: Fabrication and engineering

We report on the fabrication of molybdenum (Mo) nanopillar (NP) arrays with NP diameters down to 75 nm by means of deep-reactive ion etching at cryogenic temperatures. A variable-thickness Mo metal layer sputtered onto a Si3N4/Si substrate makes possible NPs with different lengths in a controllable manner. We demonstrate how our fabrication strategy leads to tunable cross-sections with different geometries, including hexagonal, cylindrical, square and triangular shapes, by using electron beam lithography on hydrogen silsesquioxane negative tone resist. To ensure well-defined facets and surfaces, we employ deep-reactive ion etching in a gas mixture of SF6 and O2 at cryogenic temperatures in an inductively coupled plasma reactive ion etching (ICP-RIE) system. These results represent an attractive route towards the realization of high-density Mo NP arrays for applications from nanoelectronics to quantum sensing and hydrogen evolution reaction catalysis.

Room temperature plasma-etching and surface passivation of far-ultraviolet Al mirrors using electron beam generated plasmas

The development of optical systems operating in the far ultraviolet range (FUV, λ=100-200 nm) is limited by the efficiency of passivated aluminum (Al) mirrors. Although it is presently possible to obtain high-reflectivity FUV mirrors through physical vapor deposition, the process involves deposition with substrates at high temperatures, which is technically challenging for large optical elements. A novel passivation procedure for bare Al mirrors is reported. The treatment consisted of using a low-temperature electron-beam generated plasma produced in a gas mixture of Ar and SF6 to etch away the native oxide layer from the Al film, while simultaneously promoting the generation of a thin aluminum tri-fluoride (AlF3) layer on the Al surface. In the first section we analyze the effect of varying both ion energy and SF6 concentration on the FUV reflectance, thickness, composition, and surface morphology of the resulting AlF3 protective layers. In the second section, the reflectivity of samples is optimized at selected important FUV wavelengths for astronomical observations. Notably, samples attained state-of-the-art reflectances of 75% at 108.5 nm (He Lyman γ), 91% at 121.6 nm (H Lyman α), 90% at 130.4 nm (OI), and of 95% at 155.0 nm (C IV). The stability over time of these passivated mirrors is also investigated.

Predominant reaction products and dielectric breakdown properties of gas mixtures consisting of SF6 and ablation products of C2F4/BN in the temperature range of 300–3000 K

For a high voltage SF6 circuit breaker equipped with a boron nitride (BN)-filled polytetrafluoroethylene ((C2F4)) nozzle, the predominant reaction products in a high-temperature SF6 mixture with vapor decomposition of C2F4 and BN at temperatures of 300–3000 K are determined based on calculations of equilibrium composition. The calculation is performed by considering the molecular products obtained by chemical reactions with boron atoms (e.g. BN, BS, BC, BF, BF2 and BF3), nitrogen atoms (e.g. N2, N3, CN, NS, NF, NF2 and NF3) and carbon atoms (e.g. CS, CS2, C2F4, CF2, CF3 and CF4). The results obtained for the C2F4–BN molar fraction of 20%–80% reveal that BF3, N2 and CF4, the molecules originating from the nozzle material, are produced at high molar fractions over a range of 300–3000 K. Such an evaluation of the gas composition allows the derivation of a reduced collision ionization coefficient α/N and a reduced electron attachment coefficient η/N of the gas, considering various electron impact processes such as BF3 + e BF2 + F− and CF4 + e CF3 + F−. The evaluation subsequently enables us to determine a critical reduced electric field strength causing . The results suggest that the decomposed C2F4–BN mixture at of 80% causes the reduction of to 110 Td in the range of 1000–2000 K at a gas pressure of 1.0 MPa. This strength is 20 Td lower than that determined separately for the SF6 gas mixture with only 80% C2F4 vapor decomposition, because of the CF4 molar fraction reduction in the SF6/C2F4–BN mixture.

EXPERIMENTAL DETERMINATION OF FLASHOVER PERFORMANCE OF CYLINDRICAL SPACERS IN N2:SF6 GAS MIXTURE UNDER AC VOLTAGES FOR NON-UNIFORM FIELDS

The gas mixture N2:SF6 has been used in gas insulated switchgears, gas insulated transmission lines and gas insulated cables for years in order to reduce the use of SF6 gas in electrical system. By taking into account the insulation performance and environmental effect, it is advised that the use of N2:SF6 gas mixture in practical to lower the SF6 gas content as far as possible. The present study is undertaken using different N2:SF6 gas mixture ratio to determine the flashover characteristics under AC voltages. In this paper the effect of floating particles, use of different insulating spacers namely PMMA, PP and NYLON and their combination forms hybrid spacers has been studied in non-uniform electric field (needle – plane) electrode configuration. The surface flashover performances of individual and hybrid spacers have been compared in N2:SF6 gas mixture. The experimental results shows that conventional spacer PMMA show better performance than the other spacers (PP and NYLON) at the gas pressure of 0.18MPa. As the gas pressure increases further the flashover voltage values slightly decreases. The hybrid spacer PMMA – PP are more effective at lower pressure than other spacer combinations. With floating particle, PP shows better flashover at lower pressure and the performance of PMMA – NYLON is much better at lower pressure.

Outgassing and leak test of INO-ICAL RPC detectors

Glass Resistive Plate Chambers (RPCs) of size 2 m × 2 m, operated in avalanche mode will be used as an active detector element at the INO-ICAL experiment. The RPC system of the ICAL experiment will operate with a gas mixture of C2H2F4 (94.5%), isobutane (5%) and SF6 (0.5%). To fulfil the physics goals, about 29,000 RPCs will be used which are expected to last for 20 years or more. The quality and purity of the gas plays a vital role in the stable operation of RPC detectors. The presence of impurities in a gas mixture contribute towards the degradation of detector performance. The various assembly materials like glues, button spacers, frames, etc. used in the construction of the chamber may outgas and contaminate the gas mixture. We have performed the very first study to estimate the outgassing due to various materials used in the construction of INO RPCs. The present study includes the results obtained from gas chromatography showing the generation of impurities and dangerous radicals produced due to outgassing when RPC are operated in the cosmic stand. The study also includes a test of purity and effectiveness of the gas mixing system. Since a large amount of gas mixture is to be circulated inside the RPC during the operation of the ICAL detector, a proper leak test will help to minimize gas leaks reducing operational costs and atmospheric air pollution. A proper quantitative leak test is also performed on RPC after assembly by monitoring the absolute pressure inside the chamber, along with the atmospheric pressure and temperature to estimate the leakage rate.

Investigation of the performance of CF3I/c-C4F8/N2 and CF3I/c-C4F8/CO2 gas mixtures from electron transport parameters

CF3I gas mixtures have attracted considerable attention as potential environmentally-friendly alternatives to SF6 gas, owing to their excellent insulating performance. This paper attempts to study the CF3I ternary gas mixtures with c-C4F8 and buffer gases N2 and CO2 by considering dielectric strength from electron transport parameters based on the Boltzmann method and synergistic effect analysis, compared with SF6 gas mixtures. The results confirm that the critical electric field strength of CF3I/c-C4F8/70 % CO2 is greater than that of 30% SF6/70% CO2 when the CF3I content is greater than 17%. Moreover, a higher content of c-C4F8 decreases the sensitivity of gas mixtures to an electric field, and this phenomenon is more obvious in CF3I/c-C4F8/CO2 gas mixtures. The synergistic effects for CF3I/c-C4F8/70 % N2 were most obvious when the c-C4F8 content was approximately 20%, and for CF3I/c-C4F8/70 % CO2 when the c-C4F8 content was approximately 10%. On the basis of this research, CF3I/c-C4F8/70 % N2 shows better insulation performance when the c-C4F8 content is in the 15%–20% range. For CF3I/c-C4F8/70 % CO2, when the c-C4F8 content is in the 10%–15% range, the gas mixtures have excellent performance. Hence, these gas systems might be used as alternative gas mixtures to SF6 in high-voltage equipment.

Study of flow and control of gas mixture for the Resistive Plate Chamber (RPC)

The upcoming underground INO-ICAL experiment will be instrumented with 28,800 of RPCs (glass based electrode of 3 mm thick) of size (1.85× 1.74) m2, which are the active elements and the key goal of the ICAL is to precisely measure the neutrino(s) mass. The RPCs are operated in the avalanche mode, using a gas mixture of R134a (95.2%), I-Butane (4.5%) and SF6 (0.3%). The number of RPCs that would be used is large with a total volume of gas of ∼ 200 m3, a Closed Loop gas mixing System (CLS) is mandatory. The performance of the RPCs depends on various parameters namely, the environmental conditions such as atmospheric pressure, ambient temperature, humidity etc., flow rate of gas mixture into the RPCs, concentration of the gas mixture (should be maintained constant throughout the operation), quality of gas, the RPC input gas pressure, uniformity of conductive coating on the surface, uniform gas gap thickness, nozzle positions, electrode thickness, ageing etc. We have tried to achieve the optimum flow rate of gas mixture (few SCCM) into the RPCs in the CLS without deteriorating the performance of the RPC. A modest low flow rate is ideal due to the usage of glass as electrodes and low back ground radiation (low cosmic rays only) unlike CERN-CMS experiments where the electrodes used are of Bakelite and are operated under high radiations with flow rates of few litres per hour. In this paper we present the flow resistors that are fabricated (capillary) and are suitable for final ICAL RPCs, the primitive simulation studies done to understand the distribution of the flow of gas mixture inside an RPC and also the behaviour of gas flow with different position of the input gas nozzles of an RPC. We also conclude here with results of the safe operating pressure parameters required for the operation of ICAL RPC in the CLS.

Assessment of SF6 and N2 Gas Mixtures as a Dielectric Medium in a Gas Insulated Busduct

SF6 gas is used as a dielectric medium in Gas Insulated Systems; it has an excellent dielectric strength and high arc quenching property. Despite of having these properties SF6 gas has high GWP, thus it is required to reduce the usage of the gas. In this paper SF6 and N2 gas mixtures were used as an alternate to the SF6 gas. The movement of free moving metallic particles present in a GIB has been analyzed for both the pure SF6 gas and by using various percentages of N2 gas and SF6 gas as an insulating medium

Sub-20 nm Si fins with high aspect ratio via pattern transfer using fullerene-based spin-on-carbon hard masks

We report on a novel and simple pattern transfer process into Si via fullerene-based spin-on-carbon (SOC) hard masks in this work. Electron beam lithography and extreme ultraviolet interference lithography techniques are used to pattern high-resolution and dense lines on a resist/SOC bilayer. The patterns are subsequently transferred by a low-pressure O2 plasma etching (SOC) and reactive ion etching process with a gas mixture of SF6 and C4F8 (Si). Si sidewall trimming can be controlled by modifying the Si etching rate, achieving Si fins with dimension down to 15 nm half-pitch with aspect ratio as high as of 7:1.

Experimental research on insulation properties of C 6 F 12 O/N 2 and C 6 F 12 O/CO 2 gas mixtures

This study investigates the insulation performance of the new environmental friendly insulation gas C6F12O mixed with N2 and CO2 under the AC frequency voltage under 0.10–0.30 MPa. Firstly, the mixing ratio of C6F12O in the mixed gas is discussed by the gas phase state equation. The breakdown voltage of C6F12O gas mixtures under quasi-uniform electric field and the initial voltage of partial discharge in the highly non-uniform field are measured by the experimental method, and the insulation characteristics of SF6 gas mixtures are compared. The self-recovery performance of the gas mixture is analysed as well. Experimental results show that the mixing ratio of C6F12O is lower than 7% under 0.10 MPa, such that the operation requirements of the equipment can be satisfied. The breakdown voltage of 3% C6F12O gas mixtures is the same as that of 10% SF6 gas mixtures. The partial discharge inception voltage of 6% C6F12O gas mixtures is close to that of 10% SF6 gas mixtures. After 100 times of breakdown, the breakdown voltage of C6F12O gas mixtures does not decrease. Compared with N2, the breakdown voltage of C6F12O mixed with CO2 shows a more dispersive trend.

Diffuse discharges in SF 6 and mixtures of SF 6 with H 2 , formed by nanosecond voltage pulses in non‐uniform electric field

Results of experimental study of nanosecond diffuse discharge in SF6 and gas mixtures of SF6 with H2, D2 and C2H6 are presented. The aim of this work is to study parameters of discharge between two extended electrodes with a small radius of curvature in SF6 and SF6 with additives. It was shown that diffuse discharge can be formed in SF6 at elevated pressure between blade electrodes with length of 30 cm. It was also confirmed that in a sharply non-uniform electric field a beam of runaway electrons is generated and that the gap breakdown occurs due to ionisation waves which begin on electrodes with small radius of curvature. Laser action in the infrared spectral region was obtained in SF6–H2 (D2, C2H6) mixtures. The laser output up to 110 mJ was easily achieved which corresponds to ultimate intrinsic efficiency (with respect to deposited energy) of 10%.

A review on SF[formula omitted] substitute gases and research status of CF[formula omitted]I gases

SF6 gas is widely used in electric power apparatus as an insulation and arc-quenching medium. However, the application of SF6 gas faces two serious problems, including the heavy global warming effect and high toxicity of its decomposition products. So, scholars committed to finding environmental-friendly alternative to SF6 gas and the relevant research results have been obtained. As an insulating or extinguishing medium, conventional gases, SF6 gas mixtures and electronegative gases and mixed gases exhibit different strengths and disadvantages. The current research progress of the main alternative gas has been summarized in this paper. And that is summarized in detail in terms of physical and chemical properties, insulating properties, arc characteristics, mechanism and the scope and limitations of different alternative gas in engineering applications are analyzed. Meanwhile, we forecast the development prospects of environment-friendly gas insulation, and theoretically analyzed CF3I, which has the potential to replace SF6. Finally, we analyzed the current application feasibility and scope of CF3I, and pointed out the further research direction.

Technical aspects of gas insulated transmission lines and application of new insulating gases

This publication gives information about the design, testing and long term experience of AC gas-insulated transmission lines (AC GIL). AC GIL does not include any switching devices such as disconnectors, earthing switches or circuit breakers so that the main electric focus is on the insulation capabilities only. A new installation process for reducing GIL costs with a new welding technique and a mobile factory will be reported shortly. Currently pure SF 6 and N 2 / SF 6 gas mixtures are used for GIL, but research is being done concerning the application of alternative gases for insulation purpose to reduce the global warming potential (GWP). Based on the long term experience with AC GIL the analyzed gas mixtures are discussed and evaluated.

Features of the formation of non-vertical profiles on the surface of 4H-SiC by the reactive-ion etching

The features of the formation of non-vertical profiles on 4H-SiC by reactive-ion etching (RIE) using various masking coatings are studied. The formation of 4H-SiC mesa structures was carried out using automated airlock reactive-ion etching and plasma etching system “Caroline PE 15” with the ICP-source of plasma in a gas mixture of SF6, O2 and Ar. Using photoresist AZ4533 as a mask, mesa structures with a wall inclination angle of more than 130° were obtained at the etching rate of 4H-SiC was ∼0.5 μm/min. The developed technology of dry etching can be further used in the preparation of avalanche photodiodes or power electronics devices.

Breakdown Mechanism of Different Sulphur Hexafluoride Gas Mixtures

Sulphur hexafluoride (SF6) gas and SF6 gas mixtures are widely used in gas‐insulated systems (GIS) because they have good dielectric and thermal properties. Among the various gas mixtures investigated so far, SF6‐air, SF6‐CO2, and SF6‐N2 are most used gas mixtures. Also, these mixtures have much technical superiority in GIS. These are nontoxic and nonflammable gases, and they have less sensitivity to nonuniformities and higher liquefaction temperature at high pressure. GIS, switchgears, circuit breakers, and substations are systems that work with alternating current and have nonuniform electric fields. For this reason, in this study the breakdown mechanisms of 0.125, 0.5, 1, and 20% SF6 gas mixtures with rod‐plane configuration under AC voltage are investigated and explained breakdown mechanism.

Corona Identification of Impulse Voltage and Current

The corona effect in the power system has a dominant role in reducing the efficiency of the high-voltage lines. In this study, the current and voltage impulses, which are important in corona determination, have been examined in frequecy spectrogram base from test result. The frequency spectrogram graphs have been obtained from the current-time and voltage-time variation by Matlab program. With these graphs, frequency values have been found to provide an important clarification of the insulation performance of electrical equipment for reliable and accurate diagnosis. These values have been observed according to different conditions like pressure, polarity, and insulation ambient. This spectrogram analysis can be used to find the characteristic frequencies and eliminate the disturbance effect. The time between corona steps decrease when Sulphur hexafluoride (SF6) gas pressure is increased. The time of second corona is close to leader's discharge time. Corona starts early when SF6 amount is decreased in gas mixtures. The corona currents are large in the low SF6 gas mixtures, this situation is related to the high insulation of the SF6 gas. A feature which is dependent the frequency is not found.

Gravimetric Standard Gas Mixtures for Global Monitoring of Atmospheric SF6.

In this study, standard gas mixtures of SF6 in synthetic air were gravimetrically developed as a suite consisting of 6 mixtures with mole fractions of SF6 ranging from 5 to 15 pmol/mol. For precision in weighing the gas fills, an automatic weighing system coupled with a high sensitivity mass balance was used and a gravimetry precision of 3 mg (2σ) was achieved. Impurity profiles of the raw gases were determined by various analyzers. In particular, sub pmol/mol levels of SF6 in the matrix components (N2, O2, and Ar) were carefully measured, since the mole fraction of SF6 in the final step can be significantly biased by this trace amount of SF6 in the raw gases of the matrix components. Gravimetric dilution of SF6 by purity-assessed N2 was performed in 6 steps to achieve a mole fraction of 440 pmol/mol. In the final step, O2 and Ar were added to mimic the atmospheric composition. Gravimetric fractions of SF6 and the associated standard uncertainty in each step were computed according to the ISO 6142 and JCGM 100:2008, respectively, and validated experimentally. Eventually, the SF6 fraction uncertainty of the standard gas mixtures combined by uncertainties of gravimetric preparation and verification measurements were found to be nominally 0.08% at a 95% confidence interval. A comparison with independent calibration standards from NOAA shows agreement within 0.49%, satisfying the extended WMO compatibility goal, 0.05 ppt.