SF6 Atmosphere Research Articles

High-Performance H2S Sensors to Detect SF6 Leakage.

Detecting H2S in oxygen-deficient conditions is vital for identifying leaks in SF6-insulated electrical equipment. Current infrared-based detection methods are expensive and sensitive to environmental conditions, highlighting the necessity for cost-effective and stable gas sensors. Existing gas sensors based on semiconducting metal oxides (SMOXs) are limited by redox reactions with oxygen and require high operating temperatures. Here, we introduce a room-temperature (RT) H2S sensor for oxygen-deficient environments using the intrinsic conducting two-dimensional (2D) metal-organic framework (MOF), Co1.8Ni1.2(hexaiminotriphenylene)2 [Co1.8Ni1.2(HITP)2], overcoming the limitations of SMOX gas sensors. Remarkably, Co1.8Ni1.2(HITP)2 sensors exhibit exceptional selectivity for H2S with negligible cross-responses and a sensitivity drift of less than 4.13% in an SF6 atmosphere over 60 days. The Co1.8Ni1.2(HITP)2 gas sensor shows significant promise for real-time and stable monitoring of H2S gas in oxygen-deficient environments.

Room-temperature NH3 gas sensing of S-hyperdoped silicon: Optimization through substrate resistivity

Sulfur-hyperdoped black silicon (S-BSi) prepared by femtosecond laser-assisted etching in SF6 atmosphere has dual characteristics of large specific surface area and super-doped impurities, and its physics and applications have attracted extensive attention. The room-temperature NH3 gas sensing capability of the samples is studied in the conductance mode. The S-BSi-based sensors exhibit a response to NH3 gas. Interestingly, their responsivity varies with the substrate resistance, and the sensor based on an appropriate substrate resistance shows the optimal responsivity. Additionally, the device demonstrates fast response and recovery speed, as well as good selectivity. The evolution of the responsivity and response/recovery time is recorded with natural aging for two months, showing acceptable long-term durability. The mechanism by which the responsivity of S-BSi-based sensors varies with resistivity is discussed. Based on this mechanism, there is an optimal substrate resistivity that maximizes the responsivity. The results show that S-BSi is a potential material for the fabrication of conductivity gas sensor with good NH3 detection performance.

Decomposition of SF6 by a plasma focus device

Decomposition of SF6 by a low energy plasma focus (PF) device has been investigated. The PF device performance in SF6 atmosphere was weaker than other gases such as argon, methane and nitrogen. Tungsten and vanadium anode tips were employed to generate thin layers of deposited products on substrates with different number of focused shots in SF6 atmosphere. The proper choice of the anode tip and substrate material has a direct effect on SF6 decomposition and generation of desire products. Free fluorine atoms and ions are produced during discharges that react with housing chamber and corrode the device materials such as Pyrex insulator, 304 stainless steel chamber and many more. Field emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS) were utilized to identify the deposited solid film products.

Hyperdoping-regulated room-temperature NO2 gas sensing performances of black silicon based on lateral photovoltaic effect

Black silicon co-hyperdoped with sulfur and nitrogen in different ratios is prepared by femtosecond laser-assisted chemical etching in the mixed atmosphere of SF6 and NF3 with varying gas pressure ratios. Their room-temperature NO2 gas sensing capability is studied systematically, in which the photocurrent as a readout signal is generated by the lateral photovoltaic effect of black silicon under an asymmetrical light illumination. These co-hyperdoped black silicon exhibits high response, fast response/recovery, ultrawide detection range from 29 ppb to 2000 ppm, excellent selectivity and acceptable long-term durability over 3 months. Moreover, NO2 gas sensing performances are effectively tuned or optimized by deliberately changing the co-doping ratio of sulfur and nitrogen, as different photovoltaic characteristics are induced by changes in morphology and structural defects resulting from different hyperdoping. Specifically, ultra-high relative gas response (∼3955%@20 ppm NO2) and superior selectivity are obtained at the SF6/NF3 pressure ratio of 56/14, while faster response/recovery time (17 s/ 47 s @ 20 ppm NO2) and response photocurrent with a weaker disturbance by humidity are given by the samples with SF6/NF3 of 7/63 and 63/7, respectively. Therefore, such black silicon material has good potential to meet different application needs.

Classification of different post-hyperdoping treatments for enhanced crystallinity of IR-sensitive femtosecond-laser processed silicon

Crystalline silicon becomes photosensitive and absorbing in the sub-bandgap spectral region if hyperdoped, i.e. supersaturated to a level above the solubility limit in thermal equilibrium, by deep impurities, such as sulfur. Here we apply femtosecond laserpulses to crystalline silicon in a SF6 atmosphere as hyperdoping method. The ultrashort laser pulses cause crystal damage and amorphous phases that would decrease quantum efficiency in a potential optoelectronic device application. We investigate five different post-hyperdoping methods: three etching techniques (ion beam etching IBE, reactive ion etching RIE, and wet-chemical etching HNA) as well as ns-annealing and minute-long thermal annealing and study their impact on crystallinity by Raman spectroscopy and absorptance in the visible and near infrared wavelength regime. We use femtosecond laser hyperdoped silicon (fs-hSi) with two different levels of surface roughness to study a potential dependence on the impact of post-treatments. In our investigation, ns-annealing leads to the best results, characterized by a high Raman crystallinity and a high remaining absorptance in the sub-bandgap spectral region of silicon. Within the used etching methods IBE outperforms the other etching methods above a certain level of fs-hSi surface roughness. We relate this to the specific anisotropic material removal behavior of the IBE technique and back this up with simulations of the effect of the various etching processes.

Wafer-scale synthesis of a morphologically controllable silicon ordered array as a platform and its SERS performance.

In this study, we fabricated four different structures using single crystal silicon wafers for surface-enhanced Raman spectroscopy (SERS) applications. For single crystal silicon, different crystal orientations exhibit different physical and chemical properties. In chemical etching, the etching speed of different crystal planes also exhibits significant differences. We first used reactive ion etching (RIE) to process the surface of the substrate, and subsequently used KOH anisotropic wet etching technology to modify the surface of silicon wafers with different crystal orientations and produced four different results. In the RIE stage in an O2 atmosphere, the (110) silicon wafer formed a hexagonal hole structure, and the (100) silicon wafer formed an inverted pyramid hole structure; however, in the RIE-treated substrates in O2 and SF6 atmosphere, the (110) silicon wafer formed a pyramid with a diamond-shaped base, and the (100) silicon wafer showed a columnar structure with a "straw hat" at the top. The formation mechanisms of these four structures were elucidated. We also performed structure-related SERS characterizations of the four different structures and compared their performance differences.

Study on Adhesion Reliability and Particle Inhibition of Epoxy Resin Coating in DC GIL after Thermal Ageing Experiment

The movement of metal particles is effectively inhibited when a DC GIL’s (gas-insulated transmission line) electrode is coated. This article aims to study the problem of coating falling off during GIL operation and the change in the particle-inhibitory effect after coating ageing. A closed constant temperature heating platform and a particle motion observation platform in an SF6 atmosphere were built. The epoxy resin coating was aged for 1200 h in an SF6 atmosphere at 160 °C. Pull-off and particle-lifting experiments were carried out for the samples. The experimental results show that the adhesion of the coating changes from rapid decline to slow decline, decreasing by 35.5%. The lifting voltage of particle startup gradually decreased, and the inhibition effect on particle activity decreased from 45.89% to 35.7%. The coating mass loss rate and surface morphology were tested to explain adhesion decline. Then, the dielectric constant, electrical conductivity and adhesion work between the coating and the particles, which are the key factors affecting the lifting of the particles, were measured. Compared with the adhesion work, the dielectric constant of the coating has a greater impact on the starting voltage. The dielectric constant of the coating decreases by 24.07%, and the conductivity increases, which weakens its inhibition of particles. After ageing, due to the decrease in the dielectric constant and the increase in the conductivity of the coating, the inhibition of coating on particles is weakened. This paper reveals the changes in coating adhesion reliability and particle inhibition in DC GIL, providing guidance for using and improving the performance of coatings in practical engineering.

Corrosion reaction kinetics and high‐temperature corrosion testing of contact element strips in ultra‐high voltage bushing based on the phase‐field method

For the structural characteristics of power transmission equipment such as bushings, the plug-in structure is mostly used for current-carrying connections owing to the large size and long distance at high voltage levels. The contact element strip is the key component of the current-carrying structure, carrying the combined electrical–thermal effect. Its electrical current-carrying performance is important. In recent years, discharge failures caused by the overheating failure of electrical connections have occurred from time to time. However, there have been relatively few studies on the failure mechanism of the electrical connection structures of high-voltage power transmission equipment, such as bushings. In this study, from the perspective of corrosion reaction kinetics, a two-dimensional phase-field model of high-temperature corrosion is established to obtain the carrier concentration distribution in the film. The effects of temperature, gas partial pressure, film thickness, and applied electric field on the growth rate of corrosion reaction film were calculated using the finite element method. At the same time, with the contact element strips used in the bushing as the test object, a corrosion test was carried out in SF6 atmosphere to simulate the high-temperature corrosion caused by the uneven current carrying of the contacts in actual operating conditions. Combined with the effect of temperature on the film growth rate, the contact area was the most severely corroded location. The simulation can provide a qualitative analysis for the contact degradation test and a new method for studying the factors affecting the corrosion mechanism of contact element strips.

Large-Scale Wideband Light-Trapping Black Silicon Textured by Laser Inducing Assisted with Laser Cleaning in Ambient Air.

Black silicon, which is an attractive material due to its optical properties, is prepared mainly by laser inducing in an SF6 atmosphere. Considering the effect of SF6 gas on the environment and human health, here we propose an efficient, economical, and green approach to process large-scale black silicon. In the wavelength range of 0.3–2.5 µm, the role of air could replace SF6 gas to texture black silicon by laser inducing with appropriate processing parameters. Then, to extend the working window of its excellent light-trapping status, laser-plasma shockwave cleaning was introduced to eliminate the deposition and improve the structures and morphology. The results revealed that the micro-nano structures became higher, denser, and more uniform with increasing cleaning times and deteriorating cleaning velocity, which compensated for the role of S atoms from the ambient SF6. Moreover, absorptance above 85% in the wavelength range of 0.3–15 µm was realized using our method. The effect of scanning pitch between adjacent rows on large-scale black silicon was also discussed. Our method realized the ultrahigh absorptance of large-scale black silicon fabricated in air from visible to mid-infrared, which is of significance in the field of optoelectronic devices.

All-in-One High Output Rotary Electrostatic Nanogenerators Based on Charge Pumping and Voltage Multiplying.

Electrostatic generators as a kind of effective energy harvesters have attracted intensive attention. However, the output of the generators is highly dependent on the charge density. Here, we demonstrate an all-in-one rotary electrostatic nanogenerator based on the charge pumping and voltage multiplying strategy (CV-ESG), which achieves high output power in SF6 atmosphere. CV-ESG integrates a pumping electret generator, a main generator, and a voltage multiplying and stabilization circuit on a pair of rotator and stator. We analyze the breakdown effect and its influence on the insulating layer covered on the electrodes through experiments. The breakdown voltage is high in SF6 atmosphere, and the maximum average power of CV-ESG in SF6 is 37.29 mW at 750 rpm, which is 3.29 times that in air. There is no surface friction in CV-ESG, which avoids abrasion and reduces friction damping. And the pumping generator is integrated with the main generator, making CV-ESG compact and easy to assemble. This work provides the design strategy for a high-power rotary electrostatic generator with good performance.

Dielectric Surface Flashover under Long-Term Repetitive Microsecond Pulses in Compressed Gas Environment.

As a key component of a high-power microwave (HPM) system, a multi-gap gas switch (MGS) has recently developed insulation failure due to surface flashover. Although design criteria for surface insulation have been put forward, it is still not clear how the insulation in this case deteriorated under long-term repetitive microsecond pulses (RMPs). In this paper, flashover experiments under RMPs were carried out on various dielectric surfaces between parallel-plane electrodes in SF6 and air atmospheres, respectively. Based on tests of the surface insulation lifetime (SIL), an empirical formula for SIL prediction is proposed with variations of insulator work coefficient λ, which is a more suitable parameter to characterize SIL under RMPs. Due of the accumulation effect, the relationship between E/p and ptdelay varies with the pulse repetitive frequency (PRF) and SIL recovery capability decreases with an increase in PRF and surface deterioration is exacerbated during successive flashovers. It is concluded that the flashover path plays a crucial role in surface insulation performance under RMPs due to the photoemission induced by ultraviolet (UV) radiation, signifying the necessity of reducing surface paths in future designs as well as the improvement of surface insulation.

Fabrication of freestanding Pt nanowires for use as thermal anemometry probes in turbulence measurements

We report a robust fabrication method for patterning freestanding Pt nanowires for use as thermal anemometry probes for small-scale turbulence measurements. Using e-beam lithography, high aspect ratio Pt nanowires (~300 nm width, ~70 µm length, ~100 nm thickness) were patterned on the surface of oxidized silicon (Si) wafers. Combining wet etching processes with dry etching processes, these Pt nanowires were successfully released, rendering them freestanding between two silicon dioxide (SiO2) beams supported on Si cantilevers. Moreover, the unique design of the bridge holding the device allowed gentle release of the device without damaging the Pt nanowires. The total fabrication time was minimized by restricting the use of e-beam lithography to the patterning of the Pt nanowires, while standard photolithography was employed for other parts of the devices. We demonstrate that the fabricated sensors are suitable for turbulence measurements when operated in constant-current mode. A robust calibration between the output voltage and the fluid velocity was established over the velocity range from 0.5 to 5 m s−1 in a SF6 atmosphere at a pressure of 2 bar and a temperature of 21 °C. The sensing signal from the nanowires showed negligible drift over a period of several hours. Moreover, we confirmed that the nanowires can withstand high dynamic pressures by testing them in air at room temperature for velocities up to 55 m s−1.

Highly-efficient conversion of SF6 via an eight-electron transfer process in lithium batteries

Abstract Li-SF6 battery has been proposed as a novel energy conversion device for both reducing greenhouse-effect SF6 and potentially providing large specific energy. However, the real discharge process of SF6 remains ambiguous with fast cathode passivation and large polarization. Here we report that the Li-SF6 chemistry is able to deliver an unprecedented capacity of >20,000 mA h gc−1 with reduced polarization and excellent rate capability by using a low-cost, high-surface-area porous carbon as cathode host. More importantly, by adjusting the SF6 atmosphere, an approximately eight-electron transfer process with predominant products of LiF and Li2S is realized via two voltage steps, whereas previously only a little Li2S could be found with one plateau appearing. The eight-electron process leads to a high energy density about 2,300 W h kg−1 based on the mass of carbon and discharge products, which sheds light on SF6 as a promising high-energy-density cathode.

Examination of the surface layer composition formed on ML19 magnesium alloy during melting at protective gas atmospheres

Currently the most common method of the magnesium alloys flux free melting is the melting under the gas protective atmosphere. This atmosphere consists of inert carrier gas with low addition of active gas. The ML19 casting magnesium alloy contains Y and Nd that enough active. The interaction of such alloys with gas protective atmospheres is poorly studied and has serious practical importance. Sulfur hexafluoride (SF6) has a great influence on the global warming and because of that its application is limited. As a result, the number of countries cross over to HFC-R134a as the active gas. This paper presents the investigation of the effect of gas protective mixtures consisting of carrier gas (argon of nitrogen) and active gas (SF6 or HFC-134a) on the composition of protective layer formed on the surface of ML19 magnesium alloy melt. It was developed a special laboratory setup providing the contact of the protective gas mixture with the alloy during heating, melting and solidification of the samples and preventing the influence of the surrounding atmosphere. The loss of the alloying elements was negligible but in the case of using nitrogen as a carrier gas the Y and Nd content in alloy was lower than if the argon is used. If SF6 is used as an active gas, the Zr content in alloys was lower. Composition and thickness of oxide film that formed in both SF6 and HFC-R134a protective atmospheres are mostly the same. The surface film is consist of magnesium fluoride (MgF2) with admixtures of oxides, fluorides and nitrides of zirconium, yttrium and magnesium. The key difference of protective layer phase composition if HFC-R134a used as an active gas is presence of the large amount carbon in the form of compounds and in a free state. Additionally, it was established that using of HFC-134a in protective atmosphere requires more careful dosage given the fact of its percentage in the gas mixture of more than 1 vol.% leads to severe corrosion of the crucible inner surface during the melting.

High-Performance Free-Standing Flexible Photodetectors Based on Sulfur-Hyperdoped Ultrathin Silicon.

Flexible photodetectors (PDs) prepared with silicon-based materials have received considerable attention for their use in a wide range of portable and wearable applications. In this study, we present the first free-standing flexible PD based on sulfur-hyperdoped ultrathin silicon, which was fabricated using a femtosecond laser in a SF6 atmosphere. It is found that the fabricated device exhibits excellent performance of broadband photoresponse from 400 to 1200 nm, with a peak responsivity of 63.79 A/W @ 870 nm at a low bias voltage of -2 V, corresponding to an external quantum efficiency reaching 9092%, which surpasses most values reported for silicon-based flexible PDs. In addition, the device shows a fast response speed (rise time τr = 68 μs) and stable detection performance with good mechanical flexibility. The high-performance PD described here suggests a promising way in flexible applications for sensors, imaging systems, and optical communication systems.

Decomposition Characteristics of SF6 under Flashover Discharge on the Epoxy Resin Surface.

In this paper, the flashover discharging experiment was carried out on epoxy resin surface in an SF6 atmosphere under pin-plate electrodes, with the electrodes distance from 5 mm to 9 mm. The concentration of seven characteristic gases was detected, indicating that the concentration of SOF2 and CF4 was the two highest, followed by SO2, CO2, SO2F2, CS2, and H2S. Based on the changes in the concentration of the characteristic gases, a preliminary rule was proposed to predict the occurrence of flashover discharge on epoxy resin: When the concentration of SOF2 reaches twice of CF4 concentration, and the total concentration of both SOF2 and CF4 is much higher than that of H2S, a possible flashover discharge on the epoxy resin surface in SF6-infused electrical equipment occurs. Through the simulation of decomposition of epoxy resin, it has been revealed that H2O has different generation paths that can facilitate the formation of SOF2, finally influencing the concentration variation of the seven characteristic gases.

DFT-based study on H2S and SOF2 adsorption on Si-MoS2 monolayer

H2S and SOF2 gas are two typical decomposition products of SF6 gas under partial discharge condition. Based on the density functional theory (DFT), several doping structures of Si-MoS2 and adsorption structures of gas adsorbed Si-MoS2 were investigated. The electronic property, adsorption energy, charge transfer, and electron density difference were calculated to analyze the adsorption mechanism. Compared with the structure of pristine MoS2 monolayer, Si doping improves the surface activity of MoS2 monolayer. Moreover, the introduction of Si atom increases the effects of orbital hybridization between gas molecules and MoS2 monolayer, and promotes the charge transfer. Especially, SOF2 possesses a large adsorption energy with Si-MoS2 by crossing a small energy barrier. In addition, the good compatibility of Si-MoS2 with SF6 atmosphere, makes it a promising adsorbent for SF6 decomposition products removal.

Laser-ablation-assisted SF6 decomposition for extensive and controlled fluorination of graphene

We present a safe, clean, convenient and easy-to-control method for extensive fluorination of graphene using laser-ablation-assisted decomposition of gaseous SF6 molecules. The decomposition process is based on irradiation of a silicon target using a Nd:YAG laser (532 nm) in an SF6 atmosphere. The reactive species produced in the plume above the silicon target are consequently responsible for fluorination of graphene placed in proximity to the plume. The applicability of the proposed method is demonstrated on fluorination of CVD-grown graphene on copper foil. Samples with different fluorination levels were evaluated using both X-ray photoelectron spectroscopy and Raman spectroscopy. The influence of the applied number of laser pulses on the nature of the CF bonds, fluorine and sulphur concentrations, as well as graphene damage, is discussed. The method enables control of fluorine content by simple counting of laser pulses. We show that a fluorination level corresponding to a stoichiometry up to C1.4F can be achieved. The fluorination does not lead to the formation of CuF, in contrast to previously published approaches.

Gas-enhanced triboelectric nanogenerator based on fully-enclosed structure for energy harvesting and sensing

The output performance of triboelectric nanogenerators (TENGs) in various gas atmospheres can differ because of gaseous discharge. Here, a gas-enhanced enclosed cubic-TENG is designed and fabricated using the contact electrification between a fluorinated ethylene propylene thin film and a layer of conducting fabric. The effects of different gases including He, Ar, air, N2, CO2, CHF2Cl, and SF6 on the output performance of the cubic-TENG are systematically investigated. The highest voltage and current of 500 V and 11 µA, respectively, are obtained in the SF6 atmosphere and respectively represent increases of 67% and 120% compared with an air atmosphere. The output performance of the cubic-TENG is not affected by the ambient humidity. Finally, the cubic-TENG is demonstrated herein as an energy harvester and vibration sensor.

Research on photoelectric characteristics of (S, Se) co-doped silicon fabricated by femtosecond-laser irradiation

Micro-structured (S, Se) co-doped and Se-doped silicon are prepared via femtosecond laser irradiating Si coated with Si-Se bilayer films in the presence of SF6 and N2. The S-doped silicon is prepared by irradiating crystalline silicon surface via femtosecond laser in atmosphere of SF6. The samples are made with the same thickness of Se film and the same gas pressure of SF6 or N2. The surface morphology, optical and electronic properties of S-doped, Se-doped and (S, Se) co-doped silicon are studied systematically, which show that co-doping is beneficial to improve the doping concentration and photoresponse of doped silicon. The n+–n photodiodes are fabricated from the three doped silicon annealed at 775 K for 1 h and (S, Se) co-doped silicon photodiode are fabricated for the first time. At 1064 nm, the responsivity achieves 1.60 A/W at −12 V reverse bias for the (S, Se) co-doped silicon photodiode, which is higher than that of S or Se doped silicon photodiode. This investigation demonstrates that (S, Se) co-doping has potential to improve the property of black silicon and facilitate its application in optoelectronic devices.