Template:Short description Template:Cs1 config Template:Redirect Template:Use American English Template:Chembox Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non-flammable, and non-toxic gas. Template:Chem has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule.Template:Cn
Typical for a nonpolar gas, Template:Chem is poorly soluble in water but quite soluble in nonpolar organic solvents. It has a density of 6.12 g/L at sea level conditions, considerably higher than the density of air (1.225 g/L). It is generally stored and transported as a liquefied compressed gas.<ref>Template:Citation</ref>
Template:Chem has 23,500 times greater global warming potential (GWP) than Template:CO2 as a greenhouse gas (over a 100-year time-frame) but exists in relatively minor concentrations in the atmosphere. Its concentration in Earth's troposphere reached 11.50 parts per trillion (ppt) in October 2023, rising at 0.37 ppt/year.<ref name="sf6trend"/> The increase since 1980 is driven in large part by the expanding electric power sector, including fugitive emissions from banks of Template:Chem gas contained in its medium- and high-voltage switchgear. Uses in magnesium, aluminium, and electronics manufacturing also hastened atmospheric growth.<ref name="sf6b20" /> The 1997 Kyoto Protocol, which came into force in 2005, is supposed to limit emissions of this gas. In a somewhat nebulous way it has been included as part of the carbon emission trading scheme. In some countries this has led to the defunction of entire industries.<ref name="cim11">Template:Cite book</ref>
Synthesis and reactionsEdit
Template:See also Sulfur hexafluoride on Earth exists primarily as a synthetic industrial gas, but has also been found to occur naturally.<ref name="sf6nat">Template:Cite journal</ref>
Template:Chem can be prepared from the elements through exposure of [[Sulfur|Template:Chem]] to [[fluorine|Template:Chem]]. This was the method used by the discoverers Henri Moissan and Paul Lebeau in 1901. Some other sulfur fluorides are cogenerated, but these are removed by heating the mixture to disproportionate any [[Disulfur decafluoride|Template:Chem]] (which is highly toxic) and then scrubbing the product with NaOH to destroy remaining [[Sulfur tetrafluoride|Template:Chem]].Template:Clarify
Alternatively, using bromine, sulfur hexafluoride can be synthesized from SF4 and CoF3 at lower temperatures (e.g. 100 °C), as follows:<ref>Template:Cite conference</ref>
There is virtually no reaction chemistry for Template:Chem. A main contribution to the inertness of SF6 is the steric hindrance of the sulfur atom, whereas its heavier group 16 counterparts, such as SeF6 are more reactive than SF6 as a result of less steric hindrance.<ref>Template:Cite book</ref> It does not react with molten sodium below its boiling point,<ref>Template:Cite book Extract of page 160</ref> but reacts exothermically with lithium. As a result of its inertness, Template:Chem has an atmospheric lifetime of around 3200 years, and no significant environmental sinks other than the ocean.<ref>Template:Cite journal</ref>
ApplicationsEdit
By 2000, the electrical power industry is estimated to use about 80% of the sulfur hexafluoride produced, mostly as a gaseous dielectric medium.<ref>Template:Cite journal</ref> Other main uses as of 2015 included a silicon etchant for semiconductor manufacturing, and an inert gas for the casting of magnesium.<ref>Template:Cite journal</ref>
Dielectric mediumEdit
Template:Chem is used in the electrical industry as a gaseous dielectric medium for high-voltage sulfur hexafluoride circuit breakers, switchgear, and other electrical equipment, often replacing oil-filled circuit breakers (OCBs) that can contain harmful polychlorinated biphenyls (PCBs). Template:Chem gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. The high dielectric strength is a result of the gas's high electronegativity and density. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room.
Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment. Exposure to an arc chemically breaks down Template:Chem though most of the decomposition products tend to quickly re-form Template:Chem, a process termed "self-healing".<ref>Template:Citation</ref> Arcing or corona can produce disulfur decafluoride ([[Disulfur decafluoride|Template:Chem]]), a highly toxic gas, with toxicity similar to phosgene. Template:Chem was considered a potential chemical warfare agent in World War II because it does not produce lacrimation or skin irritation, thus providing little warning of exposure.
Template:Chem is also commonly encountered as a high voltage dielectric in the high voltage supplies of particle accelerators, such as Van de Graaff generators and Pelletrons and high voltage transmission electron microscopes.
Alternatives to Template:Chem as a dielectric gas include several fluoroketones.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite book</ref> Compact GIS technology that combines vacuum switching with clean air insulation has been introduced for a subset of applications up to 420 kV.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Medical useEdit
Template:Chem is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations<ref>Template:Cite book</ref> in the form of a gas bubble. It is inert in the vitreous chamber.<ref>Template:Cite book</ref> The bubble initially doubles its volume in 36 hours due to oxygen and nitrogen entering it, before being absorbed in the blood in 10–14 days.<ref>Template:Cite journal</ref>
Template:Chem is used as a contrast agent for ultrasound imaging. Sulfur hexafluoride microbubbles are administered in solution through injection into a peripheral vein. These microbubbles enhance the visibility of blood vessels to ultrasound. This application has been used to examine the vascularity of tumours.<ref name="pmid18376462">Template:Cite journal</ref> It remains visible in the blood for 3 to 8 minutes, and is exhaled by the lungs.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Tracer compoundEdit
Sulfur hexafluoride was the tracer gas used in the first roadway air dispersion model calibration; this research program was sponsored by the U.S. Environmental Protection Agency and conducted in Sunnyvale, California on U.S. Highway 101.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Gaseous Template:Chem is used as a tracer gas in short-term experiments of ventilation efficiency in buildings and indoor enclosures, and for determining infiltration rates. Two major factors recommend its use: its concentration can be measured with satisfactory accuracy at very low concentrations, and the Earth's atmosphere has a negligible concentration of Template:Chem.
Sulfur hexafluoride was used as a non-toxic test gas in an experiment at St John's Wood tube station in London, United Kingdom on 25 March 2007.<ref>Template:Cite news</ref> The gas was released throughout the station, and monitored as it drifted around. The purpose of the experiment, which had been announced earlier in March by the Secretary of State for Transport Douglas Alexander, was to investigate how toxic gas might spread throughout London Underground stations and buildings during a terrorist attack.
Sulfur hexafluoride is also routinely used as a tracer gas in laboratory fume hood containment testing. The gas is used in the final stage of ASHRAE 110 fume hood qualification. A plume of gas is generated inside of the fume hood and a battery of tests are performed while a gas analyzer arranged outside of the hood samples for SF6 to verify the containment properties of the fume hood.
It has been used successfully as a transient tracer in oceanography to study diapycnal mixing and air-sea gas exchange.<ref>Template:Cite journal</ref> The concentration of sulfur hexafluoride in seawater (typically on the order of femtomoles per kilogram<ref>J.L. Bullister and T. Tanhua, 2010. "Sampling and measurement of chlorofluorocarbons and sulfur hexafluoride in seawater". IOCCP Report No. 14. http://www.go-ship.org/HydroMan.html</ref>) has been classified by the international oceanography community as a "level one" measurement, denoting the highest priority data for observing ocean changes.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Other usesEdit
- The magnesium industry uses Template:Chem as an inert "cover gas" to prevent oxidation during casting,<ref name=bartos02>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> and other processes including smelting.<ref name="ayres00">Template:Cite news</ref> Once the largest user, consumption has declined greatly with capture and recycling.<ref name="sf6b20" />
- Insulated glazing windows have used it as a filler to improve their thermal and acoustic insulation performance.<ref name="ecofys">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref><ref>Template:Cite book</ref>
- Template:Chem plasma is used in the semiconductor industry as an etchant in processes such as deep reactive-ion etching. A small fraction of the Template:Chem breaks down in the plasma into sulfur and fluorine, with the fluorine ions performing a chemical reaction with silicon.<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>
- Tires filled with it take longer to deflate from diffusion through rubber due to the larger molecule size.<ref name="ecofys" />
- Nike likewise used it to obtain a patent and to fill the cushion bags in all of their "Air"-branded shoes from 1992 to 2006.<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> 277 tons was used during the peak in 1997.<ref name="ecofys" />
- The United States Navy's Mark 50 torpedo closed Rankine-cycle propulsion system is powered by sulfur hexafluoride in an exothermic reaction with solid lithium.<ref>Template:Cite journal</ref>
- Waveguides in high-power microwave systems are pressurized with it. The gas electrically insulates the waveguide, preventing internal arcing.
- Electrostatic loudspeakers have used it because of its high dielectric strength and high molecular weight.<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>
- Disulfur decafluoride, a chemical weapon, is produced with it as a feedstock.
- For entertainment purposes, when breathed, Template:Chem causes the voice to become significantly deeper, due to its density being so much higher than air. This phenomenon is related to the more well-known effect of breathing low-density helium, which causes someone's voice to become much higher. Both of these effects should only be attempted with caution as these gases displace oxygen that the lungs are attempting to extract from the air. Sulfur hexafluoride is also mildly anesthetic.<ref>Template:Cite journal</ref><ref>Template:Cite AV media</ref>
- For science demonstrations / magic as "invisible water" since a light foil boat can be floated in a tank, as will an air-filled balloon.
- It is used for benchmark and calibration measurements in Associative and Dissociative Electron Attachment (DEA) experiments<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Greenhouse gasEdit
- SF6 mm.png
Sulfur hexafluoride (SF6) measured by the Advanced Global Atmospheric Gases Experiment (AGAGE) in the lower atmosphere (troposphere) at stations around the world. Abundances are given as pollution free monthly mean mole fractions in parts-per-trillion.
- AGAGE sulfur hexafluroride growth.png
main}}. File:CC-BY icon.svg Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.</ref>
- Halogenated gas concentrations 1978-present.png
Atmospheric concentration of SF6 vs. similar man-made gases (right graph). Note the log scale.
According to the Intergovernmental Panel on Climate Change, Template:Chem is the most potent greenhouse gas. Its global warming potential of 23,900 times that of [[carbon dioxide|Template:Chem]] when compared over a 100-year period.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Sulfur hexafluoride is inert in the troposphere and stratosphere and is extremely long-lived, with an estimated atmospheric lifetime of 800–3,200 years.<ref>Template:Cite journal</ref>
Measurements of SF6 show that its global average mixing ratio has increased from a steady base of about 54 parts per quadrillion<ref name="sf6nat" /> prior to industrialization, to over 11.5 parts per trillion (ppt) as of October 2023, and is increasing by about 0.4 ppt (3.5%) per year.<ref name="sf6trend">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite FTP</ref> Average global SF6 concentrations increased by about 7% per year during the 1980s and 1990s, mostly as the result of its use in magnesium production, and by electrical utilities and electronics manufacturers. Given the small amounts of SF6 released compared to carbon dioxide, its overall individual contribution to global warming is estimated to be less than 0.2%,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> however the collective contribution of it and similar man-made halogenated gases has reached about 10% as of 2020.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Alternatives are being tested.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>
In Europe, Template:Chem falls under the F-Gas directive which ban or control its use for several applications.<ref name="sfphy">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Since 1 January 2006, Template:Chem is banned as a tracer gas and in all applications except high-voltage switchgear.<ref>Template:Cite news</ref> It was reported in 2013 that a three-year effort by the United States Department of Energy to identify and fix leaks at its laboratories in the United States such as the Princeton Plasma Physics Laboratory, where the gas is used as a high voltage insulator, had been productive, cutting annual leaks by Template:Convert. This was done by comparing purchases with inventory, assuming the difference was leaked, then locating and fixing the leaks.<ref name=NYT61313>Template:Cite news</ref>
Physiological effects and precautionsEdit
Sulfur hexafluoride is a nontoxic gas, but by displacing oxygen in the lungs, it also carries the risk of asphyxia if too much is inhaled.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Since it is more dense than air, a substantial quantity of gas, when released, will settle in low-lying areas and present a significant risk of asphyxiation if the area is entered. That is particularly relevant to its use as an insulator in electrical equipment since workers may be in trenches or pits below equipment containing Template:Chem.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
As with all gases, the density of Template:Chem affects the resonance frequencies of the vocal tract, thus changing drastically the vocal sound qualities, or timbre, of those who inhale it. It does not affect the vibrations of the vocal folds. The density of sulfur hexafluoride is relatively high at room temperature and pressure due to the gas's large molar mass. Unlike helium, which has a molar mass of about 4 g/mol and pitches the voice up, Template:Chem has a molar mass of about 146 g/mol, and the speed of sound through the gas is about 134 m/s at room temperature, pitching the voice down. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol which leads to a speed of sound of 343 m/s.<ref name="Wolfe">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Sulfur hexafluoride has an anesthetic potency slightly lower than nitrous oxide;<ref>Template:Cite book</ref> it is classified as a mild anesthetic.<ref>Template:Cite journal</ref>
See alsoEdit
- Selenium hexafluoride
- Tellurium hexafluoride
- Uranium hexafluoride
- Hypervalent molecule
- Halocarbon—another group of major greenhouse gases
- Trifluoromethylsulfur pentafluoride, a similar gas
ReferencesEdit
Further readingEdit
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- Template:Cite book
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- SF6 Reduction Partnership for Electric Power Systems
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External linksEdit
- Fluoride and compounds fact sheet— National Pollutant Inventory
- High GWP Gases and Climate Change from the U.S. EPA website
- International Conference on SF6 and the Environment (related archive)
- CDC - NIOSH Pocket Guide to Chemical Hazards
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