Tungsten

Tungsten, also known as wolfram, is a chemical element with the chemical symbol W and atomic number 74.

A steel-gray metal under standard conditions when uncombined, tungsten is found naturally on Earth only combined in chemical compounds. Its important ores include wolframite and scheelite. The free element is remarkable for its robust physical properties, especially the fact that it has the highest melting point of all the non-alloyed metals and the second highest of all the elements after carbon. Also remarkable is its very high density of 19.3 times that of water. This density is slightly more than that of uranium and 71% more than that of lead.[3] Tungsten with minor amounts of impurities is often brittle[4] and hard, making it difficult to work. However, very pure tungsten is more ductile, and can be cut with a hacksaw.[5]

The unalloyed elemental form is used mainly in electrical applications. Tungsten's many alloys have numerous applications, most notably in incandescent light bulb filaments, X-ray tubes (as both the filament and target), and superalloys. Tungsten's hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are most often used industrially as catalysts.

Tungsten is the only metal from the third transition series that is known to occur in biomolecules, and is the heaviest element known to be used by living organisms.[6][7]

History

In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite (at the time named tungstenite). Scheele and Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid.[8] In 1783, José and Fausto Elhuyar found an acid made from wolframite that was identical to tungstic acid. Later that year, in Spain, the brothers succeeded in isolating tungsten by reduction of this acid with charcoal, and they are credited with the discovery of the element.[9][10]

In World War II, tungsten played a significant role in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides, because of its deposits of wolframite ore. Tungsten's resistance to high temperatures and its strength in alloys made it an important raw material for the weaponry industry.[11]
Etymology

The name "tungsten" (from the Nordic tung sten, meaning "heavy stone") is used in English, French, and many other languages as the name of the element. Tungsten was the old Swedish name for the mineral scheelite. The other name "wolfram" (or "volfram"), used for example in most European (especially Germanic and Slavic) languages, is derived from the mineral wolframite, and this is also the origin of its chemical symbol, W.[5] The name "wolframite" is derived from German "wolf rahm" ("wolf soot" or "wolf cream"), the name given to tungsten by Johan Gottschalk Wallerius in 1747. This, in turn, derives from "Lupi spuma", the name Georg Agricola used for the element in 1546, which translates into English as "wolf's froth" or "cream" (the etymology is not entirely certain), and is a reference to the large amounts of tin consumed by the mineral during its extraction.[12]
Characteristics
Physical

In its raw form, tungsten is a steel-gray metal that is often brittle and hard to work, but, if pure, it can be worked easily.[5] It is worked by forging, drawing, extruding or sintering. Of all metals in pure form, tungsten has the highest melting point (3,422 °C, 6,192 °F), lowest vapor pressure and (at temperatures above 1,650 °C, 3,000 °F) the highest tensile strength.[13] Tungsten has the lowest coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and strength of tungsten are due to strong covalent bonds formed between tungsten atoms by the 5d electrons.[14] Alloying small quantities of tungsten with steel greatly increases its toughness.[3]
Isotopes
Main article: Isotopes of tungsten

Naturally occurring tungsten consists of five isotopes whose half-lives are so long that they can be considered stable. Theoretically, all five can decay into isotopes of element 72 (hafnium) by alpha emission, but only 180W has been observed[15] to do so with a half-life of (1.8 ± 0.2)×1018 yr; on average, this yields about two alpha decays of 180W in one gram of natural tungsten per year.[16] The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be[16]

182W, T1/2 > 8.3×1018 years

183W, T1/2 > 29×1018 years

184W, T1/2 > 13×1018 years

186W, T1/2 > 27×1018 years

Another 30 artificial radioisotopes of tungsten have been characterized, the most stable of which are 181W with a half-life of 121.2 days, 185W with a half-life of 75.1 days, 188W with a half-life of 69.4 days, 178W with a half-life of 21.6 days, and 187W with a half-life of 23.72 h.[16] All of the remaining radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes.[16] Tungsten also has 4 meta states, the most stable being 179mW (T½ 6.4 minutes).
Chemical

Main article: Tungsten compounds

Elemental tungsten resists attack by oxygen, acids, and alkalis.[17]

The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6.[17][18] Tungsten typically combines with oxygen to form the yellow tungstic oxide, WO3, which dissolves in aqueous alkaline solutions to form tungstate ions, WO2−4.

Tungsten carbides (W2C and WC) are produced by heating powdered tungsten with carbon and are some of the hardest carbides, with a melting point of 2770 °C for WC and 2780 °C for W2C. WC is an efficient electrical conductor, but W2C is less so. Tungsten carbide behaves similarly to unalloyed tungsten and is resistant to chemical attack, although it reacts strongly with chlorine to form tungsten hexachloride (WCl6).[3]

Aqueous tungstate solutions are noted for the formation of heteropoly acids and polyoxometalate anions under neutral and acidic conditions. As tungstate is progressively treated with acid, it first yields the soluble, metastable "paratungstate A" anion, W7O6–24, which over time converts to the less soluble "paratungstate B" anion, H2W12O10–42.[19] Further acidification produces the very soluble metatungstate anion, H2W12O6–40, after which equilibrium is reached. The metatungstate ion exists as a symmetric cluster of twelve tungsten-oxygen octahedra known as the Keggin anion. Many other polyoxometalate anions exist as metastable species. The inclusion of a different atom such as phosphorus in place of the two central hydrogens in metatungstate produces a wide variety of heteropoly acids, such as phosphotungstic acid H3PW12O40.
Biological role

Tungsten, at atomic number 74, is the heaviest element known to be used by any living organism, with the next heaviest being iodine (Z = 53). Tungsten has not been found to be necessary or used in eukaryotes, but it is an essential nutrient for some bacteria. For example, enzymes called oxidoreductases use tungsten similarly to molybdenum by using it in a tungsten-pterin complex with molybdopterin. Molybdopterin, despite its name, does not contain molybdenum, but may complex with either molybdenum or tungsten in use by living organisms. Tungsten-using enzymes typically reduce free carboxylic acids to aldehydes— a difficult synthetic process in chemistry and biochemistry.[20] However, the tungsten oxidoreductases may also catalyse oxidations. The first tungsten-requiring enzyme to be discovered also requires selenium, and in this case the tungsten-selenium pair may function analogously to the molybdenum-sulfur pairing of some molybdenum cofactor-requiring enzymes.[21] One of the enzymes in the oxidoreductase family which sometimes employ tungsten (bacterial formate dehydrogenase H) is known to use a selenium-molybdenum version of molybdopterin.[22] Although a tungsten-containing xanthine dehydrogenase from bacteria has been found to contain tungsten-molydopterin and also non-protein bound selenium, a tungsten-selenium molybdopterin complex has not been definitively described.[23]
Other effects on biochemistry

In soil, tungsten metal oxidizes to the tungstate anion. It may substitute for molybdenum in certain enzymes, and in such cases the resulting enzyme in eukaryotes would presumably be inert. The soil's chemistry determines how the tungsten polymerizes; alkaline soils cause monomeric tungstates; acidic soils cause polymeric tungstates.[24]

Sodium tungstate and lead have been studied for their effect on earthworms. Lead was found to be lethal at low levels and sodium tungstate was much less toxic, but the tungstate completely inhibited their reproductive ability.[25]

Tungsten has been studied as a biological copper antagonist in the same role as molybdenum. It has been found that tetrathiotungstates may be used as biological copper chelation chemicals in a role similar to tetrathiomolybdates.[26]

Production
Wolframite
Tungsten output in 2005

Tungsten is found in the minerals wolframite (iron-manganese tungstate, FeWO4/MnWO4), scheelite (calcium tungstate, (CaWO4), ferberite (FeWO4) and hübnerite (MnWO4). These are mined and used to produce about 37,400 tonnes of tungsten concentrates per year in 2000.[27] China produced over 75% of this total, with most of the remaining production coming from Austria, Bolivia, Portugal, and Russia.[27]

Tungsten is extracted from its ores in several stages. The ore is eventually converted to tungsten(VI) oxide (WO3), which is heated with hydrogen or carbon to produce powdered tungsten.[8] It can be used in that state or pressed into solid bars.

Tungsten can also be extracted by hydrogen reduction of WF6:

WF6 + 3 H2 → W + 6 HF

or pyrolytic decomposition:[28]

WF6 → W + 3 F2 (ΔHr = +)

Tungsten prices are tracked on the London Metal Exchange. The price for pure metal is around $20,075 per tonne as of October 2008.[29]
Applications
Close-up of a tungsten filament inside a halogen lamp.
Tungsten carbide ring

Because it retains its strength at high temperatures and has a high melting point, elemental tungsten is used in many high-temperature applications,[30] such as light bulb, cathode-ray tube, and vacuum tube filaments, heating elements, and rocket engine nozzles.[5] Its high melting point also makes tungsten suitable for aerospace and high-temperature uses such as electrical, heating, and welding applications, notably in the gas tungsten arc welding process (also called tungsten inert gas (TIG) welding).

Because of its conductive properties and relative chemical inertia, tungsten is also used in electrodes, and in the emitter tips in electron-beam instruments that use field emission guns, such as electron microscopes. In electronics, tungsten is used as an interconnect material in integrated circuits, between the silicon dioxide dielectric material and the transistors. It is used in metallic films, which replace the wiring used in conventional electronics with a coat of tungsten (or molybdenum) on silicon.[28]

The electronic structure of tungsten makes it one of the main sources for X-ray targets,[31] and also for shielding from high-energy radiations (such as in the radiopharmaceutical industry for shielding radioactive samples of FDG). Tungsten powder is used as a filler material in plastic composites, which are used as a nontoxic substitute for lead in bullets, shot, and radiation shields. Since this element's thermal expansion is similar to borosilicate glass, it is used for making glass-to-metal seals.[13]

The hardness and density of tungsten are applied in obtaining heavy metal alloys. A good example is high speed steel, which may contain as much as 18% tungsten.[32] Tungsten high melting point makes tungsten a good material for applications like rocket nozzle, for example in the UGM-27 Polaris Submarine-launched ballistic missile.[33] Superalloys containing tungsten, such as Hastelloy and Stellite, are used in turbine blades and wear-resistant parts and coatings.

Applications requiring its high density include heat sinks, weights, counterweights, ballast keels for yachts, tail ballast for commercial aircraft, and as ballast in race cars for NASCAR and Formula One. It is an ideal material to use as a dolly for riveting, where the mass necessary for good results can be achieved in a compact bar. High-density alloys of tungsten with nickel, copper or iron are used in high-quality darts[34] (to allow for a smaller diameter and thus tighter groupings) or for fishing lures (tungsten beads allow the fly to sink rapidly). Some types of strings for musical instruments are wound with tungsten wires.

In armaments, tungsten, usually alloyed with nickel and iron or cobalt to form heavy alloys, is used in kinetic energy penetrators as an alternative to depleted uranium, in applications where uranium's additional pyrophoric properties are not required (for example, in ordinary small arms bullets designed to penetrate body armor). Similarly, tungsten alloys have also been used in cannon shells, grenades and missiles, to create supersonic shrapnel.

Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum.[5][35] Its hardness makes it ideal for rings that will resist scratching, are hypoallergenic, and will not need polishing, which is especially useful in designs with a brushed finish.[36]

Tungsten compounds are used in catalysts, inorganic pigments (e.g. tungsten oxides), and as high-temperature lubricants (tungsten disulfide). Tungsten carbide (WC) is used to make wear-resistant abrasives and cutters and knives for drills, circular saws, milling and turning tools used by the metalworking, woodworking, mining, petroleum and construction industries[3] and accounts for about 60% of current tungsten consumption.[37] Tungsten oxides are used in ceramic glazes and calcium/magnesium tungstates are used widely in fluorescent lighting, while tungsten halogen bulbs are frequently used to light indoor photo shoots, and special negative films exist to take advantage of tungsten's unique disentangling properties. Crystal tungstates are used as scintillation detectors in nuclear physics and nuclear medicine. Other salts that contain tungsten are used in the chemical and tanning industries.[13]
Precautions

The data concerning the toxicity of tungsten is limited, but cases of intoxication by tungsten compounds are known, the lethal dose is estimated to be between 500 mg/kg and 5 g/kg for humans.[38][39] Tungsten is known to generate seizure and renal failure with acute tubular necrosis.[40][41][42]

The effects of tungsten within the environment are essentially unknown, a concern that has arisen in response to increasingly widespread use of the material as a fishing sinker, some of which are inevitably lost into water bodies. The same unknown variable applies whenever tungsten may be deposited into the environment, either knowingly or inadvertently.[43]

See also

* Field emission gun
* Isotopes of tungsten

References

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36. ^ Gray, Theo (March 14, 2008). "How to Make Convincing Fake-Gold Bars". Popular Science. . Retrieved 2008-06-18.
37. ^ "The Canadian Encyclopaedia". http://www.thecanadianencyclopedia.com/index.cfm?PgNm=TCE&Params=A1ARTA0008159. Retrieved 2009-05-05.
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39. ^ Masten, Scott (2003). "Tungsten and Selected Tungsten Compounds — Review of Toxicological Literature". National Institute of Environmental Health Sciences. http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPdf/tungsten.pdf. Retrieved 2009-03-19.
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External links
* WebElements.com – Tungsten
* Properties, Photos, History, MSDS
* Picture in the collection from Heinrich Pniok
* Elementymology & Elements Multidict by Peter van der Krogt – Tungsten
* Detection of the Natural Alpha Decay of Tungsten
* International Tungsten Industry Association