Tungsten

Tungsten (pronounced /ˈtʌŋstən/), also called wolfram (/ˈwʊlfrəm/), is a chemical element that has the symbol W (German: Wolfram) and atomic number 74. A very hard, heavy, steel-gray to white transition metal, tungsten is found in several ores including wolframite and scheelite and 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. The pure form is used mainly in electrical applications but its many compounds and alloys are widely used in many applications, most notably in light bulb filaments, in X-ray tubes (as both the filament and target), and in superalloys. Tungsten is the only metal from the third transition series that is known to occur in biomolecules.^{[citation needed]}

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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. All can decay into isotopes of element 72 (hafnium) by alpha emission; ¹⁸⁰W has been observed to have a half life of 1.8 ± 0.2 Ea. The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be: ¹⁸²W, T_1/2 > 8.3 Ea; ¹⁸⁴W, T_1/2 > 29 Ea; ¹⁸⁵W, T_1/2 > 13 Ea; ¹⁸⁶W, T_1/2 > 27 Ea. ^[1] On average, two alpha decays of ¹⁸⁰W occur in one gram of natural tungsten per year.

27 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 and ¹⁷⁸W with a half-life of 21.6 days. All of the remaining radioactive isotopes have half-lives of less than 24 hours, and most of these have half-lives that are less than 8 minutes. Tungsten also has 4 meta states, the most stable being ^179mW (t_½ 6.4 minutes).

Notable characteristics

Pure tungsten is steel-gray to tin-white and is a hard metal. Tungsten can be cut with a hacksaw when it is very pure (it is brittle and hard to work when impure) and is otherwise worked by forging, drawing, extruding, or sintering. Of all metals at temperatures above 1650 °C (3000 °F), this element has the highest melting point (3422 °C) (6192 °F), lowest vapor pressure and the highest tensile strength. Tungsten has the lowest coefficient of thermal expansion of any pure metal. Its corrosion resistance is excellent and it can be attacked only slightly by most mineral acids. Tungsten metal forms a protective oxide when exposed to air but can be oxidized at high temperature. Steel alloyed with small quantities of tungsten greatly increases its toughness.

Chemical compounds

The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from -1 to +6.^[2] Tungsten typically combines with oxygen to form the yellow tungstic oxide, WO₃, which dissolves in aqueous alkaline solutions to form tungstate ions, WO₄²⁻.

Aqueous polyoxoanions

Aqueous tungstate solutions are noted for the formation of polyoxoanions under neutral and acidic conditions. As tungstate is progressively treated with acid, it first yields the soluble, metastable "paratungstate A" anion, W₇O₂₄⁶⁻, which over hours or days converts to the less soluble "paratungstate B" anion, H₂W₁₂O₄₂¹⁰⁻. Further acidification produces the very soluble metatungstate anion, H₂W₁₂O₄₀⁶⁻, after equilibrium is reached. The metatungstate ion exists as a symmetric cluster of twelve tungsten-oxygen octahedra known as the "Keggin" anion. Many other polyoxoanions 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 H₃P W₁₂O₄₀ in this example.

See also tungsten compounds.

Biological role

Tungsten is an essential nutrient for some organisms.

Enzymes called oxidoreductases use tungsten in a way that is similar to molybdenum by using it in a tungsten-pterin complex.

On August 20, 2002, officials representing the U.S.-based Centers for Disease Control and Prevention announced that urine tests on leukemia patient families and control group families in the Fallon, Nevada area had shown elevated levels of the metal tungsten in the bodies of both groups.^[3] Sixteen recent cases of cancer in children were discovered in the Fallon area which has now been identified as a cancer cluster, (it should be noted, however, that the majority of the cancer victims are not long time residents of Fallon). Dr. Carol H. Rubin, a branch chief at the CDC, said data demonstrating a link between tungsten and leukemia is not available at present.^[4]

Applications

Tungsten is a metal with a wide range of uses, the largest of which is as tungsten carbide (W₂C, WC) in cemented carbides. Cemented carbides (also called hardmetals) are wear-resistant materials used by the metalworking, mining, petroleum and construction industries. Tungsten is widely used in light bulb and vacuum tube filaments, as well as electrodes, because it can be drawn into very thin wire with a high melting point. Other uses:

• Its high melting point makes tungsten suitable for aerospace and high temperature uses which include electrical, heating, and welding applications, notably in the GTAW process (also called TIG welding).
• Hardness and density properties make this metal ideal for making heavy metal alloys that are used in armament, heat sinks, and high density applications, such as weights, counterweights, ballast keels for yachts and tail ballast for commercial aircraft.
• The high density makes it an ideal ingredient for darts, normally 80% and sometimes up to 97%. This allows darts containing tungsten to have a smaller diameter than those of other metals at the same weight, permitting tighter groupings.
• High speed steel contains tungsten and some tungsten steels contain as much as 18% tungsten.
• Superalloys containing tungsten are used in turbine blades and wear resistant parts and coatings. Examples are Hastelloy and Stellite.
• 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.
• Tungsten chemical compounds are used in catalysts, inorganic pigments, and tungsten disulfide high-temperature lubricants which are stable to 500 °C (930 °F).
• Since this element's thermal expansion is similar to borosilicate glass, it is used for making glass-to-metal seals.
• It is used in kinetic energy penetrators, usually alloyed with nickel and iron or cobalt, to form heavy alloys, used as an alternative to depleted uranium.
• Tungsten is used as an interconnect material in integrated circuits. Contact holes are etched in silicon dioxide dielectric material, filled with tungsten and polished to form connections to transistors. Typical contact holes can be as small as 65 nm.
• Tungsten carbide is one of the hardest carbides and is used in machine tools such as make milling and turning tools, and used together with cobalt and carbon is often the best choice for such applications.
• Used extensively for shielding in the radiopharmaceutical industry. It is often employed when transporting individual FDG doses (called 'pigs') - the high energy of fluorine-18 makes lead much less effective.
• Tungsten is used in the emitters of focused ion beam and electron microscopes.
• Tungsten is also beginning to be used in jewelry. Its hardness makes it ideal for rings that will never scratch, are hypoallergenic and will not need polishing. This property is especially useful in designs with a brushed finish.
• Also used in fishing lures like the Mormyshka.
• Tungsten is used as ballast in high level race cars in series such as NASCAR and Formula 1.

Miscellaneous: Oxides are used in ceramic glazes and calcium/magnesium tungstates are used widely in fluorescent lighting. Crystal tungstates are used as scintillation detectors in nuclear physics and nuclear medicine. The metal is also used in X-ray targets and heating elements for electrical furnaces. Salts that contain tungsten are used in the chemical and tanning industries. Tungsten 'bronzes' (so-called due to the colour of the tungsten oxides) along with other compounds are used in paints. Some types of strings for musical instruments are wound with tungsten wire.  

Production

  Tungsten is found in the minerals wolframite (iron-manganese tungstate, FeWO₄/MnWO₄), scheelite (calcium tungstate, CaWO₄), ferberite and hübnerite. There are major deposits of these minerals in China (with about 57% world share), Russia, Austria and Portugal, reports the British Geological Survey. The metal is commercially produced by reducing tungsten oxide with hydrogen or carbon.

World tungsten reserves have been estimated at 7 million t W. Many of these reserves are not economically viable at this time. At current rates of consumption, these reserves will last for approximately 140 years. It is estimated that 30% of the reserves are wolframite and 70% are scheelite ores. Another factor that controls the tungsten supply is scrap recycling of tungsten, which is more readily refined for reuse than raw ore.

History

Tungsten (Swedish tung sten meaning "heavy stone"), even though the current name for the element in Swedish is wolfram (sometimes spelled in Swedish as volfram), from the denomination volf rahm by Wallerius in 1747, translated from the description by Agricola in 1546 as Lupi spuma, meaning "wolf's froth" after the way tin is eaten up like a wolf after sheep in the process of its extraction.^[5]

It was first hypothesized to exist by Peter Woulfe in 1779 who examined wolframite and concluded that it must contain a new substance. In 1781 Carl Wilhelm Scheele ascertained that a new acid could be made from tungstenite. Scheele and Torbern Bergman suggested that it could be possible to obtain a new metal by reducing tungstic acid. In 1783 José and Fausto Elhuyar found an acid in wolframite that was identical to tungstic acid. In Spain later that year the brothers succeeded in isolating tungsten through reduction of this acid with charcoal. They are credited with the discovery of the element.^[6][7]

In World War II, tungsten played an enormous role in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides, because of its sources of wolframite ore. The resistance to high temperatures, as well as the extreme strength of its alloys, made the metal into a very important raw material for the weaponry industry.

See also

• Field emission gun
• Oliver Sacks: "Uncle Tungsten: Memories of a Chemical Boyhood"

References

• Los Alamos National Laboratory - Tungsten
• DC/AC Circuits and Electronics: Principles & Applications by Robert K. Herrick, Published by Delmar Learning 2003 for Purdue University

1. ^ National Nuclear Data Center table of nuclides, http://www.nndc.bnl.gov/chart/
2. ^ Emsley, John (2000). The Elements, 3rd edition. 
3. ^ http://www.cdc.gov/nceh/clusters/Fallon/study.htm
4. ^ http://www.rgj.com/news/printstory.php?id=22000
5. ^ http://elements.vanderkrogt.net/elem/w.html
6. ^ http://www.itia.info/FileLib/ITIA_Newsletter_June05.pdf
7. ^ http://www.itia.info/FileLib/ITIA_Newsletter_December05.pdf