Tungsten

A steel-gray metal, tungsten is often brittle and hard to work in its raw state. However, if pure, it can be cut with a hacksaw^[1]. 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^[2]. The pure form is used mainly in electrical applications, but its many compounds and alloys are used in many applications, most notably in light bulb filaments, X-ray tubes (as both the filament and target), and superalloys. Tungsten is also the only metal from the third transition series that is known to occur in biomolecules. ^[3] ^[4]

Etymology

"Tungsten" (from the Swedish tung sten, meaning "heavy stone") is commonly accepted as the name of the material, although some chemists (primarily in Germany but also e.g. in Sweden) refer to it as "wolfram", from its ore wolframite. The name "wolframite" was derived from "volf rahm", the word Johan Gottschalk Wallerius used to refer to it in 1747. This, in turn, was translated from "Lupi spuma", the word Georg Agricola used to refer to the element in 1546. Its English translation is "wolf's froth", so named because the mineral consumed a large amount of tin in its extraction.^[5] Its chemical symbol, W, is derived from wolfram as well.^[1] "Tungsten" is commonly accepted as the name of the metal, although some chemists (primarily in Germany and Sweden) refer to it as "wolfram", from its ore wolframite. The name "wolframite" was derived from "volf rahm", the word Johan Gottschalk Wallerius used to refer to it in 1747. This, in turn, was translated from "Lupi spuma", the word Georg Agricola used to refer to the element in 1546. Its English translation is "wolf's froth", so named because the mineral consumed a large amount of tin in its extraction.^[5] Its chemical symbol, W, is derived from wolfram as well.^[1]

Physical properties

In its raw form, tungsten is a steel-gray metal that is often brittle and hard to work. However, if pure, it is much easier to work^[1]. It is worked by forging, drawing, extruding, or sintering. Of all metals this element has the highest melting point (3,422 °C) (6,192 °F), lowest vapor pressure and the highest tensile strength^[6]. Tungsten has the lowest coefficient of thermal expansion of any pure metal. Alloying small quantities of tungsten with steel greatly increases its toughness^[2].

Isotopes

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)·10¹⁸ yr. The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be:

¹⁸²W, T_1/2 > 8.3·10¹⁸ yr;

¹⁸³W, T_1/2 > 29·10¹⁸ yr;

¹⁸⁴W, T_1/2 > 13·10¹⁸ yr;

¹⁸⁶W, T_1/2 > 27·10¹⁸ yr.

^[7] 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).

Chemical properties

Tungsten resists attack by oxygen, acids, and alkalis^[8]

Compounds

Main article: Tungsten compounds

The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from -1 to +6.^[8] 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 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, 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 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 H₃P W₁₂O₄₀ in this example.

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.^[9] 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.^[10]

Applications

Because of its ability to produce hardness at high temperatures and its high melting point (the second highest of any known element), tungsten is used in many high-temperature applications^[11]. These include light bulb, cathode-ray tube, and vacuum tube filaments, as well as heating elements and nozzles on rocket engines^[1]. It is also used in electrodes, and in the emitter tips of field emission electron-beam intruments, such as focused ion beam (FIB) and electron microscopes. The metal is also used in X-ray targets.

Tungsten carbide (W₂C or WC), produced by heating powdered tungsten with carbon, is one of the hardest carbides, with a melting point of 2770 degrees C for WC and 2780 degrees C for W₂C. WC is an efficient electrical conductor, but W₂C is not as efficient. Tungsten carbide behaves in a manner very similar to that of unalloyed tungsten and is resistant to chemical attack, although it reacts strongly with chlorine to form tungsten hexachloride (WCl₆)^[2]. Tungsten carbide 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. In these applications, tungsten carbide may be combined with cobalt, coated with titanium nitride or coated with titanium carbide^[2].

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).

The hardness and density of tungsten find uses in 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 aircraftand as ballast in high level race cars in series such as NASCAR and Formula 1. Darts may contain a high proportion of tungsten, allowing their diameter to be smaller than those made of other metals, and permitting tighter groupings. Fishing lures like the Mormyshka and many fishing flies use tungsten bead heads to sink the fly rapidly. Some types of strings for musical instruments are wound with tungsten wire.

In metal alloys, 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^[6].

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. Tungsten may be used in cannon shells, grenades and missiles to create super-sonic shrapnel.

In electronics, 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.

In the radiopharmaceutical industry, tungsten is used extensively for shielding. It is often employed when transporting individual FDG doses (called 'pigs') - the high energy of fluorine-18 makes lead much less effective.

Tungsten is sometimes used in jewelry (such as wedding bands) as an alternative to gold or platinum, as tungsten has a similar density to gold^[1]. Its hardness makes it ideal for rings that will resist scratching, are hypoallergenic and will not need polishing. This property is especially useful in designs with a brushed finish^[12].

The 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.

Calcium and magnesium tungstates are used in flourescent lighting. Other salts that contain tungsten are used in the chemical and tanning industries^[6]. Tungsten 'bronzes' (so-called due to the colour of the tungsten oxides) along with other compounds are used in paints.

Production

File:Tungsten (mined)2.PNG

Tungsten output in 2005

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^{[citation needed]}. The extraction of tungsten has several stages, the ore eventually being converted to tungsten (VI) oxide (WO₂), which is heated with hydrogen or carbon, producing powdered tungsten^[13]. It can be used in that state or converted into solid bars. Approximately 75% of the world's tungsten resources are thought to exist in China^[1], much of the remainder being supplied by Russia^[13]. About 45,000 tons of tungsten is extracted annually, and much of it is recycled^[13].

History

In 1781, Carl Wilhelm Scheele ascertained that a new acid could be made from scheelite (at the time named tungstenite): tungstic acid. Scheele and Torbern Bergman suggested that it could be possible to obtain a new metal by reducing this acid^[13]. In 1783 José and Fausto Elhuyar found an acid made from 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^[14]^[15].

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.

References

DC/AC Circuits and Electronics: Principles & Applications by Robert K. Herrick, Published by Delmar Learning 2003 for Purdue University

^ ^a ^b ^c ^d ^e ^f ^g Stwertka, Albert. A Guide to the elements. 2nd ed. New York: Oxford University Press, 2002.
^ ^a ^b ^c ^d Daintith, John. Facts on File Dictionary of Chemistry. 4th ed. New York, New York: Checkmark Books, 2005.
^ J McMaster and John H Enemark (1998). "The active sites of molybdenum- and tungsten-containing enzymes". Current Opinion in Chemical Biology. 2 (2): 201–207. doi:10.1016/S1367-5931(98)80061-6.
^ "Molybdenum and tungsten in biology". Trends in Biochemical Sciences. 27 (7): 360–367. 2002. doi:10.1016/S0968-0004(02)02107-2.
^ ^a ^b http://elements.vanderkrogt.net/elem/w.html
^ ^a ^b ^c Los Alamos National Laboratory - Tungsten
^ National Nuclear Data Center table of nuclides, http://www.nndc.bnl.gov/chart/
^ ^a ^b Emsley, John E. The elements. 2nd ed. New York: Oxford University Press, 1991.
^ Cancer Clusters: Fallon Cancer Study: Final Report CDC HSB
^ http://www.rgj.com/news/printstory.php?id=22000
^ DeGarmo, E. Paul. Materials and Processes in Manufacturing. 5th ed. New York, New York: MacMillan Publishing, 1979.
^ How to Make Convincing Fake-Gold Bars
^ ^a ^b ^c ^d Saunders, Nigel. Tungsten and the Elements of Groups 3 to 7. Chicago, Illinois: Heinemann Library, 2003.
^ http://www.itia.info/FileLib/ITIA_Newsletter_June05.pdf
^ http://www.itia.info/FileLib/ITIA_Newsletter_December05.pdf

External links

[albert-1] ^ ^a ^b ^c ^d ^e ^f ^g Stwertka, Albert. A Guide to the elements. 2nd ed. New York: Oxford University Press, 2002.

[daintith-2] Daintith, John. Facts on File Dictionary of Chemistry. 4th ed. New York, New York: Checkmark Books, 2005.

[3] J McMaster and John H Enemark (1998). "The active sites of molybdenum- and tungsten-containing enzymes". Current Opinion in Chemical Biology. 2 (2): 201–207. doi:10.1016/S1367-5931(98)80061-6.

[4] "Molybdenum and tungsten in biology". Trends in Biochemical Sciences. 27 (7): 360–367. 2002. doi:10.1016/S0968-0004(02)02107-2.

[sweetums-5] ttp://elements.vanderkrogt.net/elem/w.html

[desu-6] Los Alamos National Laboratory - Tungsten

[7] National Nuclear Data Center table of nuclides, http://www.nndc.bnl.gov/chart/

[emsley-8] Emsley, John E. The elements. 2nd ed. New York: Oxford University Press, 1991.

[9] Cancer Clusters: Fallon Cancer Study: Final Report CDC HSB

[10] ttp://www.rgj.com/news/printstory.php?id=22000

[11] DeGarmo, E. Paul. Materials and Processes in Manufacturing. 5th ed. New York, New York: MacMillan Publishing, 1979.

[12] How to Make Convincing Fake-Gold Bars

[junk-13] Saunders, Nigel. Tungsten and the Elements of Groups 3 to 7. Chicago, Illinois: Heinemann Library, 2003.

[14] ttp://www.itia.info/FileLib/ITIA_Newsletter_June05.pdf

[15] ttp://www.itia.info/FileLib/ITIA_Newsletter_December05.pdf

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