tungsten [Swed.,=heavy stone], metallic chemical element; symbol W; at. no. 74; at. wt. 183.85; m.p. about 3,410°C;; b.p. 5,660°C;; sp. gr. 19.3 at 20°C;; valence +2, +3, +4, +5, or +6. Tungsten is a very hard, silver-white to steel-gray metal with a body-centered cubic crystalline structure. In its chemical properties it resembles molybdenum, the element above it in Group 6 of the periodic table. It is sometimes called wolfram, and the chemical symbol is taken from this name; in naming compounds of tungsten, use of the name wolfram as a root is preferred. Tungsten is one of the most dense metals and has a higher melting point than any other metal. Pure tungsten is ductile, and wires made of it, even those of very small diameter, have a very high tensile strength. The element is resistant to ordinary acids and aqua regia but dissolves in a mixture of hydrofluoric and nitric acids. It forms compounds with carbon, chlorine, oxygen, sulfur, and some other elements. It is hexavalent in its most important compounds. It forms tungstic acid (H₂WO₄), or wolframic acid, which is the basis of a series of salts called tungstates, or wolframates. Tungsten metal is used extensively for filaments for light bulbs and electronic tubes. Carboloy, stellite, and tungsten steels are of importance in industry because they retain their hardness and strength at high temperatures. Tungsten is usually added to steel in the form of ferrotungsten, obtained by the reduction of ferrous tungstate in an electric furnace. Tungsten carbide is used in place of diamond for dies and as an abrasive. Sodium wolframate is used in the fireproofing of fabrics, in the weighting of silk, and as a mordant in dyeing. Tungsten does not occur uncombined in nature; large deposits of its ores are found in various parts of the world. The trioxide occurs in nature as the mineral wolfram ochre; scheelite and wolframite are the chief wolframate minerals. Tungsten is usually prepared from the trioxide by reduction with hydrogen or carbon. Tungsten was first isolated from tungstic acid in 1783 by the de Elhuyar brothers.

or wolfram

Metallic chemical element, one of the transition elements, chemical symbol W, atomic number 74. Exceptionally strong, white to grayish, and brittle, it has the highest melting point (6,170 °F [3,410 °C]), greatest high-temperature strength, and lowest thermal expansion coefficient of any metal. Its chief uses are in steels to increase hardness and strength and in lightbulb filaments (see incandescent lamp). It is also used in electrical contacts, rocket nozzles, chemical apparatus, high-speed rotors, and solar-energy devices. Tungsten is relatively inert, but compounds (in which it has various valences) are known. The most important, tungsten carbide, noted for its hardness, is used to increase the wear-resistance of cast iron and of tools' cutting edges.

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Tungsten also known as wolfram (/ˈwʊlfrəm/), is a chemical element that has the symbol W and atomic number 74.

A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite. It 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. Tungsten is often brittle and hard to work in its raw state; however, if pure, it can be cut with a hacksaw. 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.

Etymology

The name "Tungsten" (from the Swedish and Danish tung sten, meaning "heavy-" or, more accurately, "hard-stone") is used in English, French, Italian and all three modern Goidelic languages (Irish Gaelic, Scottish Gaelic, and Manx, though not in any other Celtic language) as the name of the element, although in many other languages (e.g. in German and Spanish) it is known as "wolfram" (though in modern Spanish it is commonly known as "tungsteno"), and its ore as wolframite, and this is the also the origin of its chemical symbol, W. The name "wolframite" is derived from "volf rahm", 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.

Physical properties

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. 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) the highest tensile strength. Tungsten has the lowest coefficient of thermal expansion of any pure metal. Alloying small quantities of tungsten with steel greatly increases its toughness.

Isotopes

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

¹⁸²W, T_1/2 > 8.3·10¹⁸ years

¹⁸³W, T_1/2 > 29·10¹⁸ years

¹⁸⁴W, T_1/2 > 13·10¹⁸ years

¹⁸⁶W, T_1/2 > 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. All of the remaining radioactive isotopes have half-lives of less than 3 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

Elemental tungsten resists attack by oxygen, acids, and alkalis.

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. Tungsten typically combines with oxygen to form the yellow tungstic oxide, WO₃, which dissolves in aqueous alkaline solutions to form tungstate ions, WO₄²⁻.

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 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).

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, , which over hours or days converts to the less soluble "paratungstate B" anion, . Further acidification produces the very soluble metatungstate anion, , 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. For example, 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 tungsten in the bodies of both groups. Sixteen recent cases of cancer in children were discovered in the Fallon area, which has now been identified as a cancer cluster; although the majority of the cancer victims are not longtime residents of Fallon. However, there is not enough data to support a link between tungsten and leukemia at this time.

Applications

Because of its ability to produce hardness at high temperatures and its high melting point (the second highest of any known element), elemental tungsten is used in many high-temperature applications. These include light bulb, cathode-ray tube, and vacuum tube filaments, as well as heating elements and nozzles on rocket engines. The high melting point also makes tungsten suitable for aerospace and high temperature uses which include electrical, heating, and welding applications, notably in the gas tungsten arc welding process (also called TIG welding).

Due to its conductive properties, as well as its relative chemical inertia, tungsten is also used in electrodes, and in the emitter tips of field emission electron-beam instruments, such as focused ion beam (FIB) and electron microscopes. In electronics, tungsten is used as an interconnect material in integrated circuits, between the silicon dioxide dielectric material and the transistors. Additionally, it is used in the manufacture of metallic films, which replace the wiring used in conventional electronics with a coat of tungsten (or molybdenum) on silicon.

In World War I, steel was put under restricted commercial uses, so Victor developed a tungsten gramophone needle. Called the "Tungs-tone" stylus (as Victor called it), this needle could withstand about 50 to 100 uses before wearing out the record. Victor claimed the needle could be used 300 times, but this would likely cause excessive wear/damage to a record. Victor stated that the needle be tested in the run-off (near the label) grooves and be rotated a quarter turn periodically, and then be tested in the run-off grooves. Victor stated that a Tungs-tone should be a used one for playing a record.

The electronic structure of tungsten makes it one of the main sources for X-ray targets, 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.

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. Superalloys containing tungsten, like 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 high level race cars in series, such as NASCAR and Formula 1. It is an ideal material to use as a bucking bar for riveting, where the mass necessary for good results can be achieved in a small, easy to wield bar. 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 but may also be used in cannon shells, grenades and missiles to create supersonic shrapnel. High-density alloys of tungsten may be used in darts (to allow for a smaller diameter and thus tighter groupings) or for fishing lures (tungsten bead heads allow the fly to sink rapidly). Some types of strings for musical instruments are wound with tungsten wires. Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum. Its hardness makes it ideal for rings that will resist scratching, and are hypoallergenic and will not need polishing, which is especially useful in designs with a brushed finish.

Tungsten chemical compounds are used in catalysts, inorganic pigments (e.g. tungsten oxides), and also 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. Tungsten 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. Other salts that contain tungsten are used in the chemical and tanning industries.

Lately, tungsten is used for jewelry because of its longevity and extremely high durability.

Production

Tungsten is found in the minerals wolframite (iron-manganese tungstate, FeWO₄/MnWO₄), scheelite (calcium tungstate, (CaWO₄), ferberite and hübnerite. These are mined and used to produce about 37,400 tons of tungsten concentrates per year in 2000. Over 75% of this production came from China, while most of the remaining production is done in Austria, Bolivia, Portugal, and Russia, while United States produces none.

The extraction of tungsten has several stages, the ore eventually being converted to tungsten (VI) oxide (WO₃), which is heated with hydrogen or carbon, producing powdered tungsten. It can be used in that state or converted into solid bars.

Tungsten can also be extracted by hydrogen reduction of WF₆ (WF₆ + 3H₂ = W + 6HF) or pyrolytic decomposition (WF₆ + energy = W + 3F₂).

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. 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 through reduction of this acid with charcoal. They are credited with the discovery of the element.

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 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
• Uncle Tungsten: Memories of a Chemical Boyhood by Oliver Sacks

References

Further reading

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

External links

• Tungsten Disulfide Applications WS2
• WebElements.com – Tungsten
• Properties, Photos, History, MSDS
• ScienceLab.com – Tungsten
• 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
• Comprehensive Data on Tungsten