Tungsten发表评论(0)编辑词条

Tungsten (pronounced /ˈtʌŋstən/), also known as wolfram (/ˈwʊlfrəm/), is a chemical element with the chemical 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 incandescent 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, and is the heaviest element known to be used by living organisms.

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. 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.

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.

Etymology
The name "tungsten" (from the Swedish and Danish tung sten, meaning "heavy stone") is used in English, French, Italian 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 German, Spanish, Russian and in both Swedish and Danish, is derived from the mineral wolframite, and this is also the origin of its chemical symbol, W. 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.

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, 3,002 °F) the highest tensile strength.[13] Tungsten has the lowest coefficient of thermal expansion of any pure metal. Alloying small quantities of tungsten with steel greatly increases its toughness.

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

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. All of the remaining radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes. 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.

The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from -1 to +6. An oxidation state of -2 has also been reported in 2008. 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).

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. 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.

Applications
Closeup of a tungsten filament inside a halogen lamp.Because it retains its strength at high temperatures and has a high melting point, elemental tungsten is used in many high-temperature applications,[28] 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).

Due to its conductive properties, as well as its 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.

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, 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 bucking bar for riveting, where the mass necessary for good results can be achieved in a compact 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 beads 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, are hypoallergenic, and will not need polishing, which is especially useful in designs with a brushed finish.

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 and accounts for about 60% of current tungsten consumption. 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 high durability.