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Tungsten

Tungsten (formerly Wolfram) is a chemical element in the periodic table that has the symbol W (L. Wolframium) 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. 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 and in space-age superalloys).

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, or extruding. This element has the highest melting point (3422 °C) (6192 °F) , lowest vapor pressure and the highest tensile strength at temperatures above 1650 °C (3002 °F) of all metals. Its corrosion resistance is excellent and it can only be attacked slightly by most mineral acids. Tungsten metal forms a protective oxide when exposed to air but can be oxidized at high temperature. When alloyed in small quantities with steel, it greatly increases its hardness.

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 metal wires that have a high melting point. Other uses;

- A high melting point also makes tungsten suitable for space-oriented and high temperature uses which include electrical, heating, and welding applications.
- Hardness and density properties make this metal ideal for making heavy metal alloys that are used in armaments, heat sinks, and high-density applications, such as weights and counterweights.
- The high weight while maintaining a relatively small size makes it an ideal ingredient for darts, sometimes up to 80%.
- High speed tool steels (Hastelloy®, Stellite®) are often alloyed with tungsten, with tungsten steels containing as much as 18% tungsten.
- Superalloys containing this metal are used in turbine blades, tool steels, and wear-resistant alloy parts and coatings.
- Composites are used as a substitute for lead in bullets and shot.
- Tungsten chemical compounds are used in catalysts, inorganic pigments, and tungsten disulfide high-temperature lubricants which is stable to 500 °C (932 °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 as an alternative to depleted uranium.

Miscellaneous: Oxides are used in ceramic glazes and calcium/magnesium tungstates are used widely in fluorescent lighting. 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. Tungsten Carbide has recently been used in the fashioning of jewelry due to its hypoallergenic nature and the fact that due to its extreme hardness it is not apt to lose its luster like other polished metals.

History

Tungsten (Swedish tung sten meaning “heavy stone”, even though the current name for the element in Swedish is Wolfram) 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. 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 wolframite’s (an ore of tungsten) resistance to high temperatures and extreme strength, making it a perfect metal to craft weaponry out of.

Biological role

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. 16 recent cases of cancer in children were discovered in the Fallon area which has now been identified as a “Cancer Cluster.” Dr. Carol H. Rubin, a branch chief at the CDC, said data demonstrating a link between tungsten and leukemia is not available at present.

Occurrence

Tungsten is found in the minerals wolframite (iron-manganese tungstate, FeWO4/MnWO4), scheelite (calcium tungstate, CaWO4), ferberite and huebnerite. Important deposits of these minerals are in Bolivia, California, China, Colorado, Portugal, Russia, and South Korea (with China producing about 75% of the world’s supply). The metal is commercially produced by reducing tungsten oxide with hydrogen or carbon.

Compounds

The most common oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 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−.

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 the so-called heteropolyanions.

Isotopes

Naturally occurring tungsten consists of five radioisotopes whose half-lives are so long that for most practical purposes they are considered stable. 27 other radioisotopes have been characterized, the most stable of which are W-181 with a half-life of 121.2 days, W-185 with a half-life of 75.1 days, W-188 with a half-life of 69.4 days and W-178 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 W-179m (t½ 6.4 minutes).

The isotopes of tungsten range in atomic weight from 157.974 amu (W-158) to 189.963 amu (W-190). The lighter natural isotopes, W-180, W-182, W-183, and the most abundant W-184, decay into isotopes of element 72 (hafnium) by alpha emission, whereas W-186 decays into osmium by beta decay. It should be noted, however, that their half-lives are so long that for all practical purposes they are stable.

Tungsten


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