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Main article: Isotopes of tungsten
ஓரிடத்தான்	இயற்கையில் கிடைக்கும் வளம்	அரை வாழ்வு	சிதைவுறும் முறை	சிதைவுறும் ஆற்றல் (MeV)	சிதை விளைவு
¹⁸⁰W	0.12%	1.8×10¹⁸ y	α	2.516	¹⁷⁶Hf
¹⁸¹W	syn	121.2 d	ε	0.188	¹⁸¹Ta
¹⁸²W	26.50%	W is stable with 108 neutrons
¹⁸³W	14.3%	W is stable with 109 neutrons
¹⁸⁴W	30.64%	W is stable with 110 neutrons
¹⁸⁵W	syn	75.1 d	β^-	0.433	¹⁸⁵Re
¹⁸⁶W	28.43%	W is stable with 112 neutrons

மேற்கோள்கள்

மெல்லிழையம் (Tungsten) W (L. wolframium) என குறியீடு மற்றும் அணுவெண் (atomic number) 74 கொண்டுள்ள தனிமமாகும். இது ஒரு மிக கடினமான எடையுள்ள வெள்ளி சாம்பல் வெள்ளை தன்மைமாறு மாழைtransition metal ஆகும். tungsten is found in several ores including wolframite and scheelite and is remarkable for its robust physical properties, especially the fact that it has a higher melting point than any other non-alloy in existence. 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 as both the filament and target in most X-ray tubes and in space-age superalloys).

பொருளடக்கம்

[மறை]

[தொகு] 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 (3000 °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.

[தொகு] 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, 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 armaments, 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 %.
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.
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 (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 tungsten heavy alloys, 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 65nm.
Tungsten carbide is one of the hardest substances in existence and is used in, among other things, machine tools such as milling cutters. Tungsten carbide is the most common material used to make milling and turning tools, and 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.
Because of its extreme hardness, it is used toghether with cobolt and carbon in tungsten-carbide cutting tools.

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. 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. Some types of strings for musical instruments are wound with tungsten wire.

[தொகு] History

Tungsten (Swedish, Danish and Norwegian tung sten meaning "heavy stone", even though the current name for the element in all three languages 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) 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 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.

[தொகு] 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.[1] 16 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.[2]

[தொகு] Occurrence

Tungsten is found in the minerals wolframite (iron-manganese tungstate, FeWO₄/MnWO₄), scheelite (calcium tungstate, CaWO₄), ferberite and hübnerite. There are important deposits of these minerals in Bolivia, California, China, Colorado, Portugal, Russia, Vietnam 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.

World tungsten reserves have been estimated at 7 million t W. Unfortunately, most of these reserves are not economically workable so far. At our current annual consumption rate, these reserves will only last for about 140 years. China has been the largest supplier thus far. According to further estimates, it has been suggested that 30% of the reserves are Wolframite and 70% are Scheelite ores. Another factor that controls the tungsten supply is scrap recycling of tungsten and it has been proven to be a very valuable raw material in comparison to ore.

[தொகு] 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⁻.

[தொகு] 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.

See also tungsten compounds.

[தொகு] 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 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).

[தொகு] 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

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

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