Overblog Suivre ce blog
Editer l'article Administration Créer mon blog
27 décembre 2011 2 27 /12 /décembre /2011 09:48

Microprocessors

The electronics industry discovered that hafnium-based compound can be employed in gate insulators in the 45 nm generation ofintegrated circuits from Intel, IBM and others. Hafnium oxide-based compounds are practical high-k dielectrics, allowing reduction of the gate leakage current which improves performance at such scales.Sony VAIO VGN-AR730E/B Battery

Other uses

Due to its heat resistance and its affinity to oxygen and nitrogen, hafnium is a good scavenger for oxygen and nitrogen in gas-filled and incandescent lamps. Hafnium is also used as the electrode in plasma cutting because of its ability to shed electrons into air.Sony VAIO VGN-AR73DB Battery

The high energy content of 178m2Hf was the concern of a DARPA funded program in the US. This program determined the possibility of using a nuclear isomer of hafnium (the above mentioned 178m2Hf) to construct high yield weapons with X-ray triggering mechanisms—an application of induced gamma emission, was infeasible because of its expense. See Hafnium controversy.Sony VAIO VGN-AR74DB Battery

Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known astantalium, the name comes from Tantalus, a hero in Greek mythology.[3] Tantalum is a rare, hard, blue-gray, lustroustransition metal that is highly corrosion resistant. It is part of the refractory metals group, which are widely used as minor component in alloys.Sony VAIO VGN-AR75UDB Battery

The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for platinum, but its main use today is in tantalum capacitors in electronic equipment such as mobile phones, DVD players, video game systems and computers.Sony VAIO VGN-AR790U/B Battery

Tantalum, always together with the chemically similar niobium, occurs in theminerals tantalite, columbite and coltan (a mix of columbite and tantalite).

Tantalum was discovered in Sweden in 1802 by Anders Ekeberg. One year earlier, Charles Hatchett had discovered the elementcolumbium.Sony VAIO VGN-AR820E Battery

In 1809, the English chemist William Hyde Wollaston compared the oxides derived from both columbium—columbite, with a density 5.918 g/cm3, and tantalum—tantalite, with a density 7.935 g/cm3, and concluded that the two oxides, despite their difference in measured density, were identical. He decided to keep the name tantalum.Sony VAIO VGN-AR830E Battery

AfterFriedrich Wöhler confirmed these results, it was thought that columbium and tantalum were the same element. This conclusion was disputed in 1846 by the German chemist Heinrich Rose, who argued that there were two additional elements in the tantalite sample, and he named them after the children of Tantalus:Sony VAIO VGN-AR83S Battery

niobium (from Niobe, the goddess of tears), and pelopium (from Pelops). The supposed element "pelopium" was later identified as a mixture of tantalum and niobium, and it was found that the niobium was identical to the columbium already discovered in 1801 by Hatchett.Sony VAIO VGN-AR83US Battery

The differences between tantalum and niobium were demonstrated unequivocally in 1864 by Christian Wilhelm Blomstrand,[8]andHenri Etienne Sainte-Claire Deville, as well as by Louis J. Troost, who determined the empirical formulas of some of their compounds in 1865.Sony VAIO VGN-AR84S Battery

Further confirmation came from the Swiss chemist Jean Charles Galissard de Marignac,  in 1866, who proved that there were only two elements. These discoveries did not stop scientists from publishing articles about the so-called ilmenium until 1871.Sony VAIO VGN-AR84US Battery

De Marignac was the first to produce the metallic form of tantalum in 1864, when hereduced tantalum chloride by heating it in an atmosphere of hydrogen.[12] Early investigators had been only able to produce impure tantalum, and the first relatively pure ductile metal was produced by Werner von Bolton in 1903.Sony VAIO VGN-AR85S Battery

Wires made with metallic tantalum were used for light bulb filaments until tungsten replaced it in widespread use.

The name tantalum was derived from the name of the mythological Tantalus, the father of Niobe in Greek mythology.Sony VAIO VGN-AR85US Battery

In the story, he had been punished after death by being condemned to stand knee-deep in water with perfect fruit growing above his head, both of which eternally tantalized him. (If he bent to drink the water, it drained below the level he could reach, and if he reached for the fruit, the branches moved out of his grasp.)Sony VAIO VGN-AR870EA Battery

Ekeberg wrote "This metal I call tantalum … partly in allusion to its incapacity, when immersed in acid, to absorb any and be saturated."

For decades, the commercial technology for separating tantalum from niobium involved the fractional crystallization of potassium heptafluorotantalate away from potassium oxypentafluoroniobate monohydrate,Sony VAIO VGN-AR890U Battery

a process that was discovered by Jean Charles Galissard de Marignac in 1866. This method has been supplanted by solvent extraction from fluoride-containing solutions of tantalum.[9]

Physical properties

Tantalum is dark (blue-gray),[16] dense, ductile, very hard, easily fabricated, and highly conductive of heat and electricity.Sony VAIO VGN-AR93S Battery

The metal is renowned for its resistance to corrosion by acids; in fact, at temperatures below 150 °C tantalum is almost completely immune to attack by the normally aggressive aqua regia. It can be dissolved with hydrofluoric acid or acidic solutions containing the fluoride ion and sulfur trioxide, as well as with a solution of potassium hydroxide.Sony VAIO VGN-AR93US Battery

Tantalum's high melting point of 3017 °C (boiling point 5458 °C) is exceeded only by tungsten, rhenium and osmium for metals, and carbon.

Tantalum exists in two crystalline phases, alpha and beta. The alpha phase is relatively ductile and soft;Sony VAIO VGN-AR94S Battery

it has body-centered cubic structure (space group Im3m, lattice constant a = 0.33058 nm), Knoop hardness 200–400 HN and electrical resistivity 15–60 µ??cm. The beta phase is hard and brittle; its crystal symmetry is tetragonal (space group P42/mnm, a = 1.0194 nm, c = 0.5313 nm), Knoop hardness is 1000–1300 HN and electrical resistivity is relatively high at 170–210 µ??cm.Sony VAIO VGN-AR94US Battery

The beta phase is metastable and converts to the alpha phase upon heating to 750–775 °C. Bulk tantalum is almost entirely alpha phase, and the beta phase usually exists as thin films obtained by magnetron sputtering, chemical vapor deposition orelectrochemical deposition from an eutectic molten salt solution.Sony VAIO VGN-AR95S Battery

Chemical properties

Tantalum forms oxides with the oxidation states +5 (Ta2O5) and +4 (TaO2). The most stable oxidation state is +5, tantalum pentoxide.[18] Tantalum pentoxide is the starting material for several tantalum compounds.Sony VAIO VGN-AR95US Battery

The compounds are created by dissolving the pentoxide in basic hydroxide solutions or by melting it in another metal oxide. Such examples are lithium tantalate (LiTaO3) and lanthanum tantalate (LaTaO4). In the lithium tantalate, the tantalate ion TaO?
3 does not occur;Sony VAIO VGN-AW110J/H Battery

instead, this part of the formula represents linkage of TaO7?6 octahedra to form a three-dimensional perovskite framework; while the lanthanum tantalate contains lone TaO3?4 tetrahedral groups.

The fluorides of tantalum can be used for its separation from niobium.Sony VAIO VGN-AW11M/H Battery

Tantalum forms halogen compounds in the oxidation states of +5, +4, and +3 of the type TaX5, TaX4, and TaX3, although multi core complexes and substoichiometric compounds are also known.  Tantalum pentafluoride (TaF5) is a white solid with a melting point of 97.0 °C and tantalum pentachloride (TaCl5) is a white solid with a melting point of 247.4 °C.Sony VAIO VGN-AW11S/B Battery

Tantalum pentachloride is hydrolyzed by water and reacts with additional tantalum at elevated temperatures by forming the black and highly hygroscopic tantalum tetrachloride (TaCl4). While the trihalogen compounds can be obtained by reduction of the pentahalogenes with hydrogen, the dihalogen compounds do not exist.Sony VAIO VGN-AW11XU/Q Battery

A tantalum-tellurium alloy forms quasicrystals. Tantalum compounds with oxidation states as low as ?1 have been reported in 2008.

Similar to most other refractory metals, the hardest known compounds of tantalum are its stable nitrides and carbides.Sony VAIO VGN-AW11Z/B Battery

Tantalum carbide, TaC, like the more commonly used tungsten carbide, is a very hard ceramic that is used in cutting tools. Tantalum(III) nitride is used as a thin film insulator in some microelectronic fabrication processes. Chemists at theLos Alamos National Laboratory in the United States have developed a tantalum carbide-graphite composite material that is one of the hardest materials ever synthesized.Sony VAIO VGN-AW120J/H Battery

Korean researchers have developed an amorphous tantalum-tungsten-copper alloy that is more flexible and two to three times stronger than commonly used steel alloys.[23] There are two tantalum aluminides, TaAl3 and Ta3Al. These are stable, refractory, and reflective, and they have been proposed[24] as coatings for use in infrared wave mirrors.Sony VAIO VGN-AW150Y/H Battery

Natural tantalum consists of two isotopes: 180mTa (0.012%) and 181Ta (99.988%). 181Ta is a stable isotope. 180mTa (m denotes a metastable state) is predicted to decay in three ways: isomeric transition to the ground state of 180Ta, beta decay to 180W, or electron capture to 180Hf.Sony VAIO VGN-AW160J/Q Battery

However, radioactivity of this nuclear isomer has never been observed. Only a lower limit on its half life of over 1015 years has been set. The ground state of 180Ta has a half life of only 8 hours. 180mTa is the only naturally occurring nuclear isomer (excluding radiogenic and cosmogenic short-living nuclides).Sony VAIO VGN-AW170Y/Q Battery

It is also the rarest isotope in the Universe, taking into account the elemental abundance of tantalum and isotopic abundance of 180mTa in the natural mixture of isotopes (and again excluding radiogenic and cosmogenic short-living nuclides).Sony VAIO VGN-AW180Y/Q Battery

Tantalum has been examined theoretically as a "salting" material for nuclear weapons (cobalt is the better-known hypothetical salting material). An external shell of 181Ta would be irradiated by the intensive high-energy neutron flux from a hypothetical exploding nuclear weapon.Sony VAIO VGN-AW190JAH Battery

This would transmute the tantalum into the radioactive isotope 182Ta, which has a half-life of 114.4 days and produces gamma rays with approximately 1.12 million electron-volts (MeV) of energy apiece, which would significantly increase the radioactivity of the nuclear fallout from the explosion for several months.Sony VAIO VGN-AW190NAB Battery

Such "salted" weapons have never been built or tested, as far as is publicly known, and certainly never used as weapons.[26]

Occurrence

Tantalum is estimated to make up about 1 ppm[27] or 2 ppm[20] of the Earth's crust by weight.Sony VAIO VGN-AW190NBB Battery

There are many species of tantalum minerals, only some of which are so far being used by industry as raw materials: tantalite, microlite, wodginite, euxenite,polycrase. Tantalite (Fe,Mn) Ta2O6 is the most important mineral for tantalum extraction.Sony VAIO VGN-AW190NCB Battery

Tantalite has the same mineral structure as columbite (Fe,Mn) (Ta,Nb)2O6; when there is more Ta than Nb it is called tantalite and when there is more Nb than Ta is it called columbite (or niobite). The high density of tantalite and other tantalum containing minerals makes the use of gravitational separation the best method. Other minerals include samarskite and fergusonite.Sony VAIO VGN-AW190YAB Battery

The primary mining of tantalum is in Australia, where the largest producer, Global Advanced Metals, formerly known as Talison Minerals, operates two mines in Western Australia, Greenbushes in the Southwest and Wodgina in the Pilbara region. The Wodgina mine was reopened in January 2011 after mining at the site was suspended in late-2008 due to the global financial crisis.Sony VAIO VGN-AW190YBB Battery

This mine produces a primary tantalum concentrate which is further upgraded at the Greenbushes operation before being sold to customers.  Whereas the large-scale producers of niobium are in Brazil and Canada, the ore there also yields a small percentage of tantalum.Sony VAIO VGN-AW190YCB Battery

Some other countries such as China, Ethiopia, and Mozambique mine ores with a higher percentage of tantalum, and they produce a significant percentage of the world's output of it. Tantalum is also produced in Thailand and Malaysia as a by-product of the tin mining there.Sony VAIO VGN-AW190YDB Battery

During gravitational separation of the ores from placer deposits, not only is Cassiterite (SnO2) found, but a small percentage of tantalite also included. The slag from the tin smelters then contains economically useful amounts of tantalum, which is leached from the slag.Sony VAIO VGN-AW220J/B Battery

Future sources of supply of tantalum, in order of estimated size, are being explored in Saudi Arabia, Egypt, Greenland, China, Mozambique, Canada,Australia, the United States, Finland, and Brazil.

Coltan, the industrial name for a columbite–tantalite mineral from which columbium (i.e. niobium) and tantalum are extracted,Sony VAIO VGN-AW230J/H Battery

can also be found in Central Africa, which is why tantalum is being linked to the war in the Congo. According to an October 23, 2003 United Nations report, the smuggling and exportation of coltan has helped fuel the war in the Congo, a crisis that has resulted in approximately 5.4 million deaths since 1998– making it the world’s deadliest documented conflict since World War II.Sony VAIO VGN-AW235J/B Battery

Ethical questions have been raised about responsible corporate behavior, human rights, and endangering wildlife, due to the exploitation of resources such as coltan in the armed conflict regions of the Congo Basin.  However, although important for the local economy in Congo, the coltan mining in Congo is not important for the world supply of tantalum.Sony VAIO VGN-AW290JFQ Battery

The United States Geological Survey reports in its 2006 yearbook that this region produced a little less than 1% of the world's tantalum output for the past four years, peaking at 10% in 2000 and 2008.

Production and fabrication

Several steps are involved in the extraction of tantalum from tantalite.Sony VAIO VGN-AW41JF Battery

First, the mineral is crushed and concentrated by gravity separation. This is generally carried out near the mine site. Further processing by chemical separation is usually done by treating the ores with a mixture of hydrofluoric acid and sulfuric acidat over 90 °C.Sony VAIO VGN-AW41JF/H Battery

These reagents dissolve the tantalum and niobium oxides to give complex fluorides, which can be separated from impurities and from each other:

Ta2O5 + 14 HF ? 2 H2[TaF7] + 5 H2O

Nb2O5 + 10 HF ? 2 H2[NbOF5] + 3 H2OSony VAIO VGN-AW41MF Battery

The first industrial-scale separation, developed by de Marignac, used the difference in solubility between the complex niobium and tantalum fluorides K2[NbOF5]•H2O (potassium oxypentafluoroniobate monohydrate) and K2[TaF7] (potassium heptafluorotantalate) in water.Sony VAIO VGN-AW41MF/H Battery

Newer processes use the liquid extraction of the fluorides fromaqueous solution by organic solvents such as cyclohexanone. The complex niobium and tantalum fluorides are extracted separately from the organic solvent with water, and either precipitated by the addition of potassium fluoride to produce a potassium fluoride complex, or precipitated with ammonia as the pentoxide:Sony VAIO VGN-AW41XH Battery

H2[TaF7] + 2 KF ? K2[TaF7]? + 2 HF

2 H2[TaF7] + 14 NH4OH ? Ta2O5? + 14 NH4F + 9 H2O

The purified K2[TaF7] is generally treated by reduction with molten sodium to produce a coarse tantalum powder.Sony VAIO VGN-AW41XH/Q Battery

In terms of fabrication, all welding of tantalum must be done in an inert atmosphere of argon or helium in order to shield it from contamination with atmospheric gases. Tantalum is not solderable. Grinding tantalum is difficult, especially so for annealed tantalum. In the annealed condition, tantalum is extremely ductile and can be readily formed as metal sheets.Sony VAIO VGN-AW41ZF Battery

Electronics

The major use for tantalum, as the metal powder, is in the production of electronic components, mainly capacitors and some high-powerresistors. Tantalum electrolytic capacitors exploit the tendency of tantalum to form a protective oxide surface layer,Sony VAIO VGN-AW41ZF/B Battery

using tantalum powder, pressed into a pellet shape, as one "plate" of the capacitor, the oxide as the dielectric, and an electrolytic solution or conductive solid as the other "plate". Because the dielectric layer can be very thin (thinner than the similar layer in, for instance, an aluminium electrolytic capacitor), a high capacitance can be achieved in a small volume.Sony VAIO VGN-AW50DB/H Battery

Because of the size and weight advantages, tantalum capacitors are attractive for portable telephones, personal computers, and automotive electronics.

Alloys

Tantalum is also used to produce a variety of alloys that have high melting points, are strong and have good ductility.Sony VAIO VGN-AW51JGB Battery

Alloyed with other metals, it is also used in making carbide tools for metalworking equipment and in the production of superalloys for jet engine components, chemical process equipment, nuclear reactors, and missile parts. Because of its ductility, tantalum can be drawn into fine wires or filaments, which are used for evaporating metals such as aluminium.Sony VAIO VGN-AW52JGB Battery

Since it resists attack by body fluids and is nonirritating, tantalum is widely used in making surgical instruments and implants. For example, porous tantalum coatings are used in the construction of orthopedic implants due to tantalum's ability to form a direct bond to hard tissue.Sony VAIO VGN-AW53FB Battery

Tantalum is inert against most acids except hydrofluoric acid and hot sulfuric acid, also hot alkaline solutions cause tantalum to corrode. This property makes it an ideal metal for chemical reaction vessels and pipes for corrosive liquids. Heat exchanging coils for the steam heating of hydrochloric acid are made from tantalum.Sony VAIO VGN-AW70B/Q Battery

Tantalum was extensively used in the production of ultra high frequency electron tubes for radio transmitters. The tantalum is capable of capturing oxygen and nitrogen by forming nitrides and oxides and therefore helps to sustain the high vacuum needed for the tubes.Sony VAIO VGN-AW71JB Battery

Other uses

The oxide is used to make special high refractive index glass for camera lenses. The high melting point and oxidation resistance lead to the use of the metal in the production of vacuum furnace parts. Due to its high density, shaped charge and explosively formed penetrator liners have been constructed from tantalum.Sony VAIO VGN-AW72JB Battery

Tantalum greatly increases the armor penetration capabilities of a shaped charge due to its high density and high melting point. It is also occasionally used in precious watches e.g. from Audemars Piguet, Hublot, Montblanc and Panerai. Tantalum is also highly bioinert and is used as an orthopedic implant material.Sony VAIO VGN-AW73FB Battery

Precautions

Compounds containing tantalum are rarely encountered in the laboratory. The metal is highly biocompatible and is used for body implants and coatings, therefore attention may be focused on other elements or the physical nature of the chemical compound.Sony VAIO VGN-AW80NS Battery

A single study  is the only reference in literature linking tantalum to local sarcomas. The study was quoted in IARC Monograph vol. 74 which includes the following "Note to the reader": "Inclusion of an agent in the Monographs does not imply that it is a carcinogen, only that the published data have been examined."Sony VAIO VGN-AW80S Battery

Tungsten , also known as wolfram /?w?lfr?m/ (wuul-fr?m), is a chemical element with the chemical symbol W and atomic number 74.

A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds.Sony VAIO VGN-AW80US Battery

It was identified as a new element in 1781, and first isolated as a metal in 1783. Its important ores includewolframite and scheelite. The free element is remarkable for its robustness, especially the fact that it has the highestmelting point of all the non-alloyed metals and the second highest of all the elements after carbon.Sony VAIO VGN-AW81DS Battery

Also remarkable is its high density of 19.3 times that of water, comparable to that of uranium and gold, and much higher (about 1.7 times) than that of lead.[3] Tungsten with minor amounts of impurities is often brittle[4] and hard, making it difficult to work. However, very pure tungsten, though still hard, is more ductile, and can be cut with a hard-steel hacksaw.Sony VAIO VGN-AW81JS Battery

The unalloyed elemental form is used mainly in electrical applications. Tungsten's many alloys have numerous applications, most notably in incandescent light bulb filaments, X-ray tubes (as both the filament and target), and superalloys. Tungsten's hardness and high density give it military applications in penetrating projectiles.Sony VAIO VGN-AW81YS Battery

Tungsten compounds are most often used industrially as catalysts.

Tungsten is the only metal from the third transition series that is known to occur in biomolecules, where it is used in a few species of bacteria. It is the heaviest element known to be used by any living organism. However, tungsten interferes withmolybdenum and copper metabolism, and is somewhat toxic to animal life.Sony VAIO VGN-AW82DS Battery

In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite (at the time named tungsten). Scheele and Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid.[8] In 1783, José and Fausto Elhuyar found an acid made from wolframite that was identical to tungstic acid.Sony VAIO VGN-AW82JS Battery

Later that year, inSpain, 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.Sony VAIO VGN-AW82YS Battery

Portugal, as the main European source of the element, was put under pressure from both sides, because of its deposits of wolframite ore at Panasqueira. Tungsten's resistance to high temperatures and its strengthening of alloys made it an important raw material for the arms industry.Sony VAIO VGN-AW83FS Battery

Etymology

The name "tungsten" (from the Nordic tung sten, meaning "heavy stone") is used in English, French, 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"),Sony VAIO VGN-AW83GS Battery

used for example in most European (especially Germanic and Slavic) languages, is derived from the mineralwolframite, and this is also the origin of its chemical symbol, W.[5] 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.Sony VAIO VGN-AW83HS Battery

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.Sony VAIO VGN-AW90NS Battery

Physical properties

In tungsten's raw form, it is a hard steel-gray metal that is often brittle and hard to work. However, if made very pure, although tungsten retains its hardness (which exceeds that of many steels), it becomes malleable enough that it can be worked easily. It is worked by forging, drawing, extruding or sintering.Sony VAIO VGN-AW90S Battery

Of all metals in pure form, tungsten has the highest melting point (3,422 °C, 6,192 °F), lowest vapor pressure (at temperatures above 1,650 °C, 3,000 °F) and the highest tensile strength.[13] Tungsten has the lowest coefficient of thermal expansion of any pure metal.Sony VAIO VGN-AW90US Battery

The low thermal expansion and high melting point and strength of tungsten originate from strongcovalent bonds formed between tungsten atoms by the 5d electrons.[14] Alloying small quantities of tungsten with steel greatly increases its toughness.

Tungsten exists in two major crystalline forms: ? and ?. The former has a body-centered cubic structure and is the most stable form.Sony VAIO VGN-AW91CDS Battery

The structure of the ? phase is called A15 cubic; it is metastable, but can coexist with the ? phase at ambient conditions owing to non-equilibrium synthesis or stabilization by impurities. Contrary to the ? phase which crystallizes in isometric grains, the ? form exhibits a columnar habit.Sony VAIO VGN-AW91CJS Battery

The ? phase has a three times lower electrical resistivity  and a much lower superconducting transition temperature TC than the ? phase: ca. 0.015 K vs. 1–4 K; mixing the two phases allows obtaining intermedicate TC values. The TC value can also be raised by alloying tungsten with another metal (e.g. 7.9 K for W-Tc).Sony VAIO VGN-AW91CYS Battery

Such tungsten alloys are sometimes used in low-temperature superconducting circuits.

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 years; on average, this yields about two alpha decays of 180W in one gram of natural tungsten per year.Sony VAIO VGN-AW91DS Battery

The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be 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, and187W 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.Sony VAIO VGN-AW91JS Battery

Tungsten also has 4 meta states, the most stable being 179mW (T½ 6.4 minutes).

Chemical properties

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 ?2 to +6.Sony VAIO VGN-AW91YS Battery

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. W2C is resistant to chemical attack, although it reacts strongly with chlorineto form tungsten hexachloride (WCl6).Sony VAIO VGN-AW92CDS Battery

In aqueous solution, tungstate gives the 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.Sony VAIO VGN-AW92CJS Battery

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.Sony VAIO VGN-AW92CYS Battery

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.Tungsten trioxide can form intercalation compounds with alkali metals. These are known as bronzes; an example is sodium tungsten bronze.Sony VAIO VGN-AW92DS Battery

Occurrence

Tungsten is found in the minerals wolframite (iron-manganese tungstate, (Fe,Mn)WO4), scheelite (calcium tungstate, (CaWO4), ferberite (FeWO4) and hübnerite (MnWO4).China produced 51,000 tonnes of tungsten concentrate in 2009, which was 83% of the world output.Sony VAIO VGN-AW92JS Battery

Most of the remaining production originated from Russia (2,500 t),Canada (1,964 t), Bolivia (1,023 t), Austria (900 t), Portugal (900 t), Thailand (600 t), Brazil (500 t), Peru (500 t) and Rwanda (500 t).[27] Tungsten is also considered to be a conflict mineral due to the unethical mining practices observed in the Democratic Republic of the Congo.Sony VAIO VGN-AW92YS Battery

Biological role

Tungsten, at atomic number 74, is the heaviest element known to be biologically functional, with the next heaviest being iodine (Z = 53).Sony VAIO VGN-AW93FS Battery

not in eukaryotes, tungsten is used by some bacteria. For example, enzymes called oxidoreductases use tungsten similarly to molybdenum by using it in a tungsten-pterin complex withmolybdopterin (molybdopterin, despite its name, does not contain molybdenum, but may complex with either molybdenum or tungsten in use by living organisms).Sony VAIO VGN-AW93GS Battery

Tungsten-using enzymes typically reduce carboxylic acids to aldehydes.  However, the tungsten oxidoreductases may also catalyse oxidations. The first tungsten-requiring enzyme to be discovered also requires selenium, and in this case the tungsten-selenium pair may function analogously to the molybdenum-sulfur pairing of some molybdenum cofactor-requiring enzymes.Sony VAIO VGN-AW93HS Battery

One of the enzymes in the oxidoreductase family which sometimes employ tungsten (bacterial formate dehydrogenase H) is known to use a selenium-molybdenum version of molybdopterin. Although a tungsten-containing xanthine dehydrogenase from bacteria has been found to contain tungsten-molydopterin and also non-protein bound selenium, a tungsten-selenium molybdopterin complex has not been definitively described.Sony VAIO VGN-AW93ZFS Battery

In soil, tungsten metal oxidizes to the tungstate anion. It can be selectively or non-selectively imported by some prokaryotic organisms and may substitute formolybdate in certain enzymes. Its effect on the action of these enzymes is in some cases inhibitory and in others positive. It is thought that a tungstate-containing enzyme in eukaryotes would be inert.Sony VAIO VGN-AW93ZGS Battery

The soil's chemistry determines how the tungsten polymerizes; alkaline soils cause monomeric tungstates; acidic soils cause polymeric tungstates.

Sodium tungstate and lead have been studied for their effect on earthworms. Lead was found to be lethal at low levels and sodium tungstate was much less toxic, but the tungstate completely inhibited their reproductive ability.Sony VAIO VGN-AW93ZHS Battery

Tungsten has been studied as a biological copper metabolic antagonist, in a role similar to the action of molybdenum. It has been found that tetrathiotungstates may be used as biological copper chelation chemicals, similar to the tetrathiomolybdates.

About 61,300 tonnes of tungsten concentrates were produced in the year 2009.Sony VAIO VGN-BZ11EN Battery

Tungsten is extracted from its ores in several stages. The ore is eventually converted to tungsten(VI) oxide (WO3), which is heated with hydrogen or carbon to produce powdered tungsten.  Because of tungsten's high melting point, it is not commercially feasible to cast tungsten ingots.Sony VAIO VGN-BZ11MN Battery

Instead, powdered tungsten is mixed with small amounts of powdered nickel or other metals, andsintered. During the sintering process, the nickel diffuses into the tungsten, producing an alloy.

Tungsten can also be extracted by hydrogen reduction of WF6:Sony VAIO VGN-BZ11XN Battery

WF6 + 3 H2 ? W + 6 HF

or pyrolytic decomposition:

WF6 ? W + 3 F2 (?Hr = +)

Tungsten is not traded as a futures contract and cannot be tracked on exchanges like the London Metal Exchange. The price for pure metal is around $20,075 per tonne as of October 2008.Sony VAIO VGN-BZ12VN Battery

Applications

Approximately half of the tungsten is consumed for the production of hard materials — namely tungsten carbide — with the remaining major use being its use in alloys and steels. Less than 10% is used in chemical compounds.Sony VAIO VGN-BZ12XN Battery

Hard materials

Tungsten is mainly used in the production of hard materials based on tungsten carbide, one of the hardest carbides, with a melting point of 2770 °C. WC is an efficient electrical conductor, but W2C is less so. WC is used to make wear-resistantabrasives and cutters and knives for drills,Sony VAIO VGN-BZ31VT Battery

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.

The jewelry industry makes rings of sintered tungsten carbide, tungsten carbide/metal composites, and also simply metallic tungsten.Sony VAIO VGN-BZ31XT Battery

Sometimes manufacturers or retailers refer to tungsten carbide as a metal, but it is rather a ceramic. Because of tungsten carbide's hardness, rings made of this material are extremely abrasion resistant, and will hold a burnished finish much longer than rings made of metallic tungsten. Tungsten carbide rings are brittle, however, and may crack under a sharp blow.Sony VAIO VGN-BZ560 Battery

The hardness and density of tungsten are applied in obtaining heavy metal alloys. A good example is high speed steel, which can contain as much as 18% tungsten.Tungsten's high melting point makes tungsten a good material for applications like rocket nozzles, for example in the UGM-27 Polaris Submarine-launched ballistic missile.Sony VAIO VGN-BZ560N24 Battery

Superalloys containing tungsten, such asHastelloy and Stellite, are used in turbine blades and wear-resistant parts and coatings.

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,Sony VAIO VGN-BZ560N30 Battery

in applications where uranium's additional pyrophoric properties are not required (for example, in ordinary small arms bullets designed to penetrate body armor). Similarly, tungsten alloys have also been used in cannon shells, grenades and missiles, to create supersonic shrapnel.Sony VAIO VGN-BZ560P Battery

Tungsten has also been used in Dense Inert Metal Explosives, which use it as dense powder to reduce collateral damage while increasing the lethality of explosives within a small radius.

Chemical applications

Tungsten(IV) sulfide is a high temperature lubricant and is a component of catalysts for hydrodesulfurization.[47] MoS2 is more commonly used for such applications.Sony VAIO VGN-BZ560P20 Battery

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.Sony VAIO VGN-BZ560P22 Battery

oxide (WO3) is incorporated into selective catalytic reduction (SCR) catalysts found in coal-fired power plants. These catalysts convert nitrogen oxides(NOx) to nitrogen (N2) and water (H2O) using ammonia (NH3). The tungsten oxide helps with the physical strength of the catalyst and extends catalyst life.Sony VAIO VGN-BZ560P28 Battery

Niche uses

Applications requiring its high density include 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 dolly for riveting, where the mass necessary for good results can be achieved in a compact bar.Sony VAIO VGN-BZ560P30 Battery

High-density alloys of tungsten with nickel, copper or iron are used in high-quality darts[50] (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.Sony VAIO VGN-BZ560P34 Battery

Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum. Metallic tungsten is harder than gold alloys (though not as hard as tungsten carbide), and is hypoallergenic, making it useful for rings that will resist scratching, especially in designs with a brushed finish.Sony VAIO VGN-BZ561 Battery

Electronics

Because it retains its strength at high temperatures and has a high melting point, elemental tungsten is used in many high-temperature applications,  such aslight bulb, cathode-ray tube, and vacuum tube filaments, heating elements, and rocket engine nozzles.Sony VAIO VGN-BZ561N20 Battery

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).

Because of its conductive properties and 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.Sony VAIO VGN-BZ561P20 Battery

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 (ormolybdenum) on silicon.Sony VAIO VGN-BZ562P Battery

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 theradiopharmaceutical 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.Sony VAIO VGN-BZ563P Battery

Since this element's thermal expansion is similar to borosilicate glass, it is used for making glass-to-metal seals.

Precautions

Because tungsten is rare and its compounds are generally inert, the effects of tungsten on the environment are limited.Sony VAIO VGN-BZAAFS Battery

The median lethal dose LD50 depends strongly on the animal and the method of administration and varies between 59 mg/kg (intravenous, rabbit)  and 5000 mg/kg (tungsten metal powder, intraperitoneal, rats).Sony VAIO VGN-BZAAHS Battery

Patent claim

Tungsten is unique amongst the elements in that it has been the subject of legal proceedings. In 1928, a US court rejected General Electric's attempt to patent it, overturning U.S. Patent 1,082,933 granted in 1913 to William D. Coolidge.Sony VAIO VGN-BZAANS Battery,Sony VAIO VGN-BZAAPS Battery

Partager cet article

Repost 0
Published by batterys - dans Laptop Battery
commenter cet article

commentaires