Tungsten

Tungsten, or wolfram,[8][9] is a chemical element with symbol W and atomic number 74. The name tungsten comes from the former Swedish name for the tungstate mineral scheelite, tung sten or "heavy stone".[10] Tungsten is a rare metal found naturally on Earth almost exclusively combined with other elements in chemical compounds rather than alone. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include wolframite and scheelite.

The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered, melting at 3422 °C (6192 °F, 3695 K). It also has the highest boiling point, at 5930 °C (10706 °F, 6203 K).[11] Its density is 19.3 times that of water, comparable to that of uranium and gold, and much higher (about 1.7 times) than that of lead.[12] Polycrystalline tungsten is an intrinsically brittle[13][14] and hard material (under standard conditions, when uncombined), making it difficult to work. However, pure single-crystalline tungsten is more ductile and can be cut with a hard-steel hacksaw.[15]

Tungsten's many alloys have numerous applications, including incandescent light bulb filaments, X-ray tubes (as both the filament and target), electrodes in gas tungsten arc welding, superalloys, and radiation shielding. Tungsten's hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are also often used as industrial catalysts.

Tungsten is the only metal from the third transition series that is known to occur in biomolecules that are found in a few species of bacteria and archaea. It is the heaviest element known to be essential to any living organism.[16] However, tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to more familiar forms of animal life.[17][18]

Tungsten,  74W
Wolfram evaporated crystals and 1cm3 cube
Tungsten
Pronunciation/ˈtʌŋstən/ (TUNG-stən)
Alternative namewolfram, pronounced: /ˈwʊlfrəm/ (WUUL-frəm)
Appearancegrayish white, lustrous
Standard atomic weight Ar, std(W)183.84(1)[1]
Tungsten in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Mo

W

Sg
tantalumtungstenrhenium
Atomic number (Z)74
Groupgroup 6
Periodperiod 6
Blockd-block
Element category  transition metal
Electron configuration[Xe] 4f14 5d4 6s2[2]
Electrons per shell
2, 8, 18, 32, 12, 2
Physical properties
Phase at STPsolid
Melting point3695 K ​(3422 °C, ​6192 °F)
Boiling point6203 K ​(5930 °C, ​10706 °F)
Density (near r.t.)19.3 g/cm3
when liquid (at m.p.)17.6 g/cm3
Heat of fusion52.31 kJ/mol[3][4]
Heat of vaporization774 kJ/mol
Molar heat capacity24.27 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 3477 3773 4137 4579 5127 5823
Atomic properties
Oxidation states−4, −2, −1, 0, +1, +2, +3, +4, +5, +6 (a mildly acidic oxide)
ElectronegativityPauling scale: 2.36
Ionization energies
  • 1st: 770 kJ/mol
  • 2nd: 1700 kJ/mol
Atomic radiusempirical: 139 pm
Covalent radius162±7 pm
Color lines in a spectral range
Spectral lines of tungsten
Other properties
Natural occurrenceprimordial
Crystal structurebody-centered cubic (bcc)
Body-centered cubic crystal structure for tungsten
Speed of sound thin rod4620 m/s (at r.t.) (annealed)
Thermal expansion4.5 µm/(m·K) (at 25 °C)
Thermal conductivity173 W/(m·K)
Electrical resistivity52.8 nΩ·m (at 20 °C)
Magnetic orderingparamagnetic[5]
Magnetic susceptibility+59.0·10−6 cm3/mol (298 K)[6]
Young's modulus411 GPa
Shear modulus161 GPa
Bulk modulus310 GPa
Poisson ratio0.28
Mohs hardness7.5
Vickers hardness3430–4600 MPa
Brinell hardness2000–4000 MPa
CAS Number7440-33-7
History
DiscoveryCarl Wilhelm Scheele (1781)
First isolationJuan José Elhuyar and Fausto Elhuyar (1783)
Named byTorbern Bergman (1781)
Main isotopes of tungsten
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
180W 0.12% 1.8×1018 y α 176Hf
181W syn 121.2 d ε 181Ta
182W 26.50% stable
183W 14.31% stable
184W 30.64% stable
185W syn 75.1 d β 185Re
186W 28.43% stable

Characteristics

Physical properties

In its raw form, tungsten is a hard steel-grey metal that is often brittle and hard to work. If made very pure, tungsten retains its hardness (which exceeds that of many steels), and becomes malleable enough that it can be worked easily.[15] It is worked by forging, drawing, or extruding. Tungsten objects are also commonly formed by sintering.

Of all metals in pure form, tungsten has the highest melting point (3422 °C, 6192 °F), lowest vapor pressure (at temperatures above 1650 °C, 3000 °F), and the highest tensile strength.[19] Although carbon remains solid at higher temperatures than tungsten, carbon sublimes at atmospheric pressure instead of melting, so it has no melting point. Tungsten has the lowest coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons.[20] Alloying small quantities of tungsten with steel greatly increases its toughness.[12]

Tungsten exists in two major crystalline forms: α and β. The former has a body-centered cubic structure and is the more stable form. The structure of the β phase is called A15 cubic; it is metastable, but can coexist with the α phase at ambient conditions owing to non-equilibrium synthesis or stabilization by impurities. Contrary to the α phase which crystallizes in isometric grains, the β form exhibits a columnar habit. The α phase has one third of the electrical resistivity[21] and a much lower superconducting transition temperature TC relative to the β phase: ca. 0.015 K vs. 1–4 K; mixing the two phases allows obtaining intermediate TC values.[22][23] The TC value can also be raised by alloying tungsten with another metal (e.g. 7.9 K for W-Tc).[24] Such tungsten alloys are sometimes used in low-temperature superconducting circuits.[25][26][27]

Isotopes

Naturally occurring tungsten consists of four stable isotopes (182W, 183W, 184W, and 186W) and one very long-lived radioisotope, 180W. 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;[28][29] on average, this yields about two alpha decays of 180W per gram of natural tungsten per year.[30] The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be at least 4 × 1021 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.[30] All of the remaining radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes.[30] Tungsten also has 11 meta states, with the most stable being 179mW (t1/2 6.4 minutes).

Chemical properties

Elemental tungsten resists attack by oxygen, acids, and alkalis.[31]

The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6.[31][32] 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 chlorine to form tungsten hexachloride (WCl6).[12]

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, W
7
O6–
24
, which over time converts to the less soluble "paratungstate B" anion, H
2
W
12
O10–
42
.[33] Further acidification produces the very soluble metatungstate anion, H
2
W
12
O6–
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.

Tungsten trioxide can form intercalation compounds with alkali metals. These are known as bronzes; an example is sodium tungsten bronze.

History

In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite (at the time named tungsten).[34] Scheele and Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid.[35] In 1783, José and Fausto Elhuyar found an acid made from wolframite that was identical to tungstic acid. Later that year, at the Royal Basque Society in the town of Bergara, Spain, the brothers succeeded in isolating tungsten by reduction of this acid with charcoal, and they are credited with the discovery of the element (they called it "wolfram" or "volfram").[36][37][38]

The strategic value of tungsten came to notice in the early 20th century. British authorities acted in 1912 to free the Carrock mine from the German owned Cumbrian Mining Company and, during World War I, restrict German access elsewhere.[39] In World War II, tungsten played a more 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 at Panasqueira. Tungsten's desirable properties such as resistance to high temperatures, its hardness and density, and its strengthening of alloys made it an important raw material for the arms industry,[40][41] both as a constituent of weapons and equipment and employed in production itself, e.g., in tungsten carbide cutting tools for machining steel.

Etymology

The name "tungsten" (from the Swedish tung sten, "heavy stone") is used in English, French, and many other languages as the name of the element, but not in the Nordic countries. "Tungsten" was the old Swedish name for the mineral scheelite. "Wolfram" (or "volfram") is used in most European (especially Germanic, Spanish and Slavic) languages and is derived from the mineral wolframite, which is the origin of the chemical symbol W.[15] 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 Latin "lupi spuma", the name Georg Agricola used for the element in 1546, which translates into English as "wolf's froth" and is a reference to the large amounts of tin consumed by the mineral during its extraction.[42]

Occurrence

Wolframite from Portugal
Wolframite mineral, with a scale in cm.

Tungsten is found mainly in the minerals wolframite (ironmanganese tungstate (Fe,Mn)WO4, which is a solid solution of the two minerals ferberite FeWO4, and hübnerite MnWO4) and scheelite (calcium tungstate (CaWO4). Other tungsten minerals range in their level of abundance from moderate to very rare, and have almost no economical value.

Chemical compounds

Tungsten forms chemical compounds in oxidation states from -II to VI. Higher oxidation states, always as oxides, are relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are often associated with metal clusters, and very low oxidation states are typically associated with CO complexes. Mo and W chemistry shows strong similarities. The relative rarity of tungsten(III), for example, contrasts with the pervasiveness of the chromium(III) compounds. The highest oxidation state is seen in tungsten(VI) oxide (WO3).[43] The trioxide, which is volatile at high temperatures, is the precursor to virtually all other Mo compounds as well as alloys. Molybdenum has several oxidation states, the most stable being +4 and +6.

Tungsten(VI) oxide is soluble in aqueous base , forming tungstate (WO42−). This oxyanion condenses at lower pH values, forming polyoxotungstates.[44]

JAFYAQ
Structure of W6Cl18 ("tungsten trichloride").

The broad range of oxidation states of tungsten is reflected in it various chlorides:[43]

Organotungsten compounds are numerous and also span a range of oxidation states. Notable examples include the trigonal prismatic W(CH3)6 and octahedral W(CO)6.

Production

Tungsten mined in 2013
Tungsten mined in 2012

About 61,300 tonnes of tungsten concentrates were produced in the year 2009,[45] and in 2010, world production of tungsten was about 68,000 tonnes.[46] The main producers were as follows (data in tonnes):[47]

Major producers of tungsten[46]
Country Production (tonnes)
2009 2010 2011 2012
 China 51,000 59,000 61,800 64,000
 Russia 2,665 2,785 3,314 3,537
 Canada 1,964 420 1,966 2,194
 Bolivia 1,023 1,204 1,124 1,247
 Vietnam 725 1,150 1,635 1,050
 Portugal 823 799 819 763
 Austria 887 977 861 706
 Rwanda 380 330 520 700
 Spain 225 240 497 542
 Brazil 192 166 244 381
 Australia 33 18 15 290
 Peru 502 571 439 276
 Burundi 110 100 165 190
 Myanmar 874 163 140 140
 North Korea 100 110 110 100
 DR Congo 200 25 70 95
 Thailand 190 300 160 80
 Mongolia 39 20 13 66
 Uganda 7 44 8 21
Total 61,200 68,400 73,900 76,400
A full trolly coming from one of the galleries (27072015764)
Tungsten mining in Rwanda forms an important part of the country's economy.

There is additional production in the U.S., but the amount is proprietary company information. U.S. reserves are 140,000 tonnes.[47] US industrial use of wolfram is 20,000 tonnes: 15,000 tonnes are imported and the remaining 5,000 tonnes come from domestic recycling.[48]

Tungsten is considered to be a conflict mineral due to the unethical mining practices observed in the Democratic Republic of the Congo.[49][50]

There is a large deposit of tungsten ore on the edge of Dartmoor in the United Kingdom, which was exploited during World War I and World War II as the Hemerdon Mine. With recent increases in tungsten prices, as of 2014 this mine has been reactivated.[51]

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.[35] Because of tungsten's high melting point, it is not commercially feasible to cast tungsten ingots. Instead, powdered tungsten is mixed with small amounts of powdered nickel or other metals, and sintered. During the sintering process, the nickel diffuses into the tungsten, producing an alloy.

Tungsten can also be extracted by hydrogen reduction of WF6:

WF6 + 3 H2 → W + 6 HF

or pyrolytic decomposition:[52]

WF6 → W + 3 F2 (ΔHr = +)

Tungsten is not traded as a futures contract and cannot be tracked on exchanges like the London Metal Exchange. The prices are usually quoted for tungsten concentrate or WO3. If converted to the metal equivalent, they were about US$19 per kilogram in 2009.[45]

Applications

Tungsten filament in halogen lamp
Close-up of a tungsten filament inside a halogen lamp
Tungsten ring-cropbright
Tungsten carbide ring (jewelry)
Tungsten cylinder 1kg
1 kilogram tungsten cylinder (scale shown below)

Approximately half of the tungsten is consumed for the production of hard materials – namely tungsten carbide – with the remaining major use being in alloys and steels. Less than 10% is used in other chemical compounds.[53]

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-resistant abrasives, and "carbide" cutting tools such as knives, drills, circular saws, milling and turning tools used by the metalworking, woodworking, mining, petroleum and construction industries.[12] Carbide tooling is actually a ceramic/metal composite, where metallic cobalt acts as a binding (matrix) material to hold the WC particles in place. This type of industrial use accounts for about 60% of current tungsten consumption.[54]

The jewelry industry makes rings of sintered tungsten carbide, tungsten carbide/metal composites, and also metallic tungsten.[55] A renowned Jeweler specialises in Tungsten wedding bands.[56] WC/metal composite rings use nickel as the metal matrix in place of cobalt because it takes a higher luster when polished. Sometimes manufacturers or retailers refer to tungsten carbide as a metal, but it is a ceramic.[57] Because of tungsten carbide's hardness, rings made of this material are extremely abrasion resistant, and will hold a burnished finish longer than rings made of metallic tungsten. Tungsten carbide rings are brittle, however, and may crack under a sharp blow.[58]

Alloys

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.[59] 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.[60] Tungsten alloys are used in a wide range of different applications, including the aerospace and automotive industries and radiation shielding.[61] Superalloys containing tungsten, such as Hastelloy and Stellite, are used in turbine blades and wear-resistant parts and coatings.

Quenched (martensitic) tungsten steel (approx. 5.5% to 7.0% W with 0.5% to 0.7% C) was used for making hard permanent magnets, due to its high remanence and coercivity, as noted by John Hopkinson (1849 - 1898) as early as 1886. The magnetic properties of a metal or an alloy are very sensitive to microstructure. For example, while the element tungsten is not ferromagnetic (but iron is), when present in steel in these proportions, it stabilizes the martensite phase, which has an enhanced ferromagnetism, as compared to the ferrite (iron) phase, due to its greater resistance to magnetic domain wall motion.

Tungsten's heat resistance makes it useful in arc welding applications when combined with another highly-conductive metal such as silver or copper. The silver or copper provides the necessary conductivity and the tungsten allows the welding rod to withstand the high-temperatures of the arc welding environment.

Mallory metal is proprietary name for an alloy of tungsten, with other metallic elements added to improve machining.[62]

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, in applications where uranium's radioactivity is problematic even in depleted form, or where uranium's additional pyrophoric properties are not desired (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. Germany used tungsten during World War II to produce shells for anti-tank gun designs using the Gerlich squeeze bore principle to achieve very high muzzle velocity and enhanced armor penetration from comparatively small caliber and light weight field artillery. The weapons were highly effective but a shortage of tungsten used in the shell core limited that effectiveness.

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.[63]

Chemical applications

Tungsten(IV) sulfide is a high temperature lubricant and is a component of catalysts for hydrodesulfurization.[64] MoS2 is more commonly used for such applications.[65]

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.[19] Tungsten 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.[66]

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. In Formula One nowadays, a much more advanced material is utilized: a tungsten alloy trademarked, Densamet.[67] Depleted uranium is also used for these purposes, due to similarly high density. Seventy-five-kg blocks of tungsten were used as "cruise balance mass devices" on the entry vehicle portion of the 2012 Mars Science Laboratory spacecraft. 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. High-density alloys of tungsten with nickel, copper or iron are used in high-quality darts[68] (to allow for a smaller diameter and thus tighter groupings) or for fishing lures (tungsten beads allow the fly to sink rapidly). Tungsten has seen use recently in nozzles for 3D printing; the high wear resistance and thermal conductivity of tungsten carbide improves the printing of abrasive filaments.[69] Some cello C strings are wound with tungsten. The extra density gives this string more projection and often cellists will buy just this string and use it with three strings from a different set.[70] Tungsten is used as an absorber on the electron telescope on the Cosmic Ray System of the two Voyager spacecraft.[71]

Sodium tungstate is used in Folin-Ciocalteu's reagent, a mixture of different chemicals used in the "Lowry Assay" for protein content analysis.

Gold substitution

Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum.[15][72] Metallic tungsten is hypoallergenic, and is harder than gold alloys (though not as hard as tungsten carbide), making it useful for rings that will resist scratching, especially in designs with a brushed finish.

Because the density is so similar to that of gold (tungsten is only 0.36% less dense), and its price of the order of one-thousandth, tungsten can also be used in counterfeiting of gold bars, such as by plating a tungsten bar with gold,[73][74][75] which has been observed since the 1980s,[76] or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.[77] The densities are not exactly the same, and other properties of gold and tungsten differ, but gold-plated tungsten will pass superficial tests.[73]

Gold-plated tungsten is available commercially from China (the main source of tungsten), both in jewelry and as bars.[78]

Electronics

Because it retains its strength at high temperatures and has a high melting point, elemental tungsten is used in many high-temperature applications,[79] such as light bulb, cathode-ray tube, and vacuum tube filaments, heating elements, and rocket engine nozzles.[15] 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).

TIG torch breakdown
Tungsten electrode used in a gas tungsten arc welding torch

Because of its conductive properties and relative chemical inertness, 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.[52]

The electronic structure of tungsten makes it one of the main sources for X-ray targets,[80][81] and also for shielding from high-energy radiations (such as in the radiopharmaceutical industry for shielding radioactive samples of FDG). It is also used in gamma imaging as a material from which coded apertures are made, due to its excellent shielding properties. 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.[19] In addition to its high melting point, when tungsten is doped with potassium, it leads to an increased shape stability (compared to non-doped tungsten). This ensures that the filament does not sag, and no undesired changes occur.[82]

Nanowires

Through top-down nanofabrication processes, tungsten nanowires have been fabricated and studied since 2002.[83] Due to a particularly high surface to volume ratio, the formation of a surface oxide layer and the single crystal nature of such material, the mechanical properties differ fundamentally from those of bulk tungsten.[84] Such tungsten nanowires have potential applications in nanoelectronics and importantly as pH probes and gas sensors.[85] In similarity to silicon nanowires, tungsten nanowires are frequently produced from a bulk tungsten precursor followed by a thermal oxidation step to control morphology in terms of length and aspect ratio.[86] Using the Deal–Grove model it is possible to predict the oxidation kinetics of nanowires fabricated through such thermal oxidation processing.[87]

Biological role

Tungsten, at atomic number Z = 74, is the heaviest element known to be biologically functional. It is used by some bacteria and archaea,[88] but not in eukaryotes. For example, enzymes called oxidoreductases use tungsten similarly to molybdenum by using it in a tungsten-pterin complex with molybdopterin (molybdopterin, despite its name, does not contain molybdenum, but may complex with either molybdenum or tungsten in use by living organisms). Tungsten-using enzymes typically reduce carboxylic acids to aldehydes.[89] 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.[90] 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.[91] Acetylene hydratase is an unusual metalloenzyme in that it catalyzes a hydration reaction. Two reaction mechanisms have been proposed, in one of which there is a direct interaction between the tungsten atom and the C≡C triple bond.[92] 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.[93]

In soil, tungsten metal oxidizes to the tungstate anion. It can be selectively or non-selectively imported by some prokaryotic organisms and may substitute for molybdate in certain enzymes. Its effect on the action of these enzymes is in some cases inhibitory and in others positive.[94] The soil's chemistry determines how the tungsten polymerizes; alkaline soils cause monomeric tungstates; acidic soils cause polymeric tungstates.[95]

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.[96]

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.[97]

In archaea

Tungsten is essential for some archaea. The following tungsten-utilizing enzymes are known:

A wtp system is known to selectively transport tungsten in archaea:

Health factors

Because tungsten is a rare metal[99] and its compounds are generally inert, the effects of tungsten on the environment are limited.[100] The abundance of tungsten in the Earth's crust is thought to be about 1.5 parts per million. It is one of the more rare elements.

It was at first believed to be relatively inert and an only slightly toxic metal, but beginning in the year 2000, the risk presented by tungsten alloys, its dusts and particulates to induce cancer and several other adverse effects in animals as well as humans has been highlighted from in vitro and in vivo experiments.[101][102] The median lethal dose LD50 depends strongly on the animal and the method of administration and varies between 59 mg/kg (intravenous, rabbits)[103][104] and 5000 mg/kg (tungsten metal powder, intraperitoneal, rats).[105][106]

People can be exposed to tungsten in the workplace by breathing it in, swallowing it, skin contact, and eye contact. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 5 mg/m3 over an 8-hour workday and a short term limit of 10 mg/m3.[107]

Patent claim

Tungsten is unique amongst the elements in that it has been the subject of patent 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.[108][109][110]

See also

References

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External links

Gas tungsten arc welding

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode is protected from oxidation or other atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.

Group 6 element

Group 6, numbered by IUPAC style, is a group of elements in the periodic table. Its members are chromium (Cr), molybdenum (Mo), tungsten (W), and seaborgium (Sg). These are all transition metals and chromium, molybdenum and tungsten are refractory metals. The period 8 elements of group 6 are likely to be either unpenthexium (Uph) or unpentoctium (Upo). This may not be possible; drip instability may imply that the periodic table ends around unbihexium. Neither unpenthexium nor unpentoctium have been synthesized, and it is unlikely that this will happen in the near future.

Like other groups, the members of this family show patterns in its electron configuration, especially the outermost shells resulting in trends in chemical behavior:

"Group 6" is the new IUPAC name for this group; the old style name was "group VIB" in the old US system (CAS) or "group VIA" in the European system (old IUPAC). Group 6 must not be confused with the group with the old-style group crossed names of either VIA (US system, CAS) or VIB (European system, old IUPAC). That group is now called group 16.

Halogen lamp

A halogen lamp, also known as a tungsten halogen, quartz-halogen or quartz iodine lamp, is an incandescent lamp consisting of a tungsten filament sealed into a compact transparent envelope that is filled with a mixture of an inert gas and a small amount of a halogen such as iodine or bromine. The combination of the halogen gas and the tungsten filament produces a halogen cycle chemical reaction which redeposits evaporated tungsten to the filament, increasing its life and maintaining the clarity of the envelope. For this to happen, a halogen lamp must be operated at a higher envelope temperature (250° C; 482º F) than a standard vacuum incandescent lamp of similar power and operating life; this also produces light with higher luminous efficacy and color temperature. The small size of halogen lamps permits their use in compact optical systems for projectors and illumination. The small glass envelope may be enclosed in a much larger outer glass bulb for a bigger package; the outer jacket will be at a much lower, safer, temperature, protects the hot bulb from harmful contamination, and makes the bulb mechanically more similar to a conventional lamp that it might replace.Standard and halogen incandescent bulbs are much less efficient than LED and compact fluorescent lamps, and have been banned in many jurisdictions because of this.

Incandescent light bulb

An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated to such a high temperature that it glows with visible light (incandescence). The filament is protected from oxidation with a glass or fused quartz bulb that is filled with inert gas or a vacuum. In a halogen lamp, filament evaporation is slowed by a chemical process that redeposits metal vapor onto the filament, thereby extending its life.

The light bulb is supplied with electric current by feed-through terminals or wires embedded in the glass. Most bulbs are used in a socket which provides mechanical support and electrical connections.

Incandescent bulbs are manufactured in a wide range of sizes, light output, and voltage ratings, from 1.5 volts to about 300 volts. They require no external regulating equipment, have low manufacturing costs, and work equally well on either alternating current or direct current. As a result, the incandescent bulb is widely used in household and commercial lighting, for portable lighting such as table lamps, car headlamps, and flashlights, and for decorative and advertising lighting.

Incandescent bulbs are much less efficient than other types of electric lighting; incandescent bulbs convert less than 5% of the energy they use into visible light, with standard light bulbs averaging about 2.2%. The remaining energy is converted into heat. The luminous efficacy of a typical incandescent bulb for 120 V operation is 16 lumens per watt, compared with 60 lm/W for a compact fluorescent bulb or 150 lm/W for some white LED lamps.Some applications of the incandescent bulb (such as heat lamps) deliberately use the heat generated by the filament. Such applications include incubators, brooding boxes for poultry, heat lights for reptile tanks, infrared heating for industrial heating and drying processes, lava lamps, and the Easy-Bake Oven toy. Incandescent bulbs typically have short lifetimes compared with other types of lighting; around 1,000 hours for home light bulbs versus typically 10,000 hours for compact fluorescents and 30,000 hours for lighting LEDs.

Incandescent bulbs have been replaced in many applications by other types of electric light, such as fluorescent lamps, compact fluorescent lamps (CFL), cold cathode fluorescent lamps (CCFL), high-intensity discharge lamps, and light-emitting diode lamps (LED). Some jurisdictions, such as the European Union, China, Canada and United States, are in the process of phasing out the use of incandescent light bulbs while others, including Colombia, Mexico, Cuba, Argentina and Brazil, have prohibited them already.

Isotopes of tungsten

Naturally occurring tungsten (74W) consists of five isotopes. Four are considered stable (182W, 183W, 184W, and 186W) and one is slightly radioactive, 180W, with an extremely long half-life of 1.8 ± 0.2 Ea (1018 years). On average, two alpha decays of 180W occur per gram of natural tungsten per year, so for most practical purposes, tungsten 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. The other naturally occurring isotopes have not been observed to decay, and lower bounds for their half lives have been established:

182W, t1/2 > 7.7×1021 years183W, t1/2 > 4.1×1021 years184W, t1/2 > 8.9×1021 years186W, t1/2 > 8.2×1021 yearsThirty-three artificial radioisotopes of tungsten have been characterized with mass numbers ranging from 157 to 194, 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 11 meta states with mass numbers of 158, 179, with 3, 180, with 2, 183, 185, 186, with 2, and 190, the most stable being 179m1W (t1/2 6.4 minutes).

Kinetic bombardment

A kinetic bombardment or a kinetic orbital strike is the hypothetical act of attacking a planetary surface with an inert projectile, where the destructive force comes from the kinetic energy of the projectile impacting at very high speeds. The concept originated during the Cold War.

The typical depiction of the tactic is of a satellite containing a magazine of tungsten rods and a directional thrust system. (In science fiction, the weapon is often depicted as being launched from a spaceship, instead of a satellite). When a strike is ordered, the launch vehicle would brake one of the rods out of its orbit and into a suborbital trajectory that intersects the target. As the rod approaches periapsis and the target due to gravity, it picks up immense speed until it begins decelerating in the atmosphere and reaches terminal velocity shortly before impact. The rods would typically be shaped to minimize air resistance and maximize terminal velocity.

Kinetic bombardment has the advantage of being able to deliver projectiles from a very high angle at a very high speed, making them extremely difficult to defend against. In addition, projectiles would not require explosive warheads, and—in the simplest designs—would consist entirely of solid metal rods, giving rise to the common nickname "Rods from God". Disadvantages include the technical difficulties of ensuring accuracy and the high costs of positioning ammunition in orbit.

Palm Tungsten

The Tungsten series was Palm, Inc.'s line of business-class Palm OS-based PDAs.

Refractory metals

Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group differs. The most common definition includes five elements: two of the fifth period (niobium and molybdenum) and three of the sixth period (tantalum, tungsten, and rhenium). They all share some properties, including a melting point above 2000 °C and high hardness at room temperature. They are chemically inert and have a relatively high density. Their high melting points make powder metallurgy the method of choice for fabricating components from these metals. Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, and chemical reaction vessels in corrosive environments. Partly due to the high melting point, refractory metals are stable against creep deformation to very high temperatures.

Seaborgium

Seaborgium is a synthetic chemical element with symbol Sg and atomic number 106. It is named after the American nuclear chemist Glenn T. Seaborg. As a synthetic element, it can be created in a laboratory but is not found in nature. It is also radioactive; the most stable known isotope, 269Sg, has a half-life of approximately 14 minutes.In the periodic table of the elements, it is a d-block transactinide element. It is a member of the 7th period and belongs to the group 6 elements as the fourth member of the 6d series of transition metals. Chemistry experiments have confirmed that seaborgium behaves as the heavier homologue to tungsten in group 6. The chemical properties of seaborgium are characterized only partly, but they compare well with the chemistry of the other group 6 elements.

In 1974, a few atoms of seaborgium were produced in laboratories in the Soviet Union and in the United States. The priority of the discovery and therefore the naming of the element was disputed between Soviet and American scientists, and it was not until 1997 that International Union of Pure and Applied Chemistry (IUPAC) established seaborgium as the official name for the element. It is one of only two elements named after a living person at the time of naming, the other being oganesson, element 118.

Sodium tungstate

Sodium tungstate is the inorganic compound with the formula Na2WO4. This white, water-soluble solid is the sodium salt of tungstic acid. It is useful as a source of tungsten for chemical synthesis. It is an intermediate in the conversion of tungsten ores to the metal.

Tungsten(III) oxide

Tungsten(III) oxide (W2O3) is a compound of tungsten and oxygen. It has been reported (2006) as being grown as a thin film by atomic layer deposition at temperatures between 140 and 240 °C using W2(N(CH3)2)6 as a precursor. It is not referred to in major textbooks.

Some older literature refers to the compound W2O3 but as the atomic weight of tungsten was believed at the time to be 92, i.e., approximately half the modern accepted value of 183.84, the compound actually being referred to was WO3.

Tungsten(IV) fluoride

Tungsten tetrafluoride is an inorganic compound with the formula WF4. This little studied solid has been invoked, together with tungsten pentafluoride, as an intermediate in the chemical vapor deposition of tungsten films using tungsten hexafluoride.

Tungsten(IV) oxide

Tungsten dioxide is the chemical compound with the formula WO2. The bronze-colored solid crystallizes in a monoclinic cell. The rutile-like structure features distorted octahedral WO6 centers with alternate short W–W bonds (248 pm). Each tungsten center has the d2 configuration, which gives the material a high electrical conductivity.

WO2 is prepared by reduction of WO3 with tungsten powder over the course of 40 hours at 900 °C. An intermediate in this reaction is the partially reduced, mixed valence species W18O49.

2 WO3 + W → 3 WO2The molybdenum analogue MoO2 is prepared similarly. Single crystals are obtained by chemical transport technique using iodine. Iodine transports the WO2 in the form of the volatile species WO2I2.

Tungsten(V) bromide

Tungsten(V) bromide is the inorganic compound with the empirical formula WBr5. The compound consists of bioctahedral structure, with two bridging bromide ligands, so its molecular formula is W2Br10.

Tungsten carbide

Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through a process called sintering for use in industrial machinery, cutting tools, abrasives, armor-piercing rounds, other tools and instruments, and jewelry.

Tungsten carbide is approximately twice as stiff as steel, with a Young's modulus of approximately 530–700 GPa (77,000 to 102,000 ksi), and is double the density of steel—nearly midway between that of lead and gold. It is comparable with corundum (α-Al2O3) in hardness and can only be polished and finished with abrasives of superior hardness such as cubic boron nitride and diamond powder, wheels, and compounds.

Tungsten disulfide

Tungsten disulfide is the chemical compound with the formula WS2. It occurs naturally as the rare mineral tungstenite. This material is a component of certain catalysts used for hydrodesulfurization and hydrodenitrification.

WS2 adopts a layered structure related to MoS2, with W atoms situated in trigonal prismatic coordination sphere. Owing to this layered structure, WS2 forms inorganic nanotubes, which were discovered on an example of WS2 in 1992.

Tungsten hexafluoride

Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula WF6. It is a toxic, corrosive, colorless gas, with a density of about 13 g/L (roughly 11 times heavier than air.) It is one of the densest known gases under standard conditions. WF6 is commonly used by the semiconductor industry to form tungsten films, through the process of chemical vapor deposition. This layer serves as a low-resistivity metallic "interconnect". It is one of seventeen known binary hexafluorides.

Tungsten trioxide

Tungsten(VI) oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal.

2 WO3 + 3 C → 2 W + 3 CO2 (high temperature)

WO3 + 3 H2 → W + 3 H2O (550 - 850 °C)Tungsten(VI) oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO3·H2O, meymacite WO3·2H2O and hydrotungstite (of the same composition as meymacite, however sometimes written as H2WO4). These minerals are rare to very rare secondary tungsten minerals.

Wolframite

Wolframite, (Fe,Mn)WO4, is an iron manganese tungstate mineral that is the intermediate between ferberite (Fe2+ rich) and hübnerite (Mn2+ rich). Along with scheelite, the wolframite series are the most important tungsten ore minerals. Wolframite is found in quartz veins and pegmatites associated with granitic intrusives. Associated minerals include cassiterite, scheelite, bismuth, quartz, pyrite, galena, sphalerite, and arsenopyrite.

This mineral was historically found in Europe in Bohemia, Saxony, and Cornwall. China reportedly has the world's largest supply of tungsten ore with about 60%. Other producers are Canada, Portugal, Russia, Australia, Thailand, South Korea, Rwanda, Bolivia, the United States, and the Democratic Republic of the Congo.

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