The chemical inertness and very high melting point of tantalum make it valuable for laboratory and industrial equipment such asreaction vessels andvacuum furnaces. It is used intantalum capacitors for electronic equipment such as computers. It is being investigated for use as a material for high-quality superconducting resonators in quantum processors.
Tantalum was discovered in Sweden in 1802 byAnders Ekeberg, in two mineral samples – one from Sweden and the other from Finland.[12][13] One year earlier,Charles Hatchett had discoveredcolumbium (now niobium).[14] In 1809, the English chemistWilliam Hyde Wollaston compared the oxides of columbium and tantalum,columbite andtantalite. Although the two oxides had different measured densities of 5.918 g/cm3 and 7.935 g/cm3, he concluded that they were identical and kept the name tantalum.[15] 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 chemistHeinrich Rose, who argued that there were two additional elements in the tantalite sample, and he named them after the children ofTantalus: niobium (fromNiobe), and pelopium (fromPelops).[16][17] 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.[citation needed]
The differences between tantalum and niobium were demonstrated unequivocally in 1864 byChristian Wilhelm Blomstrand,[18] andHenri Etienne Sainte-Claire Deville, as well as byLouis J. Troost, who determined the empirical formulas of some of their compounds in 1865.[18][19] Further confirmation came from the Swiss chemistJean Charles Galissard de Marignac,[20] in 1866, who proved that there were only two elements. These discoveries did not stop scientists from publishing articles about the so-calledilmenium until 1871.[21] De Marignac was the first to produce the metallic form of tantalum in 1864, when hereduced tantalum chloride by heating it in an atmosphere ofhydrogen.[citation needed] Early investigators had only been able to produce impure tantalum, and the first relatively pure ductile metal was produced byWerner von Bolton inCharlottenburg in 1903. Wires made with metallic tantalum were used forlight bulb filaments untiltungsten replaced it in widespread use.[22]
The name tantalum was derived from the name of the mythological Tantalus, the father of Niobe inGreek mythology. 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 eternallytantalized 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.)[23] Anders Ekeberg wrote "This metal I calltantalum ... partly in allusion to its incapacity, when immersed in acid, to absorb any and be saturated."[24]
Tantalum is dark (blue-gray),[25] dense, ductile, very hard, easily fabricated, and highly conductive of heat and electricity. The metal is highly resistant tocorrosion byacids: at temperatures below 150 °C tantalum is almost completely immune to attack by the normally aggressiveaqua regia. It can be dissolved withhydrofluoric acid or acidic solutions containing thefluoride ion andsulfur trioxide, as well as with moltenpotassium hydroxide. Tantalum's high melting point of 3017 °C (boiling point 5458 °C) is exceeded among the elements only bytungsten,rhenium andosmium for metals, andcarbon.
Tantalum exists in two crystalline phases, alpha and beta. The alpha phase is stable at all temperatures up to the melting point and hasbody-centered cubic structure with lattice constanta = 0.33029 nm at 20 °C.[3] It is relativelyductile, hasKnoop hardness 200–400 HN and electrical resistivity 15–60 μΩ⋅cm. The beta phase is hard and brittle; its crystal symmetry istetragonal (space groupP42/mnm,a = 1.0194 nm,c = 0.5313 nm), Knoop hardness is 1000–1300 HN and electrical resistivity is relatively high at 170–210 μΩ⋅cm. 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[26] obtained by magnetronsputtering,chemical vapor deposition orelectrochemical deposition from aeutectic molten salt solution.[27]
Natural tantalum consists of two stableisotopes:180mTa (0.012%) and181Ta (99.988%).180mTa (m denotes a metastable state) is predicted to decay in three ways:isomeric transition to theground state of180Ta,beta decay to180W, or electron capture to180Hf. However, radioactivity of thisnuclear isomer has never been observed, and only a lower limit on itshalf-life of 2.9×1017 years has been set.[28] The ground state of180Ta has a half-life of only 8 hours.180mTa is the only naturally occurringnuclear isomer (excludingradiogenic andcosmogenic short-lived nuclides). It is also the rarest primordial isotope in the Universe, taking into account the elemental abundance of tantalum and isotopic abundance of180mTa in the natural mixture of isotopes (and again excluding radiogenic and cosmogenic short-lived nuclides).[29]
Tantalum has been examined theoretically as a "salting" material fornuclear weapons (cobalt is the better-known hypothetical salting material). An external shell of181Ta would be irradiated by the intensive high-energy neutron flux from a hypothetical exploding nuclear weapon. This would transmute the tantalum into the radioactive isotope182Ta, which has ahalf-life of 114.4 days and producesgamma rays with approximately 1.12 million electron-volts (MeV) of energy apiece, which would significantly increase the radioactivity of thenuclear fallout from the explosion for several months. Such "salted" weapons have never been built or tested, as far as is publicly known, and certainly never used as weapons.[30]
Tantalum can be used as a target material for accelerated proton beams for the production of various short-lived isotopes including8Li,80Rb, and160Yb.[31]
Tantalum forms compounds in oxidation states −III to +V. Most commonly encountered are oxides of Ta(V), which includes all minerals. The chemical properties of Ta and Nb are very similar. In aqueous media, Ta only exhibit the +V oxidation state. Like niobium, tantalum is barely soluble in dilute solutions ofhydrochloric,sulfuric,nitric andphosphoric acids due to the precipitation of hydrous Ta(V) oxide.[32] In basic media, Ta can be solubilized due to the formation of polyoxotantalate species.[33]
Tantalum pentoxide (Ta2O5) is the most important compound from the perspective of applications. Oxides of tantalum in lower oxidation states are numerous, including manydefect structures, and are lightly studied or poorly characterized.[34]
Tantalates, compounds containing [TaO4]3− or [TaO3]− are numerous.Lithium tantalate (LiTaO3) adopts a perovskite structure.Lanthanum tantalate (LaTaO4) contains isolatedTaO3− 4 tetrahedra.[35]
As in the cases of otherrefractory metals, the hardest known compounds of tantalum are nitrides and carbides.Tantalum carbide, TaC, like the more commonly usedtungsten carbide, is a hardceramic that is used in cutting tools. Tantalum(III) nitride is used as a thin film insulator in some microelectronic fabrication processes.[36]
Tantalum halides span the oxidation states of +5, +4, and +3.Tantalum pentafluoride (TaF5) is a white solid with a melting point of 97.0 °C. The anion [TaF7]2- is used for its separation from niobium.[37] The chlorideTaCl 5, which exists as a dimer, is the main reagent in synthesis of new Ta compounds. It hydrolyzes readily to anoxychloride. The lower halidesTaX 4 andTaX 3, feature Ta-Ta bonds.[35][32]
Organotantalum compounds includepentamethyltantalum, mixed alkyltantalum chlorides, alkyltantalum hydrides, alkylidene complexes as well as cyclopentadienyl derivatives of the same.[38][39] Diverse salts and substituted derivatives are known for the hexacarbonyl [Ta(CO)6]− and relatedisocyanides.
Tantalum is estimated to make up about 1 ppm[40] or 2 ppm[32] of theEarth's crust by weight. There are many species of tantalum minerals, only some of which are so far being used by industry as raw materials:tantalite (a series consisting of tantalite-(Fe), tantalite-(Mn) and tantalite-(Mg)),microlite (now a group name),wodginite,euxenite (actually euxenite-(Y)), andpolycrase (actually polycrase-(Y)).[41] Tantalite (Fe,Mn)Ta2O6 is the most important mineral for tantalum extraction. Tantalite has the same mineral structure ascolumbite (Fe,Mn) (Ta,Nb)2O6; when there is more tantalum than niobium it is called tantalite and when there is more niobium than tantalum is it called columbite (orniobite). The high density of tantalite and other tantalum containing minerals makes the use ofgravitational separation the best method. Other minerals includesamarskite andfergusonite.
Tantalum producers in 2015 with Rwanda being the main producer
Australia was the main producer of tantalum prior to the 2010s, withGlobal Advanced Metals (formerly known asTalison Minerals) being the largest tantalum mining company in that country. They operate two mines in Western Australia,Greenbushes in the southwest andWodgina in thePilbara region. The Wodgina mine was reopened in January 2011 after mining at the site was suspended in late 2008 due to theglobal financial crisis.[42] Less than a year after it reopened, Global Advanced Metals announced that due to again "... softening tantalum demand ...", and other factors, tantalum mining operations were to cease at the end of February 2012.[43] Wodgina produces a primary tantalum concentrate which is further upgraded at the Greenbushes operation before being sold to customers.[44] Whereas the large-scale producers of niobium are inBrazil andCanada, the ore there also yields a small percentage of tantalum. Some other countries such asChina,Ethiopia, andMozambique 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 inThailand andMalaysia as a by-product of thetin mining there. During gravitational separation of the ores from placer deposits, not only iscassiterite (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.[19][45]
Tantalum producers in 2006 with Australia being the main producer
World tantalum mine production has undergone an important geographic shift since the start of the 21st century when production was predominantly from Australia and Brazil. Beginning in 2007 and through 2014, the major sources of tantalum production from mines dramatically shifted to theDemocratic Republic of the Congo,Rwanda, and some other African countries.[46] Future sources of supply of tantalum, in order of estimated size, are being explored inSaudi Arabia,Egypt,Greenland, China, Mozambique, Canada, Australia, theUnited States,Finland, and Brazil.[47][48]
Tantalum is considered aconflict resource.Coltan, the industrial name for acolumbite–tantalite mineral from which niobium and tantalum are extracted,[49] can also be found inCentral Africa, which is why tantalum is being linked towarfare in the Democratic Republic of the Congo (formerlyZaire). According to an October 23, 2003United Nations report,[50] 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[51] – making it the world's deadliest documented conflict sinceWorld War II. 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 theCongo Basin.[52][53][54][55] TheUnited States Geological Survey reports in its yearbook that this region produced a little less than 1% of the world's tantalum output in 2002–2006, peaking at 10% in 2000 and 2008.[45] USGS data published in January 2021 indicated that close to 40% of the world's tantalum mine production came from the Democratic Republic of the Congo, with another 18% coming from neighboringRwanda andBurundi.[56]
Several steps are involved in the extraction of tantalum from tantalite. First, the mineral iscrushed and concentrated bygravity separation. This is generally carried out near themine site.
The refining of tantalum from its ores is one of the more demanding separation processes in industrial metallurgy. The chief problem is that tantalum ores contain significant amounts ofniobium, which has chemical properties almost identical to those of Ta. A large number of procedures have been developed to address this challenge.
In modern times, the separation is achieved byhydrometallurgy.[58] Extraction begins withleaching the ore withhydrofluoric acid together withsulfuric acid orhydrochloric acid. This step allows the tantalum and niobium to be separated from the various non-metallic impurities in the rock. Although Ta occurs as various minerals, it is conveniently represented as the pentoxide, since most oxides of tantalum(V) behave similarly under these conditions. A simplified equation for its extraction is thus:
Ta2O5 + 14 HF → 2 H2[TaF7] + 5 H2O
Completely analogous reactions occur for the niobium component, but the hexafluoride is typically predominant under the conditions of the extraction.
Nb2O5 + 12 HF → 2 H[NbF6] + 5 H2O
These equations are simplified: it is suspected that bisulfate (HSO4−) and chloride compete as ligands for the Nb(V) and Ta(V) ions, when sulfuric and hydrochloric acids are used, respectively.[58] The tantalum and niobium fluoride complexes are then removed from theaqueous solution byliquid-liquid extraction intoorganic solvents, such ascyclohexanone,octanol, andmethyl isobutyl ketone. This simple procedure allows the removal of most metal-containing impurities (e.g. iron, manganese, titanium, zirconium), which remain in the aqueous phase in the form of theirfluorides and other complexes.
Separation of the tantalumfrom niobium is then achieved by lowering theionic strength of the acid mixture, which causes the niobium to dissolve in the aqueous phase. It is proposed thatoxyfluoride H2[NbOF5] is formed under these conditions. Subsequent to removal of the niobium, the solution of purified H2[TaF7] is neutralised with aqueousammonia to precipitate hydrated tantalum oxide as a solid, which can becalcined totantalum pentoxide (Ta2O5).[59]
In an older method, called theMarignac process, the mixture of H2[TaF7] and H2[NbOF5] was converted to amixture of K2[TaF7] and K2[NbOF5], which was then separated byfractional crystallization, exploiting their different water solubilities.
Tantalum can also be refined by electrolysis, using a modified version of theHall–Héroult process. Instead of requiring the input oxide and output metal to be in liquid form, tantalum electrolysis operates on non-liquid powdered oxides. The initial discovery came in 1997 when Cambridge University researchers immersed small samples of certain oxides in baths of molten salt and reduced the oxide with electric current. The cathode uses powdered metal oxide. The anode is made of carbon. The molten salt at 1,000 °C (1,830 °F) is the electrolyte. The first refinery has enough capacity to supply 3–4% of annual global demand.[61]
Allwelding of tantalum must be done in an inert atmosphere ofargon orhelium in order to shield it from contamination with atmospheric gases. Tantalum is notsolderable. Grinding tantalum is difficult, especially so forannealed tantalum. In the annealed condition, tantalum is extremelyductile and can be readily formed as metal sheets.[62]
The major use for tantalum, as the metal powder, is in the production of electronic components, mainlycapacitors and some high-powerresistors.Tantalum electrolytic capacitors exploit the tendency of tantalum to form a protectiveoxide surface layer, using tantalum powder, pressed into a pellet shape, as one "plate" of the capacitor, the oxide as thedielectric, and an electrolytic solution or conductive solid as the other "plate". Because thedielectric layer can be very thin (thinner than the similar layer in, for instance, an aluminium electrolytic capacitor), a highcapacitance can be achieved in a small volume. Because of the size and weight advantages, tantalum capacitors are attractive forportable telephones,personal computers,automotive electronics andcameras.[63]
Tantalum is also used to produce a variety ofalloys that have high melting points, strength, and ductility. Alloyed with other metals, it is also used in making carbide tools for metalworking equipment and in the production ofsuperalloys for jet engine components, chemical process equipment,nuclear reactors, missile parts, heat exchangers, tanks, and vessels.[64][63][65] Because of its ductility, tantalum can be drawn into fine wires or filaments, which are used for evaporating metals such asaluminium.
Tantalum is inert against most acids excepthydrofluoric acid and hotsulfuric acid, and hotalkaline solutions also cause tantalum to corrode. This property makes it a useful metal for chemical reaction vessels and pipes for corrosive liquids. Heat exchanging coils for the steam heating of hydrochloric acid are made from tantalum.[66] Tantalum was extensively used in the production ofultra high frequencyelectron tubes for radio transmitters. Tantalum is capable of capturing oxygen and nitrogen by forming nitrides and oxides and therefore helped to sustain the high vacuum needed for the tubes when used for internal parts such as grids and plates.[37][66]
Medical researcher Gerald L. Burke at the Los Angeles Orthopaedic Hospital first discovered in 1938 that tantalum is bio-inert in human tissue and could be used safely as an orthopaedic implant material. [[67]] Burke also demonstrated perhaps the other most appreciated characteristic of tantalum in surgical procedures:tantalum would permanently bond to bone with no degradation of the surrounding bone. Later, Burke's team working with a team from the California Institute of Technology led by John Norton Wilson showed that tantalum, while hard enough to be fabricated into surgical tools, could also be fabricated in a form sufficiently ductile, yet still sufficiently strong to be drawn into fine threads that could be used for non-scarring sutures. Burke's team in 1940 was the first to propose the use of tantalum for arthroplasty procedures, the repair of intertrochanteric fractures, and for jaw repairs and dental implants. Burke's initial biological research results were confirmed and credited in greater detail by the Harvard Medical School in a series of neurological experiments using powdered tantalum implants.[68] More than 50 years later, researchers were still refining and documenting their understanding of the basic surgical procedures developed by Burke after his pioneering discoveries.[69]
Nowadays, in spite of the cost, tantalum is still widely used in making surgical instruments and implants, and new procedures continue to be developed. For example, porous tantalum coatings are used in the construction of titanium implants due to tantalum's exceptional ability to form a direct bond to hard tissue.[70]Because tantalum is a non-ferrous, non-magnetic metal, tantalum implants are considered to be acceptable for patients undergoing MRI procedures.[71]
Tantalum was used by NASA to shield components of spacecraft, such asVoyager 1 andVoyager 2, from radiation.[72] The high melting point and oxidation resistance led to the use of the metal in the production ofvacuum furnace parts. Tantalum is extremely inert and is therefore formed into a variety of corrosion resistant parts, such asthermowells, valve bodies, and tantalum fasteners. Due to its high density,shaped charge andexplosively formed penetrator liners have been constructed from tantalum.[73] Tantalum greatly increases the armor penetration capabilities of a shaped charge due to its high density and high melting point.[74][75] It is also occasionally used in preciouswatches e.g. fromAudemars Piguet,F.P. Journe,Hublot,Montblanc,Omega, andPanerai. Tantalum oxide is used to make special highrefractive indexglass forcamera lenses.[76] Spherical tantalum powder, produced by atomizing molten tantalum using gas or liquid, is commonly used inadditive manufacturing due to its uniform shape, excellent flowability, and high melting point.[77][78]
Tantalum receives far less attention in the environmental field than it does in other geosciences. Upper Crust Concentration (UCC) and the Nb/Ta ratio in the upper crust and in minerals are available because these measurements are useful as a geochemical tool.[79] The latest value for upper crust concentration is 0.92 ppm, and the Nb/Ta(w/w) ratio stands at 12.7.[80]
Little data is available on tantalum concentrations in the different environmental compartments, especially in natural waters where reliable estimates of ‘dissolved’ tantalum concentrations in seawater and freshwaters have not even been produced.[81] Some values on dissolved concentrations in oceans have been published, but they are contradictory. Values in freshwaters fare little better, but, in all cases, they are probably below 1 ng L−1, since ‘dissolved’ concentrations in natural waters are well below most current analytical capabilities.[82] Analysis requires pre-concentration procedures that, for the moment, do not give consistent results. And in any case, tantalum appears to be present in natural waters mostly as particulate matter rather than dissolved.[81]
Values for concentrations in soils, bed sediments and atmospheric aerosols are easier to come by.[81] Values in soils are close to 1 ppm and thus to UCC values. This indicates detrital origin. For atmospheric aerosols the values available are scattered and limited. When tantalum enrichment is observed, it is probably due to loss of more water-soluble elements in aerosols in the clouds.[83]
Pollution linked to human use of the element has not been detected.[84] Tantalum appears to be a very conservative element in biogeochemical terms, but its cycling and reactivity are still not fully understood.
Compounds containing tantalum are rarely encountered in the laboratory. The metal is highlybiocompatible[67] and is used for bodyimplants andcoatings, therefore attention may be focused on other elements or the physical nature of thechemical compound.[85]
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