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= Tantalum =
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Introduction
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Tantalum is a chemical element; it has symbol Ta and atomic number 73.
It is named after Tantalus, a figure in Greek mythology. Tantalum is a
very hard, ductile, lustrous, blue-gray transition metal that is
highly corrosion-resistant. It is part of the refractory metals group,
which are widely used as components of strong high-melting-point
alloys. It is a group 5 element, along with vanadium and niobium, and
it always occurs in geologic sources together with the chemically
similar niobium, mainly in the mineral groups tantalite, columbite,
and coltan.
The chemical inertness and very high melting point of tantalum make it
valuable for laboratory and industrial equipment such as reaction
vessels and vacuum furnaces. It is used in tantalum capacitors for
electronic equipment such as computers. It is being investigated for
use as a material for high-quality superconducting resonators in
quantum processors.
History
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Tantalum was discovered in Sweden in 1802 by Anders Ekeberg, in two
mineral samples - one from Sweden and the other from Finland. One year
earlier, Charles Hatchett had discovered columbium (now niobium). In
1809, the English chemist William Hyde Wollaston compared the oxides
of columbium and tantalum, columbite and tantalite. 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. After Friedrich 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: niobium
(from Niobe), 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.
The differences between tantalum and niobium were demonstrated
unequivocally in 1864 by Christian Wilhelm Blomstrand, and Henri
Etienne Sainte-Claire Deville, as well as by Louis J. Troost, who
determined the empirical formulas of some of their compounds in 1865.
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. De Marignac was
the first to produce the metallic form of tantalum in 1864, when he
reduced tantalum chloride by heating it in an atmosphere of hydrogen.
Early investigators had only been able to produce impure tantalum, and
the first relatively pure ductile metal was produced by Werner von
Bolton in Charlottenburg in 1903. 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. 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.) Anders 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, 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.
Physical properties
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Tantalum is dark (blue-gray), dense, ductile, very hard, easily
fabricated, and highly conductive of heat and electricity. The metal
is highly resistant to corrosion by acids: 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 molten potassium hydroxide. Tantalum's high melting point
of 3017 °C (boiling point 5458 °C) is exceeded among the elements only
by tungsten, rhenium, and osmium for metals, and carbon.
Tantalum exists in two crystalline phases, alpha and beta. The alpha
phase is stable at all temperatures up to the melting point and has
body-centered cubic structure with lattice constant 'a' = 0.33029 nm
at 20 °C. It is relatively ductile, has Knoop hardness 200-400 HN and
electrical resistivity 15-60 μΩ⋅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 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 obtained by magnetron
sputtering, chemical vapor deposition or electrochemical deposition
from a eutectic molten salt solution.
Isotopes
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Natural tantalum consists of two stable isotopes: 180mTa (0.012%) and
181Ta (99.988%). 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. However,
radioactivity of this nuclear isomer has never been observed, and only
a lower limit on its half-life of 2.9 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-lived nuclides). It is also the rarest primordial
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-lived
nuclides).
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. 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. Such "salted"
weapons have never been built or tested, as far as is publicly known,
and certainly never used as weapons.
Tantalum can be used as a target material for accelerated proton beams
for the production of various short-lived isotopes including 8Li,
80Rb, and 160Yb.
Chemical compounds
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Tantalum forms compounds in oxidation states −3 to +5. 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 exhibits the +5 oxidation state. Like niobium, tantalum is
barely soluble in dilute solutions of hydrochloric, sulfuric, nitric
and phosphoric acids due to the precipitation of hydrous Ta(V) oxide.
In basic media, Ta can be solubilized due to the formation of
polyoxotantalate species.
Oxides, nitrides, carbides, sulfides
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Tantalum pentoxide (Ta2O5) is the most important compound from the
perspective of applications. Oxides of tantalum in lower oxidation
states are numerous, including many defect structures, and are lightly
studied or poorly characterized.
Tantalates, compounds containing [TaO4]3− or [TaO3]− are numerous.
Lithium tantalate (LiTaO3) adopts a perovskite structure. Lanthanum
tantalate (LaTaO4) contains isolated tetrahedra.
As in the cases of other refractory metals, the hardest known
compounds of tantalum are nitrides and carbides. Tantalum carbide,
TaC, like the more commonly used tungsten carbide, is a hard ceramic
that is used in cutting tools. Tantalum(III) nitride is used as a thin
film insulator in some microelectronic fabrication processes.
The best studied chalcogenide is Tantalum sulfide (TaS2), a layered
semiconductor, as seen for other transition metal dichalcogenides. A
tantalum-tellurium alloy forms quasicrystals.
Halides
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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. The
chloride tantalum(V) chloride, which exists as a dimer, is the main
reagent in synthesis of new Ta compounds. It hydrolyzes readily to an
oxychloride. The lower halides and , feature Ta-Ta bonds.
Organotantalum compounds
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Organotantalum compounds include pentamethyltantalum, mixed
alkyltantalum chlorides, alkyltantalum hydrides, alkylidene complexes,
as well as cyclopentadienyl derivatives of the same. Diverse salts and
substituted derivatives are known for the hexacarbonyl [Ta(CO)6]− and
related isocyanides.
Occurrence
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Tantalum is estimated to make up about 1 ppm or 2 ppm of the Earth'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)), and polycrase (actually polycrase-(Y)).
Tantalite (Fe, Mn)Ta2O6 is the most important mineral for tantalum
extraction. Tantalite has the same mineral structure as columbite (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 (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.
Australia was the main producer of tantalum prior to the 2010s, with
Global Advanced Metals (formerly known as Talison Minerals) being the
largest tantalum mining company in that country. They operate 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 2008
financial crisis. 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.
Wodgina 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. 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. 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.
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 the Democratic Republic of the Congo, Rwanda, and some
other African countries. 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.
Status as a conflict resource
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Tantalum is considered a conflict resource. Coltan, the industrial
name for a columbite-tantalite mineral from which niobium and tantalum
are extracted, can also be found in Central Africa, which is why
tantalum is being linked to warfare in the Democratic Republic of the
Congo (formerly Zaire). 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. 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. The United
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. 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 neighboring Rwanda and Burundi.
Production and fabrication
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Several steps are involved in the extraction of tantalum from
tantalite. First, the mineral is crushed and concentrated by gravity
separation. This is generally carried out near the mine site.
Refining
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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 of niobium, 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 by hydrometallurgy.
Extraction begins with leaching the ore with hydrofluoric acid
together with sulfuric acid or hydrochloric 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. The tantalum
and niobium fluoride complexes are then removed from the aqueous
solution by liquid-liquid extraction into organic solvents, such as
cyclohexanone, octanol, and methyl 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 their fluorides and other complexes.
Separation of the tantalum 'from' niobium is then achieved by lowering
the ionic strength of the acid mixture, which causes the niobium to
dissolve in the aqueous phase. It is proposed that oxyfluoride
H2[NbOF5] is formed under these conditions. Subsequent to removal of
the niobium, the solution of purified H2[TaF7] is neutralised with
aqueous ammonia to precipitate hydrated tantalum oxide as a solid,
which can be calcined to tantalum pentoxide (Ta2O5).
Instead of hydrolysis, the H2[TaF7] can be treated with potassium
fluoride to produce potassium heptafluorotantalate:
: H2[TaF7] + 2 KF → K2[TaF7] + 2 HF
Unlike H2[TaF7], the potassium salt is readily crystallized and
handled as a solid.
K2[TaF7] can be converted to metallic tantalum by reduction with
sodium, at approximately 800 °C in molten salt.
: K2[TaF7] + 5 Na → Ta + 5 NaF + 2 KF
In an older method, called the Marignac process, the mixture of
H2[TaF7] and H2[NbOF5] was converted to a 'mixture' of K2[TaF7] and
K2[NbOF5], which was then separated by fractional crystallization,
exploiting their different water solubilities.
Electrolysis
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Tantalum can also be refined by electrolysis, using a modified version
of the Hall-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 1000 C is the electrolyte. The first
refinery has enough capacity to supply 3-4% of annual global demand.
Fabrication and metalworking
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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.
Electronics
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The major use for tantalum, as the metal powder, is in the production
of electronic components, mainly capacitors and some high-power
resistors. Tantalum electrolytic capacitors exploit the tendency of
tantalum to form a protective oxide surface layer, 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. Because of the size and weight advantages, tantalum
capacitors are attractive for portable telephones, personal computers,
automotive electronics and cameras.
Alloys
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Tantalum is also used to produce a variety of alloys 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 of superalloys for jet engine components, chemical
process equipment, nuclear reactors, missile parts, heat exchangers,
tanks, and vessels. Because of its ductility, tantalum can be drawn
into fine wires or filaments, which are used for evaporating metals
such as aluminium.
Tantalum is inert against most acids except hydrofluoric acid and hot
sulfuric acid, and hot alkaline 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. Tantalum
was extensively used in the production of ultra high frequency
electron 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.
Surgical uses
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Tantalum is widely used in surgery because of two unique
characteristics of tantalum. Tantalum's hardness and ductility is
useful in making sharp, durable surgical instruments and also for
monofilament sutures. However, a completely unrelated use for tantalum
in surgery arises from its unique ability to form a lasting and
durable structural bond with human hard tissue, making it uniquely
useful for bone and dental implants. Tantalum coatings are
increasingly used in the construction of complex tantalum-coated
titanium surgical implants due to the tantalum plating's ability to
form a strong and biologically stable bond to hard tissue. An
incidental consequence of its use for durable surgical implants is
that tantalum implants are considered to be acceptable for patients
undergoing MRI procedures because tantalum is a non-ferrous,
non-magnetic metal.
Other uses
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Tantalum was used by NASA to shield components of spacecraft, such as
'Voyager 1' and 'Voyager 2', from radiation. The high melting point
and oxidation resistance led to the use of the metal in the production
of vacuum furnace parts. Tantalum is extremely inert and is therefore
formed into a variety of corrosion resistant parts, such as
thermowells, valve bodies, and tantalum fasteners. Due to its high
density, shaped charge and explosively formed penetrator liners have
been constructed from tantalum. 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, F. P. Journe, Hublot, Montblanc,
Omega, and Panerai. Tantalum oxide is used to make special high
refractive index glass for camera lenses. Spherical tantalum powder,
produced by atomizing molten tantalum using gas or liquid, is commonly
used in additive manufacturing due to its uniform shape, excellent
flowability, and high melting point.
Environmental issues
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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. The latest value
for upper crust concentration is 0.92 ppm, and the Nb/Ta(w/w) ratio
stands at 12.7.
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. 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. 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.
Values for concentrations in soils, bed sediments and atmospheric
aerosols are easier to come by. 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.
Pollution linked to human use of the element has not been detected.
Tantalum appears to be a very conservative element in biogeochemical
terms, but its cycling and reactivity are still not fully understood.
Precautions
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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.
People can be exposed to tantalum in the workplace by breathing it in,
skin contact, or eye contact. The Occupational Safety and Health
Administration (OSHA) has set the legal limit (permissible exposure
limit) for tantalum exposure in the workplace as 5 mg/m3 over an
8-hour workday. 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. There is a
paradox arising because of tantalum's ability to form a strong and
permanent bond with bone tissue: at levels of 2500 mg/m3, tantalum
dust becomes immediately dangerous to life and health if tantalum dust
accidentally bonds with the wrong tissue.
External links
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* [
http://tanb.org/ Tantalum-Niobium International Study Center]
* [
https://www.cdc.gov/niosh/npg/npgd0585.html CDC - NIOSH Pocket
Guide to Chemical Hazards]
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=========
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Original Article:
http://en.wikipedia.org/wiki/Tantalum
