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= Scandium =
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Introduction
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Scandium is a chemical element; it has symbol Sc and atomic number 21.
It is a silvery-white metallic d-block element. Historically, it has
been classified as a rare-earth element, together with yttrium and the
lanthanides. It was discovered in 1879 by spectral analysis of the
minerals euxenite and gadolinite from Scandinavia.
Scandium is present in most of the deposits of rare-earth and uranium
compounds, but it is extracted from these ores in only a few mines
worldwide. Because of the low availability and difficulties in the
preparation of metallic scandium, which was first done in 1937,
applications for scandium were not developed until the 1970s, when the
positive effects of scandium on aluminium alloys were discovered. Its
use in such alloys remains its only major application. The global
trade of scandium oxide is 15-20 tonnes per year.
The properties of scandium compounds are intermediate between those of
aluminium and yttrium. A diagonal relationship exists between the
behavior of magnesium and scandium, just as there is between beryllium
and aluminium. In the chemical compounds of the elements in group 3,
the predominant oxidation state is +3.
Chemical characteristics
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Scandium is a soft metal with a silvery appearance. It develops a
slightly yellowish or pinkish cast when oxidized by air. It is
susceptible to weathering and dissolves slowly in most dilute acids.
It does not react with a 1:1 mixture of nitric acid () and 48.0%
hydrofluoric acid (), possibly due to the formation of an impermeable
passive layer. Scandium turnings ignite in the air with a brilliant
yellow flame to form scandium oxide.
Isotopes
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In nature, scandium is found exclusively as the isotope 45Sc, which
has a nuclear spin of ; this is its only stable isotope.
The known isotopes of scandium range from 37Sc to 62Sc. The most
stable radioisotope is 46Sc, which has a half-life of 83.8 days.
Others are 47Sc, 3.35 days; the positron emitter 44Sc, 4 hours; and
48Sc, 43.7 hours. All of the remaining radioactive isotopes have
half-lives less than 4 hours, and the majority of them have half-lives
less than 2 minutes.
The low mass isotopes are very difficult to create. The initial
detection of 37Sc and 38Sc only resulted in the characterization of
their mass excess.
Scandium also has five nuclear isomers: the most stable of these is
44m2Sc ('t'1/2 = 58.6 h).
The primary decay mode of ground-state scandium isotopes at masses
lower than the only stable isotope, 45Sc, is electron capture (or
positron emission), but the lightest isotopes (37Sc to 39Sc) undergo
proton emission instead, all three of these producing calcium
isotopes. The primary decay mode at masses above 45Sc is beta
emission, producing titanium isotopes.
Occurrence
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In Earth's crust, scandium is not rare. Estimates vary from 18 to 25
ppm, which is comparable to the abundance of cobalt (20-30 ppm).
Scandium is only the 50th most common element on Earth (35th most
abundant element in the crust), but it is the 23rd most common element
in the Sun and the 26th most abundant element in the stars. However,
scandium is distributed sparsely and occurs in trace amounts in many
minerals. Rare minerals from Scandinavia and Madagascar such as
thortveitite, euxenite, and gadolinite are the only known concentrated
sources of this element. Thortveitite can contain up to 45% of
scandium in the form of scandium oxide.
The stable form of scandium is created in supernovas via the
r-process. Also, scandium is created by cosmic ray spallation of the
more abundant iron nuclei.
*28Si + 17n → 45Sc (r-process)
*56Fe + p → 45Sc + 11C + n (cosmic ray spallation)
Production
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The world production of scandium is in the order of 15-20 tonnes per
year, in the form of scandium oxide. The demand is slightly higher,
and both the production and demand keep increasing. In 2003, only
three mines produced scandium: the uranium and iron mines in Zhovti
Vody in Ukraine, the rare-earth mines in Bayan Obo, China, and the
apatite mines in the Kola Peninsula, Russia. Since then, many other
countries have built scandium-producing facilities, including 5
tonnes/year (7.5 tonnes/year ) by Nickel Asia Corporation and Sumitomo
Metal Mining in the Philippines.
In the United States, NioCorp Development hopes to raise $1 billion
toward opening a niobium mine at its Elk Creek site in southeast
Nebraska, which may be able to produce as much as 95 tonnes of
scandium oxide annually. In each case, scandium is a byproduct of the
extraction of other elements and is sold as scandium oxide.
To produce metallic scandium, the oxide is converted to scandium
fluoride and then reduced with metallic calcium.
*
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Madagascar and the Iveland-Evje region in Norway have the only
deposits of minerals with high scandium content, thortveitite ), but
these are not being exploited. The mineral kolbeckite has a very high
scandium content but is not available in any larger deposits.
The absence of reliable, secure, stable, long-term production has
limited the commercial applications of scandium. Despite this low
level of use, scandium offers significant benefits. Particularly
promising is the strengthening of aluminium alloys with as little as
0.5% scandium. Scandium-stabilized zirconia enjoys a growing market
demand for use as a high-efficiency electrolyte in solid oxide fuel
cells.
The USGS reports that, from 2015 to 2019 in the US, the price of small
quantities of scandium ingot has been $107 to $134 per gram, and that
of scandium oxide $4 to $5 per gram.
Compounds
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Scandium chemistry is almost completely dominated by the trivalent
ion, Sc3+. The radii of M3+ ions in the table below indicate that the
chemical properties of scandium ions have more in common with yttrium
ions than with aluminium ions. In part because of this similarity,
scandium is often classified as a lanthanide-like element.
: Ionic radius (pm)
|Al Sc Y La Lu
|53.5 74.5 90.0 103.2 86.1
Oxides and hydroxides
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The oxide scandium oxide and the hydroxide are amphoteric:
: + 3 → (scandate ion)
: + 3 + 3 →
α- and γ-ScOOH are isostructural with their aluminium hydroxide oxide
counterparts. Solutions of in water are acidic due to hydrolysis.
Halides and pseudohalides
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The halides , where X= Cl, Br, or I, are very soluble in water, but
scandium fluoride is insoluble. In all four halides, the scandium is
6-coordinated. The halides are Lewis acids; for example, scandium
fluoride dissolves in a solution containing excess fluoride ion to
form . The coordination number 6 is typical for Sc(III). In the larger
Y3+ and La3+ ions, coordination numbers of 8 and 9 are common.
Scandium triflate is sometimes used as a Lewis acid catalyst in
organic chemistry.
Organic derivatives
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Scandium forms a series of organometallic compounds with
cyclopentadienyl ligands (Cp), similar to the behavior of the
lanthanides. One example is the chlorine-bridged dimer, and related
derivatives of pentamethylcyclopentadienyl ligands.
Uncommon oxidation states
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Compounds that feature scandium in oxidation states other than +3 are
rare but well characterized. The blue-black compound is one of the
simplest. This material adopts a sheet-like structure that exhibits
extensive bonding between the scandium(II) centers. Scandium hydride
is not well understood, although it appears not to be a saline hydride
of Sc(II). As is observed for most elements, a diatomic scandium
hydride has been observed spectroscopically at high temperatures in
the gas phase. Scandium borides and carbides are non-stoichiometric,
as is typical for neighboring elements.
Lower oxidation states (+2, +1, 0) have also been observed in
organoscandium compounds.
History
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Dmitri Mendeleev, who is referred to as the father of the periodic
table, predicted the existence of an element 'ekaboron', with an
atomic mass between 40 and 48 in 1869. Lars Fredrik Nilson and his
team detected this element in the minerals euxenite and gadolinite in
1879. Nilson prepared 2 grams of scandium oxide of high purity. He
named the element scandium, from the Latin 'Scandia' meaning
"Scandinavia". Nilson was apparently unaware of Mendeleev's
prediction, but Per Teodor Cleve recognized the correspondence and
notified Mendeleev.
Metallic scandium was produced for the first time in 1937 by
electrolysis of a eutectic mixture of potassium, lithium, and scandium
chlorides, at 700-800 °C. The first pound of 99% pure scandium metal
was produced in 1960. Production of aluminium alloys began in 1971,
following a US patent. Aluminium-scandium alloys were also developed
in the USSR.
Laser crystals of gadolinium-scandium-gallium garnet (GSGG) were used
in strategic defense applications developed for the Strategic Defense
Initiative (SDI) in the 1980s and 1990s.
Aluminium alloys
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The main application of scandium by weight is in aluminium-scandium
alloys for minor aerospace industry components. These alloys contain
between 0.1% and 0.5% of scandium. They were used in Russian military
aircraft, specifically the Mikoyan-Gurevich MiG-21 and MiG-29.
The addition of scandium to aluminium limits the grain growth in the
heat zone of welded aluminium components. This has two beneficial
effects: the precipitated forms smaller crystals than in other
aluminium alloys, and the volume of precipitate-free zones at the
grain boundaries of age-hardening aluminium alloys is reduced. The
precipitate is a coherent precipitate that strengthens the aluminum
matrix by applying elastic strain fields that inhibit dislocation
movement (i.e., plastic deformation). has an equilibrium L12
superlattice structure exclusive to this system.
A fine dispersion of nano scale precipitate can be achieved via heat
treatment that can also strengthen the alloys through order hardening.
Recent developments include the additions of transition metals such as
zirconium (Zr) and rare earth metals like erbium (Er) produce shells
surrounding the spherical precipitate that reduce coarsening.
These shells are dictated by the diffusivity of the alloying element
and lower the cost of the alloy due to less Sc being substituted in
part by Zr while maintaining stability and less Sc being needed to
form the precipitate. These have made somewhat competitive with
titanium alloys along with a wide array of applications. However,
titanium alloys, which are similar in lightness and strength, are
cheaper and much more widely used.
The alloy is as strong as titanium, light as aluminium, and hard as
some ceramics.
Some items of sports equipment, which rely on lightweight
high-performance materials, have been made with scandium-aluminium
alloys, including baseball bats, tent poles and bicycle frames and
components. Lacrosse sticks are also made with scandium. The American
firearm manufacturing company Smith & Wesson produces
semi-automatic pistols and revolvers with frames of scandium alloy and
cylinders of titanium or carbon steel.
Since 2013, Apworks GmbH, a spin-off of Airbus, have marketed a high
strength Scandium containing aluminium alloy processed using metal
3D-Printing (Laser Powder Bed Fusion) under the trademark Scalmalloy
which claims very high strength & ductility.
Light sources
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The first scandium-based metal-halide lamps were patented by General
Electric and made in North America, although they are now produced in
all major industrialized countries. Approximately 20 kg of scandium
(as ) is used annually in the United States for high-intensity
discharge lamps. One type of metal-halide lamp, similar to the
mercury-vapor lamp, is made from scandium triiodide and sodium iodide.
This lamp is a white-light source with high color rendering index that
sufficiently resembles sunlight to allow good color-reproduction with
TV cameras. About 80 kg of scandium is used in metal-halide
lamps/light bulbs globally per year.
Dentists use erbium-chromium-doped yttrium-scandium-gallium garnet ()
lasers for cavity preparation and in endodontics.
Other
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The radioactive isotope 46Sc is used in oil refineries as a tracing
agent. Scandium triflate is a catalytic Lewis acid used in organic
chemistry.
The 12.4 keV nuclear transition of 45Sc has been studied as a
reference for timekeeping applications, with a theoretical precision
as much as three orders of magnitude better than the current caesium
reference clocks.
Scandium has been proposed for use in solid oxide fuel cells (SOFCs)
as a dopant in the electrolyte material, typically zirconia (ZrO₂).
Scandium oxide (Sc₂O₃) is one of several possible additives to enhance
the ionic conductivity of the zirconia, improving the overall thermal
stability, performance and efficiency of the fuel cell. This
application would be particularly valuable in clean energy
technologies, as SOFCs can utilize a variety of fuels and have high
energy conversion efficiencies.
Health and safety
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Elemental scandium is considered non-toxic, though extensive animal
testing of scandium compounds has not been done. The median lethal
dose (LD50) levels for scandium chloride for rats have been determined
as 755 mg/kg for intraperitoneal and 4 g/kg for oral administration.
In the light of these results, compounds of scandium should be handled
as compounds of moderate toxicity. Scandium appears to be handled by
the body in a manner similar to gallium, with similar hazards
involving its poorly soluble hydroxide.
External links
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*[
http://www.periodicvideos.com/videos/021.htm Scandium] at 'The
Periodic Table of Videos' (University of Nottingham)
*[
http://www.webelements.com/webelements/elements/text/Sc/index.html
WebElements.com - Scandium]
*
License
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Original Article:
http://en.wikipedia.org/wiki/Scandium
