Transition metal
Scandium is a chemical element; it has symbol Sc and atomic number 21. It is a silvery-white metallic element found in the d-block of the periodic table. Usually, it is 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.
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.
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 dissolve in a 1:1 mixture of nitric acid (HNO3) and 48.0% hydrofluoric acid (HF), 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.
In nature, scandium is found exclusively as the isotope 45Sc, which has a nuclear spin of 7⁄2; this is its only stable isotope.
The known isotopes of scandium range from 37Sc to 63Sc, and the most stable radioisotopes are 46Sc with a half-life of 83.76 days, 47Sc with a half-life of 3.3492 days, 48Sc at 43.67 hours, 44Sc at 4.042 hours, and 43Sc at 3.891 hours. All others have half-lives shorter than an hour, and the majority of these shorter than 15 seconds. The most stable meta state is 44m3Sc with half-life 58.6 hours; this is the lightest isotope with a long-lived isomer.
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.
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. …
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, all of which are sources of other rare earths. Thortveitite can contain up to 45% scandium oxide.
The stable form of scandium is created in supernovae via the r-process. Also, scandium is created by cosmic ray spallation of the more abundant iron-peak nuclei. Example reactions are:
28Si + 17n → 45Sc (r-process)
56Fe + p → 45Sc + 11C + n (cosmic ray spallation)
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 Al3Sc 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 Al3Sc precipitate is a coherent precipitate that strengthens the aluminum matrix by applying elastic strain fields that inhibit dislocation movement (i.e., plastic deformation). Al3Sc 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 Al3Sc 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. …
In nature, scandium is found exclusively as the isotope 45Sc, which has a nuclear spin of 7⁄2; this is its only stable isotope.
The known isotopes of scandium range from 37Sc to 63Sc, and the most stable radioisotopes are 46Sc with a half-life of 83.76 days, 47Sc with a half-life of 3.3492 days, 48Sc at 43.67 hours, 44Sc at 4.042 hours, and 43Sc at 3.891 hours. All others have half-lives shorter than an hour, and the majority of these shorter than 15 seconds. The most stable meta state is 44m3Sc with half-life 58.6 hours; this is the lightest isotope with a long-lived isomer.
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.
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 for heavier isotopes is beta emission, producing titanium isotopes.
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.