Скандий: свойства, применение и история открытия

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. To this day, 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.

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 (|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.

The halides |ScX3, where X= Cl, Br, or I, are very soluble in water, but |ScF3 is insoluble. In all four halides, the scandium is 6 coordinated. The halides are Lewis acids; for example, |ScF3 dissolves in a solution containing excess fluoride ion to form |[ScF6](3−). 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.

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, |[ScCp2Cl]2 and related derivatives of pentamethylcyclopentadienyl ligands.

Compounds that feature scandium in oxidation states other than +3 are rare but well characterized. The blue black compound |CsScCl3 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). Lower oxidation states (+2, +1, 0) have also been observed in organoscandium compounds.

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.

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. 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. These have made |Al3Sc 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 |Al20Li20Mg10Sc20Ti30 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.

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 |Sc2O3) 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 (|Er,Cr:YSGG) lasers for cavity preparation and in endodontics.

The radioactive isotope 46Sc is used in oil refineries as a tracing agent. 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.

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.