germanium(redirected from Element 32)
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Ge, a chemical element in Group IV of Mendeleev’s periodic system. Atomic number, 32; atomic weight, 72.59. It is a grayish white solid with a metallic luster. Natural germanium is a mixture of five stable isotopes with mass numbers 70, 72, 73, 74, and 76. In 1871, D. I. Mendeleev predicted the existence and properties of germanium and called the then unknown element ekasilicon, because its properties closely resembled those of silicon. In 1886 the German chemist C. Winkler discovered a new element, which he called germanium in honor of his country, in the mineral argyrodite; it was found to be entirely identical to ekasilicon. Until the second half of the 20th century, the practical use of germanium remained quite limited. Industrial production of germanium increased in connection with development of semiconductor electronics.
The total germanium content of the earth’s crust is 7 x 10-4 percent by weight—that is, greater than that of antimony, silver, or bismuth. However, the germanium minerals themselves are exceptionally rare. Almost all of them are sulfides: germanite, Cu2(Cu, Fe, Ge, Zn)2(S, As)4; argyrodite, Ag8GeS6; canfieldite, Ag8(Sn, Ce)S6; and others. Most of the germanium in the earth’s crust is scattered over a large number of rocks and minerals—in sulfide ores of nonferrous metals, in iron ores, in some oxide minerals (chromite, magnetite, and rutile), and in granites, diabases, and basalts. In addition, it is present in all silicates and in some deposits of coal and in petroleum.
Physical and chemical properties. Germanium crystallizes in the cubic system of the diamond type; unit cell constant a = 5.6575 angstroms. Density of solid germanium, 5.327 g/cm3 (25° C), and of the liquid, 5.557 g/cm3 (1000° C); melting point, 937.5° C, boiling point, about 2700° C; coefficient of thermal conductivity, ~ 60 watts/(m.° K), or 0.14 cal/ (cm. sec-deg) at 25° C. Even very pure germanium is brittle at room temperature, but above 550° C plastic deformation sets in. Its hardness on the mineralogical scale is 6-6.5; the coefficient of compressibility over the pressure range of 0-120 giganewtons per sq m (GN/m2), or 0-12,000 kilograms-force per sq mm (kgf/mm2), is 1.4 × 10-7 m2/M (1.4 × 10-6 cm2/kgf); surface tension, 0.6 N/m (600 dynes/cm). Germanium is a typical semiconductor, with a forbidden band width of 1.104 × 10-19 joule, or 0.69 electron volt (25° C); the specific electrical resistance of very pure germanium is 0.60 ohm-m (60 ohm-cm) at 25° C; the electron mobility is 3,900 cm2/volt-sec, and hole mobility is 1,900 cm2/volt-sec at 25° C (with an impurity content of less than 10-8 percent). Germanium is transparent to infrared radiation of wavelengths greater than 2 microns.
In chemical compounds germanium usually has a valence of 2 or 4; tetravalent compounds are more stable. At room temperature germanium is resistant to the action of air, water, alkali solutions, and dilute hydrochloric and sulfuric acids but dissolves readily in aqua regia and alkaline hydrogen peroxide. It is slowly oxidized by nitric acid. On heating in air to 500°-700° C it is oxidized to the oxide GeO and the dioxide GeO2. The latter is a white powder with a melting point of 1116° C; its solubility in water is 4.3 g/l (20° C). It is amphoteric; it is soluble in alkalies and with difficulty in inorganic acids. It is produced by calcining the hydrated precipitate (CeO2-nH2O), which is produced upon hydrolysis of the chloride GeCl4. Fusion of GeO2 with other oxides can be used to produce germanic acid derivatives—metal germanates (Li2GeO3, Na2GeO3, and others)—which are solids with high melting points.
The reaction of germanium with halogens gives the corresponding tetrahalides. The reaction occurs most readily with fluorine and chlorine (even at room temperature), followed by bromine (with slight heating) and iodine (at 700°-800° C in the presence of CO). One of the most important germanium compounds is the tetrachloride GeCl4, which is a colorless liquid. Melting point, -49.5° C; boiling point, 83.1° C; density, 1.84 g/cm3 (20° C). It is strongly hydrolyzed by water, with the formation of a precipitate of hydrated dioxide. It is prepared by chlorinating metallic germanium or by treating GeO2 with concentrated hydrochloric acid. Germanium dihalides of the general formula GeX2, the monochloride GeCl, hexachlorodigermane (Ge2Cl6), and germanium oxychlorides (for example, GeOCl2) are also known.
Germanium reacts vigorously with sulfur at 900°-1000° C to give the disulfide GeS2, a white solid whose melting point is 825° C. A monosulfide (CeS) and analogous compounds with selenium and tellurium, which are semiconductors, have also been described. Germanium reacts slightly with hydrogen at 1000°-1100° C, giving germane, (GeH)x, which is an unstable and readily volatile compound. Germanium hydrides of the series CenH2n + 2, up to Ce9H20, may be produced by reacting germanides with dilute hydrochloric acid. Germylene, CeH2, is also known. Germanium does not react directly with nitrogen, but a nitride, Ce3N4, produced by the action of ammonia on germanium at 700°-800° C, exists. Carbon does not react with germanium. Germanium forms compounds (germanides) with many metals.
Numerous complex compounds of germanium are known; they are becoming increasingly important both in the analytical chemistry of germanium and in processes for producing it. Germanium forms complex compounds with organic molecules containing hydroxyls (polyhydric alcohols, poly-basic acids, and so on). Germanium heteropolyacids have been prepared. It is characteristic of germanium, as of other elements of Group IV, that it forms organic-metallic compounds—for example, tetraethylgermane, (C2H5)4Ge.
Production and use. In industry, germanium is primarily produced from by-products of the processing of nonferrous metal ores (zincblende and zinc-copper-lead polymetal concentrates) containing 0.001-0.1 percent germanium. Raw materials used include coal ash, gas-generator dust, and wastes from coal-tar plants. First of all a germanium concentrate (2-10 percent germanium) is prepared from the above sources by various methods. The extraction of germanium from the concentrate usually includes the following stages: (1) Chlorination of the concentrate with hydrochloric acid, a solution of chlorine in hydrochloric acid, or other chlorinating agents, giving industrial GeCl4. The GeCl4 is purified by distillation and extraction of impurities with concentrated HC1. (2) Hydrolysis of GeCl4 and calcination of the hydrolysis products to produce GeO2. (3) Reduction of the GeO2 to germanium metal by ammonia or hydrogen. To produce the very pure germanium used in semiconductor devices, zone melting of the metal is used. The single-crystal germanium required in the semiconductor industry is usually produced by zone melting or by the Czochralski method.
Germanium is one of the most valuable materials in modern semiconductor technology. It is used to make diodes, triodes, crystal detectors, and power rectifiers. Single-crystal germanium is also used in dosimeter devices and in devices for measuring the strength of steady or variable magnetic fields. An important area of application of germanium is infrared technology, particularly in the production of infrared radiation detectors operating in the 8-14 micron range. Numerous germanium-containing alloys and glasses based on GeO2 and other germanium compounds show promise for practical use.
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B. A. POPOVKIN