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Ga, a chemical element in Group III of the Mendeleev periodic system. Atomic number, 31; atomic weight, 69.72. A silver-white, soft metal. It consists of two stable isotopes with mass numbers 69 (60.5 percent) and 71 (39.5 percent).
The existence and main properties of gallium (eka-aluminum) were predicted in 1870 by D. I. Mendeleev. The element was discovered by spectroscopic analysis of Pyreneean sphalerite and was isolated in 1875 by the French chemist P. E. Lecoq de Boisbaudran. It was named in honor of France (Latin Gallia). The exact agreement between the actual and predicted properties of gallium was the first triumph of the periodic system.
The earth’s crust has a relatively high average gallium content (1.5 × 10-3 percent by mass), which is the same as the content of lead and molybdenum. Gallium is a typical trace element. The only gallium mineral is gallite, CuGaS2, which is very rare. The geochemistry of gallium is closely associated with that of aluminum; this is caused by the closeness of their physicochemical properties. Most of the gallium in the lithosphere is included in aluminum minerals. Its content in bauxites and nephelines varies from 0.002 to 0.01 percent. Higher concentrations are also found in sphalerites (0.01-0.02 percent), coals (along with germanium), and some iron ores.
Physical and chemical properties. The gallium lattice is rhombic (pseudotetragonal), with parameters a =4.5197 angstroms (Å), b = 7.6601 Å, and c = 4.5257 Å. Density of the solid metal, 5.904 g/cm3 (at 20° C); of the liquid metal, 6.095 g/cm3 (at 29.8° C)—that is, it expands on solidifying. Melting point, 29.8° C; boiling point, 2230° C. The distinctive features of gallium are its large temperature interval in the liquid state (2200° C) and its low vapor pressure at temperatures up to 1100°-1200° C. The specific heat of solid gallium is 376.7 joules per (kg° K), or 0.09 cal/(g° C), over the temperature range from 0° to 24° C; of liquid gallium, 410 joules per (kg · °K) or 0.098 cal/(g · ° C), over the temperature range from 29° to 100° C; the specific electrical resistance (ohm cm) of solid gallium is 53.4 × 10-6 (at 0° C), and of liquid gallium, 27.2 x 10-6 (at 30° C); viscosity (poise = 0.1 newton sec/m2), 1.612 (98° C) and 0.578 (1100° C); surface tension, 0.735 newton/m, or 735 dynes/cm (at 30° C in an H2 atmosphere). The reflection coefficients for wavelengths of 4360 and 5890 Å are 75.6 and 71.3 percent, respectively. The thermal neutron capture cross section is 2.71 barns (2.7 × 10-28sq m).
Gallium is stable in air at ordinary temperatures. Slow oxidation is observed above 260° C in dry oxygen (the oxide film protects the metal). Sulfuric and hydrochloric acids dissolve gallium slowly; hydrofluoric acid dissolves it rapidly. It is resistant to cold nitric acid and dissolves slowly in hot alkali solutions. It reacts with chlorine and bromine while cold and with iodine upon heating. Above 300° C, molten gallium reacts with all construction materials and alloys.
Trivalent gallium compounds, whose properties in many ways are close to those of aluminum compounds, are the most stable. Univalent and divalent compounds are also known. The higher oxide Ga2O3 is white and insoluble in water. The corresponding hydroxide is a white, gelatinous precipitate from solutions of gallium salts. It has a well-defined amphoteric character—upon solution in alkalies, gallates such as Na[Ga(OH)4] are formed; in acids, gallium salts such as Ga2(S04)3 and GaCl3 are formed. The acidic properties of gallium hydroxide are more marked than those of aluminum hydroxide—the separation interval for Al(OH)3 is pH = 10.6-4.1; for Ga(OH)3, pH = 9.7-3.4.
Unlike Al(OH)3, gallium hydroxide dissolves not only in strong alkalies but also in ammonia solutions. When the ammonia solution is boiled, the hydroxide is reprecipitated from it.
The most important gallium salts are the chloride, GaCl3 (Tm = 78° C, Tb = 200° C), and the sulfate, Ga2 (SO4)3. The latter forms double salts of the alum type—for example, (NH4)Ga(SO4)2·12H2O—with sulfates of alkali metals and ammonium. Gallium forms a ferrocyanide, Ga4[Fe(CN)6]3, which is poorly soluble in water and dilute acids; this can be used to separate it from aluminum and a number of other elements.
Preparation and use. The main source for the production of gallium is the manufacture of aluminum. When bauxite is treated by the Bayer process, gallium collects in the circulating mother liquors after the removal of the Al(OH)3. Gallium is obtained from such solutions by electrolysis on a mercury cathode. The compound Ga(OH)3, which is dissolved in the alkali, is precipitated from the alkaline solution formed by treating the amalgam with water; the gallium is extracted by electrolysis.
In the soda-lime method of treating bauxite or nepheline ore, gallium collects in the last precipitate fractions obtained in carbonization. For further enrichment, the hydroxide precipitates are treated with milk of lime, in which case most of the aluminum remains in the precipitate, and the gallium passes into solution, from which a gallium concentrate (6-8 percent Ga203) is separated by passing C02 through it; the gallium concentrate is dissolved in an alkali, and the gallium is obtained electrolytically.
Another source of gallium is residual anode alloy from aluminum refining by three-layer electrolysis. In zinc production, the sublimates (rotary kiln oxides) formed in the treatment of the end products of the leaching of zinc cinders are a source of gallium.
After washing with water and acids (HCl and HN03), the liquid gallium obtained by electrolysis of an alkaline solution is 99.9-99.95 percent pure. Purer metal is produced by vacuum fusion, zone smelting, or drawing’a single crystal from a melt.
Gallium has not yet received extensive industrial application. So far the potential scale of production of gallium as a by-product of aluminum manufacture is much greater than the consumption of the metal. The most promising application of gallium is in the form of chemical compounds of the GaAs, GaP, and GaSb types, which have semiconducting properties. They can be used in high-temperature rectifiers and transistors, solar batteries, and other devices where the photoelectric effect in the cutoff layer can be used, as well as in infrared radiation receivers. Gallium can be used to make optical mirrors of high reflectivity. An aluminum-gallium alloy has been proposed instead of mercury for the cathode of ultraviolet lamps used in medicine. The use of liquid gallium and its alloys has been proposed in making high-temperature thermometers (600°-1300° C) and manometers. The use of these substances as a heat carrier in nuclear power reactors is of interest (the reactivity of gallium with construction materials at operating temperatures impairs its usefulness in this case; a eutectic Ga-Zn-Sn alloy is less corrosive than pure Ga).
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Eremin, N. N. Gallii. Moscow, 1964.
Zelikman, A. N., O. E. Krein, and G. V. Samsonov. Metallurgiia redkikh metallov, 2nd ed. Moscow, 1964.
Einecke, E. Das Gallium. Leipzig .
A. N. ZELIKMAN