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hafnium(hăf`nēəm), metallic chemical element; symbol Hf; at. no. 72; at. wt. 178.49; m.p. about 2,227°C;; b.p. 4,602°C;; sp. gr. 13.31 at 20°C;; valence +4. Hafnium is a lustrous, ductile, silvery metal with a hexagonal, close-packed crystalline structure. Its chemical properties are almost identical to those of zirconiumzirconium
, metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.224; m.p. about 1,852°C;; b.p. 4,377°C;; sp. gr. 6.5 at 20°C;; valence +2, +3, or +4.
Zirconium is a very strong, malleable, ductile, lustrous silver-gray metal.
..... Click the link for more information. , the element directly above it in Group 4 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. . The two elements are among the most difficult to separate—zirconium is almost always an impurity in hafnium and affects its physical properties. Finely powdered hafnium can spontaneously ignite in air; because of this reactivity the metal has found use in the manufacture of light bulbs and vacuum tubes as a scavenger for small amounts of oxygen and nitrogen. Hafnium reacts directly with the halogens to form tetrahalides, and when heated it reacts with carbon, boron, sulfur, and silicon. Hafnium carbide is a refractory material with an extremely high melting point. Hafnium metal is produced by the Kroll process, in which a hafnium tetrahalide is reacted with magnesium or sodium metal. Because it is a good neutron absorber, hafnium metal is often used for nuclear reactor control rods. It has been alloyed with several other metals, among them iron and titanium. Hafnium is found widely distributed in nature, usually in association with zirconium minerals such as zircon. The existence of hafnium was suspected for many years before it was demonstrated (1923) through X-ray spectroscopic analysis by Dirk Coster and Georg von Hevesy. They named the element for Hafn, Latin for Copenhagen, the city where they had made the discovery.
Hf, a chemical element in Group IV of the Mendeleev periodic system. Atomic number, 72; atomic weight, 178.49; a silvery-white metal. Natural hafnium consists of six stable isotopes with mass numbers 174 and 176-80. The existence of hafnium was predicted by D. I. Mendeleev in 1870. In 1921, N. Bohr demonstrated that element no. 72 must have an atomic structure similar to zirconium and that therefore it should not be sought among the rare earths, as previously believed, but among the zirconium minerals. The Hungarian chemist G. de Hevesy and the Dutch physicist D. Coster systematically studied the minerals of zirconium by X-ray spectral analysis and in 1922 discovered element no. 72, naming it hafnium after the place of discovery, the city of Copenhagen (Late Latin Hafnia).
Hafnium does not form its own minerals; it usually accompanies zirconium in nature. The earth’s crust contains 3.2 × 10−4 percent hafnium by weight. In most zirconium minerals its content varies from 1-2 to 6-7 percent and sometimes up to 35 percent in secondary minerals. The most valuable commercial sources of hafnium are marine and alluvial deposits of the mineral zircon.
Physical and chemical properties. At room temperature, hafnium has a hexagonal lattice with constants a = 3.1946 angstroms (Å) and c = 5.0511 Å. Density, 13.09 g/cm3 (20° C). Hafnium is refractory; melting point, 2222° ± 30° C; boiling point, 5400° C. Atomic heat capacity, 26.3 kilojoules per kg-atom · °K (kJ/kg-atom · ° K), or 6.27 cal/(g-atom · deg) (25°-100° C); electrical resistance, 32.4 × 10−8 ohm · m (0° C). A peculiarity of hafnium is its high emissivity; its electron work function is 5.77 × 10−19 joules (J), or 3.60 electron volts (eV) (980°-1550° C). It has a large thermal neutron capture cross section, equal to 115 × 10−28 m2, or 115 barns (for zirconium, 0.18 × 10−28 m2, or 0.18 barn). Pure hafnium is ductile and lends itself readily to cold and hot working (rolling, forging, and stamping).
In its chemical properties, hafnium is very similar to zirconium because of the almost identical ion sizes of these elements and the close resemblance of the electron structure. However, the chemical activity of hafnium is somewhat lower than that of zirconium. The most common valence of hafnium is 4. Compounds of trivalent, divalent, and univalent hafnium are also known.
Solid hafnium is completely resistant to atmospheric gases at room temperature. Upon heating to above 600° C, however, it oxidizes rapidly and, like zirconium, reacts with nitrogen and hydrogen. Hafnium is distinguished by corrosion resistance in pure water and water vapor up to 400° C. Powdered hafnium is pyrophoric. Hafnium dioxide, HfO2, is a white, refractory substance (melting point, 2780° C) with high chemical stability. Hafnium dioxide and the corresponding hydroxides [HfO2 · xH2O and HfO(OH)2] are amphoteric, with predominant basic properties. Heating HfO2 with alkali hydroxides and alkaline earth oxides leads to the formation of hafnates, such as Me2HfO3, Me4HfO4, Me2Hf2O5.
Hafnium reacts with halogens upon heating, forming compounds of the type HfX4 (tetrafluoride, HfF4, and tetrachloride, HfCl4). At high temperatures hafnium reacts with carbon, boron, nitrogen, and silicon to give metallike, refractory compounds that are very resistant to chemical reagents: HfB and HfB2 (melting point, 3250° C), HfC (melting point, 3887° C), HfN (melting point, 3310° C), Hf2Si, HfSi, and HfSi2. Metallic hafnium dissolves in hydrofluoric and concentrated sulfuric acids and in molten fluorides of alkali metals. It is virtually insoluble in nitric, hydrochloric, phosphoric, and organic acids and is highly resistant to alkali solutions. Highly water-soluble hafnium compounds, which are used in technology and analytical chemistry of hafnium, include the tetrachloride and oxychloride, HfCl4 and HfOCl2·8H2O, as well as the nitrates and sulfates, HfO(NO3)2·nH2O (n = 2 and 6), Hf(SO4)2, and Hf(SO4)2·4H2O. The formation of complexes with various organic oxygen-containing compounds is characteristic of hafnium.
Preparation and use. Hafnium compounds are usually separated at the end of the production cycle of zirconium compounds from ore raw materials. Metallic hafnium is produced at present by reducing HfCl4 with magnesium or sodium. The use of hafnium in various areas of technology began only recently. It is used in nuclear power engineering (reactor control rods and protective screens for neutron radiation) and in radio electronics (cathodes, getters, and electrical contacts). A promising application of hafnium is in the production of heat-resistant alloys for aviation and rocket technology. The solid solution of hafnium and tantalum carbides, which melt above 4000° C, is the most refractory ceramic material; it is used in the production of rocket-engine parts and crucibles for melting refractory metals.
REFERENCESMetallurgiia gafniia. Edited by D. E. Thomas and E. T. Hayes. Moscow, 1967. (Translated from English.)
Spravochnik po redkim metallam. Moscow, 1965. Pages 177-203. (Translated from English.)
L. N. KOMISSAROVA