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titanium(tītā`nēəm, tĭ–) [from Titan], metallic chemical element; symbol Ti; at. no. 22; at. wt. 47.867; m.p. 1,675°C;; b.p. 3,260°C;; sp. gr. 4.54 at 20°C;; valence +2, +3, or +4. Titanium is a lustrous silver-white metal that exhibits allotropyallotropy
[Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes.
..... Click the link for more information. ; below about 880°C; it has a hexagonal crystalline structure, but above that temperature it changes to a cubic crystalline structure. The metal is strong and has low density; it is ductile when pure and malleable when heated. Its chemical properties resemble those of zirconium, the element below 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. . When heated, it ignites and burns in air. It is the only element that burns in nitrogen. It is very corrosion resistant and is unattacked by most acids, by moist chlorine gas, or by common salt solutions. Several of its compounds are commercially important. Pure crystalline titanium dioxide (titania) is used as a gemstone. The dioxide is also widely used as a paint pigment, especially for exterior paints. Titanates are formed from the dioxide, which is weakly acidic. An interesting example is barium titanate, which is piezoelectric and can be used as a transducer for the interconversion of sound and electricity. Titanium tetrachloride, a liquid, fumes in moist air; it is used for smoke screens and in skywriting. It is also an important catalyst in the polymerization of olefins. Titanium esters, formed by the reaction of the tetrachloride with alcohols, are used as waterproofing agents on fabrics. Titanic sulfate is used as a textile mordant. Titanium metal and its alloys are light in weight and have very high tensile strength, even at high temperatures. These metals are utilized in aircraft and spacecraft construction and in naval ships, guided missiles, and lightweight armor plate for tanks. Titanium compounds are widely distributed in nature. Rutile, the native dioxide, and ilmenite, which contains, besides titanium, iron and oxygen, are its chief sources. The metal cannot be produced by reduction of the dioxide, because titanium reacts with both oxygen and nitrogen at high temperatures. One method used consists in passing chlorine over ilmenite or rutile, heated to redness with carbon. Titanium tetrachloride, which is formed, is condensed, purified by fractional distillation, and then reduced with molten magnesium at 800°C; in an atmosphere of argon. Titanium is present in the sun and certain other stars, in meteorites, and on the moon. Titanium dioxide causes the star effect in certain sapphires and rubies. The element was discovered (1791) by William Gregor and rediscovered (1795) by M. H. Klaproth, who gave it its present name.
Ti, a chemical element in group IV of Mendeleev’s periodic system. Atomic number, 22; atomic weight, 47.90. One of the light metals, titanium has a silvery white color. Natural titanium is composed of a mixture of the five stable isotopes 46Ti (7.95 percent), 47Ti (7.75 percent), 48Ti (73.45 percent), 49Ti (5.51 percent), and 50Ti (5.34 percent). Artificial radioisotopes, among them 45Ti (half-life, 3.09 hours) and 51Ti (half-life, 5.79 min), are known to exist.
History. Titanium in dioxide form was discovered in 1791 by the British amateur mineralogist W. Gregor in the magnetic ferriferous sands near the parish of Manaccan, in England. In 1795, the German chemist M. H. Klaproth established that the mineral rutile is a natural oxide of the same metal, which he named titanium. (In Greek mythology, Titans were the children of Uranus [Heaven] and Ge [Earth].) Attempts to isolate titanium in pure form were long unsuccessful, and it was only in 1910 that the American scientist M. A. Hunter obtained metallic titanium by heating a chloride of the element with sodium in an airtight steel cylinder. The metal obtained was ductile only at elevated temperatures, displaying brittleness at room temperature because of the high content of impurities. The possibility of studying the properties of pure titanium was opened in 1925, when the Dutch scientists A. van Arkel and J. deBoer employed thermal dissociation of a titanium iodide to obtain a high-purity metal that was ductile at low temperatures.
Distribution in nature. Titanium is a widely distributed element; its average content in the earth’s crust (clarke) equals 0.57 percent by weight, making it the fourth most widespread structural metal, after iron, aluminum, and magnesium. The highest concentrations of titanium are found in basic rocks of the basaltic layer (0.9 percent); lower concentrations are the rule in rocks of the granitic layer (0.23 percent), and titanium occurs in even lower concentrations in ultrabasic rocks (0.03 percent). Rocks rich in titanium include pegmatites of basic rocks, as well as alkalic rocks, syenites, and the syenites’ associated pegmatites. The 67 known titanium minerals are basically of magmatic origin, the most important being rutile and ilmenite.
Titanium is for the most part dispersed throughout the biosphere. Seawater contains 1 × 10–7 percent Ti; titanium migrates only to a slight extent.
Physical properties. Titanium exists in two allotropic forms. Below 882.5°C, the element exists in the stable ct-form, with a close-packed hexagonal lattice (a = 2.951 angstroms [Å], c = 4.679 Å); above this temperature, the elementtakes the β-form, with a body-centered cubic lattice (a = 3.269 Å). Impurities and alloying additives may substantially alter the temperature of α/β transition.
The density of the α-form is 4.505 g/cm3 at 20°C and 4.35 g/cm3at 870°C; the density of the β-form at 900°C is 4.32 g/cm3. Titanium has an atomic radius of 1.46 Å and the ionic radii Ti+ 0.94 Å, Ti2+ 0.78 Å, Ti3+ 0.69 Å, and Ti4+ 0.64 Å. It has a melting point of 1668°±5°C and a boiling point of 3227°C; thermal conductivity within the temperature range 20°–25°C is 22.065 watts/(m-°K), or 0.0527 calorie/(cm.sec.°C), and the linear coefficient of thermal expansion is 8.5 × 10–6 at 20°C and 9.7 × 10–6 in the temperature range 20°–700°C. The element has a specific heat of 0.523 kilojoule/(kg·°K), or 0.1248 calorie/(g·°C), an electrical resistivity of 42.1 × 10–6 ohm-cm at 20°C, and a temperature coefficient of resistivity of 0.0035 at 20°C. It has superconducting properties below 0.38°±0.01°K. Titanium is paramagnetic, with a magnetic susceptibility of (3.2±0.4) × 10–6 at 20°C. Ultimate strength is 256 meganewtons/m2 (25.6 kilograms-force/mm2), elongation is 72 percent, and the Brinell hardness is less than 1,000 meganewtons/m2 (100 kilograms-force/mm2). The modulus of elasticity is 108,000 meganewtons/m2 (10,800 kilograms-force/mm2). High-purity titanium metal is malleable at ordinary temperature.
The titanium used in industry contains admixtures of oxygen, nitrogen, iron, silicon, and carbon that increase strength, reduce ductility, and affect the temperature of polymorphic transition, which occurs in the range 865°–920°C. Commercial titanium of the VT1–00 and VT1–0 type has a density of 4.32 g/cm3, an ultimate strength of 300–550 meganewtons/m2 (30–55 kilograms-force/mm2), an elongation of no less than 25 percent, and a Brinell hardness of 1,150–1,650 meganewtons/m2 (115–165 kilograms-force/mm2). The electron configuration of the outer sub-shells of the Ti atom is 3d24s2.
Chemical properties. Pure titanium is a chemically active transition element. In compounds, the element has an oxidation state of +4 and, more rarely, +3 and +2. It resists corrosion at ordinary temperatures and at temperatures up to 500°–550°C because of the thin but strong oxide film on its surface.
Titanium reacts to a marked extent with atmospheric oxygen at temperatures above 600°C to form TiO2. A thin titanium chip, when insufficiently lubricated, may ignite during mechanical treatment. Given a sufficient concentration of oxygen in the surrounding medium and an oxide film damaged from impact or friction, relatively large pieces of the metal may ignite at room temperature.
The oxide film does not protect titanium in the liquid state from further interaction with oxygen (unlike aluminum); therefore melting and welding of Ti should be carried out in a vacuum, in an atmosphere of neutral gas, or under flux-shielded conditions. Titanium has the ability to absorb atmospheric gases and hydrogen, thereby forming brittle alloys unsuited for practical use. Given an activated surface, hydrogen absorption occurs at a slow rate even at room temperature, and the rate increases considerably at 400°C and higher. The solubility of hydrogen in titanium is reversible, and the gas can be almost entirely removed by annealing in a vacuum. Titanium reacts with nitrogen above 700°C to yield nitrides of the type TiN; in the form of fine powder or wire, titanium can burn in a nitrogen atmosphere. The rate of diffusion of nitrogen and oxygen in titanium is significantly lower than that of hydrogen. The film produced by the reactions with these gases is extremely hard and brittle; it is removed from the surface of titanium products by pickling or mechanical treatment. Titanium reacts vigorously with dry halogens but is stable in relation to wet halogens since moisture acts as an inhibitor.
Titanium metal is stable in nitric acid of any concentration, with the exception of red fuming nitric acid. The latter brings about the corrosive disintegration of titanium, and the reaction is sometimes accompanied by an explosion. The metal is also unaffected by weak solutions of sulfuric acid (up to 5 percent by weight). Hydrochloric, hydrofluoric, concentrated sulfuric, and certain hot organic acids (oxalic, formic, and trichloroacetic), however, do react with titanium.
Titanium resists corrosion when exposed to air, seawater, sea air, wet chlorine, chlorine water, hot and cold chloride solutions, and various solutions and reactants used in the chemical, petroleum, and paper industries, as well as in hydrometallurgy. The element reacts with C, B, Se, and Si to form metallic compounds characterized by refractoriness and high strength. The carbide TiC (melting point, 3140°C) is prepared by heating a mixture of TiO2 and carbon black to a temperature of 1900°–2000°C in a hydrogen atmosphere; the nitride TiN (melting point, 2950°C) is obtained by heating titanium powder in nitrogen at a temperature above 700°C. Titanium is known to form the suicides TiSi2, Ti5Si3, and TiSi and the borides TiB, Ti2B5, and TiB2. At 400°–600°C, titanium absorbs hydrogen to form solid solutions and hydrides (TiH,TiH2).
Melting TisO2 with alkalies produces such salts of titanic acids as metatitanates and orthotitanates, for example, Na2TiO3 and Na4TiO4, and polytitanates, for example, Na2Ti2O5 and Na2Ti3O7. The titanates include such important titanium minerals as ilmenite (FeTiO3) and perovskite (CaTiO3). All titanates are sparingly soluble in water. Titanium dioxide and titanic acids (precipitates), as well as the titanates, dissolve in sulfuric acid to form solutions containing titanyl sulfate (TiOSO4). With dilution and heating of these solutions, H2TiO3 precipitates as a result of hydrolysis; the precipitate in turn is used in obtaining titanium dioxide. The addition of hydrogen peroxide to acidic solutions containing Ti(IV) compounds yields peroxy (pertitanic) acids with the composition H4TiO5 and H4TiOg and the corresponding salts; these compounds are yellow or orange-red depending on the titanium concentration; this formation of color is used for the analytic determination of titanium.
Preparation. The most widespread method of obtaining metallic titanium is the magnesium process, involving the reduction of titanium tetrachloride with metallic magnesium or, more rarely, sodium:
TiCl4 + 2Mg = Ti + 2MgCl2
In both cases, such titanium oxide ores as rutile and ilmenite serve as starting material. For ores of the ilmenite type, titanium in slag form is separated from the iron by smelting in electric furnaces. The slag (like rutile) is subjected to chlorination in the presence of carbon to yield titanium tetrachloride, which, after purification, is charged into a reactor having a neutral atmosphere.
The titanium obtained through this process is in the form of a spongy mass. The mass is next crushed and remelted to form ingots in evacuated electric-arc furnaces; alloying agents are added if an alloy is to be obtained. The magnesium process makes possible large-scale production of titanium on the basis of a closed production cycle, since the by-product formed during reduction—magnesium chloride—is sent on for electrolysis to obtain magnesium and chlorine.
In many cases, it is advantageous to use the methods of powder metallurgy in manufacturing articles of titanium and titanium alloys. Especially fine powders, such as those used in radio electronics, may be prepared by the reduction of titanium dioxide with calcium hydride.
World production of metallic titanium has undergone rapid growth. The total was approximately 2 tons in 1948, 2,100 tons in 1953, and 20,000 tons in 1957; in 1975 it exceeded 50,000 tons.
Use. The main advantage of titanium as a structural metal is the combination of lightness, strength, and corrosion resistance. Titanium alloys surpass the majority of alloys based on such other metals as iron and nickel in both absolute strength and, even more, specific strength (strength in relation to density) at temperatures between –250° and 550°C; in corrosion resistance they are comparable to alloys of noble metals. As an independent structural material, however, titanium came into use only in the 1950’s because of the considerable technical difficulties encountered in extracting titanium from ores and processing the metal. (It was for this reason that titanium was classed as a rare metal.) Most titanium is used in the aerospace industry and in shipbuilding. Alloys of titanium and iron, known collectively as ferrotitanium (20–50 percent titanium), serve as alloying additives and reducing agents in the metallurgy of high-grade steel and special alloys.
Commercial titanium is used in the manufacture of chemical reactors, pipes, fittings, tanks, pumps, and other articles that must operate in aggressive media. The element is therefore particularly useful in the construction of chemical equipment. Fittings made from titanium are employed in nonferrous hydrometallurgy, and titanium is used to coat steel products. In many cases, the use of titanium is warranted both economically and technologically because the element not only lengthens the service life of equipment but also makes possible the intensification of certain processes, for example, those in nickel hydrometallurgy. The biological harmlessness of titanium makes the element a superior material for use in the manufacture of food-processing equipment and in restorative surgery. Under conditions of extreme cold, the strength of titanium increases and good ductility is preserved, a phenomenon making titanium a suitable structural material in cryogenics. Titanium lends itself readily to polishing, color anodizing, and other techniques of surface finishing, hence its use in art, including monumental sculpture. An example is provided by the memorial built in Moscow in honor of the launching of the first artificial earth satellite. The titanium compounds with practical importance include the oxides, halides, and suicides, the last finding use in high-temperature technology. Titanium borides and their alloys are used as moderators in nuclear power plants because of their refractoriness and large neutron-capture cross section. Titanium carbide, possessing a high degree of hardness, is a component of the hard tool alloys used as abrasives and in the manufacture of cutting tools.
Titanium dioxide and barium titanate are the basic materials of titanium ceramics; barium titanate is also an extremely important ferroelectric.
S. G. GLAZUNOV
In the organism. Titanium is always present in plant and animal tissue. Its concentration in terrestrial plants is approximately 10–4 percent; in marine vegetation, the concentration ranges from 1.2 × 10–3 to 8 × 10–2 percent. The tissue of terrestrial animals shows a concentration of less than 2 × 10–4 percent, while that of marine animals has from 2 × 10–4 to 2 × 10–2 percent. Titanium accumulates in vertebrates, primarily in horny formations and in the spleen, adrenal glands, thyroid, and placenta; it is poorly absorbed from the gastrointestinal tract. The daily human intake of titanium with food and water equals 0.85 mg; it is eliminated in the urine and feces (0.33 and 0.52 mg, respectively). The toxicity of titanium is relatively low.
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