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Titanium is a light, strong, silvery metallic element. It is classified as one of the transition elements. Titanium was discovered by the British clergyman William Gregor in 1790. Gregor's analysis of a black magnetic sand found near Falmouth, England, yielded a large proportion of a white metallic oxide. In 1795 the German chemist Martin Heinrich Klaproth realized that Gregor's description of the oxide coincided closely with the properties of an oxide that he had isolated from a sample of Hungarian rutile. Klaproth gave the name "titanium" to the metallic element in the oxide.
The high cost of titanium metal often limits its use to military purposes. Because of its lightness and strength, titanium is used as a structural material in high-speed aircraft, rockets, guided missiles, and recoil mechanisms for artillery. Titanium is often used in the chemical processing industry because of its resistance to corrosion. This resistance is probably due to a thin coating of titanium dioxide, which protects the metal from further corrosion. The metal has unusually good resistance to corrosion by salt water, and so it is used in propeller shafts and other parts exposed to the sea.
Titanium is added to other metals, such as copper, steel, and aluminum, to affect certain properties. For example, in the manufacture of stainless steel, metallic titanium is used to stabilize the carbon and nitrogen content. Ferrotita-nium is added to steel as a deoxidizer.
Titanium dioxide, a white compound, is used in the production of paint pigment, paper, plastics, glass, and ceramics. The presence of titanium dioxide produces the "stars" in star rubies and sapphires.
Titanium hydride is used in powder metallurgy, in the production of hydrogen, as a getter in vacuum tubes, and in the production of foamed metals. Barium titanate is widely used in the electronics industry because of its high dielectric constant. Organic alkali titanates are used as waterproofing agents. Titanium trioxide is used in dental porcelain. Titanium tetrachloride is used as a mordant in the textile industry, in artificial pearls, and in titanium pigments. Titanium nitride is used in cermets and semiconductor devices. Titanous sulfate is used as a reducing agent in the textile industry.
A new nickel, titanium alloy called nitinol has been developed, which has the unusual property of regaining its previous shape when heated. First the alloy is shaped and heated to a critical temperature. When the object has cooled, it can be mechanically reshaped. However, when it is then reheated, it will resume its original form. There are numerous potential uses for nitinol, particularly in construction, where areas are often difficult to reach. The alloy could be used to make rivets and cotter pins.
A number of titanium oxides can be prepared, including titanium monoxide, titanium sesquioxide, titanium dioxide, and titanium trioxide. Titanium dioxide is the most stable of these compounds and is found in nature in several minerals. Reduction of titanium dioxide with charcoal produces titanium monoxide. Titanium sesquioxide is produced from the reduction of titanium dioxide. Titanium combines readily with fluorine, chlorine, bromine, and iodine.
Of the various compounds that can be formed, titanium tetrachloride is the most common. This substance is a liquid at room temperature. Exposure to air results in its decomposition to form corrosive hydrogen chloride gas. The tetrabromide (yellow), the tetraiodide (reddish brown), and the tetrafluoride (white) are all solids at room temperature.
The extraction of titanium from its ores is a relatively slow and costly process, which makes the metal expensive. The most widely used method for obtaining metallic titanium is the Kroll process, which was developed in 1937 by the German scientist William A. Kroll. Until then, it was impossible to obtain large quantities of the pure element because the liquid titanium reacts so readily with nitrogen and oxygen from the air.
In the Kroll process, titanium tetrachloride is prepared by chlorination of the ores ilmenite or rutile. Liquid titanium tetrachloride is then reduced, using metallic magnesium under an inert atmosphere in a sealed reactor. Sometimes, sodium is used instead of magnesium.
The reaction is carried out in a large flat-bottomed steel vessel. The vessel is charged with metallic magnesium, checked for leaks, and filled with argon or helium. The reaction vessel is heated in a furnace to the melting point of magnesium. At this point, liquid titanium tetrachloride is allowed to flow into the vessel. In the reaction that follows, titanium and magnesium chloride are produced. Most of the magnesium chloride liquid is drained from the reaction vessel during the reduction process.
After the reaction is complete, the cooled vessel must be opened in a dry room to prevent contamination of the titanium sponge with moisture. The sponge is usually contaminated with some ferric chloride, as well as magnesium chloride and unreacted magnesium metal. After the sponge has been machined into chips of a manageable size, the impurities are removed by leaching with dilute hydrochloric acid. The sponge, along with any necessary additives, is melted in an electric furnace to form ingots. The ingots are remelted to form the final pure titanium ingot, which can be readily drawn, forged, or rolled.
Very pure titanium can be obtained by the decomposition of titanium tetraiodide at high temperatures and by electrolytic reduction in a fused salt system. These methods work well on a laboratory scale, but they have not been used for commercial production of the metal.
Titanium is as strong as steel but almost 50% lighter. Pure titanium is easily fabricated, but it becomes brittle when contaminated with other elements, such as carbon and nitrogen.
The metal has a strong affinity for oxygen, carbon, and nitrogen, making it difficult to obtain in the pure state. Titanium will burn in air at about 1200° C (2192° F). It is one of the few metals that will burn in a stream of nitrogen gas. Metallic titanium is readily attacked by concentrated sulfuric and hydrochloric acids, but reacts slowly with the dilute forms of these acids. It is not much affected by nitric acid.