Titanium

Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength that is resistant to corrosion in sea water, aqua regia, and chlorine.
Titanium was discovered in Cornwall, Great Britain, by William Gregor in 1791 and was named by Martin Heinrich Klaproth after the Titans of Greek mythology. The element occurs within a number of minerals, principally rutile and ilmenite, which are widely distributed in the Earth's crust and lithosphere; it is found in almost all living things, as well as bodies of water, rocks, and soils. The metal is extracted from its principal mineral ores by the Kroll and Hunter processes. The most common compound, titanium dioxide, is a popular photocatalyst and is used in the manufacture of white pigments. Other compounds include titanium tetrachloride, a component of smoke screens and catalysts; and titanium trichloride, which is used as a catalyst in the production of polypropylene.
Titanium can be alloyed with iron, aluminium, vanadium, and molybdenum, among other elements. The resulting titanium alloys are strong, lightweight, and versatile, with applications including aerospace, military, industrial processes, automotive, agriculture, sporting goods, jewelry, and consumer electronics. Titanium is also considered one of the most biocompatible metals, leading to a range of medical applications including prostheses, orthopedic implants, dental implants, and surgical instruments.
The two most useful properties of the metal are its corrosion resistance and tensile-strength-to-density ratio, the highest of any metallic element, although resistance to tensile stress does not mean it has the highest ratio for other stresses: bulk compression, shear, and pressure wave. In its unalloyed condition, titanium is as tensile-strong as some steels, but less dense. There are two allotropic forms and five naturally occurring isotopes of this element, Ti through Ti, with Ti being the most abundant.

Characteristics

Physical properties

As a metal, titanium is recognized for its high strength-to-weight ratio. It is a strong metal with low density that is quite ductile, lustrous, and metallic-white in color. Due to its relatively high melting point it has sometimes been described as a refractory metal, but this is not the case. It is paramagnetic and has fairly low electrical and thermal conductivity compared to other metals. Titanium is superconducting when cooled below its critical temperature of 0.49 K.

Commercially pure grades of titanium have ultimate tensile strength of about 434 MPa, equal to that of common, low-grade steel alloys, but are less dense. Titanium is 60% denser than aluminium, but more than twice as strong as the most commonly used 6061-T6 aluminium alloy. Certain titanium alloys achieve tensile strengths of over 1,400 MPa. However, titanium loses strength when heated above.
Titanium is not as hard as some grades of heat-treated steel; it is non-magnetic and a poor conductor of heat and electricity. Machining requires precautions, because the material can gall unless sharp tools and proper cooling methods are used. Like steel structures, those made from titanium have a fatigue limit that guarantees longevity in some applications.
The metal is a dimorphic allotrope of a hexagonal close packed α form that changes into a body-centered cubic β form at. The specific heat of the α form increases dramatically as it is heated to this transition temperature but then falls and remains fairly constant for the β form regardless of temperature.

Chemical properties

Like aluminium and magnesium, the surface of titanium metal and its alloys oxidizes immediately upon exposure to air to form a thin non-porous passivation layer that protects the bulk metal from further oxidation or corrosion. When it first forms, this protective layer is only 1–2 nm thick, but it continues to grow slowly, reaching a thickness of 25 nm in four years. This layer gives titanium excellent resistance to corrosion against oxidizing acids, but it will dissolve in dilute hydrofluoric acid, hot hydrochloric acid, and hot sulfuric acid.
Titanium is capable of withstanding attack by dilute sulfuric and hydrochloric acids at room temperature, chloride solutions, and most organic acids. However, titanium is corroded by concentrated acids. Titanium burns in normal air at temperatures lower than its melting point, so melting the metal is possible only in an inert atmosphere or vacuum. At room temperature, titanium is fairly inert to halogens, but will violently combine with chlorine and bromine at to form titanium tetrachloride and titanium tetrabromide, respectively.
Titanium readily reacts with oxygen at in air, and at in pure oxygen, forming titanium dioxide. This oxide is also formed by reaction between titanium and pure oxygen at room temperature and pressure of. Titanium is one of the few elements that burns in pure nitrogen gas, reacting at to form titanium nitride, which causes embrittlement.

Occurrence

Titanium is the ninth-most abundant element in Earth's crust and the seventh-most abundant metal. It is present as oxides in most igneous rocks, in sediments derived from them, in living things, and natural bodies of water. Of the 801 types of igneous rocks analyzed by the United States Geological Survey, 784 contained titanium. Its proportion in soils is approximately 0.5–1.5%.
Common titanium-containing minerals are anatase, brookite, ilmenite, perovskite, rutile, and titanite. Akaogiite is an extremely rare mineral consisting of titanium dioxide. Of these minerals, only rutile and ilmenite have economic importance, yet even they are difficult to find in high concentrations. About 6.0 and 0.7 million tonnes of those minerals were mined in 2011, respectively. Significant titanium-bearing ilmenite deposits exist in Australia, Canada, China, India, Mozambique, New Zealand, Norway, Sierra Leone, South Africa, and Ukraine. Total reserves of anatase, ilmenite, and rutile are estimated to exceed 2 billion tonnes.
The concentration of titanium is about 4 picomolar in the ocean. At 100 °C, the concentration of titanium in water is estimated to be less than 10 M at pH 7. The identity of titanium species in aqueous solution remains unknown because of its low solubility and the lack of sensitive spectroscopic methods, although only the 4+ oxidation state is stable in air. No evidence exists for a biological role, although rare organisms are known to accumulate high concentrations of titanium.
Titanium is contained in meteorites, and it has been detected in the Sun and in M-type stars with a surface temperature of. Rocks brought back from the Moon during the Apollo 17 mission are composed of 12.1% TiO₂. Native titanium is only found in rocks that have been exposed to pressures between roughly 2.8 to 4.0gigapascal on Earth, but it has been identified in nanocrystals on the Moon.

Isotopes

Naturally occurring titanium is composed of five stable isotopes: Ti, Ti, Ti, Ti, and Ti, with Ti being the most abundant. Twenty-three radioisotopes have been characterized, the most stable of which are Ti with a half-life of 63 years; Ti, 184.8 minutes; Ti, 5.76 minutes; and Ti, 1.7 minutes. All other radioactive isotopes have half-lives less than 33 seconds, with the majority less than half a second.
The isotopes of titanium range from Ti to Ti. The primary decay mode for isotopes lighter than Ti is positron emission, leading to isotopes of scandium, and the primary mode for isotopes heavier than Ti is beta emission, leading to isotopes of vanadium. Titanium becomes radioactive upon bombardment with deuterons, emitting mainly positrons and hard gamma rays.

Compounds

The +4 oxidation state dominates titanium chemistry, but compounds in the +3 oxidation state are also numerous. Commonly, titanium adopts an octahedral coordination geometry in its complexes, but tetrahedral TiCl₄ is a notable exception. Because of its high oxidation state, titanium compounds exhibit a high degree of covalent bonding.

Oxides, sulfides, and alkoxides

The most important oxide is TiO₂, which exists in three important polymorphs; anatase, brookite, and rutile. All three are white diamagnetic solids, although mineral samples can appear dark, as in rutile. They adopt polymeric structures in which Ti is surrounded by six oxide ligands that link to other Ti centers.
The term titanates usually refers to titanium compounds, as represented by barium titanate. With a perovskite structure, this material exhibits piezoelectric properties and is used as a transducer in the interconversion of sound and electricity. Many minerals are titanates, such as ilmenite. Star sapphires and rubies get their asterism from the presence of titanium dioxide impurities.
A variety of reduced oxides of titanium are known, mainly reduced stoichiometries of titanium dioxide obtained by atmospheric plasma spraying. Ti₃O₅, described as a Ti-Ti species, is a purple semiconductor produced by reduction of TiO₂ with hydrogen at high temperatures, and is used industrially when surfaces need to be vapor-coated with titanium dioxide: it evaporates as pure TiO, whereas TiO₂ evaporates as a mixture of oxides and deposits coatings with variable refractive index. Also known is Ti₂O₃, with the corundum structure, and TiO, with the rock salt structure, although often nonstoichiometric.
The alkoxides of titanium, prepared by treating TiCl₄ with alcohols, are colorless compounds that convert to the dioxide on reaction with water. They are industrially useful for depositing solid TiO₂ via the sol-gel process. Titanium isopropoxide is used in the synthesis of chiral organic compounds via the Sharpless epoxidation.
Titanium forms a variety of sulfides, but only TiS₂ has attracted significant interest. It adopts a layered structure and was used as a cathode in the development of lithium batteries. Because Ti is a "hard cation", the sulfides of titanium are unstable and tend to hydrolyze to the oxide with release of hydrogen sulfide.