Titanium hydride normally refers to the inorganic compound TiH₂ and related nonstoichiometric materials.^[1][2] It is commercially available as a stable grey/black powder, which is used as an additive in the production of Alnico sintered magnets, in the sintering of powdered metals, the production of metal foam, the production of powdered titanium metal and in pyrotechnics.^[3]

Production and reactions of TiH_(2-x)

In the commercial process for producing non-stoichiometric TiH_(2-x), titanium metal sponge is treated with hydrogen gas at atmospheric pressure at between 300-500 °C. Absorption of hydrogen is exothermic and rapid, changing the color of the sponge grey/black. The brittle product is ground to a powder, which has a composition around TiH_1.95.^[3] In the laboratory, titanium hydride is produced by heating titanium powder under flowing hydrogen at 700 °C, the idealized equation being:^[4]

Ti + H₂ → TiH₂

Other methods of producing titanium hydride include electrochemical and ball milling methods.^[5][6]

Reactions

TiH_1.95 is unaffected by water and air. It is slowly attacked by strong acids and is degraded by hydrofluoric and hot sulfuric acids. It reacts rapidly with oxidising agents, this reactivity leading to the use of titanium hydride in pyrotechnics.^[3]

The material has been used to produce highly pure hydrogen, which is released upon heating the solid starting at 300 °C.^[4] Only at the melting point of titanium is dissociation complete.^[3] Titanium tritiide has been proposed for the long-term storage of tritium gas.^[7]

Structure

As TiH_x approaches stoichiometry, it adopts a distorted body-centered tetragonal structure, termed the ε-form with an axial ratio of less than 1. This composition is very unstable with respect to partial thermal decomposition, unless maintained under a pure hydrogen atmosphere. Otherwise, the composition rapidly decomposes at room temperature until an approximate composition of TiH_1.74 is reached. This composition adopts the fluorite structure, and is termed the δ-form, and only very slowly thermally decomposing at room temperature until an approximate composition of TiH_1.47 is reached, at which point, inclusions of the hexagonal close packed α-form, which is the same form as pure titanium, begin to appear.

The evolution of the dihydride from titanium metal and hydrogen has been examined in some detail. α-Titanium has an hexagonal close packed (hcp) structure at room temperature. Hydrogen initially occupies tetrahedral interstitial sites in the titanium. As the H/Ti ratio approaches 2, the material adopts the β-form to a face centred cubic (fcc), δ- form, the H atoms eventually filling all the tetrahedral sites to give the limiting stoichiometry of TiH₂. The various phases are described in the table below.

If titanium hydride contains 4.0% hydrogen at less than around 40 °C then it transforms into a body-centred tetragonal (bct) structure called ε-titanium.^[8]

When titanium hydrides with less than 1.3% hydrogen, known as hypoeutectoid titanium hydride are cooled, the β-titanium phase of the mixture attempts to revert to the α-titanium phase, resulting in an excess of hydrogen. One way for hydrogen to leave the β-titanium phase is for the titanium to partially transform into δ-titanium, leaving behind titanium that is low enough in hydrogen to take the form of α-titanium, resulting in an α-titanium matrix with δ-titanium inclusions.

A metastable γ-titanium hydride phase has been reported.^[9] When α-titanium hydride with a hydrogen content of 0.02-0.06% is quenched rapidly, it forms into γ-titanium hydride, as the atoms "freeze" in place when the cell structure changes from hcp to fcc. γ-Titanium takes a body centred tetragonal (bct) structure. Moreover, there is no compositional change so the atoms generally retain their same neighbours.

Hydrogen embrittlement titanium and titanium alloys

The absorption of hydrogen and the formation of titanium hydride are a source of damage to titanium and titanium alloys (Ti /Ti alloys). This hydrogen embrittlement process is of particular concern when titanium and alloys are used as structural materials, as in nuclear reactors.

Hydrogen embrittlement manifests as a reduction in ductility and eventually spalling of titanium surfaces. The effect of hydrogen is to a large extent determined by the composition, metallurgical history and handling of the Ti /Ti alloy.^[10] CP-titanium (commercially pure: ≤99.55% Ti content) is more susceptible to hydrogen attack than pure α-titanium. Embrittlement, observed as a reduction in ductility and caused by the formation of a solid solution of hydrogen, can occur in CP-titanium at concentrations as low as 30-40 ppm. Hydride formation has been linked to the presence of iron in the surface of a Ti alloy. Hydride particles are observed in specimens of Ti /Ti alloys that have been welded, and because of this welding is often carried out under an inert gas shield to reduce the possibility of hydride formation.^[10]

Ti /Ti alloys form a surface oxide layer, composed of a mixture of Ti(II), Ti(III) and Ti(IV) oxides,^[11] which offers a degree of protection to hydrogen entering the bulk.^[10] The thickness of this can be increased by anodizing, a process which also results in a distinctive colouration of the material. Ti /Ti alloys are often used in hydrogen containing environments and in conditions where hydrogen is reduced electrolytically on the surface. Pickling, an acid bath treatment which is used to clean the surface can be a source of hydrogen.

Uses

Common applications include ceramics, pyrotechnics, sports equipment, as a laboratory reagent, as a blowing agent, and as a precursor to porous titanium. When heated as a mixture with other metals in powder metallurgy, titanium hydride releases hydrogen which serves to remove carbon and oxygen, producing a strong alloy.^[3]