Titanium hydride normally refers to the
inorganic compound and related
nonstoichiometric materials. It is commercially available as a stable grey/black powder, which is used as an additive in the production of
Alnico
Alnico is a family of iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni), and cobalt (Co), hence the acronym ''al-ni-co''. They also include copper, and sometimes titanium. Alnico alloys are ferromagnetic, ...
sintered magnets, in the sintering of powdered metals, the production of
metal foam
Regular foamed aluminium
A metal foam is a cellular structure consisting of a solid metal (frequently aluminium) with gas-filled pores comprising a large portion of the volume. The pores can be sealed (closed-cell foam) or interconnected (open-c ...
, the production of powdered titanium metal and in pyrotechnics.
Production and reactions of
In the commercial process for producing non-stoichiometric , 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 .
In the laboratory, titanium hydride is produced by heating
titanium powder Titanium powder metallurgy (P/M) offers the possibility of creating net shape or near net shape parts without the material loss and cost associated with having to machine intricate components from wrought billet. Powders can be produced by the blend ...
under flowing hydrogen at 700 °C, the idealized equation being:
[M. Baudler "Hydrogen, Deuterium, Water" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 114-115.]
:
Other methods of producing titanium hydride include electrochemical and ball milling methods.
Reactions
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.
The material has been used to produce highly pure hydrogen, which is released upon heating the solid starting at 300 °C.
[ Only at the melting point of titanium is dissociation complete.] Titanium tritide () has been proposed for the long-term storage of tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...
gas.
Structure
As 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 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 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 a 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 . 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.
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. When α-titanium hydride with a hydrogen content of 0.02-0.06% is quenched
In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ph ...
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 in titanium and titanium alloys
The absorption of hydrogen and the formation of titanium hydride are a source of damage to titanium and titanium alloys. This hydrogen embrittlement
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed ...
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
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...
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 and Ti alloy. 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 and 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.
Ti and Ti alloys form a surface oxide layer, composed of a mixture of Ti(II), Ti(III) and Ti(IV) oxides, which offers a degree of protection to hydrogen entering the bulk. The thickness of this can be increased by anodizing
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
The process is called ''anodizing'' because the part to be treated forms the anode electrode of an electr ...
, a process which also results in a distinctive colouration of the material. Ti and Ti alloys are often used in hydrogen containing environments and in conditions where hydrogen is reduced electrolytically on the surface. Pickling
Pickling is the process of preserving or extending the shelf life of food by either anaerobic fermentation in brine or immersion in vinegar. The pickling procedure typically affects the food's texture and flavor. The resulting food is cal ...
, an acid bath treatment which is used to clean the surface can be a source of hydrogen.
Uses
Common applications include ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s, pyrotechnics, sports equipment
Sports equipment, sporting equipment, also called sporting goods, are the tools, materials, apparel, and gear used to compete in a sport and varies depending on the sport. The equipment ranges from balls, nets, and protective gear like helmets. ...
, as a laboratory reagent, as a blowing agent
A blowing agent is a substance which is capable of producing a cellular structure via a foaming process in a variety of materials that undergo hardening or phase transition, such as polymers, plastics, and metals. They are typically applied wh ...
, and as a precursor to porous titanium. When heated as a mixture with other metals in powder metallurgy
Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can reduce or eliminate the need for subtractive processes in manufacturing, lowering material losses and ...
, titanium hydride releases hydrogen which serves to remove carbon and oxygen, producing a strong alloy.
The density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of titanium hydride varies based on the alloying constituents, but for pure titanium hydride it ranges between 3.76 and 4.51 g/cm3.
Even in the narrow range of concentrations that make up titanium hydride, mixtures of hydrogen and titanium can form a number of different structures, with very different properties. Understanding such properties is essential to making quality titanium hydride. At room temperature, the most stable form of titanium is the hexagonal close-packed
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
(HCP) structure α-titanium. It is a fairly hard metal that can dissolve only a small concentration of hydrogen, no more than 0.20 wt% at , and only 0.02% at . If titanium hydride contains more than 0.20% hydrogen at titanium hydride-making temperatures it transforms into a body-centred cubic
In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There are three main varieties of ...
(BCC) structure called β-titanium. It can dissolve considerably more hydrogen, more than 2.1% hydrogen at . If titanium hydride contains more than 2.1% at then it transforms into a face-centred cubic (FCC) structure called δ-titanium. It can dissolve even more hydrogen, as much as 4.0% hydrogen , which reflects the upper hydrogen content of titanium hydride.
There are many types of heat treating
Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are als ...
processes available to titanium hydride. The most common are annealing and quenching. Annealing is the process of heating the titanium hydride to a sufficiently high temperature to soften it. This process occurs through three phases: recovery
Recovery or Recover may refer to:
Arts and entertainment Books
* ''Recovery'' (novel), a Star Wars e-book
* Recovery Version, a translation of the Bible with footnotes published by Living Stream Ministry
Film and television
* ''Recovery'' (fil ...
, recrystallization, and grain growth
In materials science, grain growth is the increase in size of grains (crystallites) in a material at high temperature. This occurs when recovery and recrystallisation are complete and further reduction in the internal energy can only be achieved ...
. The temperature required to anneal titanium hydride depends on the type of annealing. Annealing must be done under a hydrogen atmosphere to prevent outgassing
Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which ...
.
Other methods of production
There is also an electrochemical method for producing titanium hydride.
A physical ball milling method
Titanium is found in the Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's crust only in the form of an ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
, usually a titanium oxide, such as anatase
Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral form ...
, brookite
Brookite is the orthorhombic variant of titanium dioxide (TiO2), which occurs in four known natural polymorphic forms (minerals with the same composition but different structure). The other three of these forms are akaogiite (monoclinic), anatas ...
, etc. Titanium is extracted from titanium ore by removing the oxygen and reacting the ore with chlorine. This process is known as the Kroll process The Kroll process is a pyrometallurgical industrial process used to produce metallic titanium from titanium tetrachloride. The Kroll process replaced the Hunter process for almost all commercial production.
Process
In the Kroll process, the TiCl ...
. The Kroll process results in a volatile chemical compound (titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds ...
). Titanium is further extracted from titanium tetrachloride by removing the chlorine and reacting the titanium tetrachloride with hydrogen. This process is known as hydrogen reduction, and results in an alloy (titanium hydride).
Hydrogen reduces the stress necessary to force dislocation
In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to s ...
s in the titanium atom crystal lattice to slide past one another. Varying the amount of hydrogen and other alloying elements and the form of their presence in the titanium hydride (solute elements, precipitated phase) controls qualities such as the hardness
In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...
, ductility
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...
, and tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
of the resulting titanium hydride. Titanium hydride with increased hydrogen content can be made softer and more ductile
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
than titanium.
Alloys with a high percentage of hydrogen (depending on other element content and possibly on processing) are known as cast titanium hydride. Because they are not malleable even when hot, they can be worked only by casting, and they have lower melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
, and good castability Castability is the ease of forming a quality casting. A very castable part design is easily developed, incurs minimal tooling costs, requires minimal energy, and has few rejections.Ravi, p. 2 Castability can refer to a part design or a material pro ...
.
Also known as titanium–hydrogen alloy, is an alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...
of titanium
Titanium is a chemical element with the Symbol (chemistry), 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, resista ...
, hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
, and possibly other elements. When hydrogen is the primary alloying element, its content in the titanium hydride is between 0.02% and 4.0% by weight. Alloying elements intentionally added to modify the characteristics of titanium hydride are gallium, iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
, vanadium, and aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
.
See also
*Machinability Machinability is the ease with which a metal can be cut (machined) permitting the removal of the material with a satisfactory finish at low cost.Degarmo, p. 542. Materials with good machinability (free machining materials) require little power to c ...
References
External links
*
{{Hydrides by group
Hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...
Metal hydrides
Reducing agents
Pyrotechnic fuels