Titanium hydride: Difference between revisions

{{about|the stable titanium hydride|the unstable molecular [[chemical compound]]|Titanium(IV) hydride}}

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| Watchedfields = changed

| verifiedrevid = 436121889

| ImageFile = Titanium hydride TiH2.jpg

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| ImageCaption = Titanium hydride powder

| IUPACName = titanium dihydride (hydrogen deficient)

| OtherNames =

| Section1 = {{Chembox Identifiers

| CASNo_Ref = {{cascite|correct|CAS}}

| CASNo = 7704-98-5

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = 8930U91840

| PubChem = 197094

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| Section2 = {{Chembox Properties

| Formula = {{chem2|TiH_{2−''x''}|}}

| MolarMass = 49.88 g/mol ({{chem2|TiH2}})

| Appearance = black powder (commercial form)

| Density = 3.76 g/cm³ (typical commercial form)

| MeltingPt = Decomposes

| BoilingPt =

| Solubility = insoluble

| Section3 = {{Chembox Hazards

| MainHazards =

| FlashPt =

| AutoignitionPt =

'''Titanium hydride''' normally refers to the [[inorganic compound]] {{chem2|TiH2}} and related [[nonstoichiometric compound|nonstoichiometric]] materials.<ref>{{Greenwood&Earnshaw2nd}}</ref><ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. {{ISBN|0-12-352651-5}}.</ref> 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.<ref name = "Ullmanns"/>

Also known as '''titanium–hydrogen alloy''',<ref name=McQuillan1950>{{cite journal|last=McQuillan|first=A. D.|title=An experimental and thermodynamic investigation of the hydrogen-titanium system|journal=Proceedings of the Royal Society A|date=22 December 1950|volume=204|issue=1078|pages=309–323|doi=10.1098/rspa.1950.0176|bibcode=1950RSPSA.204..309M |s2cid=135759594 |url=http://rspa.royalsocietypublishing.org/content/204/1078/309.abstract|access-date=10 March 2013}}</ref><ref name=Bennett1980>{{cite journal|last=Bennett|first=L. H.|title=Nuclear magnetic resonance in alloys|journal=MRS Proceedings|year=1980|volume=3|doi=10.1557/PROC-3-3|url=http://journals.cambridge.org/action/displayAbstract;jsessionid=7611C84A9BF6AE1B9B569DE4D6BE39FC.journals?fromPage=online&aid=8088647|access-date=10 March 2013}}</ref> it is an [[alloy]]<ref>{{cite journal|last=Wang|first=Xin-Quan|author2=Wang, Jian-Tao |title=Structural stability and hydrogen diffusion in TiH{{sub|x}} alloys|journal=Solid State Communications|date=15 June 2010|volume=150|issue=35–36|pages=1715–1718|doi=10.1016/j.ssc.2010.06.004|bibcode=2010SSCom.150.1715W |url=http://www.sciencedirect.com/science/article/pii/S0038109810003285|access-date=10 March 2013}}</ref> of [[titanium]], [[hydrogen]], and possibly other elements. When hydrogen is the main 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 include [[gallium]], [[iron]], [[vanadium]], and [[aluminium]].

==Production==

In the commercial process for producing non-stoichiometric {{chem2|TiH_{2−''x''}|}}, titanium [[metal sponge]] is treated with hydrogen gas at atmospheric pressure at between 300-500&nbsp;°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 {{chem2|TiH1.95}}.<ref name = "Ullmanns">{{cite book |last1=Rittmeyer |first1=Peter |last2=Weitelmann |first2=Ulrich |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH|date= 2005 |chapter=Hydrides |doi=10.1002/14356007.a13_199 |isbn=978-3-527-30673-2 }}</ref> In the laboratory, titanium hydride is produced by heating [[titanium powder]] under flowing hydrogen at 700&nbsp;°C, the idealized equation being:<ref name=Brauer>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.</ref>

:{{chem2|Ti + H2 → TiH2}}

Other methods of producing titanium hydride include electrochemical and ball milling methods.<ref name=Millenbach1982>{{cite journal|last=Millenbach|first=Pauline|author2=Givon, Meir |title=The electrochemical formation of titanium hydride|journal=Journal of the Less Common Metals|date=1 October 1982|volume=87|issue=2|pages=179–184|doi=10.1016/0022-5088(82)90086-8|url=https://dx.doi.org/10.1016/0022-5088%2882%2990086-8|access-date=10 March 2013}}</ref><ref name="ZhangKisi1997">{{cite journal|last1=Zhang|first1=Heng|last2=Kisi|first2=Erich H|title=Formation of titanium hydride at room temperature by ball milling|journal=Journal of Physics: Condensed Matter|volume=9|issue=11|year=1997|pages=L185–L190|issn=0953-8984|doi=10.1088/0953-8984/9/11/005|bibcode=1997JPCM....9L.185Z|s2cid=250853926 }}</ref>

==Reactions==

{{chem2|TiH1.95}} is unaffected by water and air.{{citation needed|date=November 2023}} It is slowly attacked by strong acids and is degraded by hydrofluoric and hot sulfuric acids. It reacts rapidly with oxidizing agents, this reactivity leading to the use of titanium hydride in pyrotechnics.<ref name = "Ullmanns"/>

The material has been used to produce highly pure hydrogen, which is released upon heating the solid.<ref name="Brauer" /> Hydrogen release in TiH_~2 starts just above 400&nbsp;°C but may not be complete until the melting point of titanium metal.<ref>{{Cite journal |last=Paulin |first=Irena |last2=Donik |first2=Črtomir |last3=Mandrino |first3=Djordje |last4=Vončina |first4=Maja |last5=Jenko |first5=Monika |date=January 2012 |title=Surface characterization of titanium hydride powder |url=https://linkinghub.elsevier.com/retrieve/pii/S0042207X11003010 |journal=Vacuum |volume=86 |issue=6 |pages=608–613 |doi=10.1016/j.vacuum.2011.07.054}}</ref><ref name="Ullmanns" /> Titanium [[Hydride#Tritides|tritide]] (Ti{{sup|3}}H{{sub|''x''}}) has been proposed for long-term storage of [[tritium]] gas.<ref name=Brown1988>{{cite journal|last=Brown|first=Charles C.|author2=Buxbaum, Robert E. |title=Kinetics of hydrogen absorption in alpha titanium|journal=Metallurgical Transactions A|date=June 1988|volume=19|issue=6|pages=1425–1427|doi=10.1007/bf02674016|bibcode=1988MTA....19.1425B|s2cid=95614680}}</ref>

==Structure==

As {{chem2|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 {{chem2|TiH1.74}} is reached.{{citation needed|date=November 2023}} 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 {{chem2|TiH1.47}} 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 a [[Close-packing of equal spheres|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 [[Cubic crystal system|face centred cubic (fcc)]], δ-form, the H atoms eventually filling all the tetrahedral sites to give the limiting stoichiometry of {{chem2|TiH2}}. The various phases are described in the table below.

{|class="wikitable" style="text-align:center"

|+ Temperature approx. 500&nbsp;°C, taken from illustration<ref name="Fukai">{{cite book |last=Fukai |first=Y |year=2005 |title=The Metal-Hydrogen System, Basic Bulk Properties, 2d edition|publisher=Springer|isbn=978-3-540-00494-3}}</ref>

!Phase!! Weight % H!!Atomic % H!!{{chem2|TiH_{''x''}|}}!!Metal lattice

|-

|α||0 – 0.2||0 – 8||{{chem2|TiH0}} – {{chem2|TiH0.1}}||[[hexagonal close packed|hcp]]

|-

|α & β||0.2 – 1.1||8 – 34||{{chem2|TiH0.1}} – {{chem2|TiH0.5}}||

|-

|β||1.1 – 1.8||34 – 47||{{chem2|TiH0.5}} – {{chem2|TiH0.9}}||[[Cubic crystal system|bcc]]

|-

|β & δ||1.8 – 2.5||47 – 57||{{chem2|TiH0.9}} – {{chem2|TiH1.32}}||

|-

|δ||2.7 – 4.1||57 – 67||{{chem2|TiH1.32}} – {{chem2|TiH2}}||[[Cubic crystal system|fcc]]

|}

If titanium hydride contains 4.0% hydrogen at less than around 40&nbsp;°C then it transforms into a [[tetragonal crystal system|body-centred tetragonal]] (bct) structure called ε-titanium.<ref name="Fukai"/>

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.<ref name="NumakuraKoiwa1988">{{cite journal|last1=Numakura|first1=H|last2=Koiwa|first2=M|last3=Asano|first3=H|last4=Izumi|first4=F|title=Neutron diffraction study of the metastable γ titanium deuteride|journal=Acta Metallurgica|volume=36|issue=8|year=1988|pages=2267–2273|issn=0001-6160|doi=10.1016/0001-6160(88)90326-4}}</ref> When α-titanium hydride with a hydrogen content of 0.02-0.06% is [[quenching|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 in titanium and titanium alloys==

[[Image:Anodized titanium colors.svg|right|200px|thumb|Selected colours achievable through anodization of titanium.]]

The absorption of hydrogen and the formation of titanium hydride are a source of damage to titanium and titanium 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 and Ti alloy.<ref name="Donachie">{{cite book |last=Donachie |first=Matthew J. |year=2000 |title=Titanium: A Technical Guide|publisher=ASM International|isbn=978-0-87170-686-7}}</ref> 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.<ref name="Donachie"/>

Ti and Ti alloys form a [[Passivation (chemistry)|surface oxide layer]], composed of a mixture of [[titanium(II) oxide|Ti(II)]], [[titanium(III) oxide|Ti(III)]] and [[titanium dioxide|Ti(IV)]] oxides,<ref name="LuBernasek2000">{{cite journal|last1=Lu|first1=Gang|last2=Bernasek|first2=Steven L.|last3=Schwartz|first3=Jeffrey|title=Oxidation of a polycrystalline titanium surface by oxygen and water|journal=Surface Science|volume=458|issue=1–3|year=2000|pages=80–90|issn=0039-6028|doi=10.1016/S0039-6028(00)00420-9|bibcode=2000SurSc.458...80L}}</ref> which offers a degree of protection to hydrogen entering the bulk.<ref name="Donachie"/> The thickness of this can be increased by [[anodizing]], 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 (metal)|Pickling]], an acid bath treatment which is used to clean the surface can be a source of hydrogen.

== Uses ==

Common applications include [[ceramic]]s, [[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.<ref name = "Ullmanns"/>

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The [[density]] 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 crystal system|hexagonal close-packed]] (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 {{convert|464|C}}, and only 0.02% at {{convert|25|C}}. If titanium hydride contains more than 0.20% hydrogen at titanium hydride-making temperatures it transforms into a [[cubic crystal system|body-centred cubic]] (BCC) structure called β-titanium. It can dissolve considerably more hydrogen, more than 2.1% hydrogen at {{convert|636|C}}. If titanium hydride contains more than 2.1% at {{convert|636|C}} 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 {{convert|37|C}}, which reflects the upper hydrogen content of titanium hydride.<ref>{{cite journal |last1=Setoyama |first1=Daigo |last2=Matsunaga |first2=Junji |last3=Muta |first3=Hiroaki |last4=Uno |first4=Masayohi |last5=Yamanaka |first5=Shinsuke |title=Mechanical properties of titanium hydride |journal=Journal of Alloys and Compounds|date=3 November 2004|volume=381|issue=1–2|pages=215–220 |doi=10.1016/j.jallcom.2004.04.073}}</ref>

There are many types of [[heat treatment|heat treating]] processes available to titanium hydride. The most common are [[annealing (metallurgy)|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 (metallurgy)|recovery]], [[recrystallization (metallurgy)|recrystallization]], and [[grain growth]]. The temperature required to anneal titanium hydride depends on the type of annealing. Annealing must be done under a hydrogen atmosphere to prevent [[outgassing]].

== See also ==

*[[Machinability]]

{{Reflist}}

==External links==

*{{Commons category-inline}}

{{Hydrides by group}}

[[Category:Titanium(II) compounds|Hydride]]

[[Category:Reducing agents]]