Titanium hydride
| Titanium hydride | ||
|---|---|---|
 Titanium hydride powder | ||
| IUPAC name titanium dihydride (hydrogen deficient) | ||
| Identifiers | ||
| CAS number | 7704-98-5 | |
| PubChem | 197094 | |
| Properties | ||
| Molecular formula | TiH2-x | |
| Molar mass | 49.88g/mol(TiH2) | |
| Appearance | black powder (commercial form) | |
| Density | 3.76 g/cc (typical commercial form) | |
| Melting point | 350 °C approx | |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa) | ||
| Infobox references | ||
Titanium hydride normally refers to the non-stoichiometric compound TiH(2-x), 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.
Production and reactions of non-stoichiometric TiH(2-x)
In the commercial process for producing non-stoichiometric TiH(2-x) titanium metal sponge is reacted 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 TiH1.95.[1] Other methods of producing titanium hydride have been reported and these include electrochemical and ball milling methods.[2][3]
TiH1.95 is unaffected by water and air, only slowly attacked by strong acids, but is soluble in hydrofluoric and hot sulfuric acids. It reacts rapidly with oxidising agents. At around 350 °C, it decomposes, losing hydrogen gas. Only at the melting point of titanium is dissociation finally complete.[1]
To produce powdered titanium the hydride is heated to 600°C under vacuum to remove hydrogen.[1]
Titanium - hydrogen system
Titanium has an hexagonal close packed (hcp) structure at room temperature, (α- form). Hydrogen is absorbed exothermically. The initial absorption involves hydrogen atoms forming a solid solution in α titanium, occupying tetrahedral interstitial sites in the metal lattice. Solubility is limited. Dissolution of hydrogen reduces the transition temperature from the hcp α- form to the bcc β- form by over 500 °C and as more hydrogen is absorbed the metal lattice starts to change to the β-. As more H atoms enter the metal the lattice then converts from 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, which has the fluorite structure. [4]
| Phase | Weight % H | Atomic % H | TiHx | Metal lattice |
|---|---|---|---|---|
| α- | 0 - 0.2 | 0 - 8 | hcp | |
| α- & β- | 0.2 - 1.1 | 8 - 34 | TiH0.1 - TiH0.5 | |
| β- | 1.1 - 1.8 | 34 - 47 | TiH0.5 - TiH0.9 | bcc |
| β- & δ | 1.8 - 2.5 | 47 - 57 | TiH0.9 - TiH1.32 | |
| δ- | 2.7 - 4.1 | 57- 67 | TiH1.32 - TiH2 | fcc |
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.[4]
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.[5] 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 of "CP" (commercially pure) 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). One obvious example of physical is spalling of the surface due to hydride formation at low temperatures. The effect of hydrogen is to a large extent determined by the composition, metallurgical history and handling of the Ti /Ti alloy.[6] Ti /Ti alloys are often coated with oxide, Ti(II) , Ti(III) and Ti(IV) oxides have been observed,[7] and this layer offers a degree of protection to hydrogen entering the bulk.[6] Ti /Ti alloys 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 sometimes used to clean the surface can be a source of hydrogen. CP-titanium (commercially pure less than about 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.[6]
Uses
Common applications include ceramics, pyrotechnics, sports equipment, as a laboratory reagent, as a blowing agent, and as a starting material for porous titanium production.
As of 1988, titanium hydride was considered to be a leading candidate as a form of long-term tritium storage.[8]
References
- ↑ 1.0 1.1 1.2 Rittmeyer, Peter; Weitelmann, Ulrich (January 1, 2001) [1st. Pub. 1986]. "Hydrides". Ullmans Fine Chemicals ,e-book. Wiley -VCH. p. 695. ISBN 978-3-527-68359-8.
- ↑ Millenbach, Pauline; Givon, Meir (1 October 1982). "The electrochemical formation of titanium hydride". Journal of the Less Common Metals 87 (2): 179–184. doi:10.1016/0022-5088(82)90086-8. Retrieved 10 March 2013.
- ↑ Zhang, Heng; Kisi, Erich H (1997). "Formation of titanium hydride at room temperature by ball milling". Journal of Physics: Condensed Matter 9 (11): L185–L190. doi:10.1088/0953-8984/9/11/005. ISSN 0953-8984.
- ↑ 4.0 4.1 4.2 Fukai, Y (2005). The Metal-Hydrogen System, Basic Bulk Properties, 2d edition. Springer. ISBN 978-3-540-00494-3.
- ↑ Numakura, H; Koiwa, M; Asano, H; Izumi, F (1988). "Neutron diffraction study of the metastable γ titanium deuteride". Acta Metallurgica 36 (8): 2267–2273. doi:10.1016/0001-6160(88)90326-4. ISSN 0001-6160.
- ↑ 6.0 6.1 6.2 Donachie, Matthew J. (2000). Titanium: A Technical Guide. ASM International. ISBN 0-87170-686-5.
- ↑ Lu, Gang; Bernasek, Steven L.; Schwartz, Jeffrey (2000). "Oxidation of a polycrystalline titanium surface by oxygen and water". Surface Science 458 (1-3): 80–90. Bibcode:2000SurSc.458...80L. doi:10.1016/S0039-6028(00)00420-9. ISSN 0039-6028.
- ↑ Brown, Charles C.; Buxbaum, Robert E. (June 1988). "Kinetics of hydrogen absorption in alpha titanium". Metallurgical Transactions A 19 (6): 1425–1427. Retrieved 16 February 2013.