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Zirconium hydride

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This article is about the zirconium-hydrogen alloy. For the chemical compound, see Zirconium(II) hydride.

Zirconium hydride describes alloys with the formula ZrHx. They are dark gray to black metallic powders.[1] These alloys behave as usual metals in terms of electrical conductivity and magnetic properties (paramagnetic.[2] Similar to other metal hydrides, different crystalline phases of zirconium hydrides are labeled with Greek letters, and α is reserved for the metal.

Known ZrHx phases are γ (x = 1), δ (x = 1.5–1.65) and ε (x = 1.75–2). Often samples are mixtures: the compositions with x = 0.8–1.5 usually contain a mixture of α, γ and δ phases, and δ and ε phases coexist for x = 1.65–1.75. As a function of increasing x, the transition between δ-Zr and ε-Zr is observed as a gradual distortion of the face-centered cubic δ (fluorite-type) to face-centered tetragonal ε lattice. This distortion is accompanied by a rapid decrease in Vickers hardness, which is constant at 260 HV for x < 1.6, linearly decreases to 160 HV for 1.6 < x < 1.75 and stabilizes at about 160 HV for 1.75 < x < 2.0.[3] This hardness decrease is accompanied by the decrease in magnetic susceptibility.[4][5] The mass density behaves differently with the increasing hydrogen content: it decreases linearly from 6.52 to 5.66 g/cm3 for x = 0–1.6 and changes little for x = 1.6–2.0.[6] The zirconium-hydrogen system exhibits temperature-dependent solubility limits in α-zirconium. When the hydrogen concentration exceeds these limits, zirconium hydride phases precipitate, as represented in the Zr-H phase diagram.[7]

Hydrogen acts as a hardening agent, preventing dislocations in the zirconium atom crystal lattice from sliding past one another. Varying the amount of hydrogen and the form of its presence in the zirconium hydride (precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting zirconium hydride. Zirconium hydride with increased hydrogen content can be made harder and stronger than zirconium, but such zirconium hydride is also less ductile than zirconium.

Material properties

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The density of zirconium hydride varies based the hydrogen and ranges between 5.56 and 6.52 g cm−3.

Even in the narrow range of concentrations make up zirconium hydride, mixtures of hydrogen and zirconium can form a number of structures, with distinct properties. At room temperature, α-zirconium dissolve no more than 0.069 wt% at 550 °C. β-zirconium can dissolve considerably more hydrogen, more than 1.2% hydrogen above 900 °C.

Approximate formula CAS number Molecular
weight		 Density
g/cm3		 Symmetry Space group No Pearson symbol
ZrH 13940-37-9 92.232 5.9[6] Orthorhombic[8] Cccm 66 oS8
ZrH1.6 5.66 Cubic[4] Fm3m 225 cF12
ZrH2 7704-99-6 93.240 5.56 Tetragonal[5] I4/mmm 139 tI6
ZrH4 15457-96-2 95.256

Preparation and chemical properties

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Zirconium hydrides form upon interaction of the metal with hydrogen gas. Hydrogen absorbed by zirconium can diffuse through metal lattice, allowing hydrogen to redistribute within the material before precipitating as hydride phases.[9] Whereas this reaction occurs even at room temperature, homogeneous bulk hydrogenation is usually achieved by annealing at temperatures of 400–600 °C for a period between several hours and a few weeks.[4]

At room temperature, zirconium hydrides quickly oxidize in air. The formed nanometer-thin layer of oxide inhibits further oxygen diffusion into the material, and thus the change in composition due to oxidation can usually be neglected. However, the oxidation proceeds deeper into the bulk with increasing temperature.[2] The hydrogen is anionic due to the electronegativity difference between Zr and H.[10] When prepared as thin films, the crystal structure can be improved and surface oxidation minimized.[11]

Zirconium hydrides are dissolve in hydrofluoric acid.[1]

Applications

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Formation of zirconium hydrides is an important factor in the operation of several types of nuclear reactors, such as boiling water reactors Fukushima I and II, which suffered from a series of explosions caused by the 2011 Tōhoku earthquake and tsunami. Their uranium fuel pellets are enclosed in metal rods made from Zircaloy – an alloy of typically about 98.25% zirconium with 1.5% tin and minor amounts of other metals. Zircaloy is used because of its small absorption cross-section for thermal neutrons and superior mechanical and corrosion properties to those of most metals, including zirconium.[12][13][14] The rods are cooled by streaming water which gradually oxidizes zirconium, liberating hydrogen. In Fukushima reactors, the reactor cooling system failed because of the tsunami. The resulting temperature increase accelerated chemical reactions and caused accumulation of significant amounts of hydrogen, which exploded upon reaction with oxygen when the gas was released to the atmosphere.[15]

In regular operation, most hydrogen is safely neutralized in the reactor systems; however, a fraction of 5-20% diffuses into the Zircaloy rods forming zirconium hydrides.[12] This process mechanically weakens the rods because the hydrides have lower hardness and ductility than metal. Only a few percent of hydrogen can dissolve in zirconium. Excess hydrogen forms voids that weaken Zircalloy.[14] Among Zircaloys, Zircaloy-4 is the least susceptible to hydrogen blistering.[12]

It is also used as a neutron moderator in thermal-spectrum nuclear reactors such as the TRIGA research reactor developed by General Atomics or the Soviet TOPAZ nuclear reactors. At neutron energies above 0.14 eV it is as effective at moderating a nuclear reactor as elemental hydrogen (the best known material), but far more dense, and therefore permits compact reactors with high power per unit volume. It has neutron resonances that prevent almost all moderation at energies below 0.14 eV. Zirconium deuteride is superior, because it has a lower neutron absorption cross-section than aneutronic hydrogen, decreasing neutron absorption in a reactor.[16][17][18]

As a pure powder, zirconium hydrides are used as hydrogenation catalysts, in powder metallurgy, and as getters in the vacuum tube industry.[1] In vacuum systems, zirconium hydrides help establish a seal between a metal and ceramic. In this method, a hydride powder is mixed with the sealing metal; heating the mixture results in decomposition of the hydride. The evolving hydrogen cleans up the surrounding area, and the produced metal flows and forms a seal even at temperatures as low as 300 °C.[19]

ZrH2 is used in powder metallurgy, as a hydrogenation catalyst, and as a reducing agent, vacuum tube getter, and a foaming agent in production of metal foams. Other uses include acting as a fuel in pyrotechnic compositions, namely pyrotechnic initiators.

Irradiation-assisted dissolution of inter-granular zirconium hydrides under 6 keV He implantation in situ in a Transmission Electron Microscope at MIAMI Facility, United Kingdom.[20]

Zirconium(II) dihydride

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Zirconium(II) dihydride (ZrH2) is an elusive molecular species. It has been generated by laser ablation and observed at low temperature.[21]

Uses

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Zirconium(II) hydride is used as a thermal neutron moderator in nuclear reactors and as a material for neutron reflectors in fast reactors.[22]

Zirconium(II) hydride in the form of a powder is used in powder metallurgy as a hydrogenation catalyst, vacuum tube getter, foaming agent in the production of metal foams and as a reducing agent.[22]

Safety

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When exposed to air, zirconium hydrides are flammable and can ignite and explode if exposed to heat, fire, or sparks. Zirconium hydrides can react violently with water, acids, oxidizers or halogenated compounds.[1]

References

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  1. ^ a b c d Occupational Safety and Health Guideline for Zirconium & Compounds Archived July 21, 2011, at the Wayback Machine, Occupational Safety & Health Administration, U.S. Department of Labor
  2. ^ a b Bowman, R.; Venturini, E.; Craft, B.; Attalla, A.; Sullenger, D. (1983). "Electronic structure of zirconium hydride: A proton NMR study". Physical Review B. 27 (3): 1474–1488. Bibcode:1983PhRvB..27.1474B. doi:10.1103/PhysRevB.27.1474.
  3. ^ Korn, C. (1983). "NMR study comparing the electronic structures of ZrHx and TiHx". Physical Review B. 28 (1): 95–111. Bibcode:1983PhRvB..28...95K. doi:10.1103/PhysRevB.28.95.
  4. ^ a b c Bowman, R.; Craft, B.; Cantrell, J.; Venturini, E. (1985). "Effects of thermal treatments on the lattice properties and electronic structure of ZrHx". Physical Review B. 31 (9): 5604–5615. Bibcode:1985PhRvB..31.5604B. doi:10.1103/PhysRevB.31.5604. PMID 9936554.
  5. ^ a b Niedźwiedź, K.; Nowak, B.; Żogał, O. (1993). "91Zr NMR in non-stoichiometric zirconium hydrides, ZrHx (1.55 ≤ x ≤ 2)". Journal of Alloys and Compounds. 194 (1): 47–51. doi:10.1016/0925-8388(93)90643-2.
  6. ^ a b Atwood, J. D.; Zuckerman, J. J. (1999). Inorganic reactions and methods: Formation of ceramics. John Wiley and Sons. pp. 377–. ISBN 978-0-471-19202-2. Retrieved 16 March 2011.
  7. ^ Motta, Arthur T.; Capolungo, Laurent; Chen, Long-Qing; Cinbiz, Mahmut Nedim; Daymond, Mark R.; Koss, Donald A.; Lacroix, Evrard; Pastore, Giovanni; Simon, Pierre-Clément A.; Tonks, Michael R.; Wirth, Brian D.; Zikry, Mohammed A. (2019-05-01). "Hydrogen in zirconium alloys: A review". Journal of Nuclear Materials. 518: 440–460. doi:10.1016/j.jnucmat.2019.02.042. ISSN 0022-3115.
  8. ^ Switendick, A. C. (1984). "Electronic structure of γ phase zirconium hydride". Journal of the Less-Common Metals. 103 (2): 309–315. doi:10.1016/0022-5088(84)90254-6.
  9. ^ Kearns, J. J (1972-06-01). "Diffusion coefficient of hydrogen in alpha zirconium, Zircaloy-2 and Zircaloy-4". Journal of Nuclear Materials. 43 (3): 330–338. doi:10.1016/0022-3115(72)90065-7. ISSN 0022-3115.
  10. ^ Quijano, Ramiro (2009). "Electronic structure and energetics of the tetragonal distortion for TiH2, ZrH2 and HfH2". Physical Review B. 80 (18) 184103. Bibcode:2009PhRvB..80r4103Q. doi:10.1103/PhysRevB.80.184103.
  11. ^ Magnuson, M. (2017). "Bonding Structures of ZrHx Thin Films by X-ray Spectroscopy". J. Phys. Chem. C. 121 (46): 25750. arXiv:1711.09415. Bibcode:2017arXiv171109415M. doi:10.1021/acs.jpcc.7b03223. S2CID 104107002.
  12. ^ a b c DOE-HDBK-1017/2-93 JANUARY 1993 Archived 2016-03-04 at the Wayback Machine, DOE FUNDAMENTALS HANDBOOK, MATERIAL SCIENCE, Volume 2 of 2, U.S. Department of Energy january 2003, pp. 12, 24
  13. ^ Nuclear Fuel Fabrication Archived July 26, 2011, at the Wayback Machine, Fuel Fabrication Archived July 26, 2011, at the Wayback Machine World Nuclear Association, March 2010
  14. ^ a b Delayed hydride cracking in zirconium alloys in pressure tube nuclear reactors, Final report of a coordinated research project 1998–2002, IAEA, October 2004
  15. ^ Japanese engineers work to contain nuclear reactor damage, Los Angeles Times, March 14, 2011
  16. ^ Baron, Matthias; Böck, Helmuth; Villa, Mario. "TRIGA Reactor Characteristics". IAEA Education and Training. IAEA. Retrieved 2 June 2016.
  17. ^ Gylfe, J. D. "US Patent 3,145,150, Aug. 18, 1954, Fuel Moderator Element for a Nuclear Reactor, and Method of Making". U.S. Patent Office. U.S. Government. Archived from the original on 24 February 2021. Retrieved 2 June 2016.
  18. ^ Massie, Mark; Dewan, Leslie C. "US 20130083878 A1, April 4, 2013, NUCLEAR REACTORS AND RELATED METHODS AND APPARATUS". U.S. Patent Office. U.S. Government. Archived from the original on 25 February 2021. Retrieved 2 June 2016.
  19. ^ Alexander Roth (1994). Vacuum sealing techniques. Springer. pp. 212–. ISBN 978-1-56396-259-2. Retrieved 16 March 2011.
  20. ^ Tunes, M. A.; Harrison, R. W.; Greaves, G.; Hinks, J. A.; Donnelly, S. E. (2017). "Effect of He implantation on the microstructure of zircaloy-4 studied using in situ TEM" (PDF). Journal of Nuclear Materials. 493: 230–238. Bibcode:2017JNuM..493..230T. doi:10.1016/j.jnucmat.2017.06.012. S2CID 102695615.
  21. ^ Chertihin, George V.; Andrews, Lester (1995). "Reactions of laser-ablated Zr and Hf atoms with hydrogen. Matrix infrared spectra of the MH, MH2, MH3, and MH4 molecules". The Journal of Physical Chemistry. 99 (41): 15004–15010. doi:10.1021/j100041a014. ISSN 0022-3654.
  22. ^ a b "Zirconium hydride". Haz-Map. Retrieved 18 February 2025.
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