Zirconium hydride

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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. In situ TEM dissolution of ZrH under 6 keV He implantation.gif
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.

Zirconium hydride describes an alloy made by combining zirconium and hydrogen. 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.

Contents

Material properties

Zirconium is found in the Earth's crust only in the form of an ore, usually a zirconium silicate, such as zircon. Zirconium is extracted from zirconium ore by removing the oxygen and silica. This process, known as the Kroll process, was first applied to titanium. The Kroll process results in an alloy containing hafnium. The hafnium and other impurities are removed in a subsequent step. Zirconium hydride is created by combining refined zirconium with hydrogen. Like titanium, solid zirconium dissolves hydrogen quite readily.

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 which make up zirconium hydride, mixtures of hydrogen and zirconium can form a number of different structures, with very different properties. Understanding such properties is essential to making quality zirconium hydride. At room temperature, the most stable form of zirconium is the hexagonal close-packed (HCP) structure α-zirconium. It is a fairly soft metallic material that can dissolve only a small concentration of hydrogen, no more than 0.069 wt% at 550 °C. If zirconium hydride contains more than 0.069% hydrogen at zirconium hydride making temperatures then it transforms into a body-centred cubic (BCC) structure called β-zirconium. It can dissolve considerably more hydrogen, more than 1.2% hydrogen above 900 °C.

When zirconium hydrides with less than 0.7% hydrogen, known as hypoeutectoid zirconium hydride, are cooled from the β phase the mixture attempts to revert to the α phase, resulting in an excess of hydrogen.

Another polymorphic form is the γ phase, is generally accepted to be a metastable phase.

Approximate formula CAS number Molecular
weight		Density
g/cm3		Symmetry Space group No Pearson symbol
ZrH13940-37-992.2325.9 [2] Orthorhombic [3] Cccm66oS8
ZrH1.65.66 Cubic [4] Fm3m225cF12
ZrH27704-99-693.2405.56 Tetragonal [5] I4/mmm139tI6
ZrH415457-96-295.256

Zirconium hydrides are odorless, dark gray to black metallic powders. [6] They behave as usual metals in terms of electrical conductivity and magnetic properties (paramagnetic, unless contaminated with ferromagnetic impurities). Their structure and composition is stable at ambient conditions. [7] Similar to other metal hydrides, different crystalline phases of zirconium hydrides are conventionally labeled with Greek letters, and α is reserved for the metal. The known ZrHx phases are γ (x = 1), δ (x = 1.5–1.65) and ε (x = 1.75–2). Fractional x values often correspond to mixtures, so 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. [8] 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. [2]

Preparation and chemical properties

Zirconium hydrides form upon interaction of the metal with hydrogen gas. 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, and even in high vacuum. The formed nanometer-thin layer of oxide stops 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. [7] The hydrogen is anionic due to the electronegativity difference between Zr and H. [9] When prepared as thin films, the crystal structure can be improved and surface oxidation minimized. [10]

Zirconium hydrides are soluble in hydrofluoric acid or alcohol; they react violently with water, acids, oxidizers or halogenated compounds. [6]

Applications

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. [11] [12] [13] 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. [14]

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. [11] 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. [13] Among Zircaloys, Zircaloy-4 is the least susceptible to hydrogen blistering. [11]

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. [15] [16] [17]

As a pure powder, zirconium hydrides are used as hydrogenation catalysts, in powder metallurgy, and as getters in the vacuum tube industry. [6] In vacuum system, zirconium hydrides help establish a seal between a metal and ceramic. In this method, a hydride powder (particularly ZrH4) 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. [18]

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.

Safety

Powdered zirconium hydrides are flammable and can ignite and explode if exposed to heat, fire, or sparks. When heated above 300 °C, they decompose releasing hydrogen gas, which is also flammable. [6]

Related Research Articles

<span class="mw-page-title-main">Zirconium</span> Chemical element, symbol Zr and atomic number 40

Zirconium is a chemical element; it has symbol Zr and atomic number 40. The name zirconium is derived from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian zargun. It is a lustrous, grey-white, strong transition metal that closely resembles hafnium and, to a lesser extent, titanium.

Palladium hydride is metallic palladium that contains a substantial quantity of hydrogen within its crystal lattice. Despite its name, it is not an ionic hydride but rather an alloy of palladium with metallic hydrogen that can be written PdHx. At room temperature, palladium hydrides may contain two crystalline phases, α and β. Pure α-phase exists at x < 0.017 whereas pure β-phase is realised for x > 0.58; intermediate x values correspond to α-β mixtures.

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<span class="mw-page-title-main">Nuclear fuel</span> Material fuelling nuclear reactors

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<span class="mw-page-title-main">Lithium hydride</span> Chemical compound

Lithium hydride is an inorganic compound with the formula LiH. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not soluble but reactive with all protic organic solvents. It is soluble and nonreactive with certain molten salts such as lithium fluoride, lithium borohydride, and sodium hydride. With a molar mass of 7.95 g/mol, it is the lightest ionic compound.

<span class="mw-page-title-main">Zirconium alloys</span> Zircaloy family

Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.

<span class="mw-page-title-main">Zirconium carbide</span> Chemical compound

Zirconium carbide (ZrC) is an extremely hard refractory ceramic material, commercially used in tool bits for cutting tools. It is usually processed by sintering.

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Zirconium(II) hydride is a molecular chemical compound with the chemical formula ZrH2. It is a grey crystalline solid or dark gray to black powder. It has been prepared by laser ablation and isolated at low temperature.

<span class="mw-page-title-main">Titanium hydride</span> Chemical compound

Titanium hydride normally refers to the inorganic compound TiH2 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 sintered magnets, in the sintering of powdered metals, the production of metal foam, the production of powdered titanium metal and in pyrotechnics.

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Scandium hydride, also known as scandium–hydrogen alloy, is an alloy made by combining scandium and hydrogen. Hydrogen acts as a hardening agent, preventing dislocations in the scandium atom crystal lattice from sliding past one another. Varying the amount of hydrogen controls qualities such as the hardness of the resulting scandium hydride. Scandium hydride with increased hydrogen content can be made harder than scandium.

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<span class="mw-page-title-main">High-entropy alloy</span> Alloys with high proportions of several metals

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References

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  5. 1 2 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. 1 2 3 4 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
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  9. ↑ 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.
  10. ↑ 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.
  11. 1 2 3 DOE-HDBK-1017/2-93 JANUARY 1993, DOE FUNDAMENTALS HANDBOOK, MATERIAL SCIENCE, Volume 2 of 2, U.S. Department of Energy january 2003, pp. 12, 24
  12. ↑ 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
  13. 1 2 Delayed hydride cracking in zirconium alloys in pressure tube nuclear reactors, Final report of a coordinated research project 1998–2002, IAEA, October 2004
  14. ↑ Japanese engineers work to contain nuclear reactor damage, Los Angeles Times, March 14, 2011
  15. ↑ Baron, Matthias; Böck, Helmuth; Villa, Mario. "TRIGA Reactor Characteristics". IAEA Education and Training. IAEA. Retrieved 2 June 2016.
  16. ↑ 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. Retrieved 2 June 2016.
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