Zirconium hydride
Encyclopedia
Zirconium hydride is an inorganic chemical compound
, a hydride
of zirconium
with the formula Zr
Hx
. Whereas x can be as large as 4, the most common values are between 1 and 2. Zirconium hydrides form upon reaction of zirconium
metal with hydrogen
gas. They behave as typical metals in terms of electrical, magnetic and mechanical properties. Those properties depend on the composition. For example, the Vickers hardness decreases with increasing hydrogen content – a phenomenon known as hydrogen embrittlement
.
Zirconium hydrides are odorless, dark gray to black metallic powders.
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. 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. This hardness decrease is accompanied by the decrease in magnetic susceptibility
. 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.
Zirconium hydrides are soluble in hydrofluoric acid
or alcohol; they react violently with water, acids, oxidizers or halogenated compounds.
s 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. 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.
Under 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. This process mechanically weakens the rods because the hydrides have lower hardness and ductility than metal. More important though is that the solubility of hydrogen in zirconium is limited to a few percent maximum. Thus the excess hydrogen forms voids which contribute more to the weakening of the alloy. Among Zircaloys, Zircaloy-4 is the least susceptible to this hydrogen blistering.
As a pure powder, zirconium hydrides are used as hydrogenation catalysts, in powder metallurgy, and as getters in the vacuum tube industry. 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.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
, a hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
of zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
with the formula Zr
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
Hx
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. Whereas x can be as large as 4, the most common values are between 1 and 2. Zirconium hydrides form upon reaction of zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
metal with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas. They behave as typical metals in terms of electrical, magnetic and mechanical properties. Those properties depend on the composition. For example, the Vickers hardness decreases with increasing hydrogen content – a phenomenon known as hydrogen embrittlement
Hydrogen embrittlement
Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
.
Physical properties
| Formula | CAS number CAS registry number CAS Registry Numbersare unique numerical identifiers assigned by the "Chemical Abstracts Service" toevery chemical described in the... | Molecular weight | Density g/cm3 | Symmetry | Space group Space group In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct... | No | Pearson symbol Pearson symbol The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8... |
|---|---|---|---|---|---|---|---|
| ZrH | 13940-37-9 | 92.232 | 5.9 | Orthorhombic Orthorhombic crystal system In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,... |
Cccm | 66 | oS8 |
| ZrH1.6 | 5.66 | Cubic Cubic crystal system In crystallography, the cubic 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.... |
Fmm | 225 | cF12 | ||
| ZrH2 Zirconium(II) hydride Zirconium hydride is a chemical compound, a hydride of zirconium. The commercial form is usually a flammable, gray-black powder.It 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... |
7704-99-6 | 93.240 | 5.56 | Tetragonal Tetragonal crystal system In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais... |
I4/mmm | 139 | tI6 |
| ZrH4 | 15457-96-2 | 95.256 |
Zirconium hydrides are odorless, dark gray to black metallic powders.
They behave as usual metals in terms of electrical conductivity and magnetic properties (paramagnetic
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
, unless contaminated with ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
impurities). Their structure and composition is stable at ambient conditions. 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
Cubic crystal system
In crystallography, the cubic 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....
δ (fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
-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. This hardness decrease is accompanied by the decrease in magnetic susceptibility
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
. 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.
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. 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.Zirconium hydrides are soluble in hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
or alcohol; they react violently with water, acids, oxidizers or halogenated compounds.
Applications
Formation of zirconium hydrides is an important factor in the operation of several types of nuclear reactors, such as boiling water reactorBoiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...
s Fukushima I
Fukushima I Nuclear Power Plant
The , also known as Fukushima Dai-ichi , is a disabled nuclear power plant located on a site in the towns of Okuma and Futaba in the Futaba District of Fukushima Prefecture, Japan. First commissioned in 1971, the plant consists of six boiling water reactors...
and II
Fukushima II Nuclear Power Plant
The , or Fukushima Dai-ni , is a nuclear power plant located on a site in the town of Naraha and Tomioka in the Futaba District of Fukushima Prefecture, Japan...
, which suffered from a series of explosions caused by the 2011 Tōhoku earthquake and tsunami
2011 Tōhoku earthquake and tsunami
The 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...
. Their uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
fuel pellets
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
are enclosed in metal rods made from Zircaloy
Zircaloy
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...
– 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. 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.
Under 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. This process mechanically weakens the rods because the hydrides have lower hardness and ductility than metal. More important though is that the solubility of hydrogen in zirconium is limited to a few percent maximum. Thus the excess hydrogen forms voids which contribute more to the weakening of the alloy. Among Zircaloys, Zircaloy-4 is the least susceptible to this hydrogen blistering.
As a pure powder, zirconium hydrides are used as hydrogenation catalysts, in powder metallurgy, and as getters in the vacuum tube industry. 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.
Safety
Powdered zirconium hydrides are flammable and can ignite and explode if exposed to heat, fire, or sparks. When heated to above 300 °C, they decompose releasing hydrogen gas, which is also flammable.External links
- Google books search results for the dedicated conference named "Zirconium in the nuclear industry"