Uranium nitrides is any of a family of several

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

materials:

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

mononitride (UN), uranium sesquinitride (U₂N₃) and uranium dinitride (UN₂). The word

nitride In chemistry, a nitride is a chemical compound of nitrogen. Nitrides can be inorganic or organic, ionic or covalent. The nitride anion, N3−, is very elusive but compounds of nitride are numerous, although rarely naturally occurring. Some nitr ...

refers to the −3

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

of the nitrogen bound to the

. Uranium nitride has been considered as a potential

nuclear fuel Nuclear fuel refers to any substance, typically fissile material, which is used by nuclear power stations or other atomic nucleus, nuclear devices to generate energy. Oxide fuel For fission reactors, the fuel (typically based on uranium) is ...

and will be used as such in the BREST-300 nuclear reactor currently under construction in Russia. It is said to be safer, stronger, denser, more thermally conductive and having a higher temperature tolerance. Challenges to implementation of the fuel include a complex conversion route from enriched UF₆, the need to prevent oxidation during manufacturing and the need to define and license a final disposal route. The necessity to use expensive, highly isotopically enriched ¹⁵N is a significant factor to overcome. This is necessary due to the (relatively) high neutron capture cross-section of the far-more-common ¹⁴N, which affects the neutron economy of a reactor.

Synthesis

Carbothermic reduction

The common technique for generating UN is carbothermic reduction of

uranium oxide Uranium oxide is an oxide of the element uranium. The metal uranium forms several oxides: * Uranium dioxide or uranium(IV) oxide (UO2, the mineral uraninite or pitchblende) * Diuranium pentoxide or uranium(V) oxide (U2O5) * Uranium trioxide or ...

(UO₂) in a 2 step method illustrated below. :3UO₂ + 6C → 2UC + UO₂ + 4CO (in argon, > 1450 °C for 10 to 20 hours) :4UC + 2UO₂ +3N₂ → 6UN + 4CO

Sol-gel

Sol-gel methods and arc melting of pure

under nitrogen atmosphere can also be used.Ganguly, C.; Hegde, P. ''J. Sol-Gel Sci. Technol.''. 1997, 9, 285.

Ammonolysis

Another common technique for generating UN₂ is the

ammonolysis In chemistry, ammonolysis (/am·mo·nol·y·sis/) is the process of splitting ammonia into NH2- + H+. Ammonolysis reactions can be conducted with organic compounds to produce amines (molecules containing a nitrogen atom with a lone pair, :N), o ...

uranium tetrafluoride Uranium tetrafluoride is the inorganic compound with the formula UF4. It is a green solid with an insignificant vapor pressure and low solubility in water. Uranium in its tetravalent ( uranous) state is important in various technological process ...

. Uranium tetrafluoride is exposed to

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

gas under high pressure and temperature, which replaces the

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

with nitrogen and generates

hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

.Silva, G. W. C.; Yeamans, C. B.; Ma, L.; Cerefice, G. S.; Czerwinski, K. R.; Sarrelberger, A. P. Chem. Mater.''. 2008, 20, 3076. Hydrogen fluoride is a colourless gas at this temperature and mixes with the ammonia gas.

Hydriding-nitriding

An additional method of UN synthesis employs fabrication directly from metallic uranium. By exposing metallic uranium to hydrogen gas at temperatures in excess of 280 °C, UH₃ can be formed. Furthermore, since UH₃ has a higher specific volume than the metallic phase, hydridation can be used to physically decompose otherwise solid uranium. Following hydridation, UH₃ can be exposed to a nitrogen atmosphere at temperatures around 500 °C, thereby forming U₂N₃. By additional heating to temperatures above 1150 °C, the sesquinitride can then be decomposed to UN. :2U + 3H₂ → 2UH₃ :2UH₃ + 1.5N₂ → U₂N₃ :U₂N₃ → UN + 0.5N₂ Use of the

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

¹⁵N (which constitutes around 0.37% of natural nitrogen) is preferable because the predominant isotope, ¹⁴N, has a significant

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

absorption cross section In physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department o ...

which affects neutron economy and, in particular, it undergoes an (n,p) reaction which produces significant amounts of radioactive ¹⁴C which would need to be carefully contained and sequestered during reprocessing or permanent storage.Matthews, R. B.; Chidester, K. M.; Hoth, C. W.; Mason, R. E.; Petty, R. L. ''Journal of Nuclear Materials''. 1988'', 151(3), 345.

Decomposition

Each uranium dinitride complex is considered to have three distinct compounds present simultaneously because of decomposing of uranium dinitride (UN₂) into uranium sesquinitride (U₂N₃), and then uranium mononitride (UN). Uranium dinitrides decompose to uranium mononitride by the following sequence of reactions: : 4UN₂ → 2U₂N₃+ N₂ : 2U₂N₃ → 4UN +N₂ Decomposition of UN₂ is the most common method for isolating uranium sesquinitride (U₂N₃).

Uses

Uranium mononitride is being considered as a potential fuel for

generation IV reactors Generation IV (Gen IV) reactors are nuclear reactor design technologies that are envisioned as successors of generation III reactors. The Generation IV International Forum (GIF) – an international organization that coordinates the development o ...

such as the

Hyperion Power Module Gen4 Energy, Inc. (formerly Hyperion Power Generation, Inc.

reactor created by

Hyperion Power Generation Gen4 Energy, Inc. (formerly Hyperion Power Generation, Inc.

. It has also been proposed as

in some

fast neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

nuclear test reactors. UN is considered superior because of its higher fissionable density,

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

, and melting temperature than the most common nuclear fuel,

(UO₂), while also demonstrating lower release of fission product gases and swelling, and decreased chemical reactivity with cladding materials. It also has a superior mechanical, thermal, and radiation stability compared to standard

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

lic uranium fuel.Mizutani, A.; Sekimoto, H. ''Ann. Nucl. Energy''. 2005, 25(9), 623–638. The thermal conductivity is on the order of 4–8 times higher than that of uranium dioxide, the most commonly used nuclear fuel, at typical operating temperatures. Increased thermal conductivity results in a smaller

thermal gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature spatial gradient is a vector quantity with dimension of temperat ...

between inner and outer sections of the fuel, potentially allowing for higher operating temperatures and reducing

macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenome ...

restructuring of the fuel, which limits fuel lifetime.

Molecular and crystal structure

The uranium dinitride (UN₂) compound has a

face-centered cubic In crystallography, the cubic (or isometric) 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. There are three main varieties o ...

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

of the

calcium fluoride Calcium fluoride is the inorganic compound of the elements calcium and fluorine with the formula CaF2. It is a white solid that is practically insoluble in water. It occurs as the mineral fluorite (also called fluorspar), which is often deeply col ...

(CaF₂) type with a

space group In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

of Fmm.Rundle, R. E.; Baenziger, N. C.; Wilson, A. S.; McDonald, R. A. ''J. Am. Chem Soc.''. 1948, 70, 99. Nitrogen forms

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

s on each side of uranium forming a linear structure.Weck P. F., Kim E., Balakrishnan N., Poineau F., Yeamans C. B., and Czerwinski K. R. ''Chem. Phys. Lett.''. 2007, 443, 82. Wang, X.; Andrews, L.; Vlaisavljevich, B.; Gagliardi, L. ''Inorganic Chemistry''. 2011, 50(8), 3826–3831. doi:10.1021/ic2003244 α-(U₂N₃) has a

body-centered cubic In crystallography, the cubic (or isometric) crystal system is a crystal system where the Crystal structure#Unit cell, unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There ...

of the (Mn₂O₃) type with a

of Ia . UN has a

of the

NaCl Sodium chloride , commonly known as edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs as the mineral hali ...

type.Mueller, M. H.; Knott, H. W.''Acta Crystallogr.''. 1958, 11, 751–752. doi:10.1107/S0365110X58002061 The

component of the bond uses the 5

f orbital F, or f, is the sixth letter of the Latin alphabet and many modern alphabets influenced by it, including the modern English alphabet and the alphabets of all other modern western European languages. Its name in English is ''ef'' (pronounc ...

of the uranium but forms a relatively weak interaction but is important for the

. The

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

portion of the bonds forms from the overlap between the 6 d orbital and 7 s orbital on the uranium and the 2

p orbitals In quantum mechanics, an atomic orbital () is a Function (mathematics), function describing the location and Matter wave, wave-like behavior of an electron in an atom. This function describes an electron's Charge density, charge distribution a ...

on the nitrogen.Évarestov, R. A., Panin, A. I., & Losev, M. V. ''Journal of Structural Chemistry''. 2008, 48, 125–135. N forms a

with uranium creating a linear structure. Image:Fluorite-unit-cell-3D-ionic.png, Uranium dinitride Image:Tl2O3structure.jpg, Uranium sesquinitride Image:Sodium-chloride-3D-ionic.png, Uranium mononitride

Uranium nitrido derivatives

Recently, there have been many developments in the synthesis of complexes with terminal uranium nitride (–U≡N) bonds. In addition to radioactive concerns common to all uranium chemistry, production of uranium nitrido complexes has been slowed by harsh reaction conditions and solubility challenges. Nonetheless, syntheses of such complexes have been reported in the past few years, for example the three shown below among others. Other U≡N compounds have also been synthesized or observed with various structural features, such as bridging nitride ligands in di-/polynuclear species, and various oxidation states.Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Molecular Octa-Uranium Rings with Alternating Nitride and Azide Bridges. ''Science'' 2005, ''309'' (5742), 1835–1838.

References

External links

New Uranium Bond
at ''

The Periodic Table of Videos ''Periodic Videos'' (also known as ''The Periodic Table of Videos'') is a video project and YouTube channel on chemistry. It consists of a series of videos about chemical elements and the periodic table, with additional videos on other topics i ...

'' (University of Nottingham)
Uranium Mononitride (UN) Handbook
(Los Alamos National Laboratory) {{Nitrides Nitrides Uranium compounds