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, porcelain ...

materials:

uranium Uranium is a chemical element with the 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. Uranium is weak ...

mononitride (UN), uranium sesquinitride (U₂N₃) and uranium dinitride (UN₂). The word nitride refers to the −3

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

of the nitrogen bound to the

. Uranium nitride has been considered as a potential nuclear fuel 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 Carbothermic reactions involve the reduction of substances, often metal oxides (O^2-), using carbon as the reducing agent. These chemical reactions are usually conducted at temperatures of several hundred degrees Celsius. Such processes are applie ...

of uranium oxide (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 of uranium tetrafluoride. Uranium tetrafluoride is exposed to

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

gas under high pressure and temperature, which replaces the

fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...

with nitrogen and generates

hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

.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, hybridation can be used to physically decompose otherwise solid uranium. Following hybridation, 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 two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

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 , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

absorption cross section 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 reactors (Gen IV) are six nuclear reactor designs recognized by the Generation IV International Forum. The designs target improved safety, sustainability, efficiency, and cost. The most developed Gen IV reactor design is the sodium ...

such as the

Hyperion Power Module Gen4 Energy, Inc (formerly Hyperion Power Generation, Inc.Hyperion Power Generation. It has also been proposed as

nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...

in some fast neutron 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 conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

, and melting temperature than the most common nuclear fuel, uranium oxide (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 (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

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 gradient is a Dimensional analysis, dimensional quantity express ...

between inner and outer sections of the fuel, potentially allowing for higher operating temperatures and reducing macroscopic restructuring of the fuel, which limits fuel lifetime.

Molecular and crystal structure

The uranium dinitride (UN₂) compound has a face-centered cubic

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

of the calcium fluoride (CaF₂) type with a space group 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 bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...

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

of the (Mn₂O₃) type with a space group of Ia . UN has a face-centered cubic

of the NaCl 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 In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any spe ...

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 atoms ...

portion of the bonds forms from the overlap between the 6

d orbital In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any spe ...

and 7 s orbital on the uranium and the 2

p orbitals In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any spe ...

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'' (University of Nottingham) {{Nitrides Nitrides Uranium(III) compounds Rock salt crystal structure