Uranium dioxide

The solid is isostructural with (has the same structure as) fluorite (calcium fluoride), where each U is surrounded by eight O nearest neighbors in a cubic arrangement. In addition, the dioxides of cerium, thorium, and the transuranic elements from neptunium through californium have the same structures.[3] No other elemental dioxides have the fluorite structure. Upon melting, the measured average U-O coordination reduces from 8 in the crystalline solid (UO₈ cubes), down to 6.7±0.5 (at 3270 K) in the melt.[4] Models consistent with these measurements show the melt to consist mainly of UO₆ and UO₇ polyhedral units, where roughly 2⁄3 of the connections between polyhedra are corner sharing and 1⁄3 are edge sharing.[4]

• 
  Uranium dioxide
• 
  Sintered uranium dioxide pellet

Uranium dioxide is oxidized in contact with oxygen to the triuranium octaoxide.

3 UO₂ + O₂ → U₃O₈ at 700 °C (970 K)

The electrochemistry of uranium dioxide has been investigated in detail as the galvanic corrosion of uranium dioxide controls the rate at which used nuclear fuel dissolves. See spent nuclear fuel for further details. Water increases the oxidation rate of plutonium and uranium metals.[5][6]

Uranium dioxide is carbonized in contact with carbon, forming uranium carbide and carbon monoxide.

${\displaystyle {\ce {UO2 \ + \ 4C -> UC2 \ + \ 2CO}}}$.

This process must be done under an inert gas as uranium carbide is easily oxidized back into uranium oxide.

UO₂ is used mainly as nuclear fuel, specifically as UO₂ or as a mixture of UO₂ and PuO₂ (plutonium dioxide) called a mixed oxide (MOX fuel), in the form of fuel rods in nuclear reactors.

Note that the thermal conductivity of uranium dioxide is very low when compared with uranium, uranium nitride, uranium carbide and zirconium cladding material. This low thermal conductivity can result in localised overheating in the centres of fuel pellets. The graph below shows the different temperature gradients in different fuel compounds. For these fuels, the thermal power density is the same and the diameter of all the pellets are the same.

The thermal conductivity of zirconium metal and uranium dioxide as a function of temperature

• 
  Uranium oxide fuel pellet
• 
  Starting material containers for uranium dioxide fuel pellet production at a plant in Russia

Uranium oxide (urania) was used to color glass and ceramics prior to World War II, and until the applications of radioactivity were discovered this was its main use. In 1958 the military in both the US and Europe allowed its commercial use again as depleted uranium, and its use began again on a more limited scale. Urania-based ceramic glazes are dark green or black when fired in a reduction or when UO₂ is used; more commonly it is used in oxidation to produce bright yellow, orange and red glazes.[7] Orange-colored Fiestaware is a well-known example of a product with a urania-colored glaze. Uranium glass is pale green to yellow and often has strong fluorescent properties. Urania has also been used in formulations of enamel and porcelain. It is possible to determine with a Geiger counter if a glaze or glass produced before 1958 contains urania.

Prior to the realisation of the harmfulness of radiation, uranium was included in false teeth and dentures, as its slight fluorescence made the dentures appear more like real teeth in a variety of lighting conditions.

Depleted UO₂ (DUO₂) can be used as a material for radiation shielding. For example, DUCRETE is a "heavy concrete" material where gravel is replaced with uranium dioxide aggregate; this material is investigated for use for casks for radioactive waste. Casks can be also made of DUO₂-steel cermet, a composite material made of an aggregate of uranium dioxide serving as radiation shielding, graphite and/or silicon carbide serving as neutron radiation absorber and moderator, and steel as the matrix, whose high thermal conductivity allows easy removal of decay heat.

Depleted uranium dioxide can be also used as a catalyst, e.g. for degradation of volatile organic compounds in gaseous phase, oxidation of methane to methanol, and removal of sulfur from petroleum. It has high efficiency and long-term stability when used to destroy VOCs when compared with some of the commercial catalysts, such as precious metals, TiO₂, and Co₃O₄ catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity.[8]

The use of uranium dioxide as a material for rechargeable batteries is being investigated. The batteries could have high power density and potential of 4.7 V per cell. Another investigated application is in photoelectrochemical cells for solar-assisted hydrogen production where UO₂ is used as a photoanode. In earlier times, uranium dioxide was also used as heat conductor for current limitation (URDOX-resistor), which was the first use of its semiconductor properties.

Uranium dioxide displays strong piezomagnetism in the antiferromagnetic state, observed at cryogenic temperatures below 30 kelvins. Accordingly, the linear magnetostriction found in UO₂ changes sign with the applied magnetic field and exhibits magnetoelastic memory switching phenomena at record high switch-fields of 180,000 Oe.[9] The microscopic origin of the material magnetic properties lays in the face-centered-cubic crystal lattice symmetry of uranium atoms, and its response to applied magnetic fields.[10]

The band gap of uranium dioxide is comparable to those of silicon and gallium arsenide, near the optimum for efficiency vs band gap curve for absorption of solar radiation, suggesting its possible use for very efficient solar cells based on Schottky diode structure; it also absorbs at five different wavelengths, including infrared, further enhancing its efficiency. Its intrinsic conductivity at room temperature is about the same as of single crystal silicon.[11]

The dielectric constant of uranium dioxide is about 22, which is almost twice as high as of silicon (11.2) and GaAs (14.1). This is an advantage over Si and GaAs in the construction of integrated circuits, as it may allow higher density integration with higher breakdown voltages and with lower susceptibility to the CMOS tunnelling breakdown.

The Seebeck coefficient of uranium dioxide at room temperature is about 750 µV/K, a value significantly higher than the 270 µV/K of thallium tin telluride (Tl₂SnTe₅) and thallium germanium telluride (Tl₂GeTe₅) and of bismuth-tellurium alloys, other materials promising for thermoelectric power generation applications and Peltier elements.

The radioactive decay impact of the ²³⁵U and ²³⁸U on its semiconducting properties was not measured as of 2005. Due to the slow decay rate of these isotopes, it should not meaningfully influence the properties of uranium dioxide solar cells and thermoelectric devices, but it may become an important factor for VLSI chips. Use of depleted uranium oxide is necessary for this reason. The capture of alpha particles emitted during radioactive decay as helium atoms in the crystal lattice may also cause gradual long-term changes in its properties.

The stoichiometry of the material dramatically influences its electrical properties. For example, the electrical conductivity of UO_1.994 is orders of magnitude lower at higher temperatures than the conductivity of UO_2.001.

Uranium dioxide, like U₃O₈, is a ceramic material capable of withstanding high temperatures (about 2300 °C, in comparison with at most 200 °C for silicon or GaAs), making it suitable for high-temperature applications like thermophotovoltaic devices.

Uranium dioxide is also resistant to radiation damage, making it useful for rad-hard devices for special military and aerospace applications.

A Schottky diode of U₃O₈ and a p-n-p transistor of UO₂ were successfully manufactured in a laboratory.[12]