Uranium dioxide 3467050 215501933 2008-05-28T14:06:54Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. {{Chembox new | Name = Uranium dioxide | ImageFile = UO2lattice.jpg | IUPACName = Uranium dioxide<br />Uranium(IV) oxide | Section1 = {{Chembox Identifiers | CASNo = 1344-57-6 }} | Section2 = {{Chembox Properties | Formula = UO<sub>2</sub> | MolarMass = 270 g/mol | Solvent = other solvents | SolubleOther = Insoluble | MeltingPt = 2846.85°C (3120 K)}} }} '''Uranium dioxide''' ({{Uranium}}{{Oxygen}}<sub>2</sub>), an [[oxide]] of [[uranium]], also known as '''urania''' or '''uranic oxide''' is a black, radioactive, crystalline powder. It occurs naturally in the mineral [[uraninite]]. It has a [[melting point]] of 2800°C and is used in [[nuclear fuel]] rods in [[nuclear reactors]]. A mixture of uranium and [[plutonium]] dioxides is used as [[MOX fuel]]. Prior to 1960 it was used as yellow and black color in [[ceramic]] [[ceramic glaze|glaze]]s and [[glass]]. ==Production== Uranium dioxide is produced by [[reducing]] [[uranium trioxide]] with [[hydrogen]]. :UO<sub>3</sub> + H<sub>2</sub> → UO<sub>2</sub> + H<sub>2</sub>O at 700°C (970K) This reaction takes part in the [[Nuclear reprocessing|reprocessing of nuclear fuel]] and [[enriched uranium|enrichment of uranium]] for nuclear fuel. ==Chemistry== [[Image:Uranium-dioxide-3D-vdW.png|thumb|right|200px|Uranium dioxide]] ===Structure=== The solid is [[isostructural]] with (has the same structure as) [[fluorite]] ([[calcium fluoride]]). In addition, the dioxides of [[plutonium]] and [[neptunium]] have the same structures ===Oxidation with oxygen=== Uranium dioxide is [[oxidized]] in contact with [[oxygen]] to the [[triuranium octaoxide]]. :3UO<sub>2</sub> + O<sub>2</sub> → U<sub>3</sub>O<sub>8</sub> at 700°C (970K) ===Aqueous electrochemistry=== 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 the [[spent nuclear fuel]] page for further details. ===Oxidation of uranium metal=== It has been reported that [[water]] causes the rate of the oxidation of both [[plutonium]] and [[uranium]] metal to increase when compared with the situation which exists when water is absent.[http://www.osti.gov/bridge/servlets/purl/756904-DrPADO/webviewable/756904.pdf] ==Uses== ===Nuclear Fuel=== UO<sub>2</sub> is used mainly as [[nuclear fuel]], specifically as UO<sub>2</sub> or as a mixture of UO<sub>2</sub> and PuO<sub>2</sub> ([[plutonium dioxide]]) called a mixed oxide ([[MOX fuel]]) for [[fuel rod]]s in [[nuclear reactor]]s. 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. [[Image:ZrUthermalcond.png|center|thumb|450px|The thermal conductivity of zirconium metal and uranium dioxide as a function of temperature]] [[Image:rim200pd250rad1000fueltemp.jpg|center|thumb|450px|This is a 20 mm diameter fuel pellet, note that the central temperature is very different for the different fuel solids, also for the different pellets it has the lowest centre line temperature, power density is 250 W per cubic meter and rim temperature of 200°C]] ===Colour for ceramics glaze=== All uranium oxides were used to colour glass and ceramics. Uranium oxide-based ceramics become green or black when fired in a reducing atmosphere and yellow to orange when fired with oxygen. Orange-coloured [[Fiestaware]] is a well-known example of a product with a uranium-based glaze. Uranium oxide has also been used in formulations of [[Vitreous enamel|enamel]], [[uranium glass]], and [[porcelain]]. Prior to 1960, uranium oxides were used as coloured glazes. It is possible to determine with a [[Geiger counter]] if a glaze or glass contains uranium oxides. ===Other use=== [[Depleted uranium|Depleted]] UO<sub>2</sub> (DUO<sub>2</sub>) 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 [[cask]]s for [[radioactive waste]]. Casks can be also made of DUO<sub>2</sub>-[[steel]] [[cermet]], a [[composite material]] made of an [[aggregate (composite)|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]], eg. for degradation of [[volatile organic compound]]s 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 [[catalyst]]s, such as [[precious metal]]s, [[titanium dioxide|TiO<sub>2</sub>]], and [[cobalt oxide|Co<sub>3</sub>O<sub>4</sub>]] catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity. {{ref|hutchings}} Use of uranium dioxide as a material for [[rechargeable battery|rechargeable batteries]] is investigated. The batteries could have high [[power density]] and potential of 4.7V per cell. Another investigated application is in [[photoelectrochemical cell]]s, for solar-assisted hydrogen production. UO<sub>2</sub> is used as a [[photoanode]]. ===Semiconductor properties=== Uranium dioxide is a [[semiconductor material]]. Its [[band gap]] is about 1.3 [[electronvolt|eV]], which lies between the band gap for [[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 cell]]s 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. Its [[dielectric constant]] is about 22, which is almost twice as high as of silicon (11.2) and GaAs (14.1), which poses an advantage over Si and GaAs for construction of [[integrated circuit]]s, as it may allow higher density integration with higher [[breakdown voltage]]s and with lower susceptibility to the [[CMOS]] [[quantum tunneling|tunneling]] 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]] (Tl<sub>2</sub>SnTe<sub>5</sub>) and [[thallium germanium telluride]] (Tl<sub>2</sub>GeTe<sub>5</sub>) and of [[bismuth]]-[[tellurium]] alloys, other materials promising for [[thermopower]] applications and [[Peltier element]]s. The [[radioactive decay]] impact of the<sup>235</sup>U and<sup>238</sup>U 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 [[very-large-scale integration|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<sub>1.994</sub> is orders of magnitude lower at higher temperatures than the conductivity of UO<sub>2.001</sub>. Uranium dioxide, like U<sub>3</sub>O<sub>8</sub>, 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 [[radiation hardening|rad-hard]] devices for special military and [[aerospace]] applications. A [[Schottky diode]] of [[yellowcake|U<sub>3</sub>O<sub>8</sub>]] and a [[p-n-p transistor]] of UO<sub>2</sub> were successfully manufactured in a laboratory. ==See also== *[[Ducrete]] ==References== {{Citationstyle|date=September 2007}} {{reflist}} #{{note|barett}}{{cite journal | author= Barrett SA, Jacobson AJ, Tofield BC, Fender BEF | title= The preparation and structure of barium uranium oxide BaUO<sub>3+x</sub>| journal= Acta Crystallographica B | year= 1982 | volume= 38| pages= 2775–2781 }} [http://dx.doi.org/10.1107/S0567740882009935 DOI] #{{note| hutchings }}{{cite journal | author= Hutchings GJ | title= A Uranium-Oxide-Based Catalysts for the Destruction of Volatile Chloro-Organic compounds| journal= Nature | year= 1996 | volume= 384| pages= 341–343 | doi= 10.1038/384341a0 }} ==External links== * [http://web.ead.anl.gov/uranium/pdf/WM01Semicond.pdf SEMICONDUCTIVE PROPERTIES OF URANIUM OXIDES] * [http://students.engr.ncsu.edu/ans/files/haire.pdf Research opportunities with uranium] * [http://www.thefreedictionary.com/uranium+dioxide Free Dictionary Listing for Uranium Dioxide] [[Category:Uranium compounds]] [[Category:Nuclear materials]] [[Category:Oxides]] [[Category:Semiconductor materials]] [[de:Urandioxid]] [[ja:二酸化ウラン]] [[pl:Dwutlenek uranu]] [[ru:Оксид урана(IV)]]