Thorium dioxide
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{{Chembox new
| Name = Thorium dioxide
| ImageFile = Thorium dioxide.jpg
| ImageSize = 200px
| ImageName = Thorium dioxide
| IUPACName = Thorium dioxide</br>Thorium(IV) oxide
| OtherNames = Thoria
| Section1 = {{Chembox Identifiers
| CASNo = 1314-20-1
}}
| Section2 = {{Chembox Properties
| Formula = ThO<sub>2</sub>
| MolarMass = 264.04 g/mol
| Appearance = white odorless solid
| Density = 9.86 g/cm<sup>3</sup>, solid
| Solubility = Insoluble
| MeltingPt = 3220 °C
| BoilingPt = 4400°C
}}
| Section3 = {{Chembox Structure
| Coordination = Octahedral
| CrystalStruct = [[Fluorite]]
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = ? kJ/mol
| Entropy = ? J.K<sup>−1</sup>.mol<sup>−1</sup>
}}
| Section7 = {{Chembox Hazards
| EUClass = not listed
}}
| Section8 = {{Chembox Related
| OtherAnions =
| OtherCations = [[Hafnium dioxide]]<br />[[Protactinium pentoxide]]<br />[[Uranium trioxide]]<br />[[Neptunium pentoxide]]<br />[[Plutonium dioxide]]
}}
}}
'''Thorium dioxide''' (ThO<sub>2</sub>), also called '''thorium(IV) oxide''' (IUPAC) is a white, crystalline powder. It was formerly known as '''thoria''' or '''thorina'''. It is produced mainly as a by-product of [[lanthanide]] and [[uranium]] production<sup>[1]</sup>.
==Chemistry==
The compound is [[radioactivity|radioactive]] due to the radioactivity of [[thorium]]. Thorium dioxide can be used as a [[nuclear reaction|nuclear]] fuel. (Refer to the article of [[thorium]] for more information on this application.)
The high [[thermal stability]] of thorium dioxide allows applications in flame spraying and high temperature ceramics. Thorium dioxide was the primary ingredient in the [[X-ray]] contrast medium [[Thorotrast]]. Use of Thorotrast was abandoned when it was found to be a [[carcinogen]], sometimes causing [[cholangiocarcinoma]]. Today, [[barium sulfate]] is the standard X-ray contrast agent.
Thoria has the [[fluorite]] crystal structure. Few other binary dioxides have this structure: [[uranium dioxide]], [[hafnium dioxide]] and [[cerium dioxide]], not to mention [[plutonium dioxide]]. The [[band gap]] of thoria is about 6 [[eV]].
==Applications==
Thorium dioxide is used as a stabilizer in [[tungsten]] electrodes in [[tungsten inert gas welding|TIG welding]], electron tubes, and aircraft engines. As an alloy, thoriated tungsten metal is not easily deformed because the high fusion material thoria augments the high temperature mechanical properties, and thorium helps stimulate the emission of [[electron]]s ([[thermion]]s). It is the most popular oxide additive because of its low cost, but is being phased out in favor of non-radioactive elements such as [[cerium]], [[lanthanum]] and [[zirconium]].
A major use in the past was in [[gas mantle]]s of lanterns, which were frequently composed of 99 percent ThO<sub>2</sub> and 1% [[cerium(IV) oxide]]. Even as late as the 1980s it was estimated that about half of all ThO<sub>2</sub> produced (several hundred tonnes per year) was used for this purpose<sup>[1]</sup>. Some mantles still use thorium, but [[yttrium oxide]] (or sometimes [[zirconium oxide]]) is used increasingly as a replacement.
Thorium dioxide was formerly added to [[glass]]es during manufacture to increase their [[refractive index]], producing ''thoriated glass'' with up to 40% ThO<sub>2</sub> content. These glasses were used in the construction of high-quality [[photographic lens]]es. However, the radioactivity of the thorium caused both a safety and pollution hazard and self-degradation of the glass (turning it yellow or brown over time). [[Lanthanum oxide]] has replaced thorium dioxide in almost all modern high-index glasses.
The melting point of thorium oxide is 3300°C - the highest of all oxides. Only a few elements (including [[tungsten]] and [[carbon]]) and a few compounds (including [[tantalum carbide]]) have higher melting points.
Thorium oxide is a Welsbach material. It has been suggested<sup>[2]</sup> that these chemicals could be sprayed into the upper atmosphere to reflex sunlight and thus lower the global temperature.
The [[Ruzicka large ring synthesis]] uses thorium oxide as catalyst.
==References==
# N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', Pergamon Press, Oxford, UK, 1984. See pages 1425, 1456.
# Chang, David B; Shih, I-fu [http://www.freepatentsonline.com/5003186.html "Stratospheric Welsbach seeding for reduction of global warming", US Patent No. 5003186].
[[Category:Oxides]]
[[Category:Thorium compounds]]
[[Category:Refractory materials]]
[[de: Thoriumdioxid]]