Thorium dioxide

Thoria exists as two polymorphs. One has a fluorite crystal structure. This is uncommon among binary dioxides. (Other binary oxides with fluorite structure include cerium dioxide, uranium dioxide and plutonium dioxide.){{clarify|date=August 2018}} The band gap of thoria is about 6 eV. A tetragonal form of thoria is also known.

Thorium dioxide is more stable than thorium monoxide (ThO).{{cite journal |first1= Heming |last1= He |first2= Jaroslaw |last2= Majewski |first3= David D. |last3= Allred |first4= Peng |last4= Wang |first5= Xiaodong |last5= Wen |first6= Kirk D. |last6= Rector |title= Formation of solid thorium monoxide at near-ambient conditions as observed by neutron reflectometry and interpreted by screened hybrid functional calculations |journal= Journal of Nuclear Materials |volume= 487 |year= 2017 |pages= 288–296 |doi= 10.1016/j.jnucmat.2016.12.046 |bibcode= 2017JNuM..487..288H |doi-access= free }} Only with careful control of reaction conditions can oxidation of thorium metal give the monoxide rather than the dioxide. At extremely high temperatures, the dioxide can convert to the monoxide either by a disproportionation reaction (equilibrium with liquid thorium metal) above {{convert|1850|K|°C °F}} or by simple dissociation (evolution of oxygen) above {{convert|2500|K|°C °F}}.{{cite journal |title= The Reaction Occurring on Thoriated Cathodes |first1= Michael |last1= Hoch |first2= Herrick L. |last2= Johnston |journal= J. Am. Chem. Soc. |year= 1954 |volume= 76 |issue= 19 |pages= 4833–4835 |doi= 10.1021/ja01648a018 }}

Thorium dioxide (thoria) can be used in nuclear reactors as ceramic fuel pellets, typically contained in nuclear fuel rods clad with zirconium alloys. Thorium is not fissile (but is "fertile", breeding fissile uranium-233 under neutron bombardment); hence, it must be used as a nuclear reactor fuel in conjunction with fissile isotopes of either uranium or plutonium. This can be achieved by blending thorium with uranium or plutonium, or using it in its pure form in conjunction with separate fuel rods containing uranium or plutonium. Thorium dioxide offers advantages over conventional uranium dioxide fuel pellets, because of its higher thermal conductivity (lower operating temperature), considerably higher melting point, and chemical stability (does not oxidize in the presence of water/oxygen, unlike uranium dioxide).

Thorium dioxide can be turned into a nuclear fuel by breeding it into uranium-233 (see below and refer to the article on thorium for more information on this). The high thermal stability of thorium dioxide allows applications in flame spraying and high-temperature ceramics.

Thorium dioxide is used as a stabilizer in tungsten electrodes in TIG welding, electron tubes, and aircraft gas turbine 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 electrons (thermions). 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.

Thoria-dispersed nickel finds its applications in various high-temperature operations like combustion engines because it is a good creep-resistant material. It can also be used for hydrogen trapping.{{cite book | url = https://books.google.com/books?id=iQQcERxsNywC&pg=PA473 | page = 473 | isbn = 978-0-471-43623-2 | title = An Introduction to Materials Engineering. and Science for Chemical and Materials. | author1 = Mitchell, Brian S | year = 2004 | publisher = John Wiley & Sons }}{{cite journal | first = Wayne M. | last = Robertson | title = Measurement and evaluation of hydrogen trapping in thoria dispersed nickel | journal = Metallurgical and Materials Transactions A | volume = 10 | issue = 4 | year = 1979 |doi = 10.1007/BF02697077 | pages =489–501 | bibcode = 1979MTA....10..489R | s2cid = 137105492 }}{{cite journal|last1=Kumar|first1=Arun|last2=Nasrallah|first2=M.|last3=Douglass|first3=D. L.|title=The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys|journal=Oxidation of Metals|volume=8|issue=4|year=1974|pages=227–263|issn=0030-770X|doi=10.1007/BF00604042|hdl=2060/19740015001|s2cid=95399863|hdl-access=free}}{{cite journal|last1=Stringer|first1=J.|last2=Wilcox|first2=B. A.|last3=Jaffee|first3=R. I.|title=The high-temperature oxidation of nickel-20 wt.% chromium alloys containing dispersed oxide phases|journal=Oxidation of Metals|volume=5|issue=1|year=1972|pages=11–47|issn=0030-770X|doi=10.1007/BF00614617|s2cid=92103123}}{{cite journal|last1=Murr|first1=L. E.|title=Interfacial energetics in the TD-nickel and TD-nichrome systems|journal=Journal of Materials Science|volume=9|issue=8|year=1974|pages=1309–1319|issn=0022-2461|doi=10.1007/BF00551849|bibcode=1974JMatS...9.1309M |s2cid=96573790}}

Thorium dioxide has almost no value as a commercial catalyst, but such applications have been well investigated. It is a catalyst in the Ruzicka large ring synthesis. Other applications that have been explored include petroleum cracking, conversion of ammonia to nitric acid and preparation of sulfuric acid.Stoll, Wolfgang (2012) "Thorium and Thorium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a27_001}}

Thorium dioxide was the primary ingredient in Thorotrast, a once-common radiocontrast agent used for cerebral angiography, however, it causes a rare form of cancer (hepatic angiosarcoma) many years after administration.[https://radiopaedia.org/articles/thorotrast Thorotrast]. radiopaedia.org This use was replaced with injectable iodine or ingestable barium sulfate suspension as standard X-ray contrast agents.

{{Main|Gas mantle}}

Another major use in the past was in gas mantle of lanterns developed by Carl Auer von Welsbach in 1890, which are composed of 99% ThO₂ and 1% cerium(IV) oxide. Even as late as the 1980s it was estimated that about half of all ThO₂ produced (several hundred tonnes per year) was used for this purpose.{{Greenwood&Earnshaw1st|pages=1425, 1456}} Some mantles still use thorium, but yttrium oxide (or sometimes zirconium oxide) is used increasingly as a replacement.

File:Yellowing of thorium lenses.jpg

When added to glass, thorium dioxide helps increase its refractive index and decrease dispersion. Such glass finds application in high-quality lenses for cameras and scientific instruments.{{cite book| last= Hammond| first= C. R.| title= The Elements, in Handbook of Chemistry and Physics| edition= 81st| publisher= CRC Press| isbn= 978-0-8493-0485-9| date= 2004| url-access= registration| url= https://archive.org/details/crchandbookofche81lide}} The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal.{{Cite web|author=Oak Ridge Associated Universities|year=1999|title=Thoriated Camera Lens (ca. 1970s)|url=https://orau.org/health-physics-museum/collection/consumer/products-containing-thorium/camera-lens.html|access-date=29 September 2017}} Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare-earth oxides such as lanthanum oxide in almost all modern high-index glasses, as they provide similar effects and are not radioactive.{{cite book |first=W. |last=Stoll |chapter=Thorium and Thorium Compounds |doi=10.1002/14356007.a27_001 |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH |year=2005 |isbn=978-3-527-31097-5 |page=32}}

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• {{cite book | editor= Haynes, William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = CRC Press | isbn = 978-1439855119| title-link = CRC Handbook of Chemistry and Physics }}

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Category:Fluorite crystal structure