Editing Thorium (section)

==Modern applications==
Non-radioactivity-related uses of thorium have been in decline since the 1950s{{sfn|Stoll|2005|p=32}} due to environmental concerns largely stemming from the radioactivity of thorium and its decay products.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}<ref name="Furuta" />

Most thorium applications use its dioxide (sometimes called "thoria" in the industry), rather than the metal. This compound has a melting point of 3300&nbsp;°C (6000&nbsp;°F), the highest of all known oxides; only a few substances have higher melting points.<ref name="Emsley2011" /> This helps the compound remain solid in a flame, and it considerably increases the brightness of the flame; this is the main reason thorium is used in [[gas mantle|gas lamp mantles]].{{sfn|Stoll|2005|p=31}} All substances emit energy (glow) at high temperatures, but the light emitted by thorium is nearly all in the [[visible spectrum]], hence the brightness of thorium mantles.<ref name="Ivey">{{cite journal |first=H.F. |last=Ivey |year=1974 |title=Candoluminescence and radical-excited luminescence |journal=Journal of Luminescence |volume=8 |issue=4 |pages=271–307 |doi=10.1016/0022-2313(74)90001-5 |bibcode=1974JLum....8..271I}}</ref>

Energy, some of it in the form of visible light, is emitted when thorium is exposed to a source of energy itself, such as a cathode ray, heat, or [[ultraviolet light]]. This effect is shared by cerium dioxide, which converts ultraviolet light into visible light more efficiently, but thorium dioxide gives a higher flame temperature, emitting less [[infrared light]].{{sfn|Stoll|2005|p=31}} Thorium in mantles, though still common, has been progressively replaced with yttrium since the late 1990s.<ref name="Matson2011">{{cite book |last=Matson |first=Tim |year=2011 |title=The Book of Non-electric Lighting: The classic guide to the safe use of candles, fuel lamps, lanterns, gaslights, & fire-view stoves |page=60 |publisher=[[Countryman Press]] |isbn=978-1-58157-829-4}}</ref> According to the 2005 review by the United Kingdom's [[National Radiological Protection Board]], "although [thoriated gas mantles] were widely available a few years ago, they are not any more."<ref>{{cite web |last1=Shaw |first1=J. |last2=Dunderdale |first2=J. |last3=Paynter |first3=R.A. |date=9 June 2005 |title=A review of consumer products containing radioactive substances in the European Union |website=NRPB Occupational Services Department |url=https://ec.europa.eu/energy/sites/ener/files/documents/139.pdf |access-date=21 July 2017 |archive-date=10 March 2021 |archive-url=https://web.archive.org/web/20210310074604/https://ec.europa.eu/energy/sites/ener/files/documents/139.pdf |url-status=live }}</ref> Thorium is also used to make cheap permanent [[negative ion generator]]s, such as in [[pseudoscientific]] health bracelets.<ref>{{cite web | title="Negative Ion" Technology—What You Should Know | website=U.S. Nuclear Regulatory Commission | date=28 July 2014 | url=https://public-blog.nrc-gateway.gov/2014/07/28/negative-ion-technology-what-you-should-know/ | access-date=16 June 2021 | archive-date=8 December 2014 | archive-url=https://web.archive.org/web/20141208010325/https://public-blog.nrc-gateway.gov/2014/07/28/negative-ion-technology-what-you-should-know/ }}</ref>

During the production of [[incandescent]] filaments, [[Recrystallization (chemistry)|recrystallisation]] of tungsten is significantly lowered by adding small amounts of thorium dioxide to the tungsten [[sintering]] powder before drawing the filaments.{{sfn|Stoll|2005|p=32}} A small addition of thorium to tungsten [[hot cathode|thermocathodes]] considerably reduces the [[work function]] of electrons; as a result, electrons are emitted at considerably lower temperatures.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} Thorium forms a one-atom-thick layer on the surface of tungsten. The work function from a thorium surface is lowered possibly because of the electric field on the interface between thorium and tungsten formed due to thorium's greater electropositivity.<ref>{{cite book |last=Pridham |first=G.J. |year=2016 |title=Electronic Devices and Circuits |series=The Commonwealth and International Library: Electrical Engineering Division |page=105 |publisher=Elsevier |isbn=978-1-4831-3979-1 |language=en}}</ref> Since the 1920s, thoriated tungsten wires have been used in electronic tubes and in the cathodes and anticathodes of X-ray tubes and rectifiers.The reactivity of thorium with atmospheric oxygen required the introduction of an evaporated [[magnesium]] layer as a [[getter]] for impurities in the evacuated tubes, giving them their characteristic metallic inner coating.<ref>{{Cite book |last=Stokes |first=John W. |title=70 years of radio tubes and valves: a guide for electronic engineers, historians, and collectors |date=1982 |publisher=Vestal Press |isbn=978-0-911572-27-8 |location=Vestal, N.Y}}</ref>{{rp|16}} The introduction of transistors in the 1950s significantly diminished this use, but not entirely.{{sfn|Stoll|2005|p=32}} Thorium dioxide is used in [[gas tungsten arc welding]] (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} Thorium oxide is being replaced in this use with other oxides, such as those of zirconium, cerium, and [[lanthanum]].<ref>{{cite book |last=Uttrachi |first=J. |year=2015 |title=Weld Like a Pro: Beginning to advanced techniques |publisher=CarTech Inc. |page=42 |isbn=978-1-61325-221-5}}</ref><ref>{{cite book |last=Jeffus |first=L. |year=2016 |title=Welding: Principles and Applications |publisher=Cengage Learning |page=393 |isbn=978-1-305-49469-5}}</ref>

Thorium dioxide is found in [[refractory]] ceramics, such as high-temperature laboratory [[crucible]]s,{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} either as the primary ingredient or as an addition to [[zirconium dioxide]]. An alloy of 90% [[platinum]] and 10% thorium is an effective catalyst for oxidising [[ammonia]] to nitrogen oxides, but this has been replaced by an alloy of 95% platinum and 5% [[rhodium]] because of its better mechanical properties and greater durability.{{sfn|Stoll|2005|p=32}}

[[File:Yellowing of thorium lenses.jpg|left|thumb|alt=Three lenses from yellowed to transparent left-to-right|Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)]]
When added to [[glass]], thorium dioxide helps increase its [[refractive index]] and decrease [[dispersion (optics)|dispersion]]. Such glass finds application in high-quality [[lens (optics)|lenses]] for cameras and scientific instruments.<ref name="CRC" /> The radiation from these lenses can darken them and turn them yellow over a period of years and it degrades film, but the health risks are minimal.<ref>{{cite web |publisher=Oak Ridge Associated Universities |year=2021 |title=Thoriated Camera Lens (ca. 1970s) |url=https://www.orau.org/health-physics-museum/collection/consumer/products-containing-thorium/camera-lens.html |access-date=11 October 2021 |archive-date=24 June 2021 |archive-url=https://web.archive.org/web/20210624165739/https://www.orau.org/health-physics-museum/collection/consumer/products-containing-thorium/camera-lens.html |url-status=live }}</ref> Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced in this application by rare-earth oxides, such as [[Lanthanum oxide#lanthanum glass anchor|lanthanum]], as they provide similar effects and are not radioactive.{{sfn|Stoll|2005|p=32}}

Thorium tetrafluoride is used as an anti-reflection material in multilayered optical coatings. It is transparent to electromagnetic waves having wavelengths in the range of 0.350–12&nbsp;μm, a range that includes near ultraviolet, visible and [[Infrared|mid infrared]] light. Its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.<ref>{{cite book |last=Rancourt |first=J.D. |year=1996 |title=Optical Thin Films |page=196 |series=User Handbook |publisher=[[SPIE Press]] |isbn=978-0-8194-2285-9}}</ref> Replacements for thorium tetrafluoride are being developed as of the 2010s,<ref>{{cite book |last1=Kaiser |first1=N. |last2=Pulker |first2=H. K. |year=2013 |title=Optical Interference Coatings |page=111 |publisher=Springer |isbn=978-3-540-36386-6 |language=en}}</ref> which include [[Lanthanum trifluoride#la fl coating anchor|Lanthanum trifluoride]].

[[Mag-Thor]] alloys (also called thoriated magnesium) found use in some aerospace applications, though such uses have been phased out due to concerns over radioactivity.