Yttralox

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| image1 = Yttrium oxide at 100X magnification with discontinuous grain growth and pores.jpg

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| image2 = Yttralox - Y203 with ThO2 at 150X magnification.jpg

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| footer = Sintering of pure Yttria leads to discontinuous grain growth and pores (left), while Yttralox has a more uniform grain size and no pores (right).

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Yttralox is a solid solution of thorium dioxide in yttria.{{Cite journal|date=1966-10-17|title=GE has transparent ceramic|journal=Chemical & Engineering News Archive|volume=44|issue=43|pages=38|doi=10.1021/cen-v044n043.p038a|issn=0009-2347}} The thorium dioxide additive affects the growth of grains during densification, leading to improved optical transparency. Uncontrolled grain growth allows a few grains to grow larger than the others, trapping pores inside them. The additive increases the grain boundary hardness more than the internal grain hardness. This causes porosity to remain on grain boundaries rather than becoming trapped inside grains, allowing them to be eliminated later in the sintering process. This greatly improves the material's optical transparency, because porosity causes light scattering.{{Cite journal|url=https://apps.dtic.mil/sti/citations/ADA460289|title=Transparent Yttria for IR Windows and Domes - Past and Present|last1=Hogan|first1=Patrick|last2=Stefanik|first2=Todd|date=2004-05-19|website=Defense Technical Information Center|publisher=DoD Electromagnetic Windows Symposium|pages=2–3|last3=Willingham|first3=Charles|last4=Gentilman|first4=Richard}} Porosities as low as one part per million were reported.{{Cite journal|url=https://apps.dtic.mil/sti/citations/ADA120588|title=Technical Information Series: Sintering|last1=Burke|first1=J. E.|last2=Rosolowski|first2=J. H.|date=September 1973|website=Defense Technical Information Center|publisher=General Electric|page=11|access-date=2017-09-16}} The resulting grain size was in the range 10–50 μm.

Yttralox was marketed as being "transparent as glass", has a melting point twice as high, and transmits frequencies in the near infrared band as well as visible light.{{cite press release|title=A space age ceramic material transparent as glass, but which can withstand temperatures twice as high, was announced today by General Electric scientists|date=October 10, 1966|publisher=Peter Van Avery, General Electric Research and Development Center Public Information}}{{cite journal|author1=Anderson, Richard C.|author2=John Barker|name-list-style=amp|date=January–February 1969|title=A unique optical ceramic|journal=Optical Spectra|issue=Optical Materials Issue}} However, it has little plasticity at high temperatures and low thermal conductivity, giving it a thermal shock performance little better than common glass.

Commercialization was limited because Yttralox required high sintering temperatures of 2000–2200°C. Yttralox was proposed for use in lamp envelopes and high-temperature windows and lenses. It was investigated for use as a low-loss window material for lasers,{{Cite journal|url=https://apps.dtic.mil/sti/citations/ADA019774|archive-url=https://web.archive.org/web/20180602143026/http://www.dtic.mil/docs/citations/ADA019774|url-status=live|archive-date=June 2, 2018|title=Low Loss Window Materials for Chemical Lasers|last=Harrington|first=James A.|date=November 1975|website=Defense Technical Information Center|publisher=Defense Advanced Research Projects Agency}} for example in conjunction with a laser Doppler velocimeter for ramjet research. It was also investigated for use with infrared equipment in missiles. Neodymium oxide–doped Yttralox was used as a proof of concept for laser gain in a polycrystalline oxide ceramic, but was not commercialized due to low efficiency.

Yttralox's competing materials were an yttria containing lanthanum oxide manufactured by GTE, and a pure yttria material manufactured by Raytheon.

File:Richard C Anderson holding up Yttralox.jpg

Yttralox was invented in 1966 by Richard C. Anderson at the General Electric Research Laboratory while sintering mixtures of rare earth minerals. The initial objective of the research was to develop ionic conductors for fuel cells using yttria–zirconium dioxide materials. Although the zirconium dioxide-rich versions were of more interest for ionic conductors, the yttria-rich versions unexpectedly produced transparent samples.{{Cite book|title=Phase Diagrams in Advanced Ceramics|last=Rhodes|first=W. H.|date=1995-02-08|publisher=Academic Press|isbn=9780080538723|editor-last=Alper|editor-first=Allen M.|pages=8|language=en|chapter=Phase Chemistry in the Development of Transparent Polycrystalline Oxides|chapter-url=https://books.google.com/books?id=nxzRd-irkBIC&pg=PA8}} Further research established that other oxides of Group 4 elements, thorium dioxide and hafnium dioxide, were also effective at producing transparent yttria, and the thorium dioxide system became the most extensively studied.{{Cite book|title=High Temperature Oxides: Oxides of Rare Earths, Titanium, Zirconium, Hafnium, Niobium and Tantalum|last=Anderson|first=Richard C.|publisher=Academic Press|year=1970|isbn=9781483271392|editor-last=Alper|editor-first=Allen M.|pages=30–32|language=en|chapter=Thoria and Yttria|chapter-url=https://books.google.com/books?id=CgUcBQAAQBAJ&pg=PA32}} Further work at GE was performed by Paul J. Jorgensen, Joseph H. Rosolowski, and Douglas St. Pierre. Fabrication of Yttralox was reported by Greskovich and Woods.Greskovich, C. and Woods, K.N., "Fabrication of Transparent ThO₂-doped Y₂O₃", Bull. Amer. Ceram. Soc., Vol. 52, p. 473 (1973)

As of 1982, Yttralox was no longer being produced.{{Cite web|url=https://apps.dtic.mil/sti/citations/ADA120576|title=A Study of the Use of Transparent Yttrium Oxide for Ramjet Combustion Research|last=Buckley|first=Paeker L.|date=August 1982 |publisher=Defense Technical Information Center, Air Force Systems Command|pages=6, 18, 23|access-date=2017-09-16}}

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Category:Ceramic materials

Category:Transparent materials

Category:Yttrium compounds