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Transparent alumina is alumina (aluminum oxide, Al2O3) which is transparent. In bulk, solid form, alumina is a colorless, transparent solid. Ruby and sapphire are two forms, containing different impurities, which occur naturally. Corundum, an opaque form typically containing greater impurities, is more common.
Manufacture
As a powder or solid formed by sintering (welding together small particles), alumina is opaque or translucent. Recently, a method of sintering very small particles of alumina has been developed by researchers at the Fraunhofer Institute for Ceramic Technologies and Sintered Materials. This sintered alumina is very hard, nearly transparent, and has a very high melting point (2303 kelvins), yet like other sintered materials it can be produced at temperatures much lower than its melting point.
In 2004, Anatoly Rosenflanz and colleagues at 3M in Minnesota used a "flame-spray" technique to alloy alumina (aluminium oxide) with rare-earth metal oxides to produce strong glass with good optical properties. The method avoids many of the problems encountered in conventional glass forming and could, say the team, be extended to other oxides (see #References).
See also
Bulk transparent alumina apparently is now practical. http://physicsweb.org/article/news/8/8/9
References
- Andreas Krell, Paul Blank, Hongwei Ma, Thomas Hutzler, and Manfred Nebelung. "Processing of High-Density Submicrometer Al2O3 for New Applications", Journal of the American Ceramic Society 86(4):546-553, April 2003. (also online: [1] (http://www.ceramicjournal.org/issues/v86n4/full/6776.html))
- A Rosenflanz et al 2004 Nature 430 761 (online abstract (http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v430/n7001/abs/nature02729_fs.html))
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