Sapphire: Difference between revisions
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The first ever [[laser]] produced was based on the ''ruby'', the red variety of corundum. While this laser has few applications, the [[Ti-sapphire laser]] is popular due to the relatively rare ability to tune the laser wavelength in the red-to-near [[infrared]] region of the [[electromagnetic spectrum]]. It can also be easily [[modelocking|modelocked]]. In these lasers, a synthetically produced sapphire [[crystal]] with chromium or [[titanium]] impurities is irradiated with intense light from a special lamp, or another laser, to create [[stimulated emission]]. |
The first ever [[laser]] produced was based on the ''ruby'', the red variety of corundum. While this laser has few applications, the [[Ti-sapphire laser]] is popular due to the relatively rare ability to tune the laser wavelength in the red-to-near [[infrared]] region of the [[electromagnetic spectrum]]. It can also be easily [[modelocking|modelocked]]. In these lasers, a synthetically produced sapphire [[crystal]] with chromium or [[titanium]] impurities is irradiated with intense light from a special lamp, or another laser, to create [[stimulated emission]]. |
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One application of synthetic sapphire is ''sapphire glass''. Sapphire is not only highly transparent to wavelengths of light between 170 nm to 5.3 μm (the human eye can discern wavelengths from around 400 nm to 700 nm), but it is also five times stronger than glass and ranks a 9 on the [[Mohs Scale]]. Sapphire glass is made from pure sapphire boules by slicing off and polishing thin wafers. Sapphire glass windows are used in high pressure chambers for [[spectroscopy]] but also for crystals in high quality [[watch |
One application of synthetic sapphire is ''sapphire glass''. Sapphire is not only highly transparent to wavelengths of light between 170 nm to 5.3 μm (the human eye can discern wavelengths from around 400 nm to 700 nm), but it is also five times stronger than glass and ranks a 9 on the [[Mohs Scale]]. Sapphire glass is made from pure sapphire boules by slicing off and polishing thin wafers. Sapphire glass windows are used in high pressure chambers for [[spectroscopy]] but also for crystals in high quality [[watch|watches]], since the material's exceptional hardness makes it very resistant to scratching. Owners of such watches should still be careful to avoid exposure to [[diamond]] jewelry, and should avoid striking their watches against artificial stone and simulated stone surfaces that often contain [[silicon carbide]] and other materials that are harder than sapphire and thus capable of causing scratches (Scheel 2003). |
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Wafers of single crystal sapphire are also used in the [[semiconductor]] industry as a [[Wafer (electronics)|substrate]] for the growth of [[gallium nitride]] based devices. |
Wafers of single crystal sapphire are also used in the [[semiconductor]] industry as a [[Wafer (electronics)|substrate]] for the growth of [[gallium nitride]] based devices. |
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Revision as of 23:36, 23 June 2008
 | This article needs additional citations for verification. (March 2008) |
 | It has been suggested that padparadscha and Talk:Sapphire#Proposed Merger with Padparadscha be merged into this article. (Discuss) Proposed since September 2007. |
| Sapphire | |
|---|---|
 | |
| General | |
| Category | Mineral Variety |
| Formula (repeating unit) | aluminium oxide, Al2O3 |
| Crystal system | Trigonal |
| Identification | |
| Color | Every color except red (which is ruby) |
| Crystal habit | massive and granular |
| Cleavage | None |
| Fracture | Conchoidal, splintery |
| Mohs scale hardness | 9.0 |
| Luster | Vitreous |
| Streak | White |
| Specific gravity | 3.95-4.03 |
| Optical properties | Abbe number 72.2 |
| Refractive index | 1.762-1.778 |
| Pleochroism | Strong |
| Fusibility | infusible |
| Solubility | insoluble |
| Other characteristics | Coefficient of thermal expansion 5e-6–6.6e-6/K |
Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide (Al2O3), when it is a color other than red. Sapphire can be found naturally or manufactured in large crystal boules. Because of its remarkable hardness sapphire is used in many applications, including infrared optical components, watch crystals, high-durability windows, and wafers for the deposition of semiconductors, such as GaN nanorods.
The mineral corundum consists of pure aluminium oxide. Trace amounts of other elements such as iron, titanium and chromium give corundum their blue, yellow, pink, purple, orange or greenish color. Sapphire includes any gemstone quality varieties of the mineral corundum except the fully saturated red variety, which is instead known as ruby, and the pinkish-orange variety known as padparadscha.
Natural sapphire
Sapphires and rubies are formed at great depth in the earth's mantle or the lowest part of the crust[1]. Although blue is considered the normal color for sapphires, they can be found across a full range of spectral colors as well as brown, colorless, grey and black. Those other than blue in color are considered fancy color sapphires. Some natural sapphires can be found as completely transparent, or "white." White sapphires usually come out of the ground as light grey or brown and are then heated to make them clear. However, in very rare circumstances they will be found in a clear state.
Blue sapphire
Various shades of blue [dark and light] result from titanium and iron substitutions in the aluminium oxide crystal lattice. Some stones are not well saturated and show tones of grey. It is common practice to bake natural sapphires to improve or enhance color. This is usually done by heating the sapphires to temperatures of up to 1800 °C for several hours, or by heating in a nitrogen deficient atmosphere oven for seven days or more. On magnification, the silk due to included rutile needles are often visible. If the needles are unbroken, then the stone was not heated; if the silk is not visible then the stone was heated adequately. If the silk is partially broken, then a process known as low tube heat may have been used. Low tube heat is the process whereby the rough stone is heated to 1300 °C over charcoal for 20 to 30 minutes. This removes grey or brown in the stone and improves color saturation.
Fancy color sapphire
Purple sapphires are lower in price than blue ones. These stones contain the trace element vanadium and come in a variety of shades. Yellow and green sapphires have traces of iron that gives them their color. Pink sapphires have a trace of the element chromium and the deeper the color pink the higher their monetary value as long as the color is going toward the red of rubies. Sapphires also occur in shades of orange and brown, and colorless sapphires are sometimes used as diamond substitutes in jewelry. Salmon-color padparadscha sapphires often fetch higher prices than many of even the finest blue sapphires. The word 'padparadscha' is Sinhalese for 'lotus flower'. Recently many sapphires of this color have appeared on the market as a result of a new treatment method called "lattice diffusion".[citation needed]
Color change sapphire
Color shift sapphires are blue in outdoor light and purple under incandescent indoor light. Color changes may also be pink in daylight to greenish under fluorescent light. Some stones shift color well and others only partially, in that some stones go from blue to bluish purple. Such color-change sapphires are widely sold as “lab” or “synthetic” alexandrite, which is accurately called an alexandrite simulant (also called alexandrium) since the latter is actually a type of chrysoberyl---an entirely different substance whose pleochroism is different and much more pronounced than color-change corundum (sapphire).
Star sapphire
A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism. Star sapphires contain intersecting needle-like inclusions (often the mineral rutile, a mineral composed primarily of titanium dioxide[2] ) that cause the appearance of a six-rayed 'star'-shaped pattern when viewed with a single overhead light source.
The value of a star sapphire depends not only on the carat weight of the stone but also the body color, visibility and intensity of the asterism.
Treatments
Some sapphires are heat-treated or otherwise enhanced to improve their appearance and color, though some people object to such practices and prefer natural untreated stones. Heat treatments tend to improve the sapphire's color and clarity, and it is unusual to find un-heated stones for sale. Controversy has arisen as certain milky white sapphire stones, called geuda, can obtain clear blue color after heat treatment, dramatically increasing the appearance and value of an otherwise nearly worthless gem. Diffusion treatments are somewhat more controversial as they are used to add elements to the sapphire for the purpose of improving colors.
Mining
Sapphires are mined from alluvial deposits or from primary underground workings. The finest specimens are mined in Sri Lanka and the disputed territory of Kashmir. Both the Logan sapphire and the Star of Bombay originate from Sri Lankan mines. Sapphires are also mined in Australia, Madagascar, Thailand and Myanmar. Madagascar leads the world in sapphire production (as of 2007) specifically in and around the city of Ilakaka. Prior to Ilakaka, Australia was the largest producer of sapphires (as of 1987). Ilakaka is prone to violence, but sapphires are found everywhere including on the ground and in the river mud. Pakistan, Afghanistan, India, Tanzania and Kenya also produce sapphires. The US state of Montana has produced sapphires from both the El Dorado Bar and Spokane Bar deposit near Helena. Well-known for their intense, pure blue color, yogo sapphires are found in Yogo Gulch, near Utica, Montana. Gem grade sapphires and rubies are also found in and around Franklin, North Carolina, USA. Several mines are open to the public.
Synthetic sapphire
Synthetic sapphire crystals can be grown in cylindrical crystal boules of large size, up to many inches in diameter. As well as gemstone applications there are many other uses:
The first ever laser produced was based on the ruby, the red variety of corundum. While this laser has few applications, the Ti-sapphire laser is popular due to the relatively rare ability to tune the laser wavelength in the red-to-near infrared region of the electromagnetic spectrum. It can also be easily modelocked. In these lasers, a synthetically produced sapphire crystal with chromium or titanium impurities is irradiated with intense light from a special lamp, or another laser, to create stimulated emission.
One application of synthetic sapphire is sapphire glass. Sapphire is not only highly transparent to wavelengths of light between 170 nm to 5.3 μm (the human eye can discern wavelengths from around 400 nm to 700 nm), but it is also five times stronger than glass and ranks a 9 on the Mohs Scale. Sapphire glass is made from pure sapphire boules by slicing off and polishing thin wafers. Sapphire glass windows are used in high pressure chambers for spectroscopy but also for crystals in high quality watches, since the material's exceptional hardness makes it very resistant to scratching. Owners of such watches should still be careful to avoid exposure to diamond jewelry, and should avoid striking their watches against artificial stone and simulated stone surfaces that often contain silicon carbide and other materials that are harder than sapphire and thus capable of causing scratches (Scheel 2003).
Wafers of single crystal sapphire are also used in the semiconductor industry as a substrate for the growth of gallium nitride based devices.
One type of xenon arc lamp, known as Cermax (original brand name - generically known as a ceramic body xenon lamp), uses sapphire output windows that are doped with various other elements to tune their emission. In some cases, the UV emitted from the lamp during operation causes a blue glow from the window after the lamp is turned off. It is approximately the same color as Cherenkov radiation but is caused by simple phosphorescence.
A transparent conductive coating (TCC) formed from gallium nitride GaN on a sapphire substrate. In order to account for the lattice mismatch between the GaN and the sapphire substrate, a nucleation layer is formed on the sapphire substrate. A mask, for example, silicon dioxide SiO2, is formed on top of the nucleation layer with a plurality of openings. GaN is grown through the openings in the mask to form a lateral epitaxial overgrowth layer upon which defect-free GaN is grown. The lateral epitaxial overgrowth compensates for the lattice mismatch between the sapphire substrate and the GaN. The use of a sapphire substrate eliminates the need for a cover glass and also significantly reduces the cost of the TCC, since such sapphire substrates are about {fraction (1/7)} the cost of germanium substrates. The TCC may then be disposed on a GaAs solar cell. In order to compensate for the lattice mismatches between the GaAs and the GaN, an indium gallium phosphate InGaP may be disposed between the GaAs solar cell and the GaN TCC to compensate for the lattice mismatch between the GaN and the GaAs. In order to further compensate for the lattice mismatch between the GaN and InGaP, the interface may be formed as a super lattice or as a graded layer. Alternatively, the interface between the GaN and the InGaP may be formed by the offset method or by wafer fusion. The TCC, in accordance with the present invention, is able to compensate for the lattice mismatches at the interfaces of the TCC while eliminating the need for a cover glass and a relatively expensive germanium substrate.
Historical and cultural references
- According to Rebbenu Bachya, and many English Bible translations, the word Sapir in the verse Exodus 28:18 means sapphire and was the stone on the Ephod representing the tribe of Issachar. However, this is extremely disputed as though it is true that the English word sapphire derives from the Hebrew sapir (via Greek sapphiros), sapphires were actually not known before the Roman Empire (and were initially considered to be forms of jacinth, rather than deserving of a word to themselves), and prior to that time sapphiros referred to blue gems in general. It is thought by scholars that the sapphire of the Bible was actually lapis lazuli - which was frequently sent as a gift between middle-eastern nations in Biblical times (Texas Natural Science Center, 2006). There is a wide range of views among traditional sources about which tribe the stone refers to.
- Blue sapphire is associated with Saturn (Wojtilla, 1973), yellow sapphire with Jupiter in Vedic astrology. It is understood that word Sapphire seems to be a corrupted form of Sanipriya(Sanskrit:- Sani = Saturn, Priya = Beloved). Buddhist monks who moved to Middle East introduced the Stone as Sani piriya and eventually become sapir and sapphire.
- Sapphire is the birthstone associated with September.
- The 45th wedding anniversary is known as the sapphire anniversary.
The theft of a sapphire known as the "Blue Water" is central to the plot of the novel Beau Geste by P. C. Wren and its various film adaptations.
References
- ^ Understanding the origin of rubies and sapphires to improve prospecting strategies http://www.eurekalert.org/pub_releases/2007-12/idrp-uto120307.php
- ^ Emsley, John (2001). Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press. pp. pp. 451 – 53. ISBN 0-19-850341-5.
{{cite book}}:|pages=has extra text (help)
- Scheel, Hans J. and Tsuguo Fukuda, Eds, 2003, Crystal Growth Technology, John Wiley & Sons ISBN 0-471-49059-8 (Available as PDF
- Wojtilla, G. Indian precious stones in the ancient East and West, Acta Orientakia (Budapest) 27, 2, 211-224.
- Sofianides, Anna S. and George E. Harlow, 1997, Gems & Crystals, Parkgate Books, pp. 44-55 ISBN 1-85585-391-4
- Texas Natural Science Center, Mineral Lore and Mythology, 2006
- Wedmineral Corundum Page Webmineral with extensive crystallographic and mineralogical information on Corundum
- Farlang Sapphire References Historical resources on Sapphires, mining, famous gems etc.
- Mindat Sapphire page Mindat with extensive locality information
- ICA's Sapphire page International Colored Stone Sapphire page