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Al2O3 is a relatively low refractive index material (1.760 @ 0.6 microns) with excellent durability and temperature range. It is non-hygroscopic with excellent abrasion and chemical resistance. Because of its hardness it is difficult to polish flat to interferometric specifications but is easily polished to window quality. It is on the GRAS list and is an excellent choice as a window for near-IR measurements on process streams. Al2O3 is bi-refringent and all applications should be carefully evaluated for the effects of bi-refringence on the intended measurement.
Melting Point: 2040 °C
Density: 4.0 g/cm3
Solubility in H2O: Insoluble
Hardness: 1370 kg/mm2
Short Wavelength Limit: 40,000 cm-1 (1 mm)
Long Wavelength Limit: 1608 cm-1 (1 mm), 1834 cm-1 (4 mm)
Al2O3 detector windows are often coated with a BBAR (broad band anti-reflective) coating covering the 2–6 micron spectral region. When used as sampling windows they are usually not coated since the coating is much less durable than the uncoated window.
Short and Long Wavelength Ranges for which transmissivity is greater than 50% of stated crystal thickness