Removal of residual chromium from aluminum oxide by containerless liquid-phase processing
- PDF / 492,870 Bytes
- 5 Pages / 576 x 792 pts Page_size
- 26 Downloads / 193 Views
J. K. Richard Weber and Paul C. Nordine Containerless Research Inc., 910 University Place, Evanston, Illinois 60201 (Received 17 October 1994; accepted 17 March 1995)
Verneuil sapphire was purified of Cr 3+ by containerless melting and processing at ca. 2550 K in argon, dry air, and pure oxygen. Recovered material was examined by laser induced fluorescence and Raman spectroscopy. The Cr 3+ fluorescence intensity decreased in processed specimens at rates proportional to the chromium concentration and p(O2)° 21 . The initial chromium concentration was ca. 5 ppm and decreased by factors of ca. 50, 3000, and 2 X 105 after processing for 300 s in argon, air, and oxygen, respectively. Evidence is presented that the Cr 3+ was removed predominantly as CrO2(g) and not by conversion to other oxidation states of chromium in the condensed phase.
I. INTRODUCTION High purity aluminum oxide (sapphire) is widely used as an optical and laser host material. Metallic ion impurities, either deliberately added as dopants, or present as residual impurities, strongly influence the optical properties of the solid and the melt.1"* Cr 3+ plays a prominent role in this regard. For example, the strong optical absorption of ruby peaked in the violet (—400 nm) and green (—543 nm)5 regions of the visible spectrum, resulting from the addition of —500 ppm of Cr 3+ in the host sapphire. The absorbed radiation is responsible for exciting strong red fluorescent emission peaked at 694.3 nm, also known as the "R lines". This emission forms the basis for operation of the ruby laser.6 High purity undoped sapphire contains residual (—1-5 ppm) of Cr 3+ as impurity. Even in such low concentrations, residual Cr 3+ causes "ruby-like" absorption-fluorescence behavior. While the fluorescence in this case is much reduced compared to ruby, it still gives rise to a strong signal7'8 especially when excited by green laser light. In principle, the high volatility of chromium oxides, CrO3(g), CrO2(g), and CrO(g), provides a means by which the chromium content of alumina can be reduced to exceedingly low values. For example, thermodynamic data9 show that the activity of Cr 2 O 3 = 4.7 X 10~7 in liquid alumina, when the total pressure of aluminumand chromium-bearing species are equal, at an ambient oxygen pressure of 1 bar and a temperature of 2600 K. The gas phase Cr:Al concentration thus exceeds the condensed phase Cr: Al ratio by a large factor, and only a small loss of alumina would be required to greatly reduce the chromium impurity content by vaporization processes. This approach is similar to that used to remove J. Mater. Res., Vol. 10, No. 7, Jul 1995 http://journals.cambridge.org
Downloaded: 13 Mar 2015
oxide impurities from liquid metals by vaporization of suboxides under containerless conditions. 1011 In the present work the removal of residual chromium from Verneuil sapphire was investigated by liquid-phase processing under the unique containerless conditions achieved in an aero-acoustic levitator (AAL).12 Rapid stirring and diffusion in the liquid phase
Data Loading...
