Removal of Residual Chromium from Aluminum Oxide by Containerless Liquid-phase Processing

Berneuil sapphire was purified of Cr3+ by containerless melting and processing at ca. 2550 K in high purity argon, dry air and pure oxygen. Recovered material was examined by laser induced fluorescence and Raman spectroscopy. The Cr3+ fluorescence intensity decreased in processed specimens at rates proportional to the chromium concentration and p(O2)0.21. The initial chromium concentration was ca. 5 ppm and decreased by factors of ca. 50, 3000, and 2x105 after processing for 300 seconds in argon, air and oxygen, respectively. Evidence is presented that the Cr3+ was removed predominantly as CrO2(g) and not by conversion to other oxidation states of chromium in the condensed phase. A comparison of the Raman peak shifts for as-received and processed samples indicates that considerable stress was retained during containerless solidification of the highly undercooled melts.