Effects of Melt Chemistry on the Spectral Absorption Coefficient of Molten Aluminum Oxide

Values of the spectral absorption coefficient (α) of liquid aluminum oxide were determined by transmission of a pulsed dye laser beam incident on continuous-wave (CW) CO₂-laser-melted pendant drops attached to sapphire filaments. Measurements were made on molten drops of Verneuil sapphire at wavelengths of 0.450 and 0.633 μm, at ambient oxygen partial pressures from 10^-10 to 1 bar in eight pure gases (Ar, CO, CO₂, H₂, H₂O, HCI, N₂ and O₂), in CO/CO₂ mixtures, and in H₂/H₂O mixtures, and at a temperature of ca. 2400 K. Specimens contaminated with iron, magnesium, silicon, and tungsten were also investigated in an oxygen atmosphere. At a wavelength of 0.633 μm, the value of α was greater than 50 cm^-1 under reducing or inert gas conditions. It decreased to a minimum at intermediate oxygen partial pressures of 5 × 10^-5 bar in CO/CO₂ mixtures and 5 × 10^-3 bar in H₂/H₂O mixtures, and increased at larger oxygen partial pressures. The specimens were opaque (α > 55 cm^-1) in hydrogen, in HCI at pressures above 0.04 bar. Specimens contaminated with 5000-10000 ppm of Fe, Mg, Si, or W were also opaque. At a wavelength of 0.45 μm the liquid aluminum oxide specimens were opaque in Ar and oxygen, and gave α= 46 cm^-1 in CO₂. The dynamic response when the ambient gas was changed from CO₂ to argon showed that the transmission maximum for = 0.45 μm was at p (O₂) < 0.1 bar.