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.