Evaporation and Energy Transfer for a Partially Molten, Laser-Heated Sapphire Filament

Molten regions were formed on 0.025-cm-diameter sapphire filaments that were heated from one side with a continuouswave CO₂ laser beam in a low-pressure flow reactor. As the laser intensity was increased, the liquid/solid interface moved from the laser-heated edge to the opposite edge of the filament, the apparent temperature measured in the molten region with an optical pyrometer increased from 1470 ± 25 to 2040 ± 30 K, and the filament evaporation rate increased by a factor of 1.6. This change in apparent temperature resulted from an increase in the spectral emittance with the liquid layer thickness. The change in evaporation rate resulted from a 1.09 times larger evaporating area, a 1.24 ± 0.09 times larger evaporation coefficient, and a 10 ± 5 K larger average temperature when the filament cross section was completely liquid than when liquid first formed on the solid filament. Optical and energy-transfer properties of sapphire and liquid Al₂O₃ were calculated from optical pyrometry, energy-balance measurements, and spectral absorption coefficient data for sapphire. At the melting temperature, the total emittance is approximately 0.051 and 0.31 ± 0.03 for the solid sapphire and liquid aluminum oxide filaments, respectively. The thermal accommodation coefficient for Ar atoms is 0.53 ± 0.07 on the solid and approximately unity on the liquid. The spectral absorption coefficient, kλ, at the optical pyrometer wavelength (0.665 μm) is 0.1 ± 0.04 cm⁻¹ for the solid and 12 ± 12 cm⁻¹ for the liquid. This value of kλ for solid sapphire at the melting point is 12 times that of pure, void-free material and reflects the influence of impurities and small voids in the sapphire filaments that were used.