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

Abstract

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.