CO2 Laser heating of c-axis sapphire filaments was used in a low pressure flow reactor in order to study evaporation and chlorination of single crystal and liquid aluminas. Reacted specimens were examined by scanning electron microscopy (SEM) to monitor the evolution of facets and other surface structures with reaction conditions.
An increase of the incident energy on a laser heated sapphire filament at the melting point does not result in complete melting, because the semi-transparent melt layer that forms has a higher effective emittance than the solid. This technique allows simultaneous reactivity study of liquid and single crystal aluminas at the melting point, as well as study of reactions of single crystal alumina at lower temperatures.
Sapphire specimens develop a hexagonal prism shape after reaction with chlorine containing gases whereas evaporation does not change the initially cylindrical specimen shape.
The primary structural characteristic of the 0.025 cm as-received sapphire specimens from Tyco is the presence of ∼1μm voids that are produced during the growth of the crystal. These voids form conical arrays 20–30μm apart along the filament axis. Specimens reacted with chlorine containing gases below ∼1900K exhibit surface structures which form arrays with the same size and pattern as the voids in the as-received specimens.