Sublimation of Aluminum Oxide in Hydrogen

The sublimation of single‐crystal Al 2 O 3 spheres and plates in pure, dry H 2 was studied. The kinetics of this process are controlled by diffusion of the volatile gaseous products through the stagnant gas boundary layer adjacent to the surface. The evaporating species are probably the lower oxides of aluminum obtained by reduction of Al 2 O 3 with hydrogen. The etching of the surface by heating in dry H 2 was also studied. The basal planes developed most easily. The facets formed were fine steps consisting of basal and prismatic planes. Condensation of the vapors in the form of small crystallites of thin plates and whiskers of Al 2 O 3 was observed in the colder regions of the furnace.