Transitions in Vapor‐Deposited Alumina from 300° to 1200°C

The transition of amorphous alumina to α‐alumina was studied by X‐ray diffraction, electron diffraction, DTA, TGA, and microscopic observation. The amorphous alumina was prepared by condensing vapor from evaporating molten alumina in vacuo onto the glass envelope of the vacuum chamber. The amorphous alumina was transformed to a poorly crystalline material by heating for 16 hr between 570° and 670°C. Between 670° and 1200°C, the poorly crystalline alumina was converted to α‐alumina via two parallel series of transition aluminas. The principal series was γ‐alumina to δ‐alumina to α‐alumina. A minor amount of θ‐alumina developed from the initial crystallization and persisted throughout the duration of the principal series as a parallel path. Some conversion of δ‐ to θ‐alumina was detected above 900°C. DTA produced an unexplained exothermic peak at 320°C and a second exothermic peak at 860°C which corresponded to formation of metastable aluminas.