Systematics of the Evaporation Coefficient Al2O3, Ga2O3, In2O3

The rate of evaporation from surfaces of Al2O3, Ga2O3, and In2O3 has been studied through a combination of thermal-imaging and mass-spectrometric techniques. A discontinuity is observed in the rate of evaporation of Al2O3 and Ga2O3 at the melting point TM. The evaporation coefficient αv of the solid at the melting point is equal to 0.3±0.05 for all gaseous species over Al2O3, Ga2O3, and In2O3. The rate of evaporation from the surface of the liquid at the melting point is the equilibrium rate (αv=1) for all gaseous species. The evaporation coefficient αv over the solid is very nearly temperature independent for monatomic vapor species but depends strongly on temperature for polyatomic species. It is shown that the data for the different systems can be directly correlated when they are plotted as logαv vs TM/T, where TM is the melting temperature. This correlation also applies to other systems and appears to be useful in predicting evaporation behavior in related systems. The equilibrium rates of evaporation are used to calculate: D0°(AlO) = 116.0±3.0 kcal mole−1, D0°(Al2O) = 241.6±7.0 kcal mole−1, ΔH°v,298(Ga) = 64.2±2.0 kcal mole−1, D0°(GaO) = 90.2±3.5 kcal mole−1, D0°(Ga2O) = 206.8±7.0 kcal mole−1, D0°(In2O) = 179.2±4.0 kcal mole−1.