Thermodynamics of the Al2O3–Al4C3 system: III. Equilibrium vapour pressures and activation energies for volatilisation

The present paper addresses the vapour phase in equilibrium with the compounds of the stable Al2O3–Al4C3 system, between 25 °C and 1827 °C. The volatilisation of each of them into any of five gaseous species referred to as [AlxOy], is described in terms of an activation energy and a pre-exponential term; for Al2O3, also sublimation is modelled on the basis of rare vapourization data. At least up to 1300 °C, the relative importance of the various [AlxOy] derives from the activation energies necessary to form them; above 1300 °C, both activation energy and pre-exponential terms are influential. Activation energies increase with increasing ratio y/x in [AlxOy], from approximately 365 kJ mol−1 for [Al] to about 900 kJ mol−1 for [AlO2]; the effects of temperature and mole fraction Al2O3 are investigated. Up to almost 1807 °C, [Al] is the predominant species regardless of carbon monoxide pressure, and thus the most likely intermediate in processes involving vapour phase reactions. Solid–vapour equilibria are illustrated in a volatility diagram at 1787 °C.