Thermodynamics of the Al_2O_3-Al_4C_3 system III. Equilibrium vapour pressures and activation energies for volatilisation

摘要

The present paper addresses the vapour phase in equilibrium with the compounds of the stable Al_2O_3-Al_4C_3 system, between 25 deg C and 1827 deg C. The volatilisation of each of them into any of five gaseous species referred to as [Al_xO_y], is described in terms of an activation energy and a pre-exponential term; for Al_2O_3, also sublimation is modelled on the basis of rare vapourization data. At least up to 1300 deg C, the relative importance of the various [Al_xO_y] derives from the activation energies necessary to form them; above 1300 deg C, both activation energy and pre-exponential terms are influential. Activation energies increase with increasing ratio y/x in [Al_XO_y], from approximately 365 kJ mol~(-1) for [Al] to about 900 kJ mol~(-1) for [AlO_2]; the effects of temperature and mole fraction Al_2O_3 are investigated. Up to almost 1807 deg 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 deg C.