Thermodynamics of the Al_2O_3-Al_4C_3 system II. Free energies of mixing, solid solubilities and activities

摘要

The phase diagram of the Al_2O_3-Al_4C_3 system is now based on unit constituents 1/5 Al_2O_3 and 1/7 Al_4C_3, and the formation of intermediate compounds 1/9 Al_4O_4C and 1/4 Al_2OC below 2100 K is addressed in terms of free energies of mixing, with corresponding partial molar quantities. Some relations are derived involving extents of solid solubilities and activities, especially at the eutectoid temperature. It is shown that the positive excess amount, relative to the stoichiometric mole fraction, of 1/7 Al_4C3 in the oxycarbide 1/9 Al_4O_4C, highest at that temperature, can be no greater than 3.98 X 10~(-10), whereas, based on areasonable assumption, the negative amount should not exceed 6.9319488 X 10~(-5). Activities a_(1/7Al_4C_3) and a_(1/5Al_2O_3) calculated at the eutectoid temperature show that both 1/7 Al_4C_3 and 1/5 Al_2O_3 present remarkably large positive deviations from Raoultian behaviour, as attested by their activity coefficients at infinite dilution.