CRITICAL-REVIEW OF THE THERMODYNAMICS OF THE NA-AL-O SYSTEM

The free energies of formation of various phases in the Na-Al-O systems are critically reviewed for the composition range between pure alumina and sodium-aluminate. The results clearly indicate that in the temperature range between 600 and 1100 K neither β-alumina nor β′'-alumina (Na2O.5Al2O3) are stable with respect to sodium; the most stable phase in that range being sodium aluminate. Least square fitting and statistical analysis of all the available data on the free energy of formation of β-alumina deemed accurate yields ΔGform0 (from oxides) = -8772+1.53 T (J/mole of O) (600 ≤T ≤ 1200K) If the composition of β-alumina is assumed to be Na20.11Al2O3 then ΔGform0 (Na1/17Al11/17O, from elements) = -565100+109.6 T (J mole of O) (600 ≤T ≤ 1200 K) If the composition is assumed to be 1:9 then: ΔGform0 (Na1/14Al9/14O, from elements) = -564000+109.6 T (J/mole of O) (600 ≤T ≤ 1200K) Based on the only study to date, the best estimate for the free energy of formation of β″-alumina is ΔGform0 (Na1/8Al5/8O, from the elements) = -567860+114 T (J/mole of O) (600 ≤T ≤ 1200 K) The uncertainty (95% confidence band) is estimated to be about ±200 J/mole of O. Based on these results β″-alumina should be thermodynamically stable at lower temperatures, but become unstable at temperatures >900 K and should, kinetics permitting, dissociate into β-alumina and sodium aluminate. Experimental evidence, however exists that conclusively shows that β″-alumina is thermodynamically stable at much higher temperatures than 900 K which leads to the conclusion that ΔG °form for β″-alumina reported to date is too high and should be more negative. © 1990 Chapman and Hall Ltd.