REACTION AND MIGRATION OP SODIUM IN THE SYSTEM SODIUM OXYGEN ~ ALUMINA
A number of investigations have been made Into the reactlon/dlffusion of Na, Na20/Na mixtures, NaOH, Na20 and Na/TiOg mixtures with single crystals, and sintered polycrystalline alumina in the temperature range 973K-1758K. No observable diffusion or reaction was observed for sodium metal in the absence of oxygen over the entire temperature range. When oxygen was Included in the above forms, a large corrosion reaction to form Na20A1203 occurred wherever liquid sodium oxide was in contact with alumina, and above I3OOK B alumina (Na205A1203) and B alumina (Na2011A12O3) were also formed together with large quantities of Na2OA12O5. Mechanisms of reaction to form B aluminas are proposed which involve a change from the < alumina structure to the G y alumina structure and which Involve the exchange of Na * Ions for AIO * ions in the surface of < alumina, equations for the formation of B and B alumina from Na20 are shown below:- 2OAI2O3 + 8Na20 —> (Na205A1203)+ 5Na2 OAI2O3 2(Na205A1203) + 3Na2OA12O3 - 3(Na20l1Al203) + 6Na20 Corrosion by sodium oxide was more rapid for polycrystalline than for single crystal alumina, and whole grains were thought to fall away from the surface of the former due to preferential grain boundary attack. The same concentration and penetration depths of B alumina were found for polycrystalline and single crystal alumina at a given temperature, and therefore grain boundaries were not involved in the mechanics of these reactions. Rates of reaction were limited by diffusion processes in c>< alumina and not by thermodynamic parameters which governed the free energy of reaction. A correlation between the formation temperature of alumina and the formation temperature at AIO was found, and B aluminas which contained MgO were found to be more stable than aluminas which did not contain MgO.
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