Investigation of Role of Beta Alumina in the System Na2O-Al2O3-SiO2

Beta alumina was once considered to be merely a metastable polymorph of corundum. Its status as a distinct and stable compound (Na₂O.-11Al₂O₅ and analogous formulas) is now quite firmly established. Yet the published phase diagram for the system soda-alumina-silica takes no cognizance of its existence. A study of the alumina-rich portion of the system was conducted to determine whether or not beta alumina should be incorporated in the diagram. Conventional methods of investigation were used, with certain innovations designed to overcome difficulties inherent in the system. Solid-state sintering and modified quenching techniques were utilized to minimize soda loss. Sintering was at 1400°C. or thereabout, in platinum envelopes which were immersed in buffering mixtures so as to reduce volatilization. To the same end, extra-large quenching charges were employed, and these were held at 1600°C., far below the liquidus temperatures of most of the mixtures studied. Phase identification was primarily by the X-ray powder method. The results of these studies confirm the status of beta alumina as a discrete compound, and are believed to justify the insertion of an additional tie line, linking beta alumina to carnegieite, in the soda-alumina-silica diagram. A large primary field of beta alumina, occupying a portion of the present corundum field, is also strongly indicated. The phase diagram accordingly has been modified so as to incorporate these latest findings. The revised diagram, although in some respects tentative, is believed to be in fuller accord with technological experience. It is believed also to be in complete agreement with all recent laboratory studies in the system lime-soda-alumina-silica.