Low-temperature synthesis of α-alumina using NaCl–Na3AlF6 flux

Abstract A low-temperature synthesis of α-alumina was studied using a flux consisting of salts and additives. Al(OH)3 was used as a precursor for the preparation of alumina; NaCl–KCl, NaCl, Na2SO4 and K2SO4 were used as the salts in the flux; and cryolite (Na3AlF6) was used as an additive. Using a flux consisting of Na3AlF6 with each salt confirmed that a phase change to α-alumina was possible even at a low temperature of 600 °C. Among the salts used, NaCl was the most effective. The flux with NaCl and Na3AlF6 could produce highly pure and highly crystalline platelet α-alumina crystals at 700 °C. Herein, NaCl exhibited interfacial activity during the phase change process and contributed to the growth of the α-alumina plate-like crystals with a clear crystal surface. Na3AlF6 was found to contribute significantly to the formation of crystal nuclei for the initiation of the phase change to α-alumina even with the addition of small amounts, namely, a 0.02–0.03 M ratio (Na3AlF6/Al(OH)3).