Gel-to-crystallite conversion technique for the syntheses of M-β/β"-alumina (M = Li, Na, K, Rb, Ca or Eu)

Various alkali and alkaline earths hydroxides arc reacted with hydrated alumina gel Al2O3 center dot nH(2)O (80 < n < 120) in hydrophilic organic solvents to yield precursor(s) with metal cation entrapped in the matrix, with the general formula M(y)AIC(y)(OH)(3-y) (where M = Li, Na, K, Rb, Ca and Eu). Depending on the cation, the precursor crystallizes as metal-inserted gibbsite or boehmite structure. The nature of the precursor depends on the size of the cation, and its concentration, solvent used, solvent/water ratio, etc. These precursors yield M-beta/beta"-alumina phase on calcination at elevated temperatures in the case of sodium, potassium and rubidium. In the case of lithium, LiAl5O8 (spinel) phase is obtained. While calcium and europium give rise to calcium hexaaluminate and EuAlO3 (perovskite), respectively, on calcination. The need of metal ion-inserted-gamma-alumina phase is emphasized in obtaining M-beta/beta"-alumina phase. (C) 2007 Elsevier Ltd. All rights reserved.