Synthesis of beta alumina from aluminum hydroxide precursor formed by gel to crystallite conversions

A novel method is reported for the preparation of phase-pure β″-(Na) alumina by the gel to crystallite conversion technique. Coarse gels of hydrated alumina, Al2O3 · Y H2O (80 < Y< 120), were reacted with sodium hydroxide solution in ethanol, yielding crystalline NaxAlOx(OH)3 − x (0.17 < x < 0.23) having the gibbsite structure with inserted sodium ions. Thermal decomposition of this precursor above 573 K yielded the intermediate NaxAlO1 + x(OH)1 − x with the boehmite structure. At higher temperatures this intermediate gave rise to γ-alumina, NaxAl2O3 − x(OH)3x and NaAl73O4 with the defect spinel structure. It is proposed that because of the cubic close packing of oxygen ions present in the intermediate alumina, formation of β″-alumina takes place above 1273 K as a continuous solid-state process with minimal rearrangement of the close packed oxide ions. The β″-alumina phase thus formed is free of intergrowths of the β-phase, as revealed by HREM and is stable up to near-melting conditions.