Solution combustion synthesis of magnesium compensated sodium-β-aluminas

Sodium β and β′′-alumina (Al11-yMgyO16) (Na1+yO) of different compositions 0.45≤y≤1.0 were synthesised through a novel rapid-solution combustion synthesis route using stoichiometric amounts of metal nitrates and urea. The redox mixture when rapidly heated to 773 K ignites and yields β-alumina in a few minutes. The powder X-ray diffraction pattern shows single-phase β-alumina for compositions 0.45≤y≤0.67. The samples of composition 0.45≤y≤0.56 on annealing at various temperatures, of 100° intervals, in the range 1273–1873 K remained as β phase whereas compositions in the range 0.57≤y≤0.67 transformed from β to β′′ phase above 1773 K. The results confirm that the stable phases in the composition ranges 0.45≤y≤0.56 and 0.57≤y≤0.67 are β and β′′, respectively. The β phase observed for compositions 0.57≤y≤0.67 below 1773 K is metastable and they transformed to β′′-phase on annealing at 1823 K for 3 h.