Synthesis of magnesium-aluminium spinel from autoignition of citrate-nitrate gel

Magnesium-aluminium spinel has been synthesized by citrate-nitrate route using hydrated nitrates of Mg and Al as the precursors with a citrate-nitrate ratio of 1:1. Self-ignition of the gel-like mass yielded a black mass, which crystallized to Mag-Al spinel on calcination at 650 degreesC for 9 h. The crystallized mass was a mixture of ordered and disordered phases of Mag-Al spinel. Phase pure-ordered spinel could be obtained for powders calcined at 1100 degreesC and above. The green compacts of the spinel powders could be sintered to near theoretical density at 1150 degreesC for 30 min. The activation energy for densification was calculated (16.4 kcal/mol) assuming Arrhenius type behaviour. The densification mechanism could be assigned to the ordering of the disordered spinel involving vacancy diffusion through the surface. (C) 2003 Elsevier B.V. All rights reserved.