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 °C 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 °C and above. The green compacts of the spinel powders could be sintered to near theoretical density at 1150 °C 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.