Mechanochemical synthesis and sintering behaviour of magnesium aluminate spinel

X-ray amorphous precursor phases for the synthesis of spinel (MgAl2O4) have been prepared by grinding mixtures of gibbsite (Al(OH)3) with brucite (Mg(OH)2) or hydromagnesite (4MgCO3·Mg(OH)2·4H2O). The mechanochemical treatment does not remove any water or carbonate, but converts some of the gibbsite octahedral Al sites into tetrahedral sites and other sites with a 27Al MAS NMR resonance at about 38 ppm. The brucite-derived precursor forms spinel on heating at ≤850°C, by contrast with unground mixtures which show little spinel formation even at 1250°C. The hydromagnesite-derived precursor transforms at about 850°C into a mixture of spinel and hydrotalcite (Mg6Al2(OH)16CO3·4H2O), the latter decomposing to spinel and MgO by 1050°C. Spinel derived from the hydromagnesite-containing precursor shows superior pressureless sintering properties at 1400–1600°C, producing a body of 97% theoretical bulk density at 1600°C. Under the same conditions, the brucite-derived spinel sintered to 72% theoretical density and showed a morphology consisting of widely disparate grain sizes.