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 (M g(OH)(2)) or hydromagnesite (4MgCO(3.)Mg(OH)(2.)4H(2)O). The mechanochemica l treatment does not remove any water or carbonate, but converts some of th e gibbsite octahedral Al sites into tetrahedral sites and other sites with a (2)7Al MAS NMR resonance at about 38 ppm. The brucite-derived precursor f orms spinel on heating at less than or equal to 850%C, by contrast with ung round mixtures which show little spinel formation even at 1250%C. The hydro magnesite-derived precursor transforms at about 850%C into a mixture of spi nel and hydrotalcite (Mg6Al2(OH)(1)6CO(3.)4H(2)O), the latter decomposing t o spinel and MgO by 1050%C. Spinel derived from the hydromagnesite-containi ng precursor shows superior pressureless sintering properties at 1400-1600% C, producing a body of 97% theoretical bulk density at 1600%C. Under the sa me conditions, the brucite-derived spinel sintered to 72% theoretical densi ty and showed a morphology consisting of widely disparate grain sizes. (C) 2000 Kluwer Academic Publishers.