Spinel formation, reaction conditions and densification properties of magnesia-spinel composites

Abstract

Densification, reaction conditions and properties of densified MgO and spinel composites were examined using both pressureless sintering and hot-pressing. Initially fine (32 nm) MgO powder, either alone or blended with mum dimension spinel particles or alumina powder, could not be sintered without extensive and large scale cracking. Bulk densities of calcined MgO discs after uniaxial and isostatic pressing, and after pressureless sintering, were thus determined as a function of powder calcination temperature. The peak temperature of 1300 degreesC for 2 h was found to be optimum for pure MgO powders, giving the highest bulk density for the MgO discs. Increasing calcination temperature and decreasing the heating rate reduced the extent of cracking, but did not eliminate it completely. However, the use of precalcined MgO powder and a slow heating rate gave dense materials with no large-scale cracks. This work helped to understand better how macrocracking caused by the fabrication stage could be reduced and/or removed. The conditions of temperature and time to produce in-situ formed spinel were conformed by XRD. The sintering parameters and conditions, based on the density, calcination temperatures, pressure, time and particle size, were determined, and the optimum sintering schedule was proposed for MgO-spinel composites.