The influence of MgO and La2O3 on the fast sintering of mullite was evaluated in this work. High-purity mullite was doped with MgO and La2O3, conventionally sintered, fast sintered, and microwave-sintered in rapid sintering cycles. The results showed that MgO and La2O3 strongly affected the densification of fast sintered samples; and only doped bodies reached high densities when fast sintered. However, the densification behavior of doped samples was dependent on the amount of sintering aids. MgO doped samples presented better densification behavior than La2O3 doped samples. Microwave-processed samples required a balance between power, time and amount of additives to obtain highly dense and homogenous microstructures.

Commercially available alumina powder with high-purity submicrometer particle size and narrow particle size distribution was fully densified by a microwave hybrid fast firing technique. The alumina compacts were surrounded by susceptor material, which helped the heating of the samples, and sintered in a microwave oven at a frequency of 2.45 GHz and a power level of 1.8 kW. The sintered samples reached densities of 99% in sintering cycles of 30 to 40 minutes, a much shorter time than conventional sintering processes. The sintered samples showed uniform microstructures with powder particle size/average grain size rations higher than 1:2.