Direct microwave sintering of pure alumina in a single mode cavity: Grain size and phase transformation effects

A comparative study between conventional and direct microwave sintering of pure alpha and gamma nanoscale alumina powders has been performed to identify possible microwave effects on densification and microstructure changes. Microwave sintering experiments have been performed in a 2.45 GHz single mode cavity furnace allowing for an accurate control of the thermal cycle and for a continuous measurement of specimen dimensional changes, so that direct comparison with conventional sintering can be achieved. Special attention has been given to the influence of particle grain size and of the gamma to alpha phase transformation occurring during heating of gamma alumina powder on the sintering behaviour. Experimental data unequivocally showed a significant effect of microwaves on a powders (lower onset temperature of densification and smaller activation energy) and on gamma powders (lower phase transformation temperature). However, the sintering trajectory in grain size vs density diagram is similar in conventional and microwave sintering. Therefore, microwave heating was not beneficial to obtain dense alumina with very fine grains. Microwave effects have been explained through the ponderomotive force induced by the electromagnetic field and acting on diffusion and phase transformation mechanisms. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.