Direct microwave sintering of alumina in a single mode cavity: Magnesium doping effects

Unexpected results have been obtained when comparing the densification behaviour of undoped and MgO-doped alpha and gamma alumina powders in conventional and direct microwave heating. Although the densification starts at lower temperature under microwaves, the addition of MgO in the raw powder slows down the densification under microwaves. This leads to samples with a lower final density compared to undoped or conventionally heated samples after similar thermal cycles. These original results have been obtained realizing microwave sintering experiments by direct heating, in a 2.45 GHz single-mode cavity. The furnace allowed for an accurate control of the thermal cycle and for a continuous measurement of specimen dimensional changes. Direct comparison with conventional sintering has been achieved. These results are explained by the combination of the MgO effect on the alumina sintering behavior at low temperature generally reported under conventional heating and of a specific microwave effect linked to a ponderomotive force.