Evidence for the microwave effect during hybrid sintering

A microwave/conventional hybrid furnace has been used to sinter three ceramics with different microwave absorption characteristics under pure conventional and a range of microwave/ conventional hybrid heating regimes. The precursor powder particle size was also varied for each material. In each case it was ensured that every sample within a series had an identical thermal history in terms of its temperature/time profile. An increase in both the onset of densification and the final density achieved was observed with an increasing fraction of microwave energy used during sintering, the effect being greatest for the materials that absorbed microwaves most readily. Twenty-three percent greater densification was observed for submicron zinc oxide powder, the material with the largest microwave absorption capability, when sintered using hybrid heating involving 1 kW of microwave power compared with pure conventional power under otherwise identical conditions. For the ceramic with the lowest microwave absorption characteristic, alumina, the increase in densification was extremely small; partially stabilized zirconia, a moderate microwave absorber, was intermediate between the two. Temperature gradients within the samples, a potential cause of the effect, were assessed using two different approaches and found to be too small to explain the results. Hence, it is believed that clear evidence has been found to support the existence of a genuine ‘‘microwave effect.’