Evidence for the microwave effect during the annealing of zinc oxide

A microwave/conventional hybrid furnace has been used to anneal virtually fully dense zinc oxide ceramics under pure conventional and a microwave/conventional hybrid heating regime with a view to obtaining evidence for the ‘microwave effect’ during the resulting grain growth. In each case it was ensured that each sample within a series had an identical thermal history in terms of its temperature/time profile. The results showed that grain growth was enhanced during hybrid heating compared to pure conventional heating; the greatest enhancement, a factor of ~3 increase in average grain size, was observed in the range 1100-1150°C. The grain growth exponent decreased from 3 during conventional heating to 1.4 during hybrid heating in this temperature range, suggesting an acceleration of the diffusional processes involved. Temperature gradients within the samples were found to be too small to explain the results. This suggests that clear evidence has been found to support the existence of a genuine ‘microwave effect’.