ABSTRACT. High heating rate is an important possible advantage of microwave sintering, due to direct, volumetric, instant absorption and the ease of delivering high microwave power into the sample. High heating rates may be especially important for the synthesis of advanced ceramics with superior properties unattainable by other means. Examples are sintering of nano powders with a strong inhibition of grain growth and net-shape, pressureless sintering of samples with complex geometry.

In this work we report on the sintering of ZnO samples with three different particle sizes (nominally, micron, submicron and nanopowder), at well-controlled heating rates from 5^oC/min to 800^oC/min in an advanced microwave processing system. Our modeling shows that heating rates >1400^oC/min may be possible. The key to very high heating rates is the use of a unique insulation system, local optical temperature measurement and computerized control of process parameters. The effect of heating rates on densification and microstructure of the three sizes of ZnO powder was studied. Both slower and faster heating rates produced fully dense samples. However, the higher heating rates resulted in fully dense samples with more uniform and finer microstructures. The final grain size tended to exponentially decrease with increasing heating rates for a 5-minute hold at sintering temperature. In addition to the experimental results, the effect of rapid sintering on coarsening and densification will be discussed in terms of classical sintering models.

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