Microwave hybrid firing of low-purity alumina

Microwave (hybrid) firing (MHF) offers many potential benefits to ceramic processing, including reductions of both the production time and temperature required to fire ceramic components. These benefits result from features of microwave processing, such as volumetric heating and enhanced diffusion within the ceramic body. Further investigation into issues such as scale-up, microwave susceptors, and mechanical properties of MHF samples is needed if this technique is to gain acceptance by industry.;An experimental study was undertaken to investigate MHF as an alternative to conventional electric or gas firing for the production of small, uniform batches of alumina samples. A MHF assembly was designed and developed so that comparisons could be made between the densification and mechanical properties (hardness, strength, and fracture toughness) of samples fired using the two techniques. In addition to these comparisons, the effect of alumina purity on MHF and the effect of energy partitioning on sample densification were also evaluated.;Through this study, it was determined that MHF was a viable alternative to conventional firing for the production of uniform batches of low-purity alumina pieces. The firing temperature required to reach identical levels of densification in the alumina samples was from 75 to 200°C lower for MHF as compared to conventional firing.;Possible reasons for the enhanced densification of MHF relative to conventional firing include the microwave volumetric heating phenomenon and a non-thermal effect of microwaves on the viscosity and/or surface energy of the glassy phase present in the low-purity alumina.;Some control of the sample densification rate was afforded through control of the energy partitioning between microwave volumetric heating and conventional surface heating. It was found that increasing the ratio of conventional surface heating to microwave volumetric heating could decrease the sample densification rate.;The mechanical properties of samples fired to identical relative densities (95% relative density) using the two techniques were statistically similar. Hardness was uniform in samples processed using either technique.