Microwave sintering of alumina at 2.45 GHz

The sintering kinetics and microstructural evolution of alumina tubes (similar to17 mm length, similar to9 mm inner diameter, and similar to11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to +/-10degreesC. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350degreesC versus 1600degreesC for conventionally heated samples. The activation energy for microwave sintering was 85 +/- 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 +/- 14 kJ/mol. Despite the difference in temperature, grains grew from similar to1.0 mum at 86% density to similar to2.6 mum at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.