Use of partially oxidized SiC particle bed for microwave sintering of low loss ceramics

A 1 kW hybrid microwave system was fabricated using a partially oxidized Si C powder bed and used to sinter small components of low-loss insulating cer amics. Samples of 3% yttria stabilized zirconia/20% alumina (3YZA) and 99% alumina were sintered to final densities of 99%(3YZA) and 95% (99% alumina) . Partially oxidized silicon carbide (beta-SiC) powder was used as suscepto r (preheater). By comparison, unoxidised beta-SiC powder, which couples wel l with microwaves at room temperature, exhibited thermal runaway above 400 degrees C. It could be possible that at high oxidation levels the connectiv ity between SiC particles in beta-SiC powder bed might become depercolated, and a model similar to one proposed for silicon nitridation under microwav es could be appropriate in understanding the microwave absorption phenomeno n. Stable temperature measurements at various positions in the system confi rmed that the maximum temperature was at the center of the 3YZA sample. The plot of the relative temperature difference between the sample and its sur roundings as a function of sample temperature resulted in a bell-shaped cur ve with a clear maximum at around 800 degrees C, associated with the rapid increase in radiation heat transfer above this temperature. Experiments als o confirmed that sintering occurred at lower temperatures in a microwave fi eld when compared to conventional sintering. (C) 1999 Elsevier Science S.A. All rights reserved.