| Microwave processing studies were carried out in a multimode microwave cavity at 2.45 GHz on alumina, mullite, hydroxyapatite (HAp), low-thermal-expansion (NZP) materials, and various partially stabilized zirconia (PSZ, tetragonal)-containing ceramic composites. Microwave enhancing effects on the densification and related processes were evaluated.;Microwave irradiation substantially enhances the densification, mullitization, and epitaxy of the diphasic mullite gel, while the effect on crystalline mullite and monophasic mullite gel was not significant even up to 1500;Significant microwave enhanced densification was also observed in HAp, alumina, NZP materials, and zirconia-containing composites. The amorphous alumina shows much stronger microwave effect than the crystalline one. For composites, the microwave enhancement decreases with increasing the PSZ content due to the better sinterability of PSZ than the matrix materials.;It is suggested that, besides the intrinsic dielectric properties, free energy potential, or activity, which is related to various higher-energy defects and surface area, is an important factor related to microwave absorption of materials irradiated. Specific surface area is an indicator of free energy. Also, a higher surface area also implies lower bulk thermal conductivity and higher scattering effect to microwaves. Therefore, the higher the surface area, the stronger the microwave absorption, the stronger the microwave enhancing effects.;It is also suggested that the microwave enhancing effect is due to an athermal effect which promotes the chemical potential as the driving force for the thermal processes. The degree of microwave enhancing effect depends on the relative magnitude of the athermal microwave effect and the total thermal energy supplied to the material. |
