Development of sintering and joining processes of high performance ceramics for next generation by using high power millimeter-wave radiation

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Sintering and joining processes based on millimeter-wave radiation were developed for fabricating high performance ceramics and fundamental properties of ceramics powders were investigated by using millimeter-wave spectroscopy. Following results were obtained.In the research on clarifying the dielectric characteristics of ceramics powders in the millimeter-wave region, power absorption of silicon carbide powder was first measured by millimeter-wave spectroscopy and it was found that the absorption coefficient increased with increasing the powder size, temperature and the content of free carbon. Secondly, the dielectric constant of alumina-zirconia showed high values with decreasing powder density and increasing zirconia content.New sintering aids containing Yb_2O_3 were developed for sintering silicon nitrides with 28GHz millimeter-wave radiation and a rapid sintering process was successfully established for fabricating bulk silicon nitrides which were sintered within only one hour lower a temperature which was lower by about 400℃ than those in conventional methods. The rapid millimeter-wave sintering process was also applied to the fabrication of aluminum nitride with high thermal conductivity of about 190 W/m・K was successfully produced at a temperature of 200℃ lower and at a sintering time of one-tenth shorter than those in conventional methods.Subsequently, as an example of fabricating nanocrystalline bulk materials, consolidation of nano-powder anatase was investigated by millimeter-wave heating, resulting in obtaining porous bulk titania with 200 nm in the average diameter with 300 Hv in the hardness.Further, focused millimeter-wave beam process enabled to crystallize amorphous SrTiO_3 films at a low temperature below 400℃ and well-crystallized SrTiO_3 films with high dielectric constant similar to polycrystalline bulk SrTiO_3 was synthesized successfully.