Abstract
Materials processing based on electromagnetic energies have recently been interested in syntheses of new bulk materials from powders. Among these electromagnetic processing, capabilities of millimeter-wave heating and pulsed high current energizing methods were reviewed by exemplifying several results on syntheses of bulk ceramics from powder compacts. From our results on millimeter-wave sintering of alumina, Si3N4 and A1N, it has indicated that millimeter-wave heating has various advantages of rapid densification, decrease of sintering temperature and improvement of properties of sintered bodies. For example, high bending strength over 800 GPa was attained in the high pure alumina without any additives, and high thermal conductivity over 180 W/(m⋅K) was obtained in the Yb2O3-added aluminum nitride irrespective of sintering in non-reducing atmosphere. These are attributed to improvements of grain boundary structure and distribution of pore size and so on, originated in so-called "microwave effect". Addition of Yb2O3was effective to obtain aluminum nitride with high thermal conductivity and possibility of aluminum nitride with more than 200 W/(m⋅K) was suggested. Subsequently, capability of pulsed high current method was indicated from the results on behaviors of densification without transformation to rutile and preferentially orientation of crystal planes in nano-powder anatase compact. Densification behavior of alumina was also compared with those by millimeter-wave heating method. KEYWORDS millimeter-wave heating, pulsed high current heating, densification, microwave effect, bending strength, thermal conductivity
