Synthesis of high-strength ceramics by centrifugal casting and microwave sinterieng and evaluation of mechanical properties

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A12O3 and Si_3 N_4 were synthesized by microwave sintering. Its properties were investigated and advantages and disadvantages of microwave sintering were evaluated. Major obtained results were the following. In both ceramics, relative density was higher with microwave sintering than with conventional sintering. It was also found that by microwave sintering, the onset temperature for sintering was lowered by 50ﾟC and the sintering rate was increased. Since microwave sintering could decrease the activation energy for diffusion by 40%, these improvemens were due to the enhanced diffusion under microwave conditions.We also found that there was no direct correlation between the sintered density and the green density. This was contradictory to the results in the case of conventional sintereing. Observation of microwave-sintered Al2O3 and Si_3N_4 by a scanning electron microscope revealed the absence of pores having a diameter of less than 2 mum. This indicated that small pores of less than 2 mum were vanished during microwave sintering. Therefore, the final density was found to be determined not by the green density but by the amount of the small pores. A possible reason for the disappearance of the small pores is that the surface energy is increased under the microwave and that the small pores having a large surface/volume ratio would shrink and disappear. The reduction of the small pores during performing becomes a key to further improve the final density and associated mechanical properties