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

采用微波烧结法制备了Al_2O_3和Si_3N_4。研究了其性能，评价了微波烧结的优缺点。取得的主要成果如下。在这两种陶瓷中，微波烧结的相对密度都高于常规烧结。微波烧结可使烧结起始温度降低50ﾟC，提高烧结速度。由于微波烧结可以使扩散活化能降低40%，这主要是由于微波促进了扩散，同时发现烧结密度与生坯密度之间没有直接的关系。这与传统烧结的结果相矛盾。用扫描电子显微镜观察了微波烧结的Al_2O_3和Si_3N_4，发现没有直径小于2微米的气孔。这表明在微波烧结过程中，小于2微米的小孔消失了。因此，最终的密度不是由毛坯密度决定的，而是由小孔的数量决定的。小孔消失的一个可能原因是在微波作用下表面能增加，比表面积大的小孔收缩消失。在加工过程中减少小孔成为进一步提高最终密度和相关机械性能的关键

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