In this study, silicon nitride doped with $Al_2O_3$ and $Y_2O_3$ was sintered by microwave heating in a multimode cylindrical cavity applicator operating at 2.45 GHz. For comparison, conventional pressureless sintering was performed using the same sintering schedule, and then the effect of microwave heating on densification and α→β phase transformation was investigated.
In chapter 4-1, an Insulation box was designed to prevent cracking and nonuniform sintering by thermal runaway during microwave sintering. The insulation box consisted of a BN crucible, a SiC plate, and Alumina fiber insulation boards. By using the SiC plate as a preheater, silicon nitride specimen was preheated to the critical temperature uniformly and could be sintered successfully without thermal runaway.
In chapter 4-2,the densification and the phase transformation behavior of microwave-sintered silicon nitride were investigated and compared with those of conventionally sintered one. By microwave heating, the densification and the phase transformation were enhanced and dense silicon nitrides with relative density of >99% could be obtained. Microwave-sintered silicon nitrides also showed microstructure with larger grain size in spite of the same sintering condition. After isothermal holding, microwave-sintered silicon nitrides showed in-situ composite microstructure composed of small equiaxed grains and large elongated grains (d>10㎛). In the case of conventional sintering, as such the in-situ composite microstructure could not be obtained despite the longer holding time.
By comparing the microwave heating behavior of specimens with different amount of additives (0,4,8 and 12wt%) under the same microwave power, it could be found that microwave heating of silicon nitride occurred predominantly by coupling between the microwave and the intergranular liquid phase that was formed from the sintering additives. From this result, two possibilities were postulated for the enhanced densification and phase transformation ; one was hotter liquid phase and the other was nonthermal effect of microwave. Through model calculation, it was revealed that the time for thermal equilibrium between hotter liquid phase and silicon nitride grains was so fast($\simeq10^{-9}$ sec). As a result,it was suggested that the nonthermal effect of microwave was responsible for the enhanced densification and phase transformation.
In chapter 4-3, quenching experiment was carried and the microstructural development was compared. The silicon nitrides quenched after microwave heating,needle-shaped grains (β-nuclei) with higher aspect ratio were formed by phase transformation after full densification. On the other hand,silicon nitrides quenched after conventional heating,need- shaped grains with lower aspect ratio were formed before full densification. From these results, it could be suggested that the higher grain growth rate combined with the larger driving force for abnormal grain growth by the formation of large β-nuclei was responsible for the in-situ composite microstructure of microwave-sintered silicon nitride. It was also expected that densification was enhanced much more than phase transformation by microwave heating. To calculate and compare the kinetic constant for the densification and the phase transformation, Isothermal holding experiment was performed. As a result, it was shown that the densification and the phase transformation were enhanced by microwave heating, 6 and 2.5 times, respectively. By considering factors related to the kinetics of densification and of phase transformation, it was postulated that the nonthermal effect of microwave was the decrease in the viscosity of liquid phase by interaction between liquid phase and microwave.
In chapter 4-4, the postulation was proved by the model experiments for $β-Si_3N_4-Al_2O_3-Y_2O_3$ and $Si_3N_4-SiO_2$ systems. Consequently, it was concluded that the densification and the phase transformation of silicon nitride were enhanced by microwave heating through the decrease in the viscosity of liquid phase.
