The $\alpha$ to $\beta$ phase transformation and correlated microstructures in silicon nitride through suitable liquid solvent have been investigated. The specimens consisting of 1) granulated $\alpha-Si_3N_4,\; \beta-Si_3N_4$ and free Si, 2) $\alpha-Si_3N_4$ whisker, $\beta-Si_3N_4$ and free Si, 3)...
The $\alpha$ to $\beta$ phase transformation and correlated microstructures in silicon nitride through suitable liquid solvent have been investigated. The specimens consisting of 1) granulated $\alpha-Si_3N_4,\; \beta-Si_3N_4$ and free Si, 2) $\alpha-Si_3N_4$ whisker, $\beta-Si_3N_4$ and free Si, 3) granulated $\alpha-Si_3N_4$, free Si and oxide glass were heat treated in the range $1650\,^\circ\!C$ to $1750\,^\circ\!C$ argon atmosphere, respectively. The $\beta$-fraction was determined by comparing the intensities of $\alpha_{210}$ and $\beta_{210}$ diffraction peaks. For the observation of the precipitated phases, the specimens were etched by conc. $HF + HNO_3$ solution and molten NaOH. In this study, $Y_2O_3$-glass was used as oxide glass and added Si had two different particle sizes. Free silicon in silicon nitride compacts could act as suitable liquid solvent for $Si_3N_4$ and assisted in the $\alpha$ to $\beta$ phase transformation. The rate of transformation depended on the size of added silicon. When the small silicon particles were added, the rate of transformation was more rapid than with large particles: the complete transformation could be achieved at $1700\,^\cric\!C$ in 1h. The size of precipitated $\beta-Si_3N_4$ depended on the heat treating temperature. Although large grains were precipitated at $1650\,^\circ\!C$, fine precipitate-grains were mainly observed with increasing heat treating temperature. This was due to the change of rate controlling factors from the nucleation controlled by the dissolution reaction to the growth controlled by the diffusion of nitrogen. The morphology in microstructure of the heat treated specimen with a prismatic shape grain was resulted from preexisting $\beta$ phase as well as $\alpha$ phase; nevertheless, the phase transformation was effected by only $\alpha$ phase, not by $\beta$ phase. Three regimes could be defined for the $\alpha$ to $\beta$ phase transformation of granulated $\alpha -Si_3N_4$ through large Si: 1) formation of Si melts and nucleation of precipitated phase, 2) growth of precipitated grains and Si melts spreaded out toward the surrounding $Si_3N_4$ region, and 3) fine grains were precipitated through the spreaded Si melts and Si melts completely ran out leaving large pores. As the granulated $\alpha-Si_3N_4$ containing oxide glass was heat treated, the $\alpha$ to $\beta$ transformation and densification were occurred. When $\alpha$-whisker with and without Si was heat treated, the $\alpha$ to $\beta$ phase transformation was occurred. In this case, the oxygen content in $\alpha$ -whisker was dominently effected on the transformation rather than added Si. When $Y_2O_3$-glass was added to silicon nitride with silicon, the $Y_2O_3$-glass predominantly influenced on the transformation of silicon nitride rather than added Si. It seems that the $Y_2O_3$-glass influenced on both transformation and densification processes, while the added silicon tended to effect on the transformation rather than densification process.
The $\alpha$ to $\beta$ phase transformation and correlated microstructures in silicon nitride through suitable liquid solvent have been investigated. The specimens consisting of 1) granulated $\alpha-Si_3N_4,\; \beta-Si_3N_4$ and free Si, 2) $\alpha-Si_3N_4$ whisker, $\beta-Si_3N_4$ and free Si, 3) granulated $\alpha-Si_3N_4$, free Si and oxide glass were heat treated in the range $1650\,^\circ\!C$ to $1750\,^\circ\!C$ argon atmosphere, respectively. The $\beta$-fraction was determined by comparing the intensities of $\alpha_{210}$ and $\beta_{210}$ diffraction peaks. For the observation of the precipitated phases, the specimens were etched by conc. $HF + HNO_3$ solution and molten NaOH. In this study, $Y_2O_3$-glass was used as oxide glass and added Si had two different particle sizes. Free silicon in silicon nitride compacts could act as suitable liquid solvent for $Si_3N_4$ and assisted in the $\alpha$ to $\beta$ phase transformation. The rate of transformation depended on the size of added silicon. When the small silicon particles were added, the rate of transformation was more rapid than with large particles: the complete transformation could be achieved at $1700\,^\cric\!C$ in 1h. The size of precipitated $\beta-Si_3N_4$ depended on the heat treating temperature. Although large grains were precipitated at $1650\,^\circ\!C$, fine precipitate-grains were mainly observed with increasing heat treating temperature. This was due to the change of rate controlling factors from the nucleation controlled by the dissolution reaction to the growth controlled by the diffusion of nitrogen. The morphology in microstructure of the heat treated specimen with a prismatic shape grain was resulted from preexisting $\beta$ phase as well as $\alpha$ phase; nevertheless, the phase transformation was effected by only $\alpha$ phase, not by $\beta$ phase. Three regimes could be defined for the $\alpha$ to $\beta$ phase transformation of granulated $\alpha -Si_3N_4$ through large Si: 1) formation of Si melts and nucleation of precipitated phase, 2) growth of precipitated grains and Si melts spreaded out toward the surrounding $Si_3N_4$ region, and 3) fine grains were precipitated through the spreaded Si melts and Si melts completely ran out leaving large pores. As the granulated $\alpha-Si_3N_4$ containing oxide glass was heat treated, the $\alpha$ to $\beta$ transformation and densification were occurred. When $\alpha$-whisker with and without Si was heat treated, the $\alpha$ to $\beta$ phase transformation was occurred. In this case, the oxygen content in $\alpha$ -whisker was dominently effected on the transformation rather than added Si. When $Y_2O_3$-glass was added to silicon nitride with silicon, the $Y_2O_3$-glass predominantly influenced on the transformation of silicon nitride rather than added Si. It seems that the $Y_2O_3$-glass influenced on both transformation and densification processes, while the added silicon tended to effect on the transformation rather than densification process.
Keyword
#Microstructure 상변태 미세 구조 Silicon nitride Phase transformations (Statistical physics) 질화규소
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