Clarification of Liquid Phase Sintering Process of Silicon Nitride from the Perspective of Physico-Chemical Properties of High Temperature Melts

用旋转坩埚粘度计和环法分别研究了RE_2O_3 （RE=Y、Gd、Nd和La）对45.2MgO-54.8SiO_2 （mol%）熔体粘度和表面张力的影响。此外，用红外光谱仪研究了这些淬火玻璃样品的结构特征。随着稀土添加量的增加，RE_2O_3-MgO-SiO_2熔体的粘度降低，表明稀土氧化物在高温熔体中作为复合硅酸盐阴离子的网络改性剂。黏度随稀土阳离子半径的增大而减小；从Y_2O_3，Gd_2O_3，Nd_2O_3到La_2O_3。随着稀土添加量的增加，MgO-SiO_2二元熔体的表面张力增大。表面张力随稀土离子半径的增加而明显增大，其变化与粘度的增加成反比。在1773 K热处理64 ~ 512 min后，研究了分散在稀土（RE=Y,Gd，Nd和La）氮化氧液体（RE- mg - si - o - n）中的(β-Si_3N_4晶体的生长规律。用电子显微镜定量分析了α- _si_3n_4和β-Si_3N_4晶体的显微结构特征。氮化氧玻璃中（β-Si_3N_4）晶体的体积分数随热处理时间的延长而增大。结果表明，添加Y_2O_3的氮化氧液体中α/β的转变速度比添加其他添加剂的氮化氧液体中快。同样，稀土元素的半径对β-Si_3N_4晶体的平均晶粒尺寸和长径比也有显著影响。在上述所有情况下，离子半径从La到y的顺序发生了明显的变化。La_2O_3添加剂热处理的针状β-Si_3N_4晶体的平均宽度在本研究热处理的时间范围内几乎是恒定的。而加入La_2O_3后，β-Si_3N_4晶体的长宽比远大于加入其他添加剂的氮化氧液体，表明β-棱柱面的生长可以忽略不计。少

The effect of RE_2O_3 (RE=Y,Gd,Nd and La) on the viscosity and the surface tension of 45.2MgO-54.8SiO_2 (mol%) melts have been investigated with rotating crucible viscometer and ring method, respectively. Additionally, structural characterizations of these quenched vitreous samples have been investigated with Infra-Red spectrometer.The viscosities of RE_2O_3-MgO-SiO_2 melts were found to decrease with increasing the content of any rare-earth additions, which suggests that rare-earth oxide behaves as a network modifier of complex silicate anions in high temperature melts. The viscosities decreased in the order of cationic radius of rare-earth ; from Y_2O_3,Gd_2O_3,Nd_2O_3 to La_2O_3. The surface tension of MgO-SiO_2 binary melts were found to increase with increasing the content of any rare-earth additions. The surface tensions obviously increased in the order of cationic radius of rare-earth, which the change of with rare-earth addition is direct antithesis of that of viscosities.Isoth … More ermal growth of (β-Si_3N_4 crystals dispersed in a series of rare-earth (RE=Y,Gd,Nd and La) oxynitride liquids (RE-Mg-Si-O-N) was studied after heat treatment at 1773 K for 64 to 512 min. The microstructural characterization of α-_and β-Si_3N_4 crystals was quantitatively analyzed by electron microscopy. The volume fraction of (β-Si_3N_4 crystals in oxynitride glasses increased with heat treatment time. It is determined that the rate of α/β transformation in the oxynitride liquid with Y_2O_3 additive can be faster than that in oxynitride liquids with other additives. Likewise, the radii of rare-earth elements make significant differences in the mean grain size and aspect ratio of β-Si_3N_4 crystals. In all cases mentioned above, a marked change was observed in the order of ionic radius from La to Y. The mean width of the needlelike β-Si_3N_4 crystals heat-treated with La_2O_3 additive was almost constant in the time range of heat treatment in this study. However, the aspect ratio of β-Si_3N_4 crystals with La_2O_3 additive was found to be much larger than that in oxynitride liquids with other additives, indicating negligible growth of β-prism plane. Less