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

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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