A Study on the Sintered Si<sub>3</sub>N<sub>4</sub>-MgO System

Abstract

Studies were made on the sintered Si3N4 with an addition of MgO, hot-pressed at the temperature of 1730°C.The following results were obtained. At low concentration of MgO, only β-Si3N4 phase was observed. When MgO concentration was increased, the formation of MgSiN2 was found. Further increase of MgO concentration gave rise to the formation of MgSiN2 and 2 MgO⋅SiO2. The X-ray diffraction angles or the lattice constants of β-Si3N4 were unchanged by the addition of MgO. The linear coefficient of the thermal expansion was increased with MgO concentration. MgO was inferred to dissolve into β-Si3N4. The solubility limit of MgO at 1730°C was estimated to be about 30mol%. In the solid solution, oxygen atoms are substituted for nitrogen atoms, a half of the dissolved magnesium atoms for silicon atoms and the rest magnesium atoms are located in the interstitial spaces, which are situated in some of the “tunnels” intrinsically present in the β-Si3N4 structure. The dissolution of four MgO molecules introduces one vacancy at the site of silicon. This interpretation for the solid solution was supported by the good agreement between calculated and observed values of both specific gravity and relative X-ray intensity. From the inferred structure of the solid solution and the agreement between the activation energy of sintering for Si3N4 containing MgO and that for Si3N4 containing Al2O3, enhanced sintering of β-Si3N4 added with MgO may be attributed to the diffusion of the silicon site vacancies introduced in the solid solution.