Yoichi Oyama

Solid Solution in the System Si3N4-Ga2O3-Al2O3

The solid solution of Si3N4 containing Ga2O3 and Al2O3 denoted as Si3N4s.s. (Ga2O3, Al2O3) has been found to be formed through sintering by hot-pressing at 1730–1800°C and 250 kg/cm2 for 20–180 min. The solubility limits of Ga2O3 and Al2O3 in Si3N4s.s. (Ga2O3, Al2O3) have been determined. From the observation of the specific gravity of Si3N4s.s. (Ga2O3, Al2O3), the solid solution is thought to be mainly of substitutional type, and most aluminium and gallium atoms are substituted for silicon atoms, and most oxygen atoms for nitrogen atoms. Small amounts of solute atoms are located in interstitial spaces. A close correlation exists between the oxygen and gallium or aluminium atoms in the solid solution. A new compound, m(Al2O3, Ga2O3) nSi3N4 (m\fallingdotseq3, n\fallingdotseq2), has also been found. The nature of the bond changes from covalent to ionic with an increase in Al2O3 concentration. The thermal expansion coefficient of the solid solution Si3N4s.s. (Al2O3) is decreased and then increased with the addition of Al2O3. This increase may be attributed to an increase of (Si, Al)N2O2 tetrahedron. The liquid phase has been observed in the region of high concentration of Al2O3. Its appearance is explained on the basis of the increased ionicity (electrostatic interaction) in the bond.