The stability during sintering of SiO
2, Y
2O
3 and Al
2O
3, well known as sintering aids of Si
3N
4 ceramics, was examined from both thermodynamical and experimental aspects. Both the heat treatment of SiO
2-Y
2O
3-Al
2O
3 glass and the sintering of Si
3N
4 ceramics were carried out. In the thermodynamical considerations, the stability of the glass composition was discussed on the basis of known volatility diagrams for SiO
2, Al
2O
3 and MgO and the thermodynamic data of Y
2O
3 available in the literature. The results suggest that Y
2O
3 is the most stable among all materials in the sintering temperature range usually selected for Si
3N
4 ceramics. SiO
2-Y
2O
3-Al
2O
3 (equi-mass%) glass was heat-treated in a BN sagger at 2023K for 2h in N
2 gas of 0.1 MPa. Si
3N
4 with addition of 5 mass% Y
2O
3 and 3 mass% Al
2O
3 was sintered in the BN sagger at 2023K for 0 to 16h in N
2 gas of 0.1MPa. The elemental contents in the glass and ceramics before and after heat treatment and sintering were determined by chemical analysis. As a result, it was found that only SiO
2 in the glass and SiO
2 and Si
3N
4 in the Si
3N
4 ceramics vaporized. On the other hand, the Y
2O
3 and Al
2O
3 contents in both the glass and Si
3N
4 ceramics did not decrease under the selected temperature.
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