Im Rahmen einer Neuuntersuchung des Titelsystems bei Temp. oberhalb 650°C werden die beiden stabilen Verbindungen (I) und (II) erstmals beschrieben.

Hot-pressed Si3N4 were prepared from two kinds of commercial Si3N4 with addition of Y2O3 and Al2O3. Oxidation behavior of hot-pressed Si3N4 bodies were investigated in pure dry oxygen at 1200•‹ and 1300•Ž, at several steps up to 30 days. Results of the weight gain, X-ray diffraction analysis, EPMA analysis and SEM observa tions on the oxidized layer are discussed in relation to the mechanism of oxidation.

Phase relations in the system PbO–PbSiO3 were determined at temperatures above 650 °C. Two new lead silicate phases, Pb5SiO7 and Pb11Si3O17 were found to be stable. The former melts incongruently at 733±1 °C to solid PbO plus liquid, and decomposes below 702±1 °C. The latter melts congruently at 728±1 °C. The compound Pb4SiO6, which has been accepted as stable, could not be confirmed.

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