C and Si02 of different types were mixed and heat-treated (1410" to 1550"C), according to a standardized temperature program, in flowing N2 at different pressures. It was found that a starting material combination of C (115 m'/g) and Si02 (50 m'/g) yielded pure SiJN4 after about 2 h at 1550°C and 1.3 MPa. By adjustment of the pressure in the range 2 to 6 MPa, irreversible evaporation of SiO(g) as well as formation of nonequilibrium phases was suppressed, and the amounts of residual C and 0 in the sample were controlled. Even a small amount of CO in the N2 was observed to retard the nitridation. Possible explanations are discussed, based on thermodynamic calculations on the Si-0-N-C system.

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Phase relations in the Si-C-0-N system were studied by subjecting mixtures of carbon and silica (C/Si02 = 2 to 3) to high temperatures (up to 2000 K) and elevated nitrogen pressures (0.5 to 2.0 MPa) in a gas autoclave. The condensedphase assemblages obtained at different temperatures, pressures, and N/Si ratios are reported and compared with thermodynamic calculations. Values of AGf"(Si20N2) consistent with the experimental data are -397 kJ/mol at 1780 K and -369 kJ/mol at 1890 K. The influence on the synthesis of seeding the starting material is discussed.

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