Experimentelle Konstitutionsuntersuchungen und thermodynamische Gleichgewichtsberechnungen im System Nd-Si-Al-O-N

The subsolidus phase relations were studied in the quasi-ternary oxide system Nd2O3-Al2O3-SiO2. Seven three phase equilibria were experimental estimated. The measured temperature of solidus was 1368 degrees centigrade of the system Si3N4-SiO2-AlN-Al2O3-NdN-Nd2O3. The apatite phase with the composition limits Nd8(SiO4)6 and Nd9.33(SiO4)6O2 in the Nd2O3-SiO2 system expands in the quasi-ternary system in the direction of the composition Nd10(Si0.67Al10.33O4)O2. The results show a maximum solubility of 5 per cent alumina in the apatite phase. The a- and c values increase with increasing content of Al2O3. In the region Si3N4-Al2O3-Al2O3:AlN-Al2O3-Nd2O3-SiO2 several compositions prepared in the temperature range 1150 - 1700 degrees centigrade. Fourteen polyedra with three or four phases were estimated. Further phase equilibria can be predict by combination of the estimated phase equilibria themselves or with known phase equilibria of the quasi-ternary systems. Furthermore results of thermody namic calculations are presented. For the liquid phase in the quasi-binary systems Nd2O3-Al2O3, Nd2O3-AlN, Nd2O3-SiO2 and Nd2O3-Si3N4 the Redlich-Kister model both and the "ionic two-sublattice" model were applied. The apatite phase in the quasi-ternary oxide system was described with the "compound energy" model.