A Study on phase equilibria in the CaO-Al2O3-SiO2-"NB2O5" (5 mass pct) system in reducing atmosphere

Abstract

Phase equilibria in 5 mass% "NB2O5" plane of CaO-Al2O3-SiO2-" NB2O5" system at 1873K in an oxygen partial pressure of 1.78×10-6 Pa have been investigated through isothermal equilibration and quenching followed by EPMA examinations. In order to characterize the effect of niobium oxide on the phase relationship of the CaO-Al2O3-SiO2 system, NB2O5-containing and NB2O5 -free samples with the same CaO-Al2O3-SiO2 weight ratio were investigated simultaneously. The ratios of CaO/Al2O3/SiO2 were selected from the CaO.2Al2O3-liquid two-phase equilibrium region in the CaO-Al2O3-SiO2 system at 1873 K. It was found that the adding of 5 mass% NB2O5 to the CaO-Al2O3-SiO2 system caused the original CaO.2Al2O3-liquid equilibrium to become three different new equilibria. The three equilibria were single liquid phase, CaO.6Al2O3-liquid and gehlenite-CaO.2Al2O3-liquid equilibrium respectively. The gehlenite phase may be a new solid solution of 2CaO.Al2O3.SiO2 and NbOx with melting point higher than 1873 K.