Phase Equilibrium of 3CoO⋅Al<sub>2</sub>O<sub>3</sub>⋅3SiO<sub>2</sub>-3Y<sub>2</sub>O<sub>3</sub>⋅5Al<sub>2</sub>O<sub>3</sub> System

Abstract

The phase equilibrium of 3CoO⋅Al2O3⋅3SiO2-3Y2O3⋅5Al2O3 system was determined by using the quenching technique and the heating and cooling method. A silicon carbide furnace, a platinum-rhodium strip furnace and a solar furnace were used to heat the specimens. The system 3CoO⋅Al2O3⋅3SiO2-3Y2O3⋅5Al2O3 is actually a quasi-two component system under the atomospheric pressure, because cobalt garnet is only stable under high pressure [T. Noda and M. Ushio, this journal, 75 125 (1967)]. Mixtures of SiO2, Al2O3, Y2O3 and CoO or premelted glass specimens having the compositions of x(3CoO⋅Al2O3⋅3SiO2)+[(1-x)/2] (3Y2O3⋅5Al2O3), where x was the weight fraction, were kept at temperatures ranging from 1100° to 1900°C for 1-60 hr under the atmospheric pressure and then quenched. The quenched specimens were examined by the X-ray diffraction method and also under a polarizing microscope. Carnet was the only crystalline phase in the quenched specimens of x=0.00-0.30. The lattice constant of the garnets decreased continuously with x, indicating that the continuous solid solution of garnet was formed.Carnet was formed coexisted with spinel, Y2O3-SiO2 mineral and cristobalite in the specimens of x=0.30-0.41 quenched from temperatures below 1250°C. Garnet and spinel were found in the specimens quenched from temperatures between 1300° and 1500°C. Above 1520°C garnet was the only stable crystalline phase.In the quenched specimens of x=0.41-0.46, spinel was found to be the only crystalline phase above 1520°C, garnet separated below 1520°C and cristobalite separated below 1300°C. Spinel, garnet and cristobalite coexisted with Y2O3-SiO2 mineral below 1250°C.In the specimens of x=0.49-0.63 quenched from temperatures below 1270°C spinel, cristobalite, Y2O3-SiO2 mineral and a small amount of unidentified mineral were obtained, but no garnet was found.In the specimens of x=0.72-0.99 quenched at temperatures below 1270°C, spinel, cristobalite, olivine and a small amount of unidentified mineral were observed. It seemed that spinel, cristobalite, olivine and Y2O3-SiO2 mineral were stable below 1270°C. in the composition range of x=0.49-0.99. Above. 1300°C, spinel was the only crystalline phase, and between 1300°C and 1270°C, spinel and cristobalite coexisted.In the case of x=1.0, i, e, cobalt garnet composition, spinel, olivine and cristobalite were found to be stable in specimens quenched at temperatures below 1270°C. Above 1270°C olivine disappeared and cristobalite disappeared above 1300°C. Spinel melted at 1424±4°C.The liquidus temperature increased with the decrease in the fraction of cobalt garnet composition, passing a maximum at 1600°C around x=0.50-0.48, also minimum at 1500°C around x=0.41-0.42, then increased to 1913±15°C of yttrium aluminum garnet.