Abstract
In the phase diagrams studies on the Al2O3-Ln2O3 systems, liquidus temperatures of Al2O3-Eu2O3 and Al2O3-Gd2O3 systems were measured from the cooling curves of specimens by the specular reflection method with a heliostat-type solar furnace. Quenched specimens from the melt were examined by X-ray diffraction technique, petrographic microscopy and chemical analysis.
EuAlO3 and GdAlO3 of the orthorhombic perovskite structure were obtained as the single phase by heating at 1300 and 1600°C in an electric resistance furnace, and also by cooling the fused specimen in the solar furnace.
The freezing point of EuAlO3 was measured as 2047±20°C and that of GdAlO3 as 2069±20°C.
The lattice parameters of these compounds were a0=5.267Å, b0=5.281Å, c0=7.450Å, and a0=5.254Å, b0=5.301Å, c0=7.447Å, respectively.
The monoclinic phases of 2Eu2O3⋅Al2O3 and 2Gd2O3⋅Al2O3 were observed as single phases in both specimens heat treated at 1600°C and those quenched from the melt.
The freezing points and the lattice parameters of 2Eu2O3⋅Al2O3 were 1950±20°C and a0=7.533Å, b0=10.696Å, c0=11.180Å, and β=108.81°. Those of 2Gd2O3⋅Al2O3 were 1951±20°C and a0=7.514Å, b0=10.580Å, c0=11.180Å, and β=108.79°, respectively.
All of the above compounds appeared to be a stable phase, because they showed no phase transition in repeated heating and cooling cycles.
The Al2O3-Eu2O3 system shows three eutectics points at 1710°C with 24mol% Eu2O3, at 1900°C with 62mol% Eu2O3 and at 1860°C with 77mol% Eu2O3 composition, respectively.
In the Al2O3-Gd2O3 system, three eutectics points were found at 1720°C, 1920°C and 1930°C with 25, 65 and 73mol% of Gd2O3 composition, respectively.
Each of cooling curves of Al2O3, EuAlO3, GdAlO3, 2Eu2O3⋅Al2O3 and 2Gd2O3⋅Al2O3 showed a single exothermic peak which was assigned to solidification point, while Eu2O3 and Gd2O3 showed three exothermic peaks related to the solidification point as well as the solid state phase transformation below the solidus temperature.
High temperature phase diagrams for the Al2O3-Eu2O3 and Al2O3-Gd2O3 systems were presented tentatively.
