Elena R. Andrievskaya

Interaction of Cerium Oxide with Hafnium, Zirconium, and Yttrium Oxides at 1500°C

Using the methods of x-ray diffraction and petrographic analysis as well as electron microscopy, phase relationships in the ternary systems ZrO2 ― Y2O3 ― CeO2 and HfO2 ― Y2O3 ― CeO2 were investigated over the entire range of concentrations at 1500°C. The accuracy of the phase relationships of the binary systems HfO2 ― CeO2, ZrO2 ― CeO2, and CeO2 ― Y2O3 at this temperature was improved. Isothermal sections of the ternary phase diagrams at 1500°C were constructed. Specimens of various compositions were prepared from nitrate solutions by evaporating, drying, and heat treating at 1500°C. These systems are characterized by the formation of broad ranges of solid solutions based on C-Y2O3, F-CeO2, F-HfO2 and ZrO2, and narrow ranges based on T-ZrO2 and M-HfO2. No new phases were detected. The course of isoparametric lines in the solid solution ranges of F-HfO2 (ZrO2) leads to the assumption that yttrium and cerium ions substitute for each other, but no appreciable substitution by them for zirconium or hafnium ions occurs.

Liquidus surface in the HfO2-Y2O3-La2O3 system

The liquidus surface in the HfO2-Y2O3-La2O3 system was studied by differential thermal analysis in helium at temperatures of up to 2500°C, derivative thermal analysis in air at temperatures of up to 3000°C, x-ray diffraction, optical microscopy, and electron microscopy. The liquidus surface was found to comprise five primary crystallization fields-those of theH-Y2O3-,C-Y2O3-,F-HfO2-, andX-La2O3-based solid solutions and the pyrochlore phase La2Hf2O7. Three invariant equilibria were identified in the system studied-two peritectics and one eutectic.

Phase Equilibria during Solidification of Alloys of the Ternary System HfO2 ― Y2O3 ― La2O3

Phase equilibria in the region of alloy solidification have been studied in the system HfO2 ― Y2O3 ― La2O3. A melting-point diagram is constructed for this system. It is established that in the system HfO2 ― Y2O3 ― La2O3 there are seven monovariant three-phase and three invariant four-phase processes with participation of liquid of eutectic (1) and peritectic (2) types. The last three correspond to isothermal planes on the solidus surface at 2230, 2170, and 2040°C.

Approximating the Liquidus Surface of the ZrO2 ― Y2O3 ― La2O3 Phase Equilibrium Diagram with Reduced Polynomials

A mathematical model of the liquidus surface of the ZrO2 ― Y2O3 ― La2O3 phase equilibrium diagram was constructed by the use of reduced polynomials. It was established that the liquidus surface of the ternary system includes five primary solidification fields: solid solutions based on the cubic (C) and hexagonal (H) modifications of Y2O3, cubic modification of ZrO2 with the fluorite structure (F), high-temperature cubic modification of La2O3 (X), and ordered phase La2Zr2O7 with a pyrochlor structure (Py). Three four-phase invariant equilibria of the eutectic and peritectic types exist in the ternary system.

Liquidus Surface of the ZrO2–Y2O3–Eu2O3 Phase Diagram

The derivative thermal analysis in air up to 3000 °C, X-ray diffraction, petrography, and electron microscopy are employed to examine phase equilibria on the liquidus surface of the ZrO2–Y2O3–Eu2O3 phase diagram. It is established that the liquidus surface of the system is formed by four primary solidification fields of the phases: solid solutions based on hexagonal (N) and cubic (C) crystalline modifications of Y2O3, cubic ZrO2 modification with fluorite (F) structure, and high-temperature cubic modification (X) of Eu2O3. Two four-phase invariant incongruent equilibria are found in the ternary system.

Phase equilibria in the HfO2-ZrO2-CeO2 system at 1250°C

The 1250°C phase equilibria in the HfO2-ZrO2-CeO2 system have been studied for the first time over the entire composition triangle, and the 1250°C section of the HfO2-ZrO2-CeO2 phase diagram has been constructed using x-ray diffraction, optical microscopy, and scanning electron microscopy. No new phases have been identified in the system. We have located three regions of solid solutions based on the constituent oxides (M-HfO2, T-ZrO2, and F-CeO2), three two-phase (T + F, T + M, and F + M) regions, and one three-phase (T + F + M) region.