Ya. S. Tishchenko

Al2O3–HfO2–Y2O3 phase diagram. I. Isothermal sections at 1250 and 1650°C

The isothermal sections of the Al2O3–HfO2–Gd2O3 phase diagram at 1250 and 1650°C are constructed for the first time and phase equilibria at these temperatures are established. No ternary compounds or appreciable solid solution regions based on components or binary compounds are found in the ternary system. Interaction in the system is determined by the most thermodynamically stable compound, HfO2, which equilibrates with all phases in the system. In the region with Gd2O3 content up to ~65%, the sections are similar, only width of the regions changes. This is connected with changes in the extension of M and F solid solutions in the HfO2–Gd2O3 binary bounding system. The presence of AL + F, GA + GH2, and G2A + F two-phase regions at the isothermal sections suggests that there are triangulating sections of the Al2O3–HfO2–Gd2O3 system in them. Since the F and GH2 phases are of variable composition, these sections can be qualified as conditionally quasibinary. In wide three-phase regions, like in the Al2O3–ZrO2–Gd2O3 system, ternary eutectic points are expected to exist.

Calorimetric Study of the La2Hf2O7 Heat Capacity in the Range 57–302 K

The heat capacity of La2Hf2O7 has been studied in the range 57–302 K by adiabatic calorimetry. The heat capacity Cp of lanthanum hafnate changes monotonically and there are no anomalies. The values of heat capacity, entropy, enthalpy, and reduced Gibbs energy have been determined in standard conditions: Cp°(298.15 K) = 229.39 ± 0.92 J · mol–1 · K–1, S° (298.15 K) = 246.9 ± 2 J × × mol–1 ∙ K–1, Ф° (298.15 K) = 114.76 ± 1.72 J ∙ mol–1 ∙ K–1, and H° (298.15 K)–H° (0 K) = 39403 ± 197 J ∙ mol–1. In the series of isostructural La2Zr2O7 → La2Hf2O7 compounds, atomic oscillation frequency in the lattice decreases and low-temperature heat capacity increases with greater mass of oscillator atoms from Zr to Hf.

Isothermal sections of the Al2O3–HfO2–Er2O3 phase diagram at 1250 and 1600°C

The isothermal sections at 1250 and 1600°C for the Al2O3–HfO2–Er2O3 phase diagram are constructed for the first time. Phase equilibria are established at these temperatures; they are determined by the most thermodynamically stable compound, HfO2. No ternary compounds or appreciable solid-solution regions based on components or binary compounds are found in the ternary system. The presence of AL + F, Er3A5 + F, ErA + F, and Er2A + F two-phase regions on the isothermal section at 1650°C suggests that they contain triangulating sections of the Al2O3–HfO2–Er2O3 ternary system.

Enthalpy of SmAlO3 in the range 472–2252 K

The enthalpy increment of SmAlO3 is measured for the first time using high-temperature drop calorimetry in the temperature range 472–2252 K. The enthalpy values are used to determine other thermodynamic functions, such as heat capacity, entropy, and reduced Gibbs energy. Phase transitions from orthorhombic to trigonal structure and from trigonal to cubic structure are observed at 1055 and 2103 K, respectively. The phase transition at 1055 K is studied by differential thermal analysis. The enthalpies and entropies of phase transitions are determined.

Phase Diagrams of Refractory Oxide Systems and Microstructural Design of Materials

It is shown that the phase diagrams of refractory oxide systems based on ZrO2, HfO2, Al2O3, and rare earth oxides underlie the microstructural design of various high-performance materials. Process steps to produce coarse-grained ceramics in the HfO2–ZrO2–Y2O3, ZrO2–Y2O3–Sc2O3, HfO2–ZrO2–Sc2O3, Y2O3–Er2O3, Y2O3–ZrO2, Y2O3–HfO2, Y2O3–Al2O3, Y2O3–SiO2, and Y2O3–La2O3 systems to perform at temperatures up to 2200°C are designed. Process steps to produce high-performance fine-grained composites in the HfO2–ZrO2–Y2O3 (Ln2O3) (Ln–Dy, Ho, Er, Tm, Yb), ZrO2–Y2O3–Sc2O3, ZrO2–Y2O3–Sc2O3, Al2O3–Zr(Hf)O2–Ln(Y)2O3 (Ln–La, Nd, Sm, Gd, Er, Yb), and ZrO2–Y2O3–CeO2–Al2O3 systems are designed as well.

The Al2O3–Zr(Hf)O2–La2O3 Phase Diagrams as a Scientific Basis for Developing New Thermal Barrier Coatings

Comparison of the isothermal sections in similar Al2O3–ZrO2–La2O3 and Al2O3–HfO2–La2O3 systems shows that equilibria between the α-Al2O3, Hf(Zr)O2, La2Zr2O7, La2Hf2O7, and LaAlO3 phases in these systems differ fundamentally. While the ZrO2−LaAlO3 equilibrium is observed in the system with ZrO2, the alternative La2Hf2O7–Al2O3 equilibrium takes place in the system with HfO2. The triangulation in the system with HfO2 changes. This phenomenon is probably due to higher thermodynamic stability of the La2Hf2O7 phase (melting point 2420°C) compared to the similar La2Zr2O7 phase (melting point 2280°C). Other equilibria in both systems are of the same type. Lanthanum hafnate can be directly deposited onto the binding coating without reacting with α-Al2O3 or destroying the thermal barrier coating. Stable performance of the lanthanum hafnate coating is expected.

Solidus and liquidus surfaces of the Al2O3–HfO2–Er2O3 phase diagram

Phase equilibria during solidification of alloys in the Al2O3–HfO2–Er2O3 system are studied, and liquidus and solidus surfaces of the Al2O3–HfO2–Er2O3 phase diagram are constructed for the first time. It is established that interaction in the system is eutectic. No ternary compounds or appreciable regions of solid solutions based on components or binary compounds are found in the ternary system.

High-Temperature Enthalpy of La 2 Hf 2 O 7 in the Temperature Range 490–2120 K

Lanthanum hafnate La2Hf2O7 was produced chemically by inverse precipitation from ammonia solution and a mixture of La and Hf nitrates, followed by hydroxide decomposition at 1250°C in air and melting of the oxide mixture in a solar furnace. The formation of La2Hf2O7 was ascertained by X-ray diffraction. The La2Hf2O7 enthalpy increment was measured in the range 490–2120 K (for the first time in the temperature ranges 490–988 K and 1740–2120 K) by drop calorimetry using a Setaram HT-1500 high-temperature differential calorimeter and a high-temperature calorimetric device. A fitted equation for the enthalpy increment was used to calculate the main thermodynamic functions (heat capacity, entropy, and Gibbs energy) in the temperature range 298–2120 K. The experimental results are compared with the published data and those assessed using the Neumann–Kopp rule.

Isothermal Section of the Al 2 O 3 –TiO 2 –Er 2 O 3 Phase Diagram at 1400°C

The nature of phase equilibria in the Al2O3–TiO2–Er2O3 system at 1400°C is established and shown in the isothermal section of the phase diagram at this temperature. The interaction in the system is determined by compound Er2Ti2O7, which participates in equilibria with most phases of the system and determines its triangulation. No new phases or appreciable solubility regions based on components and binary compounds are found in the system. Ternary eutectic points are expected in the three-phase regions and binary eutectic points in the quasibinary sections.

Isothermal Sections of the Al2O3–TiO2–Y2O3 Phase Diagram at 1550 and 1400°C

The isothermal sections of the Al2O3–TiO2–Y2O3 phase diagram are constructed for the first time at 1550 and 1400°C. New phases or appreciable homogeneity ranges based on components and binary compounds are not found in the ternary system. Triangulation of the system is determined by the Y2T2O7 phase, which is in equilibrium with compounds Al2TiO5, Y3Al5O12, YAlO3, and Y4Al2O9 and components TiO2 and Al2O3. The system is triangulated into six secondary triangles, in which three-phase eutectics are expected to exist. In five quasibinary sections, two-phase eutectics are likely to exist.