S. M. Lakiza

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.

Triangulation and Liquidus Surface in the Al2O3 ― ZrO2 ― La2O3 Phase Diagram

A projection has been constructed for the liquidus surface in the phase diagram for the Al2O3 ― ZrO2 ― La2O3 system on to the plane of the concentration triangle. There are found to be two partially quasibinary sections LaAlO3 ― (98.5 mole% ZrO2 ― 1.5 mole% La2O3) and La2O3·11Al2O3 ― (98.5 mole% ZrO2 ― 1.5 mole% La2O3) together with one quasibinary LaAlO3 ― La2Zr2O7 section, which triangulate the ternary system, and the phase diagrams have been constructed for them. There are nine fields of primary crystallization for phases based on the T and F forms of ZrO2, the X, H, and A forms of La2O3, and also the phases La2Zr2O7, LaAlO3, La2O3·11Al2O3, and Al2O3, together with two quasibinary eutectics and three ternary ones. It is not found that there are any ternary phases and solid-solution regions based on the components and the binary compounds. The minimum temperature in the system is 1665°C. The interactions are basically of eutectic type.

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 diagram of the Al2O3-ZrO2-Er2O3 system. III. Solidus surface and phase equilibria in alloy crystallization

A projection has been constructed for the solidus surface in the Al2O3 - ZrO2 - Nd2O3 phase diagram on the plane of the concentration triangle, which consists of six isothermal three-phase fields corresponding to two nonvariant equilibria of eutectic type and four nonvariant ones of peritectic type, together with five lineated surfaces for the end of crystallization of the monovariant eutectics. The highest solidus temperature in the system is 2710°C, the melting point of pure ZrO2 , and the least is 1675°C, the temperature of the ternary eutectic L ⇔ β + F + NA. No ternary phases and no appreciable regions of solid solutions based on their components and the binary compounds have been observed. Data on the adjoining binary systems, liquidus and solidus surfaces allowed for construction of the phase equilibrium diagram together with a reaction scheme for the equilibrium crystallization of alloys in the Al2O3 - ZrO2 - Nd2O3 system.

Thermal Barrier Coatings: Current Status, Search, and Analysis

The principles for selecting materials to be used as thermal barrier coatings (TBCs) are presented. The advantages and disadvantages of new methods for TBC deposition are briefly described. After measurement of the thermal conductivity and thermal expansion coefficient, it is required to ascertain that such materials do not interact with the thermally grown aluminum oxide and then to determine their strength, fracture toughness, hardness, and Young’s modulus. The thermal conductivity of TBC can be reduced by increasing its porosity and suppressing its sintering. The need for and drawbacks of multilayer coatings are shown. If TBC meets all the requirements, then TBC corrosion resistance to Na2SO4, V2O5, P2O5, sand, and volcanic ash in operation and ways to protect TBC against damage need to be determined. The prospects and areas for development of these techniques are outlined.

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.

Solidus Surface and Phase Equilibria in Al2O3 ― ZrO2 ― La2O3 Alloy Crystallization

The projection of the solidus surface in the phase diagram for the Al2O3 ― ZrO2 ― La2O3 system on the plane of a concentration triangle has been constructed, which consists of seven isothermal three-phase fields corresponding to three nonvariant equilibria of eutectic type and four nonvariant equilibria of peritectic type, as well as four lineated surfaces for the end of crystallization in the binary eutectics. The highest temperature on the solidus surface is 2710°C, and the lowest is 1665°C. No ternary phases and appreciable areas of solid solution are observed. Data on the bounding binary systems, liquidus and solidus surfaces have been used to construct the phase-equilibrium diagram together with a scheme for the reactions in equilibrium crystallization in the Al2O3 ― ZrO2 ― La2O3 system.