Preparation and thermophysical properties of Sm2YbTaO7 and Sm2YTaO7

Sm₂YbTaO₇ and Sm₂YTaO₇ ceramics were synthesized by solid reaction method at 1600 °C for 10 h. Crystal phases have been identified by X-ray diffraction, and their thermal conductivities and thermal expansion coefficients were measured using a laser flash method and the pushing-rod technology, respectively. Results indicate that Sm₂YbTaO7 and Sm₂YTaO₇ exhibit a typical defect fluorite-type crystal structure. Compared to Sm₂YTaO₇, Sm₂YbTaO₇ has lower thermal conductivity due to the higher atomic weight difference between the substituted and substituting atoms. The thermal expansion coefficient of Sm₂YbTaO₇ is greater than that of Sm₂YTaO₇ due to its elongated average interionic distance. Their thermal conductivities are much lower than that of YSZ, and their thermal expansion coefficients are very close to that of YSZ. The synthesized ceramics also exhibit excellent phase stability in the temperature range from ambient to 1200 °C.     

Optimizing the structure and properties of Y2O3 stabilized zirconia: An atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) based comparative study

The yttria stabilized zirconia (8%YSZ) is widely used to insulate the metallic components of the engine from high temperature and improve the operating temperature of gas turbine engines. With different processing parameters, 8YSZ coatings are prepared by atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) techniques and the microstructural features and thermodynamics properties are compared. The electron back scattered diffraction (EBSD) analysis indicate that the substitutional point defects (Zr_0.86Y_0.14O_1.93) in the 8YSZ APS coatings are considerably higher than the corresponding SPPS coatings. The replacement of Zr⁴⁺ by Y³⁺ disturbs the charge neutrality of the system which might be compensated by the creation of oxygen vacancy. Both the substitutional point defects and the oxygen vacancies are the sources of phonon scattering, modifying the thermal conductivity of the coating. Pores and cracks are qualitatively and quantitatively analyzed in the microstructure of 8YSZ coatings. Strain tolerant and high thermal cycling life coatings are prepared by SPPS due to the existence of vertical cracks in the microstructure. Comparing the thermal insulation properties of the coatings, the APS coating provided lower thermal conductivities relative to the SPPS coatings which might be due to the high concentration of point defects and low concentration of the mixed oxide phase.     

Thermal properties of La2Zr2O7 double-layer thermal barrier coatings

La₂Zr₂O₇ is a promising thermal barrier coating (TBC) material. In this work, La₂Zr₂O₇ and 8YSZ-layered TBC systems were fabricated. Thermal properties such as thermal conductivity and coefficient of thermal expansion were investigated. Furnace heat treatment and jet engine thermal shock (JETS) tests were also conducted. The thermal conductivities of porous La₂Zr₂O₇ single-layer coatings are 0.50–0.66?W?m^?1?°C^?1 at the temperature range from 100 to 900°C, which are 30–40% lower than the 8YSZ coatings. The coefficients of thermal expansion of La₂Zr₂O₇ coatings are about 9–10?×?10^?6?°C^?1 at the temperature range from 200 to 1200°C, which are close to those of 8YSZ at low temperature range and about 10% lower than 8YSZ at high temperature range. Double-layer porous 8YSZ plus La₂Zr₂O₇ coatings show a better performance in thermal cycling experiments. It is likely because porous 8YSZ serves as a buffer layer to release stress.     

Mechanical properties,oxygen barrier property,and chemical stability of RE3NbO7 for thermal barrier coating

Rare earth niobate (RE₃NbO₇, RE = Dy, Y, Er, Yb) ceramics have shown extremely low thermal conductivity but remain questionable in high temperature thermal barrier coating (TBC) applications with high thermal, mechanical, and chemical loads. Herein, we comprehensively characterize the properties of rare earth niobates, including mechanical properties, oxygen barrier properties, chemical stability, etc. It is found that the oxygen conductivities of the rare earth niobates are three orders of magnitude lower than 7wt.% yttria-stabilized zirconia (YSZ), indicating a remarkable oxygen barrier property to avoid oxidation of underlying metallic components. The corrosion resistance of rare earth niobate against calcium-magnesium-aluminum silicate (CMAS) is also significantly better than that of YSZ. Together with the extremely low thermal conductivity, the rare earth niobates exhibit a combination of excellent high temperature properties, which may become a promising candidate material of high temperature TBC of next generation gas turbines.     

Low thermal conductivity of atomic layer deposition yttria-stabilized zirconia (YSZ) thin films for thermal insulation applications

Thermal insulation applications have long required materials with low thermal conductivity, and one example is yttria (Y₂O₃)-stabilized zirconia (ZrO₂) (YSZ) as thermal barrier coatings used in gas turbine engines. Although porosity has been a route to the low thermal conductivity of YSZ coatings, nonporous and conformal coating of YSZ thin films with low thermal conductivity may find a great impact on various thermal insulation applications in nanostructured materials and nanoscale devices. Here, we report on measurements of the thermal conductivity of atomic layer deposition-grown, nonporous YSZ thin films of thickness down to 35 nm using time-domain thermoreflectance. We find that the measured thermal conductivities are 1.35–1.5 W m⁻¹ K⁻¹ and do not strongly vary with film thickness. Without any reduction in thermal conductivity associated with porosity, the conductivities we report approach the minimum, amorphous limit, 1.25 W m⁻¹ K⁻¹, predicted by the minimum thermal conductivity model.     

Structure and thermal properties of Al2O3-doped Gd3TaO7 as potential thermal barrier coating

In this paper, a series of solid solutions ceramics of (Al_xGd_1-x)₃TaO₇ (x = 0, 0.01, 0.03, 0.05) were synthesized via solid-state reaction. X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the crystal structure of (Al_xGd_1-x)₃TaO₇ ceramics is weberite in spite of the content of Al³⁺ is up to 5 mol.%. The thermal conductivities of (Al_xGd_1-x)₃TaO₇ ceramics range from 1.37 W?m^?1 K^?1 to 1.47 W?m^?1 K^?1 at 900 ℃, which is much lower than that of 7–8 YSZ (about 2.5 W?m^?1 K^?1). The thermal expansion coefficients (TECs) of (Al_xGd_1-x)₃TaO₇ ceramics vary in the range of 6–10 × 10^-6 K^-1 within the temperature range 100–1200 ℃, and the values are close to the TECs of 7–8 YSZ. Given the low thermal conductivity and high thermal expansion coefficients of (Al_xGd_1-x)₃TaO₇ ceramics, they have the potential to be the next generational thermal barrier coating materials.     

Effect of point defects on the thermal conductivity of Sc2O3-Y2O3 co-stabilized tetragonal ZrO2 ceramic materials

In this paper, the reduction mechanism in thermal conductivity of a series of Sc₂O₃-Y₂O₃ co-stabilized tetragonal ZrO₂ ceramics is systematically discussed. The thermal conductivity is approximately 20–28% lower than that of 6–8 wt.% yttria-stabilized zirconia (YSZ). A phonon scattering model, on account of the influence of oxygen vacancy variation and cation mass fluctuation, is optimized and utilized to depict the thermal conductivity of these materials. For the samples with the same amount of oxygen vacancy, Sc³⁺ is more effective in lowering thermal conductivity than Y³⁺ due to the large mass difference with Zr⁴⁺, as evidenced by the scattering model and phonon vibrational density of states. The experimental and calculation results suggest that this optimized model is proved to be more effective in predicting the thermal conductivity of binary or multiple rare earth oxides co-doped tetragonal ZrO₂ and guiding the compositional design of thermal barrier materials.     

The corrosion behaviors of thermal barrier material of M-YTaO4 attacked by CMAS at 1250 °C

The corrosion of YSZ TBCs attacked by calcium–magnesium–aluminosilicate (CMAS) is a serious problem. Yttrium tantalite (YTaO₄), a new kind of potential thermal barrier ceramic material, was expected to replace the YSZ to manufacture the TBCs because of its great thermophysical characteristics. In this study, porous YTaO₄ ceramic pellets, instead of actual TBCs, were used to investigate the CMAS corrosion resistance at 1250 °C. Results indicated that CMAS couldn't cover the whole surface of YTaO₄ pellets homogeneously because of low wettability between liquid CMAS and YTaO₄, in addition, there was almost no reaction layer after 4 h reaction. The XRD results showed that M-YTaO₄, M′-YTaO₄, Ca₂Ta₂O₇ and Y₂Si₂O₇ were the main four phases after reaction and there was no phase containing the elements of Mg and Al. Compared with YSZ TBCs, this new kind of potential thermal barrier ceramic material showed well resistance to CMAS corrosion.     

Lanthanum-titanium-aluminum oxide: A novel thermal barrier coating material for applications at 1300 °C

Considerable efforts are being invested to explore new thermal barrier coating (TBC) materials with higher temperature capability to meet the demand of advanced turbine engines. In this work, LaTi₂Al₉O₁₉ (LTA) is proposed and investigated as a novel TBC material for application at 1300 °C. LTA showed excellent phase stability up to 1600 °C. The thermal conductivities for LTA coating are in a range of 1.0-1.3 W m⁻¹ K⁻¹ (300-1500 °C) and the values of thermal expansion coefficients increase from 8.0 to 11.2 × 10⁻⁶ K⁻¹ (200-1400 °C), which are comparable to those of yttria stabilized zirconia (YSZ). The microhardness of LTA and YSZ coatings were in the similar level of ∼7 GPa, however, the fracture toughness value was relatively lower than that of YSZ. The lower fracture toughness was compensated by the double-ceramic LTA/YSZ layer design, and the LTA/YSZ TBC exhibited desirable thermal cycling life of nearly 700 h at 1300 °C.     

Y3Ce7Ta2O23.5 and Yb3Ce7Ta2O23.5—Two kinds of novel ceramics for thermal barrier coatings

For the development of ceramic candidates for thermal barrier coatings, two kinds of new ceramics, Y₃Ce₇Ta₂O_23.5 and Yb₃Ce₇Ta₂O_23.5, were synthesized by sintering at 1873?K for 10?h. The obtained samples were composed of a single fluorite-type phase, and their relative densities are greater than 90%. Because of phonon scattering caused by the complex lattice, the large number of oxygen vacancies, and substituted atoms, the thermal conductivity is lower than that of 8YSZ. The coefficients of thermal expansion (CTEs) of these two products are located in the range of 10.22–12.57?×?10^?6/K and 9.62–12.66?×?10^?6/K, respectively, from 323?K to 1473?K, and they also exhibit excellent phase stability up to 1473?K. However, their thermal conductivities and CTEs are lower than those of RE₂Ce₂O₇ (RE?=?La, Nd, or Sm).     

Determinations of Ce solid solution mechanism and limit thermal conductivity of YTaO4 ceramics

x mol% CeO₂-YTaO₄ (x = 0, 3, 6, 9, 12) ceramics have been synthesized by the spark plasma sintering (SPS) technique. We focus on the changes in lattice distortion, bonding length, thermal conductivity, thermal expansion, and phase stability of the prepared samples. XRD, Raman, and XPS are used to determine the chemical valence and solid solution mechanism of Ce in the lattice of YTaO₄, while its effects on thermal/mechanical properties are elucidated from microstructures. Y³⁺ is substituted via Ce³⁺, and all samples maintain a monoclinic phase. The limit thermal conductivity (1.2 W?m^?1?K^?1, 900 °C) is realized in 9 mol% CeO₂-YTaO₄, and the thermal expansion coefficients are increased to 10.2 × 10^?6 K^?1 at 1200 °C. Furthermore, the exceptional phase stability and mechanical properties of all samples indicate that they can provide good thermal insulation at high temperatures, and have higher working temperatures than the current YSZ thermal barrier coatings.     

Thermal cycling performance of La2Ce2O7/50 vol.% YSZ composite thermal barrier coating with CMAS corrosion

La₂Ce₂O₇ (LC) is receiving increasing attention due to its lower thermal conductivity, better phase stability and higher sintering resistance than yttria partially stabilized zirconia (YSZ). However, the low fracture toughness and the sudden drop of CTE at approximately 350?°C greatly limit its application. In this study, the LC/50?vol.% YSZ composite TBC was deposited by supersonic atmospheric plasma spraying (SAPS). Compared to YSZ or double layered LC/YSZ coating, the thermal cycling life of LC/50?vol.% YSZ coating with CMAS attack increased by 93% or 91%. The latter possessed higher fracture toughness (1.48?±?0.26?MPa?m^1/2) than LC (0.72?±?0.15?MPa?m^1/2) and better CMAS corrosion resistance than YSZ owing to the formation of Ca₂(La_xCe_1-x)₈(SiO₄)₆O_6–4x with <001> orientation perpendicular to the coating surface. The sudden CTE decrease of LC was fully suppressed in LC/50?vol.% YSZ coating due to the change of temperature dependent residual stresses induced by YSZ.     

Mechanical properties and thermal conductivities of 3YSZ-toughened fully stabilized HfO2 ceramics

In this work, the 3 mol% yttria stabilized zirconia (3YSZ) composed of tetragonal phase has been introduced into the 10 mol% Er₂O₃ stabilized cubic hafnia (10ErSH) matrix to improve its fracture toughness. The effects of the addition of 3YSZ on the phase composition, microstructure, mechanical properties and thermal conductivities of the 10ErSH have been investigated. The results showed that all the 3YSZ-toughened 10ErSH samples were composed of cubic phase and a little (<10 mol%) monoclinic phase. The introduced tetragonal phase of 3YSZ fully disappeared even when the volume fraction of 3YSZ reached 50%, indicating that the phase transformation occurred during 1500 °C. The fracture toughness for the sample with 50% 3YSZ was improved by 60% compared with the pure 10ErSH ceramics owing to the sub-mico/micro hybrid structure, which changed the crack propagation mode and consumed part of the crack extension energy. Additionally, the thermal conductivity slightly decreased due to the mass and radius misfits induced by substitution atoms (Zr⁴⁺, Er³⁺ and Y³⁺). Considering the improved mechanical and thermal properties, the 3YSZ-toughened 10ErSH ceramics may be alternative TBC materials.     

Low–thermal–conductivity and high–toughness CeO2–Gd2O3 co–stabilized zirconia ceramic for potential thermal barrier coating applications

Yttria partially stabilized zirconia (～4.0?mol% Y₂O₃–ZrO₂, 4YSZ) has been widely employed as thermal barrier coatings (TBCs) to protect the high–temperature components of gas–turbine engines. The phase stability problem existing in the conventional 4YSZ has limited it to application below 1200?°C. Here we report an excellent zirconia system co–doped with 16?mol% CeO₂ and 4?mol% Gd₂O₃ (16Ce–4Gd) presenting nontransformable feature up to 1500?°C, in which no detrimental monoclinic (m) ZrO₂ phase formed on partitioning. It also exhibits a high fracture toughness of ～46?J m^?2 and shows high sintering resistance. Besides, the thermal conductivity and thermal expansion coefficient of 16Ce–4Gd are more competent for TBCs applications as compared to the 4YSZ. The combination of properties suggests that the 16Ce–4Gd system could be of potential use as a thermal barrier coating at 1500?°C.     

Influence of Gd2O3 addition on thermophysical properties of La2Ce2O7 ceramics for thermal barrier coatings

In order to develop new candidate ceramic materials for thermal barrier coatings (La_1?xGd_x)₂Ce₂O₇ ceramics were prepared by pressureless-sintering at 1600 °C for 10 h in air. The phase structure, micro-morphology and thermophysical properties of (La_1?xGd_x)₂Ce₂O₇ ceramics were investigated, respectively. XRD results revealed that pure (La_1?xGd_x)₂Ce₂O₇ ceramics with defect fluorite structure were synthesized and SEM showed that their microstructures were dense and no other phases existed among the particles. With the increasing temperature, their thermal expansion coefficients increased, while the thermal conductivities decreased. The thermophysical results indicated that thermal expansion coefficients of these ceramics were higher than that of 8YSZ, and their thermal conductivities were much lower than that of 8YSZ. The lower thermal conductivities of these ceramics were mainly attributed to more oxygen vacancies and substitution atoms. These results imply that the (La_1?xGd_x)₂Ce₂O₇ ceramics can be explored as candidate materials for the ceramic layer in TBC system.     

Thermal conductivity and expansion coefficient of Ln2LaTaO7 (Ln=Er and Yb) oxides for thermal barrier coating applications

Two kinds of novel Ln₂LaTaO₇ (Ln=Er and Yb) ceramics were prepared via high-temperature solid reaction method. The phase composition, micro-morphology and thermophysical properties were investigated. Results indicate that pure Ln₂LaTaO₇ ceramics with single fluorite-type structure are synthesized successfully. The thermal conductivities of Er₂LaTaO₇ and Yb₂LaTaO₇ are in the range of 1.22–1.43 W/m K and 1.17–1.51 W/m K, respectively, which are much lower than that of YSZ. The lower thermal conductivity can be attributed to the phonon scattering caused by oxygen vacancies and the substituting atoms. The average thermal expansion coefficients of Yb₂LaTaO₇ and Er₂LaTaO₇ are 9.94×10⁻⁶/K and 9.63×10⁻⁶/K, respectively. As compared with Yb₂LaTaO₇, the higher thermal expansion coefficient of Er₂LaTaO₇ can be ascribed to its lower ionic-bond strength between cations at sites A and B.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

The mechanical and thermal properties,CMAS corrosion resistance,and the wettability of novel thermal barrier material GdTaO4

Because the CMAS corrosion and phase transformation at elevated temperatures above 1250 °C have limited the applications of traditional YSZ, the design of novel thermal barrier materials is a hotspot. GdTaO₄ is considered as a type of potential novel thermal barrier material owing to its low thermal conductivity. In this study, the mechanical and thermal properties, CMAS corrosion resistance, and the wettability of the GdTaO₄ were studied and compared with that of YSZ. The results show that the coefficient of thermal expansion and hardness of GdTaO₄ are 14.1 × 10⁻⁶ K⁻¹ (1350 °C) and 534.2 Hv_0.3 respectively. The thickness of CMAS reaction layer of GdTaO₄ is ~30.8 μm after 24 h reaction at 1350 °C, which is thinner than that of YSZ. After corrosion reaction, the CMAS glass aggregated instead of completely disappearing or continuously extending over the surface of GdTaO₄. The main reaction product is Ca₂Ta₂O₇, and the anorthite phase may not be detected, which is similar to YTaO₄. By comparison, the dense substrate of YSZ became porous and CMAS glass has disappeared after 10 h. CMAS corrosion at 1350 °C. The on-line contact angle results show that the wettability of CMAS on GdTaO₄ is worse than that on YSZ at 1350 °C, while the opposite of the work of adhesion, which indicates that GdTaO₄ can remove liquid CMAS more easily than YSZ TBCs during the service. Furthermore, the corrosion depth and areas of GdTaO₄ are smaller than those of YSZ in the same situation. These findings suggest that GdTaO₄ possesses better high-temperature properties and CMAS corrosion resistance than YSZ as a kind of potential of thermal barrier material.     

Theoretical investigation of mechanical and thermal properties of MPO4 (M = Al,Ga)

Minimum lattice thermal conductivities and mechanical properties of polymorphous MPO₄ (M = Al, Ga) are investigated by first principles calculations. The theoretical minimum thermal conductivities are found to be 1.02 W (m K)^?1 for α-AlPO₄, 1.20 W (m K)^?1 for β-AlPO₄, 0.87 W (m K)^?1 for α-GaPO₄ and 0.88 W (m K)^?1 for β-GaPO₄. The lower thermal conductivities in comparison to YSZ can be attributed to the lattice phonon scattering due to the framework of heterogeneous bonds. In addition, the low shear-to-bulk modulus ratio for both β-AlPO₄ (0.38) and β-GaPO₄ (0.30) is observed. Our results suggest their applications as light-weight thermal insulator and damage-tolerant/machinable ceramics.     

Oxygen-vacancy-mediated microstructure and thermophysical properties in Zr3Ln4O12 for high-temperature applications

Yttria-stabilized zirconia (YSZ) has been considered as state-of-the-art material for high-temperature thermal barrier coatings, which provide thermal insulation to the superalloy components in gas turbines and jet engines. Oxygen vacancies induced by yttria substitutions are believed to be mainly responsible for the low thermal conductivity of YSZ due to their phonon scattering effect. However, high mobility of oxygen vacancies in YSZ leads to a rapid oxygen diffusion at high temperatures, therefore accelerates the failure of coatings by grain coarsening, sintering, and simultaneous oxidation of the underlying metallic bondcoat. In the present research, we further explored in the ZrO₂–Ln₂O₃ binary phase diagram and synthesized a series of ceramic materials with the chemical formula of Zr₃Ln₄O₁₂ (Ln = La, Gd, Y, Er, and Yb), in which more oxygen vacancies were involved and extremely low phonon thermal conductivities (1.3-1.6 W/m·K) were obtained, even approaching to the theoretical minimum. In addition, the mobility of these oxygen vacancies was remarkably suppressed by the lattice ordering with the decrease of Ln³⁺ radius, as confirmed by X-ray diffraction, Raman and transmission electron microscopy. Thus, the oxygen barrier property and sintering resistance were significantly enhanced accordingly, which makes Zr₃Ln₄O₁₂ compounds promising thermal barrier coating materials for next generation gas turbines and jet engines.