Rare Earth Zirconates Research Articles

Study on intrinsic mechanical behavior and erosion resistance of multi-rare earth doped La2Zr2O7 coating

Lanthanum zirconate coatings (La2Zr2O7, LZ) have a high Poisson’s ratio and low fracture toughness, making it indescribable for long service life at high temperatures. Rare earth doped lanthanum zirconate (RE-LZ) is an effective way to improve its mechanical properties. In this study, the lanthanum zirconate ceramic coating modified by binary rare earth ((La1/3Gd1/3Y1/3)2Zr2O7, LGYZ; and (La1/3Sm1/3Yb1/3)2Zr2O7, LGYZ) and multi-principal rare earth modified ((La1/5Gd1/5Y1/5Sm1/5Yb1/5)2Zr2O7, HEC-LZ) was studied, aiming to improve the intrinsic mechanical properties of lanthanum zirconate ceramic coating. By comparing the mechanical properties such as fracture toughness, bonding strength, microhardness dissipation parameter, and solid erosion properties of various types of coatings, the results show that compared with binary rare earth modified coatings such as LGYZ and LSYZ, the elastic modulus of the HEC-LZ ceramic coating is reduced by 4.7–6.3%, the microhardness dissipation parameter (MDP) is increased by 4.8–10.2%, and the fracture toughness is increased by 5.2–7.4%. The HEC-LZ ceramic coating showed favorable toughness and crack resistance. Regarding the erosion resistance of the coatings, the ability of RE-LZ coatings is significantly better than that of LZ coatings, and the erosion rate at 45° and 90° is reduced by 44.9–66.0% and 45.7–62.9%, respectively.

Highly anti-sintering and toughened pyrochlore (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 high-entropy ceramic for advanced thermal barrier coatings

Single-phase high-entropy rare-earth zirconate (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 with excellent high-temperature phase stability at 1600 °C was designed and synthesized. After being heated at 1600 °C for 1 ∼ 50 h, the average grain size of the sample increases only from 0.74 μm to 2.22 μm, indicating a high anti-sintering ability. The fracture toughness is 2.07 ± 0.03 MPa m1/2 (25 °C), which is 50% higher than lanthanum zirconate. The local lattice distortions induced by the atomic interactions and atomic-size mismatch are considered to be responsible for the strong anti-sintering ability and improved fracture toughness. In addition, the thermal expansion coefficient and thermal conductivity of (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 are 10.59 × 10-6 K-1 (25 ∼ 1500 °C) and 1.14 ± 0.09 W·m-1·K-1 (1500 °C), respectively. The outstanding combination of anti-sintering ability, mechanical behavior, and thermophysical properties of (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 highlights its great potential for next-generation thermal barrier coating applications.

Mechanical, thermal and CMAS resistance properties of high-entropy (Gd0.2Y0.2Er0.2Tm0.2Yb0.2)2Zr2O7 ceramics

A high entropy zirconate ceramic (Gd0.2Y0.2Er0.2Tm0.2Yb0.2)2Zr2O7 (HEZ) with fluorite structure has been synthesized by solid state reaction and high temperature sintering successfully. The investigation indicated that HEZ has ultra-low thermal conductivity (0.82 W × m−1 × K−1 at 1200 °C), similar thermal expansion coefficient (10.61 × 10−6 K−1 at 1300–1400 °C) with that of YSZ, excellent mechanical properties (hardness, elastic modulus, fracture toughness), and great CMAS corrosion resistance. Emphasis is given to discussing the CMAS wetting, corrosion process and failure mechanisms using high-temperature in-site wetting method and transmission electron microscopy (TEM). The results show that the HEZ exhibits better corrosion resistance than single rare earth zirconate Gd2Zr2O7 due to the coupling effect of anti-wettability, gradient diffusion rate and multicomponent apatite crystallization produced by doping rare earth ion (Gd3+, Y3+) close to Ca2+ radius and rare earth ion (Er3+, Tm3+, Yb3+) with small ion radii. In the comprehensive view, (Gd0.2Y0.2Er0.2Tm0.2Yb0.2)2Zr2O7 is a competitive candidate for high reliability TBC materials.

Thermal expansion coefficient of nonstoichiometric gadolinium zirconate: First-principles calculations and experimental study

A high thermal expansion coefficient (TEC) is highly desirable for a thermal barrier coating material to prolong the life of the entire coating system. The TECs of Gd2-xZr2+xO7+0.5x (x = 0, ±0.125, ±0.25, ±0.375) have been studied by first-principles calculations and solid-state reactions. Due to the strong bond energy and interatomic interaction force between Zr and O, and Gd3+ having a higher thermal diffusion coefficient and lower solid solubility, the TEC of a material with higher Gd3+ content and lower Zr4+ content is improved. Calculated results show that the highest Gd3+ content (x = −0.375) increases the TEC of Gd2-xZr2+xO7+0.5x from 7.26 × 10−6/K (x = 0.375) to 8.95 × 10−6/K (1500K). The Debye temperature decreases with increasing Gd3+ content. The experimental TEC is highest (11.593 × 10−6/K) at a content of x = −0.375, far exceeding that at x = 0.375 (10.690 × 10−6/K). The experimental results agree well with the calculated results. This work lays a foundation for obtaining rare-earth zirconate materials with outstanding thermal physical properties, and it provides a novel method for their creation.

Oxidation Behavior of Single-Layer Gd2Zr2O7, and Gd2Zr2O7/YbSZ, and Double-Layer YSZ + Gd2Zr2O7/YbSZ Thermal Barrier Coatings

Oxidation is one of the failure mechanisms of gas turbines under operating temperature. Yttria-stabilized zirconia (YSZ) is well-known ceramic topcoat material of thermal barrier coatings (TBCs), but it has some drawbacks. From this point of view, other new materials such as rare earth zirconate are interesting. In the present research, single-layer Gd2Zr2O7 (GZO), and GZO/YbSZ (Gd2Zr2O7 + 50 wt% YbSZ), and double-layer YSZ + GZO/YbSZ (YSZ/Gd2Zr2O7 + 50 wt% YbSZ) topcoats as TBC were deposited by air plasma spraying. To estimate the phase stability of homogeneous mixtures of GZO (pyrochlore) and YbSZ (tetragonal), heat treatment was conducted at 1400 °C and showed complete conversion to a single-cubic (fluorite) phase after 100 h. The as-sprayed TBCs were subjected to oxidation for different times at 900 °C, and their features were investigated by different analysis techniques (SEM and XRD). The microstructural analysis indicated that the thickness of the thermally grown oxide layer was less for GZO coating due to reduced oxygen diffusion. Conversely, because of the lower fracture toughness and lower thermal expansion of GZO, it had a considerably less lifetime. It was concluded that coatings with the new composition GZO/YbSZ exhibited higher oxidation resistance and longer lifetime than GZO coatings.

Structural and Luminescence Analysis of Low Temperature Solution Combustion Derived White Light Emitting Y2(1-x)Dy2xZr2O7 Nanophosphors for WLEDs

White light emitting dysprosium doped rare-earth zirconate Y2(1-x)Dy2xZr2O7 (x = 0.5 to 5 mol%) crystalline phosphors in nano-regime were obtained using urea supported solution combustion methodology at low temperature 650 ºC. Presence of fluorite structure having cubic symmetry with Fm3m space group was analyzed by powder X-ray diffraction patterns. Spherical shaped nanoparticles with clear boundaries were observed by surface morphological studies. Diverse elements Y, Zr, Dy and O were present in the optimized nanophosphors as examined by energy dispersive X-ray analysis. Upon near ultraviolet (NUV) excitation at 354 nm wavelength, the Y2(1-x)Dy2xZr2O7 phosphors displayed the characteristic white light emanation resulted from blue and yellow emission owing to 4F9/2→6H15/2 (450-525 nm) and 4F9/2→6H13/2 (525-650) transitions, respectively. With the doping of 1.5 mol% amount of Dy3+ ions in Y2(1-x)Dy2xZr2O7 powders showed maximum photoluminescence emission, above which intensity was quenched due to dipole-dipole interactions. In addition, Y1.97Dy0.03Zr2O7 nanopowders exhibit CIE colour coordinates (0.2605, 0.3145) residing in white region. The closeness of this value to the white emission coordinates of the National Television System Committee (NTSC) endorsed Y2(1-x)Dy2xZr2O7 nanophosphors as fascinating candidate for displays and white light emitting diodes.

Rare-earth high-entropy aluminate-toughened-zirconate dual-phase composite ceramics for advanced thermal barrier coatings

Superb toughening is achieved by incorporating a secondary ferroelastic phase in high-entropy rare-earth zirconate 5RE2Zr2O7 (HZ). Here, we report an enhancement of 64% in fracture toughness through the addition of 30mol% high-entropy rare-earth aluminate 5REAlO3 (HA) to the HZ matrix (30HA). The aforementioned rare-earth elements RE are La, Sm, Eu, Gd, and Yb. The present dual-phase composite ceramic 30HA has a large fracture toughness of 2.77 ± 0.14 MPa m1/2, along with excellent high-temperature phase stability, resulting in good usage for potential thermal barrier coating applications. Particularly, the fracture toughness of the dual-phase composite ceramics at first increases to a maximum and then drops suddenly, as the mole fraction of HA increases from 0 to 50%. A clear definition of fitting parameters and their physical significance is provided for a better interpretation of the experimental data. The present toughening mechanism sheds light on microstructure engineering in high-entropy ceramics for excellent mechanical properties.

Fabrication, characterisation, and structure of Nd2Zr2O7 microbelts prepared by electrospinning method

• A novel Nd 2 Zr 2 O 7 microbelts. • Electrospinning technology. • A pyrochlore-type crystal structure. • A feasible formation mechanism. Nd 2 Zr 2 O 7 microbelts were successfully prepared by simple electrospinning paired with calcination. The composition, microstructure, and structure of the as-prepared Nd 2 Zr 2 O 7 microbelts were systematically characterized through a combination of characterisations, such as XRD, SEM, TEM, FT-IR, and TG-DSC techniques. Results indicated that the as-prepared Nd 2 Zr 2 O 7 microbelts had a pyrochlore-type crystal structure and uniform size with about 10 μm thickness. Moreover, a feasible formation mechanism of the Nd 2 Zr 2 O 7 microbelts was discussed. The present procedure can be expanded to develop other rare-earth zirconate materials with belt structure.

Synthesis and characterization of sol-gel derived lanthanum zirconate ceramic aerogels toward ultralow thermal conductivity

La2Zr2O7 ceramic aerogels (LZO CAs) were successfully fabricated via sol-gel method followed by heat treatment process. The morphology, microstructure, and phase composition of the as-prepared samples were systematically studied using various technological means. The results indicated that the as-prepared LZO CAs present a typical three-dimentional (3-D) porous network with a single-phase pyrochlore crystal structure. In addition, the as-prepared LZO CAs presented high specific surface area of 594.85 m2/g, low thermal conductivity of 0.033 W/(m·K) at room temperature, and remarkable high temperature heat-insulating property. This work offers a novel strategy for the development of other rare-earth zirconates ceramic aerogels which can be used as high temperature insulations.

Size disorder: A descriptor for predicting the single- or dual-phase formation in multi-component rare earth zirconates

Eighteen different 5-component equimolar rare earth zirconates were successfully synthesized by solid state reaction method. Among them, 5 of them exhibit a high-entropy fluorite structure, 8 have a high-entropy pyrochlore structure, and the remaining 5 have dual-phase structures of both pyrochlore and fluorite. For high-entropy rare earth zirconates with single-phase structure, the lattice parameter is almost linearly correlated with the average cation radius ratio. In combination with a large number of previous reports, a summary is made to study the synergistic effects of the average cation radius ratio, configuration entropy and size disorder on the single-phase formation ability of multi-component rare earth zirconates. The conclusion verifies that the size disorder of about 5% is the boundary for the multi-component rare earth zirconates to form a single-phase or dual-phase region, while the average cation radius ratio of 1.467 is the demarcation between a pyrochlore and a defective fluorite structure in the single-phase zone, and the configuration entropy had less effect. Excessive size disorder leads to the enrichment or barrenness of certain components, which is the main reason for the formation of dual phase ceramics. • 18 Different equimolar 5-component rare earth zirconates have been prepared. • The size disorder ( δ *) can predict the single-phase formation ability of zirconates. • δ * = 5% is the boundary between the single- and dual-phase rare earth zirconates. • The single-phase forming ability of rare-earth zirconates is independent of entropy.

Spin-Dependent First-Principles Study on Optoelectronic Properties of Neodymium Zirconates Pyrochlores Nd2Zr2O7 in Fd-3m and Pmma Phases.

Rare-earth zirconate pyrochlores (RE2Zr2O7) are of much fundamental and technological interest as optoelectronic, scintillator and thermal barrier coating materials. For the first time, we report the detailed optoelectronic properties of rare-earth zirconates Nd2Zr2O7 in both, i.e., for spin up and spin down states, via the use of first-principles density functional theory (DFT) procedure. To obtain the desired optoelectronic properties, we used a highly accurate method called full-potential linearized augmented plane wave (FPLAPW) within the generalized gradient approximation (GGA), parametrized with Hubbard potential U as an exchange-correlation function. The band gaps predicted for Nd2Zr2O7 were of the order 2.4 eV and 2.5 eV in Fd-3m and Pmma symmetrical phases, respectively. For both the phases, our research involved a complete examination of the optical properties of Nd2Zr2O7, including extinction coefficient, absorption coefficient, energy loss, function, reflectivity, refractive index, and real optical conductivity, analyzed in the spectral range from 0.0 eV to 14 eV. The calculated optical properties in both phases showed a considerable spin-dependent effect. The electronic bonding characteristics of different species in Nd2Zr2O7 within the two crystal symmetries were explored via the density distribution mapping of charge.

Order–disorder transition and thermal conductivities of the (NdSmEuGd)(1-x)/2Dy2xZr2O7 series

Rare-earth zirconates with pyrochlore and fluorite structures have recently been identified as promising thermal barrier coating materials owing to their low thermal conductivities. In this study, six samples with the general formula (NdSmEuGd)(1-x)/2Dy2xZr2O7 were synthesized to further reduce the thermal conductivity. X-ray diffraction and Raman spectroscopy showed that the transition from an ordered pyrochlore to a disordered fluorite structure is due to cation and anion disorder. Transmission electron microscopy showed that anion disorder occurred before cation disorder. A modified mass disorder parameter was introduced into this system, which can describe the change in thermal conductivity well. This parameter can be a basis for designing more complex materials with lower thermal conductivities.

Influence of chemical disorder on mechanical and thermal properties of multi-component rare earth zirconate pyrochlores (nRE1/n)2Zr2O7

To explore the underlying mechanism of chemical disorder in high-entropy pyrochlores, ten rare earth zirconates (nRE1/n)2Zr2O7 (n = 1, 2, and 4, RE = La, Nb, Sm, Eu, and Gd) are studied by using first-principles calculations. The mechanical and thermal properties are carefully analyzed with a special focus on local structural evolution and interatomic interaction. It is found that all three kinds of bond lengths increase linearly with lattice parameters whether the pyrochlore involves chemical disorder or not. Compared with the single-component counterparts, the multi-component pyrochlores are recognized to exhibit higher elastic constants and moduli but lower elastic anisotropy. Meanwhile, (LaSmEuGd)2Zr2O7 shows the lowest thermal conductivity, which can be attributed to the larger La atoms and the weaker La–O bonding. Furthermore, the abnormal strengthening of phonon anharmonicity in (SmEu)2Zr2O7 emphasizes the significance of fluctuation in local distortion rather than enhancement in chemical disorder on decreasing thermal conductivity for high-entropy ceramics. This work uncovers the physical origins of the chemical disorder effect on mechanical and thermal properties for pyrochlores and further shed some lights on the design of high-performance high-entropy ceramics with great potential applications including thermal barrier coatings.

Designing high-entropy rare-earth zirconates with tunable thermophysical properties for thermal barrier coatings

The development of rare-earth zirconates with tunable thermal expansion coefficient (TEC) and thermal conductivity is of great importance in the area of thermal barrier coatings. In general, TEC can be tailored by the average electronegativity difference, and the thermal conductivity can be customized by the atomic mass difference and/or ionic radius. Based on this thermophysical properties design principle, a series of high-entropy rare-earth zirconate ceramics were synthesized and characterized, and the obtained results indicate that: (1) the TEC is negatively correlated with the average electronegativity difference of cation and anion concerning its corresponding compound; (2) the thermal conductivity is negatively correlated with the atomic mass difference and ionic radius difference concerning its corresponding compound; (3) the synthesized ceramics have excellent high-temperature long-term thermal phase stability, larger TEC (RT-1500 °C, 10.20 ~ 10.39 × 10-6 K-1), lower thermal conductivity (1500 °C, 1.17 ~ 1.37 W·m-1·K-1), and higher fracture toughness (1.61 ~ 1.69 MPa·m1/2) compared with lanthanum zirconate. The tailorable design principle involved in this work sheds new insights for designing alternative ceramics with pre-assigned thermophysical properties for the next-generation thermal barrier coatings.

Synthesis and characterization of high-entropy (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2(Zr0.75Ce0.25)2O7 nanopowders

Developing a novel ceramic nanopowder is essential for thermal-sprayed high-performance nanostructured thermal barrier coatings. Recently, high-entropy rare-earth zirconates have been extensively concerned by researchers. In the present work, we synthesized high-entropy (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2(Zr0.75Ce0.25)2O7 (LNSGY) nanopowders by the sol-gel technology. In addition, the LNSGY nanopowders were investigated by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. Results show that LNSGY nanopowders have a typical defective fluorite structure. The average crystalline size of LNSGY nanopowders is approximately 15 nm. Meanwhile, the elemental composition of LNSGY nanopowders presents uniform distribution. This paper lays a foundation for high-performance high-entropy nanostructured feedstocks used for plasma spraying in the future.

Understanding the thermal barrier performance of plasma-sprayed rare-earth zirconate coating against volcanic ash ingestion

Plasma sprayed thermal barrier coatings (TBC) are widely used to protect gas turbine engine components typically exposed to different types of failures. The state-of-art yttria-stabilized-zirconia (YSZ) TBC fails above 1150 °C under calcium‑magnesium-alumino-silicate (CMAS) or volcanic ash (VA) infiltration and associated thermal cyclic degradation. Rare earth zirconate (REZ) is considered a potential candidate TBCs to perform at much higher temperatures and exhibit better CMAS infiltration resistance. Advanced REZs such as gadolinium zirconate (Gd2Zr2O7, GZ) and yttrium zirconate (Y2Zr2O7,YZ) were chosen for the present study. Free-standing coatings of REZ were deposited by atmospheric plasma spray (APS) and assessed for their VA interaction behavior at elevated temperatures (1150 °C/1250 °C). Single-layered (SL) and double-layered (DL) TBC systems were deposited on superalloy substrates to assess the thermal cycling performance. From the studies, Plasma-sprayed REZ TBCs demonstrate enhanced resistance toward CMAS infiltration and thermal cycling degradation at simulated test conditions, thereby qualifies for futuristic high-performance TBC applications.

Rattler effect on the properties of multicomponent rare-earth-zirconate ceramics

Rattler effect caused by size disorder in the cation sublattice of rare-earth zirconates has been proved to be an effective way for the thermal conductivity reduction. Herein, a series of multicomponent ((La1/3Eu1/3Gd1/3)1-xYbx)2Zr2O7 (x = 0, 0.25, 0.5, 0.75 and 1) ceramics were synthesized, their structures and the influence of rattler effect on thermal conductivity were investigated. The increasing Yb3+ doping content causes the phase transformation from pyrochlore structure to fluorite structure. The dual-phase ((La1/3Eu1/3Gd1/3)0.75Yb0.25)2Zr2O7 exhibits enhanced hardness owing to the small grain size of the dual-phase structure, which can effectively alleviate the stress to hinder deformation. Also, it has a higher coefficient of thermal expansion of 11.2 × 10−6/K (1200 °C). Since Yb3+ acts as active resonant scattering center, lower thermal conductivity of ((La1/3Eu1/3Gd1/3)0.75Yb0.25)2Zr2O7 and ((La1/3Eu1/3Gd1/3)0.5Yb0.5)2Zr2O7 caused by rattler effect makes them promising thermal barrier coatings.

Highly porous (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 ceramics with ultra-low thermal conductivity

The high-entropy rare earth zirconate (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 porous ceramics ((5RE1/5)2Zr2O7 PCs) were prepared using a foam-gel casting-freeze drying method combined with segmented calcination process. The results of SEM, TEM, and XRD analyses of the (5RE1/5)2Zr2O7 PCs indicated the formation of a defective fluorite crystal structure, with the rare earth elements homogeneously distributed. Meanwhile, the as-prepared (5RE1/5)2Zr2O7 PCs exhibited high porosity, low bulk density, low thermal conductivity, and relatively high compressive strength. Moreover, the high-temperature thermal conductivity of the samples was evaluated, and the results showed that the (5RE1/5)2Zr2O7 PCs maintain a thermal conductivity of 0.150 ± 0.002 W m−1 K−1 even at 1000 °C. The strategy used in this paper can be extended to the synthesis of other high-entropy porous ceramics with high porosity and low thermal conductivity, which is suitable for applications as thermal insulation materials.

High-entropy rare-earth zirconate ceramics with low thermal conductivity for advanced thermal-barrier coatings

The high-entropy rare-earth zirconate ((La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2Zr2O7, 5RE2Zr2O7 HEREZs) ceramics were successfully prepared by a new high-speed positive grinding strategy combined with solid-state reaction method. The microstructure, crystal structure, phase composition, and thermophysical and mechanical properties of the samples were systematically investigated through various methods. Results indicate that the samples have a single-phase defect fluorite-type crystal structure with excellent high-temperature thermal stability. The as-prepared samples also demonstrate low thermal conductivity (0.9–1.72 W·m−1·K−1 at 273–1273 K) and high coefficient of thermal expansion (CTE, 10.9 × 10−6 K−1 at 1273 K), as well as outstanding mechanical properties including large Young’s modulus (E = 186–257 GPa) and high fracture toughness (KIC). Furthermore, the formation possibility of the as-prepared samples was verified through the first-principles calculations, which suggested the feasibility to form the 5RE2Zr2O7 HE-REZs in the thermodynamic direction. Therefore, in view of the excellent multifunctional properties exhibited by the as-prepared 5RE2Zr2O7 HE-REZs, they have great potential applications in next-generation thermal-barrier coatings (TBCs).

Calculation of Thermal Expansion Coefficient of Rare Earth Zirconate System at High Temperature by First Principles

Compounds of rare earth zirconates with pyrochlore structure are candidates for the application of thermal barrier coatings of next generation. In order to modify the mechanic properties and maintain the low thermal conductivity, other trivalent rare-earth element substitution is commonly used. Presently, investigation on the evaluation of the property of thermal expansion is attracting more attention. In this paper, a feature parameter of thermal expansion coefficient at high temperature (α∞) was proposed by combining Grüneisen’s equation and the Debye heat capacity model. Using α∞ model, the thermal expansion property of different compounds can be easily figured out by first principles. Firstly, α∞ of ZrO2, HfO2, were calculated, and results are in good agreement with the experimental data from the literature. Moreover, α∞ of La2Zr2O7, Pr2Zr2O7, Gd2Zr2O7, and Dy2Zr2O7 were calculated, and results demonstrated that the model of α∞ is a useful tool to predict the thermal expansion coefficient at high temperature. Finally, Gd2Zr2O7 with 4 different Yb dopant concentrations (Gd1-xYbx)2Zr2O7 (x = 0, 0.125, 0.3125, 0.5) were calculated. Comparing with the experimental data from the literature, the calculation results showed the same tendency with the increasing of Yb concentration.