Marked reduction in the thermal conductivity of (La0.2Gd0.2Y0.2Yb0.2Er0.2)2Zr2O7 high-entropy ceramics by substituting Zr4+ with Ti4+

The (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ (x = 0–0.5) high-entropy ceramics were successfully prepared by a solid state reaction method and their structures and thermo-physical properties were investigated. It was found that the high-entropy ceramics demonstrate pure pyrochlore phase with the composition of x = 0.1–0.5, while (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂Zr₂O₇ shows the defective fluorite structure. The sintered high-entropy ceramics are dense and the grain boundaries are clean. The grain size of high-entropy ceramics increases with the Ti⁴⁺ content. The average thermal expansion coefficients of the (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics range from 10.65 × 10^?6 K^?1 to 10.84 × 10^?6 K^?1. Importantly, the substitution of Zr⁴⁺ with Ti⁴⁺ resulted in a remarkable decrease in thermal conductivity of (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics. It reduced from 1.66 W m^?1 K^?1 to 1.20 W m^?1 K^?1, which should be ascribed to the synergistic effects of mass disorder, size disorder, mixed configuration entropy value and rattlers.     

Tuning stoichiometry of high-entropy oxides for tailorable thermal expansion coefficients and low thermal conductivity

Thermal barrier coating materials with proper thermal expansion coefficient (TEC), low thermal conductivity, and good high-temperature stability are of great significance for their applications in next-generation turbine engines. Herein, we report a new class of high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x with different Ce⁴⁺ contents synthesized by a solid-state reaction method. They exhibit different crystal structures at different Ce⁴⁺ content, including a bixbyite single phase without Ce⁴⁺ doping (x = 0), bixbyite-fluorite dual-phase in the RE₂O₃-rich region (0 < x < 2), and fluorite single phase in the stoichiometric (x = 2) and CeO₂-rich region (x > 2). The high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x exhibit tailorable TECs at a large range of 9.04 × 10^–6–13.12 × 10^–6 °C^–1 and engineered low thermal conductivity of 1.79–2.63 W·m^–1·K^–1. They also possess good sintering resistance and high-temperature phase stability. These results reveal that the high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x are promising candidates for thermal barrier coating materials as well as thermally insulating materials and refractories.     

Dual-phase rare-earth-zirconate high-entropy ceramics with glass-like thermal conductivity

A series of rare earth zirconates (RE₂Zr₂O₇) high-entropy ceramics with single- and dual-phase structure were prepared. Compared with La₂Zr₂O₇ and Yb₂Zr₂O₇, the smaller “rattling” ions (Yb³⁺, Er³⁺, Y³⁺) have been incorporated into pyrochlore lattice in (La_0.2Nd_0.2Y_0.2Er_0.2Yb_0.2)₂Zr₂O₇ (LNYEY) while larger ions (La³⁺, Nd³⁺, Sm³⁺, Eu³⁺) incorporated into fluorite lattice in (La_0.2Nd_0.2Sm_0.2Gd_0.2Yb_0.2)₂Zr₂O₇ (LNSGY). Due to high-entropy lattice distortion and resonant scattering derived from smaller ions Yb³⁺, Er³⁺, and Y³⁺, LNYEY shows a lower glass-like thermal conductivity (1.62-1.59 W m^-1 K^-1, 100-600℃) than LNSGY (1.74-1.75 W m^-1 K^-1, 100-600℃). Moreover, LNYEY and LNSGY exhibit enhanced Vickers’ hardness (LNYEY, H_v = 11.47 ± 0.41 GPa; LNSGY, H_v = 10.96 ± 0.26 GPa) and thermal expansion coefficients (LNYEY, 10.45 × 10^-6 K^-1, 1000℃; LNSGY, 11.02 × 10^-6 K^-1, 1000℃). These results indicate that dual-phase rare-earth-zirconate high-entropy ceramics could be desirable for thermal barrier coatings.     

Irradiation induced structural damage and evolution of mechanical properties in high entropy fluorite oxide

Pursuing material with excellent irradiation resistance, high chemical durability, and stable mechanical properties under extreme conditions is of great significance for developing irradiation-resistant materials. Herein, a novel irradiation-resistant high-entropy fluorite oxide (Nd_0.2Sm_0.2Gd_0.2Dy_0.2Er_0.2)₂Ce₂O₇ is reported. After 9-MeV Au ion irradiation with ion fluence of 2.7 × 10¹⁵ and 4.5 × 10¹⁵ ions/cm², the high-entropy (Nd_0.2Sm_0.2Gd_0.2Dy_0.2Er_0.2)₂Ce₂O₇ shows excellent phase stability without phase decomposition and transformation. In comparison with Nd₂Ce₂O₇, the high-entropy (Nd_0.2Sm_0.2Gd_0.2Dy_0.2Er_0.2)₂Ce₂O₇ possesses much less amorphization and lattice expansion, suggesting its improved irradiation resistance. No pronounced variation in Raman spectra can be detected in the post-irradiated structure, implying rarely structural shift arises in high-entropy (Nd_0.2Sm_0.2Gd_0.2Dy_0.2Er_0.2)₂Ce₂O₇. After irradiation, there is no irradiation-induced segregation at grain boundaries or inside the grains of high-entropy (Nd_0.2Sm_0.2Gd_0.2Dy_0.2Er_0.2)₂Ce₂O₇. The nanoindentation tests reveal that the mechanical properties of the high-entropy fluorite oxide rarely degrade. The results, along with the insight into the mechanism of heavy-ion irradiation resistance, provide insight for the subsequent research on the heavy-ion irradiation of high-entropy ceramics.     

Influence of component manipulation on the structural,mechanical, thermophysical and electrical properties of La0·2Ce0.2Nd0.2(ZrxY1−x)0.4O2−δ high-entropy ceramics

A series of high-entropy ceramics (HECs) with compositions of La_0·2Ce_0.2Nd_0.2(Zr_xY_1−x)_0.4O_2−δ (x = 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0, the corresponding names being HEC(Zr_0·5/Y_0.5, Zr_0·6/Y_0.4, Zr_0·7/Y_0.3, Zr_0·8/Y_0.2, Zr_0·9/Y_0.1, Zr_1·0/Y₀)) were sintered in air at 1600 ^°C for 10 h. When x is in the range of 0.5–0.7, a fluorite phase is formed. Then, as x exceeds 0.7, a second pyrochlore-structured phase appears, and its content gradually increases with the increasing x. The grain growth of the samples is inhibited by increasing in the relative Zr content. The grain refinement and the formation of second phase reduce the thermal conductivity and reinforce the mechanical properties of the samples. HEC(Zr_0.9/Y_0.1) has the lowest thermal conductivity (50–500 °C) and brittleness index, as well as the highest fracture toughness among all samples. In addition, La_0·2Ce_0.2Nd_0.2(Zr_xY_1−x)_0.4O_2−δ ceramics have excellent thermal stability under Ar atmosphere in 50–1400 °C. The thermal expansion coefficients of the samples marginally change regardless of the variation in x. All samples show higher oxygen barrier property than Y₂O₃-stabilized ZrO₂.     

Response of structure and mechanical properties of high entropy pyrochlore to heavy ion irradiation

Material with superior damage tolerance, chemical durability, and structure stability is of increasing interest in high-level radioactive waste management and structural components for advanced nuclear systems. In this paper, high-entropy (La_0.2Ce_0.2Nd_0.2Sm_0.2Gd_0.2)₂Zr₂O₇ with pyrochlore-type structure was synthesized through conventional solid-state method. The as-synthesized high-entropy oxide maintained crystalline after being irradiated by using Au³⁺ with 9.0 MeV energy at the fluence of 4.5 × 10¹⁵ ions·cm^-2, indicating its high tolerance to heavy-ion irradiation. The irradiation-induced order-disorder transition from pyrochlore structure to defective fluorite structure occurred in high-entropy (La_0.2Ce_0.2Nd_0.2Sm_0.2Gd_0.2)₂Zr₂O₇. After irradiation, no irradiation-induced segregation was observed at grain boundary. Moreover, the mechanical properties of high-entropy pyrochlore were improved. The heavy-ion irradiation resistance mechanisms of high-entropy pyrochlore were discussed in detail. Our work identified high-entropy (La_0.2Ce_0.2Nd_0.2Sm_0.2Gd_0.2)₂Zr₂O₇ can be a promising candidate for immobilization of high-level radioactive waste as well as advanced nuclear reactor system from the perspective of irradiation resistance.     

Thermophysical and electrical properties of rare-earth-cerate high-entropy ceramics

A new series of rare-earth-cerate high-entropy ceramics with compositions of (La_0.2Nd_0.2Sm_0.2Gd_0.2Dy_0.2)₂Ce₂O₇ (HEC1), (La_0.2Nd_0.2Sm_0.2Gd_0.2Yb_0.2)₂Ce₂O₇ (HEC2), (La_0.2Nd_0.2Sm_0.2Yb_0.2Dy_0.2)₂Ce₂O₇ (HEC3), (La_0.2Nd_0.2Yb_0.2Gd_0.2Dy_0.2)₂Ce₂O₇ (HEC4), (La_0.2Yb_0.2Sm_0.2Gd_0.2Dy_0.2)₂Ce₂O₇ (HEC5) as well as a single component of Nd₂Ce₂O₇ are fabricated via sintering the corresponding sol–gel-derived powders at 1600°C for 10 h. HEC1–5 samples exhibit a single-cerate phase with fluorite structure and high configurational entropy. Compared with Nd₂Ce₂O₇, HEC1–5 samples have a lower grain growth rate owing to the sluggish diffusion effect. The chemical compositional uniformity of HEC1–5 as well as Nd₂Ce₂O₇ does not apparently change after annealing at 1500°C for different time intervals (1, 6, 12, and 18 h). Compared with 8YSZ, HEC1–5 samples display the decreased thermal conductivity and increased thermal expansion coefficient. The lattice size disorder parameter of HEC1–5 is negatively related to the thermal conductivity in 26–450°C. Furthermore, HEC1–5 and Nd₂Ce₂O₇ exhibit lower oxygen-ion conductivity, meaning an increased resistance to oxygen diffusion.     

High-entropy fluorite oxides

Eleven fluorite oxides with five principal cations (in addition to a four-principal-cation (Hf_0.25Zr_0.25Ce_0.25Y_0.25)O_2-δ as a start point and baseline) were fabricated via high-energy ball milling, spark plasma sintering, and annealing in air. Eight of the compositions, namely (Hf_0.25Zr_0.25Ce_0.25Y_0.25)O_2-δ, (Hf_0.25Zr_0.25Ce_0.25)(Y_0.125Yb_0.125)O_2-δ, (Hf_0.2Zr_0.2Ce_0.2)(Y_0.2Yb_0.2)O_2-δ, (Hf_0.25Zr_0.25Ce_0.25)(Y_0.125Ca_0.125)O_2-δ, (Hf_0.25Zr_0.25Ce_0.25)(Y_0.125Gd_0.125)O_2-δ, (Hf_0.2Zr_0.2Ce_0.2)(Y_0.2Gd_0.2)O_2-δ, (Hf_0.25Zr_0.25Ce_0.25)(Yb_0.125Gd_0.125)O_2-δ, and (Hf_0.2Zr_0.2Ce_0.2)(Yb_0.2Gd_0.2)O_2-δ, possess single-phase solid solutions of the fluorite crystal structure with high configurational entropies (on the cation sublattices), akin to those high-entropy alloys and ceramics reported in prior studies. Most high-entropy fluorite oxides (HEFOs), except for the two containing both Yb and Gd, can be sintered to high relative densities. These single-phase HEFOs exhibit lower electrical conductivities and comparable hardness (even with higher contents of softer components such as Y₂O₃ and Yb₂O₃), in comparison with 8?mol. % Y₂O₃-stabilized ZrO₂ (8YSZ). Notably, these single-phase HEFOs possess lower thermal conductivities than that of 8YSZ, presumably due to high phonon scattering by multiple cations and strained lattices.     

Multi-component strategy for remarkable suppression of thermal conductivity in strontium diyttrium oxide: The case of high-entropy Sr(Y0.2Sm0.2Gd0.2Dy0.2Yb0.2)2O4 ceramic

Anti-spinel oxide SrY₂O₄ has attracted extensive attention as a promising host lattice due to its outstanding high-temperature structural stability and large thermal expansion coefficient (TEC). However, the overhigh thermal conductivity limits its application in the field of thermal barrier coatings. To address this issue, a novel high-entropy Sr(Y_0.2Sm_0.2Gd_0.2Dy_0.2Yb_0.2)₂O₄ ceramic was designed and synthesized for the first time via the solid-state method. It is found that the thermal conductivity of Sr(Y_0.2Sm_0.2Gd_0.2Dy_0.2Yb_0.2)₂O₄ is reduced to 1.61 W·m⁻¹·K⁻¹, 53 % lower than that of SrY₂O₄ (3.44 W·m⁻¹·K⁻¹) at 1500 °C. Furthermore, reasonable TEC (11.53 ×10⁻⁶ K⁻¹, 25 °C ∼ 1500 °C), excellent phase stability, and improved fracture toughness (1.92 ± 0.04 MPa·m^1/2) remained for the high-entropy Sr(Y_0.2Sm_0.2Gd_0.2Dy_0.2Yb_0.2)₂O₄ ceramic, making it a promising material for next-generation thermal barrier coatings.     

Thermophysical performances of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 high-entropy ceramics for thermal barrier coating applications

In this study, a novel high-entropy oxide of (La_1/6Nd_1/6Yb_1/6Y_1/6Sm_1/6Lu_1/6)₂Ce₂O₇was prepared using a sol–gel and high-temperature sintering technology. Additionally, its lattice structure, micro-morphology, elemental composition, and thermophysical and mechanical properties were evaluated. The results revealed that the obtained oxide powder has a typical fluorite-type lattice with particle sizes in the range of～30–100 nm. The bulk sample has a dense microstructure and uniform elemental distribution. Owing to its low lattice order, the thermal expansion coefficient of (La_1/6Nd_1/6Yb_1/6Y_1/6Sm_1/6Lu_1/6)₂Ce₂O₇ is greater than that of Sm₂Ce₂O₇, which also exhibits excellent lattice stability up to 1200 °C. Further, owing to phonon scattering due to lattice distortion, oxygen vacancy, and cation substitution, the thermal conductivity of (La_1/6Nd_1/6Yb_1/6Y_1/6Sm_1/6Lu_1/6)₂Ce₂O₇ is lower than that of Sm₂Ce₂O₇, while its mechanical performance is inferior to that of 7YSZ.     

Effect of multi-component rare-earth doping on maintaining structure stability of RE2Zr2O7 (RE = La,Sm, Gd,Y, Yb) coatings under thermal cycling

Inspired by the high entropy effects of high-entropy components, a novel high-entropy rare-earth zirconate (La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇ (HEC-LZ) was designed and successfully synthesized in this work. In addition, two binary rare-earth doped zirconates (RE-LZ), (La_1/3Sm_1/3Yb_1/3)₂Zr₂O₇ (LSYZ) and (La_1/3Gd_1/3Y_1/3)₂Zr₂O₇ (LGYZ), were proposed using the same rare-earth elements for comparison. The thermal barrier coatings with LZ-based ceramic top layer were prepared by spray granulation, solid-phase synthesis and atmospheric plasma spraying techniques. The as-synthesized LZ-based ceramics are all dominated by the pyrochlore phase. Under 1000 °C, the thermal cycling performances of the three coatings were studied. The microstructure evolution and crack expansion during the failure process were investigated in detail. The strengthening mechanism and the cause of coating spallation are proposed in combination with mechanical properties and thermal matching analysis. The results showed that compared with the undoped LZ coating, the thermal shock life of LGYZ coating, LSYZ coating and HEC-LZ coating is improved by nearly 46%, 27% and 57%, respectively. Due to the characteristics of high randomness, HEC-LZ ceramic has a large lattice distortion than RE-LZ ceramics, resulting in a higher coefficient of thermal expansion and fracture toughness, which contributes to maintaining the structure stability of coatings under thermal stress.     

Phase structure and thermophysical properties of co-doped La2Zr2O7 ceramics for thermal barrier coatings

La₂Zr₂O₇ has high melting point, low thermal conductivity and relatively high thermal expansion which make it suitable for application as high-temperature thermal barrier coatings. Ceramics including La₂Zr₂O₇, (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ were synthesized by solid state reaction. The effects of co-doping on the phase structure and thermophysical properties of La₂Zr₂O₇ were investigated. The phase structures of these ceramics were identified by X-ray diffraction, showing that the La₂Zr₂O₇ ceramic has a pyrochlore structure while the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ exhibit a defect fluorite structure, which is mainly determined by ionic radius ratio r(A_av.³⁺)/r(B_av.⁴⁺). The measurements for thermal expansion coefficient and thermal conductivity of these ceramics from ambient temperature to 1200 °C show that the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ have a larger thermal expansion coefficient and a lower thermal conductivity than La₂Zr₂O₇, and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ shows the more excellent thermophysical properties than (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ due to the increase of Yb₂O₃ content.     

Solvothermal-derived multicomponent rare-earth cerate ceramics toward remarkable thermophysical properties

The multicomponent rare-earth cerate (Y_0.2La_0.2Nd_0.2Sm_0.2Eu_0.2)₂Ce₂O₇ (5RE₂Ce₂O₇) ceramics were successfully fabricated through solvothermal method and the following calcination process. The microstructure and phase composition of the as-obtained products were systematically characterized via SEM, TEM and XRD techniques. The results showed that the as-synthesised 5RE₂Ce₂O₇ has a single-phase fluorite-type crystal structure with the particle size of approximately 200 nm. Furthermore, the as-synthesised 5RE₂Ce₂O₇ demonstrated lower thermal conductivity (1.9–1.26 W m^?1·K^?1 at 25–1000 °C), higher thermal expansion coefficients (CTEs, 12.48 × 10^?6 K^?1 at 1000 °C), and outstanding mechanical properties including large Young's modulus (248.0 ± 0.35 GPa) and high fracture toughness (2.4 ± 0.21 MPam^1/2). The excellent properties of the as-synthesised 5RE₂Ce₂O₇ demonstrates its potential application as a new type of next-generation TBCs.     

Single-phase rare-earth high-entropy zirconates with superior thermal and mechanical properties

Emerging of high-entropy ceramics has brought new opportunities for designing and optimizing materials with desired properties. In the present work, high-entropy rare-earth zirconates (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ and (Yb_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ are designed and synthesized. Both high-entropy ceramics exhibit a single pyrochlore structure with excellent phase stability at 1600 °C. In addition, the Yb-containing system possesses a high coefficient of thermal expansion (10.52 × 10^?6 K^-1, RT～1500 °C) and low thermal conductivity (1.003 W·m^-1 K^-1, 1500 °C), as well as excellent sintering resistance. Particularly, the Yb-containing system has significantly improved fracture toughness (1.80 MPa·mm^1/2) when compared to that of lanthanum zirconate (1.38 MPa·mm^1/2), making it a promising material for thermal barrier coatings (TBCs) applications. The present work indicates that the high-entropy design can be applied for further optimization of the comprehensive properties of the TBCs materials.     

Synthesis and thermal stability of Re2Zr2O7, (Re = La,Gd) and La2(Zr1−xCex)2O7−δ compounds under reducing and oxidant atmospheres for thermal barrier coatings

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La₂Zr₂O₇, Gd₂Zr₂O₇, La₂Ce₂O_7?γ and La₂(Zr_0.7Ce_0.3)O_7?γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400 °C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar_(g)/3%H_2(g) atmosphere.     

Influence of B-site substituent Ce on thermophysical,oxygen diffusion,and mechanical properties of La2Zr2O7

Pyrochlore-type La₂Zr₂O₇ (LZ) is a promising candidate for high-temperature thermal barrier coatings (TBCs). However, its thermal expansion coefficient and low fracture toughness are not optimal for such application and thus, need to be improved. In this study, we systematically report the effect of CeO₂ addition on phase formation, oxygen-ion diffusion, and thermophysical and mechanical properties of full compositions La₂(Zr₁?_xCe_x)₂O₇ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1). La₂(Zr₁?_xCe_x)₂O₇ exhibits a pyrochlore structure at x ≤ 0.3, while a fluorite structure is observed outside this range. With the increase in CeO₂ content, thermal expansion coefficient and oxygen-ion diffusivity in La₂(Zr₁?_xCe_x)₂O₇ are increased. Oxygen-ion diffusivity of La₂(Zr₁?_xCe_x)₂O₇ is two orders of magnitude less than that of classical 8YSZ. Among La₂(Zr₁?_xCe_x)₂O₇ compounds, La₂(Zr_0.7Ce_0.3)₂O₇ and La₂(Zr_0.5Ce_0.5)₂O₇ exhibit relatively low oxygen diffusivities. The composition La₂(Zr_0.5Ce_0.5)₂O₇ presents the lowest thermal conductivity due to the strongest phonon scattering and also the highest fracture toughness due to the solid-solution toughening. The highest sintering resistance is achieved by the composition La₂(Zr_0.7Ce_0.3)₂O₇ because of its ordered pyrochlore structure and high atomic mass of Ce. Based on these results, the compositions La₂(Zr_0.5Ce_0.5)₂O₇ and La₂(Zr_0.7Ce_0.3)₂O₇ are alternatives for classical 8YSZ for TBC materials operating at ultrahigh temperatures.     

Investigation on bi-layer coating with La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7/YSZ prepared by laser cladding

Thermal barrier coatings are an effective technology for improving the high-temperature performance of hot section components in gas turbine engine. Due to their excellent properties, high-entropy oxides are considered to be promising materials for thermal barrier coatings. Laser cladding is a coating preparation technology and the top coat prepared by laser cladding technology has an important application value for thermal barrier coatings. In this work, to improve the thermal cycling behavior of the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide coating, a bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide layer and the YSZ layer was designed and fabricated by laser cladding on the NiCoCrAlY alloy surface. The microstructure, phase and mechanical properties of the coating were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness and nanoindentation tests, respectively. The results show that a bi-layer La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇/YSZ coating was successfully prepared by the laser cladding method, and shows good bonding at the interface between the layers. The high-entropy oxide layer maintains a relatively stable defective fluorite structure and its microstructure exists in the stable cellular and dendrite crystalline state after laser cladding. The high-entropy oxide layer prepared by laser cladding showed an average elastic modulus of 167 GPa and an average hardness of 1022.8HV in nanoindentation tests. Thermal cycling of the coating was carried out at 1050 °C. Failure of the bi-layer coating occurred after 60 thermal cycles at 1050 °C. Thermal stresses between different layers are calculated during thermal cycling. Due to its excellent mechanical properties, the bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide and YSZ layers is expected to become an effective high-entropy oxide thermal barrier coating.     

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

The high-entropy rare earth zirconate (La_1/5Nd_1/5Sm_1/5Gd_1/5Yb_1/5)₂Zr₂O₇ porous ceramics ((5RE_1/5)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 (5RE_1/5)₂Zr₂O₇ PCs indicated the formation of a defective fluorite crystal structure, with the rare earth elements homogeneously distributed. Meanwhile, the as-prepared (5RE_1/5)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 (5RE_1/5)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.     

Formation of single-phase multicomponent zirconate with colossal atomic radius difference via reactive flash sintering

In this study, multicomponent rare-earth zirconate ceramics (Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂Zr₂O₇ and (La_0.2Eu_0.2Gd_0.2Yb_0.2Y_0.2)₂Zr₂O₇ were synthesized via conventional sintering and reactive flash sintering, respectively. Single-phase (Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂Zr₂O₇ ceramics, with the defect fluorite structure, were successfully obtained via conventional sintering and reactive flash sintering, while secondary phase segregation and precipitation were observed only in conventionally-sintered (La_0.2Eu_0.2Gd_0.2Yb_0.2Y_0.2)₂Zr₂O₇ ceramics. This study proposes that the critical electric field of reactive flash sintering introduces defects to soften the lattice, which not only improves the mass transportation, but also relieves the lattice stress induced by the atomic radius difference, resulting in the single-phase defect fluorite structure of (La_0.2Eu_0.2Gd_0.2Yb_0.2Y_0.2)₂Zr₂O₇. Thus, reactive flash sintering is an efficient route for synthesizing and developing novel multicomponent oxides that cannot be synthesized via conventional sintering due to pronounced lattice stress.     

Influence of doping Mg2+ or Ti4+ captions on the microstructures,thermal radiation and thermal cycling behavior of plasma-sprayed Gd2Zr2O7 coatings

Gadolinium zirconate (Gd₂Zr₂O₇) coatings doped by the transition metal Ti and the alkaline earth metal Mg were expected to have improved thermal radiation performance, which could be combined with their excellent thermal barrier properties to comprehensively improve the thermal insulating performance. The results show that the parent Gd₂Zr₂O₇ powder as well as the Gd-site and Zr-site substituted powders crystallize as pyrochlore Gd₂Zr₂O₇ in Fd-3m space group, while all the as-sprayed coatings have the combination of fluorite and a little part of pyrochlore phase. Gd₂Zr₂O₇ ceramic has high mid-infrared emittance and the addition of Ti⁴⁺ into Gd₂Zr₂O₇ can enhance the infrared absorption/emittance in a specific wavenumber range, dominantly in the near-infrared (0.75–2.5 μm) band due to the enhancement of electron transition induced by the impurity energy levels linked to the widening of the conduction band. The normal spectral infrared emissivity of Gd₂Zr₂O₇-based coating was higher than 0.88 at 1073 K. The monolayered doped Gd₂Zr₂O₇ coatings present very low thermal cycling lifetime, similar with the parent coating, mainly related with their low fracture toughness, despite (Gd_1-xMg_x)₂Zr₂O₇ series display lower thermal conductivity than the parent one.