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.     

Oxidation behavior of medium-entropy (Y1/3Yb1/3Lu1/3)2O3 modified SiC ceramic at 1700 °C: Experimental and theoretical study

The medium-entropy oxide (Y_1/3Yb_1/3Lu_1/3)₂O₃ with a body-centered cubic structure was successfully synthesized by solid-state reaction process, and then it was introduced into SiC ceramic to study its effect on the oxidation behavior of SiC ceramic at 1700 °C. The (Y_1/3Yb_1/3Lu_1/3)₂O₃-modified SiC ceramic exhibited better oxidation resistance than its individual oxides (Y₂O₃, Yb₂O₃, and Lu₂O₃) modified SiC ceramic. The experimental and calculated results all indicate that the rare-earth atoms had the tendency to diffuse into the SiO₂ structure and occupy the interstitial positions within SiO₂ structure. The introduction of medium-entropy oxide (Y_1/3Yb_1/3Lu_1/3)₂O₃ reduced the initial oxidation rate of the ceramic samples (1?3 h), and enhanced the stability of SiO₂ structure, thus resulting in a better oxidation resistance at 1700 °C.     

Fabrication of Yb-doped Lu2O3-Y2O3-La2O3 solid solutions transparent ceramics by self-propagating high-temperature synthesis and vacuum sintering

A comparative analysis of sintering and grain growth processes of lutetium oxide and lutetium-yttrium-lanthanum oxides solid solutions, as well as optical properties, luminescence and laser generation of (Lu_xY_0.9-xLa_0.05Yb_0.05)₂O₃ transparent ceramics are reported. Fabrication of highly dispersed initial powders of these compounds was performed via nitrate-glycine self-propagating high-temperature synthesis (SHS) method. The powders were compacted at 300?MPa and vacuum sintered at temperatures up to 1750?°С. Optical ceramics of (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ elemental composition were shown to have the highest in-line transmittance, which achieved 78% at the wavelength of 800?nm. Generation of laser radiation at a wavelength of 1032?nm with the differential efficiency of 20% was demonstrated in the (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ ceramics.     

La2Zr2O7 based high entropy ceramic with a low thermal conductivity and enhanced CMAS corrosion resistance

Herein, five new La₂Zr₂O₇ based high-entropy ceramic materials, such as (La_0.2Ce_0.2Gd_0.2Y_0.2Er_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Gd_0.2Y_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Y_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Gd_0.2Y_0.2Sm_0.2)₂Zr₂O₇), were synthesized using a sol-gel and high-temperature sintering (1000 °C) method. The spark plasma sintered (SPS) (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇ pellet shows a low thermal conductivity of 1.33 W m^-1 K^-1 at 773 K, and it also exhibits better CaO–MgO–Al₂O₃–SiO₂ corrosion resistance than that of Y₂O₃ stabilized ZrO₂. It shows that (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇ has a promising application potential as a thermal barrier coating.     

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.     

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.     

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.     

Thermophysical properties of (La0.25Sm0.25Gd0.25Yb0.25)2Ce2+xO7+2x(x=0.1, 0.2, and 0.3) high entropy oxides

A series of high-entropy oxides (La_0.25Sm_0.25Gd_0.25Yb_0.25)₂Ce_2+xO_7+2x were synthesised adopting a improved sol-gel technique and fritting method. The crystal-lattice, microstructure, elemental constitution, and thermal-physical performances were studied. The results showed that the synthesised high-entropy oxides have a single-fluorite lattice structure. The bulk specimen exhibits a compact microstructure, and clear grain boundaries. The thermal conductivities of the obtained high-entropy oxides are lower than those of CeO₂ and 7YSZ due to lattice strains and numerous oxygen vacancies. The obtained high-entropy oxides have greater thermal expansion coefficients than 7YSZ. The thermal conductivity and expansion coefficient are elevated because of the addition of excess CeO₂. The synthesised high-entropy oxides also exhibit outstanding lattice steadiness up to 1200 °C.     

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.     

Rare-earth element doped Si3N4/SiC micro/nano-composites—RT and HT mechanical properties

Dense Si₃N₄/SiC micro/nano-composites with varying grain boundary phase composition were fabricated by hot-pressing under the same conditions. Six different sintering aids (Lu₂O₃, Yb₂O₃, Y₂O₃, Sm₂O₃, Nd₂O₃ and La₂O₃) were used. The formation of SiC nano-inclusions was achieved by in situ carbothermal reduction of SiO₂ by C during the sintering process. Room temperature, fracture toughness, hardness and strength tended to increase when the cation radius of the rare-earth element used in the oxide additive decreased (i.e. from La³⁺ to Lu³⁺). The composite material with Lu₂O₃ sintering additive showed the highest hardness and had reasonably high fracture toughness and strength. The same micro/nano-composite also possessed the highest creep resistance in the temperature range from 1250 °C to 1400 °C and with loads in the range 50–150 MPa.     

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.     

Effects of Yb3+ doping on phase structure,thermal conductivity and fracture toughness of (Nd1-xYbx)2Zr2O7

To investigate the effects of Yb³⁺ doping on phase structure, thermal conductivity and fracture toughness of bulk Nd₂Zr₂O₇, a series of (Nd_1-xYb_x)₂Zr₂O₇ (x?=?0, 0.2, 0.4, 0.6, 0.8, 1.0) ceramics were synthesized using a solid-state reaction sintering method at 1600?°C for 10?h. The phase structures were sensitive to the Yb³⁺ content. With increasing doping concentration, a pyrochlore-fluorite transformation of (Nd_1-xYb_x)₂Zr₂O₇ ceramics occurred. Meanwhile, the ordering degree of crystal structure decreased. The substitution mechanism of Yb³⁺ doping was confirmed by analyzing the lattice parameter variation and chemical bond of bulk ceramics. The thermal conductivities of (Nd_1-xYb_x)₂Zr₂O₇ ceramics decreased first and then increased with the increase of Yb³⁺ content. The lowest thermal conductivity of approximately 1.2?W?m^?1 K^?1 at 800?°C was attained at x?=?0.4, around 20% lower than that of pure Nd₂Zr₂O₇. Besides, the fracture toughness reached a maximum value of ~1.59?MPa?m^1/2 at x?=?0.8 but decreased with further increasing Yb³⁺ doping concentration. The mechanism for the change of fracture toughness was discussed to result from the lattice distortion and structure disorder caused by Yb³⁺ doping.     

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.     

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.     

Directionally solidified Al2O3/(Y0.2Er0.2Yb0.2Ho0.2Lu0.2)3Al5O12 eutectic high-entropy oxide ceramics with well-oriented structure,high hardness,and low thermal conductivity

Directionally solidified Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic high-entropy oxide ceramics (HEOCs) were successfully prepared with an optical floating zone furnace. The Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs were pure phases with uniform distribution of rare-earth elements. The preferred growth orientation relationships were <10−10 > {0001}Al₂O₃ // <110 > {211}(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂. The indentation fracture toughness and Vickers hardness were 6.8 ± 0.9 MPa·m^1/2 and 16.1 ± 0.3 GPa, which were higher than that of Al₂O₃/Y₃Al₅O₁₂ eutectic ceramics. The room temperature bending strength was 333 ± 42 MPa. Crack bridging, deflection and bifurcation were the main toughening mechanism. Hardness and reduced modulus mapping results illustrated that the hardness of (Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ was close to that of Al₂O₃. Thermal expansion coefficient of Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs was very similar to that of Al₂O₃/Y₃Al₅O₁₂ but thermal conductivity was as low as 4.9 Wm⁻¹ K⁻¹ due to strong lattice distortion. These results suggest that high-entropy Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic ceramics are promising candidates for structural components application in gas turbine engines.     

Synthesis and thermophysical performances of (Nd1-xYbx)2AlTaO7 oxides for heat-insulation coating applications

A series of novel (Nd_1-xYb_x)₂AlTaO₇ oxides for thermal barrier coating application were synthesized exploring multi-step solid state clotting technology, the phase composition and thermophysical performances were analyzed. Results show that with increasing Yb₂O₃ content, the obvious phase transformation from weberite lattice to pyrochlore structure can be found. Owning to influence of oxygen vacancy, thermal conductivity of obtained products is less than that of 8YSZ. The thermal conductivity for (Nd_1-xYb_x)₂AlTaO₇ oxides decreases gradually with increasing Yb₂O₃ content, and (Nd_0.3Yb_0.7)₂AlTaO₇ has the lowest value. For (Nd_1-xYb_x)₂AlTaO₇ (x = 0, 0.1, 0.3) oxides, the lattice-energy has primary influence on thermal expansion coefficient, and electro-negativity governs the thermal expansion coefficient of (Nd_1-xYb_x)₂AlTaO₇ (x = 0.5, 0.7, 0.9 and 1) oxides. Thermal expansion coefficient of obtained oxides is close to that of 8YSZ.     

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.     

Low Thermal Conductivity of Rare‐Earth Zirconate‐Stannate Solid Solutions (Yb2Zr2O7)1−x(Ln2Sn2O7)x (Ln = Nd,Sm)

Dense monoliths of rare‐earth zirconate‐stannate solid solutions (Yb₂Zr₂O₇)_1?x(Ln₂Sn₂O₇)_x (Ln = Nd, Sm) were prepared by solid‐state reaction. Characterized by XRD, Raman, SEM, and TEM, a double‐phase structure of Yb₂Zr₂O₇‐rich fluorite and Nd₂Sn₂O₇‐rich pyrochlore was observed in the specimens of x = 0.4 and 0.5 of (Yb₂Zr₂O₇)_1?x(Nd₂Sn₂O₇)_x series while complete solid solutions were formed within the whole composition range of (Yb₂Zr₂O₇)_1?x(Sm₂Sn₂O₇)_x series. Except for the defect phonon scattering, lattice softening caused by order–disorder phase transformation between pyrochlore and fluorite structures also plays an important role in minimizing the thermal conductivity. Low thermal conductivity with positive temperature dependence is achieved in both the series. Considering the structure stability and low thermal conductivity, rare‐earth zirconate‐stannate solid solutions may be promising materials for thermal insulating applications, such as thermal barrier coatings.     

Influence of rare-earth oxide additives on the oxidation resistance of Si3N4–SiC nanocomposites

The influence of different rare earth oxide additives (La₂O₃, Nd₂O₃, Sm₂O₃, Y₂O₃, Yb₂O₃ and Lu₂O₃) on the oxidation behaviour of carbon derived Si₃N₄–SiC micro-nanocomposites has been investigated. All investigated composites exhibited predominately parabolic oxidation behaviour indicated diffusion as the rate limiting mechanism. Except the Si₃N₄–SiC composite sintered with Lu₂O₃ the rate-limiting oxidation mechanism for all other materials was an outward diffusion of the additive cations along the grain boundary towards the surface. Such diffusion of cation has been strongly suppressed in the Lu-doped composite because of the beneficial effect of stable grain boundary phase and the presence of the SiC particles predominately located at the grain boundaries of Si₃N₄. Nanoparticles at the grain boundaries act as the obstacles for migration of cations of the additives resulting in superior oxidation resistance of Si₃N₄–SiC–Lu₂O₃ where the rate-limiting step is inward diffusion of oxygen through the oxide layer to the bulk ceramics.     

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.