Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y_2.8Yb_0.2)(Al_4.8Sc_0.2)O₁₂ (YS-YAG) showed the lowest thermal conductivity (1.58 Wm^?1K^?1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10^?6 K^?1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.     

PREPARATION AND THERMAL EXPANSION OF FE2-xYxW3O12 POWDER BY CITRATE SOL-GEL PROCESS

Fe_2-xY_xW₃O₁₂ powder has been synthesized by the citrate sol-gel process. A model was proposed to calculate the concentration of species in a citric solution. The calculated results could provide valuable information for determining the optimal molar ratio of cation to citric acid and pH value of solution for Fe_2-xY_xW₃O₁₂ preparation. The predicted parameters derived from this model are in good agreement with the experimental results. The prepared gel and the Fe_2-xY_xW₃O₁₂ powder were characterized by X-ray diffraction (XRD) and differential thermal analysis-thermogravimetry (DTA-TG). The results show that it is very difficult to obtain pure Fe₂W₃O₁₂ powder by the citrate sol-gel process in the temperature range 500°–1000°C, however, Y₂W₃O₁₂ can easily be prepared even at 500°C. Y₂W₃O₁₂ annealed at 1000°C for 10 h is favorable for absorbing moisture in air to form Y₂W₃O₁₂·3.3H₂O. The thermal expansion coefficients of Y₂W₃O₁₂·3.3H₂O are: α_a = ? 8.01 × 10^?6°C^?1, α_b = ? 2.51 × 10^?7°C^?1, and α_c = ? 5.55 × 10^?6°C^?1 in 473–1173 K.     

The effect of Gd3+ and Ti4+ co-doping on the thermal radiation performance of (Sm1-xGdx)2(Hf1-xTix)2O7 (0 ≤ x ≤ 0.2) ceramic coatings

In this study, the (Sm_1-xGd_x)₂(Hf_1-xTi_x)₂O₇ (0 ≤ x ≤ 0.2) ceramic coatings were fabricated by atmospheric plasma spraying. The chemical compositions, morphologies and thermo-optical properties of the samples were systemically investigated. It can be found that the infrared emissivity of (Sm_1-xGd_x)₂(Hf_1-xTi_x)₂O₇ ceramic coatings at the wavelength range of 0.76–15 μm increased with the increasing content of Gd³⁺ and Ti⁴⁺. The (Sm_0.8Gd_0.2)₂(Hf_0.8Ti_0.2)₂O₇ ceramic coating exhibited the highest infrared emissivity among the coatings, which was 0.773 and 0.816 at room temperature and 1400 °C, respectively. The mechanism of the increasing infrared emissivity was attributed to the Gd³⁺ and Ti⁴⁺ co-doping can improve the free carrier concentration and the frequency and mode of the lattice vibration. Moreover, the (Sm_0.8Gd_0.2)₂(Hf_0.8Ti_0.2)₂O₇ ceramic coating possessed good thermal resistance, which did not show obvious change in the phase, surface morphology and infrared emissivity after 60 h calcination at 1400 °C.     

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.     

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.     

Ionic conduction and dielectric properties of yttrium doped LiZr2(PO4)3 obtained by a Pechini-type polymerizable complex route

We report on the ion transport properties of Li_1+xZr_2-xY_x(PO₄)₃ (0.05?≤ x?≤?0.2) NASICON type nanocrystalline compounds prepared through a Pechini-type polymerizable complex method. Structural properties were characterized by means of powder X-ray diffraction, Raman spectroscopy and electron microscopy with selected area electron diffraction. Impedance spectroscopy was utilised to investigate the lithium ion transport properties. Y³⁺ doped LiZr₂(PO₄)₃ compounds showed stabilized rhombohedral structure with enhanced total ionic conductivity at 30?°C from 2.87?×?10^?7 S?cm^?1 to 0.65?×?10^?5 S?cm^?1 for x=0.05 to 0.20 respectively. The activation energies of Li_1+xZr_2-xY_x(PO₄)₃ show a decreasing trend from 0.45?eV to 0.35?eV with increasing x from 0.05 to 0.20. The total conductivity of these compounds is thermally activated, with activation energies and pre-exponential factors following the Meyer-Neldel rule. The tanδ peak position shifts to the high-frequency side with increasing yttrium content. Scaling in AC conductivity spectra shows that the electrical relaxation mechanisms are independent of temperature.     

Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu,Sm) based oxygen ion conductors

Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 °C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf_1-xEu_xO₆ octahedra in pyrochlore Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 °C for 24h in air reduces the total conductivity of the Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5–6, due to the decrease in the concentrations of oxygen vacancies and Ln²⁺ ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) at ~800 °C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.     

Study on La2(Hf1-xTix)2O7 (x ≤ 0.20) pyrochlores for potential thermal/environmental barrier coating applications

Lanthanum hafnate (La₂Hf₂O₇) with a pyrochlore structure has excellent high temperature stability and low thermal conductivity, which is promising for thermal/environmental barrier coatings (T/EBCs) applications. To reduce its thermal expansion coefficient (TEC) so as to better match SiC_f/SiC composites, a smaller tetravalent dopant Ti⁴⁺ has been introduced in the Hf-sites to form La₂(Hf_1-xTi_x)₂O₇ (x ≤ 0.20). The phase composition and microstructure confirms that La₂(Hf_1-xTi_x)₂O₇ solid solutions possess a pure pyrochlore structure. With an increase of x, their TECs are decreasing consistently, whilst their thermal conductivities of La₂(Hf_1-xTi_x)₂O₇ are slightly increasing at high temperature but still much lower than those of meta-stable yttria partially stabilized zirconia, both of which are attributing to an increase of elastic modulus after Ti⁴⁺ doping on Hf-sites. The extremely excellent high temperature stability, relatively low thermal conductivities and low TECs suggest that La₂(Hf_1-xTi_x)₂O₇ is a prospective candidate material for T/EBC applications.     

Novel mid-temperature Y3+ → In3+ doped proton conductors based on the layered perovskite BaLaInO4

The possibility of isovalent substitution in the In-sublattice of layered perovskite BaLaInO₄ was experimentally investigated. The yttrium doping leads to the increase of total and protonic conductivity up to 2 orders of magnitude. The solid solution BaLaIn_1?xY_xO₄ (0 ≤ x ≤ 0.5) is mixed ionic-electronic (hole) conductor at the dry air. The increase in the water vapor partial pressure leads to the appearance of protons in the structure below 600 °C, which is confirmed by the decrease of activation energy from ～0.75 eV to ～0.52 eV for doped samples. Both undoped and doped BaLaIn_1?xY_xO₄ samples are nearly pure (～95%) proton conductors under wet air conditions below 400 °C. The most conductive Y-doped compositions BaLaIn_0.8Y_0.2O₄ demonstrates the proton conductivity value 4?10^?5 S/cm at the 400 °C.     

New anti-ablation candidate for carbon/carbon composites: Preparation,composition and ablation behavior of Y2Hf2O7 coating under an oxyacetylene torch

Y₂Hf₂O₇ possesses low thermal conductivity and high melting point, which make it promising for a new anti-ablation material. For evaluating the thermal stability and the potential applications of Y₂Hf₂O₇ on anti-ablation protection of C/C composites, Y₂Hf₂O₇ ceramic powder was synthesized by solution combustion method and Y₂Hf₂O₇ coating was prepared on the surface of SiC coated C/C composites using SAPS. Results shown that the coating exhibits good ablation resistance under the heat flux of 2.4?MW/m² with the linear and mass ablation rates are 0.16?μm?s^?1 and ?0.028?mg?s^?1, respectively, after ablation for 40?s. With the prolonging of the ablation time, the increasing thermal stress causes the increase of cracks. Moreover, the chemical erosion from SiO₂ and the physical volatilization of low temperature molten products aggravate failure of the Y₂Hf₂O₇ coating.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Effect of Zn addition on the structure and electrochemical properties of co-doped BaCe0.6Zr0.2Ln0.2O3-δ (Ln=Y,Gd, Yb) proton conductors

In this work, BaCe_0.6Zr_0.2Y_0.2-xYb_xO_3-δ and BaCe_0.6Zr_0.2Gd_0.2-xYb_xO_3-δ (x?=?0–0.20), proton conducting materials are prepared by the freeze-drying precursor method. The sintering conditions were optimized by adding Zn(NO₃)₂·6H₂O as sintering additive. The materials are thoroughly characterized by different structural and microstructural techniques, including X-ray diffraction, scanning and transmission electron microscopy, and thermogravimetric-differential thermal analysis. The addition of Zn favours the phase formation and densification at lower sintering temperatures; however, it leads to the segregation of a Zn-rich secondary phase, with general formula BaLn₂ZnO₅ (Ln?Y, Gd and Yb), which is identified and quantified for the first time. All samples with Zn as sintering aid exhibit cubic structure; however, the samples without Zn crystallize with orthorhombic or cubic structure, depending on the composition and thermal treatment. The electrical properties are studied by impedance spectroscopy. A deep analysis of the bulk and grain boundary contributions to the conductivity has revealed that the bulk conductivity remains almost unchanged along both series over Yb-doping; however, the grain boundary resistance decreases. The highest conductivity values are found for the intermediate members of both series, BaCe_0.6Zr_0.2Y_0.1Yb_0.1O_3-δ and BaCe_0.6Zr_0.2Gd_0.1Yb₁O_3-δ, with 33 and 28?mS?cm^?1 at 750?°C, respectively.     

Investigation of layered perovskite NdBa0.5Sr0.25Ca0.25Co2O5+δ as cathode for solid oxide fuel cells

Layered perovskite oxides with and without Ca-doped NdBa_0.5Sr_0.25Ca_0.25Co₂O_5+δ (NBSCaCO) and NdBa_0.5Sr_0.5Co₂O_5+δ (NBSCO) are studied to investigate the effects of Ca doping on the crystal structure, thermal behavior, electrical and electrochemical properties. Both NBSCO and NBSCaCO are tetragonal structure with P4/mmm space group. The average thermal expansion coefficient (TEC) value is reduced from 23.3?×?10^?6 K^?1 to 19.8?×?10^?6 K^?1 during 30–800?°C. The electrical conductivities are increased by Ca doping. Both electrical conductivities of NBSCO and NBSCaCO are higher than 600?S·cm^?1 over 30–800?°C. Substitution of Sr with Ca can effectively improve the electrochemical properties of NBSCaCO. From 650?°C to 800?°C, the area specific resistance (ASR) of NBSCaCO are decreased from 0.62 to 0.062?Ω?cm² and the corresponding output power density are increased from 258 to 812?mW?cm^?2. On the basis of these results, Ca doped layered perovskite NBSCaCO can be a good cathode candidate material for SOFC application.     

Mechanical and thermal expansion studies on Ca0.5Sr0.5Zr4-xTixP6O24 ceramics

Ca_0.5Sr_0.5Zr_4-xTi_xP₆O₂₄ (x?=?0?0.2) ceramics belonging to the NZP family were prepared and dense ceramics with no microcracks were obtained. All of the ceramic samples were still composed of the typical NZP structure with a small amount of Ti⁴⁺ substitution for Zr⁴⁺. The mechanical and thermal expansion properties of the ceramics were characterized and the result showed that the flexural strength monotonically increased to 66.5?MPa. The thermal expansion coefficient varied from 1.8 to 3.4?×?10^?6/°C with Ti⁴⁺ content increasing. Thus, it was clear that the substitution of Ti⁴⁺ for Zr⁴⁺ had obvious effects on the sinterability, mechanical and thermal expansion properties of Ca_0.5Sr_0.5Zr_4-xTi_xP₆O₂₄ ceramics, which were discussed in detail.     

Preparation of (Tb1-xLux)2O3 transparent ceramics by solid solution for magneto-optical application

Novel (Tb_1-xLu_x)₂O₃ magneto-optical transparent ceramics were prepared through a solid-solution method via vacuum pre-sintering followed by hot isostatic pressing (HIP). The Lu-containing samples were solid-solution phases between Tb₂O₃ and Lu₂O₃, which effectively stabilized the phase transition of Tb₂O₃ during the sintering process. A typical component of (Tb_1-xLu_x)₂O₃ ceramic with x = 0.5 was selected to study the densification behavior and microstructure evolution in detail. The polished (Tb_0.5Lu_0.5)₂O₃ ceramics with a thickness of 3 mm vacuum pre-sintered at 1700 °C under 1.0 × 10^?3 Pa combined with HIP post-treatment at 1600 °C showed good magneto-optical property. The Verdet constant measured at 633 nm was -224.33 rad·T^?1 m^?1, 30% higher than that of Tb₃Al₅O₁₂ (TAG). However, the in-line transmittance was 73.6% at 633 nm, lower than that of TAG. The (Tb_1-xLu_x)₂O₃ ceramics will be a promising material for Faraday devices after further improving the optical quality.     

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.     

Influence of Yb3+ doping on phase stability and thermophysical properties of (Y1-xYbx)3Al5O12 under high temperature

In this work, Yb³⁺ was selected to replace the Y³⁺ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y_1-xYb_x)₃Al₅O₁₂ (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y_1-xYb_x)₃Al₅O₁₂ ceramics were comprised of a single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xYb_x)₃Al₅O₁₂ ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y_0.7Yb_0.3)₃Al₅O₁₂ had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m⁻¹ K⁻¹ at 1000 °C, around 30% lower than that of pure YAG (2.3 W m⁻¹ K⁻¹, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y_1-xYb_x)₃Al₅O₁₂ ceramics and the CTE was approximate 10.7×10⁻⁶ K⁻¹ at 1200 °C. In addition, (Y_0.7Yb_0.3)₃Al₅O₁₂ ceramic remained good phase stability when heating from room temperature to 1450 °C.     

A new type of Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3−x (Me = Y,Gd) high-entropy ceramics used in thermal barrier coatings

Seeking for new ceramics with excellent thermophysical properties as thermal barrier coatings candidate materials has become a hot research field. In this study, Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x high-entropy ceramic powders were successfully synthesized by the method of solid-state reaction, and the ceramics with single phase were prepared by pressureless sintering at 1600°C. The phase composition, microstructure, element distribution, high-temperature thermal stability, and thermophysical properties of the ceramics were studied. The results showed that Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x ceramics were composed of SrZrO₃ phase and the second phase of AB₂O₄ spinel (i.e., SrY₂O₄ and SrGd₂O₄). The content of the second phase was gradually increased after heat treatment at 1400°C, which significantly improved the thermophysical and mechanical properties of the ceramics. The microhardness and fracture toughness of the ceramics were improved compared with that of SrZrO₃. The thermal conductivities of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 1.30 and 1.28 W m⁻¹ K⁻¹ at 1000°C, which were about 35% and 40% lower than that of SrZrO₃ (1.96 W m⁻¹ K⁻¹) and yttria-stabilized zirconia (2.12 W m⁻¹ K⁻¹), respectively. The thermal expansion coefficients of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 12.8 × 10⁻⁶ and 14.1 × 10⁻⁶ K⁻¹ at 1300°C, respectively, which was more closer to the superalloys compared with SrZrO₃ ceramic (11.0 × 10⁻⁶ K⁻¹).     

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.     

Sc- and W-substitution and tuning of phase transition in the Al2Mo3O12

Al₂Mo₃O₁₂ is a typical negative thermal expansion (NTE) material, whose thermal expansion behavior depends on its crystal phase. The thermal shock caused by temperature-induced phase transition limits its wide application. The two series of Al_{2. x}Sc_xMo₃O₁₂ (0 ≤ x ≤ 1) and Al₂Mo_3-xW_xO₁₂ (0 ≤ x ≤ 2.5) solid solutions with controllable phase transition temperature were synthesized via single cation substitution at the A or B position. The problem of thermal shock caused by the change of temperature is effectively solved in the synthesized Al_1.6Sc_0.4Mo₃O₁₂ and Al₂Mo_0.5W_2.5O₁₂, showing stable NTE performance above room temperature, and the coefficients of thermal expansion of which are ?2.19 × 10^?6 °C^?1 in 100–550 °C and ?4.25 × 10^?6 °C^?1 in 85–500 °C, respectively. A-site cation substitution is a more effective way to tune the thermal expansion properties of Al₂Mo₃O₁₂, which is attributed to the fact that the bond strength of A-O is weaker than that of B–O in the compound.