Potential thermal barrier coating materials: RE3NbO7 (RE=La, Nd,Sm, Eu,Gd, Dy) ceramics

In this work, RE₃NbO₇ ceramics are synthesized via solid‐state reaction and the phase structure is characterized by X‐ray diffraction and Raman spectroscopy. The relationship between crystal structure and thermophysical properties is determined. Except Sm₃NbO₇, each RE₃NbO₇ exhibits excellent high‐temperature phase stability. The thermal expansion coefficients increase with the decreasing RE³⁺ ionic radius, which depends on the decreasing crystal lattice energy and the maximum value reaches 11.0 × 10^?6 K^?1 at 1200°C. The minimum thermal conductivity of RE₃NbO₇ reaches 1.0 W m^?1 K^?1 and the glass‐like thermal conductivity of Dy₃NbO₇ is dominant by the high concentration of oxygen vacancy and the local structural order. The outstanding thermophysical properties pronounce that RE₃NbO₇ ceramics are potential thermal barrier coating materials.     

Synthesis and thermophysical properties of RETa3O9 (RE = Ce,Nd, Sm,Eu, Gd,Dy, Er) as promising thermal barrier coatings

Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa₃O₉ compounds have a defect‐perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa₃O₉ bulk ceramics were fabricated via solid‐state reaction. The crystal structure was characterized by X‐ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermophysical properties of RETa₃O₉ were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients, and high‐temperature phase stability. The thermal conductivities of RETa₃O₉ are very low (1.33‐2.37 W/m·K, 373‐1073 K), which are much lower than YSZ and La₂Zr₂O₇; and the thermal expansion coefficients range from 4.0 × 10^?6 K^?1 to 10.2×10^?6 K^?1 (1273 K), which is close to La₂Zr₂O₇ and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high‐temperature phase stability with these oxides, RETa₃O₉ ceramics were promising candidate materials for TBCs.     

High-entropy ferroelastic rare-earth tantalite ceramic: (Y0.2Ce0.2Sm0.2Gd0.2Dy0.2)TaO4

In this study, high-entropy rare-earth tantalate ceramics (Y_0.2Ce_0.2Sm_0.2Gd_0.2Dy_0.2)TaO₄ ((5RE_0.2)TaO₄) have been successfully fabricated. The possibility of formation of (5RE_0.2)TaO₄ was verified via first-principles calculations. In addition, the phase structure, ferroelastic toughening mechanism, thermophysical, and mechanical properties were systematically investigated. The (5RE_0.2)TaO₄ ceramics have lower phonon thermal conductivity (1.2–2.6 W·m^–1·K^–1) in the entire temperature range than that of RETaO₄ and YSZ. (5RE_0.2)TaO₄ has a higher fracture toughness and lower brittleness index than YSZ. The thermal expansion coefficients of (5RE_0.2)TaO₄ are as high as 10.3 × 10^-6 K^–1 at 1200°C and Young's modulus is 66–189 GPa, and thus, (5RE_0.2)TaO₄ possesses great potential for application in thermal barrier coatings (TBCs).     

Calcium-magnesium-alumina-silicate (CMAS) resistant Ba2REAlO5 (RE = Yb,Er, Dy) ceramics for thermal barrier coatings

Calcium-magnesium-alumina-silicate (CMAS) attack has been a great challenge for the application of thermal barrier coatings (TBCs) in modern turbine engines. In this study, a series of prospective TBC candidate materials, Ba₂REAlO₅ (RE = Yb, Er, Dy), are found to have high resistance to CMAS attack. The rapid formation of a continuous crystalline layer on sample surface contributes to this desirable attribute. At 1250 °C, Ba₂REAlO₅ dissolve in the molten CMAS, accumulating Ba, RE and Al in the melt, which could trigger the crystallization of celsian, apatite and wollastonite crystals. Especially, the formation of the crystalline layer in the Ba₂DyAlO₅ sample is the fastest. This study also reveals that Ba is a useful element for altering CMAS composition to precipitate celsian. Thus, doping Ba²⁺ in yttria partially stabilized zirconia or other novel TBCs might be an attractive way of mitigating CMAS attack.     

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.     

Calcium-magnesium-alumina-silicate (CMAS) resistance characteristics of LnPO4 (Ln = Nd,Sm, Gd) thermal barrier oxides

Calcium-magnesium-alumina-silicate (CMAS) attack has been considered as a significant failure mechanism for thermal barrier coatings (TBCs). As a promising series of TBC candidates, rare-earth phosphates have attracted increasing attention. This work evaluated the resistance characteristics of LnPO₄ (Ln = Nd, Sm, Gd) compounds to CMAS attack at 1250 °C. Due to the chemical reaction between molten CMAS and LnPO₄, a dense, crack-free reaction layer, mainly composed of Ca₃Ln₇(PO₄)(SiO₄)₅O₂ apatite, CaAl₂Si₂O₈ and MgAl₂O₄, was formed on the surface of compounds, which had positive effect on suppressing CMAS infiltration. The depth of CMAS penetration in LnPO₄ (Ln = Nd, Sm, Gd) decreased in the sequence of NdPO₄, SmPO₄ and GdPO₄. GdPO₄ had the best resistance characteristics to CMAS attack among the three compounds. The related mechanism was discussed based on the formation ability of apatite phase caused by the reaction between molten CMAS and LnPO₄.     

Variable thermochemical stability of RE2Si2O7 (RE = Sc,Nd, Er,Yb, or Lu) in high-temperature high-velocity steam

Five rare-earth (RE) disilicates (RE₂Si₂O₇, RE = Sc, Nd, Er, Yb, or Lu) were synthesized and exposed to high-velocity steam (up to 235 m/s) for 125 hours at 1400°C. Water vapor reaction products, mass loss, average reaction depths, and product phase microstructural evolution were analyzed for each material after exposure. Similar to steam testing results in the literature, RE₂Si₂O₇ (RE = Er, Yb, Lu) underwent silica depletion producing gaseous silicon hydroxide species, RE₂SiO₅, and RE₂O₃ product phases. Sc₂Si₂O₇ reacted with high-velocity steam to produce only a Sc₂O₃ product layer with no stable Sc₂SiO₅ phase detected by X-ray diffraction or microscopy techniques. Further, Nd₂Si₂O₇ rapidly reacted with steam to produce with no Nd₂SiO₅ or Nd₂O₃ reaction products. All RE₂Si₂O₇ that produced a silicate reaction product (RE = Nd, Er, Yb, Lu) showed densification of the product phase at steam velocities above 150 m/s that resulted in enhanced resistance. The results presented in this work demonstrate that rare-earth silicates show diverse steam reaction products, reaction product microstructures, and total reaction depths after high-temperature high-velocity steam exposure. Of the materials in this study, RE₂Si₂O₇ (RE = Yb, Lu) were most stable in high-temperature high-velocity steam, making them most desirable as environmental barrier coating candidates.     

Dissolution,bioactivity behavior,and cytotoxicity of rare earth-containing bioactive glasses (RE = Gd,Yb)

Rare earth-containing bioactive glasses (RE-BGs) have been poorly explored in the biomaterials field, although RE has optical, nuclear, and magnetic properties that could be used in different biomedical applications. In order to verify whether these glasses can be promising as biomaterials, we studied the dissolution, bioactivity, and cytotoxicity of RE-BGs based on the SiO₂–Na₂O–CaO–P₂O₅–RE₂O₃ (RE = Gd, Yb) system. The glasses were obtained by melting-quenching and their particle size was determined by laser diffraction. Their dissolution behavior was studied in Tris-HCl, while bioactivity was performed in simulated body fluid solution under physiological conditions during several periods. The cytotoxicity test was performed using glass-derived conditioned medium and mesenchymal stem cell derived from deciduous teeth. The dissolution results showed that the glasses dissolved under two different kinetics, which are lower for rare earth-containing glasses, due to the more covalent character of Si–O–RE bonds. The bioactivity results evidenced that all glasses showed bioactivity after 24 hours. However, gadolinium and ytterbium promoted a more calcium phosphate deposition, which contrasts with the slower dissolution kinetics of rare earth-containing glasses. All the glasses were considered biocompatible, showing cell viability higher than 80%. The overall results showed that RE-BGs are promising materials for applications that require bioactivity and/or biocompatibility.     

Electrical properties of (Na2O)35.7(RE2O3)7.2(SiO2)57.1 (RE = Y,Sm, Gd,Dy, Ho,Er and Yb) glasses and ceramics

Fifteen kinds of sodium rare earth silicate glasses and ceramics with (Na₂O)_35.7(RE₂O₃)_7.2(SiO₂)_57.1 (RE = Y, Sm, Gd, Dy, Ho, Er and Yb) composition were synthesized from a mixture of Na₂CO₃, RE₂O₃ and SiO₂. The densities of the glasses were in fairly good agreement with the theoretical densities and were 0.2–0.41 g cm⁻³ larger than those of the polycrystalline ceramics. The conductivities of the glasses are 1–2 orders lower than those of the ceramics and the highest electrical conductivity was achieved for the Yb ceramic sample with the smallest ion radius of RE³⁺. The electromotive force, EMF, of the potentiometric CO₂ gas sensors using (Na₂O)_35.7(Y₂O₃)_7.2(SiO₂)_57.1 glass and ceramic increased linearly with an increase in the logarithm of CO₂ partial pressure, in accordance with Nernst's law. It was suggested from the slope of Nernst's equation that the two electron-transfer reaction associated with the carbon dioxide molecule takes place at the detection electrode above 450 °C.     

Structure and dielectric properties of Ln3NbO7 (Ln = Nd,Gd, Dy,Er, Yb and Y)

The structure and dielectric properties of Ln₃NbO₇ (Ln = Nd, Gd, Dy, Er, Yb and Y) ceramics are investigated. With decreasing ionic radius of Ln³⁺, the stable crystal structure of the compounds shifts from orthorhombic weberite to defect fluorite. It is experimentally observed that, with the exception of Gd₃NbO₇, the room temperature real part of the relative permittivity of Ln₃NbO₇ ceramics decreases from approximately 40 to 30 (at 1 MHz) with increasing ionic radius of Ln³⁺. The observed imaginary part of the relative permittivity is in order of 10⁻² to 10⁻¹ (room temperature and 1 MHz) and it is relatively stable up to 80 °C, where it increases with a rise in temperature. Interesting exceptions of these trends are Nd₃NbO₇ that crystallizes with a weberite-type structure and shows large positive temperature variation of the dielectric properties, and Gd₃NbO₇ that crystallized in a weberite related structure and displays frequency and temperature dependent dielectric relaxation behavior.     

The effect of ZrO2 alloying on the microstructures and thermal properties of DyTaO4 for high-temperature application

High fracture toughness of 8 YSZ (8 wt% yttria-stabilized zirconia) is linked to its ferroelastic toughening mechanism. In this work, the similar ferroelastic domain is detected in monoclinic Dy_1−xTa_1−xZr_2xO₄ ceramics, which derives from the ferroelastic transformation between the high-temperature tetragonal (t) and low-temperature monoclinic (m) phase. The lowest thermal conductivity of Dy_1−xTa_1−xZr_2xO₄ ceramics is reduced by 30% compared with 8 YSZ, and the largest thermal expansion coefficients (TECs) is up to 11 × 10⁻⁶ K⁻¹ at 1200°C, which is comparable to that of 8 YSZ. Notably, the systematic investigations containing phase, microstructure, thermophysical properties of Dy_1−xTa_1−xZr_2xO₄ ceramics will provide guidance for its high-temperature application, especially as thermal barrier coatings.     

Features of crystal structures and thermo-mechanical properties of weberites RE3NbO7 (RE=La,Nd, Sm,Eu, Gd) ceramics

The features of crystal structures, thermo-mechanical properties and their dominant mechanisms of weberites RE₃NbO₇ were studied as high-temperature oxides. We concentrated on connections between structures and thermo-mechanical properties, the influences of bond lengths, lattice distortion degrees and microstructures on these properties were estimated. The shortening of bond length and increment of bonding strength would lead to the increase of mechanical properties. The Vickers hardness (4.5-7.8 GPa) and toughness (0.5-1.6 MPa·m^1/2) of weberites RE₃NbO₇ are enhanced by grain refinement and increment of bond strength, while crystal structures, bond lengths, and lattice distortion degrees influenced their Young's modulus (100-170 GPa). Nano-indentation was applied to test the influence of microstructures on modulus and hardness. The dominant mechanisms for mechanical properties and thermal conductivity were proposed, which was conducive to properties tailoring and engineering applications of weberites RE₃NbO₇ oxides.     

Investigation of the thermophysical properties of (Y1-xYbx)TaO4 ceramics

As a novel candidate of thermal barrier coatings materials proposed recently, rare earth tantalates (RETaO₄) are attractive series because of its excellent attributes. With respect to RETaO₄ compound, it exists at least three types of polymorphs and two phase-transformation mechanisms. In this paper, the monoclinic (Y_1-xYb_x)TaO₄ compounds with diverse space groups are investigated systematically, and contents involve phase, microstructural and thermophysical properties’ evolution with the changes of composition and structure. The primary purpose of this paper is to optimize the thermophysical properties of (Y_1-xYb_x)TaO₄ ceramics by adjusting the Y/Yb ratio, and further physical mechanisms of its heat transfer are also revealed.     

(Sm0.7Yb0.3)2Ce2O7陶瓷的热膨胀系数及热导率

以Sm2O3、Yb2O3和CeO2为原材料,采用固相反应法制备了(Sm0.7Yb0.3)2Ce2O7陶瓷材料,用X射线衍射(XRD分析了其相结构,采用扫描电子显微镜(SEM)和电子能谱(EDS)分析其显微组织和元素组成,用推杆膨胀法和激光脉冲法测试了其热膨胀系数和热导率.结果表明,所制备的(Sm0.7Yb0.3)2Ce2O7具有典型的萤石结构,其微观组织致密,晶界清晰.Yb3+离子较小的离子半径使其热膨胀系数低于Sm2Ce2O7,基质原子与取代原子之间质量及尺寸之间的差别,使其具有比Sm2Ce2O7更低的热导率,该材料有潜力用作新型热障涂层表面陶瓷层材料.     

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

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

Thermal expansion performance and intrinsic lattice thermal conductivity of ferroelastic RETaO4 ceramics

Ferroelastic RETaO₄ ceramics are promising thermal barrier coatings (TBCs) because of their attractive thermomechanical properties. The influence of crystal structure distortion degree on thermomechanical properties of RETaO₄ is estimated in this work. The relationship between Young's modulus and TECs is determined. The highest TECs (10.7 × 10⁻⁶ K⁻¹, 1200°C) of RETaO₄ are detected in ErTaO₄ ceramics and are ascribed to its small Young's modulus and low Debye temperature. The intrinsic lattice thermal conductivity (3.94-1.26 W m⁻¹ K⁻¹, 100-900°C) of RETaO₄ deceases with increasing of temperature due to an elimination in thermal radiation effects. The theoretical minimum thermal conductivity (1.00 W m⁻¹ K⁻¹) of RETaO₄ indicates that the experimental value is able to be reduced further. We have delved deeply into the thermomechanical properties of ferroelastic RETaO₄ ceramics and have emphasized their high-temperature applications as TBCs.     

Novel high-entropy microwave dielectric ceramics Sr(La0.2Nd0.2Sm0.2Eu0.2Gd0.2)AlO4 with excellent temperature stability and mechanical properties

Novel high-entropy Sr(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)AlO₄ ceramics with a layered perovskite structure have been prepared via the standard solid-state reaction method. The design of high-entropy improves the bond valence and subsequently optimizes the large negative temperature coefficient of resonant frequency (τ_f = ?32 ppm/°C) of the simple SrLaAlO₄ ceramics. Excellent temperature stability (τ_f = ?6 ppm/°C) together with a relative permittivity (ε_r) of 18.6 and a quality factor (Qf = 14,509 GHz) are obtained in Sr(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)AlO₄ ceramics sintered at 1475 °C. It indicates that the present ceramics have great application prospects in passive microwave components such as resonators and filters. Meanwhile, significant improvements in compressive strength and strain are achieved, which are 1040 MPa and 15.7% for Sr(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)AlO₄ compared to 583 MPa and 12% in SrLaAlO₄. The enhanced mechanical properties originate from the dislocation strengthening mechanism as the intertwining of interlayer lattices is revealed from the high-resolution transmission electronic micrographs.     

Sintering and microstructure of rare earth phosphate ceramics REPO4 with RE = La,Ce or Y

Sintering of rare earth phosphates REPO₄ (RE = La, Ce or Y) was studied using dilatometry. The presence of a secondary rare earth metaphosphate phase RE(PO₃)₃ as sintering aid was investigated. It proved to accelerate the densification but it activated fast grain growth, which was very detrimental to the microstructural design of processed ceramics. A temperature of 1400–1450 °C was required to sinter pure LaPO₄ and CePO₄ ceramics with fine grains. Both compounds behave similar while YPO₄ did not densify even at 1500 °C. The influence of specific surface area of starting powders, temperature and holding time on the sintering rate and microstructures of dense REPO₄ materials is also reported.     

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

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