Impact of ZrO2 alloying on thermo-mechanical properties of Gd3NbO7

The ZrO₂ alloying effect is widely used to optimize the thermo-mechanical properties of potential thermal barrier coatings. In this study, dense x mol% ZrO₂-Gd₃NbO₇ with C222₁ space group were manufactured via a solid-state reaction. The crystalline structure was determined through X-ray diffraction and Raman spectroscopy, when the surface morphology was observed by scanning electron microscopy. ZrO₂-Gd₃NbO₇ had identical orthorhombic crystal structures, and there was no second phase. The crystalline structure of ZrO₂-Gd₃NbO₇ shrunk with the increasing ZrO₂ content as indicated by XRD and Raman results. The heat capacity and thermal diffusivity of ZrO₂-Gd₃NbO₇ were 0.31–0.43 J g⁻¹ K⁻¹ (25–900 °C) and 0.25–0.70 mm²/s (25–900 °C), respectively. It was found that ZrO₂-Gd₃NbO₇ had much lower thermal conductivity (1.21–1.82 W m⁻¹ K⁻¹, 25–900 °C) than YSZ (2.50–3.00 W m⁻¹ K⁻¹) and La₂Zr₂O₇ (1.50–2.00 W m⁻¹ K⁻¹). The thermal expansion coefficients (TECs) were higher than 10.60 × 10⁻⁶ K⁻¹ (1200 °C), which were better than that of YSZ (10.00 × 10⁻⁶ K⁻¹) and La₂Zr₂O₇ (9.00 × 10⁻⁶ K⁻¹). The mechanical properties of Gd₃NbO₇ change little with the increasing ZrO₂ content, Vickers hardness was about 10 GPa, and Young's modulus was about 190 GPa, which was lower than YSZ (240 GPa). Compared with previous work about alloying effects, much lower thermal conductivity was obtained. Due to the high melting point, high hardness, low Young's modulus, ultralow thermal conductivity and high TECs, it is believed that ZrO₂-Gd₃NbO₇ is promising TBCs candidate.     

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.     

Thermal properties of (Zr0.2Ce0.2Hf0.2Y0.2RE0.2)O1.8 (RE= La,Nd and Sm) high entropy ceramics for thermal barrier materials

The high-entropy ceramic materials (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_1.875 (H-0) and (Zr_0.2Ce_0.2Hf_0.2Y_0.2RE_0.2)O_1.8 (H-RE) (RE = La, Nd and Sm) with fluorite structure and homogeneous element distribution were prepared. With fluorite structure, fine grain size and high density, the H-0 and H-RE ceramics displayed low thermal conductivity, suitable thermal expansion coefficient, high hardness and fracture toughness. The effect of La, Nd and Sm on the mechanical, heat conductivity and heat expansion properties of high entropy ceramics were discussed. The single-phase high-entropy ceramic materials in this work are very suitable for application as thermal barrier materials.     

The thermo-mechanical properties and ferroelastic phase transition of RENbO4 (RE = Y,La, Nd,Sm, Gd,Dy, Yb) ceramics

In this work, RENbO₄ (RE = Y, La, Nd, Sm, Gd, Dy, Yb) ceramics with low density, low Young's modulus, low thermal conductivity, and high thermal expansion have been systematically investigated, the excellent thermo-mechanical properties indicate that RENbO₄ ceramics possess the potential as the new generation of thermal barrier coatings (TBCs) materials. X-ray diffraction and Raman spectroscopy phase structure identification reveal that all dense bulk specimens obtained by high-temperature solid-state reaction belonged to the monoclinic (m) phase with C12/c1 space group. The ferroelastic domains are detected in the specimens, revealing the ferroelastic transformation between tetragonal (t) and monoclinic (m) phases of RENbO₄ ceramics. The Young's modulus and hardness of the RENbO₄ ceramics measured by the NanoBlitz 3D nanoindentation method are discussed in details, and the lower Young's modulus (60-170 GPa) and higher hardness (the maximum value reaches 11.48 GPa) indicating that higher resistance of RENbO₄ ceramics to failure and damage. Lower thermal conductivity (1.42-2.21 W [m k]⁻¹ at 500°C-900°C) and lower density (5.330-7.400 g/cm³) than other typical TBCs materials give RENbO₄ ceramics the unique advantage of being new TBCs materials. Meanwhile, the thermal expansion coefficients of RENbO₄ ceramics reach 9.8-11.6 × 10⁻⁶ k⁻¹ and are comparable or higher than other typical TBCs materials. According to the first-order derivative of the thermal expansion rate, the temperature of the ferroelastic transformation of RENbO₄ ceramics can be observed.     

Towards thermal barrier coating application for rare earth silicates RE2SiO5 (RE= La,Nd, Sm,Eu, and Gd)

Rare earth (RE) silicates X1-RE₂SiO₅ (RE = La, Nd, Sm, Eu, and Gd) are comprehensively investigated as promising thermal barrier coating candidates. The mechanical, thermal, and corrosion resistance properties are evaluated by theoretical exploration and experimental measurement. Mechanical properties and corrosion resistance to calcium-magnesium alumino-silicates (CMAS) melts of X1-RE₂SiO₅ are linearly correlated with ionic radius of RE elements. Elastic moduli increase with the decrease of ionic radius of RE³⁺. X1-RE₂SiO₅ with larger RE³⁺ exhibits better resistance to molten melts corrosion. For thermal properties, they are not obviously sensitive to RE species. All X1-RE₂SiO₅ demonstrate low thermal conductivities and their magnitudes are significantly modified by concentration of defects. Thermal expansion coefficients of X1-RE₂SiO₅ are more or less close and are compatible with the value of superalloy. The results highlight X1-RE₂SiO₅ as potential thermal barrier coating candidates with overall properties.     

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.     

Structure and microwave dielectric properties of ALn4(MoO4)7 (A = Ba,Sr, Ca,Ln = La,Pr, Nd,and Sm) ceramics

ALn₄(MoO₄)₇ (A = Ba, Sr, Ca, Ln = La, Pr, Nd, Sm) ceramics are prepared by solid state ceramic route and the structural properties have been studied using powder X-ray diffraction and laser Raman spectroscopy. All the ceramics under study are phase pure except BaLn₄(MoO₄)₇ (Ln = Pr, Nd, Sm). Scanning electron micrographs of the sintered ceramics show closely packed microstructure with phase homogeneity. BaLa₄(MoO₄)₇ ceramic has a maximum density of 4.5 g/cm³ at 710°C together with ԑ_r = 11.8, Q_u x f = 39 300 GHz, and τ_f = −68 ppm/^oC. SrLa₄(MoO₄)₇ ceramic exhibited a maximum density of 4.4 g/cm³, ԑ_r = 11.7, Q_u x f = 44 200 GHz, and τ_f = −83 ppm/^°C at 740°C whereas CaLa₄(MoO₄)₇ ceramic possess a maximum density of 4.2 g/cm³, ԑ_r = 11.4, Q_u x f = 30 200 GHz and τ_f = −90 ppm/^oC at 750°C at microwave frequencies. The chemical compatibility of the BaLa₄(MoO₄)₇, SrLa₄(MoO₄)₇ and CaLa₄(MoO₄)₇ ceramics with silver electrode have been studied using powder X-ray diffraction of the co-fired samples and is further examined with energy dispersive X-ray spectroscopy.     

Thermal Conductivity of Monazite-Type REPO4 (RE=La, Ce, Nd, Sm, Eu, Gd)

Low-thermal conductivity ceramics in monazite-type REPO₄ (RE=La, Ce, Nd, Sm, Eu, Gd) ceramics are expected to have potential in structural (refractories, thermal insulator) and nuclear applications. To this end, the present study determines their thermal conductivities and examines how differences of the rare earth ions change their thermal conductivity at different temperatures. The results show that their conductivities are remarkably low from 25° to 1000°C. In addition, different conductivity variation mechanisms exist that change gradually upon altering from LaPO₄ to GdPO₄ at low and high temperatures. At relatively lower temperatures (≤400°C), the thermal conductivities of all the REPO₄ ceramics decrease nearly at first, reach a minimum value, and then rise with gradual altering from LaPO₄ to GdPO₄. It may be due to the combined effects of the increase of both the anharmonicities in lattice vibrations and the bond strength. As the temperature increases, the conductivity trends become obscure, and the conductivities of the monazite-type REPO₄ approach their minimum thermal conductivities when the temperature is above 800°C.     

Structure and dielectric properties of novel low temperature co-fired Bi2O3-RE2O3-MoO3 (RE=Pr,Nd, Sm,and Yb) based microwave ceramics

Novel monoclinic Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based low temperature co-fired ceramics (LTCC) systems with high sintering density and low microwave dielectric loss are synthesized by conventional solid state reaction technique. The structure and dielectric properties of Bi₂O₃-xRE₂O₃-yMoO₃ ceramics are investigated. Dense BiNdMoO₆ ceramics sintered at 900 °C for 8 h in air have a low dielectric constant ε_r=~7.5, a high quality factor Q×f=~ 24, 800 GHz at 7.0 GHz, and τ_f=~−16 ppm/̊C. Especially, good chemical compatibility of BiNdMoO₆ with Ag electrodes is represented as well. In contrast, BiSmMoO₆ ceramics sintered at 1000 °C for 8 h show enhanced Q×f=~43, 700 GHz at 7.8 GHz with ε_r=~8.5 and τ_f=~−27 ppm/°C. Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based ceramics could be considered as promising microwave ceramics for LTCC applications.     

Crystal structures,bond characteristics,and dielectric properties of novel middle-εr Ln3NbO7 (Ln = Nd,Sm) microwave dielectric ceramics with opposite temperature coefficients

This study synthesized two novel middle-ε_r Ln₃NbO₇ (Ln = Nd, Sm; named NNO and SNO) microwave dielectric ceramics through the classic solid-state process. The results of XRD and Rietveld refinement show that NNO and SNO ceramics formed pure phases with the space group Cmcm (63) and C222₁ (20), respectively. The properties of Ln-O and Nb–O bonds of NNO and SNO ceramics were calculated based on the P–V–L theory. The Nb–O bonds positively affect the crystal structure stability of the two ceramics. The optimum microwave dielectric properties were obtained (NNO: ε_r = 31.61, Q·f = 6,615 GHz (at 6.10 GHz), and τ_f = ?455.70 ppm/°C; SNO: ε_r = 34.55, Q·f = 11,625 GHz (at 5.77 GHz) and τ_f = 72.59 ppm/°C) when the samples sintered at 1550 °C. Notably, SNO ceramic shows a low dielectric loss and medium dielectric constant, and the opposite τ_f of NNO and SNO ceramics provide the possibility to fabricate microwave dielectric devices with good temperature stability.     

Mechanical properties,thermal expansion performance and intrinsic lattice thermal conductivity of AlMO4 (M=Ta,Nb) ceramics for high-temperature applications

The vital thermo-mechanical properties of thermal and environment barrier coatings (TBCs/EBCs) include high hardness, low Young's modulus, matching thermal expansion coefficients (TECs) with substrate and low thermal conductivity. The effect of distortion degree of crystal structure on thermo-mechanical properties of AlMO₄ (M=Ta, Nb) ceramics are assessed in this work. AlMO₄ ceramics display modest TECs and no phase transformation is detected from room temperature to 1200?℃. The experiment thermal conductivity can be dropped further as the theoretical minimum thermal conductivity of AlTaO₄ and AlNbO₄ is 1.48?W?m^?1 K^?1 and 1.05?W?m^?1 K^?1, respectively. The temperature dependent phonon thermal diffusivity of AlMO₄ ceramics has been confirmed; the intrinsic lattice thermal conductivity is determined. The extraordinary thermo-mechanical properties make it clear that AlMO₄ ceramics are suitable for high-temperature applications.     

Modulation of microwave dielectric properties in melilite-structured SrREGa3O7 (RE = La,Pr) ceramics

Gallium-based SrREGa₃O₇ (RE = La, Pr) melilite ceramics were prepared and selected to modify their microwave dielectric properties. Sintered at 1425 °C for 6 h, SrLaGa₃O₇ (SLGO) and SrPrGa₃O₇ (SPGO) ceramics exhibited high relative densities of 98.33 and 95.23%, low ε_r values of 11.8 and 10.9, Q×f values of 32,500 GHz (at 12.1 GHz) and 26,400 GHz (at 12.7 GHz), negative τ_f values of ?32 and ?54 ppm/°C. As a compensator, CaTiO₃ can tune the τ_f values of SLGO and SPGO to near zero (+2 and ?4 ppm/°C). In SrREGa₃O₇ melilite ceramics, the ε_r and τ_f values are mainly dependent on ionic polarizability, crystal structure and the “rattling” effect. The micromorphology, XPS, Raman spectrum and A-site bond valence (V_RE) of SrREGa₃O₇ (RE = La, Pr) microwave dielectric ceramics have also been comprehensively reported.     

Low temperature coefficient dielectric materials,PbRETiTaO7 (RE=Y,La, Nd,Sm, Gd,or Dy) having pyrochlore type structure

Abstract

The present work investigates the dielectric properties of pyrochlore type oxides, PbRETiTaO₇ (RE=Y, La, Nd, Sm, Gd, or Dy) in the low frequency region (100 kHZ–1 MHz) at temperatures between 30 and 100°C and the microwave frequency region. The 1 MHz dielectric constants (K) are in the range 43–99 and show somewhat low variation with temperature (30–100°C) as well as frequency (1 kHz to 1 MHz). The temperature coefficient of dielectric constant (TCK) over the temperature range 30–100°C is negative and varies in the range, ?72 to ?342 ppm °C^?1. The dielectric constant in the microwave frequency region is in the range 23–43. Among the samples, PbGdTiTaO₇ shows very good quality factor (Q×f) of 4008 in the microwave frequency region. They all have cubic pyrochlore type structure as indicated by powder X-ray diffraction patterns. The sintered microstructure shows well formed grains without much porosity.     

Crystal Structure and Microwave Dielectric Properties of LiRE9(SiO4)6O2 Ceramics (RE = La,Pr, Nd,Sm, Eu,Gd, and Er)

The crystal structure and microwave dielectric properties of apatite‐type LiRE₉(SiO₄)₆O₂ ceramics (RE = La, Pr, Nd, Sm, Eu, Gd, and Er) have been investigated. The densification of lithium apatites has been greatly improved with the addition of 1 wt% LiF. Selected area electron diffraction and X‐ray diffraction (XRD) Rietveld analysis confirm that these compounds belong to the P6₃/m (No. 176) space group with hexagonal crystal symmetry. The porosity‐corrected relative permittivity was found to decrease with decreasing ionic polarizability of RE³⁺ ions. Relationships between the structural parameters and microwave dielectric properties have been examined. The observed variation in the quality factor of LiRE₉(SiO₄)₆O₂ + 1 wt% LiF ceramics (RE = La, Pr, and Nd) was correlated with average cation covalency (%). The temperature coefficient of resonant frequency was found to depend on the bond valence sum of cations. LiEr₉(SiO₄)₆O₂ + 1 wt% LiF ceramics showed good microwave dielectric properties with ε_r = 12.8, Q_u × f = 13000 GHz and τ_f = +17 ppm/°C. All the compositions showed low coefficient of thermal expansion with thermal conductivity in the range 1.3–2.8 W (m K)^?1.     

Reactive flash sintering of high-entropy oxide (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7: Microstructural evolution and aqueous durability

Herein, the phase evolution, densification and grain growth process of the high entropy ceramics during flash sintering were systematically characterized and quantified to understand the microstructural evolution for the first time. It was demonstrated that the densification rate of (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ by flash sintering in this work was generally around 60 times that of conventional sintering at 1600 °C, while the grain growth rate by flash sintering was only around 1.5–6 times that of conventional sintering, indicating that grain growth was suppressed during flash sintering. The grain growth mechanisms by flash sintering and conventional sintering could be both attributed to surface diffusion and volume diffusion. In addition, the flash sintered high-entropy ceramics as promising immobilization materials for high-level radioactive waste (HLW) exhibited excellent aqueous durability with normalized leaching rates of Nd, Gd and Zr approximately 10^?6～10^?7 g m^?2 d^?1 after 42 days, which were much lower than most reported pyrochlore materials.     

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.     

Multiphonon scattering mechanisms to limit thermal conductivity in weberite RE3NbO7: A case study of (La1-xGdx)3NbO7 ceramics

The management of thermal conductivity is of significant scientific interest, particularly for thermal barrier coatings (TBCs). Multifarious strategies have been used to regulate heat transportation, but it is hard to achieve limit thermal conductivity at elevated temperatures. A systematical investigation of weberite (La_1-xGd_x)₃NbO₇ was thus performed, and multiphonon scattering mechanisms were introduced to achieve limit thermal conductivity (0.92 W m^?1 K^?1). Phonon point defect scattering process accounted for thermal conductivity reduction at low temperatures. Additionally, lattice softening strongly contributed to the reduction of high-temperature thermal conductivity, and solid and stiff chemical bonds were beneficial for inhibiting thermal radiative conductivity. A novel strategy was presented to modify thermal transportation property of weberite RE₃NbO₇ ceramics. Also, the hardness, toughness, and modulus were improved to promote engineering applications of weberite RE₃NbO₇. This study also illuminates novel paths for thermal management and mechanical properties manipulation of TBCs, thermoelectric materials, and microelectronics.     

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.     

Investigation of anisotropic mechanical properties of textured KSr2Nb5O15 ceramics via ab‐initio calculation and nanoindentation

Anisotropic mechanical properties of KSr₂Nb₅O₁₅ (KSN) crystals were investigated through first‐principles calculations based on density functional theory. These properties were experimentally verified via nanoindentation on textured KSN ceramics fabricated, using a reactive template grain growth method. Nanoindentation was performed in directions parallel and perpendicular to the [001] direction of samples consisting of highly oriented grains with tetragonal symmetry. Calculations revealed that Nb‐O yields a relatively strong covalent effect and Nb‐O octahedral distortions induce spontaneous polarization in the KSN crystal. The measured indentation modulus values concurred with the predictions, based on the calculated elastic constants, as indicated by an anisotropic ratio of ~10% between the 2 tested orientations. The hardness exhibited negligible anisotropy. However, the predictions revealed a pronounced anisotropy of the Young's modulus (ratio of ~40% between the [100] direction and a direction tilted by ~45° from the [001] toward the [100]).     

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.