Yb2O3 and Gd2O3 doped strontium zirconate for thermal barrier coatings

Yb₂O₃ (10 mol%) and Gd₂O₃ (20 mol%) doped SrZrO₃ was investigated as a material for thermal barrier coating (TBC) applications. The thermal expansion coefficients (TECs) of sintered bulk Sr(Zr_0.9Yb_0.1)O_2.95 and Sr(Zr_0.8Gd_0.2)O_2.9 were recorded by a high-temperature dilatometer and revealed a positive influence on phase transformations of SrZrO₃ by doping Yb₂O₃ or Gd₂O₃. The results for the thermal conductivities of Sr(Zr_0.9Yb_0.1)O_2.95 and Sr(Zr_0.8Gd_0.2)O_2.9 indicated that both dopants can reduce the thermal conductivity of SrZrO₃. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense Sr(Zr_0.9Yb_0.1)O_2.95 and Sr(Zr_0.8Gd_0.2)O_2.9 showed lower Young's modulus, hardness and comparable fracture toughness with respect to YSZ. The cycling lifetimes of Sr(Zr_0.9Yb_0.1)O_2.95/YSZ and Sr(Zr_0.8Gd_0.2)O_2.9/YSZ double layer coatings (DLC), which were prepared by plasma spraying, were comparable to that of YSZ at operating temperatures <1300 °C. However, the cycling lifetime of Sr(Zr_0.9Yb_0.1)O_2.95/YSZ DLC was 25% longer, whereas Sr(Zr_0.8Gd_0.2)O_2.9/YSZ DLC had a shorter lifetime compared to the optimized YSZ coating at operating temperatures >1300 °C.     

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

(Gd_1−xYb_x)₂Zr₂O₇ compounds were synthesized by solid reaction. Yb₂O₃ doped Gd₂Zr₂O₇ exhibited lower thermal conductivities and higher thermal expansion coefficients (TECs) than Gd₂Zr₂O₇. The TECs of (Gd_1−xYb_x)₂Zr₂O₇ ceramics increased with increasing Yb₂O₃ contents. (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) ceramic exhibited the lowest thermal conductivity among all the ceramics studied, within the range of 0.8–1.1 W/mK (20–1600 °C). The Young's modulus of GYbZ bulk is 265.6 ± 11 GPa. GYbZ/YSZ double-ceramic-layer thermal barrier coatings (TBCs) were prepared by electron beam physical vapor deposition (EB-PVD). The coatings had an average life of more than 3700 cycles during flame shock test with a coating surface temperature of ∼1350 °C. Spallation failure of the TBC occurred by delamination cracking within GYbZ layer, which was a result of high temperature gradient in the GYbZ layer and low fracture toughness of GYbZ material.     

Phase structure and thermal conductivities of Er2O3 stabilized ZrO2 toughened Gd2Zr2O7 ceramics for thermal barrier coatings

3.5 mol% Er₂O₃ stabilized ZrO₂ (ErSZ) and Gd₂Zr₂O₇ powders were produced by a chemical co-precipitation and calcination method, and ErSZ was used to toughen Gd₂Zr₂O₇. The phase structure, toughness and thermal conductivities of ErSZ toughened Gd₂Zr₂O₇ ceramics were investigated. When the ErSZ content was below 15 mol%, the compound consisted of pyrochlore phase, the ordering degree of which decreased with the increase of the ErSZ content. High ErSZ doping led to the formation of metastable tetragonal (t′) phase in the compound. The addition of ErSZ in Gd₂Zr₂O₇ benefited its toughness, mainly attributable to the presence of t′ phase in the compound. With the increase of the ErSZ content in the compound, the thermal conductivity first decreased and then showed an upward tendency, and 10 mol% ErSZ toughened Gd₂Zr₂O₇ exhibited the lowest thermal conductivity.     

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.     

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.     

Yb2O3-Gd2O3 codoped strontium zirconate composite ceramics for potential thermal barrier coating applications

Strontium zirconate (SrZrO₃) has been considered as a promising thermal barrier coating (TBC) material for application in gas turbine engines; however, the phase transition problem limits its application. In this study, an Yb₂O₃ and Gd₂O₃ codoped SrZrO₃ system with excellent properties was reported. Yb₂O₃-Gd₂O₃ codoped SrZrO₃ ceramic powders [Sr_0.8(Zr_0.9Yb_0.05Gd_0.05)O_2.75, SZYG/YGZO], [Sr(Zr_0.9Yb_0.05Gd_0.05)O_2.95, SZYG] and pure SrZrO₃ (SZO) powders were produced by a conventional solid-state reaction method. The XRD and Raman results show that, the composite SZYG/YGZO ceramics consist of the SZO and Yb_0.5Zr_0.5O_1.75 phases with a low thermal conductivity of ~1.3 W/(m·K) at 1000°C, which is at least 40% lower than that of the SZO ceramics. The TG-DSC results show that the SZYG/YGZO ceramics have no phase transition in the temperature range of 600 to 1400°C. The thermal expansion coefficient of the SZYG/YGZO ceramics reaches 10.9 × 10⁻⁶ K⁻¹ (1250°C). In addition, the fracture toughness of the SZYG/YGZO ceramics increases by more than 30% compared with the SZO ceramics, and this can be attributed to the presence of the Yb_0.5Zr_0.5O_1.75 phase.     

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.     

Comparison of hot corrosion resistance of Sm2Zr2O7 and (Sm0.5Sc0.5)2Zr2O7 ceramics in Na2SO4+V2O5 molten salt

Sm₂Zr₂O₇ and (Sm_0.5Sc_0.5)₂Zr₂O₇ ceramics were fabricated by a chemical co-precipitation and calcination method, and their hot corrosion behaviors in Na₂SO₄+V₂O₅ molten salt were investigated. Hot corrosion tests were carried at 700 °C, 800 °C and 900 °C for 4 h, and corroded surfaces were investigated using X-ray diffractometer and scanning electron microscopy. The corrosion products of Sm₂Zr₂O₇ ceramics were composed of SmVO₄ and monoclinic-ZrO₂, while those of (Sm_0.5Sc_0.5)₂Zr₂O₇ ceramic consisted of SmVO₄ and Zr₅Sc₂O₁₃. Considering the fact that Zr₅Sc₂O₁₃ is more desirable than monoclinic-ZrO₂ for thermal barrier coating applications, (Sm_0.5Sc_0.5)₂Zr₂O₇ showed better corrosion resistance to Na₂SO₄+V₂O₅ salt than Sm₂Zr₂O₇. The hot corrosion mechanisms of Sm₂Zr₂O₇ and (Sm_0.5Sc_0.5)₂Zr₂O₇ in Na₂SO₄+ V₂O₅ salt were discussed in detail.     

Thermophysical performances of (Gd1-xYbx)2AlTaO7 oxides for high-temperature heat-insulation coating applications

Newly developed (Gd_1-xYb_x)₂AlTaO₇ oxides for high-temperature heat-insulation coatings were prepared using a multi-step solid-state fritting method. The important features of synthesized oxides including phase composition, thermal conductivity and expansion performances were studied. It is investigated that the fabricated ceramics are confirmed to possess a sole pyrochlore crystal structure. Owing to the influence of the strain fields and mass fluctuations caused by Yb₂O₃ addition, thermal conductivities of (Gd_1-xYb_x)₂AlTaO₇ oxides are lower than that of Gd₂AlTaO₇ or Yb₂AlTaO₇, and the (Gd_0.9Yb_0.1)₂AlTaO₇ exhibits the lowest thermal expansion coefficient. Due to the synergic effects of the relatively high electro-negativity of Yb³⁺, decedent lattice order, and numerous oxygen vacancies, the thermal expansion coefficients increase gradually with increasing Yb₂O₃ content. The thermophysical performances of (Gd_1-xYb_x)₂AlTaO₇ oxides satisfy the conditions for high-temperature heat-insulation coatings.     

Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt at 700–1000 °C

Hot corrosion behavior of (Gd_0.9Sc_0.1)₂Zr₂O₇ ceramic exposed to V₂O₅ molten salt at 700–1000 °C was investigated, providing better understanding of its corrosion resistance as a promising thermal barrier coating. Obvious corrosion reaction occurred between (Gd_0.9Sc_0.1)₂Zr₂O₇ and V₂O₅ molten salt after 4 h heat treatment, corrosion products being temperature dependent. At 700 °C, large amount of Sc₂O₃ doped ZrV₂O₇ and GdVO₄, together with a minor amount of Sc₂O₃-stabilized tetragonal ZrO₂ (t-ZrO₂), formed on the sample surfaces. With the increase of the test temperature, Sc₂O₃ doped ZrV₂O₇ turned to decompose, leading to the formation of more t-ZrO₂. At 900 °C and 1000 °C, the corrosion products were composed of GdVO₄ and t-ZrO₂. The mechanism by which the corrosion reaction occurs is proposed based on phase diagrams and Lewis acid-base rule.     

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.     

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.     

In situ characterizations of mechanical behaviors of freestanding (Gd0.9Yb0.1)2Zr2O7 coatings by bending tests under different temperatures based on digital image correlation

It is important to investigate the high-temperature mechanical properties and failure mechanisms of Gd₂Zr₂O₇ ceramic materials for the development of novel thermal barrier coatings. Freestanding (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) coatings were prepared by supersonic plasma sprayed technique. A modified high-temperature in situ experimental system (up to 1500 °C) with the aid of digital image correlation technique was used to evaluate the fracture strength and flexural modulus of GYbZ coatings using three-point bending tests, and the fracture toughness was studied by single edge notched beam experiments. In addition, the extended finite element analysis was used to estimate the critical energy release rate of GYbZ coatings through the aforementioned experimental data. The effect of test temperature on the mechanical properties, cracking and fracture morphology of freestanding GYbZ coatings was discussed under bending loads. The results are useful for understanding high-temperature failure mechanisms of multilayered GYbZ thermal barrier coatings.     

Gd2Zr2O7 ceramics synthesized by solid-state reactive sintering: Effects of starting powders with different-scale particle sizes

In this work, the effects of starting oxide powders with different-scale particle sizes on the synthesis of gadolinium zirconate pyrochlore (Gd₂Zr₂O₇, GZO) and its physical properties were studied. Micron Gd₂O₃ (μG), micron ZrO₂ (μZ), nano Gd₂O₃ (nG), and nano ZrO₂ (nZ) powders were used. GZO ceramics were prepared by employing solid-state reactive sintering at 1300 °C, 1400 °C, 1500 °C and 1600 °C with mixed powders of different sizes (μGμZ, μGnZ, nGμZ and nGnZ). X-ray diffraction and Raman analyses of the ceramics revealed that nG has a more significant impact on the crystallization process than nZ. All ceramics synthesized with different sized oxide powders crystallized into pyrochlore phases except for those synthesized with μGnZ mixed powders, which resulted in a fluorite phase. The results indicated that decreasing the particle size of only ZrO₂ to synthesize pyrochlore-phase Gd₂Zr₂O₇ with high crystallinity may not be effective. Samples obtained at 1500 °C were further analyzed. Scanning electron microscopy results revealed that all four ceramics have a non-homogeneous grain size and that the average grain size ranges from 5.40 to 8.30 μm. In addition, the density and Vickers hardness measurements showed that the use of nanopowders significantly improves the mechanical properties.     

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.     

Electrical Conductivity Improvement of Nd2Ce2O7 Ceramic Co‐doped with Gd2O3 and ZrO2

(Nd_1–xGd_x)₂(Ce_1–xZr_x)₂O₇ (0 ≤ x ≤ 0.5) ceramic powders synthesised by the chemical‐coprecipitation and calcination method were pressureless‐sintered at 1,973 K for 10 h in air. The structure and electrical conductivity of (Nd_1–xGd_x)₂(Ce_1–xZr_x)₂O₇ ceramics were investigated by the X‐ray diffraction and impedance spectroscopy measurements. (Nd_1–xGd_x)₂(Ce_1–xZr_x)₂O₇ (0 ≤ x ≤ 0.5) ceramics exhibit a single phase of defect fluorite‐type structure. The electrical conductivity of (Nd_1–xGd_x)₂(Ce_1–xZr_x)₂O₇ ceramics increases with temperature in the range 623–1,173 K following an Arrhenius law. At identical temperature levels, the measured electrical conductivity of (Nd_1–xGd_x)₂(Ce_1–xZr_x)₂O₇ ceramics varies with doping different Gd₂O₃ and ZrO₂ contents and exhibits a maximum at x = 0.1.     

Effect of TiO2 and Ta2O5 co-doping on phase stability,fracture toughness,and sintering behavior of ZrO2 stabilized by 10mol%(Y0.4Gd0.3Yb0.3)2O3

The effect of TiO₂ and Ta₂O₅ co-doping on the phase structure, fracture toughness, and sintering behavior of 10mol%(Y_0.4Gd_0.3Yb_0.3)₂O₃-stabilized zirconia was investigated using X-ray diffraction, scanning electron microscopy, microindentation, and pressureless sintering. The results showed that 10mol%(Y_0.4Gd_0.3Yb_0.3)₂O₃–ZrO₂ had a single cubic phase structure, and an increase in the Ta₂O₅ (≥6 mol%) and TiO₂ doping concentrations resulted in a simultaneous increase in the content and stability of the tetragonal phase. The fracture toughness of TiO₂ and Ta₂O₅ co-doping 10mol%(Y_0.4Gd_0.3Yb_0.3)₂O₃–ZrO₂ decreased with an increase in the Ta₂O₅ content. On the other hand, the TiO₂ content had no significant effect on the fracture toughness of 10mol%(Y_0.4Gd_0.3Yb_0.3)₂O₃–ZrO₂. The sintering resistance of the specimens increased with an increasing in the Ta₂O₅ content; however, an increase in the TiO₂ content accelerated the densification of the specimens. When the Ta₂O₅ content was 10 mol% and the TiO₂ content was in the range of 4–8 mol%, a single non-transformable tetragonal phase structure with fracture toughness similar to that of 6–8 wt% Y₂O₃ stabilized ZrO₂ and excellent anti-sintering properties could be obtained. This structure can be explored as a thermal barrier coating material for high-temperature applications.     

Thermal behaviour of multilayer and functionally-graded YSZ/Gd2Zr2O7 coatings

Zirconates with pyrochlore structure, such as Gd₂Zr₂O₇, are new promising thermal barrier coatings because of their very low thermal conductivity and good chemical resistance against molten salts. However, their coefficient of thermal expansion is low, therefore their thermal fatigue resistance is compromised. As a solution, the combination of yttria-stabilised zirconia (YSZ) and Gd₂Zr₂O₇ can reduce the thermal contraction mismatch between the thermal barrier coating parts.In the present study, two possible designs have been performed to combine YSZ/Gd₂Zr₂O₇. On the one hand, a multilayer coating was obtained where YSZ layer was deposited between a Gd₂Zr₂O₇ layer and a bond coat. On the other hand, a functionally-graded coating was designed where different layers with variable ratios of YSZ/Gd₂Zr₂O₇ were deposited such that the composition gradually changed along the coating thickness.Multilayer and functionally-graded coatings underwent isothermal and thermally-cycled treatments in order to evaluate the oxidation, sintering effects and thermal fatigue resistance of the coatings. The YSZ/Gd₂Zr₂O₇ multilayer coating displayed better thermal behaviour than the Gd₂Zr₂O₇ monolayer coating but quite less thermal fatigue resistance compared to the conventional YSZ coating. However, the functionally-graded coating displays a good thermal fatigue resistance. Hence, it can be concluded that this kind of design is ideal to optimise the behaviour of thermal barrier coatings.     

Improvement on the phase stability,mechanical properties and thermal insulation of Y2O3-stabilized ZrO2 by Gd2O3 and Yb2O3 co-doping

Gd₂O₃ and Yb₂O₃ co-doped 3.5 mol% Y₂O₃–ZrO₂ and conventional 3.5 mol% Y₂O₃–ZrO₂ (YSZ) powders were synthesized by solid state reaction. The objective of this study was to improve the phase stability, mechanical properties and thermal insulation of YSZ. After heat treatment at 1500 °C for 10 h, 1 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (1Gd1Yb-YSZ) had higher resistance to destabilization of metastable tetragonal phase than YSZ. The hardness of 5 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (5Gd1Yb-YSZ) was higher than that of YSZ. Compared with YSZ, 1Gd1Yb-YSZ and 5Gd1Yb-YSZ exhibited lower thermal conductivity and shorter phonon mean free path. At 1300 °C, the thermal conductivity of 5Gd1Yb-YSZ was 1.23 W/m K, nearly 25% lower than that of YSZ (1.62 W/m K). Gd₂O₃ and Yb₂O₃ co-doped YSZ can be explored as a candidate material for thermal barrier coating applications.     

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.