Influence of B-site substituent Ce on thermophysical,oxygen diffusion,and mechanical properties of La2Zr2O7

Pyrochlore-type La₂Zr₂O₇ (LZ) is a promising candidate for high-temperature thermal barrier coatings (TBCs). However, its thermal expansion coefficient and low fracture toughness are not optimal for such application and thus, need to be improved. In this study, we systematically report the effect of CeO₂ addition on phase formation, oxygen-ion diffusion, and thermophysical and mechanical properties of full compositions La₂(Zr₁?_xCe_x)₂O₇ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1). La₂(Zr₁?_xCe_x)₂O₇ exhibits a pyrochlore structure at x ≤ 0.3, while a fluorite structure is observed outside this range. With the increase in CeO₂ content, thermal expansion coefficient and oxygen-ion diffusivity in La₂(Zr₁?_xCe_x)₂O₇ are increased. Oxygen-ion diffusivity of La₂(Zr₁?_xCe_x)₂O₇ is two orders of magnitude less than that of classical 8YSZ. Among La₂(Zr₁?_xCe_x)₂O₇ compounds, La₂(Zr_0.7Ce_0.3)₂O₇ and La₂(Zr_0.5Ce_0.5)₂O₇ exhibit relatively low oxygen diffusivities. The composition La₂(Zr_0.5Ce_0.5)₂O₇ presents the lowest thermal conductivity due to the strongest phonon scattering and also the highest fracture toughness due to the solid-solution toughening. The highest sintering resistance is achieved by the composition La₂(Zr_0.7Ce_0.3)₂O₇ because of its ordered pyrochlore structure and high atomic mass of Ce. Based on these results, the compositions La₂(Zr_0.5Ce_0.5)₂O₇ and La₂(Zr_0.7Ce_0.3)₂O₇ are alternatives for classical 8YSZ for TBC materials operating at ultrahigh temperatures.     

New double-ceramic-layer thermal barrier coatings based on zirconia–rare earth composite oxides

A series of La₂O₃–ZrO₂–CeO₂ composite oxides were synthesized by solid-state reaction. The final product keeps fluorite structure when the molar ratio Ce/Zr ≥ 0.7/0.3, and below this ratio only mixtures of La₂Zr₂O₇ (pyrochlore) and La₂O₃–CeO₂ (fluorite) exist. Averagely speaking, the increase of CeO₂ content gives rise to the increase of thermal expansion coefficient and the reduction of thermal conductivity, but La₂(Zr_0.7Ce_0.3)₂O₇ has the lowest sintering ability and the lowest thermal conductivity which could be explained by the theory of phonon scattering. Based on the large thermal expansion coefficient of La₂Ce_3.25O_9.5, the low thermal conductivities and low sintering abilities of La₂Zr₂O₇ and La₂(Zr_0.7Ce_0.3)₂O₇, double-ceramic-layer thermal barrier coatings were prepared. The thermal cycling tests indicate that such a design can largely improve the thermal cycling lives of the coatings. Since no single material that has been studied so far satisfies all the requirements for high temperature thermal barrier coatings, double-ceramic-layer coating may be an important development direction of thermal barrier coatings.     

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.     

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.     

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.     

Synthesis and thermal stability of Re2Zr2O7, (Re = La,Gd) and La2(Zr1−xCex)2O7−δ compounds under reducing and oxidant atmospheres for thermal barrier coatings

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La₂Zr₂O₇, Gd₂Zr₂O₇, La₂Ce₂O_7?γ and La₂(Zr_0.7Ce_0.3)O_7?γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400 °C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar_(g)/3%H_2(g) atmosphere.     

Influence of Yb Substitution for La on Thermophysical Property of La2AlTaO7 Ceramics

The (La_1−xYb_x)₂AlTaO₇ ceramics were synthesized by pressureless sintering process at 1600 °C for 10 h in air. The crystal phase, microstructure and thermophysical properties were investigated. Results show that pure (La_1−xYb_x)₂AlTaO₇ cermics with single weberite structure are prepared successfully. Owing to the reduction of crystal-lattice tolerance-factor, the thermal conductivity of (La_1−xYb_x)₂AlTaO₇ (x>0) ceramics increases with increasing Yb₂O₃ fraction at identical temperatures, which is lower than that of La₂AlTaO₇. Due to the relatively high electro-negativity of Yb element, the addition of Yb₂O₃ increases the thermal expansion coefficient of (La_1−xYb_x)₂AlTaO₇ ceramics.     

Influence of Gd2O3 addition on thermophysical properties of La2Ce2O7 ceramics for thermal barrier coatings

In order to develop new candidate ceramic materials for thermal barrier coatings (La_1?xGd_x)₂Ce₂O₇ ceramics were prepared by pressureless-sintering at 1600 °C for 10 h in air. The phase structure, micro-morphology and thermophysical properties of (La_1?xGd_x)₂Ce₂O₇ ceramics were investigated, respectively. XRD results revealed that pure (La_1?xGd_x)₂Ce₂O₇ ceramics with defect fluorite structure were synthesized and SEM showed that their microstructures were dense and no other phases existed among the particles. With the increasing temperature, their thermal expansion coefficients increased, while the thermal conductivities decreased. The thermophysical results indicated that thermal expansion coefficients of these ceramics were higher than that of 8YSZ, and their thermal conductivities were much lower than that of 8YSZ. The lower thermal conductivities of these ceramics were mainly attributed to more oxygen vacancies and substitution atoms. These results imply that the (La_1?xGd_x)₂Ce₂O₇ ceramics can be explored as candidate materials for the ceramic layer in TBC system.     

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.     

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.     

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).     

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.     

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.     

Phase composition,microstructure and thermophysical properties of the Srx(Zr0.9Y0.05Yb0.05)O1.95+x ceramics

Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9, 0.8, 0.7) ceramics were prepared by solid state reaction sintering. The sintered Sr_1.0(Zr_0.9Y_0.05Yb_0.05)O_2.95 is a single-phase solid solution while the sintered Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=0.9?0.7) are composites, and a significant grain growth inhibition is observed in the sintered Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9). Rare-earth elements distribution in the bulk materials indicates that Yb and Y preferentially substitute Zr-sites in SrZrO₃, and the highest solubility of RE₂O₃ in pure SrZrO₃ is ～0.8 mol%. The sintered Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x have high thermal expansion coefficients up to ～11.0×10^?6 K^-1 (1200°C). Sr_0.8(Zr_0.9Y_0.05Yb_0.05)O_2.75 has the lowest thermal conductivity of 1.38 W·m^-1·K^-1 at 800°C. Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9, 0.8) show no phase transition from 600 to 1400°C, whereas Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=0.9, 0.8) have excellent high-temperature phase stability over the whole investigated temperature range. Therefore, Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9, 0.8) are considered as promising TBCs materials that might be operated at higher temperatures compared to YSZ.     

Investigation on bi-layer coating with La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7/YSZ prepared by laser cladding

Thermal barrier coatings are an effective technology for improving the high-temperature performance of hot section components in gas turbine engine. Due to their excellent properties, high-entropy oxides are considered to be promising materials for thermal barrier coatings. Laser cladding is a coating preparation technology and the top coat prepared by laser cladding technology has an important application value for thermal barrier coatings. In this work, to improve the thermal cycling behavior of the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide coating, a bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide layer and the YSZ layer was designed and fabricated by laser cladding on the NiCoCrAlY alloy surface. The microstructure, phase and mechanical properties of the coating were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness and nanoindentation tests, respectively. The results show that a bi-layer La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇/YSZ coating was successfully prepared by the laser cladding method, and shows good bonding at the interface between the layers. The high-entropy oxide layer maintains a relatively stable defective fluorite structure and its microstructure exists in the stable cellular and dendrite crystalline state after laser cladding. The high-entropy oxide layer prepared by laser cladding showed an average elastic modulus of 167 GPa and an average hardness of 1022.8HV in nanoindentation tests. Thermal cycling of the coating was carried out at 1050 °C. Failure of the bi-layer coating occurred after 60 thermal cycles at 1050 °C. Thermal stresses between different layers are calculated during thermal cycling. Due to its excellent mechanical properties, the bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide and YSZ layers is expected to become an effective high-entropy oxide thermal barrier coating.     

Vacuum sintering of highly transparent La1+xYb1+yZr2O7 ceramics with excess La and Yb contents

In this study, a series of transparent ceramics with chemical composition of La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) were successfully prepared by vacuum sintering using combustion synthesized powders. The effects of excess contents on the phase composition, microstructure and in-line transmittance have been studied. The detailed results indicate that the in-line transmittance increases at first and then decreases as La content be elevated. It was also determined that the highest in-line transmittance of La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) ceramics is 84.1 % at 1100 nm when the excess amount of co-doped La-Yb is 30 %. Compared with stoichiometric LaYbZr₂O₇ ceramic, the nonstoichiometric La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) ceramics exhibit much higher transparency. In addition, the high excess amount of La, Yb and co-doped La-Yb also shows effects on the phase composition and crystal structure.     

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.     

Thermo-physical and mechanical properties of Yb2O3 and Sc2O3 co-doped Gd2Zr2O7 ceramics

Ceramic materials for the thermal barrier coating (TBC) application of Gd₂Zr₂O₇ (GZO), (Gd_0.94Yb_0.06)₂Zr₂O₇ (GYb_0.06Z), (Gd_0.925Sc_0.075)₂Zr₂O₇ (GSc_0.075Z), (Gd_0.865Sc_0.075Yb_0.06)₂Zr₂O₇ (GSc_0.075Yb_0.06Z), and (Gd_0.8Sc_0.1Yb_0.1)₂Zr₂O₇ (GSc_0.1Yb_0.1Z) were successfully synthesized by chemical co-precipitation. The effects of the doping of Sc₂O₃ and Yb₂O₃ on the phases, thermo-physical and mechanical properties of the ceramics were investigated. The results show that both Yb₂O₃ and Sc₂O₃ doping promoted the phase transition of GZO from pyrochlore to fluorite. All the Sc₂O₃-doped samples exhibited enhanced fracture toughness, as compared to the undoped sample. Furthermore, the GSc_0.075Yb_0.06Z sample revealed a thermal conductivity of ~0.8 W/mK at 1200 °C, which was nearly 30% lower than that of the undoped sample. The associated mechanisms related to the effects of the doping on the thermophysical and mechanical properties are discussed.     

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.     

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.