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.     

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.     

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.     

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.     

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.     

Effect of Ba nonstoichiometry on the phase composition,microstructure, chemical stability and electrical conductivity of BaxCe0.7Zr0.1Y0.1Yb0.1O3−δ (0.9≤x≤1.1) proton conductors

The perovskite proton conductors Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x=0.9, 0.94, 0.98, 1.0, 1.03, 1.06, and 1.1) have been successfully prepared by the conventional solid state reaction route. X-ray diffraction (XRD) patterns of the samples indicate that Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x≥1.0) samples possess a single phase orthorhombic structure, but a secondary phase (Y,Ce)O_2−δ exists in Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x<1.0) samples. SEM photographs show that the grain size of Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ increases and the porosity decreases with Ba²⁺ content varying from x=0.9 to 1.1. Because of ZnO addition as sintering aid, the sintering temperature of the samples reduces from 1550 °C to 1250 °C. The chemical stability of the samples against CO₂ decreases with the increase in Ba content from x=0.9 to 1.1. All the samples show a excellent stability against water vapor at 850 °C. The conductivities of the samples increase and the activation energies reduce with the increase in Ba content. The present results suggest that it is very important to control the stoichiometry of cations to obtain desired perovskite type high temperature proton conductors.     

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

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

Study of structure and conductivity of Li3/8Sr7/16-3x/2LaxZr1/4Nb3/4O3 solid electrolytes

Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized using the conventional solid-state reaction method. In order to increase the vacancy concentration, La³⁺ was doped on the Sr²⁺ site. Crystal structures of doped samples were characterized by X-ray diffraction. Except, perovskite-type Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0, 0.05, 0.10, 0.15) samples were fabricated by heat treatment at 1250 °C, 1275 °C, 1275 °C and 1275 °C, respectively, for 15 h. Lattice sizes decreased with the increase of doping amounts because of the smaller ion radius of La³⁺ compared to that of Sr²⁺. Ionic conductivities of the samples were measured by AC impedance spectroscopy. The results showed that the ionic conductivity increases at first and then decreases with raising doping amounts and sintering temperatures. So the optimized composition Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0.05) sintered at 1275 °C was selected with the highest total conductivity of 3.33 × 10^?5 S cm^?1at 30 °C and an activation energy of 0.27 eV. Additionally, potentiostatic polarization test was used to evaluate the electronic conductivity. The optimal composition Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0.05) as a possible Li-ion conducting solid electrolyte has an electronic conductivity of only 8.39 × 10^?9 S cm^?1.     

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

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

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.     

Structural-phase state and lasing of 5–15 at% Yb3+:Y3Al5O12 optical ceramics

Yttrium aluminum garnet (Yb³⁺:Y₃Al₅O₁₂) laser ceramics doped by 5, 10 and 15 at% of ytterbium ions were obtained by reactive sintering. Optimal sintering temperature range for the formation of highly-dense transparent Yb³⁺:Y₃Al₅O₁₂ ceramics under normal recrystallization conditions was found to be T = 1750–1800 °C. The influence of Yb³⁺ ions on structural-phase state, phase composition, microstructure, optical and luminescent properties of sintered samples was experimentally investigated. It was shown that lattice parameter a of Yb³⁺:Y₃Al₅O₁₂ ceramics decreases linearly with increasing of Yb³⁺ concentration in a good agreement with L. Vegard’s rule, that indicates to the formation of (Y_1−xYb_x)₃Al₅O₁₂ (х = 0.05–0.15) substitutional solid solutions. No concentration quenching of Yb³⁺ luminescence was observed in Yb³⁺:Y₃Al₅O₁₂ within the 5–15 at% doping range. Quasi-CW lasing of Yb³⁺:Y₃Al₅O₁₂ ceramics was studied under diode-pumping at 970 nm. A highest slope efficiency of about 50% was obtained for 15 at%-doped Yb³⁺:Y₃Al₅O₁₂ ceramics sintered at T = 1800 °C for 10 h.     

Enhanced thermal stability of piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O3 systems through tailoring phase transition behavior

Good thermal stability in lead-free BaTiO₃ ceramics is important for their applications above room temperature. In this study, thermal stable piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O₃ ceramics was enhanced by tailoring their phase transition behaviors. Comparison between (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ and (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ revealed that latter system at y?=?0.80 had much better thermal stable piezoelectric coefficient than the former at x?=?0.45. Both systems crystalized in tetragonal to orthorhombic phase boundary at room temperature. The phase transition temperature and degree of diffusion were adjusted by Ca and Zr ions contents and demonstrated great influence on temperature dependent dielectric permittivity, hysteresis loops, and in-situ domain structures. The improved thermal stability of (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ prepared at y?=?0.80 was linked to its higher paraelectric to ferroelectric phase transition temperature (T_m?=?115.7?°C) and less degree of diffusion (degree of diffusion constant γ?=?1.35). By comparison, (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ prepared at x?=?0.45 revealed T_m?=?81.3?°C and γ?=?1.65. Overall, these findings look promising for future stimulation of phase transition behaviors and design of piezoelectric materials with good thermal stabilities.     

Phase stability and thermo-physical properties of ZrO2-CeO2-TiO2 ceramics for thermal barrier coatings

The properties of ZrO₂ co-stabilized by CeO₂ and TiO₂ ceramic bulks were investigated for potential thermal barrier coating (TBC) applications. Results showed that the (Ce_0.15Ti_x)Zr_0.85-xO7 (x?=?0.05, 0.10, 0.15) compositions with single tetragonal phase were more stable than the traditional 8YSZ at 1573?K. These compositions also showed a large thermal expansion coefficient (TEC) and a high fracture toughness, which were comparable to those of YSZ. However, the phase stability, fracture toughness and sintering resistance of the CeO₂-TiO₂-ZrO₂ system showed a decline tendency with the increase of TiO₂ content. The TEC of the ceramic bulks decreased with increase of TiO₂ content as well because the crystal energy was enhanced with increasing substitution of Zr⁴⁺ by smaller Ti⁴⁺. The (Ce_0.15Ti_0.05)Zr_0.8O₂ had the best comprehensive properties among the (Ce_0.15Ti_x)Zr_0.85-xO₂ compositions as well as a low thermal conductivity. Therefore, it can be explored as a TBC candidate material for high-temperature applications.     

Structures and electrical conductivities of Gd3+ and Fe3+ co-doped cerium oxide electrolytes sintered at low temperature for ILT-SOFCs

Gd³⁺ and Fe³⁺ co-doped cerium oxide electrolytes, Ce_0.9Gd_0.1‐xFe_xO_2-δ (x?=?0.00, 0.01, 0.03, 0.05, 0.07, 0.10), were prepared by co-precipitation for ultrafine precursor powders and sintering for densified ceramic pellets. The crystal and microscopic structures were characterized by XRD, FESEM and Raman spectroscopy and their electrical properties were studied by AC impedance spectroscopy and the measurement of single cell's outputs. In comparison with Ce_0.9Gd_0.1O_1.95, the ceramic pellets of Ce_0.9Gd_0.1‐xFe_xO_2-δ with a relative density of 95% can be obtained after sintered at 1000?°C for 5?h, showing a remarkably enhanced sintering performance with a sintering temperature reduction of 500?°C, which might be ascribed to the highly activated migration of constituent species in the cerium oxide lattice doped with Gd³⁺ and Fe³⁺ions. Moreover, the electrical conductivity of Ce_0.9Gd_0.1‐xFe_xO_2-δ can be significantly enhanced depending on the mole fraction x, with Ce_0.9Gd_0.07Fe_0.03O_1.95 exhibiting the highest electrical conductivity of 38 mS/cm at 800?°C, about 36% higher than that of Ce_0.9Gd_0.1O_1.95 electrolyte sintered at 1500?°C for 5?h. So, The Gd³⁺ and Fe³⁺ co-doped cerium oxide would be an excellent candidate electrolyte for ILT SOFCs due to its prominent sintering performance and enhanced electrical conductivity.     

Highly transparent Yb:Y2O3 ceramics obtained by solid-state reaction and combined sintering procedures

(Y_0.87-xLa_0.1Zr_0.03Yb_x)₂O₃ (x?=?0.02, 0.04, 0.05) transparent ceramics were obtained by solid-state reaction and combined sintering procedures with La₂O₃ and ZrO₂ as sintering additives. A method based on two-step intermediate sintering in air followed by vacuum sintering was applied in order to control the densification and grain growth of the samples during the final sintering process. The results indicate that La₂O₃ and ZrO₂ co-additives can improve the microstructure and optical properties of Yb:Y₂O₃ ceramics at relatively low sintering temperature. On the other hand, the addition of Zr⁴⁺ ions leads to the formation of dispersed scattering volumes in the ceramic bodies. Transmittance of 78.8% was measured for the 2.0?at% Yb:Y₂O₃ ceramic sample at the wavelength of 1100?nm. The spectroscopic properties of Yb:Y₂O₃ ceramics were investigated at room temperature. The obtained results show that the absorption cross-section at 978?nm is in the range of 2.08?×?10^–20 to 2.36?×?10^–20 cm², whereas the emission cross-section at 1032?nm is ~1.0?×?10^–20 cm².     

The influence of Gd doping on thermophysical properties,elasticity modulus and phase stability of garnet-type (Y1-xGdx)3Al5O12 ceramics

In this work, Gd³⁺ was selected to partially substitute the Y³⁺ in yttrium aluminum garnet (YAG) in order to improve the thermophysical properties of YAG. A series of (Y_1-xGd_x)₃Al₅O₁₂ (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were synthesized through chemical co-precipitation route. The microstructure, thermophysical properties and elasticity modulus of (Y_1-xGd_x)₃Al₅O₁₂ were investigated. The (Y_1-xGd_x)₃Al₅O₁₂ ceramics was comprised of single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xGd_x)₃Al₅O₁₂ bulk samples decreased with increasing doping concentration to 0.2, but increased with furthering increasing the concentration to 0.4. The thermal conductivity of (Y_0.8Gd_0.2)₃Al₅O₁₂ was 1.51 W m⁻¹ K⁻¹ at 1200 °C. The average thermal expansion coefficient of (Y_0.8Gd_0.2)₃Al₅O₁₂ was slightly larger than that of Y₃Al₅O₁₂. (Y_0.8Gd_0.2)₃Al₅O₁₂ bulk sample exhibited the lowest elasticity modulus among the investigated (Y_1-xGd_x)₃Al₅O₁₂. In addition, (Y_0.8Gd_0.2)₃Al₅O₁₂ ceramic remained good phase stability from room temperature to 1600 °C.     

Sand corrosion,thermal expansion,and ablation of medium- and high-entropy compositionally complex fluorite oxides

Sand corrosion, thermal expansion, and ablation properties of a new class of medium- and high-entropy compositionally complex fluorite oxides (CCFOs) are examined as potential protective coating materials. Five binary oxides were mixed and sintered into dense, single-phase CCFOs of the general formula: [Hf_(1-2x)/3Zr_(1-2x)/3Ce_(1-2x)/3Y_xYb_x]O_2-δ (x = 0.2, 0.074, and 0.029). These CCFOs exhibit decreased molten sand infiltration and interaction at intermediate temperatures (1200-1300°C) in comparison with a cubic yttria-stabilized zirconia (YSZ) reference; however, at higher temperatures, the trend is reversed due to the increased chemical reactivity. The equimolar high-entropy (Hf_0.2Zr_0.2Ce_0.2Y_0.2Yb_0.2)O_2-δ exhibits no grain boundary penetration by molten sand at all examined temperatures (1200°C-1500°C), although reaction and precipitation are significant. Moreover, these CCFOs exhibit higher intrinsic thermal expansion coefficients (CTE) than the YSZ reference, thereby being more compatible with Ni-based superalloys. The 8YSZ-like (Hf_0.284Zr_0.284Ce_0.284Y_0.074Yb_0.074)O_2-δ exhibits the highest CTE in this series of CCFOs due to oxygen clustering effects. Finally, these CCFOs also exhibit lower emissivities and form unique faceted microstructures in ablative environments.     

Phase structure and thermophysical properties of co-doped La2Zr2O7 ceramics for thermal barrier coatings

La₂Zr₂O₇ has high melting point, low thermal conductivity and relatively high thermal expansion which make it suitable for application as high-temperature thermal barrier coatings. Ceramics including La₂Zr₂O₇, (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ were synthesized by solid state reaction. The effects of co-doping on the phase structure and thermophysical properties of La₂Zr₂O₇ were investigated. The phase structures of these ceramics were identified by X-ray diffraction, showing that the La₂Zr₂O₇ ceramic has a pyrochlore structure while the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ exhibit a defect fluorite structure, which is mainly determined by ionic radius ratio r(A_av.³⁺)/r(B_av.⁴⁺). The measurements for thermal expansion coefficient and thermal conductivity of these ceramics from ambient temperature to 1200 °C show that the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ have a larger thermal expansion coefficient and a lower thermal conductivity than La₂Zr₂O₇, and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ shows the more excellent thermophysical properties than (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ due to the increase of Yb₂O₃ content.     

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.     

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

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