Tuning stoichiometry of high‐entropy oxides for tailorable thermal expansion coefficients and low thermal conductivity

Abstract

Thermal barrier coating materials with proper thermal expansion coefficient (TEC), low thermal conductivity, and good high-temperature stability are of great significance for their applications in next-generation turbine engines. Herein, we report a new class of high-entropy (La 0.2 Sm 0.2 Er 0.2 Yb 0.2 Y 0.2 ) 2 Ce x O 3+2x with different Ce 4+ contents synthesized by a solid-state reaction method. They exhibit different crystal structures at different Ce 4+ content, including a bixbyite single phase without Ce 4+ doping (x = 0), bixbyitefluorite dual-phase in the RE 2 O 3 -rich region (0 < x < 2), and fluorite single phase in the stoichiometric (x = 2) and CeO 2 -rich region (x > 2). The highentropy (La 0.2 Sm 0.2 Er 0.2 Yb 0.2 Y 0.2 ) 2 Ce x O 3+2x exhibit tailorable TECs at a large range of 9.04 × 10 -6 -13.12 × 10 -6 • C -1 and engineered low thermal conductivity of 1.79-2.63 W⋅m -1 ⋅K -1 . They also possess good sintering resistance and high-temperature phase stability. These results reveal that the high-entropy (La 0.2 Sm 0.2 Er 0.2 Yb 0.2 Y 0.2 ) 2 Ce x O 3+2x are promising candidates for thermal barrier coating materials as well as thermally insulating materials and refractories.

Visualizing the Formation of High-Entropy Fluorite Oxides from an Amorphous Precursor at Atomic Resolution

“…These diffraction peaks in the XRD patterns correspond well to those of the HEFOs prepared by a solidstate sintering method. 24,37,38 Based on the Scherrer formula, 48 the average sizes of grains are calculated from the XRD patterns (Figure 1b) and show an increase from about 2.3 to 33 nm corresponding to annealing temperatures from 400 to 1100 °C. This also correlates well with the evolution of the morphologies imaged in STEM (Figure 1c).…”

“…Among the HEOs, high-entropy fluorite oxides (HEFOs) have been shown to exhibit exceptional resistance to hydrothermal and thermal degradation as the catalyst support for CO oxidation, ultralow thermal conductivity and good sintering resistance as thermal barrier coating materials, − and a tunable band gap for application in visible-light-absorbing materials . Among the available synthesis methods, wet chemical routes such as the sol–gel process, hydrothermal process, and nebulized spray pyrolysis method could offer better homogeneity when mixing the constituent elements at a molecular level in the solutions. ,, The use of a wet-chemical-derived method provides the opportunity to visualize the entire formation process of HEFOs at temperatures accessible in most characterization techniques.…”

Phase evolution and thermophysical properties of high‐entropy RE2(Y0.2Yb0.2Nb0.2Ta0.2Ce0.2)2O7 oxides

“…Besides, the larger difference of bond length and charge disorder of high-entropy RE 2 (Y 0.2 Yb 0.2 Nb 0.2 Ta 0.2 Ce 0.2 ) 2 O 7 oxides are also the additional factors that hinder heat conduction. [2,28,29] In the case of conventional high-entropy oxides (such as Ce-based oxides [22,24,30,50] ), the difference of bond strength between A-O and B-O bonds is quite slight due to the similar cation radii of A 3+ and B 4+ , whereas, in high-entropy RE 2 (Y 0.2 Yb 0.2 Nb 0.2 Ta 0.2 Ce 0.2 ) 2 O 7 oxides, the difference of bond strength between A-O and B-O bonds is much magnified for the larger different in bond length. The inhomogeneity of bond strength can reduce the energy transfer rate of acoustic vibration modes, thus reducing thermal conductivity.…”

“…[20] It is of great interest to discover high-entropy A 2 B 2 O 7 -type oxides with engineered thermal conductivity and TECs as well as superior high-temperature stability for applications in promising TBC and thermal insulating materials. [21][22][23][24][25][26] Unfortunately, in the present high-entropy A 2 B 2 O 7 -type oxide systems with multicomponent equivalent cations in the cation sublattice, the lattice distortions can be relieved because the mass, size, and charge of the cations are quite similar for several reasons: (i) ion radii of A-site cations (usually lanthanide cations) are close to each other because of the lanthanide contraction [27,28] ; (ii) the mass difference of A-site cations is also very small; (iii) the charge difference of cations is quite small because all of A-and B-site cations are trivalent and tetravalent positive charge, respectively. It has been established that increasing mass and charge disorder in crystal can lead to the large chemical inhomogeneity and force constant disorder, which eventually results in the enhanced phonon scattering and lower thermal conductivity.…”

Thermal conduction mechanism of ferroelastic Zr‐Y‐Yb‐Ta‐Nb‐O high‐entropy oxides with glass‐like thermal conductivity

“…Driven by the urgent demand for enhanced efficiency and decreased energy consumption, high‐entropy oxides (HEOs) with low thermal conductivity ( k ) have raised wide interests for thermal management applications such as thermal barrier coatings (TBCs), thermoelectric materials, etc 1–3 . Inspired by the high‐entropy strategy, many low‐ k HEOs (0.86–1.50 W m −1 K −1 at 1000°C), such as high‐entropy rare‐earth zirconates ((5RE 0.2 ) 2 Zr 2 O 7 ), 4,5 high‐entropy rare‐earth cerates ((6RE 1/6 ) 2 Ce 2 O 7 6 and (5RE 0.2 ) 2 Ce x O 3+2 x 7 ) and aluminates ((5RE 0.2 ) 4 Al 2 O 9 8 ), have been designed for TBCs to replace the state‐of‐the‐art TBCs material, 7–8 wt% yttria‐stabilized zirconia (7–8YSZ). It is generally accepted that the low thermal conductivity of HEOs originates from the presence of a large number of lattice defects such as oxygen vacancies and substitutional solute atoms in the material 9–11 .…”