Scheme 1. Schematic of SPRG-TIC process for ultralight HEKAs and their application as on-off thermal switches.

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.

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In this work, the dense bulk polymorphous YTaO4 ceramics with M or M' phase are synthesized by spark plasma sintering method accompanying with different tempering procedures. Combined with the nano‐indentation and theoretical calculation, their mechanical properties are systematically investigated. The identification of crystal structure reveals that the YTaO4 crystallizes into M phase (space group: I2/a) with higher tempering temperature, otherwise it crystallizes into M' phase (space group: P2/a). The results of mechanical properties indicate M‐phase YTaO4 possesses larger Young's modulus and hardness than that of M' phase. It is stemmed from the chemical bonding strength of M phase is stronger than that of M' phase, and the stronger bonding strength of M phase also results in its elastic resilience is superior to M' phase. Besides, on account of the low symmetry of monoclinic crystal system, the Young's modulus of polymorphous YTaO4 ceramics exhibit strong anisotropy.

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Recently, in order to develop novel materials with optimized properties, "high entropy" has attracted significant attention in the field of materials research relate to structural entropy within a material. [1][2] Multi-principal high-entropy alloys and ceramics can be synthesized, which exhibit several excellent properties, such as high strength and toughness, corrosion resistance, wear resistance, thermal performance, and electrical performance, on the basis of four high-entropy effects [3][4][5][6] : (1) high-entropy effect of thermodynamics; (2) lattice distortion effect of structure; (3) hysteretic diffusion effect; and (4) cocktail effect on performance.The high-entropy can significantly improve the comprehensive performance of the materials. For example, Gild et al. 7 reported high-entropy fluorite rare-earth oxides, that is, REO 2-δ , with thermal conductivities of 1.1-2.02 W•m -1 •K -1 , therefore, REO 2-δ has shown potential as low-thermal-conductivity materials. Ren et al. 8 synthesized a (Y 1/4 Ho 1/4 Er 1/4 Yb 1/4 ) 2 SiO 5 ceramic with excellent phase stability and low thermal conductivity (1.1 W•m -1 •K -1 at 1300°C), which is considered to be applied as a thermal and environmental barrier coating (TEBC) material. Yan et al. 9 reported a high-entropy ceramic (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )C, which exhibited a much lower thermal conductivity (5.42-6.45 W•m -1 •K -1 at room temperature) than the binary carbides, such as HfC, ZrC, TaC, and TiC(6.3-29.3 W•m -1 •K -1 at room temperature). Zhao et al. 10 reported a new type of rare-earth zirconate (La 0.2 Ce 0.2 Nd 0.2 Sm 0.2 Eu 0.2 ) 2 Zr 2 O 7 high-entropy ceramic with low thermal conductivity (0.76 W -1 •m -1 •k -1 ) and low grain growth speed, which consolidated its potential value in the field of TBCs. Zhao et al. 11 reported (Nd 0.2 Sm 0.2 Eu 0.2 Y 0.2 Yb 0.2 ) 4 Al 2 O 9 exhibits a close thermal expansion coefficient (6.96 × 10 −6 K −1 at 300-1473 K) to that of mullite, good phase stability from 300 to 1473 K, and low thermal conductivity (1.50 W -1 •m -1 •k -1 at room temperature).

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