Improving device lifetime and stability remains the stumbling block of the commercialization of hybrid perovskite-based devices (HPDs). Although extensive efforts have been paid, thermal property, one of the most crucial parameters in conventional solid-state electronic devices, has rarely been studied for HPDs. Here, we investigate the temperature-dependent ultralow thermal conductivity and ultrahigh thermal expansion of single-crystalline MAPbX 3 (MA = CH 3 NH 3 ), which are found distinct from traditional thin-film solar cells materials. Particularly, for MAPbI 3 , thermal conductivity is observed being only 0.3 W•m −1 •K −1 and linear thermal expansion coefficient along [100] direction is as high as 57.8 × 10 −6 K −1 (tetragonal) and much higher at the structural phase transition point. We attribute the ultralow thermal conductivity and ultrahigh thermal expansion to the weak chemical bonds associated with the soft perovskite materials. These unique properties can be very challenging for the multilayer device design, but their ultralow thermal conductivity may unveil a new thermoelectric material concept.

Ferroelastic RETaO 4 ceramics are promising thermal barrier coatings (TBCs) because of their attractive thermomechanical properties. The influence of crystal structure distortion degree on thermomechanical properties of RETaO 4 is estimated in this work. The relationship between Young's modulus and TECs is determined. The highest TECs (10.7 × 10 −6 K −1 , 1200°C) of RETaO 4 are detected in ErTaO 4 ceramics and are ascribed to its small Young's modulus and low Debye temperature. The intrinsic lattice thermal conductivity (3.94-1.26 W m −1 K −1 , 100-900°C) of RETaO 4 deceases with increasing of temperature due to an elimination in thermal radiation effects. The theoretical minimum thermal conductivity (1.00 W m −1 K −1 ) of RETaO 4 indicates that the experimental value is able to be reduced further. We have delved deeply into the thermomechanical properties of ferroelastic RETaO 4 ceramics and have emphasized their high-temperature applications as TBCs. K E Y W O R D Sband gap, intrinsic lattice thermal conductivity, rare earth tantalates, thermal barrier coatings, thermal expansion performance, Young's modulus

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.

The microstructures of high entropy alloys of the system CoCrCuFe xNi and CoCrCuFeNi x (where x indicates the molar ratio, which, where not specified, is 1) have been investigated. Many Cu rich spheres were evident in the microstructure of CoCrCuFe0.5Ni and CoCrCuFeNi0.5 alloys, which indicates that liquid phase separation had occurred before solidification. During liquid phase separation, the original liquids separated into two liquids: Cu rich and Cu depleted. In contrast, in other alloys ( x = 1.0, 1.5 and 2.0), typical dendritic and interdendritic structures are obtained. Cu and/or Cr rich precipitates, with various morphologies, can be seen in the interdendritic region. Additionally, Cu rich nanoparticles and Cr rich bird shaped structures can be observed in the Cu rich spheres. Sluggish cooperative diffusion causes the element segregation and formation of nanoprecipitates in the microstructures. The calculated positive mixing enthalpies of CoCrCuFe0.5Ni and CoCrCuFeNi0.5 alloys are likely reasons for their liquid phase separation.