High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.
Perovskite solar cells (PSCs) are highly promising next‐generation photovoltaic devices because of the cheap raw materials, ideal band gap of ≈1.5 eV, broad absorption range, and high absorption coefficient. Although lead‐based inorganic‐organic PSC has achieved the highest power conversion efficiency (PCE) of 25.2%, the toxic nature of lead and poor stability strongly limits the commercialization. Lead‐free inorganic PSCs are potential alternatives to toxic and unstable organic‐inorganic PSCs. Particularly, double‐perovskite Cs2AgBiBr6‐based PSC has received interests for its all inorganic and lead‐free features. However, the PCE is limited by the inherent and extrinsic defects of Cs2AgBiBr6 films. Herein, an effective and facile strategy is reported for improving the PCE and stability by introducing an N719 dye interlayer, which plays multifunctional roles such as broadening the absorption spectrum, suppressing the charge carrier recombination, accelerating the hole extraction, and constructing an appropriate energy level alignment. Consequently, the optimizing cell delivers an outstanding PCE of 2.84%, much improved as compared with other Cs2AgBiBr6‐based PSCs reported so far in the literature. Moreover, the N719 interlayer greatly enhances the stability of PSCs under ambient conditions. This work highlights a useful strategy to boost the PCE and stability of lead‐free Cs2AgBiBr6‐based PSCs simultaneously, accelerating the commercialization of PSC technology.
The low-temperature hydrothermal synthesis method has been drawing ever-growing attention due to the fact that it has many advantages over conventional methods for preparing promising cathode material LiFePO 4 . However, the mechanism for hydrothermal synthesis of LiFePO 4 remains unclear. Here, the hydrothermal reaction mechanism of LiFePO 4 is systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and specific surface analysis. As evidenced by apparent precursor dissolution, fast hydrothermal formation, and significant decrease in particle size with adding alcohols and/or carbon black in the reaction system, a dissolution-precipitation mechanism accounts for the hydrothermal synthesis of LiFePO 4 . Moreover, we identified tetraphosphate in the LiFePO 4 precursor. This compound undergoes hydrolysis upon heating during the hydrothermal process, resulting in a remarkable decline of pH value.
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