A brief review of microstructure design in transition metal-based magnetocaloric materials.

Magnetic cooling, a solid-state refrigeration technology based on the magnetocaloric effect, has attracted significant attention in space cooling due to its high energy-efficiency and environmental friendliness. Transition metal-based magnetocaloric materials (MCMs) with the merit of low-cost have emerged as promising candidates for efficient magnetic refrigeration applications. This review explores the intricate relationship between microstructure and multiple properties (e.g. magnetocaloric properties, mechanical stability, thermal conductivity, and functional reversibility) of these materials. A variety of microstructural manipulation approaches (e.g. crystallographic texture, precipitates, micropores, atomic-scale defects, size effect, and composites) are examined for their effects on the comprehensive performance of MCMs. We show that microstructure design provides an effective tool to achieve excellent performance in multiple aspects, which may facilitate the commercialization of transition-metal based MCMs.

Enhanced Density of States Facilitates High Thermoelectric Performance in Solution-Grown Ge- and In-Codoped SnSe Nanoplates.

SnSe single crystals have gained great interest due to their excellent thermoelectric performance. However, polycrystalline SnSe is greatly desired due to facile processing, machinability, and scale-up application. Here, we report an outstanding high average ZT of 0.88 as well as a high peak ZT of 1.92 in solution-processed SnSe nanoplates. Nanosized boundaries formed by nanoplates and lattice strain created by lattice dislocations and stacking faults effectively scatter heat-carrying phonons, resulting in an ultralow lattice thermal conductivity of 0.19 W m-1 K-1 at 873 K. Ultraviolet photoelectron spectroscopy reveals that Ge and In incorporation produces an enhanced density of states in the electronic structure of SnSe, resulting in a large Seebeck coefficient. Ge and In codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, helping to maintain a high power factor over a wide temperature range. Benefiting from an enhanced power factor and markedly reduced lattice thermal conductivity, high average ZT and peak ZT are achieved in Ge- and In-codoped SnSe nanoplates. This work achieves an ultrahigh average ZT of 0.88 in polycrystalline SnSe by adopting nontoxic element doping, potentially expanding its usefulness for various thermoelectric generator applications.

Nature of Li2O2 oxidation in a Li-O2 battery revealed by operando X-ray diffraction.

Fundamental research into the Li-O2 battery system has gone into high gear, gaining momentum because of its very high theoretical specific energy. Much progress has been made toward understanding the discharge mechanism, but the mechanism of the oxygen evolution reaction (OER) on charge (i.e., oxidation) remains less understood. Here, using operando X-ray diffraction, we show that oxidation of electrochemically generated Li2O2 occurs in two stages, but in one step for bulk crystalline (commercial) Li2O2, revealing a fundamental difference in the OER process depending on the nature of the peroxide. For electrochemically generated Li2O2, oxidation proceeds first through a noncrystalline lithium peroxide component, followed at higher potential by the crystalline peroxide via a Li deficient solid solution (Li(2-x)O2) phase. Anisotropic broadening of the X-ray Li2O2 reflections confirms a platelet crystallite shape. On the basis of the evolution of the broadening during charge, we speculate that the toroid particles are deconstructed one platelet at a time, starting with the smallest sizes that expose more peroxide surface. In the case of in situ charged bulk crystalline Li2O2, the Li vacancies preferentially form on the interlayer position (Li1), which is supported by first-principle calculations and consistent with their lower energy compared to those located next to oxygen (Li2). The small actively oxidizing fraction results in a gradual reduction of the Li2O2 crystallites. The fundamental insight gained in the OER charge mechanism and its relation to the nature of the Li2O2 particles is essential for the design of future electrodes with lower overpotentials, one of the key challenges for high performance Li-air batteries.