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Materials that undergo a magnetostructural transition (MST) usually exhibit fascinating magnetoresponsive properties, making them an important class of smart materials. However, practical application of this class of smart materials has been hindered by structural degradation as well as large irreversibility of the MST during consecutive thermal and field cycles. Here we report a significant improvement of the reversibility and mechanical stability in grain-oriented MnNiGe-based alloys that were fabricated using a directional solidification method. The preferred grain orientation enables synergistic deformations between neighboring grains during the MST, leading to a substantial reduction in the transition-induced stress concentration. As a result, in situ and ex situ microscopic observations demonstrate a good mechanical stability of the textured alloys across the MST, in strong contrast to conventional MM'X (M, M' = Mn, Fe, Co, Ni; X = Si, Ge) materials with randomly-oriented grains. The detailed transition stages of the MST have also been observed at the microscopic scale, and are reported for the first time in the MM'X family. Besides, a low thermal hysteresis (ΔT hys) of ∼ 4 K was obtained in the textured alloys, which is the lowest ΔT hys in the MM'X material family. Textured MnNiGe-based alloys show a large reversible isothermal entropy change (≥ 35.9 Jkg−1K−1) in a 5 T field change, which is the highest among typical magnetocaloric materials. Consequently, this work provides a promising strategy for enhancing the cyclic stability of materials with a MST, which may boost their practical applications in solid-state refrigerators, energy harvesters and high-precision actuators. [Display omitted] [ABSTRACT FROM AUTHOR]

The geometry of a magnetocaloric regenerator (MR) is crucial to the heat exchange efficiency and the performance of a magnetic refrigerator. Here we proposed a new and facile way to produce monolithic La 0.67 Ca 0.33 MnO 3 MRs with highly ordered and unidirectional oriented microchannels. This is realized by a self-organization process of alginate sol, which does not require specific processing equipment and hence is well-suited for large-scale production. The average diameter and total porosity of the well-defined microchannels can be adjusted in the range of 71.5–156.3 μm and 15.3%-21.5%, respectively. The orthorhombic perovskite structure is retained in the sintered MRs without any detectable secondary phase. Besides, the sintered MRs exhibit a significantly higher isothermal entropy change than the original La 0.67 Ca 0.33 MnO 3 powders due to an increase in the grain size. Consequently, our study may trigger a breakthrough in the large-scale production of monolithic MRs with desirable geometries for magnetic refrigerators. [Display omitted] [ABSTRACT FROM AUTHOR]

Magnetic refrigeration based on the magnetocaloric effect is expected to trigger technological revolution in the refrigeration industry due to the merits of high energy efficiency and complete elimination of greenhouse gas emissions. However, the implementation of this emerging technology is hindered by some challenges in the magnetocaloric materials, e.g., low thermal conductivity (λ) and poor mechanical properties. This paper reports a novel magnetocaloric composite that is characterized by continuous Cu networks within the (Mn,Fe) 2 (P,Si) magnetocaloric matrix. This unique microstructure is designed with the aid of finite-element simulations and experimentally realized by hot pressing the (Mn,Fe) 2 (P,Si)/Cu core/shell powders. High-resolution transmission electron microscope revealed good interfacial coherence between the matrix and the binder, which is highly desirable to reduce the interfacial thermal resistance. The magnetocaloric composites with such a novel microstructure exhibit a high λ of 20.4 Wm−1K−1 and a large maximum compressive strength of 570 MPa, which are the best comprehensive properties for room-temperature magnetocaloric materials ever reported. Consequently, this work offers a promising way to tackle the bottleneck problems of low thermal conductivity and the brittleness of the magnetocaloric materials, which may accelerate the practical application of magnetic refrigeration. [Display omitted] [ABSTRACT FROM AUTHOR]

In the field of nanoscale magnetocaloric materials, novel concepts like micro-refrigerators, thermal switches, microfluidic pumps, energy harvesting devices and biomedical applications have been proposed. However, reports on nanoscale (Mn,Fe) 2 (P,Si)-based materials, which are one of the most promising bulk materials for solid-state magnetic refrigeration, are rare. In this study we have synthesized (Mn,Fe) 2 (P,Si)-based nanoparticles, and systematically investigated the influence of crystallite size and microstructure on the giant magnetocaloric effect. The results show that the decreased saturation magnetization (M s) is mainly attributed to the increased concentration of an atomically disordered shell, and with a decreased particle size, both the thermal hysteresis and T c are reduced. In addition, we determined an optimal temperature window for annealing after synthesis of 300–600 °C and found that gaseous nitriding can enhance M s from 120 to 148 Am2kg−1 and the magnetic entropy change (ΔS m) from 0.8 to 1.2 Jkg−1K−1 in a field change of Δ μ 0 H = 1 T. This improvement can be attributed to the synergetic effect of annealing and nitration, which effectively removes part of the defects inside the particles. The produced superparamagnetic particles have been probed by high-resolution transmission electron microscopy, Mössbauer spectra and magnetic measurements. Our results provide important insight into the performance of giant magnetocaloric materials at the nanoscale. [Display omitted] [ABSTRACT FROM AUTHOR]