Falsaperna Mario, Saines Paul J
School of Physical Sciences, Ingram Building, University of Kent, Canterbury, CT2 7NH, UK.
Dalton Trans. 2022 Mar 1;51(9):3394-3410. doi: 10.1039/d1dt04073a.
Caloric materials have attracted significant interest as replacements for conventional refrigeration, which is becoming increasingly important in our daily lives, yet poses issues for sustainability due to both energy consumption and loss of refrigerants into the atmosphere. Among caloric materials, which are key to solid state cooling technologies, those exhibiting the magnetocaloric effect (MCE), an entropy-driven phenomenon under cycled applied magnetic fields, are promising candidates for cryogenic cooling. These have potential to replace conventional cryogenics, particularly liquid He - an increasingly scarce and expensive resource. Amongst magnetocalorics, coordination polymers containing polyatomic ligands have been shown to be very promising materials due to their large entropy changes at low temperatures. One of the contributing factors to this peformance is their unique structural flexibility, as they can adopt a wide range of structures usually not accessible for conventional materials, such as close-packed metal oxides. The most researched materials for magnetocaloric applications are those containing Gd as their magnetic centre, as the combination of structure and the weakly interacting 4f orbitals of Gd in these materials enables the fabrication of promising magnetocalorics that contain a high density of cations and thus exhibit a high entropy change as a function of their weight and volume at ultra-low cryogenic temperatures. Alongside this, there is a growing interest in magnetocaloric coordination polymers with their magnetocaloric effect optimised for lower applied fields that can be generated using permanent magnets through incorporating other magnetic cations, including lanthanides with greater magnetic anisotropy. When combined with tailored magnetic interactions this leads to promising entropy changes above 4 K, a typical base temperature for many cryogenic applications. This review discusses the most promising magnetocalorics among coordination polymers and MOFs, highlighting their structural characteristics, and concluding with a brief perspective on the future of this field.
热材料作为传统制冷的替代品已引起了广泛关注，传统制冷在我们的日常生活中变得越来越重要，但由于能源消耗以及制冷剂向大气中的排放，对可持续性构成了问题。在作为固态冷却技术关键的热材料中，那些表现出磁热效应（MCE）的材料，即在循环施加磁场下的一种熵驱动现象，是低温冷却的有前途的候选材料。这些材料有潜力取代传统低温制冷剂，特别是液态氦——一种日益稀缺且昂贵的资源。在磁热材料中，含有多原子配体的配位聚合物由于其在低温下的大熵变而被证明是非常有前途的材料。这种性能的一个促成因素是它们独特的结构灵活性，因为它们可以采用通常传统材料无法获得的多种结构，例如密堆积金属氧化物。用于磁热应用研究最多的材料是以钆作为其磁中心的那些材料，因为这些材料中结构与钆的弱相互作用4f轨道的结合使得能够制造出有前途的磁热材料，这些材料含有高密度的阳离子，因此在超低温下作为其重量和体积的函数表现出高熵变。除此之外，人们对磁热配位聚合物的兴趣日益增加，这些聚合物通过掺入其他磁性阳离子（包括具有更大磁各向异性的镧系元素），其磁热效应针对可以使用永久磁铁产生的较低施加磁场进行了优化。当与定制的磁相互作用相结合时，这会导致在4K以上有希望的熵变，4K是许多低温应用的典型基础温度。本综述讨论了配位聚合物和金属有机框架中最有前途的磁热材料，突出了它们的结构特征，并对该领域的未来进行了简要展望。
