The Changing Shape of Magnetic Refrigeration: an investigation of adaptive magnetic materials

现代制冷几乎完全基于压缩/膨胀制冷循环--这项技术自一个多世纪前发明以来或多或少没有改变。这是一个高能源需求的行业，每年消耗数十亿千瓦时。然而，现代制冷已接近其基本性能极限，远低于热力学上可能达到的极限。此外，作为制冷剂的液体化学品最终会泄漏到环境中，它们是臭氧层消耗和全球变暖气体，即危险化学品。近年来，磁制冷已成为一种新的、环保的固态制冷技术。在过去的十年里，已经展示了磁性冰箱的原型。事实证明，它们比传统冰箱能效高得多，可以跨越室温附近的广泛温度范围。但大多数原型使用昂贵的稀土金属，如Gd作为制冷剂，迫切需要替代材料。人们已经发现了几类有前途的磁性材料，但到目前为止，这一过程还是一个启发式的过程。在这项提议中，我们打算建立一个从头算量子材料模拟工具来改变这一过程，并促进从事磁性材料研究的小组对其应用。在最合适的材料中，支撑磁性的相互作用必须微妙地保持平衡，我们的建模需要能够跟踪和指示它们的温度依赖关系，它们如何随着成分和结构的变化而变化和/或何时添加掺杂。在磁性制冷剂中，当施加磁场时，材料中随机取向的磁矩排列在一起，使固体升温。通过使用水或空气等导热流体来消除这些热量，然后去除磁场，就可以使磁性材料的温度降低。然后，从被冷却的物体中提取热量，并与换热流体一起完成循环。当磁场施加到材料上时，发生的熵和温度的变化描述了磁热效应，这个提议将在定量的基础上确定它和它背后的磁相互作用。我们对几类材料的结果将根据现有的大量实验数据进行测试。这项工作的一个特别新颖和雄心勃勃的部分将是研究如何纳米结构一个大的磁热效应。为此，我们将研究一些稀土-过渡金属异质结构并优化其效应。这一物理在施加磁场时会产生强烈的升温效应，还有另一个有趣的方面。它可以解释一些最有希望的材料如何在磁场存在的情况下显著改变其形状。这种磁塑性“磁形状记忆”效应具有多种潜在的技术应用，如微泵、声纳和磁力传感器。我们将采用我们的理论纳米结构模型来研究材料中边界缺陷上的磁相互作用的强度和各向异性，以及它们是如何导致缺陷本身在施加磁场时移动的。将进行一个Ni-Mn-Ga Heusler合金的测试案例，随着合金成分的不同，效果将得到优化。

Modern cooling is based almost entirely on a compression/expansion refrigeration cycle - a technology more or less unchanged since its invention over a century ago. It is a high-energy demand industry which consumes billions of kWh every year. Yet, modern refrigeration is close to its fundamental performance limit which is well below what is thermodynamically possible. Furthermore, the liquid chemicals used as refrigerants, which eventually escape into the environment, are ozone layer depletive and global warming gases, or hazardous chemicals.Recently magnetic refrigeration has emerged as a promising way for a new and environmentally friendly solid state cooling technology. Prototype magnetic fridges have been demonstrated during the last decade. They have been proven to be much more energy efficient than conventional fridges and can span a broad temperature range around room temperature. But most prototypes use expensive rare earth metals such as gadolinium as the refrigerant and alternatives are urgently required. Several families of promising magnetic materials have been discovered but up to now this process has been a heuristic one. In this proposal we intend to establish an ab-initio quantum materials modeling tool to transform this process and to facilitate its application by groups working with magnetic materials. In the most suitable materials the interactions that underpin the magnetic properties have to be delicately poised and our modeling will need to be able to track and indicate their temperature dependence, how they vary with compositional and structural changes and/or when dopants are added. In a magnetic refrigerant randomly oriented magnetic moments in the material align when a magnetic field is applied making the solid warm up. By removing this heat using a heat transfer fluid, like water or air, and then removing the field allows the magnetic material to lower its temperature. The heat from the object being cooled is then extracted with the heat transfer fluid and the cycle completed. The changes in entropy and temperature that happen when a magnetic field is applied to a material describe the magnetocaloric effect and this proposal will determine it and the magnetic interactions behind it on a quantitative basis. Our results for several classes of materials will be tested against the extensive experimental data available. A particularly novel and ambitious part of the work will be to investigate how to nanostructure a large magnetocaloric effect. To this end we will study some rare earth - transition metal heterostructures and optimise the effect.This physics which produces a strong warming effect when a magnetic field is applied has another intriguing facet. It can explain how some of the most promising materials also change their shape significantly in the presence of a magnetic field. Such magnetoplastic, 'magnetic shape memory' effects have diverse potential technological applications, such as micropumps, sonars and magnetomechanical sensors. We will adapt our theoretical nanostructural modeling to investigate the strengths and anisotropies of the magnetic interactions across a boundary defect in the material and how they lead to the defect itself moving as a magnetic field is applied. A test case of a Ni-Mn-Ga Heusler alloy will be undertaken and the effect will be optimised as the composition of the alloy is varied.