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

点击翻译按钮获取中文摘要

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.