Enhanced Magnetic Cooling through Optimising Local Interactions

冷藏是现代社会的核心，它使炎热的气候适宜居住，保存食物，促进医疗扫描仪和量子计算。据估计，英国超过十分之一的电力用于制冷，每年的成本超过50亿英镑，全球有近1200万人从事制冷相关行业。低温制冷，提供接近绝对零度的温度，正在成为一个主要产业；研究委员会STFC预测，到2025年，低温制冷对英国经济的贡献将从3.24亿英镑增加到3300亿英镑。基于热量材料的固态冷却比目前基于流体制冷剂的技术有更高的能源效率，并且不会遭受其有效成分不可避免地以气体形式逸出的影响，例如用于低温应用的稀有液氦。热量制冷剂依赖于熵的变化，熵是衡量宇宙无序趋势的一种方法，是对外界刺激（如外加磁场、电场或压力）的反应。实际的热冷却需要新的材料，在容易实现的外部刺激下，它们的熵变化最大。在磁热学中，冷却过程是由外加磁场驱动的。这具有优点，包括与其他热量相比，材料在暴露于磁场时不会变质，因此具有高可循环性。磁热学已经被发现了一个多世纪，但它们的使用大多局限于在超低温（以毫开尔文为单位）和非常大的磁场下制冷，限制了它们的应用。开发在更高温度和更低磁场下工作的磁热学将使基于磁化的低温冷却得到更广泛的应用。我们可以通过调整这些材料中的相互作用，系统地优化新的磁热学以达到所需的低温。这实现了在关键操作温度下小磁场变化的大熵变化。虽然这种方法在近室温冷却的磁热学中是众所周知的，但它在优化低温冷却的磁热学时的重要性最近才得到证明。增强型低温磁热学将比其他冷却技术具有更高的效率，使我们能够大大减少对液氦的依赖，液氦越来越昂贵，而且容易供应中断。实现最好的磁热学需要结合化学和物理的跨学科方法；这与我们团队在材料合成、结构和物理特性表征以及原型测试方面的专业知识相匹配。磁性离子链内强相互作用和链间弱竞争相互作用的磁热学似乎特别适合取代液氦。我们将利用这一最新发现，开发框架磁热学，这是一种新型材料，由于其非常灵活的成分和结构，可以极大地自由地优化性能。我们将系统地研究如何通过调整具有强磁耦合的单元之间的竞争相互作用的程度和修改强耦合单元的维数来优化这些磁热学在4 K以上的使用。这些磁热量将通过改变磁性离子来直接调整它们的磁性相互作用来进一步优化。然后对筛选过程中产生的最佳磁热学进行评估，以确定其最大冷却能力和功率。这一关键信息将使我们能够在实际的冷却装置中建立它们的效用，这将通过结合到一个原型磁热冰箱中来证明。从该项目中获得的理解将有助于开发定制磁热学的精确设计规则。

Refrigeration is central to modern society by making hot climates habitable, preserving food, and facilitating medical scanners and quantum computing. Over a tenth of Britain's electricity is estimated to go to cooling, at a cost of over £5 billion a year, with nearly 12 million (m) people globally employed in refrigeration related industries. Cryogenic refrigeration, which provides temperatures close to absolute zero, is becoming a major industry; the research council STFC have predicted that contributions to the UK economy from cryogenic refrigeration will increase from £324m to £3300m in the decade to 2025. Solid state cooling based on caloric materials promises higher energy efficiencies than current technologies based on fluid refrigerants and do not suffer from inevitable escape of their active components as gases, such as scarce liquid helium used for cryogenic applications. Caloric refrigerants rely on a change in entropy, a measure of the universe's tendency to disorder, in response to external stimuli such as applied magnetic and electric fields or pressure. Practical caloric cooling requires new materials that exhibit the maximum change in their entropy for readily achievable external stimuli. In magnetocalorics, the cooling process is driven by applied magnetic fields. This has advantages, including high cyclability compared to other calorics as materials do not tend to deteriorate when exposed to magnetic fields. Magnetocalorics have been known for over a century, but their use has mostly been restricted to refrigeration at ultra-low temperatures, measured in milli-kelvins, and with very large magnetic fields, limiting their application. Developing magnetocalorics that work at higher temperatures and under lower magnetic fields will enable magnetisation based cryogenic cooling to be more used much more widely. We can systematically optimise new magnetocalorics for desired cryogenic temperatures by tuning interactions in these materials. This achieves large entropy changes for small magnetic field changes at key operating temperatures. While this approach is well known for magnetocalorics for near room temperature cooling its importance when optimising magnetocalorics for cryogenic cooling has only been shown recently. Enhanced cryogenic magnetocalorics will have greater efficiency than alternative cooling technologies enabling us to greatly decrease dependence on liquid helium, which is increasingly expensive and prone to supply disruptions. Realising the best magnetocalorics requires an interdisciplinary approach that combines chemistry and physics; this is well matched by our team's expertise in materials synthesis, structural and physical properties characterisation and prototype testing.Magnetocalorics with strong interactions within chains of magnetic ions and weaker competing interactions between chains appear particularly suited to replace liquid helium. We will take advantage of this recent discovery by developing framework magnetocalorics, a new class of materials that enable enormous freedom to optimise properties due to their very flexible compositions and structures. We will systematically investigate how these magnetocalorics are best optimised for use above 4 K by tuning the extent of competing interactions between units with strong magnetic coupling and modifying the dimensionality of the strongly coupled units. These magnetocalorics will be optimised further by changing the magnetic ions incorporated to directly tune their magnetic interactions. The best magnetocalorics to emerge from this screening process will then be assessed to determine their maximum cooling capacity and power. This key information will enable us to establish their utility in practical cooling devices, which will be demonstrated by incorporation into a prototype magnetocaloric refrigerator. Understanding gained from this project will enable development of precise design rules for tailored magnetocalorics.