Enhanced Magnetic Cooling through Optimising Local Interactions

冷藏是现代社会的核心，它使炎热的气候变得宜居，保存食物，并促进医疗扫描仪和量子计算。据估计，英国十分之一以上的电力用于制冷，每年的成本超过50亿GB，全球有近1200万人受雇于与制冷相关的行业。提供接近绝对零度温度的低温制冷正在成为一个主要行业；研究委员会STFC预测，在截至2025年的十年里，低温制冷对英国经济的贡献将从3.24亿GB增加到3300亿GB。基于热能材料的固态制冷承诺比目前基于流体制冷剂的技术具有更高的能源效率，并且不会不可避免地将其活性成分作为气体泄漏，例如用于低温应用的稀缺液氦。卡路里制冷剂依赖于外加磁场和电场或压力等外部刺激时，宇宙无序倾向的衡量标准--熵的变化。实际的热量冷却要求新材料在容易获得的外部刺激下表现出最大的熵变化。在磁热学中，冷却过程是由外加磁场驱动的。这具有优势，包括与其他卡路里相比具有很高的循环性，因为材料在暴露在磁场中时不会恶化。磁热已有一个多世纪的历史，但它们的使用大多被限制在超低温下的制冷，以毫开尔文为单位，并且具有非常大的磁场，限制了它们的应用。开发在较高温度和较低磁场下工作的磁热元件将使基于磁化的低温制冷得到更广泛的应用。我们可以通过调节这些材料中的相互作用来系统地优化新的磁热材料，以满足所需的低温温度。这实现了在关键工作温度下的小磁场变化的大的熵变。虽然这种方法是众所周知的磁热制冷近室温，它的重要性，在优化磁热制冷低温，直到最近才显示出来。增强型低温磁热技术将比替代冷却技术具有更高的效率，使我们能够极大地减少对液氦的依赖，液氦越来越昂贵，而且容易造成供应中断。要实现最好的磁热，需要一种结合化学和物理的跨学科方法；这与我们团队在材料合成、结构和物理性能表征以及原型测试方面的专业知识很好地匹配。磁热在磁性离子链内具有强烈的相互作用，而链之间的竞争作用较弱，似乎特别适合取代液体氦。我们将利用这一最新发现，开发框架磁热材料，这是一种新的材料类别，由于其非常灵活的组成和结构，使其能够极大地自由地优化性能。我们将系统地研究如何通过调节具有强磁耦合的单元之间的竞争相互作用的程度和改变强耦合单元的维度来优化这些磁热元件在4K以上的使用。通过改变加入的磁性离子来直接调节它们的磁性相互作用，这些磁热效应将得到进一步的优化。然后将对筛选过程中出现的最佳磁热进行评估，以确定它们的最大冷却能力和功率。这些关键信息将使我们能够确定它们在实际冷却设备中的用途，这将通过整合到原型磁热制冷机中进行演示。从这个项目中获得的理解将有助于制定定制磁热元件的精确设计规则。

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.