CAS: Using Narrow Bands and Competing Exchange Interactions as Design Principles for Magnetocaloric Materials

非技术摘要：磁性是一种迷人的物理现象，使现代传感，逻辑和存储设备。磁性的一个鲜为人知的应用是通过一种称为磁热效应（MCE）的机制在制冷技术中。磁制冷机与普通制冷机类似，除了压缩和减压气体，它磁化和退磁固态材料以产生低温。用于该应用的材料通常含有稀土元素，例如钆和铒。这些元素是昂贵的，不容易获得，特别是在美国。通过这个项目，由固态和材料化学计划以及材料研究部的凝聚态材料物理计划支持，波士顿学院的研究人员合成了具有巨大MCE并且仅含有少量稀土元素的新材料。通过这种方式，该项目通过消除消耗臭氧的制冷气体和最大限度地减少磁热材料的稀土含量，解决了可持续性的关键问题。这个跨学科项目为培养下一代材料科学家提供了一个平台，他们对量子化学有着深刻的理解，并有能力实施基础科学来解决社会问题。此外，该项目还为波士顿地区代表性不足的少数民族的大学预科学生提供职业讲习班，并为杰克逊州立大学的学生提供研究机会。技术摘要：表现出磁热效应（MCE）的材料，作为磁场的函数，温度的可逆变化，为制冷技术提供了一个可持续的平台。磁冰箱比普通冰箱能耗低得多，而且不依赖消耗臭氧层的气体。磁制冷机商业化的主要挑战是缺乏由地球丰富的元素制成的具有大制冷容量（RC）的材料。巨磁热材料要么由有毒元素如P和As制成，要么具有大量的稀土含量。因此，合成无毒且几乎没有稀土含量的巨磁热材料是可持续性的一个关键方面。在这种支持下，波士顿学院的研究人员通过两种化学设计原则合成了这种材料，即窄带宽和竞争交换相互作用。窄的带宽导致通过Stoner机制在室温附近的磁性转变。临界温度可以通过掺杂来调节，这使得多级磁制冷机的设计成为可能。竞争交换相互作用拓宽了磁转变，并导致在宽的温度范围内的熵释放，从而提高RC。所研究的材料中稀土含量不到10%。研究人员阐明了在这些材料中以最小的稀土含量实现巨大MCE的新机制。教育影响包括参与化学和物理之间的多学科研究项目的代表性不足的学生，以及在国家高磁场和同步辐射设施培训学生。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Non-technical Abstract:Magnetism is a fascinating physical phenomenon that enables modern sensing, logic, and memory devices. A less-known application for magnetism is in refrigeration technology through a mechanism called the magnetocaloric effect (MCE). A magnetic refrigerator is similar to regular refrigerators, except instead of compressing and decompressing a gas, it magnetizes and demagnetizes a solid-state material to create the low temperatures. Materials used for this application typically contain rare-earth elements, for example gadolinium and erbium. These elements are expansive and not readily available, especially not in the U.S.. With this project, supported by the Solid State and Materials Chemistry program and the Condensed Mater Physics program in the Division of Materials Research, the researchers at Boston College synthesize new materials that exhibit a giant MCE and contain only minimal amounts of rare-earth elements. In this way the project addresses critical aspects of sustainability by eliminating ozone-depleting refrigeration gases and by minimizing the rare-earth content of the magnetocaloric materials. This interdisciplinary project provides a platform for training the next generation of materials scientists with a deep understanding of quantum chemistry and the ability to implement fundamental science to solve the societal problems. Additionally, this project provides career workshops for pre-college students from underrepresented minorities in the Boston area and research opportunities for students from Jackson State University, a HBCU.Technical Abstract:Materials that exhibit a magnetocaloric effect (MCE), the reversible change of temperature as a function of magnetic field, provides a sustainable platform for refrigeration technology. Magnetic refrigerators are considerably less energy consuming than regular refrigerators and do not rely on ozone-depleting gases. The main challenge for commercializing magnetic refrigerators is the lack of materials with a large refrigeration capacity (RC) made from earth-abundant elements. Giant magnetocaloric materials are either made from toxic elements such as P and As, or have a large rare-earth content. As such, the synthesis of giant magnetocaloric materials that are non-toxic and have little to no rare-earth content is a critical aspect of sustainability. With this support researchers from Boston College synthesize such materials by following two chemical design principles, namely narrow bandwidths and competing exchange interactions. The narrow bandwidth leads to a magnetic transition near room temperature via the Stoner mechanism. The critical temperature can be adjusted by doping, which enables the design of multi-stage magnetic refrigerators. Competing exchange interactions broaden the magnetic transition and lead to entropy release over a wide temperature range, thereby enhancing RC. The investigated materials have less than ten per cent rare-earth content. The researchers elucidate novel mechanisms for achieving giant MCE in these materials with minimal rare-earth content. The educational impacts include the engagement of underrepresented students in a multidisciplinary research project between chemistry and physics as well as training students at the national high-magnetic field and synchrotron radiation facilities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.