Understanding Mechanisms in Magneto-Structural Transformations

制冷和建筑制冷占美国总用电量的17%。高效的磁性制冷剂可以根据外部磁场升温或降温，有可能取代传统的蒸汽压缩冰箱和热泵，大大减少电力消耗，从根本上改变国内能源格局。然而，这些材料目前受到每次磁场变化时发生的能量损失的限制，这与材料内部结构的突变有关。该奖项支持对造成这种能量损失的微观机制的研究，以了解其根本起源。这项研究将有助于更好地理解如何设计高效的近室温磁性制冷剂。这些知识也可以改善其他功能材料的设计，如形状记忆合金。磁滞在限制固-固功能相变效率方面起着关键作用。虽然已经开发了大量的理论来设计热弹性马氏体中的低滞后一阶相变，但仍然不清楚为什么某些材料系统比其他系统表现出明显更大的滞后。本研究旨在研究Fe2P合金的相变机制，该合金可以表现出1k的迟滞，目的是确定该系统中迟滞的起源。研究计划包括三个目标：1)确定Fe2P合金一阶磁结构相变的微观转变机制，并检测可能显著导致磁滞的过程；2)阐明迟滞的起源及其与一阶向二阶过渡行为转变的关系；3)分析低迟滞合金基于磁冷剂循环的性能。本研究旨在为设计新型低迟滞Fe2P合金提供一种合理的方法，并可能为其他固-固相变的迟滞工程提供更大的见解。

Refrigeration and building cooling account for 17 percent of total electricity consumption in the US. High efficiency magnetic refrigerants, which heat up or cool down in response to external magnetic fields, have the potential to replace traditional vapor-compression refrigerators and heat pumps, dramatically reducing electricity consumption and radically altering the domestic energy landscape. However, these materials are currently limited by energy loss that occurs every time the magnetic field is changed, associated with abrupt changes in the internal structure of the material. This award supports research into the microscopic mechanism causing this energy loss, in order to understand its fundamental origin. This research will result in a better understanding of how to design high-efficiency near-room temperature magnetic refrigerants. Such knowledge could also improve the design of other functional materials such as shape memory alloys.Hysteresis plays a critical role in limiting the efficiency of functional solid-solid phase transitions. While substantial theory has been developed to design low-hysteresis first-order phase transitions in thermoelastic martensites, it remains unclear why certain materials systems exhibit significantly larger hysteresis than other systems. This research seeks to investigate the phase transformation mechanisms in Fe2P alloys, which can exhibit hysteresis 1 K, with the objective of identifying the origins of hysteresis in that system. The research plan consists of three goals: 1) Identify microscopic transformation mechanisms in first-order magneto-structural phase transitions in Fe2P alloys, and detect processes that could significantly contribute to hysteresis; 2) Elucidate the origin of hysteresis and its relationship to a shift from first-order to second-order transition behavior; and 3) Analyze magnetic refrigerant cycle-based performance for low-hysteresis alloys. This research aims to achieve a rational approach to design new low-hysteresis Fe2P alloys, and may additionally provide greater insight into hysteresis engineering in other solid-solid phase transitions.