Magnetocaloric Effect in Alloys with Distributed Exchange Interactions

【非技术内容】磁热效应是磁性材料在施加或去除磁场时所发生的温度变化。表现出大磁热效应的材料对制冷应用很感兴趣，因为磁热效应制冷不使用传统气体压缩冰箱所必需的消耗臭氧的气体。磁制冷的效率比传统制冷高出20%，并且有可能减少对环境的影响，减少冷却的能源需求，这是美国每年能源使用量的很大一部分。早在2020年，磁热制冷冰箱就已宣布用于消费产品。磁热冷却也可以用于热管理，从而为商业和军事防御车辆提供更高效的电机。要研究的材料是新型合金，适合需要耐用材料的应用，因为它们之前已经研究过在极端温度和压力环境下使用，使它们能够用于发动机的主动冷却和热管理。该项目将参与佛罗里达理工大学的外展项目，并指导高中理科学生。技术摘要磁热效应（MCE）是指在磁场h的作用下磁性材料的温度变化，在室温附近具有较大磁热效应的材料可用于磁冷却。关键稀土金属（REs）化合物由于在室温附近具有较大的磁热效应而首次被研究。然而，不断增加的成本和稀土的稀缺性限制了其在商业制冷中的应用，因此人们开始研究过渡金属基的替代品。磁热材料的分类分为：(1)一级磁结构相变，或(2)二级磁相变。一阶相变在磁结构相变中有一个大而窄的磁熵变化峰，但这伴随着热滞后，使得它们不适合多循环冷却。二阶相变具有小而宽的磁熵变化。广义熵变使这些二阶相变材料具有较大的工作温度范围，提高了它们的制冷能力。最优的磁热材料要求具有较大的峰值熵变、较大的RC和工作温度范围、较低的热滞后和抗热机械疲劳。研究将集中在晶体无序和压力在多组分高熵合金交换相互作用中的作用，以评估这些材料中的最大磁热效应。这些多组分合金具有可调的居里温度、Tc、磁热响应和制冷能力。我们将模拟位置无序如何影响系统的磁相变和磁热响应。四元和四元Fe-Co-Ni基合金引起了人们的兴趣，因为分布的J(R)允许通过二阶跃迁的宽度对rc进行Tc调谐和控制，而Fe-Co-Ni基允许系统的较大平均磁矩，这增加了磁热响应。我们将(1)在Argonne国家实验室使用可达到p6gpa的压力电池观察压力P下的结构；(2)利用能带理论扩展随机交换模型，以理解多组分系统的Tc调谐和Bethe-Slater曲线的p依赖性；(3)利用包括完全相对论形式下自旋轨道相互作用在内的能带理论研究两两交换相互作用，以证实Bethe-Slater曲线预测的J(R)；(4)研究基于氟氯化碳的HEA？s，在同步加速器设施中测量，与计算的J(R) s一起用于预测t相关磁化和MCE；(5)通过p依赖敏感性测量证实M(T)预测；(6)利用穆斯堡尔谱法测量技术相关合金中Tc以下的超细磁场分布。该项目将参与佛罗里达理工大学的外展项目，并指导高中理科学生。

Non Technical AbstractThe magnetocaloric effect is the temperature change a magnetic material undergoes on application or removal of a magnetic field. Materials exhibiting a large magnetocaloric effect are of interest for refrigeration applications because magnetocaloric refrigeration does not use ozone depleting gases necessary in conventional gas compression refrigerators. Magnetic refrigeration is up to 20% more efficient than conventional refrigeration and has potential to lessen environmental impact and decrease energy demands of cooling, that is a significant portion of US yearly energy usage, Magnetocaloric cooling refrigerators have been announced for consumer products as early as 2020. Magnetocaloric cooling could also be used for thermal management, resulting in more efficient motors both for commercial and military defense vehicles. The materials to be studied are new alloys that fit applications requiring durable materials, as they have previously been studied for use in extreme temperature and pressure environments making them viable for active cooling and thermal management of engines. The project will engage outreach programs at Florida Polytechnic University and mentor high school science students. Technical AbstractThe magnetocaloric effect (MCE) refers to the temperature change of a magnetic material on application of a magnetic field, H. Materials with a large magnetocaloric response near room temperature can be used for magnetic cooling. Critical rare earths metals (REs)compounds were first studied because of large magnetocaloric effects near room temperature. However, the increasing cost and scarcity of REs limits their use for commercial refrigeration, so transition metal-based replacementss have been investigated. Magnetocaloric materials are classified by undergoing: (1) a 1st order magneto-structural phase transition, or (2) a 2nd order magnetic transition. 1st order transitions have a large, narrow peak magnetic entropy changes in a magneto-structural phase transition, but this is accompanied by thermal hysteresis making them less desirable for multi-cycle cooling. 2nd order phase transitions have a small but broad magnetic entropy change. The broad entropy change gives these 2nd order phase transition materials a larger working temperature range increasing their refrigeration capacity, RC. Optimal magnetocaloric materials require a relatively large peak entropy change, a large RC and working temperature range, low thermal hysteresis, and resistance to thermomechanical fatigue. The studies will focus on the role of crystallographic disorder and pressure on exchange interactions in multi-component high entropy alloys to assess the maximum magnetocaloric effect in these materials. These multi-component alloys have tunable Curie temperatures, Tc, magnetocaloric response, and refrigeration capacity. We will model how positional disorder affects the magnetic phase transition and magnetocaloric response of the system. Quaternary and quinternary Fe-Co-Ni-based alloys are of interest because distributed J(R) allows Tc tuning and control of RCs, through the breadth of the 2nd order transition, while the Fe-Co-Ni basis allows for a larger average magnetic moment of the system, which increases the magnetocaloric response. We will (1) observe structure under pressure, P, at Argonne National Lab with pressure cells that can reach P 6 GPa.; (2) extend random exchange models with band theory to understand Tc tuning in multicomponent systems and P-dependence of the Bethe-Slater curve; (3) investigate pairwise exchange interactions using band theory that includes spin-orbit interactions in fully relativistic formalisms to confirm J(R)'s predicted by the Bethe-Slater curve; (4) study pressure dependent atomic spacing in fcc-based HEA?s, measured at synchrotron facilities, to be used with calculated J(R)'s to predict T-dependent magnetization and MCE; (5) confirm M(T) predictions by P-dependent susceptibility measurements; and (6) measure magnetic hyperfine field distributions using Mossbauer spectroscopy below Tc in technologically relevant alloys. The project will engage outreach programs at Florida Polytechnic University and mentor high school science students.