Magnetocaloric Effect in Alloys with Distributed Exchange Interactions

磁热效应是磁性材料在施加或去除磁场时所经历的温度变化。表现出大的磁热效应的材料对于制冷应用是令人感兴趣的，因为磁热制冷不使用常规气体压缩制冷机中所必需的臭氧消耗气体。磁制冷的效率比传统制冷高出20%，并且有可能减轻对环境的影响并减少冷却的能源需求，这是美国每年能源使用的重要组成部分，磁热冷却冰箱最早已宣布用于消费品2020年。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。要研究的材料是适合需要耐用材料的应用的新合金，因为它们以前已经被研究用于极端温度和压力环境，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。 磁热效应（MCE）是指磁性材料在磁场H作用下的温度变化。在室温附近具有大磁热响应的材料可用于磁冷却。临界稀土金属化合物由于在室温附近具有较大的磁热效应而被首次研究。然而，稀土的成本增加和稀缺性限制了它们在商业制冷中的使用，因此过渡金属基制冷剂已经被研究。磁热材料通过经历：（1）第一阶磁结构相变，或（2）第二阶磁转变来分类。第一阶转变在磁结构相变中具有大的、窄的峰值磁熵变化，但是这伴随着热滞后，使得它们对于多循环冷却不太理想。第二阶相变具有小但宽的磁熵变。宽的熵变给予这些第二阶相变材料更大的工作温度范围，增加了它们的制冷能力RC。最佳的磁热材料需要相对大的峰值熵变、大的RC和工作温度范围、低热滞后和抗热机械疲劳性。 这些研究将侧重于多组分高熵合金中晶体学无序和压力对交换相互作用的作用，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。四元和五元Fe-Co-Ni基合金是令人感兴趣的，因为分布的J（R）允许通过第二阶转变的宽度对RC进行Tc调谐和控制，而Fe-Co-Ni基允许系统的更大平均磁矩，这增加了磁热响应。我们将（1）在阿贡国家实验室用可达P6 GPa的压力盒观察压力P下的结构。(2)扩展随机交换模型与能带理论，以了解在多组分系统的Tc调谐和P-依赖的Bethe-Slater曲线;（3）调查成对交换相互作用，使用能带理论，包括自旋轨道相互作用在完全相对论形式主义，以确认J（R）的预测的Bethe-Slater曲线;（4）研究压力依赖的原子间距在fcc为基础的HEA？s，在同步加速器设备上测量，与计算的J（R）的，以预测T依赖的磁化强度和MCE;（5）确认M（T）的预测由P依赖的磁化率测量;和（6）测量磁超精细场分布使用穆斯堡尔谱低于Tc在技术相关的合金。 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.