Magnetocaloric Effect in Alloys with Distributed Exchange Interactions

非技术摘要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。表现出大磁热效应的材料在制冷应用中很有价值，因为磁热制冷不使用传统气体压缩冰箱所需的臭氧消耗气体。磁制冷的效率比传统制冷高达20%，并有可能减少对环境的影响和减少制冷的能源需求，这是美国年度能源使用量的重要组成部分，早在2020年，磁热制冷冰箱就已宣布用于消费产品。磁热冷却还可以用于热管理，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型合金，因为它们以前曾被研究用于极端温度和压力环境，使其适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)是指磁性材料在磁场作用下的温度变化，H。在室温附近具有较大磁热响应的材料可用于磁冷却。由于在室温附近有很大的磁热效应，临界稀土金属(RES)化合物首次被研究。然而，日益增长的成本和稀缺性限制了稀土在商业制冷中的应用，因此人们开始研究过渡金属基替代材料。磁热材料可分为：(1)一阶磁-结构相变，或(2)二阶磁相变。在磁结构相变中，一阶相变有一个很大、很窄的峰值磁熵变化，但这伴随着热滞，这使得它们不太适合多循环冷却。二阶相变具有较小但较宽的磁熵变化。广义的熵变使这些二级相变材料有了更大的工作温度范围，提高了它们的制冷量Rc。理想的磁热材料需要相对较大的峰值熵变、较大的RC和工作温度范围、较低的热滞和抗热机械疲劳性能。这些研究将集中在多组分高熵合金中晶体无序和压力对交换作用的作用，以评估这些材料的最大磁热效应。这些多元合金具有可调的居里温度、T_c、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基合金很有价值，因为分布的J(R)允许T_c调谐和控制RCS，通过二级转变的宽度，而Fe-Co-Ni基允许系统更大的平均磁矩，这增加了磁热响应。我们将(1)在Argonne国家实验室用可达P6 Gpa的压力室观察压力下的结构P；(2)用能带理论扩展随机交换模型，以了解多元体系的T_c调谐和Bethe-Slate曲线的P依赖性；(3)用完全相对论形式中包括自旋-轨道相互作用的能带理论研究成对交换相互作用，以证实Bethe-Slate曲线预测的J(R)‘S；(4)研究面心立方HEA中原子间距与压力的关系？S，在同步加速器装置上测量，与计算的J(R)‘S一起用于预测T依赖磁化强度和平均磁化率；(5)通过P依赖磁化率测量证实M(T)预测；以及(6)在技术相关的合金中使用低于T_c的穆斯堡尔谱测量磁超精细场分布。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.