Coupling phenomena in magnetocaloric materials: From thin layers to composites

在这个联合项目中，我们的目标是了解和改进Heusler合金系统的磁热和物理性质，以便能够执行从模型系统到应用的步骤。作为起点，我们将使用Ni-Co-Mn-Al，这是一种在SPP的第一个资助期被确定为非常有前途的磁热Heusler合金。基于已经获得的关于这一材料系统的知识，现在将在两个长度的尺度上继续理解和改进。在薄膜的长度尺度上，化学、晶体结构和微结构可以很容易地控制，并将执行从完美晶体到具有缺陷的材料的步骤，即成分变化、界面、点缺陷、晶界。在这里，我们的目标是将我们以前发展的关于晶格和自旋自由度耦合的知识推广到这些情况，并借助实验和理论概念来提高磁热性能，特别是RCP。在器件的长度尺度上，我们将构建新颖的复合结构，以实现磁热蓄热器内的最佳换热。选择的概念是基于不同转变温度的熔体快淬NiCoMnAl型薄带的层状复合材料，以实现大工作温度范围的梯度再生器。特别感兴趣的是通过制备具有高导热系数的附加组分的复合材料来实现向热传递介质的增强热流。这个项目的魅力在于多方面的知识转移：首先，以前获得的关于Heusler材料耦合以及熵贡献耦合的知识，被应用到最近发现的系统的挑战中。其次，将为薄膜作为模型系统开发的知识转移到新的演示器和器件概念，并用于表征其性能和长期稳定性。第三，磁热效应机理的DFT模拟知识及其与化学、结构和热导率的关系将被用作实验研究的指导。此外，负责薄膜制备和各种表征技术的实验小组与负责层状复合材料的制备和表征的实验小组之间的密切合作将加速Heusler合金在磁热再生器和设备中的使用。

In this joint project, we aim to understand and improve the magnetocaloric and physical properties of a Heusler alloy system such that the step from model systems to applications can be performed. As a point of departure we will use Ni-Co-Mn-Al, which has been identified as a very promising magnetocaloric Heusler alloy during the first funding period of the SPP. Based on the already achieved knowledge about this material system, the understanding and improvement will now continue on two length scales. On the length scale of thin films, where the chemistry, crystal structure, and microstructure can be readily controlled, and the step from perfect crystals to materials with imperfections, i.e. composition changes, interfaces, point defects, grain boundaries will be performed. Here, it is our goal to generalize our previously developed knowledge about the coupling of lattice and spin degree of freedom to these situations, and to increase the magnetocaloric performance, in particular RCP, with the help of experimental and theoretical concepts. On the length scale of devices we will construct novel composite architectures in order to achieve an optimum heat transfer within the magnetocaloric regenerator. The concept of choice are layered composites based on melt spun NiCoMnAl-type ribbons with different transition temperatures in order to achieve graded regenerators with large span of operating temperature. Particular interest is on achieving enhanced heat flow towards the heat transfer medium by preparing composites with additional components with high thermal conductivity. The charm of this project is a multifold knowledge transfer: First, the previously obtained knowledge about the coupling of Heusler materials as well as the coupling of entropy contributions, is applied to the challenges of the recently identified system. Second, the knowledge developed for thin films as model systems is transferred to novel demonstrator and device concepts and used to characterize their performance and long-term stability. Third, the knowledge of DFT-based simulations of the mechanisms of the magnetocaloric effect and their relation to the chemistry, structure and thermal conductivity will be used as a guideline for experimental investigations. In addition the strong collaboration between the experimental groups of which one is responsible for the thin film preparation and a variety of characterization techniques, and the other for the preparation and characterization of the layered composites will accelerate the use of Heusler alloys in magnetocaloric regenerators and devices.