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合金系统的磁热学和物理性质，以便从模型系统到应用程序的步骤可以执行。作为起点，我们将使用Ni-Co-Mn-Al，它在SPP的第一个资助期被确定为非常有前途的磁热Heusler合金。基于对该材料系统已经获得的知识，现在将继续在两个长度尺度上进行理解和改进。在薄膜的长度尺度上，化学、晶体结构和微观结构可以很容易地控制，从完美晶体到有缺陷的材料，即成分变化、界面、点缺陷、晶界将被执行。在这里，我们的目标是将我们之前关于晶格和自旋自由度耦合的知识推广到这些情况，并在实验和理论概念的帮助下提高磁热性能，特别是RCP。在设备的长度尺度上，我们将构建新的复合结构，以实现磁热再生器内的最佳传热。为了实现工作温度跨度大的分级蓄热器，选择的概念是在熔融纺丝nicomnal型带状材料的基础上，采用不同转变温度的层状复合材料。特别感兴趣的是通过制备具有高导热性的附加成分的复合材料来增强对传热介质的热流。该项目的魅力在于多重知识转移：首先，将先前获得的关于Heusler材料耦合以及熵贡献耦合的知识应用于最近确定的系统的挑战。其次，将薄膜作为模型系统开发的知识转移到新的演示器和设备概念中，并用于表征其性能和长期稳定性。第三，基于dft的磁热效应机理模拟及其与化学、结构和导热系数的关系将作为实验研究的指导。此外，一个负责薄膜制备和各种表征技术的试验组与另一个负责层状复合材料制备和表征的试验组之间的强有力合作将加速Heusler合金在磁热再生器和器件中的应用。

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.