In-situ XRD for studying the effects of external fields on the structure of magnetocaloric materials

点击翻译按钮获取中文摘要

The properties of a magnetocaloric material depend very sensitively on its structure. Alterations at all scales, ranging from atomic distances through microstructure to macroscale, can have a large impact on the magnitude of the magnetocaloric effect, its hysteresis and the long-term properties of the material. In this project proposal, I will explore how structural changes which accompany a magnetostructural transition can be used most efficiently for magnetocaloric cooling applications. In-situ X-ray techniques will be applied to learn how changes in the lattice upon application of an external field can benefit the magnitude of the magnetocaloric effect while at the same time effects reducing the materials life span can be eliminated or lessened. In the project, two established magnetocaloric materials (La(Fe,Si)13 and NiMnGa-type Heusler alloys) will form the basis of the investigations, while in the long run, novel YCo5- and Fe-rich CeFe-based alloys will be explored for their room temperature magnetocaloric cooling potential.The structure of the Heusler alloy thin films with intrinsically different states of stress will be studied using stress and temperature-dependent XRD. Using this approach, we will be able to infer the structural state close to a bilayer interface from the state of the single layers and its dependence on temperature. In the best case, the coupling of lattice and magnetic degrees of freedom close to an interface will lead to a substantial increase of the magnetic entropy change. In the long run we will attempt to transfer this principle of generating additional contributions at interfaces to a twophase bulk system.The abrupt changes in the structure of a first-order-type magnetocaloric material brings – alongside an increased magnetocaloric performance - attributes which can make it challenging to apply these materials in actual devices, like deterioration under cyclic application of external fields and hysteresis. Again, knowing how the structure changes together with the magnetic properties is the key for improvement. In this context, we are going to study the effect of external fields upon the structure of magnetocaloric bulk materials. For La(Fe,Si)13, we will cover two length scales which are intimately connected. On one hand, in-situ X-ray diffraction will probe the atomic distances as a function of magnetic field and temperature, probing the microscopic volume changes during the transition. On the other hand, in-situ X-ray tomography will explore the morphology of the material upon field application, imaging cracks and pores which often appear during the magnetostructural transition. These techniques together will give a complementary insight into how an external field acts upon the magnetocaloric material, giving a unique opportunity to tackle the issues that currently hamper the application of magnetocaloric materials in practice.