Lünser Klara, Diestel Anett, Nielsch Kornelius, Fähler Sebastian
Institute for Metallic Materials, Leibniz IFW Dresden, 01069 Dresden, Germany.
Institute of Materials Science, TU Dresden, 01062 Dresden, Germany.
Materials (Basel). 2020 Aug 20;13(17):3674. doi: 10.3390/ma13173674.
Magnetocaloric materials based on field-induced first order transformations such as Ni-Mn-Ga-Co are promising for more environmentally friendly cooling. Due to the underlying martensitic transformation, a large hysteresis can occur, which in turn reduces the efficiency of a cooling cycle. Here, we analyse the influence of the film microstructure on the thermal hysteresis and focus especially on large angle grain boundaries. We control the microstructure and grain boundary density by depositing films with local epitaxy on different substrates: Single crystalline MgO(0 0 1), MgO(1 1 0) and Al2O3(0 0 0 1). By combining local electron backscatter diffraction (EBSD) and global texture measurements with thermomagnetic measurements, we correlate a smaller hysteresis with the presence of grain boundaries. In films with grain boundaries, the hysteresis is decreased by about 30% compared to single crystalline films. Nevertheless, a large grain boundary density leads to a broadened transition. To explain this behaviour, we discuss the influence of grain boundaries on the martensitic transformation. While grain boundaries act as nucleation sites, they also lead to different strains in the material, which gives rise to various transition temperatures inside one film. We can show that a thoughtful design of the grain boundary microstructure is an important step to optimize the hysteresis.
基于场诱导一级相变的磁热材料，如镍-锰-镓-钴合金，有望实现更环保的冷却方式。由于其潜在的马氏体相变，可能会出现较大的磁滞现象，进而降低冷却循环的效率。在此，我们分析了薄膜微观结构对热滞的影响，并特别关注大角度晶界。我们通过在不同衬底上进行局部外延沉积薄膜来控制微观结构和晶界密度：单晶氧化镁（0 0 1）、氧化镁（1 1 0）和氧化铝（0 0 0 1）。通过将局部电子背散射衍射（EBSD）和全局织构测量与热磁测量相结合，我们发现较小的磁滞与晶界的存在有关。在含有晶界的薄膜中，与单晶薄膜相比，磁滞减小了约30%。然而，较大的晶界密度会导致转变变宽。为了解释这种行为，我们讨论了晶界对马氏体相变的影响。虽然晶界作为形核位点，但它们也会导致材料内部产生不同的应变，从而在同一薄膜内产生各种转变温度。我们可以证明，精心设计晶界微观结构是优化磁滞的重要一步。
