Temperature-dependent magneto-optical imaging is applied to study the thermal hysteresis of magnetocaloric MnFePSi spherical powder-packed beds across their magneto-elastic transition. Cooling and heating imaging series are used to analyze the transition of such a complex powder ensemble. The magnetization versus temperature behavior reconstructed from these local measurements shows very good agreement with integral measurements of the magnetization of the whole packed bed. Hence, local magneto-optical imaging measurements represent the ensemble behavior well if the number of measurements is large enough. Furthermore, we analyzed the Curie temperature (T C ) distribution of layers with different T C values and observed that the spread of T C within one layer is larger than the spacing between different layers, leading to a gradual switching behavior of the layer ensemble. Additionally, high resolution light microscopy was applied to observe the transition of individual particles, and correlate it to the local magnetic measurements.
Magnetic Cooling Becoming a Hot Topic: How Novel Magnetocaloric Materials Pave the Way for First Applications of Energy-Efficient CoolingFor more than 100 years the vacuum compressor has been the benchmark in refrigeration technology. Meanwhile, the discovery of new materials with a giant magnetocaloric effect allows the construction of a novel heat pump with a much higher efficiency. Current prototypes that have been realized recently allow testing novel materials in heat exchangers with tailor-made geometry such that the energy efficiency can be optimized on the device level. Absolutely essential is the development of a computer-based model that predicts the performance of the complete heat pump based on the physical properties of the active magnetocaloric materials.
Magnetocaloric refrigeration is a cooling technology expected to offer significant energy savings. For optimal performance, magnetic materials close to a first‐order magneto‐structural transition are employed for this application. To understand hysteresis of the first‐order transition in these materials, the study of both internal and external factors is required, bridging scales from atomic interactions towards macroscopic ensemble behavior. In order to understand the origin of the volume change and magneto‐elastic interaction occurring at the firstorder transition and its impact on hysteresis in magnetocaloric systems, first‐principles calculations are carried out (see bottom right image and article no. http://doi.wiley.com/10.1002/pssb.201700465 by Gruner et al.). In‐situ measurements are another valuable tool for studying phase transitions and associated hysteresis in magnetocaloric materials, as shown in the top image on the cover. Here, the authors apply temperature‐dependent magnetooptical imaging to study the thermal hysteresis of individual magnetocaloric spheres inside a packed bed (see bottom left image and article no. http://doi.wiley.com/10.1002/pssb.201700345 by Funk et al.). Hence, using local measurements on a large number of spheres, it is possible to study local interactions between spheres and to derive ensemble behavior from individual measurements.
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