Magnetic Composites and Applications
Magnetocaloric materials change their temperature upon the application and removal of an external magnetic field. This behavior is the basis for the development of an alternative magnetic cooling system free of ozone-depleting gases, which in addition exceeds the Carnot efficiency of conventional refrigeration processes.
In our group, we study the magnetic, thermal and 3D structural properties of these materials, and develop novel magnetocaloric materials and composites to make the best possible use of the extraordinary properties of this material class.
Our activities comprise close cooperation with both industry and research community:
- Magnetocaloric Air Conditioning: BMWi-Project "SOMAK"
- Characterization and development of novel materials: Projects with BASF and Kitech
- In-situ XRD for studying the effects of external fields on the structure of magnetocaloric materials: Project A7 in the framework of DFG SPP 1599 (www.ferroiccooling.de)
Magneto-optical imaging paper on cover page of PSS-B
Our recent paper "Hysteresis of MnFePSi Spherical Powder Ensembles Studied by Magneto-Optical Imaging" has been selected for the cover of Physica Status Solidi B's special issue "Hysteresis in Magnetocaloric, Electrocaloric and Elastocaloric Refrigeration"! In the article, we show that in-situ magneto-optical measurements are a valuable tool for studying phase transitions and associated hysteresis in magnetocaloric materials. By 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.
Introducing magnetocaloric air conditioning
Watch our youtube video to learn how magnetocaloric air conditioning works! In the BMWi-funded project SOMAK, we study how to best combine a magnetocaloric "booster" unit and a conventional DEC (= desiccative evaporative cooling) air conditioner. In conventional DEC-systems energy losses ususally occur during the adiabatic dehumidification of the sorption wheel. A magnetocaloric "booster" unit will be implemented into the DEC-system in order to alleviate these losses by providing a certain heat level for the wheel regeneration on the hot side of the regenerator, whereas the cool side can be used to complementary increase the performance of the air condition device. Together with energy and electrical engineers from TU Dresden and industrial partners (Glen Dimplex, Innius GTD) we adress key questions regarding thermal and electrical dimensioning of the magnetocaloric booster unit and the development of novel production technologies for regenerator geometries.
Hysteresis of MnFePSi Spherical Powder Ensembles Studied by Magneto-Optical Imaging
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 (TC) distribution of layers with different TC values and observed that the spread of TC 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.
A. Funk, M. Zeilinger, F. Dötz, I. Soldatov, R. Schäfer, A. Waske, Physica Status Solidi B, 2017 URL
Packed beds of spherical particles are a commonly used geometry to realize heat exchangers for magnetocaloric refrigeration. In order to increase the temperature span obtained for cooling, layers of materials with different magnetocaloric transition temperatures are used. However, despite their slight chemical difference, the spheres in all layers look very similar and it is nearly impossible to identify the boundary between them from a micrograph through the packed bed (see figure, top). In this work, we show that X-ray computed tomography (XCT) allows for non-destructive detection of the boundaries by analysing minute details in defect and particle size distributions for each batch. This "signature" of each layer unambiguously reveals the layer interfaces (see figure, b-d). In this way, XCT can help to understand structural details of fabricated magnetocaloric regenerators and their influence on the performance in a cooling device.
A. Funk, M. Zeilinger, A. Miehe, D. Sopu, J. Eckert, F. Dötz, A. Waske, Chemical Engineering Science 175, 84-90 (2018), URL
Symposium at Intermag 2017 in Dublin
We organize an invited speaker symposium at the Intermag in Dublin on the topic "Magnetocaloric Materials: New Concepts for Energy Application". Magnetocaloric materials already proved their huge potential in room-temperature magnetic refrigeration. While refrigeration motivated the development of magnetic materials with a first order transformation, recent developments show that these first order materials also paved the way for novel applications, which range from gas liquefaction, humidity control towards thermomagnetic energy harvesting. This symposium focuses on these emerging applications and novel cooling concepts and the material developments required for these.
First order magnetocaloric materials reach high entropy changes but at the same time exhibit hysteresis losses which depend on the sample’s microstructure. We use non-destructive 3D X-ray microtomography to understand the role of surface morphology for the magnetovolume transition of LaFe11.8Si1.2. The technique provides unique information on the spatial distribution of the volume change at the transition and its relationship with the surface morphology. Complementary Hall probe imaging confirms that on a morphologically complex surface minimization of strain energy dominates. Our findings sketch the way for a tailored surface morphology with low hysteresis without changing the underlying phase transition.
Anisotropic thermal conductivity in epoxy-bonded magnetocaloric composites
As thermal managment is one of the crucial issues for effective magnetic cooling processes, this paper comprises a numerical study of the influence of the particle orientation on the thermal conductivity in polymer bonded magnetocaloric composites. The 3D inner structure of composite plate was studied by X-ray computed tomography and revealed a preferred orientation of the particles perpendicular to the compaction direction. This anisotropic particle arrangement influences the thermal conductivity in all three directions of the composite plate. Based on that finding different scenarios of particle arrangements are studied numerically and show that anisotropy of the thermal conductivity can be enhanced by optimizing the direction of particle orientation and their distribution.
Bruno Weise, Kai Sellschopp, Marius Bierdel, Alexander Funk, Manfred Bobeth, Maria Krautz and Anja Waske, Journal of Applied Physics 120 (2016) 125103 URL
Exploring corrosion protection of La-Fe-Si magnetocaloric alloys by passivation
Magnetocaloric La(Fe,Si)13-based alloys are promising materials for magnetic cooling systems but their limited corrosion resistance in water-based heat transfer fluids is critical. Most fundamental corrosion studies are performed under stagnant conditions. However, a magnetocaloric regenerator bed is constantly perfused by a heat exchanger fluid. Therefore, electrochemical studies were conducted in defined electrolytes under forced flow electrolyte conditions in order to assess corrosion behavior under more realistic conditions. Moreover, the applicability of a phosphate conversion coating treatment in 0.15 M NaH2PO4 (pH = 4) was evaluated and prospects of this approach are discussed.
Annett Gebert, Maria Krautz, Anja Waske, Intermetallics 75 (2016) 88-95 URL
BMWi funded Project "SOMAK" launched
„Solar-magnetic air conditioning of buildings (SOMAK)“ is an interdisciplinary project between Dresden University of Technology and IFW Dresden that aims to increase the potential and flexibility of dessicant and evaporative air conditioning (DEC-systems) by magnetocaloric cooling.
Our group is involved in the work packages of materials development and processing.
The project duration is four years with a volume of 1.72 million Euros funded by the Federal Ministry for Economic Affairs and Energy (BMWi).
A new type of La(Fe,Si)13 based magnetocaloric composites with amorphous metallic matrix
This work shows a promising way towards the production of compact refrigerant bodies by a novel combination of an amorphous matrix and brittle giant magnetocaloric material. Magnetocaloric performance in such materials is crucially affected by mechanical integrity, i.e. depending on particle size. We show that hot-compaction at the glass transition temperature of the amorphous material helps to buffer the applied stresses to the brittle magnetocaloric alloy and therefore optimal particle size for large magnetocaloric effect can be retained.
M. Krautz, A. Funk, K.P. Skokov, T. Gottschall, J. Eckert, O. Gutfleisch, A. Waske, Scripta Materialia 95 (2015) 50 URL
Asymmetric first-order transition and interlocked particle state in magnetocaloric La(Fe,Si)13
Computed tomography and in-situ synchrotron XRD measurements of the magnetocaloric material LaFe11.8Si1.2 are used to understand virgin effects and asymmetry of the underlying first order magnetovolume transition. A two-step disintegration process of the temperature-cycled sample was revealed: crack formation governs the first step, leading to an interlocked-state of the sample, where lumps of particles are still interconnected. Further cycling leads to crack propagation and the sampel disintegrates into separated particles. Each step shows characteristic transition properties indicated by magentometry.
A. Waske, L. Giebeler, B. Weise, A. Funk, M. Hinterstein, M. Herklotz, K. Skokov, S. Fähler, O. Gutfleisch, J. Eckert, Phys. Status Solidi RRL, 1–5 (2015) URL