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Thermal management is one of the crucial issues in the development of magnetocaloric refrigeration technology for application. In order to ensure optimal exploitation of the materials "primary" properties, such as entropy change and temperature lift, thermal properties (and other "secondary" properties) play an important role. In magnetocaloric composites, which show an increased cycling stability in comparison to their bulk counterparts, thermal properties are strongly determined by the geometric arrangement of the corresponding components. In the first part of this paper, the inner structure of a polymer-bonded La(Fe, Co, Si)13-composite was studied by X-ray computed tomography. Based on this 3D data, a numerical study along all three spatial directions revealed anisotropic thermal conductivity of the composite: Due to the preparation process, the long-axis of the magnetocaloric particles is aligned along the xy plane which is why the in-plane thermal conductivity is larger than the thermal conductivity along the z-axis. Further, the study is expanded to a second aspect devoted to the influence of particle distribution and alignment within the polymer matrix. Based on an equivalent ellipsoids model to describe the inner structure of the composite, numerical simulation of the thermal conductivity in different particle arrangements and orientation distributions were performed. This paper evaluates the possibilities of microstructural design for inducing and adjusting anisotropic thermal conductivity in magnetocaloric composites. [ABSTRACT FROM AUTHOR]

Abstract: Strong coupling effects in magnetocaloric materials are the key factor to achieve a large magnetic entropy change. Combining insights from experiments and ab initio calculations, we review relevant coupling phenomena, including atomic coupling, stress coupling, and magnetostatic coupling. For the investigations on atomic coupling, we have used Heusler compounds as a flexible model system. Stress coupling occurs in first‐order magnetocaloric materials, which exhibit a structural transformation or volume change together with the magnetic transition. Magnetostatic coupling has been experimentally demonstrated in magnetocaloric particles and fragment ensembles. Based on the achieved insights, we have demonstrated that the materials properties can be tailored to achieve optimized magnetocaloric performance for cooling applications. [ABSTRACT FROM AUTHOR]

Abstract: First‐order, magnetocaloric materials are promising for energy efficient solid‐state cooling because of their high entropy changes, but they have the drawback of a hysteresis due to the nucleation and growth of the involved phases. Here, synchrotron based X‐ray diffraction is used to investigate epitaxial Ni−Mn−Ga−Co thin films grown on Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMN‐PT). This integral method is used to test several hypotheses regarding structure and orientation of the martensite, whose nucleation was proposed to be the decisive contribution to hysteresis in Heusler materials. The measurements are compared to a recently proposed model of the martensitic nuclei, combining the phenomenological martensite theory with the adaptive concept. This investigation demonstrates that the diffraction patterns are consistent with a martensitic nucleus built from 8 variants forming a‐b laminates. Additionally, it is shown that in accordance with the surface morphology of these films only a subset of nuclei occur, namely those needed to form type X martensite. [ABSTRACT FROM AUTHOR]

Abstract: Magnetocaloric composites made from La−Fe−Co−Si particles and an epoxy binder matrix exhibit mechanical stability and good magnetocaloric properties, but also a large characteristic time for thermal transport. Here, the origin of this large time constant is examined by comparing two measurement techniques, direct and contactless, to finite‐element simulations based on a tomographic dataset of the sample. The combination of the low thermal conductivity of the epoxy matrix and a thermal resistance at the interface between epoxy and La−Fe−Co−Si is shown to be in good agreement in simulations and experiments. The findings help to disentangle the role of the thermal conductivity and the interfacial thermal resistance for the heat flow in magnetocaloric composites. It is shown that the low thermal conductivity of the epoxy alone cannot explain the large time constant and possibilities for using the interfacial thermal resistance to tailor anisotropic thermal conductivity for directional heat transfer in magnetocaloric composites are presented. [ABSTRACT FROM AUTHOR]

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 remarkable change of the transition kinetics occurs after the first cycle, which we attribute to the formation of cracks originating from the volume change. Tomographic imaging revealed that the bulk material disintegrates via an interlocked state where fragments are loosely connected. Though cracks have opened between the fragments, the transition is sharp, which we attribute to magnetostatic interactions. In the cycled sample we find a strong asymmetry between the transition interval upon heating and cooling, which we explain by isostatic pressure acting on parts of the sample during the cooling transition. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

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 remarkable change of the transition kinetics occurs after the first cycle, which we attribute to the formation of cracks originating from the volume change. Tomographic imaging revealed that the bulk material disintegrates via an interlocked state where fragments are loosely connected. Though cracks have opened between the fragments, the transition is sharp, which we attribute to magnetostatic interactions. In the cycled sample we find a strong asymmetry between the transition interval upon heating and cooling, which we explain by isostatic pressure acting on parts of the sample during the cooling transition. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Composites of two magnetocaloric components have been prepared by melt spinning. A special nozzle modification allows to separate the melts of two different materials which solidify simultaneously as rapidly quenched composite. Two materials combinations were chosen: (a) Fe-based amorphous materials with different Cr-contents and (b) Fe-based alloys with different Si-content resulting in crystalline La(Fe,Si)13 phase with large magnetocaloric effect after subsequent annealing. By melt spinning crystalline segments of several cm length and long amorphous ribbons were obtained. The magnetocaloric properties of the composites were assessed by magnetometry and direct measurements of the adiabatic temperature change. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase. [ABSTRACT FROM AUTHOR]

A concept for the application of a magnetocaloric device in energy-efficient air conditioners is introduced. In order to evaluate this concept, a test stand has been developed equipped with a magnetic field source providing about a 1.5-T flux density change into which different regenerator geometries can be implemented and evaluated. A processing route for the production of profiled magnetocaloric LaFeSiMn-based composite plates by tape casting is presented. The processed plates show a maximum isothermal entropy change of about 3.8 J kg − 1 K − 1 at a magnetic field change of 1.5 T at 285 K. The hydraulic and thermal performance of regenerator geometries that can be realized by profiled plates is assessed by calculations. [ABSTRACT FROM AUTHOR]

In this study, the optimized fabrication and evolution of the microstructure and magnetic transition behavior of the melt-extraction LaFe 11.2 Si 1.8 microwires have been studied. After the optimization of extraction technique (heating power 22 KW, feeding rate 30–50 μm/s, rotation velocity 1700 r/min), the content of La(Fe,Si) 13 phase in the as-extracted microwires was 54 wt% due to the high solidification velocity, which was increased to 85 wt% via annealing at 1373 K for 20 min. The amount of La(Fe,Si) 13 phase was increased and the composition of La(Fe,Si) 13 phase became more homogenized through peritectic reaction and short-distance diffusion in the microwires during annealing process. The coexistence of the nanocrystalline and amorphous structures contributed to the broad magnetic transition temperature range of the as-extracted and annealed microwires. The annealed microwires exhibited a second-order magnetic transformation behavior and showed a maximum magnetic entropy change | ΔS M | max of 6.2 J/kg·K and working temperature interval of 36.0 K under a magnetic field of 20 kOe. Image 1 • LaFe 11.2 Si 1.8 microwires were prepared by melt-extraction technique. • The content of La(Fe,Si) 13 phase in the as-extracted microwire reached 54 wt%. • Nanoscale La(Fe,Si) 13 phase and α-Fe grains were formed in the extracted microwire. • 85 wt% La(Fe,Si) 13 phase was obtained in microwire annealed at 1373 K for 20 min. • The annealed microwire exhibited.| ΔS M | max 6.2 J/kg·K and ΔT FWHM 36.0 K under 20 kOe. [ABSTRACT FROM AUTHOR]

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. The corrosion behavior of as-cast and annealed La-Fe-Si alloy samples was analyzed in comparison to that of La and Fe for evaluation of the impact of alloy chemistry and microstructure. Electrochemical studies were conducted in defined electrolytes starting with aerated distilled water (pH = 6) for assessing the influence of pH value changes and anion contaminations. Specifically, forced flow electrolyte conditions were applied which are closer to operation conditions of real magnetocaloric regenerator beds than stagnant ones. Corroded sample surfaces were analyzed with SEM to assess damage mechanisms. The reactive nature of the alloy constituents determines the high corrosion activity and limited passivation ability of La-Fe-Si alloys. Their exposure to distilled water is particularly detrimental under stagnant conditions as local fluid acidification enhances corrosion processes. These are based on galvanic coupling between the phases with different corrosion activities: La-rich phases > La(Fe,Si) 13 -based matrix > alpha-Fe(Si). Laminar fluid flow is beneficial for alloy surface passivation. But anion contaminants like sulfate or hydrogen phosphate ions counteract the weak passivity in flowing distilled water. While acidic conditions lead to instable corrosive states, a pH value control of the heat transfer fluid at alkaline conditions is effective for stable passivity of the alloy surface. Also, the applicability of a phosphate conversion coating treatment in 0.15 M NaH 2 PO 4 (pH = 4) was evaluated and prospects of this approach are discussed. [ABSTRACT FROM AUTHOR]

Abstract In this work the global and local mechanical properties of the magnetocaloric intermetallic LaFe 11.2 Si 1.8 alloy are investigated by a combination of different testing and characterization techniques in order to shed light on the partly contradictory data in recent literature. Macroscale compression tests were performed to illuminate the global fracture behavior and evaluate it statistically. LaFe 11.2 Si 1.8 demonstrates a brittle behavior with fracture strains below 0.6% and widely distributed fracture stresses of 180–620 MPa leading to a Weibull modulus of m = 2 to 6. The local mechanical properties, such as hardness and Young's modulus, of the main and secondary phases are examined by nanoindentation and Vickers microhardness tests. An intrinsic strength of the main magnetocaloric phase of at least 2 GPa is estimated. The significantly lower values obtained by compression tests are attributed to the detrimental effect of pores, microcracks, and secondary phases. Microscopic examination of indentation-induced cracks reveals that ductile α-Fe precipitates act as crack arrestors whereas pre-existing cracks at La-rich precipitates provide numerous 'weak links' for the initiation of catastrophic fracture. The presented systematic study extends the understanding of the mechanical reliability of La(Fe, Si) 13 alloys by revealing the correlations between the mechanical behavior of macroscopic multi-phase samples and the local mechanical properties of the single phases. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Magnetic refrigeration is considered as an alternative to conventional gas-compression-expansion cooling techniques. While the physical properties of magnetocaloric materials have been studied extensively, the question of how to process and shape these materials into active magnetic regenerator (AMR) heat exchangers for application in magnetocaloric cooling devices is less explored. Packed beds made from gas atomized spherical particles offer attractive shaping options to realize an AMR. In this work, we show that X-ray computed tomography (XCT) allows for non-destructive testing of the packed bed magnetic regenerators, including 3D visualization of the internal bed structure, measurement of the particles’ packing density and homogeneity, detection of defects, cracks and interfaces between the layers with different transition temperatures. In this way, XCT can help to understand structural details of fabricated magnetocaloric regenerators and their influence on the performance in a cooling device. [ABSTRACT FROM AUTHOR]

We report the processing, analysis and testing of magnetocaloric composite materials consisting of La–Fe–Co–Si particles of various size fractions and a polymer matrix. All of the composites have working temperatures close to room temperature. The composites were pressed into thin plates, a geometry favorable for testing the composites in an active magnetic regenerator (AMR). In order to investigate the influence of particle size and binder type (epoxy), eight different epoxy-bonded La–Fe–Co–Si plates were made and analyzed. We found that the higher filling factor that can be achieved by using a mixture of several particle size fractions has beneficial influence on the thermal conductivity. Tests in the AMR revealed that a maximum temperature span of approximately Δ T =10 K under magnetic field change of μ 0 H =1.15 T can be obtained at no cooling load conditions. The stability of the measured Δ T values and the mechanical integrity of sample after cyclic application of a magnetic field have been monitored for 90,000 cycles and showed no significant changes. We therefore conclude that epoxy-bonded La–Fe–Co–Si magnetocaloric composites have good magnetocaloric properties at low material-processing costs and hence represent a competitive way to produce magnetocaloric materials to be used in AMR. [ABSTRACT FROM AUTHOR]

We report on novel magnetocaloric composites based on La(Fe,Si) 13 particles in an amorphous metallic matrix. Hot compaction at the glass transition temperature, T g , of the matrix prevents crack formation as commonly occurs in conventional hot-compacted La(Fe,Si) 13 material. This damage can affect the magnetocaloric performance. The approach in the present paper shows that the La(Fe,Si) 13 particles stay intact due to the buffer-effect of the amorphous and ductile matrix at T g . Therefore, the magnetocaloric properties of the composites are almost independent of the compaction pressures. [ABSTRACT FROM AUTHOR]

Highlights: [•] The influence of Mn on the transition temperature is independent from Si-content. [•] The change of transition temperature upon hydrogenation depends on the Si-content. [•] Suitability for room-temperature application of LaFe13 − x − y Si x Mn y H alloys is evaluated. [•] Optimal conditions for a maximum temperature change are given for chosen samples. [ABSTRACT FROM AUTHOR]