Magnetocaloric composite wires for regenerators

Lukas Beyer^1,2, Bruno Weise¹, Tino Gottschall³, Jens Freudenberger^1,2, Julia Hufenbach^1,2, Maria Krautz^1
1 Leibniz IFW Dresden, Institute for Complex Materials, Helmholtzstr, 20, 01069, Dresden, Germany
² TU Bergakademie Freiberg, Institute of Materials Science, Gustav-Zeuner-Str. 5, 09599, Freiberg, Germany
³ Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, 01328, Dresden, Germany

Ever-increasing energy demand in the cooling sector and its growing contribution to global warming drives the need to develop novel cooling technologies and innovations [1]. Magnetic refrigeration based on the magnetocaloric effect is seen as a promising substitute to conventional cooling solutions [2]. One aspect of current research in this field is the shaping of magnetocaloric materials into suitable heat exchanger geometries [3]. These geometries need to satisfy two opposing demands. Coarse structures for a low pressure drop on the one side and a high surface-to-volume ratio for effective heat exchange on the other side. Different approaches have been performed to get the most promising material families for room-temperature applications into shape. Due to the brittleness of the intermetallic phases in such magnetocaloric alloys as La(Fe,Si)₁₃-based or (Mn,Fe)₂(P,Si)-based, the variety of resulting geometries and successful shaping options is limited [4-7]. The Powder-In-Tube (PIT) process, a method for the production of superconducting electric wires and therefore industrially established, provides a further opportunity to produce magnetocaloric composites. The PIT process allows to produce wires with magnetocaloric cores as filling. The obtained wires are a pre-product that can be readily assembled in caloric devices offering novel designs when arranged differently. Also, when combining to pin-like heat-exchanger geometries optimized thermal exchange and pressure drop performance can be achieved. The proof of concept has been published in 2018 by A. Funk et. al. in [8] with a magnetocaloric La(Fe,Co,Si)₁₃-core cladded by a non-magnetocaloric austenitic steel jacket.

With “Magnetocaloric composite wires for regenerators” we present an overview on the Powder-In-Tube process as a shaping route for magnetocaloric composites as well as recent results with magnetocaloric composite wires. Therefore, the presented poster is divided into two main sections. The first one showing the state of art in shaping of magnetocaloric materials and the PIT process route to produce wires. The second section presents results of experiments and simulations on magnetocaloric composite wires, e.g. magnetic pulsed-field measurements and corresponding findings.
The first section called `Powder-In-Tube forming of magnetocaloric materials` presents firstly a comparison between the most common shapes for regenerators namely particle beds and plates with wires by PIT. A categorization of the commonly used shapes by a pressure-heat transfer ratio vs. Re number diagram is provided. The advantage of wires as pre-product to build different set-ups optimized in pressure-drop and heat transfer can be seen. A second area describes the route to produce magnetocaloric composite wires in detail including the geometrical aspects that can be achieved in those PIT-wires. Depending on the starting parameter set wires with diameters down to 0.5 mm, filling factors up to 69 vol% and wall thickness below 100 µm can be achieved. Lastly, results of composite wires with a La(Fe,Co,Si)₁₃-core and a stainless-steel jacket are shown. It can be seen, that the initial magnetocaloric properties of the core material could be restored and stresses in the jacket compensated by a post-annealing step at 1050°C for only 10 minutes. The annealing time is drastically reduced from hours to minutes due to a refined microstructure after deformation. Also, 2D images of computed tomography are shown to present the shape of core and jacket of the composite wires after deformation to different diameters, specific 1 mm and 0.5 mm.
The second section called `Thermal performance of magnetocaloric composite wires` features results of different magnetic measurements and simulations, as well as a second PIT composite wire generation. Part one displays the thermal performance of La(Fe, Co, Si)₁₃-filled wires that has been evaluated by magnetic pulsed-field and heat capacity measurements. The magnetic pulsed-field experiments allowed direct measurement of the temperature change on the inside (at the core) and outside (at the jacket) of the composite wires. These results are of great interest since they reveal the usable effective temperature change of the magnetocaloric composite wires in a regenerator. Simulations to assess influences on the thermal performance of the composite wires have been drawn out and the findings are shown on the poster. As an example, the field-frequency influence on the adiabatic temperature change indicating reasonable frequencies up to 2 Hz is given. Further, a second magnetocaloric composite wire produced by PIT is viewed. This second generation contains Gadolinium as core material within the same austenitic steel tube as used for theLa(Fe,Co,Si)₁₃-filled wires. 2D images and the geometrical aspects, with 1 mm in diameter and a filling factor of 62 vol%, of these magnetocaloric composite wires are presented. The successful heat treatment to restore the magnetocaloric effect of the Gd-core after the PIT deformation is shown by a ΔST-T-diagram. Next to it, the comparison between magnetic pulsed-field measurements for  La(Fe,Co,Si)₁₃-filled and Gd-filled wires is given. It can be seen, that the achieved temperature change of these Gd-filled composite wires in comparison to wires with La(Fe,Co,Si)₁₃-filling is higher, but the heat transfer between active core and passive jacket is reduce due to their heat capacity ratio. Lastly, the poster has an outlook on the next generations of magnetocaloric composite wires produced by the Powder-In-Tube process.

 
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