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Xinmin You1 Michael Maschek1 Niels Dijk1 Ekkes Bruck1

1, TU Delft, Delft, , Netherlands

Magnetocaloric materials (MCMs) show great potential for utilization in magnetic refrigeration techniques and energy conversion methods in thermomagnetic generators (TMGs).[1] A thermomagnetic motor (TMM) prototype utilizing Gd was built by Swiss Blue Energy and is able to convert low-temperature waste heat to electrical or mechanical energy.[2] In order to enhance the efficiency appropriate advanced magnetocaloric materials having strong magnetization changes with low hysteresis should be used to replace Gd. In contrast to the refrigeration application, energy conversion with MCMs requires a low latent heat for high efficiencies.
Materials with suitable Curie temperatures and small hysteresis can be found in the quaternary (Mn,Fe)2(P,Si) system. In order to find suitable compounds, a large fraction of the phase diagram of MnxFe2-xPi-ySiy [3] (0.5≤Mn≤1.8, 0.33≤Si≤0.6) has been investigated. Polycrystalline samples of MnxFe2-xPi-ySiy have been prepared by the ball-milling method. After ball-milling, the samples were sintered at 1373K for 25h then quenched into water.
Suitable materials for applications having Curie temperatures around room temperature, large magnetization changes and small hysteresis can be found in the Mn-rich (x>1.2) and Fe-rich (x<0.7) region. Furthermore, we observe an antiferromagnetic (AFM) to paramagnetic (PM) phase transition for MnxFe2-xP2/3Si1/3 with 1.1≤x≤1.7. X-ray diffraction shows that all the MnxFe2-xP2/3Si1/3 compounds with 0.5≤x≤1.8 crystallize in Fe2P-based hexagonal structure. The lattice parameter a (in plane) expands and c shrinks (out of plane) with increasing manganese content, while only a small volume expansion is observed. In the Fe2P type region (0.5≤x≤0.9), Tc and hysteresis rise with increasing manganese concentration. In contrast, in the region with AFM-PM transition (1.1≤ x≤1.7), TN and hysteresis drop while the manganese concentration increases.

References

[1] Kitanovski, A., et al., Magnetocaloric Energy Conversion. 2015: Springer International Publishing.
[2] http://www.swiss-blue-energy.ch/
[3] Dung, N.H., et al., Mixed Magnetism for Refrigeration and Energy Conversion. Advanced Energy Materials, 2011. 1(6): p. 1215-1219.

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