Magnetocaloric Effects of Binary Rare Earth Nitrides
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1040-Q09-05
Magnetocaloric Effects of Binary Rare Earth Nitrides Yusuke Hirayama1, Takashi Nakagawa2, Takafumi Kusunose3, and Takao A. Yamamoto1 1 Graduate School of Engineering, Osaka univesity, 2-1 Yamada-oka, Osaka, Suita, 565-0871, Japan 2 Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro, Tokyo, 152-8552, Japan 3 Institute of Scientific and Industrial Research, Osaka university, Osaka, Ibaraki, 567-0047, Japan ABSTRACT We synthesized HoxEr1-xN (x =0.1, 0.3, 0.5, 0.7, 0.9) by the carbothermic reduction method. The magnetic entropy change, ∆S, which is an indicator of performance of magnetocaloric effect, MCE, was obtained from data sets of magnetization M (H, T) measured at various temperature, T, and magnetic field, H, through the Maxwell equations. The Thus obtained ∆S vs. T curves have peaks at temperatures depending on x in a range of 6–19 K. ∆S values of HoxEr1− xN expressed in terms of J K-1 m-3 were higher than those of intermetallic compounds of rare earth and transition metals previously reported. These ∆S peak plots are along a convex curve. INTRODUCTION Hydrogen is expected as the most promising energy material in near future society because of its small environment load. The most efficient form for transportation and storage of hydrogen would be liquid hydrogen from viewpoint of energy density. However, an efficient and economic liquefaction system is necessary to realize the infrastructure with the liquid hydrogen. The magnetic refrigeration has been intensively studied as a new cooling method with advantages of chlorofluorocarbon-free, solid state.1 This method is based on the magnetocaloric effect in which a heat changes ∆Q =T∆S is given from the magnetic substance, where ∆S is magnetic entropy change driven by the external field. 2 In general, the lower temperature is, the higher its efficiency is. The application of this method to hydrogen liquefaction is, therefore, worth studying, especially below the liquid nitrogen temperature and near the hydrogen liquefaction temperature 20 K. To enhance the efficiency much more, it is necessary to develop a new material which has large ∆S in the relevant temperature. In general, ∆S of the ferromagnet is maximized in the vicinity of its Curie’s temperature TC. Mononitirides of rare-earth elements, Gd, Tb, Dy, Ho and Er, possess the NaCl type crystal structure and are ferromagnets with TC varying with the atomic number in a range ca. 70 – 10 K.3-5 Binary compounds consisting of neighboring or next-neighboring elements LnxLm1-xN (Ln, Lm: rare-earth) form continuous solid solutions with intermediate lattice constants and intermediate TC. It is remarkable that the packing density of rare-earth atoms in this nitride phase is significantly higher than those of metallic phase of the hcp structure, which leads to a high packing density of magnetic moments undergoing the ferro-para magnetic transition. This high density and this TC range are very beneficial to magnetic refrigerants and/or regenerators for the general cryogenic cooli
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