Segmentation and Alignment of Nd 2 Fe 14 B Platelets in Nd-Cu Eutectic Alloys Using the Electromagnetic Vibration Techni
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IN comparison with an isotropic magnet, an anisotropic magnet has attracted increasing interest from scientific and technical viewpoints. This has been of particular importance since the invention of the Nd2Fe14B magnet[1,2] that has been witnessed a wide application in hybrid electric vehicles and electric vehicles over the past decades. The pioneering work of fabricating a bulk anisotropic Nd2Fe14B magnet may be attributed to Lee,[3] who used a die-upset technique to press the melt-spun ribbons, during which the Nd2Fe14B platelets were well aligned and thus an anisotropic magnet was produced. From then on, there have been a number of studies[4–6] to improve the technique to align the Nd2Fe14B platelets and nowadays, the technique, frequently termed as powder metallurgy, has been predominant in fabricating the anisotropic Nd2Fe14B
MINGJUN LI and TAKUYA TAMURA are with the National Institute of Advanced Industrial Science and Technology (AIST), Magnetic Powder Metallurgy Research Center, 2266-98, Anagahora, Shimo-Shidami, Moriyama, Nagoya 463-8560 Japan. Contact email: [email protected] Manuscript submitted October 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
magnet, which mainly involves the preparation of fine powder, the alignment and pressing the powder in a static magnetic field, the densification of the pre-aligned bulk by sintering, and final treatment and tailoring into a product. Here arises a basic concern; why the Nd2Fe14B phase exhibits anisotropy in magnetism? The magnetic anisotropy mainly originates from the crystalline anisotropy because of the coupling of orbital motion of the electrons, i.e., spinning of electrons, with crystal electric field in a lattice unit. As far as the crystal lattice of the Nd2Fe14B phase is concerned, it has a tetragonal structure (space group P42/mnm)[7] in which four Nd2Fe14B formula are included with 68 atoms. Figure 1(a) depicts the schematic of an Nd2Fe14B unit cell. One can see that along the c-axis direction, each Nd2Fe14B unit cell consists of an eight-layer structure, indicating that the atomic stacking sequence along the c-axis is more complex than that along the a-axis in terms of the atomic sites, occupancies, and coordinates.[8] The Nd2Fe14B crystal is a well-ordered stoichiometric intermetallic compound, in which each atom has its well-determined position, as depicted in Figure 1(a). The growth of the Nd2Fe14B crystal requires each atom to sort its correct position and then incorporate into the
Fig. 1—(a) The tetragonal unit cell of the Nd2Fe14B crystal. The actual a/c ratio is 1.385 (a = 8.80 A˚ and c = 12.19 A˚)[7] that is exaggerated to illustrate the stacking of the hexagonal iron nets. (b) The schematic drawing of an actual Nd2Fe14B crystal exhibiting a platelet morphology when considering the pronounced difference in growth kinetics along the a and c directions.
corresponding lattice site, which is a short-range diffusion-limited process at the solid/liquid interface.[9] By reviewing the linear kinetic coefficients of various compounds and oxide
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