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Fabrication, Characterization, and Development of Hot-Deformed Magnets

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Materials Physics".

Deadline for manuscript submissions: 20 October 2024 | Viewed by 4923

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Special Issue Editors

Dr. Tae-Hoon Kim

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Guest Editor

Korea Institute of Materials Science, Changwon 51508, Korea
Interests: magnetic materials; permanent magnets; rare-earth magnets; rare-earth free magnets; sintering; hot deformation; microstructure characterization

Dr. Xin Tang

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Guest Editor

Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba, Japan
Interests: Nd-Fe-B; hot-deformed magnets

Special Issue Information

Dear Colleagues,

1) High-performance permanent magnets are a core material in the production of motors of next-generation mobility driven by environmentally friendly electric energy. In particular, among various magnet manufacturing processes, the hot-deformation process is a promising next-generation industrial process for the production of high-coercivity permanent magnets comprising nano-sized grains. Therefore, for sustainable R&D in the permanent magnet field, significant research results regarding the fabrication, characterization, and development of hot-deformed magnets should be accumulated and shared.

2) The aim of this Special Issue, entitled “Fabrication, Characterization, and Development of Hot-Deformed Magnets“, is to share the state-of-the-art development in high-performance hot-deformed permanent magnets. We cordially invite you to contribute to the permanent magnet field both academically and industrially by submitting your meaningful research results regarding hot-deformed magnets. We are sure that your contribution will be of great help to the growth and expansion of the worldwide permanent magnet field. Manuscripts in the form of full research papers, communications, and review articles are encouraged.

Dr. Tae-Hoon Kim
Dr. Xin Tang
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

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Keywords

permanent magnets
rare-earth based magnets
rare-earth free magnets
hot deformation
microstructure
magnetic properties

Published Papers (4 papers)

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Research

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11 pages, 3282 KiB

Open AccessArticle

Design of High-Remanence Nd-Fe-B Hot-Pressed Magnets by Manipulating Coercivity of Hydrogenation-Disproportionation-Desorption-Recombination Treated Anisotropic Precursors

by Jae-Gyeong Yoo, Tae-Hoon Kim, Hee-Ryoung Cha, Yang-Do Kim and Jung-Goo Lee

Materials 2023, 16(24), 7599; https://doi.org/10.3390/ma16247599 - 11 Dec 2023

Viewed by 942

Abstract

We propose a method of manipulating the coercivity of anisotropic hydrogenation-disproportionation-desorption-recombination (HDDR) powders to fabricate high-remanence and fine-grained Nd-Fe-B magnets using only hot-pressing without a subsequent hot-deformation process. By reducing the Nd content of anisotropic HDDR precursors such that their coercivity (H_cj) is lowered, the c-axis of each HDDR particle is well-aligned parallel to the direction of the applied magnetic field during the magnetic alignment step. This is because the magnetic repulsive force between adjacent particles, determined by their remanent magnetization, decreases as a result of the low coercivity of each particle. Therefore, after hot-pressing the low-H_cj HDDR powders, a significantly higher remanence (11.2 kG) is achieved in the bulk than that achieved by hot-pressing the high-H_cj HDDR powders (8.2 kG). It is clearly confirmed by the large-scale electron backscatter diffraction (EBSD) analysis that the alignment of the c-axis of each anisotropic HDDR particle in the bulk is improved when low-H_cj HDDR powders are used to fabricate hot-pressed magnets. This coercivity manipulation of HDDR powders can be a helpful method to expand the use of HDDR powders in fabricating anisotropic Nd-Fe-B bulk magnets. Full article

(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)

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12 pages, 2967 KiB

Open AccessArticle

Inhomogeneity of the Backward Extruded NdFeB Ring Magnet Prepared from Amorphous Powders

by Weida Huang, Ke Xu, Xuefeng Liao, Bang Zhou, Hongya Yu, Xichun Zhong and Zhongwu Liu

Materials 2023, 16(14), 5117; https://doi.org/10.3390/ma16145117 - 20 Jul 2023

Viewed by 773

Abstract

Back extrusion is an important process to prepare radially oriented NdFeB ring magnets. In this work, we fabricate the ring magnets using amorphous magnetic powders as the raw material. The microstructure, magnetic properties, corrosion resistance, and mechanical properties of the backward extruded magnet at different positions along the axial direction have been investigated, and the inhomogeneity of the magnet is clarified. The results showed that the grains in the middle region of the ring magnet exhibit a strong c-axis orientation, whereas the grains at the bottom and top regions are disordered with random orientation. The microstructure variation is related to the distribution of the grain boundary phase and the degree of grain deformation. Due to the microstructure difference, the magnetic properties, temperature stability, corrosion resistance, and mechanical properties in the middle region of the magnet are higher than those in the top and bottom regions. The exchange coupling between grains also varies in different regions, which is related to the grain size and grain boundary thickness. In addition, different Co element segregations were observed in different regions, which has a crucial effect on the Curie temperature and thermal stability of the magnet. The microstructure difference also leads to the variation of corrosion resistance and mechanical properties for the samples from different regions of the magnet. This work suggests that the amorphous powder can be used to directly prepare radially oriented ring magnets, and the inhomogeneity of the magnet should be fully understood. Full article

(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)

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Review

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17 pages, 7478 KiB

Open AccessReview

Development of High-Performance Hot-Deformed Neodymium–Iron–Boron Magnets without Heavy Rare-Earth Elements

by Keiko Hioki

Materials 2023, 16(19), 6581; https://doi.org/10.3390/ma16196581 - 06 Oct 2023

Cited by 1 | Viewed by 977

Abstract

Neodymium–iron–boron magnet is an essential material for the traction motors of green vehicles because it exhibits the highest maximum energy product, (BH)_max, out of all permanent-magnet materials. However, heavy rare-earth elements such as dysprosium and terbium, which are scarce resources, are added to these magnets to improve their heat resistance. To address this resource problem, considerable efforts have been made to reduce the composition of heavy rare-earth elements in these magnets without causing a significant reduction in coercivity. Hot-deformed Nd-Fe-B magnets are a category of Nd-Fe-B magnets where precious materials such as heavy rare-earth elements can be eliminated or reduced to maintain high coercivity owing to their fine microstructure. Although they are not often used for the fabrication of high-performance magnets due to their complicated production process and the difficulty in controlling their fine microstructure, after the rare-earth crisis in 2020, these magnets have begun to attract attention as a material that could increase coercivity when controlling their microstructures. This paper provides an overview of hot-deformed magnets and the efforts made to improve their properties by controlling their microstructures. Full article

(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)

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25 pages, 23142 KiB

Open AccessEditor’s ChoiceReview

Significant Progress for Hot-Deformed Nd-Fe-B Magnets: A Review

by Renjie Chen, Xianshuang Xia, Xu Tang and Aru Yan

Materials 2023, 16(13), 4789; https://doi.org/10.3390/ma16134789 - 03 Jul 2023

Viewed by 1618

Abstract

High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due to their fine grain sizes of 200–400 nm, which are smaller by more than an order of magnitude compared to the traditional sintered Nd-Fe-B magnets. However, the current available coercivity of HD magnets is not as high as expected according to an empirical correlation between coercivity and grain size, only occupying about 25% of its full potential of the anisotropy field of the Nd₂Fe₁₄B phase. For the sake of achieving high-coercivity HD magnets, two major routes have been developed, namely the grain boundary diffusion process (GBDP) and the dual alloy diffusion process (DADP). In this review, the fundamentals and development of the HD Nd-Fe-B magnets are comprehensively summarized and discussed based on worldwide scientific research. The advances in the GBDP and DADP are investigated and summarized based on the latest progress and results. Additionally, the mechanisms of coercivity enhancement are discussed based on the numerous results of micromagnetic simulations to understand the structure–property relationships of the HD Nd-Fe-B magnets. Lastly, the magnetization reversal behaviors, based on the observation of magneto-optic Kerr effect microscopy, are analyzed to pinpoint the weak regions in the microstructure of the HD Nd-Fe-B magnets. Full article

(This article belongs to the Special Issue Fabrication, Characterization, and Development of Hot-Deformed Magnets)

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