SIPS 2018

DETAILLED PROGRAM OVERVIEW

Novel Multicomponent Magnets Using Less Critical Raw Materials for Sustainable Use
Tomaž Tomše¹; Jean-marie Dubois²; Spomenka Kobe³;
¹JOžEF STEFAN INSTITUTE, Ljubljana, Slovenia; ²INSTITUT JEAN LAMOUR, Nancy, France; ³JOSEF STEFAN INSTITUTE, Ljubljana, Slovenia;
PAPER: 306/SISAM/Invited (Oral)
SCHEDULED: 17:10/Wed./Copacabana A (150/1st)

ABSTRACT:
This talk will address the general issue of energy production and use, while limiting the consumption of critical raw materials and restraining the impact on the environment that are the pillars of sustainability, which is derived from the framework of this summit series. The magnetic properties of Nd-Fe-B magnets originate in the intrinsic properties of the hard magnetic Nd2Fe14B phase (exhibiting large magnetocrystalline anisotropy) [1], but the overall performance of the bulk Nd-Fe-B magnets heavily depend on the material's microstructure, which is determined by the manufacturing route and chemical composition of the alloy [1], [2]. The maximum energy product ((BH)max) of anisotropic sintered Nd-Fe-B magnets is often used as a figure of merit, which is much higher than the (BH)max values of other magnetic materials developed in the past (cobalt magnet steel) and other permanent magnets, that are currently on the market (hard ferrites, alnicos and Sm-Co). For the same performance, less magnetic material is needed, which effectively leads to miniaturization of devices [3]. The disadvantage of sintered Nd-Fe-B magnets is the fact that their intrinsic coercivity (Hci), which is a measure of the magnet's capability to withstand the external demagnetizing fields, especially at high temperatures, heavily depends on the use of heavy rare earth (HRE) elements, dysprosium, and terbium. These elements substitute the Nd atoms in the Nd2Fe14B phase and adds up to 10 wt. % are common in the high-coercivity magnets [4]. As a result, the magnetocrystalline anisotropy, a property of hard magnetic phases, is increased. But at the same time, the saturation magnetization of the system is lowered due to the antiferromagnetic coupling of the magnetic moments of Dy and Tb atoms with the moments of iron, which weakens the field produced by the magnet. The values of remnant magnetization (Br) and maximum energy product are thus reduced [5]. In addition, the HRE elements are far less abundant than Nd (except in China), and therefore much more expensive [4][6]. In some applications, like traction motors of (hybrid) electric vehicles and electric power steering (EPS) motors, magnets suffer from demagnetization due to the large reverse magnetic fields. Although the standard approach is to use HREs throughout the magnet body to increase its Hci value, certain parts of the magnet are more exposed than others [7][8]. Based on the facts shown above, the purpose of this work is to address the issue of the growing need for critical HRE elements. This was achieved in two ways. Firstly, nanostructured materials that employ less HREs to achieve high intrinsic coercivity, compared to the microcrystalline materials, were considered for the preparation of the bulk magnets. Secondly, novel Nd-Fe-B magnets were prepared by using a combination of magnetic powders obtained with one or more of the established manufacturing techniques and condense them into a so-called multicomponent bulk magnet in a fast consolidation step by using a special kind of a hot pressing technique called Pulsed Electric Current Sintering (PECS). By minimizing the process time and using low consolidation temperatures, the magnetic properties of the respective hard magnetic materials were preserved and tailored. During this study, we optimized the PECS process parameters for each type of the Nd-Fe-B magnetic powders in order to avoid the deterioration of the magnetic properties. We found a suitable combination of magnetic powders with different magnetic properties that can be processed together in a single consolidation step, and we prepared a multicomponent magnet by using a HRE-free and HRE-containing powder. We avoided the diffusion of the HRE element from the HRE-containing into the HRE-free part of the magnet during consolidation. Based on numerical simulations, we have shown that the demagnetizing fields have significant effects only on certain parts of the magnet that are located near their edges. As a consequence, only those parts need to be protected against demagnetization. Therefore, the highly inventive idea was to develop a magnet with a large volume fraction of its body HRE-free and only using HREs in the exposed parts to significantly improve the performance and to address the issue of the resource efficiency at the same time.

References:
[1] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, and Y. Matsuura, "New material for permanent magnets on a base of Nd and Fe (invited)," Journal of Applied Physics, vol. 55, pp. 2083-2087, 1984.
[2] O. Gutfleisch, "Controlling the properties of high energy density permanent magnetic materials by different processing routes," Journal of Physics D: Applied Physics, vol. 33, p. R157, 2000.
[3] B. D. Cullity and C. D. Graham, Introduction to Magnetic Materials. Hoboken: Wiley, 200
[4] D. Brown, Z. Wu, F. He, D. Miller, and J. Herchenroeder, "Dysprosium-free melt-spun permanent magnets," Journal of Physics: Condensed Matter, vol. 26, p. 064202, 2014.
[5] J. Coey, "Intrinsic magnetic properties of compounds with the Nd2Fe14B structure," Journal of the Less Common Metals, vol. 126, pp. 21-34, 1986.
[6] K. Binnemans and P. T. Jones, "Rare earths and the balance problem," Journal of Sustainable Metallurgy, vol. 1, pp. 29-38, 2015.
[7] Y. Matsuura, "Recent development of Nd-Fe-B sintered magnets and their applications," Journal of Magnetism and Magnetic Materials, vol. 303, pp. 344-347, 2006.
[8] D. D. TREMELLING, "Layered permanent magnet with conductive cage rotor construction," ed: Google Patents, 2015.
[9] J. Jaćimović, F. Binda, L. G. Herrmann, F. Greuter, J. Genta, M. Calvo, T. Tomše, R. Simon, "Net Shape 3D Printed NdFeB Permanent Magnets" Advanced Engineering Materials, pp. 1700098-n/a, 2017
[10] T. Tomše, J. Jaćimović, L. Herrmann, F. Greuter, R. Simon, S. Tekavec, J.-M. Dubois, S. Kobe, "Properties of SPS-processed permanent magnets prepared from rapidly solidified Nd-Fe-B powders" (ACCEPTED TO JOURNAL OF ALLOYS AND COMPOUNDS)