United States

The Effects of Zr + Fe2B additions on NdFeB-type Material during the Hydrogen Ductilisation Process (HyDP)

F. Burkhardt1, O.P. Brooks1, A. Walton1, I.R. Harris1
1School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

It is well known that NdFeB-type alloys are extremely brittle at room temperature. Therefore, in order to shape the material into fully dense magnets, it can either be reduced to a powder and sintered at high temperatures between 1000-1200°C [1][2], or hot pressed at temperatures between 800-900°C [1]. Recent studies have shown that it is possible to overcome the brittle nature of NdFeB-type starting cast alloys by pre-processing the material in hydrogen prior to mechanical deformation, using a process termed the „Hydrogen Ductilisation Process (HyDP)“[3][4][5].
During the HyDP the alloy is processed in hydrogen to produce a solid disproportionated (s-HD) microstructure that can be deformed at room temperature. However, during pressing trials it has been shown that fracturing occurs in the minority NdFe4B4 phase [4], which will lower the density of the final magnet and possibly limits the range of plastic deformation techniques that can be applied.
One way to reduce the amount of NdFe4B4 phase in book mould cast NdFeB alloys, is to move towards a stoichiometric Nd2Fe14B alloy. However, this is associated with the formation of free α-Fe dendrites which in turn are detrimental to the magnetic properties. It has previously been shown that by adding zirconium and Fe2B to cast NdFeB alloys it is possible to avoid the formation of free α-Fe dendrites [6]. This study investigates the effect of Zr and Fe2B additions on the microstructure and deformation behaviour of NdFeB alloys during the Hydrogen Ductilisation Process (HyDP).

References
[1] Müller, K.-H. et al. (1998a) ‘Hot deformed NdFeB permanent magnets’, in Materials Letters, pp. 50–54. doi: 10.1016/S0167-577X(97)00141-9.
[2] Croat, J. J. (2018b) Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets. 1. Edition. Elsevier Ltd. ISBN: 978-0-08-102225-2.
[3] Harris, I. R., Walton, A. and Brooks, O. P. (2017c) ‘Patent: Magnet Production’. United Kingdom: World Intellectual Property Organization.
[4] Brooks, O. P. et al. (2017d) ‘The Hydrogen Ductilisation Process (HyDP) for shaping NdFeB magnets’, Journal of Alloys and Compounds. Elsevier B.V, 703, pp. 538–547. doi: 10.1016/j.jallcom.2016.12.177.
[5] Brooks, O. P. et al. (2018e) ‘Complete ductility in NdFeB-type alloys using the Hydrogen Ductilisation Process (HyDP)’, Acta Materialia. Acta Materialia Inc., 155, pp. 268–278. doi: 10.1016/j.actamat.2018.04.055.
[6] Kirby, K. et al. (2018f) The effects of AB2 (A=Zr,Hf,Ti) additions on the free-iron content in ingots with the initial composition of Nd2.2Fe14B1.1. Birmingham. (Awaiting publication in JMMM)



REPM 2021: The 26th International Workshop on Rare-Earth and Future Permanent Magnets and their Applications

Sustainable Magnets Production using Hydrogen - Challenges and Chances

hydrogen

hddr

novel processing

ndfeb

hydp

It is our great pleasure to invite you to attend the 26th International Workshop on Rare Earth and Future Permanent Magnets and Their Applications (REPM2021) which will be held virtually on June 7-10, 2021. Please note, this is an updated schedule due to COVID-19 travel restrictions which forced us to postpone the conference from summer of 2020. The REPM workshop takes place every two years and is the premier conference on permanent magnets covering a wide range of topics including:

* Rare earth magnets
* Rare earth-free magnets
* Raw materials, mining and recycling
* Advanced synthesis, processing and characterization techniques
* Theoretical modeling of coercivity
* Fundamental properties of advanced magnetic materials
* Thin film magnets
* Applications of permanent magnets

***Workshop***
The workshop will be chaired by Professor George C. Hadjipanayis, University of Delaware, Newark, DE and Dr. Jinfang Liu, President & COO of Electron Energy Corporation, Landisville, PA.
The objectives of the workshop are to bring together engineers and scientists who work on permanent magnets and their applications to present their latest results and exchange ideas on a wide range of relevant topics including resources, raw materials, recycling, processing, fundamental and hard magnetic properties and applications of rare earth and future magnets.

To regsiter for this event, please follow the link below.

poster

Usually, the production of rare-earth sintered magnets requires some care in their manufacturing process, as powder manipulation at controlled atmosphere to avoid oxidation. Oxygen-free environments are required to prevent oxidation of rare earth-based powders. In this work, sintered Nd-Fe-B permanent magnets were prepared using a commercial alloy, following the traditional powder metallurgy route. The alloy was hydrogen decrepitated (HD process), and the resultant coarse powder was mechanically milled. This step was performed employing several reduced graphene oxide (rGO) additions (0.02 to 0.10 wt %). It was observed that rGO acts as a lubricant and effective milling medium, preventing agglomeration and welding of the particles. Additionally, it was possible to handle the milled powders without the need for an inert atmosphere. Magnetic properties of sintered magnets decreased with increasing rGO addition, showing that carbon is deleterious to the magnetic properties at high concentrations (0.10 wt %). The highest values of remanence (Br) and intrinsic coercivity (iHc) values were obtained for the sintered magnet using 0.02 wt% rGO addition, Br = 1.1 T and iHc = 764.0 kAm-1.

Nd-Fe-B sintered magnets obtained by mechanical ball milling with rGO addition

Phase stability and magnetocrystalline anisotropy of Bi-Co co-substituted M-type Sr ferrite for a rare-earth-free permanent magnet 
Mohamed A. Kassem, Takeshi Waki, Yoshikazu Tabata, Hiroyuki Nakamura 
Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan 

The performance of the M-type Sr ferrite (SrF) permanent magnet (PM) is significantly improved by the Co2+ substitution for Fe3+, assisted by the simultaneous substitution by La3+ for Sr2+ [1]. After a number of early studies on the Co2+ occupation sites, microscopic investigations have revealed that Co2+ ions mainly occupy the tetrahedrally coordinated 4f1 site and partly the octahedrally coordinated 2a and 12k sites [2–4]. Based on recent results of 59Co-NMR experiments as well as calculations of the electronic structure and magnetic anisotropy (MA) for the Fe3+ clusters in La-Co co-substituted M-SrF, it becomes clear that the enhancement in MA is ascribed to the orbital moment of Co2+ occupying at the 4f1 sites [5–8]. Furthermore, the enhancement in MA becomes more significant when the c/a ratio is smaller, which means that a uniaxial chemical pressure is desirable to aggregate Co2+ in the 4f1 site [7,8]. Other available trivalent substituents with the ionic size comparable to La3+ are required to develop new ferrite PMs with high performance. 
Here, we introduce Bi3+-Co2+ co-substituted Sr-ferrites as a new rare-earth-free PM. Investigations by XRD, SEM and WDX following systematic syntheses of the compounds with starting compositions SrFe12−zBizO19 and Sr1−xBixFe12O19 show that Bi3+ prefers to occupy the Sr site rather than for the Fe site. Interestingly, we have found that the limited solubility of Bi can be increased by the Bi-Co co-substitution in Fe2+-free Sr1−xBixFe12−yCoyO19 with almost identical Bi and Co compositions up to x = y = 0.3 in an open-air synthesis. Magnetization measurements using oriented polycrystalline samples showed that the Bi-Co co-substitution markedly enhances MA, which supports the uniaxial stain-induced Co-site selectivity mechanism. The effect of O2 potential has been also examined by syntheses of both Sr1−xBixFe12O19 and Sr1−xBixFe12−yCoyO19 under O2-pressures up to 10 atm.


Phase stability and magnetocrystalline anisotropy of Bi- and Bi-Co substituted M-type Sr ferrites as a rare-earth-free permanent magnet

AC losses in segmented rare earth magnets
Gerhard Martinek, Maximilian Lanz, Gerhard Schneider, Dagmar Goll
Aalen University, Materials Research Institute, Aalen, Germany

In permanent magnet machines, significant eddy current losses may occur in the electrically conducting rare earth magnets. It has become common practice to reduce these losses by segmenting the magnets into thin, electrically insulated slices. Magnets with varying degrees of segmentation, often referred to as “laminated” magnets, are now readily available from numerous permanent magnet suppliers. 
The reduction of eddy currents is straightforward and can be evaluated using numerical modeling or analytical formulae. However, the developers of the machines often pay little attention to the disadvantages of segmenting. One obvious consequence is the loss in magnet volume due to the gaps between the magnet segments: In magnets with very high degrees of segmentation, adhesive and spacers (for electrical insulation) may make up several percent of the magnet volume. Further negative consequences arise from the degradation of the magnetic properties at the surfaces of rare earth magnets. The well-known formation of surface layers with reduced coercivity will not only cause a further reduction of the useful magnetization levels. It may also give rise to hysteresis losses in alternating fields. The creation of additional magnet surfaces by segmenting is therefore not without risk. Determining the degree of segmentation without considering possible surface losses can potentially lead to severely incorrect decisions in the design of an electrical machine. 
In order to assess these dangers, segmented rare earth magnets were characterized by two kinds of magnetic measurements [1]: Static hysteresis curves, including minor loops, were determined in a conventional hysteresisgraph. Dynamic loss measurements at frequencies up to 2000 Hz were performed using a custom-made laminated yoke.
The static demagnetization curves of segmented NdFeB magnets show a pronounced drop at small demagnetizing fields. The amplitude of this drop corresponds to low-coercive surface layers with a thickness of about 15 µm. In Sm2Co17 magnets, the curve shape is not affected by segmentation.
AC loss measurements show the expected reduction of eddy current losses by segmentation in the NdFeB and in the Sm2Co17 samples. However, the NdFeB magnets also show hysteresis losses, which increase with finer degrees of segmentation. In an AC field of ± 80 kA at 1000 Hz, the total losses in NdFeB samples thus show a minimum for a segmentation into 2 mm slices. For finer segmentation, the losses increase again, and the total losses at 0.5 mm segmentation were of the same magnitude as in the samples without any segmentation. 
Segmentation did not lead to hysteresis losses in Sm2Co17. However, this material did show substantial hysteresis losses at larger field amplitudes. These losses were independent of the degree of segmentation, showing the same level in bulk and in heavily segmented samples.
The strong hysteresis losses in NdFeB are observed for AC fields oscillating around H = 0. DC bias fields of >100 kA/m reduce the hysteresis losses by more than one order of magnitude.
Determining the hysteresis losses from quasi-static minor loop measurements gives similar results as found in the AC measurements. An evaluation of the hysteresis losses is thus possible by hysteresisgraphs, which are more widely available than AC measuring systems for permanent magnets.
[1] G. Martinek, M. Lanz, G. Schneider, D. Goll; J. Appl. Phys. 128, 113902 (2020), doi.org/10.1063/5.0010031


AC losses in segmented rare earth magnets

Sm-Co hard magnetic compound has attractive intrinsic properties of high anisotropy field and high Curie temperature. To develop high coercivity and high magnetization anisotropic magnets for high temperature environmental applications based on the Sm-Co compound, achieving a high coercivity in monocrystalline powder is necessary. It is well known that the coercivity of monocrystalline powder strongly depends on the particle size. As a new process, we focused on the induction thermal plasma (ITP) process as a physical bottom up process which enables us to prepare nanopowder with single crystalline. In this work, we explored the potential and the possibility to apply this ITP process to prepare Sm-Co alloy nanopowder.
We used two powders of the mixture of Sm and Co metal powder with an atomic ratio of Sm:Co = 1:5 and SmCo5 alloy powder as raw material and tried to identify the obtained phases through this alloying process. The processed nanopowders from mixed powder and alloy powder were denoted as S1 and S2, respectively.
The SEM images show that the particle shape was nonangular. The mean particle sizes of the S1 and S2 were estimated to be 61 nm and 31 nm, respectively. According to the XRD profile, magnetic alloy phases of SmCo5 and Sm2Co17 were obtained for both S1 and S2. Therefore, we successfully prepared the Sm-Co alloy fine particles by the ITP process. The maximum coercivity of 5.6 kOe was achieved for S1, although the large amount of the soft magnetic phase of Co was also found in the XRD profile. By controlling the alloying process actively, such as temperature distribution in the chamber, cooling rate and powder feed rate, much higher coercivity can be expected when the only Sm-Co hard magnetic phases were obtained through the ITP process.

Sm-Co hard magnetic alloy nanopowder prepared by induction thermal plasma process

The rapid development of green technologies like electric vehicles has spurred 
burgeoning interest in searching for new permanent magnetic materials which could 
be utilized as motors and generators. SmFe12-based compounds, one of the most 
promising candidates of next-generation permanent magnets from the ThMn12-structure family, exhibit excellent intrinsic hard magnetic properties such as high saturation magnetization, large anisotropic field, and high Curie temperature. 
However, the coercivity of Sm-Fe-based alloys is insufficient for practical application,
so it is necessary to control phases and microstructures to increase the coercivity.
In this work, we modified Sm-Fe-Ti-based powders by changing alloy composition 
and optimizing preparation conditions, and investigated the consequent changes in 
magnetic properties, phases, and microstructures.
The Sm-Fe-Ti-based powders were prepared by induction melting and ball-milling. 
The largest coercivity we obtained is 606 kA/m for Sm-rich compositional fine 
powders, which is presumably related to the SmFe3-like phases.

Achievement of high coercivity in Sm-Fe-Ti-based powders

Development of Composition and Powder Fabrication for the Additive Manufacturing of (Pr,Nd)-Fe-Cu-B Permanent Magnets

NdFeB-based magnets are key components widely used in the automotive industry, electronics, energy conversion systems and acoustic products. Owing to the unstable supply and high cost of less abundant rare earths (e.g. Dy and to some extent Nd), it is important to reduce cost while maintaining performance. In order to address this problem, the partial replacement of Nd with La and Ce, which are cheaper and more abundant than Nd, is actively being pursued. In this work, a triple target geometry is used for a high throughput study of NdLaCeFeB films. By using movable target shields, a set of films with different composition gradients have been deposited and post-annealed under different conditions. 2D composition maps of as-deposited films were made using EDX (Energy Dispersive X-Ray) analysis in an SEM. High throughput magnetic characterisation was carried out using an in-house developed high field scanning MOKE system operated at room temperature[1]. High throughput XRD measurements were also made to investigate changes of phase and texture. More detailed magnetic characterisation of certain sample parts was carried out using SQUID-VSM. Also, machine learning based data analysis enabled us to show the correlation among compositions, microstructures and magnetic properties. In this work we demonstrate the potential offered by the combinational high throughput approach to the development of NdLaCeFeB films and reveal the influence of La and Ce content on the magnetic and structural properties of such films.

Sustainable Magnets Production using Hydrogen - Challenges and Chances

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Nd-Fe-B sintered magnets obtained by mechanical ball milling with rGO addition

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