index

Permanent Magnets

Development of energy density in permanent magnets [
Development of energy density in permanent magnets

Nd-Fe-B magnets have the highest energy density (BH)max at room temperature of all magnets and are therefore most efficient for the usage in high-performance electric hybrid vehicles (EHVs) and generators for wind turbines [1]. Their better performance compared to induction motors and generators will further increase the demand of Nd-Fe-B magnets in the coming years. Several topics play an important role in the design of permanent magnets which are all covered within our working group. Some major interests are:

 

Hot-deformation – net-shape Processing

Due to the low melting point of the Nd-rich phase, Nd-Fe-B melt spun ribbons can be hot-compacted in order to produce isotropic fully dense green compacts with nanocrystalline structure. A subsequent die-upsetting or backward-extrusion process is applied to prepare uniaxial or radial textured tablets and ring magnets with energy densities above 300 kJ/m³, respectively [2].

Die-upsetting (left) and backward-extrusion (middle) of green compacts induces textured platelet-shaped grains (right)
Die-upsetting (left) and backward-extrusion (middle) of green compacts induces textured platelet-shaped grains (right)
 

HDDR – Recycling

Grain structure of HDDR processed Nd-Fe-B
Grain structure of HDDR processed Nd-Fe-B

The HDDR (hydrogenation disproportionation desorption and recombination) process can be employed in order to process highly anisotropic nanocrystalline Nd-Fe-B powder which can be aligned in a magnetic field and bonded with resin in order to prepare highly texture magnets [3]. Furthermore the HD (hydrogen decrepitation) and HDDR methods are very promising routes for functional recycling of Nd-Fe-B magnets.

 

Dy-reduction – Grain boundary diffusion process (GBDP)

Schematic of GBDP [
Schematic of GBDP

Commonly up to 8 wt.% of heavy rare earths (HRE such as Dy and Tb) are required to compensate the deterioration in coercivity of Nd-Fe-B magnets at elevated operation temperatures. Because of the high price volatility and the forecasted long term criticality of these elements this amount is a considerable cost and supply risk in the production process of permanent magnets. Therefore the reduction of HRE without losing or even with improving the performance is one main subject of permanent magnet research. A very promising approach to achieve this goal is the grain boundary diffusion process (GBDP) which we investigate in our work group.

 
Demagnetizing curves of grain boundary diffused sintered magnets
Demagnetizing curves of grain boundary diffused sintered magnets

GBDP in microcrystalline sintered magnets

During the process HRE are deposited on the surface of the magnet, which is subsequently subjected to a heat treatment at moderate temperatures for several hours. This way the HRE diffuses into the magnet over the grain boundaries (see schematic). The attainable increase in coercivity can be as high as 35% without a significant loss of remanence, as shown in the diagram. With the GBDP the amount of needed HRE can be reduced by an order of magnitude [4].

 

GBDP in nanocrystalline hot-pressed magnets

Nanocrystalline materials are interesting for applications because of their better temperature coefficient of coercivity compared to sintered magnets. On the other hand the process parameters have to be adjusted in order to promote a Dy diffusion without inducing grain growth. A comprehensive study combining high resolution TEM and macroscopic analysis with a systematic study parameter study considering annealing time, degree of deformation, strain rate and melting point of the Dy-containing eutectic, respectively, demonstrated that it is possible to form Dy-rich shells providing a better magnetic decoupling of the Nd-Fe-B grains via a Nd-diffusion in order to increase coercivity without reducing remanence [5-7].

Nd-Cu-Ga-rich grain boundary and a Dy-rich shell in the upper grain for a hot-compacted magnet with DyCu-additions
Nd-Cu-Ga-rich grain boundary and a Dy-rich shell in the upper grain for a hot-compacted magnet with DyCu-additions
 

Rare-earth free magnets

Hysteresis loops of a Mn-Ga sample at several temperatures
Hysteresis loops of a Mn-Ga sample at several temperatures

Severe plastic deformation

Mn-based magnetic materials have been receiving much attention in the aftermath of the recent rare-earth crisis, as the rocketing prices of Nd and Dy pushed researchers to embark upon a search of alternative permanent-magnet materials containing no critical elements. Together with Mn–Al and Mn–Bi alloys, the Mn–Ga alloys are scrutinized for use as hard magnetic materials. Different advanced processing routes can be applied to the samples and in this work we focus mainly on severe plastic deformation and magnetic field assisted processing. The obtained results indicate the strong effect of deformation and field assisted annealing on the formation of the metastable phases of the Mn-Ga binary system. With this combination of methods we managed to obtain high coercivity values of Mn-Ga samples [8].

 
An example of a band energy calculation result
An example of a band energy calculation result

Materials Science: Computational and Experimental Approaches

Computational and experimental materials science provide effective screening of different material classes. First principles calculations predict possible interesting materials with promising properties which makes experimental screening much effective and fast. Using density functional theory based calculations the effects of doping on the magnetic properties were estimated for (Fe,Co,X)2B system. The experimental findings were well supported by the first principles calculations. In addition, the effect of substitutional doping the transition metal sublattices has been explored and possible candidates were found. The experimental results agree well with the theoretical predictions which indicate that the computational materials and experimental materials sciences can be a powerful combination for the rational design of novel magnet materials [9].

 

Other main topics are as follows [10-17]:

• Coercivity mechanism and advanced multi-scale characterization and modeling

• RE free permanent magnets by developing new highly anisotropic phases, utilizing shape anisotropy and inducing tetragonality

• Novel processing routes such as surfactant assisted ball milling (exchange coupling and nanoflakes) and severe plastic deformation

Electrical resistivity and eddy current losses

[1] O. Gutfleisch, M. A. Willard, E. Bruck, C. H. Chen, S. G. Sankar, J. P. Liu, Magnetic materials and devices for the 21st century: Stronger, lighter, and more energy efficient, Advanced Materials 23 (2011) 821–842.

[2] I. Dirba, S. Sawatzki, and O. Gutfleisch, Net-shape and crack-free production of Nd-Fe-B magnets by hot deformation, J. Alloy. Compd. 589 (2014) 301-306

[3] O. Gutfleisch, K. Güth, T. G. Woodcock, and L. Schultz, Recycling Used Nd-Fe-B Sintered Magnets via a Hydrogen-Based Route to Produce Anisotropic, Resin Bonded Magnets, Advanced Energy Materials, Volume 3, Issue 2 (2013) 151-155

[4] K. Löwe, Ch. Brombacher, M. Katter, and O. Gutfleisch, Temperature-dependent Dy diffusion processes in Nd-Fe-B permanent magnets, Acta Mater. 83 (2015) 248-255

[5] S. Sawatzki, A. Dirks, B. Frincu, K. Löwe, and O. Gutfleisch, Coercivity enhancement in hot-pressed Nd-Fe-B permanent magnets with low melting eutectics, J. Appl. Phys. 115 (2014) 17A705

[6] S. Sawatzki, I. Dirba, H. Wendrock, L. Schultz, and O. Gutfleisch, Diffusion processes in hot-deformed Nd-Fe-B magnets with DyF3 additions, J. Magn. Magn. Mater. 358-359 (2014), 163-169

[7] S. Sawatzki, Chr. Kübel, S. Ener, and O. Gutfleisch, Grain boundary diffusion in nanocrystalline Nd-Fe-B permanent magnets with low-melting eutectics, Acta. Mater. 115 (2016) 354-363

[8] S. Ener, K. Skokov, D. Karpenkov, M. Kuz'min, and O. Gutfleisch, Magnet properties of Mn70Ga30 prepared by cold rolling and magnetic field annealing, , J. Magn. Magn. Mater. 382 (2015) 265-270

[9] A. Edström, M. Werwiński, D. Iuşan, J. Rusz, O. Eriksson, K. P. Skokov, I. A. Radulov, S. Ener, M. D. Kuz'min, J. Hong, M. Fries, D. Yu. Karpenkov, O. Gutfleisch, P. Toson, and J. Fidler, Magnetic properties of (Fe1−xCox)2B alloys and the effect of doping by 5d elements, Phys. Rev. B 92 (2015) 174413.

[10] I. Dirba, P. Komissinskiy, O. Gutfleisch, and L. Alff, Increased magnetic moment induced by lattice expansion from α-Fe to α′-Fe8N, J. Appl. Phys. 117 (2015) 173911

[11] I. Dirba, B. Yazdi, A. Radetinac, P. Komissinskiy, S. Flege, O. Gutfleisch, and L. Alff, Growth, structure, and magnetic properties of γ′-Fe4Nγ′-Fe4N thin films, , J. Magn. Magn. Mater. 379 (2015) 151-155

[12] T.G. Woodcock, F. Bittner, T. Mix, K.-H. Müller, S. Sawatzki, and O. Gutfleisch, On the reversible and fully repeatable increase in coercive field of sintered Nd–Fe–B magnets following post sinter annealing, J. Magn. Magn. Mater. 360 (2014) 157-164

[13] S. Sawatzki, T.G. Woodcock, K. Güth, K.-H. Müller, and O. Gutfleisch, Calculation of remanence and degree of texture from EBSD orientation histograms and XRD rocking curves in Nd-Fe-B sintered magnets, J. Magn. Magn. Mater. 382 (2015) 219-224

[14] S. Sawatzki, I. Dirba, L. Schultz, and O. Gutfleisch, Electrical and magnetic properties of hot-deformed Nd-Fe-B magnets with different DyF3 additions, J. Appl. Phys. 114, (2013) 133902.

[15] T.G. Woodcock, Y. Zhang, G. Hrkac, G. Ciuta, N.M. Dempsey, T. Schrefl, O. Gutfleisch , and D. Givord, Understanding the microstructure and coercivity of high performance NdFeB-based magnets, Scripta Materialia 67 (2012) 536–541.

[16] R. Gauss und O. Gutfleisch, Magnetische Materialien – Schlüsselkomponenten für neue Energietechnologien, in Rohstoffwirtschaft und gesellschaftliche Entwicklung, ed. P. Kausch und J. Matschullat, März 2016, Springer Spektrum Heidelberg, Springer-Verlag GmbH, ISBN 978-3-662-48854-6, pp. 99-118.

[17] R. Gauß, O. Diehl, E. Brouwer, A. Buckow, K. Güth, O. Gutfleisch, Verfahren zum Recycling von seltenerdhaltigen Permanentmagneten, Chem. Ing. Tech. 87 no. 11 (2015) 1477-1485, DOI: 10.1002/cite.201500061.