Permanent Magnets

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 known at the present and are therefore most promising for the use in high-performance electric hybrid vehicles (EHVs) and generators for wind turbines [1]. Their better efficiency compared to induction motors and generators predicts a strong increase in the demand of Nd-Fe-B magnets. 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 [2] or backward-extrusion [3] process is applied to prepare uniaxial or radial textured tablets and ring magnets with energy densities above 300 kJ/m³, respectively.

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 [4]. Furthermore the HD (hydrogen decrepitation) method is a very promising candidate for recycling of Nd-Fe-B magnet.

Dy-reduction – Grain boundary diffusion process (GBDP)

Schematic of the grain boundary diffusion process
Schematic of the grain boundary diffusion process

Commonly 10-15 wt.%Dy is required to compensate the deterioration in coercivity of Nd-Fe-B magnets due to elevated operation temperatures of 200°C. Because of the high price volatility and the forecasted long term criticality of Dy this amount will be a not negligible cost and safety aspect in the supply permanent magnets. Therefore the reduction of Dy without losing the performance is one main subject of permanent magnet research. One very promising approach to use the amount of HRE more efficient is the grain boundary diffusion process (GBDP) which we investigate in our working group [5].

Other main topics are as follows [6-11]:

• Investigation of coercivity mechanisms through 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

• Measurements of electrical resistivity and eddy current losses

References:

Some selected publications from the group, ( for the complete list please follow this link: Publications) :

[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] A. Kirchner, J. Thomas, O. Gutfleisch, D. Hinz, K. H. Muller, L. Schultz, HR-TEM studies of grain boundaries in die-upset Nd-Fe-Co-Ga-B magnets, Journal of Alloys and Compounds 365 (2004) 286–290.

[3] D. Hinz, A. Kirchner, D.N. Brown, B.M. Ma, O. Gutfleisch, Near net shape production of radially oriented NdFeB ring magnets by backward extrusion, Journal of Materials Processing Technology 135 (2003)

[4] K. Güth, J. Lyubina, B. Gebel, L. Schultz, O. Gutfleisch, Ultra-fine grained Nd–Fe–B by high pressure reactive milling and desorption, Journal of Magnetism and Magnetic Materials 324 (2012) 2731–2735

[5] S. Sawatzki, M. Moore, H. Wendrock, L. Schultz, and O. Gutfleisch, Thermal stability of hot-pressed NdFeB magnets with Dy-fluoride additions, Proc. 22nd Int. Workshop on RE Magnets and their Applications (Nagasaki, Japan)(2012) 409-412

[6] K. Löwe, J. Liu, K. Skokov, J. D. Moore, H. Sepehri-Amin, K. Hono, M. Katter, and O. Gutfleisch, The effect of the thermal decomposition reaction on the mechanical and magnetocaloric properties of La(Fe,Si,Co)13, Acta Materialia 60 (2012) 4268–4276

[7] 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

[8] O. Gutfleisch and V. Franco, Preface to the Viewpoint Set on: Magnetic Materials for Energy, Scripta Mat. 67 (2012) 521-523.

[9] 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

[11] S. K. Pal, L. Schultz, O. Gutfleisch, Effect of milling parameters on SmCo5 nanoflakes prepared by surfactant-assisted high energy ball milling, Appl. Phys. 113 (2013) 013913_1-6.

M D Kuz'min, K P Skokov, H Jian, I Radulov and O Gutfleisch ,Towards high-performance permanent magnets without rare earths

Journal of Physics: Condensed Matter 26 (2014) 064205

Magnetic Properties of (Fe,Co)2B Alloys With Easy-Axis Anisotropy

Hong Jian, K P Skokov, M D Kuz'min, I Radulov, and O Gutfleisch, Magnetic Properties of (Fe,Co)2B Alloys With Easy-Axis Anisotropy

IEEE TRANSACTIONS ON MAGNETICS 50 (2014) 2104504