George C. Hadjipanayis Department of Physics & Astronomy University of Delaware Newark, DE 19716, USA hadji@udel.edu

1. Trans-Atlantic Workshop on Rare-Earth Elements and Other Critical Materials for a Clean Energy Future Cambridge, Massachusetts, December 3, 2010 Moving Beyond Neodymium-Iron Permanent Magnets for Electric Vehicle Motors George C. Hadjipanayis Department of Physics & Astronomy University of Delaware Newark, DE 19716, USA hadji@udel.edu

2. Modern Motors for HEV and EV Applications • Electrical motors for the drive-train of HEVs and EVs are required to have a high starting torque and a constant-power wide speed range. • At the present, there requirements are best met by the Interior Permanent Magnet Synchronous Motors (IPMSMs) in which powerful permanent magnets (almost exclusively Nd-Dy-Fe-B) are embedded deep into the rotor. • IPMSMs are energy-efficient, they provide high torque values and they can operate in a wide speed range. Nd-Dy-Fe-B magnets • In the IPMSM design, the permanent magnets are subjected to strong demagnetizing fields and moderately high temperatures. • Thus, the magnets must have a high coercivity and an operating temperature of at least 200 oC. Y. Matsuura. J. Magn. Magn. Mater. 303 (2006).

3. Permanent Magnets and Measure of Their Strength ( Figure of Merit=(BH)max ) 1952 1735 1985 (BH)m~ H2agVag / Vm The higher the (BH)m the smaller the Vm! • Generally, a good permanent magnet must have: • (a) a high Curie temperature TC to maintain its magnetic order. • (b) a high remanence Mr to produce a large magnetic field. • (c) a high coercive force Hc to resist demagnetization. • (BH)max, which is proportional to the maximum stored magnetic energy, is the best integrated measure of the magnet strength. • If Fe-Co had HcMr/2 (12 kOe), its (BH)m would be (4πMs/2)2 = 144 MGOe!

4. Permanent Magnet Materials: Fundamentals • Coercive force (magnetic hardness) always arises from magnetic anisotropy which in practical magnets is caused by a crystal electric field (RE-TM, CoPt) or by the crystal shape (Alnico magnets). It can also be caused by stress and by ordering of impurity atoms. • To "convert" the magnetic anisotropy into Hc one has to assure a proper microstructure, which either inhibits the emergence (nucleation magnets, Nd-Fe-B) or re-arrangement of magnetic domains (domain wall pinning magnets, Sm(Co,Fe,Cu,Zr)z). Sintered Nd-Fe-B Sm(Co,Fe,Cu,Zr)z

5. Permanent Magnet Materials: Manufacturing • A large remanence Mr is obtained by the alignment of all grains/particles. This important requirement for the magnet texture and fine microstructure can be best fulfilled through powder metallurgy/sintering. Virtually all commercially available magnets with (BH)max > 25 MGOe are sintered from oriented powders. Additional heat treatment may be necessary, especially for Sm(Co,Fe,Cu,Zr)z magnets. Shin-Etsu website • Polymer-bonded magnets with inferior properties are manufactured from ground, rapidly solidified or hydrogen-treated permanent magnet alloys. The binder dilutes the magnetization; most of these magnets are not textured. • Some other manufacturing methods, such as hot pressing or hot extrusion, are known but rarely used. • Several recent attempts of direct chemical synthesis were reported, but so far without much progress.

6. Permanent Magnet Materials: Overview EV Motors • Alnico magnets have very low Hc ( 2 kOe). • CoPt and FePt magnets are prohibitively expensive.

7. Permanent Magnet Materials for EV Motors • At the present, magnets for EV motors are being made from Nd-Dy-Fe-B. • Dysprosium strongly increases the magnetic anisotropy (coercivity) of the Nd2Fe14B phase and it is added to offset the rapid decline of Hc when the magnets are heated to ≈200 oC. • Since Dy is among the most scarce REs, many ongoing efforts (particularly in Japan) are aimed to optimizing its amount/distribution. • From the performance point of view, the Sm-Co magnets are superior to the "high-temperature" Nd-Dy-Fe-B and they even contain slightly less REs (Sm-Co drawbacks: more brittle, difficult to magnetize, complex heat treatment, based on cobalt). Hitachi Neo Magnets %Dy → * The properties at 200 oC are of NEOMAX-28EH; the properties at 240 oC are of VACODYM 688AP. ** All the properties are of EEC 2:17-31.

8. Rare Earth-Lean (Nanocomposite) Magnets • The amount of RE in Nd-Fe-B and Sm-Co magnets is 25-30 wt.%. One way to decrease it is to dilute the RE-TM phase with a RE-free magnetic phase like Fe-Co. • The phenomenon of magnetic exchange coupling allows us to combine the magnetic hardness of rare-earth compounds with the high magnetization of soft magnetic materials. • The predicted (BH)max of the hard-soft composites exceeds 100 MGOe(59 MGOe is the present record for sintered Nd-Fe-B). • Because the exchange interaction has very short range, the phase structure must be of a nanoscale(size of soft phase ≤ 20 nm). This already makes the development of exchange-coupled magnets difficult; it is even more difficult to obtain crystallographic alignment in the nanoscale.

9. Development of New Advanced Permanent Magnets • At the present, permanent magnets based on Nd2Fe14B, SmCo5, Sm2Co17 and Sm2Fe17Nxhave reached their potential limits. • University of Delaware leads a concerted program that involves four universities, one government lab and one industrial company aimed toward the development of High-Energy Permanent Magnets for Hybrid Vehicles and Alternative Energy Uses. This program is supported by DOE ARPA-E.

10. Flow Chart of ARPA-E Supported Program Synthesis of high-Ms nanoparticles Synthesis of core/shell nanoparticles Blending Comminuting Alignment Alignment Consolidation Nanocomposite Magnets Novel Hard Magnetic Materials Nd-Fe-B, Sm-Co, Sm-Fe-N Fe, Fe-Co Search for RE-TM-X compound with superior properties Inducing anisotropy in Fe-Co intermetallics Synthesis of high-Hc nanoparticles Modeling New High-Performance Magnet

11. Bottom-Up Fabrication of Nanocomposite Magnets Arrangement & Alignment Consolidation • The hard/soft nanoparticles must be assembled together in an aligned structure and then consolidated to obtain a dense bulk magnet. • Although the nanocomposite magnets may lead to a reduced consumption of the REs, their primary advantage is seen in the high (BH)max which is increased, essentially, at the expense of the Hc.

12. Superior Rare Earth-Free Magnets? • Since late 1960s nearly all the R&D efforts were focused on perfecting the RE magnets. • Recent years/months saw a renewed interest in the development of the RE-free alternatives. • RE-free hard magnetic compounds exist: FePt, CoPt, MnBi, MnAl, Zr2Co11, ε-Fe2O3 • Even the Alnico-type magnets still have a room for improvement; their theoretical (BH)max is 49 MGOe and they have excellent temperature stability!

13. Superior Rare Earth-Free Magnets? • Since the late 1960s nearly all the R&D efforts were focused on perfecting the RE magnets. • A comprehensive and concerted effort is needed to search for rare earth free magnets. • Such program needs to include scientists and engineers with a wide expertise from materials design (theory), phase diagrams, design of microstructures, applied magnetics and fabrication techniques (combinatorial approach). Possible Approaches • Shape • Anisotropy • Materials • Fe(Co) • Fe(Ni) Nanorods (Nanowires) • Change • cubic symmetry • of high-Ms materials • to uniaxial • Fe-Co-X • Fe-Ni-X Non-equilibrium techniques • New • uniaxial compounds • Fe-V(Cr) • Tetragonal Heusler alloys • TC > 400 oC • 4πMs > 10 kG • K1 > 107 erg/cm3 • Nanocomposite • magnets • X/Y (hard/soft) • Chemical deposition • Core-shell structures