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

Modern Motors for HEV and EV Applications Nd-Dy-Fe-B magnets Y. Matsuura. J. Magn. Magn. Mater. 303 (2006). ● 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 o C. ● 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.

(BH) m ~ H 2 ag V ag / V m The higher the (BH) m the smaller the V m ! ● Generally, a good permanent magnet must have: (a) a high Curie temperature T C to maintain its magnetic order. (b) a high remanence M r to produce a large magnetic field. (c) a high coercive force H c to resist demagnetization. Permanent Magnets and Measure of Their Strength ( Figure of Merit=(BH) max ) ● (BH) max, which is proportional to the maximum stored magnetic energy, is the best integrated measure of the magnet strength. ● If Fe-Co had H c  M r /2 (12 kOe), its (BH) m would be (4πM s /2) 2 = 144 MGOe!

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 H c 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

Permanent Magnet Materials: Manufacturing ● 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. ● A large remanence M r 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

Permanent Magnet Materials: Overview ● Alnico magnets have very low H c ( 2 kOe). ● CoPt and FePt magnets are prohibitively expensive. EV Motors

Permanent Magnet Materials for EV Motors B r (kG) i H c (kOe)(BH) max (MGOe) 25 o C200 o C o C 25 o C200 o C o C 25 o C200 o C o C Nd-Dy-Fe-B* > < 20< 16 Sm(Co,Fe,Cu,Zr) z ** * The properties at 200 o C are of NEOMAX-28EH; the properties at 240 o C are of VACODYM 688AP. ** All the properties are of EEC 2: Hitachi Neo Magnets ● At the present, magnets for EV motors are being made from Nd-Dy-Fe-B. ● Dysprosium strongly increases the magnetic anisotropy (coercivity) of the Nd 2 Fe 14 B phase and it is added to offset the rapid decline of H c when the magnets are heated to ≈200 o C. ● 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). %Dy →

Rare Earth-Lean (Nanocomposite) Magnets ● The amount of RE in Nd-Fe-B and Sm-Co magnets is 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.

● At the present, permanent magnets based on Nd 2 Fe 14 B, SmCo 5, Sm 2 Co 17 and Sm 2 Fe 17 N x have 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. Development of New Advanced Permanent Magnets

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

Arrangement & Alignment Consolidation Bottom-Up Fabrication of Nanocomposite Magnets ● 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 H c.

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, Zr 2 Co 11, ε-Fe 2 O 3 ● Even the Alnico-type magnets still have a room for improvement; their theoretical (BH) max is 49 MGOe and they have excellent temperature stability! CompoundStructureSaturation magnetization Curie temperature ( o C) Anisotropy constant K 1 (MJ/m 3 ) Cohexagonal17.6 kG FePttetragonal14.3 kG CoPttetragonal10.0 kG Co 3 Pthexagonal13.8 kG MnAltetragonal6.2 kG MnBihexagonal7.8 kG BaFe 12 O 19 hexagonal4.8 kG Zr 2 Co 11 orthorhombic(?)≈70 emu/g500? (H A = 34 kOe) ε-Fe 2 O 3 orthorhombic≈16 emu/g?? (H c = 23.4 kOe) SmCo 5 hexagonal11.4 kG Nd 2 Fe 14 Btetragonal16.0 kG3125.0

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-M s materials to uniaxial ● Fe-Co-X ● Fe-Ni-X Non-equilibrium techniques New uniaxial compounds ● Fe-V(Cr) ● Tetragonal Heusler alloys T C > 400 o C 4πM s > 10 kG K 1 > 10 7 erg/cm 3 Nanocomposite magnets ● X/Y (hard/soft) Chemical deposition Core-shell structures