George C. HadjipanayisDepartment of Physics & Astronomy

University of DelawareNewark, DE 19716, USA

hadji@udel.edu

Trans-Atlantic Workshop on Rare-Earth Elements and Other Critical Materialsfor 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 oC.

●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.

17351735

19521952

19851985

(BH)m ~ H 2

ag Vag / 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.

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 Hc Mr/2 (12 kOe), its (BH)m would be

(4πMs/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 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

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 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

Permanent Magnet Materials: Overview

●Alnico magnets have very low Hc ( 2 kOe).

●CoPt and FePt magnets are prohibitively expensive.

EV Motors

Permanent Magnet Materials for EV Motors

Br (kG) iHc (kOe) (BH)max (MGOe)

25 oC 200 oC 240 - 250 oC

25 oC 200 oC 240 - 250 oC

25 oC 200 oC 240 - 250 oC

Nd-Dy-Fe-B* 10.8 9.0 8.0 >30 7.0 4.0 28 < 20 < 16

Sm(Co,Fe,Cu,Zr)z** 11.5 10.7 10.5 24.7 13.1 10.3 31.3 26.7 25.4

* 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.

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 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).

%Dy →

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.

●At the present, permanent magnets based on Nd2Fe14B, SmCo5, Sm2Co17 and Sm2Fe17Nx 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 forRE-TM-X

compound with superior

properties

Inducing anisotropy in

Fe-Co intermetallics

Comminuting

Synthesisof high-Hc

nanoparticles

Synthesisof high-Ms

nanoparticles

Synthesisof core/shell

nanoparticles

Nanocomposite Magnets

Blending

Alignment

Nd-Fe-B, Sm-Co,Sm-Fe-N

Fe, Fe-Co

NewHigh-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 Hc.

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!

Compound Structure Saturation magnetization

Curie temperature (oC)

Anisotropy constant K1 (MJ/m3)

Co hexagonal 17.6 kG 1115 0.53

FePt tetragonal 14.3 kG 477 6.6

CoPt tetragonal 10.0 kG 567 4.9

Co3Pt hexagonal 13.8 kG 727 2.0

MnAl tetragonal 6.2 kG 377 1.7

MnBi hexagonal 7.8 kG 357 1.2

BaFe12O19 hexagonal 4.8 kG 450 0.33

Zr2Co11 orthorhombic(?) ≈70 emu/g 500 ? (HA = 34 kOe)

ε-Fe2O3 orthorhombic ≈16 emu/g ? ? (Hc = 23.4 kOe)

SmCo5 hexagonal 11.4 kG 681 17.0

Nd2Fe14B tetragonal 16.0 kG 312 5.0

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

ShapeAnisotropyMaterials

●Fe(Co)●Fe(Ni)

Nanorods(Nanowires)

Changecubic symmetry

of high-Ms materials to uniaxial

●Fe-Co-X●Fe-Ni-X

Non-equilibriumtechniques

Newuniaxial compounds

●Fe-V(Cr) ●Tetragonal

Heusler alloys

TC > 400 oC4πMs > 10 kG

K1 > 107 erg/cm3

Nanocompositemagnets

●X/Y (hard/soft)

Chemical depositionCore-shell structures