Transcript Session_B1_Hadjipanayis.ppt (1.21 MB)

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
[email protected]
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).
Permanent Magnets and Measure of Their Strength ( Figure of Merit=(BH)max )
(BH)m ~ H 2ag 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.
● (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!
1952
1735
1985
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
● 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.
Permanent Magnet Materials: Overview
EV Motors
● Alnico magnets have very low Hc ( 2 kOe).
● CoPt and FePt magnets are prohibitively expensive.
Permanent Magnet Materials for EV Motors
● At the present, magnets for EV motors are
being made from Nd-Dy-Fe-B.
Hitachi Neo Magnets
● 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 SmCo magnets are superior to the "hightemperature" Nd-Dy-Fe-B and they even
%Dy →
contain slightly less REs (Sm-Co drawbacks:
more brittle, difficult to magnetize, complex
heat treatment, based on cobalt).
Br (kG)
i Hc
(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.
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 exchangecoupled magnets difficult; it is even more difficult to obtain crystallographic alignment in
the nanoscale.
Development of New Advanced Permanent Magnets
● 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 HighEnergy Permanent Magnets for Hybrid Vehicles and Alternative Energy Uses. This
program is supported by DOE ARPA-E.
Flow Chart of ARPA-E Supported Program
Nanocomposite Magnets
Novel Hard Magnetic Materials
Search for
RE-TM-X
compound with
superior
properties
Nd-Fe-B, Sm-Co,
Sm-Fe-N
Inducing
anisotropy in
Fe-Co
intermetallics
Synthesis
of high-Hc
nanoparticles
Fe, Fe-Co
Synthesis
of high-Ms
nanoparticles
Blending
Comminuting
Modeling
Alignment
Alignment
Consolidation
New
High-Performance
Magnet
Synthesis
of core/shell
nanoparticles
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.
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
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