IEEE Electrification Magazine - March 2017 - 27

magnets in the Future

The method of slicing
small pieces of the
diffused magnet
and stacking them
together to measure
the BH curves of
the representative
regions inherently
contains multiple
sources of
possible error.

Here we discussed in great detail
grain-boundary-diffused magnets
and the challenges in obtaining reliable and representative BH curves for
electromagnetic motor design. The
development of GBD NdFeB magnets
was spurred by the need to reduce
the utilization and associated cost of
the HREs incorporated to improve
high-temperature performance. The
same impetus is motivating efforts to
improve coercivity through grain size
reduction in sintered NdFeB magnets,
where the modest but significant
boost in coercivity in fine-grained
NdFeB allows for some reduction in
the HRE content. It may also lead to a
resurgence of hot deformed magnets
(hot pressed, die upset, or extruded
magnets) made from rapidly solidified NdFeB precursor ribbons. Optimized hot deformed magnets can have temperature coefficients of coercivity and remanence that are
superior to those in sintered magnets, at least relative to
the room temperature values, and thus experience less
reduction in their magnetic properties at high temperatures. Equally important, there is renewed focus on motor
design optimization, and on design of novel motor configurations, that make best use of NdFeB magnets, achieving
targeted motor performance values using the minimum
possible magnet sizes.
Looking further toward the future, the need to reduce
rare-earth content has stimulated considerable research
efforts into alternative permanent magnet materials.
Cerium (Ce)-based magnets are one such option, as Ce is
a fraction of the cost of Nd. For example, magnets based
on Ce2Fe14B have limited coercivity and temperature
performance but could be suitable for less demanding
applications. For higher temperature uses, compounds
based on a different chemistry having a rare-earth:Fe
ratio of 1:12 are interesting because they contain onethird less rare earth than NdFeB compounds. For example, CeFe12-xSix has the curious property that its Curie
temperature (TC) goes up as the Si content increases,
even though the magnetic Fe content is diluted by nonmagnetic Si. CeFe10Si2 has TC = 310 °C, equivalent to the
Curie temperature of NdFeB.
Rare-earth-free magnet materials represent another
set of options. In part this is a revival of research on materials such as manganese-aluminum-carbon (MnAlC) and
manganese-bismuth (MnBi), where earlier development
was largely abandoned after the discovery of NdFeB. However, there are also new Fe-rich materials under investigation that hold potential as permanent magnets. Iron
nitride (Fe16N2) has very large magnetization (2.6-2.9
Tesla, or even higher). FeNi has high magnetization

(1.5 Tesla), high Curie temperature
(TC >500 °C), and coercivities of at
least 300 kA/m should be achievable
(similar to ferrite, but with much
higher magnetization). Both of these
compounds are marginally stable relative to magnetically soft compounds,
so formation of the desired hard magnetic phase is the current research
challenge. It is unlikely that any rareearth-free magnet will equal the
excellent properties of NdFeB or
samarium-cobalt, whose high coercivity derives primarily from the rareearth ions. Rather, it is anticipated
that new low-cost rare-earth-free
magnets with magnetic properties in
the gap between low-coercivity, lowmagnetization ferrite magnets and
high-performance rare-earth magnets
will find numerous ready applications and thus reduce
the pressure on high-performance rare-earth materials.

For Further reading
H. Nakamura, K. Hirota, M. Shimao, T. Minowa, and M. Honshima, "Magnetic properties of extremely small Nd-Fe-B sintered magnets," IEEE Trans. Magn., vol. 41, no. 10, pp.
3844-3846, Oct. 2005.
K. Hirota, H. Nakamura, T. Minowa, and M. Honshima,
"Coercivity enhancement by the grain boundary diffusion
process to Nd-Fe-B sintered magnets," IEEE Trans. Magn., vol.
42, no. 10, pp. 2909-2911, Oct. 2006.
S. Jurkovic, K. M. Rahman, B. Bae, N. Patel, and P. Savagian,
"Next generation Chevy Volt electric machines; design, optimization and control for performance and rare-earth mitigation," in Proc. IEEE Energy Conversion Congr. Exposition, Montreal,
Quebec, Canada, 2015, pp. 5219-5226.
F. Momen, K. M. Rahman, Y. Son, and P. Savagian, "Electric
motor design of General Motors' Chevrolet Bolt electric vehicle," SAE Int. J. Alt. Power, vol. 5, no. 2, pp. 286-293, July 2016.
R. W. McCallum, L. H. Lewis, R. Skomski, M. J. Kramer, and I.
E. Anderson, "Practical aspects of modern and future permanent magnets," Annu. Rev. Mater. Res, vol. 44, pp. 451-477, 2014.

biographies
Margarita P. Thompson (margarita.thompson@gm.com) is
with General Motors.
Edwin Chang (edwin.chang@gm.com) is with General
Motors.
Aldo Foto (aldo.foto@gm.com) is with General Motors.
Jorge G. Citron-Rivera ( jorge.cintron-rivera@gm.com)
is with General Motors.
Daad Haddad (daad.haddad@gm.com) is with General Motors Global Research and Development.
Richard Waldo is with General Motors Research Laboratories.
Frederick E. Pinkerton (frederick.e.pinkerton@gm.com)
is with General Motors Research and Development Center.

IEEE Electrific ation Magazine / march 2 0 1 7

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