Development of Dy-free high coercivity Nd-Fe-B permanent magnets
The interest in the coercivity mechanism of Nd-Fe-B sintered magnets has
recently been revived in Japan due to the increasing demand for Dy-free
high coercive Nd-Fe-B sintered magnets for automotive applications.The
operating temperature of Nd-Fe-B magnets increases to 200C in traction
motors for hybrid and electric vehicles (HEVs). In order to overcome the
demagnetization problem of the Nd-Fe-B magnets at the oeperating temperature,
the current Nd-Fe-B permanent magnets for traction motors of HEVs contains
10% of Dy in (Nd,Dy)-Fe-B magnets. As shown in Figure 1, the room temperature
coercivity (Hc) of (Nd,Dy)-Fe-B magnet is three times higher than that
of Nd-Fe-B magnets.
Figure 1 Permanent magnets in a HV motor and the temperature dependence
of Hc of Nd-Fe-B and (Nd,Dy)-Fe-B sintered magents.
As shown in Fig. 2, there are ranges of Nd-Fe-B sintered magnets. The magnet
without Dy (Fe80Nd14B6 in atomic ratio) show the highest maximum energy
product (BH)max value, which is the figure of merit of permanet magnets.
However the coercivity is only 10 kOe, which is too low for traction motor
applictions. As Dy concentration in the alloy increases, the coercivit
of the magnet increases at the expense of (BH)max. This is because of the
ferrimagnetic coupling of spins between Dy and Nd (Fe). Thus the coercivity
increase by the Dy substitution with Nd can be achieved only at the expense
of (BH)max.
Figure 2 Coercivity (Hc) and maximum energy product (BH)max of commercial sintered magnets.
Our goal is to enhance the coercivity of the Nd-Fe-B magnets by optimizing
the microstructure without using Dy. Coercivity is not an intrisic value
of materials, but is structural sensitive properties. Assuming that the
magnetization reversal occurs by the perfect coherent rotation of magnetically
isolaated single domain particles of a few tens nanometer, the coercivity
can be as large as 77 kOe theoretically. However, due to various defects
on the crystal surface and magnetic coupling of particles, the actual coercivity
never reach this level. On the other hand, the coercivity of the commercial
Nd-Fe-B magnets are only 10 - 15% of the theoretical limit. This suggests
that there is enough room to improve the coercivity by controlling the
microstructure of the Nd-Fe-B magnets. Thus, in this study, we investigate
the microstructure-property relationships of Nd-Fe-B permanent magnets
to understand the reason why the coercivity of the commercial magnets are
much lower than the ideal value. By understanding the microstructure-coercivity
relationships, we should be able to obtain an idea how to optimize the
microstructure to achieve higher coercivity without using Dy.
Figure 2 Coercivity (Hc) and maximum energy product (BH)max of commercial sintered magnets.
Based on this idea, we carry out multi-scale characterization of Nd-Fe-B sintered magnets. Using scanning electron microscopy (SEM), we can observe the overall picture of the microstructure of Nd-Fe-B magnets. By using focused ion beam (FIB9 technique, we prepare transmission electron microscopy (TEM) specimens from specific areas in the sample. Then, do HREM observation from grain boundaries and other interfaces formed among Nd2Fe14B grains and various Nd-rich phases. We can also observe the magnetic domains from the same view using the Lorenz microscopy. For quantitative chemical analysis of the grain boundaries and other interfaces, we employ three-dimensional atom probe (or atom probe tomography). Based on the experimental observations, we construct model microstructure and do micromagnetic simulations using a finite micromagnetic simulator developed by Prof. Schrefl at St. Poelten University.
As shown in the following list of publications, our understanding on the
microstructure-coercivity is advancing and we apply some of the concept
in enhancing the coercivity of Nd-Fe-B magnets.
Related Publications
Microstructure optimization for achieving
high coercivity in anisotropic Nd-Fe-B thin films
W. B. Cui, Y. K. Takahashi
and K. Hono, Acta
Mater. 59, 7768 (2011).
Microstructure of fine grained Nd-Fe-B
sintered magnets with high coercivity
H. Sepehri-Amin, Y. Une, T. Ohkubo, K.
Hono and M. Sagawa, Scripta Mater. 65, 396
- 399 (2011).
Distribution of Dy in high-coercivity (Nd,Dy)-Fe-B sintered magnets
W.F. Li, H. Sepehri-Amin, T. Ohkubo, N. Hase, and K. Hono,
Acta Materialia 59,
3061 (2011)..
Coercivity enhancement of hydrogenation-disproportionation-desorption-recombination
processed Nd-Fe-B powders by the diffusion of Nd-Cu eutectic alloys
H. Sepehri-Amin, T. Ohkubo,
T. Nishiuchi, N. Nozawa, S. Hirosawa and K. Hono, Scripta Mater, 63,
1124 - 1127 (2010)..
Coercivity enhancement of HDDR-processed Nd-Fe-B permanent magnet by rapid
hot-press consolidation process
N. Nozawa,
H. Sepehri-Amin, T. Ohkubo, K. Hono, T. Nishiuchi, and S. Hirosawa, J. Mag. Mag. Mater. 323, 115
- 121 (2010).
Fabrication and characterization of highly textured Nd?Fe?B thin film with
a nanosized columnar grain structure
C. Y.
You, Y. K. Takahashi, and K. Hono,
J. Appl. Phys. 108, 043901 (2010).
Grain boundary structure and chemistry of
Dy-diffused Nd-Fe-B sintered magnets,
H. Sepehri-Amin, T. Ohkubo, and K.
Hono, J. Appl. Phys. 107, 09A745
(2010).
Effect of Ga addition on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination processed Nd-Fe-B powder
H. Sepehri-Amin, W. F. Li, T. Ohkubo, T. Nishiuchi, S. Hirosawa, and
K. Hono, Acta Mater.
58, 1309 (2010).
The effect of oxygen on the surface coercivity of Nd-coated Nd-Fe-B sintered magnets
T. Fukagawa, S. Hirosawa, T. Ohkubo, and
K. Hono, J. Appl. Phys. 105,
07A724 (2009).
The origin of coercivity decrease in fine grained Nd-Fe-B sintered magnets
W. F. Li, T. Ohkubo, K. Hono, and M. Sagawa,
J. Mag. Mag. Mater.
(2009) in press.
Effect of post-sinter annealing on the coercivity and microstructure of Nd-Fe-B permanent magnets
W. F. Li, T. Ohkubo, and K. Hono,
Acta Mater. 57,
1337-1346 (2009).
The role of Cu addition in the coercivity enhancement of sintered Nd-Fe-B permanent magnets
W. F. Li, T. Ohkubo, T. Akiya, H. Kato,
and K. Hono, J. Mater. Res.
24, 413 (2009).