Development of high performance anisotropic nanocomposite permanent magnets
The final goal of this project is to develop high coercivity permanent magnets with low rare earth consumption for hybrid electric vehicles and electric vehicles. The nanocomposite magnets that are composed of exchange coupled hard and soft magnetic phases have been a subject of recent studies for their potential to achieve the maximum energy product (BH)max that is
higher than those achieved in the existing sintered magnets [1,2]. Although
various types of nanocomposite magnets were reported in the Nd-Fe-B system,
their coercivity (Hc), remanence (Br), and
(BH)max are lower than those for commercial sintered magnets of the same system
due to their isotropic feature; thus, nanocomposite magnets have been considered
as economical medium performance materials for bonded magnet applications
[3]. However, crystallographically textured anisotropic nanocomposites
still have a great technological potential to achieve a higher (BH)max. The aim of this investigation is to challege to the development of bulk
anisotropic nanocomposite magnets that are composed of strongly textured
nanocrystalline Nd2Fe14B grains and nanosized Fe particles.
Figure 1 Concenpt of the preparation process of anisotropic nanocomposite
magnets.
Figure 1 summarizes the research plan. To process ultrafing grained structure with a strong crystallographic texture, we adopt the hydrogenation disproportionation desorption recombination (HDDR) process originally developed by Takeshita and Nakayama in 1989 [3]. The HDDR process can produce a strongly textured ultrafine grained microstructure with a single domain size (~200 nm). The HDDR powder can show an exceptional microstructural feature with a strong crystallographic texture provided that the right processing conditions are applied, i.e., the easy axis of the Nd2Fe14B grains is aligned to the same orientation as that of an original monograin
particle. The coercivity increases sharply after a critical time in the
DR process. The typical coercivity values reported for HDDR powders range
from 10 to 13 kOe as shown in Figure 2. However, if each Nd2Fe14B grain is magnetically isolated, a much higher coercivity is expected
since the grain size is close to the single domain size of the Nd2Fe14B phase and the anisotropy field of the Nd2Fe14B phase is ~73 kOe. In fact, the coercivity Hc of sintered magnets follows Hc=24-2.6ln(D2) [7], from which a coercivity of around 32 kOe is expected for D=200 nm
if good magnetic isolation is kept. However, as shown in Fig. 2, the coercivyt
of sintered magnets start to decrease below the critical size which is
10 times larger than the sigle domain size. Hence, the low coercivity of
the ultrafine grain structure of the HDDR powder suggests that a magnetic
isolation of the hard phase is not sufficient and the coercivity mechanism
is not the nucleation but pining.
Therefor the first goal of this project is to achieve much higer coercivity
in anisotropic HDDR powder, then to make composites with nanocrystalline
Fe, thereby a permanent magnets with improved (BH)max and low rare earth
contents may be achieved.
Figure 2 Grain size dependence of coercivity of various Nd-Fe-B based permanent
magnets.
Related Publications
Anisotropic Nd-Fe-B nanocrystalline magnets processed by
spark plasma sintering and in-situ hot pressing of HDDR powder
R Gopalan, H.
Sepehri-Amin, K. Suresh, T.Ohkubo, K.Hono, T. Nishiuchi, N. Nozawa and S.
Hirosawa, Scripta
Mater. (2009), in press.
Consolidation of
hydrogenation-disproportionation-desorption-recombination processed Nd-Fe-B
magnets by spark plasma sintering
K. Suresh, T. Ohkubo, Y. K. Takahashi, K.
Oh-ishi, R. Gopalan,K. Hono, T. Nishiuchi, N. Nozawa, and S. Hirosawa, J. Mag. Mag. Mater., (2009)
in press.
The role of grain boundaries in the coercivity of
hydrogenation disproportionation desorption recombination processed Nd-Fe-B
powder
W. F. Li, T. Ohkubo, K. Hono, T. Nishiuchi, and S. Hirosawa, J. Appl. Phys. 105, 07A706
(2009).
Purpose and objectives of "Project for
high performatnce anisotropic nanocomposite permanent magnets with low
rare-earth content" (in Japanese)
S. Hirosawa, T. Nishiuchi, T. Ohkubo, W. F.
Li, K. Hono, J. Yamazaki, M. Takezawa, K. Sumiyama, and S. Yamamuro, J. Jpn. Inst.
Metals, 73, 135 - 140 (2009).
Nanostructure and properties of permanent
magnet materials (in Japanese)
K. Hono, Magnetics Jpn, 4, 136 - 142
(2009).
Coercivity
mechanism of hydrogenation disproportionation desorption recombination processed
Nd-Fe-B based magnets
W. F. Li, T. Ohkubo, K. Hono, T. Nishiuchi, and S.
Hirosawa,Appl. Phys. Lett.
93, 052505 (2008).
Copyright (2008), American Institute of
Physics