Development of Dy-free high coercivity Nd-Fe-B permanent magnets

Nd-Fe-B sintered magnets exhibit the highest maximum energy product of all permanent magnets (55 MGOe). However, their relatively low coercivity of Hc~12 kOe puts a limit on certain applications such as traction motors of (hybrid) electric vehicles and wind power generators, for which a higher coercivity of 30 kOe is required. This level of coercivity is currently achieved by the partial substitution of Dy for Nd, with a typical composition of (Nd0.7Dy0.3)14Fe80B6. However, because of the limited natural resources of Dy that are exclusively produced only in China, the sustainable supply of Dy for the Nd-Fe-B permanet magnets for HV and EV applications is becoming difficult. This spurred worldwide intensive researches on the development of high performance permanet magnets that can substitute currely used (Nd,Dy)-Fe-B magnets.

Some even claim that rare earth free permanent magnets should be dveleped, but considering the required combination of the high magnetization (>1.5 T) and anisotropy field (20 kOe), it is very unlikely that any ferromagnetic compounds without rare earth elements can surpass the performance of Nd-Fe-B magnets even in the future. It should be noted that the natural resources of Nd is one of the most aboundant among rare earth elements, and its supply is expected to be stabilized after the rare earth panic cause by the recent export control of rare earth element by China is ceased. Thus, it would be more sensible to keep using Nd for developing high performance magnets for HV and EVs. However, the reduction of the usage of Dy from high coercivity magnets is essential. Hence, the objective of this research is to enhance the coercivity of Nd-Fe-B magnets without using Dy while keeping the (BH)max to the current level of 30MGOe.

Why do we need high coercivity magnets for HV and EV 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, a high coercivity of at least 25 kOe is needed. The current Nd-Fe-B permanent magnets for traction motors of HEVs contains 10% of Dy in the (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 3 Multiscale characterization of Nd-Fe-B permanet mangets.

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.


Reference
See "JRC Report on Critical Metals in Strategic Energy Technologies" for the assessment of demand and supply forcast of strategic materials for green technologies.

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).