Small angle neutron scattering study of interface nanostructure in sintered Nd-Fe-B magnets processed under high magnetic fields

Takahiro Akiya 1Hiroaki Kato 2Masayasu Takeda 3Jun-ichi Suzuki 3Daisuke Yamaguchi 4Satoshi Koizumi 4Masato Sagawa 5Keiichi Koyama 6

1. Department of Applied Physics, Tohoku University, Aoba-yama 6-6-05, Sendai 980-8579, Japan
2. Yamagata University, Graduate school of science and engineering, 4-3-16, Jounan, Yonezawa 992-8510, Japan
3. Japan Atomic Energy Agency, Quantum Beam Science Directorate (JAEA), 2-4 Sirane Sirakata Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
4. Japan Atomic Energy Agency, Advances Science Research Center (ASRC), 2-4 Sirane Sirakawa Tokai-mura, Naka-gun, Ibaraki, Ibaraki 311-195, Japan
5. Intermetallics Co., Ltd, Kyoto 615-8206, Japan
6. Institute for Materials Research, Tohoku University (IMR), Sendai 980-8577, Japan

Abstract
Sintered Nd-Fe-B magnets are the most promising materials for a driving motor of the hybrid electric vehicle (HV). In this application, a large value of coercivity Hc is required, because the operating temperature of magnets reaches beyond 450 K. As a symptomatic treatment, Nd has been replaced partly by Dy to enhance Hc. But the problem is an estimated short supply of Dy in the HV mass-production stage due to its low natural abundance. We have recently shown[1] that Hc of Nd-Fe-B magnets can be enhanced by applying a high magnetic field during the annealing. In order to investigate the reason for the Hc increase, we started a small angle neutron scattering (SANS) study. SANS technique has an advantage over microscopic methods. That is, we can non-destructively obtain a quantitative information averaged over a bulk sample. In the present study, two kinds of Nd-Fe-B samples with and without Cu and Al additives were prepared. The 99% enriched 11B isotope was used to avoid high neutron absorption of 10B in natural boron. SANS experiments were performed with SANS-J and PNO apparatus equipped at JAEA in which the incident wave length of neutrons were 0.65 and 0.2 nm, respectively. In the samples without Al and Cu additives, the intensity of scattered neutrons was proportional to the q-4 where q is the scattering vector. When the sample contains Al and Cu additives, however, we observed a marked deviation from the simple relation above for q > 0.3 nm-1. We have reproduced these experiments by a calculation, assuming a structure such that a core of Nd2Fe14B phase is surrounded by a Cu-rich shell with a thickness of 4-8 nm. We already reported[1] that the DSC measurements suggest an existence of Nd-Cu and Al-Cu eutectic phases at the grain boundary. The DSC and SANS results thus suggest that an existence of the Cu-containing liquid phase is strongly correlated with the Hc enhancement by the high field annealing.

[1] H. Kato et al., J. Magn. Magn. Mater. 310, 2596 (2007).

 

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Presentation: Oral at E-MRS Fall Meeting 2007, Symposium J, by Takahiro Akiya
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-21 11:13
Revised:   2009-06-07 00:44