Effect of high-magnetic-field processing on the coercivity of sintered Nd-Fe-B magnets

Hiroaki Kato 2Takahiro Akiya 2Masato Sagawa 3Keiichi Koyama 1Terunobu Miyazaki 2

1. Institute for Materials Research, Tohoku University (IMR), Sendai 980-8577, Japan
2. Department of Applied Physics, Tohoku University, Aoba-yama 6-6-05, Sendai 980-8579, Japan
3. Intermetallics Co., Ltd, Kyoto 615-8206, Japan

Abstract

Sintered Nd-Fe-B magnets are the most promising material for a driving motor of the electric vehicle (EV). In this kind of application, very large value of room-temperature coercivity Hc is required owing to the high-temperature circumstance of the EV-motor. Current Nd-Fe-B magnets commercially available for such high Hc applications, therefore, contains huge amount (~10 wt%) of Dy to enhance Hc. But, this method has a crucial disadvantage. That is, the magnetization must be sacrificed owing to the antiparallel coupling of Dy and Fe moments, resulting in smaller energy product values.

It is well known that a heat treatment around 600 C is indispensable to obtain a high coercivity in the Nd-Fe-B sintered magnets. We have been investigating an effect of high magnetic field in this annealing process on the magnetic properties of Nd-Fe-B sintered magnets. We already reported that the 37% increase of Hc occurs, when the sample containing 3.1 wt% Dy and 0.1 wt% Al was annealed at Ta = 550 C under a magnetic field of Ha = 140 kOe [1]. Here, we report on the high-magnetic-field processing work for a series of Dy-free Nd-Fe-B magnets.

The annealing treatments were carried out for samples with the dimensions 5x5x5 mm3 by using a furnace installed in a cryocooled superconducting magnets which generate magnetic fields of up to 140 kOe. Six different values of Ta were selected between 500 and 625 C. In the sample containing small amount of Al and Cu, highest Hc value of 14.7 kOe was obtained for Ta = 550 C and Ha=140 kOe. This value of Hc is 16% larger than that of the control sample (Ta=550 C and Ha=0). These results were discussed in terms of a field-induced change in the interface matching between the main Nd2Fe14B and intergranular Nd-rich phases.

[1] H. Kato, M. Sagawa, K. Koyama and T. Miyazaki, Appl. Phys. Lett. 84 (2004) 4230.

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Presentation: oral at E-MRS Fall Meeting 2005, Symposium B, by Hiroaki Kato
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-20 09:08
Revised:   2009-06-07 00:44
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