Journal of Magnetism and Magnetic Materials 101 (1991)375-376 North-Holland
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Diminution of permanent magnet properties of polymer bonded N d 4 F e 77 B 19 magnets K.-H. Miiller, D. E c k e r t a n d A. H a n d s t e i n Institut fiir Festk6rperphysik und Festk6rperchemie im Z F W Dresden, Helmholtzstr. 20, 0-8027 Dresden, Germany
The magnetic properties of polymer bonded nd4Fe77B19 magnets are diminished in comparison with those of annealed melt-spun flakes of this composition. This deterioration is mainly caused by a simple dilution and by internal demagnetization fields due to the polymer bonding. No diminution is found for the coercivity.
1. Introduction A two phase permanent magnet material made from Nd-poor alloys with a composition near Nd4Fe77B19 was obtained by Coehoorn et al. [1,2]. After melt spinning and subsequent annealing a metastable Fe3B phase and small precipitations of NdzFe14B were found (see e.g. refs. [3,4]). The material shows an isotropic magnetic behaviour. Optimally prepared ribbons or flakes should be candidates for a cheap permanent magnet material. In this paper we compare the magnetic properties of melt-spun and annealed flakes with polymer bonded magnets made from a Nd4Fe77BI9 alloy.
2. Experimental Amorphous flakes of an Nd-poor NdFeB alloy of the nominal composition Nd4Fe78B19 were prepared by rapid quenching using a single roller technique in air analogously to ref. [3]. The wheel speed was 20 m / s at surface. The annealing of as-quenched samples was performed in a vacuum chamber at temperatures from TA = 580 ° C up to 700 o C. The flakes were coarsely ground by a mortar. Different powder fractions obtained by sieving were polymer bonded by Araldit AT1. The samples were compacted by a die-press and hardened at 160 ° C for 2 h. The samples were cylinders with a diameter of 3.6 mm and a length of about 3 mm. Their density was about 5.4 g / c m 3.
ity /~0jHc = 0.35 T and energy density (BH)max = 100 k J / m 3. Initial magnetization curves of the flakes and of bonded magnets are shown in fig. 2. It should be noted that B r is more reduced in the bonded material than expected from a simple dilution law. The additional reduction is caused by internal magnetostatic fields which can nearly be taken into account by an internal demagnetization factor
v)/3,
Dint = (1 -
where v is the volume fraction of Nd4Fe77B w in the bonded magnets, and v = 0.7 was estimated from the measured values of saturation polarization and density, respectively. For comparison we prepared bonded magnets from particles with different size fractions: i) coarsely ground flakes; ii) > 160 Ixm and iii) < 160 I~m (both sieved from i)). As in the case of bonded MQ1 powders [5], larger-particle fractions yield weakly increased values of B r not drawn in fig. 2.
311] 4.5
.. ii. j
..11 .'If .'1I"
3. Results and discussion Fig. 1 shows the dependence of the demagnetization curves on annealing time for the original flakes. Due to precipitation of soft magnetic phases the dip in the second-quadrant demagnetization curves increases with increasing annealing time. For preparing of polymer bonded magnets we used only material annealed at 630 o C for 30 min. Typical values of magnetic characteristics of the NdaFe77B19 flakes are saturation polarization Js = 1.6 T, remanence Br = 1.2 T, coerciv-
(1)
;'.i 0.3
~0 min ..... 30 rain --- 60 rain --- 420rain -
-
i !
-0.5
01s ,°.th
Fig. 1. Influence of annealing time on the demagnetization curves of melt-spun NdaFe77B19 flakes (TA= 630 ° C).
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
376
K.-H. Miiller et al. / Bonded N d 4Fe77B19 magnets
]IT] ,1.5:
flakes
3 [T]
/ /
ftok~s
f
1.0,
bonded
bonded
/
/•l
/ / /
J Nd+FenB++
/
Nd+F++,+,B++
/
"0.5
0.5
0'5
po H
+b
~oH [T] Fig. 3. Influence of water corrosion on bonded Nd4Fe77B19 magnets.
Fig. 2. Demagnetization curves and remagnetization curves starting from the thermally demagnetized state (th) and from the dc-field demagnetized state (dc) for NdaFe77B19 flakes and for bonded magnets made from them.
It is easier to remagnetize dc-field demagnetized samples than thermally demagnetized ones, because the two interacting ferromagnetic phases in Nd4Fe77 B 19 material differ considerably in their values of anisotropy field [6]. The difference between initial susceptibility Xf = 2.3 of thermally demagnetized flakes and Xb = 1.5 of thermally demagnetized bonded samples is caused by internal demagnetization fields. In an improved mean-field approximation we find vXf( 1 + x f D i i )
1
Xb = 1 - vxfDii(1 + xfDii) -l
'
(2)
where the bar indicates averaging with respect to orientation of the axes of local demagnetization factors Dzi with diagonal elements Dla = 1, D22 = D33 = 0. The coercivity of the bonded samples agrees with that of nonbonded flakes. Contrary to this result, other authors obtained a remarkable diminution of j H c for bonded magnets prepared from Nd-rich melt-spun or sintered materials [7-9] and from Nd-poor materials similar to our Nd4Fe77B19 flakes [10]. W e found such a behaviour only in the case of bonded Nd4Fe77B19 magnets of larger dimensions, which were cut into smaller samples by a water-cooled diamond saw. As shown in fig. 3, the magnetic properties are now
stronger diminished than in the case of fig. 2. Obviously, the water corrosion produces additional phases reducing the saturation polarization and the critical fields for nucleation of internal demagnetization processes. References [1] R. Coehoorn, D.B. de Mooij, J.P. Duchateau and K.H.J. Buschow, J. de Phys. 49 (1988) C8-669. [2] R. Coehoorn and C. de Waard, J. Magn. Magn. Mater. 83 (1990) 228. [3] J. Schneider, D. Eckert, K.-H. Miiller, A. Handstein, H. Miihlbach, H. Sassik and H.R. Kirchmayr, Mater. Lett. 9 (1990) 201. [4] D. Eckert, K.-H. Miiller, A. Handstein, J. Schneider, R. Gr6ssinger and R. Krewenka, IEEE Trans. Magn. MAG26 (1990) 1834. [5] A. Handstein, K.-H. Miiller, R. Gr6ssinger, H.R. Kirchmayr and R. Krewenka, J. Magn. Magn. Mater. 101 (1991) paper 429K. [6] K.-H. Mfiller, J. Schneider, A. Handstein, D. Eckert, P. Nothnagel and H.R. Kirchmayr, Mater. Sci. Eng. A 133 (1991) 151.. [7] J. Yamasaki, H. Soeda, M. Yanagida, K. Mohri, N. Teshima, O. Kohmoto, T. Yoneyama and N. Yamaguchi, IEEE Trans. Magn. MAG-22 (1986) 763. [8] H. Yamamoto, M. Nagakura, T. Katsuno and T. Yamamoto, IEEE Trans. Magn. MAG-26 (1990) 2595. [9] H.H. Stadelmaier and N.C. Liu, Mater. Lett. 4 (1986) 304. [10] M.G. Gr6nefeld, Thesis, Universit~it Stuttgart (1990).