Metal treatment – Stock – Magnetic
148101, 148104, 419 12, 419 19, 419 20, 75233, 75244, H01F 1053
Sinter magnets of the Fe-Nd-B type are characterised at room temperature by especially high magnetic characteristic values: Their temperature stability--mainly the coercive field strength H.sub.CJ --is, however, unsatisfactory and prevents the use of the magnets in temperature-stressed machines.
Therefore, for technical uses, it is necessary to improve the magnets to such an extent that their use up to 200.degree. C. is possible in the case of strong counterfields. In order to achieve this, especially the coercive field strength of the magnets must be further improved and the temperature dependency of the coercive field strength are reduced in order to ensure still sufficient values at comparatively high temperatures.
One has already attempted to achieve this improvement by additions of further elements to the Fe-Nd-B alloy. Thus, with additions of Dy, Tb, Al and Nb, a clear improvement of the H.sub.CJ could be achieved.
Dy and Tb, as expensive, heavy rare earth metals, influence the crystal anisotropy of the Fe.sub.14 Nd.sub.2 B phase and thus also the coercive field strength in favourable way.
From M. H. Ghandehari, Appl. Phys. Lett., 48 (8) 1986, pp. 548-550, it is known that by reaction sintering of Fe.sub.77 Nd.sub.15 B.sub.8 with (in comparison with the pure elements the cheaper) oxides Dy.sub.2 O.sub.3 and Tb.sub.4 O.sub.7, the increase of H.sub.CJ achieved by addition of the corresponding amounts of the pure elements Dy and Tb is reduced. This allows an impairment of the positive action of the Dy or Tb addition by oxygen addition to be recognised.
Nb addition brings about separations in the Fe.sub.14 Nd.sub.2 B grains which are to act as obstacles in the case of the domain wall movement. The cause of the influence of Al on H.sub.CJ has not yet been fully elucidated.
From the U.S. Pat. No. 4,588,439, it is further known that the resistance of rare earth metal-containing permanent magnets against corrosion is improved when the pre-alloy is ground in oxygen-containing atmosphere. Nothing is hereby reported regarding an improvement of the coercive field strength.
Therefore, the task forming the basis of the invention is to improve the coercive field strength in the case of sinter magnets of the type Fe-Nd-B and to reduce the temperature dependency thereof without having to add heavy rare earth metals, such as Dy and Tb.
According to the invention, this task is solved by a sinter magnet based on Fe-Nd-B which is characterised in that it consists of 25 to 50 wt. % Nd, 0.5 to 2 wt. % B, 0 to 5 wt. % Al, 0.5 to 3 wt. % O, remainder Fe and usual impurities and the oxygen content is adjusted by the addition of at least one Al and/or Nd oxide before the dense sintering.
Surprisingly, it has been shown that by introduction of oxygen in the form of Al and/or Nd oxide, there can be achieved not only a considerable increase of the coercive field strength but also a clear improvement of the temperature dependency of these properties.
Composition, production and properties of the sinter magnets according to the invention are described in more detail in the following in conjunction with the drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 a graphic illustration of the relationship between H.sub.CJ and the Al oxide content for 4 different Fe:Nd ratios;
FIG. 2 a comparison of the H.sub.CJ values for a base alloy in dependence upon the addition as Al.sub.2 O.sub.3 and as Al;
FIG. 3 the temperature dependency of H.sub.CJ of a sinter magnet according to the invention with Al.sub.2 O.sub.3 addition;
FIG. 4 a graphic illustration corresponding to FIG. 1 for a base alloy and Nd.sub.2 O.sub.3 addition.
Sinter magnets based on Fe-Nd-B normally already contain small amounts of oxygen as impurity, depending upon the production process. Thus, the oxygen content of the Fe-Nd-B pre-alloys normally produced as intermediate products for the production of sinter magnets usually amounts to about 0.02 wt. %. By the grinding of the pre-alloys, a further increase of the oxygen content can be obtained if this is not carefully excluded
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Ghandehari, "Reactivity Dy.sub.2 O.sub.3 and Tb.sub.4 O.sub.7 with Nd.sub.15 Fe.sub.77 B.sub.8 Powder and the Coercivity of the Sintered Magnets", Appl. Phys. Lett., 48(8), Feb. 24, 1986, pp. 548-550.
Ghandehari, "Microstructural Evidence for the Magnetic Surface Hardening of Dy.sub.2 O.sub.3 -Doped Nd.sub.15 Fe.sub.77 B.sub.8 Magnets", Materials Letter, Jul., 1987, vol. 5, pp. 285-287.
501 Max-Planck-Gesellschaft zur Forderung der Wissenschaften E.V
502 Robert Bosch GmbH
Sheehan John P.
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