EP0237416A1

EP0237416A1 - A rare earth-based permanent magnet

Info

Publication number: EP0237416A1
Application number: EP19870400473
Authority: EP
Inventors: Ken Ohashi
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1986-03-06
Filing date: 1987-03-04
Publication date: 1987-09-16
Also published as: DE3760962D1; JPS62206802A; EP0237416B1; JPH07105289B2

Abstract

A power-metallurgically sintered rare earth-based permanent magnet, e.g. neodymium-iron-boron magnet, can be imparted with a great increased coercive force with little decrease in the residual magnetization when the powder to be subjected to sintering is a mixture of a powder of the first alloy of neodymium, iron and boron with a minor amount of a powder of the second alloy composed of a heavy rare earth element, e.g. dysprosium, and an alloying element selected from the group consisting of aluminum, niobium, zirconium, vanadium, tantalum and molybdenum in a limited proportion.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a rare earth-based permanent magnet or, more particularly, to a rare earth-based permanent magnet prepared by the powder metallurgical method.
Among the various types of rare earth-based permanent magnets, those based on neodymium, iron and boron, referred to as the Nd-Fe-B magnets hereinbelow, are prominently highlighted in recent years by virtue of their outstandingly high magnetic properties in comparison with the rare earth-based permanent magnets of other types. These Nd-Fe-B magnets are expected to be a material used in large quantities, especially, in electric motors for which more and more powerful permanent magnets are required from the standpoint of energy saving or when more compact but more powerful motors are desired.
One of the problems in these Nd-Fe-B permanent magnets, however, is the relatively large temperature dependency of the magnetic properties thereof so that the Nd-Fe-B magnets at an elevated temperature cannot exhibit the magnetic properties as high as at low or room temperature. For example, it is not rare that the temperature of an electric motor under continuous running is increased to reach 100 to 150 °C so that the Nd-Fe-B magnets built in the motor can no longer exhibit the desired magnetic properties. In this regard, it is an important technical problem to develop a rare earth-based permanent magnet which can retain the high magnetic properties even at elevated temperatures.
To give a more detailed explanation of this problem, it is known that the Nd-Fe-B magnets have coefficients of temperature dependency of about -0.13%/°C and -0.6%/°C for the residual magnetization B_r and coercive force iH_c, respectively. Namely, the temperature dependency of the coercive force is considerably larger than that of the residual magnetization and should desirably be much smaller. A coefficient of temperature dependency of - 0.6%/°C means that the coercive force of the magnet at 100 °C is only about a half of the value at room temperature. This large temperature dependency is the reason for the relatively low upper limit of 50 to 70 °C at the highest of the temperature range in which the Nd-Fe-B magnets can be practically used.
Various attempts and proposals have been made hitherto in order to improve the large temperature dependency of the magnetic properties or, in particular, coercive force of the Nd-Fe-B magnets. Two types of means have been proposed for the improvement of the Nd-Fe-B magnets. For example, as is taught in Japanese Patent Kokai 59-89401 and 60-32306 and practiced with some success, the temperature dependency in the coercive force of the magnets can be improved by the admixture of the magnet alloy with a so-called heavy rare earth element such as dysprosium and terbium, light metal element such as aluminum or transition met al element such as niobium and vanadium. Alternatively, the magnetic properties of the Nd-Fe-B magnets could be improved when the metallographic structure of the magnet alloy be converted to the precipitation-hardening type although no successful results have yet been obtained by this means.
The principle of the former method by the additional alloying elements is to impart the Nd-Fe-B magnet with a further increased coercive force so that the magnet can retain a value of the coercive force still in an acceptable range even at an elevated temperature to cause decrease in the coercive force. This improvement in the coercive force is naturally obtained at the sacrifice of the residual magnetization B_r. Namely, aluminum, niobium and vanadium as an additional alloying element are each non-magnetic so that the addition thereof to the magnet alloy is necessarily accompanied by the decrease in the residual magnetization in proportion to the added amount of these elements or even larger. The heavy rare earth elements, such as dysprosium and terbium, have a magnetic moment aligned anti-parallel to that of the transition metal elements so that the decrease in the residual magnetization of the Nd-Fe-B magnets by the addition thereof is even larger than by the addition of aluminum, niobium or vanadium. As a consequence, the neodymium-based permanent magnets of the high coercive force type in the prior art unavoidably have a greatly decreased residual magnetization in comparison with conventional Nd-Fe-B magnets.
As is taught in Journal of Applied Physics, volume 55, page 2083 (1984), the coercive force of the neodymium-based magnets is exhibited by the mechanism of nucleation. A presumption of the reason therefor is that, while the extremely smooth and clean grain boundaries of the crystallites inhibit incipience of reverse magnetic domains even under impression of a magnetic field in a direction reverse to that of the magnetization, the magnetic domain walls are strongly constrained to the narrow region in the vicinity of the grain boundaries.
According to the disclosure by Hiraga, et al. in Japanese Journal of Applied Physics, volume 24, L30 (1985), an electron microscopic examination of a neodymium-based permanent magnet revealed that the magnet had a structure in which the crystallite grains of Nd₂Fe_I4B were, so to say, enveloped by a magnetically soft, thin b.c.c. phase and the interface therebetween is in a very clean condition without distortion. This fact suggests that the large coercive force of the magnet is produced as a result of the fact that the magnetic domain walls are constrained to the outermost layer formed of the magnetically soft b.c.c. phase. Accordingly, it is presumable that the coercive force of the neodymium-based magnet is increased by the addition of the above mentioned additive elements such as the heavy rare earth elements, aluminum, niobium and the like because these additive elements have an effect to increase the anisotropic magnetic field of the N d2Fe14B compound and to influence on the morphology at the proximity of the grain boundaries of the crystallites.
The above described information and consideration have provided the inventors with a guide principle that the key factor for the improvement in the coercive force of a neodymium-based permanent magnet is to control the condition on and around the grain boundaries alone of the crystallites leading to the completion of the present invention.

SUMMARY OF THE INVENTION

Thus, the present invention provides a rare earth-based permanent magnet which is a sintered body of a powdery mixture comprising or, rather, essentially composed of:

(a) from 90 to 99.9 parts by weight of a first alloy containing from 25 to 35% by weight of a light rare earth element selected from the group consisting of the elements of the atomic number from 57 to 62, of which at least a half by weight is neodymium, praseodymium or a combination thereof, and from 0.7 to 1.5% by weight of boron, the balance being iron, cobalt or a combination of iron and cobalt; and
(b) from 10 to 0.1 part by weight of a second alloy containing from 30 to 86% by weight of a heavy rare earth element selected from the group consisting of the elements of the atomic number from 64 to 71, of which at least a half by weight is terbium, dysprosium, holmium or a combination thereof, the balance being an element selected from the group consisting of aluminum, niobium, zirconium, vanadium, tantalum and molybdenum.

The powdery mixture should preferably have a particle size distribution in the range from 2 to 8 pm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is understood from the above given summary of the invention, the rare earth-based permanent magnet of the invention is a magnet prepared by the powder metallurgical process from a magnetic alloy powder which is characteristically a mixture of two kinds of alloys defined above. Different from the conventional method in which the additive elements contributing to the increase of the coercive force are uniformly admixed beforehand with the principal magnet alloy of a light rare earth element, e.g. neodymium, iron and boron, the invention proposes that the alloying elements are divided into two groups which are separately converted into the first alloy for the principal magnetic constituent and the second alloy for the additive elements and these two alloys are concurrently pulverized or separately pulverized followed by mixing of the powders together to give a powdery mixture to be subjected to shaping and sintering.
In the conventional method in which all of the alloying elements are melted together into a single alloy for the powder metal-(urgical process, the coercive force of the resultant permanent magnet cannot be sufficiently improved unless the amounts of the effective additive elements are considerably large to affect the residual magnetization in order to have a sufficient influence on or in the proximity of the grain boundaries of the crystallites. In contrast thereto, the additive elements in the invention form the second alloy separately from the first alloy for the matrix phase of the magnet and the powdery mixture for the powder metallurgical process is formed of the particles of these two types of alloys. In the sintering procedure, accordingly, the additive elements diffuse into the particles of the matrix phase from the surface of the particles but never reach the core portions of the particles. Therefore, the concentration of the additive elements is inhomogeneous in the inventive magnet as sintered. Namely, the concentration is high only at the surface of the matrix particles while the additive elements are substantially absent in the core portion of the matrix particles exhibiting a great influence on the anisotropic magnetic field and morphology at or in the vicinity of the grain boundaries even when the overall amount of the additive elements is so low that the residual magnetization of the magnet is little affected and consequently the magnet has a high maximum energy product (BH)_max. The above mentioned inhomogeneous distribution of the additive elements could be confirmed by the X-ray microprobe analysis.
The first alloy, which is pulverized and mixed with a powder of the second alloy, is a ternary alloy composed of a light rare earth element, iron and/or cobalt and boron. The light rare earth element here implied as the first component of the first alloy includes the rare earth elements having an atomic number of 57 to 62, i.e. lanthanum to samarium, but it is preferably neodymium or praseodymium although combinations of these two elements without or with a minor amount of the other light rare earth elements can be used equally. In particular, at least 50% by weight of the light rare earth component should be neodymium, praseodymium or a combination of the two. Neodymium is preferred. The amount of the light rare earth element or elements in the first alloy should be in the range from 25 to 35% by weight.
The second component in the first alloy is boron, of which the content in the first alloy should be in the range from 0.7 to 1.5% by weight. The balance of the above mentioned light rare earth elements and boron in the first alloy is iron, cobalt or a combination thereof although iron is preferred mainly for the economical reason while replacement of a part of iron with cobalt has an effect of increasing the Curie point of the magnet contributing to the improvement of the reversible temperature coefficient. The amount of this third component, i.e. iron and/or cobalt, in the first alloy should accordingly be in the range from 63.5 to 74.3% by weight including unavoidable impurity elements, the amount of which should be as small as possible.
The second alloy, which is pulverized and mixed with the powder of the first alloy, is a binary alloy composed of a heavy rare earth element and an alloying element selected from the group consisting of aluminum, niobium, zirconium, vanadium, tantalum and molybdenum. The heavy rare earth element here implied is an element having an atomic number of 64 to 71, i.e. gadolinium to lutetium, and terbium, dysprosium and holmium are preferred, of which dysprosium is more preferable. These heavy rare earth elements are preferred to the light rare earth elements, e.g. neodymium, for the reason that these heavy rare earth elements give a R₂Fe₁₄B type compound (R: a rare earth element) having a larger anisotropic magnetic field than Nd₂Fe₁₄B so that the improvement on the coercive force of the magnet could be obtained with addition of a smaller amount thereof. The above mentioned six kinds of alloying elements can exhibit an effect of increasing the coercive force of the magnet even in an unalloyed condition while alloying thereof with a heavy rare earth element may have a synergistic effect. It is noteworthy that the alloy is more resistant against oxidation than the heavy rare earth element alone. The amount of the heavy rare earth element or elements in the second alloy should be in the range from 30 to 86% by weight, the balance, i.e. from 70 to 14% by weight, being one or a combination of the above mentioned alloying elements including unavoidable impurity elements, the amount of which should be as small as possible. When the amount of the heavy rare earth element or elements is too small, the alloy can be pulverized with great difficulties due to the increased tenacity of the alloy. When the amount of the heavy rare earth element is too large, on the other hand, the alloy would be more susceptible to oxidation.
Among the many possible combinations of the heavy rare earth elements and the six kinds of the alloying elements, the most preferred is an alloy of dysprosium and aluminum, which should have a composition of DyAI₂ in the so-called Laves phase. This is because the Laves phase of the DyAl_z alloy is brittle and can be easily pulverized and the powder thereof is little susceptible to oxidation in addition to the relatively large effect on the magnet properties by the addition thereof.
The elementary materials forming the first or the second alloy should be melted together to prepare the first and second alloys separately. The method for the preparation of the alloy can be conventional without particular limitations. The two alloys may be separately pulverized into powders which are weighed and mixed together subsequently. It is, however, a convenient way that each of the alloys in the form of an ingot is crushed into coarse granules having a particle size distribution of, for example, 10 to 500 µm which should be mixed with the granules of the other alloy in a calculated proportion followed by concurrent fine pulverization so that the pulverization and mixing can be performed in one step. The fine powder of the two alloys should have a particle size
distribution in the range from 1 to 10 µm or, preferably, from 2 to 8 pm.
The thus prepared mixed powder should be composed of from 90 to 99.9 parts by weight of the first alloy and from 10 to O.lpart by weight of the second alloy. When the amount of the second alloy is smaller than 0.1 part by weight in 100 parts by weight of the mixed powder, no sufficient improvement can be obtained in the coercive force of the resultant sintered magnet. When the amount of the second alloy is too large, on the other hand, the residual magnetization of the sintered magnet would be unduly decreased.
In the following, the sintered rare earth-based permanent magnet of the invention is described in more detail by way of examples. The values of percentage appearing below are all in % by weight.

Example 1.

A first alloy ingot was prepared by melting together, in a high frequency induction furnace under an inert atmosphere, metallic neodymium having a purity of 99.4%, iron having a purity of 99.5% and boron having a purity of 99.5% in such a proportion that the alloy was composed of 34.0% of neodymium, 64.9% of iron and 1.1% of boron.
Separately, a second alloy ingot was prepared from metallic dysprosium having a purity of 99.4% and aluminum having a purity of 99.9% in a weight proportion of 75.1% dysprosium and 24.9% aluminum.
Each of the alloy ingots was crushed in a disc mill separately from the other into granules having a fineness to pass a screen of 20 meshes by the Tyler standard. The granules of the first alloy were admixed with the granules of the second alloy in four different weight proportions as indicated in Table 1 below and each of the mixtures as well as the granules of the first alloy alone for comparative purpose was finely pulverized in a jet mill using nitrogen as the ject gas into a powder having an average particle diameter of 3.0 µm.
The powder was molded into a shaped body in a magnetic field of 10 kOe under a compressive pressure of 1.5 tonsfcm² into a green body which was subjected to sintering at 1050 °C for 1 hour in an atmosphere of argon followed by aging at 550 °C for 1 hour and then quenching with a cold inert gas. Table 1 below shows the residual magnetization B_r and coercive force iHc of the thus prepared sintered magnets. It is understood from these results that the addition of the second alloy to the first alloy was very effective in increasing the coercive force of the magnets with little adverse influence on the residual magnetization of the magnets.

Example 2.

The experimental procedure in each of the experiments (Experiments No. 1 to No. 5) was substantially the same as in Example 1 excepting modifications in the compositions of the first and second alloys and the mixing ratio thereof. Namely, the first alloy was composed of of 31% neodymium, 68% iron and 1% boron as prepared using the same materials as used in Example 1. The second alloy was one of the four alloys having compositions of:

(1) 46% dysprosium and 54% niobium (Experiment No. 1);
(2) 61% dysprosium and 39% vanadium (Experiment No. 2);
(3) 45% dysprosium and 55% molybdenum (Experiment No. 3); and
(4) 74.7% terbium and 25.3% aluminum (Experiment No. 4), each prepared from materials having a purity of 99.9%. The mixing ratio of the first and second alloys was 99:1 by weight in each of Experiments No. 1 to No. 4. For comparison in Experiment No. 5, the first alloy alone was processed into a sintered magnet in the same manner by omitting the second alloy. Table 2 below shows the residual magnetization B_r and coercive force _iH_c of the thus obtained sintered magnets. It is clear from these results that the addition of the second alloy is very effective in increasing the coercive force with little decrease in the residual magnetization.

Claims

1. A rare earth-based permanent magnet which is a sintered body of a powdery mixture comprising:

(a) from 90 to 99,9 parts by weight of a first alloy containing from 25 to 35% by weight of a light rare earth element and from 0.7 to 1.5% by weight of boron, the balance being iron, cobalt or a combination of iron and cobalt;

and

(b) from 10 to 0.1 part by weight of a second alloy containing from 30 to 86% by weight of a heavy rare earth element, the balance being an element selected from the group consisting of aluminum, niobium, zirconium, vanadium, tantalum and molybdenum.

2. The rare earth-based permanent magnet as claimed in claim 1 wherein the light rare earth element is selected from the group consisting of lanthanum, cerium, praseodymium, neodymium and samarium.

3. The rare earth-based permanent magnet as claimed in claim 1 wherein the heavy rare earth element is selected from the group consisting of gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

4. The rare earth-based permanent magnet as claimed in claim 1 wherein at least 50% by weight of the light rare earth element in the first alloy is neodymium, praseodymium or a combination thereof.

5. The rare earth-based permanent magnet as claimed in claim 1 wherein at least 50% by weight of the heavy rare earth element in the second alloy is terbium, dysprosium, holmium or a combination thereof.

6. The rare earth-based permanent magnet as claimed in claim 1 wherein the second alloy is an alloy of dysprosium and aluminum.

7. The rare earth-based permanent magnet as claimed in claim 1 wherein the powdery mixture has a particle size distribution in the range from 2 to 8 pm.