IE921347A1

IE921347A1 - PROCESS FOR THE MANUFACTURE OF A NITROGEN-CONTAINING¹PERMANENT MAGNET, ESPECIALLY Sm-Fe-N

Info

Publication number: IE921347A1
Application number: IE921347A
Authority: IE
Inventors: Georg-Werner Reppel; Werner Rodewald
Original assignee: Vacuumschmelze Gmbh
Priority date: 1991-05-25
Filing date: 1992-04-27
Publication date: 1992-12-02
Also published as: EP0515854A2; EP0515854A3; DE4117104A1

Abstract

To produce bound, nitrogen-containing permanent magnets, it is proposed first to compact a pulverulent, substantially nitrogen-free master alloy to give a porous moulding and to carry out the nitriding on the already compacted moulding by reaction-annealing in a nitrogen-containing atmosphere. The process is suitable especially for permanent magnets of the SE-TM-N type, where SE designates at least one rare earth element and TM designates at least one transition element. Preferably, the permanent magnets have the composition Sm2-Fe17-Nx or Sm2-Fe17-(C,N)x, wherein the iron can be partially replaced by cobalt and/or nickel. Further alloy elements can also be present.

Description

The invention relates to a process for the manufacture of a nitrogen-containing permanent magnet.

From European laid-open Specification No. 369 097, nitrogen-containing permanent magnets are known which contain from 5 to 20 atoms % of at least one rare earth element, 5 to 30 atoms % nitrogen, 0.01 to 10 atoms % hydrogen, the remainder being iron and optionally 0.1 to 40 atoms % of further additive elements. For manufacturing a permanent magnet of this type, the previously nitrated magnet powders are, for example, first pressed and then, in a nitrogen and oxygen-containing atmosphere, subjected to a heat treatment which is there referred to as sintering. The temperature in the so-called sintering should be between 100 and 650°C. A temperature lower than 450°C is preferred, since then the magnetic material is sufficiently stable.

If the temperature is higher than 650°C, this leads to a rapid decomposition of the hard magnetic compound. For this reason it is not possible to increase the sintering temperature to above 650°C on account of the instability of the magnetic material. Accordingly the actual objects of sintering, namely an increase in hardness and/or in density compared with the compressed density of the moulded body is not achieved to a sufficient extent.

As a result of the limited sintering possibilities of the named alloys there arises as an alternative the manufacture of bound magnets from the magnet alloys.

This approach is also described in EP-OS 369097. This document starts, by way of example, from a pulverulent pre-alloy of composition Sm2-Fe17, which is subjected to a heat treatment in a nitrogen- and hydrogen-containing atmosphere for taking up these elements. The Sm-Fe-N-H alloy obtained is further comminuted in a nitrogen atmosphere and then mixed with an artificial resin binder, poured into a mould and afterwards cured. As an alternative the Sm-Fe-N-H magnet powder can also be compressed in a magnetic field and then impregnated.

Since the magnetic particles can no longer orient themselves unimpeded in a magnetic field, when the magnet powder is mixed with the binder, the above described process for the manufacture of bound permanent magnets from anisotropic magnet powder leads to a lowering of the residual magnetism. Furthermore the magnet particles can be damaged during the compressing process. It is known that this kind of damage of the magnet material can be made good by an annealing treatment of for example about 2 hrs at 600-1000°C in a vacuum, using the known SmCo5 permanent magnets. In the case of magnets bound with artificial resins, such treatment is not possible, since the annealing temperature has to be so high that the plastics material would be decomposed.

Furthermore, the publication of J.M.D. Coey and Hong Sun in Journal of Magnetism and Magnetic Materials, 87 (1990) pages L251 - L254 makes known nitrogen-containing SE-Fe-N permanent magnet alloys which contain carbon as a further alloy component. For producing the alloy powder - 4 there described, the nitrogen is introduced into SE2-Fe17 or SE2-Fe17-C alloys by heat treatment in a nitrogencontaining atmosphere. It is also mentioned there, that these compounds decompose at temperatures of higher than 550°C, and that at 850°C there is present a mixture of various decomposition products. For this reason these alloys are also suggested especially for the manufacture of bound permanent magnets.

Conventional process techniques for the manufacture of plastics-bound magnets consist of mixing or compounding the magnet powder with plastics before compacting the mixture. The powder particles are coated with a plastics melt or with a plastics substance dissolved in a solvent. In the latter case the solvent is removed by evacuation. Afterwards the compounded powder must be comminuted and sieved for better workability.

An object of the invention is, therefore, to provide a simplified and more economical process for the manufacture of bound nitrogen-containing permanent magnets, in which damage to the powder particles, caused for example by selective oxidation, is largely avoided. This object is achieved by a process for the manufacture of a nitrogen-containing permanent magnet by compression of a pulverulent, substantially nitrogen-free pre-alloy to a porous moulded body and subsequent nitration of the compressed moulded body by annealing reaction in a nitrogen-containing atmosphere. The process according to the invention further permits an unimpeded orientation of the magnetic particles.

According to the process of the invention a pre-alloy which is essentially free from nitrogen is first compressed to a porous moulded body. By a substantially - 5 nitrogen-free pre-alloy is to be understood a pre-alloy which contains not more than about 10% of the nitrogen content of the finished permanent magnet.

The compressed slug which is at first nitrogen-free, can be manufactured by pressing in a press tool, by isostatic compression, wire extrusion of powder in a capsule or similar compression techniques. The nitration does not take place until after the compression, by means of a heat treatment in a nitrogen-containing atmosphere. Preferably this treatment takes place in N2 or in a mixture of N2 and H2 or in an NH3 atmosphere.

As a result of the porosity, i.e. through the bound pore channels of the moulded body, a rapid diffusion of the nitrogen is guaranteed. For further acceleration of the annealing reaction, the nitration can take place, in particular, under a raised nitrogen pressure (more than 1 bar). Nitration takes place for preference at a temperature between 250°C and the decomposition temperature of the nitrogen-containing compound. The nitration is thus carried out in the process according to the invention not on the powder but only on the compressed slug. Surprisingly the compressed moulded body survives the uptake of nitrogen in spite of lattice expansion .

By the process according to the invention selective oxidation of the powder particles, which can lead to reduction of the coercive field strength by magnetic nucleus formation, is largely avoided. Furthermore, by means of powdered metallurgical additions of hydrides of the rare earth elements, any alpha-iron present, or partially oxidised particle surfaces, can be converted to the SE2-Fe17 compound during the nitration. - 6 By an additional impregnation of the moulded body with a plastics or metal binder, a further increase in the hardness and the corrosion resistance can be achieved.

The impregnation is carried out, for example, in the form of a vacuum impregnation of the nitrated slug with artificial resin. Other forms of impregnation can be pressure impregnation with plastics or a metallic melt. Suitable metals are, for example Hg, Sn or Zn.

According to a particular embodiment the compression can also take place in such manner that the pre-alloy, which is substantially nitrogen-free, is already compressed together with the metallic binder. For this purpose the metallic binder can be mixed into the pre-alloy as a powder. Alternatively the pre-alloy powder can also be coated with the metallic binder.

The process according to the invention is particularly suitable for the manufacture of permanent magnets of the SE—TM—N type, in which SE designates at least one rare earth element and TM at least one transition element. Samarium has become known in this connection as a preferred rare earth element. The transition element TM is represented especially by iron, but a portion of the iron can also be replaced by cobalt and/or nickel. In those cases the permanent magnets manufactured according to the invention are, in particular, magnets of the composition Sm2-Fe17-Nx or Sm2-Fe17-(C,N)x wherein 2 < x < 3. The permanent magnet alloy can furthermore contain up to 9 atoms % of at least one of the elements Sn, Ga, In, Bi, Pb, Zn, Al, Zr, Cu, Mo, Ti, P, Si and B. Further elements, especially oxygen, can be contained in concentrations which correspond to common impurities.

The preferred pre-alloy Sm2-Fe17 possesses planar anisotropy. Approximately 70% of the residual magnetism of the anisotropic solid magnet can be achieved by compression of the pulverulent pre-alloy in a magnetic field and subsequent formation of the nitrogen-containing Sm2-Fe17-Nx phase with uniaxial anisotropy. For example the alloy Sm2-Fe17-N2.5, in the form of a solid magnet, has a saturation magnetisation of 1 .54 T. At a packing density of 75%, accordingly, a residual megnetism of the compacted magnet of about 0.8 T can be achieved with this alloy in the process according to the invention.

According to a further preferred embodiment, the starting compound Sm2-Fe17 is alloyed with carbon. The intermetallic compound Sm2-Fe17-Cy, when y > 1, has uniaxial isotropy. If a pulverulent pre-alloy of this kind is compressed in a magnetic field according to the process of the invention, and then nitrated, a bound magnet with a residual magnetism of 0.95 T can be achieved, with a residual magnetism of the solid compound Sm2-Fe17(C,N)x of, for example, 1.27 T, and a packing density of the magnetic material of 75% of the volume. According to the degree of orientation, however, the achieved residual magnetism of the magnet will as a rule be somewhat lower, so that for example for bound magnets, which are oriented perpendicularly to the direction of compression in the magnetic field, in practice about 95% of this value is achieved. In this way, with the process according to the invention, magnets can be manufactured from the above-mentioned alloys, the magnetic properties of which are close to the magnetic values of solid sintered magnets of the Sm-Co5 type.

In a special embodiment pre-alloys of Sm2-Fe17 were fused in the vacuum induction oven. The resulting ingots were then comminuted and pre-milled. The coarse powder thus obtained was finally milled to particle sizes of 2.5 or 2.8 urn. An isotropic moulded body was produced from these alloy powders by isostatic compression. - 8 The nitration of the moulded body took place at a temperature of 500°C in a nitrogen atmosphere under a pressure of 0.7 bar. After cooling, the moulded body was steeped with a thermosetting methacrylate impregnating agent and cured at 120°C.

Claims

1. Process for the manufacture of a nitrogen-containing permanent magnet by compression of a pulverulent, substantially nitrogen-free pre-alloy to a porous moulded body and subsequent nitration of the compressed moulded body by annealing reaction in a nitrogen-containing atmosphere.

2. Process according to claim 1, characterised in that the moulded body is impregnated with a plastics or metallic binding agent.

3. Process according to claim 2, characterised in that the impregnation is carried out after nitration.

4. Process according to one of the preceding claims, characterised in that the pre-alloy is already compressed together with a metallic binding agent.

5. Process according to claim 4, characterised in that the metallic binding agent is mixed with the pre-alloy in the form of a powder. - 10

6. Process according to claim 4, characterised in that the pre-alloy powder is coated with the metallic binding agent.

7. Process according to one of the preceding claims, characterised in that the pre-alloy is oriented during or before the compression in a magnetic field .

8. Process according to one of the preceding claims, characterised in that nitration takes places at a temperature between 250°C and the decomposition temperature of the nitrogen-containing compound.

9. Process according to one of the preceding claims, characterised in that the annealing reaction takes place under elevated nitrogen pressure of more than 1 bar.

10. Process according to one of the preceding claims, characterised in that the permanent magnet is of the SE—TM—N type, in which SE designates at least one rare earth element and TM at least one transition element.

11. Process according to claim 10, characterised in that the permanent magnet contains samarium as the rare earth element.

12. Process according to claim 11, characterised in that the magnet is an Sm2Fe17Nx- or Sm2Fe17(C,N)xpermanent magnet in which 2 < x < 3.

13. Process according to claim 12, characterised in that the iron is partially replaced by cobalt and/or nickel. Process according to one of the claims 10-13, characterised in that the magnet alloy without its binding agent component contains up to a total of 9 atoms % of at least one of the elements Sn, Ga, In, Bi, Pb, Zn, Al, Zr, Cu, Mo, Ti, P, Si and B. 15. 15. 16. 16. Process for the manufacture of a nitrogen-containing permanent magnet, substantially as herein described and exemplified. A nitrogen-containing permanent magnet whenever prepared by a process claimed in a preceding claim.