Method Of Densifying Magnetically Anisotropic Powders

Abstract

A method of densifying magnetically anisotropic powders under a magnetic field by the use of isostatic pressures of at least 10 kb.

Description

The invention relates to a method of densifying magnetically anisotropic powders, for example, powders of ferromagnetic metals, metal alloys or ceramic materials into solid magnetic bodies, in which method a powdery mass is subjected to an external magnetic field which orientates the powder particles, said mass being densified by compression.

It is common practice to subject the previously orientated powder, as the case may be, subsequent to preliminary densification at low pressure, to a high pressure in a mould by one die or one lower and one upper die. It has to be endeavored to maintain the orientation of the particles in order to obtain satisfactory permanent magnetic properties.

Such a densifying method has various disadvantages. In the first place the orientated powder is not densified homogeneously so that the relative orientation of the powder particles (alignment) is changed and hence the magnetization in the direction of magnetization is adversely affected. During compression the powder mass is locally deformed to a great extent mainly due to friction along the wall. This has an unfavorable influence on the orientation of the particles and hence on the magnetization in the direction of magnetization. It has, moreover, been found that in the known compression method the attainable density and hence the attainable magnetization of the compressed powder are limited. There appears a strange phenomenon in that however high the pressure is raised the density of the compressed powder does not exceed a given value. For example, in compression barium hexaferrite it has not been possible to attain a density exceeding 60 percent of the theoretical density.

This phenomenon has not yet been satisfactorily accounted for. A fact is, however, that the attempts to increase magnetization by increasing the density by compression have so far failed due thereto.

The present invention has for its object to obviate said disadvantage and to overcome the barrier to the increase in magnetization by an increased density obtained by compression.

For this purpose the method according to the invention is characterized in that the powdery mass is compressed by an isostatic pressure of at least 10 kb., and that it is slightly deformed anisotropically plactically, the said isostatic pressure being maintained.

It has been found that by isostatic compression a homogeneous densification of the powdery mass is obtained, while the orientation of the particles is maintained. During the subsequent slight anisotropic, plastic deformation, while the isostatic pressure is maintained, a surprisingly considerable increase in density is obtained, whereas the orientation of the particles is hardly affected by this deformation. These two measures provide a magnet having a considerably higher energy product (BH).sub.max than could hitherto be obtained.

The invention furthermore relates to a permanent magnet manufactured of magnetically anisotropic powder by the method described above. The permanent magnet is characterized in that the density is at least 85 percent of the theoretic density and the magnetization in the direction of magnetization is at least 90 percent of the saturation magnetization.

An advantageous permanent magnet embodying the invention is characterized in that the essential constituent of the powder is a compound of hexagonal structure, the existence range of which is integral with the existence range of the compound in the system M--R that is M.sub.5 R, wherein M is Co or a combination of Co with one or more of the elements Fe, Ni and Cu and R designates one or more of the elements of the rare earth metals and/or Th and/or Y.

Such powders are known from Dutch Pat. application No. 6,608,335 and are particularly suitable for providing in conjunction with the method according to the invention permanent magnets having very high (BH).sub.max values.

In a further advantageous permanent magnet embodying the invention M is Co and R is Sm, while the energy product (BH).sub.max has a value of at least 14.10.sup.6 Gauss Oersted. The energy product (BH).sub.max of this magnet is considerably higher than that of the SmCo.sub.5 magnets hitherto known. From the review "Journal of Applied Physics", Vol. 39, No. 3, 1968, pages 1,719-1,720 is known, for example, an SmCo.sub.5 magnet on which a (BH).sub.max value of 8.1.times.10.sup.6 Gauss Oersted has been measured, whereas the present magnet attains values of 15.times.10.sup.6 Gauss Oersted and higher.

By way of example the invention will be described more fully with reference to the drawing.

Magnetically anisotropic powder is put into a rubber bag and disposed in a magnetic field so that the powder particles are orientated. While the magnetic field is maintained, the powder is compressed until a coherent block of particles is obtained.

Then the rubber bag with its contents is evacuated and sealed in an airtight manner. The block is then isostatically predensified (for example at a pressure of 8 kb.) and after the compression the rubber bag is removed.

The isostatic predensification may be carried out by hydrostatic means.

The block is then introduced in a container of ductile material. The container is sealed by a covering plate, which may be soldered to the container. Then the container is compressed at a high isostatic pressure of, for example, 20 kb. and while this high pressure is maintained it is slightly deformed anisotropically and plastically. This compression may be carried out hydrostatically in a press having, in addition, the means for performing said plastic deformation. Such a press may be constructed as is shown schematically in the FIGURE.

Reference numeral 1 designates a compression vessel having a shoulder 2 on the inner side. The vessel has a space 3 filled with liquid. In order to prevent the liquid from changing into the solid state at the high pressures of the order of 7 kb., petrol is used as a pressure transmitting medium. Other appropriate liquids may be chosen for this purpose.

The space 3 is bounded on the lower side by a plunger 4, which is adapted to reciprocate in the compression vessel by means of a hydraulic worm 5, connected therewith, and on the upper side by a plunger 6, which is connected with a hydraulic worm 7 and is also adapted to reciprocate in the compression vessel. The hydraulic worms 5 and 7 can be driven independently of each other.

In the space 3 between the shoulder 2 and the plunger 6 two loose dies 8 and 9 are provided, between which is arranged the aforesaid, hermetically closed container 10 containing the block of magnetically orientated material. The container may be introduced into the vessel by removing the plunger 6. The assembly of the compression vessel, the dies and the hydraulic worms is arranged in a frame 11.

The press operates as follows:

By actuating the worms 5 and 7 each of the plungers 4 and 6 exert equal high pressures on the liquid in the space 3. The hydrostatic pressure is exerted on the container 10, which is thus compressed, the orientated powder contained therein being homogeneously densified. Then the pressure exerted by the plunger 6 is slightly raised, whereas the pressure exerted by the plunger 4 is kept constant. The assembly of plungers 4 and 6 and the liquid column thus move slowly downwards. At a given instant the plunger 6 touches the die 8 and the container 10 is slightly deformed. By means of the die 8 an anisotropic plastic deformation is obtained. It has, of course, to be ensured that liquid can always flow along the dies 8 and 9, for example, by providing holes therein.

After the hydrostatic compression of the container 10 and the slight anisotropic, plastic deformation, the pressure of the plungers is obviated and the container can be removed from the compression vessel and be opened.

Although only one form of the method according to the invention is described herein, presses of different constructions may of course also be employed.

EXAMPLE

Powder of SmCo.sub.5 having an average particle size of less than 10 .mu.m. was orientated in a magnetic field of 30,000 Oersted and then hydrostatically predensified to 70 percent of the theoretic density. The resultant material was hermetically enclosed in a lead container then hydrostatically compressed and slightly deformed anisotropically and plastically in the manner described above. The hydrostatic pressure was 20 kb. The result was an SmCo.sub.5 magnet having a density amounting to 93 percent of the theoretic density. The energy product (BH).sub.max was 18.5.times.10.sup.6 Gauss Oersted.

Claims

What is claimed is:

1. A method of densifying magnetically anisotropic powders of ferromagnetic metals, metal alloys or ceramic materials to solid magnetic bodies, comprising the steps of subjecting a powdery mass to an external magnetic field which orients the powder particles, and compressing said mass under an isostatic pressure of at least 10 kb. to slightly deform said mass anisotropically and plastically.