US3490703A - Permanent magnet and method of manufacturing the same - Google Patents

Description

Jan. '20, 1970 F. TOMHOLT ET AL 3,490,703

PERMANENT MAGNET AND METHOD OF MANUFACTURING THE SAME Original Filed March '28, 1963 INVENTORfi TOMHGLT WILLBW E. Wt

,EVEEN AGENT United States Patent M 3,490,703 PERMANENT MAGNET AND METHOD OF MANUFACTURING THE SAME Frits Tomholt and Willemeise Witteveen, Emmasmgel,

Eindhoven, Netherlands, assignors, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Original application Mar. 28, 1963, Ser. No. 268,782 pow Patent No. 3,341,940, dated Sept. 19, 1967. Divided and this application May 29, 1967, Ser. No. 694,352 Int. Cl. 1507b 13/00; B07c /344; H01f 3/08 US. Cl. 241--79.1 5 Claims ABSTRACT OF THE DISCLOSURE Apparatus for the manufacture of permanent anisotropic magnets, particularly ceramic permanent magnets, including a hermetically sealed receptacle for receivmg a finely-divided permanent magnet material. Within the receptacle are provided means for whirling the materlal into a dust cloud and magnetic means which separate particles forming conglomerates which, after being removed from the receptacle, are processed into permanent magnets.

This application is a division of application Ser. No. 268,782, filed Mar. 28, 1963, now Patent No. 3,341,940.

Our invention relates to a method of manufacturing a magnetically anisotropic body and to a magnetically anisotropic body maunfactured by this method.

Magnetically anisotropic magnets have been made by compressing a finely divided permanent magnetic material in a magnetic field which orients the magnetic material, after which the compressed oriented material is sintered into a highly coherent body. Naturally, it is desirable in this case that the magnetic orientation produces a maximum effect, so that the compressed mass is indeed strongly magnetically anisotropic. However, it has been found that the particles which should be oriented mutually in parallel with their easy direction of magnetization show a tendency to conglomerate because they attract each other magnetically while forming magnetically isotropic conglomerates, as a result of which the efiect of the magnetic orientation is decreased.

It is a principal object of our invention to minimize the tendency of the particles to form isotropic conglomerates during orientation.

A further object of our invention is to provide a method of manufacturing anistropically permanent magnets having improved anisotropic properties. These and further objects of the invention will appear as the specification progresses.

According to the invention, we completely disintegrate the material from which a permanent magnet is to be made, in a closed receptacle, to a powder having an average particle size under 1 micron. This powder is then whirled or centrifuged to form a cloud of dust from which the powder particles are separated under the influence of a magnetic field produced within the closed receptacle and united to form magnetically anisotropic threads, chains or skeins hereinafter referred to as magnetically anisotropic conglomerates. The latter, after removal from the closed receptacle, are compressed into a body which then is sintered to form a highly-coherent body.

These magnetically anisotropic threads, chains or skeins are unexpectedly quite resistant to the disorienting influence of external mechanical forces which are exerted on them when they are transferred to a die; in fact, forma- 3,490,703 Patented Jan. 20, 1970 tion of magnetically isotropic conglomerates virtually does not take place.

The invention is of particular importance for the manufacture of ceramic anisotropic magnets having as the constituent essential for the permanent magnetic properties at least one material, in the form of hexagonal crystals, with a chemical composition according to the formula MO.6Fe O where M is at least one of the metals Ba (barium), Sr (strontium) and Pb (lead). The proportions of the Weiss domains of the said crystals generally lie within the range of approximately 1 to 3 microns, so that for an average particle size under 1 micron, for example approximately 0.3 micron, by far most of the particles each are single Weiss domains. That is to say, these particles are micro-magnets magnetized spontaneously and up to the saturation, which have a strong tendency to conglomerate while forming the above-mentioned magnetically isotropic conglomerates. However, if it is possible to substantially prevent this conglomeration, a maximum magnetic orientation effect can actually be obtained just in this case. The present invention offers a means which is excellently suited to this purpose.

According to a particular embodiment of the invention, the permanent magnetic material from which the magnet body to be manufactured is constituted, is disintegrated and whirled to form a cloud of dust in a container which is closed with a cover on the inside of which the abovementioned magnetically anisotropic conglomerate, i.e. threads, chains or skeins, deposit.

The magnetic field, under the influence of which the magnetically anisotropic threads, chains or skeins are formed, is usually produced by an electro-magnet which is rendered operative after at least a part of the permanent magnetic material is disintegrated to an average prticle size under 1 micron and is whirled to form a cloud of dust. Preferably, the electromagnet is provided in the cover of the container.

The invention will now be described with reference to a particular example which is illustrative only and the accompanying drawing the sole figure of which shows an apparatus for carrying out the invention.

Finely-divided permanent magnet material 8, e.g. barium hexaferrite, BaO.6Fe O initially ground in a ball mill, is placed in a metal container 1 provided with a metal cover 4 which hermetically seals the container. Cover 4 is provided with a neck 5 in which a soft iron magnet core 6 is provided which is wound with wire turns 7, the core and the wire turns together forming an electromagnet. Extending through the bottom of the container is a shaft 2 which supports a set of blades 3 for rotation.

After the apparatus has been switched on, the permanent magnetic powder is disintegrated by the metal blades, rotating at high speed, to an average particle size of approximaterial 0.5 micron, and whirled to form a cloud of dust. Two minutes later the electromagnet is switched on. In the neck of the cover the magnetically anisotropic threads, chains or skeins 9 of powder particles soon deposit in large numbers. After another two minutes grinding is discontinued, the cover is removed from the container, its lower side placed over a filling funnel, and the electromagnet switched off. The magnetically anisotropic chains are no longer attracted by the cover and fall into the filling funnel. Form there the mass is transferred to a die, in which by means of an electromagnet a substantially homogeneous magnet field is produced. The mass is then compressed in the magnetic field under a pressure of 0.5 ton/cm. The compressed body is removed from the die and sintered by heating it for 24 hours at a maXimum temperature of 1290" C. The resulting magnet body at room temperature has a (BH) of 3.8)(10 gauss oersted and a magnetic anisotropy, expressed in the value of the quotient ill of 4.0 (where B,]] is the maximum remanence and B i the remanence measured in the directon at right angles to the direction in which the maximum remancnce B, is measured).

While we have described the invention with reference to a particular apparatus and a particular material, it will be obvious to those skilled in the art that other forms of apparatus and other materials could be used equally well. For instance, instead of barium hexaferrite, other well-known magnetic materials such as magnetic alloys which can be formed into sintered bodies may be subjected to a similar treatment. Likewise, the material could be ground initially to the required fine state of subdivision and centrifuged by a conventional centrifuge, the magnetic field being applied from an external source.

Therefore, we wish it to be clearly understood that the foregoing example is illustrative only, the invention itself being limited only and defined by the appended claims,

We claim:

1. Apparatus for manufacturing magnetically anisotropic bodies comprising a hermetically sealed receptacle, means to disintegrate and whirl a finely-divided material within said container into a cloud of dust of said material, and means to produce a magnetic field in said container for separating particles in said dust cloud and uniting said particles into magnetically anisotropic conglomerates.

2. Apparatus for manufacturing magnetically anisotropic bodies comprising a sealed receptacle having a cover which hermetically seals the receptacle, means to disintegrate and whirl a finely-divided material within said container into a cloud of dust of said material, and means to produce a magnetic field in said container for separating particles in said dust cloud and uniting said particles into magnetically anisotropic conglomerates which are deposited on said cover.

3. Apparatus for manufacturing magnetically anisotropic bodies comprising a receptacle provided with a cover for hermetically sealing the receptacle, means to disintegrate and whirl a finely-divided material within said container into a cloud of dust of said material, and magnetic-field producing means associated with said cover for separating particles in said dust cloud and uniting said particles into magnetically anisotropic conglomerates which are deposited on said cover.

4. Apparatus for manufacturing magnetically anisotropic bodies comprising a receptacle provided with a removable cover for hermetically sealing the receptacle, means to disintegrate and whirl a finely-divided material within said container into a cloud of dust of said material, and an electromagnet associated with said cover to produce a magnetic field in said container for separating particles in said dust cloud and uniting said particles into magnetically anistoropic conglomerates.

5. Apparatus for manufacturing magnetically anisotropic bodies comprising a receptacle provided with a removable cover for hermetically sealing the receptacle, rotatable blade means to disintegrate and whirl a finelydivided material within said container into a cloud of dust of said material, and an electromagnet associated with said cover to produce a magnetic field in said container for separating particles in said dust cloud and uniting said particles into magnetically anisotropic conglomerates.

References Cited ROBERT C. RIORDON, Primary Examiner MARTIN G. RASKIN, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Nou 3 ,490 ,703 January 20 197 Frits Tomholt et a1.

It is certified that error apyears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 line 5 after "remanence" insert measured in a direction parallel to the principal direction of magnetizati Signed and sealed this 6th day of October 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER,

Edward M. Fletcher, Jr.

Commissioner of Patel Attesting Officer

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