US3558372A

US3558372A - Method of making permanent magnet material powders

Info

Publication number: US3558372A
Application number: US701840*A
Authority: US
Inventors: Joseph J Becker
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-01-31
Filing date: 1968-01-31
Publication date: 1971-01-26
Anticipated expiration: 1988-01-26
Also published as: FR2001038A1; GB1210332A; ES362575A1; NL6900668A; DE1901056A1; BE726341A

Abstract

THE COERCIVE FORCE OF A COBALT-RARE EARTH INTERMETALLIC COMPOUND IS GREATLY ENHANCED BY CHEMICALLY TREATING THE COMPOUND IN FINELY-DIVIDED FORM SO AS TO SMOOTH THE SURFACES OF THE INDIVIDUAL PARTICLES.

Description

United States Patent Oifice 3,558,372 METHOD OF MAKING PERMANENT MAGNET MATERIAL POWDERS Joseph J. Becker, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Jan. 31, 1968, Ser. No. 701,840 Int. Cl. H01f 1/06 U.S. Cl. 148-101 8 Claims ABSTRACT OF THE DISCLOSURE The coercive force of a cobalt-rare earth intermetallic compound is greatly enhanced by chemically treating the compound in finely-divided form so as to smooth the surfaces of the individual particles.

The present invention relates generally to the art of making permanent magnets and is more particularly concerned with new magnetic material powders having unique characteristics and with a novel method for producing these powders.

It is generally recognized that the permanent magnet properties of bulk magnetic materials having large magnetocrystalline anisotropies can be enhanced by reducing them to powders. It is also common knowledge that such powders can be incorporated in bonding media to provide composite permanent magnets having properties substantially superior to those of the bulk source materials. These advantages are, however, offset to a substantial degree in some instances when the particle size reduction is accomplished by grinding. Thus, a comparatively low value of coercive force can substantially diminish the advantages to be gained by converting the bulk body to a powder and fabricating a composite finished article from the powder.

In accordance with the present invention, the detrimental eiTects of grinding upon the magnetic characteristics of the cobalt-rare earth materials can be eliminated and the coercive force of mechanically-reduced materials of this kind can be enhanced to a surprising extent. In essence, the method of this invention centers in the key step of chemically smoothing the surfaces of finely-divided, ground, magnetic material. This step is carried out with an acid mixture in a contact period of from a few seconds to 30 minutes during which time the individual powder particles preferably are not substantially reduced in size but their sharp edges and points are rounded and smoothed by acid attack.

This invention thus centers in the concept of subjecting these ground materials to a chemical treatment to eliminate the degrading effects of the grinding upon their magnetic properties. The invention is also based upon my discoveries that such treatment can result in surprisingly large increases in coercive force of these cobalt-rare earth materials, and that this result can be obtained without incurring a significant product yield penalty or other substantial dis advantage.

As a general proposition, the chemical treatment is carried out according to this invention by contacting cobalt-rare earth magnetic material with a suitable acid solution, the material in powder form preferably being immersed in the acid solution for the required period of contact time and then promptly removed and rinsed free from that solution in order to arrest the acid attack.

Time and temperature are interrelated factors in this method, I have found, however, that practical operating ranges of these variables are so broad that the necessity for precise control of the method can readily be avoided. Thus, except for threshold or marginal conditions of time and temperature, the operating conditions or a combination of these conditions is not critical to the success or failure of the method in terms of the products obtained.

3,558,372 Patented Jan. 26, 1971 In accordance with my preference, the acid mixture will be at rooom temperature but it may be at any temperature at which the acid mixture is a liquid. The period of immersion or contact may likewise be anywhere from one or two seconds to 30 minutes, my present preference being about 30 seconds. Prolonged acid contact may lead to significantly diminishing magnet material yields, particularly if the treating solution temperature is substantially above room temperature. At the other end of the time scale it is the mechanical or manipulative limitation which generally is the important one. Acid solutions I prefer for this process act rapidly enough that momentary contact is sufficient to insure consistently good results and it remains only to device means enabling immersion and removal of the powder materials at the rate desired in a batchwise or a continuous chemical treatment operation. In any event a second or two would appear to be a reasonable immersion contact period and five to 10 seconds would be an easier goal to reach in production operations.

For experimental purposes, I have used cobalt-rare earth magnet material of particle size in the range between 250 mesh and +325 mesh (U.S. standard screen sizes). Materials of this kind may, however, be treated according to this invention with the foregoing results when the particles are twice as large, but the maximum coercive force obtainable is lower because of the fact that particle size is generally inversely proportional to coercive force. Much finer particles may likewise be treated in accordance with this method but at the cost of smaller product yields because of the relatively larger proportion of each particle dissolved in the acid attack.

This invention method has been found in actual practice to be particularly beneficial in actual practice in the treatment of cobalt-base permanent magnet materials including Co Y, Co Sm and Co M (cerium-rich misch metal). The results obtained by applying the method to materials of this kind are summarized in Table I.

TABLE I Coercive Initial force of coercive treated Particle force, Acid Time in material, size 1 oerstcds 2 solutions 4 seconds oersteds -250, +325 105 Acid A 10 1, 840 250, +325 20 2, 720 -250, +325 30 3, 340 -250, +325 7 s05 -5 30 4, 720 4 -20 10 6,400 250, +325 30 5, 250 -250, +325 30 4, 870 -250, +325 30 4, e -250, +325 2, 490 Acid 13.... 30 5, 520 -250, +325 1, 600 do 30 5, 230 -250, +325 6 1,290 do 30 4, 800 325 5 8, 100 Acid A 20 8, 450 325 6, 430 do' 20 8, 300 -325 t 5, 200 d 20 s, 220 325 870 30 2,940 325 870 .do.. 60 3, 370

1 U.S. standard screen sizes except where otherwise specified. 2 Coercive force measured in field Hn=2 1,000 oersteds except where otherwise stated.

3 Chemical polishing reagents A and O as given on page 328 of Smithell's Metals Reference Book (Plenum Press, 1907) of composition as follows:

Acid A: Three parts nitric acid One part sulfuric acid One part orthophosphoric acid Five parts glacial acetic acid One part nitric acid One part sulfuric acid One part orthophosphoric acid Five parts glacial acetic acid 200 grams chromic oxide grams nitric acid 10 grams sodium sulfate Water to one liter volume. 4 Microns.

5 Hm=30,000 oersteds. 5 Hm=l8,000 oersteds. 1 M=Cerium-rich misch metal.

Acid B:

Acid 0:

applying this method to cobalt-rare earth permanent magnet materials can be explained on the basis that the chemically-treated powders of these materials are smooth and free from sites for the nucleation of domains of reverse magnetization. The improvement in the coercive force characteristic of powders of such materials treated in accordance with this method is such as to give support to this theory and there does not appear to be any alternative explanation for this remarkable change in this key property of these matrials. Moreover, there does not appear to be any chemical change produced in the magnet materials by the acid mixture treatment of this method, the only detectable difierences being the rounding of sharp edges and points and the great increase in coercive force of the powder particles.

The following illustrative, but not limiting example of an operation embodying this invention which I have carried out is oifered by way of further describing the present novel method to those skilled in the art:

EXAMPLE Co Y was prepared by arc-melting cobalt and yttrium together under an argon atmosphere and then casting the melt to form an ingot. A piece of the resulting very brittle ingot weighing about one half gram was ground with mortar and pestle and the resulting powder was screened and the fraction passing a 250 mesh screen and return on a 325 mesh screen was selected for test. A part of this fine powder fraction was introduced intoa body of molten parafiin wax and the wax was cooled in an aligning magnetic field of about 21,000 oersteds until it was solidified. The coercive force of this powder sample was measured as =being105 oersteds. The remainder of the -250 +325 mesh powder fraction was placed in a dish with ml. of a mixture of 3 parts nitric acid, one part sulfuric acid, one part orthophosphoric acid and five parts glacial acetic acid. The acid mixture was at C., and the powder was permitted to remain in the acid mixture for seconds whereupon the powder was removed from the acid mixture and rinsed with water and with acetone and permitted to dry in air. This sample was then mounted inparaffin and tested as described above with the result that the coercive force was found to be 3340 oersteds, as stated in the third entry of Table I.

Whatever percentages or proportions are stated in this specification, results are made to the volume basis rather than weight basis.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

-What I claim as new and desire to secure by Letters Patent of the United States is:

1. In the method of producing a colbalt-rare earth intermetallic compound denoted by Co R where R is a rare earth metal in a form in which it has superior coercive force including the steps of forming a bulk body of said compound and grinding the bulk body to a powder, the combination of the step of contacting the resulting fine-particle material with a chemical polishing reagent which is an acid solution and thereby substantially increasing the coercive force of the fine-particle material by smoothing the surfaces of the individual powder particles.

2. The method of claim 1 in which the bulk body is reduced to a powder by a grinding operation and in which the particle size of the final powder product is substantially the same as the particle size of the fineparticle material prior to contact with acid solution.

3. The method of claim 1 in which the acid solution consists essentially of three parts of nitric acid, one part of sulfuric acid, one part of orthophosphoric acid and five parts of glacial acetic acid.

4. The method of claim 1 in which the compound is Co Y.

5. The method of claim 1 in which the compound is Co Sm.

6. The method of claim 1 in which the compound is Co M wherein M is cerium-rich misch metal.

References Cited UNITED STATES PATENTS 1,878,589 9/1932 Marris 148-104X 1,932,639 10/1933 Roseby 148104 1,998,840 4/1935 Legg etval. 148100 3,421,889 l/1969 Ostertag et al. -170 3,424,578 1/ 1969 Strnat et a1. 75-213 3,463,678 8/1969 Becker 148105 3,501,358 3/1970 Becker 148-105X L. DEWAYNE RUTLEDGE, Primary Examiner G. -F. WHITE, Assistant Examiner U.S. C1. X.R.