US 3664892 A
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 (OCR text may contain errors)
United States Patent Becker 1 May 23, 1972 54 PERMANENT MAGNET MATERIAL 3,546,030 12/1970 Buschow et al 148 3157 POWDERS HAVING SUPERIOR 3,523,836 8/1970 Buschow et a1 148/3 1 .57 MAGNETIC CHARACTERISTICS OTHER PUBLICATIONS Division of Ser. No. 701,840, Jan. 31, 1968, Pat. No.
us. c1 ..14s/31.s7, 75/o.5 AB, 75/170 .......no11 l/06 FieldofSearch ..14s/100,1o1,102,103,105,
References Cited UNITED STATES PATENTS Stmat et a1. ..75/213 Stmat et aL, A Family of New Cobalt-Base Permanent Magnet Materials, Journal of Applied Physics, (38), 1967, Pages 1001 and 1002.
Primary Examinerl.. Dewayne Rutledge Assistant ExaminerG. K. White Attorney-Charles T. Watts, Paul A. Frank, Jane M. Binkowski, Frank L. Neuhauscr, Oscar B. Waddell and Joseph B. Forman  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.
8 Claims, No Drawings PERMANENT MAGNET MATERIAL POWDERS HAVING SUPERIOR MAGNETIC CHARACTERISTICS This is a division of copending application, Ser. No. 701,840, filed Jan. 31, 1968, now US. Pat. No. 3,558,372 entitled Permanent Magnet Material Powders Having Superior Magnetic characteristics And Method.
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 enchanced 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 su perior 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 effects 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 disadvantage.
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 inorder 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. In accordance with my preference, the acid mixture will be at room 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 1 or 2 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 devise 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 LII be a reasonable immersion contact period and 5 to seconds would be an easier goal to reach in production operations.
For experimental purposes, I have used cobalt-rear 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 of materials of this kind are summarized in Table i.
TABLE 1 Coercive Initial force of coercive treated Particle force, Acid Solu- Time in material, size 1 oersteds 2 tions 3 seconds oersteds 250F325 105 10 1, 840 250+325 105 2, 720 250+'326 105 3, 340 -250+325 101 7 805 325 364 30 4, 720 20 780 10 6, 400 250+325 b 2, 490 30 5, 250 250+325 1, 600 30 4, 870 250+325 1, 290 30 4, 680 250+325 b 2, 490 30 5, 520 260+325 1,600 30 5, 230 250+325 1, 290 30 4, 800 325 b 8, 100 20 8, 450 325 6, 430 20 8, 300 325 B 5, 200 20 8, 220 325 870 30 2, 940 325' 870 3, 370
1. U.S. standard screen sizes except where otherwise specified.
2. Coercive force measured in field Hm=21,000 oersteds except where otherwise stated.
3. Chemical polishing reagents A and C as given on page 328 of Smithelfis Metals Reference Book (Plenum Press, 1967) of composition as to ows:
Acid A: Three parts nitric acid, one part sulfuric acid, one part orthophosphorlc acid, five parts glacial acetic acid.
Acid B: One part nitric acid, one part sulfuric acid, one part orthophosphoric acid, live parts glacial acetic acid. Acid 0: 200 grams chromic oxide, 86 grams nitric acid, 10 grams sodium sulfate, water to one liter volume. 4. M=Cerium-rlch misch metal. v Microns. 4 lII =30,0QQ ee. H Ilm=18,000o1-..
While I do not intend any limitation on the claims, it is my belief and theory that the results obtained by 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 materials. 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 differences 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 offered 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 returned on a 3 2 5 nesh screen was selected for test. A part of this fine powder fraction was introduced into a body of molten parafi'in 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 being 105 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, 1 part sulfuric acid, 1 part orthophosphoric acid and 5 parts glacial acetic acid. The acid mixture was 25 C., and the powder was permitted to remain in the acid mixture for 30 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 in paraffin and tested as described above with the result that the coercive force was found to be 3,340 oersteds, as stated in the third entry of Table l.
Wherever 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 v 'ations are considered to be within the purview and scope 6? die invention and the appended claims.
What I claim as new and desire to secure by Letters Patent of the United States is: y
1. A permanent magnet materials which is a Co R intermetallic compound, where R is a rare earth metal, in the form of a ground powder wherein the particles have acid-polished substantially smooth and rounded surfaces resulting in a permanent magnet with a substantially increased coercive force when compared with as-ground particles.
2. The material of claim 1 wherein said Co R intermetallic compound is C0,,Y.
3. The material of claim 1 wherein said Co R intermetallic compound is C0,,Sm. i
4. The material of claim 1 wherein said Co R intermetallic compound is Co M where M is cerium-rich mischmetal.
5. A permanent magnet comprised of the magnetized material of claim 1.
6. A permanent magnet comprised of the magnetized material of claim 2.
7. A permanent magnet comprised of the magnetized material of claim 3.
8. A permanent magnet comprised of the magnetized material of claim 4.