US3558371A - Method of making permanent magnet material powders - Google Patents

Abstract

THE GRINDING OF BULK PERMANENT MAGNET MATERIAL TO POWDER CAN RESULT IN A RELATIVELY LOW VALUE OF THE MAXIMUM COERCIVE FORCE OBTAINABLE. BY THE PRESENT METHOD INVOLVING IMMERSION OF SUCH BULK MATERIAL FOR A SHORT TIME IN NITRIFIC ACID, HYDROCHLORIC ACID, SULFURIC ACID, PHOSPHORIC ACID, OR ACETIC ACID, POWDER PARTICLES ARE PRODUCED WHICH HAVE HIGH COERCIVE FORCES.

Description

United States Patent 3,558,371 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 May 20, 1968, Ser. No. 730,577 The portion of the term of the patent subsequent to Mar. 17, 1987, has been disclaimed Int. Cl. H01f 1/06 US. Cl. 148-101 6 Claims ABSTRACT OF THE DISCLOSURE The grinding of bulk permanent magnet material to powder can result in a relatively low value of the maximum coercive force obtainable. By the present method involving immersion of such bulk material for a short time in nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid, or acetic acid, powder particles are produced which have high coercive forces.

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 eifects of grinding upon the magnetic characteristics of the cobalt-rare earth materials can be eliminated and the coercive forceof 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 treating the finely-divided, ground, magnetic material with an acid in a contact period sulficient to increase the coercive force of the powder significantly. Although the exact mechanism is not certain, it seems likely that the acid produces holes or channels in the particles in such a way that the form of the particle is changed with an attendant increase in coercive force.

This invention thus centers in the concept of subjecting these ground materials to an acid treatment to eliminate the degrading effects of the grinding upon their magnetic properties. The invention is also based upon my discovery that such treatment can result in surprisingly large increases in coercive force of these cobalt-rare earth materials.

The acids which are employed in this case are different in composition and apparently in effect from those disclosed and claimed in my copending patent application, Ser. No. 701,840, filed Ian. 31, 1968, and assigned to the present assignee. The polishing eifect leading to sub stantial increase in coercive force of the treated powder in accordance with my prior invention stands in contrast to the acid attack and erosion on the particles which leads to substantial increase in coercive force value in accordance with the process of my present invention.

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

Time, temperature and acid concentration 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, temperature and concentration, 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 will be at room temperature but it may be at any temperature at which the acid is a liquid. At room temperature, i.e. about 25 C., the contact period generally ranges from about a few seconds to about ninety minutes. The specific contact period necessary to obtain a maximum coercive force for a particular powder is determinable empirically and depends largely on particle size, the specific acid used and its concentration, and acid temperature. Generally, shorter contact periods are required when smaller particles are used or when more concentrated acids or acids substantially above room temperature are used. However, prolonged acid contact may lead to significantly diminishing magnet material yields, particularly if the treating acid temperature is substantially above room temperature.

Representative of the acids useful in the present invention are hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid and acetic acid. The concentration of the acid used may vary widely. Generally, the more dilute an acid, the longer is the contact time required to produce a maximum coercive force.

For experimental purposes, I have used cobalt-rare earth magnet material of particle size ranging from less than 30 microns to mesh (between 88 and 147 microns) (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 coercive force generally varies inversely with particle size. 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 the treatment of cobaltbase permanent magnet materials including Co Y, Co Sm and Co M (cerium-rich misch metal).

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 acid produces holes and channels in the particles in such a way that the form of the particle is changed with an attendant increase in coercive force. 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 treatment of this method.

The following illustrative, but not limiting examples of operations embodying this invention which I have carried out are offered by way of further describing the present novel method to those skilled in the art:

EXAMPLE 1 An ingot of cobalt-Samarium (Co Sni) was ground with mortar and pestle. The resulting powder was screened and the fraction passing through a 325 mesh screen was selected for test. Substantially equal portions of this fine powder fraction were used in the preparation of four samples.

One sample was prepared by introducing a portion of the powder into a body of molten paraffin wax and cooling the wax in an aligning magnetic field of 21,000 oersteds until it was solidified.

Another portion of the powder was immersed in 2% nitric acid at room temperature, i.e. about 25 C. At the end of 30 seconds, the powder was removed from the acid, rinsed with water and with acetone, and permitted to dry in air. Additional portions of the powder were treated in the same manner except that they were immersed in the 2% nitric acid for 60 seconds and 90 seconds, respectively. Each of the treated, dried, powder portions was then mounted in parafiin as described above.

The coercive force of each sample was measured after magnetization in a field of 30,000 oersteds. The results 4 EXAMPLE 4 Cobalt-yttrium (Co Y) was used in this example. The procedure was the same as that disclosed in Example 1 except that 1% nitric acid was used and the coercive force was measured after magnetization in a magnetic field of 21,000 oersteds.

The results are given in Table IV.

As illustrated in Table IV, the coercive force of the cobalt-yttrium powder is increased significantly after relatively short period of immersion in the acid.

EXAMPLE 5 Cobalt-yttrium (Co Y) was used in this example. The procedure in this example was the same as that disclosed in Example 1 except that the acids given in Table V were used, and the coercive force was measured after magare given in Table I. netization in a field of 21,000 oersteds.

TABLE I Coercive Time force Material Size Acid (seconds) (ocrsteds) Sample No.1

1 Co S1n 325 mesh 431nierons) None 0 500 d an 8,450 60 1, 650 90 11,450

As illustrated in Table I, the coercive force increased TABLE v significantly after relatively short periods of immersion in the nitric acid. 2322 EXAMPLE 2 (oersteds) In this example, the procedure was the same as that 460 disclosed in Example 1 except that the cobalt-samarium 489 was ground to a size less than microns and 1% nitric acid was used. The results are given in Table II. 21770 1,775 TABLE II 2 E88 Coercive 1 Size, A Tine force Materia microns cid (secon s) (oersteds) 27 CHBCOOH minutes" 1,250

Co5Sm 30 None 0 10,200

cotsm 30 1% HNO; 30 11,430 50 s E E g: 1% H2 58 Table V illustrates the number of ditferent acids which As illustrated in Table II, the coercive force of cobaltsamarium powder as fine as less than 30 microns can be increased significantly by short periods of immersion in an acid as dilute as 1% nitric acid.

EXAMPLE 3 In this example the procedure was the same as that disclosed in Example 1 except that the ground cobaltsamarium fraction passing a 250 mesh and returned on a 325 mesh screen was selected for test and 1% nitric acid was used. The results are given in Table III.

As illustrated in Table III, the coercive force of cobaltsaniarium powder as coarse as the 250+325 mesh was increased significantly after a short immersion in an acid as dilute as 1% nitric acid.

may be used to increase the coercive force of the powder, and the approximate rates at which these particular acids increase such force. Specifically, a comparison of Sample 22 of Table V, where a 20% nitric acid was used, with Sample 13 of Table IV, where a 1% nitric acid was used, shows how much faster the coercive force was raised by the more concentrated acid.

EXAMPLE 6 In this example cobalt-yttrium (Co Y) was used. The procedure in this example was the same as that disclosed in Example 5 except that the ground fraction passing through a mesh screen and returned on a mesh screen was used for test and the acid was 2% nitric acid.

A sample of the powder as ground, i.e. not treated with acid, had a coercive force of 71 oersteds. A sample of the powder was immersed in 2% nitric acid at room temperature for six minutes. At the end of this time its coercive force was measured as being 1100 oersteds.

This data illustrates that even in this instance, where the initial particle size was so large, the coercive force was still significantly increased after a short period of immersion in the acid.

EXAMPLE 7 Co M, where M is cerium-rich misch metal, was used in this example. The procedure in this example was the same as that disclosed in Example 1 except that the coercive force was measured after magnetization in a field of 21,000 oersteds. A sample of the powder as ground, i.e. not treated with acid, had a coercive force of 795 oersteds. A sample of the powder was immersed in 2% nitric acid at room temperature for 30 seconds. At the end of this time its coercive force was measured as being 1725 oersteds.

EXAMPLE 8 In this example cobalt-yttrium (Co -Y) was used. The procedure used in this example was the same as that disclosed in Example 5 except that the ground fraction passing through a 250 mesh screen and returned on a 325 mesh screen was used for test and the acid was 2% nitric acid.

A sample of the powder as ground, i.e. not treated with acid, had a coercive force of 1 03 oersteds. This sample was placed under an optical microscope and a magnified picture of it was made. A sample of the powder was immersed in the 2% nitric acid at room temperature for two minutes. At the end of this time its coercive force was measured as being 1860 oersteds. This sample was also placed under the optical microscope and a magnified picture of it was made. A comparison of the two pictures shows the untreated sample to consist of solid particles whereas in the acid treated sample the particles were porous to some extent and their surfaces were pitted.

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 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 cobalt-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 fineparticle material with an acid for a period of time sufficient to increase the coercive force of said compound, said acid being selected from the group consisting of nitric acid, hydrochloric acid, sulphuric acid, phosphoric acid and acetic acid.

2. The method of claim 1 in which the compound is C05Y.

3. The method of claim 1 in which the compound is C0581.

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

5. The method of claim 1 in which the ground, fineparticle material ranges in size from less than 30 microns to up to about 147 microns.

6. The method of claim 1 in which the acid is at room temperature.

References Cited UNITED STATES PATENTS 1,878,589 9/1932 Marris 148104X 1,932,639 10/1933 Roseby 148104 1,998,840 4/1935 Legg et a1. 148l00 3,421,889 1/1969 Ostertag et al 170 3,424,578 1/ 1969 'Strnat et a1. 75-2l3 3,501,358 3/1970 Becker l48105 L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. XJR.