US3359152A - Machinable anisotropic magnet - Google Patents

Description

D 1967' w. s. BLUME. JR 3,

MACHINABLE ANI SOTROPIC MAGNET Original Filed July 15, 1958 INVENTOR WALTER S. BLUME, JR.

BY \NMAW F ATTORNEY United States Patent 3,359,152 MACHINABLE ANIS TROPIC MAGNET Walter S. Blume, Jr., Cincinnati, Ohio, assignor to Leyman Corporation, Cincinnati, Ohio, a corporation of Ohio Original application July 15, 1958, Ser. No. 748,705, now Patent No. 2,999,275, dated Sept. 12, 1961. Divided and this application May 7, 1959, Ser. No. 817,018

2 Claims. (Cl. 161-162) This application is a division of my copending application Ser. No. 748,705, filed July 15, 1958 now Patent No. 2,999,275, and is a continuation-in-part of my copending application Ser. No. 477,241, filed Dec. 23, 1954.

This invention relates to permanent magnets and is directed particularly to improvements in the manufacture of permanent magnets from magnetically anisotropic materials.

A principal objective of the invention has been to provide permanent magnets having excellent magnetic properties but which are readily machinable whereby they may be cut to desired shapes as required by the purposes which the magnets serve. Products displaying excellent or good permanent magnetic properties heretofore have been available from metal alloys such as Alnico, but such materials are so hard that they cannot be cut except by grinding with abrasive wheels. For that reason the conventional mode of fabrication has been to cast the molten alloy composition into a mold conforming to the ultimate shape desired. Where dimensional accuracy is requisite, the casting is then ground to form or size. The cost of this mode of fabrication obviously is appreciable, the surface finish of the unground casting generally is poor, and there is considerable variation from piece to piece in all unground dimensions.

More recently, it has been experimentally determined that barium ferrite corresponding to the general chemical formula BaFe O and similar ferrites of lead or strontium can be made to possess desirable permanent magnetic properties by compressing particles thereof and sintering the compressed particle mass by subjecting the compressed mass to high temperature. This technique for preparing magnets of such non-metallic or ceramic materials of necessity entails the use of expensive dies. In addition, the compressed powder structure, after removal from the die and before sintering, is extremely fragile and must be handled with great care. There is a high percentage of rejects. The sintered product is itself rather brittle. It cannot be machined, nor can it be subjected to rough usage Without chipping at its edges. Furthermore, unless the sintering is performed very carefully, undue crystal growth occurs which reduces coercivity and thereby defeats the improvement of magnetic qualities which the sintering is intended to provide. In addition, the products tend to fracture during sintering.

Machinable magnets have been produced by the compression or injection molding of mixtures of subdivided magnetic material and plastic, but no method of orientation has been known to permit the utilization of the superior properties peculiar to ultrafine anisotropic materials in such procedures, and the magnets inevitably display low energy products.

Permanent magnets of the ferrite materials are potentially less expensive than metal alloys such as Alnico because the materials from which the ferrites are made are much more abundant and readily available. However, because the ferrites are of a crystalline refractory nature to begin with, the pressing and sintering technique is not even as well suited to the production of permanent magnets in a variety of shapes as is the casting method.

The permanent magnets of the present invention embody magnetically anisotropic materials and display permanent magnet properties comparable to or exceeding those of the ferromagnetic materials previously known, but they also possess qualities of machinability, workability, or cutability which makes them amenable to fabrication in simple or intricate shapes, as desired, by the use of ordinary cutting tools or instrumentalities as distinguished from the grinding to which past products have been limited. The products of the invention preferably are made from particles of barium, strontium, or lead ferrite, or mixtures thereof, but the methods of fabrication which this invention provides also may be used in the preparation of readily machinable permanent magnets made from various elements, compounds, or alloys such as manganese-bismuth, finely divided iron, etc. In substance, the products of this invention possess the improved permanent magnet properties of past materials plus the quality of machinability in which the past materials have been deficient, and the finished products are limited as to shape only by the nominal costs involved in the production machining, punching, or cutting of bulk solids.

In order adequately to describe the invention, it is convenient to refer to certain recent discoveries in the physical nature of magnetism. For many years, according to th classical theory of magnetism, it was believed that the individual atoms or molecules of a magnetic substance were in themselves elemental magnets, and that the substance was magnetized when these elemental magnets were aligned in a certain fashion, for example, in a manner similar to that in which iron filings align themselves when scattered on a paper placed over the poles of a common horseshoe magnet. However, as described in Bozorths Ferromagnetism, D. Van Nostrand Co., Inc., 1951, it is now known that all ferromagnetic materials are composed of many small magnets or domains, each of which consists of many atoms. Within a domain all of the atoms are aligned in parallel and the domain is thus saturated, even when no field is applied. The material is therefore said to be spontaneously magnetized. When the magnetization of the material is changed, the atoms turn together in groups (each atomic magnet about its own axis), the atoms in each group remaining parallel to each other so that they are aligned more nearly with the magnetic field applied to the material. So far as is known presently, the exact size or configuration of a domain varies with the material; with respect to barium ferrite the domain size is of the order of one micron.

In the case of certain fine grain permanent magnet materials, particularly the ferrites of barium, lead, and strontium, these domains are strongly magnetically anisotropic, that is, they are magnetized more easily (and their residual inductance and coercivity are better) if they the aligned in a certain so-called preferred crystallographic direction with respect to the magnetizing field. The crystal structure of barium, strontium, and lead ferrites is hexagonal with the direction of easy magnetization being along the 00.1 axis. In the absence of such alignment, the magnetizing force which must be applied to saturate the material, i.e., to effect its full magnetization, is greater and the characteristics of the magnet upon removal of the field are not as good as if the particles had been properly aligned. Extensively ball milled or attrition milled ferrites of barium, strontium, or lead have been shown by electron microscopy to fracture along the basal plane into plate-shaped particles having two substantially parallel surfaces and an irregular edge perimeter. The diameter of these plates when properly cornminuted is in the range of approximately .5 micron. Peculiarly, the preferred direction of magnetization of the ferrite plates is normal to the two parallel surfaces; i.e., the domain plates are more easily magnetized if the magnetic lines of force of the applied external field are per- 3 pendicular to the plate. Apparently, in the case of ferrites at least, the effective domain energy is relatively independent of plate thickness.

Alignment of particles of domain size so that the preferred directions of all of the particles are parallel has heretofore been done magnetically. The domains, being themselves elementary magnets, are acted upon and tend to be aligned by an externally applied magnetic field. Where the domains are embedded or embodied in a matrix material, however, the frictional stresses or the formation of interlocking dipoles between adjacent domains and the general immobility of the domains contained in a matrix tends to resist the orienting force of the externally applied field. While that field exerts a torque about the domains when they are not aligned and thereby tends to align them, still, as the domains are very small, so is the torque in relation to the inter-particle forces and may overcome or orient them only to a small degree, if at all. For that reason, alignment accomplished by application of an external field at best is small or only partial, and the method is incapable of enabling the full magnetic potentialities to be realized.

The essence of the present invention lies in the concept of mechanically orienting or aligning the preferred mag netic axes of the domains with respect to each other, rather than doing it magnetically by means of an external field. It has been found that much better orientation can be achieved in this manner and this method can be practiced with great economy since a magnetic field need not be maintained nor high temperature utilized.

In accordance with this invention, alignment of the particles and the property of machinability are obtained in a permanent magnet of the consolidated powder type by a method wherein particles ground to a suitable state of fineness, preferably domain size, are disposed in an elastomeric or plastic medium, such as rubber, polyethylene, plasticized polyvinyl chloride, or the like. Dispersed heterogeneously in this medium, the particles are relatively immobile and cannot be made favorably to respond, just as the medium itself is relatively immobile. But I have discovered that the particles or domains can be made to disarrange themselves from a heterogeneous pattern of disorganization into an orderly pattern of orientation and alignment by subjecting the composition to strong mechanical force in the nature of shearing stress such as is exerted internally and externally upon a mass as it passes through one or a series of closely spaced rollers or an extrusion plate. Various preferred methods for achieving proper orientation are subsequently disclosed in detail, but to illustrate one practice of the method, by way of example, the orientation may be conducted by adding domain-sized ferrite powder to a natural rubber base and milling the resulting composition into thin sheets by means of a conventional roller-type rubber mill wherein the composition is subjected to the shearing action of differentially speeded rolls between which the material is passed, preferably a number of times. The milling process disperses the magnetic material evenly in and throughout the rubber base, but as an incident thereof also orients the domains, so that the preferred directional axes of the individual particles are a parallel to one another.

Apparently what happens is that such an operation rotates the plate-like particles Within the composition as it forms the composition into a' sheet whereby the plane surfaces of the plates assume positions parallel to the plane surfaces of the sheets with the preferred magnetic axes of the plate normal to the sheet surface. After milling is completed, a plurality of the sheets may be stacked on top of each other until a desired thickness is obtained. The stacked sheets may then be consolidated by the application of pressure and heat to cure the matrix material thereof, after which the products are magnetized. In the alternative, shapes may be punched from the sheets and the shapes may be stacked for consolidation to produce a given form which may then be cured and magnetized as desired. Moreover, the indivdual sheets themselves when cured may be used individually to furnish thin permanent magnets as desired for specialized purpose or use. Such magnets are durable, easily machinable, and possess excellent magnetic qualities, comparable even with those of the Alnico alloys. They are inexpensive to produce since the raw materials are themselves inexpensive, and the process involves no unusually costly methods.

The immobilizing matrix may be a resinous or plastic composition, or elastomeric semi-solid, or viscous liquid in which the powder can be evenly dispersed and which is capable of hardening, setting, or being cured to a solid state. According to one method, for example, the ferritic or potentially magnetic powder is dispersed in uncured rubber which, upon being milled, is cured to immobilize the particles within it. Application of heat and pressure to the mass after orientation cures or stabilizes the rubber to provide the desired coherence without disturbing directional alignment. In general, the base material may be any of that class of materials which preferably: (a) are themselves non-magnetic; (b) have no amorphous or adverse crystallizing elfect upon the ferromagnetic material dispersed therein; -(c) are viscous enough in their soild, semi-solid, liquid, or plastic state to maintain the immobility of the magnetic powder therein at least through the curing or setting period; and (d) are sufficiently workable readily to transmit internal shear forces y-et plastic enough to permit the heterogeneously disposed domain particles to move relative to one another in response to the shear forces exerted 'by milling or extrusion.

While the invention is disclosed in relation to the use of barium, lead, or stontium ferrite by way of illustration on account of their low cost and abundancy, the method of orientation provided bythe present invention is equally applicable to any anisotropic magnetic material having domain-sized particles, which particles are capable of being acted upon by internal shear stresses in a manner achieving orientation. The only limitation on the material, in other words, is that the particles possess a preferred magnetic axis which will lie consistently on a geometrically unique axis such that the mechanical shearing forces or turning moments acting upon the particles during the orienting step will not act in any one of several directions with equal probability.

The desirable orientation, once obtained, is not disturbed by subsequent handling of the composition once it has been cured or set, as the case may be, nor does subsequent cutting or working of magnets formed from laminated sheets of the composition cause the magnets to lose their orentiation or magnetic properties. Localized surface shearing forces, such as are set up during machining of the material, may disturb the orientation of particles in a thin layer near the surface, but such effects are negligible Where the magnet is other than of very small size, since the portion of the magnet in which orientation is effected is inconsequentially small in comparison with the total volume of the magnet.

Although the individual ceramic particles constituting the magnetic phase of the finished product possess their usual hardness, the application of a cutting tool to the finished product severs the matrix and thereby readily permits the product to be shaped. The product may be cut, punched, drilled, turned, or machined to a desired shape or configuration.

The following examples illustrate typical practice of the invention:

Example I To prepare barium ferrite for use in the process of the present invention, barium carbonate is admixed with ferric oxide, for example, in the proportion of one mol BaCO With six mols Fe O The mixture is fired at a temperature of 1250 C. for one hour, thereby to produce BaFe O This raw material is next reduced to domain size. If the comminution is to be effected by ball mill, the preferred practice is to ball mill the barium ferrite in water for 90 hours, then remove the powder from the ball mill, dry it, and heat treat it for minutes at a temperature of approximately 1000" C. after which the powder is again subjected to ball milling for another 90 hours. In the alternative, barium ferrite may be comminuted in an attrition mill, for example, a standard Szegvari attrition mill using stainless steel shot, for a length of time sufficient to reduce the particles to domain size. In general, the attrition mill is in the order of 10-20 times faster than the ball mill and therefore is preferred. The heat treating step is desirable because this treatment increases the coercivity of the final product; in the case of lead ferrite the increase may be as much as 100%, although the effect of the heat treatment is somewhat less in the case of barium and strontium ferrite.

It will be recognized that the foregoing method of preparing barium ferrite is offered only by way of illustration and that other methods are known in the art which may be used in place of the procedures shown. It will also be recognized that powdered barium ferrite and other ferromagnetic materials adapted for use in the practice of this invention are available from commercial supply houses.

In general, the milled particle size should be in the range of .5 micron, although magnets having good properties have been obtained using particles which in average size were somewhat larger. After final milling, the powder is dried, any lumpy agglomerations are reduced, and the powder is then ready for use. The attainment of domain size may be determined by means of periodic inspection of the material with an electron microscope or, more easily, though somewhat less accurately, by comparing the color of a smear of powder of unknown particle size with that of a smear of powder known to be of domain size, which in the case of barium ferrite has a deep red color. As the size reduction continues, the color of a barium ferrite powder initially fired at a high temperature, for example, 1250 C., changes from black to purple to reddish brown.

A suitable rubber base or matrix to which this ferrite may be added has a composition as follows:

Parts by weight Natural rubber 12.5 Stearic acid 0.1 Zinc oxide 0.3 Barium ferrite 136.0 Phenyl-naphthylamine 0.2 Sulfur 0.3 Tetramethylthiura-m disulfide 0.3 Zinc salt of mercaptobenzothiozole 0.1

Those skilled in the art of compounding elastomeric compositions or the like readily will understand that a wide variety of compounding agents, plasticizers, vulcanizing agents, and the like are available to provide variations in workability, curability, or hardness of the matrix composition to adapt it to special purposes within the purview of the present invention.

The rubber used is suitably No. l Ribbed Smoked Sheet, a standard quality rubber. The zinc oxide is active in the subsequent vulcanization of the rubber, while the stearic acid component helps to activate the accelerators. Sulfur, of course, is the primary curing agent in the vulcanizing process. The remaining organics are accelerators which enter into the vulcanization as well as accelerate the action of the sulfur. The various substances are added in the order given. By volume the barium ferrite may be added to the extent of 65% of the total volume of the mixture.

In the blending and milling process, the uncompounded natural rubber is first run through a standard two-roll rubber mill geared, for example, so that the speeds of the two rolls bear a 1.1 :1 ratio to each other. The speed differential causes a shearing stress to be exerted on the rubber as it sheets between the two rolls, one surface of the rubber being accelerated relative to the other surface, whereby a masticating effect is achieved. The milling thus serves to work the rubber to soften it and make it somewhat plastic. Thoughout the mixing and milling, water preferably is circulated through the mill rolls to maintain the rubber mix at an operating temperature in the range from about 180 F. Above the latter temperature the rubber mix may tend to vulcanize prematurely.

The crude rubber is mixed with itself for approximately 5 minutes, during which time it forms a smooth band with an even bank between the two rolls. After this period the other materials are added. This blending conveniently may take place roughly over a 20 minute period. The materials are poured or sprinkled evenly along the sheet just prior to its repassage between the rolls. As the magnetic material is added, it initially tends to make the rubber softer than before. It is not known whether this change in the physical consistency of the rubber may be due to the increased heat of friction resulting from the working of the ferrite particles against themselves and the rubber or possibly from an actual chemical interaction with the rubber. However, as additional quantities of ferrite are added, the increased softness disappears, and the product becomes relatively stiffer. The sheets produced acquire a degree of toughness which makes them self-sustaining even when the sheet thickness is very low.

After all of the ingredients have been added, the rubber composition is sheeted off the mill, the sheets preferably being thin, e.g., about say, .02.03 inch. As a rule of thumb, the thinner the sheet, the more easily the desired degree of orientation is obtained.

In the accompanying drawings, FIGURE 1 illustrates a presently offered explanation of the process through which mechanical domain orientation is achieved in the practice of this invention. The figure is a vertical section through a conventional rubber mill. Matrix-ferrite mixture is indicated generally at 1, where it collects prior to passing between the respective rolls 2 and 3. Roll 2 is rotatifig at a slightly greater rate than roll 3. Barium ferrite plates 4 in the mixture are randomly oriented in the mixture ahead of the rolls, as at 1. At 5, where the mixture passes between the rolls, the shearing forces acting on the rubber due to the differential in the speed of the two rolls, and perhaps additionally the compressional forces acting as the rubber is squeezed between the two, coact so as to tip over the plates, so to speak, so that the plane surfaces of all plates are approximately parallel to the surface of the rubber sheet. This result may not be achieved in a single pass but is progressive in repeated passages of the material through the rolls.

In the drawing the relative size of the plates is, of course, greatly exaggerated for purposes of illustration. However, it will readily be seen that when one roll is moving at a greater surface speed than the other, the material between the rolls is subjected to shearing forces which presumably are transmitted across or through the entire thickness as the material internally accommodates itself to the speed gradient. It is this effect, apparently, which causes particles not symmetrically disposed in the plane of movement through the rolls to move within the matrix into that attitude wherein they are subjected to the least turning moment or that position wherein the turning moments at the opposite faces of the particles are opposite and equal. At least in passage through the rolls the anisotropic particles become oriented magnetically. This is confirmed by both magnetic and X-ray diffraction analyses.

Although differential speeding of rolls produces a speed gradient within the material as it passes between them, a sec-0nd aligning effect is believed to be conferred upon the magnetic particles in the material by reason of the reduction in thickness of the material as it passes through the rolls, whether or not they are differentially speeded.

In this case, as is exemplified by calender rolls, the material is dragged frictionally from the mass or accumulation existing at the roll nip, and the reduction in thickness from this mass produces a speed gradient internally of the material which may be greater or less depending upon the amount of reduction in thickness.

7 Similar orientation is effected wherein the passageway through which the ferrite-matrix composition is forced is in the form of an extrusion orifice having draft and feed favorably disposed to the plane of the desired alignment rather than in the form of an opening between mill rolls. In this case one explanation for the desirable result may reside in the fact that the composition moving along but in contact with the throat of the extrusion orifice is subjected to more drag or at least is moving at a rate which is different from the rate at which the composition at the interior of the stream is moving whereby differential forces occur internally of the material to cause those anisotropic particles which are not disposed in the plane of the stream to assume that attitude and thereby become aligned with the others. Again, it must be noted that this explanation is by way of illustration and not limitation and comprises no part of the invention. It is merely theorization about an empirically obtained result which has been found to be particularly useful.

In a typical milling operation, barium ferrite to the extent of 65% by volume of the mix is incorporated into the rubber, although a still greater quantity may be introduced. A theoretical limit on ferrite concentration, i.e., loading factor, is reached when the mix contains such a concentration of ferrite particles that they tend to interlock with each other. When this condition is reached the inter-particle frictional forces then prevent the impinging shear forces from aligning the particles. Experimentally, it has been found possible to obtain loadings as high as 70% by volume. However, the uncured composition is then difiicult to process and does not have good strength after curing, there being a tendency to crumble. The greater resiliency of the 65% volume materials makes such materials the more suitable for general purpose usage.

After the milling and sheeting processes are completed, the thin sheets resulting therefrom may be cured and magnetized as such or stacked up until a laminate of the desired thickness is obtained. Since the ferrite domain particles of each of the sheets are aligned so that their preferred directions are normal to the sheet, when the sheets are stacked in facial juxtaposition, the resulting laminate has a preferred magnetic direction normal to its plane surfaces. This is so, it will be seen, regardless of the number of sheets in the laminate.

While the invention has been disclosed particularly in relation to plate-like particles as exemplified by barium ferrite and the like, orientation is obtained with equal facility where the particles are elongated as in the caseof iron, but here it will be understood that their preferred axis is longitudinal of the particles and therefore the preferred axis of the sheet will be in the plane of the sheet rather than in a direction normal to the plane.

To bind or affix the laminated sheets to each other to form a unitary whole, the laminate is placed under a pressure of about 100 pounds per square inch for example and heated to a temperature of about 300 F. or Whatever temperature is required to effect curing of the particular matrix composition. In this operation the laminated sheets are integrated. Magnets of the desired configuration may then be cut from the composite. During these operations the orientation of the particles is not disturbed because they are held immobile in the matrix.

The product thus formed is permanently magnetized by placing it in a magnetic field with respect to which it is located so that the applied field is parallel to the preferred direction of the magnet. FIGURE 2, for example, shows a proper method of magnetizing a small right cylindrical magnet manufactured according to this invention. In the figure, 10 and 11 are the pole pieces of an electromagnet which, upon being energized, magnetizes the ferrite particle magnets in the laminate. The dashed lines 12 indicate the magnetic field between the pole pieces. The laminated magnet 13 is shown in the proper magnetizing position in this field, the arrow 14 indicating the preferred direction of magnetization. The arrow 14, it will be observed, is parallel to the lines of force 12 of the external field. Thus, if the pole 10 is the'north pole of the electromagnet, the opposing face 15 of the laminated magnet 13 will be the south pole of that magnet, and so on.

Rather than cutting the magnets from the cured laminated sheets, as was above described, the magnet may alternatively be formed by punching forms of the desired cross-section out of a single sheet and then laminating and curing the stacked punched-out forms. This method is desirable to eliminate waste since the uncured trim readily may be reworked. FIGURE 3 illustrates this procedure. The punched-out forms 40 from the single sheets are stacked in a cavity 41 within a mold 42 having end pieces 43 which may be moved so as to compress the sheets within the mold. Heat is then applied in any suitable manner so as to cure the sheets.

A suitably magnetized specimen containing 65% barium ferrite by volume made in accordance With the method of this invention had a residual induction of about 2100 gauss, a coercivity of 1200 oersteds, and a maximum energy product of .9 10 gauss-oersteds. The magnet can be handled and worked freely without danger of breakage and may readily be cut with a knife or other edged tool. The same material measured at right angles to the preferred direction of mechanical alignment had a maximum energy product of 28x10 gauss-oersteds, a residual induction of 1200 gauss, and a coercivity of 800 oersteds. In place of magnetizing after curing, a magnetizing field as illustrated by the lines 44 may be applied while the magnet there formed is being cured in the mold by making the end caps 43 of the mold themselves serve as the pole pieces of an electromagnet 13.

Example 2 This example generally follows the preceding example except that lead ferrite is substituted for barium ferrite as the magnetic material. Lead ferrite may be produced as follows: 17.5 parts by weight of lead monoxide (1.5 mol PhD) is intimately mixed with 50 parts by weight of ferric oxide (6.0 mol Fe O This mixture is fired in a surrounding atmosphere of air, starting from 700 C. and increasing the temperature gradually to 900 C. over a period of six hours in order to produce crystalline lead ferrite. After quenching in air, the lead ferrite so produced is then milled to domain size (for example, by grinding two hours in the attrition mill, then heat treated for 15 minutes at 850 C. and reground for one hour), after which it is dried.

The matrix composition to which the lead ferrite is added may be the same as that described above, except that the lead ferrite is added in the amount of 116.0 parts by weight. In this amount the ferrite comprises 57% by volume of the composite. Other matrix materials may be used in place of rubber as previously described.

In respect to this and other examples concerning the practice of the invention, it is to be noted that the maximum energy product, coercivity, and other magnetic qualities exhibited by the final material varies as to the nature of the particular ferrite selected, the manner in which it is prepared, the grinding period, and the nature of the matrix material, etc.

Example 3 ture thus prepared is fired in an air atmosphere for approximately one hour at a temperature of 1250 C. and then milled and treated as described in Example 1.

Strontium ferrite so produced is incorporated with the rubber in the amount of 123.0 parts by weight, the weights of the other components being as specified previously. In this amount it is 62% by volume of the composite.

Example 4 Those skilled in the art will readily understand that a wide variety of thermo-plastic or thermo-setting materials may be used to form the matrix in place of rubber. For example, the ferromagnetic material may be incorporated into a plastic of the polyvinyl chloride type. In such cases a material is selected which is susceptible of being sheeted between rolls or of being extruded through a narrow orifice into elongated shapes Whose surface area is great relative to their volume. The shearing forces set up within the composition during the extrusion or milling process cause local orienting movement of one portion of the composition relative to another portion, and apparently therein lies the reason for the observed orientation which takes place.

It will be understood that the chemical and physical characteristics of the particular matrix material selected will determine the exact nature of the milling or orientation process, but the fundamental conception remains the same in that the milled composition, whatever its nature, enables the potentially permanently magnetic domain particles tobe oriented or aligned in a mechanical way which affords excellent utilization of their potential properties.

Having described my invention, I claim:

1. A permanent magnet material comprising a laminate of sheet forms each sheet comprising roughly plate-like, substantially domain size anisotropic particles of at least one permanent magnet substance selected from the class consisting of the ferrites of barium, strontium and lead, said particles individually having single predominantly preferred magnetic axes which are consistently normal to their respective general planes, said particles being adhered by a physically coherent, non-magnetic binder selected from the class consisting of rubbers, elastomers, and resins, said particles being so oriented with respect to each other and to said material that the planes of said plate-like particles are substantially parallel to each other and the preferred magnetic axes of said particles are substantially parallel to each other and are substantially perpendicular to the surface of said material, said laminate having a preferred direction of magnetization which is substantially normal to the planes of the sheet material comprising the laminate.

2. A permanent magnet produced by magnetizing a laminate in accordance with claim 1.

References Cited UNITED STATES PATENTS 2,566,441 9/1951 Camras 25262.5 2,589,766 3/1952 Bradley 317198 2,655,195 10/1953 Curtis 252-625 2,825,670 3/1958 Adams et al. 25262.5 2,837,483 6/1958 Hakker et al. 252-625 2,959,832 11/1960 Baermann 252-625 2,999,275 9/1961 Blume 252--62.5

FOREIGN PATENTS 746,492 3/ 1956 Great Britain. 758,320 10/1956 Great Britain.

OTHER REFERENCES Article: Fine-Particle Magnets, pages 851-857; Electrical Engineering, October 1957.

TOBIAS E. LEVOW, Primary Examiner.

SAM'UEL BERNSTEIN, JULIUS GREENWALD,

MAURICE A. BRINDISI, Examiners.

A. C. MARMOR, J. B. MILSTEAD, S. R. BRESCH,

R. D. EDMONDS, Assistant Examiners.

Claims (1)

1. A PERMANENT MAGNET MATERIAL COMPRISING A LAMINATE OF SHEET FORMS EACH SHEET COMPRISING ROUGHLY PLATE-LIKE, SUBSTANTIALLY DOMAIN SIZE ANISOTROPIC PARTICLES OF AT LEAST ONE PERMANENT MAGNET SUBSTANCE SELECTED FROM THE CLASS CONSISTING OF THE FERRITES OF BARIUM, STRONTIUM AND LEAD, SAID PARTICLES INDIVIDUALLY HAVING SINGLE PREDOMINANTLY PREFERRED MAGNETIC AXES WHICH ARE CONSISTENTLY NORMAL TO THEIR RESPECTIVE GENERAL PLANES, SAID PARTICLES BEING ADHERED BY A PHYSICALLY COHERENT, NON-MAGNETIC BINDER SELECTED FROM THE CLASS CONSISTING OF RUBBERS, ELASTOMERS, AND RESIN, SAID PARTICLES BEING SO ORIENTED WITH RESPECT TO EACH OTHER AND TO SAID MATERIAL THAT THE PLANES OF SAID PLATE-LIKE PARTICLES ARE SUBSTANTIALLY PARALLEL TO EACH OTHER AND THE PREFERRED MAGNETIC AXES OF SAID PARTICLES ARE SUBSTANTIALLY PARALLEL TO EACH OTHER AND ARE SUBSTANTIALLY PERPENIDCULAR TO THE SURFACE OF SAID MATERIAL, SAID LAMINATE HAVING A PREFERRED DIRECTION OF MAGNETIZATION WHICH IS SUBSTANTIALLY NORMAL TO THE PLANES OF THE SHEET MATERIAL COMPRISING THE LAMINATE.

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