Method Of Fabricating Radially Oriented Magnets

Abstract

A method of fabricating radially oriented particle disc or arc magnets comprising forming a flexible sheet containing magnetic particles with their easy magnetic axes perpendicular to the sheet surface, winding the sheet, cutting to the desired configuration, laminating each cut portion, and sintering each laminated portion to form the finished permanent magnet. The fabricated anisotropic magnets produced by the process of this invention can be used in eddy current devices and permanent magnet DC motors.

Description

BACKGROUND OF THE INVENTION

Permanent magnets which are used in DC motors or eddy current devices characteristically have an arc or disc shape. It is well known that magnets, in which the individual particles are oriented, have magnetic properties in the direction of orientation superior to isotropic magnets, i.e., magnets having a random particle distribution. Arc or disc magnets for use in DC motors or eddy current devices ideally would have a radial particle orientation. That is, the easy axis of magnetization of each particle would be perpendicular to the curved surface of the magnet. However, only partially oriented arc or disc magnets are commercially available. These are generally made by extrusion wherein the orientation is obtained mechanically or by compacting in a magnetic field.

It is an object of this invention to provide a novel method of fabricating ceramic magnets having a desired configuration with improved radial particle orientation.

SUMMARY OF THE INVENTION

This invention relates to a method of fabricating ceramic magnets having a radial particle orientation comprising, forming a flexible sheet containing organically bonded magnetic particles in which the easy magnetic axes of the particles is perpendicular to the sheet surface, winding said sheet in the form of a cylinder to provide radial particle orientation, cutting from said cylinder portions having the desired configuration, laminating each cut portion, and sintering each laminated cut portions to complete the formation of the ceramic magnets. The magnets produced by this process are considered to be novel and superior to those of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first step of the process involves forming a flexible sheet of organically bonded magnetic particles in which the easy magnetic axes of the particles is perpendicular to the sheet surface. This may be accomplished by any conventional procedure such as described in U.S. Pat. Nos. 3,110,675 and 3,163,922. A simpler method involves forming a slurry of the magnetic material, doctor blading a sheet of the slurry in a magnetic field to orient the magnetic particles, and drying the sheet.

The magnetic material utilized can be any of the well-known ceramic materials, but the materials must have an easy axis of magnetization which will orient parallel to an applied magnetic field. The high coercivities of the ceramic magnetic compounds commonly used in permanent magnets are indicated by their high crystalline anisotropy constants. These constants are a measure of the preference for the individual electron spin moments to be aligned in a specific crystallographic direction (easy direction of magnetization). To gain the full benefit of this high crystalline anisotropy, the particle size of the individual grains must be small enough such that by energy considerations, the electron spin moments in the particle are all aligned parallel to the easy axis. Therefore, it is preferable, although not necessary, that the magnetic material be composed of single domain particles; such ceramic particles will usually have a particle size of about 3 microns or less. If a particle is larger than single domain, the lowest energy configuration will yield regions of opposing magnetic moments separated by a domain boundary (multidomain particles). The hexagonal ferrites, e.g., (Ba, Sr, Pb) Fe.sub.12 0.sub.19, are ideally suited for the process of this invention, but other magnetic materials fulfilling the above requirements can also be used.

The magnetic materials are made by conventional techniques. For example, the raw materials e.g. iron oxide, barium oxide, etc. are measured out and mixed with a sufficient amount of fluid and blended in a ball mill as a slurry. This slurry is very thoroughly mixed and then filtered to remove most of the fluid. The mixed oxide is then dried and fired to form the magnetic material. The magnetic material is then ground in a ball mill until the desired particle size is obtained.

In the preferred embodiment of this invention, the finely divided magnetic material is dispersed in an organic binder system. This system must contain a suitable binder material which will hold the magnetic particles together so that a sheet of magnetic material may be formed, and will provide the necessary flexibility to the sheet containing magnetic particles. Also, the binder must be volatilizable so that it will burn off during the sintering operation. Suitable binders include thermoplastic resins, thermosetting resins and other resins which may be formulated to cross-link during the drying operation. Typical resins include polymers of ethylene, polymers of vinyl acetate, copolymers of ethylene and vinyl acetate, acid terpolymers of ethylene and vinyl acetate, acrylic resins, esterified epoxy resins, polyurethane resins, acetals of polyvinyl alcohol, and blends of the above. The binder is usually dispersed or dissolved in a suitable liquid media, e.g. water, organic solvents, etc.

The dispersion of magnetic material in the organic binder systems can be made, for example, by ball milling the mixture for several days. It is important that particle agglomeration does not occur in the dispersion since this will inhibit orientation in an applied magnetic field. A thin sheet is formed by doctor blading the dispersion onto a smooth surface e.g. plastic, glass metal) with an applied magnetic field perpendicular to the surface of the sheet The magnetic field orients the easy magnetic axis of the particles perpendicular to the surface of the sheet. The magnetic field strength should be at least 400 oersted. This sheet is thereafter dried so that it can be handled in further processing steps.

The next step is very significant in that it provides the desired radial particle orientation. The sheet of magnetic material is wound into a cylinder, for example, on a suitable mandrel. This cylinder may be described as a "jellyroll" of magnetic material. The cylinder or jellyroll is then cut to form portions having the desired configuration or shape. In this cutting process various shaped magnets such as discs or arcs can be formed. These cut portions are then laminated to form a rigid structure. It has been found that laminating is necessary to provide magnets which have a unitary structure and do not exhibit separation between the various layers. In a preferred laminating procedure, both heat and pressure are applied to form a rigid structure having a density greater than 3.5 g/cc. Characteristically, a pressure greater than 5,000 p.s.i. and a temperature greater than 150.degree. C. are used in the laminating step.

Lastly, the "green" magnet is sintered under carefully controlled conditions to form the dense radially oriented magnet. Well-known sintering techniques may be utilized. Care must be taken to insure that the organic binder burns off without damaging the formed structure. This can be accomplished by slowly heating the laminated bodies to the sintering temperature. For hexagonal ferrite radially oriented magnets, sintering is normally carried out at about 1,250.degree. C. for 1 hour.

In order to further illustrate this invention, the following example is given. A dispersion was prepared by ball milling a mixture containing 64 grams SrFe.sub.12 0.sub.19 (average particle size 1 micron) and an organic binder system for 2 days in a steel mill jar with steel grinding media. The organic binder system contained 4.6 grams Elvax 4260 (ethylene/vinyl acetate/acid terpolymer), 112 grams perchloroethylene, 2 grams soya lecithin dispersing agent and 3 grams of an antifoam agent. This dispersion was prepared by ball milling the mixture for 2 days in a steel mill jar with steel grinding media. The dispersion was doctor bladed onto a Mylar surface with a 600 oersted field perpendicular to the sheet surface and the sheet was allowed to dry. The flexible magnetic sheet was wound by hand on a 1/4-inch-diameter rubber cylinder to form a 2 inch diameter cylinder of magnetic sheet. The magnetic sheet was cut into discs having a height of about three-eighths inch. The discs were laminated at 200.degree. C. and under 10,000 p.s.i. Then the laminated discs were sintered by heating to 1,250.degree. C. for 1 hour.

The degree of orientation in the magnets of this invention was tested as follows:

Several 3/4-inch-diameter magnets were formed by stacking numerous disc portions which were cut from a magnetic sheet prepared as described above; the stacked discs were laminated and sintered as above. The measured magnetic properties of the magnet were Br 3,500 gauss and Hc 2,000 oe.

The process of this invention produces permanent magnets having highly radially oriented particles. Consequently, these magnets can be used wherever disc or arc magnets are required, such as in automobiles, electric razors, electric knives, etc.

Claims

I claim:

1. A method of fabricating ceramic magnets having a radial particle orientation comprising, forming a flexible sheet containing organically bonded magnetic particles in which the easy magnetic axes of the particles is perpendicular to the sheet surface, winding said sheet in the form of a cylinder to provide radial particle orientation, cutting from said cylinder portions having the desired configuration, laminating each cut portion, and sintering each laminated cut portion to complete the formation of the ceramic magnets.

2. A method of fabricating ceramic magnets having a radial particle orientation comprising, forming a dispersion of magnetic particles in an organic binder system, applying the dispersion to a smooth surface to form a thin sheet while maintaining a magnetic field to align the particles with their easy axis perpendicular to the sheet surface, drying the sheet, winding the sheet to form a cylinder, cutting from said cylinder portions having the desired configuration, laminating each cut portion, and sintering each laminated cut portion to complete the formation of the ceramic magnets.

3. A method in accordance with claim 2 wherein the magnetic particles are selected from the group consisting of BaFe.sub.12 0.sub.19, SrFe.sub.12 0.sub.19, PbFe.sub.12 0.sub.19 and mixtures thereof.

4. A method in accordance with claim 3 wherein the magnetic particles are single domain particles.

5. A method of fabricating ceramic magnets in accordance with claim 2 wherein the laminating is carried out by the action of heat and pressure.

6. A method of fabricating ceramic magnets in accordance with claim 2 wherein the magnetic field used to align the particles is within the range of 400-4,000 oersted.

7. A method of fabricating ceramic magnets in accordance with claim 2 wherein the sheet is formed by doctor blading.

8. A method of fabricating ceramic magnets in accordance with claim 2 wherein the sheet is wound on a mandrel.