Flexible Ferrite Permanent Magnet And Methods For Its Manufacture

Abstract

A flexible permanent magnet is described that contains single-crystalline ferrite particles embedded in an elastomeric binder. The ferrite particle size is much larger than that of the ferrites in the prior-art flexible magnets which results in greatly improved magnetic and mechanical properties. The large-size, single-crystalline ferrite particles are obtained by preparing a highly oriented, fully sintered hexaferrite and by pulverizing the hexaferrite into a relatively coarse powder.

Description

During the last ten years, a new type of permanent magnet has found widespread use. It is a flexible magnet made by mixing substantially domain-size particles of a hexaferrite with a flexible binder and by molding the mixture, usually by extrusion. Hexaferrites are permanent magnets that can be described by the formula M O .sup.. 6 Fe.sub.2 O.sub.3 wherein M stands for at least one element out of the group Ba, Sr, Pb, and Ca, the Ca - content being usually limited to 2 wt% of the ferrite. Other names for these hexaferrites are ceramic magnets, ferrite permanent magnets,hard ferrites, magnetoplumbites, etc. The hexaferrite for flexible magnets has generally been a lead-modified barium ferrite in most applications. The flexible binder has usually been either a natural rubber or a mixture of chlorosulfonated polyethylene and polyisobutylene. The properties of these magnets and methods for their manufacture have been carefully investigated and are disclosed in numerous patents and publications. The principal application for these magnets has been the so-called magnetic gasket which is used today, for example, for sealing and closing most refrigerator doors.

The usual procedure for making such flexible magnets has been the following: 18 wt% barium carbonate BaCO.sub.3, 2 wt% leadmonosilicate Pb0 .sup.. SiO.sub.2 are mixed with 80 wt% chemically prepared ferric oxide Fe.sub.2 O.sub.3. (The term chemically prepared ferric oxide is used here in contrast to the terms natural ferric oxide or iron ore, the chemically prepared ferric oxides being far more expensive than the natural ones. The ferric oxide generally used in the manufacture of the prior-art flexible magnets is an oxide produced by the calcination of ferrous sulfate FeSO.sub.4 .sup.. H.sub.2 O). The mixture of Fe.sub.2 O.sub.3, BaCO.sub.3 and PbO .sup.. SiO.sub.2 is heated in air to approximately 2150 .degree.F, usually in a rotary furnace. The barium carbonate decomposes into barium oxide BaO which then reacts with the ferric oxide Fe.sub.2 O.sub.3 to form the hexaferrite crystals of the type BaO .sup.. 6 Fe.sub.2 O.sub.3. The lead-monosilicate PbO.sup.. SiO.sub.2 is added to promote the formation of plate-shape barium ferrite particles and to insulate the ferrite crystals from each other so that the intrinsic coercive force stays high. The expensive, chemically prepared ferric oxide is used because it was found with the prior-art procedures that flexible magnets made by using natural ferric oxides did not exhibit the desired magnetic properties. The reacted ferrite is cooled to room temperature and is milled with steel balls until essentially all ferrite particles are of single-domain size. The critical size, below which a single-domain particles has a lower energy than a multi-domain particle, is approximately 1.0 .mu.m for the hexaferrites as is described in the prior art. Such small-size particles have the characteristics of a dust. During the milling into single-domain size, the lattices of the ferrite crystals become distorted which results in a substantial loss in magnetic properties. Therefore, the single-domain size powder is usually annealed at a temperature of approximately 1700.degree.F to heal out the lattice defects and to restore the magnetic properties. Approximately 91 wt% of the annealed ferrite powder is mixed with 9 wt% elastomeric binder, usually between a pair of rolls heated to 200.degree.F. The flexible sheets of mixed ferrite and binder are taken off the rolls and are then further processed, usually in a short screw extruder that extrudes the ferrite-binder mix at a temperature of approximately 220.degree.F into flexible magnet strips of various sizes, one typical size being 0.350 inches wide and 0.150 inches thick. The magnet strip is magnetized usually with two poles on only one side of the strip.

One problem with these prior-art, extruded, flexible magnets is their low maximum energy product (BH).sub.max which has been less than 0.7 MGOe in most applications -- far lower than that of the other permanent magnets in use today. Another problem has been the lead content in the prior-art flexible magnets. The lead was found necessary in order to obtain the optimum magnetic properties. When no lead additions were tried, it was found that the (BH).sub.max of the flexible magnet strip was greatly reduced. Because of the toxic nature of the lead-containing material, great precautions had to be taken in the prior-art procedure, particularily during the heating steps, during which toxic, lead-containg vapors are generated. Also, great precautions had to be taken so that the lead-containing single-domain-size dust would not be inhaled by the workers during the mixing operation with the elastomeric binder. Furthermore, the lead-containing flexible magnets could generally not be used in applications, such as certain toys, where there is a chance that small children may eat them.

Another problem with the prior-art flexible magnets has been the high raw material cost because the basic raw material has been the expensive, chemically prepared ferric oxide. Because of this, the prior-art flexible magnets have been relatively expensive. Another problem with the prior-art flexible magnets is that they break relatively easily when bent because their mechanical properties are poor compared to those of the binder without ferrite. This is related to the use of the single-domain-size dust, such dust not only being difficult to handle and difficult to mix with the flexible binder, but also causing the mechanical properties to deteriorate at the high volume percentages of ferrite that are needed for obtaining optimum magnetic properties. Of course, it was known that a larger ferrite particle size would be highly desirable because this would allow higher volume percentages of ferrite, improve the mechanical properties, and simplify the processing. However, all previous attempts of obtaining suitable flexible magnets with larger ferrite particles have failed. With the prior-art procedure it was found necessary to use the single-domain-size dust in order to prepare flexible magnets with optimum magnetic properties.

A still further problem with the prior-art procedure is the requirement of annealing the single-domain-size dust after the milling operation. This processing step has been difficult to carry out because of the problems involved in moving a small-particle dust through a high-temperature furnace. The rotary tube furnaces used for obtaining the inital ferrite reaction can not be used bacause the single-domain-size dust is not free-flowing and stickes to the walls of the furnace tube. Obviously, a combustion gas can not be used directly to heat the single-domain-size dust because such gases would blow the dust out through the furnace stack. For these reasons, elaborate furnaces had to be used, and the annealing operation has been a very costly processing step.

It is the primary object of this invention to provide a flexible, extruded ferrite permanent magnet strip having greatly improved magnetic properties.

It is another object of this invention to provide a flexible ferrite permanent magnet that is safe to use and contains no lead or any other toxic ingredient.

A still further object of this invention is to provide lower-cost, flexible magnets by using iron ore as the basic raw material in place of the chemically prepared ferric oxide.

It is a further object of this invention to provide a flexible ferrite permanent magnet with greatly improved mechanical properties.

Another object of this invention is to provide a process of making flexible ferrite permanent magnets in which no powder dust has to be heat-treated, in particular a process in which the expensive annealing step is eliminated.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. For a better understanding of the nature and objects of this invention, reference should be had to the following detailed description and drawings, in which:

FIGS. 1 and 1a provide is a schematic comparison of a multi-domain particle having single crystal characteristic with a single-domain particle.

FIG. 2 is a schematic diagram, partially in cross section, of portions of a pressing and orienting apparatus arranged for making multi-domain particles having single crystal characteristic.

FIG. 3 is a schematic diagram, partially in cross section, of portions of an extrusion apparatus for making one type of magnet of this invention.

FIG. 4 is a schematic drawing of a magnet of the invention.

FIGS. 5 and 5a illustrate alternative distribution of the lines of flux in the magnet of FIG. 4.

FIG. 6 is a plot of the maximum enenrgy product against the ferrite particle size for the extruded, flexible magnets of this invention.

FIG. 7 is a plot of the maximum energy product against the SiO.sub.2 -content of the ferrite component of the extruded, flexible magnets of this invention.

FIG. 8 is a plot of the maximum energy product against the mole ratio Fe.sub.2 O.sub.3 /BaO for the extruded, flexible magnets of this invention.

FIG. 9 is a comparison of the intrinsic demagnetization curves of various flexible magnets of this invention with the curve for the prior-art magnet.

Referring to FIG. 1 and FIG. 1a, there is shown a schematic comparison of a multi-domain ferrite permanent magnet particle 1 having single-crystal characteristics with a single-domain particle 2. The term "domain" is used here, as it generally is, to describe the volume portions of the material in which the ferromagnetic alignment is in one direction. Particle 1 is made up of many domains 3, the alignment in each domain indicated by an arrow 4 pointing up or pointing down whereas the particle 2 is made up of only one domain. (Prior to magnetization, the net moment of magnetization is essentially zero in particle 1 because there are an equal number of domains aligned up and down which tends to cancel out the magnetization). The term "single crystal" is used here, as it usually is, to describe a crystal structure where all crystal planes are parallel to the corresponding planes through the elementary lattice cell and where all atoms are arranged in the same ordered structure throughout the material. Of course, it is understood that the actual shapes of particles 1 and 2 may be different than the ones schematically shown in FIG. 1 and FIG. 1a. Also, the actual shapes and numbers of the domains 3 may be different than the ones shown schematically for particle 1 in FIG. 1. Particle 1 has essentially single crystal characteristics because all arrows are essentially parallel. The present invention is primarily directed toward barium ferrite and strontium ferrite flexible permanent magnets, and - as is known from numerous publications -- such ferrite materials are hexagonal and are strongly anisotropic having a preferred direction perpendicular to the basal plane. For barium ferrite and for strontium ferrite all arrows would be perpendicular to the basal, hexagonal lattice and would point in the so-called easy direction. The prior-art flexible ferrite permanent magnets are made from single-domain-size particles, such as particle 2 in FIG. 1a, and it was generally believed that such particles are needed for obtaining flexible ferrite permanent magnets with optimum properties. Surprisingly, we have discovered that greatly improved flexible ferrite permanent magnets can be made by using multi-domain particles having single-crystal characteristics, such as particle 1 in FIG. 1. Whereas the single-domain particles of critical size have a length of approximately 1.0 .mu.m, the multi-domain particles having single crystal characteristics preferably have a length of approximately 5 .mu.m. Thus the volume of the multi-domain particles having single crystal characteristics is considerably larger than that of the single-domain particles.

The magnets of the invention and the processes for their manufacture will be described with particular reference to FIGS. 2 through 5a. Multi-domain ferrite permanent magnet particles having predominantly single crystal characteristics can be prepared in several ways. One of them consists of preparing first the single-domain-size particles in essentially the same way as has been practiced in the prior art -- except, preferably, with no lead addition and by using natural iron ore as the basic raw material. Beneficiated or unbenficiated iron ore powder is mixed with barium carbonate or strontium carbonate, and the mixture is heated to a temperature higher than 2000.degree.F to form the hexaferrite. The ferrite clinkers are broken up and milled until the ferrite particles are essentially of single-domain size. The single-domain particles are suspended in a liquid to form a slurry as is common practice in the manufacture of the anisotropic hexaferrites, and several methods can be used of preparing highly oriented, dense, ferrite agglomerates from the ferrite flurry. One method is the one described schematically in FIG. 2 which is essentially FIG. 1 of U.S. Pat. No. 3,412,461. In this method, which is widely practiced today in the manufacture of the fully sintered hexaferrites, a highly anisotropic green ferrite magnet body having the proper density is obtained with relatively small amounts of mechanical and electric power. An upper press member 4 and lower press member 5 are shown which, with the die wall 6, define a cavity 7 for containing a ferrite slurry. Two coils 8 and 8' are shown in part in cross section. These coils have the function of establishing the orienting magnetic field, the lines of force of which are shown schematically in the cavity at 9. It will be noted that the lines of force are straight in the inner portions of the die cavity and are curved only at the edges of the cavity as at 10. Accordingly, the single-domain-size ferrite particles are all oriented strongly and uniformly in parallelism at all portions of the body except near the edges. In operation, the punch member 5 is raised to operative position and the punch member 4 is lowered into the die member 6 to form the cavity 7 into which a ferrite slurry is pumped through means not shown. Simultaneously with the pressing action, coils 8 and 8' are energized producing the magnetic field indicated at 9 and 10 which operates to orient the ferrite particles. Following the orientation operation, the punch member 4 is raised and the die member 6 is lowered exposing the green ferrite plate.

The green ferrite plates are broken up, and the ferrite pieces are sintered in one of several ways, the most economical one being in a directly gas-fired rotary kiln that is lined with a refractory brick. Because of the high density and relatively large size of the ferrite pieces, very high production rates are obtainable with only a small rotary kiln. The sintered ferrite pieces are crushed into a suitable size, such as to an average size of approximately 5 .mu.m. Ferrite particles processed in this manner are multi-domain and are almost perfect single crystals. Of course, there are other methods for the preparation of such multi-domain ferrite particles having predominantly single-crystal characteristics.

The ferrite particles are mixed with an elastomeric binder in a conventional mixer, such as between rolls or in a Banburry mixer. The mixture of ferrite and binder can then be brought into suitable shape for feeding into the hopper of a screw extruder, for example, by pelletizing in a chopper.

Referring now to FIG. 3, there is seen the cross section of a portion of a screw extruder 11 with the usual heating coils 12 and 13 which provide the heat for keeping the mixture 14 at a temperature of 200.degree.F or higher. A flexible magnet strip 15 is extruded through the nozzle 16 through the action of the screw 17.

Referring to FIG. 4, the flexible magnet strip 18 can be magnetized with one pair of poles on side 19 only, Of course, it is understood that, when the multi-domain particles having predominantly single crystal characteristics are magnetized, the domain structure changes compared to the domain structure shown for particle 1 in FIG. 1.

Referring to FIG. 5 and FIG. 5a, there are seen two basic types of flux distribution in a cross section of the magnet strip. The lines of flux 20 are curved and go from the one pole to the other whereas the lines of flux 21 are straight with no flux in the small nonmagnetized portion 22. When the latter flux distribution is used, it is desirable to fasten a steel back-up strip 23 to the back of the magnet strip providing a return path for the flux. Although this greatly reduces the flexibility of the magnet arrangement, such a method can be used at an advantage because the holding force is increased, particularly if there is a preferred orientation of the ferrite particles, with the basal plane parallel to the two main surfaces of the strip. Of course, other flux distributions and pole configurations can be used. The extruded magnet strip with or without steel back-up strip can be inserted in a balloon gasket composed of, for example, polyvinylchloride. Such a gasket can be used to close and seal doors, such as refrigerator or storm doors.

Flexible magnets made in accordance with the described process have not only greatly improved magnetic and mechanical properties, but are also lower in cost than the prior-art flexible magnets when natural iron ore is used as the basic raw material. They are safer to use because they can be made without lead. We believe that the surprising improvement in the magnetic and mechanical properties of the flexible magnets is due to a combination of several effects. The mechanical properties are improved because there is less ferrite surface in contact with the elastomeric binder. Also, the shape of the large particles having single crystal characteristics is better suited for the elastomeric binder systems than that of other ferrite particles. The magnetic properties are improved for several reasons. There are fewer, nonmagnetic gaps in the flexible magnets through which the magnetic flux must pass. Because of the fewer non-magnetic gaps, the hysteresis loop is less sheared, and the remanence B.sub.r and the maximum energy product (BH).sub.max are increased. The large particles having predominantly single crystal characteristics have a considerably higher physical density than other types of ferrite particles at the same level of intrinsic coercive force H.sub.ci, particularily when natural iron ore is used as the basic raw material for the flexible magnets. Because of the high physical density of the particles, the remanence B.sub.r and the maximum energy product (BH).sub.max are further increased. The shape of the large particles having single crystal characteristics is also particularily suited for the proper magnetic stacking in the elastomeric binder system, particularly when an anisotropic or partially aniotropic flexible magnet is desired. Of course, it is readily understood that the flexible ferrite permanent magnets of this invention can be made anisotropic with considerably less effort than the prior-art flexible magnets because the friction forces on the interfaces between ferrite and binder, which restrict the alignment of the crystals, are far smaller for large than for small ferrite particles.

Although we have found that excellent properties can be obtained without lead additions, we have found also that some additions to the binary ferrite system are highly desirable, the preferred addition being the silicon oxide SiO.sub.2. In the preferred embodiment of this invention, the ferrite componet of the flexible magnet should contain between 0.1 and 0.5 wt% SiO.sub.2. Such an addition is safe to use, among others, because SiO.sub.2 has a far lower vapor pressure than PbO at high tempertures. The magnetic stacking in the elastomeric binder system is improved when the described SiO.sub.2 -- addition is used in the flexible magnet of this invention. This increases the remanence B.sub.r and the maximum energy product (BH).sub.max. Other beneficial additives that can be used in place of the lead compound - with and without SiO.sub.2 -- are the sulfates SrSO.sub.4, BaBO.sub.3, CaSO.sub.4, and Na.sub.2 SO.sub.4 and other refractory oxides, particularily CaO and Al.sub.2 O.sub.3. The preferred ranges for these additives are the following: 0.05 to 1.0 wt% SO.sub.4 ; 0.1 to 1.0 wt% CaO; 0.1 to 1.0 wt% Al.sub.2 O.sub.3. A combination of SiO.sub.2 and Al.sub.2 O.sub.3 is particularily beneficial. Surprisingly, the expensive annealing step is not needed. Evidently, during the milling of the multi-domain particles having single crystal characteristics very few crystal imperfections are introduced. As a result, the remanence B.sub.r and the intrinsic coercive force H.sub.ci do not deteriorate noticeably during the milling, in contrast to the conditions of the procedures of the prior art.

We have further found that, when optimum magnetic properties are desired, the mole ratio between the ferric oxide Fe.sub.2 O.sub.3 and the earth alkaline oxide BaO or SrO should be between 4.5 and 5.5, with the best properties obtained at a mole ratio of 5.2 or 5.3. This is surprising because the optimum mole ratio for obtaining the highest possible saturation moment for barium ferrite or strontium ferrite is approximately 5.9 and the optimum mole ratio for obtaining the highest maximum energy product (BH).sub.max on the sintered hexaferrites is 5.6 or 5.7. Evidently, a lower mole ratio than is normally used for these ferrites gives a better combination of properties for making the flexible magnets of this invention. It apperas that a more plate-like particle shape is formed at a mole ratio of 5.2 or 5.3 than at the higher ratios normally used for these ferrites and that the improved particle shape enhances the ferrite distribution in the elastomeric binder system.

As has already been mentioned, iron ore is the preferred basic raw material for the flexible magnets of this invention. By "basic" is meant here that at least 50 wt% of the flexible magnet can be derived from iron ore -- and preferably more. When barium ferrite flexible magnets are made, the content of the flexible magnet derived from the natural iron oxide can be approximately 76 wt%. This amount is calculated as follows: The preferred starting mix for the ferrite reaction consists of essentially 19 wt% BaCO.sub.3, 80 wt% natural iron oxide Fe.sub.2 O.sub.3 and 1 wt% additions, mostly SiO.sub.2, sulfates, CaO and/or Al.sub.2 O.sub.3. 4.3 wt% of this mix is lost during calcining, mostly in the form of CO.sub.2 which goes out through the furnaces stack. This increases the Fe.sub.2 O.sub.3 -- content in the barium ferrite clinker to 83.2 wt%. During further processing there is a small further increase in the Fe.sub.2 O.sub.3 -- content of the barium ferrite because there is some wear of the steel balls during the ball milling, the wear of the steel balls being oxidized to essentially Fe.sub.2 O.sub.3 during the process. There is also a slight loss in BaO during the pressing of the ferrite slurry, the BaO being lost through the filters. For these reasons, the final preferred Fe.sub.2 O.sub.3 -- content in the ferrite component is approximately 83.5 wt%. To make the flexible magnet, the preferred starting mix is 91 wt% ferrite and 9 wt% elastomeric binder. This means that approximately 76 wt% of the flexible barium ferrite magnet consists of Fe.sub.2 O.sub.3. Obviously, when strontium ferrite flexible magnets are made, the weight percentage of Fe.sub.2 O.sub.3 in the flexible magnet is still higher because SrO is lighter than BaO, the preferred mole ratio being essentially the same for both ferrites, namely 5.2 to 5.3, as was pointed out above. A similar calculation shows that the Fe.sub.2 O.sub.3 -- content in the flexible strontium ferrite magnet is between 80 and 81 wt%.

These calculations show that, despite the relatively low mole ratios in the ferrites, the basic raw material for the flexible magnets can be iron ore if iron ore is used as the source for the ferric oxide. It is not yet clearly understood why the flexible magnets of this inventions have such particularily outstanding properties when iron ore is used as the basic raw material. It appears that a silica-containing phase forms a film along the grain boundaries of the iron ore particles and that this film is being carried over into the barium ferrite or strontium ferrite. The effect of this film apparently is that particularily beneficial ferrite particle shapes are formed during the sintering and milling which in combination with the elastomeric binder system give flexible magnets with particularily useful properties.

There are several methods available for determining whether a hexaferrite particle is a single crystal or a polycrystal or whether it is a single-domain size or of multi-domain size. Direct optical measurements are difficult to carry out because of the tendency of the ferrite particles to cling together due to the magnetic attraction. Indirect methods are generally far easier to use. One method for determining the amount of single crystals in a ferrite powder aggregate is to try to align and fix the ferrite particles in a strong magnetic field and then measure the magnetic properties parallel and perpendicular to the direction of intended alignment. If the powder aggregate consists of perfect polycrystals, the properties are essentially the same in both directions. If the powder aggregate consists of perfect single crystals, the properties are very different in both directions due to the high crystalline anisotropy. By comparing the results of such measurements, the degree of single crystal characteristics can be determined quickly. The term "having predominantly single crystal characteristics," as defined here, is based on the results of such measurements, meaning that at least 50 percent of the ferrite particles behave like single crystals in this type of test.

Whether a particle is of single-domain size or of multi-domain size is determined by calculating the critical domain size and by measuring the size of the particle in question. The size of the particles can most easily be determined by indirect methods, the method used here being the Fisher Subsieve Sizer. In this method air is pumped through a powder sample. The average particle size is found from the air flow resistance of the sample. If the Fisher size is 1.0 .mu.m (1 micron) or less, the powder is of single-domain size for the purpose of this disclosure. For a preferred type of magnet of this invention, the Fisher size is 5 .mu.m. Such particles have more than one hundred times the volume of a single-domain size particle. However, a significant improvement in the properties of the flexible magnets of this invention is already found when the Fisher size is 2 .mu.m which means an average ferrite particle volume approximately ten times the volume of a single-domain particle. Of course, when the ferrite particles are much larger, as is desired for certain magnets of this invention, a conventional wire screen can be used to determine the average ferrite particle size. From the volume fraction of the ferrite powder aggregate retained on , for example, a 400-mesh screen and from the method used in preparing the ferrite powder, the average ferrite particle size can often be estimated quite accurately. For example, we have found that the Fisher subsieve size is approximately 6 .mu.m when 2 percent of the ferrite powder is retained on a 400-mesh screen and when we use 1 inches diameter steel balls in a conventional dry ball mill to pulverize single-crystalline barium ferrites. In order that the invention may be more clearly understood, a number of examples of the practice of the invention are now offered. As has already been pointed out, only one type of ferric oxide has been found suitable for the basic raw material of the prior-art flexible ferrite permanent magnets although many other types of iron oxide are available. This ferric oxide, which is the one used in the Examples 1 through 23, is produced by the calcination of ferrous sulfate. For a better understanding of the significance of some aspects of the present invention, a description will first be given for the preparation of this type of ferric oxide: Metallic iron is dissolved in sulfuric acid, and the liquid obtained in this manner is heated in a vacuum to precipitate out crystals of FeSO.sub.4 .sup.. 7 H.sub.2 O. The mixture of liquid and crystals is filtered, and the filter cake is heated to 300.degree.F to form FeSO.sub.4 .sup.. H.sub.2 O. The dried and partially dewatered filter cake is pulverized and screened. A certain particle size fraction of the dried cake is heated in a special acid-proof, high-temperature furnace at 1800.degree.F. The H.sub.2 SO.sub.4 formed thereby is seperated from the solids which consist of essentially 90 wt% Fe.sub.2 O.sub.3 and 10 wt% undecomposed FeSO.sub.4. The solid mixture is ball-milled in water to reduce the particle size. The slurry is washed several times to remove most of the FeSO.sub.4. The clean slurry is filtered, and the filter cake is dried at 300.degree.F to remove the water. Finally, the dried cake is pulverized to form the powder used for making today's ferrite permanent magnets.

Example 1A

An extruded, flexible ferrite magnet strip was made as follows: 800 g of ferric oxide Fe.sub.2 O.sub.3 produced by the calcination of ferrous sulfate were mixed in a steel ball mill in water for 4 hrs with 180 g barium carbonate BaCO.sub.3 and 20 g leadmonosilicate PbO .sup.. SiO.sub.2. The slurry was dried, and the dried cake was calcined at 2150.degree.F for 15 min. The calcined clinkers were pulverized, and the powder was milled in a steel ball mill in water for 16 hrs, after which the ferrite particles were found to be essentially of single-domain size. The ferrite slurry was pressed in a magnetic field of approximately 10 kOe at an end pressure of 3000 psi using the arrangement schematically shown in FIG. 2. The pressed green plates were broken up and sintered for 15 min at 2250.degree.F. The sintered pieces had an average degree of anisotropy (1 - B.sub.r / B.sub.r)= 0.81 where B.sub.r and B.sub.r are the remanences perpendicular to the direction of magnetic alignment. This indicates that they were predominantly single crystals. They were pulverized dry in a steel ball mill with steel balls into a multi-domain powder having predominantly single crystal characteristics, the average particle size being 4.6 Fisher microns. 1.9 g chlorosulfonated polyethylene (HYPALON 45) and 2.7 g polyisobutylene (VISTANEX L-140) were mixed at 200.degree.F on a standard two-roll rubber mill until the mixture was in sheet form whereupon 273 g of the ferrite powder was added and worked in. The spacing between the two rolls was 1/16 inches. Total milling time was about 10 minutes. The mixture was sheeted off the mill and granulated. The granules were fed into a conventional rubber extruder heated to 240.degree.F, with the die heated to 220.degree.F. A rectangular, flexible magnet strip was extruded having a width of 0.300 inches and a thickness of 0.150 inches. The strip was tested magnetically and mechanically. The test results are summarized in Table I.

Example 1B

An extruded, flexible magnet strip was made exactly as in Example 1A, except that the prior-art process was used. The ferrite slurry was dried after the 16-hr-ball-milling, and the dried powder was annealed for 2 hrs at 1700.degree.F, after which the powder was added to the HYPALON-VISTANEX binder exactly as in Example 1A, and an extruded, flexible magnet strip was made as in Example 1A. The average ferrite particle size was 0.9 Fisher microns. The properties of the extruded strip are described in Table I. A comparison between the test results of Example 1A and 1B shows that the flexible magnet of the invention has greatly improved magnetic and mechanical properties over the prior-art flexible magnet. Contrary to the teachings of the prior art, the strip with the large, single-crystalline-type particles is magnetically far superior to the prior-art magnets with the small, single-domain-size particles. Also, the expensive powder annealing step, which has caused so many problems in the past, is no longer needed because the powder of Example 1A was not annealed whereas the powder of Example 1B was annealed.

Example 2 through 8

Extruded, flexible magnet strip was prepared exactly as in Example 1A, except that the particle size and the weight percentages of the ferrite component of the strip was varied. The test results are summarized in Table I. The maximum energy product (BH).sub.max of the strip is plotted in FIG. 6 against the Fisher particle size. Two curves are seen, one for 91 wt% and one for 94 wt% ferrite. It is seen that the strip of Example 2 having a particle size of 2.1 Fisher microns is significantly improved over the prior-art strip -- Example 1B. The ferrite particles in the strip of Example 2 have a volume at least ten times the volume of the single-domain size particles. The particle size in Example 3 is 1.0 Fisher microns which means essentially single-domain size. (BH).sub.max has dropped to 0.8 MGOe which is still higher than the value for the Example 1B strip. This means that an improved strip -- and without the expensive annealing step -- can be made by using the process of this invention along with the teachings of the prior art of using single-domain size particles. However, as the data clearly show, a ferrite particle volume of at least ten times, and preferably of the order of one hundred times the volume of the single-domain particle is to be preferred. If the particle is too large (Examples 4 and 5), the mechanical properties are improved, but (BH).sub.max is decreased as FIG. 6 shows. When the percentage of ferrite in the strip is increased to 94 wt%, still higher (BH).sub.max values are obtained (Example 7) at the expense of the mechanical properties. As is seen from FIG. 6, the 94-wt%-curve peaks at a particle size of 6.6 Fisher microns (Example 7). (BH).sub.max is decreased at lower particle size (Example 6) and at larger particle size (Example 8). The mechanical properties of the extruded strip always improve with increasing ferrite particle size and with decreasing weight percentages of ferrite.

Example 9 through 15

The ferrite component of the extruded, flexible magnet strips of Example 1 through 8 contained approximately 1.6 wt% Pb. As has been mentioned, such a large amount of lead is undesirable for several reasons. Non-toxic additives were tried out in an effort to find a substitute for the lead compound. Flexible magnet strips were prepared exactly as in Example 1A, except that no leadmonosilicate was used and that the starting mix was slightly changed. The weight ratio Fe.sub.2 O.sub.3 /BaCO.sub.3 was kept at 4.26 in order to keep the mole ratio Fe.sub.2 O.sub.3 /BaCO.sub.3 in the extruded strip constant at 5.2 to 5.3. Additions of the SiO.sub.2 were used at the following levels: 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6 wt%. The SiO.sub.2 was added at the start of the first ball-milling in the form of finely pulverized quartz (5-MICRON Min-U-Sil). Table I gives the ferrite starting compositions and the test results on the extruded strip. The maximum energy product (BH).sub.max is plotted in FIG. 7 against the SiO.sub.2 -- addition. As is seen the curve peaks at 1.02 MGOe at 0.20 wt% SiO.sub.2 -- the same value of (BH).sub.max that was obtained in Example 1A with an addition of 2.0 wt% PbO .sup.. SiO.sub.2. Surprisingly, excellent magnetic properties can be obtained without lead with 0.20 wt% SiO.sub.2 and at a mole ratio Fe.sub.2 O.sub.3 /BaO of 5.2 to 5.3. As is seen from the data, a significant improvement in the properties of the flexible magnet strip is obtained when the SiO.sub.2 -- content lies in the range 0.1 wt% (Example 10) to 0.5 wt% (Example 14). At 0.6 wr% SiO.sub.2 (Example 15) the (BH).sub.max -- value has dropped to almost the level obtained with no addition (Example 9).

Examples 16 through 22

Extruded, flexible magnet strip was prepared exactly as in Example 11, except that the mole ratio Fe.sub.2 O.sub.3 /BaO in the strip was varied by varying the weight ratio Fe.sub.2 O.sub.3 /BaCO.sub.3 in the ferrite starting mix. Table I gives the ferrite starting compositions and summarizes the test results. The mole ratio of the ferrite starting composition was chosen 0.05 points lower in order to account for the steel pick-up and BaO-loss during processing. As is seen from FIG. 8, the highest (BH).sub.max -- value is obtained at mole ratios of 4.5 (Example 21), 4.75 (Example 20), 5.0 (Example 19), and 5.5 (Example 18). When the mole ratio is further increased or decreased, (BH).sub.max drops sharply. Thus, the data show that the preferred mole ratio for the flexible magnets of this invention is 4.5 to 5.5 when 0.2 wt% SiO.sub.2 are used as the only additive.

Example 23

Magnets were prepared exactly as in Example 13, except that 0.2 wt% SiO.sub.2 and 0.2 wt% Al.sub.2 O.sub.3 were added in place of 0.4 wt% SiO.sub.2. Table I summarizes the results of the tests. As is seen, the combination of SiO.sub.2 and Al.sub.2 O.sub.3 is particularly beneficial in obtaining outstanding properties. A (BH).sub.max -- value of 1.04 MGOe could be obtained without lead and at a ferrite percentage of only 91 wt%.

Example 24

An iron ore known as "Itabira Blue Dust" was used as the basic raw material. It is one of the purest hematite ores available, and it is mined in the Vale do Rio Doce in Brazil. Its impurity was 0.38 wr% SiO.sub.2 and 0.37 wt% Al.sub.2 O.sub.3. It costs ten times less than the expensive, chemically prepared ferric oxide. 810 g of the "Blue Dust" and 190 g of barium carbonate BaCO.sub.3 were milled in a ball mill in water for 8 hrs. The further processing into an extruded, flexible ferrite magnet strip was carried out exactly as in Example 1A. The average particle size in the strip was 4.8 Fisher microns. The ferrite component of the flexible strip contained 0.26 wt% SiO.sub.2 and 0.22 Al.sub.2 O.sub.3. The extruded magnet strip had a remanence B.sub.r = 2350 Gauss, an intrinsic coercive force H.sub.ci = 2700 Oe, a maximum energy product (BH).sub.max = 1.15 MGOe and a tensile strength of 1450 psi. These properties are considerably better than those of any of the strips of the above examples.

Example 25

Extruded flexible magnet strip was prepared exactly as in Example 24, except that "Lac Jeannine Superconcentrate" iron ore was used in place of "Blue Dust." The Lac Jeannine ore is a specular hematite that is mined in Canada and contains quartz as a major impurity. Most of the quartz can be removed easily. The resulting beneficiated ore is the superconcentrate which had 0.21 wt% SiO.sub.2 and 0.39 wr% Al.sub.2 O.sub.3 as the only significant impurities. The ferrite component of the extruded flexible strip contained 0.15 wt% SiO.sub.2 and 0.27 wt% Al.sub.2 O.sub.3. The ferrite particle size in the strip was 4.7 Fisher microns. The strip had a remanence B.sub.r = 2330 Gauss, an intrinsic coercive force H.sub.ci = 2650 Oersted, a maximum energy product (BH).sub.max = 1.13 MGOe, and a tensile strength of 1400 psi. The data show that extruded, flexible magnet strip with excellent properties can be prepared with the Canadian ore with essentially the same properties obtained with the Brazilian ore. Such strip is superior to any of the strip that could be prepared using the expensive, chemically prepared ferric oxide as the basic raw material.

Example 26

Multi-domain strontium ferrite particles having single crystal characteristics were prepared by the so-called iron-ore-celestite-soda-ash process. 2500 g Itabira Blue Dust were ball-milled in 2100 cc water with 560 g Mexican celestite and 450 g soda ash for 8 hrs. The resulting slurry was washed until the Na.sub.2 SO.sub.4 -- content was reduced to 0.5 wt% of the solids. The slurry was dried. The further processing was carried out exactly as in Example 1A. The Mexican celestite that was used had the following composition in wt%: 95.1 SrSO.sub.4 -- 2.5 CaSO.sub.4 - 0.5 BaSO.sub.4 -- 1.2 CaCO.sub.3 -- 0.15 SiO.sub.2 -- 0.12 Al.sub.2 O.sub.3. The soda ash was 98 percent Na.sub.2 CO.sub.3. The ferrite component of the extruded, flexible magnet strip contained 0.22 wt% SiO.sub.2, 0.28 wt% Al.sub.2 O.sub.3, 0.3 wt% Na.sub.2 SO.sub.3, and 0.2 wt% SrSO.sub.4. The ferrite particle size in the strip was 4.8 Fisher microns. The mole ratio Fe.sub.2 O.sub.3 /SrO was 5.1 in the strip. The strip had a remanence B.sub.r = 2340 Gauss, an intrinsic coercive force H.sub.ci = 3100 Oersted, a maximum energy product (BH).sub.max = 1.14 MGOe, and a tensile strength of 1350 psi. These data show that very much improved, extruded, flexible ferrite magnet strip can be prepared from multi-domain strontium ferrite particles having predominantly single crystal characteristics if ore is used as the basic raw material. Higher coercive forces at essentially the same level of (BH).sub.max can be obtained with the strontium ferrite flexible strip.

In other experiments, the optimum particle size, SiO.sub.2 -content, mole ratio, etc. were determined for the strontium ferrite extruded, flexible strip. Curves essentially the same as those shown in FIGS. 6, 7, and 8 were obtained. The optimum ferrite particle size is near 5 Fisher microns. The optimum SiO.sub.2 -content of the ferrite in the strip is near 0.2 wt%. The optimum mole ratio Fe.sub.2 O.sub.3 /SrO in the strip is near 5.2.

Example 27

Calendered and laminated, flexible barium ferrite magnets were made as follows: Multi-domain barium ferrite powder having predominantly single crystal characteristics was prepared exactly as in Example 24. A binder consisting of 4.9 g HYPALON 45 and 2.1 g VISTANEX L-140 was mixed at 200.degree.F in a standard two-roll rubber mill until the mixture was in sheet form. However, unlike the conditions for Examples 1 through 26, the spacing between the two rolls was only 15 mils. 93 g of barium ferrite powder was added and worked in. Total milling time was about 15 min. The mixture was sheeted off the mill. Twenty pieces were cut from the sheet and were laminated to form one single test sample which had a remanence B.sub.r = 2750 Gauss, an intrinsic coercive force H.sub.ci = 2400 Oe, and a maximum energy product (BH).sub.max = 1.65 MGOe. The reason for this large increase in B.sub.r and (BH).sub.max over that of the strip of Example 24 is the much higher degree of ferrite crystal orientation in the flexible magnet due to the small spacing between the two rolls. The flexible magnets of this example are particularly suited when the flux distribution is that of 21 in FIG. 5. The resulting holding force with a steel back-up strip is approximately twice that of a strip made, for example, in accordance with Example 24 and magnetized so that the flux distribution is that of 20 in FIG. 5. Of course, it will be appreciated that the strip of Example 24 can be extruded at a rate of approximately 60 feet per min. whereas the rate of making the strip of this example is small in comparison.

Example 28

Unlike the magnets of all of the above Examples, which were made using ferric oxide Fe.sub.2 O.sub.3 as a source for the iron oxide, magnetite was used in the form of a "Missouri magnetite super-concentrate" which had the following impurities: 0.3 wt% CaO, 0.2 wt% SiO.sub.2, 0.15 wt% Al.sub.2 O.sub.3, 0.1 wr% MgO. Extruded, flexible magnet strip was prepared exactly as in Example 24, except that the Missouri magnetite superconcentrate was used in place of the Itabira Blue Dust. The ferrite component of the flexible strip contained 0.15 wt% SiO.sub.2, 0.23 wt% CaO and 0.10 wt% Al.sub.2 O.sub.3. The ferrite particle size in the strip was 4.9 Fisher microns. The strip had a remanence B.sub.r = 2340 Gauss, an intrinsic coercive force H.sub.ci = 2600 Oe, and a maximum energy product (BH).sub.max = 1.14 MGOe. These data show that magnetite ore can be used in place of hematite.

The most important findings of Examples 1 through 27 are summarized in FIG. 9 which shows demagnetization curves B--H versus H. The curve for Example 1B is typical for an extruded strip of the expensive and toxic prior-art process. The curve for Example 1A shows that very much improved, extruded strip can be made in accordance with this invention, but with the practice of Example 1A the strip is still relatively expensive and toxic. The curves for Examples 24 and 26, which are almost indentical, show that the extruded strip can be further improved by using natural iron ore as the basic raw material. As an additional benefit it is found that such strip is inexpensive and non-toxic. The curve for Example 27 shows that other types of flexible magnets, in addition to the extruded strips, can be made which may have certain advantages in some applications.

Although only one type of binder system was used in the examples, those skilled in the art of compounding elastomeric compositions or the like will readily understand that a wide variety of compounding agents, plasticizers, vulcanizing agents, and the like is available to provide variations in workability, flexi-bility, and hardness of the binder system to adapt to special purposes within the scope of this invention.

As the examples and FIG. 9 show, it is possible to manufacture magnets having exceptionaly good characteristics with the simple procedure encompassed by the invention. The properties of the magnets are very much improved over those of the prior-art magnets. No toxic ingredients are needed. The magnets can be made at considerably lower cost than the prior-art magnets. While we have described our invention in connection with specific examples and specific embodyments, other modifications thereof will be readily apparant to those skilled in the art without departing from the spirit and scope of this invention as defined in the appended claims. ##SPC1##

Claims

We claim as our invention:

1. In the method of making flexible permanent magnets, the steps comprising reacting iron ore powder with a divalent metal oxide MO, wherein M stands for at least one element from the group Ba and Sr and which may also include small amounts of the elements Pb and Ca to form a ferrite, Pb, when present, not exceeding about 1.6 wt% of the ferrite and Ca, when present, not exceeding 2 wt% of the ferrite, pulverizing said ferrite into particles of near single-domain size, aligning said particles in a magnetic field, sintering the aligned particle agglomerate into a multi-domain ferrite having predominantly single crystal characteristics, pulverizing said ferrite into a powder, preparing a mxiture of elastomeric binder with at least a portion of said ferrite powder, molding the mixture, and magnetizing at least a portion of the molded pieces.

2. In the method of making flexible permanent magnets, the steps comprising aligning ferrite particles in a magnetic field, said ferrite particles having the composition Mo .sup.. k Fe.sub.2 O.sub.3 wherein M represents at least one element selected from the group consisting of barium and strontium and wherein k lies between 4.5 and 5.5, sintering the aligned particle agglomerate thereby producing multidomain ferrites having predominantly single crystal characteristics, pulverizing said ferrites into a powder, preparing a mixture of elastomeric binder with at least a portion of said powder, molding the mixture, and magnetizing at least a portion of the molded pieces.

3. In the method of making lead-free flexible permanent magnets, the steps comprising reacting iron ore powder with a divalent metal oxide MO, wherein M stands for at least one element selected from the group consisting of Ba and Sr to form the ferrite MO .sup.. k Fe.sub.2 O.sub.3 wherein k lies between 4.5 and 5.5, milling said ferrite in water with steel balls, pressing the slurry in a magnetic field and aligning the ferrite particles, sintering the aligned particle agglomerate into a multi-domain ferrite having predominantly single crystal characteristics, pulverizing said ferrite into a powder, preparing a mixture of elastomeric binder with at least a portion of said ferrite powder, extruding the mixture into a strip, and magnetizing at least a portion of the extruded strip.

4. The method of claim 3, wherein the ferrite component of the flexible strip has an average particle size of from at least 2 .mu.m up to about 10 .mu.m and contains at least one addition selected from the group consisting of from 0.1 to 1 wt% SiO.sub.2, 0.1 to 1 wt% CaO, from 0.1 to 1 wt% Al.sub.2 O.sub.3 and from 0.05 to 1 wt% of a sulfate from the group consisting of SrSO.sub.4, BaSO.sub.4, CaSO.sub.4 and NaSo.sub.4.

5. A flexible permanent magnet containing multi-domain ferrite particles embedded in an elastomeric binder, said ferrite particles having predominantly single crystal characteristics and having an average particle size of from at least 2.mu.m up to about 10 .mu.m, said ferrite particles having the composition MO .sup.. k Fe.sub.2 O.sub.3 wherein M represents an element selected from the group consisting of barium and strontium and wherein k lies between 4.5 and 5.5.

6. The flexible magnet of claim 5 wherein M includes small amounts of one or both of lead and calcium, lead, when present, not exceeding about 1.6wt% of the ferrite and calcium, when present, not exceeding 2 wt% of the ferrite.

7. The flexible magnet of claim 5 wherein the ferrite contains at least one addition selected from the group consisting of from 0.1 to 1 wt% SiO.sub.2, 0.1 to 1 wt% CaO, from 0.1 to 1 wt% Al.sub.2 O.sub.3 and from 0.05 to 1 wt% of a sulfate from the group consisting of SrSO.sub.4, BaSO.sub.4, CaSO.sub.4 and Na.sub.2 SO.sub.4.

8. The flexible magnet of claim 5 wherein the ferrite contains from 0.1 to 0.5 wt% SiO.sub.2.

9. The flexible magnet of claim 5 wherein the ferrite contains from 0.1 to 1 wt% Al.sub.2 O.sub.3.

10. The flexible magnet of claim 8 wherein the ferrite contains from 0.1 to 1 wt% Al.sub.2 O.sub.3.

11. The flexible magnet of claim 7 wherein the ferrite particles have an average particle size of about 5 .mu.m.

12. The magnet of claim 7, wherein said elastomeric binder contains a mixture of chlorosulfonated polyethylene and polyisobutylene.