US3764539A - Flexible ferrite permanent magnet and methods for its manufacture - Google Patents

Flexible ferrite permanent magnet and methods for its manufacture Download PDF

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US3764539A
US3764539A US00080580A US3764539DA US3764539A US 3764539 A US3764539 A US 3764539A US 00080580 A US00080580 A US 00080580A US 3764539D A US3764539D A US 3764539DA US 3764539 A US3764539 A US 3764539A
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ferrite
flexible
strip
magnet
particles
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P Cochardt
A Cochart
A Cochardt
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COMMUNITY BUILDING ASS OF WASH
COMMUNITY BUILDING ASS OF WASHINGTON INDIANA INC US
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F13/00—Apparatus or processes for magnetising or demagnetising
    • H01F13/003—Methods and devices for magnetising permanent magnets
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
    • H01F1/10—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure
    • H01F1/11—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure in the form of particles
    • H01F1/113—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure in the form of particles in a bonding agent
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
    • H01F1/10—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure
    • H01F1/11—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure in the form of particles
    • H01F1/113—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure in the form of particles in a bonding agent
    • H01F1/117—Flexible bodies

Definitions

  • l-lexaferrites are permanent magnets that can be described by the formula M O 6 Fe O 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.
  • 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 2 wt% leadmonosilicate PbO SiO2 are mixed with 80 wt% chemically prepared ferric oxide Fe O (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 H O).
  • the mixture of Fe O BaCO and PbO SiO is heated in air to approximately 2150 F, usually in a rotary furnace.
  • the barium carbonate decomposes into barium oxide BaO which then reacts with the ferric oxide F6 0, to form the hexaferrite crystals of the type BaO 6 Fe O
  • the leadmonosilicate PbO'SiO- 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 singledomain size.
  • the critical size, below which a singledomain particles has a lower energy than a multidomain particle, is approximately 1.0 pm for the hexaferrites as is described in the prior art. Such small-size particles have the characteristics of a dust.
  • the singledomain size powder is usually annealed at a temperature of approximately l700F 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 200F.
  • 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 220F 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.
  • the lead-containing singledomain-size dust would not be inhaled by the workers during the mixing operation with the elastomeric binder.
  • 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.
  • 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 hightemperature 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.
  • 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.
  • 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.
  • 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.
  • 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 -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 O /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.
  • FIG. 1 and FIG. la 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.
  • 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).
  • 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.
  • the actual shapes of particles l and 2 may be different than the ones schematically shown in FIG. 1 and FIG. la.
  • the actual shapes and numbers of the domains 3 may be different than the ones shown schematically for particle l 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.
  • the multi-domain particles having single-crystal characteristics such as particle 1 in FIG. 1.
  • the single-domain particles of critical size have a length of approximately 1.0 pm
  • the multi-domain particles having single crystal characteristics preferably have a length of approximately 5 pm.
  • the volume of the multi-domain particles having single crystal characteristics is considerably larger than that of the singledomain particles.
  • 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 singledomain-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. Beneflciated or unbenficiated iron ore powder is mixed with barium carbonate or strontium carbonate, and the mixture is heated to a temperature higher than 2000F 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 manu facture 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 US. Pat. No. 3,412,461.
  • the singledomain-size ferrite particles are all oriented strongly and uniformly in parallelism at all portions of the body except near the edges.
  • 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.
  • coils 8 and 8 are energized producing the magnetic field indicated at 9 and 10 which operates to orient the ferrite particles.
  • 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 pm. 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.
  • 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.
  • 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 200F or higher.
  • a flexible magnet strip 15 is extruded through the nozzle 16 through the action of the screw 17.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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, and the maximum energy product (BI-U 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 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, and the maximum energy product (Bl-U 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.
  • 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.
  • the ferrite componet of the flexible magnet should contain between 0.1 and 0.5 wt% SiO Such an addition is safe to use, among others, because SiO, 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 addition is used in the flexible magnet of this invention. This increases the remanence B, and the maximum energy product (BH),,,,,,,.
  • beneficial additives that can be used in place of the lead compound with and without SiO are the sulfates SrSO BaBO CaSO and Na SO and other refractory oxides, particularily CaO and Al O
  • the preferred ranges for these additives are the following: 0.05 to 1.0 wt% S0 0.1 to 1.0 wt% CaO; 0.1 to 1.0 wt% A1 0
  • a combination of SiO, and Al O is particularily beneficial.
  • the expensive annealing step is not needed.
  • very few crystal imperfections are introduced.
  • the remanence B, and the intrinsic coercive force H do not deteriorate noticeably during the milling, in contrast to the conditions of the procedures of the prior art.
  • the mole ratio between the ferric oxide Fe O 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.
  • 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 (BI-1),, on the sintered hexaferrites is 5.6 or 5.7.
  • 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.
  • iron ore is the preferred basic raw material for the flexible magnets of this invention.
  • basic is meant here that at least 50 wt% of the flexible magnet can be derived from iron ore and preferably more.
  • 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 80 wt% natural iron oxide Fe O and 1 wt% additions, mostly SiO sulfates, CaO and/or A1 4.3 wt% of this mix is lost during calcining, mostly in the form of CO which goes out through the furnaces stack.
  • the F e 0 content in the barium ferrite clinker increases to 83.2 wt%.
  • the Fe- O 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 re o during the process.
  • the final preferred Fe O content in the ferrite component is approximately 83.5 wt%.
  • the preferred starting mix is 91 wt% ferrite and 9 wt%'elastomeric binder.
  • 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.
  • 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 ferthe properties are essentially the same in both direc- I tions. 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 um (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 pm. Such particles have more than one hundred times the volume of a single-domain size particle.
  • the Fisher size is 2 pm which means an average ferrite particle volume approximately ten times the volume of a single-domain particle.
  • 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.
  • the Fisher subsieve size is approximately 6 um 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.
  • a number of examples of the practice of the invention are now offered.
  • This ferric oxide which is the one used in the Examples 1 through 23, is produced by the calcination of ferrous sulfate.
  • the H SO formed thereby is separated from the solids which consist of essentially 90 wt% Fe O and wt% undecomposed FeSO
  • 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
  • the clean slurry is filtered, and the filter cake is dried at 300F to remove the water.
  • the dried cake is pulverized to form the powder used for making todays ferrite permanent magnets.
  • Example lA An extruded, flexible ferrite magnet strip was made as follows: 800 g of ferric oxide Fe O 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 and 20 g leadmonosilicate PbO SiO The slurry was dried, and the dried cake was calcined at 2l50F for 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 2250F.
  • Example 18 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-ballmilling, and the dried powder was annealed for 2 hrs at 1700F, 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
  • 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),, 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.
  • Example 3 The particle size in Example 3 is 1.0 Fisher microns which means essentially single-domain size. (BI-U has dropped to 0.8 MGOe which is still higher than the value for the Example 13 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 (BI-I) is decreased as FIG. 6 shows.
  • 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 O /BaCO was kept at 4.26 in order to keep the mole ratio Fe O /BaCO in the extruded strip constant at 5.2 to 5.3. Additions of the SiO were used at the following levels: 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6 wt%.
  • the SiO was added at the start of the first ball-milling in the form of finely pulverized quartz (S-MICRON Min-U- Sil).
  • Table I gives the ferrite starting compositions and the test results on the extruded strip.
  • the maximum energy product (BH), is plotted in FIG. 7 against the SiO addition. As is seen the curve peaks at 1.02 MGOe at 0.20 wt% SiO the same value of (BH),,,,,,,, that was obtained in Example 1A with an addition of 2.0 wt% PbO SiO Surprisingly, excellent magnetic properties can be obtained without lead with 0.20 wt% SiO and at a mole ratio Fe O /BaO of 5.2 to 5.3.
  • Examples 16 through 22 Extruded, flexible magnet strip was prepared exactly as in Example 11, except that the mole ratio Fe O /BaO in the strip was varied by varying the weight ratio Fe O /BaCO 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 (Bl-1), value is obtained at mole ratios of 4.5 (Example 21), 4.75 (Example 5.0 (Example 19), and 5.5 (Example 18).
  • (BH) drops sharply.
  • the preferred mole ratio for the flexible magnets of this invention is 4.5 to 5.5 when 0.2 wt% SiO are used as the only additive.
  • Example 23 Magnets were prepared exactly as in Example 13, except that 0.2 wt% SiO and 0.2 wt% A1 0 were added in place of 0.4 wt% SiO Table I summarizes the results of the tests. As is seen, the combination of SiO and A1 0 is particularly beneficial in obtaining outstanding properties. A (BH),,,,, 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% S10 and 0.37 wt% A1 0 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 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 a particle size in the strip was 4.8 Fisher microns.
  • the ferrite component of the flexible strip contained 0.26 wt% SiO, and 0.22 A1 0
  • the extruded magnet strip had a remanence B, 2350 Gauss, an intrinsic coercive force H 2700 De, a maximum energy product (BI-I),,, 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 and 0.39 wr% A1 0 as the only significant impurities.
  • the ferrite component of the extruded flexible strip contained 0.15 wt% SiO and 0.27 wt% A1 0
  • the ferrite particle size in the strip was 4.7 Fisher microns.
  • B remanence B
  • BI-I maximum energy product
  • 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 socalled 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 SO 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, 2.5 CaSO, 0.5 BaSO, 1.2 CaCO 0.15 SiO 0.12 A1 0
  • the soda ash was 98 percent Na C0
  • the ferrite component of the extruded, flexible magnet strip contained 0.22 wt% SiO 0.28 wt% A1 0 0.3 wt% Na SO and 0.2 wt% SrSO
  • the ferrite particle size in the strip was 4.8 Fisher microns.
  • the mole ratio Fe O /SrO was 5.1 in the strip.
  • the strip had a remanence B, 2340 Gauss, an intrinsic coercive force H 3100 Oersted, a maximum energy product (BI Dinar 1.14 MGQe, and a tensile strength of 1350 psifThese data show that very much improved, ex-
  • truded, 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),,,,,, can be obtained with the strontium ferrite flexible strip.
  • O -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 -content of the ferrite in the strip is near 0.2 wt%.
  • the optimum mole ratio Fe O /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- was mixed at 200F 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.
  • Example 28 Unlike the magnets of all of the above Examples, which were made using ferric oxide Fe O 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 0.15 wt% A1 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 ltabira Blue Dust.
  • the ferrite component of the flexible strip contained 0.15 wt% SiO 0.23 wt% C210 and 0.10 wt% A1 0
  • the ferrite particle size in the strip was 4.9 Fisher microns.
  • the strip had a remanence B, 2340 Gauss, an intrinsic coercive force H 2600 Oe, and a maximum energy product (BH 1.14 MGOe. These data show that magnetite ore can be used in place of hematite.
  • binder system 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.
  • FIG. 9 shows demagnetization curves BH 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 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, pulveriz ing 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.
  • MO divalent metal oxide
  • the steps comprising aligning ferrite particles in a magnetic field, said ferrite particles having the composition Mo k mo, 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.
  • the method of making lead-free flexible permanent magnets 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 k Fe O 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.
  • MO divalent metal oxide
  • M stands for at least one element selected from the group consisting of Ba and Sr to form the ferrite MO k Fe O wherein k lies between 4.5 and 5.5
  • the ferrite component of the flexible strip has an average particle size of from at least 2 um up to about um and contains at least one addition selected from the group consisting of from 0.1 to 1 wt% SiO 0.1 to 1 wt% CaO, from 0.1 to
  • M includes small amounts of one or both of lead and calcium, lead, when present, not exceeding about l.6wt% of the ferrite and calcium, when present, not exceeding 2 wt% of the ferrite.
  • said elastomeric binder contains a mixture of chlorosulfonated polyethylene and polyisobutylene.

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Cited By (26)

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US3897355A (en) * 1973-04-26 1975-07-29 Gen Electric Method of making permanent ferrite magnets
US4028255A (en) * 1973-01-31 1977-06-07 Ici Australia Limited Preparation of polymer composites
US4042516A (en) * 1974-03-01 1977-08-16 Sakai Chemical Industry Co., Ltd. Bonded magnets containing single crystalline ferrite particles
US4067810A (en) * 1976-02-09 1978-01-10 Ofco, Inc. Fluid filter magnet assembly
US4120806A (en) * 1976-08-30 1978-10-17 Dowa Mining Co., Ltd. Hexagonal-system ferrite powder, composite plastic-ferrite magnet comprising same and process for production thereof
US4126567A (en) * 1975-12-02 1978-11-21 Kuraray Co., Ltd. Permanent magnet composites and method therefor
US4308155A (en) * 1976-11-24 1981-12-29 Tdk Electronics Co., Ltd. Rubber or plastic magnet and magnetic powder for making the same
WO1983003920A1 (en) * 1982-04-23 1983-11-10 Sintef Magnetic polymer particles and process for the preparation thereof
WO1984002031A1 (en) * 1982-11-10 1984-05-24 Sintef Magnetic polymer particles and process for the preparation thereof
DE3305816A1 (de) * 1983-02-19 1984-08-23 Thomas J. Arlington Va. Greer jun. Gliederpuppe mit beweglichem gesicht
US4824587A (en) * 1985-03-18 1989-04-25 The Dow Chemical Company Composites of coercive particles and superparamagnetic particles
US4881988A (en) * 1987-11-16 1989-11-21 Rjf International Corporation Novel flexible magnet for use in small dc motors
US5055155A (en) * 1989-10-12 1991-10-08 Texstyle, Inc. Method and apparatus for laminating flexible magnetic strips onto flexible plastic substrates
US5715841A (en) * 1993-07-06 1998-02-10 Utecht; Leo J. Personal protection apparatus with adhesive
RU2157013C2 (ru) * 1998-12-24 2000-09-27 Никитин Лев Васильевич Магнитоуправляемый эластичный композиционный материал
US6394946B1 (en) * 1998-11-09 2002-05-28 Mir Magnetic Institute Research S.A. Treatment of cellulite
US6416458B1 (en) 2000-07-12 2002-07-09 Therion Research Inc. Therapeutic flexible magnetic sheet and method
US6464894B1 (en) * 1998-02-09 2002-10-15 Vacuumschmelze Gmbh Magnetic film and a method for the production thereof
US6790378B2 (en) 2001-10-05 2004-09-14 R. William Graham Coating composition having magnetic properties
US7326170B1 (en) 1999-06-18 2008-02-05 Miller Wendell E Method and apparatus for restoring or enhancing an ambient magnetic field
WO2008091297A3 (en) * 2006-08-11 2008-10-09 Univ Northeastern Method of manufacturing thick-film, low microwave loss, self-biased barium-hexaferrite having perpendicular magnetic anisotropy
US20100176678A1 (en) * 2009-01-09 2010-07-15 James Ching Sik Lau Electric motor
US20110025572A1 (en) * 2009-07-28 2011-02-03 Samsung Electronics Co. Ltd. Y-type hexagonal ferrite, fabrication method thereof, and antenna apparatus using the same
US8893955B2 (en) 2010-10-27 2014-11-25 Intercontinental Great Brands Llc Releasably closable product accommodating package
CN114538910A (zh) * 2020-11-26 2022-05-27 和也健康科技有限公司 一种智能健康家居用磁条及其制备方法
US11654349B2 (en) * 2019-04-24 2023-05-23 Scale Up The Fun, Llc Hobby projectile

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DE3268714D1 (en) * 1981-11-20 1986-03-06 Goodrich Co B F Permanent magnets
GB2134128A (en) * 1983-01-27 1984-08-08 Oilfield Inspection Services Magnetically conductive materials and articles and methods using them
GB8402360D0 (en) * 1984-01-30 1984-02-29 Picker Int Ltd Nmr shims
EP0310223A1 (en) * 1987-07-27 1989-04-05 Maghemite Inc. Magnet composition

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GB860220A (en) * 1958-07-15 1961-02-01 Leyman Corp Mechanical orientation of magnetically anisotropic particles
US3115469A (en) * 1959-06-22 1963-12-24 Monsanto Chemicals Production of single crystals of ferrites
US3337461A (en) * 1962-08-01 1967-08-22 Westinghouse Electric Corp Two-phase ferrite magnet composition and method for preparing same
US3379643A (en) * 1964-11-27 1968-04-23 Allied Chem Flexible magnetic plastic composition
US3597357A (en) * 1968-03-19 1971-08-03 Westinghouse Electric Corp Method of making strontium-ferrite magnets and the products so produced

Cited By (29)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4028255A (en) * 1973-01-31 1977-06-07 Ici Australia Limited Preparation of polymer composites
US3897355A (en) * 1973-04-26 1975-07-29 Gen Electric Method of making permanent ferrite magnets
US4042516A (en) * 1974-03-01 1977-08-16 Sakai Chemical Industry Co., Ltd. Bonded magnets containing single crystalline ferrite particles
US4126567A (en) * 1975-12-02 1978-11-21 Kuraray Co., Ltd. Permanent magnet composites and method therefor
US4067810A (en) * 1976-02-09 1978-01-10 Ofco, Inc. Fluid filter magnet assembly
US4120806A (en) * 1976-08-30 1978-10-17 Dowa Mining Co., Ltd. Hexagonal-system ferrite powder, composite plastic-ferrite magnet comprising same and process for production thereof
US4308155A (en) * 1976-11-24 1981-12-29 Tdk Electronics Co., Ltd. Rubber or plastic magnet and magnetic powder for making the same
WO1983003920A1 (en) * 1982-04-23 1983-11-10 Sintef Magnetic polymer particles and process for the preparation thereof
WO1984002031A1 (en) * 1982-11-10 1984-05-24 Sintef Magnetic polymer particles and process for the preparation thereof
DE3305816A1 (de) * 1983-02-19 1984-08-23 Thomas J. Arlington Va. Greer jun. Gliederpuppe mit beweglichem gesicht
US4824587A (en) * 1985-03-18 1989-04-25 The Dow Chemical Company Composites of coercive particles and superparamagnetic particles
US4881988A (en) * 1987-11-16 1989-11-21 Rjf International Corporation Novel flexible magnet for use in small dc motors
US5055155A (en) * 1989-10-12 1991-10-08 Texstyle, Inc. Method and apparatus for laminating flexible magnetic strips onto flexible plastic substrates
US5732716A (en) * 1993-07-06 1998-03-31 Utecht; Leo J. Personal protection method
US5715841A (en) * 1993-07-06 1998-02-10 Utecht; Leo J. Personal protection apparatus with adhesive
US6464894B1 (en) * 1998-02-09 2002-10-15 Vacuumschmelze Gmbh Magnetic film and a method for the production thereof
US6394946B1 (en) * 1998-11-09 2002-05-28 Mir Magnetic Institute Research S.A. Treatment of cellulite
RU2157013C2 (ru) * 1998-12-24 2000-09-27 Никитин Лев Васильевич Магнитоуправляемый эластичный композиционный материал
US7326170B1 (en) 1999-06-18 2008-02-05 Miller Wendell E Method and apparatus for restoring or enhancing an ambient magnetic field
US6416458B1 (en) 2000-07-12 2002-07-09 Therion Research Inc. Therapeutic flexible magnetic sheet and method
US6790378B2 (en) 2001-10-05 2004-09-14 R. William Graham Coating composition having magnetic properties
WO2008091297A3 (en) * 2006-08-11 2008-10-09 Univ Northeastern Method of manufacturing thick-film, low microwave loss, self-biased barium-hexaferrite having perpendicular magnetic anisotropy
US20100173101A1 (en) * 2006-08-11 2010-07-08 Northeastern University Method of manufacturing thick-film, low microwave loss, self-biased barium-hexaferrite having perpendicular magnetic anisotropy
US20100176678A1 (en) * 2009-01-09 2010-07-15 James Ching Sik Lau Electric motor
US20110025572A1 (en) * 2009-07-28 2011-02-03 Samsung Electronics Co. Ltd. Y-type hexagonal ferrite, fabrication method thereof, and antenna apparatus using the same
US8893955B2 (en) 2010-10-27 2014-11-25 Intercontinental Great Brands Llc Releasably closable product accommodating package
US11654349B2 (en) * 2019-04-24 2023-05-23 Scale Up The Fun, Llc Hobby projectile
CN114538910A (zh) * 2020-11-26 2022-05-27 和也健康科技有限公司 一种智能健康家居用磁条及其制备方法
CN114538910B (zh) * 2020-11-26 2023-03-07 和也健康科技有限公司 一种智能健康家居用磁条及其制备方法

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IT939984B (it) 1973-02-10

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