Flexible ferrite-particle magnets

Abstract

Process for making barium ferrite particles which are especially adapted to mechanical orientation in admixture with a workable nonmagnetic matrix material to provide flexible magnets of extraordinarily high magnetic values. Starting with acicular alpha-Fe.sub.2 O.sub.3 particles, BaCO.sub.3, a fluxing agent such as NaF and a lead compound such as PbO, magnetic particles are obtained which provide, when oriented in a rubber matrix, permanent magnet material having a maximum energy product of at least 1.4 .times. 10.sup.6 gauss-oersteds.

Parent Case Text

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of applicants' copending application Ser. No. 63,299, filed Aug. 12, 1970 and now abandoned.

Description

FIELD OF THE INVENTION

The present invention relates to flexible permanent magnets consisting of aligned ferrite particles in a non-magnetic matrix of flexible resin or rubber.

BACKGROUND OF THE INVENTION

Flexible permanent magnets have been manufactured for many years by mixing ferrite particles with a flexible resin or rubber. The most versatile such magnets are made by the process of U.S. Pat. No. 2,999,275 (Blume). As there disclosed, plate-shaped domain-size particles of barium ferrite which have a preferred direction of magnetization normal to the two parallel surfaces may be mixed with rubber on a rubber mill in amounts up to about 70% of the total volume of the mixture. Mechanical forces incident to the rolling progressively cause the ferrite platelets to become parallel to the surface of the rubber sheet. The resulting thin sheets may be cured and magnetized as such or laminated to a desired thickness. Magnets punched out from the sheets or laminates may have a residual induction of 2100 gauss, a coercivity H.sub.c of 1200 oersteds and a maximum energy product of 0.9 .times. 10.sup.6 gauss-oersteds in the preferred direction of magnetization. Magnets presently being produced commercially in this manner have somewhat higher values, perhaps in part due to treatment of the ferrite with an aqueous acid solution as described in U.S. Pat. No. 3,387,918 (Moore et al.). Such magnets typically have a residual induction of 2150 gauss, a coercivity H.sub.c of 1750 oersteds, an intrinsic coercivity H.sub.ci of 3000 oersteds and a maximum energy product BH.sub.max of about 1.1 .times. 10.sup.6 gauss-oersteds.

The Blume patent also teaches that the ferrite platelets may be incorporated into a thermoplastic resin of the polyvinyl chloride type and extruded through a narrow orifice into elongated shapes. The shearing forces during the extrusion process cause the ferrite platelets to become oriented or aligned in a mechanical way. If desired, the extruded material is squeezed between rolls while being subjected to a magnetic field such that the lines of force are perpendicular to the top surface of the material as disclosed in U.S Pat. No. 3,312,763 (Peccerill et al.).

Magnets made by the process of the Blume patent have achieved considerable commercial success. To a large extent this is due to their flexibility and toughness and to their amenability to be shaped and cut to precise dimensions. However, some important potential uses for flexible magnets call for higher magnetic values than have been attainable prior to the present invention.

THE PRESENT INVENTION

The present invention concerns barium ferrite particles which are especially adapted to mechanical orientation in admixture with a workable nonmagnetic matrix material. The novel barium ferrite particles are prepared from a mixture of:

a. acicular alpha-Fe.sub.2 O.sub.3 of high surface area, i.e., at least 15 and preferably more than 20 square meters per gram,

b. BaCO.sub.3 or equivalent source of barium oxide in amount to provide upon reaction with the alpha-Fe.sub.2 O.sub.3 a ferrite of the generalized formula BaFe.sub.12 O.sub.19,

c. about 1- 12 weight percent, and preferably 3-10 weight percent, of fluxing agent such as NaF, and

d. if the amount of fluxing agent is less than about 6 weight percent, a lead compound such as PbO in an amount ##EQU1## where "x" is the weight percent of fluxing agent but not more than about 2 weight percent.

Regardless of the amount of fluxing agent, flexible magnets of highest maximum energy products are attained where the amount of the lead compound exceeds 3/4 weight percent and is less than 13/4 weight percent.

The straightforward procedure of:

1. homogeneously mixing the above materials,

2. calcining the mixture at 850.degree.-1100.degree.C, and

3. treating the resultant ferrite with aqueous acid solution to remove undesirable reaction products and any unreacted material

provides particles which are especially adapted for the manufacture of flexible permanent magnets by the process of the above-discussed Blume patent, that is, by the further steps of:

4. mixing the elongated platelets with a workable rubber or thermoplastic matrix material to provide a mixture comprising about 55-70% ferrite by volume,

5. rolling or extruding the mixture to align the ferrite platelets by mechanical shearing forces, and

6. then vulcanizing the rubber or cooling the thermoplastic matrix material to lock the ferrite platelets in place.

By employing this process to make flexible magnets, maximum energy products exceeding 1.4 .times. 10.sup.6 gauss-oersteds have been consistently attained along with other high magnetic values, e.g., B.sub.r at least 2500 gauss, H.sub.c above 2000 oersteds and H.sub.ci above 3000 oersteds. If a sheet of the magnet material is subjected to a magnetic field extending perpendicular to the surface of the sheet while the matrix is in a semi-fluid state, a small degree of further enhancement of magnetic values is realized, as evidenced by maximum energy products on the order of 1.5 to 1.6 .times. 10.sup.6 gauss-oersteds.

When magnetic orientation is combined with mechanical orientation, the magnet material should be subjected to the magnetic field at a time when the matrix material is semifluid and maintained until the matrix material has set to a consistency locking the ferrite particles in place. When the matrix material is a thermoplastic resin, it is convenient to apply the magnetic field while the resin is hot, simultaneously with mechanical orientation, as in the aforementioned Peccerill patent. The thermoplastic resin should then be cooled quickly to prevent the particles from rotating out of alignment.

Maximum energy products above 1.5 .times. 10.sup.6 gauss-oersteds can be attained without the need for magnetic orientation by employing at least 5 weight percent of fluxing agent plus at least 3/4 weight percent of the lead compound. The highest energy products are realized at about 7 to 10 weight percent of fluxing agent. If the lead compound is omitted and the fluxing agent content exceeds 5 weight percent, maximum energy products of 1.4 .times. 10.sup.6 gauss-oersteds can still be attained. Even though the use of relatively large percentages of fluxing agent involve large weight losses, e.g., even 30% or higher, the greater energy products that can be attained create new and valuable uses for flexible magnets.

The achievement of these results requires starting materials which are reasonably free from impurities. For example, some commercial sources of alpha-Fe.sub.2 O.sub.3 contain undesirably high sulfur content which can readily be reduced to satisfactory levels by roasting the particles at 700.degree.C until the sulfur content is 0.3 weight percent or less.

EXAMPLE 1

Fifty pounds of acicular alpha-Fe.sub.2 O.sub.3 of high surface area (Columbian Carbon Co. "Mapico Red 516-M"), 10.9 pounds of BaCO.sub.3, 0.64 pound of PbO and 2.56 pounds of NaF were charged into a dry blender. The alpha-Fe.sub.2 O.sub.3 had a surface area of 26.4 sq. meters per gram. After mixing for 2 minutes at 1300 rpm, 8 pounds of the dry mixture were placed in a gas-fired rotary calciner at 950.degree.C for 1 hour to provide lead-modified barium ferrite particles.

Forty-eight hundred ml of 5% HCl solution were heated to 95.degree.C in a glass flask, and 1440 grams of the barium ferrite particles were added. After stirring for 15 minutes, with the temperature at 87.degree.-93.degree.C, the acidic solution was decanted and the ferrite was rinsed 5 times in warm water, once in acetone, and then was dried in an air-circulating oven at 150.degree.C. The product ferrite particles were elongated platelets of generally submicron size.

A 13-15% weight loss occurred during the acid treatment, which is considered satisfactorily small in view of the enhanced results attributable to the acid treatment. Although a more severe acid treatment might further improve magnetic values, a weight loss above about 20% is considered economically unjustifiable in the practice of this example.

To produce flexible magnets, the following composition was prepared:

Grams The acid-treated lead-modified barium ferrite 3250 Butadiene-acrylonitrile copolymer, medium-level nitrile ("Chemigum N-608") 260 Zinc stearate 7 Sulfur 2.5 Benzothiazyl disulfide accelerator ("Altax") 5.2

The rubber and zinc stearate were mixed in a Banbury mixer with gradual addition of the ferrite. The mixture was pulverized and placed in a dry blender with the sulfur and accelerator. After 5 minutes this was sheeted between cold rolls (0.015 inch roll spacing, roll ratio 1:1). The sheet was repeatedly doubled over and put through the rolls at gradually increased roll spacing until a laminate of 32 layers was obtained having a thickness of about 0.140 inch. This laminate was put through the rolls again at gradually reduced roll spacing until its thickness was reduced to 0.125 inch.

A day later this was again passed through the rolls repeatedly at gradually reduced roll spacing to a thickness of 0.050 inch and then repeatedly doubled over and put through the rolls at gradually increased roll spacing until a laminated sheet of 32 layers of laminate was obtained at a thickness of 0.140 inch, which was brought down to a finished thickness of 0.125 inch by additional passes. The final laminated sheet was vulcanized in an air-circulating oven at 150.degree.C for one hour.

Two 1/2-inch diameter plugs were punched from the cured sheet and stacked for testing, with the following results (averaged from three identical preparations):

B.sub.r 2520 gauss H.sub.c 2100 oersteds H.sub.ci 3310 oersteds BH.sub.max 1.45 .times. 10.sup.6 gauss-oersteds.

EXAMPLE 2

The following composition was prepared:

Grams The product ferrite particles of Example 1 135.0 Cis-1,4-polyisoprene ("Natsyn 200") 12.62 Stearic acid 0.255 Paraffin wax 0.48 Sulfur 0.194 Activated dithiocarbamate accelerator ("Butyl-eight") 0.505

The rubber with the stearic acid and wax was banded on a cold rubber mill (two mill rolls 3 inches in diameter and 8 inches in length, roll ratio 1:1), initially at a roll spacing of about 0.015 inch. The ferrite was added to the banding rubber while the roll spacing was gradually increased to accommodate the increased bulk. When all the ferrite was incorporated, the sulfur and accelerator were added. The sheet, which at this point had a thickness of about 0.070-0.075 inch, was repeatedly doubled over and put through the rolls until a laminate of 32 layers was obtained at a thickness of about 0.090 inch, and this was again put through the rolls without folding to bring its thickness down to about 0.082 inch.

A piece of the sheet was placed in an aluminum box wrapped in a heating jacket and subjected to a 13.5 kilogauss field perpendicular to the surface of the sheet. The direction of the field was reversed six times and then maintained constant while the temperature was brought up to 150.degree.C over about 40 minutes. After additional 30 minutes at 150.degree.C, the sheet was allowed to cool to 50.degree.C over a period of about 45 minutes, at which time the field was removed and the sheet was taken from the box.

Three 1/2-inch diameter plugs were punched from the cured sheet and stacked for testing, with the following results:

B.sub.r 2570 gauss H.sub.c 2260 oersteds H.sub.ci 3900 oersteds BH.sub.max 1.55 .times. 10.sup.6 gauss-oersteds.

In Examples 1 and 2, barium ferrite particles comprised about 64 percent by volume of the magnet material, a level providing excellent magnetic values. As in the Blume patent, the ferrite particles should comprise at least 55% and preferably more than 60% of the magnet material by volume to provide desirably high magnetic values. Above about 65% the resultant magnets may have less integrity and flexibility than desired, but adequate physical properties for many uses have been attained at 70% ferrite by volume. In any event, a flexible magnet of one-eighth inch thickness ought to withstand bending over a 5-inch mandrel without breaking.

EXAMPLE 3

Charged to a dry blender were 1761.4 grams of the same acicular alpha-Fe.sub.2 O.sub.3 used in Example 1, 385.8 grams of BaCO.sub.3 and 186.6 grams of NaF. 500 grams of the dry blended raw material mix were placed in an alumina sagger which was held in a globar furnace at 1050.degree.C for 80 minutes. The product barium ferrite particles were acid treated in the same manner as in Example 1. A 20 percent weight loss occurred during the acid treatment. The acid-treated particles were elongated platelets of generally submicron size.

To produce flexible magnets, the following composition was prepared:

Grams The acid-treated barium ferrite 3075 Butadiene-acrylonitrile copolymer ("Chemigum N-608") 283 Stearic acid 3.1 Sulfur 2.5 Benzothiazyl disulfide accelerator ("Altax") 5.2 Tetramethylthiuram disulfide accelerator ("Methyl tuads") 1.4 Zinc oxide 8.4 Polyether plasticizer ("TP-90B") 2.8

The rubber, stearic acid, sulfur and polyether plasticizer were mixed in a Banbury mixer with gradual addition of the ferrite. The mixture was pulverized and placed in a dry blender with the zinc oxide and the two accelerators. After a 5-minute blend, this was sheeted and laminated between cold rolls and then vulcanized by the same procedure as in Example 1. The cured sheet had the following properties:

B.sub.r 2475 gauss H.sub.c 2260 oersteds H.sub.ci 4300 oersteds BH.sub.max 1.47 .times. 10.sup.6 gauss-oersteds.

EXAMPLE 4

1761.4 grams of acicular alpha-Fe.sub.2 O.sub.3 particles ("516-M") were combined with 385.8 grams of BaCO.sub.3, 187.8 grams NaF and 23.6 grams PbO in a dry blender. 500 grams of the dry blended raw material mix were placed in an alumina sagger which was introduced into a globar furnace at 1100.degree.C and held at this temperature for 85 minutes to form lead-modified barium ferrite particles. The particles were acid-treated as in Example 1 except using an 8.8 percent HCl solution. Weight loss was 27%.

To provide flexible magnets, the acid-treated, lead-modified barium ferrite particles were processed in the same way and with the same composition as in Example 3 except that the amount of ferrite particles was increased to 3600 grams so that the resultant magnet sheet was 69.5 volume percent ferrite (as compared to 65% in Example 3). Also, the polyether plasticizer was omitted. The magnets evidenced:

B.sub.r 2690 gauss H.sub.c 2425 oersteds H.sub.ci 3625 oersteds BH.sub.max 1.72 .times. 10.sup.6 gauss-oersteds.

These examples illustrate the effectiveness of NaF as the fluxing agent. Other fluxing agents are listed in the tables at pages 9 and 10 of Australian Pat. No. 284,335.

Claims

We claim:

1. Process comprising the steps of:

1. homogeneously mixing

a. acicular alpha-Fe.sub.2 O.sub.3 of high surface area,

b. BaCO.sub.3 or equivalent source of barium oxide in amount to provide upon reaction with the alpha-Fe.sub.2 O.sub.3 a ferrite of the generalized formula BaFe.sub.12 O.sub.19,

c. about 1-12 weight percent of NaF fluxing agent, and

d. where the amount of fluxing agent in weight percent "x" is less than 6, at least ##EQU2## weight percent of a lead compound up to about 2 weight percent of the mixture,

2.

2. calcining the mixture at about 850.degree.-1100.degree.C, and

3. treating the ferrite with aqueous acid solution to remove undesirable reaction products and any unreacted material to provide elongated platelets which are especially adapted to mechanical orientation in

admixture with a workable nonmagnetic matrix material. 2. Process for making barium ferrite particles which are especially adapted to mechanical orientation in admixture with a workable nonmagnetic matrix material, which process comprises the steps of:

1. homogeneously mixing

a. acicular alpha-Fe.sub.2 O.sub.3 of high surface area,

b. BaCO.sub.3 or equivalent source of barium oxide in amount to provide upon reaction with the alpha-Fe.sub.2 O.sub.3 a ferrite of the generalized formula BaFe.sub.12 O.sub.19,

c. about 1-6 percent of NaF fluxing agent, and

d. a lead compound in an amount exceeding 3/4 weight percent and less than 13/4 weight percent of the mixture,

2. calcining the mixture at about 850.degree.-1100.degree.C, and

3. treating the ferrite with aqueous acid solution to remove undesirable reaction products and any unreacted material to provide elongated platelets.

3. Process comprising the steps of:

1. homogeneously mixing materials which are reasonably free from impurities and comprise

a. acicular alpha-Fe.sub.2 O.sub.3 having a surface area of at least 15 square meters per gram,

b. BaCO.sub.3 in amount to provide upon reaction with the alpha-Fe.sub.2 O.sub.3 a ferrite of the generalized formula BaFe.sub.12 O.sub.19,

c. 5-12 weight percent of NaF, and

d. PbO in an amount exceeding 3/4 weight percent and less than 13/4 weight percent of the mixture,

2. calcining the mixture at about 850.degree.-1100.degree.C,

3. treating the ferrite with aqueous acid solution to remove undesirable reaction products and any unreacted material to provide elongated platelets which are especially adapted to mechanical orientation in admixture with a workable nonmagnetic matrix material,

4. mixing the elongated platelets with a workable rubber matrix material to provide a mixture comprising about 55-70% ferrite by volume,

5. rolling or extruding the mixture to align the ferrite platelets by mechanical shearing forces, and

6. then vulcanizing the matrix material to provide a permanent magnet having a maximum energy product greater than 1.5 .times. 10.sup. 6 gauss-oersteds.

4. Process comprising the steps of:

1. homogeneously mixing

a. acicular alpha-Fe.sub.2 O.sub.3 particles having a surface area of at leat 15 square meters per gram,

b. BaCO.sub.3 or equivalent source of barium oxide in amount to provide upon reaction with the alpha-Fe.sub.2 O.sub.3 particles a ferrite of the generalized formula BaFe.sub.12 O.sub.19,

c. about 1-12 weight percent of NaF fluxing agent, and

d. where the amount of fluxing agent in weight percent "x" is less than 6, at least ##EQU3## weight percent of a lead compound up to about 2 weight percent of the mixture,

2. calcining the mixture at about 850.degree.-1100.degree.C,

3. treating the ferrite with aqueous acid solution to remove undesirable reaction products and any unreacted material to provide elongated platelets,

4. mixing the elongated platelets with a workable rubber or thermoplastic matrix material to provide a mixture comprising about 55-70% ferrite by volume,

5. rolling or extruding the mixture to align the ferrite platelets by mechanical shearing forces, and

6. then vulcanizing the rubber or cooling the thermoplastic matrix material to lock the ferrite platelets in place to provide permanent magnet material having a maximum energy product of at least 1.4 .times. 10.sup.6 gauss-oersteds.

5. In an process comprising the steps of:

1. homogeneously mixing alpha-Fe.sub.2 O.sub.3 particles with BaCO.sub.3 or equivalent source of barium oxide in amount to provide upon reaction with the alpha-Fe.sub.2 O.sub.3 particles a ferrite of the generalized formula BaFe.sub.12 O.sub.19,

2. calcining the mixture to provide barium ferrite platelets,

3. mixing the ferrite platelets with a workable rubber or thermoplastic matrix material to provide a mixture comprising about 55-70% ferrite by volume,

4. rolling or extruding the mixture to align the ferrite platelets by mechanical shearing forces, and

5. then vulcanizing the rubber or cooling the thermoplastic matrix material to lock the ferrite platelets in place to provide permanent magnet material,

the improvement comprising:

a. step (1) employs acicular alpha-Fe.sub.2 O.sub.3 particles having a surface area of at least 15 square meters per gram,

b. included in step (1) is about 1-12 weight percent of NaF fluxing agent and where the amount of fluxing agent in weight percent "x" is less than 6, at least ##EQU4## weight percent of a lead compound up to about 2 weight percent of the mixture,

c. the calcining step (2) is carried out at 850.degree.-1100 C,

d. step (2) is followed by treating the ferrite with aqueous acid solution to remove undesirable reaction products and any unreacted material to provide elongated platelets.

6. Process as defined in claim 5 wherein PbO is added in step (1) in an amount exceeding 3/4 weight percent.

7. Process as defined in claim 4 wherein a magnetic field is employed as part of step (5) to supplement the mechanical shearing forces in order to improve the alignment of the ferrite platelets.

8. Process as defined in claim 1 wherein the fluxing agent is in the amount of 3-10 weight percent.

9. Process as defined in claim 4 wherein the fluxing agent is in the amount of 3-10 weight percent.

10. Process as defined in claim 1 wherein PbO is present in an amount exceeding 3/4 weight percent.