US3903228A

US3903228A - Flexible ferrite-particle magnets

Info

Publication number: US3903228A
Application number: US229200A
Authority: US
Inventors: Kenneth M Riedl; Karl E Nelson
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1970-08-12
Filing date: 1972-02-24
Publication date: 1975-09-02
Anticipated expiration: 1992-09-02

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-Fe2O3 particles, BaCO3, 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 X 106 gauss-oersteds.

Description

United States Patet Riedl et a1. Sept. 2, 1975 [54] EXIBLE FERRITE-PARTICLE MAGNETS 2,999,275 9/1961 Blume. Jr 264/DIG. 5s 3,093,589 6 1963 DO t 1.. 264 DIG. 5s

[75] lnvemors: Kenneth Ried'; Karl 3,278,440 10/1966 252/6263 both of Paul 3,387,918 6/1968 Moore Ct 211. 264/D1G. 5s

[ Assigneer Minnesota Mining and FOREIGN PATENTS OR APPLICATIONS Wmufacturmg Company Paul 284,335 4/1966 Australia 252/6263 Mmn.

[22] Filed: Feb. 24, 1972 Primary Examiner-Donald .1. Arnold pp NO: 229,200 Attorney, Agent, or firm-Alexander, Sell, Steldt &

Related US. Application Data Continuation-impart of Ser. No. 63,299, Aug. 12, 1970, abandoned.

References Cited UNITED STATES PATENTS 1/1957 Crowley 252/625 8/1958 Peterman 264/D1G. 58

DeLaHunt [5 7] 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 a1pha-Fe O particles, BaCO 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 X 10 gaussoersteds.

10 Claims, N0 Drawings FLEXIBLE FERRITE-PARTICLE MAGNETS REFERENCE TO RELATED APPLICATION This application is a continuationin-part of applicants copending application Ser. No. 63,299, filed Aug. 12, 1970 and now abandoned.

FIELD OF THE INVENTION The present invention relates to flexible permanent magnets consisting of aligned ferrite particles in a nonmagnetic 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 2 l gauss, a coercivity H of 1200 oersteds and a maximum energy product of 0.9 X 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 gaus s, a coercivity H 1750 oersteds, an intrinsic coercivity H of 3000 oersteds and a maximum energy product BH of about 1.1 X 10 gausseoersteds.

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 nar row 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 parti' cles 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:

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 weight percent and is less than 1% weight percent.

The straightforward procedure of:

1. homogeneously mixing the above materials,

2. calcining the mixture at 850-l 100C, 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 X 10 gaussoersteds have been consistently attained along with other high magnetic values, e.g., B at least 2500 gauss, H above 2000 oersteds and H 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 X 10 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 Maximum energy products above 1.5 X 10 gaussoersteds can be attained without the need for magnetic orientation by employing at least weight percent of fluxing agent plus at least weight percent of the lead compound. The highest energy products are realized at about to weight percent of fluxing agent. If the lead compound is omitted and the fluxing agent con tent exceeds 5 weight percent, maximum energy products of -l.4 X 10 gauss-oers'teds can still be attained. Evenlthough 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-F0 0 contain undesirably high sulfur content which can readily be reduced to satisfactory levels by roasting the particles at 700C untilthe sulfur content is 0.3 weight percent or less.

EXAMPLE 1. A

pounds of the dry mixture were placed in agas-fired rotary calciner at 950C for l hour to. provide leadmodified barium ferrite particles.

-Forty-eight hundred ml of 5% l-lCl solution were heated to 95C in a glass flask, and 1440 grams of the barium ferrite particles were added. After stirring for vr'n inutes,--with the temperature at 8793C, the

acidic solution 'was decanted and the ferrite was rinsed 5;.timeszin warm water, once in acetone, and then was dried in an air-circulating oven at 150C. The product ferrite particles were elongated platelets of generally submicron $17.0. 7

. 1 3-1 5% weight loss occurred during the acid treat- ,mentgwhich is consideredsatisfactorily small in view of the enhanced results attributable to the acid treatment. Although. .a moresevere acid treatment might further improve magneticwalues, a weight loss above, about 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 v 3250 Butadicne-acrylonitrile copolymer. I medium-level-nitrile (Chemigum N-608") 260 Zinc stearate I 7 Sulfur 2.5

Benzothiazyl disulfide accelerator (Altax") The rubber and zinc stearate .were mixed in a Banbury.

mixer with gradual addition of the ferritesThe mixture .was pulverized and placed in a dry blender with the sul fur and accelerator. After 5 minutes this was sheeted between cold rolls (0.015 inch roll spacing, roll ratio 1:1). Thesheet 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 ata 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 vulca nized in an air-circulating oven at C for one hour.

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

2520 gauss H i v 2100 oersteds H,.,- r 3310 oersteds nmJ' 1.45 X 10" gauss-oersteds.

v EXAMPLE 2 The following composition was prepared:

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 0.0700.075 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. 1

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 150C over about 40 minutes. After additional 30 minutes at 150C, the sheet was allowed to cool to 50C over a period of about 45 minutes, at which time the field was removed and the sheet was taken from the box. Three /2-inchdiameter plugs were punched from the cured sheet and stacked for testing, with the following results:

2570 gauss r 2260 oersteds 0! 3900 oersteds BH 1 .55 X 10 gauss-oersteds.

the Blume patent,the fcrri te par ticles should comprise at least 55% and preferablymorc 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 usesvha v e been attained at 70% ferrite by volume. In any-event, a flexible magnet of one-eighth inch thickness ought to withstand bending over a SinCh'mandrel without breaking.

EXAMPLE "3 y Charged to a dry blender were 1761.4 grams of the same acicular alpha1-Fc O -used in Example*l,,385r8 grams of BaCO 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 lO50C for80 minutes. The produetbarium ferrite particles were acid treated in the same manner as in Example'l. A-2O percent, weight loss occur'redduring the acid treatment. The acid-treated particles were elongated plateletsrof generally submicrorrsize;

To produce flexible magnets, the following composition was prepared:

The rubber, stearic acid, sulfur andpolyether'plastieizer were mixed in a Banbury mixer with gradual addi tion of the ferrite. The mixture pulverized and placed in a dry blender with the zinc oxide and the two accelerators, After a 5-minute blend, this was stieeted and laminated between cold rolls and then vulcanized by the same procedurea s in Example li The cured sheet, had the following properties;

2475 gauss 2260 oersteds m 4300 oersteds I 1.47 X gauss-ocrsteds.

mIIcc EXAMPLE 4 1761.4 grams of acicular alpha-Fe Q, particles (5 l6-M) were combined with 385.8 grams of BaCQ 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 1 100C 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, leadmodified 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:

,. i 2690 gauss i 2425 oerstcds 3625 oersteds 1.72 X It)" gauss-oersteds.

r'l HIH'H'. 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.

We claim: v i l i v 1. Process comprising the steps of:

l. homogeneously mixing 5 v a. acicular alpha-Fe O ,of high surface area; i b. BaCQ, or equivalent source of barium oxide in amount to provide upon reaction with the alpha- Fe O a ferrite of the generalized formula BaFe j 2 1; i p c. about 1- an y d. where the amount of fluxing agent in weight percent .r is less than 6, at least 1 l2 weight percent of NaFfluxin g agent,

weight percent of a lead compound up to about 2 weight percent of the mixture, 2. calcining the mixture at about 850l C, and

:3. treating the ferrite with aqueous acid solution to remove undesirable reaction products and any un' reacted material to provide elongated platelets which areespecially adapted .to mechanical orien i tation in admixture with a workable nonmagnetic matrix material, I 7

2. Pr0cess for makingbarium ferrite particles which are especiallyadapted to mechanical orientation in admixture with a workable nonmagnetic matrix material, which process comprises the steps of:

' l. homogeneously mixing 1 al. acicular alphaFe O of high surface area,

V b. BaGO or equivalent source of barium oxide in amount to provide upon reaction with the alpha- Fe O a ferrite of the generalized formula BaFe c. about 1-6 percent of NaF, fluxing agent, and d. a lead compound in an amount exceeding 4 weight percent and less than 1% weight percent of the mixture, 2. calcining the mixture at about 850-l 100C, 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: l. homogeneously mixing materials which are reasonably free from impurities and comprise a. acicular alpha-Fe O having a surface area of at least 15 square meters per gram,

b. BaCO in amount to provide upon reaction with the alpha-Fe- O a ferrite of the generalized formula BaFe O c. 5-12 weight percent of NaF, and d. PhD in an amount exceeding weight percent and less than 1% weight percent of the mixture, 2. calcining the mixture at about 850l 100C, 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,

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

4. Process comprising the steps of:

l. homogeneously mixing a. acicular alpha-Fe O particles having a surface area of at leat 15 square meters per gram,

b. BaCO or equivalent source of barium oxide in amount to provide upon reaction with the alpha- Fe O particles a ferrite of the generalized formula BaFe O c. about l-l2 weight percent of NaF fluxing agent,

and

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

2. calcining the mixture at about 850l 100C,

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 5570% ferrite by volume,

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 X gauss-oersteds.

5. In an process comprising the steps of:

l. homogeneously mixing alpha-Fe O particles with BaCO or equivalent source of barium oxide in amount to provide upon reaction with the alpha- Fe O particles 21 ferrite of the generalized formula IZ ISv 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 5570% 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 O particles having a surface area of at least 15 square meters per gram,

b. included in step (1) is about 1l2 weight percent of NaF fluxing agent and where the amount of fluxing agent in weight percent x is less than 6, at least weight percent of a lead compound up to about 2 weight percent of the mixture, c. the calcining step (2) is 850-110() c, (1. 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 carried out at in step (1) in an amount exceeding 4 weight percent.

present in an amount exceeding weight percent.

Claims

1. PROCESS COMPRISING THE STEPS OF,

1. HOMOGENEOUSLY MIXING A. ACICULAR ALPHA-FC2O3 OF HIGH SURFACE AREA, B. BACO3 OR EQUIVALENT SOURCE OF BARIUM OXIDE IN AMOUNT TO PROVIDE UPON REACTION WITH THE ALPHA-FE2O3 A FERRITE OF THE GENERALIZED FORMULA BAFE12O19, 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

2. CALCINING THE MIXTURE AT ABOUT 850*-1100*C, AND

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:

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

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

2. calcining the mixture to provide barium ferrite platelets,

2. calcining the mixture at about 850*-1100*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.

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

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,

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:

3. TREATING THE FERRITE WITH AQUEOUS ACID SOLUTION TO REMOVE UNDESIRABLE REACTION PRODUCTS AND ANY UNCREATED MATERIAL TO PROVIDE ELONGATED PLATELETS WHICH ARE ESPECIALLY ADAPTED TO MECHANICAL ORIENTATION IN ADMIXTURE WITH A WORKABLE NONMAGNETIC MATRIX MATERIAL.

4. Process comprising the steps of:

5. In an process comprising the steps of:

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-Fe2O3 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

6. then vulcanizing the matrix material to provide a permanent magnet having a maximum energy product greater than 1.5 X 106 gauss-oersteds.

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 X 106 gauss-oersteds.

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.