US 2980617 A
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April 18, 1961 Hc -OERSTEDS OR Br GAUSS J. R. IRELAND Filed March 15, 1956 4200 l zooo ,/1
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atent l FERRITE COMPOSITIONS AND METHOD F MAKING SAME James R. Ireland, Valparaiso, Ind., assignor to Indiana General Corporation, a corporation of Indiana Filed Mar. 13, 1956, Ser. No. 571,210
Y I 4 Claims. (Cl. 252-625) I' The'present invention is concerned with a method for `improving the magnetic properties of certain magnetic compositions, and to compositions thus produced.
The invention deals specifically with methods for improving the coercive forceY and the maximum energy product of permanent magnets of the ferrite type represented by the formula MO.xFe2O3 wherein M is a ,energy product of such materials can be improved by including during their manufacture a step which might be called an orienting process. In order to make this :process fullyeifective, the `oxide .powder has to be brought into- `a state where .every powder particle' rep- Vresents a single crystal.V ThisV is usually accomplished by thorough milling of thepowdered materials, sinter- Ying, and then milling to the desired particle size. When Vsuch a powder. is oriented and compacted `in a magtion.` Y, In some cases, the -maximum netic eld, its particles orient themselves with a certain crystallographic axis more or lessparallel to the applied field This preferred orientation persists after sintering the sample, giving it a high value of residual inducenergy product can ,be brought toa value approximating BXlOS'gauss oersteds by the orientation procedure.
1 One of the difliculties vaccompanying suchrincreasel in maximum energy product, however, isV the fact that as the residual induction VisY increased, itfis usually accomplished -by a decrease in the coercive force of thema- -teriaL Thus,
raising the sintering temperature of ferrite mixtures Yusually, increases the residual induction while lower-ingr the coercive force due to the increase ,inA apparenpdensity of the specimen due to grain growth. .For
.-manyapplications, apreferable solution lwould be to provide` some means of .increasing the coercive force r whichv automatically would increase the maximum energy product, provided, the ,residufal induction-was keptu more` or 1es sconstant 'during the process.
' Accordingly, an1 Pobject, vof the present invention to ,provider Ian improved method for increasing .the..maxi
z'provedf,method; forl increasingl v vthe maximum energylproduct offerrite typel permanent' magnet compositione without substantially affecting the ,y y
f A further object of 'theinvention 1s tions.'
.,.ffnothe'r-objeet of-V the linvention is. to provide an'firn#V Y Ythe' coercivel force and residual induction ofthe material...
maximum energy products'at lea'stgashighgas, and ,igenvferall'y .higher thanr comparable materials" presentlyavailr.
. tor-provide improved magnetic corn'posltionsrv of the ferrite. type having a '701small; amounts`4 ofother'metallic oxides. and compounds. n `The beneficial; effects ofthe refractory a'ldi'tionare v 4:evidentjeven whe're.as .little'` @190.10% :byweiglit ofthe'- Another object of the invention is toprovide an improved composition for the manufacture of permanent magnet materials in which the sintering conditions are not so critical as they are presently in lthe manufacture of ferrite magnets.
The present invention provides a magnetic material of increased maximum energy product comprising a fer rite composition containing a minor amount of a refractory ceramic composition distributed through the ferrite composition. The addition of relatively small amounts of such refractory ceramic materials, particularly combinations of alumina and silica, has been found to increase rather substantially the maximum energy product and the coercive force values of ferrite compositions and, as another advantage, has been found to render the condi-tions of sinteringA less criticalthan previously.v
The ferrite compositions usable in the practiceof the Vpresent invention Vmay be any of ia wide variety of ferrites, and preferably consist of those ferrites which Ihave maximum energy products on the o-rder of l 106 gauss oersteds or more. A particularly preferred ferrite composition for use in the present invention consists of noncubic crystals of ferrites in which theferric oxide lattice is substituted by a metal such as barium, strontium, or lead. PerhapsY the best example of a compositionv of this type is one prepared by combining one molecular proportion of bariumoxide with six molecular proportionsof ferrie oxide to produce materials having high coercive forcevvalues and high maximumenergy products. The preparation of crystals of this type usually Vconsists in mixing thoroughly a powdery mixture of Aferrie oxide and an oxide or carbonate of` one or more of the other metals, sintering at an elevated temperature and lmilling the sintered material -to the desired particle size. The resulting particles will then repre- ,seut substantially single crystals. This powder is then -compacted while under the inuence Vof a magnetic field and the resulting compactY is heated to atemperature in the range from about 2000 to about 2500 Erin a non-reducing atmosphere so that the ferrie oxide is not reduced to a lower state'of oxidation.
During the mixing or milling operation, the powders are combined with a refractory ceramic powder. All of the following materials have been found to produce the desired eife'ct in varying degrees of effectiveness:
(2) Alumina (3) Titania" '(4) Boric oxide ,Y A,
M The following vmixtures have also beenV found effec- ,tive the .mixtures being employed in any proportions,
either 'as a physical mixture or as a chemical combination:,,-
"(5) Alumina-silica (6) Calcium oxide-silica '1(7) Calcium oxide-ftitania ,I (8) .Alumina-titania I (9)Boric oxide-silica v (l 0) lBoric oxidetitania ,1(,1 1') Chromic oxidesilica f ('12). C hromic oxide-ftitaniai l Where physicavlmixtures Aarejemploye'd,the two vcompounds Yare preferably combined in equal parts-by weight. .The preferred materiallis an alumina-silicacombinaa silica-alumina porcelain,
sis'tin'g 1 essentially of aluminum silicates andk Irelatively ytionfof the type-represented by angaluminumsilicate such -V l K. K kaolin ('Al2O3.2SiO2.-2H2O),* mu1li'te.:(3&l203.2SiO2) or other-clays vor.inineralsfconi.jisilia-anminaporcelain'constituig 7% by Wfl-bibi t oxide, mixture, or compound is included in the composi tion. Amounts of the additive in excess of about 2% by weight appear to provide no additional effect insofar as raising the maximum energy product is concerned.
The mechanism by which the refractoryY additions operate to improve the magnetic properties is not cornpletely understood. It is possible that some form of nonsmagnetic` material is formed karound each crystal, thereby causing some slight separation between the crystals and reducing the possibility of domain walls crossing from one crystal to another. The effect of this factor would be to increase the coercive force. It seems clear that the improvement is not a simple mechanical phenomenon because magnets made without the additions have resistivities on the order of a few hundred thousand to a few million ohm-cm, whereas magnets using such additions havefresistivities on the order of a few hundred to a few thousand ohm-cm.
The process of the present invention has been used successfully to produce permanent magnet materials havingmaximum energy products in excess of V3 l0s gauss oersteds. This improvement is obtained by achieving an increased coercive force at a given residual induction. The increase in coercive force-is of a considerable importance not only because it is accompanied by an in'- crease in maximum energy product but also because it makes the magnet less susceptible to the demagnetizing influences of stray fields, changes.
In the past, some magnet compositions have been prepared with energy products on the order of 3.5 million gauss oersteds, but these were achieved by grinding away the misoriented material on the surfaces of such 'magnets Evenwiththis process, entirely inconsistent results were obtained, whereas with the process of the present invention, it is possible to produce consistently magnets with energy products of 3.5 million gauss oersteds without resorting to any grinding Whatever.
The attached sheet of drawing illustrates a graph in which the coercive force, remanence, and maximum energy product of two identical materials are compared, the difference being thatthecurve drawn with the dashed vlines representa bariumoxide-ferricoxide (l to 6 mole vratio) ferrite composition without the addition of the refractory material, while vthe solid line curve represents the Values obtained with the same composition'combined with 0.5% of powdered kaolin. As evidenced'from these curves, an increase of. theisintering temperature shocks, and temperature very substantially reducesV the coerciv'eforceof the materialwitho'ut the kaoli'n addition, While the reduction of coercive force` in` the preferred sample isfmuchmore gradual. It will also be noted that the maximum energy product of the sample with the kaolin addition increasesy despitejan increase in the sintering temperature, whereas thevfmaxifmumenergy product of the material without V tliefadrlitionl decreases after sinteringmtemperature of about 2250 F.
.t w-ill also be lnoted that the, coercive force .values for the materialwith the additiveare higher y in all instances at the various .psintering temperaturesfand vthat the remanence characteristics are substantially', A higherat the lower sinteringtemperatures for the improved material of the preseptinvention, and substantially the same at thehighersintering temperatures.
- The following-specitic examples illustratev the results? achieved in tlietcompositicmsV of .thepreSent inyention. 'Y
one molecular proportion. of barium oxide and six molecitypical'example of `aY barium oxide ferrite containing Y the mixture, the remanence observed n the iinal product was 4010, the coercive force was 1900 oersteds and the maximum energy product was 355x106. Thus, the addition of even a small amount of the refractory material was effective to raise the maximum energy product by about 25%.
Example VIl The same starting material as in Example I was combined with of kaolin and ball-milled to produce a very linely divided powder. This material after sintering had a remanence of 3900, a coercive force of 1860 oersteds, and a maximum energy product 0f 3.47)(106.
Example III A barium ferrite sample rof thetype used in the preceding examples was made with the addition of 1/2% silica. The materials had a remanence of 3940, a coercive force of 1700 oersteds and a maximum energy product of 339x106.
Example 1V modications canjbe madey to the described embodiment without departing from the scope of the `present iu- `Vention.
I claim as my invention:
1. In a method of producing a permanent magnet of the mixed ferrite type having a non-cubic crystalline lattice and an empirical formula MO.6Fe2O3 where M is a metal kselected from the grouppconsisting of barium, strontium, and lead wherein said magnet is prepared by mixing iron oxide with a source of the oxideMQfollowed by sintering the mixture at a temperature in the range from 2,000 to2,500 F. and compacting the mixture while under the inuence of a magnetic eld, the improvement whereby the maximum energy product is increasedwhich comprises adding to said `mixture prior t0 said compacting from 0.1 to 2.0% by weightrof a refracsilica, boric oxide and titania, chromic oxide andsilica, and 'chromic oxideV and titania, the alumina t0 s1lica molar ratio, beingfgcnerally between l to 24 to3 to2, andthe other oxides ,of the remaining mixtures being substantially in ,equal proportionsby weight. t
2. In amethod of producing apermanentrnagnet of` -the mixed'ferrite type having anon-cubic crystalline latf tice 'andan empirical formulaiMO-6Fe2O3 vwhere M is a metal selected from thejgroup vconsisting yofgharium,
' strontium, aiidleadwher'e'nv saidmagnet' is prepared by Vmixing iron oxide with 'a source of the oxidewMOfollowed i byk sintering themixture ata temp 'eratuiiefof A2,000 to 2,500?V fand vcompacting the,mixtnr'efwhiley under the intiuence of faA magnetic `field," the'` imprbvemenewliereby 'the maximum energyproduct'is increased wh1chfcorn fprises adding to said'mixturefpriorto'compactingfrom l -,0.1 to -2.0%,by Weightof a mixture ofalumin'aiandfsilica Y Vhaving ka molar rattio'of from about lgto. 2 to -.3 -Ato Y2'.
Y3. "A permanent'magnet material comprising-famon- "'cubiomixed ferrite composition havingthe empirical for` mula MOiGezOa where Mfis'a `metalselected from' a groupconsisting offbarium, strontium,v and lead in comv This material had a remaalumina and silica, calcium oxide and silica, calcium oxide and titania, alumina and titania, boric oxide and silica, -boric oxide and titania, chromic oxide and silica, and chromic oxide and titania, the alumina and silica being in relative molar proportions of between about 1 to 2 to 3 to 2, and the other oxides of the remaining mixtures being substantially in equal proportions by weight.
4. A permanent magnet material comprising a barium Iferrite having the empirical formula BaO.6Fe2O3 in cornbination with from 0.1 to 2.0% by Weight of a mixture of silica and alumina having a molar ratio of from about 1 to 2 to 3 to 2, said material having a maximum energy product in excess of 3 X 10a gauss oersteds.
References Cited in the file of this patent UNITED STATES PATENTS 2,551,711 Snoek et al. May 8, 1951 2,565,111 Albers-Schoenberg Aug. 21, 1951 2,565,861 Leverenz et al Aug. 28, 1951 6 2,677,663 Jonker -l May 4, 1954 2,762,778 Gorter et al Sept. l1, 1956 2,828,264 Medvedietr Mar. 25, 1958 2,837,483 Hakker et al. June 3, 1958 2,900,344 Stuyts et al. Aug. 18, 1959 FOREIGN PATENTS 513,734 Canada June 14, 1955 515,205 Belgium Nov. 14, 1952 521,244 Belgium --.Q Jan. 6, 1954 683,722 Great Britain Dec. 3, 1952 A697,219 Great Britain Sept. 16, 1953 OTHER REFERENCES R.C.A. Review, September 1950, vol. XI, No. 3, page 346.
Electrical Engineering, July 1952, page 646.
Phillips Technical Review, Vol. 13, No. 7, page 201 (1951).