US2703787A - Method for preparing magnetic compositions - Google Patents

Description

United States Patent METHOD FOR PREPARING MAGNETIC COMPOSITIONS Henry L. Crowley, South Orange, N. J., assignor, by mesne assignments, to Henry L. Crowley & Company, Inc., West Orange, N. J., a corporation of New Jersey No Drawing. Application November 18, 1952, Serial No. 321,291

Claims. (Cl. 252-625) This invention relates to magnetic compositions and articles formed therefrom, and has for its object the provision of certain improvements in the method of making the same. More particularly, the invention contemplates certain improvements in those methods of making magnetic compositions, and articles formed therefrom, described in my United States Letters Patent No. 2,575,099, patented November 13, 1951, in my copending patent application Ser. No. 226,545, filed May 15, 1951 and in the copending patent application of Arthur M. Hossenlopp and myself Ser. No. 230,706, filed June 8, 1952. More specifically, the present invention involves certain improvements in the preparation of the initial oxide mixture for the pre-firing or first stage heat-treatment in those methods of making magnetic compositions.

The methods disclosed in the aforementioned patent and patent applications involve subjecting an initial oxide mixture composed of at least 50% by weight of ferric oxide and containing electrical and magnetic modifying metal oxides (e. g. zinc oxide and nickel oxide, respectively), to a two-stage heat-treatment. The first heattreatment is carried out at a temperature of l000 to 1450 C. and the second heat-treatment is usually carried out at a somewhat higher temperature than that of the first heat-treatment. The properties of the magnetic composition are influenced by the heat-treatment temperatures, and the respective temperatures of the two heat-treatments are determined (by experience or test) to impart to the composition the optimum electrical and magnetic prop-- erties for its intended use. The environments in which the initial oxide mixture and the product of the first heattreatment are respectively heat-treated are so controlled that ferrous oxide is present in the resulting product at the conclusion of the second heattreatment, and the product is gradually cooled through at least the greater part of the temperature range in which ferrous oxide spontaneous ly dissociates into metallic iron and magnetite. Throughout this specification and the appended claims magnetic composition means the final and finished composition and includes articles made therefrom.

I have now discovered that the electrical and magnetic properties of the magnetic compositions of the aforementioned patent and patent applications can be usefully enhanced by substantially deaerating and densely compacting the intimately-mixed initial oxide mixture preparatory to the pie-firing or first-stage heat-treatment. Based on that discovery, the present invention involves compacting and densifying the initial oxide mixture under considerable pressure after and preferably while the mixture is subjected to a relatively high vacuum. The thuscompacted and densified mixture, in the shape or form resulting from compacting, is then subjected to heattreatment at a temperature within the range of 1000 to 145 0 C., and the resulting heat-treated product is crushed and ground to a relatively fine powder. 7

Compacting of the initial oxide mixture may be done by pressure alone, as for example by briquetting in a mold or press, by extrusion, or by pressure followed by extrusion, all under evacuated conditions. A very con siderable pressure is used in compacting, since the purpose is to density the deaerated mixture a-s greatly as practical. To this end, a compacting pressure of at least 5000 pounds per square inch is employed, and preferably the compacting pressure is about 20,000 to 40,000 pounds per square inch. Moreover, it has been found, for reasons not clearly understood, that extrusion, or a combination of pressure and extrusion, imparts superior physical and electncal properties to the magnetic composition, as contrasted with briquetting alone. Compacting and densifymg may advantageously be carried out by extruding the deacrated mixture in the form of bars or rods approxi mately one-half /2) inch in diameter under a pressure approximating 20,000 pounds per square inch. The extruded bars break off in lengths of about 3 to 4 inches, suitable for handling before, during and following prermg.

Prior to compacting, the initial oxide mixture is subected to a relatively high vacuum, sufficient to substantially deaerate the mixture, so that the compacted and densified mixture is substantially devoid of air pores. The high vacuum should be maintained during compactmg to avoid re-entrance of air into any of the already evacuated pores of the mixture. In practice, .it is usually preferable to carry out the steps of evacuation and compacting simultaneously. The vacuum should be as high as practical, and of the. order of at least 25 inches of mercury, and preferably as high as 28 inches of mercury, and even higher if practical.

In my aforementioned Patent No. 2,575,099, I have described agglomerating the initial oxide mixture preparatory to pre-firing, but such agglomeration was primarily for convenience in handling the mixture, and was not carried out under evacuated conditions. In later commercial practice of the method of that patent, it was frequently customary to pre-fire the initial oxide mixture in granular form, that is as a more or less loose powder. The discovery upon which the present invention is based was made in the course of an investigation of certain deficient and unpredictable physical characteristics, both macroand microscopic, which were frequently present in structures of'the magnetic composition. It now appears that when the initial oxide mixture is pre-fired in the form of a more or less loose powder, or in the form of ordinary agglomerates, the inevitable presence of innumerable air spaces Within the mass of the mixture results in a product which, even after grinding, is so permeated with internal air pores as to exercise a. deleterious effect not only upon the physical but also upon the magnetic and electrical characteristics of the magnetic material. Irrespective of the degree of subsequent grinding, it appears impossible to sufiiciently eliminate internal air pores in the pre-fired product to prevent these deleterious effects. On the other hand, when the initiai oxide mixture is pre-fired in the form of the deaerated and densely compacted shapes of the invention, the pre-fired product is substantially solid and for all practical purposes is substantially devoid of internal pores. Even after grinding, as well as throughout all subsequent processing steps, this characteristic freedom from internal pores or voids is maintained, with all of its attendant advantages upon the magnetic and electrical properties of the magnetic composition.

In addition to substantially eliminating internal air pores, practice of the invention results in other benefits and advantages. When the initiai oxide mixture is pre fired in a more or less loose powdery form, substantial loss of certain constituents of the mixture, notably zinc oxide, takes place during pre-firing. Since these losses are not always uniform, the chemical composition of the prefired product will vary, depending upon the losses, and such variations in chemical composition cause undesirable variations in the magnetic and electrical properties of the magnetic composition. By deaerating, compacting and densifying the initial oxide mixture, such losses during pre-firing become practically negligible. Additionally, the heat-treatment of the initial oxide mixture in the form of deaerated and densely compacted shapes promotes uniform and complete chemical reaction between the metal oxide constituents of the mixture whereby the heattreated product has greater uniformity of chemical com? position, and hence the magnetic and electrical properties of the magnetic material are more uniform. Further,

heat-treatment of the initial oxide mixture in the form of deaerated and densely compacted shapes imparts certain physical propertiesv to the heat-treated product which beneficially influence shrinkage during the second-stage of heat-treatment. Taken in their entirety, these advantages enable and insure repeated and continuous commercial production of magnetic compositions of more consistent-- ly uniform magnetic and electrical properties than has heretofore been attained.

Heat'treatment of the deaerated and densely compacted shapes is carried out in any suitable type of kiln or furnace at a temperature between about l000 C. and about 1450 C., depending upon the desired characteristics of the magnetic material. The temperature of heat-treatment is preferably somewhat above the softening temperature of at least one of the constituents of the initial oxide mixture, but not high enough to result in an appreciable vapor pressure. In order to definitely allow the interior of all the shapes undergoing heat-treatment to reach a uniform elevated temperature before the exteriors of the shapes shrink sufficiently to close the pores, heating is leveled-off at a temperature substantially lower (e. g. 50200 C. lower) than the ultimate temperature of heat-treatment, and maintained at the leveling-off temperature until the entire mass of the shapes have uniformly attained that temperature, whereupon the shapes are gradually heated to the ultimate temperature of heat-treatment. In practice, the shapes may be permitted to shrink about 2 to 4% before reaching the leveling-off temperature. At the completion of heat-treatment, shrinkage of the shapes may amount to from 10 to 20%. In a typical heattreatment cycle, the shapes are progressively heated to a levelling-off temperature of about 10004250 C., in about 2 hours, are maintained at that temperature for approximately 2 hours, and are then gradually heated over a period of approximately 2 hours to the ultimate temperature of heat-treatment of about 1200l350 C.

In practicing the methods of the aforementioned patent and applications, magnetic compositions of superior mag netic and electrical properties are produced when prefiring or first-stage heat-treatment is conducted under conditions promoting the removal of some, but not all, of the oxygen content of the ferric oxide component of the initial oxide mixture. Although heating to a temperature within the aforementioned range in an indifferent ambient atmosphere will effect a significant removal of oxygen from the ferric oxide, it has been found generally preferable to effect removal of a more substantial and definite amount of oxygen from the ferric oxide by a reducing environment such as that provided by a reducing ambient atmosphere or by a reducing agent present in the initial oxide mixture, or by both. Reducing agents suitable for incorporation in the initial oxide mixture are preferably such carbonaceous materials as dextrin, starch, flour, cellulosic substances etc., and may advantageously be added in amounts of from about 1 to 5% by weight of the initial oxide mixture.

In compacting and densifying the deaerated initial oxide mixture, it is desirable to incorporate therein a binder and sufficient moisture to impart plasticity. Preferably, the binder is one of the aforementioned carbonaceous materials which serves additionally as a reducing agent. A moisture (water) content of about by weight usually imparts the desired amount of plasticity to the mixture. The deaerated and densely compacted shapes are dried in any suitable manner, as for example in an oven at a temperature slightly above 100 C. to eliminate moisture, and are then pre-fired as hereinbefore described.

At the conclusion of the pre-firing or first-stage heattreatment, the heat-treated shapes are preferably quenched to minimize dissociation of ferrous oxide into metallic iron and magnetite, since the presence of magnetic materials is detrimental in the subsequent grinding operations. Quenching is conveniently effected by directly discharging the heat-treated shapes from the furnace into water. The quenched shapes are then crushed and ground, preferably to as uniform a size as practicable. Grinding should produce a relatively fine powder, say of an average particle size not exceeding 25 microns. However, in the preferred practice of the aforementioned methods, much finer grinding is desirable, preferably such that at least 75% of the ground powder is of a particle size between 2 and 10 microns. Particles of 1 micron and smaller are undesirable, since they present too great surface areas and tend to chemically impair the magnetic composition. On the other hand, too large a proportion of particles above 10 to microns in size is undesirable.

Particle size determinations may conveniently be made with a Cenco-Sheard-Sanford Photclometer, obtained from the Central Scientific Company, of Chicago. The determinations are made by allowing a portion of the ground product to settle through a transparent liquid of known viscosity and depth. A beam of light is passed through the liquid and a photoelectric cell arrangement records the amount of light passed in a given time. As the particles settle through the liquid they interrupt the light. Since the particles settle in accordance with Stokes law, the amount of light that is cut off during a given time period is correlated to the percentage of particles of a certain particle size or within a particular range of particle size.

The finely ground product of the pre-firing or firststage heat-treatment (with added binder) is compacted. and pressed by any suitable apparatus into shaped articles of the shape and size predetermined by the shape and size of the final articles (e. g. a magnetic core), and the shaped articles are subjected to the secondstage of heat-treatment or final-firing. Due allowance should be made for shrinkage during final-firing. As a consequence of pre-firing the deaerated and densely compacted initial oxide mixture in accordance with the present invention, shrinkage during final-firing is very considerably less than occurs when the initial oxide mixture is pre-fired in the form of ordinary agglomerates or a more or less loose powder. Heretofore, pressures of the order of 25,000 pounds per square inch have customarily been employed in compacting the product of pre-firing an initial oxide mixture in powdery form or in the form of ordinary agglomerates, and shrinkage during final-firing is about 15%. Shaped articles made from products of the pro-firing practice of the invention and compacted under the same pressure shrink less than 10% during final-firing. Alternatively, where a predetermined shrinkage during final-firing is desired, a very considerably lower compacting pressure is employed with products of the pre-firing practice of the invention than with products pre-fired in powdery form or as ordi nary agglomerates. Thus, using existing dies designed for a shrinkage of 15% during final-firing, a compacting pressure of only 5,000 pounds per square inch is required with products of the pro-firing practice of the invention as contrasted with a compacting pressure of about 25,000 pounds per square inch with products prctired in powdery form or as ordinary agglomerates.

While the invention is of special advantage and particularly applicable to the methods of making the magnetic compositions described in the aforementioned patent and patent applications, it can be practiced with advantage in the heat-treatment of any mixture of finely divided metallic oxides in the course of preparing magnetic materials therefrom. It is well known that air gaps (e. g. voids or pores) have a pronounced effect on the electrical and magnetic properties of magnetic materials, and that usually such effects are undesirable, particularly in magnetic cores. By the practice of the invention, internal air pores are practically non-existent in the heat-treated product, even after grinding to a relatively fine powder, and this absence of air pores remains characteristic of the product through all subsequent processing steps in the production of the contemplated magnetic material. It thus becomes possible to commercially produce by heat-treatments magnetic compositions or materials of uniform magnetic and electrical properties from finely divided mixtures of metal oxides, which may include sub-oxides, such as ferrous oxide, metallic constituents, such as metallic iron, and the like, all in a finely divided state of subdivision.

I claim:

1. In a method of preparing a magnetic composition in which an initial oxide mixture composed of at least 50% by weight of ferric oxide and containing at least two other metal oxides capable of modifying the magnetic and electrical properties of the composition is subjected to heat-treatment at a temperature within the range of 1000 to 1450 C., the improvement which comprises substantially deaerating the initial oxide mixture by subjecting it to a vacuum of at least 25 inches of mercury, compacting the deaerated mixture under a pressure of at least 5,000 pounds per square inch into dense shapes substantially devoid of internal air pores, subjecting the thus-compacted and dense shapes to heat-treatment at a temperature within the aforementioned range, and grinding the resulting heattreated product to a relatively fine powder in which at least 75 by weight is of a particle size between 2 and microns.

2. The improvement according to claim 1 in which the heat-treated product is quenched in water directly followmg heat-treatment.

3. The improvement according to claim 1 in which the heat-treating temperature to which the shapes are subected is progressively raised until the shapes have undergone a shrinkage of from 2 to 4% at a heat-treating tem perature substantially lower than the ultimate temperature of heat-treatment, continuing heating of the shapes w1thout further increase in the heat-treating temperature until they are uniformly heated throughout to the aforesa d lower heat-treating temperature, and then gradually ra1 s1ng the heat-treating temperature until the shapes are uniformly heated throughout to the ulimate temperature of heat-treatment.

4. In a method of preparing a magnetic composition in which an initial oxide mixture composed of at least 50% by weight of ferric oxide and containing at least two other metal oxides capable of modifying the magnetic and electrical properties of the composition is subjected to heattreatment at a temperature within the range of 1000 to 1450 C., the improvement which comprises substantially deaerating the initial oxide mixture by subjecting it to a vacuum of at least 28 inches of mercury, compacting the deaerated mixture under a pressure of at least 5,000 pounds per square inch into dense shapes substantially devoid of internal air pores, subjecting the thus-compacted and dense shapes to heat-treatment at a temperature within the aforementioned range in the course of which the heat-treating temperature is progressively raised until the shapes have undergone a shrinkage of from 2 to 4% at a heat-treating temperature from 50 to 200 C. lower than the ultimate temperature of heat-treatment, continuing heating of the shapes without further increase in the heattrcating temperature until they are uniformly heated throughout to the aforesaid lower heat-treating temperature, then gradually raising the heat-treating temperature until the shapes are uniformly heated throughout to the ultimate temperature of heat-treatment, quenching the resulting heat-treated shapes in water, and grinding the quenched shapes to a relatively fine powder in which at least 75 by weight is of a particle size between 2 and 10 microns.

5. In a method of preparing a magnetic composition in which an initial oxide mixture composed of at least 50% by weight of ferric oxide and containing at least two other metal oxides capable of modifying the magnetic and electrical properties of the composition is subjected to heat treatment at a temperature within the range of 1000 to 1450 C., the improvement which comprises compacting and densifying the intimately-mixed initial oxide mixture under a pressure of at least 5,000 pounds per square inch while maintained under a vacuum of at least 25 inches of mercury, subjecting the thus-compacted and densified oxide mixture to heat-treatment at a temperature within the aforementioned range, and grinding the resulting heattreated product to a relatively fine powder in which at least 75% by weight is of a particle size between 2 and 10 microns.

6. The improvement according to claim 5 in which the heat-treating temperature to which the compacted and densified oxide mixture is subjected is progressively raised until the compacted mixture has undergone a shrinkage of from 2 to 4% at a heat-treating temperature substantially lower than the ultimate temperature of heat-treatment, continuing heating of the compacted mixture without further increase in the heat-treating temperature until the compacted mixture is heated throughout to the aforesaid lower heat-treating temperature, and then gradually raising the heat-treating temperature until the compacted mixture is uniformly heated throughout to the ultimate temperature of heat-treatment.

7. The improvement according to claim 5 in which the heat-treated product is quenched in water directly following heat treatment.

8. The improvement according to claim 6 in which the heat-treated product is quenched in water directly following heat-treatment.

9. In a method of preparing a magnetic composition in which an initial oxide mixture composed of at least 50% by weight of ferric oxide and containing at least two other metal oxides capable of modifying the magnetic and electrical properties of the composition is subjected to heattreatment at a temperature within the range of 1000 to 1450 C., the improvement which comprises compacting the initial oxide mixture into densified bars about onehalf inch in diameter by extruding the mixture at a pressure of at least 5,000 pounds per square inch and while maintained under a vacuum of at least 25 inches of mercury, subjecting the compacted and densified bars to heat-treatment at a temperature within the aforementioned range, and grinding the resulting heat-treated bars to a relatively fine powder in which at least by weight is of a particle size between 2 and 10 microns.

10. In a method of preparing a magnetic composition in which an initial oxide mixture composed of at least 50% by weight of ferric oxide and containing at least two other metal oxides capable of modifying the magnetic and electrical properties of the composition is subjected to heat-treatment at a temperature within the range of 1000 to 1450 C., the improvement which comprises compacting the initial oxide mixture into densified bars about one-half inch in diameter by extruding the mixture at a pressure approximating 20,000 pounds per square inch and while maintained under a vacuum of at least 28 inches of mercury, subjecting the compacted and densified bars to heat-treatment at a temperature within the aforemen tioned range in the course of which the heat-treating temperature is progressively raised until the bars have undergone a shrinkage of from 2 to 4% at a heat-treating temperature substantially lower than the ultimate temperature of heat-treatment, continuing heating of the bars without further increase in the heat-treating temperature until they are uniformly heated throughout to the aforesaid lower heat-treating temperature, then gradually raising the heat-treating temperature until the bars are uniformly heated throughout to the ultimate temperature of heat-treatment, quenching the resulting heat-treated bars in water, and grinding the quenched bars to a relatively fine powder in which at least 75% by weight is of a particle size between 2 and 10 microns.

Fessler Jan. 26, 1937 Lecuir June 6, 1950 Crowley Nov. 13, 1951

Claims (1)

1. IN A METHOD OF PREPARING A MAGNETIC COMPOSITION IN WHICH AN INITIAL OXIDE MIXTURE COMPOSED OF AT LEAST 50% BY WEIGHT OF FERRIC OXIDE AND CONTAINING AT LEAST TWO OTHER METAL OXIDES CAPABLE OF MODIFYING THE MAGNETIC AND ELECTRICAL PROPERTIES OF THE COMPOSITION IS SUBJECTED TO HEAT-TREATMENT AT A TEMPERATURE WITHIN THE RANGE OF 1000 TO 1450* C., THE IMPROVEMENT WHICH COMPRISES SUBSTANA VACUUM OF AT LEAST 25 INCHES OF MERCURY, COMPACTING THE DEAERATED MIXTURE UNDER A PRESSURE FO AT LEST 5,000 POUNDS PER SQUARE INCH INTO DENSE SHAPES SUBSTANTIALLY DEVOID OF INTERNAL AIR PORES, SUBJECTING THE THUS-COMPACTED AND DENSE SHAPES TO HEAT-TREATMENT AT A TEMPERATURE WITHIN THE AFOREMENTIONED RANGE, AND GRINDING THE RESULTING HEATTREATED PRODUCT TO A RELATIVELY FINE POWER IN WHICH AT LEAST 75% BY WEIGHT IS OF A PARTICLE SIZE BETWEEN 2 AND 10 MICRONS.