US3661556A - Method of making ferromagnetic metal powders - Google Patents

Abstract

The magnetic properties of ferromagnetic metal powders made by the borohydride reduction of appropriate mixtures of iron, cobalt, nickel and optionally chromium salts can be modified by the addition of complex forming agents to the reaction medium.

Description

United States Patent Jolley et al.

[ 51 May 9,1972

[54] METHOD OF MAKING FERROMAGNETIC METAL POWDERS [72] Inventors: John Eric Jolley; Ernest Lewis Little, Jr.,

both of Wilmington, Del.

[73] Assignee: E. l. du Pont de Nemours and Company,

Wilmington, Del.

[22] Filed: Mar. 3, 1969 [21] App1.No.: 803,985

[52] U.S.Cl ..75/0.5 AA, 75/108,75/1 19,

75/121, 252/6255, 252/6258, 148/105 [51] Int. Cl. ..C22b 23/04, C22b 39/00 [58] Field of Search ..75/0.5 AA, 05, 101, 108,119,

[56] References Cited UNITED STATES PATENTS 3,567,525 3/1971 Graham et a1 ..75/.5 AA X 1,783,662 12/1930 Marx et a1 ...75/108 UX 2,999,770 9/1961 Gutzeit ..106/1 Kirk- Othmer, Encyclopedia of Chemical Technology, 2nd Edn., V01. 6,1nterscience Publishers, NY. 1965, p. 21.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. T. Ozaki AtI0rneyD. R. .1. Boyd [5 7] ABSTRACT The magnetic properties of ferromagnetic metal powders made by the borohydride reduction of appropriate mixtures of iron, cobalt, nickel and optionally chromium salts can be modified by the addition of complex forming agents to the reaction medium.

6 Claims, N0 Drawings METHOD OF MAKING FERROMAGNETIC METAL POWDERS FIELD OF THE INVENTION This invention relates to a method of making ferromagnetic metal powder by the borohydride reduction of iron, nickel and cobalt salts and optionally chromium salts. More particularly this invention relates to a method of controlling the magnetic properties of ferromagnetic powders containing iron, nickel, cobalt and optionally chromium made by the borohydride reduction process by adding chelating agents to the reaction medium.

DESCRIPTION OF THE PRIOR ART In the US. Pat. No. 3,206,338, Miller & Oppegard describe the preparation of acicular, ferromagnetic, metal alloy powders of high coercivity by reducing iron, cobalt, and nickel salts with metal borohydrides in a magnetic field. Preparation of related chromium-containing products is described by E. L. Little and .l. D. Wolf U.S. Pat. No. 3,535,104. A thermal method for improving the magnetic properties of borohydride-precipitated ferromagnetic powders is described in copending patent application US. Pat. No. 3,567,525.

ln the above processes the magnetic properties of the products are controlled by the chemical composition, the reaction conditions, and subsequent thermal treatment.

The utility of magnetic particles depends inter alia upon the coercivity and other magnetic properties of the particles. More present day magnetic tape recording equipment, for example, is designed for powders that have a coercivity of about 250600 oersteds and a saturation magnetization of about 60-90 emu./g. previously known borohydride reduction procedures usually give particles with magnetic properties outside the optimum range, for example with too high or too low coercivity and with less than optimum saturation magnetization and remanence. It has now been found that powders with altered magnetic properties, for example with coercivity, saturation magnetization, and remanence in a more useful range, may be obtained by carrying out the borohydride reduction step in the presence of certain complex forming agents.

SUMMARY OF THE INVENTION The present invention can be defined as the improvement in making ferromagnetic metal powders by the reduction of salts of iron, cobalt, nickel and optionally chromium with a metal borohydride which comprises conducting the reaction in the presence of from 0.25 to 55 percent by weight, based on the weight of the metal salts of a complexing agent that forms a chelate complex soluble in the reaction medium with at least one of the metal reactants, said complexing agent being an organic or organo-inorganic compound containing at least two functional groups selected from primary amino, secondary amino, tertiary amino, imino, carbonyl, carboxyl or hydroxyl groups, or alkali metal salts thereof.

DETAILED DESCRIPTION OF THE INVENTION The procedure for reducing salts of iron, cobalt, and nickel with metal borohydrides is described by Miller and Oppegard in US. Pat. No. 3,206,338. As described by Little and Wolf, Ser. No. 739,860 filed June 25, 1968, and now abandoned, chromium can be co-reduced with the Group VIII ferromagnetic metal salts to obtain alloy particles containing chromium which have superior resistance to oxidation. The process of the present invention is an improvement in the above processes in that the reaction mixture contains, in addition to the metal salts and borohydride reducing agent, 0.25-55 percent by weight based on the weight of metal salt(s) of a complexing (chelating agent) that contains at least two functional groups selected from primary amino (-NI-I,), secondary amino NH), tertiary amino N), imino (=NI-l), carbony! CO), carboxyl (COOH). and hydroxyl (OH) or their alkali metal salts.

Usage ofthe term chelating agent herein follows that of Martell and Calvin who state on page 1 of Chemistry of the Metal Chelate Compounds," Prentice-Hall, Inc, New York, N. Y., 1952:

When a metal ion combines with an. electron donor, the

resulting substance is said to be a complex, or coordination compound. Ifthe substance which combines with the metal group contains two or more donor groups so that one or more rings are formed, the resulting structure is said to be a chelate compound, or metal chelate, and the donor is said to be a chelating agent.

The terms complexing agent and "complex as used herein are interchangeable with chelating agent and "chelate. It is known that chelate compounds form in several ways, e.g., by (1) simple addition of bidentate chelating agents, e.g., urea and ethylenediamine, to metal ions, (2) by reaction of metal ions with bidentate chelating agents containing both coordinating and acidic functional groups, e.g., NI-I CH COOI-I and the salts of ethylenediaminetetraacetic acid, and (3) by reaction of metal ions with bidentate chelating agents containing two acidic groups, e.-g., o-hydroxy compounds such assalicylic acid and o-dihydroxybenzene.

The chelating agents of this invention are polydentate, i.e., they contain two or more functional groups, each of which contains an oxygen or nitrogen atom separated in the chain by one to six carbons, preferably one to three carbons, said nitrogen and oxygen atoms being capable of simultaneous attachment to iron, nickel, cobalt or chromium ions by simple addition or formation of a primary bond, thereby producing four to nine membered rings, preferably four to seven membered rings. In general the chelating agents contain up to 12 carbon atoms, although this is not critical.

Preferred complexing (chelating) agents are:

Lactic acid: CH CIIOIIC 0 ()ll Sodium glycollate: HO CH2C O ONa Sodium citrate: NaO O C CHzC (OH) C 0 O Na C IIQC 0 t) Nil-XII O Sodium salts of ethylencdiaminetetraacetic acid, e.g.:

HOOCCH CIIQCOOII NCIIzCIIz-N 2IIQ() NaOOCCIIz CIIQCOONa Urea: I'IzNC ONIIz Sodium succinatc: NaO O C C HZCIIZC O O Na Ascorbic acid:

In addition to the above, a wide variety of chelatingjagents containing at least two functional groups selected from primary amino, secondary amino, tertiary amino, imino, carboxyl, carbonyl, and hydroxyl or their water-soluble (usually alkali metal) salts may be employed. It is important that the chelating agents be soluble in water or in aqueous mixtures of watermiscible organic liquids, e.g., ethanol or acetone, and that they do not precipitate iron, cobalt, nickel, or chromium chelate compounds in the reaction medium in which the borohydride reduction reaction is effected. Suitable chelating agents include:

Primary amino, COOH, COOM aand fl-alanjne and their alkali metal salts glycylalanine, asparagine, aspartic acid, and and H N-CHCONHOHC H and their water-soluble salts.

Ilydroxyl Mannitol, glycerol, o,0-diphen0l, glycols, pyrocateehol, dimethylglyoxime.

COOH Oxalic, malonic, ethylmalonic, phthalic, and

' maleic acids, and their alkali metal salts.

0 O Acstylaceton, benzoylacetone and benzoyltrl- H I fluoroacetone, and their alkali metal salts; C-C- ethylacatoacetate.

Primary amino, OH HOCH2CH2NH2, o-amlnophenol.

Tertiary amino, OH Tertiary amino, primary amino.

CHO, OH

s hydroxyquinoline. r N (CHzCHzNH2)3.

(IIHO Other examples of useful chelating compounds containing the above functional groups will be found in Martell and Calvin, Chemistry of the Metal Chelate Compounds, Prentice- Hall, Inc., New York, N. Y., 1952, pp. 514-558.

The mechanism through which these agents operate to control coercivity and other magnetic properties is not known. Presumably, the effect is related in some obscure way to the known ability of the agents to form multi-membered rings via formation of primary and/or secondary valence bonds with ferromagnetic metal ions. In the case of iron, the agents act to stabilize its divalent form, thereby preventing troublesome oxidation followed by hydrolysis and precipitation of non-magnetic hydrous ferric oxide.

Although water is a convenient and readily available medium for carrying out the process of this invention, it will be appreciated that the aforementioned chelating agents, the metal salts, and the metal borohydrides are soluble in other media, particularly aqueous solutions of water-miscible organic liquids such as methanol, ethanol, acetone, and the like, and that solvents of this sort may be substituted for water and, in fact, may be preferable in certain instances. The proportion of the organic component in such aqueous media is primarily governed by its effect on the solubility of the various components of the reaction mixtures, and the organic component usually will not exceed the volume of water employed.

The products are obtained by adding, preferably slowly, aqueous solutions of metal borohydrides to an aqueous solution, preferably at about 4560 C., of the metal salt(s) and chelating agent. This may be done in a magnetic field to obtain acicular forms of some of the powders. Reaction mixtures are preferably agitated to insure mixing, but the degree of agitation is held to a minimum when reaction is effected in a magnetic field to prevent breakage of the acicular assemblages. Products are isolated for example by magnetic settling and decantation of supernatant liquid, by filtration, etc., and washed with water and with acetone to remove by-products and, preferably, allowed to stand in acetone for about -24 hours before final isolation and drying. Aging in acetone or, altematively, drying of the particles in an inert gas, e.g., argon, to which gradually increasing concentrations of air (oxygen) are added, eliminates any tendency the particles may have to be pyrophoric. Limited oxidation taking place during isolation of the products, possibly coupled with the presence of boron and the effect of use of chelating agents, virtually eliminates any tendency for the products to be pyrophoric.

While the proportion of metal borohydride to metal salts( s) can be varied considerably, it is preferred to employ one mole of metal borohydride to two moles of metal salt(s). The proportion of complexing agent to metal salt can be varied widely as shown in the examples and, in general, depends upon the magnetic properties desired, particularly coercive force and saturation magnetization. The preferred quantity of complexing agent is 0.25-55 percent by weight based on weight of the metal salt(s).

A variety of metal borohydrides may be employed, but sodium and potassium borohydride are preferred because of ready availability and lower cost. Other useful metal borohydrides include lithium borohydride, magnesium borohydride, calcium borohydride, tetramethylammonium borohydride, tetraethylammonium borohydride, and the like.

Salts of iron, cobalt and nickel, singly or jointly, or in combination with chromium, may be used as the metal salt reactants. Choice depends upon the particular combination of magnetic properties desired. Chromium salts alone are not reduced to elementary chromium by metal borohydrides under the described conditions, If present, chromium salts should be in an amount of from 0-20 percent by weight based on the total weight of the magnetic metal salts of iron, cobalt and nickel. Preferred salts are soluble in water, and for this reason halides, sulfates, nitrates, fluoroborates, and acetates (or other short or branched-chain metal acylates) are customarily employed. Ferrous salts are preferred to ferric salts because less reductant is required and, as indicated earlier, troublesome by-products are avoided. Chromous salts oxidize rapidly in air, consequently it is preferred to use chromic salts. Specific, useful metal salts include ferrous sulfate, ferrous chloride, ferrous fiuoroborate [Fe(BF cobalt sulfate, cobalt chloride, cobalt nitrate, nickel chloride, nickel nitrate, nickel sulfate, chromic sulfate, chromic chloride, and potassium chromic sulfate.

Concentrations of the metal salt and metal borohydride solutions used may range up to the limiting solubilities of the metal salts and metal borohydrides in water. Use of concentrated solutions of metal salts is preferred, primarily for reasons of economy.

Reaction may be effected at ordinary temperature and pressure though temperatures of about 4560 C. usually give superior products. While it is preferred to operate at atmospheric pressure because equipment requirements are less stringent,

pressures of 0.5 atmosphere to 5 atmospheres or higher are operable.

The soluble metal borohydrides reduce dissolved salts of ferromagnetic metals in the presence of the complexing (chelating) agents of the invention, precipitating finely divided metal powders that contain boron and oxygen. When the salts of more than one ferromagnetic metal are present in a chelate-containing solution, the alloy-like precipitate includes each of the ferromagnetic metals in addition to boron and oxygen. Iron-containing alloys, notably iron-chromium-boron, iron-cobalt-boron, and iron-cobalt-chromium-boron have particularly desirable magnetic properties and constitute preferred reaction products. Iron-boron, iron-nickel-boron, cobalt-boron and cobalt-boron alloys containing other ferromagnetic metals, e.g., nickel, are also useful magnetic materials. The particular ferromagnetic metals employed and the relative proportions in which they are used depend upon the specific combination of magnetic properties desired.

The boron present in the products cannot be detected by X- ray diffraction; the boron may be present in elementary form, dissolved in the metal(s), or as amorphous metal boride. X-ray diffraction data suggest that in the case of iron-chromiumboron, the principal phase is a solution of both chromium and boron in body-centered cubic alpha-iron. Oxygen present in the products is believed to be combined as metal oxide, metal hydroxide, and/or adsorbed water.

When produced in the absence of a magnetic field, the products produced by borohydride reduction in the presence of chelating agents are roughly spherical in shape and about 100-600 angstrom units in diameter. Especially in case of iron-containing solutions, the small particles may aggregate into acicular assemblages about 0.01-0.15 micron in crosssectional diameter by about 0.02-5 microns in length when reaction is affected in a magnetic field of at least 100 gauss and preferably of at least 1,000 gauss strength. Dimensions of particles produced in this way in some of the examples described hereinafter were as follows:

Synthesis variables that effect shape include the strength of the magnetic field applied during precipitation, the rate of addition of borohydride solution, the chemical composition of the reactants, and the precipitation temperature. Synthesis in the absence of a magnetic field usually results in essentially equi-axed particles.

The very fine size and high coercivity and remanance ratio 5 of many of the alloy powders produced by the process of this invention clearly show that they are largely single domain, that is that they have no domain walls. For example, Charles Kittel, Physical Review, 70, 965 (1946) estimates that particles and grains less than about 2 X 10 cm. in cross-section are single domain. The much larger, acicular, magnetically oriented assemblages of these small particles are also single domains in which most of the atomic moments are parallel except at the moment of magnetization reversal. It is in this sense that the term single domain is applied to the acicular assemblages see A. L. Oppegard, F. .I. Darnell, and H. C. Miller, J. Appl. Phys., Suppl, 32, 1848 (1961) and 1. S. Jacobs and C. P. Bean, Phys. Rev., 100, 1060(1955).

This invention is further illustrated by the following examples which should not, however, be construed as fully delineating the scope of this discovery. In these examples, parts are given by weight unless otherwise specified. Sigma values were determined using packed tubes of the powders in a 4,400 gauss (oersted) field and apparatus similar to that described by T. R. Bardell on pp. 226-228 of Magnetic Materials in the Electrical Industry, Philosophical Library New York (1955).

Values of intrinsic coercive force were determined on a DC. ballistic-type apparatus which is a modified form of the apparatus described by David & Hartenheim in the Review of Scientific Instruments 7, 147 (1936).

EXAMPLES 1-9 PREPARATION OF IRON-CHROMlUM-BORON (NO MAGNETIC FIELD) Examples 1 to 9, Table 1, illustrate the preparation of nonpyrophoric, single domain iron-chromium-boron powders in the absence of a magnetic field but in the presence and absence of complexing agents. In each case a cold solution of 7.6 g. of NaBH, in 250 ml. of cold distilled water was added over 3-10 minute period to a solution in 500 ml. of water of 20 g. of reagent-grade Cr (SO,);,-xH,O and the specified quantity of ferrous salt and chelating agent. Vigorous exothermic reaction occurred as the NaBH solution was added, and magnetic black solid precipitated, which was separated by filtration,

washed with water and with acetone and dried in air. The products contained oxygen, believed to be present as oxides, hydroxides, and/or adsorbed water.

EXAMPLES 10-12 PREPARATION OF IRON-CHROMIUM-BORON m PRESENCE AND ABSENCE OF A MAGNETIC FIELD) Examples 10-12, Table 11, show the preparation of nonpyrophoric, single domain iron-chromium-boron powders in the presence and absence of l-ascorbic acid as complexing agent and in the presence and absence of a magnetic field. In each case 500 ml. of a chilled solution of 19 g. of NaBI-l. was

TABLE I.PREPARATION AND PROPERTIES OF IRON-CHROMIUM-BORON FERROMAGNETIC POWDERS 1N0 magnetic field] Characterization of product Analysis, percent Magnetic properties Yield Example No. Iron salt Complexing agent (g.) Fe Or B H. v. v. Ur/da 1 43.5 g. FeSO4-7HzO None 6 59.85 16.95 5.3 253 41.8 10.8 0.258 3. 5 79. 5 2. 91 3. 2 250 88.8 25.8 0.291 as above 2.0 139 95. 3 16.8 0.130 5.2 69.8 12.7 2. 2 232 91. 6 20. 2 0.22 1.5 77.2 9.0 3.05 96.6 18.2 0.19 2. 5 69. 1 9. 3 1. 9 735 90. 6 38. 2 0. 42 3.0 60.1 4.3 2.0 600 54.8 20. 2 0.38 2.0 66.8 1 7 4.0 487 59. 8 33. 0 0.57 2.0 577 75. 8 28. 4 0. 37

\ Sm. formula below:

TABLE II.P REPA RATION were separated by filtration, washed with water followed by acetone, aged in acetone (except for Example 17), and dried in air. Conditions were similar in Examples 25 and 26 except that (l) the metal salts and, if employed, the chelating agent were dissolved in 200 ml. of water, (2) the sodium borohydride solution was added over a 10-minute period, and (3) the products, after washing with 500 ml. of water and then with 500 ml. of acetone, were suspended overnight in l25 ml. of acetone before they were finally separated and dried in air.

AND PROPERTIES OF IRON-CHROMIUM BORON IN PRESENCE AND ABSENCE OF A MAGNETIC FIELD Characterization of product Analysis, Grams of percent Magnetic properties 'nsvorhic Magnetic Yield lhnnlple Nu. nvld nsml l'wld (grams) Fo Cr ill, a. a. m/n

ill Noun X. 0 ill). ll. 4 425 119.0 33. 0 0.333 ll 0 VHS. II." 74.0 10.4 450 90.0 36.0 0.375 1'. l Yrs 9. 3 70. 4 l0. 4 445 105. 0 38. 0 U. 30l

EXAMPLES 13-26 PREPARATION OF lRON-COBALT-BORON Preparation of non-pyrophoric, single domain, iron-cobaltboron ferromagnetic powders is shown in Examples 13-26 of Table III. In Examples l3-24 a solution of 3.8 g. of NaBH in 100 ml. of cold distilled water was added over a 15-minute period to a solution of CoSO -7H,O, FeSO -7H,O and, where shown, the specified quantity of chelating agent in 100 ml. of water. Reaction was effected in a 2-liter beaker resting on the poles of a horseshoe magnet of 1,300 gauss field strength or in EXAMPLES 27-30 PREPARATION OF IRON-COBALT-CHROMIUM-BORON Examples 27-30 in Table IV show the preparation of ironcobalt-chromium-boron alloy powders in the absence and presence of the sodium salt of ethylenediaminetetraacetic acid as chelating agent. A cold solution of 3.8 g. of NaBl-l, in. ml. of pure water was added to 200 ml. of a solution of 33.4 g. of FeSO '7H,O, 22.4 g. of CoSO -7H,O, 10 g. of KCr(SO 121-],0 and, if used, the indicated quantity of chelating agent. The reaction vessel rested on the poles of a 1,500 gauss horseshoe magnet. The precipitated iron-cobalt-chromium-boron TABLE TIL-PREPARATION AND PROPERTIES OF IRON-COBALT-BORON FERROMAGNETIC POWDERS Characterization of products Analysis, percent Magnetic properties Example No. (g.) Complexing (chelating) agent a, v.- (Y /o,

22. 3 5. 6 92. 0 44 0. 488 22. 3 5. 6 5 g. urea. 62 0. 485 22. 3 5. 6 5 g. sod. citrate 100 37 0.370 22. 3 5. 6 5 g. sod. succinate- 104 30 0. 294 22. 3 5. 6 2.5 g. sod. ED'IA 124 48. 3 0. 390 22.3 5. 6 5 g. sod. EDTA' L... 142 56 0.395 22. 3 5. 6 10 g. Sod. EDTAL 51 0. 340 22. 3 5. 6 15 g. sod. EDTAZ 147 50 0. 340 16. 7 11.2 None 91. 5 45. 2 0. 459 16. 7 11.2 5 g. sod. citrate" 96. 0 28.0 0. 292 16. 7 11. 2 116 42. 0 0. 362 16. 7 11. 2 99 42. 0 0. 425 33. 4 22. 4 97 46 0. 476 33. 4 22.4 95 75 0. 267

1 Magnetic Field-Reactions effected in gap of electromagnet with field strength of 2,000 gauss.

3 Magnetic Field-Reaction efiected in beaker resting on poles of horseshoe magnet of 1,300 gauss field strength.

TABLE IV.PREPARATION AND PROPERTIES OF Fe-Co-Cr-B FERROMAGNETIC POWDE RS Characterization of product Analysis, ercent Ma netie r0 erties Example Yield p g p p N o. (Chelating) agent (g.) Fe Co Cr B ;H,, a. a, ruler,

27. None 4. 0 37. 3 16. 5 7. 4 3. 3 570 62 25 0. 403 28 1 g. sod. salt of EDTA 3. 5 36. 9 19. 8 9. 6 1. 8 425 59. 3 23. 1 0. 394 29. 2.5 g. sod. salt of EDTA 3. 5 34.6 19. 8 9. 7 2. 0 335 63. I 22. 7 0. 359 30. 5 g. sod. salt of EDTA 8. 0 34. 4 18. 1 10. 5 l. 9 265 52. 1 17. 3 0. 331

1 See formula below:

HOOCCH: CH2COOH NCHzCHzN zmo NaOOCCHz CHzCOONa a smaller beaker located in the gap of an electromagnet with a field strength of 2,000 gaussThe resulting black precipitates ferromagnetic powders were isolated by filtration and washed and dried as described in previous examples.

PREPARATION OF IRON-BORON This example illustrates the preparation of non-pyrophoric, ferromagnetic single domain iron-boron particles in a magnetic field in the presence and absence of the sodium salt of ethylenediaminetetraacetic acid. Precipitation was effected in a 2-liter beaker resting on the poles of a horseshoe magnet with a field strength of 1,300 gauss.

(A) A solution of 3.8 g. of sodium borohydride, NaBH in 100 ml. of cold, distilled water was slowly added over a minute period to 27.8 g. of ferrous sulfate heptahydrate dissolved in 100 ml. of distilled water. The black precipitate was separated by filtration and washed with water and with acetone. The iron-boron powder had a coercivity of 625 oersteds, a saturation magnetization of 102 emu./g., a remanence of 47.2 emu./g., and a remanence ratio of 0.463.

(B) Experiment A was repeated but with 15 g. of the sodium salt of ethylenediamine tetraacetic acid dissolved in the solution of FeSO -7l-1 O. The product had a coercivity of 120 oersteds, a saturation magnetization of 138 emu./g., a remanence of 27.2 emu./g., and a remanence ratio of 0.197.

EXAMPLES 32-40 PREPARATION OF lRON-NlCKEL-BORON, COBALT- BORON AND COBALT-NICKEL-BORON Examples 32-40 in Table V show the preparation and properties of ferromagnetic powders of iron-nickel-boron, cobaltboron, and cobalt-nickel-boron obtained by the process of this invention. The indicated quantities of metal salts and chelating agent were dissolved in 200 ml. of water, and beakers containing the solutions were placed on the poles of a magnetically strong, upright horseshoe magnet. A solution of 3.8 g. of sodium borohydride in 100 ml. of chilled distilled water was added to each over a IO-minute period with gentle agitation. The ensuing mixture was filtered, and the precipitate was washed in turn with 500 ml. of water and 500 ml. of acetone, suspended overnight in 125 ml. of acetone, and finally separated by filtration and dried in air.

The ferromagnetic boron-containing products of this invention frequently undergo a marked improvement in magnetic properties when they are heated below their sintering temperature in hydrogen at about 300600 C. in the manner described in the copending application, Ser. No. 739,732, filed June 25, 1968, now abandoned.

The products of this invention, especially those of lower Curie temperature, may also be used as the magnetic component of recording members employed for reflux thermomagnetic imaging as taught by Nacci in Belgian Pat. No. 627,017.

EXAMPLE A An acicular, ferromagnetic iron-nickel-boron powder prepared as described in Example 34 was formed into a permanent magnet by compacting l g. in a mold into a 1.5 inch by 0.1 inch bar at 80,000 lbs/sq. in. pressure and room temperature. The resulting bar was further magnetized by placing it in a magnetic field of 1,700 gauss. The bar displayed the properties of a permanent magnet. It had a coercivity of 620 oersteds, a saturation magnetization of 79 emu./g., a remanence of 41 emu./g. and a remanence ratio of 0.5 1 8. It is obvious that permanent magnets may also be prepared using other procedures conventional in the art, e.g., by compaction in magnetic fields which magnetize the particles and by compaction of mixtures of the powders with binders of various sorts which may or may not be removed after compaction. These binders may be thermo-setting or thermoplastic organic polymers or air-drying, film-forming organic materials.

EXAMPLE B The preparation of iron-chromium-boron used in magnetic recording tape is described in this example. Ferrous sulfate heptahydrate (55.6 g.), KCr(SO 'l2I-l,O (20 g.), and the sodium salt of ethylenediaminetetraacetic acid (5 g.) were dissolved in water (100 ml.) and the solution was placed in a magnetic field of 1,300 gauss. A solution of NaBI-I, (3.8 g.) in water (100 ml.) was added over a 15 minute period. The precipitate was washed with water and with acetone, slurried TABLE V.-PREPARATION AND PROPERTIES OF IRON-NICKEL-BO RON, COBALT-BORON, AND COBALT-NICKELBORON Characterization of products Magnetic properties Example Yield No. Product Metal salts used Chelating agent used (g.) Analysis i c r r/h 32 Fc-Ni-B 41.8 g. FeSOflHzO, 10.4 g. None 6. 07 58.35% Fe, 22.4% Ni, 7.0% B.. 1,030 88 41 0 38 M80 -6H 0. Same as abo ro 10 g. urea 3. 06 58.9% Fe, 22.1% Ni, 4.0% B. 800 88 38 0. 431 ....d 70.4% Fe, 6.8% Ni, 3.4% B 805 86 0. 465 56.2 g. COSO4-7H2 3. 2 76.8% Co, 7.6% B 31. 8 8. ft 0. 280 Same as above. 10 g. urea... 4.1 76.9% Co, 7.5% B 70 32.0 8. 0 0. 252 .-...do 10 g. sodium EDTA 3. 5 74.8% Co, 7.0% B 105 39. 0 11. 0 0.28; 3-0 g- C0SO4-7Hz0, 10.6 1;. None 5.4 58.9% C0, 16.1% Ni, 7.0% B--. 35 14 2 0- l NISO4-6H20. Same as above 10 urea 4.8 ($3.57 Co, 12.47 Ni, 8.3% B... 35 23 3 0- 130 .....do 10 2. sodium EDTA 3 a 65.2% C0, 7.4% Ni, 5.0% 13.-.. 1o 35 s 0. 228

1 See formula below:

HOOCCH: CHZCOOH NCH2CH2'N\ -2H O NnOOCCH: 7 CHQQOONa v overnight in acetone, isolated and dried. The procedure was repeated 10 times, thereby affording 22 grams of black magnetic powder that was tumbled in a rotating canister containing short polytetrafluoroethylene cylinders. This increased the apparent bulk density of the powder from about 0.1 g./ml. to 0.8 g./ml. The compacted powder had a coercivity of 200 oersteds, a saturation magnetization of 98 emu./g., a remanence of 20 emu./g., and a remanence ratio of 0.204. By analysis it contained 67.93% Fe, 7.89% Cr, and 2.04% B.

A 6-gram sample of the compacted powder, 20 ml. of tetrahydrofuran, 20 ml. of 20-30 mesh Ottawa sand, and 0.24 g. of Alcolec 329 soya lecithin dispersant were stirred with ice-bath cooling-for 60 minutes, using a l iii-inch disk stirrer operating at a speed of 1,200 peripheral feet/minute. To the dispersion were then added 7 g. of a percent by weight tetrahydrofuran solution of a polyester-polyurethane resin made from diphenylmethane diisocyanate/adipic acid/butanediol and 3.5 g. of a 30 percent by weight methyl isobutyl ketone solution of an 80/20 vinylidene chloride/acrylonitrile copolymer. After stirring for another 30 minutes at 1,200 feet/minute, the dispersion was pressure-filtered through nylon felt and a S-micron stainless steel screen to separate sand and undispersed particles. The filtered dispersion had a viscosity of 7 poises.

The filtered dispersion was spread under a 3-mil doctor knife over a 4-foot by 3.5 inch area of polyethylene terephthalate film. While the coating was still fluid, the film was passed through a 800l ,000 gauss magnetic field to align the Fe-Cr-B particles in the direction of passage. The dry film was cured 4 for 4 hours at 65 C. and 12 inches vacuum, calendered three times at W)" (./l .200 pounds per linear inch/3S feet per minute. and again cured for 24 hours in nitrogen at 65 C. The finished film. i.e.. magnetic tape, had the properties: coating thickness. 0.29 mil; oh inch, the residual flux per one-half inch, 0.835 maxwells; E the residual flux density, 890 gauss; iHc the intrinsic coercivity (direct current), 175 oersteds; iHc the intrinsic coercivity (alternating current), 165 oersteds; B,-/B,, where B, is the saturation flux density, 0.54; and ratio of peak-to-waist of 7. The peak-to-waist ratio was measured on the first derivation of a hysteresis loop curve generated by a 60 cps alternating electric field. The derivative curve was obtained as an oscilliscope display on a standard B/l-l meter, e.g., a meter provided by Scientific Atlanta, Model 6518. The reported P/W value is the ratio of the peak amplitude of the derivative curve to the waist amplitude at zero field in the derivative curve.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the manufacture of non-pyrophoric ferromagnetic metal powders by the reduction. In aqueous solution, of salts of magnetic metals selected from at least one of iron, cobalt and nickel, together with from 0 to 20 percent by weight of said salts of magnetic metals of chromium salts, with metal borohydrides, said metal powders having up to about 30 percent by weight of oxygen present in the form of oxides, hydroxides or adsorbed water, the improvement which comprises conducting the reaction in the presence of from 0.25 to 55 percent by weight based on said salts of a chelating agent that forms a chelate complex soluble in the reaction medium with at least one of the magnetic metals, said chelating agent consisting of an organic or organo-inorganic compound containing at least two functional groups selected from primary amino, secondary amino, tertiary amino, imino, carbonyl, carboxyl or hydroxyl and alkali metal salts thereof.

2. The process of claim 1 wherein said complexing agent is selected from glycine, lactic acid, sodium glycollate, sodium citrate, sodium salts of ethylenediaminetetraacetic acid, urea, sodium succinate and ascorbic acid.

3. The process of claim 2 wherein said complexing agent is selected from the sodium salts of ethylenediaminetetraacetic acid.

4. Process of claim 2 wherein said complexing agent is ascorbic acid.

5. Process of claim 2 wherein said complexing agent is sodium citrate.

6. Process of claim 2 wherein said complexing agent is sodium glycollate.

Claims (5)

1. 2. The process of claim 1 wherein said complexing agent is selected from glycine, lactic acid, sodium glycollate, sodium citrate, sodium Salts of ethylenediaminetetraacetic acid, urea, sodium succinate and ascorbic acid.
2. 3. The process of claim 2 wherein said complexing agent is selected from the sodium salts of ethylenediaminetetraacetic acid.
3. 4. Process of claim 2 wherein said complexing agent is ascorbic acid.
4. 5. Process of claim 2 wherein said complexing agent is sodium citrate.
5. 6. Process of claim 2 wherein said complexing agent is sodium glycollate.