US3663318A - Process for making ferromagnetic metal powders - Google Patents

Abstract

Disclosed herein is a process for making ferromagnetic powders from salts of iron, cobalt, nickel, and from 0 to 35 percent of a chromium salt, by total weight of the salts. The process comprises reacting the metal salt or salts with a reducing agent of (a) an amine-borane and (b) a tetrahydroborate.

Description

United States Patent Little, Jr. et al.

[451 May 16, 1972 [54] PROCESS FOR MAKING F ERROMAGNETIC METAL POWDERS [72] Inventors: Ernest L. Little, Jr.; Jack D. Wolf, both of [21] App1.No.: 78,182

[52] US. Cl. ..l48/105, 75/0.5 AA, 75/108, 75/119,148/103,148/108 [51} Int. Cl ..H0lf l/06, HOlf l/ZO, C22b 23/04 [58] Field ofSearch ..148/100, 103, 105,108,3155, 148/3157; 75/0.5 AA, 108, 119

[56] References Cited UNITED STATES PATENTS 3,206,338 9/1965 Miller et al.. ..148/105 3,369,886 2/1968 Metzger ..75/108 X 3,535,104 10/1970 Little, Jr. et a1 ..75/0.5 AA 3,567,525 3/1971 Graham et a1. 148/105 X FOREIGN PATENTS OR APPLICATIONS 843,367 6/1970 Canada ..75/108 OTHER PUBLICATIONS R. Paul et al., Catalytic Activity of Nickel Bodies, Industrial and Engineering Chemistry, Vol. 44, No. 5, 1952), Pages 1006- 1010.

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-G. K. White Attorney.1amcs A. Costello [57] ABSTRACT Disclosed herein is a process for making ferromagnetic powders from salts of iron, cobalt, nickel, and from 0 to 35 percent of a chromium salt, by total weight of the salts. The process comprises reacting the metal salt or salts with a reducing agent of (a) an amine-borane and (b) a tetrahydroborate.

12 Claims, No Drawings PROCESS FOR MAKING FERROMAGNETIC METAL POWDERS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a new process for making ferromagnetic metal powders by the reduction of ferromagnetic metal ions.

2. Description of the Prior Art Amine-borane reducing agents are known to reduce metal salts to form metal films on catalytic surfaces such as the surfaces of objects to be plated. Metal powders, heretofore, could not be made using amine-borane reducing agents. Although it is known to reduce salts of magnetic metals to metal powders using tetrahydroborates such as sodium tetrahydroborate, the powders are often non-magnetic. See, for instance, Raymond Paul et al., Industrial and Engineering Chemistry, 44, 1006-1010, 1952).

SUMMARY OF THE INVENTION Salts containing the Bl-lfanion are referred to herein as tetrahydroborates. They are also known in the literature as borohydrides. The tetrahydroborate salts have now been found to promote or trigger the reduction of ferromagnetic metal salts by amine-boranes. It is believed that this triggering effect is the in situ generation of catalyst sites. Subsequent reduction by amine-boranes takes place, selectively, at these sites to form homogeneous ferromagnetic metal powders.

The novel process comprises reacting, in solution, a salt or salts of at least one of the metals selected from the group consisting of iron, cobalt and nickel, and from to 35 percent, by total weight of the salts, of a chromium salt, with a reducing agent comprising (a) an amine-borane and (b) a tetrahydroborate; wherein the molar ratio of the amineborane to the tetrahydroborate is from about 3 to l to 700 to 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A wide variety of iron, cobalt, nickel and chromium salts may be employed herein. Preferred salts are soluble in water, and for this reason halides, sulfates, nitrates, fluoroborates, and acetates are customarily used. Ferrous salts are preferred to ferric salts because less reductant is required. Chromous salts oxidize rapidly in air, consequently it is preferred to use chromic salts.

The process of this invention produces ferromagnetic metal powders of low boron content. The low boron content of these powders helps explain their excellent magnetic properties. See Tables I, II, III and IV in this regard. Furthermore, the use of metallic salts of lower alkanoic acids favors formation of products of especially low boron content. Consequently, salts of lower alkanoic acids constitute a preferred class. Within this class, formates, acetates (Ac), and propionates are especially preferred because of their solubility in water. Other useful metal salts include ferrous sulfate, ferrous chloride, ferrous fluoroborate, ferrous propionate, cobalt sulfate, cobalt chloride, cobalt nitrate, cobalt fonnate, nickel chloride, nickel sulfate, nickel acetate, chromic sulfate, chromic chloride, and potassium chromic sulfate.

Any ratio of iron, cobalt and/or nickel salts can be used. The alloy composition of the powder product depends on the particular metal salts employed. The relationship between metal salt composition and alloy composition can be readily determined for any system. The amount of chromium salt, if one is used, should be up to about 35 percent by weight of the mixture of salts.

In general, any amine-borane can be employed herein. In amine-boranes, the nitrogen of an organic amine is coordinately bonded to a EH moiety, for example, as in dimethylamine-borane, (CI-l NH'Bl-l The amine portion of the amine-borane can be a primary, secondary, or tertiary amine. It can be aliphatic, aromatic, or heterocyclic, or can have more than one of these characteristics. It can be a monoamine, a diamine, or a polyamine. In amine-boranes formed from diamines and polyamines, normally each amine nitrogen will be coordinately bonded to a BI-l moiety.

Examples of operable amine-boranes are methylamine-borane ethylamine-borane isopropylamine-borane t-butylamine-borane cyclohexylamine-borane aniline-borane p-toluidine-borane m-anisidine-borane p-chloroaniline-borane p-bromoaniline-borane diethylamine-borane morpholine-borane di-t-butylamine-borane diisopropylamine-borane dipropylamine-borane diisopentylamine-borane dibutylamine-borane piperidine-borane pyridine-borane N, N-dimethylcyclohexylamine-borane dimethyloctadecylamine-borane N ,N ,N,N '-tetramethylenediamine-bisborane triethylamine-borane N-methylmorpholine-borane dimethylethylamine-borane N,N-dibutylcyclohexylamine-borane diethylbutylamine-borane N,N-diethylcyclohexylamine-borane N,N-dimethylaniline-borane N,N-dimethyl-p-toluidine-borane Useful amine-boranes derived from secondary and tertiary amines are readily available and stable. Preferred are amineboranes containing aliphatic or heterocyclic amines, or amines having both these characteristics such as N-methylmorpholine-borane. Monoamine-boranes are also preferred. Especially preferred, because of their stability and solubility in the solvents ordinarily used for the process, are amine-boranes of the formula R ,,NH,,-BH where the R's are the same or different, preferably the same, and are lower alkyl, and n is 0 or 1. Of these, trimethylamine-borane dimethylamine-borane, and triethylamine-borane are most preferred.

The volume, Boron, Metallic-boron Compounds, and Boranes, lnterscience, 1964, R. M. Adams, is a general reference to the preparation of the amine-boranes and tetrahydroborates which are useful in the practice of this invention.

Contemplated tetrahydroborates include alkali-and alkaline-eanh-metal tetrahydroborates, tetraalkylammonium tetrahydroborates and alkali-metal tris (lower alkoxy) tetrahydroborates. Sodium and potassium tetrahydroborates are preferred. Other useful tetrahydroborates are lithium tetrahydroborate, magnesium tetrahydroborate, calcium tetrahydroborate, tetramethylammonium tetrahydroborate, tetraethylammonium tetrahydroborate, tetraisopentylammonium, tetrahydroborate, ethyl tri-s-butylammonium tetrahydroborate, dihexyldimethylammonium tetrahydroborate, and potassium tributoxytetrahydroborate.

The amine-borane/tetrahydroborate weight ratio can vary widely and can be from about 5/1 to 1000/1. Preferably it is between about lO/l and 600/1. 'Altematively, the amineborane/tetrahydroborate mole ratio can range from about 3/1 to 700/ l and preferably is between about 6/1 and 400/ l.

The ratio of amine-borane/metal salt is not critical; up to percent stoichiometric excess, or even greater, of either reactant can be used. It is preferred to use a slight stoichiometric excess, say, 10-50 percent of metal salt over amineborane. In the stoichiometry of the reaction, it is assumed that the amine-borane has three equivalents of reducing power per BI-l moiety. That is, each hydrogen bonded to boron can reduce the oxidation number of a metal ion by l.

Although there is no necessity for a particular order or reactant addition, it is preferred to add the tetrahydroborate to a mixture of the metal salts and amine-borane. Addition of the metal salt solution to a mixture of the reducing agents may result in less eflicient reduction. The addition of the amineborane to a mixture of the tetrahydroborate and metal salts, after a time lapse of, say, 30 minutes or more, may also result in loss of efficiency.

In most cases, much more powder product is produced than is stoichiometrically possible from the relatively small amounts of tetrahydroborate employed. However, iron-rich metal salts are especially difficult to reduce, by any reductant, and consequently it is necessary to employ more than an equivalent amount of the tetrahydroborate to initiate the formation of iron-rich alloys.

Although water is a convenient medium for carrying out the process of the invention, the metal salts, amine-boranes, and tetrahydroborates are soluble in other media, particularly aqueous solutions of water-miscible organic liquids such as methanol, ethanol, acetone, tetrahydrofuran, l, Z-dimethoxyethane, 2-ethoxyethyl ether, propyl alcohol, and isopropyl alcohol. Solvents of this sort may be substituted for water and in fact may be preferable in certain instances. An example of such an instance is one in which the amine portion of the amine-borane has a relatively high carbon content. The proportion of the organic solvent in mixed aqueous-organic media is primarily governed by its effect on the solubility of components of the reaction mixtures; the organic solvent usually will not exceed the volume of the water used.

To improve the magnetic properties of the powders produced, the novel process can be carried out in a magnetic field. This will generally increase coercivity and remanence by fostering chaining of the particles. Reaction mixtures are generally agitated manually or magnetically. It is important when the process is carried out in a magnetic field, to control agitation so as to enhance the formation of chained particles. A rotating magnet can be used to promote mixing and to provide the desired magnetic field.

The process is most conveniently carried out at ordinary temperatures (2030 C.), although temperatures of up to about 45 60" C. can be used. Temperatures below 20 C. can be used, but no advantage results. A practical low limit is imposed by the freezing point of the particular mixture being used. Pressure is not a critical variable. Atmospheric pressure is usually used for convenience, but higher or lower pressures can be used.

Under ordinary conditions, the reaction begins immediately upon mixing the reactants, as shown by precipitation of a dark solid product. Reaction is substantially complete within a few minutes, but the mixture is usually allowed to stand for about 15 minutes to insure complete reaction.

The product is isolated by filtration and washed with water and with acetone to remove by-products. Then, it may be allowed to stand in acetone for about to 24 hours before final isolation and drying. Aging in acetone eliminates any tendency of the particles to be pyrophoric. Alternatively, aging can be accomplished by drying the product in a flow of an inert gas such as argon, to which gradually increasing concentrations of air (oxygen) are added.

The ferromagnetic powders made herein may contain oxygen in the form of metal oxides, hydroxides or adsorbed moisture. The presence of oxygen in one of these forms is helpful eliminating pyrophoric tendencies in the powders. In most cases, the elimination of the pyrophoric tendencies by the oxygen is accomplished without unduly affecting magnetic properties. All the powders are useful in the preparation of magnetic tapes and permanent magnets.

SPECIFIC EMBODlMENTS The following Examples illustrate the invention and are included within the limits of the invention without setting those limits.

All magnetic properties reported in the Examples were determined by packing the powders in tubes and placing the tubes in an extraction magnetometer with an applied field of about 4,400 Oersteds (Oe). The apparatus is similar to that described in T. R. Bardell on pp. 226-228 of "Magnetic Materials in the Electrical Industry," Philosophical Library, New York (1955 Saturation magnetization 0-,, and remanent magnetization, 0,, are given in the Examples as emu/g. Intrinsic coercive force, il-lc, is expressed in Oersteds. Intrinsic coercive force is defined in Special Technical Publication No. of the American Society for Testing Materials, entitled "Symposium of Magnetic Testing" (1948), pp. 191-198. 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 lnstruments 7, 147(1936). V 1 EXAMPLE 1 A solution of 0.5 g of sodium tetrahydroborate in 50 ml of water was added to a solution of 56.2 g of (3080 -711 0 and 5.9 g of dimethylamine-borane in 400 ml of water. The resulting mixture was manually stirred only enough to insure complete mixing. After 15 minutes at room temperature, the reaction mixture was filtered. The dark solid on the filter was washed with water (500 ml), washed with acetone (500 ml), and suspended in 125 ml of acetone overnight. It was then separated by filtration and air0dried, to give 3.9 g of a ferromagnetic solid that contained 89.36 percent cobalt, 3.72 percent boron, and 5.26 percent oxygen.

EXAMPLES 2-19 By the method of Example 1, a number of other reductions of various cobalt salts by dimethylamine-borane in the TABLE I NBBHI, Magnetic Product, Percent Percent;

Ex Cobalt salt 5. field 3. Co 1H0 v 0. b0 3. 9 89. 4 3. 72 175 98 24 0. 25 3. 6 92. 0 3. 47 18 0. 10 2. 4 91. 8 3. 40 65 111 19 0. 05 l. 4 91. 8 3. 64 75 106 14 0. 01 1. 4 93. 0 3. 16 35 118 8 0. 60 3. 1 89. 6 3. 24 185 104 20 0. 26 2. 6 85. 0 3. 39 175 102 25 0.10 1. 0 71. 8 3. 39 90 109 '20 0. 05 l. 0 90. 6 3. 32 75 65 13 0- do 0. 01 0. 6 90. 6 85 109 20 11..." 50 g. of COAczAHzO 0. 60 4. 7 81.3 1. 82 55 121 12 I2.. Same as above 0.10 4. 2 92. 7 0. 96 40 11 13 d 0.06 2.7 99.3 1.03 40 123 13 0.01 0. 5 88. 2 1. 08 50 13 0. 60 6.1 88. 8 1. 74 85 102 14 0. 05 1. 8 93. 7 1. 14 60 130 16 do 0. 01 1. 4 93. 8 1.10 46 128 15 47.8 g. 0! (30011111110 0. 50 4. 5 90. 4 3. 20 104 22 Same as above 0. 50 3. 3 89. 8 3. 61 220 102 26 EXAMPLES 20-25 The embodiments of the invention in which an exclusive Examples 20 to 25 represent experiments carried out by the property or pnvllege ls clam-led are defined as follows:

method of Example 1. In Examples 22 to 25 mixtures of meta] A for making ferromagnetic metal Powders salts were reduced. In each experiment the metal salt/- 5 pnsmg reactmg amineOborane solution was made up by adding a solution of Salts of at least one Ofthe metals iron, Cobalt, nickel, and 5.9 g of dimethylamine-borane in 200 ml of water to a solution from 0 to 35 1361136"t of a chromium Salt, based t01111 of the metal salt or salts in 200 ml of water. In none of the exwelght 9 the Salts amples was there any evidence of reduction of the metal salts wlth a redufimg agent of until the tetrahydroborate was added. The final products were amme'borane and all black powders having magnetic properties. These experiafietrahydroborate mems are Summarized in Table 1L wherein the molar ratio of the amine-borane to the TABLE II Product, Percent Percent Ex. Metal salts g. metal B 1H 5. a,

50 g. NlAcz-4HzO 4. 6 93.1 Ni 3. 68 -30 13 5 21 5g 8 K $0284 $518. 0. 9 go 1. 30 405 99 42 g. e g o 22.. g g sg' g g 2.1 C0 1.74 735 77 35 55. g e 4'7 42.9 Fe z3 {gO g g gyg 0.8 $28 1.73 435 3s 11 B- 10 ("2- 2 0 1L ,8 g g g 5.7 02m 140 55 11s 10 5 R. lAcr'i z 1 94.1 NI .5 K cmcrzmu 5 o Cr 3.41 50 11 2 EXAMPLES 26-34 tetrahydroborate is from about 3 to l to 700 to l.

. 2. A rocess accordin to claim 1, wherein one of the metals The method of Example I was used to reduce a series of isiron p g iron and n ckel salt mixtures. In each experiment, a solution of A process according to claim 1, wherein one of the metals 5.9 g of dimethylamine-borane in 200 ml of water was added is cobalt to a Solunon of and mckel Salts 200 ml of water In a 4. A process according to claim. 1, wherein one of the metals vessel on the poles of a permanent lSOO-Oe magnet. There 30 is nickel. was no evldeflce of redlfcnon of the mefal over a P 5. A process according to claim 1, wherein one of the metals of about 5 minutes at this stage. A solution of 0.5 g of sodium is chromium.

tetrahydroborate in 50 ml of water was then added, and the reaction was initiated and the process carried out as in Example l. The details of these experiments are summarized in Table III. All the products were black solids having magnetic properties. In this series the ratio Fe/Ni in the product was tetrahydroborateis sodium tetrahydroborata similar to the Fe/Ni ratio in the original salt mixture. As can be A process according to claim 1, wherein the seen from the Table, both a, and 0,. tend to decrease as nickel tetrahydroborate is potassium tetrahydroborate is increased.

6. A process according to claim 1, wherein the salts are selected from the group consisting of halides, sulfates, nitrates, fluoroborates, formates, acetates and propionates.

7. A process according to claim 1, wherein the TABLE III FeSOflHzO, NiSOa-fiHzO, Product, Percent Percent Percent g. g. g. Fe Ni B .11., tr, a,

EXAMPLES 35-36 9. A process according to claim 1 carried out in a magnetic field.

10. A process according to claim 1, employing an amineborane of the formula,

By the general procedure of Example 1, tetrahydroborates other than sodium tetrahydroborate were used to promote the 55 reduction of cobalt ion by dimethylamine-borane. In each experiment, the solution 0.5 of the tetrahydroborate in 50 ml of water was added to a solution of 56.2 g of CoSO '7H O and 5.9 g of dimethylamine-borane in 400 ml of water in a vessel placed on the poles of a permanent 1,500-Oe magnet. The when?! theR Sarelowei T g or h details of these experiments, which produced black solids, are A process according to c alm w erem t e amulesummarized in Table borane is dimethylamineborane. I

12. A process according to claim 10, wherein the amine- TABLE IV borane is dimethylamine borane and the tetrahydroborate is Tetrahydrosodium tetrahydroborate and the mole ratio is between about Ex. borate Product Co B iHc 0', 0', 6 to 1 to 400 to 1, respectively.

NBH4 36 KBI-l 3.l g. 90.4 3.25 165 98 22 t

Claims (11)

1. 2. A process according to claim 1, wherein one of the metals is iron.
2. 3. A process according to claim 1, wherein one of the metals is cobalt.
3. 4. A process according to claim 1, wherein one of the metals is nickel.
4. 5. A process according to claim 1, wherein one of the metals is chromium.
5. 6. A process according to claim 1, wherein the salts are selected from the group consisting of halides, sulfates, nitrates, fluoroborates, formates, acetates and propionates.
6. 7. A process according to claim 1, wherein the tetrahydroborate is sodium tetrahydroborate.
7. 8. A process according to claim 1, wherein the tetrahydroborate is potassium tetrahydroborate.
8. 9. A process according to claim 1 carried out in a magnetic field.
9. 10. A process according to claim 1, employing an amine-borane of the formula, R3 nNHn.BH3, wherein the R''s are lower alkyl and n is 0 or 1.
10. 11. A process according to claim 10, wherein the amine-borane is dimethylamine borane.
11. 12. A process according to claim 10, wherein the amine-borane is dimethylamine borane and the tetrahydroborate is sodium tetrahydroborate and the mole ratio is between about 6 to 1 to 400 to 1, respectively.