US2140889A - Method for producing magnetic powder - Google Patents

Description

Patented METHOD FOR PRODUCING MAGNETIC POWDER Hans Volt. Berlin-'Steglits, Germany, aslinior to Micro Products Corporation, Hastings on Hudson, N. Y., a corporation of Delaware No Drawing. Application January 27, 1938, Serial No. 61,111. In Germany January 80, 1935 9 Claim.

This invention relates to the production of mag-' netic powder. more particularly for making high frequency iron' cores. It is an object of the invention to render possible the production of such powder from cheap and easily available initial material. Another object is to improve the electric properties oi such powders.

Bo far, the only iron powder material giving iron powder cores 01' maximum eiiiciency and minimum losses more particularly for high irequency work and iron cored tuning coils of very small size and high Q-value, was the carbonyl iron. This material, however, is dii'ilcuit to produce and very expensive. thus handicapping the 16 introduction in radio sets of highly efiicient iron cored tuning coils which are technically superior to air coils and to iron cored coils using inferior iron powder. Now, by the novel process, it is possible to make from very cheap initial material I such as red iron oxide etc. and by simple and cheap methods, iron powders which are electrically equivalent if not superior to carbonyl iron I and easier to mold to solid cores.

The particular advantage of magnetic powders as made by the present process consists in their high specific resistance due to the silicon content; not only is the interior resistance of each particle increased in this way but also the contact resistance from particle to particle is greatly increased, so thus reducing eddy currents both within each particle and through the whole core structure from particle to particle. Moreover the hardness of the particles isincreased, thus facilitating the moulding or compressing of the powder to highly as eiiicient cores.

Further objects of the invention will be apparent from the following detailed description. 5 The method according to the invention comprises intimately mixing small particles contain- 40 ing'iron and a resistance-increasing material such as silicon and/or aluminium and heating this mixture in a reaction chamber under action of non-oxidizing gases to a temperature sufllcient to cause the silicon and/or aluminium to substan- 45 tially alloy with the iron particles but insufllcient to cause substantial sintering together 01 the iron particles. The silicon content in the initial mixture should preferably be more than 3%.

The mixing may be eflected prior to the heat 50 treatment, in mechanical mills such as ball mills,

centrifugal mills, colloidal mills etc. and gaseous or liquid agents may be used in the mills so as to protect the particles from oxidizing and to cause intimate mixing and adherence between the dif- 5 ferent particles. The mixing may also be eilfected or improved by mixing the ingredients continuously during the heating and alloying process, for instance by carrying out the process in a rotary tube furnace or another reaction chamber enabling continuous movement and agitation of the 5 material.

The initial iron material may be pureiron disintegrated to sufilcient fineness or preferably iron oxide which is reduced in the reaction chamber and simultaneously alloyed with the silicon. Var- 10 ious iron oxides may be used, such as red iron oxide. iron hydroxide (Fe(0I-i)a) having proved to be particularly suitable for this process.

The material containing silicon may be ferrosilicon, such as used in iron foundries and particularly with a silicon content oi the order of 98%, which should be ground to very fine particles of the order of 1 micron. Also it may be in the form of a silicon compound facilitating the alloying process in liquid form, such as silicon tetrachlo- 0 ride and/or in gaseous form such as silicon-hydrogen compounds (silanes). The mixture should be so effected and treated that 4 to 12% of silicon are diflused into the iron particles.

According to the kind of ingredient the nonoxidizing gases used in the process may be inert gases, such as nitrogemand/or reducing gases, such as hydrogen, and/or gases having a catalytic eflect, such as chlorine gas and silanes. For example, if the material already is reduced a neutral gas may be used, but if reduction of the material is necessary a reducing gas or a mixture of reducing gas with the neutral gas may be used depending upon the intensity of reducing action required. These gases may continuously stream through the reaction chamber, for instance through the rotary tube furnace. Further the reaction chamber may be kept under reduced pressure or under elevated pressure. Catalytic gases, and/ or solid or liquid catalytic substances may be added. Silicon tetrachloride or other salts, dis,- sociating in the reaction chamber and liberating the silicon, may also be used. Gaseous catalysts may be added to the hydrogen used in the process.

The electric properties of the powder thus obtained may be considerably improved by suitable heat treatment thereof, more particularly by very slowly cooling down the powder after the process. This may be eflected by passing the powder from the furnace into a heated vessel, which is very slowly cooled down and preferably slightly moved.

The initial material should be of such kind and the process so conducted that the resulting powv der is of great fineness below 20 microns. For

high frequency cores the particles should substantially have a size of 3 to 5 microns or, particularly for short waves, even 0.5 to 2 microns.

The powder may be further improved by mechanical treatment after the alloying, e. g. by hammering in such manner that the density of the particles is augmented and the size reduced. More particularly any lumps or aggregations are disintegrated. Ball mills using hard balls of high specific weight may be employed therefore and grinding agents, more particularly hydrocarbons, esters, chlorides, solvents etc. may be added.

Small quantities of insulating substances preferably in dissolved form may be added which are suitable to form an insulating coating on the surface of the particles during grinding. This insulating coating may be hardened or oxidized or, if it is not hardened, a binder should be used the solvent of which does not dissolve the insulating skin.

It is understood that, in the whole process, aluminium or the corresponding aluminium compounds may be used alternatively or additionally to silicon and its compounds. Other substances, such as carbon, boron, titanium, zirconium, thorium, vanadium, antimony, arsenic, phosphorus, tellurium, may likewise be applied alone or in combination with silicon and/or aluminium.

Some forms of the novel process will be described hereunder in detail as follows:

Example 1 I' e(OH)a is intimately mixed with 2 to 6% by weight of powdered silicon and treatedin a rotary tube furnace during about 30 minutes at 600 to 800 0., preferably 700, in a preheated hydrogen current, to which 0.5% chlorine gas may be added. The iron oxide is thus reduced to iron and immediately alloyed with the silicon. The powder is subsequently cooled .down during 12 hours in a hydrogen atmosphere and treated in a ball mill-to increase the permeability.

Example 2 Finely disintegrated iron oxide is distributed in a reduction furnace in a thin layer and heated to GOO-700 C. in a preheated hydrogen gas current as under 1), the powder is thus treated during 1 hour and cooled down as under 1). The reduced power is then'ground for 1 to 3 hours with about e6% by weight of silicon and heated to 500-700 C., preferably evacuating the heating chamber. A small quantity of chlorine gas is then added and the powder moved during 30 minutes, then kept at this temperature during 2 hours and cooled down.

Example 3 100 parts by weight of iron oxide hydrate are intimately mixed with 2.5% by weight of finely divided 99% ferrosilicon and exposed to a hydrogen gas current containing 1% chlorine gas. during 30 minutes and at 600 C. The powder is cooled down to room temperature during 6 hours; according to analysis, it consists of 4.2% 8i; 95.8% Fe. without free Bi. The same process,

but without chlorine shows 1.7% free Si in the final product. 1

Example 4 100 parts by weight of dry iron oxide hydrate are heated to 650 C. in a rotary drum and treated with preheated hydrogen gas to which before entering the furnace 17% by weight of gaseous silicon tetrachloride are added during a reaction time of 30 minutes. Further treatment as before.

In most practical cases the completeness of reaction can be easily determined by compressing the treated powder in a mould without a binder. If the powder forms a coherent body, it is not suitable. while, if falling to pieces, the reaction is finished.

The powder may be moulded to high frequency cores with an insulating binder. Mostly, it is not necessary to form an individual insulating skin upon the particles.

I claim:-

1. A method of producing magnetic powder which comprises heating an intimate mixture of a finely divided solid iron-containing material of the group consisting of iron, iron oxides, andiron hydroxides with a finely divided solid material of the group consisting of silicon and aluminum and their alloys in a non-oxidizing atmosphere to a temperature sumcient to cause the silicon or aluminum to alloy with the iron but insufiicient to cause sintering together of the particles.

2. Method as defined in claim 1 in which the finely divided iron is supplied to the reaction mixture as such.

3. Method as defined in claim 1 in which the finely divided iron is supplied to the reaction mixture by incorporating an oxidic iron compound in the reaction mixture and heating the reaction mixture in the presence of a reducins gas.

4. Method as defined in claim 1 in which the reaction mixture is continuously mixed during the heating and alloying operation.

5. Method as defined in claim 1 in which the reaction is carried out in the presence of a catalytic agent of the group consisting of the halogens.

6. Method as defined in claim 1 in which the alloy is subjected to heat treatment involving a slow cooling thereof.

'7. Method as defined in claim 1 in which the alloy is subjected to hammering to increase the density and reduce the size of the particles thereof.

8. Method as defined in claim 1 in which silicon is supplied to the mixture as ferrosilicon,

9. Method of producing magnetic powder as defined in claim 1 in which silicon is supplied to the mixture as such and iron is supplied to the mixture as an oxidic compound of iron and the mixture is heated to a temperature of 600' to 800' C. in a current of hydrogen containing a small proportion of chlorine and the mixture is slowly cooled and subjected to impact disintegration.

HANS V001.