US2828264A - Manufacture process of permanent magnets from sintered mixtures of oxides - Google Patents

Description

March 25,-- 1958 s. MEDVEDIEFF 2,328,254

MANUFACTURE PROCESS OF PERMANENT MAGNETS FROM SINTERED MIXTURES OF OXIDES Filed Sept. 12, 1955 I Fl-G.2

FIGS

SCRGE an/501s FF NYVE/V TOE MM A r-roomy United States Patent MANUFACTURE PROCESS OF PERMANENT MAGIEISTITS FROM SINTERED MIXTURES OF 0X11) Serge Medvedieif, Boulogne sur Seine, France, assignor to Society Audax, Montreuil sous Bois, France, a French society Application September 12, 1955, Serial No. 533,577

Claims priority, application France November 9, 1954 Claims. (Cl. 252-625) The invention relates to permanent magnets constituted by sintered mixed oxides of iron and of one or more of the metals barium, strontium, calcium and lead, incorporating oxide of chromium.

The ferromagnetism of the ferrites of the alkaline-earth metals and of lead has been known for several decades.

It has been shown that these ferrites are very ferromagnetic and that they have an hexagonal structure.

The employment of the ferrites of Ba, Sr, Ca and Pb of the formula nFe O MO, in which n is a number between 2 and 9 and M denotes one of the aforesaid metals, for the manufacture of permanent magnets has been proposed in the specification of French Patent No. 1,048,792 of July 18, 1951. In that specification, it appears that the appearance of the properties of a permanent magnet in these ferrites is attributed to the .fact that the majority of the grains of the sintered aggregate would be of small dimensions as they comprise only a single Weiss domain; since these ferrites are characterized by a considerable magnetocrystalline anisotropy, the dimensions of the single domain particles might be of the order of microns.

This interpretation appears to be erroneous, because the powder of barium ferrite, for example, in the sintered state renders it possible to obtain compact magnets having a density-of approximately 5, the theoretical density corresponding to zero porosity'being 5.3 gms. per cc. In this advanced state of aggregation, it may be supposed that there could not be grains which are magnetically insulated from one another.

It is more probable that the magnetic properties of the ferrites depend, to-a certain extent, on the residual porosity of the sintered pieces. It has already been observed that, in the case of the magnets made of cobalt ferrite, a too advanced sintering involved a rapid diminution of the coercive force and of the. residual magnetisation (S. Medvedieff, These 'dIngenieur-Docteur, Paris, 1952).

In order to obtain a high residual magnetisation B,, it is necessary to give the sintered material a density that is as great as possible. In fact, if 0 denotes the magnetic moment per unit of mass, the magnetisation I per unit of volume is equal to the product 1rd, where 0! denotes the density, and the residual induction B is equal to that is to say, isproportional to d.

Now, experience shows that,'for large values of the density (of the order of about 5), an increase thereof if accompanied by a rapid diminution of the coercive force 1 The heat treatments then become extremely critical. By way of example, on heating the same powder of 6Fe O BaO for one hour at temperatures of 1150" C. and 1200 C., residual inductions of 2350 and 2300 gauss respectively and coercive magnetisation forces (values of the magnetic field eliminating the magnetisation) dispersed between 1800 and 1050 oersteds have been found.

N 2,828,264 Patented Mar. 25, 1958 From applicants experiments it has been found that the mixed oxides of iron, of chromium and of an alkalineearth metal or lead, render it possible, owing to the presence of chromium oxide, to produce permanent magnets,

having, at the same time, highvalues of residual magnetisation and of the coercive force. These magnets have,

for a density of approximately 5, verygreat coercive in the mass of the material and favouring the existence of a considerable coercive force owingto 'a mechanism such as that known in the theory of Kerstens foreign bodies (M. Kersten,,Hirzel, Leipzig, 1943, pp. 1-88).

On the contrary, the chromium ions form an integral part of the crystalline lattice of the mixed oxides formed at temperatures of between 900 C. and 1400 C. as is confirmed by the displacement of the Curie point.v Oxides of .iron (Fe O and of chromium (Cr O and barium carbonate (BaCO have been mixed in proportions that are suitable for obtaining the molecular ratios Fe/Cr hereinafter indicated, and then finely ground in a ball mill for fourteen hours. The mixture was dried, sifted and compressed into the form of rods, having a diameter of 1 cm. and a length of from 1 to 2 cms., which were baked at 1200 C. for one hour. With the proportion of barium oxide remaining the same, the Curie point was taken for different proportions Fe/Cr:

Curie point, degrees C.

In the case in which the alkaline-earth metal is barium, the magnets according to the invention can be prepared by using, as initial material, ferric oxide (Fe O and barium chromate. An intimate mixture of these compounds in the suitable proportions .is made by grinding The powder is dried,

for fourteen hours in a ball mill. sifted, a binder being added, if required, which is entirely eliminated on heating, and then compressed by an hydraulic press under a pressure of 2 to 4 tons per square centimeter into the form of rods having a diameter of 1 cm. and a length of from 1 to 2cms. These rods are baked for one hour at 1200 C;

By way of example; the following four mixtures have been prepared:

Example 1.-.Fe O 192 grams; BaCrO 51 grams,

Fe/Cr=12. Example 2.Fe' O 240 grams; BaCrO 76 grams,

Fe/Cr=10.

Example 3.Fe O 192 grams; BaCrO 76 grams,

Fe/C r= 8. p v Example 4.--Fe O 144 grams; BaCrO 76' grams,

In .order to show that the conditions of sintering are not critical, one portion of the rods obtained with the powders of the different examples were baked for one" I hour. at 1150 Cflandfanother portion were baked for 3 l the same period at 1250* C. The features of the magnets obtained are set forth in the following table:

The range of molecular proportions Fe/Cr hereinbefore given as between 6 and 12 may be greater, for example between 4 and 20.

From the magnetic point of view,'the crystals of the mixed oxides of iron, of chromium and of an alkalineearth metal are magnetically anisotropic and have (1) A magnetic anisotropy due to the existence of an axis of easy magnetisation which, in the present case, is the senary axis.

- (2) An anisotropy of shape, which, in the case of needle-shaped crystals, is proportional to the difference between the demagnetising shape factors along the geometrical axis of the needle and in a perpendicular direction.

Also, grains of the compound according to the invention, which are placed in an homogeneous magnetic field, become aligned in a direction which is the resultant of the two above anisotropy axes; if the crystallisation of the initial mixture has been suitably regulated, the product obtained is composed of essentially mono-crystalline fine particles for which the axes are coincident.

As has been shown by numerous studies and, in particular, by those of Charles Guillaud, Proprits daimant des poudres ferromagntiques, Journal des Recherches du Centre National de la Recherche Scientifique, Nov. 9, 1949, it appears, and experience confirms, that the properties of magnets made of magnetic material in highly anisotropic fine grains are greatly improved if the grains are aligned.

C. Guillaud has demonstrated it on a powder of the alloy Mn-Bi. This alloy has the same hexagonal structure as the compound of the invention and is characterised by a high magnetic anisotropy along the senary axis. He has, by making the following experiments, shown that this alloy, reduced to the state of a fine powder, lent itself to the production of excellent permanent magnets by compressing the powder.

(1) The powder was compressed without regard to parallel alignment of the magnetic axes of the particles of the alloy.

(2) The powder was compressed (or, more accurate- 1y, set by cooling a mixture of powder and wax) in the presence of an homogeneous magnetic field.

In the second case, the residual magnetism is three times as great as in the first case.

According to an improvement of the invention, permanent magnets are made with the powders hereinbefore described by aligning the grains of these powders before the sintering treatment.

The manufacture of the permanent magnets according to the invention is carried out as follows:

Ferric oxide and barium chromate are mixed in one of the foregoing proportions, for example in the proportion Fe O 1BaCrO The mixture is ground in water in .a ball mill for to 14 hours. The ground mixture is drained and dried and then compresed into the form of cakes having, for example, the following dimensions: diameter 8 centimeters, height 2 to 3 centimeters.

These cakes are baked at 1250 C. for one hour in air and then soaked in water. The lump of oxides bursts and the fragments are friable and can be crushed easily to a fineness for enabling it to pass through a No. 25 sieve.

This coarse powder is ground with water for a time of between 4 and 14 hours, for example, for six hours.

After grinding, the paste or sludge is kept and, after addition of a suitable binder, is used for preparing oriented magnets.

Two methods of orientation may be employed.

In order to obtain'a radial magnetisation, that is to say magnets 14 having the shape of a cylindrical ring (torus having a rectangular cross-section) oriented with a given polarity on the internal cylinder 15 and with opposite polarity on the external cylinder 16 (Fig. 1), the apparatus represented in Fig. 2 is employed.

The apparatus comprisesa coil 1, through which a direct current passes, and a magnetic circuit comprising a central hollow post 2 constituting the core of the coil, a cylindrical part 3 coaxial with the said post, a base 4 connecting the post 2 with the cylinder 3, and a lid 5 having therein a hole in which a ring 6 is fitted. The central post 2 is surmounted by a pole head 7 which, with the ring 6, bounds an air gap in which the lines of force are radial.

, Beneath the lid 5 there is arranged a plate 8 made of bronze or other non-magnetic metal and having holes 9 therein. Arranged above the coil is a second plate 10 also made of non-magnetic metal and, together with the former plate, defining an exhaust chamber 11. The plate 10 is funnel-shaped and is joined to the central post 2 in a zone in which holes 12 bring the exhaust chamber 11 into communication with the passage 13 provided in the central post. The passage 13 may be put into communication with a vacuum pump.

The bottom of the annular space between the members 6 and 7 is covered with a filter cloth and the mould thus formed is filled with paste. The radial magnetic field is applied by passing current through the coil '1 and the water is removed by putting into operation the vacuum pump connected with the passage 13.

Experience has shown that, in order to ensure the appropriate alignment of the grains of the powder along the lines of force of the magnetic field, it was necessary that the grains should be in suspension in a liquid, for example water, so that they should be sufficiently mobile. The grains are thus oriented in the liquid phase. As the water is removed, the position of the grains remains fixed by the field. When the greater part of the water hasbeen removed, there is applied, to the top part, with the aid of a piston 28 made of non-magnetic metal, a pressure of some hundreds of kgs. per square centimeter which compresses the thick paste.

The field is cut 0E and the assemblage comprising the members 6 and 7 and the paste is removed; After a partial drying of the compressed paste, the latter is subjected, in the mould itself, to a compression of the order of one ton per square centimeter. -After removal from the mould, a piece in the form of a ringis obtained, one of the poles of which is spread over the inner cylindrical surface and the other is spread over the outer cylindrical surface. The particles are therefore oriented radially. The compressed piece is then subjected to baking in an electric furnace for a time varying from half an hour to one hour and to a slow cooling. A sintered ceramic piece is thus obtained which has a density of between 4.5 and 5 gms. per cubic centimeter and which, if'the radial field applied is between 4000 and 5000 oersteds, gives a powerful magnet with radial magnetisation, the characteristics of which are substantially Residual induction: 13,:2900-3300 gauss. V Coercive force of'magnetisation I ==16002500 oerteds. p

(BH) max.=1.7 to 2.2.10 gauss-oersteds.

In order to obtain a parallel magnetisation, that is to say magnets in the form of parallelepipeds or of discs 17 (Fig. 3), the north and south poles of which are situated respectively on two parallel faces 18 and 19, the apparatus represented in Fig. 4 is employed.

The apparatus comprises a magnetic circuit 20 which produces a magnetic field of some thousands of oersteds between the plane faces opposite an upper piston 22 and a bottom cylindrical stand 23 made of steel. The piston 22 slides inside a thick tube 24 which is made of nonmagnetic metal and in which the stand 23 is encased. The bottom stand 23 is provided with holes 25 and is connected to a vacuum pump through piping 26. A filter cloth 27 is arranged on the top face of this perforated stand; the fluid paste is poured into the mould constituted by the tube 24; the piston 22 is engaged in the tube, the field is applied and the vacuum pump is set in operation. The forces of magnetic attraction in the ari gap exert a first compression on the paste and accelerate the filtration. As the length of the air gap diminishes, the fields, to which the particles are subjected, increases with the compression. At the end of a few minutes, the force of an hydraulic press, giving a pressure of one ton per square centimeter, is applied to the piston. After removal from the mould, the disc of oriented material, although possessing a cohesion which is sufficient for the manipulations (owing to the previously incorporated binder), is still damp. It is dried in order to eliminate first the moisture and then the binder. Finally, it is subjected to baking under the same conditions as in the case of the radial magnetisation. The magnetic characteristics are the same.

The temperature and pressure values which have been given depend upon the molecular ratio which, as has been seen, may vary from 4 to 20 and are not critical. A pressure of between 1 and 4 tons per square centimeter and a temperature of between 900 and 1400 C. are allowable.

What I claim is:

1. A process for preparing an anisotropic permanently magnetic barium chromium ferrite having a density of at least aboutv 4.45 comprising mixing Fe O with a member of the group consisting of BaCr and a mixture of BaCO and Cr O as will form BaCrO the molecular proportion of iron to chromium being between 4 and 20, pressing said mixture to a temperature of between about 900" C. and about 1400 C. and subjecting said pressed mixture to a steady magnetic field to provide a shaped permanent magnet having a residual induction of at least about 1800 gauss and a coercive force of mag netization of at least about 3200 oersteds.

2. A process for preparing an anisotropic permanently magnetic barium chromium ferrite having a density of at least about 4.45 and comprising mixing finely divided Fe O and finely divided BaCrO the molecular proportion of iron to chromium being between 4 and 20 and pressing said mixture at a temperature of between about 900 C. and about 1400 C. to sinter said mixture and to decompose said barium chromate into barium oxide and chromium oxide and subjecting said sintered mixture to a steady magnetic field to provide a shaped permanent magnet having a residual induction of at least about 1800 gauss and a coercive force of magnetization of at least about 3200 oersteds.

3. A process for manufacturing a permanently magnetic barium chromium ferrite made of mixed oxides as claimed in claim 1, comprising mixing finely divided Fe O and fine divided BaCrO in proportions to obtain a mix-ture in which the molecular proportion between iron and chromium lies between 4 and 20, sintering said mixture while said mixture is subjected to a steady magnetic field, said mixture being sintered by pressing and heating at a temperature of between 1150 and 1400 C. under the said field.

4. The product produced by the process of claim 1.

5. The product produced by the process of claim 2.

References Cited in the file of this patent UNITED STATES PATENTS 1,997,193 Kato et al. Apr. 9, 1935 2,579,267 Leverenz et al. Dec. 18, 1951 2,762,777 Went et al Sept. 11, 1956 2,762,778 Gorter et al Sept. 11, 1956 OTHER REFERENCES Journal of Chemical Physics, vol. 15, No. 4, April 1947, pages 181-187, page 184 pertinent,

Claims (1)

1. A PROCESS FOR PREPARING AN ANISOTROPIC PERMANENTLY MAGNETIC BARIUM CHROMIUM FERRITE HAVING A DENSITY OF AT LEAST ABOUT 4.45 COMPRISING MIXING FE2O3 WITH A MEMBER OF THE GROUP CONSISTING OF BACRO4, THE MOLECULAR PROPORATION OF IRON TO CHROMIUM BEING BETWEEN LECULAR PROPOTION OF IRON TO CHROMIUM BEING BETWEEN 4 AND 20, PRESSING SAID MIXTURE TO A TEMPERATURE OF BETWEEN ABOUT 900*C. AND ABOUT 1400*C. AND SUBJECTING SAID PASSING MIXTURE TO A STEADY MAGNETIC FIELD TO PROVIDE A SHAPED PERMANENT MAGNET HAVING A RESIDUAL INDUCTION NETIZATION OF AT LEAST ABOUT 3200 OERSTEDS.

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