GB1589355A - Acicular cobalt magnetic iron oxide and its manufacture - Google Patents

Description

(54) ACICULAR COBALT/MODIFIED MAGNETIC IRON OXIDE AND ITS MANUFACTURE (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: The present invention relates to acicular cobalt-modified gamma-iron (III) oxide, as well as to a process for its manufacture and to its use as magnetizable material in the production of magnetic recording media.

Acicular gamma-iron (III) oxide has for a long time been employed extensively as a magnetizable material in the manufacture of magnetic recording media. However, as a result of continuing development in recording technology, there is an increasing need for magnetic materials which, above all, have a higher coercive force than gamma-iron (III) oxide.

Although it is known that the coercive force of iron oxides can, under certain circumstances, be substantially increased by modification with cobalt, the increase depending on the amount of cobalt added, such modified materials have a number of undesirable properties, in particular the residual induction and coercive force thereof are unstable to thermal and mechanical factors. Materials having such disadvantages are unsuitable for use in magnetic recording media.

Cobalt-modified acicular iron oxides may be obtained by a number of processes. According to German Patents 1,112,725 and 1,226,997, modification with cobalt is easily achievable by co-precipitation of iron hydroxide and cobalt hydroxide in the manufacture of goethite, since the subsequent conversion of the cobalt-containing goethite to the corresponding gamma-iron (III) oxide may be carried out in the conventional manner. German Laid-Open Application DOS 2,100,390 employs a similar procedure.

German Laid-Open Application DOS 2,036,612 discloses that a cobalt compound can also be precipitated onto the surface of a gamma-iron (III) oxide. The cobalt is introduced into the oxide lattice by subsequent heating at from 380 to 400"C. According to our GB Patent Specification No. 1432643, the acicular iron oxide or hydrate thereof is provided, prior to the reduction to magnetite, with a protective coating to avoid sintering of the needles, during the reduction, and is subjected to a heat treatment at 80 to 3000C after application of the cobalt compound to the magnetite or the gamma-iron oxide. German Laid-Open Application DOS 2,022,013 discloses a further method of obtaining cobalt-containing acicular iron oxides, in which acicular FeOOH or Fe203 is mixed with a cobalt-containing liquid, the slurry is dried and the mass is then either converted to gamma-iron (III) oxide by conventional methods or, if gamm-iron(III) oxide was employed as starting material, is merely sintered. If magnetite is used as the starting material in this process, it must first be oxidized to gamma-iron (III) oxide which is then treated with the cobalt compound.

In the process of German Published Application DAS 2,221,218, a cobalt-containing goethite or alpha-Fe203 is converted in the conventional manner to gamm-iron (III) oxide, but only after it has first been heated at from 600 to 8000 C. According to German Published Application DAS 1,907,236, another method of manufacturing cobalt-containing acicular gamma-iron (III) oxide is to suspend gamma-iron (III) oxide itself, or an oxide obtained as an intermediate in its manufacture, in an aqueous medium, precipitate a cobalt compound onto the oxide and then heat the material at a temperature sufficiently high to decompose the cobalt compound. Preferably, this is done in an inert gas atmosphere at above 370"C. Such modified iron oxides are then, if necessary, subjected to reduction and oxidation, in the conventional manner, to obtain the end product. The partial reduction of the cobalt-modified gamma-iron (III) oxides obtained by this process, in order to increase their coercive force, has also been proposed. In addition to these processes it has also already been proposed, in German Laid-Open Application DOS 2,308,791, to apply metallic cobalt to magnetite and treat the resulting particles in an inert atmopshere at from 100 to 6000C.

Most cobalt-containing acicular iron oxides do not have entirely saitsfactory magnetic properties and are thermally and/or mechanically unstable. The desired high coercive force is in most cases only achieved by a relatively high cobalt content which then results in the known disadvantages. Reducing the amount of cobalt and at the same time increasing the treatment temperature does not give products which, when employed in magnetic recording media, conform to the expected requirements.

It would be desirable to produce in a simple and economical manner a cobalt-modified gamma-iron (III) oxide suitable for use as magnetizable material in magnetic recording media which, although having a very low cobalt content, gives a very high coervice force, whilst the effect of temperature on the coercive force and residual induction is very slight, and which, when used for the manufacture of magnetic recording media, gives a high signal-toprint-through ratio and a high ratio of the recorded signal to the residual signal after erasure as well as good thermal and mechanical stability.

According to the prcsent invention, there is provided a cobalt modified gamma-iron (III) oxide having a cobalt content of from 0.5 to 10 per cent by weight, based on iron oxide, a TA value (as herein defined) of less than 2.5 and at the same time a coercive force of at least 38 kiloamps/m, measured at 25"C in a saturation field of 160 kiloamps/m and at a tap density of 1.2 g/cm3.

The oxide may be manufactured, according to a further aspect of the invention, by applying an appropriate amount of a cobalt hydroxide precipitate to an unprotected magnetite (as hereinafter defined) produced by reducing optionally hydrated iron OII) oxide, and oxidizing the resulting modified magnetite in an oxygen-containing atmosphere, provided that a temperature of 200"C is not exceeded after formation of the modified magnetite.

By the term "unprotected magnetite" is used herein is meant magnetite which has not had its surface protected by the provision of a sintering preventing coating thereon as is described in our GB Patent Specification No. 1432643 mentioned above either before or after its formation by reduction of optionally hydrated iron (III) oxide.

In the process according to the invention, the magnetite employed may be manufactured in the conventional manner by reduction of acicular hydrated iron oxide, especially in the form of goethite, or by reduction of the alpha-iron (III) oxide obtained therefrom by dehydration at from 200 to 600"C, the reduction being carried out by means of a gaseous reducing agent, preferably hydrogen, at from 300 to 400"C. Advantageously, the magnetite thus obtained is suspended in water. A cobalt salt solution is added to this suspension and the cobalt hydroxide precipitated onto the magnetite by adding an alkali, particularly an alkali metal hydroxide. The cobalt salt solution used is, as a rule, an aqueous cobalt sulfate solution from which the cobalt hydroxide is precipitated by means of alkali metal hydroxide solution at a pH above 8. The amount of cobalt compound added is chosen so that the resulting modified magnetite contains from 0.5 to 10 per cent by weight of cobalt based on iron oxide. After filtering off and drying, the modified magnetite (magnetite/cobalt hydroxide composite) is oxidized in the presence of oxygen at less than 200"C, preferably from 120 to 195"C.

The oxidation can also be carried out in the same way in the course of drying the filtered-off magnetite/cobalt hydroxide mixture. It can alternatively be advantageous to dry the magnetite coated with cobalt hydroxide, heat the product in an inert gas atmosphere and then oxidize it. However, it is essential that in none of the procedural steps following the manufacture of the modified magnetite (magnetite/cobalt hydroxide composite) should the stated temperature of 200"C be exceeded. Overheating during the oxidation can be avoided by employing an air-nitrogen mixture instead of pure air as the oxygen-containing atmosphere, or by using a material which has already been partially oxidized during the drying process.

In developing the process of the invention it has been found that, surprisingly, the initial stage of heating of the magnetite/cobalt hydroxide mixture is decisive for the magnitude of the TA value, whereas, after a certain degrce of oxidation has been reached, slightly exceeding the prescribed temperature limit no longer substantially affects the TA value, but greatly lowers the coercive force.

Magnetic materials within the invention are distinguished by a high coercive force, and the slight temperature dependence thereof deserves particular mention. Thus, these materials have temperature dependence values, ie. TA values, of less than 2.5, whereas the absolute value of the coercive force at + 25"C is at least 38 kiloamps/m. The TA value is the sum of the quotient of the coercive force at - 1 000C to the coercive force at + 25 0C and of the relative remanence MR/Ms at - 1 000C to that at + 25 C. A further advantage of this material is that it retains a high proportion of shape anisotropy, as a result of which magnetic recording media having a particularly high orientation ratio, ie. a high ratio of residual induction in the playing direction to that in the crosswise direction, can be obtained.

The Examples which follow illustrate the invention and should be studied in conjunction with the accompanying drawings, which show the dependence of the coercive force and relative remanence on the temperature for materials within the scope of the invention and comparative materials.

The magnetic properties, ie. the coercive force Hc [kiloamps/m], the specific remanence MR/S [nTm3/g] and the specific saturation magnetization Ms/f [nTm3/g], of the powder samples in the Examples were measured in a vibrating sample magnetometer at a field strength of 160 kiloamps/m. The magnetic orientability of the particles was determined by means of a preliminary test and quoted as the RFE.

For this test, about 1 g of the magnetic powder is introduced into a two-component epoxy resin, so that the proportion by volume of the magnetic powder is about 10%. The powder is thoroughly mixed with the two-component resin between counter-rotating glass discs and is then spread in a thin layer and cured at 700C in a homogeneous magnetic field of 800 kiloamps/m. This results in different degrees of orientation of the magnetic particles parallel to the magnetic field, the degree depending on their process of manufacture. This orientation is measured in terms of the orientation ratio, RFE, the quotient of the residual induction along, and crosswise to, the direction of orientation.

To measure the temperature dependence of the coercive force and the relative remanence and to determine the TA value, the powder samples were examined in a field of 800 kiloamps/m at from -100 to + 140"C, in each case carrying out a heating and cooling cycle.

EXAMPLE 1 Goethite which has been manufactured in alkaline medium and has a specific surface area of 29.6 m2/g is reduced to magnetite in a stream of hydrogen at 3806C. 400 parts of this magnetite are suspended in 8,000 parts of water and an aqueous solution of 48 parts of CoC12.6 H2O is added, whilst stirring vigorously. 0.5-normal NaOH is then added until the pH reaches 11.0. After stirring thoroughly for a further half-hour, the solid material, consisting of magnetite and cobalt hydroxide, is filtered off, washed with water until the filtrate shows a neutral reaction, and dried at 400C under a pressure of 100 mm Hg. The magnetite/ cobalt hydroxide mixture thus obtained is then oxidized in a 1:4 mixture of air and nitrogen at 1500C to give cobalt-containing gamma-iron (III) oxide (sample A). The values measured are shown in Table 1. The temperature dependence of the coercive field strength Hc and relative remanence MR/ Ms is shown in Figure 1 of the drawings.

COMPARATIVE EXPERIMENT 1 The procedure described in Example 1 is followed, except that the magnetite/cobalt hydroxide filter cake is dried for 2.5 hours in a rotary furnace at 2500C whilst passing over nitrogen at 1500C, and is then oxidized, still at 2500C, with a 1:4 mixture of air and nitrogen.

This sample B gives the values shown in Table 1, whilst Figure 2 of the drawings shows the temperature dependence of Hc and MR/Ms.

COMPARATIVE EXPERIMENT 2 The procedure described in Comparative Experiment 1 is followed, except that the magnetite/cobalt hydroxide mixture is dried at 1500C. This cobalt-containing gamm-iron (III oxide, referred to as sample C, gives the values shown in Table 1, whilst the temperature dependence of Hc and MR/ Ms is shown in Figure 3 of the drawings.

COMPARATIVE EXPERIMENT 3 A part of the goethite referred to in Example 1 is reduced at 320"C, reoxidized at 2500 C, modified with cobalt hydroxide as described in Example 1, filtered off, washed and then oxidized for 20 hours at 200"C (sample D; the measurements obtained are shown in Table 1 and Figure 4 of the drawings).

COMPARATIVE EXPERIMENT 4 A part of the goethite referred to in Example 1 is modified with cobalt hydroxide as described in Example 1 in the case of magnetite, filtered off, washed and then dried at 1000C and 100 mm Hg. It is then reduced at 3200C and reoxidized at 250"C (sample E; the measurements obtained are shown in Table 1 and Figure 5 of the drawings).

TABLEI Sample Ms/9 MR/j' MR/MS Hc(f = 1.2g/cm3) TA RFE [nTm3/g] [nTm3/g] [kiloamps/m] value A 76 41 0.54 40.7 2.3 3.3 B 84 46 0.55 33.5 3.9 2.6 C 84 46 0.55 32.2 4.1 2.6 D 82 40 0.49 29.5 2.3 3.2 E 75 39 0.52 37.0 4.3 2.1 EXAMPLE 2 AND COMPARATIVE EXPERIMENT 5 Goethite which has been manufactured in alkaline medium and has a specific surface area of 29.6 m2/g is reduced to magnetite in a stream of hydrogen at 3800C.

In each case 350 parts of this magnetite are suspended in 7,000 parts of water. An aqueous solution of 42 parts of CoC12.6 H20 is added to this suspension and the pH is then brought to 11 with 0.5-normal NaOH. After stirring thoroughly for a further half-hour, the solid material, consisting of magnetite and cobalt hydroxide, is filtered off and washed with water until the filtrate shows a neutral reaction.

The filter cake is then stored and dried for 108 hours at 50"C and 100 mm Hg, after which it still contains 15.8 per cent by weight of Fe2+. Samples G1 to G3 are oxidized in accordance with the invention below 200"C, and samples G4 to G6, for comparison, at above 200"C, in a stream of air, until the content of Fe2+ is less than 0.5 per cent by weight. The oxidation temperature and the other measurements are shown in Table 2.

TABLE 2 Temperature Fe2+ Ms/Y MR/! MR/Ms Hc(f =1.2 g/cm3) TA value content [0C] [No] jnTm3/g] [nTm3/g] [kiloamps/m] G1 150 0.4 77 41 0.53 39.4 2.19 G 2 170 0.3 76 41 0.54 39.3 2.20 G3 190 0.3 76 40 0.53 38.8 2.23 G4 210 0.2 77 41 0.53 37.2 2.24 G5 230 0.2 76 40 0.53 36.8 2.25 G 6 250 0.2 77 41 0.53 35.8 2.24 EXAMPLE 3 Geoethite having a specific surface area of 28.5 m2/g is reduced as in Example 2. 330 parts are suspended in 6,600 parts of water. After adding 39.6 parts of CoC12.6 H2O, the pH is brought to 11.2 with 0.5-normal NaOH. After stirring vigorously for half an hour, the mixture is filtered and the product is washed and dried for 120 hours at 40 C/ 100 mm Hg.

It is then oxidized at 150"C by passing air over it (sample H). The measurements are listed in Table 3.

COMPARATIVE EXPERIMENT 6 A gocthite as described in Example 3 is heated for 3 hours at 500"C and then reduced at 450"C. After preparing a suspension of 400 parts of the resulting material in 7,000 parts of water, 10.5 parts of H3 O4 are added and after stirring vigorously for a further half-hour the product is filtered off and dried for 35 hours at 120"C/ 100 mm Hg. It is then again suspended in 7,000 parts of water, a solution of 48 parts of Cowl2.6 H20 in water is addled, the pH is brought to 11 with 0.5-normal NaOH, the mixture is stirred for half an hour and the product is filtered off, washed with water until the filtrate shows a neutral reaction, dried at 120"C/ 100 mm Hg for 36 hours and then oxidized as described in Example 3 (sample J). The measured values are listed in Table 3.

TABLE3 Sample Ms/P MR/! MR/MS Hc(f=1.2g/cm3) RFE TA [nTm3/gj [nTm3/g] [kiloamps/m] H 74 40 0.54 43.8 3.9 2.30 J 75 39 0.52 33.5 4.5 2.34 This Table shows that the results obtained when the surface of the magnetite is protected by the provision of a surface coating of the type described in GB Patent Specification 1432643 prior to precipitation of cobalt hydroxide are markedly inferior, as far as the coercive force He is concerned, to the results obtained with an unprotected magnetite.

EXAMPLE 4 Following the method described in Example 1, a mixture of magnetite and cobalt hydroxide is prepared and heated in a rotary furnace at 1500C whilst passing their through it. The oxidation time is 5 hours. Table 4 shows the values measured.

COMPARATIVE EXPERIMENT 7 The experiment is carried out as described in Example 4, except that the oxidation is carried out at 210 C instead of 1500C.

TABLE 4 Ms/f MR/Y MR/MS Hc(f = 1.2 cm3/g) TA value [nTm3/g] [nTm3/g] [kiloamps/m] Comparative 80 42 0.53 35.8 2.35 Experiment 7 Example 4 81 43 0.53 40.1 2.28 EXAMPLE 5 Using the method described in Example 1, magnetite having a specific surface area of 13.8 m2/g is modified with cobalt hydroxide, filtered off and washed. The filter cake is suspended to form a slurry which can be pumped and this is dried in a spray dryer in air, the gas inlet temperature being 147"C and the gas outlet temperature 66"C. The product is then oxidized with air at 1500C.

The measured values are listed in Table 5.

COMPARATIVE EXPERIMENT 8 The procedure described in Example 5 is followed but the resulting product is subsequently treated in air at 2500C. The measured values are listed in Table 5.

TABLE5 Sample Tempera- Ms/g MR/Y MR/MS Hc(r =1.2 g/cm3) TA ture [ C] [nTm3/g] [nTm3/gj [kiloamps/m] value Example 5 150 77 39 0.51 40.9 2.28 Comparative Experiment8 250 76 38 0.50 35.6 2.25 WHAT WE CLAIM IS: 1. A cobalt-modified gamma-iron (III) oxide having a cobalt content of from 0.5 to 10 per cent by weight, based on iron oxide, a TA value (as hereinbefore defined) of less than 2.5 and at the same time a coercive force of at least 38 kiloamps/m, measured at 250C in a saturation field of 160 kiloamps/m and at a tap density of 1.2 g/cm3.

2. A process for the manufacture of a cobalt-modified gamma-iron (III) oxide as claimed in claim 1, wherein an appropriate amount of a cobalt hydroxide precipitate is applied to an unprotected magnetite (as hereinbefore defined) produced by reducing optionally hydrated iron (III) oxide, and the resulting modified magnetite is oxidized in an oxygen-containing atmosphere, provided that a temperature of 200"C is not exceeded after formation of the modified magnetite.

3. A process as claimed in claim 2, wherein the oxidation is carried out at from 120 to 195"C.

4. A process as claimed in claim 2 or 3, wherein the magnetite is produced by direct reduction of goethite or by reduction of the alpha-iron (III) oxide obtained by dehydration of goethite at from 200 to 6000C, the reduction being carried out by means of gaseous hydrogen at from 300 to 400"C.

5. A process as claimed in any of claims 2 to 4, wherein a cobalt salt solution is added to an aqueous suspension of the magnetite and cobalt hydroxide is precipitated onto the magnetite by addition of alkali.

6. A process as claimed in any of claims 2 to 5, wherein the modified magnetite is dried and heated in an inert gas atmosphere prior to oxidation.

7. A process for the manufacture of a cobalt-modified gamma-iron (III) oxide carried out substantially as described in any of the foregoing Examples 1 to 5.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. by the provision of a surface coating of the type described in GB Patent Specification 1432643 prior to precipitation of cobalt hydroxide are markedly inferior, as far as the coercive force He is concerned, to the results obtained with an unprotected magnetite. EXAMPLE 4 Following the method described in Example 1, a mixture of magnetite and cobalt hydroxide is prepared and heated in a rotary furnace at 1500C whilst passing their through it. The oxidation time is 5 hours. Table 4 shows the values measured. COMPARATIVE EXPERIMENT 7 The experiment is carried out as described in Example 4, except that the oxidation is carried out at 210 C instead of 1500C. TABLE 4 Ms/f MR/Y MR/MS Hc(f = 1.2 cm3/g) TA value [nTm3/g] [nTm3/g] [kiloamps/m] Comparative 80 42 0.53 35.8 2.35 Experiment 7 Example 4 81 43 0.53 40.1 2.28 EXAMPLE 5 Using the method described in Example 1, magnetite having a specific surface area of 13.8 m2/g is modified with cobalt hydroxide, filtered off and washed. The filter cake is suspended to form a slurry which can be pumped and this is dried in a spray dryer in air, the gas inlet temperature being 147"C and the gas outlet temperature 66"C. The product is then oxidized with air at 1500C. The measured values are listed in Table 5. COMPARATIVE EXPERIMENT 8 The procedure described in Example 5 is followed but the resulting product is subsequently treated in air at 2500C. The measured values are listed in Table 5. TABLE5 Sample Tempera- Ms/g MR/Y MR/MS Hc(r =1.2 g/cm3) TA ture [ C] [nTm3/g] [nTm3/gj [kiloamps/m] value Example 5 150 77 39 0.51 40.9 2.28 Comparative Experiment8 250 76 38 0.50 35.6 2.25 WHAT WE CLAIM IS:

1. A cobalt-modified gamma-iron (III) oxide having a cobalt content of from 0.5 to 10 per cent by weight, based on iron oxide, a TA value (as hereinbefore defined) of less than 2.5 and at the same time a coercive force of at least 38 kiloamps/m, measured at 250C in a saturation field of 160 kiloamps/m and at a tap density of 1.2 g/cm3.

2. A process for the manufacture of a cobalt-modified gamma-iron (III) oxide as claimed in claim 1, wherein an appropriate amount of a cobalt hydroxide precipitate is applied to an unprotected magnetite (as hereinbefore defined) produced by reducing optionally hydrated iron (III) oxide, and the resulting modified magnetite is oxidized in an oxygen-containing atmosphere, provided that a temperature of 200"C is not exceeded after formation of the modified magnetite.

3. A process as claimed in claim 2, wherein the oxidation is carried out at from 120 to 195"C.

4. A process as claimed in claim 2 or 3, wherein the magnetite is produced by direct reduction of goethite or by reduction of the alpha-iron (III) oxide obtained by dehydration of goethite at from 200 to 6000C, the reduction being carried out by means of gaseous hydrogen at from 300 to 400"C.

5. A process as claimed in any of claims 2 to 4, wherein a cobalt salt solution is added to an aqueous suspension of the magnetite and cobalt hydroxide is precipitated onto the magnetite by addition of alkali.

6. A process as claimed in any of claims 2 to 5, wherein the modified magnetite is dried and heated in an inert gas atmosphere prior to oxidation.

7. A process for the manufacture of a cobalt-modified gamma-iron (III) oxide carried out substantially as described in any of the foregoing Examples 1 to 5.

8. A cobalt-modified gamma-iron (III) oxide as claimed in claim 1 when manufactured

by a process as claimed in any of claims 2 to 7.

9. Magnetic recording media manufactured with a cobalt-modified gamma-iron (III) oxide as claimed in claim 1 or 8. - -