EP0024694B1 - Process for preparing ferromagnetic acicular iron particles, and their use - Google Patents

Description

The invention relates to a process for the production of acicular ferromagnetic iron particles by tempering a goethite provided with a shape-stabilizing surface coating to form α-iron (III) oxide and reduction with hydrogen at 275 to 425 ° C., and to the use of the iron particles thus obtained as a magnetic material in the production of magnetic recording media.

Because of their high saturation magnetization and the high coercive field strength achieved, ferromagnetic metal powders and metal thin layers are of particular interest for the production of magnetic recording media, since in this way the energy product and the information density can be increased considerably and such recording media compared to the current state result in narrower signal widths and better signal amplitudes .

When using needle-shaped ferromagnetic metal powders as magnetizable materials in the production of magnetic recording media, in contrast to homogeneous metal thin layers, the mechanical properties of such information media can be influenced within a wide range by a suitable selection of the polymeric organic binder systems, but the magnetic properties are then excluded to place further demands on the shape, size and dispersibility of the metal particles.

High coercive field strength and high remanence are prerequisites for materials for magnetic storage layers. Therefore, the corresponding metal particles must exhibit magnetic single-range behavior. In addition, the anisotropy that is present or can additionally be achieved by the magnetic alignment in the strip should be caused by external influences, such as, B. temperature or mechanical stress, to be little affected, ie the small particles should be shape-anisotropic, in the preferred case needle-shaped, and they should generally be between 10 and 10 ³ nm in size.

It is known to produce iron particles of the type described by reducing finely divided acicular iron compounds, such as the oxides, with hydrogen or another gaseous reducing agent. In order for the reduction to take place at a practical speed, it must be carried out at temperatures above 300 ° C. However, this brings with it the difficulty that the metal particles formed sinter. As a result, however, the particle shape no longer corresponds to that required for the magnetic properties.

To reduce the reduction temperature, it has already been proposed to catalyze the reduction by applying silver or silver compounds to the surface of finely divided iron oxide (DE-OS 2014500). The treatment of iron oxide with tin (11) chloride has also been described (DE-OS 1 907 691).

However, the catalytic acceleration of the reduction of preferably needle-shaped starting compounds generally results in needles which are far smaller than the starting product, and which also has a low length / thickness ratio. As a result, the end product has a fairly large particle size spectrum. However, it is known that the particle size dependence of coercive force and remanence in magnetic substances is very strong in the order of magnitude of the single-range particles. If there are also the influences which arise from a proportion of superparamagnetic particles which can arise as fragments in the above-mentioned procedure, such magnetic materials are unsuitable for use in the production of magnetic recording media. In such heterogeneous mixtures, the magnetic field strength, which is necessary to remagnetize the particles, is very different, and the distribution of the remanent magnetization as a function of the applied external field also results in a less steep remanence curve.

Attempts could also be made to provide the iron oxides to be reduced with a surface coating layer in order to prevent the sintering of the individual particles which occurs due to the required reduction temperature, such as e.g. B. in DE-OS 2 434 058, 2 434 096, 2 646 348 and 2 714 588, do not fully satisfy.

The object of the invention was therefore to provide a method for producing acicular ferromagnetic iron particles, with which it is easy to produce distinctly shape-anisotropic particles with high values for coercive field strength and in particular remanence and relative remanence.

It has now been found that acicular ferromagnetic iron particles can be obtained by reacting an aqueous solution of an iron (II) salt with aqueous solutions of alkali metal hydroxides, oxidizing the resulting suspensions of iron (II) hydroxide with oxygen-containing gases to goethite, and applying a shape-stabilizing coating on the surface of the goethite, tempering the goethite treated in this way to the x-iron (111) oxide and then reducing it with hydrogen at 275 to 425 ° C to acicular ferromagnetic iron particles with the required properties if the goethite provided with a shape-stabilizing coating is used 250 to 450 ° C in a water vapor atmosphere with a water vapor partial pressure of is annealed at least 30 mbar for 10 minutes to 10 0 hours.

In a particularly advantageous manner, the goethite provided with a shape-stabilizing coating is annealed for 10 minutes to 10 hours at 250 to 450 ° C. in a water vapor-containing atmosphere with a water vapor partial pressure of 30 to 1013 mbar.

The production of this goethite used in the process according to the invention by the so-called alkaline process is known and is described in detail, for example, in DE-AS 1 204 644, 2 550 225, 2 550 307 and 2 550 308. These goethite needles are characterized by a BET specific surface area of 20 to 75 m ² / g, an average particle length between 0.2 and 1.5 and preferably between 0.3 and 1.2 μm and a length-to-thickness Characterized ratio of at least 10, advantageously 10 to 40.

These goethite particles required for the process according to the invention are now provided in a known manner with a shape-stabilizing surface coating which helps to maintain the outer shape during the further reworking steps. The treatment of goethite with an alkaline earth metal and a carboxylic acid or another organic compound which has at least two groups capable of chelating with the alkaline earth metal is suitable for this purpose. These processes are described in DE-OS 2434058 and 2434096.

Also known and described in DE-OS 2 646 348 is the shape-stabilizing finishing of goethite on its surface with hydrolysis-resistant oxygen acids of phosphorus, their salts or esters and aliphatic mono- or polybasic carboxylic acids. Possible hydrolysis-resistant substances are phosphoric acid, soluble mono-, di- or triphosphates such as potassium, ammonium dihydrogen phosphate, disodium or dilithium orthophosphate, trisodium phosphate, sodium pyrophosphate and metaphosphates such as sodium metaphosphate. The compounds can be used alone or as a mixture with one another. Advantageously, the esters of phosphoric acid with aliphatic monoalcohols with 1 to 6 carbon atoms, such as. B. use tert-butyl ester of phosphoric acid. Carboxylic acids in the process are saturated or unsaturated aliphatic carboxylic acids with up to 6 carbon atoms and up to 3 acid groups, it being possible for one or more hydrogen atoms in the aliphatic chain to be substituted by hydroxyl or amino radicals. Oxidic and oxitricarboxylic acids such as oxalic acid, tartaric acid and citric acid are particularly suitable.

The goethite, which has been given a shape-stabilizing effect in the manner described, is then annealed for 10 minutes to 10 hours at temperatures between 250 to 450 ° C. in a water vapor-containing atmosphere with a water vapor partial pressure of at least 30 mbar. The end product is a needle-shaped α-iron (III) oxide provided with the surface coating formed according to the previous equipment.

This tempering can be carried out discontinuously or continuously. Reactors such as muffle furnaces, rotary tube furnaces or vortex furnaces are suitable for batch drainage. For better mixing, air, inert gases or air-inert gas mixtures can be passed over or through the stationary or moving iron oxide, these gases being loaded beforehand with the appropriate amount of water vapor. The gases or gas mixtures are expediently saturated with water vapor at temperatures between 40 ° C. and the boiling point of the water, in particular between 50 ° C. and the boiling point of the water, and are introduced into the tempering reactors in this state. The water can of course also be used in the form of steam itself or in a mixture with other gases. The tempering can be particularly favorable in continuous reactors, e.g. B. in a continuous rotary kiln, because here, in addition to the water vapor in the gas passed through, water vapor from the tempering reaction of goethite is constantly supplied in the same amount. It is therefore also possible to work here with little or no inert gas streams or air streams. After a short setting time, the corresponding required water vapor partial pressure of preferably 70 to 1013 mbar in the reaction space is reached.

To produce the needle-shaped ferromagnetic iron particles, the α-iron (III) oxide provided with a shape-stabilizing surface coating is reduced in a manner known per se with hydrogen at 275 to 425, preferably at 300 to 400 ° C. It is advisable to passivate the finely divided iron powders obtained in this way by passing an air or oxygen-inert gas mixture over them, since the pyrophoric character of the needle-shaped iron particles with a length between 0.1 to 0.8 J-Lm and a length to thickness ratio of 5 to 25: 1 can be controlled.

With the aid of the method according to the invention, it is possible to produce acicular ferromagnetic iron particles which are distinguished by a pronounced shape anisotropy. This is achieved in that the starting products are both largely dendrite-free and treated to maintain the external shape and, moreover, are good by the inventive tempering. crystallized iron (III) oxide for the subsequent reduction reaction. The resulting iron particles are characterized by markedly improved values for coercive field strength, specific remanence and relative remanence.

Are the iron particles obtained according to the invention in the usual way for the production of Magnetogram carriers used, the needle-shaped particles are particularly easy to orient magnetically, and important electroacoustic values, such as depth and height controllability, are improved.

To produce magnetic layers, the iron particles produced according to the invention are dispersed in a known manner in polymeric binders. Known compounds such as homopolymers and copolymers of polyvinyl derivatives, polyurethanes, polyesters and the like are suitable as binders for this purpose. The binders are used in solutions in suitable organic solvents, the further additives such. B. to increase the conductivity and abrasion resistance of the magnetic layers. By grinding the magnetic pigment, the binders and any additives, a uniform dispersion is obtained, which is applied to rigid or flexible carrier materials such as foils, plates or cards, the magnetic particles contained therein are aligned by a magnetic field and the layer is solidified by drying.

The process according to the invention is illustrated by the following examples and the achievable technical progress is shown by comparison tests.

The nitrogen surface S _N2 determined according to BET was primarily used to characterize the acicular iron (III) oxide hydroxides used. Electron microscope images provide information on the appearance and dimensions (L / D ratio) of the iron oxide hydroxide particles.

The magnetic values of the iron powder were measured with a vibration magnetometer at a magnetic field of 160 or 800 kA / m. The values of the coercive field strength, H _c , measured in kA / m, were based on a stuffing density of _O = 1.6 g / cm 3 in the powder measurements. Specific remanence (M _{r / p} ) and saturation (M _{m / p} ) are given in nTm ³ / g.

und

In addition to high coercive field strength Hc and high remanence, so-called remanent coercive field strength H _{R is} an important assessment variable. In the case of constant field demagnetization, half of the particles are remagnetized at the field strength H _{R in} terms of volume. It thus represents a characteristic quantity for recording processes, which in particular determines the operating point in magnetic recording. The more non-uniform the remanent coercive field strength of the individual magnetic particles in the recording layer, the wider the distribution of the magnetic fields, which can remagnetize a limited volume of the recording layer. This is particularly effective if the border area between oppositely magnetized areas should be as narrow as possible due to high recording densities or short wavelengths. To characterize the distribution of the switching field strengths of the individual particles, a value h ₅ for the total width of the remanence curve and h ₂₅ for the steepness of the remanence curve is determined from the constant field demagnetization curve. The values are determined according to

and

The number index at the letter H indicates how many of the particles are magnetized in percent.

example 1

500 parts of a goethite prepared in accordance with DE-AS 1 204 644 are suspended in 16 times the amount of water by vigorous stirring for 3 hours. For this, dissolved in 45 parts of water, 5 parts of phosphoric acid and 5 parts of oxalic acid (H ₂ C ₂ 0 ₄ - 2 H ₂ 0) are added. After stirring for a further seven hours, the solid is filtered off and dried in air at 170 ° C. The goethite treated in this way had a content of 0.9% by weight of phosphate and 0.08% by weight of carbon and a surface area (S _N2 ) of 36.9 m ² / g.

70 parts of this product are now annealed in a tube furnace with a mixture of air and water vapor at a water vapor _partial pressure ( _PH20 ) of 840 mbar at 350 ° C. The resulting surface-finished α-iron (III) oxide with a surface S _N2 of 64.2 m ² / g is then reduced in a rotary tube with hydrogen at 350 ° C within 8 hours to the needle-shaped iron. The magnetic values measured on the acicular iron particles are shown in Table 1.

Comparative experiment 1

70 parts of a goethite finished on the surface according to Example 1 are also annealed in a tube furnace at 350 ° C. for one hour, but under a pressure of 25 mbar.

The negative pressure in the reaction chamber is generated by a vacuum pump and kept constant by metering in air dried over silica gel via a vacuum valve. The resulting surface-finished α-iron (III) oxide with a surface S _N2 of 50 m ² / g is then reduced to metal in the same way as described in Example 1. The magnetic values measured on the acicular iron particles are shown in Table 1.

Example 2

70 parts of a goethite finished on the surface in accordance with Example 1 are annealed for one hour at 350 ° C. and p _H20 of 762 mbar and the resulting product is then reduced in a whirling oven at 350 ° C. for 6 hours with hydrogen (excess factor 63) to the needle-shaped iron. The pyrophoric iron particles are finally passivated by passing an air-nitrogen mixture (1% by volume oxygen) and at a temperature below 50 ° C. The magnetic properties of the pyrophoric and passivated sample are listed in Table 1. In addition, the passivated sample was examined in an external magnetic field of 800 kA / m. These results are shown in Table 2.

Comparative experiment 2

The procedure is as described in Example 2, but the surface finish of goethite is reduced without additional treatment as in Example 2. The magnetic properties of the resulting pyrophoric and passivated iron particles are listed in Tables 1 and 2.

Comparative experiment 3

500 parts of a goethite produced in accordance with the specifications of DE-AS-1 204 644 with a surface area S _N2 of 39 m ² / g are annealed in a tube furnace at 350 ° C. for one hour under a pressure of 25 mbar. The vacuum in the reaction chamber is generated by a vacuum pump and dried over silica gel by metering air via a Vakuumven _t il kept constant. The resulting α-iron (III) oxide with a surface S _N2 of 48.7 m ² / g is then reduced in the same way as described in Example 1. The magnetic values measured on the acicular iron particles are shown in Table 1.

Comparative experiment 4

500 parts of a goethite prepared according to the specifications of DE-AS-1 204 644 with a surface area S _N2 of 39 m ² / g are in a tube furnace while passing a mixture of air and water vapor with a water vapor partial pressure (pH ₂ 0) of 840 mbar annealed at 350 ° C for one hour. The resulting α-iron (III) oxide is then reduced in the same way as described in Example 1. The magnetic values measured on the acicular iron particles are shown in Table 1.

Comparative experiment 5

45 parts of an α-iron (III) oxide produced by tempering according to comparative experiment 3 are suspended in 450 parts of water with vigorous stirring. Then 0.35 part of 85% phosphoric acid and 0.5 part of oxalic acid (H ₂ C ₂ 0 ₄ - 2 H ₂ 0) are dissolved in 20 parts of water and added to the suspension. After further stirring (20 minutes), the solid is filtered off and dried in air at 170 ° C. The surface-finished α-iron (III) oxide has a surface S _N2 of 69.1 m ² / g, a phosphate content of 1.6 and a carbon content of 0.08% by weight. The subsequent reduction is carried out as described in Example 1. Finally, the pyrophoric acicular iron particles are passivated by passing an air-nitrogen mixture with 1% by volume oxygen at a temperature below 60 ° C. The magnetic properties of the pyrophoric and passivated sample are listed in Table 1.

Comparative experiment 6

45 parts of an α-iron (III) oxide produced by tempering according to comparative experiment 4 are suspended in 450 parts of water with vigorous stirring. Then 0.35 part of 85% phosphoric acid and 0.5 part of oxalic acid (oxalic acid. 2 H ₂ 0) in 20 parts of water are added to the suspension. After further stirring (20 minutes), the solid is filtered off and dried in air at 170 ° C. The surface-finished α-iron (III) oxide has a surface S _N2 of 38.8 m ² / g, a phosphate content of 1.4 and a carbon content of 0.07% by weight. The subsequent reduction is carried out as described in Example 1. Finally, the pyrophoric acicular iron particles are passivated by passing an air-nitrogen mixture with 1% by volume oxygen at a temperature below 60 ° C. The magnetic properties of the pyrophoric and passivated sample are listed in Table 1.

Example 3

800 parts of the passivated iron particles produced according to Example 2 are in a 600-volume steel cylinder mill, which contains 9000 parts of steel balls with a diameter between 4 and 6 mm, with 456 parts of a 13 percent solution of a thermoplastic polyester urethane from adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane in a solvent mixture of equal parts of tetrahydrofuran and dioxane, 296 parts of a 10 percent solution of a polyvinylformal binder containing 82 percent vinyl formal, 12 percent vinyl acetate and 6 percent vinyl alcohol units, in the solvent mixture mentioned, 20 parts butyl stearate and a further 492 Parts of the solvent mixture mentioned are mixed and dispersed for 4 days. Then another 456 parts of the specified polyester urethane solution, 296 parts of the polyvinyl formal solution used, 271 parts of the solvent mixture and 2 parts of a commercially available silicone oil are added and dispersed for a further 24 hours and by a Filtered cellulose / asbestos fiber layer. The magnetic dispersion thus produced is applied to a 11.5 μm thick polyethylene terephthalate carrier film on a conventional coating machine and, after passing through a magnetic field, is dried at 80 to 100 ° C. within 2 minutes. The magnetic layer is smoothed and compacted by pulling over heated and polished rollers at temperatures from 60 to 80 ° C. The finished magnetic layer is 3.5 µm thick. The coated film is cut into strips 3.81 mm wide.

The electroacoustic properties of these tapes are measured in accordance with DIN 45 512 with a tape speed of 4.75 cm / sec, a bias current J _H ε of 23 mA and an equalization of 70 µsec.

Table 3 shows the values for the modulation at 333 Hz (A _T ) and at 10 kHz (A _H ). The values for the magnetic tape were set to 0 dB according to comparative experiment 7.

Comparative experiment 7

Acicular iron particles produced according to comparative experiment 2 are processed into a magnetogram carrier as described in example 3. The measurement results are shown in Table 3.

Claims (3)

1. A process for the manufacture of acicular ferromagnetic iron particles by reacting an aqueous solution of an iron(II) salt with an aqueous solution of an alkali metal hydroxide, oxidizing the resulting suspension of iron(II) hydroxide with an oxygen-containing gas to give goethite, applying a shape-stabilizing coating to the surface of the goethite, heating the so-treated goethite to give alpha-iron(III) oxide and then reducing the latter with hydrogen hat 275―425°C to give acicular ferromagnetic iron particles, wherein the goethite provided with a shape-stabilizing coating is heated at 250-450°C in an atmosphere containing water vapor at a partial pressure of not less than 30 mbar for from 10 minutes to 10 hours.

2. A process as claimed in claim 1, wherein the surface of the goethite used is coated with hydrolysis-resistant oxyacids of phosphorus, or their salts or esters in an amount of from 0.2 to 2% by weight, based on the goethite, of phosphorus, and with aliphatic monobasic or polybasic carboxylic acids of 1 to 6 carbon atoms in an amount of from 0.02 to 1.2% by weight, based on the goethite, of carbon, and the so-coated goethite is then heated at 250-450°C in an atmosphere containing water vapor at a partial pressure of from 30 to 1013 mbar for from 10 minutes to 10 hours.

3. The use of the acicular ferromagnetic iron particles, manufactured in accordance with claim 1 or 2, for the production of magnetic recording media.