JPH03212821A - Production of metal magnetic powder - Google Patents

Abstract

PURPOSE:To improve corrosion resistance of a metal magnetic powder by forming an oxide layer containing cobalt on the surface of the metal magnetic powder which is obtained by reduction of goethite of specified shape in a hydrogen gas flow. CONSTITUTION:Needle-type goethite is reduced by heating in hydrogen gas to produce a metal magnetic powder essentially comprising iron and cobalt, and then this metal magnetic powder is oxidated to form an oxide layer on the surface of the powder. Then the powder is heated in a solution in which Co<2+> is dissolved to incorporate cobalt into the oxide layer. For example, the metal magnetic powder having the oxide layer on its surface is dispersed in a polyvalent alcohol in which cobalt chloride is dissolved, and this suspension is heated to make a solid solution of cobalt in the surface oxide layer of the metal magnetic power. By this method, the fine metal magnetic powder having high saturation magnetization and excellent corrosion resistance can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing metal magnetic powder mainly containing iron and cobalt.

(Prior Art) Metal magnetic powder is characterized by having larger saturation magnetization and coercive force than conventional iron oxide magnetic powder, and has been put into practical use as a magnetic powder suitable for high-density recording.

However, because the surface of metal magnetic powder particles is active, it is extremely susceptible to corrosion, making it inconvenient to handle, and the output characteristics of magnetic recording media using this powder deteriorate in high temperature and humidity environments. There are drawbacks. This is clear from the fact that when a metal magnetic powder is left in an environment with a temperature of 60° C. and a humidity of 90%, the saturation magnetization rapidly decreases within a few hours.

In order to improve the corrosivity of metal magnetic powder, attempts have been made to form a passive film on the particle surface by alloying iron with a metal such as cobalt.

For example, a standard method for manufacturing metal magnetic powder is to reduce acicular goethite powder obtained from an aqueous suspension of iron salt and alkali to obtain up to 1.6 particles. A method is known in which metals such as cobalt are alloyed by adding cobalt salt or the like.

However, although the metal magnetic powder obtained using this method has a certain degree of corrosion resistance compared to standard metal magnetic powder, it has not yet reached a sufficient level of corrosion resistance. Although the reason for this is not clear, it is thought that the amount of cobalt in the metal magnetic powder is insufficient, making it impossible to form a sufficient passive film on the particle surface.

Therefore, the present inventor attempted to increase the cobalt content of the resultant product by adding an excessive amount of cobalt salt to the aqueous suspension, but the shape of the cassite would collapse, and the goethite powder would have an amorphous shape. The uniformity of the shape and composition of the goethite particles was impaired due to the presence of particles, and cobalt could not be sufficiently dissolved in the metal magnetic powder.

The inventor of the present invention conducted various studies on the cause of this problem, and found that a fundamental solution could not be achieved permanently by following the conventional method of adding cobalt salt into the suspension during goethite formation. I noticed that.

Firstly, the iron that makes up goethite is trivalent and is not equivalent to divalent cobalt, so it cannot freely undergo ion exchange reactions.Secondly, the cobalt concentration in the aqueous suspension is is thought to control the growth rate of goethite crystals. Thirdly, the shape of the goethite particles determines the shape of the metal magnetic powder particles through subsequent processing, so the formation stage of goethite particles Therefore, it is preferable that cobalt ions, which affect the crystal growth rate, do not exist.Furthermore, corrosion resistance will not be effective unless a large amount of cobalt is added, but the solid solution amount of cobalt is limited to about 7%. In addition, because cobalt is expensive, it is more preferable to dissolve cobalt only in the surface of the powder and not in the interior. Therefore, we realized that it would be desirable to change only the surface portion of the metal magnetic powder to one that has the property of receiving cobalt.

Based on this basic idea, we discovered a method in which a metal magnetic powder with a uniform particle shape is obtained in advance, and cobalt is dissolved as a solid solution in the oxide layer formed on the surface of this powder. By heating the oxidized metal magnetic powder in a solution containing Co2+, cobalt ions are diffused into the oxide layer formed on the surface of the metal magnetic powder, and C is added to the surface.

It has been found that metal magnetic powder with extremely excellent corrosion resistance can be obtained by forming a passive film of an oxide containing .

This is because the oxide layer formed on the surface of conventional metal magnetic powder for passivation contains a large amount of unstable Fe2+, whereas the surface of the metal magnetic powder of the present invention contains a large amount of unstable Fe2+. , it is believed that this is due to the uniform content of stable C02+ instead of Fe2+. That is, when Fe2+ is oxidized to Fe3+, the crystal structure changes to Fe50. a to γ-F
It is thought that the oxide becomes e2O3, vacancies are generated in the oxide layer, oxygen enters through these vacancies, and oxidation progresses. On the other hand, in the magnetic powder of the present invention, since C02+ is extremely stable and does not oxidize, it is considered that no pores are generated in the surface layer and an extremely dense layer is formed.

For example, the metal magnetic powder obtained by the present invention has a temperature of 6
After being left for 7 days in an environment of 0°C and 90% humidity, the saturation magnetization is as high as 120 emu/g or more, making it extremely resistant to corrosion. This is because a large amount of cobalt exists on the surface, forming a passive state of cobalt. The weight ratio of cobalt is 10% by weight or more based on iron, and this amount of cobalt is convenient for maintaining excellent corrosion resistance and appropriate saturation magnetization, as will be described later.

(Problems to be Solved by the Invention) This invention solves the problem that the conventional metal magnetic powder has poor corrosion resistance, and provides a fine particle metal magnetic powder with higher corrosion resistance. The purpose is to

(Means for Solving the Problems) In order to solve these problems, the present invention provides that it is particularly important to obtain a group of metal magnetic powders having uniform particle sizes and containing cobalt in the surface oxide layer. (1) First, it is effective to obtain ketite with a uniform particle size, reduce it to make a metal magnetic powder, and then incorporate cobalt into the surface oxidation layer. (2) It is effective to add cobalt to the oxidation layer on the particle surface As a result of various studies from the viewpoint that it is preferable for the solid solution reaction to proceed uniformly in order to make the solid solution uniformly in the material layer, we found that a metal obtained by reducing goethite in a predetermined shape in a hydrogen gas stream. 10% by weight or more based on iron on the surface of magnetic powder
By forming an oxide layer containing cobalt, the corrosion resistance of the metal magnetic powder is significantly improved.

In order to obtain metal magnetic powder with high saturation magnetization and excellent corrosion resistance, it is important to uniformly dissolve cobalt in the oxide layer on the surface of the metal magnetic powder. If cobalt segregates, local batteries may occur. This makes it difficult to obtain high saturation magnetization, such as inducing corrosion.

Further, in order to provide sufficient corrosion resistance, the content of cobalt must be 10% by weight or more based on iron. If it is less than 10% by weight, oxidation progresses due to the presence of Fe'', making it impossible to obtain sufficient corrosion resistance.

As a method for uniformly dissolving cobalt in the oxide layer on the surface of the metal magnetic powder, for example, a metal magnetic powder with an oxide layer formed on the surface is dispersed in a polyhydric alcohol in which cobalt chloride is dissolved. This suspension is then heated to uniformly dissolve cobalt in the surface oxide layer of the metal magnetic powder.

Examples of polyhydric alcohols include polyethylene L, Nukuba Coal,
Diethylene glycol, ethylene glycol, propylene glycol, glycerin, etc. can be used.

In addition, nickel, chromium,
It is possible to add metal elements other than iron and cobalt, such as manganese, or to coat the surface of the particles with silica or alumina.

As described above, according to the present invention, it is possible to obtain a fine particle metal magnetic powder that has high saturation magnetization, has a coercive force suitable for magnetic recording, and has excellent corrosion resistance.

(Example> Examples of the present invention will be described below.

Example 1 0.72 mol/Q of ferrous sulfate and 0.03 mol of 1.59 aqueous sodium hydroxide solution with 5 mol/Ω drift
．． / fl of a mixed solution with nickel sulfate is added at room temperature with stirring to react to obtain a co-precipitate of ferrous hydroxide and nickel hydroxide. This precipitate suspension was
While keeping the temperature at °C, air was sucked in at a rate of 1.6R/min, stirred for 8 hours, filtered, washed with water, dried, and BET 6
5 m", / g of caesite was obtained. Next, 100 g of this caesite was dispersed in water 39, and in this suspension 1 mol/g of a sodium hydroxide aqueous solution 22 of >14 degrees and 1 mol/g of aqueous sodium hydroxide solution 22 of >14 degrees were added. 26 m2 of a sodium orthosilicate T5 aqueous solution with a concentration of 2 was added and carbon dioxide gas was blown in to neutralize it until the pH reached 8, followed by washing with water and drying to deposit a silicon compound on the surface of the cassite particles.Next, this silicate compound was deposited. Goethite was dispersed in 3 g of water, 1 mol/Q?!4 degrees of sodium hydroxide aqueous solution 29 and 135 mR of a 0.5 mol/Ω concentration of sodium aluminate aqueous solution were added to this solution, and carbon dioxide gas was blown into it. After neutralization to pH 8, it was washed with water and dried, and alumina was deposited on the surface of the goethite particles coated with the silicate compound.Next, the silicate compound and alumina were coated on the goethite particles]
・After baking at 750°C for 4 hours, 4 hours in a hydrogen gas stream.
Reduction was performed at 50°C for 8 hours to obtain metal magnetic powder. This metal magnetic powder was left at 60° C. for 2 hours in a nitrogen gas flow containing 11,000 pp of oxygen to form a thin oxide layer on the surface of the metal magnetic powder.

Next, 10 g of cobalt chloride 6 permanent was dissolved in 300 mΩ of polyethylene glycol, and 20 g of the metal magnetic powder whose surface had been slowly oxidized obtained by the above method was dispersed therein.
The mixture was heated at 180° C. for 3 hours with stirring to obtain a metal magnetic powder in which cobalt was uniformly dissolved in an oxide layer on the surface of the metal magnetic powder. Next, this metal magnetic powder was washed and dried. Using the metal magnetic powder mainly composed of iron and cobalt obtained in this way, 100 parts of metal magnetic powder VAGH
(Manufactured by CU, C, C, 10 parts by weight of vinyl chloride - Vinyl acetate - vinyl alcohol copolymer) Pandex T-5201 (6 parts by weight, manufactured by Dainippon Co., Ltd., Polyurethane, manufactured by Ink Kagaku Kogyo Co., Ltd.) 5 parts by weight of myristic acid MS-
500 (manufactured by Asahi Denka, 1 part by weight of carbon, 85 parts by weight of black methyl isobutyl ketone, 85 parts by weight of toluene) was placed in a 32-solution steel ball mill,
This was rotated for 72 hours to ensure good dispersion and prepare a magnetic paste. Then add 40% toluene to this magnetic paste.
parts by weight and 2 parts by weight of Coronate L (manufactured by Teishi Pharmaceutical Co., Ltd., a trifunctional low molecular weight isocyanate compound),
A magnetic paint was prepared. This magnetic paint was applied onto a polyester film with a thickness of 12 μm so that the coating thickness after drying was 4 μm, dried, and mirror-finished.
I cut it into 2-inch width pieces to make magnetic tape.

Example 2 In Example 1, the amount of cobalt chloride hexahydrate added was 10
A metal magnetic powder and a magnetic tape were obtained in the same manner as in Example 1 except that the amount was changed from 5 g to 5 g.

Example 3 In Example 1, the amount of cobalt chloride hexahydrate added was 10
A metal magnetic powder and a magnetic tape were obtained in the same manner as in Example 1, except that the amount was changed from g to 15 g.

Example 4 In Example 1, 1.52 to lΩ of nickel sulfate
A metal magnetic powder and a magnetic tape were obtained in the same manner as in Example 1 except that the following was changed.

Comparative Example 1 Metal magnetic powder and magnetic tape were prepared in the same manner as in Example 1, except that after providing a thin oxide layer on the surface of the metal magnetic powder in Example 1, metal magnetic powder was obtained without solid solution of cobalt. Obtained.

Comparative Example 2 In Example 1, 5 mol/2? 0, 72 mol, /Ω in 1.52 aqueous solution of sodium hydroxide at 4 degrees I
A mixed solution of 0.03 mol/Q of ferrous sulfate, 0.03 mol/Q of nickel sulfate, and 0.08 mol/Q of cobalt sulfate was added with stirring at room temperature and reacted to form ferrous hydroxide and hydroxide. A co-precipitate of nickel and cobalt hydroxide was obtained, and in the same manner as in Example 1, a silicate compound and alumina were coated on the particle surface, and the Ni and Co-containing goethite coated with the silicate compound and alumina was heated at 750°. f & was calcined at C for 4 hours and then heated and reduced in hydrogen gas at 450°C for 2 hours to obtain metal magnetic powder. A magnetic tape was made using the metal magnetic powder thus obtained in the same manner as in Example 1.

The results of the investigation on the magnetic powders obtained in the above Examples and Comparative Examples and magnetic tapes using the same are shown in the table at the end. The content of metal elements was examined using a fluorescent X-ray device, and the magnetic properties were examined using VSM.

In addition, as an evaluation of corrosion resistance performance, the saturation magnetization and saturation magnetic flux density were measured after the magnetic powder and magnetic tape were left in an environment of 60° C. and 90% humidity for one week.

(Effects of the Invention) As explained above, the metal magnetic powder that does not contain cobalt (Comparative Example 1) has inferior corrosion resistance compared to the metal magnetic powder that contains cobalt. In addition, metal magnetic powder obtained by reducing a coprecipitate of ferrous hydroxide, nickel hydroxide, and cobalt hydroxide (
Comparative Example 2) has a low coercive force and poor corrosion resistance.

On the other hand, the metal magnetic powder according to the present invention exhibits excellent corrosion resistance, and it is clear that the magnetic tape using this magnetic powder has excellent corrosion resistance.

Claims (2)

[Claims] (1) Needle-shaped goethite is heated and reduced in hydrogen gas to produce a metal magnetic powder mainly composed of iron and cobalt, and this metal magnetic powder is gradually oxidized in an oxidizing atmosphere to form oxides on the surface of the powder particles. A method for producing a metal magnetic powder, which comprises forming a layer and then heating the layer in a solution containing Co^2^+ to cause the surface oxide layer to contain cobalt. (2) The method for producing metal magnetic powder according to claim (1), wherein the solution is a polyhydric alcohol.