JP2897794B2 - Method for producing cobalt-coated magnetic iron oxide particles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a high coercive force suitable for magnetic iron oxide particles for high-density recording, has a good coercive force distribution and squareness ratio, and has excellent blackness. And a method for producing cobalt-coated magnetic iron oxide particles having excellent chemical and magnetic stability over time.

[0002]

2. Description of the Related Art In recent years, as the size and weight of magnetic recording and reproducing devices have been reduced, it has been required to improve the recording density characteristics of magnetic recording media such as magnetic tapes and magnetic disks.

In order to improve the recording density characteristics of a magnetic recording medium, the magnetic material particles used must be as fine as possible and have a high coercive force. This fact can be seen, for example, in the "Magnetic Recording Media Comprehensive Data Book" (Showa 60), pages 185 to 187, published by Sogo Electronic Research. Whether audio equipment, video equipment, or floppy disk drives, emphasis is placed on miniaturization, weight reduction, and operability based on high-density, short-wavelength recording technology. In addition, as a magnetic coating technology that matches this, a super-smooth thin film with a thickness of 1 to 2 microns using fine magnetic particles with a high coercive force was used. High-grade (HG) type using powder. In the future development of higher image quality, ultra-fine magnetic iron oxide powder is used on the tape side. Dramatic improvement in image quality is expected. ” It is as described that, even in the current,
There is no end to the demand for finer magnetic iron oxide particles.

On the other hand, video equipment usually employs a mechanism for detecting the end of a running tape with light, so that if the light transmittance of a video tape used increases, the possibility of malfunctions increases. . However, Co-coated γ-Fe _2, which is currently most frequently used for video tapes, is used.
The O ₃ particle powder has a problem that the light transmittance increases as the particle size increases.

Attempts have been made to improve the light transmittance by adding a large amount of non-magnetic filler such as carbon black. However, when a non-magnetic material such as carbon black is used, high-density recording is hindered.

Therefore, as a method for improving the light transmittance without adding a non-magnetic filler, a magnetic oxidation method in which Co is coated on the surface of acicular magnetite particles or acicular beltride compound particles having excellent blackness is used. Iron particle powders have been used.

[0007]

The above-mentioned acicular magnetite particles and acicular beltride compound particles have a surface of Co.
The technical means for applying is well known, and it is also well known that magnetic iron oxide particles having high coercive force and excellent blackness are used.

[0008] However, magnetic iron oxide particles having Co-coated on the surface of acicular magnetite particles or acicular beltride compound particles have a characteristic that the ferrous iron contained therein is gradually oxidized, and the magnetic properties of the iron oxide particles with time change. However, there is a problem that excessive deterioration occurs.

This fact is described, for example, in Japanese Patent Application Laid-Open No. Hei 3-1912.
No. 7, "‥‥ When the Fe ²⁺ / Fe ³⁺ of the ferromagnetic iron oxide powder is increased, the magnetic stability is reduced and the Hc change of the magnetic recording medium is increased. There is a problem that the conversion characteristics are also unstable, and a magnetic recording medium using ferromagnetic iron oxide powder having a large Fe ²⁺ / Fe ³⁺ has not been widely used. .

Accordingly, Co-coated magnetic iron oxide particles for the purpose of magnetic stability over time are disclosed in Japanese Patent Publication No.
44254, JP-B-63-23137, JP-A-51-23697, JP-A-59-15222.
No. 6, JP-A-61-17426, JP-A-61-17426
-252605, JP-A-62-197322 and the like.

However, in the technical means disclosed in JP-B-63-23137 and JP-A-59-152226, when the deposition treatment is performed at a low temperature as shown in the embodiments, Fe ( Since the OH) ₂ colloid is hardly adsorbed on the nucleus crystal, the magnetic stability is insufficient because the Fe (OH) ₂ colloid floats other than the nucleus crystal. In the case where the stability is improved by adding a heat treatment such as the above, the effect is still insufficient.

Further, in the technical means disclosed in Japanese Patent Application Laid-Open No. 51-23697, deposition is performed in an oxidizing atmosphere. Undesirable things such as diffusion into the nucleus crystal and an increase in coercive force distribution along with magnetic aging occur.

Further, in the technical means disclosed in Japanese Patent Publication No. 60-44254 and Japanese Patent Application Laid-Open No. Sho 62-197322, even if an oxidation treatment is performed in a liquid after the deposition, the surface of the nucleus crystal particles is not removed. Since Co or Co and Fe are deposited, Co ions diffuse into the nucleus and magnetic temporal change cannot be suppressed.

Further, in the technical means disclosed in JP-A-61-17426 and JP-A-61-252605, an aqueous solution of ferrous salt is added to add a ferrous salt solution having a boiling point of not more than 3%.
The stirring is performed for 0 minutes or more, and as shown in each example, the mixture is added at 70 ° C. and stirred at 100 ° C. When added at such a high temperature, Fe (O
Before H) ₂ colloids are uniformly mixed, non-uniformity of the reaction for rapid chemical reaction proceeds is amplified, coercivity distribution is increased.

In view of the above-mentioned problems, the present invention has a high coercive force, a good coercive force distribution, a good squareness ratio, and good electric resistance, and is excellent in light transmittance, that is, excellent in blackness. Another object of the present invention is to obtain cobalt-coated magnetic iron oxide particles having excellent chemical and magnetic stability over time.

[0016]

The above technical object can be achieved by the following method of the present invention.

That is, according to the present invention, an aqueous alkali hydroxide solution is added to an aqueous dispersion of acicular magnetite particles containing 15 to 24% by weight of ferrous iron to form an alkaline suspension. Under an atmosphere, an aqueous ferrous salt solution is added and stirred in a temperature range of 40 to 60 ° C. to form a magnetite layer on the surface of the acicular magnetite particles, and then the aqueous cobalt salt solution is added to the suspension. The spinel-type ferrite layer comprising magnetite and a cobalt compound is formed on the surface of the acicular magnetite particles by adding and stirring under heating in a temperature range of more than 60 ° C. and less than the boiling point for more than 180 minutes in a non-oxidizing atmosphere. And then
The surface of the spinel-type ferrite layer was oxidized by heating and stirring the suspension in an oxidizing atmosphere at a temperature in the range of higher than 60 ° C. and lower than the boiling point. The present invention relates to a method for producing cobalt-coated magnetic iron oxide particles, which comprises obtaining cobalt-coated magnetic iron oxide particles having excellent blackness and containing ferrous iron of at least%.

Next, various conditions for implementing the present invention will be described.

The acicular magnetite particles in the present invention include acicular or spindle-shaped magnetite particles containing about 15 to 24% by weight of ferrous iron, or beltride compound particles which are intermediate oxides between maghemite and magnetite. Can be used. These include Ni, Si, Al, Zn,
Particles containing one or more of P, Ba, Sr, Ca, Pb and the like can also be used.

As the aqueous alkali hydroxide solution in the present invention, an aqueous solution of sodium hydroxide, potassium hydroxide, aqueous ammonia or the like can be used.

The aqueous ferrous salt solution in the present invention includes:
An aqueous solution of ferrous sulfate, ferrous chloride or the like can be used.

As the aqueous solution of the cobalt salt in the present invention, an aqueous solution of cobalt sulfate, cobalt chloride, cobalt nitrate or the like can be used.

In the present invention, an aqueous alkali hydroxide solution is added to an aqueous dispersion of acicular magnetite particles to form an alkaline suspension. The addition of an aqueous alkali hydroxide solution prior to the aqueous ferrous salt solution or the aqueous cobalt salt solution to form an alkaline suspension avoids the progress of the chemical reaction due to a rapid temperature change after the addition of the aqueous ferrous salt solution. Therefore, the pH is preferably set to 11 or more.

In the present invention, the addition amount of the aqueous ferrous salt solution is 5 to 15 as Fe ^{2+ with} respect to the acicular magnetite particles.
%, And added and stirred in a temperature range of 40 to 60 ° C. to form a magnetite layer. When the content is less than 5% by weight, the magnetite layer on the surface of the nucleus crystal grains becomes thin, so that it is difficult to obtain a high coercive force when a cobalt compound is applied. Although it may exceed 15% by weight, when forming a spinel type ferrite layer with cobalt, this is practically sufficient.

When the temperature is lower than 40 ° C., the Fe (OH) ₂ colloid deposited on the surface of the nucleus crystal is slowly magnetized, and the Fe (OH) ₂ colloid floats in addition to the nucleus crystal. As in the case where the salt aqueous solution and the ferrous salt aqueous solution are added simultaneously, it is difficult to obtain magnetic stability. 60
If the temperature exceeds ℃, the reaction is rapid, and the coercive force distribution is undesirably increased because the reaction proceeds before the mixture is uniformly mixed.

In the present invention, the stirring time for forming the magnetite layer is preferably selected from the range of 5 to 120 minutes. The required time varies depending on the specific surface area of the acicular magnetite particles serving as nuclei, the reaction temperature, and the amount of ferrous salt added, so it is difficult to specify the exact time. If the reaction time exceeds 120 minutes, the reaction has already been sufficiently performed, so that it has no industrial significance.

In the present invention, the amount of the aqueous solution of the cobalt salt added is 0.1% in terms of Co with respect to the acicular magnetite particles.
The amount is more than 1% by weight, preferably more than 0.5% by weight. In order to obtain a high coercive force, it is sufficient to add a large amount of a cobalt salt, and there is no particular limitation. Usually, however, it is added up to about 10.0% by weight.

In this case, the addition temperature is not more than 60 ° C. in order to uniformly precipitate and mix the Co (OH) ₂ colloid, and the stirring time is preferably selected from the range of 10 to 60 minutes. If the time is less than 10 minutes, there is a possibility that sufficient mixing cannot be achieved. Even if it exceeds 60 minutes, there is no industrial significance.

In the present invention, the heating and stirring for forming a spinel-type ferrite layer composed of a magnetite layer and a cobalt compound on the surface of the magnetite particles is performed at a temperature higher than 60 ° C. and lower than the boiling point, preferably higher than 90 ° C. and lower than the boiling point. It is. If the temperature is lower than 60 ° C., the deposition reaction becomes extremely slow, and sufficient magnetic properties cannot be obtained. When the boiling point is exceeded, it is possible to adhere, but since an apparatus such as an autoclave is required, it is industrially preferable to carry out at a temperature below the boiling point.

The stirring time in this case is a time exceeding 180 minutes, and is preferably selected from a range of 900 minutes or less. If the time is less than 180 minutes, the adhesion is not sufficient, and if the time exceeds 900 minutes, there is no industrial significance. A practically desirable range is 180 to 600 minutes.

In the present invention, each addition and each stirring treatment for forming a magnetite layer and a spinel type ferrite layer are as follows:
Performed in a non-oxidizing atmosphere. This is for suppressing the oxidation of ferrous iron in a non-oxidizing atmosphere, effectively performing magnetite formation, and obtaining a high coercive force and excellent coercive force distribution in deposition. It is also to prevent independent generation of goethite particles and granular magnetite particles. It is also to prevent ferrous iron from being oxidized into ferric iron in each reaction, and to leave magnetic iron oxide particles having excellent blackness while leaving as much ferrous iron as possible. The non-oxidizing atmosphere is desirably performed under a flow of an inert gas such as N ₂ or Ar gas.

In the present invention, after the spinel-type ferrite layer is formed and the deposition is completed, the suspension is set to an oxidizing atmosphere, and the suspension is heated and stirred at a temperature in the range of 60 ° C. to a boiling point or lower to obtain the spinel-type ferrite. Oxidize the surface of the layer. If the temperature is lower than 60 ° C., the effect of chemical and magnetic stability over time is insufficient, and if the temperature exceeds the boiling point, an apparatus such as an autoclave is required, which is not industrially desirable.

In this case, the stirring time varies depending on the specific surface area of the particles to be treated, the reaction concentration, the flow rate of the oxidizing gas, and the like, so it is difficult to specify exactly, but it is preferable to select from the range of 30 to 180 minutes. If the time is less than 30 minutes, a sufficient oxide film may not be obtained, and if the time exceeds 180 minutes, it is not economical.

As the method, there are a method of blowing an oxidizing gas (for example, air) into the suspension and a method of adding an oxidizing agent (for example, nitric acid, sodium nitrate, sodium nitrite, etc.).

The content of ferrous iron contained in the cobalt-containing ferromagnetic iron oxide particles obtained in the present invention is 15% by weight.
That is all. If the amount is less than 15% by weight, a sufficient degree of blackness cannot be obtained, and the amount may be as high as practically up to about 24% by weight.

[0036]

As described above, it is known that the particle powder obtained by coating Co on the acicular magnetite particles is excellent in blackness, but has a problem in chemical and magnetic stability over time.

The reason is that the ferrous iron obtained by each heat treatment of heat dehydration and reduction of acicular goethite particles is 24% by weight.
The needle-like magnetite particles containing about 15 to 24% by weight of ferrite obtained by heating and oxidizing the magnetite particles contained therein and performing difficult control during the rapid oxidation reaction. This also has a high activity and causes chemical and magnetic deterioration with time in air.

The technical means for obtaining magnetic iron oxide particles having excellent chemical and magnetic stability over time even when Co deposition treatment is performed using such high-activity acicular magnetite particles is studied. Was.

The present inventor has disclosed, for example, JP-A-63-295.
No. 441, "‥‥ The rate of transformation of magnetite is
It can be seen that the gradient is clearly different at 30 ° C.
‥‥ The temperature condition is desirably 30 ° C or less throughout the deposition and aging. Attention was paid to the description “{the more the state of the adsorbed ferrous hydroxide existing without contributing to the magnetite reaction, the more}”.

Then, Fe (OH) ₂
When the colloid is adsorbed, Fe (OH) ₂ becomes Fe ₃
Unless it is transformed into O ₄ , Fe (O) ₂
H) ₂ colloid does not adsorb, so newly precipitated F
It has been found that the e (OH) ₂ colloid is floating on the surface other than the particle surface of the nucleus.

[0041] As a result of Fe (OH) ₂ is transformed into Fe ₃ O _4, newly precipitated Fe (OH) ₂ colloids were investigated temperature for one after another deposited on the particle surfaces of the seed crystals, 40
６０60 ° C. was found to be best.

A spinel-type ferrite layer of magnetite and a cobalt compound in the magnetite layer is formed by adding a cobalt salt aqueous solution, adsorbing the cobalt compound on the layer on which magnetite is multiply deposited, and stirring with heating. Even if this is done, Co ions of the cobalt compound are unlikely to diffuse into the nucleus crystal because magnetite is deposited many times.

Further, the suspension is made into an oxidizing atmosphere, and an oxidizing gas is blown thereinto and stirred at a high temperature to form a thin and uniform oxide film on the surface of the spinel-type ferrite layer, thereby providing chemical and magnetic stability over time. Was able to enhance the character.

As a method of performing the heat treatment after the Co deposition, the deposition slurry is deposited in an autoclave after the Co deposition.
Technical methods such as a method of performing a wet heat treatment at 00 to 250 ° C., a method of performing a heat treatment on a wet cake washed with water in the presence of steam, and a method of performing a dry heat treatment after drying the wet cake are also known. Have been.

However, in each of the above technical means, it is not industrially necessary to use an apparatus such as an autoclave, and it is difficult to obtain a uniform oxide film by heat treatment of a wet cake or dry heat treatment.

Therefore, a technical means by wet oxidation for blowing an oxidizing gas into the suspension after completion of the deposition reaction to obtain an oxide film is most preferable.

As described above, spinel ferrite composed of magnetite and a cobalt compound obtained by depositing magnetite on the surface of acicular magnetite particles in multiple layers, depositing a cobalt compound on the magnetite, and stirring with heating. By forming a layer and further forming a thin and uniform oxide film thereon, Co ions do not diffuse into the nucleus, and the outermost layer is formed as an oxide film, and the inside is a spinel ferrite of Co. By forming a layer, the chemical and magnetic stability over time could be significantly improved.

[0048]

Next, the present invention will be described with reference to examples and comparative examples.

The major axis diameter and the axial ratio (major axis diameter / minor axis diameter) of the particles in the following Examples and Comparative Examples are average values measured from electron micrographs, and the specific surface area is BE.
It is shown by the value measured by the T method. The magnetic properties of the needle-like magnetic iron oxide particles are described in "Vibration sample magnetometer VSM-3S-1".
5 "(manufactured by Toei Industry Co., Ltd.) and an external magnetic field of 10 KO
e.

The content of ferrous iron was measured by charging a magnetic iron oxide particle powder into a flask, replacing with an inert gas, adding a mixed acid of sulfuric acid and phosphoric acid while heating and dissolving the mixture, and then heating the solution. Was determined by a redox titration method.

ΔFe ^2; and ΔHc are values obtained by leaving the samples in a constant temperature bath at a temperature of 60 ° C. and a relative humidity of 90% for two weeks and subtracting the initial values.

For the measurement of the coercive force distribution (SFD), a differential curve of the coercive force was obtained using a sheet sample piece and a differentiating circuit of the magnetometer, and the half width of this curve was measured. Then, this value was obtained by dividing the peak value by the coercive force.

The light transmittance of the magnetic sheet was represented by a linear absorption coefficient measured using a “photoelectric spectrophotometer UV-2100” (manufactured by Shimadzu Corporation). The linear absorption coefficient is defined by the following equation, and the larger the value, the more difficult it is to transmit light. Linear absorption coefficient (μm ⁻¹ ) = ln (1 / t) / FT t: Light transmittance at λ = 900 nm (−) FT: Thickness of magnetic layer of film used for measurement (μm) Note that linear absorption coefficient is 1 If it is 0.2 or more (film thickness 4.0 μm), it is possible to satisfy the light transmittance of 0.8% or less defined by the VHS standard. Further, the linear absorption coefficient is 1.4.
If it is above, it can be said that the blackness of the Co-coated magnetic iron oxide particles is sufficiently excellent. The blackness was evaluated by the linear absorption coefficient, which is a substantial evaluation measure, as described above.

The sheet-shaped sample piece was prepared by adding a magnetic iron oxide particle powder, a resin and a solvent in the following ratio to a 100 cc polybin, and then mixing and dispersing the mixture for 6 hours with a paint conditioner to obtain a 25 μm-thick magnetic paint. Is applied to a thickness of 50 μm using an applicator on a polyethylene terephthalate film of
s in a magnetic field. 3 mmφ still ball 800 parts by weight Magnetic iron oxide particle powder 100 parts by weight Polyurethane resin having sodium sulfonate group 20 parts by weight Cyclohexanone 83.3 parts by weight Methyl ethyl ketone 83.3 parts by weight Toluene 83.3 parts by weight

Example 1 Acicular magnetite particle powder (average major axis diameter 0.22 μm,
Axial ratio (major axis diameter / minor axis diameter) 8.0, BET specific surface area 35 m
² / g, coercive force 370 Oe, ferrous iron 17.5% by weight) 8
1 g of an aqueous 18 mol / l NaOH solution was added to an aqueous dispersion obtained by dispersing 00 g in 10700 ml of water.
(Corresponding to an alkali ion concentration of 2.0 N), N ₂ gas was flowed to make a non-oxidizing atmosphere, and the temperature of the suspension was adjusted to 40 ° C. Thereafter, the treatment was performed in a non-oxidizing atmosphere. To the suspension, 446 ml of 1.8 mol / l FeSO ₄ aqueous solution (the amount of Fe corresponds to 5.6% by weight based on the powder of the acicular magnetite particles) was added, and the mixture was maintained for 5 minutes and the black-brown precipitate particles were maintained. Was generated.

1.8 mol / l CoSO was added to the suspension.
( ₄₎ 271 ml of an aqueous solution (the amount of Co corresponds to 3.6% by weight based on the powder of the acicular magnetite particles) was added over 1 minute. Subsequently, the stirring treatment was continued for 30 minutes after the addition. Further, the temperature was raised to 100 ° C. in a non-oxidizing atmosphere, kept for 240 minutes, and heated and stirred.

Next, the N ₂ gas was stopped, and air was blown into the suspension at 100 ° C. at a rate of 500 ml / min for 60 minutes to produce black precipitate particles.

The black precipitate particles were separated by filtration, washed with water and dried by a conventional method to obtain black particle powder. As a result of X-ray diffraction, the obtained black particle powder was found to contain C and ferrous iron.
o It was an adhered magnetic iron oxide particle powder.

The obtained Co-coated magnetic iron oxide particles had an average major axis diameter of 0.22 μm and an axial ratio (major axis diameter / minor axis diameter).
7.7, coercive force 687 Oe, saturation magnetization value 82.9 emu
/ G, ferrous iron 16.8% by weight. Also, ΔFe ²⁺
Was -6.3% by weight, ΔHc was -9Oe, and the deterioration with time was small.

The sheet characteristics obtained by preparing a sheet sample using the obtained Co-coated magnetic iron oxide particles were as follows:
Coercive force 690 Oe, squareness ratio (Br / Bm) 0.833,
1. Coercive force distribution (SFD) 0.381, electrical resistance
0 × 10 ⁹ Ω / sq, linear absorption coefficient (λ = 900 nm)
1.67.

Examples 2 to 8, Comparative Examples 1 to 6 The type of magnetic iron oxide particles to be treated, the temperature of the suspension, the amount of the ferrous salt aqueous solution and the stirring time after addition, the amount of the cobalt salt aqueous solution, Except for variously changing the deposition processing temperature and the stirring time after the temperature rise and the temperature and the stirring time of the air blowing,
In the same manner as in Example 1, Co-coated magnetic iron oxide particles were obtained.

Tables 1 to 3 show main manufacturing conditions and various characteristics at this time.

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

[0066]

The cobalt-coated magnetic iron oxide particles produced according to the present invention have a high coercive force, a coercive force distribution, a squareness ratio and an electric resistance, as shown in the preceding examples. It is suitable as a magnetic material for high-density recording because it has excellent blackness and excellent chemical and magnetic stability over time.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01F 1/11 C01G 49/00

Claims (1)

(57) [Claims] An aqueous alkali hydroxide solution is added to an aqueous dispersion of acicular magnetite particles containing 15 to 24% by weight of ferrous iron to form an alkaline suspension. A magnetite layer is formed on the surface of the acicular magnetite particles by adding and stirring an aqueous ferrous salt solution in a temperature range of ６０60 ° C., and then adding an aqueous cobalt salt solution to the suspension to form a non-magnetic layer. 60 in oxidizing atmosphere
By heating and stirring for more than 180 minutes in a temperature range of more than ° C and less than the boiling point, a spinel-type ferrite layer composed of magnetite and a cobalt compound is formed on the surface of the acicular magnetite particles. The surface of the spinel-type ferrite layer is oxidized by heating and stirring in a temperature range of higher than 60 ° C. and lower than the boiling point in an oxidizing atmosphere. A method for producing cobalt-coated magnetic iron oxide particles, which comprises obtaining iron-containing cobalt-coated magnetic iron oxide particles having excellent blackness.