JP2945456B2 - Method for producing ferromagnetic iron oxide particles containing cobalt - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention has a high coercive force which is suitable as magnetic iron oxide particles for high density recording, has a good coercive force distribution, and has an erasure effect. The present invention relates to a method for producing cobalt-coated magnetic iron oxide particles having excellent characteristics and transfer characteristics.

[Conventional technology]

In recent years, as the size and weight of magnetic recording / reproducing devices have been reduced, the need for higher performance for magnetic recording media such as magnetic tapes and magnetic disks has been increasing. That is, the recording density characteristics are improved.

In order to improve the recording density characteristics of the magnetic recording medium, it is necessary that the magnetic material particles used have as high a coercive force as possible. This fact can be explained, for example, by the IEICE “Technical Research Report of the Institute of Electronics and Communication Engineers” MR77-36 (issued in 1978), and in Section 37, “In order to increase the recording density of magnetic tapes, It is necessary to increase the coercive force. "

At present, so-called Co-doped needle-like magnetic iron oxide particles and so-called Co-coated magnetic iron oxide particles are known as magnetic iron oxide particles having a high coercive force. The coercive force tends to increase as the amount of Co increases. In the former, a Co-containing acicular goethite particle is generated by adding a Co salt in advance in a reaction for producing acicular goethite particles as a starting material, and then reduced to obtain a Co-containing acicular magnetite particle. By further oxidizing to form Co-containing acicular maghemite particles, the latter reduces acicular goethite particles as a starting material or, if necessary, further acquires acicular magnetite particles or acicular maghemite particles obtained by further oxidation. It is obtained as a precursor particle by coating the particle surface of the precursor particle with a Co compound.

On the other hand, since a magnetic recording medium is used repeatedly for a long period of time, it is strongly demanded that its magnetic properties be stable over time and over time, and that it have excellent erasing properties.

In order to satisfy the above requirements for the magnetic recording medium, the magnetic properties of the magnetic iron oxide particles used are thermal,
It must be stable over time and have excellent erasing characteristics.

[Problems to be solved by the invention]

Magnetic iron oxide particles having a high coercive force, and being stable over time and thermally, and having excellent erasing characteristics,
Most demanded, but as mentioned above Co
Doped magnetic iron oxide particles have a high coercive force, but on the other hand, the coercive force distribution becomes wider due to the diffusion of Co into the crystal, and as a result, thermal and temporal And has the disadvantage that the erasing characteristics are poor.

This phenomenon is due to the fact that "Co-solid solution type (doped type) iron oxide magnetic powder in the" Technical Research Report of the Institute of Electronics and Communication Engineers, " In addition, it has a major drawback of poor erasing characteristics.
It is believed that these disadvantages are due to Co ions moving within the crystal even at room temperature. ".

Also, as described above, Co-coated magnetic iron oxide particles are:
At the same time as having a high coercive force, it is thermally and temporally stable and has excellent erasing characteristics as compared with Co-doped magnetic iron oxide. This phenomenon can be explained by the fact that “‥‥ Co epitaxial (Co-coated) iron oxide magnetic powder has a double structure, which eliminates these drawbacks, It is stable over time, and a tape using this magnetic powder has excellent transfer characteristics and erasing characteristics.

However, recently, there has been no end to the demand for improving the erasing characteristics, and even in the Co-coated magnetic iron oxide particles, the spread of the coercive force distribution is large, and it cannot be said that the erasing characteristics are excellent. It is pointed out.

This fact is described, for example, in Japanese Patent Application Laid-Open No.
磁性 In the magnetic powder using the above γ-Fe ₂ O ₃ particles, the coercive force distribution expands as the γ-Fe ₂ O ₃ particles become fine particles. It has been found that it tends to spread further.と If the cobalt-adhered γ-Fe ₂ O ₃ particles are refined for high-density recording, even if a predetermined coercive force Hc is obtained, the magnetic powder with poor coercive force distribution and poor erasing characteristics is obtained. I can only get it. ‥
‥ ”.

The spread of the coercive force distribution of the Co-coated magnetic iron oxide particles increases as the amount of Co deposited increases, and as a result, the erasing characteristics tend to deteriorate, which is inversely correlated with the improvement of the coercive force. In a relationship.

On the other hand, the technical means for controlling the addition time of the ferrous salt and the addition time of the cobalt salt in the above-mentioned Japanese Patent Application Laid-Open No. Sho 61-17426 shows that the improvement of the coercive force distribution is still insufficient. No. 295441, "‥‥ As a deposition method, a method of first depositing ferrous iron and then depositing cobalt is preferable, and a deposition method which has been favorably used as a means for increasing the coercive force until now. It has been found that the operation of heating to a temperature of 50 ° C or higher during the deposition and aging reaction causes the coercive force distribution to broaden. This is considered to indicate that the iron oxide proceeds not only to the surface but also to the inside of the iron oxide. If the cobalt to be deposited next is added in such a state, the cobalt is also moved to the inside. It is easy to diffuse and partially magnetite and edge It is thought that non-uniform reaction products of the tilt easily occur, and such a phenomenon leads to deterioration of coercive force distribution due to generation of an ultra-high coercive force component which causes deterioration of erasing characteristics and deterioration of other magnetic characteristics. "".

Further, ferrous iron addition in JP-A-63-295441 →
In the technical means of the partially overlapping deposition method of ferrous and cobalt addition → cobalt addition, "‥‥ Ferrous hydroxide is first adsorbed on the surface of magnetic iron oxide to convert to magnetite Although the reaction occurs, since the temperature is as low as 30 ° C. or less, the reaction is stopped only in a very thin layer on the surface, and a layer in the state of adsorbed unreacted ferrous hydroxide is formed. The adhered formation layer is dried and finished to complete an inner layer that is an iron oxide layer containing cobalt and an outer layer that is a cobalt compound layer on the surface of the magnetic iron oxide.
As described in “‥”, it is treated at a temperature of 30 ° C. or less through deposition and aging, and dried at about 100 to 150 ° C.

However, due to the dry heat treatment at 100 to 200 ° C. during and after drying, the unreacted layer of ferrous hydroxide is in contact with the same iron oxide, magnetite, rather than reacting with cobalt hydroxide. Therefore, it is considered that it is more easily absorbed by the magnetite layer than cobalt and diffuses into the magnetic iron oxide particles.

Therefore, thermal and temporal stability are not sufficient, and
The coercive force distribution is also inadequate.

As described above, cobalt-coated magnetic iron oxide particles having a high coercive force, a good coercive force distribution, and excellent erasing characteristics and transfer characteristics are affected by heat treatment such as drying. There is a strong demand for the establishment of a technical means that is stable over time without heat.

[Means for solving the problem]

The present inventor has made various studies on the above-mentioned technical problems, and as a result, has arrived at the present invention.

That is, the present invention provides a suspension having a pH exceeding 11 by adding an aqueous solution of alkali hydroxide to an aqueous dispersion of acicular magnetic iron oxide particles, and adding the suspension to a non-oxidizing atmosphere in a temperature range of 30 to 40 ° C. Then, a magnetite layer is formed on the surface of the needle-shaped magnetic iron oxide particles by adding an aqueous solution of ferrous salt containing ferrous iron in a range determined by the following formula (1) and performing stirring treatment. Add an aqueous solution of cobalt salt to the suspension
By performing a stirring treatment in a temperature range of 30 to 40 ° C., a cobalt compound layer is formed on the magnetite layer.
By performing a stirring treatment in a temperature range of 40 ° C., a cobalt compound layer and an iron compound layer are formed on the magnetite layer, and then the suspension is heated to a temperature exceeding 70 ° C. and less than the boiling point in a non-oxidizing atmosphere. After being stirred in a temperature range, separately, washed with water and dried to obtain cobalt-containing ferromagnetic iron oxide particles whose surface is coated with a spinel-type ferrite layer containing cobalt and iron on a magnetite layer. A method for producing a cobalt-containing ferromagnetic iron oxide particle powder, characterized by the following formula: (1): 100 (S × M) / (B × A) ≧ x (wt%) It is.

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

As the acicular magnetic iron oxide particles in the present invention, it is preferable to use acicular or spindle-shaped maghemite particles, and in some cases, magnetite particles and a beltlite compound (FeO _x .Fe ₂ O ₃₀ < _x ≦ 1). ) Particles and these
One or two of Ni, Si, Al, Zn, P, Ba, Sr, Ca, Pb, etc.
Particles containing more than one species can also be used.

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

As the aqueous cobalt salt solution in the present invention, an aqueous solution of cobalt sulfate, cobalt chloride, cobalt nitrate or the like can 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 amount of ferrous iron that forms a magnetite layer in the acicular magnetic iron oxide particles in the present invention is 100 (S × M) / (B × A) ≧ Fe in terms of Fe with respect to the acicular magnetic iron oxide particles. x
(Wt%). x> 100 (S × M) / (B × A) (wt
%), The magnetite is formed not only on the surface of the nucleus but also relatively inside the nucleus, which may lead to deterioration of crystallinity of the nucleus. However, when x <0.1 (wt%), the effect of forming the magnetite layer on the surface of the nucleus is small, and even if the effects of improving the coercive force distribution, the erasing characteristics, and the transfer characteristics are obtained, they are negligible.

The stirring process for generating the magnetite layer in the present invention is performed in a temperature range of 30 to 40 ° C. When the temperature is lower than 30 ° C., even if a required amount of an aqueous ferrous salt solution is added, the reaction rate is low, and the magnetite layer on the surface of the nucleus crystal may not be sufficient. If the temperature is higher than 40 ° C., the magnetite may be formed inside the nucleus in the formation of the magnetite layer on the surface of the nucleus in the initial stage, which is not preferable.

Hereinafter, the treatment is performed in a temperature range of 30 to 40 ° C. until the particle surface forms a cobalt compound layer and an iron compound layer on the magnetite layer.

The stirring treatment for generating the magnetite layer in the present invention can be selected from the range of 10 to 120 minutes, and the surface of the nucleus crystal can be formed into a thin magnetite layer (11 of a magnetite structure).
(1) Equivalent to the surface spacing).

The required time depends on the surface area of the iron oxide particles serving as nuclei, the reaction temperature and the amount to be magnetized.
At a low temperature of 30 to 40 ° C., the reaction rate is low, so that even if a required amount of an aqueous ferrous salt solution is added, a sufficient magnetite layer may not be formed on the nucleus crystal surface. When the time exceeds 120 minutes, the progress of the reaction hardly occurs even if it takes more time, and its industrial significance is small. In practice, a desirable range is 30 to 60 minutes.

The addition amount of the aqueous cobalt salt solution in the present invention is an amount exceeding 0.1% by weight, preferably 0.5% by weight, in terms of Co with respect to the acicular magnetic iron oxide particles.

In order to obtain a high coercive force, it is sufficient to add a large amount of Co, and there is no particular limitation. Usually, it is added up to about 10 wt%.

In the present invention, the amount of the ferrous salt aqueous solution added again to the suspension containing the needle-shaped magnetic iron oxide particles whose particle surfaces are coated on the magnetite layer with the cobalt compound layer depends on the amount of the needle in the liquid. 2.Fe-based magnetic iron oxide particle powder 2.
0 to 15.0 wt%, preferably 4.0 to 10.0 wt%.
If the content is less than 2.0 wt%, the coercive force will subsequently deteriorate, and the electrical resistance of the obtained magnetic powder tends to increase. If it exceeds 15.0 wt%, the needle-like properties as magnetic powder will be greatly deteriorated. Further, there is a possibility that the magnetic distribution represented by the erase characteristic, the transfer characteristic and the coercive force distribution may be deteriorated.

The stirring process in this case is preferably selected from a range of 30 to 120 minutes. If the time is less than 30 minutes, there is a possibility that the mixture cannot be sufficiently mixed. Even if it exceeds 120 minutes, there is no industrial significance.

The heating for spinelization in the present invention is performed in a temperature range of more than 70 ° C. and less than the boiling point. When the temperature is lower than 70 ° C., spinel formation of cobalt and iron is unlikely to occur on the thin magnetite layer on the surface of the nucleus, and it takes a long time to obtain a desired coercive force. When the temperature exceeds the boiling point, spinel formation occurs, but an apparatus such as an autoclave is required.

In this case, it is preferable to select the stirring process from a range of 30 to 900 minutes. If the time is less than 30 minutes, spinel formation hardly occurs, and if it exceeds 900 minutes, there is no industrial significance. A practically desirable range is 30 to 600 minutes.

Each addition and each stirring treatment in the present invention are preferably performed in a non-oxidizing atmosphere. Under an oxidizing atmosphere, there is a possibility that magnetite particles independent of needle-like magnetic iron oxide particles may be generated. In particular, care must be taken when first forming a magnetite layer on the surface of the acicular magnetic iron oxide particles. The non-oxidizing atmosphere is desirably performed under a flow of an inert gas such as N2 or Ar gas.

[Action]

First, the most important point in the present invention is that the surface of the needle-like magnetic iron oxide particles has a size of about 5 ° (magnetite structure (11
1) Generate a magnetite layer with a thickness less than or equal to the plane spacing), then sequentially generate a cobalt compound layer and an iron compound layer, and control the diffusion of the cobalt compound into the magnetic iron oxide particles by the magnetite layer. In addition, the diffusion of the iron compound layer added to obtain a higher coercive force into the inside of the magnetic iron oxide particles by the cobalt compound layer inside is controlled, and then, the surface of the particle is coated with cobalt on the magnetite layer. By heating and stirring the acicular magnetic iron oxide particles covered with the compound layer and the iron compound layer,
Since the cobalt compound and the iron compound can be reacted without being diffused into the inside of the particles by the magnetite layer, a desired spinel ferrite containing cobalt and iron can be obtained. Therefore, it is a fact that cobalt-coated magnetic iron oxide particles having a high coercive force and a good coercive force distribution can be obtained.

In addition, since the treatment is performed at a high temperature exceeding 70 ° C., it is not affected by heat treatment such as drying, so that it is stable over time and thermally.

The present invention will be described in detail with reference to FIG. 1 from among many experiments performed by the present inventors.

FIG. 1 shows that a needle-like γ-Fe ₂ O ₃ particle powder is dispersed in water, an aqueous alkali hydroxide solution is added to form a suspension having a pH of more than 11, and a non-oxidizing atmosphere is formed. Was set to 32 ° C.

An aqueous solution of FeSO ₄ was added to the suspension at a different ratio to the needle-like γ-Fe ₂ O ₃ particles, and the mixture was stirred for 10 to 60 minutes after the addition.

Next, 2.6 wt% of a CoSo ₄ aqueous solution was added to the needle-like γ-Fe ₂ O ₃ particles in terms of Co, followed by stirring.

Further, the total amount of Fe added to the aqueous solution of FeSO ₄ is
An amount obtained by subtracting the first generation amount of the magnetio layer from 8.5 wt% (the amount added to the needle-like γ-Fe ₂ O ₃ particles in terms of Fe) was added, and a stirring treatment was performed for 30 minutes after the addition.

Subsequently, the mixture was heated and stirred at a temperature of 100 ° C. for 5 hours in a non-oxidizing atmosphere, and then separated, washed with water, and dried using a cobalt-coated magnetic iron oxide particle powder obtained by a conventional method.
Coercive force distribution (SFD) by preparing each sheet sample
Is measured, and the amount of ferrous iron that produces the magnetite layer is
It is a diagram showing the relationship with SFD.

As a result, it has been found that an excellent coercive force distribution can be obtained by generating a magnetite layer in an amount equal to or less than the amount indicated by the broken line.

From the amount corresponding to the broken line, the thickness of the coating of the acicular γ-Fe ₂ O ₃ particles is about 5 ° (corresponding to the spacing between the (111) planes). is there.

Next, the squareness ratio (Br / Bm) of the sheet sample of the cobalt-coated magnetic iron oxide particles obtained in FIG. 1 was measured. As shown in FIG. 2, the squareness ratio was proportional to the SFD. Has also been obtained.

From the above results, when the amount of ferrous salt shown in Examples in the technical means disclosed in JP-A-61-17426 was added, the coercive force distribution in the present invention was not obtained,
Also, when the amount of ferrous salt shown in Examples in the technical means disclosed in JP-A-63-295441 was added, the magnetite layer and the ferrous hydroxide layer were in contact with each other, so It turns out that the coercive force distribution in the invention cannot be obtained.

Further, when the addition amount of the ferrous salt shown in the above two publications is smaller than the amount for forming the magnetite layer of the present invention, only the cobalt compound layer is formed on the magnetite layer, and the present invention No cobalt-coated magnetic iron oxide particles having a high coercive force and a good coercive force distribution as shown in (1) can be obtained.

〔Example〕

 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 of values measured from electron micrographs, and the specific surface area is measured by a BET method. The values are shown below. The magnetic properties of the needle-shaped magnetic iron oxide particles were measured using an "oscillating sample magnetometer VSM-3S-15" (manufactured by Toei Kogyo Co., Ltd.) up to an external magnetic field of 10KOe.

The coercivity distribution (SFD) was measured using a sheet sample.
The derivative circuit of the magnetometer was used to obtain a derivative curve of the coercive force, the half width of this curve was measured, and the value was obtained by dividing the peak value of this value by the coercive force.

The erasing characteristics are shown by values measured in accordance with "Method for Measuring Erasing Magnetization of Magnetic Powder" described on pages 152 to 153 of the Association of Powder and Powder Metallurgy "Summary of the Spring Meeting of 1986". That is, the erasing characteristics are as follows:
After measuring the residual magnetization Mr, and then setting it in the erasing device to apply an erasing magnetic field from 800 Oe to zero, the residual magnetization Me was measured and indicated as a value of 20 log Me / Mr (dB).

The transfer characteristics are shown as values obtained by inserting the measured transfer value and the major axis diameter into the following equation and correcting the major axis diameter to 0.2 μm.

Q = 40 × (0.2−C) + D The actual measurement values are based on the results of “Powder and Powder Metallurgy” (1979), Vol. 26, No. 4, page 149, published by the Japan Society of Powder and Powder Metallurgy, and “The Technical Report of the Institute of Electronics and Communication Engineers,” MR77-27.
This was performed according to the method described on page 2. That is, 6mm in diameter,
50O magnetic iron oxide particles packed in a 5 mm high cylindrical container
After magnetizing by holding at 60 ° C. for 80 minutes in the magnetic field of e, cooling to room temperature, measuring the remanent magnetization Irp, then applying a DC magnetic field to the sample to obtain the saturation remanence value Irs, the following equation It is calculated by

Transfer measured value PT = −20log Irp / Irs Sheet sample was prepared by adding the following components to a 100 cc polybin at the following ratio, and then mixing and dispersing for 8 hours with Red Devil to prepare a magnetic paint. It was obtained by coating on a 25 μm polyethylene terephthalate film with an applicator to a thickness of 50 μm, and then drying in a magnetic field of 3 to 5 KGauss.

3 mmφ still ball 800 parts by weight Co-coated 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 γ-Fe ₂ O ₃ particle powder (average major axis diameter 0.22 μm, axial ratio (major axis / minor axis) 8, BET specific surface area 37 m ² / g, coercive force 355 Oe)
800 g of an aqueous dispersion obtained by dispersing in 10
A 3000 ml mol / NaOH aqueous solution was added to prepare a suspension having a pH of 14 or more. A 10 / min N ₂ gas was flowed to make a non-oxidizing atmosphere, and the temperature of the suspension was set to 32 ° C. Thereafter, the temperature was maintained at 32 ° C.

68 ml of a 1.8 mol / FeSO ₄ aqueous solution (Fe amount:
It corresponds to 0.85 wt% with respect to the acicular γ-Fe ₂ O ₃ particles. ) Was added in 5 minutes, and stirring was continued for 20 minutes after the addition to produce dark brownish precipitated particles.

To this suspension, 235 ml of a 1.5 mol / aqueous solution of CoSO ₄ (the amount of Co corresponds to 2.6 wt% based on the acicular γ-Fe ₂ O ₃ particles) was added over 10 minutes.

Subsequently, 618 ml of a 1.8 mol / FeSO ₄ aqueous solution (the Fe content is equivalent to 7.65 wt% with respect to the acicular γ-Fe ₂ O ₃ particles).
Was added over 5 minutes, and stirring was continued for 30 minutes after the addition.

Further, the temperature was raised to 100 ° C. in a non-oxidizing atmosphere, kept for 5 hours, and heated and stirred to produce black-brown precipitate particles.

The above black-brown precipitate particles were separately washed with water and dried by a conventional method to obtain black-brown particle powder.

As a result of fluorescent X-ray diffraction, the obtained black-brown particle powder was a Co-coated magnetic iron oxide particle powder in which a spinel-type ferrite layer containing Co and Fe was formed on the magnetite layer.

The obtained Co-coated magnetic iron oxide particle powder has an average major axis diameter.
0.22μm, axial ratio (major axis diameter / minor axis diameter) 7, coercive force 665Oe,
Saturation magnetization value 78.2 emu / g, erase characteristic 55.0 dB, transfer characteristic 57.1 d
B.

Using the obtained Co-coated magnetic iron oxide particle powder, a sheet sample was prepared and the sheet properties determined were as follows: coercive force 667O
e, squareness ratio (Br / Bm) 0.844, coercive force distribution (SFD) 0.38
1. The electric resistance was 8.6 × 10 ⁹ Ω / sq.

Examples 2 to 4, Comparative Examples 1 to 6 Types of magnetic iron oxide particles to be treated, temperature of generation treatment,
Amount of ferrous salt aqueous solution for generating magnetite layer and stirring time after addition, amount of cobalt salt aqueous solution, amount of ferrous aqueous solution for forming iron compound layer and stirring time after addition, and spinel type ferrite layer forming treatment Co-coated magnetic iron oxide particles were obtained in the same manner as in Example 1 except that the temperature and the stirring time were variously changed.

Tables 1 and 2 show the main production conditions and various characteristics at this time.

[Effects of the Invention] The cobalt-containing ferromagnetic iron oxide particles produced according to the present invention have a high coercive force, as shown in the preceding examples, and have a good coercive force distribution, and furthermore, an erasing property, It is suitable for high-density recording because it has excellent transfer characteristics and is stable over time and thermally, and also has an excellent squareness ratio.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the amount of ferrous iron for forming a magnetite layer and the coercive force distribution (SFD) after forming cobalt-coated magnetic iron oxide particles. FIG. 2 is a graph showing the relationship between the amount of ferrous iron that forms a magnetite layer and the squareness ratio (Br / Bm) of the cobalt-coated magnetic iron oxide particles.

Claims (1)

(57) [Claims] An aqueous suspension of needle-like magnetic iron oxide particles is added with an aqueous alkali hydroxide solution to form a suspension having a pH of more than 11, and the suspension is added to the suspension in a non-oxidizing atmosphere at a temperature of 30 to 40 ° C. By adding a ferrous salt aqueous solution containing ferrous iron in a range required by the following formula (1) and performing a stirring treatment, a magnetite layer is formed on the surface of the needle-like magnetic iron oxide particles, Add an aqueous solution of cobalt salt to the suspension and add 30-40 ° C
By performing a stirring treatment in a temperature range of, a cobalt compound layer is formed on the magnetite layer, and then, an aqueous ferrous salt solution is added again to the suspension and stirred in a temperature range of 30 to 40 ° C. By performing the treatment, a cobalt compound layer and an iron compound layer are formed on the magnetite layer, and then the suspension is stirred under a non-oxidizing atmosphere at a temperature range of more than 70 ° C. and less than the boiling point. Separately, washing with water and drying to obtain cobalt-containing ferromagnetic iron oxide particles whose particle surface is coated on a magnetite layer with a spinel-type ferrite layer containing cobalt and iron. Manufacturing method of magnetic iron oxide particles. Formula (1): 100 (S × M) / (B × A) ≧ x (wt%)