JPS5860624A - Ferromagnetic powder and its preparation - Google Patents

Abstract

PURPOSE:To prepare ferromagnetic powder having particular particle shape, by using ferromagnetic iron oxide powder produced by a specific method as seed crystal, dispersing the crystal in a solution containing a cobalt salt and a ferrous salt, and adding an alkaline solution to the dispersion. CONSTITUTION:Ferric hydroxide is prepared by adding an aqueous solution containing trivalent iron ions to more than equivalent amount of an alkaline aqueous solution at <=30 deg.C. After aging at about <=100 deg.C for about >=10min, the ferric hydroxide is subjected to the hydrothermal reaction at about 120-250 deg.C in an autoclave to form the powder of iron alpha-oxyhydroxide, which is filtered, dried, reduced in hydrogen stream at about 250-400 deg.C, and oxidized in air at about >=200 deg.C. The produced gamma-Fe2O3 powder is dispersed in an aqueous solution of a cobalt salt and a ferrous salt, and an alkaline aqueous solution is added to the dispersion to obtain ferromagnetic powder having a surface layer composed mainly of cobalt. The powder has a major diameter of <=300nm, an aspect ratio of <=5, a specific surface area of <=40m<2>/g, and a coercive force of >=23.9KA/m.

Description

Detailed Description of the Invention This DB relates to a ferromagnetic powder and a method for producing the same, and its purpose is to provide a cobalt-containing iron oxide ferromagnetic powder with good coercive force distribution and excellent thermal stability. 〇 Cobalt-containing iron oxide ferromagnetic powder has excellent magnetic properties such as high coercive force, so it is widely used as a recording element in high-performance magnetic recording media, and is also used as a magnet material. .

Various methods for producing and releasing such cobalt-containing iron oxide ferromagnetic powder have been proposed so far, and one of the most useful methods is to use r-Fe203 powder as a nucleus crystal, A method has been proposed in which iron oxide is dispersed in a solution containing a salt and a ferrous salt, and then an alkaline solution is added thereto to form an iron oxide layer containing mainly cobal F on the core crystals.

However, the γ-Fe205 powder used in this method is usually prepared by heating a ferrous salt and an alkaline aqueous solution at 30 to 60°C.
The raw material is α-iron oxyhydroxide powder, which is obtained by mixing and reacting at a humidity of about 100 mL, and is then heated to reduce and further oxidized. However, this stiff γ-Fe205 powder has a large axial ratio and uneven particle size distribution, so the coercive force distribution is wide.
When attempting to drill minute objects, it lacks thermal stability in transfer, heating demagnetization, etc., and the properties of cobalt-containing iron oxide using this as a nucleus crystal were also unsatisfactory.

The inventors conducted various studies to improve this drawback, and as a result, they found that γ-Fe20. In producing ferromagnetic powder, an aqueous solution phase containing trivalent iron ions is added to an alkaline aqueous solution in an amount equivalent to or more than the iron ions at a temperature of 30°C or less and reacted to produce ferric hydroxide. After aging, this is subjected to a hydrothermal reaction in an autoclave to produce α-iron oxyhydroxide powder. After filtering and drying, the resulting powder is reduced by heating and further oxidized to produce γ-Fe20. When it comes to ferromagnetic powder, it is fine, has a small axial ratio, and has a uniform particle size distribution.
Fe206 ferromagnetic powder is obtained, and this r-Fe205 ferromagnetic powder is dispersed in a solution containing a cobalt salt, and an alkaline aqueous solution is added to this to form a surface containing mainly cobalt on the surface of the r-Fe20° powder. When the layer is formed, the major axis diameter is 300 nm or less, the axial ratio is 5 or less, and the BET
Specific surface area by method is 40-/L or less, coercive force is 23.9
We have discovered that it is possible to obtain a cobalt-containing iron oxide ferromagnetic powder that has a uniform particle size distribution of KA/m or more, a narrow coercive force distribution, good thermal stability, and little transfer and heat demagnetization, and has led to the creation of this invention. .

In this invention, when adding an aqueous solution containing trivalent iron ions to an alkaline aqueous solution to produce ferric hydroxide, the reaction temperature is 30"C or higher to moderate the growth of ferric hydroxide. Since it is difficult to obtain fine α-ferric oxyhydroxide due to a uniform particle size distribution, it is preferable to carry out the process at a temperature below 30°C, and the lower the temperature, the easier it is to control the growth of ferric hydroxide. Therefore, it is more preferable to carry out the aging at a temperature of 20°C or lower.Furthermore, the aging is preferably carried out at a temperature of 100°C or lower for 10 minutes or more, usually at room temperature for 30 minutes or more, preferably for 3 to 70 hours, and the aging time is too short. The growth of the hydrated ferric iron is insufficient, and if it is not allowed to grow for a long time, the growth will proceed excessively, making it difficult to obtain a uniform particle size distribution.

The aqueous solution containing trivalent iron ions may be prepared by dissolving one or more of various soluble ferric salts such as ferric cyclization, ferric sulfate, and ferric nitrate in water. , or l'
i' Dissolve one or two or more of various soluble ferrous salts such as ferrous salts, ferrous sulfates, and ferrous nitrates in water, and then use an oxidizing agent etc. It is prepared by oxidation and used in a state containing trivalent iron ions. These iron salts have a concentration of 0.5 mol/! ! It is best to use the following aqueous solution.

The alkaline aqueous solution to which the aqueous solution containing trivalent iron ions is added is preferably a caustic alkaline aqueous solution such as sodium hydroxide or potassium hydroxide, and the amount used is such that ferric hydroxide is produced well. , and in order to make the particle size of ferric hydroxide appropriate, it is sufficient that the amount is equivalent to or more than the trivalent iron ion; Since it becomes heterogeneous and expands the particle size distribution, it is preferable to use it within a range where the free alkali concentration is 1 mol/l or less. In this way, an aqueous solution containing trivalent iron ions is mixed with an aqueous alkali solution containing an equivalent amount or more. When added to the liquid at a temperature of 30"C or less and reacted to produce ferric hydroxide, and further aged at room temperature, a suspension in which the growth of ferric hydroxide is appropriately controlled is obtained. When this suspension is placed in an autoclave and subjected to a hydrothermal reaction, the particle size distribution is uniform and fine σ particles with a small axial ratio are produced.
- Iron oxyhydroxide powder is obtained. And then this α
- After the iron oxyhydroxide powder is washed with water, filtered, and dried, it is reduced by heating at a temperature of 250 to 400°C in a stream of reducing gas, such as hydrogen, and further oxidized, for example, at a temperature of 200°C or higher in air. A fine needle-shaped γ-F e 20 s ferromagnetic powder with a small axial ratio and a uniform particle size distribution is obtained.

In the hydrothermal reaction in an autoclave to produce α-iron oxyhydroxide powder, crystallization takes a long time if the reaction is carried out at a temperature of 120°C or lower, while -Fe20. 120-250° as powder is mixed in.
It is preferable to carry out at a temperature in the range of 150 to 220 C.
It is more preferable to carry out the reaction within the range of °C.

In addition, when heating and reducing the temperature at 250°C or higher, a product with effective magnetic properties can be obtained, and the reduction is accelerated as the heating temperature increases, but when heated to 400°C or higher, sintering occurs and the coercive force decreases. It is preferable to carry out the temperature within the range of 250 to 400°C. Further, the temperature at which the oxidation is performed after heating reduction is preferably 200° C. or higher, since at low temperatures the oxidation becomes insufficient or takes a long time.

The γ-Fe205 boron powder thus obtained is
Next, by dispersing it in a solution containing a cobalt salt and a ferrous salt, and further adding an alkaline aqueous solution to this and reacting it, the resultant material has a major axis diameter of 300 nm or less, an axial ratio of 5 or less, and a specific surface area of 40 m by the BET method. '／! 7 or less, coercive force is 23.9K
Iron ferromagnetic powder is obtained from cobalt-containing acid with a uniform particle size distribution of A/m or more.

Suitable cobalt salts include cobaltous sulfate, cobaltous chloride, cobaltous sulfate, etc., and ferrous salts include ferrous sulfate, ferrous chloride, ferrous nitrate, etc. As the alkaline aqueous solution, a caustic alkali aqueous solution such as sodium hydroxide or potassium hydroxide is preferably used.

The concentration of the alkaline aqueous solution must be such that at least cobalt and ferrous hydroxides precipitate, and the reaction temperature is preferably 50° C. or lower in order to allow the reaction to proceed uniformly.

The cobalt-containing iron oxide ferromagnetic powder obtained in the above manner has a major axis diameter of 300 nm or less, an axial ratio of 5 or less, a specific surface area of 40 -/- or less by the BET method, and a coercive force of 1y.
Z of 23.9 KA/m or more, the particle size distribution is uniform, the retention force distribution is narrow, the thermal stability is good, the transfer characteristics are excellent, and there is little heating demagnetization.

Next, embodiments of the invention will be described.

Example 1 10 mol of ferric chloride (FeC1, -6H20) was added to 30 mol of water.
Prepare an aqueous solution of ferric chloride dissolved in 1 and 60 moles of sodium hydroxide dissolved in 60 moles of water, and add the aqueous ferric chloride solution to the aqueous sodium hydroxide solution at a humidity of 10°C. In addition, 1ql of brown precipitate was obtained. After aging this at room temperature for 18 hours, this suspension was placed in an autoclave and subjected to a hydrothermal reaction at 180°C for 1 hour. After the reaction is complete, wash the yellow precipitate with water, filter it,
After drying (1), σ-iron oxyhydroxide powder was obtained.

Next, the obtained α-iron oxyhydroxide powder was placed in air for 60 min.
Heated at 0°C for 1 hour to form α-Fe20. Grind the powder,
800 g of this α-Fe205 powder was spread on a quartz board, placed in a tubular electric furnace, hydrogen gas was passed through at a rate of 5/min, and reduced at 300°C to obtain Fe 504 powder. This was oxidized in air at a temperature of 250°C to obtain a major axis diameter of 200 nm, a % axial ratio of 4, and a specific surface area of 2 by the BET method.
5-/ri, coercive force 23.9KA/m N saturation magnetization (
σs) 9.11 X 10-'Wb1m/)Qi
γ-Fe20. A powder was obtained.

Next, this γ-Fe20. 3 tons of an aqueous solution in which 0.4 mol of cobalt sulfate and 1.2 mol of ferrous sulfate are dissolved in powdered soog! An aqueous sodium hydroxide solution 31 in which 16 mol of sodium hydroxide was dissolved was added. The temperature of this dispersion was raised to 45°C, and stirring was continued for 6 hours while maintaining this temperature. Next, the magnetic powder was taken out, thoroughly washed with water to remove the reaction solution, and then dried to obtain a cobalt-containing iron oxide ferromagnetic powder. .

The thus obtained cobalt-containing iron oxide ferromagnetic powder (-!, long axis diameter is 20' Onm, short axis diameter is 501 m
% axial ratio is 4, specific surface M22.1rrf by BET method
/, coercive force was 51.7 KA/m, and scales containing cobalt atoms were 2.57% by weight.

Using the cobalt-containing γ-Fe205 powder thus obtained, Co-containing r-Fe,,05 powder sof<i
Part VAGH (manufactured by LE&lt3.C, C, USA, chloride 1
1 Vinyl-vinyl acetate-vinyl alcohol copolymer) Bandex T-5250 (Dainippon 7 Ingi Rei, urethane elastomer) Coronate L (Nippon Polyurethane 2 parts by weight, Saiki, trifunctional low molecular weight isocyanate compound) Cyclohexanone A magnetic paint was prepared by dispersing a composition consisting of 60 liters of toluene and 60 liters of toluene for 72 hours in a ball mill. This magnetic paint was applied onto a 12 μm thick polyester base film to a dry thickness of 4 μm, dried, surface treated, and then cut to a predetermined width to produce a magnetic tape.

Example 2 In Example 1, the humidity when adding the ferric chloride aqueous solution to the sodium hydroxide aqueous solution was set to 0°C to obtain a brown precipitate, and this suspension was placed in an autoclave and heated at 180°C.
The procedure was the same as in Example 1 except that a hydrothermal reaction was carried out for 1 hour to obtain α-iron oxyhydroxide.The major axis diameter was 120 nm, the minor axis diameter was 30 nm, the uniaxial ratio was 4, and the specific surface fv was determined by the BET method.
! 12B, 7rrf/9, coercive force 50. Cobalt-containing γ-Fe20. A powder is obtained, and this cobalt-containing γ-
A magnetic tape was made in the same manner as in Example 1 using Fe203 powder.

Example 3 Example 2 except that in the process of dissolving the cobalt-containing iron oxide powder in Example 2, the amount of cobalt sulfate used was 0.8 mol and the amount of ferric sulfate was 2.4 mol. Similarly, the major axis diameter is 120 nm, the minor axis diameter is 30 nm, the s axis ratio is 4, the specific surface area is 28.1 m'/m by the BET method, the cobalt atom content is f)! with a coercive cuff of 5.6 KA/m! 4.57
wt% cobalt-containing γ-Fe20! A magnetic tape was prepared in the same manner as in Example 2 using the cobalt-containing γ-Fe 203 powder.

Comparative Example A ferrous sulfate aqueous solution was prepared by dissolving 10 moles of sulfuric acid iR'it (FeSO4,7H2O) in 40 parts of water, and an aqueous sodium hydroxide solution was prepared by dissolving 70 moles of sodium hydroxide in 40 parts of water, and the temperature was 25°. A lithium aqueous solution was added to obtain a pale green precipitate.

This suspension was then heated to 40°C in a constant temperature water bath while 1Ol of air was pumped in a! The mixture was blown into the cloudy liquid and subjected to an oxidation reaction for 6 hours to obtain a yellow precipitate, which was washed with water, transferred to filtration I, and dried to obtain α-iron oxyhydroxide powder.

Next, α-iron oxyhydroxide was heated and reduced in the same manner as in Example 1, and further oxidized to obtain γ-Fe205 powder, which was then subjected to cobalt deposition treatment to obtain a powder with a major axis diameter of 200 nm.
1 minor axis diameter 3 Onm s axis ratio 6.7, ratio table by BET method [Ti product 39.5yrs'/9, coercive force 39.8KA
cobalt-containing γ-Fe20. A powder was obtained. Furthermore, a magnetic tape was produced in the same manner as in Example 1 using this cobalt-containing r-Fe203 powder.

In order to investigate the coercive force distribution of the cobalt-containing iron oxide ferromagnetic powders obtained in Example 1 and Comparative Example, the anisotropic magnetic field distribution was measured using a sample vibrating magnetometer.

Figure 1 shows this result graphically, and graph A
Graph B shows the anisotropic magnetic field distribution of the magnetic powder obtained in Example 1, and graph B shows the anisotropic magnetic field distribution of the magnetic powder obtained in Comparative Example. The cobalt-containing iron oxide ferromagnetic powder obtained in this invention has a narrower anisotropic magnetic field distribution than that obtained in the comparative example, and from this, the cobalt-containing iron oxide elastomagnetic powder obtained by this invention has a good coercive force distribution. I understand that.

Furthermore, in order to investigate the thermal stability of the cobalt-containing iron oxide ferromagnetic powders obtained in each of the Examples and Comparative Examples, the addition and depletion ratio was measured and the transfer characteristics were tested. For the heating zone magnet, a container with a W diameter of 5 mm and a height of 3 mm was filled with the obtained magnetic powder, and it was saturated with a magnetic field of 796 KA/m at room temperature (
After measuring the saturation residual magnetization meter Irs, the saturated magnetized sample was held at 60°C for 2 hours, taken out at room temperature, and the residual magnetization amount Tr was measured.
The force/rather heating demagnetization amount sr was calculated and measured as a percentage. In addition, in the transfer property test, a container with a diameter of 6 mm and a height of 3 PII was filled with the obtained magnetic powder, and after being held in a magnetic field of 3.2 KA/m at 60°C for 1 hour, the residual magnetization amount Ira was measured at room temperature. z was measured and then calculated in decibels from the saturation magnetization in a magnetic field of 796 KA/m.

Table 1 below shows the results.

As is clear from the above Table 1, the cobalt-containing iron oxide ferromagnetic powders (Examples 1 to 3) obtained in this investigation are superior to the conventional cobalt-containing iron oxide reinforced powders (comparative examples). , all of them showed little thermal demagnetization and transfer, which shows that the cobalt-containing iron oxide ferromagnetic powder obtained by the present invention has excellent thermal stability.

Furthermore, regarding the magnetic tapes obtained in each example and comparative example, coercive force (Hc), residual magnetic flux density (Br), square shape (
Br/Bs), DC5/N and AC5/N,
The erasure properties were tested. Erasing property test is +5 for magnetic tape
Record the IKHz signal with dB manual power and record it on a reference tape (
The difference between the reproduced output signals before and after erasing was measured when erasing was performed using an erase current that was 20 times higher than the erase current that gave the erase characteristic of BASF C401R 05 dB, and was expressed in dB.

Table 2 below shows the results.

As is clear from Table 2, the magnetic tape obtained using the cobalt-containing iron oxide ferromagnetic powder of the present invention (Example 1)
-3) have higher coercive force, higher residual magnetic flux density, higher squareness, lower DC5/N and AC5/N, and better erasing characteristics than conventional magnetic tapes (comparative example). It can be seen that the magnetic recording medium obtained using the cobalt-containing iron oxide ferromagnetic powder of the present invention has excellent magnetic properties, has less noise, and has improved erasing properties.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the anisotropic magnetic field of the cobalt-containing iron oxide ferromagnetic powder obtained by the present invention and the volume percent of the powder particles. Patent applicant: Makoto Ishizaka, Director of the Institute of Industrial Science and Technology - Patent applicant: Representative of Hitachi Maxell Co., Ltd. Atsushi Mizuben Designated representative: Director of the Osaka Institute of Technology, Koshiba Institute of Technology Osaka Naito

Claims (1)

[Claims] 1. Iron oxide ferromagnetic powder is used as a core crystal, and the major axis diameter is 300 mm with a surface layer containing mainly cobalt on the core crystal.
ferromagnetic powder having an axial ratio of 5 or less, a specific surface area of 4 O-nf/m or less by the BET method, and a coercive force of 23.9 KA/m or more. is added to an alkaline aqueous solution at a temperature below 30°C and reacted to produce ferric hydroxide. After further aging, this ferric hydroxide was hydrothermally reacted in an autoclave to form α-oxyhydroxide. After producing iron oxide powder, filtering and drying, the produced powder is reduced by heating and further oxidized to obtain γ-Fe 20
x, After powdering, this γ-Fe205 powder is dispersed in a solution containing cobalt salt and ferrous salt, and alkaline water and solution are added to this to mainly coat cobalt on the surface of the γ-Fe203 powder. A method for producing a ferromagnetic powder characterized by forming a surface layer containing