JPH0688795B2 - Method for producing ferromagnetic fine powder for magnetic recording - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for producing a ferromagnetic fine powder for magnetic recording, which is composed of hexagonal ferrite crystal grains and is suitable for a medium for high density magnetic recording, particularly perpendicular magnetic recording.

[Technical background of the invention and its problems]

Magnetic recording generally employs a method of magnetizing in the longitudinal direction of the recording medium. However, in the case of this method, if the recording density is increased, the demagnetizing field in the recording medium increases and it is difficult to achieve sufficient high-density recording. In contrast to such a longitudinal recording method, a so-called perpendicular magnetic recording method, which aims at high density recording by reducing the demagnetizing field in the recording medium by magnetizing in the direction perpendicular to the surface of the recording medium layer, has been noticed in recent years. ing.

By the way, the perpendicular magnetic recording medium has been attempted to be put into practical use. In addition to the Co-Cr-based alloy film method, a so-called coating type recording medium has been proposed in which hexagonal ferrite crystal particle powder such as barium ferrite is dispersed in a binder and coated on a base film. In the case of the coating type, since the durability of the recording medium is excellent as well as the productivity can be economically advantageously produced in the same manner as in the case of the production of the conventional longitudinal recording type recording medium, Its practical application is urgent.

On the other hand, as the magnetic powder composed of hexagonal ferrite crystal particles used for the perpendicular magnetic recording medium, a coercive force (Hc: 400
~ 2000 Oe) and a large saturation magnetization, and has an easy axis of magnetization in the direction perpendicular to the surface of the particle plate,
It is said that it is important that the particles have a fine particle size of 0.3 μm or less, especially 0.2 μm or less, and have good dispersibility in the magnetic layer. Recently, however, the characteristics required for the magnetic powder are, in combination with the trend toward higher recording density,
It is further desired that the perpendicular magnetic recording medium can satisfy the reduction of the noise level and the high output in the short wavelength region. Therefore, the particle size is finer and the particle size distribution is sharper, and the dispersibility is good and the coating surface is excellent in smoothness, high orientation, and high filling property. There is an urgent need to develop hexagonal ferrite particles that exhibit

Conventionally, various methods have been known for producing hexagonal ferrite particle powder, and many proposals have been made for making the particles finer, but in general, the dispersibility and the fineness of the particles have been improved. The orientation is liable to be greatly damaged, and therefore the above-mentioned demand has not yet been fully satisfied, and a solution thereof is strongly desired. For example, as a method for producing fine powder of hexagonal ferrite particles, a method in which a precursor substance suspension obtained by mixing an aqueous metal salt solution containing constituents of hexagonal ferrite and an aqueous alkali solution is heat-treated and then fired is well known. Has been. This method is relatively easy to produce, and it is easy to obtain fine and highly saturated magnetization, but as the ferrite precursor particles become finer, interparticle sintering and particle shape in the firing process Crumbling is liable to occur, which may prevent reduction in orientation, filling properties, dispersibility, and the like.

Generally, hexagonal ferrite particles have high coercive force,
For this reason, it is necessary to control the coercive force within an appropriate range as a recording medium. Usually, by adding an appropriate amount of elements such as Co, Ti, Ni, Mn, and Zr, a part of Fe ³⁺ of ferrite constituent atoms is replaced. The method is carried out, but the effect of reducing coercive force is not sufficient unless it is uniformly and efficiently replaced and the sintering is not suppressed. In the end, when trying to obtain a desired low coercive force powder, the substitution amount of these elements is increased. However, as a result, there is a problem that the saturation magnetization is lowered and the perpendicular anisotropy of the easy magnetization axis is easily damaged.

[Object of the Invention]

The present invention is a method that can substantially avoid inter-particle sintering and easy control of coercive force, saturation magnetization is sufficiently high,
To provide a method for producing a hexagonal ferrite crystal powder having a fine particle size, a well-shaped hexagonal plate, and excellent in vertical orientation, which is suitable for highly dispersive magnetic recording, particularly for perpendicular magnetic recording. With the goal.

[Outline of Invention]

The present inventors have carried out various investigations to solve the above problems in the production of hexagonal plate-shaped hexagonal ferrite crystal particles, and as a result, a specific combination of firing treatments when firing a fine ferrite precursor substance. Surprisingly, the use of the agent makes it possible to arrange the particle shape well and substantially avoid interparticle sintering, and moreover, the partial substitution of the composition by Co, Ti, etc. is efficiently performed, and the coercive force control is easy. The present invention has been completed based on the knowledge that the above can be achieved.

That is, the present invention is to prepare an alkaline suspension by mixing an aqueous solution containing at least one metal compound selected from the group of barium, strontium, and lead and an iron compound with an alkaline aqueous solution, and then preparing the alkaline suspension. To obtain a ferrite precursor substance by heat treatment at 60 to 250 ° C., and the obtained precursor substance contains a silicon compound, an alkali metal and an alkaline earth metal chloride, a sulfate, a carbonate, and a nitrate. At least one of the above, and then 650-950
A method for producing a ferromagnetic fine powder for magnetic recording, which comprises firing hexagonal ferrite crystal particles in a temperature range of ° C.

In the method of the present invention, first, at least one metal compound Ma selected from the group consisting of barium, strontium, and lead, an iron compound, and optionally Co, Ti, Ni, Mn, or Zr as a substitution element Mb for controlling coercive force. , Zn, Ge, Nb, and V compound are prepared in an aqueous solution containing respective predetermined amounts. As these compounds, various water-soluble compounds can be used, but chlorides, nitrates and the like are preferable. The Ma component is Fe
1/4 to 1/12, preferably 1/6 to the molar ratio of + Mb component
It is 1/10. When the molar ratio is smaller than the above range, the obtained ferrite crystal particle powder is liable to be coarsened, and the dispersibility is deteriorated, and the characteristics such as orientation in the recording medium and surface smoothness are unavoidable. On the other hand, if the molar ratio is larger than the above range, a crystal phase different from the magnetoplumbite type crystal may be mixed and the saturation magnetization may not be reduced or the shape may not be uniform, which is not preferable. The substitution component Mb
Can use at least one of Co, Ti, Ni, Mn, Zr, Zn, Ge, Nb, and V in an amount of 0.2 mol or less, preferably 0.17 mol or less, based on 1 mol of Fe. Substitution with Ti element is preferred.

Next, in the above metal compound aqueous solution, for example, NaOH, KOH, NH ₄ OH
An aqueous solution such as is contacted and mixed to form an alkaline suspension. The amount of the alkali added is equivalent to or more than the metal salt contained in the aqueous metal compound solution, particularly when trying to obtain a fine ferrite ferromagnetic powder, the alkaline concentration of the suspension is 1.5 on the free OH basis. Mol / l, more preferably 2 mol / l
If it is 1 or more and is lower than the above range, the reaction does not proceed sufficiently and non-plate-like particles are often generated, which easily form sintered particles in the firing process, and orientation, dispersibility, etc. I can't help lowering.

The alkaline suspension is then used in a reaction vessel equipped with a heating device or placed in a pressure vessel such as an autoclave, and subjected to a heat reaction treatment at 60 to 250 ° C, preferably 100 to 200 ° C to obtain a ferrite precursor of plate-like particles. Form body material. When the temperature during the hydrothermal treatment is lower than the above range,
Amorphous bodies are easily formed and aggregates are easily formed. Therefore, it is difficult to obtain a ferrite particle powder having a uniform shape, and a decrease in orientation is unavoidable. On the other hand, if it is higher than the above range, formation of coarse particles and spread of the particle size distribution are unavoidable, which is not preferable.

In the present invention, when the hexagonal ferrite precursor material obtained as described above is fired, a chloride, a sulfate, a carbonate or a nitrate of a silicon compound and an alkali metal or an alkaline earth metal is previously used as a firing treatment agent. At least one of the above compounds is added, and examples of the silicon compound used as the treating agent include sodium orthosilicate, sodium metasilicate, potassium metasilicate, calcium metasilicate, magnesium silicate, and other silicates, and silicone oil. Examples thereof include silicone resins, silanes such as chlorosilanes and alkoxysilanes, siloxanes, and the like, but it is usually preferable to use silicate aqueous solutions of water glass having various compositions. Various methods can be used to add and process the treating agent comprising the silicon compound to the ferrite precursor substance. For example, an aqueous silicate solution is added to an aqueous suspension containing the ferrite precursor substance particles. By adding and neutralizing this with an acidic substance, the surface of the particles is coated with a hydrated silicate (SiO ₂ · nH ₂ O), or in a solution in which an organic silicon compound is dissolved in an organic solvent, It is carried out by suspending the ferrite precursor substance particles to adsorb the silicon compound on the surface of the particles, and further by spray-adsorbing the solution of the silicate aqueous solution or the organosilicon on the surface of the ferrite precursor substance particles. be able to. The amount of the silicon compound added is Si as a weight basis with respect to the ferrite precursor substance.
It is 0.1 to 1.5%, preferably 0.2 to 1%. If the addition treatment amount is less than the above range, a desired effect such as sintering prevention cannot be sufficiently obtained, while if it is more than the above range, the magnetic properties are deteriorated such as a decrease in saturation magnetization, which is preferable. Absent.

In addition, examples of the alkali metal used as the baking treatment agent include sodium, potassium and lithium.
Examples of alkaline earth metals include barium and strontium. These metals can be used as chlorides, sulfates, carbonates, nitrates and the like. The alkali metal and alkaline earth metal chlorides, sulfates, carbonates and nitrates can be added by various methods. For example, a ferrite precursor obtained by heating, filtering and washing. A washing cake of the substance particles is added to an aqueous solution of the chloride or salt of the metal and suspended and then dried, or an aqueous solution of the chloride or salt of the metal is added to the washing cake of the ferrite precursor substance and kneaded. It can be carried out by drying if necessary. The processing amount of the metal compound added is
5 to 120% by weight with respect to the ferrite precursor material.
If the addition treatment amount is less than the above range, the effect of suppressing interparticle sintering or adjusting the particle shape into a hexagonal plate shape is not sufficient, and if the treatment amount is too much than the above range, it is economical. Is not even advantageous.

In the method of the present invention, after the ferrite precursor material obtained by the heat reaction treatment as described above is subjected to the addition treatment of the firing treatment agent, it is normally 650 to 950 for firing.
C., preferably 700 to 900.degree. If the firing temperature is lower than the above range, the crystallization of the ferrite particles does not proceed sufficiently and the saturation magnetization is low, and if the firing temperature is higher than the above range, the ferrite particles are fixed to each other or sintered to form agglomerates. It is easy to do so, and the dispersibility in making a coating material is greatly impaired, and the magnetic characteristics and surface smoothness of the recording medium are unavoidably deteriorated. The calcination can be carried out usually for about 0.5 to 5 hours using various types of devices such as a rotary furnace and a fluidized bed furnace. In the present invention,
By performing the addition treatment of the baking treatment agent, it is possible to obtain the effect of suppressing inter-particle sintering and the effect of suppressing particle growth. Therefore, it becomes possible to perform firing in a relatively high temperature range, high saturation magnetization, and fine graining. Particles having a particle size can be obtained.

The ferrite crystal particle powder obtained as described above,
The excess barium and impurities are pickled and removed by immersion treatment in an aqueous medium or, if necessary, an acidic aqueous medium. In the above case, the dispersibility may be further enhanced by treating the aqueous medium with a strongly acidic medium.

As described in detail above, the ferromagnetic fine powder obtained by the manufacturing method of the present invention is a magnetoplumbite-type ferrite crystal particle powder having a saturation magnetization of about 45 to 60 emu / g and a coercive force of about 400 to 2,000 Oe. , Which has a hexagonal plate shape, an average particle size of approximately 0.05 to 0.15μ, and a small particle size distribution, and has excellent dispersibility in the magnetic layer of the magnetic recording medium, and is an excellent material for high density perpendicular magnetic recording. It is suitable.

Example of Invention

The present invention will be further described below with reference to Examples and Comparative Examples.

Example 1 360 ml of 1 mol / l BaCl ₂ aqueous solution, 1 mol / l FeCl ₃ aqueous solution 2
472 ml, 1 mol / l CoCl ₂ aqueous solution 204 ml and 1 mol / l TiC
l ₄ aqueous solution (204 ml) was mixed (Ba / (Fe + Co + Ti) molar ratio: 1/8 Ba / Fe molar ratio: 1.5 / 10.3), and then this mixed solution was mixed with 10 mol.
An alkaline suspension containing a brown precipitate was prepared by adding it to 5112 ml of a 1 / l NaOH aqueous solution. The free OH group concentration at this time is 5
Mol / l. Subsequently, the suspension was placed in an autoclave and heated at 150 ° C. for 3 hours to produce ferrite precursor material particles (the precursor material particles have low crystallinity according to X-ray diffraction). It has a magnetoplumbite structure, and was an irregular plate-shaped particle with an average particle size of about 0.06μ).

Then, the obtained ferrite precursor substance particles are filtered, washed with water, and repulped with water to obtain the ferrite precursor substance particles 50.
It was a slurry of g / l. 50 ml of a water glass aqueous solution (Si concentration 10 g / l) was added to 2 l of this slurry, and neutralized to pH 7.3 with hydrochloric acid (0.1 N) while stirring (at this time, the amount of Si compound coated on the particles was 0.5% by weight in terms of Si).

The Si compound-coated ferrite precursor substance particles thus obtained were again filtered and washed with water, and a washing cake (containing 50 g of the Si compound-coated ferrite precursor substance particles) was added to 350 ml of an aqueous solution in which 50 g of NaCl was dissolved, and after stirring well It was placed in a dryer and heated at 110 ° C. to evaporate the water content (content of NaCl was 100% by weight based on the Si compound-coated ferrite precursor material particles).

The dry powder thus obtained was calcined at 800 ° C. for 1 hour. Then, the obtained powder was immersed in a dilute hydrochloric acid aqueous solution, filtered, washed with water to remove NaCl, and then dried to obtain a ferromagnetic powder of the present invention. (Sample A) Example 2 A ferromagnetic powder of the present invention was obtained in the same manner as in Example 1 except that BaCl ₂ was used in place of NaCl. (Sample B) Comparative Example 1 Similar to Example 1, the ferrite precursor material particles obtained by heat treatment at 150 ° C. for 3 hours were coated with a Si compound and
A comparative sample was obtained by treating in the same manner as in Example 1 except that NaCl was not added. (Sample C) Comparative Example 2 As in Example 1, the precursor material particles were coated with the Si compound, filtered, washed, and then dried, and then calcined at 800 ° C. for 1 hour. The powder after firing was treated in the same manner as in Example 1 to obtain a comparative sample. (Sample D) Comparative Example 3 A comparative sample was obtained in the same manner as in Example 1 except that the Si compound was not coated. (Sample E) In addition, each sample obtained in the above-mentioned Examples and Comparative Examples is X
As a result of the line diffraction, all were magnetoplumbite type barium ferrite.

The average particle diameter (Dp: electron microscopy), coercive force (Hc), and saturation magnetization (σs) of each sample were measured by a conventional method,
Further, a magnetic coating material was prepared with the following composition, and the coating material was applied onto a polyester film and subjected to orientation treatment perpendicular to the coated surface to prepare a magnetic recording medium.

Magnetic powder 100 parts by weight vinyl acetate vinyl chloride copolymer resin 16.2 〃 Surfactant 4 〃 Methyl ethyl ketone 186 〃 Coercive force (Hc⊥: perpendicular to the medium surface), orientation ratio (OR) , Squareness ratio (SQ⊥:
It is the direction perpendicular to the medium surface and the value after demagnetizing field correction)
Was measured.

The results are shown in Table 1.

As is clear from the results shown in Table 1, the orientation of the precursor substance particles by adding a Si compound and an alkali metal compound or an alkaline earth metal compound to the precursor substance particles, followed by firing,
It can be seen that the dispersibility is improved and the generation of aggregated particles due to interparticle sintering is suppressed.

When the shapes of the sample powders of Examples and Comparative Examples were observed by an electron microscope photograph, both of Examples 1 and 2 were hexagonal plate-shaped and no interparticle sintering was observed. On the other hand, in Comparative Example 1, although the particles were plate-shaped, the shape was somewhat irregular and the primary particle diameter was about 0.08 μ, but inter-particle sintering was observed. The shape of the coprecipitate particles was well maintained, and almost no agglomerated particles due to inter-particle sintering were observed, but there were many irregular shapes.
Further, in Comparative Example 3, although it had a hexagonal plate shape, coarsening was observed.

[Effect of the Invention] According to the present invention, generation of aggregated particles due to inter-particle sintering is suppressed, and a hexagonal plate-like powder having excellent shape and having a high dispersibility of hexagonal ferrite crystal powder having excellent vertical orientation Ferromagnetic powder can be easily manufactured industrially, and
The coercive force can be easily controlled within a desired range without impairing the saturation magnetization, and is useful for high density recording, especially for reducing the noise level and increasing the output of a perpendicular magnetic recording medium. .

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Claims (1)

[Claims] 1. An alkaline suspension is prepared by mixing an alkaline solution with an aqueous solution containing at least one metal compound selected from the group consisting of barium, strontium and lead, and an iron compound. A ferrite precursor substance is obtained by heat treatment at ˜250 ° C., and the obtained precursor substance contains at least a chloride, a sulfate, a carbonate and a nitrate of a silicon compound and an alkali metal and an alkaline earth metal. A method for producing a ferromagnetic fine powder for magnetic recording, characterized by adding 1 type and then firing in a temperature range of 650 to 950 ° C. to obtain hexagonal ferrite crystal particles.