JPH0545529B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Field of Application) The present invention relates to the production of iron oxide ferromagnetic powder, and more particularly to a method of producing cobalt-containing iron oxide ferromagnetic powder that enables high recording density. (Prior art and problems to be solved) Acicular γ-Fe ₂ O ₃ particles, which have a high coercive force due to shape anisotropy, have traditionally been used as magnetic powder for magnetic recording, but in recent years, Furthermore, magnetic powder for magnetic recording with high coercive force capable of high recording density is required, and research on iron oxide magnetic powder containing cobalt is being actively conducted. By the way, various methods for producing cobalt-containing γ-Fe ₂ O ₃ particles have been proposed so far, but one of the most useful methods is to produce acicular γ-Fe ₂ O ₃ particles in an alkaline solution. There is a method of dispersing and adding an alkaline earth metal salt such as a strontium salt, a cobalt salt, and a ferrous salt to obtain cobalt-containing γ-Fe ₂ O ₃ particles. Cobalt-containing γ- obtained by this method
Fe ₂ O ₃ particles have an iron oxide layer containing cobalt formed on the particle surface, so although the coercive force and saturation magnetization per unit weight increase, they have poor dispersibility, poor coercive force distribution, and The drawback was that the specific surface area was greatly reduced. The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art, to have high coercive force and high saturation magnetization, and to reduce the specific surface area less than nucleated acicular γ-Fe ₂ O ₃ . The object of the present invention is to provide a method for producing cobalt-containing iron oxide ferromagnetic powder with uniform distribution and excellent dispersibility. (Means for Solving the Problems) In order to achieve the above object, the present inventor investigated and analyzed the causes of the drawbacks of the conventional method for producing cobalt-containing iron oxide magnetic powder, and found that the conventional method: Acicular γ-Fe ₂ O ₃ particles are first dispersed in an alkaline solution, and then an alkaline earth metal salt, a cobalt salt, and a ferrous salt are added thereto.
It is thought that an iron oxide layer containing ferrous ions (Fe ² +) was formed on the surface of cobalt-containing γ-Fe ₂ O ₃ particles to suppress the decrease in saturation magnetization σs per unit weight. However, if the amount of ferrous ions added increases, the above σs of cobalt-containing γ-Fe ₂ O ₃ increases, but the coercive force decreases considerably compared to before the addition of ferrous ions, and the specific surface area also decreases compared to that of the starting material. γ−
It was found that the amount decreased considerably compared to Fe ₂ O ₃ , making it difficult to obtain the desired high coercive force ferromagnetic powder. Therefore, the present inventor has developed a material that has high coercive force and high saturation magnetization, has less decrease in specific surface area than nucleated acicular γ-Fe ₂ O ₃ , has a uniform coercive force distribution, and has excellent dispersibility. As a result of intensive research to develop magnetic powder, we discovered that by adding sodium citric oxide and alkaline earth metal salts to acicular γ-Fe ₂ O ₃ particles, and then adding cobalt salts and ferrous salts, the first The present invention was made based on the discovery that it is possible to suppress the decrease in coercive force and specific surface area due to the addition of iron ions and obtain the desired magnetic powder. That is, the present invention uses acicular γ-Fe ₂ O ₃ particles.
It is dispersed in an aqueous solution containing an alkali in an amount such that the OH ^- ion concentration is 1.6 to 3.0 mol/, to which sodium citrate and alkaline earth metal salts are added, and then cobalt salts and ferrous salts are added. Acicular γ-
The gist of the present invention is a method for producing a ferromagnetic powder for magnetic recording, which is characterized by forming an iron oxide layer containing cobalt on the surface of Fe ₂ O ₃ particles. The present invention will be explained in detail below based on examples. As mentioned above, the greatest feature of the present invention is that by adding sodium citrate and alkaline earth metal salts, the reduction in coercive force and specific surface area is suppressed, and this makes it possible to suppress the decrease in coercive force and specific surface area. A cobalt-containing iron oxide magnetic powder with particularly high coercive force can be obtained with a relatively small amount of cobalt added. These treatments are carried out at temperatures below the boiling point,
Since a longer treatment time becomes necessary as the treatment temperature becomes lower, it is preferable to conduct the treatment at a temperature as high as possible, close to the boiling point. In the present invention, first, the acicular γ-Fe ₂ O ₃ particles serving as the core are sufficiently dispersed in an aqueous solution containing an alkali, and at that time, the amount of alkali is such that the OH ^- ion concentration is 1.6 to 3.0 mol/ It is necessary to add such amount. Sodium hydroxide, potassium hydroxide, lithium hydroxide, etc. can be used as the alkali. If the amount of alkali added is outside the above range,
It becomes difficult to expect the above effects. Next, after heating the dispersion to an appropriate temperature (e.g., 80°C), sodium citrate is added, the temperature is further raised (e.g., 90°C), and an aqueous solution containing an alkaline earth metal salt is added. The temperature is further increased (e.g., 100°C), an aqueous solution containing cobalt salt is added, and the temperature is increased for the required time (e.g., 70°C).
time) Stir thoroughly. As the alkaline earth metal salt, in addition to strontium chloride, barium chloride, calcium chloride, etc. can be used, and as the cobalt salt, cobalt chloride, cobalt sulfate, etc. can be used. Thereafter, an aqueous solution containing a ferrous salt is added and stirred for the required time (eg, 1 hour). Note that as the ferrous salt, ferrous chloride, ferrous sulfate, etc. can be used. Note that these treatments, especially before and after adding the ferrous salt, are performed in a non-oxidizing atmosphere because it is necessary to prevent air from entering. According to this process, a cobalt-containing iron oxide layer is formed on the surface of the acicular γ-Fe ₂ O ₃ nucleus crystal, which has high saturation magnetization, high coercive force, and uniform coercive force distribution. Moreover, a cobalt-containing iron oxide magnetic powder with less decrease in specific surface area and excellent dispersibility can be obtained. Next, an embodiment of the present invention will be described. It goes without saying that the present invention is not limited to this embodiment. (Example) Acicular γ-Fe ₂ O ₃ powder produced by a conventional method (coercive force: 360 Oe, saturation magnetization: 74 emu/g, specific surface area:
34m ² /g) 4000g, 3200g NaOH in 24ml water
was added to an aqueous solution of caustic soda and sufficiently dispersed, the temperature of this dispersion was raised to 80°C, 100g of sodium citrate was added, and the temperature was further increased to 90°C.
Add 660 ml of an aqueous solution containing 60.8 g of strontium chloride to 100°C, add aqueous solution 2 containing 333 g of cobalt chloride at 100°C, and continue stirring at this temperature while preventing air incorporation as much as possible. Approximately 30 ml samples were taken after 3 hours, 5 hours, and 7 hours. After sampling after 7 hours, aqueous solution 2 containing 569 g of ferrous chloride was added and stirred for 1 hour to complete the reaction. Thoroughly wash each sample collected and the slurry after the reaction,
Dehydrated and dried. The obtained sample was subjected to an external magnetic field using a vibrating sample magnetometer (Model VSM-3S, manufactured by Toei Kogyo).
The magnetic properties were measured using 10KOe, and the specific surface area was measured using an automatic specific surface area measuring device (Model 2200, manufactured by Micromeritics). The results are shown in Table 1, Table 2, and Figures 1 and 2. The cobalt-containing iron oxide magnetic powder obtained after 8 hours of reaction treatment had a coercive force Hc of 667 Oe,
The saturation magnetization σs was 78 emu/g and the specific surface area was 32 m ² /g. (Comparative Example 1) As a comparative example, cobalt-containing iron oxide magnetic powder was produced in exactly the same manner as in the above example except that it was treated without adding the sodium citrate used in the above example, and sampled in the same manner. The magnetic properties and specific surface area of each sample obtained were measured. The results are also shown in Table 1, Table 2, and Figures 1 and 2. The cobalt-containing iron oxide magnetic powder obtained after 8 hours of reaction treatment had a coercive force Hc of 653 Oe,
The saturation magnetism σs was 77 emu/g and the specific surface area was 29 m ² /g. (Comparative Example) A cobalt-containing iron oxide magnetic powder was produced in exactly the same manner as in the above example except that the treatment was performed without adding strontium chloride, and the magnetic properties of each sample obtained by sampling in the same manner were The properties and specific surface area were measured. The results are shown in Tables 1 and 2.
It is also shown in the table, Figures 1 and 2. Like this 8
The cobalt-containing iron oxide magnetic powder obtained after the time reaction treatment has a coercive force Hc of 605 Oe and a saturation magnetization σs.
The specific surface area was 76.8 emu/g and 32.7 m ² /g.

【table】

[Table] Using the cobalt-containing iron oxide magnetic powder obtained in the above Examples and Comparative Examples, the following components were added and kneaded in a ball mill for about 48 hours to prepare a magnetic paint. Cobalt-containing iron oxide magnetic powder...75 parts by weight Vinyl chloride-vinyl acetate copolymer...19 Dioctyl phthalate...4 Lecithin...2 Toluene...100 Methyl isobutyl ketone...100 This magnetic paint A magnetic tape was manufactured by applying the mixture to a polyester film with a thickness of 12 μm to a dry thickness of approximately 6 μm, and drying while applying magnetic field orientation. The squareness ratio (Br/Bm) of each magnetic tape obtained,
When the switching field distribution (SFD) was measured, the third
The results shown in the table were obtained.

[Table] As is clear from Table 3, the magnetic tape manufactured using the cobalt-containing iron oxide magnetic powder obtained by the method of the present invention has lower squareness ratio and reversal magnetic field distribution than the comparative example. Are better. In addition, the coercive force distribution is uniform and the dispersibility is excellent.
It can be seen that the decrease in specific surface area is also small. (Effects of the Invention) As detailed above, according to the present invention, the acicular γ-
To easily and economically produce a cobalt-containing iron oxide magnetic powder that has a smaller specific surface area reduction than Fe ₂ O ₃ , has a uniform coercive force distribution, and has excellent dispersibility, and moreover, with a small amount of cobalt added. be able to. Therefore, it is possible to manufacture a high quality magnetic recording medium.

[Brief explanation of the drawing]

Figure 1 shows the coercive force of cobalt-containing iron oxide magnetic powder.
FIG. 2 is a characteristic diagram showing the relationship between Hc and reaction treatment time, and FIG. 2 is a characteristic diagram showing the relationship between the specific surface area of the magnetic powder and reaction treatment time.

Claims (1)

[Claims] 1 Acicular γ-Fe ₂ O ₃ particles with an OH ^- ion concentration of 1.6
Disperse in an aqueous solution containing an alkali in an amount of ~3.0 mol/s, add sodium citrate and an alkaline earth metal salt, and then
By adding an aqueous solution containing a cobalt salt and a ferrous salt and treating it at a temperature below the boiling point of the dispersion in a non-oxidizing atmosphere, iron oxide containing cobalt is formed on the surface of the acicular γ-Fe ₂ O ₃ particles. A method for producing ferromagnetic powder for magnetic recording, which comprises forming a layer.