JPH0545528B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] The present invention relates to a method for producing hexagonal ferrite magnetic powder suitable for use in magnetic recording media suitable for high-density recording. [Background technology] Hexagonal ferrite magnetic powder has conventionally been prepared using Ba salt,
An alkaline aqueous solution is added to an aqueous solution of one or more of Sr salt and Pb salt and iron salt, and the coprecipitate obtained by adding this alkaline aqueous solution is hydrothermally treated using an autoclave [T.Takada ，
M.Kiyama, Proc, ICF.Conf.P69 (1971)], etc., and because it is plate-shaped and has an axis of easy magnetization perpendicular to the plate surface, this type of hexagonal ferrite Magnetic powder is used in magnetic recording media, oriented so that the plate surface is parallel to the magnetic layer surface, and the residual magnetization component in the perpendicular direction is utilized. This is attracting attention as a method suitable for high-density recording. . However, hexagonal ferrite magnetic powders obtained by these conventional methods have the disadvantage that when the particle size is reduced to make them suitable for high-density recording, the amount of saturation magnetization decreases. If the particle size is 0.3μ or less, which is suitable for the purpose, a sufficiently large saturation magnetization cannot be obtained, and if the particle size is 0.1μ or less, the saturation magnetization decreases significantly. Therefore, in order to improve this and obtain a high amount of saturation magnetization, the coprecipitate obtained by the above-mentioned hydrothermal treatment was washed with water, dried, and then heat-treated in air at 450 to 1100 °C [Hiroshi Kojima, Choji Miyagawa, Powder Metallurgy Association,
1972 Spring Conference Lecture Abstracts P118 (1972)] However, although such heat treatment increases the amount of saturation magnetization, the particle size is extremely small, and these fine particles do not come into contact with each other at high temperatures. As a result, sintering between particles is likely to occur, resulting in a decrease in dispersibility and orientation, and it is still not possible to obtain hexagonal ferrite magnetic powder that is fully satisfactory for use in magnetic recording media suitable for high-density recording. Not yet. [Object of the Invention] The present invention eliminates these drawbacks and provides a hexagonal ferrite magnetic powder suitable for use in high-density recording magnetic recording media, which has a small particle size, a large amount of saturation magnetization, and excellent dispersibility and orientation. This was done for the purpose of obtaining. [Summary of the Invention] This invention involves adding an alkaline aqueous solution to an aqueous solution of a metal salt containing one or more metal salts selected from Ba salts, Sr salts, and Pb salts and an iron salt; The coprecipitate thus obtained is hydrothermally treated to produce hexagonal ferrite particles, and water is added to the hexagonal ferrite particles obtained in this step to form an aqueous suspension, and a flux is added. It is characterized by including a step of dissolving it in this suspension, and a step of evaporating water from this suspension and then heat-treating it at a temperature higher than the melting point of the flux. Using this method, fine hexagonal ferrite particles with extremely small particle sizes are produced, and then water is added to this to form an aqueous suspension, a flux is dissolved in this suspension, and water is removed from this suspension. After evaporation, this is melted at a high temperature and heat-treated at a temperature above the melting point of the flux in a flux that does not dissolve in any way with the hexagonal ferrite particles, thereby effectively suppressing sintering between particles. The obtained hexagonal ferrite magnetic powder is suitable for use in high-density recording magnetic recording media, with extremely small particle size, high saturation magnetization, and excellent dispersibility and orientation. be. In this invention, hexagonal ferrite particles are generated by adding an alkaline aqueous solution to an aqueous solution of a metal salt containing at least one metal salt selected from Ba salt, Sr salt, and Pb salt and an iron salt. It is carried out by hydrothermally treating the coprecipitate obtained by adding an aqueous solution, and the Ba salt, Sr salt, Pb salt and iron salt include chlorides, sulfates, and sulfates of these metals.
Nitrates, carbonates, etc. are preferably used. At this time, if appropriate amounts of metal ions such as Co, Ti, Zn, and Mn are added together with these, the coercive force can be controlled arbitrarily, the particle size can be reduced, and hexagonal ferrite particles with a sharp particle size distribution can be obtained. Therefore, chlorides, sulfates, nitrates, carbonates of these Co, Ti, Zn, Mn, etc. are also added and used at the same time, if necessary. Caustic soda is usually used as the alkali, and its preferred amount is at least the molar equivalent of the metal salt to be added, with an excess alkali concentration of 0.1 mol/mole.
It is preferable to set the above value, and in particular, in order to make the particle size of the obtained hexagonal ferrite particles extremely small, it is preferable to set the excess alkali concentration to 2 mol/or more. Hydrothermal treatment is performed using an autoclave,
Heat treatment in an autoclave is used to extremely reduce the particle size of hexagonal ferrite particles.
It is carried out by heating at a temperature of 200 to 350°C for 1 to 6 hours. The extremely fine hexagonal ferrite particles produced in this way are then made into an aqueous suspension by adding water, and after dissolving the fluxing agent in this suspension and evaporating the water from this suspension. If heat treatment is performed at a temperature higher than the melting point of the flux, the molten flux will be present between the hexagonal ferrite particles during the heat treatment and the hexagonal ferrite particles will not come into contact with each other, so even if the hexagonal ferrite particles Even if the particles are very small in size, sintering between the particles does not occur. Therefore, by heat treatment in this flux, the crystals of hexagonal ferrite particles are formed without sintering between the particles. A hexagonal ferrite magnetic powder with improved properties, extremely small particle size, large saturation magnetization, and excellent dispersibility and orientation can be obtained. The flux used here is 500 to 1000℃.
It is preferable to use a material that melts at a temperature of 100° C. and does not form a solid solution with the hexagonal ferrite particles at all. If the melting temperature is lower than this, the heat treatment of the hexagonal ferrite particles will be insufficient, and the crystallinity of the hexagonal ferrite particles will deteriorate. If the magnetization is sufficiently improved and the saturation magnetization cannot be made sufficiently large, on the other hand, if the saturation magnetization is too high, the crystal growth of the hexagonal ferrite particles in the flux becomes noticeable and the particles become coarse, which is undesirable. . Further, it is not preferable to use a material that forms a solid solution with the hexagonal ferrite particles even to a small extent, since the amount of saturation magnetization cannot be sufficiently improved. Suitable examples of such fluxes include sulfates, chlorides, bromides, and iodides of Na, K, and Li.
In particular, NaCl and KCl dissolve well in water, so
After heat treatment, these fluxes can be easily removed by washing with water and do not remain as impurities in the powder particles, so they are preferably used. It is preferable to carry out the heat treatment with this flux at a temperature within the range of 800 to 850°C for 1 to 4 hours. If the treatment temperature is too low or the treatment time is too short, the heat treatment will be insufficient and the hexagonal ferrite particles If the crystallinity of the particles cannot be sufficiently improved and the saturation magnetization cannot be sufficiently increased, and the processing temperature is too high or the processing time is too long, the flux will adhere to the particle surface and the saturation magnetization will decrease. There is a risk of causing As described above, in the present invention, first, fine hexagonal ferrite particles having extremely small particle sizes are produced by a hydrothermal treatment method, and then,
Water is added to this to form an aqueous suspension, a fluxing agent is dissolved in this suspension, water is evaporated from this suspension, and then this is melted at a high temperature and completely solidified with hexagonal ferrite particles. Because it is heat-treated in an insoluble flux at a temperature above the melting point of the flux, the crystallinity of the hexagonal ferrite particles is improved without sintering between particles, resulting in extremely small particle size and high saturation magnetization. Thus, a hexagonal ferrite magnetic powder suitable for use in high-density recording magnetic recording media, which has excellent dispersibility and orientation, is obtained. [Example] Next, an example of the present invention will be described. Example 1 <Production of Ba ferrite particles> A mixed solution of 1 mole of ferric chloride, 1/8 mole of barium chloride, and 1/20 mole of cobalt chloride dissolved in 1 part of water was mixed with 1 part of water in which 5 moles of caustic soda was dissolved. It was added to a caustic soda aqueous solution and stirred. After aging this suspension for one day, the precipitate was placed in an autoclave and reacted by heating at 280°C for 4 hours.
Ferrite particles were obtained. <Heat treatment of Ba ferrite particles in a flux> The Ba ferrite particles obtained by the above method were heated to a pH of 8.
After thorough washing with water until the concentration of Ba ferrite particles was reduced to below, a suspension containing Ba ferrite particles with a total volume of 1 was prepared, and 300 g of NaCl was added to this suspension, stirred, and dissolved. Next, put this Ba ferrite particle suspension in which NaCl has been dissolved into a large vat, and dry it in a dryer for 100 min.
The water was evaporated by heating to °C. The Ba ferrite particles obtained in this way and
The NaCl mixture was placed in a crucible, heated at 830°C for 2 hours, and then cooled to room temperature. Next, NaCl was dissolved and removed by washing with water, and only Ba ferrite particles were taken out to obtain Ba ferrite magnetic powder. The particle size of the obtained Ba ferrite magnetic powder is
It was 0.10μ. Example 2 In the production of Ba ferrite particles in Example 1, Ba ferrite particles were produced in the same manner as in Example 1, except that cobalt chloride was omitted and the amount of barium chloride used was changed from 1/8 mol to 1/6 mol. The particles were synthesized and heat treated in a flux to obtain Ba ferrite magnetic powder. The particle size of the obtained Ba ferrite magnetic powder was 0.15μ. Example 3 Ba ferrite particles were produced in the same manner as in Example 1, except that the same amount of KCl was used instead of NaCl in the heat treatment in a flux of the Ba ferrite particles in Example 1, and the heat treatment in a flux was performed. Then, Ba ferrite magnetic powder was obtained. The particle size of the obtained Ba ferrite magnetic powder was 0.08μ. The coercive force, saturation magnetization, and square shape of the Ba ferrite magnetic powder obtained in each example were measured before and after heat treatment in a flux. The table below shows the results.

〔Effect of the invention〕

As is clear from the above table, the coercive force, saturation magnetization, and square shape of the products after heat treatment in a flux significantly increased. Therefore, according to the manufacturing method of the present invention, the coercive force and It can be seen that a hexagonal ferrite magnetic powder with large saturation magnetization and excellent dispersibility and orientation can be obtained. In addition, the increase in saturation magnetization was particularly remarkable.Furthermore, when the shape of Ba ferrite particles was observed before and after heat treatment in a flux using an electron microscope, the shape of the particles was hexagonal plate-like, and no interparticle sintering was observed. Almost no change in shape was observed. Therefore, it can be seen that according to the manufacturing method of the present invention, sintering between particles is effectively suppressed, and as a result, a particularly high amount of saturation magnetization is obtained. Furthermore, as a result of X-ray diffraction, the diffraction lines of the particles before heat treatment in a flux were only due to Ba ferrite particles, but the diffraction lines were broad, whereas the diffraction lines of the particles before heat treatment in a flux were In this case, the diffraction lines due to Ba ferrite particles were sharp. Therefore, according to the production method of the present invention, the crystallinity of Ba ferrite magnetic powder is improved,
It is thought that the amount of saturation magnetization has been significantly improved.

Claims (1)

[Claims] 1. A coprecipitate obtained by adding an alkaline aqueous solution to an aqueous solution of a metal salt containing one or more metal salts selected from Ba salts, Sr salts, and Pb salts and iron salts, and adding this alkaline aqueous solution. A step of hydrothermally treating to produce hexagonal ferrite particles, a step of adding water to the hexagonal ferrite particles obtained in this step to form an aqueous suspension, and dissolving a fluxing agent in this suspension. A method for producing a hexagonal ferrite magnetic powder, comprising the steps of: evaporating water from this suspension, and then heat-treating the suspension at a temperature equal to or higher than the melting point of the flux.