JPH0216248B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for producing magnetoplumbite-type barium ferrite powder by a hydrothermal synthesis method. In recent years, with the demand for higher density magnetic recording, development of perpendicular magnetic recording systems using barium ferrite (magnetoplumbite type) as a magnetic recording medium has been progressing. Barium ferrite used in perpendicular magnetic recording has an appropriate coercive force (500 to 1500e).
Therefore, it is desired that the saturation magnetization is as high as possible, the particles are small and uniform, there is no agglomeration or sintering of the particles, and the dispersibility is good. Conventionally, various methods are known for producing barium ferrite, such as coprecipitation method, flux method, and hydrothermal synthesis method.
For example, Japanese Patent Publication No. 46-3545, Japanese Patent Publication No. 56-149328
Publication No. 160328, Japanese Patent Application Publication No. 160328, Japanese Patent Application Publication No. 1982-
This is proposed in publications such as Publication No. 2224. Barium ferrite produced by hydrothermal synthesis has relatively good dispersibility without particle agglomeration, but conventionally known methods can only produce particles with large diameters or with a saturation magnetization of about 50 emu/g or lower. In other cases, high-temperature firing is required to advance the crystallization of barium ferrite, and furthermore, high-temperature firing tends to cause sintering between particles, resulting in poor dispersibility. There are some difficulties. In view of these circumstances, the present inventors conducted research with the aim of producing barium ferrite powder with small particle size, no particle agglomeration or sintering, good dispersibility, and excellent electromagnetic properties. As a result, hydrothermal synthesis was performed under specific conditions such as the content ratio of barium ions and iron ions and the amount of alkali added, and after adding sodium chloride and hydrochloric acid to adjust the pH to 7 to 10 and drying, the product was heated to 650 ml. Baked at ~850℃,
The inventors have discovered that a barium ferrite powder that can achieve the above-mentioned objectives can be obtained by washing and drying a fired product, and have thus arrived at the present invention. The present invention uses 5 iron for 1 gram atom of barium.
To an aqueous solution of a metal salt containing ~10 g atoms in the ionic state, a solution containing an alkali equivalent of 1 to 5 times the total amount of the metal salt was added, and after heating and holding at 140 to 230°C, sodium chloride and Add hydrochloric acid to adjust pH
is maintained at 7-10 and then dried to obtain a product at 650
The present invention relates to a method for producing barium ferrite powder, which is characterized by firing at ~850°C, followed by washing and drying. According to the present invention, a method in which heating during hydrothermal synthesis is performed only once and does not involve a calcination operation, for example,
Barium ferrite powder particles exhibiting particularly higher saturation magnetization than those obtained by the methods described in JP-A No. 3545 and JP-A-56-149328 were obtained;
It is possible to obtain smaller particles than in the method described in Japanese Patent No. 3545. Further, according to the present invention,
It is easier to prevent particles from sintering than in the method described in JP-A-56-160328. In the present invention, the preparation of an aqueous solution of a metal salt containing 5 to 10 gram atoms of iron in the form of ions per 1 gram atom of barium is generally carried out using water such as ferric nitrate, ferric chloride, barium nitrate, barium chloride, etc. This method is carried out by dissolving a relatively highly soluble metal salt in water so that barium ions and iron ions are in the above range. At that time, various elements added to conventional barium ferrite, such as
Co, Ni, Mn, Zn, Ca, Pb, Sr, Ti, In, Nb
A small amount of water-soluble salts such as Co, Ti, Mn, Ca, etc. may be added, and addition of Co, Ti, Mn, Ca, etc. is particularly preferable for improving magnetic properties and controlling particle size. When the atomic ratio Fe/Ba of barium and iron is smaller than 5, the amount of magnetoplumbite barium ferrite produced is small and the hexagonal plate-like shape becomes poor, and when Fe/Ba is larger than 10, α-
Fe ₂ O ₃ is generated, and the barium ferrite grain size is also large, resulting in poor magnetic properties. In addition, when preparing an aqueous solution of metal salts containing barium and iron in the ionic state, the concentration of barium salt should be
It is desirable to keep the content within the range of 150 mmol/l in order to obtain barium ferrite with a good hexagonal plate shape. An aqueous solution of a metal salt containing barium and iron in the ionic state is heated, preferably to 30 to 90°C,
Add a solution containing an alkali to this. The amount of the alkali-containing solution added is 1 to the total amount of the metal salt.
Make it ~5 times equivalent. If the amount of alkali is too small, γ-Fe ₂ O ₃ will be produced, and if it is too large, it is not economical, which is not preferable. As the alkali component of the alkali-containing solution to be added, sodium hydroxide, potassium hydroxide, etc. are suitable. In the present invention, after adding the alkali-containing solution, it is first heated and maintained at 140 to 230°C, preferably 150 to 220°C. A heating and holding time of about 0.5 to 5 hours is sufficient, and barium ferrite crystals (precipitates) as nuclei are formed by heating and holding. Although this crystal has the same chemical composition as magnetoplumbite-type barium ferrite, its magnetic properties are poor. If the heating and holding temperature is lower than the above temperature, the formation of core crystals will not be sufficient, and if it is too high, the grain size of the barium ferrite finally obtained will become large, which is not appropriate. Further, this heating and holding is preferably carried out by quickly raising the temperature to the above temperature after addition of the alkali-containing solution, and an autoclave is generally employed for heating and holding. After heating and holding at 140 to 230°C to generate a precipitate that will become fine seed crystals of barium ferrite, sodium chloride and hydrochloric acid are added to bring the pH to 7.
Hold at ~10. This sodium chloride is 10 to 80% by weight, preferably 20 to 70% by weight based on the above precipitate.
It is desirable to add If the amount added is too small, sintering tends to occur, which is undesirable, and if the amount is too large, it is uneconomical. Hydrochloric acid is added to neutralize excess alkaline content, and is preferably added to adjust the pH of the slurry to 7 to 10 using a dilute aqueous solution of hydrochloric acid. In particular, an aqueous solution of hydrochloric acid is 1 to 5%
It is desirable to have a concentration of . Although the order of addition of sodium chloride and hydrochloric acid is not limited, it is preferable to add sodium chloride first. The slurry, brought to a pH of 7-10 by adding sodium chloride and hydrochloric acid, is dried as quickly as possible without filtration and washing. The main purpose of this drying is to remove moisture, and it is usually at a temperature of 50 to 200 degrees Celsius.
It is preferable to carry out the treatment for about 1 to 20 hours. As the drying method, generally known methods are employed, but it is particularly preferable to dry quickly using a drum dryer or a spray dryer. The product (dried product) containing barium ferrite seed crystals obtained by drying is 650~
Calcinate at a temperature of 850°C, preferably 700-830°C.
If the firing temperature is too low, it may take a long time to obtain the desired product, or the obtained product may have low saturation magnetization. On the other hand, if the temperature is too high, the growth of particles will be large and particles with large diameters will be obtained. The firing time is suitably about 1 to 30 hours, and the firing atmosphere is not limited, and the firing can be performed in an air atmosphere.
Further, when firing, two-stage firing is preferred, in which firing is performed once at a low temperature (650 to 750°C) and then fired at a higher temperature (750 to 850°C). The fired product (barium ferrite) fired at 650-850°C is washed and dried. The cleaning may be carried out by any method as long as impurities such as alkali metal ions and excess barium hydroxide in the fired product can be sufficiently removed. As the cleaning liquid, water, acetic acid, nitric acid, hydrochloric acid, etc. can be used. The baked product that has been sufficiently washed is then dried, but the drying method is not particularly limited. By drying, it is possible to obtain the desired small, uniform barium ferrite powder having a hexagonal plate-like magnetoplumbite type particle size of 0.3 μm or less and having a good crystalline state. Moreover, this barium ferrite powder has good dispersibility, has a plate ratio in the range of 1/3 to 1/10, and exhibits a coercive force of 700 to 12,000 e and high saturation magnetization. Furthermore, the coercive force can be freely controlled by adding the various elements described above (eg, Co, Ti, etc.) that are conventionally added to barium ferrite. Example 1 57.9 g of barium nitrate [Ba(NO ₃ ) ₂ ] and ferric nitrate [Fe(NO ₃ ).
9H ₂ O〕581.8g, cobalt nitrate〔Co(NO ₃ )・
Add 33.2 g of 6H ₂ O, 24.1 g of titanium tetrachloride [TiCl ₄ ], and 800 ml of water, and heat to approximately 80°C while stirring in a nitrogen gas atmosphere to dissolve barium nitrate and ferric nitrate, and stir. A solution of 218.3 g of sodium hydroxide (NaOH) dissolved in 400 ml of water was gradually added dropwise to the bottom. A precipitate was formed by dropping the aqueous sodium hydroxide solution. The solution containing the precipitate was kept under stirring at 80°C for 10 minutes, then placed in an autoclave, and the temperature was raised to 200°C in about 1 hour, kept at the same temperature for 1 hour, and the slurry was transferred to 2 beakers. Sodium chloride
364 g of the slurry was added thereto, and then a 5% aqueous hydrochloric acid solution was gradually added to bring the pH of the slurry to about 8. The slurry thus obtained was dried (concentrated to dryness) using a drum dryer. The obtained dried product was placed in an electric furnace and fired at 800° C. for 10 hours in an air atmosphere. This calcined product was washed with water until all soluble materials were removed, and then overdried to obtain barium ferrite powder. The results of measuring the particle shape (particle size, thickness) of this barium ferrite powder using a transmission electron microscope (TEM) (average value of 20 particles) and the magnetic properties using a vibrating sample magnetometer are shown in the first table. Shown in the table.
In addition, to examine the dispersibility, the filtration rate when the ink after milling the barium ferrite powder with the binder and solvent in a ball mill was filtered through a sieve with a sieve size of 3μ (the filtration rate is 100% when the entire amount of ink passes through the sieve). The results of the measurements are shown in Table 1. Example 2 The firing temperature and time of Example 1 were 800℃ and 750℃ for 10 hours.
Barium ferrite powder was produced in the same manner as in Example 1, except that the temperature was changed to ℃ for 20 hours, and the particle shape and
The magnetic properties and filtration rate were measured. The results are shown in Table 1. Comparative Example 1 Except that the amount of ferric nitrate used in Example 1 was changed and the Fe/Ba (atomic ratio) in Example 1 was changed from 6 to 12,
Barium ferrite powder was produced in the same manner as in Example 1, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. Comparative Example 2 Barium ferrite powder was prepared in the same manner as in Example 1, except that the amount of sodium hydroxide used in Example 1 was changed from 1.1 times equivalent (OH/NO ₃ ) to 0.9 times equivalent in Example 1. The particle shape, magnetic properties and filtration rate were measured. The results are shown in Table 1. Comparative Example 3 Barium ferrite powder was produced in the same manner as in Example 1, except that it was fired without adding sodium chloride or adjusting the pH with hydrochloric acid, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. Comparative Example 4 Barium ferrite powder was produced in the same manner as in Example 1, except that the heating and holding at 200°C as in Example 1 was not carried out, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. Comparative Example 5 Barium ferrite powder was produced in the same manner as in Example 1, except that the heating and holding temperature at 200°C in Example 1 was changed to 130°C, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. Comparative Example 6 Barium ferrite powder was produced in the same manner as in Example 1, except that the firing temperature of 800°C in Example 1 was changed to 600°C, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. Comparative Example 7 Barium ferrite powder was produced in the same manner as in Example 1, except that the firing temperature of 800°C in Example 1 was changed to 900°C, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. Comparative Example 8 Barium ferrite powder was produced in the same manner as in Example 1, except that the firing at 800°C as in Example 1 was not performed, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. Examples 3 to 9 Fe/Ba (atomic ratio) in Example 1 was changed from 6 to 8 (Example 3), OH/NO ₃ was changed from 1.1 to 4 (Example 4), and the heating holding temperature was changed. The temperature was changed from 200°C to 180°C (Example 5) and 220°C (Example 6), and the amount of sodium chloride added was changed from 70% by weight to 50% by weight (Example 7) and 20% by weight (Example 8). Ta,
Firing temperature was changed from 800°C to 750°C (Example 9)
Otherwise, barium ferrite powder was produced in the same manner as in Example 1, and the particle shape, magnetic properties, and filtration rate were measured. The results are shown in Table 1. 【table】

Claims (1)

[Claims] 1. In an aqueous solution of a metal salt containing 5 to 10 g atoms of iron in the form of ions per 1 g atom of barium,
Add a solution containing an alkali equivalent of 1 to 5 times the total amount of the metal salts, heat and maintain at 140 to 230°C, and then add sodium chloride and hydrochloric acid to bring the pH to 7 to 7.
10 and then dry the product at 650-850℃
A method for producing barium ferrite powder, which is characterized by firing, washing and drying.