Method for producing cobalt ferrite particles and cobalt ferrite particles produced thereby

Abstract

To provide magnetic particles (cobalt ferrite) having an average particle size of μm order and uniform particle size.SOLUTION: Cobalt ferrite magnetic particles are obtained by heat-treating a cobalt ferrite precursor in the presence of sulfite.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing cobalt ferrite particles and to cobalt ferrite particles produced thereby, and in particular to provide cobalt ferrite particles having a relatively large average particle size and a uniform particle size distribution. [Background technology]

[0002] Ferrite particles are known as high-permeability materials and permanent magnet materials, and today magnetic powders are being used in new materials such as copy toner, magnetic ink, and MR fluids, and improvements in their quality and performance are expected. Cobalt ferrite, in particular, is known as a magnetic material with large crystalline anisotropy and high coercive force among spinel ferrites. Furthermore, cobalt has a chemical behavior similar to that of iron, which makes it easy to control various aspects of its manufacturing process.

[0003] Known methods for producing ferrite particles include coprecipitation, wet oxidation, and hydrothermal methods. Coprecipitation is a reaction in which two or more types of ions are precipitated simultaneously. When producing cobalt ferrite particles, Fe 3+ and Co 2+ After adding alkali to an aqueous solution containing ions, the reaction is accelerated by heating to obtain nano-sized ferrite particles. In this method, the reaction is carried out at a temperature of 80 to 100°C, and the average particle size of the obtained particles is about 20 to 50 nm, with only particles having a relatively wide particle size distribution (Patent Document 1).

[0004] The wet oxidation method is 2+ and Co 2+This method involves reacting an oxidizing agent such as air with a raw material aqueous solution containing ions while heating it. When air is used as the oxidizing agent, the reaction temperature is about 60 to 100°C, and particles of about 0.05 to 0.3 μm are obtained (Patent Document 2, Patent Document 3). In addition, in a method in which the raw material aqueous solution and the oxidizing agent solution are reacted continuously, the reaction is carried out at a temperature of 30 to 100°C, and ferrite particles of 3 to 20 nm are obtained (Patent Document 4).

[0005] The hydrothermal method is 2+ In an aqueous solution containing Co ions 2+ Ferrite particles with a relatively large particle size of 0.3 to 8 μm are produced by mixing an aqueous solution containing ions and carrying out hydrothermal synthesis in an autoclave at a high temperature of 160 to 300° C. (Patent Document 5).

[0006] When ferrite particles are produced by conventional techniques, coprecipitation and wet oxidation methods can be used to produce them at relatively low temperatures, but the resulting ferrite particles are only fine particles on the order of nanometers. Furthermore, the hydrothermal method can produce relatively large particles on the order of micrometers, but requires a hydrothermal reaction (siccol reaction) at high temperatures and pressures, which poses problems in terms of equipment and cost. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 4138344 [Patent Document 2] Special Publication No. 3-24412 [Patent Document 3] Special Publication No. 60-47722 [Patent Document 4] Patent No. 5504399 [Patent Document 5] Japanese Patent Application Publication No. 5-275224 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention overcomes the problems of the conventional techniques and provides a production method capable of synthesizing cobalt ferrite particles having a larger and more uniform average particle size than conventional particles at a lower energy level, and also provides cobalt ferrite particles produced by this method that have a spherical shape and a uniform particle size. [Means for solving the problem]

[0009] To solve the above problems, the present invention employs the following means. (1) A method for producing cobalt ferrite particles, which comprises heat-treating a ferrite precursor formed from a ferrous salt and a cobalt salt in the presence of a sulfite. (2) The method for producing cobalt ferrite particles according to (1), wherein the heat treatment in (1) is carried out in a pressure vessel under hydrothermal conditions at a temperature in the range of 100°C or higher and 190°C or lower. (3) The method for producing cobalt ferrite particles according to (1), wherein the heat treatment in (1) is carried out at normal pressure in a temperature range of 60°C or higher and lower than 100°C. (4) A method for producing cobalt ferrite particles, wherein the ferrous salt and cobalt salt in (1) are iron (II) sulfate and cobalt (II) sulfate. (5) Cobalt ferrite particles having a spherical shape and an average particle size of 0.5 to 2 μm. (6) Cobalt ferrite particles having a spherical shape and an average particle size of 2 to 5 μm. (7) A copier toner comprising the cobalt ferrite particles of (5) or (6). (8) A magnetic ink comprising the cobalt ferrite particles of (5) or (6). (9) An MR fluid consisting of cobalt ferrite particles according to (5) or (6). (10) A white powder having a titanium oxide film and a metallic silver film in this order on the surface of the cobalt ferrite particles of (5) or (6). (11) A white powder of (10) having a brightness L* of 75 or more. [Effects of the Invention]

[0010] By adopting the manufacturing method of the present invention, magnetic particles made of cobalt ferrite with a uniform particle size can be manufactured with lower energy than magnetic particles manufactured by conventional methods. Furthermore, by adding a ferrite precursor to the reaction solution during the reaction and performing a heat treatment, it is possible to adjust the particle size of the manufactured magnetic particles. The cobalt ferrite particles obtained by the production method of the present invention are spherical and have a uniform particle size, and are therefore expected to be used as copy toners, magnetic inks, and MR fluids. Furthermore, the cobalt ferrite particles of the present invention can be whitened by a known method, or can be further provided with a colored layer to form a highly bright white powder or a vividly colored colored powder. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an SEM photograph of a powder sample of Example 2. [Figure 2] 1 is an SEM photograph of a powder sample of Example 4. [Figure 3] 1 is an SEM photograph of a powder sample of Comparative Example 2. [Figure 4] 1 is an SEM photograph of a powder sample of Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0012] The method for producing cobalt ferrite particles of the present invention will be described below step by step. (Production of ferrite precursor) First, a raw material solution is prepared by dissolving a ferrous salt and a cobalt salt in desalted and deaerated water. The ferrous salt used in the method of the present invention is not particularly limited, and examples thereof include iron(II) chloride, iron(II) sulfate, and iron(II) nitrate. Iron washing wastewater from blast furnaces and electric furnaces can also be used as an inexpensive raw material. Iron(II) sulfate is most preferred because it is stable when used as a precursor. Furthermore, the cobalt salt is also not particularly limited, and examples thereof include cobalt(II) chloride and cobalt(II) nitrate. For similar reasons, cobalt(II) sulfate is most preferred. The reason for using desalted and deaerated water here is to prevent the charge state of metal ions, such as iron, dissolved in the solution from being affected by dissolved salts and oxygen. For example, it is known that the presence of free oxygen in the reaction system can oxidize divalent iron to trivalent iron, resulting in the generation of fine particles with an undesired particle size.

[0013] Next, an alkaline aqueous solution is prepared by dissolving an alkali such as sodium hydroxide, potassium hydroxide, calcium hydroxide, or ammonia, and sulfite in desalted and deaerated water. The raw material solution and alkaline solution are then mixed. Mixing the raw material components in this order ensures complete dissolution of the raw material components and prevents unintended reactions from occurring. By mixing the raw material solution with the alkaline solution, hydroxides of the added metal elements are coprecipitated to form a gel precursor.

[0014] (sulfites) The present invention is characterized by heat-treating a gel precursor composed of a ferrous salt and a cobalt salt in the presence of a sulfite. The inventors of the present invention believe that the sulfite has the following effect in the ferrite synthesis reaction. However, the following effect of the sulfite was predicted by the inventors based on the course of various reactions, and the present invention is not limited to this interpretation.

[0015] Sulfites are generally known to have a reducing effect and are used as antioxidants for various products. As described above, the gel precursor of the present invention is a co-precipitate of hydroxides of metal elements. Before heating begins, sulfites exert their usual reducing effect on the gel precursor, preventing the oxidation of the metal hydroxides. This prevents the generation of unintended oxides other than ferrite.

[0016] Next, when the gel precursor solution is heated to a high temperature, the sulfite ions and sulfate ions formed by oxidizing some of the sulfite ions are thought to act as oxygen sources. This is supported by the fact that iron-cobalt sulfide was confirmed to be produced when cobalt ferrite was synthesized using a hydrothermal method at 180°C. Metal sulfides are produced when sulfite or sulfate ions are reduced and bonded with metal ions. Therefore, it is believed that the oxygen in the sulfite ions or sulfate ions is also supplied to the ferrite synthesis reaction.

[0017] As a result, it is believed that the sulfite gradually loses its reducing power as heating begins and instead acts as an oxidizing agent. Therefore, even when the gel precursor is heated, the oxidation rate due to heating can be slowed down, and the particle size of the synthesized ferrite particles can be increased. Furthermore, ferrite particles with a uniform particle size can be produced. The present invention was made based on the above-mentioned findings newly discovered by the inventors.

[0018] (Heat treatment) In the present invention, two types of heat treatment methods are possible: heat treatment by hydrothermal method using a pressure vessel, and heat treatment under normal pressure using a water bath. The heating temperature is 60°C or higher and lower than 100°C when heat treatment is performed under normal pressure, and 100°C or higher and up to about 190°C when heat treatment is performed by a hydrothermal method. By changing the heat treatment conditions, it is possible to produce cobalt ferrite particles with different average particle sizes. That is, when produced under high temperature and high pressure conditions by a hydrothermal method using a pressure vessel, relatively fine particles with an average particle size of about 0.5 to 2 μm are obtained, whereas when produced by heating under normal pressure using a water bath, cobalt ferrite particles with a relatively large average particle size of about 2 to 5 μm are obtained. Each synthesis method will be briefly described below.

[0019] (hydrothermal method) The pressure vessel used in the present invention may be any ordinary high-pressure reaction vessel, such as an autoclave, a pressure cooker, or a boiler, but an autoclave is preferred for its versatility. In the normal Siccol method, the reaction proceeds at high temperatures of 200°C or higher, but in the present invention, magnetic particles made of cobalt ferrite can be synthesized at temperatures in the range of 100 to 190°C. In the pressure vessel, an oxidation reaction proceeds, rather than the Siccol reaction which involves oxidative hydrolysis. This can be confirmed by the fact that hydrogen gas is inevitably generated when the siccol reaction proceeds, but no hydrogen gas is generated in the present invention. As the generation of sulfides has been confirmed as described above, it is believed that an oxidation reaction using sulfite ions as an oxidizing agent is occurring. The resulting magnetic particles are washed to remove non-magnetic by-products.

[0020] (Normal pressure method) The gel precursor obtained in the previous step is placed in a container and immersed in a water bath maintained at 60-100°C for heat treatment. This condition is maintained for approximately 48 hours to allow a gradual synthesis reaction of cobalt ferrite to occur. It is believed that the same oxidation reaction as in the hydrothermal method occurs. The resulting product is washed, dried in the air and heat treated to form magnetic particles.

[0021] (cobalt ferrite particles) The cobalt ferrite particles produced by the present invention are relatively large particles with an average particle size on the order of μm. In addition, because they are nearly spherical and have a narrow particle size distribution, they have little tendency to aggregate with each other and can be compacted into close-packed shapes, which can improve the magnetic properties of the compact or increase the bulk density. Therefore, when used in copy toner, magnetic ink, and MR fluid applications, the properties of this material can be fully utilized.

[0022] (white powder) The cobalt ferrite particles of the present invention can be whitened to form a white powder, or can be whitened and then provided with a colored layer to form a colored powder. The whitening can be carried out by known methods, but it is preferable to use, for example, the whitening method patented by the applicant of the present application (Japanese Patent No. 4113045). This whitening method involves providing a titanium oxide film between the base particles and the metallic silver film. Specifically, this can be achieved by forming a titanium oxide film on the surface of the base particles by hydrolysis of titanium alkoxide (e.g., WO 96 / 28269) or by reaction with an aqueous titanium salt solution (e.g., JP 11-131102 A), and then forming a metallic silver film by a known method such as electroless plating. This method makes it possible to produce a white powder of the present invention having a titanium oxide film and a metallic silver film in this order on the surface of the cobalt ferrite particles, and to improve the lightness L* to 75 or more. [Example]

[0023] The present invention will be described in more detail below using examples, but the present invention is not limited to these examples. The average particle size and particle size distribution of the cobalt ferrite particles produced in the examples were measured by the following methods.

[0024] (Average particle size) The average particle size was measured as follows. An image of a powder sample with 16 vertical and 16 horizontal lines evenly arranged in a grid was printed on the SEM image, and the diameters of 256 particles at or closest to the intersections of the vertical and horizontal lines were measured with a vernier caliper and the average value was calculated. The length of the scale bar on the SEM image was also measured, and the particle size measured in mm was converted to μm using this value to obtain the average particle size.

[0025] (particle size distribution) The uniformity of the particle diameter of the cobalt ferrite of the present invention was determined by the CV value, which is the coefficient of variation of the particle diameter. That is, in statistics, standard deviation is used as a measure of the variability of data distribution, and this is normalized by dividing it by the arithmetic mean value of the data to evaluate the variability of the data. This is the CV value, which is the coefficient of variation, and in the present invention, the CV value is also used to evaluate the small variability in the particle size of the formed cobalt ferrite particles. A small CV value indicates small variability in the particle size distribution, and particles with a CV value of 0.1 or less are considered to be monodisperse particles, and their characteristics are attracting attention.

[0026] [Example 1] Heat treatment by atmospheric pressure method I (1) Preparation of desalted and degassed water 480 g of desalted water was degassed with N2 at 2.5 L / min for 30 minutes to prepare desalted and degassed water. (2) Preparation of raw material aqueous solution A raw material aqueous solution was prepared by dissolving 45 g of iron (II) sulfate heptahydrate (FeSO4·7H2O) and 9 g of cobalt (II) sulfate heptahydrate (CoSO4·7H2O) in 118 g of desalted and degassed water. (3) Preparation of alkaline aqueous solution Dissolve 14g of sodium hydroxide (NaOH) and 50g of sodium sulfite (Na2SO3) in 240g of desalted and deaerated water. The mixture was dissolved to prepare an alkaline aqueous solution. (4) Preparation of gel precursor The raw material aqueous solution and the alkaline aqueous solution were mixed in a container purged with N2 and stirred for 5 minutes to prepare a gel precursor. (5) Preparation of low-magnetic particles by heat treatment of gel precursor The gel precursor was placed in a container whose air had been replaced with N2, and heat-treated by immersing it in a water bath at 60°C for 20 days to prepare low-magnetic particles. (6) Washing of low magnetic particles The low-magnetic particles were filtered, washed with water, dispersed in 4 L of demineralized water at 80°C, and stirred for 24 hours. They were then dispersed in 1 L of demineralized water whose pH had been adjusted to 2.5 to 3 with 1:1 sulfuric acid, and subjected to ultrasonic waves while stirring. Non-magnetic by-products were then removed by solid-liquid separation using a magnet. (7) Drying of low magnetic particles The washed low magnetic particles were dried in an air atmosphere at 110° C. for 2 hours. (8) Heat treatment of low magnetic particles The dried low magnetic particles were heat treated at 600°C for 3 hours in an N2 atmosphere to obtain magnetic particles.

[0027] [Example 2] Heat treatment by atmospheric pressure II Magnetic particles were prepared in the same manner as in Example 1, except that the heat treatment of the gel precursor in Example 1 was carried out by immersing it in a water bath at 90° C. for 48 hours.

[0028] [Comparative Example 1] Magnetic particles were prepared in the same manner as in Example 1, except that the heat treatment of the gel precursor in Example 1 was carried out by immersing it in a water bath at 30°C.

[0029] Comparative Example 2 Magnetic particles were prepared in the same manner as in Example 2, except that sodium sulfite was not used in the heat treatment of the gel precursor in Example 2.

[0030] [Example 3] Heat treatment by hydrothermal synthesis I (1) Preparation of desalted and degassed water 480 g of desalted water was degassed with N2 at 2.5 L / min for 30 minutes to prepare desalted and degassed water. (2) Preparation of raw material aqueous solution A raw material aqueous solution was prepared by dissolving 45 g of iron (II) sulfate heptahydrate (FeSO4·7H2O) and 9 g of cobalt (II) sulfate heptahydrate (CoSO4·7H2O) in 118 g of desalted and degassed water. (3) Preparation of alkaline aqueous solution An alkaline aqueous solution was prepared by dissolving 14 g of sodium hydroxide (NaOH) and 50 g of sodium sulfite (Na2SO3) in 240 g of desalted and deaerated water. (4) Preparation of gel precursor The raw material aqueous solution and the alkaline aqueous solution were mixed in a container purged with N2 and stirred for 5 minutes to prepare a gel precursor. (5) Preparation of magnetic particles by hydrothermal treatment of gel precursor The gel precursor was placed in an autoclave purged with N2 and subjected to hydrothermal treatment at 100°C for 20 hours while stirring to obtain magnetic particles. (6) Washing of magnetic particles The magnetic particles were filtered, washed with water, dispersed in 4 L of demineralized water at 80°C, and stirred for 24 hours. They were then dispersed in 1 L of demineralized water whose pH had been adjusted to 2.5 to 3 with 1:1 sulfuric acid, and subjected to ultrasonic waves while stirring. Non-magnetic by-products were then removed by solid-liquid separation using a magnet. (7) Drying of magnetic particles The washed magnetic particles were dried in an air atmosphere at 110° C. for 2 hours.

[0031] [Example 4] Heat treatment by hydrothermal synthesis II Magnetic particles were prepared in the same manner as in Example 3, except that the hydrothermal treatment of the gel precursor in Example 3 was carried out at 190° C. for 4 hours.

[0032] Comparative Example 3 Magnetic particles were prepared in the same manner as in Example 4, except that sodium sulfite was not used in the hydrothermal treatment of the gel precursor in Example 4.

[0033] The various properties of the magnetic particles produced in Examples 1 to 4 and Comparative Examples 1 to 3 are as shown in Table 1 below. [Table 1]

[0034] The cobalt ferrite particles of Examples 1 to 4 produced under the production conditions employed in the production method of the present invention were ferrite particles with a large average particle size. On the other hand, when produced under the conditions of Comparative Example 1, no ferrite particles were produced, and under the conditions of Comparative Examples 2 and 3, only particles with a small average particle size were obtained. The CV values ​​of the particles of Examples 1 to 4 were comparable to those of Comparative Examples 1 to 3.

[0035] The shape of the produced ferrite particles, when observed with an SEM, is as shown in FIGS. FIG. 1 is an SEM image of the powder sample of Example 2. 4 2 shows an SEM image of the powder sample of Example 4, which corresponds to the ferrite particles obtained by heat treatment under normal pressure. 3 The ferrite particles obtained by heat treatment under the hydrothermal conditions corresponding to those in Fig. 1 are shown. All of them are spherical in shape. 3 and 4 are SEM images of the powder samples of Comparative Examples 2 and 3, both of which are ferrite particles obtained by heat treatment in the absence of sulfite. Both are cubic particles, and it can be seen that they are completely different ferrite particles from the ferrite particles in the Examples. From the results of these Examples and Comparative Examples, the technical significance of the presence of sulfite and the establishment of appropriate production conditions corresponding thereto is clear.

[0036] [Example 5] A yellow, transparent peroxotitanic acid solution was prepared by mixing 2.2 mL of titanium tetrachloride solution (16.0-17.0% as Ti), 5.84 g of ammonia water, and 10.0 g of hydrogen peroxide water with 19.8 g of deionized water. 9.92 g of boric anhydride, 11.72 g of potassium chloride, and 2.55 g of sodium hydroxide were dissolved in 535.81 g of deionized water, and 16.75 g of the ferrite particles obtained in Example 2 were suspended in the solution. The peroxotitanic acid solution was added dropwise to the suspension while stirring, and the suspension was then dried to obtain a titanium oxide-coated powder. A reducing solution was prepared by dissolving 4.78 g of glucose, 0.48 g of tartaric acid, and 8.50 g of ethanol in 106.24 g of deionized water. A silver ammine complex solution was prepared by mixing 5 g of sodium hydroxide, 7.00 g of silver nitrate, and 12.00 g of ammonia water with 360 g of deionized water, and 10.40 g of titanium oxide-coated powder was suspended in this solution. The reducing solution was mixed into the suspension while irradiating it with ultrasound, and the suspension was dried to obtain a silver-coated powder. The resulting white powder had a lightness L* of 78.62. [Industrial Applicability]

[0037] The cobalt ferrite particles obtained by the production method of the present invention are spherical in shape and have a uniform particle size, and are therefore expected to be used as copy toners, magnetic inks, and MR fluids.

Claims

1. Cobalt ferrite particles having a spherical shape (excluding hollow shapes) with an average particle size of 0.5 to 2 μm and a CV value of 0.35 to 0.

68.

2. 2. A cobalt ferrite particle according to claim 1, which is a toner for copying.

3. A magnetic ink comprising the cobalt ferrite particles according to claim 1.

4. An MR fluid comprising the cobalt ferrite particles according to claim 1.

5. 2. A white powder comprising the cobalt ferrite particles according to claim 1, and a titanium oxide film and a metallic silver film formed in this order on the surface of the particles.

6. 6. The white powder according to claim 5, having a lightness L* of 75 or more.

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