Composite particles of seed particles and cobalt ferrite particles and method for producing the same

Abstract

The present invention provides a manufacturing method for synthesizing composite particles of seed particles and cobalt ferrite particles, which have a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties, at lower energy and in a simpler manner, and provides composite particles of seed particles and cobalt ferrite particles with uniform particle diameters, which have a rounded shape with a predetermined average circularity, and specific magnetic properties, manufactured thereby. [Solution] A method for producing composite particles of seed particles and cobalt ferrite particles is provided, comprising the steps of: preparing a ferrite precursor by stabilizing a divalent iron salt and a cobalt salt with a complexing agent in an aqueous solution; adding spherical or substantially spherical seed particles to the ferrite precursor; and further heat-treating the ferrite precursor.

Description

[Technical Field]

[0001] The present invention relates to composite particles of seed particles and cobalt ferrite particles and a method for producing the same. The present invention provides composite particles of seed particles and cobalt ferrite particles having a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties. [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 copier toners, magnetic inks, and MR fluids, with expectations for improvements in their quality and performance. Cobalt ferrite, in particular, is known as a magnetic material with high coercivity due to its large crystalline magnetic anisotropy among spinel-type ferrites. Furthermore, because cobalt has similar chemical behavior to iron, it offers the advantage of easy control during its manufacturing process.

[0003] Methods known for producing ferrite particles include coprecipitation, wet oxidation, and hydrothermal methods. The coprecipitation method is a reaction in which two or more types of ions precipitate simultaneously, and when producing cobalt ferrite particles, Fe 3+ and Co 2+ Nano-sized ferrite particles are obtained by adding an alkali to an aqueous solution containing ions and then heating it to accelerate the reaction. In this method, the reaction is carried out at a temperature of 80-100°C, and the average particle size of the resulting particles is about 20-50 nm, meaning that only particles with a relatively broad particle size distribution can be obtained (Patent Document 1).

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

[0005] The hydrothermal method uses Fe 2+ Co 2+ A method is used in which ferrite particles with a relatively large particle size of 0.3 to 8 μm are produced by mixing aqueous solutions containing ions and performing hydrothermal synthesis in an autoclave, through a high-temperature reaction at 160 to 300°C (Patent Document 5).

[0006] Conventional ferrite particle manufacturing methods, such as coprecipitation and wet oxidation, allow for production at relatively low temperatures, but only fine particles on the nanometer order are obtained. Hydrothermal methods, on the other hand, can produce relatively large particles on the micrometer order, but require high temperature and pressure for the hydrothermal reaction (sikkol reaction), which presents 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 [Overview of the project] [Problems that the invention aims to solve]

[0008] In conventional cobalt ferrite particle manufacturing, reducing the particle size resulted in a loss of roundness and an irregular shape. When using cobalt ferrite particles in copier toner, magnetic ink, and MR fluids, a rounded particle surface allows for uniform coating with the minimum necessary amount of surface treatment agent. However, an irregular particle surface increases the amount of surface treatment agent required, resulting in unnecessary costs. One measure for evaluating the degree of roundness of a particle's surface is the average circularity of the particle. Therefore, there was a need for cobalt ferrite particles or composite particles containing cobalt ferrite that possessed a predetermined average circularity and had a rounded surface, allowing for uniform coating with the minimum necessary amount of surface treatment agent. Based on the above, the present invention overcomes the problems of the prior art and provides a manufacturing method that enables the synthesis of composite particles of seed particles and cobalt ferrite particles, having a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties, at lower energy and in a simpler manner. Furthermore, the present invention provides composite particles of seed particles and cobalt ferrite particles manufactured by the above manufacturing method, having a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties. [Means for solving the problem]

[0009] To solve the above problems, the present invention employs a means consisting of the following configuration. (1) A method for producing composite particles of seed particles and cobalt ferrite particles, comprising the steps of: preparing a ferrite precursor by stabilizing a divalent iron salt and a cobalt salt with a complexing agent in an aqueous solution; adding spherical or substantially spherical seed particles to the ferrite precursor; and further heating the ferrite precursor under hydrothermal conditions, wherein the heat treatment is performed by heating in a pressure vessel to a temperature range of 190°C to 210°C, and then further heating to a temperature 3°C to 10°C higher than the said temperature. (2) A method for producing composite particles of seed particles and cobalt ferrite particles as described in (1), wherein the heat treatment is performed by heating in a pressure vessel to a temperature range of 190°C to 210°C, holding at the first attained temperature, and then further heating at a temperature 3°C to 10°C higher than the first attained temperature. (3) A method for producing composite particles of seed particles and cobalt ferrite particles as described in (1) or (2), wherein the heat treatment is performed by heating in a pressure vessel to a temperature range of 190°C to 210°C, holding at the first attained temperature, and then heating at a temperature 3°C to 10°C higher than the first attained temperature and holding at the second attained temperature. (4) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (3), wherein the time of the heat treatment step is 1 to 50 hours. (5) A method for producing composite particles of seed particles and cobalt ferrite particles as described in (2) or (3), wherein the particle is held at the first attainable temperature for 1 to 18 hours. (6) A method for producing composite particles of seed particles and cobalt ferrite particles as described in (3), wherein the mixture is held at the second attainable temperature for 0.5 to 18 hours. (7) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (6), wherein at least one selected from iron, iron alloys, iron oxide, spinel ferrite, or silica is used as the seed particles. (8) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (7), wherein the divalent iron salt and the cobalt salt are iron(II) sulfate and cobalt(II) sulfate. (9) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (8), wherein the divalent iron salt and the cobalt salt are iron(II) chloride and cobalt(II) chloride. (10) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (9), using one selected from citrate, nitrilotriacetate, or malate as the complexing agent. (11) The method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (10), wherein the heat treatment step is performed in the presence of an oxidizing agent in addition to a complexing agent. (12) The method for producing composite particles of seed particles and cobalt ferrite particles according to (11), wherein the oxidizing agent is a nitrate. (13) The method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (12), wherein a trivalent iron salt is further added to the aqueous solution containing the ferrite precursor. (14) The method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (13), wherein a pH buffer is further added to the aqueous solution containing the ferrite precursor. (15) The method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (14), wherein an alkaline aqueous solution or a ferrite precursor is injected into the pressure vessel during or after the heat treatment step, and further heat treatment is performed. (16) Composite particles of seed particles and cobalt ferrite particles having a rounded shape with an average circularity of 0.7 to 1.0, wherein the particle diameter corresponding to the cumulative value of 50% based on the particle size distribution is 1 to 30 μm, and the particle diameter corresponding to the cumulative value of 95% based on the particle size distribution is 5 to 50 μm. (17) Composite particles of seed particles and cobalt ferrite particles according to (16), having a residual magnetic moment of 10 emu / g or more and a coercive force of 100 to 1000 Oe. (18) A toner for copying comprising the composite particles of seed particles and cobalt ferrite particles according to (16). (19) A magnetic ink comprising the composite particles of seed particles and cobalt ferrite particles according to (16). (20) An MR fluid comprising the composite particles of seed particles and cobalt ferrite particles according to (16). (21) A white powder having a titanium oxide film and a metallic silver film in this order on the surface of the composite particles of seed particles and cobalt ferrite particles according to (16). (22) The white powder according to (21), having a lightness L* of 75 or more.

Advantages of the Invention

[0010] By adopting the manufacturing method of the present invention, compared with magnetic particles manufactured by the conventional method, magnetic particles composed of composite particles of seed particles and cobalt ferrite particles can be manufactured with lower energy and in a simpler method, having a rounded shape with a particle diameter within a specific range, having a predetermined average circularity, and having specific magnetic properties. The composite particles of seed particles and cobalt ferrite particles obtained by the manufacturing method of the present invention have a rounded shape and uniform particle diameters, so they are expected to be used as copy toner, magnetic ink, and MR fluid. Further, the composite particles of seed particles and cobalt ferrite particles of the present invention can be whitened by a known method to be white, or further provided with a colored layer to be a white powder with high brightness or a colored powder colored in bright colors.

Brief Description of the Drawings

[0011] [Figure 1] FIG. 1A is a SEM photograph of the composite particles of Example A1. FIG. 1B is a SEM photograph of iron (Fe) used as the seed particles of Example A1. [Figure 2] FIG. 2A is a SEM photograph of the composite particles of Example A2. FIG. 2B is a SEM photograph of iron (Fe) used as the seed particles of Example A2. [Figure 3] FIG. 3A is a SEM photograph of the composite particles of Example A3. FIG. 3B is a SEM photograph of iron (Fe) used as the seed particles of Example A3. [Figure 4] FIG. 4A is a SEM photograph of the composite particles of Example A4. FIG. 4B is a SEM photograph of iron (Fe) used as the seed particles of Example A4. [Figure 5] FIG. 5A is a SEM photograph of the composite particles of Example A5. FIG. 5B is a SEM photograph of iron (Fe) used as the seed particles of Example A5. [Figure 6] FIG. 6A is a SEM photograph of the composite particles of Example A6. FIG. 6B is a SEM photograph of zinc ferrite, which is a kind of spinel ferrite, used as the seed particles of Example A6. [Figure 7] Figure 7A is an SEM image of the composite particles of Example A7. Figure 7B is an SEM image of Alnico, a type of iron-based alloy, used as seed particles in Example A7. [Figure 8] Figure 8A is an SEM image of the composite particles of Example A8. Figure 8B is an SEM image of magnetite, a type of iron oxide, used as seed particles in Example A8. [Figure 9] Figure 9A is an SEM image of the composite particles of Example A9. Figure 9B is an SEM image of the silica used as seed particles in Example A9. [Figure 10] Figure 10A is an SEM image of the composite particles of Example A10. Figure 10B is an SEM image of iron (Fe) used as seed particles in Example A10. [Figure 11] Figure 11A is an SEM image of the composite particles of Reference Example A1. Figure 11B is an SEM image of iron (Fe) used as seed particles in Reference Example A1. [Figure 12] Figure 12A is an SEM image of the composite particles of Reference Example A2. Figure 12B is an SEM image of iron (Fe) used as seed particles in Reference Example A2. [Figure 13] Figure 13A is an SEM image of the composite particles of Comparative Example A1. Figure 13B is an SEM image of the non-spherical magnetite used as seed particles in Comparative Example A1. [Figure 14] Figure 14A is an SEM image of the composite particles of Comparative Example A2. Figure 14B is an SEM image of the non-spherical magnetite used as seed particles in Comparative Example A2. [Figure 15] Figure 15A is an SEM image of the composite particles of Comparative Example A3. The area within the white frame in Figure 15A indicates impurities. Figure 15 is an SEM image of iron (Fe) used as seed particles in Comparative Example A3. [Figure 16] Figures 16A and 16B are conceptual cross-sectional diagrams of the composite particles of Example A1 and Example B1, based on TEM observations. In this specification, scanning electron microscopes may be referred to as "SEM" and transmission electron microscopes as "TEM". [Modes for carrying out the invention]

[0012] [Method for producing composite particles of seed particles and cobalt ferrite particles] The method for producing composite particles of seed particles and cobalt ferrite particles according to this embodiment (hereinafter also simply referred to as "composite particles") is as follows: (i) A step of preparing a ferrite precursor by stabilizing a divalent iron salt and a cobalt salt with a complexing agent in an aqueous solution, (b) A step of adding spherical or substantially spherical seed particles to the ferrite precursor, (h) The process further comprises a step of heat-treating the ferrite precursor under hydrothermal conditions, wherein the heat treatment is performed by heating in a pressure vessel to a temperature range of 190°C to 210°C, and then further heating to a temperature 3°C to 10°C higher than the aforementioned temperature. The manufacturing method for composite particles will be explained below, step by step.

[0013] (i) (Preparation process of ferrite precursor) First, the complexing agent is dissolved in desalted water. This is because complexing the ferrite precursor with the complexing agent before heat treatment protects it from oxidation by the oxidizing agent. The complexing agent is not particularly limited, but citrate, nitrilotriacetate, and malate are preferred. When citrate is used, composite particles of seed particles with a large particle size of about 1 to 30 μm on average and cobalt ferrite particles are obtained. When nitrilotriacetate or malate is used, fine particles with an average particle size of 1 μm or less are obtained. Next, a cobalt salt is dissolved in the aqueous solution. The cobalt salt is not particularly limited, and divalent cobalt salts such as cobalt(II) sulfate, cobalt(II) chloride, cobalt(II) nitrate, and cobalt(II) acetate can be used as examples. Due to their availability, cobalt(II) sulfate or cobalt(II) chloride are preferred. An oxidizing agent may be added at that time. The oxidizing agent is not particularly limited, and examples include nitrates, hypochlorites, chlorates, perchlorates, permanganates, dichromates, chromates, manganeseates, and peroxides. Nitrates are preferred from the viewpoint of improving stability. Subsequently, the aqueous solution containing the dissolved cobalt salt is degassed by reacting it with an inert gas to reduce the dissolved oxygen concentration. Degassing is performed here to prevent the charge state of metal ions such as iron dissolved in the aqueous solution in a later step from being affected by the dissolved oxygen. For example, it is known that if free oxygen is present in the reaction system, divalent iron will be oxidized to trivalent iron, resulting in the generation of particles or impurities with unintended properties.

[0014] Next, a raw material aqueous solution is prepared by dissolving a divalent iron salt in the degassed aqueous solution. The divalent iron salt is not particularly limited, and examples include iron(II) sulfate, iron(II) chloride, iron(II) nitrate, iron(II) acetate, etc. Iron washing wastewater from blast furnaces and electric furnaces can also be used as an inexpensive raw material. Iron(II) sulfate or iron(II) chloride is preferred due to its availability. Furthermore, as will be explained in the section on particle size adjustment below, a trivalent iron salt may be added to the aqueous solution. The trivalent iron salt is not particularly limited, and examples include iron(III) chloride, iron(III) sulfate, iron(III) nitrate, etc.

[0015] Examples of combinations of divalent iron salts and the aforementioned cobalt salts include iron(II) sulfate and cobalt(II) sulfate, or iron(II) chloride and cobalt(II) chloride.

[0016] Next, an alkaline solution is prepared by dissolving the alkali in desalinated water. Any alkali can be selected as the alkali, such as sodium hydroxide, potassium hydroxide, calcium hydroxide, or ammonia. Alternatively, an alkaline solution with the alkali already dissolved can be prepared. Furthermore, a ferrite precursor is prepared by adjusting the pH of the raw material aqueous solution by adding an alkaline aqueous solution. To ensure stable complex formation, it is preferable to dissolve and mix the raw materials in the order described above. In addition, to synthesize ferrite particles with good properties after the complex decomposes, it is preferable to adjust the pH to approximately 6 to 13 after adding the alkaline aqueous solution.

[0017] (b) (Seed particle addition process) Seed particles are added to the ferrite precursor described above. By adding seed particles to the ferrite precursor, a nucleus can be provided that serves as a substrate for the precipitation of cobalt ferrite particles, thereby allowing for adjustment of the particle size of the composite particles of seed particles and cobalt ferrite particles. The seed particles used here are not particularly limited and can be selected from water-insoluble inorganic compounds such as metals, alloys, and oxides. In particular, due to their affinity with cobalt ferrite, iron, iron-based alloys, iron oxides, spinel ferrite, and silica are preferred as seed particles. Magnetite, hematite, goethite, and lepidocrocite can be used as iron oxides. Furthermore, by using spherical or nearly spherical seed particles, the shape of the composite particles of the synthesized seed particles and cobalt ferrite particles can be made closer to spherical.

[0018] In this specification and in the claims, “spherical” means a shape like a sphere. “Approximately spherical” includes shapes that are close to a sphere and polyhedra consisting of multiple faces. Furthermore, in this specification and in the claims, anything that is neither “spherical” nor “approximately spherical” is referred to as “non-spherical.” Examples of “non-spherical” shapes include cubes and rectangular prisms.

[0019] The particle size of the seed particles is 0.1 μm to 50 μm, preferably 0.5 μm to 30 μm, and more preferably 1 μm to 20 μm. Laser diffraction and scattering methods can be used to measure the particle size of the seed particles.

[0020] (h) (heat treatment process) Heat treatment is performed using a hydrothermal method with a pressure vessel. The pressure vessel can be any ordinary high-pressure reaction vessel, such as an autoclave, pressure vessel, or boiler, but an autoclave is preferred due to its versatility. In the conventional high-temperature sikkol method, the reaction often proceeds at high temperatures of 200°C or higher. However, in this embodiment, by selecting a complexing agent, magnetic particles consisting of composite particles of seed particles and cobalt ferrite particles can be synthesized in a temperature range of approximately 130 to 300°C, preferably 130 to 280°C, more preferably 130 to 260°C, and even more preferably 130 to 240°C.

[0021] From the viewpoint of expecting a high reaction rate, the heat treatment temperature is preferably higher than 190°C and below 300°C. On the other hand, impurities tend to be introduced when the reaction temperature exceeds around 270°C. Therefore, from the viewpoint of balancing purity and reaction rate, the reaction temperature is preferably 190-260°C, more preferably 190-240°C, even more preferably 190-210°C, and even more preferably 195-210°C. The heat treatment time is preferably 1 to 50 hours, more preferably 1 to 45 hours, and even more preferably 1 to 40 hours.

[0022] The heat treatment temperature does not need to be constant; it may be performed in multiple stages at different temperatures. For example, it is preferable to heat to a temperature range of 190-210°C as the first stage and hold at that temperature. In one embodiment, the first temperature is preferably held for 1-14 hours, more preferably for 1-12 hours, and even more preferably for 4-12 hours. In another embodiment, the first temperature is preferably held for 1-18 hours, more preferably for 14-18 hours, and even more preferably for 15-17 hours. Furthermore, as a second step, the heat treatment can be performed by heating to a temperature range 3°C to 10°C higher than the 190 to 210°C temperature range, preferably 6°C to 10°C higher than the 190 to 210°C temperature range, more preferably 6°C to 8°C higher than the 190 to 210°C temperature range, and even more preferably 4°C to 6°C higher than the 190 to 210°C temperature range, and then holding at the second achieved temperature. In one embodiment, the second achieved temperature is preferably held for 0.5 to 18 hours, 0.5 to 16 hours, more preferably 0.5 to 12 hours, 0.5 to 8 hours, and even more preferably 0.5 to 6 hours, 0.5 to 5 hours. In another embodiment, the second achieved temperature is preferably held for 16 to 18 hours, 14 to 16 hours, more preferably 10 to 12 hours, 6 to 8 hours, and even more preferably 4 to 6 hours, 2 to 5 hours.

[0023] When heat treatment is performed in multiple stages at different temperatures, the relatively low-temperature stage suppresses excessive oxidation of the ferrite precursor, thereby preventing excessive aggregation of seed particles and contributing to the formation of composite particles of seed particles and cobalt ferrite particles closer to a spherical shape. The relatively high-temperature stage promotes the oxidation of the ferrite precursor, contributing to improved magnetic properties, increased yield, and adjustment of particle size. Furthermore, by performing heat treatment in multiple stages, the total heat treatment time can be shortened. In addition, by setting the difference between the first and second target temperatures to approximately 3-10°C, temperature adjustment during the transition from the first to the second target temperature is easier, and the time to reach the second target temperature can be shortened. Moreover, by setting the difference between the first and second target temperatures to approximately 3-10°C, the precipitation reaction rate of cobalt ferrite at the second target temperature can be controlled, preventing the particle size distribution from becoming broader or the average circularity of the particles from decreasing due to the precipitation of new particles.

[0024] (Role of complexing agents) In ferrite formation reactions, complexing agents play a major role, and the reaction is thought to proceed as follows. Before the start of hydrothermal treatment, oxidation reactions by the oxidizing agent do not occur, and the ferrite precursor remains stable in the aqueous solution due to the complexing action of the ligands in the complexing agent. This prevents the formation of easily oxidized and unstable hydroxides, thus stably protecting the precursor. Next, when heating is started, the complexing agent that protected the metal ions gradually decomposes, making the metal ions more susceptible to oxidation. At this point, an oxidizing agent such as sodium nitrate may be added to uniformly promote the oxidation reaction for ferrite formation. If the ferrite precursor is in an environment with an oxidizing agent, it will be oxidized by the oxidizing agent; even in the absence of an oxidizing agent, it will be oxidized by the hydrothermal environment, and ferrite will be formed.

[0025] The complexing action of the complexing agent can slow down the oxidation reaction of the ferrite precursor during heat treatment under hydrothermal conditions. This allows for a larger particle size of the synthesized ferrite particles, and enables the production of particles with a uniform particle size. Furthermore, by controlling the progress of the oxidation reaction of the ferrite precursor, excessive aggregation of seed particles can be suppressed, resulting in the production of composite particles of seed particles and cobalt ferrite particles with a shape close to spherical.

[0026] (Particle size adjustment 1: Addition of trivalent iron salt) In the method for producing composite particles of seed particles and cobalt ferrite particles according to this embodiment, means for adjusting the particle size of the composite particles of seed particles and cobalt ferrite particles to be produced can be employed in each step of the manufacturing process. Several particle size adjustment means are listed below. These means can be employed individually or in combination. By adding a trivalent iron salt to the raw material aqueous solution (an aqueous solution of divalent iron salt and cobalt salt), the trivalent iron salt is added to the aqueous solution containing divalent iron salt and cobalt salt stabilized by the complexing agent, thereby adjusting the particle size of the composite particles of seed particles and cobalt ferrite particles. As a result, the trivalent iron ions of the trivalent iron salt act as nuclei for ferrite particle formation, promoting the ferrite formation reaction regardless of the presence or absence of an oxidizing agent, and making it possible to adjust the particle size of the composite particles of seed particles and ferrite particles produced. The trivalent iron salts used here are not particularly limited, and examples include iron(III) chloride, iron(III) sulfate, and iron(III) nitrate. Iron washing wastewater from blast furnaces and electric furnaces can also be used as inexpensive raw materials.

[0027] (Particle size adjustment 2: Addition of pH buffer) By adding a pH buffer to an alkaline aqueous solution (an aqueous solution of alkali and a complexing agent), the pH buffer can be added to the aqueous solution containing divalent iron salt and cobalt salt stabilized by the complexing agent. This allows for adjustment of the particle size of the composite particles of seed particles and cobalt ferrite particles. The ferrite formation reaction is a reaction accompanied by a decrease in pH, and when the pH decreases, the decomposition of the ferrite precursor is suppressed, inhibiting the ferrite formation reaction. Therefore, by adding a pH buffer to suppress the decrease in pH, the growth of ferrite particles can be promoted. The pH buffer used here is selected from boric acid, sodium carbonate, sodium bicarbonate, and others.

[0028] (Particle size adjustment 3: Injection of alkaline aqueous solution and ferrite precursor during heat treatment) By injecting an alkaline aqueous solution or a ferrite precursor (complex) into a pressure vessel during or after the heat treatment process, and continuing the heat treatment thereafter, the particle size of the composite particles of seed particles and cobalt ferrite particles can be adjusted. When an alkaline aqueous solution is injected under pressure, the pH in the reaction vessel increases, which promotes the decomposition of unreacted ferrite precursors and accelerates the ferrite formation reaction. This allows for particle growth (coarser particle size). The alkaline aqueous solution is not particularly limited and can be appropriately selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, etc. On the other hand, injecting a ferrite precursor (complex) under pressure adds ferrite-forming raw materials, which can promote the growth of ferrite particles. The method for producing the ferrite precursor is as described above.

[0029] [Composite particle of seed particle and cobalt ferrite particle] For composite particles of seed particles and cobalt ferrite particles, the particle size corresponding to 50% or 95% of the cumulative value based on the particle size distribution can be used as an indicator of particle size. The particle size distribution of composite particles can be measured, for example, using a laser diffraction / scattering particle size distribution analyzer (Microtrac-Bell, model number "MT3300EXII"). The specific particle size distribution is as follows: For composite particles of seed particles and cobalt ferrite particles, the particle size corresponding to 50% of the cumulative value based on the particle size distribution is in the range of 1 to 30 μm, and the particle size corresponding to 95% of the cumulative value based on the particle size distribution is in the range of 5 to 50 μm. The composite particles of seed particles and cobalt ferrite particles are particles having magnetic properties such as an average circularity in the range of 0.7 to 1.0, more preferably 0.70 to 0.95, a remanent magnetic moment of 10 emu / g or more, and a coercivity of 100 to 1000 Oe. The composite particles, consisting of seed particles and cobalt ferrite particles, have a rounded shape and uniform particle size. The aspect ratio of the composite particles is approximately 1. As explained in the descriptions of Figures 16A and 16B below, examples of composite particles of seed particles and cobalt ferrite particles include core-shell type composite particles having a single seed particle as shown in Figure 16A, and sea-island type composite particles B-1 to B-4 having multiple seed particles as shown in Figure 16B. There is no limit to the number of seed particles, but examples include 2 to 15. The composite particles of seed particles and cobalt ferrite particles in this embodiment have a relatively large particle size, are rounded, and have a narrow particle size distribution. As a result, there is little aggregation between particles, and when molded, close packing is possible, which improves the magnetic properties of the molded body or increases its bulk density. Furthermore, because the composite particles of seed particles and cobalt ferrite particles in this embodiment have a rounded shape, the amount of surface treatment agent used when treating the surface of the composite particles can be reduced. Therefore, when used in copier toner, magnetic ink, and MR fluid, their properties can be fully demonstrated.

[0030] (white powder) The composite particles of seed particles and cobalt ferrite particles can be whitened to produce a white powder, or a colored powder can be produced by adding a colored layer after whitening. While whitening can be achieved by known methods, it is preferable to use, for example, the whitening method for which the applicant holds a patent (Patent No. 4113045). This whitening method involves forming a titanium oxide film between substrate particles and a metallic silver film to whiten powder. Specifically, it can be carried out by forming a titanium oxide film on the surface of cobalt ferrite particles by hydrolysis of titanium alkoxide (e.g., International Publication No. 96 / 28269) or a reaction from an aqueous titanium salt solution (e.g., Japanese Patent Application Publication No. 11-131102), 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 having a titanium oxide film and a metallic silver film in that order on the surface of the composite particles of seed particles and cobalt ferrite particles of this embodiment, and as a result, the brightness L* of the composite particles of seed particles and cobalt ferrite particles can be improved to 75 or higher.

[0031] The present applicant has filed a separate application for composite particles of seed particles and cobalt ferrite particles and a method for producing the same. The invention in the separate application provides composite particles of seed particles and cobalt ferrite particles that have a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties. In the separate application, the inventors solved the technical problems by performing a heat treatment of the ferrite precursor in multiple stages, for example, two stages (first temperature reached, second temperature reached), and arrived at the invention in the separate application. In an embodiment of a separate application, the difference between the first and second temperatures reached was approximately 20°C. However, after further intensive research, the inventors discovered that the technical problem could be solved by controlling the difference between the first and second temperatures to approximately 3-10°C, thus completing the present invention. The control of the difference between the first and second arrival temperatures has become significantly easier, resulting in a substantial improvement in workability compared to the separate application and an expected improvement in energy efficiency for manufacturing. The composite particles obtained by the present invention are expected to have performance equivalent to or better than the composite particles obtained by the separate application. This is truly amazing. For reference, the descriptions in the column of examples of the separate application are listed below as Examples A1 to A10, Comparative Examples A1 to A3, Reference Example A1, and Reference Example A2.

Examples

[0032] The present invention will be described in more detail below using examples, but the present invention is not limited to these examples. Also, the particle size distribution, average circularity, and magnetic properties of the manufactured composite particles of the seed particles and cobalt ferrite particles were measured by the following methods.

[0033] (Measurement of particle diameter) The particle size distribution of the composite particles of the seed particles and cobalt ferrite particles was measured using a laser diffraction / scattering particle size distribution measuring device (manufactured by Microtrac Bell, model number "MT3300EXII"), and the particle diameter D 50 corresponding to 50% of the cumulative value based on the particle size distribution, and the particle diameter D 95 corresponding to 95% of the cumulative value based on the particle size distribution were evaluated.

[0034] (SEM observation) The surface of the obtained composite particles was observed using a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, model number "S-4800").

[0035] (Measurement of average circularity) The average circularity of the composite particles of the seed particles and cobalt ferrite particles was evaluated by image analysis of SEM photographs (software manufactured by Mountech Co., Ltd., "Mac-View Ver.4"). For 100 or more composite particles per sample, the circularity 4πS / L 2 was calculated using the projected area S and the perimeter L on the SEM photograph, and the average value was taken as the average circularity.

[0036] (Measurement of magnetic properties) The magnetic properties of composite particles of seed particles and cobalt ferrite particles were evaluated using a vibrating sample magnetometer (VSM) (Tamagawa Seisakusho, model number "TM-VSM101483N7-MRO"). Hysteresis curves or demagnetization curves were obtained at a maximum magnetic field of 10,000 Oe, and the saturation magnetic moment, remanent magnetic moment, and coercivity were measured.

[0037] [Example A1] (Production of composite particles of seed particles and cobalt ferrite particles) (1) Preparation of aqueous solution of raw materials 604.73 g of trisodium citrate dihydrate (C6H5Na3O7·2H2O), 95.38 g of cobalt(II) sulfate heptahydrate (CoSO4·7H2O), and 17.30 g of sodium nitrate (NaNO3) were dissolved in 1537.55 g of desalinated water. After degassing with N2, 471.72 g of iron(II) sulfate heptahydrate (FeSO4·7H2O) was dissolved to prepare the raw material aqueous solution. (2) Preparation of ferrite precursors While continuing degassing with N2, a 48.5% sodium hydroxide aqueous solution was added to the raw material aqueous solution, and the pH was adjusted to 7.5 to prepare a ferrite precursor. (3) Preparation of magnetic particles by hydrothermal treatment 50.00 g of iron (Fe) as seed particles was placed in an N2-substituted autoclave along with a ferrite precursor, and the mixture was subjected to hydrothermal treatment at 200°C for 16 hours while stirring to obtain magnetic particles. (4) Cleaning of magnetic particles The magnetic particles were filtered and washed with desalinated water. (5) Drying of magnetic particles The cleaned magnetic particles were dried at 250°C for 1 hour in an air atmosphere.

[0038] [Example A2] In Example A1, magnetic particles were produced under the same conditions as in Example A1, except that the amount of iron (Fe) was changed to 100.00 g in (3) preparation of magnetic particles by hydrothermal treatment.

[0039] [Example A3] In Example A1 (3) Preparation of magnetic particles by hydrothermal treatment, the amount of iron (Fe) was changed to 75.00 g, and the hydrothermal treatment was changed to a two-stage process: the first stage at 200°C for 12 hours, and the second stage at 220°C for 2 hours. Otherwise, magnetic particles were produced under the same conditions as in Example A1.

[0040] [Example A4] In Example A1, the magnetic particles were produced under the same conditions as in Example A1, except that the hydrothermal treatment was changed to a two-stage process: the first stage at 200°C for 4 hours, and the second stage at 240°C for 30 minutes.

[0041] [Example A5] In Example A1, the magnetic particles were produced under the same conditions as in Example A1, except that the amount of iron (Fe) was changed to 25.00 g and the hydrothermal treatment was performed at 190°C for 40 hours.

[0042] [Example A6] In Example A1, the amount of trisodium citrate dihydrate (C6H5Na3O7·2H2O) was changed to 778.36 g in (1) preparation of the raw material aqueous solution, and in (3) preparation of magnetic particles by hydrothermal treatment, the amount of iron (Fe) used as seed particles was changed to 14.10 g of zinc ferrite, a type of spinel ferrite. Otherwise, magnetic particles were produced under the same conditions as in Example A1.

[0043] [Example A7] In Example A1, (3) Preparation of magnetic particles by hydrothermal treatment, the magnetic particles were produced under the same conditions as in Example A1, except that the iron (Fe) used as seed particles was changed to alnico, a type of iron-based alloy.

[0044] [Example A8] In Example A1, (3) Preparation of magnetic particles by hydrothermal treatment, magnetic particles were produced under the same conditions as in Example A1, except that 50.00 g of iron (Fe) as seed particles was replaced with 25.00 g of magnetite, a type of iron oxide.

[0045] [Example A9] In Example A1, the magnetic particles were produced under the same conditions as in Example A1, except that in (3) preparation of magnetic particles by hydrothermal treatment, 50.00 g of iron (Fe) as seed particles was replaced with 6.04 g of silica.

[0046] [Example A10] In Example A1, the amount of iron (Fe) was changed to 14.10 g in the preparation of the raw material aqueous solution (1) from 471.72 g of iron(II) sulfate heptahydrate (FeSO4·7H2O) to 337.29 g of iron(II) chloride tetrahydrate (FeCl2·4H2O), and the amount of cobalt(II) sulfate heptahydrate (CoSO4·7H2O) to 80.73 g of cobalt(II) chloride hexahydrate (CoCl2·6H2O). In Example A1, the amount of iron (Fe) was changed to 14.10 g in the preparation of the magnetic particles by hydrothermal treatment. Otherwise, magnetic particles were produced under the same conditions as in Example A1.

[0047] [Comparative Example A1] In Comparative Example A1, magnetic particles were produced under the same conditions as in Example A1, except that in (3) preparation of magnetic particles by hydrothermal treatment, 50.00 g of iron (Fe) was replaced with 25.00 g of magnetite (Fe3O4), a type of iron oxide. Non-spherical magnetite was used in Comparative Example A1.

[0048] [Comparative example A2] In Comparative Example A1, magnetic particles were produced under the same conditions as in Example A1, except that in (3) preparation of magnetic particles by hydrothermal treatment, 50.00 g of iron (Fe) was replaced with 50.00 g of magnetite (Fe3O4), a type of iron oxide. Non-spherical magnetite was used in Comparative Example A2.

[0049] [Comparative example A3] Magnetic particles were produced under the same conditions as in Example A1, except that trisodium citrate dihydrate (C6H5Na3O7·2H2O) was not used as a complexing agent and sodium nitrate (NaNO3) was not used as an oxidizing agent in (1) preparation of the raw material aqueous solution.

[0050] Table 1 summarizes the various properties of the composite particles obtained in each example, reference example, and comparative example. Figures 1A, 1B to 15A, and 15B show SEM images of the composite particles and their seed particles from Examples A1 to A10, Reference Example A1, Reference Example A2, and Comparative Examples A1 to A3.

[0051] [Table 1]

[0052] The reaction rates for Examples A1-A3, A5-A10, Reference Example A1, Reference Example A2, Comparative Example A1, and Comparative Example A2 in the table were calculated according to formula A below. The reaction rate for Example A4 was calculated according to formula B. The reaction rate for Comparative Example A3 could not be calculated because impurities were present in the reaction product. Formula A: (Mass of powder actually obtained by hydrothermal treatment of ferrite precursor - Amount of seed particles charged) / {Raw material charged} *2 Cobalt ferrite (Co) is obtained when all the iron and cobalt inside react. x Fe (3-x) Theoretical mass of O4) × 100 Equation B: {1 - (mass concentration of iron and cobalt dissolved in the aqueous solution after hydrothermal treatment) / (theoretical mass concentration of iron and cobalt in the ferrite precursor before hydrothermal treatment)} × 100 *2: The iron and cobalt in the raw materials used do not include the mass of iron and cobalt contained in the seed particles.

[0053] The composite particles of Example A1 had an average circularity of 0.81, with a particle size of 5.58 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 9.81 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 1A, the composite particles of Example A1 had a rounded shape.

[0054] The composite particles of Example A2 had an average circularity of 0.79, with a particle size of 4.63 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 8.10 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 2A, the composite particles of Example A2 had a rounded shape.

[0055] In Example A3, the composite particles were subjected to a two-stage heat treatment with a first temperature of 200°C and a second temperature of 220°C. The average circularity was 0.80, the particle size corresponding to 50% of the cumulative value based on the particle size distribution was 5.85 μm, and the particle size corresponding to 95% of the cumulative value based on the particle size distribution was 11.32 μm. As shown in Figure 3A, the composite particles of Example A3 had a rounded shape.

[0056] In Example A4, the composite particles were subjected to a two-stage heat treatment with a first temperature of 200°C and a second temperature of 240°C. The average circularity was 0.76, the particle size corresponding to 50% of the cumulative value based on the particle size distribution was 6.23 μm, and the particle size corresponding to 95% of the cumulative value based on the particle size distribution was 11.89 μm. As shown in Figure 4A, the composite particles of Example A4 had a rounded shape.

[0057] The composite particles of Example A5, which were heat-treated at 190°C, had an average circularity of 0.80, with a particle size of 5.17 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 9.38 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 5A, the composite particles of Example A5 had a rounded shape.

[0058] In Example A6, the composite particles, which used zinc ferrite, a type of spinel ferrite, as seed particles, had an average circularity of 0.92, a particle size of 29.73 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 47.31 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 6A, the composite particles of Example A6 had a rounded shape.

[0059] In Example A7, which used Alnico, an iron-based alloy, as seed particles, the composite particles had an average circularity of 0.86, a particle size corresponding to 50% of the cumulative value based on the particle size distribution of 11.27 μm, and a particle size corresponding to 95% of the cumulative value based on the particle size distribution of 24.41 μm. As shown in Figure 7A, the composite particles of Example A7 had a rounded shape.

[0060] In Example A8, where approximately spherical iron oxide was used as seed particles, the composite particles had an average circularity of 0.80, with a particle size of 23.16 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 38.43 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 8A, the composite particles of Example A8 had a rounded shape.

[0061] In Example A9, where silica was used as seed particle, the composite particles had an average circularity of 0.70, with a particle size of 9.06 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 15.47 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 9A, the composite particles of Example A9 had a rounded shape.

[0062] In the preparation of the raw material aqueous solution, the composite particles of Example A10, in which iron(II) chloride tetrahydrate was added as a divalent iron salt and cobalt(II) chloride hexahydrate as a cobalt salt, had an average circularity of 0.88, a particle size corresponding to 50% of the cumulative value based on the particle size distribution of 14.21 μm, and a particle size corresponding to 95% of the cumulative value based on the particle size distribution of 25.58 μm. As shown in Figure 10A, the composite particles of Example A10 had a rounded shape.

[0063] In comparative example A1, which used non-spherical magnetite as seed particles, the composite particles had an average circularity of 0.69, with a particle size of 5.26 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 9.46 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 13A, the composite particles of Comparative Example A1 were not rounded and had an irregular shape.

[0064] In comparative example A2, which used non-spherical magnetite as seed particles, the composite particles had an average circularity of 0.63, with a particle size of 3.96 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 6.98 μm corresponding to 95% of the cumulative value based on the particle size distribution. As shown in Figure 14A, the composite particles of Comparative Example A2 were not rounded and had an irregular shape.

[0065] The composite particles of Comparative Example A3, which contained no complexing agent or oxidizing agent, had an average circularity of 0.53, with a particle size of 4.48 μm corresponding to 50% of the cumulative value based on the particle size distribution, and a particle size of 19.57 μm corresponding to 95% of the cumulative value based on the particle size distribution. Furthermore, as shown within the white frame in Figure 15A, the reaction product of Comparative Example A3 contained impurities. As shown in Figure 15A, the composite particles of Comparative Example A3 were not rounded and had an irregular shape.

[0066] The results in Table 1 show that in all of Examples A1 to A10, by adding seed particles to a ferrite precursor stabilized with a complexing agent and then heat-treating it, composite particles of seed particles and cobalt ferrite particles were formed, having an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties.

[0067] In Example A1, the temperature was 200°C and the holding time was 16 hours. In Example A3, which involved heating at 200°C in the first stage and 220°C in the second stage, the total holding time was 14 hours. In Example A4, which involved heating at 200°C in the first stage and 240°C in the second stage, the total holding time was 4 hours and 30 minutes. This demonstrates that the total heating time is shortened when the heating is performed in two stages.

[0068] The composite particles of Example A5 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. From the results of Example A5, it was found that even when the heating temperature is 190°C, composite particles of seed particles and cobalt ferrite particles can be obtained that have an average circularity of 0.7 to 1.0, a rounded shape, and specific magnetic properties.

[0069] The composite particles of Example A6 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. Based on the particle size distribution of Example A6, the particle size corresponding to 50% of the cumulative value was 29.73 μm, and the particle size corresponding to 95% of the cumulative value was 47.31 μm. This indicates that the manufacturing method of composite particles of seed particles and cobalt ferrite particles can produce particles with a particle size of 30 μm or less corresponding to 50% of the cumulative value and particles with a particle size of 50 μm or less corresponding to 95% of the cumulative value.

[0070] The composite particles of Examples A7 to A9 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. From the results of Examples A1 and A6 to A9, it was found that when iron, iron-based alloys, iron oxide, spinel ferrite, and silica are used as seed particles, composite particles of seed particles and cobalt ferrite particles can be obtained that have an average circularity of 0.7 to 1.0, a rounded shape, and specific magnetic properties.

[0071] The composite particles of Example A10 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. From the results of Example A10, it was found that even when iron(II) chloride is used as the divalent iron salt and cobalt(II) chloride is used as the cobalt salt in the ferrite precursor preparation process, composite particles of seed particles and cobalt ferrite particles can be obtained that have an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties.

[0072] As shown in Figures 1B to 10B, the shapes of the iron (Fe), zinc ferrite (a type of spinel ferrite), alnico (a type of iron alloy), magnetite (a type of iron oxide), and silica used as seed particles in Examples A1 to A10 were spherical or nearly spherical. On the other hand, as shown in Figures 13B and 14B, the shape of the magnetite used as seed particles in Comparative Examples A1 and A2 was not spherical or nearly spherical. Comparing Examples A1-A10 with Comparative Examples A1 and A2, it was found that by using spherical or nearly spherical seed particles, composite particles of seed particles and cobalt ferrite particles can be obtained that have an average circularity in the range of 0.7-1.0, a rounded shape, and specific magnetic properties.

[0073] Transmission electron microscopy (TEM) observation was performed to confirm the cross-sectional structure of the obtained composite particles. Figures 16A and 16B show conceptual cross-sectional diagrams of the composite particles of Example A1 and Example B1, respectively, as representative examples with seed particles. It was confirmed that the obtained composite particles were either core-shell type composite particles A as shown in Figure 16A, or sea-island type composite particles B (B-1 to B-4) with multiple seed particles, as shown in Figure 16B, formed by the aggregation of composite particles A with each other. In this specification, when the term "particle diameter" is used, it refers to the diameter of the outermost shell of the composite particle, as indicated by arrow a in Figure 16A and arrow b in Figure 16B.

[0074] From the results of Reference Example A1 in Table 1, it was found that composite particles of seed particles and cobalt ferrite particles can be obtained even without the addition of an oxidizing agent. The reaction rate of Reference Example A1 was 8.6%, which was lower than the reaction rate of 79.9% in Example A1.

[0075] The test conditions for Reference Example A1 were the same as those for Example A1, except that sodium nitrate (NaNO3), which was added as an oxidizing agent in (1) preparation of the raw material aqueous solution in Example A1, was not used. From this, a comparison between Example A1 and Reference Example A1 revealed that the reaction rate was significantly improved by adding an oxidizing agent.

[0076] The composite particles in Reference Example A2 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. The reaction rate of Reference Example A2 was 63.5%, which was higher than that of Reference Example A1. The test conditions for Reference Example A2 were the same as those for Example A1, except that (1) sodium nitrate (NaNO3), which was added as an oxidizing agent in the preparation of the raw material aqueous solution, was not used; and (3) in the preparation of magnetic particles by hydrothermal treatment, the hydrothermal treatment was changed to two stages: the first stage at 200°C for 12 hours and the second stage at 220°C for 10 hours. By comparing Reference Example A1 and Reference Example A2, it was found that even without adding an oxidizing agent, increasing the heat treatment temperature and performing the process in two stages improved the average circularity and reaction rate of the composite particles. Composite particles were obtained with an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties, consisting of seed particles and cobalt ferrite particles.

[0077] [Example A11] (Whitening of cobalt ferrite particles) A yellow, transparent peroxotitanic acid solution was prepared by mixing 4.49 g of tetra-i-propoxytitanite solution (27.9-28.5% asTi), 14.40 g of aqueous ammonia, and 12.50 g of aqueous hydrogen peroxide in 23.30 g of deionized water. 4.95 g of anhydrous boric acid, 5.96 g of potassium chloride, and 1.28 g of sodium hydroxide were dissolved in 267.81 g of deionized water, and 122.50 g of the composite particles from Example A1 were suspended in the solution. The peroxotitanic acid solution was added dropwise while stirring the suspension, and then the suspension was dried to obtain titanium dioxide coated powder. A reducing solution was prepared by dissolving 9.08 g of glucose, 1.01 g of tartaric acid, 8.15 g of ethanol, 0.92 g of n-propanol, and 0.47 g of i-propanol in 100.38 g of deionized water. A silver ammine complex solution was prepared by mixing 5.00 g of sodium hydroxide, 7.00 g of silver nitrate, and 12.00 g of aqueous ammonia in 360.00 g of deionized water, and 29.70 g of titanium dioxide 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 film coated powder. The obtained white powder had a lightness L* of 79.73.

[0078] [Reference example A1] Magnetic particles were produced under the same conditions as in Example A1, except that sodium nitrate (NaNO3), which was added as an oxidizing agent in (1) preparation of the raw material aqueous solution, was not used. [Reference example A2] Except for (1) not using sodium nitrate (NaNO3) as an oxidizing agent in the preparation of the raw material aqueous solution in Example A1, and (3) changing the preparation of magnetic particles by hydrothermal treatment to a two-stage process: the first stage at 200°C for 12 hours and the second stage at 220°C for 10 hours, magnetic particles were produced under the same conditions as in Example A1.

[0079] Examples of this application are described below as Examples B1 to B4 and Comparative Examples B1 and B2. Unless otherwise specified, the measurement and observation methods, such as particle size measurement, SEM observation, and magnetic property measurement, are the same as those in Example A.

[0080] [Example B1] (Production of composite particles of seed particles and cobalt ferrite particles) (1) Preparation of aqueous solution of raw materials Dissolve 604.73g of trisodium citrate dihydrate (C6H5Na3O7·2H2O), 95.38g of cobalt(II) sulfate heptahydrate (CoSO4·7H2O), and 17.30g of sodium nitrate (NaNO3) in 1537.55g of desalinated water, degas with N2, and then dissolve 471.72g of iron(II) sulfate heptahydrate (FeSO4·7H2O) to prepare the raw material aqueous solution. (2) Preparation of ferrite precursors While continuing degassing with N2, a 48.5% sodium hydroxide aqueous solution is added to the raw material aqueous solution to adjust the pH to 7.5, thereby preparing a ferrite precursor. (3) Preparation of magnetic particles by hydrothermal treatment 75.00 g of iron (Fe) as seed particles, along with a ferrite precursor, was placed in an N2-substituted autoclave. After hydrothermal treatment at 200°C for 12 hours with stirring, the mixture was further hydrothermally treated at 203°C for 4 hours to obtain magnetic particles. (4) Cleaning of magnetic particles The magnetic particles are filtered and then washed with desalinated water. (5) Drying of magnetic particles The cleaned magnetic particles are dried at 250°C for 1 hour in an air atmosphere. [Example B2] In Example B1, the magnetic particles were produced under the same conditions as in Example B1, except that the first step was changed to two steps: 12 hours at 200°C and 4 hours at 204°C. [Example B3] In Example B1 (3) Preparation of magnetic particles by hydrothermal treatment, the magnetic particles are manufactured under the same conditions as in Example B1, except that the first step is changed to two steps: 12 hours at 200°C and 3 hours at 206°C. [Example B4] In Example B1, the magnetic particles are produced under the same conditions as in Example B1, except that the first step is changed to two steps: 12 hours at 200°C and 2 hours at 210°C.

[0081] [Comparative Example B1] In Example B1 (3) Preparation of magnetic particles by hydrothermal treatment, the magnetic particles are manufactured under the same conditions as in Example B1, except that the first step is changed to two steps: 12 hours at 200°C and 4 hours at 202°C.

[0082] [Comparative example B2] In Example B1, the magnetic particles are produced under the same conditions as in Example B1, except that the first step is changed to two steps: 12 hours at 200°C and 2 hours at 211°C.

[0083] Examples B1 to B4 yield composite particles of seed particles and cobalt ferrite particles with an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. On the other hand, Comparative Example B2 does not yield composite particles of seed particles and cobalt ferrite particles with an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. Comparative Example 1 yielded composite particles of seed particles and cobalt ferrite particles with an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties, but the heat treatment took longer than in Examples B1 to B4.

[0084] A comparison of Examples B1-B4 with Examples A1-A10 shows that the composite particles of seed particles and cobalt ferrite particles in Examples B1-B4 are rounded and of excellent quality. Therefore, a method comprising the steps of preparing a ferrite precursor by stabilizing a divalent iron salt and a cobalt salt with a complexing agent in an aqueous solution, adding spherical or substantially spherical seed particles to the ferrite precursor, and further heat-treating the ferrite precursor under hydrothermal conditions, wherein the heat treatment involves heating in a pressure vessel to a temperature range of 190°C to 210°C, and then further heating to a temperature 3°C to 10°C higher than the aforementioned temperature, allows for the production of composite particles of seed particles and cobalt ferrite particles with a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties, using lower energy and a simpler method. [Industrial applicability]

[0085] The composite particles of seed particles and cobalt ferrite particles obtained by the manufacturing method of the present invention have a rounded shape and uniform particle size, making them promising for use as copier toner, magnetic ink, and MR fluid.

Claims

1. A step of preparing a ferrite precursor by stabilizing a divalent iron salt and a cobalt salt with a complexing agent in an aqueous solution, A step of adding spherical or substantially spherical seed particles to the ferrite precursor, Furthermore, the process includes a step of heat-treating the ferrite precursor under hydrothermal conditions. A method for producing composite particles of seed particles and cobalt ferrite particles, wherein the heat treatment is performed by heating in a pressure vessel to a temperature range of 190°C to 210°C, and then further heating to a temperature 3°C to 10°C higher than the aforementioned temperature.

2. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1, wherein the heat treatment is performed by heating in a pressure vessel to a temperature range of 190°C to 210°C, holding at the first achieved temperature, and further heating at a temperature 3°C to 10°C higher than the first achieved temperature.

3. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein the heat treatment is performed by heating in a pressure vessel to a temperature range of 190°C to 210°C, holding at the first achieved temperature, and then heating at a temperature 3°C to 10°C higher than the first achieved temperature, and holding at the second achieved temperature.

4. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein the time of the heat treatment step is 1 to 50 hours.

5. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, wherein the particle is held at the first attainable temperature for 1 to 18 hours.

6. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 3, wherein the second temperature to be reached is maintained for 0.5 to 18 hours.

7. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein at least one selected from iron, iron alloys, iron oxide, spinel ferrite, or silica is used as the seed particles.

8. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein the divalent iron salt and the cobalt salt are iron(II) sulfate and cobalt(II) sulfate.

9. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein the divalent iron salt and the cobalt salt are iron(II) chloride and cobalt(II) chloride.

10. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein one selected from citrate, nitrilotriacetate, or malate is used as the complexing agent.

11. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein the heat treatment step is carried out in the presence of an oxidizing agent in addition to the complexing agent.

12. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 11, wherein the oxidizing agent is a nitrate.

13. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, further comprising adding a trivalent iron salt to the aqueous solution containing the ferrite precursor.

14. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, further comprising adding a pH buffer to the aqueous solution containing the ferrite precursor.

15. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1 or 2, wherein an alkaline aqueous solution or a ferrite precursor is injected under pressure into the pressure vessel during or after the heat treatment step, and further heat treatment is performed.

16. It has a rounded shape with an average roundness of 0.7 to 1.

0. The particle size corresponding to 50% of the cumulative value based on the particle size distribution is 1 to 30 μm. The particle size corresponding to 95% of the cumulative value based on the particle size distribution is 5 to 50 μm. A composite particle consisting of a seed particle and a cobalt ferrite particle.

17. A composite particle of seed particles and cobalt ferrite particles according to claim 16, wherein the residual magnetic moment is 10 emu / g or more and the coercivity is 100 to 1000 Oe.

18. A copier toner comprising composite particles of seed particles and cobalt ferrite particles as described in claim 16.

19. A magnetic ink comprising composite particles of seed particles and cobalt ferrite particles as described in claim 16.

20. An MR fluid comprising composite particles of seed particles and cobalt ferrite particles as described in claim 16.

21. A white powder having a titanium oxide film and a metallic silver film in that order on the surface of a composite particle of seed particles and cobalt ferrite particles as described in claim 16.

22. The white powder according to claim 21, wherein the brightness L* is 75 or higher.

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