Composite particles consist of cobalt ferrite and cobalt ferrite seeds, and this is the method of producing these particles.

Abstract

The invention relates to a method capable of producing composite particles of cobalt ferrite seeds and particles with lower energy, the composite particles having a diameter within a defined range, a round shape with a predetermined mean roundness, and characteristic magnetic properties; and a composite particle of cobalt ferrite seeds and particles produced by this method having a round shape with a predetermined mean roundness, characteristic magnetic properties, and uniform particle diameter. The method for producing composite particles of cobalt ferrite seeds and particles described is to include the following steps: preparation of ferrite precursor in aqueous solution with divalent iron salt and cobalt salt stabilized by a complexing agent; addition of spherical or nearly spherical seeds to the ferrite precursor; and further heat treatment of the ferrite precursor.

Description

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

[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, which have a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties.

[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-type ferrites. Furthermore, cobalt's chemical behavior is similar to that of iron, which allows for easy control of various aspects of its manufacturing process.

[0003] Known methods for producing ferrite particles include a coprecipitation method, a wet oxidation method, and a hydrothermal method. The coprecipitation method 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 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 Documents 2 and 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 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, the coprecipitation method or wet oxidation method can be used to produce ferrite particles at a relatively low temperature, 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 (a sicol reaction) at high temperature and pressure, which poses problems in terms of equipment and cost.

[0007] Patent No. 4138344 Publication of Japanese Patent Publication No. 3-24412 Publication of Japanese Patent Publication No. 60-47722 Publication of Patent No. 5504399 Publication of Japanese Patent Publication No. 5-275224

[0008] In previous cobalt ferrite particle production studies, reducing the particle size resulted in a loss of roundness and an irregular shape. When using cobalt ferrite particles in copy toners, magnetic inks, and MR fluids, a rounded particle surface allows for uniform coating of the particle surface with a minimal amount of surface treatment agent. However, an irregular particle surface increases the amount of surface treatment agent used, resulting in unnecessary costs. The average particle circularity can be considered as one measure for evaluating the degree of roundness of the particle surface. Therefore, there has been a need for cobalt ferrite particles or composite particles containing cobalt ferrite that have a predetermined average circularity and a rounded particle surface, allowing for uniform coating of the surface treatment agent with a minimal amount. Based on the above, the present invention overcomes the problems of the prior art and provides a production method that can synthesize composite particles of seed particles and cobalt ferrite particles with a specific particle size range, a rounded shape with a predetermined average circularity, and specific magnetic properties, using lower energy. The present invention also provides composite particles of seed particles and cobalt ferrite particles, which are produced by the above-mentioned production method, and which have a particle diameter within a specific range, a rounded shape with a predetermined average circularity, and specific magnetic properties.

[0009] To solve the above problems, the present invention employs the following means. (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 in an aqueous solution with a complexing agent, adding spherical or nearly spherical seed particles to the ferrite precursor, and further heat-treating the ferrite precursor. (2) A method for producing composite particles of seed particles and cobalt ferrite particles according to (1), in which the heat treatment is carried out under hydrothermal conditions in a temperature range of 130°C to 260°C in a pressure vessel. (3) A method for producing composite particles of seed particles and cobalt ferrite particles according to (2), in which the heat treatment is carried out under hydrothermal conditions in a temperature range of 190°C to 240°C in a pressure vessel. (4) A method for producing composite particles of seed particles and cobalt ferrite particles according to (2) or (3), in which the heat treatment time is 1 to 50 hours. (5) A method for producing composite particles of seed particles and cobalt ferrite particles according to (3), wherein the heat treatment is carried out under hydrothermal conditions by heating to a temperature range of 190°C to 210°C, maintaining the temperature at the first temperature reached, and further heating to a temperature range of 210°C to 240°C, and maintaining the temperature at the second temperature reached. (6) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (5), wherein the first temperature reached is maintained for 1 to 14 hours. (7) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (6), wherein the second temperature reached is maintained for 0.5 to 12 hours. (8) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (7), wherein the seed particles are at least one selected from iron, iron-based alloys, iron oxide, spinel ferrite, and silica. (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 cobalt salt are iron(II) sulfate and cobalt(II) sulfate. (10) 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 cobalt salt are iron(II) chloride and cobalt(II) chloride.(11) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (10), wherein the complexing agent is one selected from citrate, nitrilotriacetate, and malate. (12) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (11), wherein the heat treatment step is carried out in the presence of an oxidizing agent in addition to the complexing agent. (13) A method for producing composite particles of seed particles and cobalt ferrite particles according to (12), wherein the oxidizing agent is a nitrate. (14) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (13), wherein a trivalent iron salt is further added to the aqueous solution containing the ferrite precursor. (15) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (14), wherein a pH buffer is further added to the aqueous solution containing the ferrite precursor. (16) A method for producing composite particles of seed particles and cobalt ferrite particles according to any one of (1) to (15), comprising pressurizing an alkaline aqueous solution or a ferrite precursor into a pressure vessel during or after the heat treatment step, and further heat treatment. (17) Composite particles of seed particles and cobalt ferrite particles, having a rounded shape with an average circularity of 0.7 to 1.0, a particle diameter corresponding to 50% of the cumulative value based on the particle diameter distribution of 1 to 30 μm, and a particle diameter corresponding to 95% of the cumulative value based on the particle diameter distribution of 5 to 50 μm. (18) Composite particles of seed particles and cobalt ferrite particles according to (17), having a remanent magnetic moment of 10 emu / g or more and a coercive force of 100 to 1000 Oe. (19) A toner for copying comprising composite particles of seed particles and cobalt ferrite particles according to (18). (20) A magnetic ink comprising composite particles of the seed particles and cobalt ferrite particles according to (18). (21) An MR fluid comprising composite particles of the seed particles and cobalt ferrite particles according to (18). (22) A white powder having a titanium oxide film and a metallic silver film in this order on the surface of the composite particle of the seed particles and cobalt ferrite particles according to (18). (23) The white powder according to (22), having a lightness L* of 75 or more.

[0010] By adopting the manufacturing method of the present invention, magnetic particles consisting of composite particles of seed particles and cobalt ferrite particles can be produced with lower energy than magnetic particles produced by conventional methods, having a particle size within a specific range, a rounded shape with a predetermined average circularity, and 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 a uniform particle size, and are therefore expected to be used as copy toners, magnetic inks, and MR fluids. Furthermore, the composite particles of seed particles and cobalt ferrite particles of the present invention can be whitened by a known method, or can be further provided with a colored layer to produce a bright white powder or a vividly colored colored powder.

[0011] FIG. 1A is an SEM photograph of the composite particles of Example 1. FIG. 1B is an SEM photograph of iron (Fe) used as seed particles in Example 1. FIG. 2A is an SEM photograph of the composite particles of Example 2. FIG. 2B is an SEM photograph of iron (Fe) used as seed particles in Example 2. FIG. 3A is an SEM photograph of the composite particles of Example 3. FIG. 3B is an SEM photograph of iron (Fe) used as seed particles in Example 3. FIG. 4A is an SEM photograph of the composite particles of Example 4. FIG. 4B is an SEM photograph of iron (Fe) used as seed particles in Example 4. FIG. 5A is an SEM photograph of the composite particles of Example 5. FIG. 5B is an SEM photograph of iron (Fe) used as seed particles in Example 5. FIG. 6A is an SEM photograph of the composite particles of Example 6. FIG. 6B is an SEM photograph of zinc ferrite, a type of spinel ferrite, used as seed particles in Example 6. FIG. 7A is a SEM photograph of composite particles of Example 7. FIG. 7B is a SEM photograph of alnico, a type of iron-based alloy, used as seed particles in Example 7. FIG. 8A is a SEM photograph of composite particles of Example 8. FIG. 8B is a SEM photograph of magnetite, a type of iron oxide, used as seed particles in Example 8. FIG. 9A is a SEM photograph of composite particles of Example 9. FIG. 9B is a SEM photograph of silica used as seed particles in Example 9. FIG. 10A is a SEM photograph of composite particles of Example 10. FIG. 10B is a SEM photograph of iron (Fe) used as seed particles in Example 10. FIG. 11A is a SEM photograph of composite particles of Reference Example 1. FIG. 11B is a SEM photograph of iron (Fe) used as seed particles in Reference Example 1. FIG. 12A is a SEM photograph of composite particles of Reference Example 2. FIG. 12B is a SEM photograph of iron (Fe) used as seed particles in Reference Example 2. Fig. 13A is an SEM photograph of the composite particles of Comparative Example 1. Fig. 13B is an SEM photograph of the non-spherical magnetite used as seed particles in Comparative Example 1. Fig. 14A is an SEM photograph of the composite particles of Comparative Example 2. Fig. 14B is an SEM photograph of the non-spherical magnetite used as seed particles in Comparative Example 2. Fig. 15A is an SEM photograph of the composite particles of Comparative Example 3. The white frame in Fig. 15A indicates impurities. Fig. 15 is an SEM photograph of iron (Fe) used as seed particles in Comparative Example 3. Figs. 16A and 16B are cross-sectional conceptual diagrams of the composite particles of Example 1 based on TEM observation.In this specification, a scanning electron microscope may be abbreviated as "SEM" and a transmission electron microscope as "TEM."

[0012] [Method for producing composite particles of seed particles and cobalt ferrite particles] A method for producing composite particles of seed particles and cobalt ferrite particles (hereinafter simply referred to as "composite particles") according to an embodiment includes the steps of: (a) preparing a ferrite precursor by stabilizing a divalent iron salt and a cobalt salt in an aqueous solution with a complexing agent; (b) adding spherical or nearly spherical seed particles to the ferrite precursor; and (c) further heat-treating the ferrite precursor. Hereinafter, the method for producing the composite particles will be described step by step.

[0013] (A) (Ferrite Precursor Preparation Step) First, a complexing agent is dissolved in demineralized water. Complexing the ferrite precursor with a complexing agent before heat treatment can protect it from oxidation by an oxidizing agent. The complexing agent is not particularly limited, and citrate, nitrilotriacetate, malate, and the like are preferred. When citrate is used, composite particles of seed particles and cobalt ferrite particles with a large average particle size of approximately 1 to 30 μm 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 above aqueous solution. The cobalt salt is also not particularly limited, and examples include divalent cobalt salts such as cobalt(II) sulfate, cobalt(II) chloride, cobalt(II) nitrate, and cobalt(II) acetate. Cobalt(II) sulfate or cobalt(II) chloride is preferred due to its availability. An oxidizing agent may be added during this process. The oxidizing agent is not particularly limited, and examples thereof include nitrates, hypochlorites, chlorates, perchlorates, permanganates, dichromates, chromates, manganates, and peroxides. Nitrates are preferred from the viewpoint of improving stability. The aqueous solution containing the cobalt salt is then degassed by applying 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 subsequent step from being affected by the dissolved 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 particles or impurities with undesired properties.

[0014] Next, a divalent iron salt is dissolved in the degassed aqueous solution to prepare a raw material aqueous solution. The divalent iron salt is not particularly limited, and examples thereof include iron(II) sulfate, iron(II) chloride, iron(II) nitrate, and iron(II) acetate. 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 ease of availability. As described below in the section on particle size adjustment, a trivalent iron salt may also be added to the aqueous solution. The trivalent iron salt is not particularly limited, and examples thereof include iron(III) chloride, iron(III) sulfate, and iron(III) nitrate.

[0015] Examples of combinations of a divalent iron salt and the above-mentioned cobalt salt include iron(II) sulfate and cobalt(II) sulfate, or iron(II) chloride and cobalt(II) chloride.

[0016] Then, an alkali is dissolved in demineralized water to prepare an alkaline aqueous solution. Any alkali can be selected as the alkali, such as sodium hydroxide, potassium hydroxide, calcium hydroxide, or ammonia. An alkaline aqueous solution with the alkali dissolved therein may be prepared from the beginning. Furthermore, the ferrite precursor is prepared by adding an alkaline aqueous solution to the raw material aqueous solution to adjust the pH. To stably form the complex, it is preferable to dissolve and mix the raw materials in the above-mentioned order. Furthermore, to synthesize ferrite particles with good properties after the complex is decomposed, it is preferable to adjust the pH to about 6 to 13 after adding the alkaline aqueous solution.

[0017] (ii) (Seed Particle Addition Step) Seed particles are added to the ferrite precursor. Adding seed particles to the ferrite precursor provides nuclei that serve as substrates for the precipitation of cobalt ferrite particles, thereby adjusting the particle size of composite particles of the seed particles and cobalt ferrite particles. The seed particles used here are not particularly limited and are selected from water-insoluble inorganic compounds such as metals, alloys, and oxides. In particular, iron, iron-based alloys, iron oxide, spinel ferrite, and silica are preferred as seed particles due to their affinity with cobalt ferrite. As iron oxide, magnetite, hematite, goethite, and lepidocrocite can be used. Furthermore, by using spherical or nearly spherical seed particles, the shape of the synthesized composite particles of the seed particles and cobalt ferrite particles can be made closer to a sphere.

[0018] In this specification and claims, "spherical" refers to a shape similar to a sphere. "Nearly spherical" includes shapes close to a sphere and polyhedrons consisting of multiple faces. Furthermore, in this specification and claims, anything that is not "spherical" or "nearly spherical" is referred to as "non-spherical." Examples of "non-spherical" shapes include cubes and rectangular parallelepipeds.

[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. The particle size of the seed particles can be measured by a laser diffraction / scattering method.

[0020] (C) (Heat Treatment Step) Heat treatment is performed by a hydrothermal method using a pressure vessel. The pressure vessel may be a conventional high-pressure reaction vessel, such as an autoclave, a pressure cooker, or a boiler, but an autoclave is preferred for its versatility. In a conventional high-temperature Siccol method, the reaction often proceeds at a high temperature 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 at a temperature range of about 130 to 300°C, preferably about 130 to 280°C, more preferably about 130 to 260°C, and even more preferably about 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 not higher than 300°C. On the other hand, impurities tend to be introduced when the reaction temperature exceeds about 270°C. Therefore, from the viewpoint of balancing purity and reaction rate, the reaction temperature is preferably 190 to 270°C, more preferably 190 to 260°C, even more preferably 190 to 240°C, and even more preferably 195 to 240°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, and treatment may be performed at different temperatures in multiple stages. For example, in the first stage, it is preferable to heat to a temperature range of 190 to 210°C and hold at the first temperature. It is preferable to hold at the first temperature for 1 to 14 hours, more preferably for 1 to 12 hours, and even more preferably for 4 to 12 hours. Furthermore, in the second stage, heat treatment can be performed under conditions of heating to a temperature range of 210 to 240°C, preferably to a temperature range of 220 to 240°C, and holding at the second temperature. It is preferable to hold at the second temperature for 0.5 to 12 hours, more preferably for 0.5 to 10 hours, and even more preferably for 0.5 to 2 hours.

[0023] When heat treatment is performed in multiple stages at different temperatures, the relatively low-temperature stages suppress excessive oxidation of the ferrite precursor, thereby suppressing excessive aggregation of the seed particles and contributing to making the composite particles of the seed particles and cobalt ferrite particles closer to a spherical shape. The relatively high-temperature stages promote oxidation of the ferrite precursor, thereby contributing to improving magnetic properties, increasing yield, and adjusting particle size. Furthermore, heat treatment in multiple stages can shorten the total heat treatment time.

[0024] (Role of Complexing Agent) Complexing agents play a major role in the ferrite formation reaction, which is thought to proceed as follows: Before the start of hydrothermal treatment, no oxidation reaction by the oxidizing agent occurs, and the ferrite precursor remains stable in the aqueous solution due to the complexing action of the complexing agent's ligands. This prevents the formation of hydroxides, which are easily oxidized and unstable, and the precursor is stably protected. Next, when heating begins, the complexing agent that protected the metal ions gradually decomposes, making the metal ions more susceptible to oxidation. At this time, 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 oxidizing environment, it is subjected to the oxidizing action of the oxidizing agent; even if there is no oxidizing agent, it is oxidized by the action of the hydrothermal environment, and ferrite is formed.

[0025] The complexing effect of the complexing agent can slow the progress of the oxidation reaction of the ferrite precursor during heat treatment under hydrothermal conditions. This allows the particle size of the synthesized ferrite particles to be increased, and particles with a uniform particle size can be produced. Furthermore, by controlling the progress of the oxidation reaction of the ferrite precursor, excessive aggregation of the seed particles can be suppressed, and as a result, composite particles of seed particles and cobalt ferrite particles with a nearly spherical shape can be produced.

[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, a 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 production method. Several particle size adjustment means are listed below. These means can be employed alone or in combination. By adding a trivalent iron salt to a raw material aqueous solution (aqueous solution of a divalent iron salt and a cobalt salt), or by adding a trivalent iron salt to an aqueous solution containing a divalent iron salt and a cobalt salt stabilized with a complexing agent, the particle size of the composite particles of seed particles and cobalt ferrite particles can be adjusted. In this way, the trivalent iron ions of the trivalent iron salt act as nuclei for ferrite particle formation, thereby accelerating the ferrite formation reaction regardless of the presence or absence of an oxidizing agent, and it is possible to adjust the particle size of the composite particles of seed particles and ferrite particles to be produced. The iron trivalent salt used here is not particularly limited, and examples include iron chloride (III), iron sulfate (III), and iron nitrate (III). Iron washing wastewater from blast furnaces and electric furnaces may also be used as an inexpensive raw material.

[0027] (Particle Size Adjustment 2: Addition of pH Buffer) By adding a pH buffer to an alkaline aqueous solution (aqueous solution of alkali and complexing agent), or by adding a pH buffer to an aqueous solution containing a divalent iron salt and a cobalt salt stabilized by a complexing agent, the particle size of composite particles of seed particles and cobalt ferrite particles can be adjusted. The ferrite formation reaction is accompanied by a decrease in pH, and a decrease in pH suppresses the decomposition of the ferrite precursor, thereby 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, etc.

[0028] (Particle size adjustment 3: Pressurizing an alkaline aqueous solution or a ferrite precursor during heat treatment) By pressurizing an alkaline aqueous solution or a ferrite precursor (complex) into a pressure vessel during or after the heat treatment step and continuing the heat treatment thereafter, the particle size of the composite particles of seed particles and cobalt ferrite particles can be adjusted. When the alkaline aqueous solution is pressurized, the pH in the reaction vessel increases, which promotes the decomposition of unreacted ferrite precursor and accelerates the ferrite formation reaction. This allows particle growth (coarsening of grains). 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, when the ferrite precursor (complex) is pressurized, the ferrite-forming raw material is added, which can promote the particle growth of ferrite particles. The method for producing a ferrite precursor is as described above.

[0029] [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 index of the particle size of the composite particles of seed particles and cobalt ferrite particles. The particle size distribution of the composite particles can be measured, for example, using a laser diffraction / scattering particle size distribution analyzer (manufactured by Microtrac-Bell, model number "MT3300EXII"). Specific particle size distributions are as follows. The composite particles of seed particles and cobalt ferrite particles have a particle size corresponding to 50% of the cumulative value based on the particle size distribution in the range of 1 to 30 μm, and a particle size corresponding to 95% of the cumulative value based on the particle size distribution in the range of 5 to 50 μm. The composite particles of seed particles and cobalt ferrite particles have magnetic properties such as an average circularity 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 coercive force of 100 to 1,000 Oe. The composite particles of seed particles and cobalt ferrite particles have a rounded shape and a uniform particle size. The aspect ratio of the composite particles is approximately 1. As will be described later in connection with FIGS. 16A and 16B , examples of composite particles of seed particles and cobalt ferrite particles include core-shell composite particles having a single seed particle as shown in FIG. 16A and sea-island composite particles B-1 to B-4 having a plurality of seed particles as shown in FIG. 16B . The number of seed particles is not limited, but examples include 2 to 15. The composite particles of seed particles and cobalt ferrite particles of this embodiment have a relatively large particle size, are rounded, and have a narrow particle size distribution. Therefore, they have little inter-particle cohesion and can be closely packed when molded, which allows for improved magnetic properties of the molded body or an increased bulk density. Furthermore, since the composite particles of the seed particles and the cobalt ferrite particles of this embodiment have a rounded shape, the amount of surface treatment agent used can be reduced when treating the surfaces of the composite particles of the seed particles and the cobalt ferrite particles. Therefore, when used in copy toners, magnetic inks, and MR fluids, the properties of the composite particles can be fully exhibited.

[0030] (White Powder) Composite particles of seed particles and cobalt ferrite particles can be whitened to form white powders, or can be whitened and then further coated with a colored layer to form colored powders. Whitening can be achieved by known methods, but a preferred method is the whitening method patented by the present applicant (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 cobalt ferrite particles by hydrolysis of titanium alkoxide (e.g., WO 96 / 28269) or a reaction from an aqueous titanium salt solution (e.g., JP 11-131102 A), followed by 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 this order on the surface of the composite particle of the present embodiment and the cobalt ferrite particle, and as a result, it is possible to improve the lightness L* of the composite particle of the seed particle and the cobalt ferrite particle to 75 or more.

[0031] The present invention will be described in more detail below using examples, but the present invention is not limited to these examples. The particle size distribution, average circularity, and magnetic properties of the produced composite particles of seed particles and cobalt ferrite particles were measured by the following methods.

[0032] (Measurement of particle size) The particle size distribution of the composite particles of 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 D ５０ and D, which is the particle size corresponding to 95% of the cumulative value based on the particle size distribution. ９５ was evaluated.

[0033] (SEM Observation) The surfaces of the obtained composite particles were observed using a scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, model number "S-4800").

[0034] (Measurement of Average Circularity) The average circularity of composite particles of seed particles and cobalt ferrite particles was evaluated by image analysis of SEM photographs (software "Mac-View Ver. 4" manufactured by Mountec Co., Ltd.). For 100 or more composite particles per sample, the circularity 4πS / L was calculated using the projected area S and perimeter L on the SEM photograph. ２ The average value was taken as the average circularity.

[0035] The magnetic properties of the composite particles of seed particles and cobalt ferrite particles were evaluated using a vibrating sample magnetometer (VSM) (manufactured by Tamagawa Seisakusho, model number "TM-VSM101483N7-MRO"). Hysteresis loops or demagnetization curves were obtained at a maximum magnetic field of 10,000 Oe, and the saturation magnetic moment, remanent magnetic moment, and coercive force were measured.

[0036] [Example 1] (Production of composite particles of seed particles and cobalt ferrite particles) (1) Preparation of raw material aqueous solution Trisodium citrate dihydrate (C ６ H ５ Na ３ O ７ ・2H ２ O) 604.73 g, cobalt(II) sulfate heptahydrate (CoSO ４ ・7H ２ 0) 95.38 g, sodium nitrate (NaNO ３ ) 17.30 g was dissolved and N ２ After degassing, iron(II) sulfate heptahydrate (FeSO ４ ・7H ２ (2) Preparation of ferrite precursor N ２ While continuing degassing at 40°C, a 48.5% aqueous solution of sodium hydroxide was 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 ２50.00 g of iron (Fe) as seed particles were placed in the purged autoclave together with the ferrite precursor, and hydrothermal treatment was carried out at 200°C for 16 hours with stirring to obtain magnetic particles. (4) Washing of magnetic particles The magnetic particles were filtered and washed with demineralized water. (5) Drying of magnetic particles The washed magnetic particles were dried in air at 250°C for 1 hour.

[0037] Example 2 Magnetic particles were produced under the same conditions as in Example 1, except that in (3) Preparation of magnetic particles by hydrothermal treatment in Example 1, the amount of iron (Fe) was changed to 100.00 g.

[0038] [Example 3] Magnetic particles were produced under the same conditions as in Example 1, except that in (3) Preparation of magnetic particles by hydrothermal treatment in Example 1, the amount of iron (Fe) was changed to 75.00 g and the hydrothermal treatment was changed to a two-stage process, with the first stage being at 200°C for 12 hours and the second stage being at 220°C for 2 hours.

[0039] [Example 4] Magnetic particles were produced under the same conditions as in Example 1, except that in (3) Preparation of magnetic particles by hydrothermal treatment in Example 1, the hydrothermal treatment was changed to a two-stage process, with the first stage being at 200°C for 4 hours and the second stage being at 240°C for 30 minutes.

[0040] [Example 5] Magnetic particles were produced under the same conditions as in Example 1, except that in (3) Preparation of magnetic particles by hydrothermal treatment in Example 1, the amount of iron (Fe) was changed to 25.00 g and the hydrothermal treatment was changed to 190°C for 40 hours.

[0041] [Example 6] In the preparation of the raw material aqueous solution in (1) of Example 1, trisodium citrate dihydrate (C ６ H ５ Na ３ O ７ ・2H ２ Magnetic particles were produced under the same conditions as in Example 1, except that (1) the amount of iron (Fe) used as seed particles was changed to 14.10 g of zinc ferrite, a type of spinel ferrite, in (3) the preparation of magnetic particles by hydrothermal treatment, and (4) the amount of zinc ferrite was changed to 14.10 g of zinc ferrite, a type of spinel ferrite, in (4) the preparation of magnetic particles by hydrothermal treatment.

[0042] [Example 7] Magnetic particles were produced under the same conditions as Example 1, except that in (3) Preparation of magnetic particles by hydrothermal treatment in Example 1, iron (Fe) as seed particles was changed to alnico, a type of iron-based alloy.

[0043] [Example 8] Magnetic particles were produced under the same conditions as in Example 1, except that in (3) Preparation of magnetic particles by hydrothermal treatment in Example 1, 50.00 g of iron (Fe) as seed particles was changed to 25.00 g of magnetite, a type of iron oxide.

[0044] Example 9 Magnetic particles were produced under the same conditions as in Example 1, except that in (3) Preparation of magnetic particles by hydrothermal treatment in Example 1, 50.00 g of iron (Fe) as seed particles was changed to 6.04 g of silica.

[0045] [Example 10] In the preparation of the raw material aqueous solution in (1) of Example 1, iron (II) sulfate heptahydrate (FeSO 4 ) was used as the divalent iron salt. ４ ・7H ２ 471.72 g of iron(II) chloride tetrahydrate (FeCl ２ ・4H ２ 0), 337.29 g of cobalt (II) sulfate heptahydrate (CoSO ４ ・7H ２ 95.38 g of cobalt(II) chloride hexahydrate (CoCl ２ ・6H ２ Magnetic particles were produced under the same conditions as in Example 1, except that in (3) preparation of magnetic particles by hydrothermal treatment, the amount of iron (Fe) was changed to 14.10 g.

[0046] [Reference Example 1] In the preparation of the raw material aqueous solution in (1) Example 1, sodium nitrate (NaNO ３ Magnetic particles were produced under the same conditions as in Example 1, except that sodium nitrate (NaNO ), which was added as an oxidizing agent in the preparation of the raw material aqueous solution in (1) of Example 1, was not used. ３(3) In preparing the magnetic particles by hydrothermal treatment, the hydrothermal treatment was changed to a two-stage process in which the first stage was at 200°C for 12 hours and the second stage was at 220°C for 10 hours. The magnetic particles were produced under the same conditions as in Example 1, except that the magnetic particles were not used; (4) the magnetic particles were prepared by hydrothermal treatment in which the first stage was at 200°C for 12 hours and the second stage was at 220°C for 10 hours.

[0047] Comparative Example 1 In the step (3) of preparing magnetic particles by hydrothermal treatment in Example 1, 50.00 g of iron (Fe) was replaced with magnetite (Fe), a type of iron oxide. ３ O ４ Magnetic particles were produced under the same conditions as in Example 1, except that the amount of the ferrite powder was changed to 25.00 g. In Comparative Example 1, magnetite particles having a non-spherical shape were used.

[0048] Comparative Example 2 In the step (3) of preparing magnetic particles by hydrothermal treatment in Example 1, 50.00 g of iron (Fe) was replaced with magnetite (Fe), a type of iron oxide. ３ O ４ Magnetic particles were produced under the same conditions as in Example 1, except that the amount of the ferrite powder was changed to 50.00 g. In Comparative Example 2, magnetite particles having a non-spherical shape were used.

[0049] Comparative Example 3 In the preparation of the raw material aqueous solution in (1) of Example 1, trisodium citrate dihydrate (C ６ H ５ Na ３ O ７ ・2H ２ O) and sodium nitrate (NaNO) as an oxidizing agent ３ Magnetic particles were produced under the same conditions as in Example 1, except that the ferromagnetic material 1 was not used.

[0050] The various properties of the composite particles obtained in each of the Examples, Reference Examples, and Comparative Examples are summarized in Table 1. In addition, Figures 1A, 1B to 15A and 15B show SEM photographs of the composite particles and their seed particles in Examples 1 to 10, Reference Examples 1 and 2, and Comparative Examples 1 to 3.

[0051]

[0052] The reaction rates of Examples 1 to 3, Examples 5 to 10, Reference Example 1, Reference Example 2, Comparative Example 1, and Comparative Example 2 in the table were calculated according to the following formula A. The reaction rate of Example 4 was calculated according to formula B. The reaction rate of Comparative Example 3 could not be calculated because impurities were mixed into the reaction product. Formula A: (mass of powder actually obtained by hydrothermal treatment of ferrite precursor - amount of seed particles charged) / {charged raw materials *２ Cobalt ferrite (Co) is obtained when all the iron and cobalt in the ｘ Fe (３-ｘ) O ４ )} × 100 Formula B: {1 - (mass concentration of iron and cobalt dissolved in the aqueous solution after the hydrothermal treatment) / (theoretical mass concentration of iron and cobalt in the ferrite precursor before the hydrothermal treatment)} × 100 *2: The iron and cobalt in the charged raw materials do not include the mass of iron and cobalt contained in the seed particles.

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

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

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

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

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

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

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

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

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

[0062] The composite particles of Example 10, in which iron (II) chloride tetrahydrate and cobalt (II) chloride hexahydrate were added as the divalent iron salt and cobalt (II) chloride hexahydrate, respectively, in the preparation of the raw material aqueous solution, 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 FIG. 10A, the composite particles of Example 10 had a rounded shape.

[0063] The composite particles of Comparative Example 1, which used non-spherical magnetite as seed particles, had an average circularity of 0.69, a particle diameter corresponding to 50% of the cumulative value based on the particle diameter distribution of 5.26 μm, and a particle diameter corresponding to 95% of the cumulative value based on the particle diameter distribution of 9.46 μm. As shown in Fig. 13A, the composite particles of Comparative Example 1 were not rounded and had an irregular shape.

[0064] The composite particles of Comparative Example 2, which used non-spherical magnetite as seed particles, had an average circularity of 0.63, a particle diameter corresponding to 50% of the cumulative value based on the particle diameter distribution of 3.96 μm, and a particle diameter corresponding to 95% of the cumulative value based on the particle diameter distribution of 6.98 μm. As shown in Fig. 14A, the composite particles of Comparative Example 2 were not rounded and had an irregular shape.

[0065] The composite particles of Comparative Example 3, which did not contain a complexing agent or an oxidizing agent, had an average circularity of 0.53, a particle diameter corresponding to 50% of the cumulative value based on the particle size distribution of 4.48 μm, and a particle diameter corresponding to 95% of the cumulative value based on the particle size distribution of 19.57 μm. Furthermore, as shown within the white frame in Figure 15A, the reaction product of Comparative Example 3 contained impurities. As shown in Figure 15A, the composite particles of Comparative Example 3 were not rounded and had an irregular shape.

[0066] The results in Table 1 show that in all of Examples 1 to 10, by adding seed particles to a ferrite precursor stabilized with a complexing agent and then subjecting it to heat treatment, composite particles of seed particles and cobalt ferrite particles were formed that had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties.

[0067] In Example 1, the temperature was 200°C and the holding time was 16 hours, whereas in Example 3, which was heat-treated at 200°C in the first stage and 220°C in the second stage, the total holding time was 14 hours, and in Example 4, which was heat-treated at 200°C in the first stage and 240°C in the second stage, the total holding time was 4 hours and 30 minutes. It was shown that when heat-treatment was performed in two stages, the total heat-treatment time was shortened.

[0068] The composite particles of Example 5 had an average circularity of 0.7 to 1.0, a rounded shape, and specific magnetic properties. The results of Example 5 show 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 6 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. The particle diameter corresponding to 50% of the cumulative value based on the particle size distribution of Example 6 was 29.73 μm, and the particle diameter corresponding to 95% of the cumulative value was 47.31 μm. This indicates that the method for producing composite particles of seed particles and cobalt ferrite particles can produce particles with a particle diameter of 30 μm or less corresponding to 50% of the cumulative value based on the particle size distribution and particles with a particle diameter of 50 μm or less corresponding to 95% of the cumulative value.

[0070] The composite particles of Examples 7 to 9 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. The results of Examples 1 and 6 to 9 show that when iron, an iron-based alloy, iron oxide, spinel ferrite, or silica is 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 10 had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties. The results of Example 10 show 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 step, 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] 1B to 10B, the shapes of the iron (Fe), zinc ferrite which is a type of spinel ferrite, alnico which is a type of iron-based alloy, magnetite which is a type of iron oxide, and silica used as seed particles in Examples 1 to 10 were spherical or approximately spherical. On the other hand, as shown in FIGS. 13B and 14B, the shape of the magnetite used as seed particles in Comparative Examples 1 and 2 was not spherical or approximately spherical. Comparing Examples 1 to 10 with Comparative Examples 1 and 2, it was found that by using spherical or approximately spherical seed particles, composite particles of seed particles and cobalt ferrite particles could be obtained which had an average circularity in the range of 0.7 to 1.0, a rounded shape, and specific magnetic properties.

[0073] Transmission electron microscope (TEM) observation was performed to confirm the cross-sectional structure of the obtained composite particles. Figures 16A and 16B show cross-sectional conceptual diagrams of the composite particles of Example 1 as a representative example with seed particles. It was confirmed that the obtained composite particles were core-shell type composite particles A as shown in Figure 16A, or islands-in-the-sea type composite particles B (B-1 to B-4) having a plurality of seed particles as shown in Figure 16B, formed by aggregation of composite particles A. In this specification, when simply referring to "particle diameter," 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] The results of Reference Example 1 in Table 1 show that composite particles of seed particles and cobalt ferrite particles can be obtained even without adding an oxidizing agent. The reaction rate in Reference Example 1 was 8.6%, which was lower than the reaction rate of 79.9% in Example 1.

[0075] The test conditions for Reference Example 1 were as follows: (1) In the preparation of the raw material aqueous solution in Example 1, sodium nitrate (NaNO ) was added as an oxidizing agent. ３ The conditions were the same as in Example 1, except that no oxidizing agent was used. From this, a comparison between Example 1 and Reference Example 1 shows that the reaction rate is significantly improved by adding an oxidizing agent.

[0076] The composite particles of Reference Example 2 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 2 was 63.5%, which was higher than that of Reference Example 1. The test conditions for Reference Example 2 were as follows: (1) Preparation of raw material aqueous solution in Example 1: 1. The amount of sodium nitrate (NaNO ) added as an oxidizing agent was 1.0; ３ (3) In the preparation of magnetic particles by hydrothermal treatment, the hydrothermal treatment was changed to a two-stage process in which the first stage was at 200°C for 12 hours and the second stage was at 220°C for 10 hours. Comparing Reference Example 1 with Reference Example 2, it was found that even without adding an oxidizing agent, by increasing the heat treatment temperature and using two stages, the average circularity and reaction rate of the composite particles were improved, and 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 could be obtained.

[0077] Example 11 (Whitening of Cobalt Ferrite Particles) A ​​yellow, transparent peroxotitanic acid solution was prepared by mixing 4.49 g of tetra-i-propoxytitanium solution (27.9-28.5% asTi), 14.40 g of aqueous ammonia, and 12.50 g of aqueous hydrogen peroxide with 23.30 g of deionized water. 4.95 g of boric anhydride, 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 1 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 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 with 360.00 g of deionized water, and 29.70 g of titanium oxide-coated powder was suspended in the solution. The reducing solution was mixed with the suspension while irradiating it with ultrasound, and the suspension was dried to obtain a silver film-coated powder. The resulting white powder had a brightness L* of 79.73.

[0078] The composite particles of seed particles and cobalt ferrite particles obtained by the production method of the present invention have a rounded shape and a uniform particle size, and are therefore expected to be used as copy toners, magnetic inks, and MR fluids.

Claims

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 in an aqueous solution with a complexing agent; adding spherical or nearly spherical seed particles to the ferrite precursor; and further heat-treating the ferrite precursor.

2. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 1, wherein the heat treatment is carried out in a pressure vessel under hydrothermal conditions in the temperature range of 130°C to 260°C.

3. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2, wherein the heat treatment is carried out in a pressure vessel under hydrothermal conditions at a temperature in the range of 190°C to 240°C.

4. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, wherein the time for the heat treatment step is 1 to 50 hours.

5. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 3, wherein the heat treatment is carried out under hydrothermal conditions by heating to a temperature range of 190°C to 210°C, holding the first reached temperature, and further heating to a temperature range of 210°C to 240°C, and holding the second reached temperature.

6. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 5, wherein the first attained temperature is maintained for 1 to 14 hours.

7. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 5 or 6, wherein the second attained temperature is maintained for 0.5 to 12 hours.

8. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, wherein the seed particles are at least one selected from the group consisting of iron, iron-based alloys, iron oxide, spinel ferrite and silica.

9. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, wherein the divalent iron salt and the cobalt salt are iron (II) sulfate and cobalt (II) sulfate.

10. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, wherein the divalent iron salt and the cobalt salt are iron (II) chloride and cobalt (II) chloride.

11. A method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, wherein the complexing agent is one selected from the group consisting of citrate, nitrilotriacetate, and malate.

12. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, wherein the heat treatment step is carried out in the presence of an oxidizing agent in addition to a complexing agent.

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

14. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, further comprising adding a trivalent iron salt to the aqueous solution containing the ferrite precursor.

15. The method for producing composite particles of seed particles and cobalt ferrite particles according to claim 2 or 3, further comprising adding a pH buffer to the aqueous solution containing the ferrite precursor.

16. A method for producing composite particles of seed particles and cobalt ferrite particles as described in claim 2 or 3, wherein an alkaline aqueous solution or a ferrite precursor is pressurized into the pressure vessel during or after the heat treatment step, and further heat treatment is performed.

17. Composite particles of seed particles and cobalt ferrite particles, having a rounded shape with an average circularity of 0.7 to 1.0, a particle diameter corresponding to a cumulative value of 50% based on the particle size distribution of 1 to 30 μm, and a particle diameter corresponding to a cumulative value of 95% based on the particle size distribution of 5 to 50 μm.

18. The composite particle of seed particles and cobalt ferrite particles according to claim 17, which has a remanent magnetic moment of 10 emu / g or more and a coercive force of 100 to 1000 Oe.

19. A toner for copying comprising composite particles of the seed particles according to claim 18 and cobalt ferrite particles.

20. A magnetic ink comprising composite particles of the seed particles according to claim 18 and cobalt ferrite particles.

21. An MR fluid comprising composite particles of the seed particles according to claim 18 and cobalt ferrite particles.

22. A white powder comprising composite particles of the seed particles and cobalt ferrite particles according to claim 18, the surfaces of which are provided with a titanium oxide film and a metallic silver film in this order.

23. The white powder according to claim 22, having a lightness L* of 75 or more.

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