Core-shell type composite granules, inorganic composite materials produced using the core-shell type composite granules, and methods for producing the same

Abstract

To provide composite powder particles capable of suitably utilizing raw powder of inorganic materials such as ceramics with controlled structure, and structurally-controlled inorganic composite material produced thereby.SOLUTION: Core-shell type composite granules have a core that is composed of particles of matrix material and a shell that is formed in a rod or plate shape oriented in the circumferential direction and is composed of functional particles and particles of matrix material. A composite inorganic material has a sea-island structure formed by forming a green body with the core-shell type composite granules and then sintering and is structurally controlled in two steps such that the functional particles in a sea part are oriented.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to core-shell type composite granules that can be used as raw material powders for sintered bodies of ceramics, metals, etc., inorganic composite materials prepared using the core-shell type composite granules, and methods for producing them. More specifically, the present invention relates to core-shell type composite granules in which functional particles are oriented in the shell portion, inorganic composite materials prepared using the core-shell type composite granules in which the arrangement and orientation of functional particles are controlled, and methods for producing them. [Background technology]

[0002] Ceramic materials are used in a variety of fields as structural and heat-resistant materials, and are widely used in integrated circuit substrates and packages, firing vessels and setters, and furnace tubes for electric furnaces. These ceramic materials are generally produced by forming raw material powders into green bodies using dry or wet forming methods and then sintering the green bodies. Furthermore, in recent years, there has been active research into improving the functionality and properties of ceramic materials by mixing functional particles (functional particles) with the ceramic powder matrix.

[0003] Generally, in a composite material with functionality in which functional particles are added to a matrix, imparting properties such as electronic conductivity or thermal conductivity requires that the functional particles come into contact with each other in the matrix to form paths. When the ratio of functional particles in the matrix exceeds a threshold, the function is suddenly manifested (percolation transition). The shape and arrangement of the functional particles have a significant effect on the occurrence of percolation transition. As an example of a composite material made of a polymer material, Patent Document 1 discloses an acicular conductive tin oxide fine powder with an average minor axis particle diameter of 0.005 to 0.05 μm, an average major axis particle diameter of 0.1 to 3 μm, and an aspect ratio of 5 or more, and describes how even a small amount of conductivity-imparting agent can effectively form conductive paths. Non-Patent Document 1 discloses composite particles made from PMMA (polymethyl methacrylate) and h-BN (hexagonal boron nitride) as a thermally conductive filler, with the surfaces of the PMMA particles adjusted to opposite charges using a polymer electrolyte, and with h-BN particles adsorbed onto the PMMA particle surface. The paper discloses that molding the resulting composite particles results in high thermal conductivity even when only a very small amount of h-BN particles is added.

[0004] In ceramic materials, too, the arrangement of functional particles in the green body is extremely important for achieving this functionality. Non-Patent Document 2 discloses that core-shell composite granules are produced by granulating alumina particles with a positive surface charge and zirconia particles whose surface charge has been adjusted to a negative value using an anionic polymer electrolyte, and then adding CNTs (carbon nanotubes) whose charge has been adjusted using an activator to the alumina and zirconia particles, followed by granulation to form a shell made of alumina particles, zirconia particles, and CNT particles on the surface of the core. The composite material sintered using this core-shell composite granules also shows that CNTs are locally and continuously introduced, forming a segregated three-dimensional network, and that functionality is achieved with a small amount of CNTs. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 08-217445 [Non-patent literature]

[0006] [Non-Patent Document 1] Taichi Kuroda, Nguyen Huu Huy Phuc, Tsuyoshi Kawamura, Atsunori Matsuda, Hiroyuki Muto, Ceramic Society of Japan 2014 Annual Meeting "Preparation of high thermal conductive polymer composites containing h-BN", Proceedings 1P005 [Non-patent document 2] Yusaku Sato, Atsushi Yokoi, Tan Wai Kian, Tsuyoshi Kawamura, Hiroyuki Muto, Atsunori Matsuda, "Development of Ceramic Composite Materials with Segregated Three-Dimensional Conductive Network Structure Using Composite Granules," Proceedings of the 2021 Autumn Meeting (128th Lecture Meeting) of the Japan Society of Powder and Powder Metallurgy, Vol. 2-7A Summary of the Invention [Problem to be solved by the invention]

[0007] However, Non-Patent Document 1 is a technology related to polymer matrices and cannot be simply applied to inorganic materials such as ceramics. For example, it is possible to produce composite particles by attaching h-BN particles to ceramic mother particles, but even if a green body is made from these composite particles and heated, it does not sinter. In the case of a polymer matrix, when heated to high temperatures, the polymer has a certain degree of fluidity, so it flows between the h-BN particles and even onto the surface of the composite particles, holding the h-BN particles and bonding the composite particles together. However, in inorganic materials such as ceramics, the matrix material does not flow, and cannot contribute to holding the functional particles or bonding adjacent granules.

[0008] The CNTs in the shell portion of the core-shell composite granules described in Non-Patent Document 2 are arranged randomly, and their arrangement and orientation cannot be controlled. Furthermore, in a sintered body made from these core-shell composite granules, CNTs are introduced locally and continuously, and a segregated three-dimensional network can be formed, but the CNTs within the three-dimensional network exist randomly, and their arrangement and orientation cannot be controlled.

[0009] An object of the present invention is to provide composite powder particles that allow suitable use of raw powder of an inorganic material such as a structure-controlled ceramic, and a structure-controlled inorganic composite material produced therefrom. More specifically, the present invention provides powder particles that have a core-shell structure and in which the arrangement and orientation of functional particles contained in the shell portion are controlled, and also provides a two-stage structure-controlled inorganic composite material in a sintered body produced from the powder particles, which has a sea-island structure consisting of island portions made of a matrix material and a sea portion in which a functional material is segregated as a first stage, and a structure in which the functional particles in the sea portion are oriented in the direction of the boundary lines of the island portions as a second stage. [Means for solving the problem]

[0010] The present invention is illustrated below. [1] A core-shell type composite granule having a core portion consisting of particles of a first inorganic material and a shell portion attached to the surface of the core portion, wherein the core portion is a granular form in which a plurality of particles of the first inorganic material are spherically agglomerated, and the shell portion is composed of a mixture of a plurality of particles of the first inorganic material and a plurality of particles of a second inorganic material formed in a rod-like or flat shape, and the particles of the second inorganic material are attached with their long axes oriented circumferentially to the core portion. [2] A method for producing the core-shell type composite granules according to [1], The method includes a core portion forming step and a shell portion attaching step, The core portion forming step includes: adjusting the surface charge of particles of a first inorganic material to be positive; adjusting the surface charge of particles of a first inorganic material to be negative; a step of mixing an aqueous solvent dispersion of the positively charged particles of the first inorganic material with an aqueous solvent dispersion of the negatively charged particles of the first inorganic material, placing the mixed solution in a cylindrical container, and rotating the cylindrical container in a circumferential direction to form core granules of composite granules, The shell portion attachment step includes: a step of adjusting the surface charge of rod-shaped or flat particles of a second inorganic material to be positive or negative; and mixing, in an aqueous solvent, an aqueous dispersion of rod-shaped or flat particles of a second inorganic material whose surface charge has been adjusted to be positive or negative with an aqueous dispersion of particles of a first inorganic material whose surface charge has been adjusted to at least the opposite charge, adding this mixture to the aqueous dispersion of the core granules produced in the core formation step, and then placing this mixture in a cylindrical container and rotating the cylindrical container in the circumferential direction to form shell portions on the surfaces of the core granules. A method for producing core-shell type composite granules, characterized in that [3] An inorganic composite material using the core-shell type composite granules according to [1], which is formed by a plurality of the composite granules and has a sea-island structure having islands made of the cores and seas made of the shells, An inorganic composite material, characterized in that the long axes of rod-shaped or flat-plate-shaped particles of a second inorganic material contained in the sea portion are oriented in the direction of the boundary lines of the island portions. [4] The rod-shaped or flat particles of the second inorganic material are anisotropic in their properties, and the properties in the short axis or thickness direction are different from the properties in the long axis or plane direction. [3] An inorganic composite material according to [3]. [5] The inorganic composite material according to [4], characterized in that the characteristic is thermal conductivity or electronic conductivity, and the thermal conductivity or electronic conductivity in the long axis or plane direction is higher than the characteristic in the short axis or thickness direction. [6] A method for producing the inorganic composite material according to [3], A method for producing an inorganic composite material, comprising the steps of: appropriately agglomerating the core-shell type composite granules to produce a green body; and sintering the green body. [Effects of the Invention]

[0011] The core-shell composite granules of the present invention are used as a raw powder of an inorganic material. A green body of this raw powder is then produced and sintered to produce a composite material with a sea-island structure consisting of islands made of a matrix material and a sea region where a functional material is segregated. The composite material has a two-stage structure in which the functional particles in the sea region are oriented along the boundary line between the islands. In other words, by designing the raw powder, it is possible to obtain a composite material with a two-stage structure in which the functional particles in the sea region are oriented along the boundary line between the islands while simultaneously controlling the particle arrangement and orientation of the functional particles in the sea region. Furthermore, the composite material obtained by the present invention exhibits good sintering of the shell region, sintering of the interface between the shell and core regions, and sintering of the shell regions of adjacent composite granules. Therefore, the composite material of the present invention can achieve a material strength close to that of the matrix material alone.

[0012] Furthermore, in functional materials prepared by simply mixing functional microparticles and matrix particles, the level of functionality expression changes drastically before and after the percolation transition, making it difficult to control the expression level. However, in the present invention, the level of functionality expression of the composite material can be freely adjusted by adjusting the ratio of the core and shell parts of the core-shell composite granules. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a cross-sectional schematic diagram of a core-shell type composite granule of the present invention. [Figure 2] 1 is a scanning electron microscope photograph of a cross section of a crushed sintered body prepared in Non-Patent Document 2. [Figure 3] 1 is an optical microscope photograph of the core granules of Example 1. [Figure 4] 1 is an optical microscope photograph of the core-shell type composite granules of Example 1. [Figure 5] 1 is a scanning electron microscope photograph showing an enlarged view of the surface of a core-shell type composite granule of Example 1. [Figure 6] 1 is an optical microscope photograph of a fracture surface of the composite material of Example 1. [Figure 7] 1 is a scanning electron microscope photograph of a fracture surface of the composite material of Example 1. [Figure 8]1 is a scanning electron microscope photograph showing an enlarged fracture surface of the composite material of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail.

[0015] First, the core-shell type composite granules according to the present invention will be described. The core-shell type composite granules according to the present invention are used as raw material powder for producing a sintered body, and a green body is formed from the core-shell type composite granules, and the green body is sintered to obtain an inorganic composite material.

[0016] In the present invention, a composite granule refers to an accumulation composed of different types of particles, and the composite granule of the present invention has a spherical shape. Here, "spherical" does not necessarily mean a perfect sphere, but refers to an approximately spherical shape. Figure 1 shows a schematic diagram of a core-shell composite granule of the present invention. The core-shell type refers to a spherical shape in which the particle components change along the radial direction from the center of the sphere, with the central portion being the core portion and the surface portion being the shell portion. The core portion is composed of multiple particles of a first inorganic material. The shell portion is composed of a mixture of multiple particles of the first inorganic material and multiple rod-shaped or flat-shaped particles of a second inorganic material, and the long axes of the rod-shaped or flat-shaped particles of the second inorganic material are oriented in the circumferential direction of the composite granule. Here, "the long axes of the particles of the second inorganic material being oriented in the circumferential direction of the composite granule" means that the normal direction at the interface between the center of the long axis of each particle of the second inorganic material and the core portion closest to the center of the long axis of the particle of the second inorganic material is approximately perpendicular to the long axis direction of the particle of the second inorganic material. The term "approximately right angle" refers to a range of 60 degrees to 120 degrees, and 70% or more of the particles of the second inorganic material are in this range.

[0017] The particles of the first inorganic material are particles made of ceramics or metals, and may be one type of particle or two or more types of particles. When using multiple types of particles, it is preferable that the ratio of the average particle diameters is within 10 times, considering the ease of forming granules, as described below. The types of ceramics and metals are not particularly limited, and examples of ceramics include oxides, carbides, and nitrides. The oxides may be single oxides or composite oxides. Examples of such ceramics include alumina, zirconia, silicon nitride, silicon carbide, magnesia, calcia, titania, vanadium oxide, spinel, and ferrite, which may be used alone or in mixtures. Furthermore, solid solutions of these may also be used.

[0018] Furthermore, examples of metals include, but are not limited to, iron-based, copper-based, aluminum-based, nickel-based, molybdenum-based, titanium-based, and tungsten-based powders. These may be used alone or in mixtures, or may be alloys. Various alloys may be used, including, for example, iron alloys, alloy steels, copper alloys, nickel alloys, aluminum alloys, and cemented carbide alloys. Examples of cermets include, but are not limited to, TiC-Ni cermets, Al2O3-Cr cermets, and Al2O3-Fe cermets.

[0019] There is no limitation on the geometric shape of the particles of the first inorganic material, and spherical, needle-like, blocky, columnar, flat, plate-like, fibrous, amorphous, etc. are applicable.

[0020] The rod-shaped or flat particles of the second inorganic material that make up the shell portion are described below. In this invention, rod-shaped refers to a shape in which the radial dimension is smaller than the longitudinal dimension, and is also referred to as rod-shaped, needle-shaped, or rod-shaped. The radial dimension is also referred to as the minor axis, and the longitudinal dimension as the major axis. The diameter of rod-shaped particles is preferably 0.05 μm to 5 μm, and the length is preferably 0.5 μm to 50 μm. For ease of orientation, as described below, the aspect ratio is preferably 3 or greater, and more preferably 5 or greater. Note that, in this invention, rod-shaped particles do not include filament-shaped particles. Here, filament-shaped refers to a material with an aspect ratio of 100 or greater. The CNTs used in Non-Patent Document 2 fall under the filament-shaped category. Filament-shaped materials are generally highly flexible and become entangled with the particles of the first inorganic material during the granulation process of the shell portion, as described below, and are therefore not oriented. Rigid materials adhere to the granules only at points, resulting in weak adhesion and preventing incorporation into the granules. Figure 2 shows an SEM photograph of a cross section of a crushed sintered body prepared in Non-Patent Document 2. It can be seen that the CNTs are not oriented but random.

[0021] In the present invention, the term "flat particles" refers to particles having a shape in which the thickness is smaller than the length or width direction, and is also referred to as plate-like, thin, scale-like, or flake-like. The size of the flat particles is preferably an average particle diameter in the major axis direction of 0.5 μm to 50 μm, and from the viewpoint of ease of orientation described below, the aspect ratio is preferably 3 or more, and more preferably 5 or more.

[0022] The particles of the second inorganic material are particles made of ceramics or metal, and the materials listed as the particles of the first inorganic material can be used. In a preferred embodiment of the present invention, the particles of the second inorganic material are a material having remarkably high or low specific properties. Examples of specific properties include electronic conductivity, thermal conductivity, magnetism, dielectricity, elasticity, etc. In a particularly preferred embodiment of the present invention, the particles are a material having high electronic conductivity and high thermal conductivity, which exhibit high performance through percolation.

[0023] Materials with high electronic conductivity include SnO2, ZnO, TiO2, CeO2, etc., as well as materials doped with heteroatoms. Materials with high thermal conductivity include oxide particles such as Al2O3, MgO, ZnO, SiO2, TiO2, mica, potassium titanate, iron oxide, and talc; nitride particles such as boron nitride, silicon nitride, and aluminum nitride; carbide particles such as silicon carbide; metal particles such as copper and aluminum; and magnesium carbonate. Magnetic materials include iron, nickel, and cobalt. Furthermore, materials used in multiferroic materials can also be used, such as BiFeO3, BaTiO3, Pd(Zr, Ti)O3, CaMnO3, LaFeO3, and TbMnO3.

[0024] Furthermore, the structural control of the present invention is effective when the radial and longitudinal properties differ in a rod-like material, and when the thickness and longitudinal or width properties differ in a plate-like material, and is particularly effective when the longitudinal properties are higher than the width and thickness properties in a rod-like material, and when the length or width properties are higher than the thickness properties in a plate-like material.

[0025] A method for producing core-shell composite granules of the present invention will be described. The method for producing core-shell composite granules of the present invention comprises a core-forming step and a shell-attaching step. The core-forming step includes a step of adjusting the surface charge of particles of a first inorganic material to be positive, a step of adjusting the surface charge of particles of the first inorganic material to be negative, and a step of mixing an aqueous solvent dispersion of the positively charged particles of the first inorganic material with an aqueous solvent dispersion of negatively charged particles of the first inorganic material, placing the mixture in a cylindrical container, and rotating the cylindrical container in the circumferential direction to form core granules of the composite granules. The shell attachment step includes a step of adjusting the surface charge of rod-shaped or plate-shaped particles of a second inorganic material to positive or negative, and a step of mixing an aqueous solvent dispersion of rod-shaped or plate-shaped particles of the second inorganic material adjusted to a positive or negative surface charge with an aqueous solvent dispersion of particles of the first inorganic material adjusted to at least the opposite charge in an aqueous solvent, adding this mixture to the aqueous dispersion of the core granules prepared in the core formation step, and then placing this mixture in a cylindrical container and rotating the cylindrical container circumferentially to form shells on the surfaces of the core granules. Here, the aqueous solvent refers to water or a mixed solvent of a water-compatible solvent and water, and examples of water-compatible solvents include alcohol-based solvents, ketone-based solvents, etc.

[0026] Surface charge refers to the apparent potential of a particle. When a polar layer is layered on the particle surface, the charge of the outermost layer is the surface charge. To adjust the surface charge, cationic polyelectrolytes that ionize to a positive charge in aqueous solvents and anionic polyelectrolytes that ionize to a negative charge in aqueous solvents are preferably used. Examples of cationic polyelectrolytes that ionize to a positive charge in aqueous solvents include cationic polymers such as poly(diallyldimethylammonium chloride) (PDDA), polyethyleneimine (PEI), polyvinylamine (PVAm), and poly(vinylpyrrolidone-N,N-dimethylaminoethyl acrylic acid) copolymer. Examples of anionic polyelectrolytes that ionize to a negative charge in aqueous solvents include anionic polymers such as polystyrene sulfonate (PSS), polyvinyl sulfate (PVS), polyacrylic acid (PAA), polymethacrylic acid (PMA), and polycarboxylic acid (PCA).

[0027] Particles of the first inorganic material and particles of the second inorganic material have a positive or negative surface charge when dispersed in an aqueous solvent. If the particles have a positive surface charge, a strong negative surface charge can be imparted by adsorbing an anionic polyelectrolyte. Conversely, if the particles have a negative surface charge, a strong positive surface charge can be imparted by adsorbing a cationic polyelectrolyte. Furthermore, after adjusting the surface charge with a cationic or anionic polyelectrolyte, the charge can be reversed by adsorbing a polyelectrolyte with the opposite charge. This operation allows the surface charge of the particles to be adjusted to the desired charge. This operation may be repeated multiple times, such as positive-negative-positive or negative-positive-negative, to more firmly adsorb the polyelectrolyte. If the particles have a sufficient surface charge when dispersed in an aqueous solvent without treatment with a polyelectrolyte, the dispersion in an aqueous solvent can be used as a surface charge adjustment step.

[0028] When the dispersibility of particles is impaired by bridging aggregation caused by the adsorption of the polyelectrolyte, an ionic surfactant may be used instead of the polyelectrolyte, which can also impart a charge to the particle surface by adsorption.

[0029] For example, the polymer electrolyte can be adsorbed onto the particle surface by adding, stirring, and dispersing charge-adjusting particles in a solution containing the polymer electrolyte dissolved in an aqueous solvent. To ensure sufficient polymer electrolyte is adsorbed onto the particles, it is preferable to add an excess amount of polymer electrolyte to the solution compared to the amount of particles added. In this case, the excess polymer electrolyte is removed by a procedure such as precipitation, centrifugation, or filtration to separate the liquid and particles. The particles are then redispersed in the aqueous solvent to obtain a dispersion in which the particles are dispersed. The solids content of this dispersion is not particularly limited, but is typically 1 to 20% by volume.

[0030] Alternatively, as described in Japanese Patent Application No. 2013-507747, a method is also preferably used in which a mixture of particles and an aqueous solvent is stirred while monitoring the liquid properties (zeta potential, viscosity, etc.), and the addition of an aqueous solvent containing a dissolved polymer electrolyte is stopped when it is determined from the change in the liquid properties that a sufficient amount of polymer electrolyte has been adsorbed onto the particles. By this method, a dispersion in an aqueous solvent can be obtained.

[0031] Whether the desired surface charge has been obtained can be confirmed by measuring the zeta potential of the obtained particles. If the zeta potential is positive, the surface charge is positive, and if it is negative, the surface charge is negative. Furthermore, having a positive surface charge is also referred to as positively charged, and having a negative surface charge is also referred to as negatively charged. The magnitude of the surface charge of the particles after surface charge adjustment is preferably 20 mV to 150 mV, and more preferably 30 mV to 100 mV, in terms of the absolute value of the zeta potential.

[0032] Next, the preparation of the core portion of the core-shell composite granules will be described. A dispersion liquid of particles of a first inorganic material adjusted to a positive surface charge in the surface charge adjustment step and a dispersion liquid of particles of a first inorganic material adjusted to a negative surface charge are mixed so that the particle amounts are equal. If the particles of the first inorganic material consist of multiple types of particles, a dispersion liquid of particles whose surface charges have been adjusted so that the amounts of particles whose surface charges have been adjusted to a positive and particles whose surface charges have been adjusted to a negative are prepared and mixed. For example, it is sufficient to prepare dispersion liquids in which half of each type of particle has been adjusted to a positive surface charge and half of the other has been adjusted to a negative surface charge and mix them. If there are two types of particles and the amounts are equal, it is also possible to prepare dispersion liquids in which one type has been adjusted to a positive surface charge and the other has been adjusted to a negative surface charge and mix them.

[0033] When a dispersion liquid with a positive surface charge is mixed with a dispersion liquid with a negative surface charge, the particles aggregate due to electrostatic interactions and settle as aggregates of various sizes. This mixture is placed in a cylindrical container and rotated circumferentially using a rotator or similar device. Rotating the container circumferentially refers to rotating the container around an axis that passes through the center of the circle on the top and bottom surfaces of the container. The rotation speed depends on the size of the container, but is typically 10 to 50 rpm. Although there are no particular restrictions, it is preferable that the volume of the mixture be at least 60% of the container's volume, and that the height of the particles when left to settle after settling be less than 50% of the container's height. The particles that form aggregates in this process are repeatedly pulled up in the direction of rotation and then fall due to gravity. The aggregates adhere to each other due to electrostatic interactions, increasing in size, and the protrusions break down, resulting in spherical growth. Furthermore, because external forces are received differently depending on the aggregate size, the external forces become uniform, i.e., the particle diameter becomes uniform. As a result, the aggregates become spherical with a relatively uniform particle diameter. The spherical aggregates thus obtained, with a relatively uniform particle size, are used as the core granules of the core-shell composite granules. The rotation time is 0.1 to 10 days, depending on the type of particle, the size of the container, and the rotation speed. The longer the rotation time, the higher the monodispersity and sphericity of the core. The surface of the core thus produced contains a mixture of a first inorganic material with a positively adjusted surface charge and a first inorganic material with a negatively adjusted surface charge, enabling the adsorption of the shell, described below.

[0034] The shell attachment step for the core-shell type composite granules will be described.

[0035] Desired amounts of particles of a first inorganic material and particles of a second inorganic material are prepared, and a dispersion of particles whose surface charges have been adjusted for the first inorganic material and the second inorganic material is prepared and mixed so that the amounts of particles whose surface charges have been adjusted to positive and particles whose surface charges have been adjusted to negative are equal. The first and second inorganic materials can be used in both positive and negative forms. That is, the surface charges of the first and second inorganic materials are adjusted so that they are uniform across the entire mixture, without being biased toward either positive or negative. For example, dispersions of half the particles of the first inorganic material and half the particles of the second inorganic material are prepared and mixed. If the amounts of particles of the first inorganic material and the second inorganic material are equal, one may be adjusted to positive and the other to negative, and then mixed. If one of the particles is present in a larger amount, half of the total shell particles of the larger amount may be adjusted to positive or negative, and the remaining particles and the other particles may be adjusted to opposite charges and then mixed.

[0036] The mixture thus obtained is mixed with a dispersion in which a desired amount of core particles of core-shell type composite granules are dispersed in an aqueous solvent.

[0037] This mixture is placed in a cylindrical container, and the container is rotated circumferentially using a rotator or the like, in the same manner as for preparing the core portion. The surface of the core portion is composed of multiple first inorganic materials, the surface charges of which are either adjusted to positive or negative, and the first or second inorganic material constituting the shell portion is adsorbed to the surface of the core portion depending on the polarity of the adjusted surface charge. In this way, aggregates of particles of the first inorganic material and particles of the second inorganic material adhere to the surface of the core portion particles, forming granules. During this process, the granules repeatedly rotate and fall. When rod-shaped or flat-shaped particles of the second inorganic material are adsorbed with their major axis aligned in the radial direction of the spherical granule, the rotational flow rate exerts a force that tilts them circumferentially, and also exerts a force that tilts them circumferentially upon collision during falling. Particles oriented in the circumferential direction have a large contact area with the granule, so they are strongly adsorbed and fixed to the granule. If they cannot be oriented in the circumferential direction due to their positional relationship with surrounding particles, they fall off the granule due to the force that tries to tilt them, and then repeatedly adhere to the granule again. As a result, a shell portion consisting of an aggregate of particles of the first inorganic material and particles of the second inorganic material is formed on the surface of the core particle, with the long axis of the second inorganic material oriented in the circumferential direction of the composite granule. The rotation time for forming the shell portion is 1 to 100 hours.

[0038] The core-shell composite granules of the present invention may contain other components as needed. For example, a polymer resin may be included to increase the strength of the green body (unsintered compact) at a level that does not interfere with the debinding process. In this case, the polymer resin particles, whose surface charge has been adjusted, are mixed into the granules during the granulation process.

[0039] Next, a method for producing an inorganic composite material using the core-shell composite granules of the present invention will be described. The core-shell composite granules of the present invention are used as raw material powder for an inorganic material, and a green body of this raw material powder is produced and sintered to obtain an inorganic composite material. The green body molding is a process in which raw material powder particles are molded into a desired shape before sintering. Examples include press molding, in which the raw material is poured into a mold and pressed to form the green body, slip casting using a slurry containing the raw material powder particles, tape casting using a doctor blade, and additive manufacturing using a 3D printer. When a slurry is used, the slurry is dried to produce a green body having the original shape of the sintered body.

[0040] Next, we will explain the process of sintering the green body. Sintering is a process in which the surfaces of the particles that form the green body are bonded together by heating, resulting in a densified sintered body. Conventional sintering methods can be used and are selected appropriately depending on the inorganic material. The sintering temperature can be selected appropriately depending on the material; in this invention, a typical sintering temperature for the particles of the first inorganic material is selected. The firing furnace can be a combustion furnace that burns flammable gas, or an electric furnace that heats by passing electricity through a heating element such as a graphite heating element, a metal heating element, or a ceramic heating element. Sintering can be performed in air, vacuum, or under a controlled atmosphere such as an inert, oxidizing, or reducing atmosphere, as needed. A degreasing process to remove organic matter may be performed before the sintering process. To remove organic matter, the sample is heated slowly to approximately 400°C, and then sintered at a high temperature.

[0041] Pressure sintering, in which sintering is performed while pressure is applied, can also be preferably used as a sintering process. For example, a hot press method can be used, in which sintering is performed while pressure is applied using a mold. Furthermore, a spark plasma sintering method can also be preferably used, in which a pulse current is applied to the workpiece while pressure is applied, and the driving force for sintering is a combination of electromagnetic energy, the self-heating of the workpiece, and the spark plasma energy generated between particles. These methods of sintering while pressure is applied can also serve as pressure molding of the green body. The material is molded by pressure, and the pressure is continued without releasing the pressure for sintering. The pressure is generally 20 to 100 MPa.

[0042] Next, the inorganic composite material of the present invention will be described. The inorganic composite material of the present invention is an inorganic composite material using the core-shell composite granules of the present invention. It is formed from a plurality of the composite granules. After sintering, the shell portion forms a continuous phase to become a sea portion, and each core portion forms an independent island portion, forming a sea-island structure. In this case, the sea portion is formed by the shell portion. However, as described above, since the shell portion is composed of a mixture of a first inorganic material and a second inorganic material, the sea portion is formed by a region where the two are mixed. Therefore, the second inorganic material is present in the region that forms the sea portion. Furthermore, in the sea portion, the long axes of rod-shaped or flat particles of the second inorganic material are oriented in the direction of the boundary line of the island portion. Here, "the long axes of the second inorganic material particles being oriented in the direction of the boundary line of the island portion" means that the normal direction at the interface point with the sea portion closest to the center of the long axis of each second inorganic material particle is approximately perpendicular to the long axis of the second inorganic material particle. "Approximately perpendicular" means a range of 60 to 120 degrees, and more than 70% of the particles of the second inorganic material are in this range.

[0043] In the sea region, the long axes of the rod-shaped or plate-shaped particles of the second inorganic material are oriented toward the boundary lines of the island regions, resulting in a high contact frequency between particles of the second inorganic material, even with a small amount of the second inorganic material, resulting in a percolation transition. Furthermore, because the islands of the sea-island structure do not contain particles of the second inorganic material, the properties of the second inorganic material can be exhibited even with a significantly reduced amount of the second inorganic material. Furthermore, in a typical percolation transition, the performance exhibited increases rapidly with the particle ratio, making performance control difficult. In the inorganic composite material of the present invention, if the percolation transition occurs in the sea region, the performance of the entire composite material can be controlled by adjusting the island-to-sea ratio. Thus, the use of the core-shell composite granules of the present invention enables two-stage structural control: the introduction of a sea-island structure and particle orientation in the sea region, which is a major feature of the present invention. [Example]

[0044] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to these examples.

[0045] Example 1 (Core formation of composite granules) 140 nm alumina particles (TM-DAR, manufactured by Taimei Chemical Industry Co., Ltd.) were prepared as the first inorganic material particles. The alumina particles were dispersed in deionized water to a particle solids content of 9% by volume, and the zeta potential of this dispersion was measured using an ELS-Z1 (manufactured by Otsuka Electronics Co., Ltd.). Since the zeta potential was +51 mV, indicating sufficient charge, this dispersion was designated as an aqueous dispersion of positively charged alumina particles. Next, the alumina particles were placed in a 1% by mass aqueous solution of polystyrene sodium sulfonate (PSS, manufactured by Wako Fujifilm Co., Ltd.), a polyanion (polymer electrolyte; anionic polymer), and stirred for 30 minutes using a rotator (manufactured by Taitec Co., Ltd.) to adsorb PSS onto the alumina particle surface. The alumina particles were then settled using a Model 7000 refrigerated centrifuge (manufactured by Kubota Co., Ltd., rotation speed 10,000 rpm) to remove the supernatant. The particles were then washed with deionized water to remove unadsorbed PSS, and deionized water was added to a particle solids content of 9% by volume to form a dispersion. The zeta potential of this dispersion was measured in the same manner and found to be -39 mV, which was a sufficient amount of charge, and this dispersion was used as an aqueous dispersion of negatively charged alumina particles.

[0046] Equal amounts of the positively charged and negatively charged aqueous dispersions were mixed, and 2 mL of the mixture was placed in a cylindrical glass container (mouth inner diameter x body diameter x total length: φ10 x φ21 x 45 mm). 8 mL of deionized water was added to adjust the solids content to 1.8% by volume. The core granules were then rotated circumferentially at 15 rpm for 3 hours, 20 rpm for 1 day, and 30 rpm for 6 days using a rotating roller (ROLAAUV1S, IBI Scientific). Figure 3 shows an optical microscope image of the core granules. The core granules were approximately spherical and had a diameter of 300–600 μm.

[0047] Tabular boron nitride (BN) particles (HGP, manufactured by Denka Co.) were prepared as rod-shaped or flat particles. The average diameter of the particle planes was 5 μm, the thickness was 0.2 μm, and the aspect ratio was 25. The particles were placed in an aqueous solution (0.5% by mass) of the surfactant sodium deoxycholate (SDC, manufactured by Sigma-Aldrich) and stirred for 30 minutes using a rotator (manufactured by Taitec Co.) to adsorb SDC onto the particle surface. The rod-shaped or flat particles were then recovered from the aqueous solution and washed with deionized water to remove any unadsorbed SDC. The resulting SDC-coated particles were then placed in a 1% by mass aqueous solution of poly(diallyldimethylammonium chloride) (PDDA), a polycation (polymer electrolyte; cationic polymer), and stirred for 30 minutes using a rotator (manufactured by Taitec Co.) to adsorb PDDA onto the outermost surface of the particles. Then, the BN particles were precipitated using a refrigerated centrifuge Model 7000 (KUBOTA, rotation speed 10,000 rpm). height After removing the supernatant, the unadsorbed PDDA was removed by washing with deionized water, and deionized water was added to give a particle solids content of 9% by volume to obtain a dispersion. The zeta potential of this dispersion was measured using the same method and found to be +60 mV, which was a sufficient charge, making this dispersion an aqueous dispersion of positively charged BN particles.

[0048] A 9% by volume aqueous dispersion of positively charged BN particles, a 9% by volume aqueous dispersion of positively charged alumina particles, and a 9% by volume aqueous dispersion of negatively charged alumina particles were mixed at a particle volume ratio of 1:4:5. 1.25 mL of this mixture was mixed with 8.75 mL of a 1.8% by volume core granule dispersion (a core / shell particle volume ratio of 8:5). The mixture was placed in a cylindrical glass container (mouth inner diameter x body diameter x total length: φ10 x φ21 x 45 mm) and rotated circumferentially at 30 rpm for 72 hours using a rotating roller (ROLAAUV1S, manufactured by IBI Scientific) to deposit the shell particles onto the surface of the core granules. The composite granule suspension was then dried in a constant temperature and humidity oven (YAMATO, DKM600) at 60 °C for 3 hours to obtain composite granules. The composite granules were observed with a scanning electron microscope (Hitachi, S-4800) and an optical microscope (Leica, DMS1000). An optical microscope photograph is shown in Figure 4. The composite granules were roughly spherical, with diameters of 300 to 600 μm. An enlarged surface photograph of the composite granules taken with a scanning electron microscope is shown in Figure 5. The plate-like particles (black particles) were oriented in the circumferential direction, and no plate-like particles standing in the normal direction were observed.

[0049] (Green body preparation and sintering) The resulting composite granules were packed into a 10 mm diameter graphite die and sintered by electric current plasma sintering using a CSP-KIT-02121 die manufactured by SS Alloy Co., Ltd. A green body was then prepared and sintered. Sintering was performed at 1250°C for 10 minutes under a pressure of 40 MPa, yielding the inorganic composite material of the present invention. The fracture surface of the resulting composite material was observed using a scanning electron microscope (Hitachi S-4800) and an optical microscope (Leica DMS1000). An optical microscope photograph is shown in Figure 6. It can be seen that the composite has a sea-island structure. A scanning electron microscope photograph is shown in Figure 7, and an enlarged photograph is shown in Figure 8. It can be seen that the long axes of the BN particles in the sea region are oriented along the boundary lines of the island regions.

[0050] (Comparative Example 1) Aqueous dispersions of positively charged BN particles, aqueous dispersions of positively charged alumina particles, and aqueous dispersions of negatively charged alumina particles were mixed in amounts of 1.5 mL, 1.625 mL, and 0.125 mL, respectively, of a 9% by volume positively charged alumina dispersion, a negatively charged alumina dispersion, and a 9% by volume BN dispersion, so that the ratio of alumina particles to BN particles in the composite granules as a whole was the same as in Example 1. 3.25 mL of this mixture was placed in a glass container (mouth inner diameter x body diameter x total length: φ10 x φ21 x 45 mm), 6.75 mL of deionized water was added, and the mixture was rotated in the circumferential direction at a rotation speed of 20 rpm for 1 week using a rotating roller (ROLAAUV1S manufactured by IBI Scientific Co., Ltd.) to produce composite granules in which alumina particles and BN particles were uniformly dispersed. Furthermore, the mixture was sintered in the same manner as in Example 1 to obtain a comparative inorganic composite.

[0051] (Measurement of thermal conductivity) The thermal conductivity of the composite materials of Example 1 and Comparative Example 1 was evaluated by a xenon flash method (XFA300, manufactured by LINSEIS) at a thickness of 1.8 mm and a temperature of 30°C. The composite material of Example 1 had a thermal conductivity of 14.9 W / mK, while the homogeneous composite material of the comparative example had a thermal conductivity of 11.7 W / mK, and the composite material of the present invention exhibited high thermal conductivity. [Explanation of symbols]

[0052] 10: Core-shell type composite granules 11: Particles made of a first inorganic material 12: Particles of a second inorganic material formed into rod-like or flat shapes

Claims

1. A core-shell type composite granule having a core portion consisting of particles of a first inorganic material and a shell portion attached to the surface of the core portion, wherein the core portion is a granule formed by spherically agglomerating multiple particles of the first inorganic material, and the shell portion is composed of a mixture of multiple particles of the first inorganic material and multiple particles of a second inorganic material formed in a rod-like or flat shape, and the particles of the second inorganic material are attached with their long axes oriented circumferentially to the core portion.

2. A method for producing the core-shell type composite granules according to claim 1, The method includes a core portion forming step and a shell portion attaching step, The core portion forming step includes: adjusting the surface charge of particles of a first inorganic material to be positive; adjusting the surface charge of particles of a first inorganic material to be negative; a step of mixing an aqueous solvent dispersion of the positively charged particles of the first inorganic material with an aqueous solvent dispersion of the negatively charged particles of the first inorganic material, placing the mixed solution in a cylindrical container, and rotating the cylindrical container in a circumferential direction to form core granules of composite granules, The shell portion attachment step includes: a step of adjusting the surface charge of rod-shaped or flat particles of a second inorganic material to be positive or negative; and mixing, in an aqueous solvent, an aqueous dispersion of rod-shaped or flat particles of a second inorganic material whose surface charge has been adjusted to be positive or negative with an aqueous dispersion of particles of a first inorganic material whose surface charge has been adjusted to at least the opposite charge, adding this mixture to the aqueous dispersion of the core granules produced in the core formation step, and then placing this mixture in a cylindrical container and rotating the cylindrical container in the circumferential direction to form shell portions on the surfaces of the core granules. A method for producing core-shell type composite granules, characterized in that

3. 2. An inorganic composite material using the core-shell type composite granules according to claim 1, wherein the inorganic composite material is formed by a plurality of the composite granules, and has a sea-island structure having island portions consisting of the core portions and a sea portion consisting of the shell portions, An inorganic composite material, characterized in that the long axes of rod-shaped or flat-plate-shaped particles of a second inorganic material contained in the sea portion are oriented in the direction of the boundary lines of the island portions.

4. The inorganic composite material described in claim 3, characterized in that the rod-shaped or plate-shaped particles of the second inorganic material have anisotropic properties, and the properties in the short axis or thickness direction are different from the properties in the long axis or plane direction.

5. The inorganic composite material according to claim 4, characterized in that the property is thermal conductivity or electronic conductivity, and the thermal conductivity or electronic conductivity in the major axis or plane direction is higher than the property in the minor axis or thickness direction.

6. A method for producing the inorganic composite material according to claim 3, A method for producing an inorganic composite material, comprising the steps of: appropriately agglomerating the core-shell type composite granules to produce a green body; and sintering the green body.

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