JP2023078711A - Core-shell type composite granule, inorganic composite material prepared using core-shell type composite granule and manufacturing method thereof - Google Patents

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

The present invention relates to core-shell type composite granules that serve as raw material powder for sintered bodies such as ceramics and metals, inorganic composite materials produced using core-shell type composite granules, and methods for producing them. The present invention relates to core-shell type composite granules in which particles are oriented, inorganic composite materials prepared using the core-shell type composite granules in which arrangement and orientation of functional particles are controlled, and methods for producing the same.

Ceramic materials are used in various fields as structural materials and heat-resistant materials, and are widely used as substrates and packages for integrated circuits, firing vessels and setters, and core tubes of electric furnaces. This ceramic material is generally produced by molding raw material powder into a green body by a dry or wet molding method and sintering the green body. Furthermore, in recent years, studies have been actively conducted to improve the functionality and properties of ceramic materials by mixing fine particles having functionality (functional fine particles) with ceramic powder as a matrix.

In general, functional composite materials with functional fine particles added to the matrix need to form paths by contacting the functional fine particles in the matrix in order to impart electronic or thermal conductivity, for example. When the ratio of functional fine particles in the matrix exceeds a threshold value, the function suddenly appears (percolation transition). The shape and arrangement of functional fine particles have a great influence on the occurrence of percolation transition. As an example of a composite material of a polymer material, in Patent Document 1, the short axis average particle size is 0.005 to 0.05 μm, the long axis average particle size is 0.1 to 3 μm, and the aspect ratio is 5 or more. is disclosed, and it is stated that even a small amount of a conductivity-imparting agent can effectively form a conductive path. In Non-Patent Document 1, PMMA (polymethyl methacrylate) and h-BN (hexagonal boron nitride) as a thermally conductive filler are used as raw materials, and each is adjusted to have an opposite surface charge with a polymer electrolyte, and the PMMA particle surface Composite particles to which h-BN particles are adsorbed are disclosed, and by molding the resulting composite particles, it is disclosed that high thermal conductivity is exhibited even if the amount of h-BN particles added is extremely small. there is

In ceramic materials as well, the arrangement of functional fine particles in the green bodies is very important in order to develop such functionality. In Non-Patent Document 2, alumina particles having a positive surface charge and zirconia particles whose surface charge is adjusted to be negative by an anionic polymer electrolyte are used to granulate the core portion, and in addition to the alumina particles and zirconia particles, Core-shell type composite granules in which a shell portion composed of alumina particles, zirconia particles and CNT particles is formed on the surface of the core portion by adding CNTs (carbon nanotubes) charge-adjusted with an activator and performing a granulation operation, and sintering using the same. It is disclosed that CNTs are locally and continuously introduced into the resulting composite material to form a segregated three-dimensional network, and that a small amount of CNTs functions.

Japanese Patent Laid-Open No. 08-217445

Taichi Kuroda, Nguyen Huu Huy Phuc, Tsuyoshi Kawamura, Atsunori Matsuda, Hiroyuki Muto The Ceramic Society of Japan 2014 Annual Meeting "Fabrication of h-BN Addition High Thermal Conductive Polymer Composite Materials" Proceedings 1P005 Yusaku Sato, Atsushi Yokoi, Tan Wai Kian, Tsuyoshi Kawamura, Hiroyuki Muto, Atsunori Matsuda Japan Society of Powder and Powder Metallurgy 2021 Autumn Meeting (128th Lecture Meeting) "Ceramics with Segregated Three-Dimensional Conductive Network Structure Using Composite Granules" "Development of Composite Materials" Proceedings 2-7A

However, Non-Patent Document 1 is a technique relating to a polymer matrix and cannot be simply applied to inorganic materials such as ceramics. For example, it is possible to produce composite particles in which h-BN particles are attached to ceramic base particles, but even if a green body is produced from these composite particles and heated, it will not be sintered. In the case of a polymer matrix, since the polymer heated to a high temperature has a certain degree of fluidity, it flows between the h-BN particles and on the surface of the composite particles to hold the h-BN particles and bond the composite particles together. In the case of inorganic materials such as ceramics, the matrix material does not flow and cannot contribute to the retention of the functional particles or the adhesion of adjacent granules.

The CNTs in the shell portion of the core-shell type composite granules described in Non-Patent Document 2 are randomly arranged, and their arrangement and orientation cannot be controlled. Furthermore, in the sintered body produced from this core-shell type composite granule, CNTs can be locally and continuously introduced to form a segregated three-dimensional network, but the CNTs in the three-dimensional network are randomly present and arranged. , the orientation cannot be controlled.

An object of the present invention is to provide composite powder particles that can suitably use raw material powders of inorganic materials such as ceramics with a controlled structure, and structure-controlled inorganic composite materials produced therefrom. has a core-shell structure and provides powder particles in which the arrangement and orientation of the functional particles contained in the shell part are controlled, and in a sintered body produced from this powder particle, the first stage is from the matrix material It has a sea-island structure consisting of an island part and a sea part where the functional material is segregated, and the second stage is a structure in which the functional particles in the sea part are oriented in the boundary line direction of the island part. It is an object of the present invention to provide an inorganic composite material.

The present invention is shown below.
[1] A core-shell type composite granule having a core part made of particles of a first inorganic material and a shell part attached to the surface of the core part, wherein the core part is made of the first inorganic material The shell portion is a mixture of a plurality of particles made of the first inorganic material and a plurality of particles made of the second inorganic material formed into a rod or plate shape. wherein the particles of the second inorganic material are attached with their major axes oriented in the circumferential direction of the core portion.
[2] A method for producing the core-shell type composite granules according to [1],
including a core portion forming step and a shell portion attaching step,
The core portion forming step includes:
adjusting the surface charge of the particles of the first inorganic material to be positive;
adjusting the surface charge of the particles of the first inorganic material to be negative;
mixing the aqueous solvent dispersion of the positively charged particles of the first inorganic material and the aqueous solvent dispersion of the negatively charged particles of the first inorganic material, and placing the mixture in a cylindrical container; and rotating the cylindrical container in a circumferential direction to form core granules of composite granules;
The shell attachment step includes:
adjusting the surface charge of the rod-like or plate-like particles of the second inorganic material to be positive or negative;
an aqueous solvent dispersion of rod-like or plate-like particles of a second inorganic material whose surface charge has been adjusted to be positive or negative, and an aqueous solvent dispersion of particles of the first inorganic material whose surface charge has been adjusted to at least the opposite is mixed in an aqueous solvent, this mixed solution is added to the aqueous dispersion of core granules prepared in the core part forming step, and this mixed solution is further placed in a cylindrical container, and the cylindrical container is rotated in the circumferential direction and a step of forming a shell on the surface of the core granule by rotating.
[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 an island portion composed of the core portion and a sea portion composed of the shell portion. It has a sea-island structure,
An inorganic composite material, wherein long axes of rod-like or plate-like particles of the second inorganic material contained in the sea portion are oriented in the boundary line direction of the island portion.
[4] The rod-shaped or plate-shaped particles of the second inorganic material are anisotropic in their characteristics, and characterized in that the characteristics in the minor axis or thickness direction and the characteristics in the major axis or plane direction are different from each other. The inorganic composite material according to [3], characterized in that
[5] The characteristic is thermal conductivity or electronic conductivity, and the thermal conductivity or electronic conductivity in the long axis or surface direction is higher than in the short axis or thickness direction. The inorganic composite material according to [4].
[6] A method for producing an inorganic composite material according to [3],
A method for producing an inorganic composite material, comprising the steps of appropriately aggregating the core-shell type composite granules to produce a green body, and sintering the green body.

A composite material obtained by using the core-shell type composite granules of the present invention as a raw material powder of an inorganic material, preparing a green body from the raw material powder, and sintering the raw material powder consists of an island portion composed of a matrix material and a sea portion in which a functional material is segregated. and a two-step structure in which the functional particles in the sea portion are oriented in the boundary line direction of the island portion. In other words, by designing the raw material powder, it is possible to obtain a composite material with a sea-island structure and a two-step structural control in which the arrangement and orientation of the functional particles in the sea are also controlled by the conventional sintering process. Become. In addition, the composite material obtained in the present invention is excellent in sintering of the shell portion, sintering of the interface between the shell portion and the core portion, and sintering of the shell portions of adjacent composite granules. A material strength close to that of a single material can be obtained.

Furthermore, in a functional material prepared by simply mixing functional fine particles and matrix particles, the expression level of functionality changes rapidly before and after percolation transition, making it difficult to control the expression level. The expression level of the functionality of the composite material can be freely adjusted by adjusting the ratio of the core part and the shell part of the type composite granule.

1 is a schematic cross-sectional view of a core-shell type composite granule of the present invention. FIG. 2 is a scanning electron micrograph of a cross section obtained by crushing a sintered body produced in Non-Patent Document 2. FIG. 1 is an optical micrograph of core granules of Example 1. FIG. 1 is an optical micrograph of core-shell type composite granules of Example 1. FIG. 1 is an enlarged scanning electron micrograph of the surface of core-shell type composite granules of Example 1. FIG. 1 is an optical micrograph of a fracture surface of the composite material of Example 1. FIG. 1 is a scanning electron micrograph of a fracture surface of the composite material of Example 1. FIG. 1 is an enlarged scanning electron micrograph of a fracture surface of the composite material of Example 1. FIG.

Embodiments of the present invention will be described in detail below.

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 a raw material powder for producing a sintered body. Get a composite material.

In the present invention, composite granules represent aggregates composed of different types of particles, and the composite granules of the present invention have a spherical morphology. Here, the term “spherical” means not limited to true spheres but roughly spherical. FIG. 1 shows a schematic diagram of the core-shell type composite granules of the present invention. The term "core-shell type" means that the composition of the particles changes in the radial direction from the center of the sphere in the spherical shape, and the center side is the core portion and the surface side is the shell portion. The core portion is composed of a plurality of particles of the first inorganic material. The shell portion is composed of a mixture of a plurality of particles of the first inorganic material and a plurality of rod-shaped or plate-shaped particles of the second inorganic material, and the long axis of the rod-shaped or plate-shaped particles of the second inorganic material are oriented in the circumferential direction of the composite granules. Here, the expression that the long axis of the particles made of the second inorganic material is oriented in the circumferential direction of the composite granule means the interface with the core part closest to the center of the long axis of each particle made of the second inorganic material. It represents that the normal direction at the point and the long axis direction of the particles of the second inorganic material are approximately perpendicular. The term "approximately perpendicular" means that the angle ranges from 60 degrees to 120 degrees, and 70% or more of the particles of the second inorganic material fall within this range.

The particles made of the first inorganic material are particles made of ceramics or metal, and may be particles of one kind or particles of two or more kinds. In addition, when a plurality of types of particles are used, it is preferable that the ratio of the average particle size is within 10 times from the viewpoint of ease of forming granules, which will be described later. There are no particular restrictions on the types of ceramics and metals, but examples of ceramics include oxides, carbides, and nitrides. Incidentally, the oxide may be a single oxide or a composite oxide. Examples of such ceramics include alumina, zirconia, silicon nitride, silicon carbide, magnesia, calcia, titania, vanadium oxide, spinel, and ferrite, and these may be used singly or in combination. Furthermore, a solid solution of these may be used.

Examples of metals include powders of iron, copper, aluminum, nickel, molybdenum, titanium, and tungsten, but are not limited to these. Moreover, these may be used singly or in a mixture, and may be alloys. Various alloys can be used, and examples thereof include iron alloys, steel alloys, copper alloys, nickel alloys, aluminum alloys, and super steel alloys. Examples of the cermet include TiC--Ni cermet, Al ₂ O ₃ --Cr cermet, and Al ₂ O ₃ --Fe cermet, but are not limited to these.

The geometric shape of the particles of the first inorganic material is not limited, and spherical, needle-like, massive, columnar, flat, plate-like, fibrous, amorphous, and the like can be applied.

The rod-like or plate-like particles of the second inorganic material that constitute the shell portion will be described. In the present invention, the term "rod-like" refers to a shape having a smaller size in the radial direction than in the longitudinal direction, and is also referred to as a rod-like shape, a needle-like shape, or a rod-like shape. The radial direction is also expressed as the short axis and the longitudinal direction as the long axis. The rod-like particles preferably have a diameter of 0.05 μm to 5 μm and a length of 0.5 μm to 50 μm. The aspect ratio is preferably 3 or more, and more preferably 5 or more, for ease of orientation, which will be described later. In the present invention, the rod shape does not include the filament shape. Here, filamentous means a material having an aspect ratio of 100 or more. The CNT used in Non-Patent Document 2 corresponds to filamentous. The filamentous material is generally highly flexible, and is not oriented by being entangled with the particles of the first inorganic material during the process of granulating the shell portion, which will be described later. If it is rigid, it will not be incorporated into the granules due to its weak adsorptive power due to point contact with the granules. FIG. 2 shows an SEM photograph of a cross section obtained by crushing the sintered body produced in Non-Patent Document 2. As shown in FIG. It can be seen that the CNTs are not oriented and are random.

In the present invention, the term “flat” refers to particles having a shape smaller in thickness than in the length or width direction, and is also called plate-like, flaky, scale-like, or flake-like. As for the size of the tabular grains, the average grain size in the major axis direction is preferably 0.5 μm to 50 μm, and the aspect ratio is preferably 3 or more, preferably 5 or more, in terms of ease of orientation, which will be described later. is more preferable.

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 materials with remarkably high or low specific properties. Specific properties can include, for example, electronic conductivity, thermal conductivity, magnetism, dielectricity, elasticity, and the like. In the present invention, a particularly preferred embodiment is a material having high electronic conductivity and high thermal conductivity that exhibits high performance by percolation.

Materials having high electron conductivity include SnO ₂ , ZnO, TiO ₂ , CeO ₂ and the like, and materials obtained by doping these with heteroatoms. Materials with high thermal conductivity include Al ₂ O ₃ , MgO, ZnO, SiO ₂ , TiO ₂ , mica, potassium titanate, iron oxide, oxide particles such as talc, boron nitride, silicon nitride, aluminum nitride, and the like. nitride particles, 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 for multiferroic substances can also be used, such as BiFeO ₃ , BaTiO ₃ , Pd(Zr, Ti)O ₃ , CaMnO ₃ , LaFeO ₃ and TbMnO ₃ .

In addition, the structure control of the present invention is effective when the characteristics in the radial direction and the length direction are different in the rod shape, and the characteristics in the thickness direction and the length or width direction are different in the plate shape. It is particularly effective when the characteristics in the length direction are higher than in the direction, and when the characteristics in the length or width direction are higher than in the thickness direction in the case of a plate.

A method for producing the core-shell type composite granules of the present invention will be described. The method for producing core-shell type composite granules of the present invention comprises a core forming step and a shell adhering step. The core portion forming step includes a step of adjusting the surface charge of the particles of the first inorganic material to be positive, a step of adjusting the surface charge of the particles of the first inorganic material to be negative, and the positively charged first An aqueous solvent dispersion of inorganic material particles and an aqueous solvent dispersion of negatively charged first inorganic material particles are mixed, the mixture is placed in a cylindrical container, and the cylindrical container is rotated in the circumferential direction. to form core granules of composite granules. The step of adhering to the shell portion includes a step of adjusting the surface charge of the particles of the rod-shaped or plate-shaped second inorganic material to be positive or negative, and the rod-shaped or plate-shaped second inorganic material whose surface charge is adjusted to be positive or negative. and an aqueous solvent dispersion of the particles of the first inorganic material adjusted to at least the opposite charge, in an aqueous solvent, and this mixed solution was prepared in the core part forming step. In addition to the aqueous dispersion of the core granules, the mixed liquid is placed in a cylindrical container, and the cylindrical container is rotated in the circumferential direction to form the shell on the surface of the core granules. Here, the term "aqueous solvent" refers to water or a mixed solvent of a water-compatible solvent and water, and the term "water-compatible solvent" includes alcohol-based solvents, ketone-based solvents, and the like.

The surface charge is the apparent potential of the particles, and when polar layers are laminated on the particle surface, the surface charge is the charge of the outermost layer. A cationic polymer electrolyte that ionizes in an aqueous solvent and becomes positively charged, and an anionic polymer electrolyte that ionizes in an aqueous solvent and becomes negatively charged can be preferably used to adjust the surface charge. Examples of cationic polymer electrolytes that ionize and positively charge in aqueous solvents include poly(diallyldimethylammonium chloride) (PDDA), polyethyleneimine (PEI), polyvinylamine (PVAm), poly(vinylpyrrolidone N, N-dimethylaminoethylacrylic acid) can be mentioned as cationic polymers such as copolymers. Examples of anionic polymer electrolytes that are ionized and negatively charged in aqueous solvents include polystyrene sulfonic acid (PSS), polyvinyl sulfate (PVS), polyacrylic acid (PAA), polymethacrylic acid (PMA), polycarboxylic acid Anionic polymers such as acid (PCA) can be mentioned.

The particles of the first inorganic material and the particles of the second inorganic material have a positive or negative surface charge when these materials are dispersed in an aqueous solvent. If it has a positive surface charge, it can impart a strong negative surface charge by adsorbing an anionic polyelectrolyte. Conversely, when it has a negative surface charge, a strong positive surface charge can be imparted by adsorbing a cationic polymer electrolyte. After adjusting the surface charge with a cationic polymer electrolyte or anionic polymer electrolyte, the charge can be reversed by further adsorbing a polymer electrolyte having an opposite charge. This operation allows the surface charge of the particles to be adjusted to the desired charge. This operation may be repeated many times such as positive/negative/positive/negative/positive/negative to adsorb the polymer electrolyte more firmly. In the case where sufficient surface charge is obtained by just dispersing in an aqueous solvent without performing treatment with a polymer electrolyte or the like, the treatment of dispersing in an aqueous solvent can be used as the surface charge adjustment step.

If the dispersibility of the particles is hindered by bridging aggregation due to adsorption of the polymer electrolyte, an ionic surfactant may be used instead of the polymer electrolyte. Even if this ionic surfactant is used, it is possible to charge the particle surface by its adsorption.

As a method for adsorbing the polymer electrolyte to each particle, for example, the polymer electrolyte is adsorbed on the particle surface by adding, stirring, and dispersing the charge-adjusting particles in a solution in which the polymer electrolyte is dissolved in an aqueous solvent. can be done. Here, in order to allow the particles to adsorb a sufficient amount of the polymer electrolyte, it is preferable that the solution contains an excess amount of the polymer electrolyte relative to the amount of the particles charged. In this case, an operation for removing the excess polymer electrolyte, for example, sedimentation, centrifugation, filtration, or the like, separates the liquid from the particles, and removes the excess polymer electrolyte. After that, the particles are re-dispersed in the aqueous solvent to obtain a dispersion liquid dispersed in the aqueous solvent. The solid content of this dispersion is not particularly limited, but is from 1 to 20% by volume.

Alternatively, as described in Japanese Patent Application No. 2013-507747, while stirring a mixed solution in which particles are added to an aqueous solvent and monitoring the liquid physical properties (zeta potential, viscosity, etc.), an aqueous solvent in which a polymer electrolyte is dissolved is added dropwise. However, it is also possible to preferably use a method of stopping the dropping at a point where it can be judged from changes in liquid properties that a sufficient amount of polymer electrolyte has been adsorbed to the particles. A dispersion in an aqueous solvent is obtained by this technique.

Whether or not the desired surface charge is obtained can be confirmed by measuring the zeta potential of the obtained particles. A positive zeta potential indicates a positive surface charge, and a negative zeta potential indicates a negative surface charge. Moreover, having a positive surface charge is expressed as being positively charged, and having a negative surface charge is expressed as being negatively charged. The magnitude of the surface charge of the particles after surface charge adjustment is preferably 20 mV to 150 mV, more preferably 30 mV to 100 mV, in absolute value of zeta potential.

Next, preparation of the core portion of the core-shell type composite granules will be described. The dispersion liquid of the particles of the first inorganic material positively adjusted by the surface charge adjusting step and the dispersion liquid of the particles of the first inorganic material negatively adjusted are mixed so that the amount of the particles is equal. In addition, when the particles of the first inorganic material are composed of a plurality of types of particles, the amount of the particles whose surface charge is adjusted to be positive and the amount of particles whose surface charge is adjusted to be negative are equal to each other. Prepare and mix the dispersion. For example, it is possible to prepare dispersion liquids in which the surface charge is adjusted to be positive or negative by half for each of the different kinds of particles, and then mix them. If two types are equal in amount, a dispersion may be prepared in which one is positive and the other is negative, and mixed.

When a dispersion in which the surface charge is adjusted to be positive and a dispersion in which the surface charge is adjusted to be negative are mixed, the particles aggregate due to electrostatic interaction and settle as aggregates of various sizes. This mixture is placed in a cylindrical container, and the container is rotated in the circumferential direction by a rotator or the like. Rotating the container in the circumferential direction means rotating around a line passing through the centers of the circles on the upper and lower surfaces of the container. The rotation speed depends on the size of the container, but is, for example, 10-50 rpm. Although there is no particular limitation, it is preferable that the amount of the mixed liquid is 60% or more of the volume of the container, and that the height of the particles when the particles are allowed to settle is 50% or less of the height of the container. Particles aggregated in this process are pulled up in the direction of rotation and fall repeatedly due to gravity. Aggregates adhere to each other due to electrostatic interaction, and as the size increases, the projections collapse and grow into a spherical shape. Furthermore, since the way of receiving the external force differs depending on the aggregate size, the external force becomes uniform, that is, the particle diameter becomes uniform. As a result, the aggregates have a spherical shape with relatively uniform particle diameters. The thus obtained spherical aggregates with relatively uniform particle diameters are used as core granules of core-shell type composite granules. The rotation time is 0.1 to 10 days, depending on the type of particles, the size of the container and the speed of rotation. The longer the rotation time, the higher the monodispersity and sphericity of the core. On the surface of the core portion thus produced, the first inorganic material having a positive surface charge and the first inorganic material having a negative surface charge are mixed, which will be described later. The adsorption of the shell part of is possible.

The step of adhering the core-shell type composite granules to the shell will be described.

A desired amount of the particles of the first inorganic material and the particles of the second inorganic material are prepared, and first A dispersion of surface-charge-adjusted particles of the first inorganic material and the second inorganic material is prepared and mixed. The first and second inorganic materials can be used both positively and negatively tuned, respectively. That is, when the first inorganic material and the second inorganic material are mixed, adjustment is made so that the surface charge of the entire mixture is not biased to either positive or negative and is in a uniform state. For example, the particles of the first inorganic material and the particles of the second inorganic material are prepared and mixed with half of each of the particles of the second inorganic material, the surface charges of which are adjusted to be positive and negative. When the amount of particles of the first inorganic material and the second inorganic material is equal, one may be adjusted to be positive and the other to be negative and mixed. If one of the particles is more than one, half of the total shell portion particles may be adjusted to be positive or negative, and the remaining particles and the other particles may be adjusted to have opposite charges and mixed.

The mixed liquid thus obtained is mixed with a dispersion liquid obtained by dispersing a desired amount of core particles of core-shell type composite granules in an aqueous solvent.

This mixed liquid is placed in a cylindrical container, and the container is rotated in the circumferential direction by a rotator or the like in the same manner as in the production of the core portion. The surface of the core portion is composed of a plurality of first inorganic materials, the surface charges of which are both positively adjusted and negatively adjusted, and constitute the shell portion. The first or second inorganic material is adsorbed on the surface of the core portion according to the adjusted polarity of the surface charge. In this way, aggregates of the particles of the first inorganic material and the particles of the second inorganic material adhere to the surface of the core particles, and the granules grow. At this time, the granules are repeatedly rotated and dropped. When rod-shaped or plate-shaped particles of the second inorganic material are adsorbed with their major axis directed in the radial direction of the spherical granules, the flow velocity of the rotation causes a force to fall in the circumferential direction. Even in the collision, a force acts in the circumferential direction. Particles oriented in the circumferential direction have a large contact area with granules, and thus are strongly adsorbed and fixed to granules. If it cannot be oriented in the circumferential direction due to the positional relationship with surrounding particles, etc., it falls off from the granules due to the force of falling down, and adheres to the granules again. As a result, the surface of the core part particle is composed of aggregates of the particles of the first inorganic material and the particles of the second inorganic material, and the long axis of the second inorganic material is oriented in the circumferential direction of the composite granule. is formed. The rotation time for forming the shell part is from 1 hour to 100 hours.

The core-shell type composite granules of the present invention may contain other ingredients as necessary. For example, a polymer resin may be included for the purpose of increasing the strength of the green body (unsintered molded body) to a level that does not hinder the degreasing process. In this case, the granules are incorporated by mixing particles of a polymer resin with a controlled surface charge during the granulation process.

Next, a method for producing an inorganic composite material using the core-shell type composite granules of the present invention will be described. Using the core-shell type composite granules of the present invention as a raw material powder of an inorganic material, a green body of this raw material powder is produced and sintered to obtain an inorganic composite material. Forming the green body is the process of forming the raw material powder particles into a desired shape before firing. Examples include press molding in which the material is placed in a mold and pressed to form a mold, cast molding using a slurry containing raw material powder particles, tape molding using a doctor blade, and layered molding using a 3D printer. When a slurry is used, drying is performed to form a green body having the original shape of the sintered body.

Next, the step of sintering the green body will be described. Sintering is a process in which surfaces of particles forming a green body are bonded to each other by heating to form a densified sintered body. A conventionally known method can be used as the sintering method, and the method is appropriately selected according to the inorganic material. The sintering temperature may be appropriately selected according to the material, and in the present invention, a general sintering temperature for particles of the first inorganic material may be selected. As the firing furnace, a combustion furnace that burns combustible gas, an electric furnace that heats by energizing a heating element such as a graphite heating element, a metal heating element, a ceramics heating element, or the like can be used. Sintering can be carried out under controlled atmospheres such as air, vacuum, inert, oxidizing and reducing atmospheres, if desired. A degreasing step for removing organic substances may be provided before the sintering step. In order to remove the organic substances, the temperature is slowly raised to about 400° C. to heat the sample, and then the sample is sintered by heating at a high temperature.

As the sintering step, pressure sintering in which sintering is performed while applying pressure can also be preferably used. For example, a hot press method of sintering while applying pressure using a mold can be used. Furthermore, there is also a discharge plasma sintering method in which a pulse is applied to the workpiece while pressurizing it, and electromagnetic energy, self-heating of the workpiece, and discharge plasma energy generated between particles are used as driving forces for sintering. Can be used preferably. These methods of sintering while pressurizing can also serve as press molding of the green body. It is shaped by pressurization and sintered by continuing the pressurization without releasing the pressurization. The pressure is generally 20-100 MPa.

Next, the inorganic composite material of the present invention will be explained. The inorganic composite material of the present invention is an inorganic composite material that uses the core-shell type composite granules of the present invention, is formed by a plurality of the composite granules, and after sintering, the shell portion forms a continuous phase and becomes a sea portion, Each core part has a sea-island structure that becomes an independent island part by the sea part. At this time, the sea portion is formed by the shell portion, and as described above, the shell portion is composed of a mixture of the first inorganic material and the second inorganic material. It is formed by the area where Therefore, the second inorganic material is present in the region forming the sea. Further, in the sea portion, the long axes of the rod-like or plate-like particles of the second inorganic material are oriented in the boundary line direction of the island portion. Here, the expression that the long axis of the particles of the second inorganic material is oriented in the boundary line direction of the island portion means that the interface with the sea portion closest to the center of the long axis of each particle of the second inorganic material and the long axis direction of the particles of the second inorganic material are approximately perpendicular to each other. The term "approximately perpendicular" means that the angle ranges from 60 degrees to 120 degrees, and 70% or more of the particles of the second inorganic material fall within this range.

In the sea part, the long axes of the rod-like or plate-like particles of the second inorganic material are oriented in the boundary line direction of the island part, so that the frequency of contact between the particles of the second inorganic material increases even if the amount is small, resulting in percolation transition. . Furthermore, since the island portions 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 if the amount of the second inorganic material is greatly reduced. Furthermore, in the normal percolation transition, the performance is difficult to control because the performance rises rapidly with respect to the particle ratio. In the inorganic composite material of the present invention, if percolation transition occurs in the sea portion, the performance of the composite material as a whole can be controlled by the ratio of the island portion to the sea portion. Thus, it is a major feature of the present invention that the use of the core-shell type composite granules of the present invention enables two-stage structural control of introduction of a sea-island structure and particle orientation of the sea portion.

The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples.

(Example 1)
(Formation of core portion of composite granules)
Alumina particles of 140 nm (TM-DAR, manufactured by Taimei Chemical Industry Co., Ltd.) were prepared as particles of the first inorganic material. Alumina particles were dispersed in deionized water so that the solid content of the particles was 9% by volume, and the zeta potential of this dispersion was measured using ELS-Z1 manufactured by Otsuka Electronics Co., Ltd. Since the zeta potential was +51 mV and it was sufficiently charged, this dispersion was used as an aqueous dispersion of positively charged alumina particles. Next, alumina particles are added to a 1% by mass aqueous solution of sodium polystyrene sulfonate (PSS, manufactured by Wako Fuji Film Co., Ltd.), which is a polyanion (polymer electrolyte; anionic polymer). The PSS was adsorbed on the surface of the alumina particles by stirring. After that, the alumina particles are sedimented by a refrigerated centrifuge Model 7000 (manufactured by KUBOTA, rotation speed 10,000 rpm), and the supernatant is removed, followed by washing with deionized water to remove unadsorbed PSS and deionize. Water was added so that the solid content of the particles was 9% by volume to prepare a dispersion. When the zeta potential of this dispersion was measured by the same method, it was −39 mV and had a sufficient amount of charge, and this dispersion was used as an aqueous dispersion of negatively charged alumina particles.

Equal amounts of the aqueous dispersion of positively charged alumina particles and the aqueous dispersion of negatively charged alumina particles were mixed, and the mixture was placed in a cylindrical glass container (inner diameter x body diameter x total length: φ10 x φ21 x 45 mm), 2 mL and 8 mL of deionized exchange water are added to make the solid content 1.8% by volume, and a rotating roller (ROLAAUV1S manufactured by IBI Scientific) is used to rotate in the circumferential direction at a rotation speed of 15 rpm for 3 hours and 20 rpm. Core granules were prepared by rotating at a rotation speed of 1 day and at a rotation speed of 30 rpm for 6 days. FIG. 3 shows an optical micrograph of this core granule. The core granules were approximately spherical and had a diameter of 300-600 μm.

(Adherence to shell of composite granules)
Tabular boron nitride (BN) particles (HGP, manufactured by Denka Co., Ltd.) were prepared as rod-like or tabular particles. The particles have an average plane diameter of 5 μm, a thickness of 0.2 μm, and an aspect ratio of 25. The particles were put into an aqueous solution (0.5% by mass) of sodium deoxycholate (SDC, manufactured by Sigma-Aldrich), which is a surfactant, and stirred for 30 minutes with a rotator (manufactured by Taitec). SDC was adsorbed. Thereafter, the rod-shaped or tabular grains were recovered from the aqueous solution and washed with deionized water to remove unadsorbed SDC. Next, the obtained SDC-coated particles are put into a 1% by mass aqueous solution of poly(diallyldimethylammonium chloride) (PDDA), which is a polycation (polyelectrolyte; cationic polymer), and a rotator (manufactured by Taitec Co., Ltd. ) for 30 minutes to adsorb PDDA on the outermost surface of the particles. After that, the BN particles are sedimented by a refrigerated centrifuge Model 7000 (manufactured by KUBOTA, rotation speed 10,000 rpm), and the supernatant is removed, followed by washing with deionized water to remove unadsorbed PDDA and deionize. Water was added so that the solid content of the particles was 9% by volume to prepare a dispersion. When the zeta potential of this dispersion was measured by the same method, it was +60 mV and had a sufficient amount of charge, and this dispersion was used as an aqueous dispersion of positively charged BN particles.

An aqueous dispersion of positively charged BN particles (9% by volume), an aqueous dispersion of positively charged alumina particles (9% by volume), and an aqueous dispersion of negatively charged alumina particles (9% by volume) were added to the particles. Mix so that the volume ratio is 1:4:5, and mix 1.25 mL of this mixed liquid with 8.75 mL of the core part granule dispersion liquid having a solid content of 1.8% by volume (the volume of the core part particles and the shell part particles ratio 8:5), placed in a cylindrical glass container (inner diameter x body diameter x total length: φ10 x φ21 x 45 mm), and rotated at 30 rpm in the circumferential direction using a rotating roller (ROLAAUV1S manufactured by IBI Scientific). for 72 hours to attach the shell to the surface of the core granules. Thereafter, the suspension containing the composite granules was dried at 60° C. for 3 hours in a constant temperature and humidity blower (DKM600, manufactured by YAMATO Co., Ltd.) 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 micrograph is shown in FIG. The composite granules were roughly spherical with a diameter of 300-600 μm. FIG. 5 shows an enlarged photograph of the surface of the composite granules taken by a scanning electron microscope. Plate-like particles (particles seen in black) are oriented in the circumferential direction, and plate-like particles standing in the normal direction are not seen.

(Green body preparation and sintering)
The obtained composite granules were filled in a φ10 mm graphite die, and CSP-KIT-02121 manufactured by S.S.Alloy Co., Ltd. was used to prepare a green body by an electric plasma sintering method and sintered. The pressure was set to 40 MPa and sintered at 1250° C. for 10 minutes to obtain the inorganic composite material of the present invention. A fracture surface of the resulting composite material was observed with a scanning electron microscope (Hitachi, S-4800) and an optical microscope (Leica, DMS1000). An optical micrograph is shown in FIG. It can be seen that it has a sea-island structure. A scanning electron microscope photograph is shown in FIG. 7, and an enlarged photograph is shown in FIG. It can be seen that the major axis of the BN grains in the sea portion is oriented in the boundary line direction of the island portion.

(Comparative example 1)
An aqueous dispersion of positively charged BN particles, an aqueous dispersion of positively charged alumina particles, and an aqueous dispersion of negatively charged alumina particles are compared with the ratio of alumina particles and BN particles in the entire composite granules of Example 1. 1.5 mL, 1.625 mL, and 0.125 mL of 9 vol.% positively charged alumina dispersion, negatively charged alumina dispersion, and 9 vol.% BN dispersion, respectively. 3.25 mL of this mixture was placed in a glass container (inner diameter x body diameter x total length: φ10 x φ21 x 45 mm), 6.75 mL of deionized exchange water was added, and a rotating roller (IBI Scientific Co. Composite granules in which alumina particles and BN particles are uniformly dispersed were prepared by rotating in the circumferential direction at a rotational speed of 20 rpm for 1 week using a ROLAA UV1S manufactured by Manufacture Co., Ltd. Further, sintering was carried out in the same manner as in Example 1 to obtain a comparative inorganic composite.

(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) under conditions of a thickness of 1.8 mm and a temperature of 30°C.
The composite material of Example 1 exhibited high thermal conductivity of 14.9 W/mK and the homogeneous composite material of Comparative Example 11.7 W/mK.

10: Core-shell type composite granules 11: Particles of the first inorganic material 12: Particles of the second inorganic material formed into rods or flat plates

Claims (6)

A core-shell type composite granule having a core composed of particles of a first inorganic material and a shell attached to the surface of the core, wherein the core comprises a plurality of particles composed of the first inorganic material is in the form of spherically agglomerated granules, 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 the second inorganic material formed in the shape of rods or plates. A core-shell type composite granule characterized in that said second inorganic material particles are adhered with their long axes oriented in the circumferential direction of said core portion.
A method for producing the core-shell type composite granules according to claim 1,
including a core portion forming step and a shell portion attaching step,
The core portion forming step includes:
adjusting the surface charge of the particles of the first inorganic material to be positive;
adjusting the surface charge of the particles of the first inorganic material to be negative;
mixing the aqueous solvent dispersion of the positively charged particles of the first inorganic material and the aqueous solvent dispersion of the negatively charged particles of the first inorganic material, and placing the mixture in a cylindrical container; and rotating the cylindrical container in a circumferential direction to form core granules of composite granules;
The shell attachment step includes:
adjusting the surface charge of the rod-like or plate-like particles of the second inorganic material to be positive or negative;
an aqueous solvent dispersion of rod-like or plate-like particles of a second inorganic material whose surface charge has been adjusted to be positive or negative, and an aqueous solvent dispersion of particles of the first inorganic material whose surface charge has been adjusted to at least the opposite is mixed in an aqueous solvent, this mixed solution is added to the aqueous dispersion of core granules prepared in the core part forming step, and this mixed solution is further placed in a cylindrical container, and the cylindrical container is rotated in the circumferential direction and a step of forming a shell on the surface of the core granule by rotating.
An inorganic composite material using the core-shell type composite granules according to claim 1, which has a sea-island structure formed by a plurality of the composite granules and having an island portion composed of the core portion and a sea portion composed of the shell portion. can be,
An inorganic composite material, wherein long axes of rod-like or plate-like particles of the second inorganic material contained in the sea portion are oriented in the boundary line direction of the island portion.
The rod-like or plate-like particles of the second inorganic material have anisotropic properties, and are characterized in that the properties in the short axis or thickness direction and the properties in the long axis or plane direction are different from each other. The inorganic composite material according to claim 3, characterized by:
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. 5. The inorganic composite material according to 4.
A method for producing an inorganic composite material according to claim 3,
A method for producing an inorganic composite material, comprising the steps of appropriately aggregating the core-shell type composite granules to produce a green body, and sintering the green body.

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