Core-shell structure particle and method for manufacturing same

Abstract

[Problem] To provide: a core-shell structure particle that has insulation properties and is thermally conductive; and a method for manufacturing the same. [Solution] The core-shell structure particle is provided with: a core part made of metal particles; and a shell part coating the surface of the core part. The shell part is formed from an inorganic insulator and does not contain a binder resin. Boron nitride and a fibrous metal compound are contained as the inorganic insulator of the shell part. In addition, copper particles can be used as the metal particles, and fibrous alumina can be used as the fibrous metal compound. Furthermore, the core-shell structure particle can be manufactured by using an aqueous slurry containing boron nitride and fibrous alumina to coat the copper particles with boron nitride and fibrous alumina.

Description

Core-shell structured particles and method for producing the same

[0001] The present invention relates to core-shell structured particles and a method for producing the same.

[0002] A heat dissipation member having heat conductivity is used to dissipate heat from heat-generating electronic devices and the like. As a material for the heat dissipation member, a resin in which a heat-conductive filler is dispersed may be used. Also, depending on the application, electrical insulation may be required in addition to heat conductivity. As a filler that achieves both heat conductivity and electrical insulation, a spherical (granular) filler having a core-shell structure is known. The core-shell structure has a core portion at the center and a shell provided so as to cover the surface of the core portion.

[0003] Patent Document 1 discloses a filler having a core-shell structure having a core portion in which flaky boron nitride is aggregated into a spherical shape and a shell portion made of flaky boron nitride covering the core portion. Patent Documents 2 and 3 disclose a filler having a core-shell structure that employs a spherical inorganic material as the core portion. As a material for the core, it is also possible to employ an inexpensive material instead of expensive boron nitride. Also, by adopting a core-shell structure and configuring the shell portion with an electrically insulating material, it becomes possible to use an electrically conductive material for the core portion.

[0004] JP-A-2018-145090 JP-A-2020-63179 JP-A-2020-164591

[0005] In any of the documents, the shell portion is formed by covering an insulating material around the core via a resin binder. Therefore, in the process of manufacturing the core-shell structured particles, there is a concern about the risk of adverse effects on the environment due to VOCs and the like, and adverse effects on workers due to organic solvents. Also, since the resin has low heat conductivity, it may reduce the heat conduction between the shell portion and the core portion.

[0006] In view of the above problems, an object of the present invention is to provide core-shell structured particles having electrical insulation and heat conductivity without using a binder resin for the shell portion.

[0007] The core-shell structure particle according to the present invention comprises a core portion made of metal particles and a shell portion covering the surface of the core portion, wherein the shell portion is made of an inorganic insulator, and the inorganic insulator includes boron nitride and a fibrous metal compound.

[0008] By configuring the core-shell structure particles in this way, it is possible to obtain core-shell structure particles that are insulating and heat-conducting particles of the core-shell type that do not contain binder resin.

[0009] Furthermore, in the core-shell structure particle according to the present invention, the metal particles may be copper particles in the above configuration.

[0010] Furthermore, in the core-shell structure particles according to the present invention, the fibrous metal compound may be fibrous alumina.

[0011] Furthermore, in the core-shell structure particle according to the present invention, the fibrous alumina may be feathery alumina in the above configuration.

[0012] By using this core-shell particle structure, highly thermally conductive copper particles can be employed in the core, preventing the boron nitride in the shell from separating from the copper particles without the need for a binder resin. Furthermore, by employing feather-like alumina, it becomes possible to eliminate the need for surfactants.

[0013] Furthermore, in the core-shell structure particle according to the present invention, the metal particles in the core portion may have a particle size distribution in which the frequency of particles in the diameter range of 0.75 to 22 μm is 98 to 100%.

[0014] Furthermore, in the core-shell structure particle according to the present invention, the metal particles in the core portion may have a particle size distribution in which the frequency of particles in the range of 1.5 μm to 13 μm in diameter is 91 to 98%.

[0015] By using this core-shell structure particle configuration, it is possible to obtain fine core-shell structure particles, which can be applied to a variety of uses.

[0016] The method for producing core-shell structured particles according to the present invention is characterized by comprising: a slurry preparation step of preparing an aqueous slurry containing boron nitride and fibrous alumina; a dispersion step of dispersing the aqueous slurry; and a coating step of coating the surface of copper particles with the boron nitride and fibrous alumina.

[0017] In the method for producing core-shell structured particles according to the present invention, the amount of boron nitride added to the aqueous slurry may be 0.2 to 7% by mass.

[0018] In the method for producing core-shell structured particles according to the present invention, the amount of fibrous alumina added to the aqueous slurry may be 0.235 to 2.35% by mass.

[0019] By using this method for manufacturing core-shell structured particles, it is possible to produce core-shell structured particles without using binder resin, while preventing adverse effects on workers from organic compounds such as VOCs.

[0020] According to the present invention, it is possible to provide core-shell structure particles that possess insulating and thermally conductive properties without using a binder resin in the shell portion.

[0021] Figure 1(A) is a schematic cross-sectional view of core-shell structured particles 1, and Figure 1(B) is a TEM photograph showing the state of boron nitride dispersed in a slurry to which CNF has been added. Figure 2 is a schematic diagram showing the configuration of the fluidized bed apparatus 10. Figure 3 is a schematic diagram showing an example of a transmittance measuring device 20. Figure 4 shows an optical photograph showing the results of a delamination inspection and the measured value of total light transmittance. Figure 5 is an SEM photograph showing the surface state of core-shell structured particles 1 produced under different conditions. Figure 6 is a table showing the manufacturing conditions of core-shell structured particles 1. Figure 7 is a table showing the manufacturing conditions of core-shell structured particles 1 and the characteristics of the manufactured core-shell structured particles 1. Figure 8(A) is a table showing the manufacturing conditions of core-shell structured particles 1 using feathery alumina, and Figure 8(B) is a table showing the manufacturing conditions of the manufactured core-shell structured particles 1 and the characteristics of the manufactured core-shell structured particles 1. Figure 9 is a schematic flow diagram showing the manufacturing process of core-shell structured particles 1.

[0022] Embodiments of the present invention will be described below with reference to the drawings. However, none of the following embodiments are intended to provide a restrictive interpretation in determining the gist of the present invention. Furthermore, the same or similar reference numerals may be used for identical or similar components, and their descriptions may be omitted.

[0023] Furthermore, terms used in this specification to specify shapes, geometric conditions, and their degrees, such as "parallel," "orthogonal," and "identical," as well as values ​​of length and angle, shall not be strictly interpreted, but shall be interpreted to include a range that can be expected to perform similar functions.

[0024] (Embodiment 1) Figure 1(A) is a schematic cross-sectional view of a core-shell type insulating thermal conductive particle 1 (referred to as core-shell structure particle 1). Figure 1(B) is a TEM photograph showing the state of boron nitride dispersed in a slurry to which CNF has been added. As shown in Figure 1(A), the core-shell structure particle 1 has a core portion 2 and a shell portion 3. The shell portion 3 covers the surface of the core portion 2. The core portion 2 is composed of one core particle. The shell portion 3 is composed of an inorganic insulator. The statement that the shell portion 3 is "composed of an inorganic insulator" means that it does not contain a binder resin. Since the shell portion 3 has electrical insulating properties, conductive material particles such as metal particles with excellent thermal conductivity can be used for the core portion 2.

[0025] Core-shell structure particles 1 with this configuration achieve both electrical insulation and thermal conductivity. Therefore, to improve thermal conductivity, for example, in resin products, they can be filled into resin and molded, or filled into materials such as flexible heat dissipation sheets.

[0026] <Method for Manufacturing Core-Shell Structured Particles> The following describes a method for manufacturing core-shell structured particles 1, particularly a core portion 2 having copper particles as metal core particles, and a shell portion 3 having two types of inorganic insulators: boron nitride (first thermally conductive insulator) and fibrous alumina (second thermally conductive insulator), which is a fibrous metal compound.

[0027] (Slurry preparation process) A slurry (sometimes called shell slurry) containing boron nitride and fibrous alumina (insulating fibrous metal compound) is prepared. The slurry may also contain boron nitride, fibrous alumina, ion-exchanged water, and cellulose nanofiber (hereinafter referred to as CNF). The slurry is an aqueous slurry (sometimes called an aqueous dispersion slurry) in which the solid components are dispersed in water, without using a resin binder or an organic solvent to dissolve the resin. Note that "aqueous slurry" and "aqueous dispersion" mean that no organic solvents are included. Therefore, adverse environmental impacts due to VOCs, etc., and adverse effects on workers due to organic solvents can be prevented.

[0028] The boron nitride, which is the first inorganic insulator of the shell portion 3, is in the form of flakes. For example, a water-dispersible slurry (SL-170-20-WA, manufactured by MARUKA Corporation) with an average primary particle size of 50 nm and an average secondary particle diameter of 2-3 μm, containing 20% ​​by mass of solid components, was used. The aggregated flake-shaped boron nitride (secondary particles) can be confirmed by SEM imaging.

[0029] The second inorganic insulator of the shell portion 3 is aluminum oxide, which is fibrous alumina (Al ２ O ３ For example, a particle with a diameter of 4 nm and a length of 1400 nm, and a specific surface area of ​​285 m². ２ A water-dispersible sol (F-1000, manufactured by Kawaken Fine Chemical Co., Ltd.) containing 4.5-5.1% fibrous alumina as solid content is used. Therefore, it does not contain organic solvents. Acetic acid may be used as a stabilizer. It is thought that the surface of the alumina becomes positively charged in the acidic range, which prevents aggregation. Furthermore, longer fibrous alumina, for example, fibrous alumina with a length of 3000 nm, may also be used.

[0030] Fibrous alumina has the effect of strengthening the bond between boron nitride and core particles (copper particles). Fibrous inorganic insulators (specifically fibrous metal compounds) are thermally stable and can improve the heat resistance of the resulting core-shell structure. As described above, since the diameter of fibrous alumina is smaller than the particle size of boron nitride, it can easily penetrate between boron nitride pieces and between boron nitride pieces and core particles. As a result, it is thought that the effect of strengthening the bond between boron nitride pieces and core particles, and between boron nitride pieces within the shell portion 3, can be obtained. As the fibrous inorganic insulator, fibrous alumina is preferably used because it is inexpensive and provides a fine structure, but fibrous insulators such as aluminum nitride, silicon oxide, and silicon carbide may also be used.

[0031] CNF can be supplied as an aqueous dispersion of CNF. For example, an aqueous dispersion of CNF with a solid content of 2% by mass and a fiber width of 3 nm can be used. The CNF is added to prevent the re-aggregation of the crushed boron nitride during the dispersion process. As shown in Figure 1(B), after the agglomerated boron nitride particles (secondary particles) are crushed, the CNF surrounds the monodisperse boron nitride particles (black dots shown in Figure 1(B)). In this way, CNF can prevent the re-aggregation of boron nitride particles. Note that CNF may decompose due to heat treatment in the coating process, so it is not expected to have an effect of preventing peeling from the boron nitride core particles (copper particles). If there is no re-aggregation of boron nitride, CNF is not essential, and other additives that prevent the re-aggregation of boron nitride may be mixed into the slurry instead of CNF.

[0032] The sizes of boron nitride, fibrous alumina, and CNF are not limited to those specified above.

[0033] (Mixing process) For example, the slurry and copper particles are placed in a container such as a stainless steel cup and mixed. Mixing may be done using equipment such as a stirrer, but it can also be done manually. The slurry mixed with copper particles is sometimes called core-containing slurry.

[0034] The copper particles constituting the core particles were samples such as those with a D50% content of 3 μm (Cu-HWQ 3 μm manufactured by Fukuda Metal Foil & Powder Industry Co., Ltd.) and those with a D50% content of 5 μm (Cu-HWQ 5 μm manufactured by Fukuda Metal Foil & Powder Industry Co., Ltd.). However, the size of the copper particles is not limited to those mentioned above.

[0035] By using copper particles in the core portion 2, the amount of expensive boron nitride used can be reduced, contributing to a reduction in manufacturing costs. While copper particles are shown as an example of core particles, the invention is not limited to them. Particles made of aluminum, gold, silver, iron, platinum, tungsten, molybdenum, nickel, cobalt, or alloys thereof may also be used as core particles.

[0036] (Dispersion Process) The obtained core-containing slurry is processed in a high-pressure homogenizer to break down the solid components and uniformly disperse them within the slurry. For simplicity, the process of breaking down or dispersing solid components in the slurry is sometimes referred to as breaking down the slurry or dispersing the slurry. In a slurry in which boron nitride is dispersed, even if the primary particles are, for example, 50 nm in average particle size, which is significantly smaller (more than an order of magnitude smaller) than the copper particles of the core, they may aggregate to form larger secondary particles, for example, 2-3 μm, which are the same size as the copper particles of the core. Boron nitride in an aggregated state due to weak force is broken down in a high-pressure homogenizer. The processing conditions for the high-pressure homogenizer are, for example, 80 MPa x 2 passes.

[0037] (Coating Process) A nonionic surfactant (for example, an amount of 0.5% by mass) is mixed into the core-containing slurry. Then, the core-containing slurry is heated (for example, to 230°C) and stirred to apply shear force and evaporate the water. For example, the core-containing slurry can be placed in a stainless steel container and stirred with a stirring rod or the like while heating. As a result, a shell portion containing boron nitride and fibrous alumina can be coated onto the surface of the copper particles (core particles). The copper particles constitute the core, and a shell portion containing boron nitride and fibrous alumina is formed on the surface of the core, resulting in boron nitride-coated copper particles with a core-shell structure. Since the slurry used is an aqueous slurry, it is possible to stir the slurry in the container in an open atmosphere, eliminating the need for a decontamination device for organic components such as VOCs. This coating process may be performed manually, but a machine such as a bean paste kneading device may also be used (see Example 6). Similar results have been obtained with either method.

[0038] Nonionic surfactants are added to improve the adhesion between boron nitride and copper particles. In the case of shells containing boron nitride in a binder resin, as disclosed in patent literature, silane coupling agents can be suitably used. However, in this embodiment, a shell structure without resin is adopted, so silane coupling agents are not used. Therefore, adverse effects on workers from organic components such as VOCs can be prevented, and small amounts of core-shell structure particles can be manufactured manually without the use of large-scale equipment.

[0039] Alternatively, copper particles may be coated with boron nitride and fibrous alumina using a fluidized bed apparatus. Figure 2 is a schematic diagram showing the configuration of the fluidized bed apparatus 10. In Figure 2, the white arrow indicates the heated gas G. As shown in Figure 2, core particles CP (copper particles) are placed in the processing chamber 11 of the fluidized bed apparatus 10. The heated gas G supplied from the air supply pipe 12 is blown onto the core particles CP from below via the gas dispersion plate 13, causing the core particles CP to float and flow. Inside the processing chamber 11, the aqueous slurry S (shell slurry) is sprayed from above the floating core particles CP using a spray nozzle 14, mixed with the core particles CP, and heated. In the example shown in Figure 2, the spray nozzle 14 is provided on the side of the processing chamber 11, and the aqueous slurry S is sprayed tangentially to the core particles CP as they are floating in the heated gas G. When using a fluidized bed apparatus, only shell slurry that does not contain copper particles is processed in the crushing and dispersion process described above.

[0040] The water in the aqueous slurry S is evaporated by a heated gas G (e.g., heated dry air), and a shell containing boron nitride and fibrous alumina is coated onto the surface of the core particles CP. The temperature of the heated gas G is set to, for example, 80°C, without limitation. The thickness of the shell can be controlled by the amount of solid components in the slurry S, the supply amount, and the processing time.

[0041] The fluidized bed apparatus 10 may also include a blade rotor 17. The blade rotor 17 is rotated by a motor 18, stirring and swirling the core particles CP, and the shell may be rolled onto the core particles CP to form a fluidized bed coating.

[0042] The heated gas G containing water vapor is exhausted through the exhaust pipe 16 via the bag filter 15. Since the water-based slurry S does not contain binder resin, there is no blockage of the spray nozzle 14 or bag filter 15 by binder resin, which also has the effect of extending the maintenance cycle of the fluidized bed apparatus 10. Furthermore, a VOC abatement device can be eliminated.

[0043] Through the above steps, core-shell structured particles composed of a core part of copper particles and a shell part of an insulating heat conductor that contacts and coats the surface of the core part can be obtained. Note that the device used for coating may be appropriately selected according to the production amount of the core-shell structured particles, the materials used, etc.

[0044] Note that the order of the mixing step of mixing the core particles CP into the shell slurry and the dispersion step is not limited to the above production method. As described below, after the dispersion step of the shell slurry, a mixing step of mixing the core particles CP into the shell slurry may be performed. FIG. 9 is a flow chart schematically showing the production process of the core-shell structured particles 1. For the sake of understanding, FIG. 9 shows together the flow of the former production method (indicated by a broken line in the figure) in which the dispersion step is performed after the mixing step and the flow of the latter production method (indicated by a solid line in the figure) in which the mixing step is performed after the dispersion step.

[0045] (1) Slurry (shell slurry) preparation step Weigh each material of the slurry (shell slurry) (boron nitride slurry, CNF, alumina sol, ion-exchanged water) and store it in a container (weighing cup). Then, stir the stored materials using a spatula or the like to prepare a shell slurry.

[0046] (2) Dispersion step The obtained shell slurry is processed (for example, 80 MPa × 2 passes) with a high-pressure homogenizer to crush the solid components and disperse them uniformly.

[0047] (3) Surfactant addition step Add a surfactant (for example, a nonionic surfactant) to the shell slurry and stir it with a spatula or the like. Note that feather-shaped alumina may be adopted as fibrous alumina, and the addition of the surfactant may be omitted.

[0048] (4) Mixing step Put copper particles, which are the core particles CP, into the shell slurry and stir to obtain a core-containing slurry. Stirring may be performed manually, or a device such as a stirrer may be used. For example, it is put into a container such as a stainless steel cup at a ratio of copper powder: shell slurry = 2: 1. Note that in the coating step, when the fluidized bed device 10 is used, it is not necessary to mix the core particles CP into the shell slurry.

[0049] (5) Coating process The container containing the core-containing slurry is placed on a hot plate heated to, for example, 230°C, and the water is evaporated. Shear force is applied by stirring with a spatula or the like until all moisture is gone. As described above, a shell portion containing boron nitride and fibrous alumina can be coated onto the surface of the copper particles (core particles CP) (manual stirring process).

[0050] Alternatively, the coating process may be performed using the fluidized bed apparatus 10 as described above (fluidized bed process). Copper powder, which is the core particle CP, is weighed and set in the processing chamber 11 of the fluidized bed apparatus 10. The shell slurry is set on the weighing scale of the fluidized bed apparatus 10. (Preparation for operation of the fluidized bed apparatus 10) The fluidized bed apparatus 10 is operated, and the shell slurry is sprayed from the spray nozzle 14 onto the core particle CP suspended by the heated gas G, and heated while mixing with the core particle CP. The fluidized bed apparatus 10 is stopped when, for example, 10% of the weight of the set core particle CP has been sprayed with shell slurry. A shell portion containing boron nitride and fibrous alumina can be coated onto the surface of the core particle CP. Core-shell structure particles 1 are obtained by the above manufacturing method. The results of evaluating the characteristics of the core-shell structure particles 1 (examples and comparative examples) produced by the latter manufacturing method will be described later.

[0051] Furthermore, in conventional technology, when boron nitride is fixed to the core via a resin, the low thermal conductivity of the resin reduces heat conduction from the shell to the core. As described above, by creating a structure in which boron nitride coats the surface of the copper particles without the need for a binder resin, it becomes possible to effectively utilize the thermal conductivity of the copper particles in principle.

[0052] <Necessity of evaluating peelability> In the case of conventional core-shell structure particles disclosed in patent documents, the shell portion is fixed to the core portion via resin. As the amount of resin increases, the thermal conductivity decreases, so reducing the amount of resin makes the shell portion more prone to peeling. Also, if used as a filler such as a resin containing a solvent that dissolves the binder resin, the shell portion will peel off. Furthermore, in the case of boron nitride aggregates, there are almost no functional groups on the large surface area and the aggregates are held together by weak van der Waals forces. Therefore, they may peel off even with weak shear forces.

[0053] For example, when filling a core-shell structure into a resin, if a portion of the shell peels off, the peeled fragments can disperse in the resin, increasing its viscosity. As a result, uniform mixing of the core-shell structure becomes difficult, and in applications involving resin molding, fine molding may become difficult. Therefore, it is necessary to evaluate the peeling of the shell portion of the obtained core-shell structure particles. However, it is difficult to evaluate the peeling of minute shell portions, and there is no method for quantitative evaluation in particular. For this reason, the peeling status of this embodiment and comparative example was quantitatively evaluated using the novel evaluation method described below.

[0054] <Method for Evaluating Peelability> The outline of the peelability evaluation method is as follows: A solvent and core-shell structure particles are placed in a transparent container and stirred to uniformly mix the core-shell structure particles into the solvent. Next, an external force of ultrasonic vibration is applied to the solvent containing the core-shell structure particles. After that, the peeling state of the shell portion is quantitatively confirmed using an optical method. The core-shell structure particles settle due to gravity, but the fine detached fragments of the shell portion remain suspended in the solvent. The suspended detached fragments scatter light, reducing the transparency of the solvent. After ultrasonic treatment, the transparency of the solvent is measured after a predetermined time (e.g., 1 hour), allowing for quantitative evaluation of peelability.

[0055] The transparency of a solvent can be quantified by its light transmittance. For measuring transmittance, a haze meter, for example, can be used. Preferably, the transparency of the solvent can be measured optically and quantitatively by measuring the total light transmittance using a haze meter, and the degree of delamination can be evaluated as a result. Total light transmittance represents the percentage of light that passes through an object, with the amount of light passing through empty space being set at 100%. As described above, transmittance can preferably be measured using a haze meter, but other optical devices may also be used.

[0056] Figure 3 is a schematic diagram showing an example of a transmittance measuring device 20. As described above, a haze meter, for example, can be used as the transmittance measuring device 20. The transmittance measuring device 20 comprises a light source 21, an optical system 22 (lenses, etc.), and an integrating sphere 23. Light L emitted from the light source 21 is guided by the optical system 22 to the incident aperture 24 of the integrating sphere 23. The light is then scattered by a sample T placed in front of the incident aperture 24 and incident into the integrating sphere 23. The light receiver 25 detects all of the parallel component L1 and diffuse component L2 of the light incident into the integrating sphere 23, and the total light transmittance is measured. The integrating sphere 23 has a reference white plate 26 that serves as a white reference.

[0057] The following describes a specific procedure and example of peel evaluation using the transmittance measuring device 20. A sample of core-shell structure particles is uniformly mixed with the target solvent in a transparent container. Then, an external force such as ultrasonic vibration is applied to the solvent containing the core-shell structure particles for a predetermined time, and then the ultrasonic vibration is stopped. After that, the container is left to stand for a predetermined time, and the total light transmittance of the portion corresponding to the supernatant of the solvent is measured by the transmittance measuring device 20. The container is placed at the position of the sample T shown in Figure 3, and light L from the light source 21 is irradiated onto the portion corresponding to the supernatant of the solvent. The light L1 and L2 that have passed through the solvent are collected by the integrating sphere 23, and all of the light L1 and L2 that have passed through the solvent is detected by the light receiver 25, and the total light transmittance is measured.

[0058] The supernatant portion (measurement area) can be defined as the portion in the solvent where the core-shell structure particles, which are the object to be evaluated, are not visible after mixing them with a solvent and allowing them to settle without sonication. Alternatively, the core particles can be mixed with a solvent only and allowed to settle without stirring or sonication, resulting in the portion in the solvent where the core particles are not visible. The solvent can be selected according to the intended use and application of the core-shell structure particles. For example, when filling core-shell structure particles into a resin, using the solvent used in the resin allows for evaluation of the effect of delamination due to the solvent. Water may also be used as the standard solvent.

[0059] If the shell portion of a core-shell structure particle detaches, the detached fragments in the solvent scatter light, resulting in a decrease in total light transmittance. If there is no shell detachment, gravity causes the core-shell structure particle to settle at the bottom of the container, and the total light transmittance in the supernatant region will be a high value close to 100%. The predetermined time is defined as the time it takes for the non-detached core-shell structure particle stirred in the solvent to completely settle, which can be, for example, one hour, but is not limited to this. Alternatively, the measurement time may be shortened by measuring the time dependence of total light transmittance and calculating the value after the predetermined time has elapsed from the time dependence of total light transmittance.

[0060] Figure 4 shows optical photographs and total light transmittance measurements of the peeling test results. For three samples, the total light transmittance (hereinafter referred to as T.T. value) was measured after 1 hour following the application of ultrasonic vibration. The solvent used was water (ion-exchanged water). The T.T. value for Example 5 was 88.46%, for Example 3 it was 64.96%, and for Comparative Example 17 it was 27.69%. It can be seen that a higher T.T. value indicates higher transparency and less peeling material. In Examples 5 and 3, the core-shell structure particles 1 can be visually observed settling at the bottom of the container. However, in Comparative Example 17, the entire solvent in the container is milky white, and the precipitate at the bottom of the container cannot be visually observed. In this way, the peeling state can be quantitatively and easily evaluated using optical methods. Below, a T.T. value of 31% or higher was considered a normal value.

[0061] Thus, this evaluation method makes it possible to optically evaluate the degree of delamination (amount of delamination) of the covering material (shell) that covers the base material (core).

[0062] <Coverage Evaluation> To confirm the shell's coverage, the surface of the obtained core-shell structure particle 1 was observed using a SEM. If copper particles are exposed on the surface, it can be determined that there was delamination or poor coating. Preferably, an FE-SEM device can be used as the SEM device.

[0063] Figure 5 is an SEM image showing the surface state of core-shell structure particles 1 produced under different conditions. In Figure 5, the areas indicated by the white arrows show areas where the surface of the core portion 2 is exposed. In Example 5, no exposure of the core portion 2 surface is observed, but in Comparative Examples 4 and 17, exposure of the core portion 2 surface is observed. Note that only Comparative Example 17 has a different magnification of the SEM image.

[0064] <Evaluation of Core-Shell Structured Particles> The evaluation results of the fabricated core-shell structured particles 1 are described below. Figures 6 and 7 are tables showing the manufacturing conditions for core-shell structured particles 1 and the characteristics of the core-shell structured particles 1 produced under each manufacturing condition, and show the manufacturing conditions and evaluation results for Examples 1 to 13 and Comparative Examples 1 to 24 according to the present invention. For some samples, the thermal conductivity was measured after mixing with resin as a filler and curing.

[0065] (Effect of Core Particles) The thermal conductivity of Examples 1 to 5, in which the core-shell structure was filled into the resin, was lower than that of Comparative Example 8, in which only copper particles were filled into the resin, but higher than that of Comparative Example 2, in which boron nitride particles (corresponding only to the shell portion) were filled into the resin. From this, it can be concluded that the core particles of the core-shell structure have the effect of improving thermal conductivity.

[0066] (Effect of fibrous alumina) By comparing Example 1 with Comparative Example 3, it can be seen that the T.T value increases when fibrous alumina is added to the slurry which is the material for the shell. Therefore, it can be seen that fibrous alumina has an effect of preventing the peeling of boron nitride. Furthermore, when using core-shell structure particles as a filler, the thermal conductivity decreases compared to the case in Comparative Example 8 where only copper particles are used as a filler, but it can be seen that the thermal conductivity improves compared to the case in Comparative Example 2 where only boron nitride is used as a filler. In other words, it has been confirmed that using core-shell structure particles of the embodiment of the present invention as a filler improves the thermal conductivity compared to the case where boron nitride is used as a filler.

[0067] (Boron Nitride Dependence) From Examples 1, 2, 4, and 13, it can be understood that the suitable amount of boron nitride to add is in the range of 0.2 to 7% by mass (0.2% by mass or more, and 7% by mass or less). This is because if the amount of boron nitride slurry added is outside the above range, the T.T value becomes low.

[0068] (Dependence on the amount of fibrous alumina added) From Examples 2, 3, 4, and 5, it can be understood that the suitable amount of fibrous alumina to add is in the range of 0.235 to 2.35 mass% (0.235 mass% or more and 2.35 mass% or less). This is because if the amount of fibrous alumina added to the slurry is outside the above range, the T.T value becomes low. Fibrous alumina has the effect of preventing the peeling of boron nitride. However, if the amount of fibrous alumina in the slurry is excessive, there is an excess of fibrous alumina between the boron nitride and copper particles, which reduces thermal conductivity. Also, if the amount of fibrous alumina in the slurry is too low, the effect of preventing the peeling of boron nitride is reduced.

[0069] (Particle Size Range) The core particles (metal particles) of core-shell structure particle 1 have a particle size distribution in which the frequency of particles in the diameter range of 0.75 to 22 μm is 98 to 100%, and the frequency of particles in the diameter range of 1.5 μm to 13 μm is 91 to 98% (see Figure 6). In this case, the peel evaluation by T.T value and the determination of coating defects by SEM observation are satisfied. As shown in Comparative Examples 9 to 24, if the particle size of the copper particles in the core is too large, the determination of peelability and coating performance of the shell is not satisfied.

[0070] As described above, since the core-shell structure particles 1 have a fine size, when filled into a resin, they have the effect of enabling the molding of fine products. The resin may also be flexible, or it may be a curable resin that hardens by heat treatment or the like for molding. Because the core-shell structure particles 1 have a fine size, the filling rate can be up to 70-80%.

[0071] (Dependence on coating conditions) Examples 1, 6, and 8 show the influence of the equipment used for coating (dependence on coating conditions). Compared to Example 1, the T.T values ​​of Examples 6 and 8 are increased, and it can be understood that the T.T values ​​can be further increased by optimizing the coating process. Examples 7 and 9 are samples in which shells were coated using a fluidized bed apparatus. Compared to Example 1, the T.T values ​​of Examples 7 and 9, which used a fluidized bed apparatus, are increased, and the T.T value of Example 7 is very high. It can also be seen that when using a fluidized bed apparatus, the slurry ratio to copper particles can be small.

[0072] (Feathered Alumina) The fibrous alumina shown in the table in Figures 6 and 7 uses linear (or needle-shaped) alumina, but feathered alumina, in which multiple fibrous alumina molecules are aggregated in a feather-like manner, may also be used as fibrous alumina. Feathered alumina has a structure in which alumina is branched in two or three dimensions. Figure 8 is a table showing the manufacturing conditions for core-shell structure particles 1 using feathered alumina and the characteristics of the manufactured core-shell structure particles 1. For the feathered alumina, Nissan Chemical Corporation's Alumina Sol 200 (AS-200) was used, with a boron nitride addition amount of 4% by mass and feathered alumina of 0.25% by mass.

[0073] Comparative Example 25 shows an example in which Amizet 10C manufactured by Kawaken Fine Chemicals Co., Ltd. was used as the nonionic surfactant. This nonionic surfactant was used in the production of core-shell structure particles 1 shown in Figures 6 and 7. As shown in Figure 8(B), Comparative Example 25 has a small T.T value, indicating that peeling occurred. Furthermore, Comparative Example 26, which used Sanamide C-3 (anionic) manufactured by NOF Corporation, and Comparative Example 27, which used Nimid MF-210 (nonionic) manufactured by NOF Corporation, also have small T.T values, indicating that peeling occurred.

[0074] On the other hand, Examples 15 and 16 show examples in which Amicol CDE-1 and Amicol CDE-G, manufactured by Miyoshi Oil & Fat Co., Ltd., are used as nonionic surfactants. In Examples 15 and 16, it can be seen that high T.T values ​​are achieved and peeling of the core-shell structure particles 1 is prevented. Furthermore, Example 14 shows an example in which no surfactant is used. In Example 14, it can be seen that a high T.T value is achieved and peeling of the core-shell structure particles 1 is prevented. When the above linear fibrous alumina is used and no surfactant is used, peeling of the core-shell structure particles 1 was observed. However, by using feather-like alumina as fibrous alumina, peeling of the core-shell structure particles 1 can be prevented without using a surfactant. Feather-like alumina can be even more preferably used in that it does not require a surfactant.

[0075] As described above, the core-shell structure particles 1 achieve both insulating and thermal conductivity, and by incorporating them into a heat dissipation sheet, heat from electronic devices and other equipment can be dissipated. Furthermore, by filling the resin with the core-shell structure particles 1, the thermal conductivity of the resin can be improved, and as a result, the thermal conductivity of the resin molded product can be enhanced. In particular, because the size of the core-shell structure particles 1 is fine, it can be suitably used in the manufacture of resin molded products with fine structures.

[0076] According to the present invention, it is possible to provide core-shell type insulating thermal conductive particles having a metal particle core and an inorganic insulating shell, and by incorporating a fibrous metal compound in the shell, the binder resin can be eliminated. In the manufacturing stage of insulating thermal conductive particles, the impact of organic components such as VOCs caused by the binder resin on workers can be prevented. The obtained core-shell type insulating thermal conductive particles can be applied to various products and have high industrial applicability.

[0077] 1 Core-shell structured particles (core-shell type insulating heat-conducting particles) 2 Core section 3 Shell section 10 Fluidized bed apparatus 11 Processing chamber 12 Air supply pipe 13 Gas dispersion plate 14 Spray nozzle 15 Bag filter 16 Exhaust pipe 17 Blade rotor 18 Motor 20 Transmittance measuring device 21 Light source 22 Optical system 23 Integrating sphere 24 Entrance aperture 25 Photodetector 26 Reference whiteboard CP Core particles G Heated gas S Water-based slurry T Sample L Light L1 Parallel component of light L2 Diffuse component of light

Claims

1. A core-shell structure particle comprising a core portion made of metal particles and a shell portion covering the surface of the core portion, wherein the shell portion is made of an inorganic insulator, and the inorganic insulator includes boron nitride and a fibrous metal compound.

2. The core-shell structure particle according to claim 1, characterized in that the metal particles are copper particles.

3. The core-shell structure particle according to claim 2, characterized in that the fibrous metal compound is fibrous alumina.

4. The core-shell structure particle according to any one of claims 1 to 3, characterized in that the metal particles in the core portion have a particle size distribution in which the frequency of particles in the range of 0.75 to 22 μm in diameter is 98 to 100%.

5. The core-shell structure particle according to any one of claims 1 to 3, characterized in that the metal particles in the core portion have a particle size distribution in which the frequency of particles in the range of 1.5 μm to 13 μm in diameter is 91 to 98%.

6. A method for producing core-shell structured particles, comprising: a slurry preparation step of preparing an aqueous slurry containing boron nitride and fibrous alumina; a dispersion step of dispersing the aqueous slurry; and a coating step of coating the surface of copper particles with the boron nitride and fibrous alumina.

7. The method for producing core-shell structured particles according to claim 6, characterized in that the amount of boron nitride added to the aqueous slurry is 0.2 to 7% by mass.

8. The method for producing core-shell structured particles according to claim 7, characterized in that the amount of fibrous alumina added to the aqueous slurry is 0.235 to 2.35% by mass.

9. The core-shell structure particle according to claim 3, characterized in that the fibrous alumina is feathery alumina.

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