Composite material and method for producing the same

Abstract

To provide a composite material having high thermal conductivity.SOLUTION: A composite material is provided, which is composed of resin and placoid thermal conductive particles and in which a content rate of the thermal conductive particles is inclined in at least one direction. For example, 0.1 to 50 vol.% of the thermal conductive particles are included with respect to the whole composite material. For example, the thermal conductive particles have different thermal conductivities between an in-plane direction and a plate thickness direction, and an orientation angle determined from an inclination angle in the in-plane direction with respect to a reference direction is 0 to 50° or less. For example, the resin is a thermosetting resin such as an epoxy resin. For example, the thermal conductive particle is composed of a hexagonal boron nitride. For example, the composite material is obtained through an adjustment step in which a flowable mixture is obtained from a liquid resin and thermal conductive particles, and a solidification step in which a flowable mixture is solidified while centrifugal force is applied thereto.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for producing a composite material using boron nitride. [Background technology]

[0002] Electronic devices (such as semiconductor modules) that have become highly dense and have high performance require sufficient heat dissipation (including thermal diffusion) to maintain their functionality and lifespan. Heat dissipation from electronic devices is usually achieved through heat dissipation components (heat sinks, housings, etc.) made of metal or other materials. In this case, a heat dissipation sheet (thermal conductive sheet, thermally conductive insulating sheet, etc.) is often inserted between the electronic device (heat source) and each surface of the heat dissipation component to absorb unevenness and undulations.

[0003] The heat dissipation sheet uses, for example, a composite material (including a composition) made of a filler with high thermal conductivity and a resin (including elastomer, rubber, etc.) with excellent flexibility (elasticity). Various proposals have been made regarding such composite materials, and for example, there are related descriptions in the following patent documents. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-128940 [Patent Document 2] Patent Publication No. 2016-79353 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 proposes the following composite insulating board. First, a slurry of composite particles, in which polymethyl methacrylate (PMMA) is adsorbed onto hexagonal boron nitride particles (BN particles), dispersed in ion-exchanged water is placed in a mold, and the water is removed by centrifugation (10 minutes) to obtain a deposit. This deposit is compressed in the same direction as the centrifugal force used during centrifugation to obtain a compact, which is then hot-pressed in a direction perpendicular to the centrifugal force at or above the melting point of PMMA. In this way, a composite insulating board is obtained in which hexagonal boron nitride particles oriented in the in-plane direction are bound by PMMA (

[0076] to

[0082] ).

[0006] The deposit after centrifugation consists of composite particles in which PMMA remains in the solid phase and adheres to BN particles (solid phase), and there is no sparse or dense distribution of PMMA and BN particles themselves. PMMA only melts in the subsequent hot press, and cools and solidifies with the BN particles uniformly dispersed. Therefore, the distribution of BN particles in the composite insulating plate is also uniform. Note that the composite insulating plate in Patent Document 1 contains approximately 60 volume % BN particles, which is also expensive.

[0007] In Patent Document 2, a paste of hexagonal boron nitride powder and polyvinyl butyral powder (thermoplastic resin) is formed into a sheet using a blade, and then centrifugal force (2000-3000 rpm x 60-120 seconds) is applied to produce a heat dissipation sheet (organic-inorganic composite material) (

[0036] -

[0039] ). The application of centrifugal force is done to orient the hexagonal boron nitride during forming using the blade, and to remove the solvent (ethanol) and air bubbles contained in the paste.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a new composite material etc. having excellent thermal conductivity properties etc. [Means for solving the problem]

[0009] As a result of intensive research into solving this problem, the inventor came up with the idea of ​​a composite material in which the distribution of particles made of hexagonal boron nitride (simply referred to as "BN particles") varies depending on the site (region), and actually succeeded in obtaining a composite material with a gradient in the content of BN particles. By expanding on this result, the present invention, which will be described below, was completed.

[0010] <<Manufacturing method of composite material>> The present invention relates to a composite material made of a resin and thermally conductive particles, the composite material having a gradient portion in which the content of the thermally conductive particles in the composite material changes in at least one direction.

[0011] The composite material of the present invention has a sloped portion where the content of thermally conductive particles is at least partially changed. The sloped portion makes it possible to impart suitable properties to each part (region) while suppressing the amount of thermally conductive particles mixed (used) in the composite material as a whole. For example, a region where thermally conductive particles are distributed at high density (high-density region) may be used on the side where thermal properties (thermal diffusivity, thermal diffusivity, etc.) are prioritized. Also, a region where thermally conductive particles are distributed at low density (low-density region) may be used on the side where mechanical properties (rigidity (flexibility), strength, etc.) are prioritized.

[0012] <<Manufacturing method of composite material>> The present invention can also be understood as a method for producing a composite material. For example, the present invention may be a method for producing the composite material described above, comprising a preparation step of obtaining a fluid mixture from a resin and thermally conductive particles, and a solidification step of solidifying the fluid mixture while applying centrifugal force.

[0013] "others" (1) In this specification, the term "material" means "material" or "component." A composite material may be, for example, a material with an indefinite shape, or a component that has been molded or processed into a desired shape.

[0014] (2) In this specification, "x to y" includes a lower limit value x and an upper limit value y, unless otherwise specified. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit or upper limit value to establish a new range such as "a to b." In this specification, "x to y μm" means x μm to y μm, unless otherwise specified. Other unit systems (mm 2 The same applies to / s, etc. [Brief description of the drawings]

[0015] [Figure 1A] 1 is a photograph showing the appearance of a composite material (sample 5) produced by applying centrifugal force. [Figure 1B] This is a photograph of the appearance of a composite material (sample C5) produced by natural sedimentation. [Diagram 2] 1 is a bar graph showing the content of BN particles in each part of the composite materials (sample 5 and sample C5). [Diagram 3] These are SEM images of the cross sections of the composite materials (samples 2 and 5) taken along the centrifugal direction. [Figure 4] This is an example of image analysis of SEM images of composite materials (sample 1 and sample C1). [Diagram 5] FIG. 1 is a scatter diagram showing the relationship between the orientation rate of BN particles determined by image analysis of each sample and the amount of BN particles added. [Figure 6] 1 is a graph showing the relationship between the thermal diffusivity of each sample and the amount of BN particles charged. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] One or more components selected arbitrarily from this specification may be added to the components of the present invention. The contents described in this specification apply not only to composite materials (materials, members, etc.) but also to their manufacturing methods, etc. as appropriate. Even method-related components can become product-related components. Which embodiment is best depends on the target, required performance, etc.

[0017] <Heat conductive particles> The thermally conductive particles may be of one type, or may be a mixture of multiple types of particles with different component compositions and forms (sizes, particle shapes, etc.).

[0018] The thermally conductive particles preferably contain at least hexagonal crystal structure boron nitride (h-BN) particles (referred to as "BN particles"). BN particles are plate-like (flat, scale-like) and have thermal conductivity anisotropy, in which the thermal conductivity differs greatly between the in-plane direction (a-axis direction) and the plate thickness direction (c-axis direction). A composite material in which BN particles are oriented in the in-plane direction can exhibit high thermal diffusivity, etc. In the following, the BN particles will be mainly described. Note that other thermally conductive particles (cubic boron nitride (c-BN) particles, ceramic particles, metal particles, spherical particles, etc.) may be contained in the composite material in addition to the BN particles.

[0019] The particle size of the BN particles is, for example, 1 to 100 μm, 20 to 60 μm, or 30 to 50 μm. In this specification, the term "particle size" refers to the size of the particles regardless of their shape, and unless otherwise specified, refers to the 50% diameter (D50: median diameter) determined from the particle size distribution of the powder. The particle size distribution is determined by a laser diffraction method.

[0020] The BN particles may be a single layer of h-BN having a hexagonal lattice structure, or a laminate or aggregate (aggregate, secondary particle) thereof. The thickness (plate thickness) is, for example, 0.1 to 5 μm, 0.3 to 3 μm, 0.5 to 2.5 μm, or even 1 to 2 μm. The "thickness" of the BN particles is determined as the arithmetic average value of the thicknesses measured for multiple BN particles arbitrarily extracted from the field of view observed under a microscope.

[0021] The aspect ratio of the BN particle is, for example, 10 to 300, 20 to 100, or 30 to 75. The aspect ratio (particle size / thickness) is calculated based on the "thickness" and "particle size" of the BN particle determined by the above-mentioned method.

[0022] "resin" The resin serves as a binder for the thermally conductive particles or as a matrix (base material) for the composite material. The resin may be of a single type or of multiple types, and is appropriately selected according to the specifications of the composite material.

[0023] The resin may be a thermosetting resin or a thermoplastic resin. Examples of the thermosetting resin include epoxy resin, phenol resin, and silicone resin. Examples of the thermoplastic resin include polystyrene, polymethyl methacrylate, polycarbonate, and polyphenylene sulfide. In addition, rubbers such as ethylene-propylene-diene rubber (EPDM) and butyl rubber, and elastomers may be used as the resin (part thereof).

[0024] 《Content rate》 The composite material has at least portions (regions, layers) with different content (distribution density) of thermally conductive particles. In particular, it is preferable for the composite material to have a gradient portion where the content increases or decreases along at least one side. The gradient portion may be one where the content changes continuously or in steps. If the content changes continuously, there are no portions where the characteristics change suddenly, and the desired characteristics are more likely to be exhibited stably.

[0025] The inclined portion may be formed in the entire composite material, or may be present only in a part of the composite material. The composite material may have a portion that does not substantially contain thermally conductive particles (a region that is composed almost only of resin).

[0026] The composite may have a high density region (high density region) where the content of thermally conductive particles is, for example, more than 50 mass%, 55 mass% or more, or 60 mass% or more, or may have a low density region (low density region) where the content is less than 50 mass%, 45 mass% or less, or 40 mass% or less. There may be three or more layers of regions with different contents. Note that the "content" in this specification refers to the proportion (mass proportion, volume proportion, etc.) of BN particles contained in the entire target region (part). The target region may be the entire composite, or a selected or extracted partial region (part, layer, etc.). In the latter case, the target region may be an image observed by a microscope.

[0027] Orientation Irregularly shaped (non-spherical) thermally conductive particles may be oriented in the composite, which can improve the thermal conductivity properties in a particular direction (e.g., the direction of the main heat flux (heat transfer)).

[0028] For example, in the case of plate-shaped (flat, scale-shaped, etc.) thermally conductive particles (such as BN particles) with different thermal conductivities in the in-plane direction and the plate thickness direction, it is preferable that the in-plane direction is oriented to one side. The degree of orientation is indicated by the angle (tilt angle) of the in-plane direction with respect to a reference direction. The average value of the inclination angles (orientation angle) obtained for a plurality of thermally conductive particles is preferably, for example, 0 to 50°, 5 to 45°, or 10 to 43°. A portion with an orientation angle of 50° or less (less than this) is appropriately called a highly oriented portion. The reference direction can be determined arbitrarily and appropriately.

[0029] The orientation angle can be determined, for example, by image processing an observation image (SEM image, etc.) of a cross section obtained by cutting the composite material along a desired direction. The orientation angle may be constant throughout the composite material, or may vary for each region (site) of the composite material. Typically, the orientation angle can change depending on the content (distribution density) of thermally conductive particles. The fewer the thermally conductive particles in a region (region with more resin), the easier it is for the thermally conductive particles to move or change their position.

[0030] The degree of orientation may be evaluated using the orientation rate instead of the orientation angle or together with the orientation angle. The orientation rate is calculated, for example, as the ratio (Np / Nv) of the number of heat conductive particles oriented in a reference direction (Np) to the number of heat conductive particles oriented in a direction perpendicular to the reference direction (Nv). The orientation rate is calculated, for example, by image processing of the observed image, similar to the orientation angle. The orientation rate is, for example, 1 or more, 3 or more, or 4 or more. The above-mentioned contents regarding the orientation angle may also be appropriately applied to the orientation rate.

[0031] <<Manufacturing of Composite Materials>> The composite material having a gradient portion can be obtained, for example, through the following preparation and solidification steps.

[0032] (1) Preparation process In the preparation step, a fluid mixture is obtained from the resin and the thermally conductive particles. The resin may be fluid before being mixed with the thermally conductive particles, or may become fluid during mixing. An example of the former resin is a two-component resin that starts curing when the base agent and the curing agent are mixed. The curing agent may be added when the base agent and the thermally conductive particles start to be mixed, or during the mixing. Both the base agent and the curing agent should be in a fluid liquid state in the room temperature range (for example, 0 to 50°C).

[0033] The resin may become fluid by heating or adding a solvent during mixing (after the start of preparation). The resin may be solid at room temperature, but may soften or melt when heated. In any case, it is preferable for the resin to have a degree of fluidity that allows the thermally conductive particles to move or change position during the preparation process.

[0034] The thermally conductive particles (powder) are contained in an amount of, for example, 0.1 to 50 volume %, 1 to 40 volume %, or 3 to 35 volume % relative to the entire mixture. A composite material having a gradient portion can achieve both high thermal conductivity and a reduced blending amount (charge amount) of the thermally conductive particles.

[0035] In addition, the volume ratio (volume %) in this specification is determined from the true density and blending amount of the raw materials (for example, thermally conductive particles and resin) during the manufacture of the composite material. In the case of a composite material after manufacture, the volume ratio of the thermally conductive particles separated and extracted from the composite material may be obtained, or the volume ratio of the thermally conductive particles may be calculated from the area ratio obtained by image processing of an observed image of the composite material.

[0036] (2) Solidification process In the solidification process, the prepared fluid mixture is solidified (including hardened) to obtain a composite material. When a two-component resin is used, the fluid mixture hardens as the crosslinking reaction progresses. If the resin is heated to a liquid state, the fluid mixture solidifies (hardens) by cooling or thermal curing. If a solvent is added to dissolve the resin, the fluid mixture solidifies when the solvent is removed (by evaporation, etc.). When a two-component resin is used, the preparation and solidification processes can be carried out at room temperature, making it easy to work with.

[0037] The change in the content of the thermally conductive particles (the formation of the inclined portion) may occur in the preparation process or in the solidification process. Here, the case where the content of the thermally conductive particles is changed in one direction in the solidification process will be described as an example.

[0038] When an external force greater than gravity is applied to the thermally conductive particles, the content of the thermally conductive particles can change along the direction of the force. Examples of such external forces include centrifugal force, electric force, and magnetic force. Centrifugal force can be applied to various thermally conductive particles regardless of the electrical or magnetic properties of the thermally conductive particles. The centrifugal force, which is determined by the rotation speed (angular velocity), the distance from the center of rotation to the thermally conductive particles, and the mass of the thermally conductive particles, can be freely adjusted by changing the rotation speed and distance. By using such centrifugal force, the content of the thermally conductive particles and the shape of the inclined portion can also be adjusted according to the specifications of the composite material.

[0039] The centrifugal force may be constant or may vary during the process. For example, the content may be adjusted by varying the centrifugal force while taking into account the fluidity of the mixture (degree of progress of hardening).

[0040] 《Application》 The composite material of the present invention can exhibit excellent thermal conductivity properties. For example, its thermal diffusivity is 1.5 to 10 mm 2 / s, 2~7mm 2 / s, 2.5~5mm 2 / s. Thermal diffusivity and thermal conductivity are along the principal heat flux direction or the orientation of the thermally conductive particles unless otherwise specified.

[0041] The composite material of the present invention is used, for example, in heat dissipation members, substrates, housings, etc. of electronic devices, etc. The inclined portion of the composite material may be arranged, for example, such that the side with the higher content is disposed on the heat source side, and the side with the lower content is disposed on the heat dissipation side. EXAMPLES

[0042] A number of samples (composite materials) made of thermally conductive particles and resin were fabricated and their properties were evaluated. The present invention will be described in more detail with reference to such specific examples.

[0043] 《Raw materials》 (1) Thermally conductive particles As the thermally conductive particle source (raw material), a commercially available powder made of h-BN (PT110 manufactured by Momentive Corp. / referred to as "BN powder") was used. The average particle size (catalog value) of this powder was 40 μm.

[0044] (2) Resin The resin (raw material) used was a commercially available two-component curing epoxy resin (base: HERZOG Epoxy low viscosity base (1571), hardener: HERZOG Epoxy low viscosity hardener / manufactured by HERZOG Japan Co., Ltd.).

[0045] <<Production of composite materials>> The samples shown in Table 1 were prepared as follows.

[0046] (1) Fluid mixture The weighed BN powder and the epoxy resin base were mixed by hand. The epoxy resin curing agent was then added to the mixture and similarly mixed by hand (preparation process). The mixing ratio of the curing agent to the base was 1 / 10 (100 parts by mass of base and 10 parts by mass of curing agent). In this way, a fluid mixture (precursor) was prepared in which the BN particles constituted 5-30% by volume (charge amount) of the entire composite material (BN particles + base + curing agent).

[0047] (2) Solidification 8.1 cc of the fluid mixture was placed in a polypropylene spit. The spit was set in a swing rotor and rotated (5000 rpm x 5 hours) in a centrifuge (Model 4000 / Kubota Manufacturing Co., Ltd.). This applied centrifugal force from the cylindrical opening (rotation center side) of the spit to the tapered bottom (diameter expansion side) to the fluid mixture in the spit (solidification process).

[0048] In this way, samples 1 to 5 shown in Table 1 were obtained. An example of the appearance of the sample (sample 5) taken out of the flask after centrifugation is shown in FIG. 1A. All samples were visually separated into two layers, a white part and a transparent part. In addition, both the white part and the transparent part were in a hardened (solidified) state.

[0049] (3) Comparison sample 8.1 cc of the above-mentioned fluid mixture was placed in a flask and allowed to stand (settle) vertically for 5 hours or more. In this way, samples C1 to C5 shown in Table 1 were produced. An example of the appearance of the sample removed from the flask (sample C5) is shown in Figure 1B. All samples were in a hardened state and had turned white.

[0050] 《Content rate》 (1) The mass ratio (content) of BN particles contained in each part (parts I to V) of the hardened material generated in the tube was determined as shown in Figure 2. The results for sample 5 and sample C5 are also shown in Figure 2.

[0051] The content of each part was calculated as follows. First, the cured product removed from the spit was cut into each part with a band saw. Next, samples (approximately 5 mg) taken from each part were kept at 500°C for more than 20 minutes in an air atmosphere to completely burn off only the epoxy. The ratio (%) of the weight of the resulting residue to the initial weight was taken as the BN particle content. The weight of the residue was calculated from the weight loss using a differential thermobalance (TG-DTA: TG8120 made by Rigaku Corporation).

[0052] (2) As can be seen from FIG. 2, in sample 5 which was hardened while applying centrifugal force, the BN particle content increased along the direction in which the centrifugal force was applied (centrifugal direction), and it was confirmed that the BN particle content was inclined from the low-density region to the high-density region.

[0053] In addition, the amount of BN particles in sample 5 was 46 mass% (30 volume%), but the content in the portion taken out from the tip side of the spitz (parts IV and V) was about 60 mass%. In this way, a high-density region (part) of BN particles was formed while suppressing the amount of BN particles used.

[0054] Furthermore, the BN particle content differed by approximately 20% by mass between the low-density region (part I) formed at the center of the cured product of sample 5 and the high-density region (parts IV and V) formed at its tip side. Such a high-density region exhibits excellent thermal properties (thermal conductivity, thermal diffusivity, etc.), while the low-density region exhibits, in addition to high thermal properties, for example, excellent mechanical properties (strength, rigidity, etc.).

[0055] "observation" (1) A section cut out from the hardened material at the tip of the spitz (the test piece shown in Fig. 2) was embedded in resin, polished, etc., and observed under a scanning electron microscope (SEM). Samples 1 to 5 were embedded in resin so that the observation surface faced the centrifugal direction, and samples C1 to C5 faced the gravity direction (vertical direction). SEM images of samples 2 and 5 are shown in Fig. 3.

[0056] As can be seen in Figure 3, the BN particles were held in contact with each other by the epoxy resin and in a dense state. Furthermore, most of the plate-like BN particles were oriented so as to cross the centrifugal direction.

[0057] (2) The SEM images of each sample were analyzed to determine the orientation angle of the BN particles as follows. First, 10 SEM images were taken for each sample. Each SEM image was analyzed using self-made software (see FIG. 4) to calculate the pixel angle (tilt angle) of each BN particle. The arithmetic average value of these pixel angles was taken as the orientation angle (°) in this specification. This orientation angle indicates the average inclination of the in-plane direction of the BN particles relative to the direction perpendicular to the centrifugal direction (reference direction), or the average inclination of the plate thickness direction (normal direction) of the BN particles relative to the centrifugal direction. The orientation angle was calculated by analyzing images with 100 or more BN particles in the field of view. For this reason, the orientation angle was not calculated for samples C1 to C5, which had only sparse BN particles in one field of view (see FIG. 4).

[0058] The orientation angles determined for samples 1 to 5 are also shown in Table 1. As can be seen from Table 1, the orientation angles were 50° or less. As the amount of BN particles added increased, the number of BN particles per field of view in the SEM image also increased, which is considered to have led to an increase in the average orientation angle.

[0059] In addition, the ratio (orientation ratio: Np / Nv) between the number of BN particles oriented in the reference direction (parallel direction) (Np) and the number of BN particles oriented in the perpendicular direction (perpendicular direction) (Nv) was determined from the image analysis of the SEM images described above. The number of BN particles was calculated by counting the number of BN particles with inclination angles of 0° to 90° with respect to each direction.

[0060] The orientation rate of each sample is also shown in Table 1. The relationship between the orientation rate and the amount of BN particles charged is also shown in Figure 5. As is clear from Table 1 and Figure 5, if the amount of BN particles charged is less than 30 volume %, the orientation rate can be increased while reducing the amount.

[0061] Thermal properties (1) Using the portion cut out from the cured material at the tip of the spitz (test portion shown in Figure 2), the thermal diffusivity (α) of each sample was measured by the nano-flash method (measuring device: NETZSCH LFA447). The thermal diffusivity was measured in the reference direction of each sample (the direction perpendicular to the centrifugal direction or the direction of gravity). The thermal diffusivity of each sample is also shown in Table 1. The relationship between thermal diffusivity and the amount of BN particles added is also shown in Figure 6.

[0062] (2) As is clear from Table 1 and Figure 6, even though the amount of BN particles used was the same, Samples 1 to 5 had much higher thermal diffusivities and superior thermal conductivity than Samples C1 to C5. This tendency was particularly noticeable when the amount of BN particles used was 20 volume % or less.

[0063] Thus, it has been confirmed that, according to the present invention, it is possible to obtain a composite material that exhibits high thermal conductivity while suppressing the amount of thermally conductive particles used.

[0064] [Table 1]

Claims

1. A composite material made of resin and thermally conductive particles, A composite having a gradient portion in which the content of the thermally conductive particles in the composite changes in at least one direction.

2. The composite material according to claim 1, wherein the thermally conductive particles are contained in an amount of 0.1 to 50% by volume of the entire composite material.

3. The thermally conductive particles are plate-shaped with different thermal conductivities in the in-plane direction and the plate thickness direction, 2. The composite material according to claim 1, which has a highly oriented portion in which the orientation angle calculated from the in-plane direction with respect to a reference direction is 0° to 50°.

4. The composite material of claim 1 , wherein the resin is a thermosetting resin.

5. 2. The composite of claim 1, wherein said thermally conductive particles comprise hexagonal boron nitride.

6. 2. The composite material of claim 1, wherein the gradient portion has a high density region where the content is greater than 50% by mass and / or a low density region where the content is less than 50% by mass.

7. 2. The composite material according to claim 1, wherein the ratio (Np / Nv) of the number of thermally conductive particles oriented in a reference direction (Np) to the number of thermally conductive particles oriented in a direction perpendicular to the reference direction (Nv) is 1 or more.

8. preparing a flowable mixture from a resin and thermally conductive particles; a solidification step of solidifying the flowable mixture while applying centrifugal force; The method for producing a composite material according to any one of claims 1 to 7, comprising:

9. The resin is a two-component resin, The method for producing a composite material according to claim 8, wherein the solidification step is carried out in a room temperature range.

Images