Thermal conductive material and method for manufacturing the same

Abstract

To provide a thermal conductive material that exhibits the desired thermal conductivity at a low cost. [Solution] The present invention relates to a thermal conductive material in which a filler containing hematite-based iron oxide particles is dispersed in a matrix made of resin. The hematite-based iron oxide particles make up, for example, 55 to 95% by volume of the total thermal conductive material. The hematite-based iron oxide particles include, for example, coarse particles, fine particles, and even fine particles of different particle sizes. The fine particles make up, for example, 5 to 25% by mass of the total hematite-based iron oxide particles. The relative density of the composite material (ratio of bulk density to theoretical density) is, for example, 0.8 to 1. Such a composite material can be obtained, for example, by pressure molding a mixture of hematite-based iron oxide particles and resin at 0.01 to 500 MPa.

Description

Technical Field

[0001] The present invention relates to a heat conductive material and the like.

Background Art

[0002] Elements, devices, apparatuses, etc. generate an increasing amount of heat with the increase in density and performance, etc. In order to ensure their functions, lifespan, etc., sufficient heat dissipation is required. For example, electronic devices (such as semiconductor modules) promote heat dissipation through heat dissipation materials (sheets, films, greases, pads, tapes, heat sinks, housings, etc.) with excellent thermal conductivity. As the heat dissipation material, in addition to a single metal, a composite material with excellent flexibility, formability, etc. is often used. The composite material generally consists of a filler with excellent thermal conductivity and a matrix that holds the filler (for example, a resin containing an elastomer, rubber, etc.).

[0003] As such a filler, for example, ceramic particles (including fibers) such as alumina (Al2O3), silica (SiO2), aluminum nitride (AlN), etc. are generally used. Since these fillers achieve both high thermal conductivity and electrical insulation, they are suitable for heat dissipation materials for electronic devices. Descriptions related to these are, for example, in the following patent documents.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non-Patent Documents

[0005] [Non-Patent Document 1] "Thermal conductivity of dense iron oxide molded bodies" (Akiyama et al., Iron and Steel, vol. 77, No. 2, 1991) [Overview of the project] [Problems that the invention aims to solve]

[0006] The alumina (filler) used in Patent Documents 1-4 is not inexpensive, and increasing the heat dissipation surface area can increase the manufacturing cost of components and products. Furthermore, while alumina alone can exhibit high thermal conductivity (approximately 30 W / m·K), the thermal conductivity of composite materials made of alumina and resin (gap filler materials (GF materials), sheet materials, etc.) is generally only about 2-3 W / m·K, which is not necessarily high.

[0007] Incidentally, Patent Documents 2-4 describe a thermally conductive adhesive material containing 100 parts by weight of silicone polymer, 200 parts by weight of alumina, and 5 parts by weight of iron oxide. However, there is no description whatsoever regarding the specific type or crystal structure of the iron oxide, and the significance of adding such a small amount of iron oxide to the main filler (alumina) remains unclear.

[0008] Patent Document 5 describes iron oxide as an example of infrared heat dissipation powder, and Patent Document 6 describes iron oxide as an example of an inorganic filler for a heat dissipation sheet. However, these documents only contain the term "iron oxide," and do not provide any details about it or examples of its use.

[0009] Non-patent document 1 merely measures the thermal conductivity of iron oxide as a physical property, based on the premise of heat transfer analysis of the steelmaking process. Naturally, there is absolutely no mention of using iron oxide as a filler in a thermal conductive material in non-patent document 1.

[0010] This invention has been made in view of these circumstances, and aims to provide a new thermal conductive material that can ensure desired thermal conductivity while reducing costs. [Means for solving the problem]

[0011] The inventors diligently researched this problem and, as a result, unexpectedly discovered that a thermal conductive material in which hematite particles (α-Fe2O3) are dispersed in a resin (matrix) can exhibit good thermal conductivity. By further developing this result, the inventors completed the present invention described below.

[0012] Thermal conductive material The present invention relates to a thermal conductive material comprising a filler containing hematite-based iron oxide particles dispersed in a matrix made of resin, wherein the hematite-based iron oxide particles constitute 55 to 95% of the total volume of the thermal conductive material.

[0013] According to the present invention, it is possible to provide a thermal conductive material that can achieve both high thermal conductivity and cost reduction. Hematite-based iron oxide basically has high electrical resistivity and is non-magnetic (antiferromagnetic). For this reason, a thermal conductive material in which hematite-based iron oxide particles are dispersed in a resin (insulating material / non-magnetic material) has excellent insulating properties and is non-magnetic, making it highly versatile for use in electronic devices and the like. In addition, hematite-based iron oxide particles are resistant to oxidation and are stable over the long term, suppressing deterioration of the thermal conductive material over time. Of course, hematite-based iron oxide (particles) is easy to obtain and prepare (manufacture), and can be procured stably at low cost.

[0014] Method for manufacturing thermal conductive materials The present invention can also be understood as a method for manufacturing a thermal conductive material. For example, the present invention may be a manufacturing method comprising a preparation step of obtaining a mixture (including a kneaded product) of hematite-based iron oxide particles and a resin, and a molding step of molding the mixture, thereby obtaining the thermal conductive material described above.

[0015] Heat conductive material The present invention can also be understood as a heat conductive member. The heat conductive member may be, for example, a raw material (bulk material) before processing, or a product that has been molded or processed into a desired form (heat dissipation member, substrate, case, sheet, film, etc.). Including these, in this specification, it is simply referred to as "heat conductive material".

[0016] Others As used herein, "x to y" includes the lower limit value x and the upper limit value y unless otherwise specified. For any numerical value included in the various numerical values or numerical ranges described herein, a new range such as "a to b" may be newly established with the new numerical value as the new lower limit value or upper limit value. As used herein, "x to y μm" means x μm to y μm unless otherwise specified. The same applies to other unit systems (such as W / mK, Ωm, etc.).

Embodiments for Carrying Out the Invention

[0017] One or more components arbitrarily selected from this specification may be added to the components of the present invention. The content described herein may apply not only to the heat conductive material but also to its manufacturing method and the like. Even a component related to a method can become a component related to an object. Whether any embodiment is the best depends on the object, required performance, etc.

[0018] Filler (1) Type The filler contains at least hematite-based iron oxide particles. Hematite (α-Fe2O3) is a type of iron(III) oxide, is the most stable among iron oxides, and has parasitic ferromagnetism (antiferromagnetism) in the temperature range of use of the heat conductive material (for example, 250°C or lower).

[0019] Incidentally, iron(III) oxide includes β-Fe2O3, γ-Fe2O3 (maghemite), ε-Fe2O3, etc. which have different crystal structures and properties from hematite (corundum structure: hexagonal system or trigonal system). The "filler" as used herein may include iron oxide particles other than such hematite-based ones.

[0020] As used herein, "hematite-based iron oxide (particles)" is not limited to pure hematite (α-Fe2O3), and may be, for example, those in which a part of Fe is substituted with a transition metal such as Mn while maintaining its crystal structure.

[0021] As used herein, "filler" may include, in addition to hematite-based iron oxide particles, ceramic particles such as other iron oxides (Fe2O3(β, γ, ε), FeO, Fe3O4, etc.), alumina (Al2O3), silica (SiO2), aluminum nitride (AlN), boron nitride (hexagonal: h-BN, cubic: c-BN), compound particles, and metal particles. The content of particles other than hematite-based iron oxide is not specified, but for example, their total may be 0.1 to 15% by mass, 0.5 to 10% by mass, or 1 to 5% by mass relative to the total filler.

[0022] In this specification, "particles" refer to any shape, including granular (approximately spherical, irregularly shaped, plate-like, flattened, etc.), fibrous, etc.

[0023] (2) Particle size The particle size (average particle size) of the filler is adjusted as appropriate according to the specifications of the thermal conductive material. In this specification, "particle size" refers to the range of particle sizes that can be determined, for example, by sieving (classification) using a sieve of a predetermined mesh size. In this specification, particle size (size) is also referred to as "particle size," regardless of whether the particle shape is spherical (circular in cross-section) or not.

[0024] In this specification, particles (powder) with particle size x~y (μm / x>y) refer to particles (powder) that pass through a sieve with a mesh size of x (μm) but do not pass through a sieve with a mesh size of y (μm). In other words, particles (powder) with particle size x~y (μm) mean that, assuming classification by sieving, they belong to the range of maximum particle size: less than x μm to minimum particle size: greater than y μm.

[0025] However, depending on the particle size of the filler, particle size determination may be substituted or supplemented by methods such as sieving (JIS Z8815, etc.), laser diffraction / scattering (JIS Z8825, etc.), or dynamic light scattering, as appropriate. Alternatively, particle size may be determined by analyzing the observed image obtained by microscopic observation of powdered filler or filler in a thermal conductive material.

[0026] The overall particle size (minimum particle size to maximum particle size) of the filler (particularly hematite-based iron oxide particles) according to the present invention is, for example, 0.01 to 200 μm, 0.05 to 150 μm, 0.1 to 100 μm, 0.5 to 80 μm, and 1 to 70 μm. The particle size of the filler contained in the thermal conductive material can be determined, for example, by measuring the filler extracted after separating and removing the matrix resin (the same applies hereinafter).

[0027] The filler may be a mixture of particles (powder) belonging to multiple particle sizes. By mixing particles (powder) belonging to different particle sizes, the filling rate (occupied volume ratio) of the filler within the thermal conductive material can be efficiently increased. In this specification, particles (powder) belonging to larger particle sizes are referred to as coarse particles (powder), and particles (powder) belonging to smaller particle sizes are referred to as fine particles (powder) and / or fine particles (powder). When distinguishing between the two, particles (powder) belonging to a particle size smaller than fine particles are referred to as fine particles (powder). In either case, these are merely convenient designations based on relative particle sizes and do not specify concrete particle sizes.

[0028] For example, the particle size of coarse particles is 15-100 μm or 20-70 μm, the particle size of fine particles is 3-50 μm or 5-40 μm, and the particle size of fine particles is 0.01-2 μm or 0.1-1.5 μm. Furthermore, the fine particles (powder) and / or fine particles (powder) constitute, for example, 5-25%, 8-20%, or 12-18% by mass relative to the total filler (especially hematite-based iron oxide particles). By using coarse particles (powder) for the majority (remainder) of the filler, the filling rate of the filler and, consequently, the thermal conductivity of the thermal conductive material can be ensured efficiently and at low cost.

[0029] Whether or not the fillers in the thermal conductive material have multiple particle sizes is determined by measuring the particle size distribution (correlation between particle size and number of particles) of the extracted fillers and observing the number of peaks that appear in that distribution. For example, if fillers (groups of particles) with different particle sizes are contained in the thermal conductive material, multiple peaks will appear in its particle size distribution. The particle size distribution can be determined, for example, using commercially available measuring devices based on laser diffraction.

[0030] (3) Filling rate The filler content is adjusted according to the thermal conductivity of the thermal conductive material. Too little filler results in insufficient thermal conductivity of the thermal conductive material. Too much filler reduces the shape retention and moldability of the thermal conductive material.

[0031] The filling rate of the filler (especially hematite-based iron oxide particles) may be, for example, 55-97%, 60-95%, 65-92%, 70-90%, or 75-85% by volume relative to the total thermal conductive material.

[0032] The volume ratio is determined from the true density and blending amount (mass) of the filler and resin. If the thermal conductive material is not yet molded (raw material stage), the packing efficiency is determined as the volume ratio of the filler to the total of these components. If the packing efficiency is within the thermal conductive material, it is determined as the volume ratio of the extracted filler to the total volume (including voids) of the measured thermal conductive material. If the filler cannot be separated and extracted from the thermal conductive material, the packing efficiency may be determined indirectly or alternatively from an observation image (SEM image, etc.) of the thermal conductive material (cross-section).

[0033] (4) Processing The filler, in whole or in part, may be surface-treated to enhance its affinity with the matrix. Surface treatment improves the dispersibility, filling, and adhesion of the filler within the matrix, thereby improving the thermal conductivity of the thermal conductive material.

[0034] Surface treatments include, for example, hydrophobic treatment or coupling treatment. Specifically, these include silane coupling treatment and fluorine plasma treatment. Surface treatment may be applied directly to the filler before mixing (including kneading), or a surface treatment agent (such as a coupling agent) may be added during the mixing (kneading) of the matrix and filler.

[0035] "matrix" The filler is dispersed and held substantially uniformly in a matrix made of resin (including rubber, elastomer, etc.). The resin may be a thermosetting resin or a thermoplastic resin. The thermosetting resin may be subjected to a heat-curing treatment as appropriate.

[0036] Thermosetting resins include, for example, epoxy resins, phenolic resins, and silicone resins. Thermoplastic resins include, for example, polypropylene, polystyrene, polymethyl methacrylate, polycarbonate, and polyphenylene sulfide. Rubbers include, for example, ethylene-propylene-diene rubber (EPDM) and butyl rubber.

[0037] 《Manufacturing method》 The thermal conductive material is obtained, for example, through a preparation step of obtaining a mixture of a filler containing at least hematite-based iron oxide particles and a resin, and a molding step of forming the mixture into a molded body. The term "mixture" as used herein includes not only simple mixtures but also kneaded products and the like.

[0038] (1) Preparation process The mixing (including kneading) of the filler (powder) and resin (powder, granules, or liquid, etc.) may be carried out in one step or in multiple steps. For example, if the filler consists of multiple types of particles (powder) with different types and particle sizes, the mixing (kneading) may be carried out by gradually changing the type, particle size, or amount of each. Similarly, the resin may be added in multiple stages. By increasing the number of stages in the preparation process, for example, the porosity of the thermal conductive material can be reduced (relative density can be increased) and the thermal conductivity can be improved.

[0039] Mixing is performed using, for example, a ball mill, vibratory mill, or V-type mixer. Kneading is performed using, for example, a pressure kneader, Banbury mixer, or kneading extruder. The preparation process (mixing process, kneading process) may be carried out by adding solvents to adjust the viscosity of the resin. Solvents may be removed by volatilization or evaporation (drying process). The mixture (kneaded material) may be crushed, pulverized, etc. as appropriate and used as a compound in the molding process.

[0040] (2) Molding process The molded article is obtained, for example, by pressure molding a mixture or compound. The molding pressure is, for example, 0.01 to 500 MPa, 0.1 to 400 MPa, 1 to 350 MPa, or 10 to 300 MPa. The molding process may include compression molding, injection molding, transfer molding, or other methods such as film deposition or painting. When the molding thickness is small, such as in films, sheets, or coatings, the porosity can be reduced (relative density improved) even with low pressure application, and a thermal conductive material with the desired thermal conductivity can be obtained.

[0041] The molding process may be cold molding performed at room temperature or hot molding performed under heating. Hot molding should, for example, be performed at a temperature at which the resin softens or melts.

[0042] The molded body (thermal conductive material) may be the final product or a shape close to it, or it may be the material to be processed (bulk material) or intermediate material. If the matrix is ​​made of a thermosetting resin, a thermosetting process (curing treatment) may be performed after molding (and even after processing) to harden the resin.

[0043] 《Application》 Thermal conductive materials are used in applications such as heat dissipation sheets (films), fillers, substrates, and cases. Their thermal conductivity may be, for example, 2-10 W / mK, 2.5-8 W / mK, 3-7 W / mK, or 3.5-6.5 W / mK.

[0044] The denser a thermal conductive material is, the fewer voids and imperfections it has, the higher its thermal conductivity can be. Such thermal conductive materials typically have a relative density (the ratio of bulk density to theoretical density) of 0.8–1, 0.85–0.98, or 0.9–0.96. Theoretical density is calculated from the true density and blending amount (or content) of the constituent materials (e.g., fillers and resins). The bulk density of a thermal conductive material is determined from its measured mass and volume (or dimensions).

[0045] Thermal conductive materials may have either anisotropic or isotropic thermal conductivity. When the filler does not contain flattened particles or the like (for example, when the filler consists only of hematite-based iron oxide particles), its thermal conductivity is usually isotropic.

[0046] Thermal conductive materials used in electronic devices, for example, have a resistivity of 10 5 ~10 12 Ωm or 10 8 ~10 10 It would be good to have Ωm. [Examples]

[0047] Various composite materials (thermal conductive materials) in which fillers are dispersed and held in a matrix were fabricated, and their thermal conductivity properties were evaluated. The present invention will be explained in more detail with such specific examples.

[0048] [First Embodiment] 《Raw materials》 (1) Filler Three types of hematite powder with different particle sizes (coarse powder, fine powder, and very fine powder) were prepared as follows.

[0049] The coarse and fine powders were prepared as follows: First, commercially available magnetite powder (Fe3O4) was placed in an alumina boat and heated in a muffle furnace in the atmosphere (approximately 800°C for 12 hours). This was then cooled in the atmosphere to obtain iron oxide powder. This iron oxide powder was sieved to prepare coarse powder with a particle size of 30-60 μm and fine powder with a particle size of 30-10 μm.

[0050] For the fine powder, we used commercially available reagent (ferric oxide (α-Fe2O3) powder manufactured by Kojun Chemical Laboratory Co., Ltd.) as is. Its particle size was approximately 0.3 μm (<1 μm).

[0051] Each powder was weighed, and several mixed powders with different formulations were prepared, as shown in Table 1. The percentages of fine powder and ultrafine powder shown in Table 1 represent the mass ratio (remainder: coarse powder) to the total filler (coarse powder + fine powder + ultrafine powder).

[0052] (2) Matrix A one-component, heat-curing epoxy resin (EP106, manufactured by Cemedyne Co., Ltd.) was used as the matrix for holding the filler.

[0053] Sample preparation (1) Preparation process A titanium-based coupling agent and epoxy resin were added to the filler (mixed powder) of each sample and mixed in a mortar. The total amount of filler was adjusted to the volume ratio shown in Table 1 relative to the total amount of the mixture. The amount of titanium-based coupling agent added was small (approximately 2% by mass relative to the total amount of filler), and in this specification, it is treated as part of the resin.

[0054] (2) Molding process A mixture of filler and resin was placed in the cavity (φ10mm × t50mm) of a cemented carbide mold and heated (warm) molded. The molding pressure was varied for each sample within the range of 0.025 to 250 MPa, as shown in Table 1. For all samples, the molding temperature (mold temperature) was 140°C and the molding time (pressure holding time) was 1000 seconds.

[0055] (3) Heat curing process The molded body obtained after the molding process was cured in a heating furnace (150°C for 1 hour). The composite material (thermal conductive material) obtained in this way was then evaluated as follows.

[0056] "measurement" (1) Density and relative density The bulk density (ρ) of each sample (composite material) was determined from the measured mass and volume (Archimedes method). The results are shown in Table 1.

[0057] The theoretical density (ρth) of the composite material was determined based on the true density and proportion of the raw materials (filler and resin) used in each sample. For each sample, the ratio of the apparent density to the theoretical density, ρ / ρth, is shown as the relative density in Table 1. Note that (1-ρ / ρth) represents the porosity of the composite material.

[0058] (2) Thermal conductivity The thermal diffusivity (α) of each sample (composite material) was measured, and its thermal conductivity was calculated. The results are shown in Table 1. The measurements and calculations were performed as follows.

[0059] The thermal conductivity (λ) was determined by the nanoflash method (measurement device: NETZSCH LFA447). Specifically, the thermal conductivity was calculated using the formula λ = α·Cp·ρ, based on the thermal diffusivity (α) measured by the nanoflash method, the specific heat (Cp) determined by differential scanning calorimeter (DSC), and the density (ρ) determined by the Archimedes method.

[0060] "evaluation" As is clear from Table 1, it was found that composite materials using hematite particles (hematite-based iron oxide particles) as fillers can achieve thermal conductivity equivalent to or better than that of composite materials using alumina particles as fillers.

[0061] For example, samples containing approximately 80% (75-85% by volume) of filler relative to the total composite material, with a relative density of 0.8 or higher, exhibited a thermal conductivity of 2 W / m·K or higher. Furthermore, samples containing fine particles (e.g., particle size of 1 μm or less) in the filler showed a significantly higher thermal conductivity. It was also found that such fine particles were sufficient if they constituted approximately 5-25% of the total filler.

[0062] It was also found that, with the sample size described in this embodiment, increasing the molding pressure can reduce the voids within the composite material, thereby increasing the relative density and thus the thermal conductivity.

[0063] [Second Example] 《Raw materials》 (1) Filler Hematite powders with different particle sizes (coarse powder and fine powder) were prepared as follows.

[0064] For the coarse powder, as in the first example, commercially available magnetite powder was heated in air and then classified by sieving to obtain a particle size of 60-106 μm.

[0065] For the fine powder, the same commercially available reagent as in Example 1 was used as is. The coarse powder and fine particles were weighed and blended in a ratio of 75:25 (by mass).

[0066] (2) Matrix For the matrix that holds the filler, we used polypropylene (PP / Novatec® PP MA1B manufactured by Nippon Polypropylene Co., Ltd.), which is a thermoplastic resin.

[0067] Sample preparation (1) Preparation process The PP was mixed with the filler (total amount) in the volume proportions shown in Table 1 and a titanium-based coupling agent, and then mixed in a twin-screw kneader at 200°C for approximately 15 minutes. In this example as well, the titanium-based coupling agent was treated as part of the resin.

[0068] (2) Molding process A mixture of filler and resin was placed into the cavity (φ10mm × t50mm) of a cemented carbide mold and heated (warm molded). The molding pressure was varied for each sample within the range of 0.025 to 25 MPa, as shown in Table 2. The molding temperature (mold temperature) for samples 21 to 27 was 160°C, and the molding temperature for sample 28 was 180°C. The molding time (pressure holding time) for each sample was 300 seconds.

[0069] "measurement" Table 2 shows the measured (calculated) density, relative density, thermal diffusivity, and thermal conductivity for each sample.

[0070] "evaluation" As is clear from Table 2, all samples exhibited high density and excellent thermal conductivity. Because thermoplastic resin was used, it is thought that even samples with low molding pressure (e.g., samples 25-28) demonstrated high performance.

[0071] At the same molding pressure, samples with more filler exhibited higher thermal conductivity (e.g., samples 21-24). As can be seen from the comparison between sample 26 and sample 28, higher molding temperatures improved both density and thermal conductivity. However, at molding temperatures above the melting point of the matrix (approximately 160°C for PP), this effect is considered to be small.

[0072] From the above, it has become clear that, according to the present invention, a thermal conductive material exhibiting a desired thermal conductivity can be obtained at low cost.

[0073] [Table 1]

[0074] [Table 2]

Claims

1. A thermal conductive material comprising a filler containing hematite-based iron oxide particles dispersed in a matrix made of resin, The aforementioned hematite-based iron oxide particles are present in a thermal conductive material in a volume ratio of 55 to 95% of the total thermal conductive material.

2. The thermal conductive material according to claim 1, wherein the hematite-based iron oxide particles include at least coarse particles and fine particles of different particle sizes.

3. The thermal conductive material according to claim 2, wherein the hematite-based iron oxide particles include fine particles with an even smaller particle size than the fine particles.

4. The thermal conductive material according to claim 1, wherein the hematite-based iron oxide particles include at least coarse particles and fine particles of different particle sizes.

5. The thermal conductive material according to claim 1, wherein the relative density, which is the ratio of bulk density to theoretical density, is 0.8 to 1.

6. The thermal conductive material according to claim 3 or 4, wherein the fine particles are present in an amount of 5 to 25% by mass relative to the total hematite-based iron oxide particles.

7. Preparation steps to obtain a mixture of hematite-based iron oxide particles and resin, The process includes a molding step for molding the mixture, A method for producing a thermal conductive material according to any one of claims 1 to 5.

8. The method for manufacturing a thermal conductive material according to claim 7, wherein the molding step is a molding pressure of 0.01 to 500 MPa.

9. The method for producing a thermal conductive material according to claim 7, wherein the resin is a thermoplastic resin.

10. The aforementioned resin is a thermosetting resin, The method for manufacturing a thermal conductive material according to claim 7, further comprising a thermosetting step of curing the resin after the molding step.

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