Heat dissipation element and heat sink

Abstract

heat dissipation element (13) comprising a ceramic heat radiation material (20), wherein the ceramic thermal radiation material (20) contains silicon nitride and boron nitride as its main components, and a ratio of the mass of boron nitride to the mass of silicon nitride and boron nitride is 10 mass-% to 40 mass-%, The boron nitride has an average particle size of 0.05 µm to 1 µm, and the ceramic thermal radiation material (20) exhibits an average emissivity of higher or equal to 70% in wavelength ranges from 3 µm to 25 µm at temperatures up to 200°C.

Description

Area

[0001] The present disclosure relates to a heat dissipation element and a heat sink for use in the heat dissipation of electrical and electronic devices. background

[0002] Electrical / electronic devices containing heat-generating components such as light-emitting diodes (LEDs) or integrated circuits (ICs) typically employ heat dissipation techniques based on either natural air cooling with aluminum heat sinks or forced air cooling with electric fans. It is difficult to use aluminum heat sinks or electric fans, which require air convection, for electrical devices in vehicles used in sealed enclosures for dust and water resistance, or for spacecraft used in a vacuum. For information devices, including laptops, which tend to generate increasing amounts of heat as the power of their main processors (CPUs) increases, the ongoing miniaturization and high-density assembly make it difficult to find space to accommodate a large-volume aluminum heat sink.Furthermore, aluminum heat sinks, which are made of metal, generate electromagnetic interference that can lead to malfunctions in electrical / electronic devices. Therefore, conventional heat dissipation techniques based on aluminum heat sinks or electric fans are difficult to apply as heat dissipation measures for some types of electrical / electronic devices, for which ceramic heat sinks based on infrared thermal radiation have attracted attention.

[0003] Patent document 1 discloses a magnetic storage device in which a heat dissipation element is provided in contact with a surface of a seal with which a magnetic direct-access memory is sealed. Patent document 1 discloses that the heat dissipation element is made of a metal with good thermal conductivity or of a highly thermally conductive ceramic. The highly thermally conductive ceramic is shown by example to be aluminum oxide, aluminum nitride, boron nitride, silicon nitride, or silicon carbide.

[0004] Patent reference 2 discloses a method for manufacturing a novel printed circuit board characterized by very good board thickness accuracy, a low coefficient of thermal expansion, high heat dissipation and heat resistance, and used for high-frequency or direct mounting of semiconductor chips. It can preferably be used for applications.

[0005] Patent literature 3 relates to a ceramic silicon nitride composite material used as a material for a protective tube for a temperature sensor for measuring the temperature of molten steel, and a protective tube for a temperature sensor for molten steel that uses this material, and in particular to a ceramic silicon nitride composite material comprising 3.0 to 20.0 wt. parts of a sintering agent, 2.0 to 15.0 wt. parts of aluminum nitride (AlN), 2.0 to 10.0 wt. parts of SiAlON and 2.0 to 10.0 wt. parts of boron nitride (BN) per 100 wt. parts of a silicon nitride base containing silicon nitride (Si3N4) and silicon (Si), and a protective tube for a temperature sensor that uses this. List of patent literature Patent literature 1: JP 2005 - 78 693 A Patent literature 2: JP H07-336 001 A Patent literature 3: KR ​​10 2009 0 129 275 A Summary Technical Problem

[0006] The emissivity of a ceramic material is determined by the radiation spectrum characteristic of the crystal structure of each substance, and there is a wavelength range with high emissivity and a wavelength range with low emissivity. Therefore, it is generally difficult to increase the average emissivity, i.e., the average emissivity across all wavelengths of the infrared, using a single ceramic material. That is to say, with the technique described in patent literature 1, where aluminum oxide, aluminum nitride, boron nitride, silicon nitride, or silicon carbide are used alone as the highly thermally conductive ceramic, it is difficult to further improve the average emissivity in the infrared range.

[0007] The present disclosure was made in consideration of the above, and one of its objectives is to obtain a heat dissipation element with a better average emissivity in infrared ranges than conventional ones. Solution to the problem

[0008] To solve the problems described above and to fulfill the task, a heat dissipation element according to claim 1 and a heat sink according to claim 13 are provided. Advantageous effects of the invention

[0009] The present disclosure is advantageous in achieving a better average emissivity in infrared ranges than conventional ones. Brief description of the drawings Fig. Figure 1 is a cross-sectional view that schematically represents an exemplary configuration of an electrical / electronic device containing a heat dissipation element according to a first embodiment. Fig. Figure 2 is a cross-sectional view that schematically represents an exemplary configuration of the heat dissipation element according to the first embodiment. Fig. Figure 3 is a cross-sectional view that schematically represents another exemplary configuration of the heat dissipation element according to the first embodiment. Fig. Figure 4 is a cross-sectional view that schematically represents an exemplary configuration of a heat dissipation element according to a second embodiment. Fig. Figure 5 is a diagram showing examples of the raw materials, ceramic heat radiation materials and properties of the heat dissipation elements according to examples 1 to 8 and comparative examples 1 to 3. Description of embodiments

[0010] A heat dissipation element and a heat sink according to embodiments of the present disclosure are described in detail below with reference to the drawings. The present disclosure is not limited to these embodiments. First embodiment.

[0011] Fig. Figure 1 is a cross-sectional view schematically illustrating an exemplary configuration of an electrical / electronic device containing a heat dissipation element according to the first embodiment. The electrical / electronic device 1 is an electrical or electronic device equipped with a heat-generating component, such as an LED or an integrated circuit (IC). The electrical / electronic device 1 comprises a housing 10 containing a substrate 11 and components arranged on the substrate 11. In one example, the housing 10 encloses the substrate 11 for dust and water resistance. In another example, the substrate 11 is a printed circuit board. The components include a circuit component connected to the substrate 11 via solder, a semiconductor package containing a semiconductor element, and the like. Some components generate heat during operation.In the following, a component that generates heat is referred to as heat-generating component 12.

[0012] The electrical / electronic device 1 further comprises a heat dissipation element 13, which is arranged in contact with the heat-generating component 12 in the housing 10. The heat dissipation element 13 is an element that radiates heat from the heat-generating component 12 using infrared thermal radiation. Examples of a cooling device that uses the heat dissipation element 13 are a heat sink, a heat spreader, and a heat dissipation substrate. That is, the heat sink, the heat spreader, and the heat dissipation substrate contain the heat dissipation element 13. Details about the heat dissipation element 13 are described later.

[0013] Fig. Figure 2 is a cross-sectional view that schematically illustrates an exemplary configuration of the heat dissipation element according to the first embodiment. The heat dissipation element 13 comprises a sintered body made of a ceramic radiant heat material 20. The heat dissipation element 13 containing the ceramic radiant heat material 20 exerts a cooling effect by dissipating heat generated by a heat source, such as a semiconductor element contained in the heat-generating component 12, to the surroundings via infrared radiation. Therefore, the ceramic radiant heat material 20 preferably has the highest possible emissivity. However, the emissivity of a ceramic material is determined by the radiation spectrum typical for the crystal structure of each substance, and there is a wavelength range with high emissivity and a wavelength range with low emissivity.Therefore, it is generally difficult to increase the average emissivity, i.e., the average emissivity across all infrared wavelength ranges, using a single ceramic material. In light of this, the heat dissipation element 13 in the first embodiment incorporates the ceramic thermal radiation material (Si3N4-BN) 20, which is a sintered body obtained by combining silicon nitride (Si3N4) particles 21 and boron nitride (BN) particles 22 exhibiting different radiation spectra. Accordingly, a relatively high thermal emissivity can be obtained, and a high average emissivity can be achieved in the infrared range with wavelengths from 3 µm to 25 µm. Here, the term "average emissivity" refers to the average emissivity across the infrared range with wavelengths from 3 µm to 25 µm.

[0014] In the first embodiment, the mass ratio of the boron nitride particles 22 to the mass of the silicon nitride particles 21 and the boron nitride particles 22 of the ceramic heat radiation material 20 forming the heat dissipation element 13, i.e., the mass ratio of boron nitride to the mass of silicon nitride and boron nitride, is 10 wt% to 40 wt%. Hereinafter, the mass ratio of boron nitride particles 22 to the mass of silicon nitride particles 21 and boron nitride particles 22 will also simply be referred to as the mass ratio of boron nitride particles 22. The mass ratio of boron nitride particles 22 is preferably 20 wt% to 30 wt%.

[0015] If the mass ratio of the boron nitride particles 22 is too low, i.e., if the mass ratio of the boron nitride particles 22 is less than 10 mass-%, there is a wavelength range with low emissivity, resulting in an unimpeded average emissivity. This means that if the ceramic thermal radiation material 20 with such a mass ratio of silicon nitride particles 21 and boron nitride particles 22 is used as a heat dissipation element 13, sufficient cooling performance cannot be achieved. Therefore, the mass ratio of the boron nitride particles 22 is preferably higher than or equal to 10 mass-%.

[0016] On the other hand, if the mass ratio of the boron nitride particles 22 is too high, i.e., if the mass ratio of the boron nitride particles 22 is higher than 40 wt%, the ceramic thermal radiant material 20 has high porosity, which drastically reduces its thermal conductivity. This means that the heat generated by the heat-generating component 12 as the heat source is less likely to be transferred to the heat dissipation element 13, thus preventing an improvement in cooling performance. Furthermore, the mechanical strength of the ceramic thermal radiant material 20 is significantly reduced. Therefore, if the ceramic thermal radiant material 20 with such a mass ratio of silicon nitride particles 21 and boron nitride particles 22 is used as a heat dissipation element 13, fractures or cracks may occur. Therefore, the mass ratio of the boron nitride particles 22 is preferably lower than or equal to 40 wt%.If the mass ratio of the boron nitride particles 22 is 20 mass-% to 30 mass-%, both the emissivity and the thermal conductivity are further improved with respect to the cooling performance of the heat dissipation element 13.

[0017] The boron nitride particles 22 contained in the ceramic thermal radiation material 20 can be turbostratic boron nitride (t-BN), in which hexagonal BN layers are randomly stacked, but are preferably hexagonal boron nitride (h-BN), in which hexagonal BN layers are regularly stacked. By containing hexagonal boron nitride, the ceramic thermal radiation material 20 is likely to exhibit improved thermal conductivity and improved average thermal emissivity.

[0018] The porosity of the ceramic radiant heating material 20 is related to the thermal conductivity and mechanical strength of the heat dissipation element 13. That is, if the porosity of the ceramic radiant heating material 20 is too high, cavities within the material become interconnected, leading to a decrease in mechanical strength. Additionally, the air layers in the cavities act as thermal insulators, hindering heat transfer and resulting in reduced thermal conductivity. Therefore, to achieve the desired thermal conductivity and mechanical strength, the porosity of the ceramic radiant heating material 20 is preferably less than or equal to 40%. The porosity of the ceramic radiant heating material 20 is further preferably less than or equal to 35%, and even more preferably less than or equal to 30%.

[0019] The porosity of the ceramic thermal radiation material 20 tends to decrease as the mass ratio of the boron nitride particles 22 decreases. However, if, as described above, the mass ratio of the boron nitride particles 22 is less than 10 mass-%, the average emissivity of the ceramic thermal radiation material 20 is not improved. Similarly, if the mass ratio of the boron nitride particles 22 is less than 10 mass-%, the porosity is also less than 10%. Therefore, the porosity of the ceramic thermal radiation material 20 should ideally be between 10% and 40%.

[0020] The "porosity" of the ceramic thermal radiation material 20, as used here, will now be described. The "porosity" is calculated according to Archimedes' principle. In particular, the "porosity" can be calculated using the formula (1) below, taking into account the measured values ​​of the mass and dimensions of the ceramic thermal radiation material 20 cut into a rectangular parallelepiped shape. Note that the dimensions of the ceramic thermal radiation material 20, which has a rectangular parallelepiped shape, are length, width, and height. Porosity={1−[Wdry / (L×W×T) / ρtheory]}×100

[0021] In formula (1) W dry The mass (g) of the ceramic heat radiation material 20, which was dried for 2 hours at 150°C. In formula (1), L, W and T are the length, width and height (cm) of the ceramic heat radiation material 20, which has a rectangular parallelepiped shape, and ρ theoryis the theoretical density (g / cm³) 3 ) of the ceramic thermal radiation material 20.

[0022] The average emissivity of the ceramic radiant heat transfer material 20 is greater than or equal to 70%. Generally, the emissivity of the ceramic radiant heat transfer material 20 varies with temperature, but the ceramic radiant heat transfer material 20, which has an average emissivity of 70% or more in the temperature range up to 200°C, preferably in the temperature range up to 150°C, in which the heat dissipation element 13 of the electrical / electronic device 1 is normally used, can achieve sufficient cooling performance as a heat dissipation element 13. Furthermore, the thermal conductivity of the ceramic radiant heat transfer material 20 is preferably greater than or equal to 40 W / (m K). This is because, if the thermal conductivity is 40 W / (m K) or higher, the heat generated by the heat source is efficiently transferred to the heat dissipation element 13, so that a significantly higher cooling performance can be expected.

[0023] The silicon nitride and boron nitride contained in the ceramic thermal radiation material 20 are present as particles. With a view to standardizing the cooling performance of the ceramic thermal radiation material 20 and improving its mechanical strength, the boron nitride particles 22 are preferably uniformly dispersed among the silicon nitride particles 21.

[0024] From the point of view of ensuring that the boron nitride particles 22 are uniformly dispersed, the average particle size of the boron nitride particles 22 is desirable to be 0.05 µm to 1 µm.

[0025] If the average particle size of the boron nitride particles 22 exceeds 1 µm, it can be difficult to achieve a state in which the boron nitride particles 22 are uniformly dispersed among the silicon nitride particles 21. Conversely, if the average particle size of the boron nitride particles 22 is less than 0.05 µm, the boron nitride particles 22 can form strong aggregations, making it difficult to achieve a state in which the boron nitride particles 22 are uniformly distributed among the silicon nitride particles 21. As a result, regions with many boron nitride particles 22 and regions with many silicon nitride particles 21 are generated unevenly within the ceramic thermal radiation material 20. In the following, regions with many boron nitride particles 22 are referred to as boron nitride-rich regions and regions with many silicon nitride particles 21 as silicon nitride-rich regions.Such a non-uniformity leads to a non-uniformity in the cooling performance of the heat dissipation element 13. Additionally, boron nitride-rich areas have high porosity, which reduces mechanical strength and causes fractures and cracks. Therefore, if the boron nitride particles 22 are not uniformly distributed among the silicon nitride particles 21, the cooling performance and mechanical strength of the entire heat dissipation element 13, including the ceramic heat radiation material 20, tend not to be sufficiently improved. Therefore, the average particle size of the boron nitride particles 22 should ideally be between 0.05 µm and 1 µm.

[0026] The average particle size of the silicon nitride particles 21 is not particularly limited, but is preferably 2 µm to 30 µm.

[0027] The average particle size of the particles in the ceramic thermal radiation material 20 can be determined by examining a cross-section of the ceramic thermal radiation material 20 with a scanning electron microscope (SEM). Specifically, the average particle size can be obtained by sectioning the ceramic thermal radiation material 20, magnifying the cross-section (e.g., 15,000 times) with the SEM, measuring the principal axis diameters of at least 20 particles, and averaging the measured values.

[0028] In addition to the silicon nitride particles 21 and the boron nitride particles 22, the ceramic thermal radiation material 20 can contain a sintering aid for densification. The sintering aid is not particularly limited, and those known in the art can be used. Examples of sintering aids include oxides of rare-earth elements such as yttrium, oxides of aluminum, titanium, magnesium, or silicon, and nitrides of aluminum or titanium. These can be used alone or in combination with two or more. From the perspective of the average emissivity and the mechanical strength of the ceramic thermal radiation material 20, an oxide of a rare-earth element is preferably used as the sintering aid.

[0029] The amount of sintering aid contained in the ceramic thermal radiation material 20 is not particularly limited, but preferably ranges from 2 wt% to 20 wt%. If the amount of sintering aid is less than 2 wt%, the ceramic composite cannot be sufficiently densified. Conversely, if the amount of sintering aid is greater than 20 wt%, the amount of silicon nitride particles 21 and boron nitride particles 22 is reduced, and thus the average emissivity of the ceramic thermal radiation material 20 cannot be sufficiently improved. Therefore, the amount of sintering aid is preferably between 2 wt% and 20 wt%.

[0030] In addition to the components mentioned above, the ceramic thermal radiation material 20 can contain 20 different components known in the art, so that a desired effect can be achieved. The quantity of such components contained in the ceramic thermal radiation material 20 is not particularly limited, as long as the effect of the present disclosure is not impaired.

[0031] Fig. Figure 3 is a cross-sectional view that schematically illustrates another exemplary configuration of the heat dissipation element according to the first embodiment. In one example, the heat dissipation element 13 has the form of a flat plate. As shown in Fig. As shown in Figure 3, the heat dissipation element 13 can have a metal oxide layer 23 on the surface of at least part of the ceramic heat radiation material 20. The example of Fig. Figure 3 shows that the metal oxide layer 23 is provided on a surface of the ceramic heat radiation material 20 in the form of a flat plate. The metal oxide layer 23 has a radiation spectrum that differs from the radiation spectra of the silicon nitride particles 21 and the boron nitride particles 22. The metal oxide layer 23 with this property enables a further improvement in the average emissivity of the heat dissipation element 13. In particular, the metal oxide layer 23 is preferably an oxide layer containing R₂Si₂O₇, a type of rare-earth silicate, where R is a rare-earth element.Since R₂Si₂O₇ has a coefficient of thermal expansion corresponding to that of the silicon nitride particles 21, thermal stresses at the interface between the ceramic radiant heating material 20 and the metal oxide layer 23 are prevented, which helps to prevent flaking and cracking when the ceramic radiant heating material 20 and the metal oxide layer 23 are heated to a high temperature. R₂Si₂O₇ is not particularly limited here; Y₂Si₂O₇, Lu₂Si₂O₇, or Yb₂Si₂O₇ can also be used.

[0032] The metal oxide layer 23 can be formed on the surface of the ceramic thermal radiation material 20 by oxidizing the ceramic thermal radiation material 20 at high temperature in air. In this case, the amount of rare earth oxide contained in the ceramic thermal radiation material 20 is preferably 3 wt% to 20 wt%. If the amount of rare earth oxide is less than 3 wt%, the proportion of rare earth silicate contained in the metal oxide layer 23 is extremely reduced, and the metal oxide layer 23 may detach upon heating to a high temperature. On the other hand, if the amount of rare earth oxide is greater than 20 wt%, the amount of silicon nitride particles 21 and boron nitride particles 22 is reduced, as in the case of the sintering aid described above, and thus the average emissivity of the ceramic thermal radiation material 20 cannot be sufficiently improved.From the above, the desirable amount of rare earth oxide contained in the ceramic thermal radiation material 20 is 3% to 20% by mass.

[0033] The heat dissipation element 13, containing the ceramic thermal radiation material 20 according to the first embodiment, can be used as a heat dissipation measure for the electrical / electronic device 1. Specific examples of possible applications are heat sinks, heat distributors, and heat dissipation substrates. In particular, when the heat dissipation element 13 is used as a heat sink, it is desirable that the heat sink, which contains the heat dissipation element 13 in the form of a flat plate, has an uneven area on at least one of its side surfaces with a height difference that is greater than or equal to the wavelength of the infrared radiation to be emitted. In particular, since the ceramic thermal radiation material 20, which generates thermal radiation in wavelength ranges from 3 µm to 30 µm, is used, the heat sink preferably has an unevenness of 25 µm or more, and more preferably an unevenness of 30 µm or more.By providing the surface with an irregularity that is greater than or equal to the wavelength of infrared radiation, the surface area effective for infrared radiation is increased. Consequently, the apparent average emissivity is improved and the cooling performance of the heat sink is increased.

[0034] Next, a method for manufacturing the heat dissipation element 13 is described. The heat dissipation element 13 according to the first embodiment can be manufactured using a method known in the art. For example, the heat dissipation element 13 according to the first embodiment can be manufactured in the following way.

[0035] First, silicon nitride powder, boron nitride powder, a sintering aid, a dispersion agent, a binder, and water are mixed to produce a slurry. The average particle size of the silicon nitride powder, the boron nitride powder, and the sintering aid is not particularly limited, but is preferably less than or equal to 1 µm, more preferably less than or equal to 0.8 µm, and even more preferably less than or equal to 0.5 µm. In particular, if the average particle size of the boron nitride powder exceeds 1 µm, it can be difficult to achieve a state in which the boron nitride particles 22 are uniformly dispersed among the silicon nitride particles 21, which can cause inconsistencies in the cooling performance of the heat dissipation element 13.Additionally, if the average particle size of the boron nitride powder is less than 0.05 µm, it can form strong aggregations, making it difficult to achieve a state in which the boron nitride powder is uniformly dispersed among the silicon nitride powder. Therefore, the average particle size of the boron nitride powder is 0.05 µm to 1 µm.

[0036] The dispersant is not particularly limited as long as it can be used for an aqueous slurry, and those known in the trade can be used. Examples of dispersants include: anionic surfactants such as alkyl sulfate ester salts, polyoxyethylene alkyl ether sulfate ester salts, alkylbenzenesulfonates, reactive surfactants, fatty acid salts, and naphthalenesulfonate-formalin condensates; cationic surfactants such as alkylamine salts, quaternary ammonium salts, alkyl betaines, which are amphoteric surfactants, and alkylamine oxides; and nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyalkylene derivatives, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, glycerol fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene castor oil fatty acid, polyoxyethylene alkylamines, and alkylalkanolamides. These can be used alone or in combination with two or more.

[0037] The binder is not particularly limited, and those known in the trade can be used. The binder is exemplified by acrylic, cellulose-based, polyvinyl alcohol-based, polyvinyl acetal-based, urethane-based, or vinyl acetate-based resin. These can be used alone or in combination with two or more.

[0038] The type of water is not particularly limited, and pure water, reverse osmosis water (RO), deionized water or similar can be used.

[0039] The mixing process for slurry production is not particularly restricted and can be carried out using a method known in the art. The mixing process is exemplified by the use of a kneader, a ball mill, a planetary ball mill, a kneading mixer, or a bead mill.

[0040] The slurry is then granulated to produce granulated powder. The granulation process is not particularly restricted and can follow a method known in the trade. Granulated powder can be obtained, for example, by spray drying with a spray dryer or similar equipment. The spray drying conditions are adapted according to the equipment used and are not particularly limited.

[0041] Next, granulated powder is placed in a mold of the desired shape, and pressure forming is performed to produce a shaped body. When the heat dissipation element 13 is applied to a heat sink, the desired shape is exemplified by a mold in the form of a flat plate. The pressure forming process is not particularly restricted and can follow any method known in the art. Examples of pressure forming processes include cold isostatic pressing (CIP), warm isostatic pressing (WIP), and uniaxial pressure forming.

[0042] The pressure applied during pressure forming is adjusted appropriately according to the type of granulated powder, the device to be used, and the like, and is not particularly limited, but is generally in the range of 30 MPa to 500 MPa.

[0043] The molded body then undergoes a degreasing treatment. The method of degreasing is not particularly restricted and can follow any method known in the industry. For example, the degreasing treatment can be carried out by heating the molded body in an atmospheric environment. The heating temperature is not particularly limited as long as the binder can be thermally decomposed and is generally in the range of 300°C to 800°C.

[0044] Next, the degreased body is fired. The firing process is not particularly restricted and can follow a method known in the art. For example, the degreased body is fired in a nitrogen atmosphere. The pressure of the nitrogen gas during firing can be normal pressure, but is preferably 0.2 MPa to 1.0 MPa to prevent thermal decomposition of Si3N4. The firing temperature is not particularly limited, but is generally in the range of 1700°C to 2100°C, preferably 1750°C to 2050°C, and even more preferably 1800°C to 2000°C.

[0045] Afterwards, the surface of the fired body can be ground to adjust its shape. The grinding process is not particularly limited and can follow a method known in the art. An example of a grinding process is grinding with a diamond drill bit. Additionally, the fired body can be subjected to heat treatment in an oxygen atmosphere. This forms the metal oxide layer 23 on the surface. In the manner described above, the heat dissipation element 13 is formed according to the first embodiment.

[0046] In the first embodiment, the heat dissipation element 13 contains the ceramic heat radiation material 20, which is obtained by combining silicon nitride particles 21 and boron nitride particles 22 with different heat radiation spectra in the infrared range. Consequently, the heat dissipation element 13 has a higher average emissivity in the infrared range than conventional elements. As a result, the heat dissipation element 13 has better cooling performance than conventional elements. Second embodiment.

[0047] Fig. Figure 4 is a cross-sectional view that schematically illustrates an exemplary configuration of a heat dissipation element according to the second embodiment. The differences from the first embodiment are described below. Note that components identical to those of the first embodiment are identified with the same reference numerals, and their description is omitted.

[0048] The heat dissipation element 13 according to the second embodiment comprises a base material 30 and a coating layer 25 containing the ceramic heat radiation material 20. The coating layer 25 contains a filler and a binder 26. The filler is the ceramic heat radiation material 20, which contains silicon nitride particles 21 and boron nitride particles 22. The mass ratio of the boron nitride particles 22 to the mass of the silicon nitride particles 21 and the boron nitride particles 22 in the coating layer 25 according to the second embodiment is the same as in the ceramic heat radiation material 20 according to the first embodiment: 10% by mass to 40% by mass.

[0049] The heat dissipation element 13, which contains the coating layer 25 containing the silicon nitride particles 21 and the boron nitride particles 22 in a predetermined mass ratio, has a higher average emissivity and better cooling performance than conventional elements.

[0050] The binder 26 contained in the coating layer 25 is not particularly limited, as long as it serves to uniformly disperse the silicon nitride particles 21 and the boron nitride particles 22 and fix them as the coating layer 25. For example, an organic binder and an inorganic binder can be suitably selected and used as the binder 26 contained in the coating layer. One criterion for selecting the binder 26 is its heat resistance. That is, the binder 26 is selected with the desired heat resistance depending on the temperature at which the heat dissipation element 13 is used.

[0051] Examples of organic binders include, but are not limited to, epoxy resins, unsaturated polyester resins, phenolic resins, melamine resins, silicone resins, and polyimide resins. Among these, epoxy resins are preferred due to their good adhesion. Examples of epoxy resins include bisphenol-A epoxy resin, bisphenol-F epoxy resin, o-cresol novolac epoxy resin, phenol novolac epoxy resin, alicyclic aliphatic epoxy resin, and glycidylaminophenol-based epoxy resin. These resins can be used alone or in combination with two or more.

[0052] When epoxy resin is used as a thermosetting resin, examples of curing agents include: alicyclic anhydrides such as methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, and cis-5-norbornene-exo-2,3-dicarboxylic anhydride; aliphatic anhydrides such as dodecenyl succinic anhydride; aromatic anhydrides such as phthalic anhydride and trimellitic anhydride; organic dihydrazides such as dicyandiamide and adipic dihydrazide; tris-(dimethylaminomethyl)phenol; dimethylbenzylamine; 1,8-diazabicyclo(5,4,0)-undecene and derivatives thereof; and imidazoles such as 2-methylimidazole, 2-ethyl-4-methylimidazole, or 2-phenylimidazole. These curing agents can be used alone or in combination with two or more.

[0053] The amount of hardener to be mixed is determined appropriately according to the thermosetting resin to be used, the type of hardener, and the like, but in general the amount of hardener to be mixed is 0.1 parts by mass to 200 parts by mass in relation to 100 parts by mass of the thermosetting resin.

[0054] The coating layer 25 in the heat dissipation element 13 may contain an adhesion promoter to increase adhesion at the interface between the silicon nitride particles 21 and the boron nitride particles 22 and the cured product of the thermosetting resin. Examples of adhesion promoters are γ-glycidoxypropyltrimethoxysilane, N-β(aminoethyl)γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, and γ-mercaptopropyltrimethoxysilane. These adhesion promoters may be used alone or in combination.

[0055] The amount of adhesion promoter to be mixed is determined appropriately according to the thermosetting resin to be used, the type of adhesion promoter, and the like. Generally, the amount of adhesion promoter to be mixed is 0.01 parts by mass to 1 part by mass per 100 parts by mass of thermosetting resin.

[0056] The inorganic binder is preferably the liquid binder 26, which is compatible with the silicon nitride particles 21 and the boron nitride particles 22 and can be dispersed uniformly. Many inorganic binders have higher curing temperatures than organic binders, but from the perspective of processability and the prevention of denaturation by heat treatment of the base material 30, the curing temperature of the inorganic binder is lower than or equal to 250°C, preferably lower than or equal to 200°C, and even more preferably lower than or equal to 180°C. By using the inorganic binder with such a property, the coating layer 25 can be formed efficiently without thermal degradation of the base material 30.Examples of inorganic binders include, but are not limited to: sol-gel glass, organic-inorganic hybrid glass, water glass, inorganic one-component adhesives, and inorganic two-component adhesives. These can be used alone or in combination.

[0057] The base material 30 in the heat dissipation element 13 is not particularly limited, but is preferably a metal or ceramic with high thermal conductivity for efficient heat transfer from the heat-generating component 12. Examples of metals include aluminum, copper, stainless steel, iron, and other alloys. Examples of ceramics include aluminum oxide, magnesium oxide, zirconium dioxide, aluminum nitride, and silicon carbide. These can be used alone or in combination.

[0058] In the second embodiment, the heat dissipation element 13 comprises the base material 30 and the coating layer 25 containing the silicon nitride particles 21 and the boron nitride particles 22 with different thermal radiation spectra, as well as the binder 26. This composition is advantageous in achieving a higher average heat emission rate and better cooling performance than conventional compositions, such as the first embodiment. Examples

[0059] Details of the present disclosure are described below with reference to examples and comparative examples, but the present disclosure is not limited thereto. [Example 1]

[0060] The raw materials of the mixed powder are silicon nitride (Si3N4) powder, boron nitride (BN) powder, and a sintering aid. The silicon nitride powder has an average particle size of 0.1 µm, and the boron nitride powder also has an average particle size of 0.1 µm. Yttrium oxide (Y2O3) powder and aluminum oxide (Al2O3) powder, both with an average particle size of 1 µm, are used as sintering aids. The mixing ratio of the powders is 77 parts by mass of silicon nitride powder, 19 parts by mass of boron nitride powder, 3 parts by mass of yttrium oxide powder, and 1 part by mass of aluminum oxide powder. To 100 parts by mass of the mixed powder, 1 part by mass of polyoxyethylene lauryl ether as a dispersant, 1 part by mass of polyvinyl alcohol as a binder and 50 parts by mass of water are added and mixed with a ball mill for about 5 hours to produce a slurry.

[0061] The resulting slurry is then spray-dried to obtain granulated powder. The granulated powder is then placed into a radome-shaped mold, and CIP molding is performed using a cold isostatic press to produce a molded part. The pressure is 98 MPa.

[0062] The resulting molded body is then subjected to a degreasing treatment by heating it at 600°C in an air atmosphere for 2 hours. Afterwards, the degreased body is fired at 1900°C for 2 hours in a nitrogen atmosphere. The nitrogen gas pressure during firing is 0.9 MPa. In this way, the heat dissipation element 13 containing the ceramic heat-radiating material 20 is formed. [Example 2]

[0063] The process is similar to that in Example 1, except that the mixed powder contains silicon nitride powder with a mixture quantity of 86 parts by mass and boron nitride powder with a mixture quantity of 10 parts by mass. [Example 3]

[0064] The process is similar to that in Example 1, except that the mixed powder contains silicon nitride powder with a mixture quantity of 67 parts by mass and boron nitride powder with a mixture quantity of 29 parts by mass. [Example 4]

[0065] The process is similar to that in Example 1, except that the mixed powder contains silicon nitride powder with a mixture quantity of 58 parts by mass and boron nitride powder with a mixture quantity of 38 parts by mass. [Example 5]

[0066] After firing in a nitrogen atmosphere, the sintered body undergoes a heat treatment at 1300°C for 1 hour in air, i.e., an oxidation treatment, to form the metal oxide layer 23 on the surface of the sintered body. The other parts of the process are similar to those in Example 1. [Example 6]

[0067] The mixed powder consists of silicon nitride powder in a mixture of 71 parts by mass, boron nitride powder in a mixture of 18 parts by mass, and yttrium oxide powder in a mixture of 10 parts by mass. After firing in a nitrogen atmosphere, the sintered body is subjected to a heat treatment at 1300°C for 1 hour in air, i.e., an oxidation treatment, to form the metal oxide layer 23 on the surface of the sintered body. The other parts of the process are similar to those in Example 1. [Example 7]

[0068] The mixed powder consists of silicon nitride powder at a ratio of 67 parts by mass, boron nitride powder at a ratio of 17 parts by mass, and yttrium oxide powder at a ratio of 15 parts by mass. After firing in a nitrogen atmosphere, the sintered body undergoes a heat treatment at 1300°C for 1 hour in air, i.e., an oxidation treatment, to form the metal oxide layer 23 on the surface of the sintered body. The other parts of the process are similar to those in Example 1. [Example 8]

[0069] The mixed powder consists of silicon nitride powder in a mixture of 63 parts by mass, boron nitride powder in a mixture of 16 parts by mass, and yttrium oxide powder in a mixture of 20 parts by mass. After firing in a nitrogen atmosphere, the sintered body undergoes a heat treatment at 1300°C for 1 hour in air, i.e., an oxidation treatment, to form the metal oxide layer 23 on the surface of the sintered body. The other parts of the process are similar to those in Example 1. [Comparison example 1]

[0070] The process is similar to that in Example 1, except that the mixed powder contains silicon nitride powder with a mixture quantity of 96 parts by mass and boron nitride powder with a mixture quantity of 0 parts by mass. [Comparative example 2]

[0071] The process is similar to that in Example 1, except that the mixed powder contains silicon nitride powder with a mixture quantity of 91 parts by mass and boron nitride powder with a mixture quantity of 5 parts by mass. [Comparative example 3]

[0072] The process is similar to that in Example 1, except that the mixed powder contains silicon nitride powder with a mixture quantity of 48 parts by mass and boron nitride powder with a mixture quantity of 48 parts by mass.

[0073] The porosity of the heat dissipation element 13, including the ceramic heat radiation material 20 obtained in each of Examples 1 to 8 and Comparison Examples 1 to 3, is measured. The porosity is calculated using Archimedes' principle as described above.

[0074] For the heat dissipation element 13 including the ceramic heat radiation material 20 obtained in each of Examples 1 to 8 and Comparative Examples 1 to 3, (1) the cooling capacity of the heat dissipation element 13, (2) mechanical strength, (3) thermal conductivity and (4) average emissivity are evaluated. (1) Cooling capacity of the heat dissipation element 13

[0075] A ceramic heater is attached to a side surface of the ceramic radiant heating material 20, which has a length of 100 mm, a width of 100 mm, and a thickness of 7 mm. A power of 20 W is continuously applied to the attached ceramic heater for several hours until the temperature of the ceramic radiant heating material 20 and the ceramic heater reaches a saturation temperature. The surface temperature of the ceramic heater is then measured using a thermocouple. The saturation temperature of the ceramic heater, corresponding to the power input of 20 W, is the cooling capacity of the heat dissipation element 13. The lower the saturation temperature, the higher the cooling capacity of the heat dissipation element 13. (2) Mechanical strength

[0076] The three-point bending strength is measured as the mechanical strength of the heat dissipation element 13. The three-point bending strength is measured using a universal testing machine. At this point, the ceramic heat radiation material 20 is cut into a test piece with a length of 4 mm, a width of 3 mm, and a span of 40 mm. (3) Thermal conductivity

[0077] The thermal conductivity is measured using a laser flash method. At this point, the ceramic thermal radiation material 20 is cut into a test piece with a diameter of 10 mm and a thickness of 1 mm. (4) Average emission level

[0078] The average emissivity is determined by measuring the emissivity in each of the wavelength ranges from 3 µm to 25 µm using an emissivity meter and calculating the average emissivity across all wavelength ranges. At this point, the ceramic thermal radiation material 20 is cut into a test piece with a length of 20 mm, a width of 20 mm, and a thickness of 2 mm.

[0079] Fig. Figure 5 is a diagram illustrating examples of raw materials, ceramic heat radiation materials, and heat dissipation element properties according to Examples 1 to 8 and Comparative Examples 1 to 3. The raw materials entry shows the mass percentage of silicon nitride powder, boron nitride powder, and sintering aid comprising the powder raw material, and the mass parts of dispersion agent, binder, and water per 100 mass parts of the powder raw material. The ceramic heat radiation material entry 20 shows the total content of silicon nitride and boron nitride, the mass ratio of silicon nitride and boron nitride, the porosity of the ceramic heat radiation material 20, and the presence or absence of the metal oxide layer 23. The properties entry shows the results of the four evaluation points mentioned above.The four evaluation points are the mechanical strength [MPa], the thermal conductivity [W / (m K)], the average emissivity [%] of the ceramic thermal radiation material 20 in the wavelength ranges from 3 µm to 25 µm and the cooling capacity of the heat dissipation element 13, i.e. the saturation temperature [°C] corresponding to the power input of 20 W.

[0080] As in Fig.As shown in Figure 5, the heat dissipation elements 13 of Examples 1 to 8 have a high average emissivity greater than or equal to 75%. The saturation temperature corresponding to a power input of 20 W is in the range of 120°C to 133°C. The porosity of Examples 1 to 8 is in the range of 12% to 39%. The mechanical strength is in the range of 152 MPa to 309 MPa. The thermal conductivity is in the range of 29 W / (m K) to 51 W / (m K). The heat dissipation element 13 with the metal oxide layer 23 tends to have a higher average emissivity than the heat dissipation element 13 without the metal oxide layer 23, and as a result, tends to have a lower saturation temperature corresponding to a power input of 20 W than the heat dissipation element 13 without the metal oxide layer 23.Furthermore, Examples 1, 3, 5, 6, 7 and 8, where the mass ratio of boron nitride powder to the total content of silicon nitride powder and boron nitride powder is 20 wt% to 30 wt%, exhibit a thermal conductivity of more than 30 W / (m K) and an average emissivity of more than 80%, i.e., they show high values ​​for both thermal conductivity and average emissivity compared to Example 2, where the mass ratio of boron nitride powder to the total content of silicon nitride powder and boron nitride powder is 10 wt%, and to Example 4, where the mass ratio of boron nitride powder to the total content of silicon nitride powder and boron nitride powder is 40 wt%.Therefore, in order to improve both the emissivity and the thermal conductivity with respect to the cooling performance of the heat dissipation element 13, the mass ratio of boron nitride powder to the total content of silicon nitride powder and boron nitride powder is desirablely 20 wt% to 30 wt%.

[0081] On the other hand, the heat dissipation elements 13 of comparison examples 1 and 2 have an average emissivity of approximately 65% ​​and a saturation temperature corresponding to a power input of 20 W in the range of 156°C to 168°C. This may be because no boron nitride is present, as in comparison example 1, or because boron nitride is present, but the amount is small, as in comparison example 2. That is to say, it is assumed that if the Si3N4:BN ratio is in the range of 90:10 to 100:0, the average emissivity is low compared to the cases in examples 1 to 8, and as a result, the heat dissipation element 13 has a lower cooling capacity.

[0082] The heat dissipation element 13 of comparative example 3 has an average emissivity of approximately 80%, but exhibits a saturation temperature of 148°C corresponding to a power input of 20 W, which is higher than in examples 1 to 8. This may be because the amount of BN (non-metallic resin) it contains is higher than in examples 1 to 8, resulting in a high porosity of 53%. This means that heat from the heat-generating element is not efficiently transferred to the heat-dissipating element 13, resulting in an extremely low thermal conductivity compared to examples 1 to 8. Consequently, the heat dissipation element 13 in comparative example 3 is expected to have a lower cooling capacity. Furthermore, the high porosity results in extremely low mechanical strength, so there is a high probability of fractures or cracks occurring when the heat dissipation element 13 is used.

[0083] As described above, in order for the heat dissipation element 13 to exhibit a higher cooling capacity than in Comparative Examples 1 to 3, the mass ratio of the boron nitride particles 22 to the mass of the silicon nitride particles 21 and the boron nitride particles 22 of the ceramic thermal radiation material 20 of which the heat dissipation element 13 is composed should be 10 wt% to 40 wt%. Referring to the results of Examples 1 to 8 and Comparative Examples 1 to 3, the heat dissipation element 13, which has an average emissivity of 70% or higher in the wavelength ranges from 3 µm to 25 µm, can exhibit a higher cooling capacity at temperatures up to 200°C. In this case, the porosity is preferably 10% to 40%. Furthermore, if a rare earth oxide such as yttrium oxide powder is used as a sintering aid, the amount of rare earth oxide contained only needs to be 3% to 20% by mass.Under these conditions it is possible to provide the heat dissipation element 13 with a high average emissivity and good cooling performance.

[0084] The configurations described in the embodiments mentioned above show examples of the content of this disclosure. The configurations can be combined with other known techniques, and some of the configurations can be omitted or modified in an area that does not deviate from the core of this disclosure. List of reference symbols 1 electrical / electronic device; 10 cases; 11 Substrate; 12. Heat-generating component; 13 Heat dissipation element; 20 ceramic heat radiation material; 21 silicon nitride particles; 22 boron nitride particles; 23 Metal oxide layer; 25 coating layers; 26 binders; 30 basic materials.

Claims

Heat dissipation element (13) comprising a ceramic thermal radiation material (20), wherein the ceramic thermal radiation material (20) contains silicon nitride and boron nitride as main components, and the ratio of a mass of boron nitride to a mass of silicon nitride and boron nitride is 10 mass-% to 40 mass-%, the boron nitride has an average particle size of 0.05 µm to 1 µm, and the ceramic thermal radiation material (20) has an average emissivity of higher than or equal to 70% in wavelength ranges from 3 µm to 25 µm at temperatures up to 200°C. Heat dissipation element (13) according to claim 1, wherein the silicon nitride and the boron nitride of the ceramic heat radiation material (20) are uniformly dispersed. Heat dissipation element (13) according to claim 1 or 2, wherein the heat dissipation element (13) does not contain resin. Heat dissipation element (13) according to one of claims 1 to 3, wherein the boron nitride is hexagonal boron nitride. Heat dissipation element (13) according to one of claims 1 to 4, wherein the ceramic heat radiation material (20) is a sintered body containing silicon nitride particles (21) and boron nitride particles (22). Heat dissipation element (13) according to one of claims 1 to 4, further comprising a base material (30), wherein the ceramic heat radiation material (20) is a coating layer (25) with which a surface of the base material (30) is coated. Heat dissipation element (13) according to claim 6, wherein the coating layer (25) comprises a filler including the ceramic heat radiation material (20) and a binder (26). Heat dissipation element (13) according to one of claims 1 to 7, wherein the ceramic heat radiation material (20) has a thermal conductivity of greater than or equal to 40 W / (m K). Heat dissipation element (13) according to one of claims 1 to 8, wherein the ceramic heat radiation material (20) contains 3 mass-% to 20 mass-% rare earth oxide. Heat dissipation element (13) according to one of claims 1 to 9, further comprising a metal oxide layer (23) on a part of a surface of the ceramic heat radiation material (20). Heat dissipation element (13) according to claim 10, wherein the metal oxide layer (23) is a rare earth silicate represented by R2Si2O7, where R is a rare earth element. Heat dissipation element (13) according to claim 10 or 11, wherein the metal oxide layer (23) is a layer formed on the surface of the ceramic heat radiation material (20) by oxidizing the ceramic heat radiation material (20) at high temperature in air. Cooling sink comprising the heat dissipation element (13) according to one of claims 1 to 12 . Heat sink according to claim 13, wherein a surface of the heat dissipation element (13) has an uneven part with a height difference greater than or equal to 25 µm.

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