CN115725273A

CN115725273A - Diamond-based heat-conducting filler, preparation method thereof, composite heat-conducting material and electronic equipment

Info

Publication number: CN115725273A
Application number: CN202110991018.2A
Authority: CN
Inventors: 徐焰; 方浩明; 傅博
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2021-08-26
Filing date: 2021-08-26
Publication date: 2023-03-03
Also published as: WO2023024570A1

Abstract

The application provides a composite heat conduction material, which comprises an organic matrix, a heat conduction filler and an adhesion medium. The thermally conductive filler includes a plurality of large-particle-size particles and a plurality of small-particle-size particles distributed in the organic matrix. The average particle diameter of the small-particle-diameter particles is smaller than the average particle diameter of the large-particle-diameter particles. The plurality of small-particle-size particles includes a plurality of first small-particle-size particles and a plurality of second small-particle-size particles. The first small-particle-size particles are adhered to the surfaces of the large-particle-size particles, and the second small-particle-size particles are randomly distributed in the organic matrix. The bonding medium is attached to the surface of the large-particle-diameter particle so that the first small-particle-diameter particle is bonded to the surface of the large-particle-diameter particle through the bonding medium. The application also provides electronic equipment comprising the composite heat conduction material, a diamond-based heat conduction filler and a preparation method thereof. The composite heat conduction material has high heat conductivity coefficient and maintains good applicability.

Description

Diamond-based heat-conducting filler, preparation method thereof, composite heat-conducting material and electronic equipment

Technical Field

The application relates to the field of heat conduction materials, in particular to a composite heat conduction material, a diamond-based heat conduction filler in the composite heat conduction material, a preparation method of the diamond-based heat conduction filler and electronic equipment applying the composite heat conduction material.

Background

Heat generated by heat-generating power devices, such as chips, in electronic equipment is usually dissipated to the outside by means of a heat sink. From a microscopic view, the contact interface between the chip and the heat sink is uneven, and the interface heat conduction material is usually filled between the chip and the heat sink, so that the contact heat resistance is reduced. The interface heat conduction material generally comprises heat conduction silicone grease, a heat conduction pad, heat conduction gel, phase change heat conduction material, heat conduction glue and the like; and interface heat conduction materials of different types and different heat conduction coefficients can be used according to different application scenes.

The interface heat conduction material can adopt composite heat conduction material, and in order to improve the heat conductivity coefficient of the composite heat conduction material, the conventional method adopts heat conduction filler with higher heat conductivity coefficient. Currently, when an alumina filler system with a thermal conductivity of about 27W/mk is adopted, the highest thermal conductivity of the thermal conductive gel is about 6W/mk when the weight percentage of the filler content is close to 96% and the volume percentage is close to 86%. When the aluminum nitride filler system is adopted, the thermal conductivity of the thermal conductive gel can be improved to 10-12W/mk when the weight percentage of the filler content is close to 96% and the volume percentage is close to 86%, and the capacity limit of the aluminum nitride filler system is close to. The artificial diamond powder has the insulation characteristic, the heat conductivity coefficient is usually over 1000W/mk, and theoretically, the heat conductivity coefficient of the heat-conducting gel can be improved to over 12W/mk by adopting the diamond filler. However, the diamond heat-conducting filler is easy to agglomerate, and when the volume percentage of the diamond heat-conducting filler exceeds 15%, the defects of incapability of dispersing, poor colloid gelling molding, easy phase separation and the like can occur, so that the back-end processing cannot be performed. And the improvement effect of the small amount of the diamond heat-conducting filler on the heat conductivity of the heat-conducting gel is not great, so that the heat conductivity of the heat-conducting gel is difficult to improve to more than 12W/mk.

Disclosure of Invention

A first aspect of an embodiment of the present application provides a composite heat conduction material, including:

an organic matrix;

a thermally conductive filler comprising:

a plurality of large-sized particles distributed in the organic matrix;

a plurality of small-sized particles, and,

the average particle size of the small-particle size particles is smaller than the average particle size of the large-particle size particles, and the plurality of small-particle size particles include a plurality of first small-particle size particles and a plurality of second small-particle size particles; the first small-particle-size particles are adhered to the surfaces of the large-particle-size particles, and the second small-particle-size particles are randomly distributed in the organic matrix;

and the bonding medium is attached to the surfaces of the large-particle-size particles so that the first small-particle-size particles are bonded to the surfaces of the large-particle-size particles through the bonding medium.

The composite heat conduction material of the first aspect of the present application can greatly reduce the probability of microscopic voids appearing inside the composite heat conduction material by closely combining the first small-particle-size particles on the surface of the large-particle-size particles, so that the composite heat conduction material has a better heat conduction coefficient. The microscopic heat conduction path structure of the composite heat conduction material is formed by mutual contact or lap joint of large-particle-size particles, bonding medium, first small-particle-size particles and other heat conduction filler particles (such as another large-particle-size particle).

In an embodiment of the present invention, the large-sized particles have an average particle size of 20 μm or more.

In an embodiment of the present application, the average particle size of the small-particle-size particles is 10 μm or less.

In an embodiment of the present application, the thermal conductivity of the large-sized particles is higher than that of the small-sized particles.

The thermal conductivity coefficient of the large-particle-size particles is usually the highest thermal conductivity coefficient in the thermal conductive filler, wherein the large-particle-size particles play a main thermal conductive role in the composite thermal conductive material, and the small-particle-size particles play a key role in rheological property and stability of the composite thermal conductive material.

In an embodiment of the present application, the large-sized particles are diamond particles.

The large-particle-size particles can be artificial diamond and graphitized diamond; the artificial diamond is preferably raw material, either polycrystal or monocrystal, usually has a thermal conductivity of over 600W/mk and a polyhedral morphology; the large-particle-size particles can also use crushed artificial diamond, and the thermal conductivity of the diamond filler is about 200-400W/mk and is in an irregular particle shape.

In an embodiment of the present application, the nitrogen content of the diamond particles is 500ppm or less.

Generally, the lower the nitrogen content of diamond, the higher the thermal conductivity thereof, and thus controlling the nitrogen content of the diamond particles to 500ppm or less ensures a higher thermal conductivity thereof.

In an embodiment of the present invention, the material of the small-particle-size particles is at least one selected from the group consisting of an oxide, a nitride, a carbide, a metal, and a carbon material.

When the composite heat conduction material has the requirement on insulation, the small-particle-size particles are selected from oxides, carbides and nitrides, and the oxides can comprise aluminum oxide, zinc oxide and the like; the nitride includes boron nitride, silicon nitride, and the like; the carbide includes silicon carbide and the like; when the composite heat conduction material does not have the requirement on insulation, the small-particle-size particles can also be made of metals such as aluminum, silver, gold, tin, copper, indium and the like and metal compounds thereof. The material of the small particle size particles may also be a carbon material such as graphite, graphene, carbon fiber, and the like.

In an embodiment of the present application, the organic matrix is selected from at least one of a silicone system, an epoxy system, an acrylic system, a polyurethane system, and a polyimide system.

In embodiments herein, the organic matrix is selected from addition polymerization curing type silicone systems.

In an embodiment of the present application, the adhesive medium is an inorganic adhesive material.

The inorganic adhesive can be an adhesive assistant such as silicate, and is coated after a solvent is added to adjust viscosity.

In the embodiment of the application, the bonding medium is an organic bonding material, the same polymer system is selected for the bonding medium and the organic matrix, and the molecular weight of the bonding medium is lower than that of the organic matrix.

In an embodiment of the present invention, a ratio of the large-particle-size particles to the small-particle-size particles is greater than 20.

The larger the difference between the particle sizes of the large-particle-size particles and the small-particle-size particles is, the more the first small-particle-size particles can adhere to the surface of the same large-particle-size particle, which generally increases the packing density of the heat conductive filler in the composite heat conductive material.

In the embodiment of the present application, the heat conductive filler further includes medium-sized particles, and the particle size of the medium-sized particles is smaller than the particle size of the large-sized particles and larger than the particle size of the small-sized particles.

The average particle diameter of the medium-diameter particles is preferably 10 to 20 μm, and usually 60 μm or less. The kind of the medium-size particles can be at least one of carbide, nitride, oxide and metal powder.

In the present embodiment, the specific surface area of the small-particle-size particles is greater than 1m ² /g。

The larger the specific surface area of the small-sized particles, the smaller the particle size is generally represented.

In the embodiment of the application, the large-particle-size particles are subjected to surface treatment to increase the hydroxyl content on the surfaces of the large-particle-size particles, and the surface oxygen content of the large-particle-size particles is more than 10%.

Before adding the large-particle-size particles to the organic matrix, performing surface treatment on the diamond particles, such as acidification, oxidation, oxide deposition or aluminum nitride deposition, to enable the surfaces of the diamond particles to contain more active functional groups, such as hydroxyl groups, so that the oxygen content of the surfaces of the diamond particles is increased. Such large particle size particles with more reactive functional groups can be more compatible with organic substrates, such as silicone oils, for high loading mixing.

In the present embodiment, the surface of the small-particle-size particles contains hydroxyl groups.

The small-particle-size particles are selected from oxide systems with a large number of hydroxyl groups on the surface, such as aluminum oxide, zinc oxide and iron oxide. Preferably the small particle size particles have a surface oxygen content of greater than 30% as measured using X-ray photoelectron spectroscopy.

In the embodiment of the present application, the total weight percentage of the heat conductive filler is 87% or more.

In the embodiment of the application, the volume percentage of all the heat-conducting fillers in the composite heat-conducting material is more than 76%.

In the embodiment of the present invention, the number of small-sized particles bonded to the surface of the large-sized particles by the adhesive agent is 50% or less of the number of small-sized particles having a particle size of 10 μm or less in the heat conductive filler.

A second aspect of the embodiments of the present application provides an electronic device, which includes an electronic component and a cured product of the composite thermal conductive material according to the first aspect of the embodiments of the present application, disposed on the electronic component.

In an embodiment of the present application, the electronic component is a chip, the electronic device further includes a heat sink disposed on the electronic component, an interface heat conduction material is disposed between the electronic component and the heat sink, and the interface heat conduction material is a cured product of the composite heat conduction material according to the first aspect of the embodiment of the present application.

The composite heat conduction material has high heat conductivity coefficient and good applicability, and can be used as an interface heat conduction material to ensure that the heat dissipation effect of an electronic element is good.

A third aspect of the embodiments of the present application provides a composite heat conductive material, including:

an organic matrix;

a thermally conductive filler distributed in the organic matrix, the thermally conductive filler comprising:

a plurality of large particle size particles;

the self-fusion heat conduction particles are alloy particles or metal particles, and can be fused with each other at a temperature not higher than the curing reaction temperature of the organic matrix so as to form metal bond combination with the large-particle-size particles.

In the using process, the composite heat conduction material is heated to a specified temperature, the self-fusion heat conduction particles are combined with each other, and meanwhile, metal bond combination is formed between the fused self-fusion heat conduction particles and the particles with large particle size. The microscopic heat conduction path structure is the combination or close combination of large-particle-size particles (such as diamond) and other heat conduction fillers (self-fusion heat conduction particles), and has a certain probability of forming the combination of the large-particle-size particles (such as diamond) and the self-fusion heat conduction particles or another large-particle-size particle (such as diamond) instead of being bonded through an organic bonding medium (such as silicone oil). Different large-particle-size particles can be mutually connected through the fused self-fused heat-conducting particles to form a heat-conducting passage. The self-fusion filling scheme can reduce the interface thermal resistance in the system in an order of magnitude, and can realize larger or longer heat conduction path with ultra-low interface thermal resistance.

In an embodiment of the present application, the thermal conductivity of the large-sized particles is higher than the thermal conductivity of the self-fused thermal conductive particles.

In an embodiment of the present application, the large-sized particles have an average particle size of more than 5 μm.

The smaller the particle size of the metal particles, the better the feasibility and extent of the self-fusion reaction of the metal particles in general. In an embodiment of the present application, the metal particles include at least one of nano silver, nano copper, nano gold, micro silver, micro copper, and micro gold.

In an embodiment of the present invention, the sintering temperature of the metal particles is 120 to 250 degrees.

In the embodiment of the application, the surface of the metal particle is coated with a surfactant.

In order to ensure good reliability of the metal particles before sintering, it is generally necessary to coat the metal particles with a surfactant, such as polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), sodium Dodecyl Sulfate (SDS), oleic acid, etc. The surfactant is volatile at a temperature, such as during a curing reaction, so as not to interfere with the mutual fusion of the metal particles.

In an embodiment of the present application, the alloy particles have a melting point of 150 degrees or less.

In the embodiment of the present application, the first and second, the alloy particles are at least one of Sn-Cu, sn-Al, sn-Zn, sn-Pt, sn-Mn, sn-Mg, sn-Ag, sn-Au, sn-Bi, sn-In, sn-Pd, sn-Bi-In, bi-Pb-Sn, bi-Pb, al-Li, ga-In-Sn, ga-In, ga-Bi-Pb-In and Zn-Li.

In the embodiment of the present application, an adhesive medium layer is provided on the surface of the large-size particles to combine the large-size particles with the melted self-fusing heat-conducting particles.

In the embodiment of the application, when the self-fusing heat conducting particles are metal particles, the bonding medium layer is made of a metal material of the metal particles; when the self-fusion heat conduction particles are alloy particles, the material of the bonding medium layer is an intermetallic compound of at least one metal in the alloy particles. In the embodiment of the present application, when the self-fusing heat conducting particles are metal particles, the volume ratio of the self-fusing heat conducting particles in the heat conducting filler is 50% to 85%.

In the embodiment of the present application, when the self-fusing heat conducting particles are alloy particles, the volume ratio of the self-fusing heat conducting particles in the heat conducting filler is less than 50%.

In the use process of electronic equipment, in order to avoid the failure problems of interface delamination or internal cracking of the cured composite heat conduction material, the self-fusing heat conduction particles need to be controlled to be in a proper amount.

In the embodiment of the application, the thermal conductivity of the cured composite thermal conductive material is more than 5 times higher than that of the cured composite thermal conductive material.

A fourth aspect of the embodiments of the present application provides a heat conductive material which is a cured product of the composite heat conductive material according to the third aspect of the embodiments of the present application, wherein the heat conductive material includes a fused body formed by fusing the self-fusing heat conductive particles with each other, and the fused body is bonded to the large-particle-diameter particles through a metallic bond.

A fifth aspect of the embodiments of the present application provides an electronic device, including an electronic component and the heat conductive material according to the fourth aspect of the embodiments of the present application disposed on the electronic component.

In the embodiment of the present application, the electronic device further includes a heat sink disposed on the electronic component, an interface heat conduction material is disposed between the electronic component and the heat sink, and the interface heat conduction material is the heat conduction material in the fourth aspect of the embodiment of the present application.

The composite heat conduction material has high heat conductivity coefficient, and can be used as an interface heat conduction material, so that the heat dissipation effect of the electronic element is good.

A sixth aspect of the embodiments of the present application provides a diamond-based thermally conductive filler, including:

diamond particles, the diamond particles being in the shape of a polyhedron;

a plurality of small heat conductive particles coated on the surface of the diamond particles, the small heat conductive particles having a particle size smaller than that of the diamond particles, the diamond particles being formed in a shape that tends to be spherical in combination with the plurality of small heat conductive particles coated thereon;

and the bonding medium is positioned between the diamond particles and the small heat conduction particles and between the small heat conduction particles so that the plurality of small heat conduction particles coat the surfaces of the diamond particles.

According to the diamond-based heat-conducting filler, the small heat-conducting particles are coated on the surfaces of the diamond particles, so that the whole diamond-based heat-conducting filler tends to be spherical, the surface appearance of polyhedral diamond filler particles is improved, and the problems of serious abrasion of a dispensing component and frictional damage of a chip heat dissipation component caused by the factor of overhigh surface hardness are solved; meanwhile, the problems that the heat conduction material prepared by adopting the diamond filler with high filler content has poor liquidity and the compression stress is overlarge when the material is pressed to be thin in the application process are solved.

In an embodiment of the present application, a ratio of the average particle size of the diamond particles to the average particle size of the small thermally conductive particles is greater than 20.

In the embodiments of the present application, the material of the small thermally conductive particles is selected from at least one of an oxide, a nitride, a carbide, a metal, and a carbon material.

The small heat conducting particles can be selected from oxides, carbides and nitrides; the oxide may include aluminum oxide, zinc oxide, and the like; the nitride includes boron nitride, silicon nitride, and the like; the carbide includes silicon carbide and the like. The small heat conducting particles can also be made of metals such as aluminum, silver, gold, tin, copper, indium and the like and metal compounds thereof. The material of the small thermally conductive particles may also be a carbon material, such as graphite, graphene, carbon fiber, and the like.

In an embodiment of the present application, the small thermally conductive particles have an average particle diameter of 10 μm or less.

In an embodiment of the present application, the adhesive medium is an inorganic adhesive medium.

In an embodiment of the present invention, the adhesive medium is an organic adhesive medium, and the adhesive medium contains a coupling agent added to be compatible with the organic adhesive medium.

The coupling agent is used for realizing good infiltration of the bonding medium and the surface of the diamond particles.

In the embodiment of the application, the thickness of the bonding medium coating the diamond particles does not exceed the thickness of the small heat conducting particles coating the diamond particles.

The thickness of the bonding medium covering the diamond particles does not exceed the thickness of the small heat conducting particles covering the diamond particles, so that the condition that the outer surface of the diamond-based heat conducting filler is mainly the bonding medium with low heat conducting performance but not the heat conducting particles with high heat conducting performance is avoided. The most ideal situation is that in the manufacturing processes of granulation, calcination and the like of the diamond-based heat-conducting filler, the bonding medium is well filled at the part close to the diamond particles, and the bonding medium is poorly filled at the exposed part of the diamond-based heat-conducting filler, namely, the bonding medium does not fill the small heat-conducting particles at the outermost layer, so that the uneven outer surface of the diamond-based heat-conducting filler is formed by the small heat-conducting particles which are relatively raised.

In the present embodiment, the plurality of small thermally conductive particles consists of a plurality of particles having an average particle size of <10 μm or a mixture of a plurality of particle size distributions having an average particle size of <10 μm.

In the embodiment of the application, the thermal conductivity of the diamond particles is more than 600W/mk, and the diamond particles are artificial diamond or graphitized diamond.

In the present embodiment, the sphericity of the shape of the combination of the diamond particles and the plurality of small heat conductive particles covering the diamond particles is 0.7 or more.

In the embodiment of the application, the surface oxygen content of the diamond-based heat-conducting filler is more than 5%.

Because the surfaces of the diamond particles are coated with the alumina and the organic adhesive, the oxygen content of the surfaces of the fillers is obviously higher than that of the original diamond particles, so that the uniform dispersion in an organic matrix can be easily realized, and the phase separation is avoided.

In the embodiment of the application, the outer surface of the diamond-based heat-conducting filler is formed by exposed small heat-conducting particles.

In the embodiment of the application, the thickness of the diamond particles coated by the small heat conduction particles is not more than 10 times of the particle size of the small heat conduction particles.

In the embodiment of the application, the outer surface of the diamond-based heat-conducting filler has a concave-convex fluctuating microstructure, and the specific area of the diamond-based heat-conducting filler is more than 3 times of the specific area of the diamond particles.

The specific area of the diamond-based heat-conducting filler can be effectively increased by coating a plurality of small heat-conducting particles on the surfaces of the diamond particles.

A seventh aspect of the present embodiment provides a composite heat conductive material, including an organic matrix and the diamond-based heat conductive filler according to the sixth aspect of the present embodiment dispersed in the organic matrix.

An eighth aspect of the present embodiment provides an electronic device, including an electronic component and a cured product of the composite thermal conductive material according to the seventh aspect of the present embodiment, provided on the electronic component.

In an embodiment of the present application, the electronic device further includes a heat sink disposed on the electronic component, an interface heat conduction material is disposed between the electronic component and the heat sink, and the interface heat conduction material is a cured product of the composite heat conduction material according to the sixth aspect of the embodiment of the present application.

A ninth aspect of the present application provides a method for preparing a diamond-based thermally conductive filler, including:

dispersing a plurality of diamond particles and a plurality of small heat conducting particles in a bonding medium to form slurry, wherein the diamond particles are polyhedral, and the particle size of the small heat conducting particles is smaller than that of the diamond particles;

granulating and balling by using the slurry, so that the small heat-conducting particles are coated on the surfaces of the diamond particles through the bonding medium;

and removing the redundant bonding medium to combine the small heat-conducting particles with the diamond particles.

Drawings

Fig. 1 is a schematic diagram of a chip provided with a heat sink.

Fig. 2 is a schematic diagram of a packaged chip.

Fig. 3 is a schematic view of a composite heat conductive material according to a first embodiment of the present application.

Fig. 4 is a schematic view of a composite heat conductive material according to a second embodiment of the present application.

Fig. 5 is a schematic process diagram of the processing of the heat conductive filler in the composite heat conductive material according to the second embodiment of the present application.

Fig. 6 is a schematic view of a diamond-based thermally conductive filler contained in a composite thermally conductive material according to a third embodiment of the present application.

Fig. 7 is a flow chart of the preparation of the composite heat conductive material according to the third embodiment of the present application.

Description of the main elements

Circuit board 51

Chip 53

Heat sink 55

Interfacial thermally conductive material 57

Heat dissipation cover 59

Composite heat

conductive material

100a, 100b

Organic matrix 10

Large-size particles 31

Adhesive medium 32

Small particle size particles 33

First small-sized particles 33a

Second small-sized particles 33b

Medium size particles 35

Self-fusing thermally conductive particles 37

Adhesive medium layer 34

Diamond-based heat-conducting filler 200

Diamond particles 20

Small thermally conductive particles 40

Detailed Description

The embodiments of the present application will be described below with reference to the drawings. The data ranges referred to in this application are to be understood as being inclusive, unless specifically stated otherwise.

Electronic devices are often provided with a large number of heat generating electronic components, such as chips. High temperatures can have deleterious effects on the stability, reliability and life of electronic components, such as excessive temperatures that can compromise semiconductor junctions, damage circuit connections, increase conductor resistance and cause mechanical stress damage. As shown in fig. 1, a heat generating power device or a heat generating module is disposed on a circuit board 51, and in this embodiment, the heat generating power device is taken as an example of a chip 53, and a heat sink 55 is disposed on the chip 53. However, a fine uneven gap is generally present between the contact interfaces of the chip 53 and the heat sink 55, and if the chip 53 and the heat sink 55 are directly mounted together, a lot of air gaps are present between the chip 53 and the heat sink 55. Since the thermal conductivity of air is only 0.024W/(m · K), which is a poor conductor of heat, the thermal contact resistance between the electronic component and the heat sink is very large, which seriously hinders the heat conduction, and ultimately results in low performance of the heat sink 55. Therefore, the interface heat conduction material 57 is filled between the chip 53 and the heat sink 55 to eliminate the air gap between the chip 53 and the heat sink 55, and an effective heat conduction channel is established between the chip 53 and the heat sink 55, so that the contact thermal resistance can be greatly reduced, and the function of the heat sink 55 can be fully exerted.

The chip 53 may be a bare chip or a Ball Grid Array (BGA) packaged chip with a heat dissipation cover disposed thereon. As shown in fig. 2, the BGA package chip also has an interface thermal conductive material 57 filled between the chip 53 and the heat dissipation cover 59, where the interface thermal conductive material 57 is used to reduce the thermal contact resistance between the chip 53 and the heat dissipation cover 59, so that the heat generated by the chip 53 can be effectively conducted to the heat dissipation cover 59.

However, if the above-mentioned interface thermal conductive material 57 has poor heat conduction effect, the heat dissipation effect of the chip 53 will be greatly affected.

Therefore, the present application provides a composite thermal conductive material with high thermal conductivity and maintaining good applicability, which can be used as an interface thermal conductive material.

First embodiment

As shown in fig. 3, the composite thermal conductive material 100a according to the first embodiment of the present application includes an organic matrix 10 and a thermal conductive filler distributed in the organic matrix 10. The thermally conductive filler includes a plurality of large-sized particles 31 and a plurality of small-sized particles 33. The large-sized particles 31 are randomly and irregularly distributed in the organic matrix 10. The plurality of small-sized particles 33 includes a plurality of first small-sized particles 33a and a plurality of second small-sized particles 33b. The first small-sized particles 33a are bonded to the surfaces of the large-sized particles 31 by the adhesive medium 32, and the second small-sized particles 33b are randomly distributed in the organic matrix 10. The second small-sized particles 33b randomly distributed in the organic matrix 10 are not bonded by the bonding medium 32 and thus are not bonded to the surfaces of the plurality of large-sized particles 31. The adhesive agent 32 is attached to the surface of the large-sized particles 31 so that the first small-sized particles 33a are adhered to the surface of the large-sized particles 31 by the adhesive agent 32.

Generally, in the process of stirring and dispersing the organic matrix and the heat-conducting filler, local enrichment or local agglomeration is easy to occur between small-particle-size particles, a phase separation phenomenon occurs between large-particle-size particles and small-particle-size particles, and local gaps/holes are easy to exist between part of large-particle-size particles and peripheral filler particles thereof, and between large-particle-size particles and the organic matrix. And the gaps/holes in the composite heat conduction material can greatly reduce the heat conductivity coefficient, so that the reduction of the porosity of the composite heat conduction material is beneficial to improving the heat conductivity coefficient, and the lower the porosity is, the better the thermal conductivity coefficient is theoretically.

The composite heat conduction material 100a is tightly combined on the surface of the large-particle-size particle 31 through the first small-particle-size particle 33a, so that the probability of local gaps existing around the large-particle-size particle 31 is remarkably reduced, namely, the probability of microscopic gaps occurring inside the composite heat conduction material 100a is greatly reduced, and therefore the composite heat conduction material 100a has a better heat conduction coefficient.

In the composite heat conductive material 100a of the present application, the microscopic heat conductive path structure is formed by contacting or overlapping large-sized particles 31 (e.g., diamond), a bonding medium 32, small-sized particles 33, and other heat conductive filler particles (e.g., another large-sized particle 31).

The large-size particles 31 have an average particle diameter larger than that of the small-size particles 33, and are generally the largest in the thermally conductive filler. In some embodiments, for example, when the composite heat conductive material 100a is used as the interface heat conductive material 57 shown in fig. 1, the average particle size of the large-particle-size particles 31 is 20 μm or more. In some embodiments, the large-sized particles 31 have an average particle size of 40 μm to 250 μm; in other embodiments, the large-sized particles 31 have an average particle size of 60 μm to 160 μm.

In some embodiments, the large-sized particles 31 have a higher thermal conductivity than the small-sized particles 33; usually the highest thermal conductivity of the thermally conductive filler. The large-particle size particles 31 may be diamond particles such as artificial diamond, graphitized diamond. The artificial diamond is preferably raw material, polycrystalline or single crystal, and has a thermal conductivity of over 600W/mk and a polyhedral morphology. The large-particle-size particles 31 can also be made of crushed artificial diamond, and the diamond filler has a thermal conductivity of about 200-400W/mk and is in an irregular particle shape. The diamond particles are not generally spherical, and the sphericity thereof is preferably 0.5 to 1, more preferably 0.7 to 1.

In theory, the material of the large-particle-size particles 31 may be, in addition to diamond, a nitride having a high thermal conductivity such as aluminum nitride, boron nitride, or silicon nitride, a carbide having a high thermal conductivity such as silicon carbide, or a metal compound thereof such as copper, silver, aluminum, gold, indium, or tin.

In the embodiment of the present application, the nitrogen content of the artificial diamond is 500ppm or less, for example, 100ppm to 300ppm or less. The nitrogen content was measured by the elemental analysis dumas method (combustion method). Generally, the lower the nitrogen content of diamond, the higher its thermal conductivity.

In the present embodiment, the particle size of the diamond particles is strictly snap-off treated, and it is recommended that the difference between the maximum particle size and the average particle size is less than 30 μm, the difference is less than 20 μm, and more preferably the difference is less than 10 μm. The particle size distribution was measured by a dry laser particle size analyzer (marvensapanece, mastersizer 3000).

The small-size particles 33 have a particle size of 10 μm or less. In some embodiments, the small-particle size particles 33 have an average particle size of 3 μm or less; in other examples, the small-sized particles 33 have an average particle size of 1 μm or less.

The material of the small-particle-size particles 33 is at least one selected from the group consisting of an oxide, a nitride, a carbide, a metal, and a carbon material. The oxides may include aluminum oxide, zinc oxide, and the like; the nitride includes boron nitride, silicon nitride, and the like; the carbide includes silicon carbide and the like; the metal includes aluminum, silver, gold, tin, copper, indium and other metals and metal compounds thereof, and the carbon material includes, but is not limited to, graphite, graphene, carbon fiber and the like. The small-particle-size particles 33 may generally employ sub-micron-grade alumina, nano-zinc oxide, nano-boron nitride, nano-silicon nitride, and the like, and mixtures thereof. The first small-diameter particles 33a and the second small-diameter particles 33b may be made of the same material or different materials. The plurality of first small-diameter particles 33a may be made of one material or a plurality of materials. The plurality of first small-sized particles 33a may have one particle size distribution or a plurality of particle size distributions. The plurality of second small-diameter particles 33b may be made of one material or a plurality of materials. The plurality of second small-sized particles 33b may have one particle size distribution or a plurality of particle size distributions.

When the composite heat conduction material has a requirement on insulation, the small-particle-size particles 33 are selected from oxides such as aluminum oxide and zinc oxide, or nitrides such as aluminum nitride. When the composite heat conductive material does not require insulation, the small-particle-size particles 33 may be made of metal such as aluminum, silver, gold, tin, copper, indium, or a metal compound thereof.

In some embodiments, the small-particle-size particles 33 are selected from oxide systems having a large number of hydroxyl groups on the surface, such as aluminum oxide, zinc oxide, oxygenAnd (4) melting iron. Preferably the small particle size particles 33 have a surface oxygen content of greater than 30% as measured using X-ray photoelectron spectroscopy. The specific surface area of the small-particle-diameter particles 33 should be larger than 1m ² Per g, preferably greater than 2m ² /g。

The morphology of the small-particle-size particles 33 may be plate-like, needle-like, fibrous, spherical, or spheroidal. Preferably, the small-particle size particles 33 are spherical in shape.

The organic matrix 10, which may also be referred to as a polymer matrix, serves as a continuous phase to fix dispersed phases (such as various heat conductive fillers) in the composite heat conductive material in the organic matrix 10, so as to form a macroscopic composite heat conductive material. The organic matrix 10 is selected from at least one of an organosilicon system, an epoxy system, an acrylic system, a polyurethane system, and a polyimide system. When the composite heat conduction material 100a is used in a product, for example, in the application scenario in fig. 1, the composite heat conduction material 100a is a cured product, and curing mainly refers to curing of the organic matrix 10.

The organic matrix 10 as a curable polymer includes silicone polymers, epoxy polymers, urethane polymers, phenolic polymers, unsaturated polyesters, polyimide-based polymers, acrylonitrile butadiene rubber, ethylene-propylene-diene rubber, ethylene-propylene rubber, natural rubber, polybutadiene rubber, polyisoprene rubber, polyesters, polyurethanes, and the like.

In some embodiments, the material of the organic matrix 10 is preferably silicone such as silicone rubber, silicone oil, or silicone resin, or epoxy resin, and more preferably silicone. The silicone may be any of a condensation-curable silicone system and an addition reaction-curable silicone system, and is preferably an addition reaction-curable silicone system, and more preferably an addition polymerization-curable silicone rubber. In this application, silicone oil is a popular name for organopolysiloxanes.

The addition reaction curing type silicone rubber includes two main types of base silicone oil components, such as alkenyl-containing organopolysiloxane, hydrogen-containing (Si-H group) organopolysiloxane, and the like, and functional auxiliaries, such as a cross-linking agent, a coupling agent, a catalyst, an inhibitor, and the like, are usually added.

The alkenyl group-containing organopolysiloxane may include vinyl both-terminal organopolysiloxanes such as vinyl both-terminal polydimethylsiloxane, vinyl both-terminal polyphenylmethylsiloxane, vinyl both-terminal dimethylsiloxane-diphenylsiloxane copolymer, vinyl both-terminal dimethylsiloxane-phenylmethylsiloxane copolymer, and vinyl both-terminal dimethylsiloxane-diethylsiloxane copolymer.

The viscosity of the alkenyl group-containing organopolysiloxane at 25 ℃ is preferably 5 to 10000 mPas, preferably 30 to 500 mPas.

Among these, the number of hydrogen atoms bonded to silicon atoms in the molecule of the hydrogen-containing organopolysiloxane is 2 or more, and preferably 2 to 50. Examples of the hydrogen-containing organopolysiloxane include hydrogen-containing organopolysiloxanes such as methylhydrosiloxane-dimethylsiloxane copolymer, polymethylhydrosiloxane, polyethylhydrosiloxane, and methylhydrosiloxane-phenylmethylsiloxane copolymer. Wherein the molar ratio of the hydrogen group-containing organopolysiloxane to the alkenyl group-containing organopolysiloxane is preferably 0.3 to 3.

The viscosity of the hydrogen-containing organopolysiloxane at 25 ℃ is not particularly limited, but is preferably 1 to 1000 mPas, and the hydrogen-containing organopolysiloxane can be mixed with an alkenyl-containing organopolysiloxane to cure, and forms a polymer having good physical properties. The viscosity of the organopolysiloxane was measured by a rotational viscometer.

When a curable silicone polymer is used as the organic substrate 10, a noble metal catalyst is used in combination. The noble metal catalyst may be a platinum-based catalyst, a palladium-based catalyst, a rhodium-based catalyst, or the like. Preferably, a platinum-based catalyst such as elemental platinum, oxoplatinic acid, platinum-olefin complexes, platinum-alcohol complexes, platinum coordination compounds, and the like is used. The content of the catalyst is 0.1ppm to 300ppm, preferably 0.1ppm to 200ppm.

In order to increase the shelf life and pot life of the composite heat conductive material 100a and to prevent the active functional groups such as Si — H groups from being consumed at room temperature by a side reaction (hydrosilylation reaction), an inhibitor needs to be added to the composite heat conductive material 100 a. The inhibitor may be acetylene compounds such as 1-ethynyl-1-cyclohexanol and 3-butyn-1-ol, various nitrogen compounds such as triallyl isocyanurate and triallyl isocyanurate derivatives, organic phosphorus compounds such as triphenylphosphine, and the like. The content of the inhibitor is 0.01wt% to 5wt%, preferably 0.1wt% to 1wt% of the composite heat conduction material 100 a.

The bonding medium 32 tightly bonds the small-sized particles 33 to the surfaces of the large-sized particles 31. The process of tightly fixing the bonding medium 32 may be performed before the large-size particles 31 are added to the organic matrix 10, for example, the small-size particles 33 are coated on the surfaces of the large-size particles 31 by the bonding medium 32 in advance, or may be performed after the large-size particles 31 are added to the organic matrix 10.

The thickness of the binder 32 covering the large-particle-diameter particles 31 is usually 10 μm or less, and preferably 1 μm or less.

The bonding medium 32 may be the same material as the organic substrate 10 or different materials. The adhesive medium 32 may be an inorganic adhesive. The inorganic binder may be clay, phosphate, silicate, etc. The inorganic binder may be added with a solvent to adjust the viscosity, and then the small-particle-size particles 33 may be coated. The adhesive medium 32 may be an organic adhesive, which may be an adhesive material commonly used for ceramic powder granulation, such as Polyvinyl Alcohol (PVA), ethylene Vinyl Acetate (EVA), polyvinyl Butyral (PVB), etc., and a solvent may be added to significantly reduce the viscosity of the organic adhesive, thereby achieving uniform dispersion of particles with large and small particle sizes in an organic adhesive solution. The adhesive medium 32 may adhere the small-sized particles 33 to the surfaces of the large-sized particles 31 by a chemical bonding force such as a covalent bond, an ionic bond, or a metallic bond.

In some embodiments, the bonding medium 32 is a material that is homogenous with the organic matrix 10, i.e., the bonding medium 32 and the organic matrix 10 are made of the same polymer system. For example, if the organic substrate 10 is made of silicone, the adhesive medium 32 is also made of silicone. Taking the silicone organic matrix 10 as an example, the adhesive medium 32 is an organosiloxane having a predetermined number of repeating-O-Si-bonds.

In some embodiments, the number of siloxane bonds of the organosiloxane material bonding the first small-particle-diameter particles 33a is lower than the number of siloxane bonds of the silicone host molecule as the organic matrix 10, that is, the molecular weight of the bonding silicone oil is lower than the molecular weight of the silicone oil matrix. That is, the molecular weight of the organic bonding medium 32 is lower than the molecular weight of the organic matrix 10.

In some embodiments, the surfaces of the large-size particles 31 (e.g., diamond particles) and the small-size particles 33 each contain a certain amount of hydroxyl (-OH) reactive functional groups. Preferably, the bonding medium 32, such as a bonding silicone oil, forms a bond with the diamond particles via a C-O-Si bond, wherein the carbon atoms originate from the diamond. The bonding silicone oil forms adhesion with the small-particle-size particles 33 through-O-Si bonds.

In some embodiments, the silicone oil of the bonding medium 32 may have the same terminal reactive functional group as the terminal of the silicone molecular chain of the silicone oil of the organic matrix 10, such as vinyl silicone oil or hydrogen-based silicone oil for addition polymerization silicone system. When the silicone oil of the bonding medium 32 is vinyl silicone oil, at least two types of vinyl silicone oil with different molecular weights are added to the silicone matrix. When the silicone oil of the bonding medium 32 is hydrogen-based silicone oil, at least two kinds of hydrogen-based silicone oils with different molecular weights are added to the silicone base.

In some embodiments, the bonding silicone oil may be a functional group reactive with an — OH functional group, such as a carboxyl group, an epoxy group, a carbonyl group, a double bond, an amine group, an acid chloride group, an ester group, a hydroxyl group, a halogen group, or the like, at the other end of the silicone molecular chain.

In some embodiments, the bonding silicone oil may also be an inactive functional group such as an alkyl group at the other end of the silicone molecular chain. In this case, a silane coupling agent, such as a vinyl silane coupling agent or a hydrogen silane coupling agent, which is reactive with the terminal functional group of the binding silicone oil, may be further added to the silicone base.

The surface oxygen content of the large-sized particles 31 is greater than 10%. Taking diamond particles as an example, in order to improve the dispersibility of the diamond particles, the diamond particles may be subjected to a surface treatment, such as acidification, oxidation, oxide deposition or aluminum nitride deposition, before being added to the organic matrix 10, so that the surfaces of the diamond particles contain more active functional groups, such as hydroxyl groups, and the oxygen content of the surfaces of the diamond particles is increased. Such large-sized particles 31 with more reactive functional groups can be more compatible with organic substrates 10, such as silicone oils, to achieve high loading levels of mixing. Meanwhile, the silane coupling agent can more easily coat the surface of the large-particle-size particles 31 with more active functional groups on the surface, so that the large-particle-size particles 31 subjected to surface treatment by the silane compound can be easily mixed with the high-molecular organic matrix 10, and the amount of the large-particle-size particles 31 in the composite heat conduction material can be increased.

In some embodiments, the silicone adhesive may also include a functional group which is reactive with an end functional group and is reactive with an — OH functional group, such as a carboxyl group, an epoxy group, a carbonyl group, a double bond, an amine group, an acid chloride group, an ester group, a hydroxyl group, a halogen group, and the like.

The adhesion and fixation of the adhesive silicone oil with the large-particle-size particles 31 and the small-particle-size particles 33 can be performed simultaneously with the curing reaction of the main silicone substrate, or can be performed separately in advance.

In order to improve the adhesion between the organic adhesive medium 32 and the large-sized particles 31, a coupling agent compatible with the organic adhesive may be added to achieve good wetting of the surfaces of the organic adhesive medium 32 and the large-sized particles 31.

In some embodiments, the large-sized particles 31 have an average particle size that is 20 times or more larger than an average particle size of the small-sized particles 33. The ratio of the average particle diameter of the large-particle-diameter particles 31 to the average particle diameter of the small-particle-diameter particles 33 is preferably in the range of 30 to 1000, and more preferably in the range of 50 to 500. The larger the difference in the particle size between the large-particle-size particles 31 and the small-particle-size particles 33 is, the more the first small-particle-size particles 33a can adhere to the surface of the same large-particle-size particle 31, which generally increases the packing density of the heat conductive filler in the composite heat conductive material 100 a.

The heat conductive filler may optionally include other heat conductive particles in addition to the above-described large-sized particles 31 and small-sized particles 33.

For example, when the ratio of the large-particle-diameter particles 31 to the small-particle-diameter particles 33 is greater than 20, the thermally conductive filler further includes medium-particle-diameter particles 35, as shown in fig. 3. The average particle diameter of the medium-diameter particles 35 is smaller than the average particle diameter of the large-diameter particles 31 and larger than the average particle diameter of the small-diameter particles 33.

For example, when the ratio of the large-diameter particles 31 to the small-diameter particles 33 is greater than 60, the thermally conductive filler further includes two or more kinds of medium-diameter particles 35 having different diameters.

The average particle diameter of the medium-diameter particles 35 is preferably 10 to 20 μm, and is usually 60 μm or less. The medium-sized particles 35 may be at least one of carbide, nitride, oxide, and metal. The medium-sized particles 35 may also preferably be a thermally conductive filler having a thermal conductivity greater than 10W/mK, such as a material system of diamond, aluminum nitride, cubic boron nitride, or the like.

In the heat-conducting filler, the heat-conducting filler is sorted and grouped according to the average particle size, and in two groups of particles with adjacent average particle sizes, the particle size ratio of the group with larger particle size to the group with smaller particle size is less than 40, preferably less than 30, and more preferably less than 20. More specifically, when the ratio of the large-diameter particles 31 to the small-diameter particles 33 is greater than 60, it is preferable to use heat conductive fillers having four or more particle size distributions, and in one embodiment, the heat conductive fillers are grouped from large to small in particle size and include large-diameter particles 31, first medium-diameter particles, second medium-diameter particles, and small-diameter particles, wherein the ratio of the large-diameter particles 31 to the first medium-diameter particles is less than 40, the ratio of the first medium-diameter particles to the second medium-diameter particles is less than 40, and the ratio of the second medium-diameter particles to the small-diameter particles is less than 40.

The total content of the heat conductive filler refers to the total content of the large-particle-size particles 31, the medium-particle-size particles 35 (if any), and the small-particle-size particles 33 in the composite heat conductive material.

The total weight of the heat-conducting filler accounts for more than 87%, preferably more than 90%, more preferably more than 93% of the composite heat-conducting material, and the limit weight percentage does not exceed 98%.

The volume percentage of all the heat-conducting fillers in the composite heat-conducting material is more than 76%, preferably more than 80%, more preferably more than 83%, and usually the volume percentage does not exceed 88%.

The number of small-diameter particles 33 coated on the surface of the large-diameter particles 31 is 50% or less, preferably 20% or less, of the number of small-diameter particles 33 having a particle diameter of 10 μm or less in the heat conductive filler.

The large-particle-size particles 31 play a main role in heat conduction in the composite heat conduction material 100a, the volume fraction of the large-particle-size particles 31 accounts for 20% -60% of the composite heat conduction material 100a, and generally does not exceed 60%, otherwise, the problems of phase separation and poor mechanical properties of the composite heat conduction material can be caused. The volume fraction of the large-particle-size particles 31 is preferably 40 to 55%, and more preferably 45 to 50% of the composite heat-conductive material 100 a.

The medium-size particles 35 and the small-size particles 33 play a key role in the rheological property and the stability of the composite heat conduction material. The total volume fraction of the medium-size particles 35 and the small-size particles 33 is generally recommended to be 5-25%, preferably more than 8-20%, and more preferably 12-18% of the composite heat conduction material, so as to obtain a better filling effect.

In the heat conductive filler in the composite heat conductive material 100a, the volume percentage of the small-sized particles 33 having a particle size of less than 10 μm is required to be less than 50%, and from the viewpoint of better fluidity, the total volume of the small-sized particles 33 having a particle size of less than 10 μm is preferably 20% or less.

When the addition polymerization curing type organic silicon rubber is used as the organic matrix 10 to prepare the heat-conducting composite material, the heat-conducting composite material can be prepared into a double-component system or a single-component system. The two-component system is that the various heat-conducting fillers are added into the organic matrix and then stored separately as two components, and when the two components are used, the two components are mixed, for example, the two components are mixed and then applied between a chip and a radiator of electronic equipment, and then the two components are heated at normal temperature or high temperature for curing. The single-component system refers to that the various heat-conducting fillers are added into the organic matrix and then made into single components to be stored, and the single components can be cured before the composite heat-conducting material leaves a factory or cured when the heat-conducting material is used, for example, when the heat-conducting material is applied between a chip and a heat radiator.

In order to improve the wettability and interaction force between the heat conductive filler and the organic matrix 10 and prevent the heat conductive filler from agglomerating to increase viscosity, a surface treatment agent is added during the mixing process for treatment. The surface treatment agent may be a surface treatment agent such as a silane compound, an organotitanium compound, an organoaluminum compound, or a phosphoric acid compound, and is preferably a surface treatment agent using a silane compound.

The silane compound used as the surface treatment agent is not particularly limited, and may be alkoxysilanes or chlorosilanes, and preferably alkoxysilanes. Examples thereof include 2- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3- (meth) acryloxypropylmethyldimethoxysilane, 3- (meth) acryloxypropyltrimethoxysilane, 3- (meth) acryloxypropylmethyldiethoxysilane, 3- (meth) acryloxypropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropylmethyldimethoxysilane, trialkoxysilanes such as aryltrialkoxysilanes and alkyltrialkoxysilanes, dialkyldialkoxysilanes such as dialkyldialkoxysilanes and diaryldialkoxysilanes, methyltrimethoxysilanes, methyltriethoxysilane, N-propyltrimethoxysilane, N-propyltriethoxysilane, N-hexyltrimethoxysilane, N-hexyltriethoxysilane, N-octyltriethoxysilane, and N-decylsilane, and the like, and the number of alkyl trialkoxysilanes having 1 to 10 carbon atoms.

In addition, the composite heat conductive material 100a of the present application may further include additives generally used such as an antioxidant, a heat stabilizer, a colorant, a flame retardant, and an antistatic agent, as needed.

Second embodiment

As shown in fig. 4, the composite thermal conductive material 100b according to the second embodiment of the present application includes an organic matrix 10 and a thermal conductive filler distributed in the organic matrix 10. The heat conductive filler includes a plurality of large-sized particles 31 and a plurality of self-fused heat conductive particles 37. At a temperature not higher than the solidification temperature of the organic matrix 10, the self-fusing heat conducting particles 37 are alloy particles capable of fusing with each other and generating a welding reaction, or metal particles capable of fusing with each other and generating a sintering growth effect. The self-fusing heat conductive particles 37 may be fused to each other at a designated temperature (not higher than the curing temperature of the organic matrix 10) to form a metallic bond with the large-sized particles 31. The metal bond bonding in the embodiment of the present application means that the large-particle-size particles 31 are bonded to the fused self-fused heat conducting particles 37 without an organic bonding medium therebetween, and the bonding medium therebetween is a metal or an intermetallic compound.

When the composite heat conduction material 100b is used, for example, in the application scenario in fig. 1, the composite heat conduction material 100b needs to be heated to a specified temperature for curing, the curing mainly refers to curing of the organic matrix 10, and during the curing process, the self-fusing heat conduction particles 37 are fused with each other, so that the fused self-fusing heat conduction particles 37 are in contact bonding with each other, and the fused self-fusing heat conduction particles 37 are in contact bonding with other fillers such as large-particle-diameter particles 31. The self-fusing thermally conductive particles 37 fuse with each other to form a fused body (not shown) that is connected into a whole, and may not be microscopically independent particles. The thermal conductive material formed by curing the composite thermal conductive material 100b comprises: the cured organic matrix 10, the large-particle-diameter particles 31 dispersed in the cured organic matrix 10, and the self-fused heat conductive particles 37 are fused together to form a fused body, and the fused body is bonded to the large-particle-diameter particles 31 by a metallic bond. Different large-sized particles 31 can be connected with each other through the fused body to form a heat conduction path.

The large-particle-diameter particles 31 have a larger particle diameter than the self-fusing heat conductive particles 37, and are generally the largest in particle diameter among the heat conductive fillers. The large-sized particles 31 have an average particle diameter of more than 5 μm. In some embodiments, the large-sized particles 31 have an average particle size of 20 μm or more. In some embodiments, the large-sized particles 31 have an average particle diameter of 40 to 250 μm; in other embodiments, the large-sized particles 31 have an average particle size of 60 μm to 160 μm.

In some embodiments, the thermal conductivity of the large-sized particles 31 is higher than the thermal conductivity of the self-fusing thermally conductive particles 37; usually the highest thermal conductivity of the thermally conductive filler. The large-particle-diameter particles 31 are diamond particles such as artificial diamond and graphitized diamond. The artificial diamond is preferably raw material, polycrystalline or single crystal, and has a thermal conductivity of over 600W/mk and a polyhedral morphology. The large-particle-size particles 31 may also be made of crushed artificial diamond, and the diamond filler has a thermal conductivity of about 200-400W/mk and is in an irregular particle shape. The diamond particles are not generally spherical, and the sphericity thereof is preferably 0.5 to 1, more preferably 0.7 to 1.

In the embodiment of the present application, the nitrogen content of the artificial diamond is 500ppm or less, for example, 100ppm to 300ppm or less. The nitrogen content was measured by the elemental analysis dumas method (combustion method).

In the embodiment of the present application, the particle size of the diamond particles is strictly snap-broken, and it is recommended that the difference between the maximum particle size and the average particle size is less than 30 μm, the difference is less than 20 μm, and more preferably the difference is less than 10 μm. The particle size distribution was measured by a dry laser particle size analyzer (marvensapanece, mastersizer 3000).

The characteristics of the organic matrix 10 in the second embodiment of the present application, such as material, composition, etc., are the same as the organic matrix 10 in the first embodiment, and are not described herein again.

In the technical solution of the self-fusing filler of this embodiment, as shown in fig. 4, the microscopic heat conducting path structure is formed by large-particle-size particles 31 (e.g. diamond particles) and other heat conducting fillers (fused self-fusing filler)Fused thermally conductive particles 37) and there is a certain probability that a large-sized particle 31 (e.g., diamond) is bonded to another large-sized particle 31 (e.g., diamond) through a fusion of self-fused thermally conductive particles 37, rather than being bonded through an organic bonding medium (e.g., silicone oil). The bonding interface does not have interface layers with low heat-conducting property such as organic bonding media and the like, and belongs to metal bond bonding. Since the heat conductivity coefficient of the organic bonding medium (such as silicone oil) is low, the difference between a phonon heat transfer mode (lattice configuration) and inorganic non-materials and metal materials is large, the interface thermal resistance in the system can be reduced by orders of magnitude through the self-fusion packing scheme of the embodiment, and a larger or longer heat conduction path with ultralow interface thermal resistance can be realized. From theoretical calculation, the interface thermal resistance is from 10 ^-6 m ² K/W is reduced to 10 ^-7 m ² K/W, the heat conductivity coefficient of the composite heat conduction material can be improved by times. By adopting the technical scheme of the self-fusion filler, the thermal conductivity coefficient of the composite heat conduction material can be expected to be improved to more than 40W/mK, even more than 100W/mK.

Taking the self-fusing heat conducting particles 37 as an example, the nano metal particles are used as the self-fusing heat conducting particles, and the large-particle-size particles 31 (such as diamond particles), the nano metal particles and other particle-size fillers are uniformly dispersed in the organic matrix 10, when the composite material system is heated to a specified temperature, sintering reaction can be spontaneously generated among the nano metal particles, the nano metal particles are fused together to form a larger heat conducting path structure, and part of the fused nano metal is combined with the surface of the diamond particles to form a larger heat conducting path structure.

The metal particles may include at least one of nano silver, nano copper, nano gold, micro silver, micro copper, micro gold. In one embodiment, the metal particles may be divided into metal particles coated around the large-sized particles 31 and metal particles randomly distributed on the organic matrix 10. In some embodiments, the particle size of the metal particles coated around the large-particle-size particles 31 is recommended to be less than 5 μm, preferably less than 1 μm; the particle size of the randomly distributed metal particles is recommended to be below 25 μm, preferably below 10 μm. The average particle diameter of the metal particles is about 1/50 or less of the average particle diameter of the large-particle-diameter particles 31 (e.g., diamond particles), i.e., the size is 10nm to 1000nm, more preferably 10nm to 500nm. Generally, the smaller the particle size of the metal particles, the better the feasibility and extent of the self-fusion reaction of the metal particles generally occurs.

The sintering temperature of the metal particles is 120-250 ℃, and the more preferable sintering temperature is 150-200 ℃. The composite heat conductive material 100b is heated and sintered to generate sintering growth effect of the metal particles. During sintering, a pressure may be applied, preferably greater than 10psi, and more preferably greater than 20psi for better sintering of the bond.

For good reliability of the metal particles prior to sintering, it is generally necessary to add a surfactant to coat the metal particles. The surfactant can be polyvinylpyrrolidone (PVP), cetyl Trimethyl Ammonium Bromide (CTAB), sodium Dodecyl Sulfate (SDS), oleic acid, etc. The surfactant is volatile at a temperature, such as during a curing reaction, so as not to interfere with the interfusion of the metal particles.

The volume fraction ratio of the self-fusing particles to the other heat-conducting filler is that the self-fusing particles/the other heat-conducting filler = 1/50-5/1.

Taking the self-fusing heat-conducting particles 37 as an example, alloy particles are added to the organic matrix 10 such as organic silicon, and the organic matrix 10 contains particles 31 with large particle size (e.g., diamond particles), alloy particles with low melting point, and other heat-conducting fillers, and various heat-conducting fillers are uniformly dispersed in the organic matrix 10 to prepare the heat-conducting composite material. Such material systems are generally designed to cure during use of the actual electronic device, and the curing means is usually heat curing, typically at a temperature of 100 to 150 degrees.

The melting point of the alloy particles is 150 degrees or less, preferably in the range of 50 to 150 degrees, and more preferably in the range of 70 to 150 degrees. The composite heat conductive material 100b is heated to fuse the alloy particles to each other and cause a welding reaction. The temperature at which the alloy particles fuse with each other is not higher than the curing reaction temperature of the organic matrix 10.

The alloy particles are at least one of Sn-Cu, sn-Al, sn-Zn, sn-Pt, sn-Mn, sn-Mg, sn-Ag, sn-Au, sn-Bi, sn-In, sn-Pd, sn-Bi-In, bi-Pb-Sn, bi-Pb, al-Li, ga-In-Sn, ga-In, ga-Bi-Pb-In and Zn-Li, but not limited thereto. In some embodiments, the alloy particles have a melting point below 150 degrees, preferably In the range of 50 to 150 degrees, more preferably In the range of 70 to 150 degrees, such as Sn-Bi, sn-In, ga-containing low melting point alloys.

Under the heating and solidifying temperature, the alloy particles with low melting point can be melted, and form good wetting and combination with the peripheral heat conduction particles, or form tight combination through welding reaction, and finally form a heat conduction path with ultra-low interface thermal resistance, larger size and longer length. The main difference compared to the self-sintered metal particle solution is that the direct bonding heat conducting path of diamond/heat conducting filler is realized by using low melting point alloy instead of nano metal particles.

Preferably, the large-size particles 31 in the composite heat conduction material are diamond particles, and the small-size particles 33 are alloy particles with low melting point. In some embodiments, however, the low melting point alloy particles may also have a larger particle size than the diamond particles.

In order to improve the bonding or soldering of the melted low melting point alloy component with the surface of the large particle size particle 31 and to improve the probability of bonding the low melting point alloy component with the large particle size particle 31, a certain amount of flux, such as rosin, phenol resin, acrylic resin, vinyl chloride resin, polyurethane, or the like, may be added to the organic matrix 10.

The volume fraction ratio of the low-melting-point alloy to other heat-conducting fillers is that the low-melting-point alloy/the heat-conducting fillers = 1/50-5/1.

As shown in fig. 5, the surface of the large-particle-diameter particle 31 is previously coated with a bonding medium layer 34. In this embodiment, the bonding medium layer 34 is a bonding medium of a soldering type or a sintering type, but not limited thereto. The adhesive medium layer 34 enables the fused self-fused heat conductive particles 37 to form a metallic bond with the large-particle-diameter particles 31.

When the self-fused heat conducting particles 37 are metal particles capable of being fused with the large-particle-size particles 31 by a self-sintering reaction, the material of the bonding medium layer 34 is a metal compatible with the small-particle-size particles 33, preferably the same metal material as the small-particle-size particles 33, for example, the material of the bonding medium layer 34 is the metal material of the metal particles. For example, when the nano metal particles are nano silver, the material of the adhesive medium layer 34 is metal silver.

When the self-fusing heat conducting particles 37 are alloy particles that can be fused with the large-sized particles 31 through a welding reaction, the bonding medium 32 may be at least one metal in the alloy particles (homogeneous compatibility), a metal that is welding compatible with the alloy particles (welding compatible means that solid phase reactions such as intermetallic diffusion and fusion can occur), or an intermetallic compound generated by the welding reaction, but is not limited thereto. The intermetallic compound may be an intermetallic compound of one of the metals In the alloy particles, such as an intermetallic compound containing Sn, ga, in, and the intermetallic compound may also be indium tin, gallium tin, or the like. The surface of the large-particle-size particles 31 is coated with a metal material having welding compatibility with the low-melting-point alloy particles; for example, when the low melting point alloy particles are made of a material system such as AuSn, snPd, snAg, snBi, snIn, etc., the metal material on the surfaces of the large particles 31 may be Au, ag, ni, sn, al, etc.

The bonding medium layer 34 on the surface of the large-particle-size particles 31 can be formed by deposition, evaporation, ion plating and the like, or can be wrapped by a coating method and the like.

When the surface material of the large-sized particles 31 is a metal material that is the same as the metal nanoparticles, it is recommended that the metal material on the surface of the large-sized particles 31 is in a micro-scale or even nano-scale uneven shape. As shown in fig. 5, specifically, the following steps are performed: firstly, a bonding medium layer 34 is sputtered on the surface of the large-particle-size particles 31 (such as diamond particles), for example, metals such as aluminum, silver, titanium, etc. are used as the bonding medium layer 34, and then a layer of self-fusing heat-conducting particles 37 such as nano silver powder, nano copper powder or nano gold powder, etc. are deposited and wrapped on the bonding medium layer 34. After the composite heat conduction material is sintered and solidified at a low temperature, the self-fusing heat conduction particles 37 are fused with each other, and the large-size particles 31 (such as diamond) can be combined with another large-size particle 31 (such as diamond) through the fusion of the self-fusing heat conduction particles 37.

Although the composite heat conduction material realized by the two self-fused heat conduction particles 37 can realize ultrahigh heat conduction coefficient, the modulus of the cured heat conduction material is too high, and failure such as interface delamination or internal cracking of the heat conduction material can be more easily caused in the use process of electronic equipment. Therefore, in order to avoid the above failure problem, when the self-fusing heat conducting particles 37 are metal particles, the volume ratio of the self-fusing heat conducting particles 37 in the heat conducting filler is 50% to 85%, preferably 70% to 80%, and more preferably 72% to 78%. When the self-fusing heat conducting particles 37 are alloy particles, the volume ratio of the self-fusing heat conducting particles 37 in the heat conducting filler is less than 50%, preferably less than 30%.

The large-particle-size particles 31 are randomly and irregularly distributed in the organic matrix 10. Before the composite heat conduction material 100b is cured, the self-fused heat conduction particles 37 are distributed in the organic matrix 10, and may be randomly and irregularly distributed around the large-particle-size particles 31. After the composite heat conductive material 100b is cured, the self-fused heat conductive particles 37 are bonded to each other and to the large particle size particles 31.

In this embodiment, the thermal conductivity of the self-fluxing composite thermal conductive material 100b is significantly different before and after curing, and the thermal conductivity after curing is more than 5 times higher than the thermal conductivity before curing.

Third embodiment

The composite heat conductive material (not shown) of the third embodiment of the present application includes an organic matrix and a specific diamond-based heat conductive filler 200 distributed in the organic matrix. As shown in fig. 6, the diamond-based thermally conductive filler 200 includes diamond particles 20 and a plurality of small thermally conductive particles 40 coated on the surfaces of the diamond particles 20. The diamond particles 20 have a polyhedral shape. The small thermally conductive particles 40 have a particle size smaller than that of the diamond particles 20. In one embodiment, the small thermally conductive particles 40 have a particle size of 10 μm or less. The shape of the diamond particles 20 in combination with the plurality of small thermally conductive particles 40 coating them tends to be spherical.

The diamond-based heat conductive filler 200 further includes a bonding medium 32, and the bonding medium 32 is located between the diamond particles 20 and the small heat conductive particles 40 and between the small heat conductive particles 40 so that the plurality of small heat conductive particles 40 are connected and coated on the surfaces of the diamond particles 20.

It is understood that the composite heat conductive material may also include heat conductive fillers of other particle sizes. The composite heat conduction material of the third embodiment of the present application, the characteristics of the material, the components, and the like of the organic matrix, and the functional additives and the heat conduction fillers with other particle sizes that can be added to the composite heat conduction material can all refer to the first embodiment, and are not described herein again.

The hardness of diamond filler particles is high and the diamond filler particles are of a polyhedral structure, and the heat conducting materials adopting high-proportion diamond fillers can cause glue blocking and cannot be extruded when high-pressure glue extruding is carried out, and even the situation of dispensing pipeline components can be abraded. When applied between the chip and the heat sink, the assembly gap between the chip and the heat sink may be continuously changed by mechanical stress, thermal stress, etc., and the diamond filler may be too hard under such local dislocation, which may cause scratches on the surface of the chip or the heat sink. Particularly for bare chip scenarios, the use of such diamond-filled thermally conductive materials may result in chip damage.

According to the diamond-based heat-conducting filler 200, the small heat-conducting particles 40 are coated on the surfaces of the diamond particles 20, so that the whole diamond-based heat-conducting filler 200 tends to be spherical, the surface appearance of the polyhedral diamond filler particles 20 is improved, and the problems of serious abrasion of a dispensing component and friction damage of a chip heat dissipation component are solved; meanwhile, the problems that the heat conduction material prepared by adopting the diamond filler with high filler content has poor liquidity and the compression stress is overlarge when the material is pressed to be thin in the application process are solved. It is understood that in other embodiments, the diamond particles 20 may be spherical by ball milling.

The coating of the small heat conducting particles 40 on the diamond particles 20 can adopt the means of granulation, supercritical deposition, surface coating, sputtering, chemical deposition, film-forming coating, rolling ball coating and the like, and the surface of the diamond particles 20 is coated with a coating layer consisting of the small heat conducting particles 40 with the diameter of 0.1-10 microns. Preferably adopts the processes of spray granulation, film-forming coating, rolling ball coating and the like and the process combination thereof.

The thermal conductivity of the diamond particles 20 is 600W/mk or more, and the diamond particles 20 are artificial diamond or graphitized diamond.

The average particle size ratio of the diamond particles 20 to the small heat conducting particles 40 is greater than 20, in some embodiments, the average particle size ratio of the diamond particles 20 to the small heat conducting particles 40 is 30 to 1000, and in other embodiments, the average particle size ratio of the diamond particles 20 to the small heat conducting particles 40 is 50 to 500.

The material of the small heat conductive particles 40 is selected from at least one of an oxide, a nitride, a carbide, a metal, and a carbon material. The oxide may include aluminum oxide, zinc oxide, and the like; the nitride includes boron nitride, silicon nitride, and the like; the carbide includes silicon carbide and the like; the metal can include aluminum, silver, gold, tin, copper, indium and other metals and metal compounds thereof; the carbon material includes, but is not limited to, graphite, graphene, carbon fiber, and the like. In some embodiments, the small thermally conductive particles 40 have an average particle size of 3 μm or less.

The bonding medium 32 is an inorganic bonding medium or an organic bonding medium. The bonding auxiliary agent may be an inorganic bonding medium 32, such as clay, phosphate, silicate, or an organic bonding medium, and may be an organic polymer material such as PVA, EVA, PVB, or a silane molecular structure having a polymerizing ability, such as alkoxy-terminated polysiloxane.

In some embodiments, the bonding medium 32 is a material that is homogeneous with the organic matrix of the composite thermal conductive material, i.e., the bonding medium 32 and the organic matrix are made of the same polymer system. For example, if the organic substrate is made of silicone, the adhesive medium is also made of silicone. Taking an organic matrix of silicone as an example, the adhesive medium 32 is organosiloxane having a predetermined number of repeating-O-Si-bonds.

In some embodiments, the number of siloxane bonds of the organosiloxane material binding the small thermally conductive particles 40 is lower than the number of siloxane bonds as the host molecule of the silicone matrix, i.e., the molecular weight of the binding silicone oil is lower than the molecular weight of the silicone oil matrix. That is, the molecular weight of the organic bonding medium 32 is lower than the molecular weight of the organic matrix.

In some embodiments, the silicone oil of the bonding medium 32 may have the same terminal reactive functional group as the terminal of the silicone molecular chain of the silicone oil of the organic matrix, such as vinyl silicone oil or hydrogen-based silicone oil for addition polymerization silicone system. When the silicone oil of the bonding medium 32 is vinyl silicone oil, at least two types of vinyl silicone oil with different molecular weights are added to the silicone matrix. When the silicone oil of the bonding medium 32 is hydrogen-based silicone oil, at least two kinds of hydrogen-based silicone oils with different molecular weights are added to the silicone base.

In order to improve the adhesion between the organic bonding medium 32 and the diamond particles 20, a coupling agent compatible with the organic bonding medium 32 system may be added to achieve good wetting of the bonding medium 32 with the diamond particle 20 surface.

The thickness of the bonding medium 32 covering the diamond particles 20 does not exceed the thickness of the small heat conductive particles 40 covering the diamond particles 20, thereby avoiding that the outer surface of the diamond-based heat conductive filler 200 is mainly the bonding medium 32 with low heat conductive performance rather than the heat conductive particles with high heat conductive performance. Most preferably, during the manufacturing processes of the diamond-based heat-conducting filler 200, such as granulation and calcination, the bonding medium 32 is well filled at the portion close to the diamond particles 20, and the bonding medium 32 is poorly filled at the exposed portion of the diamond-based heat-conducting filler 200, that is, the bonding medium 32 does not fill the space between the outermost small heat-conducting particles 40, so that the uneven outer surface of the diamond-based heat-conducting filler 200 is formed by the small heat-conducting particles 40 which are relatively raised. That is, the outer surface of the diamond-based heat conductive filler 200 is constituted of the exposed small heat conductive particles 40.

The plurality of small thermally conductive particles 40 consist of a plurality of particles having an average particle size of <10 μm or a mixture of a plurality of particle size distributions having an average particle size of <10 μm.

The diamond particles 20 combined with the plurality of small heat conductive particles 40 covering the diamond particles have a spherical or spheroidal shape, the sphericity of which is higher than that of the diamond particles 20 inside, and the sphericity of the diamond-based heat conductive filler 200 is 0.7 or more, preferably 0.8 or more. Wherein, the sphericity is a parameter for characterizing the morphology of the particles, and the more the particles are morphologically close to a sphere, the closer the sphericity is to 1. Compared with the original polyhedral diamond particles 20, the diamond-based heat-conducting filler 200 has the advantages that the steric hindrance can be remarkably reduced, the abrasion and scratch to equipment and application components in the application of the filler can be effectively avoided, and the compressive stress of the composite heat-conducting material can be remarkably reduced.

The surface oxygen content of the diamond-based heat-conducting filler 200 is greater than 5%. In some embodiments, the diamond-based thermally conductive filler 200 has a surface oxygen content of >10%; in other embodiments, the surface oxygen content of the diamond-based thermally conductive filler 200 is 15% to 30%. Because the surfaces of the diamond particles 20 are coated with alumina and organic binder, the oxygen content of the filler surface is significantly higher than that of the original diamond particles 20, and uniform dispersion in the organic matrix 10 can be easily achieved, avoiding phase separation.

The coating thickness of the small heat conducting particles 40 is different at different positions on the surface of the diamond particles 20, the coating thickness is thinner at the connecting position of the two surfaces of the polyhedral diamond particles 20, the thinnest position may not be covered by the small heat conducting particles 40, and the coating thickness is preferably 1 to 5 times, more preferably 1 to 3 times, of the particle size of the small heat conducting particles 40. The coating thickness is relatively thick at the flat portion of the surface of the polyhedral diamond particles 20, and preferably, the coating thickness is not more than 10 times, more preferably 3 to 5 times, the particle diameter of the small thermally conductive particles 40. In combination with the particle size characteristics of the small thermally conductive particles 40, the small thermally conductive particles 40 coat the diamond particles to a thickness of no more than 20 μm, preferably less than 10 μm. The specific coating thickness can be comprehensively controlled by the proportion of the diamond particles 20 and the small heat-conducting particles 40, the viscosity of the bonding medium 32, the granulation molding process and the like.

Microscopically, the surface of the diamond-based heat conductive filler 200 is uneven, that is, the outer surface has an uneven microstructure. The specific surface area of the diamond-based thermally conductive filler 200 is significantly higher than that of the original diamond particles 20. In some embodiments, the specific surface area of the diamond-based thermally conductive filler 200 is more than 3 times that of the original diamond particles 20; in other embodiments, the diamond-based thermally conductive filler 200 has a specific surface area 5 to 10 times that of the original diamond particles 20.

The diameter of the diamond-based heat conductive filler 200 is greater than the diameter of the original diamond particles 20, and the average particle diameter of the diamond-based heat conductive filler 200 is preferably 20 μm or more, more preferably 40 to 250 μm, and even more preferably 60 to 160 μm.

Referring to fig. 7, the preparation method of the diamond-based heat conductive filler 200 includes the following steps.

Dispersing: uniformly dispersing a plurality of diamond particles and a plurality of small heat conducting particles in a bonding medium to form slurry, wherein the diamond particles are polyhedral, and the particle size of the small heat conducting particles is smaller than that of the diamond particles.

Granulating and balling: and granulating and balling by using the slurry, so that the small heat conduction particles are coated on the surfaces of the diamond particles through the bonding medium.

Rubber discharging: and removing redundant bonding media to enable the small heat conduction particles to be tightly combined with the diamond particles. The binder removal can be carried out by means of high-temperature calcination.

Size screening: the diamond-based heat-conducting filler with the size and the particle size distribution meeting the requirements is screened out through modes of gauze filtration, airflow classification and the like.

The diamond-based heat-conducting filler and other heat-conducting fillers are dispersed in an organic polymer matrix, and the composite heat-conducting material can be prepared.

With continued reference to fig. 7, in one embodiment, an exemplary process for making a composite thermally conductive material includes the following steps.

Mixing materials: the diamond-based heat-conducting filler with the large particle size, other fillers (such as fillers with small particle size and medium particle size), functional additives and the like are added into the organic matrix according to the specified formula design.

Stirring and dispersing: the random and uniform dispersion of the filler in the organic matrix is realized by adopting high-speed stirring processes such as double-planet mixing, meshing dispersion, a homogenizer and the like. Usually, during or after stirring, a vacuum is applied to remove air bubbles from the paste mixture. The temperature setting in the mixing process is not particularly limited, and may be 10 ℃ or higher and 150 ℃ or lower.

And (3) curing: according to the formula design, the composite material is cured under the specified curing condition, mainly the organic matrix is cured to prepare the composite heat conduction material. The curing process is not particularly limited, and is usually a heat curing, and typically the heat curing temperature is in the range of 100 to 250 degrees, and the heating time is in the range of half an hour to several hours. Before curing, the coating can be coated into a pad or a film according to the product requirements. After solidification, packaging such as split charging or cutting can be carried out according to the product requirements.

The present application also provides an electronic device, an electronic component that generates heat during operation, and a cured product of the composite heat conductive material of any one of the first to third embodiments that covers the electronic component. In some embodiments, as shown in fig. 1, the electronic device further comprises a circuit board 51 and a heat sink 55. The electronic component is a chip 53, the chip 53 is disposed on the circuit board 51, the heat sink 55 is disposed on a side of the chip 53 facing away from the circuit board 51, and the interface heat conduction material 57 between the chip 53 and the heat sink 55 is a cured product of any one of the composite heat conduction materials in the first to third embodiments. The interface heat conductive material 57 shown in fig. 2 may also be a cured product of any of the composite heat conductive materials of the first to third embodiments.

It can be understood that the composite heat conductive material described herein can also be used for interface heat conduction between a heat conductive structural member (such as the above-mentioned chip temperature equalization substrate and the heat conductive plate) and another heat conductive structural member, that is, heat conduction between the structural shell of one functional module and the structural shell of another functional module in the electronic device.

The technical solution of the embodiments of the present application is further described below by specific examples.

Examples 1 to 7

Preparing the diamond-based heat-conducting filler: by carrying out surface coating on the diamond, the raw materials comprise:

artificial diamond, unprocessed, henan Huanghe whirlwind, inc., particle size range 120-150 μm, sphericity 0.9, polyhedral;

alumina, unprocessed, suzhou brocade new material science and technology limited, with an average particle size of 0.4 μm, a sphericity of 1, a spherical body;

alumina, unprocessed, suzhou brocade new material science and technology limited, with an average particle size of 4 μm, a sphericity of 1, a spherical body;

bonding medium: long carbon chain polysiloxanes.

The preparation process comprises the following steps: dispersing the artificial diamond and the alumina nanoparticles in an acetone solution containing 2wt% of long carbon chain polysiloxane, stirring for 30 minutes at 30 ℃, granulating into balls, heating for 12 hours at 70 ℃, and removing the solvent to obtain the alumina-coated artificial diamond. After surface modification, the sphericity of the obtained coated artificial diamond is improved to 0.95, the oxygen content of the surface is improved to 50 percent, and the specific surface area is improved to 0.5m ² /g。

Preparing a composite heat conduction material:

organic matrix 1: vinyl-containing polyorganosiloxanes, divinylpolydimethylsiloxanes, jiangxi Lanxing-Starfire silicones Ltd., viscosity 100 pas.

Organic matrix 2: the organic hydrogenpolysiloxane, the silicone oil, the methyl hydrogenpolysiloxane, jiangxi Lanxing Xinhuo organosilicon Co., ltd, and the viscosity of 30Pa s.

Platinum catalyst: platinum-1,2-divinyltetramethyldisiloxane complex, jiangxi Lanxing starfire Silicone Co.

Inhibitor (B): ethynyl-1-cyclohexanol, jiangxi Lanxing Sihuo organosilicon Co.

Surface treating agent: dodecyl trimethoxysilane, jiangxi Lanxing Sihuo organosilicon Co.

The A-component heat conduction material is prepared by adopting addition reaction type organic silicon resin as a polymer matrix, wherein organopolysiloxane at two ends of vinyl and dodecyl trimethoxy silane as a surface treatment agent, adding diamond coated with nano particles and other heat conduction fillers into the organic matrix 1 according to the volume fraction shown in the table I, and further adding a reaction inhibitor and a platinum catalyst.

In addition, the organic matrix 2 is adopted, and the diamond coated with the nano particles and the heat conducting filler are added according to the volume fraction shown in the table I to prepare the B component heat conducting material, wherein the B component is different from the A component in organic matrix.

Mixing the component A and the component B in a mass ratio of 1:1, mixing to prepare the composite heat conduction material. Wherein the curing condition is that the curing is carried out for 2 hours at the temperature of 80 ℃. The compounding ratio of the composite heat conduction materials of examples 1 to 7 and comparative example 1 is shown in table one, and the physical property test is carried out, specifically shown in table one.

The evaluation methods and measurement methods in examples 1 to 7 and comparative example 1 are as follows.

Test of Heat conductivity

The test is carried out according to the ASTM D5470 standard by using a Longwin interface thermal resistance tester. Coating the heat-conducting composite material on one section of a copper bar, gradually heating from normal temperature to 80 ℃ under the pressure of 40psi, measuring by adopting a steady-state heat transfer means, measuring the application thermal resistance of the heat-conducting material with different thicknesses (0.5 mm, 1.0mm and 1.5 mm), and fitting the intrinsic heat conductivity coefficient of the colloid.

Fluidity test

The weight of the gum flowing out within 1min was measured using a 30cc syringe, 2.54. + -.5% inner diameter by mm, 90. + -.5% PSI pressure. The fluidity test was evaluated by the following evaluation criteria.

A, the fluidity is more than 25g/min;

b, the fluidity is 15 to 25g/min;

c, the fluidity is less than 15g/min;

surface oxygen content

The diamond particles before and after coating were analyzed and determined by XPS, and the C/O ratio of the diamond surface was mainly focused. The evaluation was made by defining the oxygen content as the surface oxygen content of diamond.

Void fraction

The actual density was determined by measuring each component of the heat conductive filler such as diamond, alumina, etc. and the silicone additive, etc. using a true density meter (Quantachrome, full-automatic true density meter Ultrapyc 1200e, instruments of Kang Da, usa). The theoretical density (without gaps) of the heat conducting material is obtained according to the mixing formula. And then, using a true density meter to measure the density of the mixed heat-conducting colloid to obtain the actual density. The ratio of the actual density divided by the theoretical density gives the internal porosity of the composite.

Watch 1

As can be seen from the results of table one, examples 1 to 7 can effectively reduce the interfacial thermal resistance between diamonds by using the coated diamond particles as the thermally conductive filler, as compared with comparative example 1, thereby obtaining a highly thermally conductive material having a thermal conductivity significantly greater than that of comparative example 1.

The microstructure of the surface of the diamond particles is improved, and a layer of nano particles is tightly coated on the surface of the diamond by a granulation process, so that the diamond is changed into a spherical shape from a polyhedron shape. Therefore, the diamond is connected with the diamond through a small amount of nano particles, and the diamond/silicon oil/diamond interface structure in the traditional process is replaced; the key measure for improving the heat conduction material is to reduce the interface thermal resistance between the large-particle diamonds through analysis on the whole heat conduction link, and the interface thermal resistance mainly comes from the middle silica gel layer.

In addition, the sphericity of the diamond is further improved, and the friction force between the fillers in a flowing state can be reduced, so that the technical problems that the heat conduction material is blocked in the glue dispensing process and is difficult to extrude and the like, and the problems that the instantaneous compressive stress is overlarge in the assembling process, the long-term compressive stress is overlarge and the like are solved.

Examples 8 to 11

Examples 8-11 this effect was achieved directly in the fabrication of the composite heat conductive material without prior surface coating of the diamond particles. In order to achieve the effect, the key point is that the organic silicon oil filling medium can strongly adhere and fix small-particle-size particles around the large-particle-size filler. Generally, the organic silicon oil can only infiltrate the surface of the filler, more specifically, the silane coupling agent at the interface of the diamond particles and the organic silicon oil realizes compatibility/fusion, the silane coupling agent is combined with the surface of the diamond particles through a Si-O chain, the molecular chain at the other end of the silane coupling agent reacts with the nano-particles, and the high-performance heat-conducting gel is prepared by a one-step blending method.

Preparation of composite heat conduction material

Heat-conducting filler:

the artificial diamond is unprocessed, and is prepared by Henan Huanghe cyclone Co., ltd, specification type MGD90, particle size range of 120-150 μm, sphericity of 0.9 and polyhedron shape.

Alumina, untreated, suzhou brocade new materials science and technology ltd, average particle size 0.4 μm, sphericity 1, spheroid.

Alumina, unprocessed, suzhou brocade new materials science and technology ltd, with an average particle size of 4 μm, a sphericity of 1, a spherical body.

Organic matrix 1: vinyl group-containing polyorganosiloxane: divinyl polydimethylsiloxane, jiangxi Lanxing Sihuo Silicone Co., ltd., viscosity 100 pas.

Organic matrix 2: an organohydrogenpolysiloxane: silicone oil: methyl hydrogen polysiloxane, jiangxi Lanxing Xinhuo organosilicon Co., ltd., viscosity 30 pas.

Platinum catalyst: platinum-1,2-divinyltetramethyldisiloxane complex, jiangxi Lanxing starfire Silicone Ltd.

Inhibitor (b): ethynyl-1-cyclohexanol, jiangxi Lanxing Sihuo organosilicon Co.

Surface treating agent: dodecyl trimethoxy silane, jiangxi Lanxing starfire silicone, inc.

The preparation method comprises the steps of adopting addition reaction type organic silicon resin as a polymer matrix, adding nano alumina, artificial diamond and dodecyl trimethoxy silane into an organic matrix 1 according to the volume fraction shown in the table II, further adding a reaction inhibitor and a platinum catalyst, and mixing by double planets to prepare the A-component heat conduction material, wherein the vinyl two-end organopolysiloxane (the viscosity at 25 ℃ is 100mPa & s) and the surface modifier is dodecyl trimethoxy silane.

Further, a heat conductive material of B component, which is different from a component a in the organic matrix, was prepared by using the organic matrix 2 and adding the untreated diamond and the heat conductive filler in the volume fractions shown in table two, with respect to the organohydrogenpolysiloxane (viscosity at 25 ℃ c: 100mPa · s) constituting the curing agent of the addition reaction type silicone resin.

Mixing the component A and the component B in a mass ratio of 1:1, mixing to prepare a novel heat conduction material, and carrying out the physical property test, wherein the curing condition is 80 ℃ and the curing time is 2 hours. The compounding ratio of the composite heat conduction materials of examples 8 to 11 and comparative example 2 is shown in table two, and the physical property test is carried out, specifically shown in table two.

Watch 2

As can be seen from the results of table two, examples 8 to 11 can coat diamond particles to some extent by adding a surface bonding agent, compared to comparative example 2, thereby obtaining a highly thermally conductive material having a thermal conductivity significantly greater than that of comparative example 2.

The surface coating treatment of the diamond filler is not required to be carried out in advance, but the effect of coating small-particle-size particles on the surface of the large-particle-size diamond filler can be achieved in the mixing and stirring process of the filler and the organic silicon oil through reasonable multiple filler compound design and interface coupling agent material selection, and the problem existing in the prior art can be improved to a certain extent. Although the final technical effect of the technical scheme is not optimal, the diamond filler surface coating treatment process with higher technical implementation requirements is avoided, and the diamond filler surface coating treatment process is easier to implement in batches in the industry.

Examples 12 to 15

Examples 12-15 the diamond/thermally conductive filler/diamond interface structure was substituted for the conventional path diamond/silicone oil/diamond interface structure described above to enhance thermal conductivity, primarily by means of low temperature semi-sintering. The contact between the filler and the filler is changed from van der waals force to chemical bond bonding, so that the contact is tighter, and the interface thermal resistance is reduced by orders of magnitude.

Surface coating of diamond

Heat conductive filler

The artificial diamond, untreated, henan Huanghe Xuanfeng GmbH, particle size range 120-150 μm, sphericity 0.9, polyhedral.

Silver nanoparticles, untreated, guangzhou Macrowu materials science and technology, with an average particle size of 0.1 μm, a sphericity of 1, a spherical shape.

Micron silver powder, untreated, guangzhou Macrowu materials science and technology, 5 μm average particle size, 1 sphericity, spheroid shape.

Surface film forming agent: long carbon chain polysiloxanes.

And sputtering the artificial diamond particles, and performing magnetron sputtering by using aluminum as a target material to coat a layer of metallic aluminum film on the surface of the artificial diamond particles. And then dispersing the particles and the nano silver powder in an acetone solution containing 2wt% of long carbon chain polysiloxane, stirring for 30 minutes at 30 ℃, granulating, heating for 12 hours at 70 ℃, and removing the solvent to obtain the artificial diamond coated with the nano silver powder. After surface modification, the sphericity of the obtained coated artificial diamond is improved to 0.95.

Preparation of high-thermal-conductivity composite material

Organic matrix 1: polyorganosiloxane containing vinyl group

Divinylpolydimethyl siloxane, jiangxi Lanxing Xinhuo Silicone Co., ltd., viscosity 100 pas.

Organic matrix 2: organic hydrogenpolysiloxane

Silicone oil: methyl hydrogen polysiloxane, jiangxi Lanxing Sihuo Silicone Co., ltd., viscosity 30 pas.

Inhibitor (b): ethynyl-1-cyclohexanol, jiangxi Lanxing Sihuo organosilicon Co.

The preparation method comprises the steps of adopting addition reaction type organic silicon resin as a polymer matrix, adding nano silver powder, micron silver powder, artificial diamond, long carbon chain polysiloxane and dodecyl trimethoxy silane into an organic matrix 1 according to volume fractions shown in the table III, further adding a reaction inhibitor and a platinum catalyst, and mixing by double planets to prepare the A-component heat conduction material.

Further, a heat conductive material of B component, which is different from the a component in the organic matrix, was prepared by adding nano silver-coated diamond and a heat conductive filler to an organohydrogenpolysiloxane (viscosity at 25 ℃ c: 100mPa · s) constituting a curing agent of addition reaction type silicone resin using the organic matrix 2 in the volume fractions shown in table three.

Mixing the component A and the component B in a mass ratio of 1:1, the mixture was cured at a high temperature of 20psi and 150 c under pressure to prepare a novel heat conductive material, the compounding ratios of the composite heat conductive materials of examples 12 to 15 are shown in table three, and physical properties were measured and specifically shown in table three.

Watch III

The thermal conductivity of the paste composite material before curing is generally lower than 6W/mk, and the thermal conductivity of the paste composite material before curing in the examples 12-15 of the application is lower than 4W/mk, but the thermal conductivity is increased by more than 5 times after curing.

Theoretically, the self-fusion type technical scheme can realize the heat-conducting gel with the concentration of more than 20W/mk, more than 40W/mk and more than 100W/mk.

Examples 16 to 20

In the composite heat conductive material system of embodiments 16 to 20, a certain amount of low melting point alloy particles are added, and in the use process, the low melting point alloy is melted by heating to be fused with the peripheral filler particles or directly combined with the peripheral filler particles, so that a dense low thermal resistance heat conductive path is constructed in the heat conductive material, thereby improving the heat conductive performance of the heat conductive material.

Preparation of high-thermal-conductivity composite material

Heat conductive filler

Low-melting-point alloy powder: sn (tin) ₄₃ Bi ₅₇ YAMATO Metal, sn, japan ₄₃ Bi ₅₇ The melting point was 138 ℃ and the average particle diameters were 5 μm, 20 μm and 100. Mu.m.

Organic matrix 1: polyorganosiloxane containing vinyl group

Divinyl polydimethylsiloxane, jiangxi Lanxing Sihuo Silicone Co., ltd., viscosity 100 pas.

Organic matrix 2: organic hydrogenpolysiloxane

Inhibitor (B): ethynyl-1-cyclohexanol, jiangxi Lanxing Sihuo organosilicon Co., ltd.

Adopting addition reaction type organic silicon resin as a polymer matrix, wherein organopolysiloxane at two ends of vinyl and surface modifier are dodecyl trimethoxy silane, adopting an organic matrix 1, and adding artificial diamond and Sn according to the volume fractions shown in the table IV ₄₃ Bi ₅₇ The component A heat conduction material is prepared by melting alloy powder at low temperature, dodecyl trimethoxy silane, further adding a reaction inhibitor and a platinum catalyst and mixing by double planets.

Further, a heat conductive material of B component, which is different from a component a in the organic matrix, was prepared by using the organic matrix 2 and adding diamond and a heat conductive filler in the volume fractions shown in table four with respect to an organohydrogenpolysiloxane (viscosity at 25 ℃ is 100mPa · s) constituting a curing agent of addition reaction type silicone resin.

Mixing the component A and the component B in a mass ratio of 1:1, the mixture was cured at a high temperature of 20psi and 150 c to prepare a novel heat conductive material, the compounding ratio of the composite heat conductive materials of examples 16 to 20 is shown in table four, and physical properties were measured and specifically shown in table four.

Watch four

It should be noted that the above is only a specific embodiment of the present application, but the scope of the present application is not limited thereto, and any person skilled in the art can easily think of the changes or substitutions within the technical scope of the present application, and all should be covered by the scope of the present application; in the present invention, the embodiments and features of the embodiments may be combined with each other without conflict. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A composite thermally conductive material, comprising:

an organic matrix;

a thermally conductive filler comprising:

a plurality of large particle size particles distributed in the organic matrix;

a plurality of small-particle-size particles having an average particle size smaller than that of the large-particle-size particles, the plurality of small-particle-size particles including a plurality of first small-particle-size particles and a plurality of second small-particle-size particles; the first small-particle-size particles are adhered to the surfaces of the large-particle-size particles, and the second small-particle-size particles are randomly distributed in the organic matrix;

2. The composite heat conductive material according to claim 1, wherein the large-particle-diameter particles have an average particle diameter of 20 μm or more.

3. The composite heat conductive material according to claim 1 or 2, wherein the small-particle-size particles have an average particle size of 10 μm or less.

4. The composite heat conductive material according to any one of claims 1 to 3, wherein the large-sized particles have a thermal conductivity higher than that of the first small-sized particles.

5. The composite heat conductive material according to any one of claims 1 to 4, wherein the large-particle size particles are diamond particles.

6. The composite heat conductive material of claim 5, wherein the diamond particles have a nitrogen content of 500ppm or less.

7. The composite heat conductive material according to any one of claims 1 to 6, wherein the material of the small-particle-size particles is at least one selected from the group consisting of oxides, nitrides, carbides, metals, and carbon materials.

8. The composite heat conductive material according to any one of claims 1 to 7, characterized in that the organic matrix is selected from at least one of silicone systems, epoxy systems, acrylic systems, polyurethane systems, polyimide systems.

9. The composite heat conductive material according to any one of claims 1 to 8, characterized in that the organic matrix is an addition polymerization curing type silicone system.

10. The composite heat conductive material of any one of claims 1 to 9, wherein the bonding medium is an inorganic bonding material.

11. The composite heat conduction material according to any one of claims 1 to 9, wherein the bonding medium is an organic bonding material, the bonding medium and the organic matrix are made of the same polymer system, and the molecular weight of the bonding medium is lower than that of the organic matrix.

12. The composite heat conductive material according to any one of claims 1 to 11, wherein the ratio of the particle size of the large-particle size particles to that of the small-particle size particles is greater than 20.

13. The composite heat conductive material according to claim 12, wherein the heat conductive filler further comprises medium-sized particles having an average particle diameter smaller than that of the large-sized particles and larger than that of the small-sized particles.

14. The composite heat conductive material of any one of claims 1 to 13, wherein the small-sized particles have a specific surface area of greater than 1m ² /g。

15. The composite heat conductive material of any one of claims 1 to 14, wherein the large-particle size particles are surface treated to increase hydroxyl content at the surface of the large-particle size particles, the surface oxygen content of the large-particle size particles being >10%.

16. The composite heat conductive material according to any one of claims 1 to 15, wherein the surface of the small particle size particles contains hydroxyl groups.

17. The composite heat conductive material of any one of claims 1 to 16, wherein the total weight percentage of the heat conductive filler in the composite heat conductive material is 87% or more.

18. The composite heat conductive material of any one of claims 1 to 17, wherein a volume percentage of all heat conductive fillers in the composite heat conductive material is 76% or more.

19. The composite heat conductive material according to any one of claims 1 to 18, wherein the number of the first small particle size particles accounts for 50% or less of the number of small particle size particles having a particle size of 10 μm or less in the heat conductive filler.

20. An electronic device comprising an electronic component and a cured product of the composite thermal conductive material of any one of claims 1 to 19 disposed on the electronic component.

21. The electronic device of claim 20, further comprising a heat sink disposed on the electronic component, wherein an interfacial thermal conductive material is disposed between the electronic component and the heat sink, and wherein the interfacial thermal conductive material is a cured product of the composite thermal conductive material of any one of claims 1 to 19.

22. A composite thermally conductive material, comprising:

an organic matrix;

a plurality of large particle size particles;

23. The composite heat conductive material of claim 22, wherein the large-sized particles have a thermal conductivity higher than that of the self-fusing heat conductive particles.

24. The composite heat conductive material of claim 22 or 23, wherein the large-particle size particles are diamond particles.

25. The composite heat conductive material of any one of claims 22 to 24, wherein the large-sized particles have an average particle size of greater than 5 μ ι η.

26. The composite heat conductive material of any one of claims 22 to 25, wherein the metal particles comprise at least one of nanosilver, nanogold, microsilver, microcapike, microsilold.

27. The composite heat conductive material of any one of claims 22 to 26, wherein the sintering temperature of the metal particles is 120 to 250 degrees.

28. The composite heat conductive material of any one of claims 22 to 27, wherein the metal particles are surface coated with a surfactant.

29. The composite heat conductive material of any one of claims 22 to 25, wherein the alloy particles have a melting point of 150 degrees or less.

30. The composite heat conductive material of any one of claims 22 to 25 and 29, wherein the alloy particles are at least one of Sn-Cu, sn-Al, sn-Zn, sn-Pt, sn-Mn, sn-Mg, sn-Ag, sn-Au, sn-Bi, sn-In, sn-Pd, sn-Bi-In, bi-Pb-Sn, bi-Pb, al-Li, ga-In-Sn, ga-In, ga-Bi-Pb-In, and Zn-Li.

31. The composite heat conductive material according to any one of claims 22 to 30, wherein the surface of the large-particle-size particles is provided with a bonding medium layer for realizing the bonding of the large-particle-size particles and the fused self-fusing heat conductive particles.

32. The composite heat conducting material of claim 31, wherein when the self-fusing heat conducting particles are metal particles, the material of the bonding medium layer is a metal material of the metal particles; when the self-fusion heat conduction particles are alloy particles, the material of the bonding medium layer is an intermetallic compound of at least one metal in the alloy particles.

33. The composite heat conductive material according to any one of claims 22 to 25, wherein when the self-fused heat conductive particles are metal particles, the self-fused heat conductive particles occupy 50 to 85% of the volume of the heat conductive filler.

34. The composite heat conductive material of any one of claims 22 to 25, wherein when the self-fusing heat conductive particles are alloy particles, the volume proportion of the self-fusing heat conductive particles in the heat conductive filler is less than 50%.

35. The composite heat conductive material of any one of claims 22 to 34, wherein the thermal conductivity of the composite heat conductive material after curing is more than 5 times higher than the thermal conductivity of the composite heat conductive material before curing.

36. A thermally conductive material which is a cured product of the composite thermally conductive material as claimed in any one of claims 22 to 35, said thermally conductive material comprising fused bodies formed by fusing said self-fusing thermally conductive particles to each other, said fused bodies being bonded to said large-size particles.

37. An electronic device comprising an electronic component and the thermally conductive material of claim 36 disposed on the electronic component.

38. The electronic device of claim 37, further comprising a heat sink disposed on the electronic component, wherein an interfacial thermally conductive material is disposed between the electronic component and the heat sink, and wherein the interfacial thermally conductive material is the thermally conductive material of claim 36.

39. A diamond-based thermally conductive filler, comprising:

diamond particles, the diamond particles being in the shape of a polyhedron;

40. The diamond based heat conductive filler according to claim 39, wherein the average particle size ratio of the diamond particles to the small heat conductive particles is greater than 20.

41. The diamond based heat conductive filler according to claim 39 or 40, wherein the material of the small heat conductive particles is at least one selected from the group consisting of oxides, nitrides, carbides, metals, and carbon materials.

42. The diamond-based heat conductive filler according to any one of claims 39 to 41, wherein the small heat conductive particles have an average particle diameter of 10 μm or less.

43. The diamond based heat conductive filler according to any one of claims 39 to 42, wherein the bonding medium is an inorganic bonding medium.

44. The diamond-based heat conductive filler according to any one of claims 39 to 42, wherein the bonding medium is an organic bonding medium containing a coupling agent added thereto so as to be compatible with the organic bonding medium.

45. The diamond based heat conductive filler according to any one of claims 39 to 44, wherein the bonding medium coats the diamond particles by a thickness not exceeding a thickness of the small heat conductive particles coating the diamond particles.

46. The diamond based heat conductive filler according to any one of claims 39 to 45, wherein the plurality of small heat conductive particles consists of a plurality of particles having an average particle size <10 μm or a mixture of particle size distributions having an average particle size <10 μm.

47. The diamond-based thermally conductive filler according to any one of claims 39 to 46, wherein the thermal conductivity of the diamond particles is 600W/mk or more, and the diamond particles are artificial diamond or graphitized diamond.

48. The diamond based heat conductive filler according to any one of claims 39 to 47, wherein the sphericity of the shape of the combination of the diamond particles and the plurality of small heat conductive particles coating it is 0.7 or more.

49. The diamond based heat conductive filler according to any one of claims 39 to 48, wherein the surface oxygen content of the diamond based heat conductive filler is >5%.

50. The diamond based heat conductive filler according to any one of claims 39 to 49, wherein an outer surface of the diamond based heat conductive filler is constituted of exposed small heat conductive particles.

51. The diamond based heat conductive filler according to any one of claims 39 to 50, wherein the thickness of the small heat conductive particles coating the diamond particles is not more than 10 times the particle size of the small heat conductive particles.

52. The diamond based heat conductive filler according to any one of claims 39 to 51, wherein an outer surface of the diamond based heat conductive filler has a rugged microstructure, and a specific area of the diamond based heat conductive filler is 3 times or more a specific surface area of the diamond particles.

53. A composite heat conductive material comprising an organic matrix and the diamond-based heat conductive filler of any one of claims 39 to 52 dispersed in the organic matrix.

54. An electronic device comprising an electronic component and a cured product of the composite thermal conductive material of claim 53 disposed on the electronic component.

55. The electronic device of claim 54, further comprising a heat sink disposed on the electronic component, wherein an interfacial thermally conductive material is disposed between the electronic component and the heat sink, and wherein the interfacial thermally conductive material is a cured product of the composite thermally conductive material of claim 53.

56. A preparation method of a diamond-based heat-conducting filler is characterized by comprising the following steps:

57. The method for preparing a diamond-based heat conductive filler according to claim 56, wherein the material of the small heat conductive particles is at least one selected from the group consisting of oxides, nitrides, carbides, metals, and carbon materials.

58. The method for preparing a diamond-based heat conductive filler according to claim 56 or 57, wherein the small heat conductive particles have an average particle diameter of 10 μm or less.

59. The method for preparing a diamond-based heat conductive filler according to any one of claims 56 to 58, wherein the bonding medium coats the diamond particles to a thickness not exceeding the thickness of the small heat conductive particles coating the diamond particles.