JP2014133669A - Thermal interface material and thermal interface method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a graphite-made TIM that solves problems of conventional polymer/inorganic complexes and has high performance and exhibits high thermal resistance.SOLUTION: This invention relates to a thermal interface material having a graphite film with a thickness of 10-15 μm. It is preferable that a thermal conductivity of the graphite film in film surface direction is 500 W/mK or more, and an anisotropy in thermal conductivity between film surface direction and thickness direction is 100 or more.

Description

The present invention relates to an interlayer thermal connection member and an interlayer thermal connection method for quickly transferring heat from a heat generation source to a cooling / radiating part.

In recent years, heat generation in electronic devices such as mobile phones, personal computers, PDAs, and game machines, and LED lighting should be solved in the electronics field due to an increase in heat generation accompanying higher speeds of microprocessors and higher performance of LED chips. It has become a big issue. There are methods that use heat conduction, heat radiation, and heat convection for heat dissipation and cooling. A heat sink or an air cooling fan is used as a cooling method that uses heat convection, and a ceramic plate is used as a method that uses heat radiation. Examples of materials that use conduction include various heat conduction (diffusion) sheets and heat conductive resins. To effectively dissipate and cool the heat from the heat source, it is necessary to combine these heat dissipating and cooling methods to efficiently transfer the heat from the heat generating part to the heat dissipating and cooling parts such as circuit boards, cooling fins, and heat sinks. For this purpose, it is important to reduce the thermal resistance between the heat generating part and the heat radiating / cooling part.

As shown in FIG. 1, even if solid layers (metals or metals and ceramics) are simply connected, the contact between the layers becomes point contact due to the unevenness of the surface of the member, resulting in thermal conductivity between the layers. Since the low air layer (thermal conductivity: 0.02 W / mK) exists, the thermal resistance increases. An interlayer thermal connection member (Thermal Interface Material, hereinafter abbreviated as TIM) is used to lower the thermal resistance between such layers, and is used by being sandwiched between metals or between a metal and a ceramic. In this case, it is important that the TIM itself has a high thermal conductivity and that the thermal resistance at the interface (between the member and the TIM) is small. In order to reduce the thermal resistance at the interface, it is necessary to increase the contact area of the interface (that is, the contact at the interface is a surface contact), and flexibility is required. Conventionally, a mixture of a flexible polymer material for making the interface a surface contact and a high thermal conductivity inorganic filler for achieving a high thermal conductivity has been used (hereinafter abbreviated as a polymer / inorganic composite). . FIG. 2 shows a connection state between the layers. The interface is in surface contact by the polymer / inorganic composite, and the air layer is removed from the layers, so that the thermal resistance between the layers can be reduced. In general, the polymer / inorganic composite TIM has a thermal conductivity of 1 to 5 W / mK and a thickness of about 0.5 to 5 mm.

However, if the amount of the high thermal conductive inorganic filler added is increased due to the high thermal conductivity, there is a problem that the flexibility is impaired and the thermal resistance of the interface increases. Therefore, at present, only a product having a thermal conductivity of about 1 to 2 W / mK for a normal polymer / inorganic composite and about 5 W / mK for a high thermal conductivity product has been commercialized. The practical thermal resistance value between layers is the sum of the thermal resistance of the TIM itself and the thermal resistance at the interface, and is usually about 0.4 to 3.0 K · cm ² / W in many cases. Since this thermal resistance value also changes depending on the magnitude of the pressure applied to the connecting surface, it is necessary to indicate the magnitude of the pressure in order to display the thermal resistance value.

In addition, in the case of a polymer / inorganic composite, a flexible polymer material, which is a matrix resin for flexibility, is limited to several types of polymers such as acrylic resin or silicone resin. Has a problem in heat resistance, and in the case of a silicone resin, a silicone monomer generated at a high temperature causes a contact failure of an electronic circuit.

By the way, because of its excellent heat resistance, chemical resistance, high thermal conductivity, and high electrical conductivity, graphite has a wide range of energy, space, medical, etc. as structural materials, reinforcing materials, sliding materials, conductive materials, heat dissipation sheets, etc. A graphite film is known as a thermal diffusion film although it is used in the field and occupies an important position as an industrial material and is not used for TIM. The basic structure of graphite crystal is a layered structure in which the basal planes of carbon atoms connected in a hexagonal network are stacked regularly (the direction in which the layers are stacked is called the c-axis direction, and the carbon atoms connected in a hexagonal network The direction in which the basal plane formed by is referred to as the ab plane direction). The carbon atoms in the basal plane are strongly bonded by a covalent bond, while the bonds between the stacked basal planes are due to weak van der Walls forces, and the basal plane spacing is 0.3354 nm. Reflecting such anisotropy, the electrical conductivity and thermal conductivity of graphite are large in the ab plane direction, and the electrical conductivity and thermal conductivity in this direction are good indicators for determining the quality of graphite. The thermal conductivity in the ab plane direction of the highest quality graphite known conventionally is 1800 W / mK, and the thermal conductivity in the c-axis direction is 5 W / mK (Non-Patent Documents 1 and 2).

As a method for obtaining a film-like high-quality, high-heat-conductive graphite, a method of directly heat-treating a special polymer to carbonize / graphite (hereinafter abbreviated as a polymer firing method) has been developed. Examples of the polymer used include polyoxadiazole, polyimides, and polyphenylene vinylene (Non-Patent Documents 3 and 4). High thermal conductivity graphite films obtained by such a method are widely used as thermal conduction (thermal diffusion) sheets (for example, Patent Documents 1 to 3). The heat conductive sheet is used for the purpose of improving heat dissipation / cooling efficiency by spreading the heat of a heat source such as a CPU or LED over a wide range. The reason why the graphite sheet is used for such a purpose is that it has a very large thermal conductivity in the ab plane direction and is optimal as a heat countermeasure material in a small electronic device such as a cellular phone. These high thermal conductive graphite films are usually often combined with various polymer films and metals for imparting adhesion, improving mechanical strength, or imparting insulating properties (Patent Documents). 4-6).

However, the application of graphite as a heat conduction (thermal diffusion) sheet is fundamentally different from TIM applications, and at present, graphite TIM has not been put into practical use. The reason is that the thermal conductivity in the thickness direction, which is important when using graphite as a TIM, is at most about 5 W / mK. Since graphite is a solid, it does not come into surface contact at the interface and has high thermal resistance at the interface. Because it has been considered to be.

As a countermeasure, it is conceivable that the ab plane having a high thermal conductivity is oriented in a direction perpendicular to the film surface of the graphite film, and that the graphite film is used as a TIM. If such an orientation can be realized, in principle, high thermal conductivity (1800 W / mK) in the surface direction of graphite can be used for interlayer thermal connection (Patent Documents 7 to 9). However, these are produced by mechanically cutting laminated or cylindrical graphite blocks, and it is not only impossible to increase the area and reduce the thickness to 1 mm or less. However, this is an extremely difficult manufacturing method and is not a practical method.

Also, as a contrivance to reduce the thermal resistance at the interface in order to use the graphite film as a TIM, a method of reducing the thermal resistance between the layers by using an expanded (that is, having a reduced density) compressed graphite film has been reported. (Patent Document 10). However, in the graphite film subjected to such a treatment, an air layer is formed inside the graphite film by foaming, so that the thermal resistance of the graphite film itself becomes high and the purpose as a TIM cannot be achieved.

Furthermore, a thermal management assembly formed from a graphite layer and structurally supported by a metal, a polymer, a ceramic, or the like is considered (Patent Document 11). However, this thermal management assembly is not essentially different from conventional polymer / inorganic composites formed from highly thermally conductive inorganic fillers and flexible polymeric materials, and the disadvantages of conventional TIMs are completely improved. Not. That is, in the case of a structure with metal or ceramic, the point contact at the interface cannot be basically a surface contact, and in the case of a composite with a polymer, even if the polymer is flexible, it is It is not different from a molecular / inorganic composite.

A film that is generally available as a graphite film by a polymer baking method has a thickness of 17.5 to 75 μm. For example, the interlayer thermal resistance of a 17.5 μm graphite film is 3.2 K · cm ² / W (pressure: 1.0 kgf / cm ² ), and the properties as a TIM are a polymer / inorganic having a relatively high thermal resistance value. It was only comparable to the complex.

JP 2003-158393 A JP 2005-314168 A JP 2006-0044999 A JP 2008-171030 A JP 2010-001191 A JP 2011-023670 A JP 2009-295921 A JP 2010-189244 A JP 2011-23670 A JP 2007-217206 A JP 2007-273934 A

L. Spain, A. R. Ubbelohde, and D. A. Young. "Electronic properties of oriented graphite" PHILOSOPHICAL TRANSACTIONS OF THE ROYALSOCIETY T. C. Chieu, M. S. Dresselhaus and M. Endo, Phys. Rev. B26, 5867 (1982) M. Murakami, N. Nishiki, K. Nakamura, J. Ehara, H. Okada, T. Kouzaki, K. Watanabe, T. Hoshi, and S. Yoshimura, Carbon, vol. 30, 2, 255 (1992) M. Inagaki, T. Takechi, Y. Hishiyama, and A. Oberin, Chem. Phys. Carbon, 26, 245 (1999)

An object of the present invention is to solve the problems of conventional polymer / inorganic composites and to provide a high-performance, high-heat-resistant graphite TIM.

As a result of diligent studies regarding the use of a graphite film as a TIM, the present inventors have found that the graphite film TIM can realize a thermal connection with a low thermal resistance exceeding that of a polymer / inorganic composite, thereby completing the present invention. did.

That is, the present invention relates to an interlayer thermal connection member having a graphite film having a thickness of 10 nm to 15 μm.

The thermal conductivity in the film surface direction of the graphite film is preferably 500 W / mK or more, and the anisotropy of the thermal conductivity in the film surface direction and the thickness direction is preferably 100 or more.

It is preferable that the density of the graphite film is 1.2 to 2.26 g / cm ³ .

The graphite film is preferably obtained by heat-treating a polymer film having a thickness of 50 nm to 50 μm at a temperature of 2000 ° C. or higher.

The polymer film preferably contains a condensed aromatic polymer.

The condensed aromatic polymer is polyamide, polyimide, polyquinoxaline, polyoxadiazole, polybenzimidazole, polybenzoxazole, polybenzthiazole, polyquinazolinedione, polybenzoxazinone, polyquinazolone, benzimidazolobenzophenanthroline ladder polymer And at least one selected from these derivatives.

It is preferable that the interlayer thermal connection member further contains 1 to 100% by weight of a fluid substance having fluidity at room temperature or higher with respect to the weight of the graphite film.

The fluid material is preferably oil having a boiling point of 200 ° C. or higher.

The present invention also relates to an interlayer thermal connection method including a step of installing the interlayer thermal connection member of the present invention between members to be thermally connected.

According to the present invention, an interlayer heat connection member having excellent heat connection characteristics equivalent to or better than conventional polymer / inorganic composites and having excellent environmental stability including heat resistance is provided. Furthermore, a low thermal resistance interlayer thermal connection can be realized.

It is a conceptual diagram between the layers when not using TIM. It is a conceptual diagram of the interlayer connection state at the time of using a polymer / inorganic composite. Black circles (●) in the figure are high thermal conductive inorganic fillers. It is a conceptual diagram of the interlayer connection state at the time of using the graphite film TIM of this invention. It is a photograph of the graphite film (F) manufactured in the Example. It is a limited visual field electron diffraction image of a graphite film (F).

The interlayer thermal connection member of the present invention has a graphite film having a thickness of 10 nm to 15 μm. Since the graphite film is thin, the film itself is flexible, and if it is 15 μm or less, it can be compressed in the same way as a conventional graphite film of thickness, and the interface is not a point contact, even if it is not softened. A situation close to contact can be realized.

Here, in this specification, a member between members that may cause a temperature difference by use is defined as an interlayer. Further, it is defined that an interlayer thermal connection is to thermally connect layers and transfer heat from one member to the other member, and an interlayer thermal connection member is a member for that purpose. That is, in order to dissipate the heat from the heat generation source efficiently, the thermal resistance between the heat generation source or the layer thermally connected to the heat generation source and the second layer at a lower temperature is reduced. The member sandwiched between the layers for reduction is an interlayer thermal connection member.

There are two reasons why a conventional graphite film cannot be used as a TIM.
(1) Although graphite is flexible, it is harder than a polymer / inorganic composite, and therefore has a high thermal resistance at the interface.
(2) As a method of softening a graphite film having a thickness of 17.5 μm, there is known a method of compressing graphite after foaming, and such a softening treatment has an effect of increasing a contact surface. However, there are many air layers inside the graphite film, resulting in an increase in the thermal resistance of the graphite film. That is, the method of softening the graphite film by reducing the density and reducing the connection interface thermal resistance increases the bulk thermal resistance of the graphite film as the TIM because an air layer is formed inside the graphite film. The heat resistance cannot be reduced.

The basic idea of the present invention is as follows.
(1) The thermal conductivity in the thickness direction of the graphite film is 5 W / mK, and the value is particularly superior to the bulk thermal conductivity of the high thermal conductivity (high performance) type polymer / inorganic composite. If not, the characteristics are equivalent or better.
(2) Since graphite is a solid, the thermal resistance at the interface tends to be higher than that of the polymer / inorganic composite. On the other hand, since graphite is a flexible and elastic material, the interface is somewhat affected. Improve from contact to surface contact.
(3) Since the thermal conductivity in the film surface direction of a graphite film is extremely large (500 to 1800 W / mK), if the film is sandwiched between layers, heat is transferred from one contact point (contact surface) to another contact point (contact Surface) and effectively has the same effect as increasing the number of contact points (contact surfaces).
(4) In the case of such a graphite film, it is preferable that the graphite film is as thin as possible in order to reduce the thermal resistance due to the graphite film itself. Thus, characteristics superior to the polymer / inorganic composite can be realized.

The principle of use of the graphite film TIM is shown in FIG. As shown in FIG. 3, the connection surface is not completely covered (the air layer at the interface is not completely removed), but graphite is a flexible and elastic material, so that a certain degree of surface contact can be realized. In addition, since the thermal conductivity of the graphite film is extremely high in the surface direction, heat spreads from a single contact surface (point) over a wide area in the film surface direction, and the interlayer thermal resistance is equivalent to interface contact on many (wide) surfaces. There is a reduction effect. Furthermore, since the thickness is thinner than the polymer / inorganic composite, the bulk thermal resistance in the direction perpendicular to the graphite film surface can be made extremely small.

(Graphite film)
Although the minimum of the thickness of a graphite film is 10 nm, 20 nm or more is preferable and 50 nm or more is more preferable. Moreover, although an upper limit is 15 micrometers, 10 micrometers or less are preferable, 5 micrometers or less are more preferable, and 3 micrometers or less are especially preferable. If it exceeds 15 μm, the graphite film that contains almost no air layer inside is hard and has little flexibility, and when used as a TIM, it tends to crack and crack due to pressure, and because it is hard, it creates an air layer at the interface. There is a problem that the interfacial thermal resistance is increased. In addition, in the graphite film subjected to the softening treatment, the value of the thermal resistance of the graphite film itself becomes too large due to the air layer existing inside the film, and it exceeds the characteristics of the polymer / inorganic composite TIM. Have difficulty. On the other hand, if it is less than 10 nm, the thermal resistance of the film itself becomes small, but it becomes extremely difficult to handle as a self-supporting film. If it is 10 nm-15 micrometers in thickness, it has the heat resistance characteristic far superior to the conventional polymer / inorganic filler composite, and is excellent in heat resistance and durability compared with the conventional TIM.

The thermal conductivity in the plane direction of the graphite film is preferably 500 W / mK or more, and more preferably 1000 W / mK or more. If it is less than 500 W / mK, there exists a tendency for a softness | flexibility and compressibility to fall. Further, the anisotropy of the thermal conductivity in the thickness direction and the plane direction is preferably 100 or more, and more preferably 200 or more. If it is less than 100, there exists a tendency for a softness | flexibility and compressibility to fall. Here, the thermal conductivity anisotropy refers to the value of the thermal conductivity in the film surface direction and the value of the thermal conductivity in the thickness direction, the larger thermal conductivity value, the smaller thermal conductivity value. Divided by the value of.

The density of the graphite film is preferably 1.2~2.26g / cm ^3, more preferably 1.4~2.26g / cm ^3, more preferably 1.6~2.26g / cm ^3. If it is less than 1.2 g / cm ³ , the air layer in the graphite film increases and the thermal conductivity of the graphite film tends to decrease. The density of 2.26 g / cm ³ is the density of graphite that does not contain any air layer, and whether or not the air layer is contained in the graphite film can be confirmed by measuring the density of the graphite film. Since the thermal conductivity in the thickness direction of the graphite film can be improved, it is desirable that there is not much air layer inside the graphite film.

When such a value of thermal conductivity and its anisotropy are satisfied, a graphite film having both flexibility and compressibility preferable as TIM can be provided. High quality graphite is basically soft and compressed in the thickness direction of the film, and a high performance TIM can be realized by utilizing the inherent properties of the graphite film and the flexibility developed by its thinness.

(Method for producing graphite film)
The manufacturing method of a graphite film is not specifically limited, For example, it can obtain by the method of heat-processing a polymer film. The polymer film is preferably a condensed aromatic polymer from the viewpoint of a high carbon content and a carbocyclic structure similar to the ab plane of graphite. Among them, selected from polyamide, polyimide, polyquinoxaline, polyoxadiazole, polybenzimidazole, polybenzoxazole, polybenzthiazole, polyquinazolinedione, polybenzoxazinone, polyquinazolone, benzimidazolobenzophenanthroline ladder polymer, and derivatives thereof It is preferable that it is at least one kind. The polymer film can be produced by a known production method.

Among the above polymer films, a polyimide film is preferable because it can be converted into high-quality graphite, and the effect of increasing the thermal conductivity appears remarkably. Among polyimide films, a film having a controlled molecular structure and higher order structure, that is, a film excellent in orientation, is preferable because it can be easily converted into graphite. In general, in order for the graphitization reaction to proceed smoothly, it is necessary to rearrange the molecules in the carbon precursor, but a polyimide film with excellent orientation requires less rearrangement. However, it is presumed that this is because the conversion to sufficiently high thermal conductivity graphite proceeds.

The polyimide film can be produced, for example, by casting an organic solvent solution of polyamic acid, which is a polyimide precursor, on a support such as an endless belt or a stainless drum, followed by drying and imidization. It can also be produced by heating and imidizing a polyamic acid thin film formed by vapor deposition polymerization.

An organic solvent solution of polyamic acid can be produced by a known method. Usually, at least one aromatic dianhydride and at least one diamine are dissolved in an organic solvent in substantially equimolar amounts, and the resulting solution is subjected to controlled temperature conditions. The polyamic acid solution is prepared by stirring until the polymerization of the acid dianhydride and the diamine is completed. These polyamic acid solutions are usually obtained at a concentration of 5 to 35% by weight, preferably 10 to 30% by weight. When the concentration is in this range, an appropriate molecular weight and solution viscosity can be obtained.

In the polyamic acid solution, graphite such as calcium hydrogen phosphate (CaHPO ₄ ), calcium phosphate (Ca ₃ (PO ₄ ) ₂ ), calcium dihydrogen phosphate (Ca (H ₂ PO ₄ ) ₂ ) is optionally added. An additive having an effect of foaming may be added.
These additives having a foaming effect are preferably added in an amount of 0 to 0.1 parts by weight based on 100 parts by weight of the total weight of the raw material components of the polyamic acid. If the amount added is large, the air layer in the graphite film may increase.

The polyamic acid thin film can be produced by a known method such as vapor deposition polymerization. In the vapor deposition polymerization, at least one kind of aromatic acid dianhydride as a raw material monomer and at least one kind of diamine are separately heated in advance, and a vacuum state tank in which a substrate for forming a vapor deposition polymerization film is installed. A polyamic acid thin film is formed on the substrate by introducing respective vapors so as to be approximately equimolar at the same time. As the monomer, those used for producing a normal polyimide can be used, but monomers that are difficult to use in polymerization by a general solution method and hardly soluble in a solvent can also be used. In addition, unlike ordinary solution methods, no impurities such as solvents and additives remain in the formed polyamic acid thin film, which makes it easy to obtain a graphite film having desired characteristics such as high crystallinity and high electrical conductivity. There are advantages.

Examples of the aromatic acid dianhydride include pyromellitic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride. 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,2 ′, 3,3′-biphenyltetracarboxylic dianhydride, 3,3 ′, 4,4′-benzophenonetetracarboxylic dianhydride Anhydride, 2,2-bis (3,4-dicarboxyphenyl) propane dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, bis (3,4-dicarboxyphenyl) propane Anhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, 1,1-bis (3,4-dicarboxyphenyl) ethane dianhydride, bis (2,3-dicarboxyphenyl) ) Methane dianhydride Bis (3,4-dicarboxyphenyl) ethane dianhydride, oxydiphthalic dianhydride, bis (3,4-dicarboxyphenyl) sulfone dianhydride, p-phenylenebis (trimellitic acid monoester acid anhydride ), Ethylene bis (trimellitic acid monoester acid anhydride), bisphenol A bis (trimellitic acid monoester acid anhydride) and the like. These may be used alone or in combination of two or more.

Examples of the diamine include 4,4′-oxydianiline, p-phenylenediamine, 4,4′-diaminodiphenylpropane, 4,4′-diaminodiphenylmethane, benzidine, 3,3′-dichlorobenzidine, 4,4 ′. -Diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 1,5 -Diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, 4,4'-diaminodiphenyl N-methylamine, 4,4 ' -Diaminodiphenyl N-pheny Amine, 1,4-diaminobenzene (p- phenylene diamine), 1,3-diaminobenzene, it may be mentioned 1,2. These may be used alone or in combination of two or more.

The method of imidizing the polyamic acid is not particularly limited, and a thermal curing method in which the polyamic acid as a precursor is converted to an imide by heating, a dehydrating agent typified by an acid anhydride such as acetic anhydride, polycholine, quinoline, or the like. There may be mentioned a chemical cure method in which tertiary amines such as isoquinoline and pyridine are used as an imidization accelerator and imide conversion is performed. The resulting polyimide film has a low coefficient of linear expansion, high modulus of elasticity, a high birefringence, and is not damaged even if tension is applied during the firing of the film. The chemical curing method is preferable from the viewpoint that it can be performed.

The following method is mentioned as a concrete manufacturing method of the film by a chemical curing method. First, a stoichiometric or higher stoichiometric dehydrating agent and a catalytic amount of an imidization accelerator are added to a polyamic acid solution, and cast or coated on a support plate, an organic film such as PET, a drum or an endless belt. A film having a self-supporting property is obtained by forming a film and evaporating the organic solvent. Next, this is further heated and dried to be imidized to obtain a polyimide film made of a polyimide polymer. The temperature during heating is preferably in the range of 150 ° C to 550 ° C. There is no particular limitation on the rate of temperature increase during heating, but it is preferable to gradually or intermittently heat so that the maximum temperature becomes the above temperature. Furthermore, it is preferable to include a step of fixing or stretching the film in order to prevent shrinkage during the polyimide manufacturing process.

Next, techniques for carbonization and graphitization of polymer films are described. The carbonization method is not particularly limited, and for example, the carbonization is performed by preheating a polymer film as a starting material in an inert gas. As the inert gas, nitrogen, argon or a mixed gas of argon and nitrogen is preferably used. Preheating is usually performed at a temperature of about 1000 ° C. In order to prevent the orientation of the starting polymer film from being lost in the preheating stage, it is effective to apply a tension in the plane direction that does not cause the film to break.

The method of graphitization is not particularly limited. For example, the film carbonized by the above method is set in a high temperature furnace and graphitization is performed. Graphitization is performed in an inert gas. Argon is most suitable as the inert gas, and a small amount of helium may be added to argon. The higher the treatment temperature, the higher the quality of graphite that can be converted, preferably 2000 ° C or higher, more preferably 2400 ° C or higher, and most preferably 2800 ° C or higher. In order to create such a high temperature, an electric current is usually passed directly through a graphite heater, and heating is performed by using the Joule heat.

The thickness of the finally obtained graphite film is generally about 60 to 30% of the thickness of the original polymer film when the thickness of the starting polymer film is 1 μm or more, and 40% to 20 when the thickness is less than 1 μm. It is often about%. On the other hand, the length is often about 100 to 60%. Therefore, in order to finally obtain a graphite film having a thickness of 10 nm to 15 μm, the thickness of the starting polymer film is preferably 50 nm to 50 μm, more preferably 50 nm to 25 μm, and particularly preferably 100 nm to 10 μm.

(Fluid material)
It is preferable that the interlayer thermal connection member further includes a fluid substance having fluidity at room temperature or higher. By wetting the surface of the graphite film with a fluid material, the thermal resistance of the interface can be lowered, the thermal resistance between the layers can be reduced, and excellent interlayer thermal connection can be realized. The content of the fluid substance is preferably 1 to 100% by weight and more preferably 2 to 50% by weight with respect to the weight of the graphite film. If it is less than 1% by weight, the effect of adding tends to be hardly obtained. If it exceeds 100% by weight, the thermal connection resistance of the composite tends to increase due to the thermal resistance of the flowable substance. Here, “fluidity” means a liquid state that easily flows at room temperature, or a paste that is highly viscous and does not flow easily, but can be deformed or spread by applying pressure. Means state.

Although it does not specifically limit as a fluid substance, An oily substance and a fluid polymer can be mentioned. As the oily substance, mineral oil, vegetable oil, synthetic oil, essential oil, edible oil, animal oil, and mixtures thereof are preferable. As the fluid polymer, a silicone resin is preferable. Above all, in order not to lose the high heat resistance and high durability, which are one of the characteristics of TIM, a substance having a low vapor pressure is desirable. The oily substance preferably has a boiling point at 1 atm of 200 ° C. or more, more preferably 300 ° C. or more, and the silicone resin preferably has fluidity between 20 ° C. and 100 ° C.

The method of including the fluid substance is not particularly limited, and it can be included by applying the fluid substance on the surface of the graphite film or immersing the graphite film in the fluid substance.

(Interlayer thermal connection member)
The interlayer thermal connection member of the present invention may be composed only of a graphite film, and may further include the above-described fluid substance, a metal thin film, an inorganic thin film, a plurality of graphite films, and the like.

(Interlayer thermal connection method)
The interlayer thermal connection method of the present invention includes a step of installing the interlayer thermal connection member between the members to be thermally connected. By sandwiching between the layers, the heat generation source or a member thermally connected to the heat generation source can perform interlayer heat connection for transferring heat to the second member having a temperature lower than that. The graphite film is sandwiched and installed between a member close to the heat source and a member far from the heat source, and the graphite film and each member are in direct surface contact. In order to realize excellent heat-resistant interlayer thermal connection, it is preferable to thermally connect the layers only with a graphite film without using various adhesive layers. As a method for realizing the interlayer thermal connection without using the adhesive layer, it may be simply fixed by mechanical pressure. Mechanically caulking with screws, screws, springs or the like is effective and preferable for direct thermal connection.

The layer for sandwiching the graphite film is not particularly limited, but aluminum (thermal conductivity: 237 W / mK), copper (thermal conductivity: 398 W / mK), silver (thermal conductivity: 428 W / mK), nickel ( Metal materials with excellent thermal conductivity such as thermal conductivity: 90 W / mK, silica (thermal conductivity: 1.5 W / mK), alumina (thermal conductivity: 20 W / mK), magnesium oxide (MgO) (heat Conductivity: 40 W / mK), boron nitride (BN) (thermal conductivity: 60 W / mK), aluminum nitride (AlN) (thermal conductivity: 70-270 W / mK), silicon carbide (SiC) (thermal conductivity: Ceramic materials such as 88-128 W / mK (depending on the crystal system), or a combination of these can be used.

Hereinafter, the present invention will be described specifically by way of examples. In addition, although an Example demonstrates the graphite film from which thickness, the thermal conductivity of a surface direction, and a density differ, this invention is not limited to these Examples.

In addition, the physical property value of the graphite film was measured by the method shown below.
<Thermal conductivity>
The thermal diffusivity of the graphite film was measured at 23 ° C. under vacuum at 10 Hz using a thermal diffusivity measuring device (“LaserPit” manufactured by ULVAC-RIKO). Thermal conductivity was calculated from the measured thermal diffusivity using values of density and specific heat.

<Density>
The density of the graphite film was calculated by dividing the weight (g) of the graphite film by the volume (cm ³ ) calculated by the product of the vertical, horizontal and thickness of the graphite film. The thickness of the graphite film was determined by observing the cross section of the graphite film with a scanning electron microscope (SEM).

<Interlayer thermal resistance>
A graphite model heater (simulating a CPU as a heating element) and a copper block (15 cm x 15 cm in length and width), each having a shape of 10 mm x 10 mm in length and width of 1.8 mm, cut into a 10 mm x 10 mm size graphite film. A pressure of 1.0 kgf / cm ^{2 to} 4.5 kgf / cm ² was applied. Next, 2.0 W of electric power was supplied to the model heater, and the temperature of the model heater and the temperature of the copper block after 5 minutes were measured using thermocouples embedded therein. The thermal resistance value R (K · cm ² / W) between the model heater and the copper block was calculated using the following equation, where the temperature of the model heater was T ₁ and the temperature of the copper block was T ₂ .
R = (T ₁ −T ₂ ) / 2

[Examples 1-7]
To 100 g of an 18% by weight DMF solution of polyamic acid prepared from pyromellitic dianhydride, 4,4′-diaminodiphenyl ether and p-phenylenediamine (4/3/1 in molar ratio), 20 g of acetic anhydride and A curing agent consisting of 10 g of isoquinoline was mixed and stirred, defoamed by centrifugation, and cast onto an aluminum foil. The process from stirring to defoaming was performed while cooling to 0 ° C. The laminate of the aluminum foil and the polyamic acid solution was heated at 120 ° C. for 150 seconds to obtain a gel film having self-supporting properties. This gel film was peeled off from the aluminum foil and fixed to the frame. This gel film was heated at 300 ° C., 400 ° C., and 500 ° C. for 30 seconds each, and an average linear expansion coefficient of 100 to 200 ° C. was 1.6 × 10 ⁻⁵ cm / cm / ° C., a birefringence of 0.14, and a thickness. Seven kinds of polyimide films having different thicknesses were produced. The film thicknesses were 25.5 μm, 12.1 μm, 5.3 μm, 2.1 μm, 1.0 μm, 0.6 μm, and 0.3 μm, respectively.

The obtained polyimide film was pretreated by heating to 1000 ° C. at a rate of 10 ° C./min in nitrogen gas using an electric furnace and keeping at 1000 ° C. for 1 hour. Next, as a graphitization treatment, the obtained carbonized film is sandwiched between surface-polished graphite blocks, set inside a cylindrical graphite heater, and heated to 2800 ° C. at a temperature rising rate of 20 ° C./min. The temperature was kept for one minute, and then the temperature was lowered at a rate of 40 ° C./min. The graphitization treatment was performed under a pressure of 0.5 kg / cm ^{2 in} an argon atmosphere. It was found that this polyimide film can be converted to high-quality graphite at 2800 ° C.

The thickness, thermal conductivity, and density of the obtained seven types (A to G) of graphite films were as follows.
(A) Thickness 13 μm, film surface direction thermal conductivity: 1200 W / mK, thickness direction thermal conductivity: 4.5 W / mK, density 1.9 g / cm ³
(B) Thickness: 4.5 μm, film surface direction thermal conductivity: 1300 W / mK, thickness direction thermal conductivity: 4.7 W / mK, density: 1.9 g / cm ³
(C) Thickness: 2.6 μm, film surface direction thermal conductivity: 1470 W / mK, thickness direction thermal conductivity 4.8 W / mK, density 2.0 g / cm ³
(D) Thickness: 0.9 μm, film surface direction thermal conductivity: 1500 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density 2.0 g / cm ³
(E) Thickness, 0.3 μm, film surface direction thermal conductivity: 1580 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density 2.0 g / cm ³
(F) Thickness, 105 nm, film surface direction thermal conductivity: 1600 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density 2.1 g / cm ³
(G) Thickness, 18 nm, film surface direction thermal conductivity: 1680 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density 2.1 g / cm ³

FIG. 4 shows a photograph of the graphite film (F). FIG. 4 is a photograph taken by attaching a graphite film (F) having a thickness of 105 nm on a PET substrate. Since the film is thin, the light from the fluorescent lamp behind it is transmitted. FIG. 5 is an electron diffraction image of the limited field of view, and it can be seen that it is a high-quality graphite.

Table 1 shows thermal resistance values of the seven types of graphite films (A) to (G) having different thicknesses. The pressure is 1.0 to 4.5 kgf / cm ² , and-is not measured. As the graphite film thickness was reduced, the thermal resistance value was reduced, and it was found that the TIM of the present invention has extremely excellent thermal resistance characteristics.

Note that when the graphite films (F) and (G) were compared, the thermal resistance of (G) was larger despite the film thickness being 105 nm and 18 nm, respectively. (G) is extremely difficult to handle and it is difficult to accurately measure its thermal resistance, but it is thought that the thermal resistance at the interface has increased. From this result, it was found that a graphite film having a thickness of 10 nm to 15 μm was preferable in practice.

[Examples 8 to 11]
Four types of graphite films were produced by changing the temperature of the graphitization treatment using a 2.1 μm-thick polyimide film produced by the same method as described in Examples 1 to 7 above. The temperatures of the graphitization treatment are 2600 ° C. (D-1), 2200 ° C. (D-2), 2000 ° C. (D-3), and 1700 ° C. (D-4), respectively.

The thickness, thermal conductivity, and density of the obtained graphite film are shown below.
(D-1) Thickness: 0.9 μm, film surface direction thermal conductivity: 1400 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density: 2.0 g / cm ³
(D-2) Thickness: 1.0 μm, film surface direction thermal conductivity: 1100 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density: 2.0 g / cm ³
(D-3) Thickness: 1.1 μm, film surface direction thermal conductivity: 500 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density 2.1 g / cm ³
(D-4) Thickness: 1.3 μm, film surface direction thermal conductivity: 390 W / mK, thickness direction thermal conductivity: 5.0 W / mK, density 2.1 g / cm ³

Using the graphite film (D) and the graphite films (D-1) to (D-4) in which the thermal conductivity in the surface direction is changed by changing the graphitization treatment temperature with a polyimide film having substantially the same thickness, In the same manner as in Examples 1 to 7, interlayer thermal resistance was measured when each graphite film was TIM. The results are shown in Table 2. In the graphite film (D-4) having a thermal conductivity in the plane direction of 390 W / mK, the thermal resistance between the layers could not be reduced to 0.5 K · cm ² / W or less regardless of the pressure. However, the thermal resistance between the layers is close to the thermal resistance (0.5 K · cm ² / W) of the conventional high-performance TIM, and further excellent in heat resistance and durability. (D-4) Is still a very useful TIM. On the other hand, the surface direction of the heat conductivity can be realized the characteristics of 500 W / mK which is (D-3) In 0.43K · ^cm 2 / W at a pressure of 4.5 kgf / cm ^2. From this, it was found that the TIM of the present invention preferably has a thermal conductivity in the plane direction of 500 W / mK or more and an anisotropy of thermal conductivity of 100 or more.

[Examples 12 to 16]
To a DMF solution of 4,4′-diaminodiphenyl ether and p-phenylenediamine (3/1 in molar ratio), calcium hydrogen phosphate (CaHPO ₄ ) and pyromellitic dianhydride (4,4′- Diaminodiphenyl ether and equimolar amount of the total amount of p-phenylenediamine) were added to prepare a 18% by weight DMF solution of polyamic acid. A curing agent composed of 20 g of acetic anhydride and 10 g of isoquinoline was mixed with 100 g of the DMF solution, stirred, defoamed by centrifugation, and cast onto an aluminum foil. The process from stirring to defoaming was performed while cooling to 0 ° C. The laminate of the aluminum foil and the polyamic acid solution was heated at 120 ° C. for 150 seconds to obtain a gel film having self-supporting properties. This gel film was peeled off from the aluminum foil and fixed to the frame. This gel film was heated at 300 ° C., 400 ° C., and 500 ° C. for 30 seconds each to produce a 2.1 μm thick polyimide film. The obtained polyimide film was pretreated by heating to 1000 ° C. at a rate of 10 ° C./min in nitrogen gas using an electric furnace and keeping at 1000 ° C. for 1 hour.
Next, as a graphitization treatment, the pretreated carbonized film obtained is sandwiched between surface-polished graphite blocks, set inside a cylindrical graphite heater, heated to 2800 ° C. at different heating rates, and 10 minutes. By holding and then lowering the temperature at a rate of 40 ° C./min, five types of graphite films having different densities were produced. The graphitization treatment was performed under a pressure of 0.5 kg / cm ^{2 in} an argon atmosphere.
The amount of calcium hydrogen phosphate added to each film and the rate of temperature increase in the graphitization treatment are shown below.
Amount of calcium hydrogen phosphate added: 0.01 parts by weight of (D-5) to 100 parts by weight of the total weight of pyromellitic dianhydride, 4,4′-diaminodiphenyl ether, and p-phenylenediamine, (D-6) 0.02 parts by weight, (D-7) 0.05 parts by weight, (D-8) 0.10 parts by weight, and (D-9) 0.10 parts by weight.
Temperature increase rate: (D-5) to (D-8) are 20 ° C./min, and (D-9) is 30 ° C./min.

The thickness, thermal conductivity, and density of the obtained graphite film are shown below.
(D-5) Thickness: 1.0 μm, film surface direction thermal conductivity: 1380 W / mK, thickness direction thermal conductivity: 4.6 W / mK, density: 1.8 g / cm ³
(D-6) Thickness: 1.2 μm, film surface direction thermal conductivity: 1320 W / mK, thickness direction thermal conductivity: 4.3 W / mK, density: 1.5 g / cm ³
(D-7) Thickness: 1.5 μm, film surface direction thermal conductivity: 1300 W / mK, thickness direction thermal conductivity: 4.2 W / mK, density: 1.2 g / cm ³
(D-8) Thickness: 2.0 μm, film surface direction thermal conductivity: 1240 W / mK, thickness direction thermal conductivity: 4.0 W / mK, density: 0.9 g / cm ³
(D-9) Thickness: 2.4 μm, film surface direction thermal conductivity: 1210 W / mK, thickness direction thermal conductivity: 3.8 W / mK, density: 0.75 g / cm ³

The thermal resistance between layers was measured by the same method as in Examples 1 to 7 using the above five types of graphite films (D-5) to (D-9) produced by changing the density (degree of foaming). The experimental results are shown in Table 3 together with the results of the graphite film (D). In graphite film density of graphite film (D-8) and 0.75 g / cm ³ is ^{0.9g / cm 3 (D-9} ), 0.5K · heat resistance regardless of the size of the pressure It was not possible to make it below cm ² / W. From this result, it was found that the density of the graphite film is preferably 1.2 to 2.26 g / cm ³ . However, with regard to the graphite films (D-8) and (D-9), the thermal resistance between the layers is a value close to that of the conventional high-performance TIM, and the heat resistance and durability are overwhelmingly superior. It remains a very useful TIM.

[Examples 17 to 19]
The graphite film (D-6) is immersed in a commercially available canola oil (smoke point 204 ° C.) and then placed on an oil-absorbing paper to absorb the canola oil, thereby producing graphite films having different canola oil impregnation amounts. did. The amount of impregnation was measured by weight measurement. Using these, the thermal resistance between the layers was measured in the same manner as in Examples 1-7. The experimental results are shown in Table 4. When the oil is impregnated in an amount of 2 to 57% by weight, the thermal resistance between the layers can be reduced as compared with the thermal resistance without impregnation with the canola oil. It was remarkable. This is because the canola oil having a thermal conductivity higher than that of air was inserted into the voids in the graphite film (D-6) having a density of 1.5 g / cm ³ , and that the canola oil on the surface of the graphite film was interface air. This is considered to be the effect of reducing the thermal resistance at the interface by reducing the number of layers.

Claims (9)

An interlayer thermal connection member having a graphite film having a thickness of 10 nm to 15 μm. The interlayer thermal connection member according to claim 1, wherein the thermal conductivity in the film surface direction of the graphite film is 500 W / mK or more, and the anisotropy of the thermal conductivity in the film surface direction and the thickness direction is 100 or more. The interlayer thermal connection member according to claim 1 or 2, wherein the density of the graphite film is 1.2 to 2.26 g / cm ³ . The interlayer thermal connection member according to any one of claims 1 to 3, wherein the graphite film is obtained by heat-treating a polymer film having a thickness of 50 nm to 50 µm at a temperature of 2000 ° C or higher. The interlayer thermal connection member according to claim 4, wherein the polymer film contains a condensed aromatic polymer. The condensed aromatic polymer is polyamide, polyimide, polyquinoxaline, polyoxadiazole, polybenzimidazole, polybenzoxazole, polybenzthiazole, polyquinazolinedione, polybenzoxazinone, polyquinazolone, benzimidazolobenzophenanthroline ladder polymer And an interlayer thermal connection member according to claim 5, which is at least one selected from these and derivatives thereof. Furthermore, 1-100 weight% of fluid substances which have fluidity | liquidity at normal temperature or more are included with respect to the weight of the said graphite film, The interlayer heat connection member in any one of Claims 1-6 characterized by the above-mentioned. The interlayer heat connecting member according to claim 7, wherein the fluid substance is an oil having a boiling point of 200 ° C. or higher. The interlayer thermal connection method including the process of installing the interlayer thermal connection member in any one of Claims 1-8 between the members to thermally connect.