JP7732196B2 - Composite sheet and method for producing the same - Google Patents

Description

The present invention relates to a composite sheet and a method for manufacturing the same.

In recent years, the amount of heat generated by electronic components such as power semiconductors (e.g., IGBT modules) and integrated circuit (IC) chips has increased as their performance has improved. As a result, electronic devices that use these components need to take measures to prevent malfunctions caused by temperature increases in the components.

To prevent malfunctions caused by temperature rise in electronic components, a common method is to promote heat dissipation by attaching a heat sink, heat sink plate, heat dissipation fin, or other heat sink made of metal to the heat-generating element of the electronic component. When using a heat sink, in order to efficiently transfer heat from the heat-generating element to the heat sink, a highly thermally conductive sheet-like member is placed between the heat-generating element and the heat sink, and a certain amount of pressure is applied to this sheet-like member to bring the two into close contact.

Thermal conductive sheets have been studied as such sheet-like components. However, when pressure is applied to thermal conductive sheets, they can tear. As a result, technologies to increase the strength of thermal conductive sheets have been investigated. For example, Patent Document 1 proposes a thermal conductive sheet in which thermal conductive layers made of a thermally conductive filler and a thermoplastic resin are provided on both sides of a metal foil. The thermal conductive sheet disclosed in Patent Document 1 can prevent the sheet from tearing when pressure is applied.

Japanese Patent Application Laid-Open No. 2020-6581

Here, composite sheets used for the purpose of heat conduction are required to be resistant to tearing when pressure is applied, as well as to be easily crushed when pressure is applied, i.e., to have excellent compressibility.
However, the above-mentioned conventional composite sheets had room for improvement in terms of achieving a high level of both resistance to tearing when sandwiched between a heat generating element and a heat dissipating element and excellent compressibility when pressed from both sides.

The present invention aims to provide a composite sheet that is resistant to tearing under pressure and has excellent compressibility, as well as a method for manufacturing the same.

The inventors conducted extensive research to achieve the above-mentioned objective. They discovered that, when manufacturing a composite sheet containing a carbon material, it is possible to achieve high levels of both resistance to tearing and compressibility under pressure by ensuring that the resulting composite sheet does not contain resin or contains resin in an amount of 15 volume percent or less, orienting the carbon material in a specific direction within the composite sheet, and designing the composite sheet to have a thermal conductivity of 10 W/mK or greater. Based on this discovery, the inventors completed the present invention.

That is, the present invention aims to advantageously solve the above-mentioned problems, and the composite sheet of the present invention is characterized in that the composite sheet contains a carbon material, and the composite sheet does not contain a resin, or if it contains a resin, the resin content is 15 volume % or less, the angle of the major axis direction of the carbon material with respect to the surface of the composite sheet is 60° to 90°, and the thermal conductivity of the composite sheet is 10 W/mK or more. If the composite sheet does not contain a resin, or if it contains a resin, the content is 15 volume % or less, the carbon material contained in the composite sheet is oriented in a specific direction, and the thermal conductivity of the composite sheet is 10 W/mK or more, such a composite sheet can achieve high levels of both resistance to tearing under pressure and compressibility.
The "angle of the major axis direction of the carbon material relative to the surface of the composite sheet" and the "thermal conductivity of the composite sheet" can be measured according to the method described in the examples of this specification.

In the composite sheet of the present invention, the carbon material is preferably a particulate carbon material. If the carbon material contained in the composite sheet is a particulate carbon material, the particulate carbon material can be appropriately spaced apart, thereby achieving both resistance to tearing under pressure and compressibility at an even higher level.

In the composite sheet of the present invention, the particulate carbon material is preferably flake graphite. If the particulate carbon material contained in the composite sheet is flake graphite, the flake-shaped graphite particles can be appropriately packed together, thereby achieving both resistance to tearing under pressure and compressibility at an even higher level.

Furthermore, in the composite sheet of the present invention, it is preferable that the flake graphite is expanded graphite. If the flake graphite contained in the composite sheet is expanded graphite, an even higher level of thermal conductivity can be achieved.

The density of the composite sheet of the present invention is preferably 1.2 g/cm ³ or less. If the density of the composite sheet is 1.2 g/cm ³ or less, it is possible to achieve both resistance to tearing under pressure and compressibility at an even higher level.
The density of the composite sheet can be measured according to the method described in the examples of this specification.

The present invention also aims to advantageously solve the above-mentioned problems, and the composite sheet manufacturing method of the present invention is characterized by comprising: a primary sheet forming step in which a composition containing the resin and the carbon material is pressurized and molded into a sheet to obtain a primary sheet; a laminate forming step in which a plurality of the primary sheets are stacked in the thickness direction or folded or rolled up to obtain a laminate; a slicing step in which the laminate is sliced at an angle of 45° or less to the stacking direction to obtain a secondary sheet; and a firing step in which the secondary sheet is fired. The composite sheet manufacturing method of the present invention makes it possible to efficiently produce composite sheets that are resistant to tearing under pressure and have excellent compressibility.

In the composite sheet manufacturing method of the present invention, it is preferable that the volume fraction of the carbon material in the secondary sheet is 31% by volume or more. If the volume fraction of the carbon material in the secondary sheet is 31% by volume or more, the strength and thermal conductivity of the resulting composite sheet can be improved.

Furthermore, in the composite sheet manufacturing method of the present invention, it is preferable that the resin contains a liquid resin. If the resin contains a liquid resin, it becomes easier to increase the filling rate of the carbon material in the primary sheet, thereby reducing the thermal resistance of the resulting composite sheet and increasing its thermal conductivity.

Furthermore, in the method for producing a composite sheet of the present invention, it is preferable that the value of the sintered state index (%) calculated according to the following formulas (1) to (3) is 0% or more and 15% or less.
Theoretical firing residual ratio=(amount of carbon material in composition)/(total solid content in composition) (1)
Measured firing residual rate = (mass of composite sheet) ÷ (mass of secondary sheet) (2)
Firing condition index (%) = {1 - (theoretical firing residue rate) ÷ (measured firing residue rate)} × 100 (3)
By adjusting various conditions so that the firing condition index falls within the above range, it is possible to achieve a higher level of both resistance to tearing and compressibility when pressed in the resulting composite sheet.

The present invention provides a composite sheet and a method for manufacturing the same that is resistant to tearing under pressure and has excellent compressibility.

Hereinafter, embodiments of the present invention will be described in detail.
Because the composite sheet of the present invention has thermal conductivity, it can be sandwiched between a heat-generating body and a heat-dissipating body. That is, the composite sheet of the present invention can function as a heat-conducting sheet as a heat-dissipating member, and can be used together with a heat-dissipating body such as a heat sink, a heat-dissipating plate, or a heat-dissipating fin to form a heat-dissipating device. Furthermore, the composite sheet of the present invention can be efficiently produced using the composite sheet production method of the present invention.

(composite sheet)
The composite sheet of the present invention contains a carbon material, is resin-free, or if it contains a resin, the content is 15 volume % or less, the angle of the long axis direction of the carbon material with respect to the surface of the composite sheet is 60° to 90°, and further, the thermal conductivity is 10 W/mK or more. Because the composite sheet of the present invention contains a carbon material oriented in a predetermined direction and is resin-free or contains only a small amount of resin, it has low thermal resistance and excellent thermal conductivity. When placed as a thermally conductive sheet in the gap between a heat generating element and a heat dissipating element and pressurized, the resin can be prevented from being extruded by pressure and spilling out of the sheet's installation area. Furthermore, because the gaps between the carbon materials in the composite sheet of the present invention are not filled with resin, the carbon material can be inserted into the gaps between the carbon materials when pressurized, which is thought to result in excellent compressibility.

<Carbon materials>
Here, the carbon material is not particularly limited, and fibrous carbon materials and particulate carbon materials can be used. Among them, from the viewpoint of improving the resistance to tearing and compressibility of the composite sheet under pressure, it is preferable that the carbon material be particulate carbon materials. Note that "fibrous carbon material" means a carbon material having an aspect ratio of more than 20, and "particulate carbon material" means a carbon material having an aspect ratio of 20 or less.

<<Carbon fiber material>>
The fibrous carbon material is not particularly limited, and examples thereof include carbon nanotubes, vapor-grown carbon fibers, carbon fibers obtained by carbonizing organic fibers, and cut products thereof. These may be used alone or in combination of two or more.

<<Carbon material particles>>
The particulate carbon material is not particularly limited, and examples thereof include artificial graphite and natural graphite. Artificial graphite includes carbon black and pyrolytic graphite. Natural graphite includes flake graphite such as expanded graphite and spherical graphite, as well as flake graphite. These may be used alone or in combination of two or more.

Of the above, it is preferable to use flake graphite as the particulate carbon material, and among flake graphite, it is even more preferable to use expanded graphite. If flake graphite is used as the particulate carbon material, the particulate carbon material can be appropriately spaced apart, achieving both resistance to tearing under pressure and compressibility at an even higher level. Furthermore, among flake graphite, if expanded graphite is used, the thermal conductivity of the composite sheet can be further increased.

<<Orientation of carbon material>>
The orientation of the carbon material in the composite sheet must satisfy the following requirement: the angle of the long axis direction of the carbon material relative to the surface of the composite sheet (hereinafter sometimes referred to as the "orientation angle of the carbon material") must be 60° or more and 90° or less. Furthermore, the orientation angle of the carbon material is more preferably 65° or more, even more preferably 70° or more, and preferably 90° or less. If the orientation angle of the carbon material is within the above-mentioned specified range, the thermal resistance of the composite sheet can be reduced and the thermal conductivity can be increased.

<<Characteristics of carbon materials>>
The volume average particle diameter of the carbon material is preferably 3 μm or more, more preferably 5 μm or more, even more preferably 8 μm or more, even more preferably 12 μm or more, even more preferably 16 μm or more, and preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 100 μm or less. If the volume average particle diameter of the carbon material is equal to or greater than the lower limit, the density of the composite sheet is appropriately improved, thereby further enhancing compressibility. On the other hand, if the volume average particle diameter of the carbon material is equal to or less than the upper limit, appropriate filling is possible, thereby further enhancing the thermal conductivity of the composite sheet.
In the present invention, the "volume average particle size" can be measured in accordance with JIS Z8825, and represents the particle size at which the cumulative volume calculated from the smallest diameter side becomes 50% in the particle size distribution (volume basis) measured by a laser diffraction method.

Furthermore, the aspect ratio (major axis/minor axis) of the particulate carbon material used as the carbon material preferably exceeds 1.2, more preferably exceeds 2, even more preferably exceeds 4, and even more preferably exceeds 6, and is preferably 20 or less, more preferably 15 or less, and even more preferably 10 or less. If the aspect ratio of the particulate carbon material is within the above-mentioned specified range, the orientation angle of the particulate carbon material with respect to the surface of the composite sheet can easily fall within the desired range described below, thereby further improving thermal conductivity.

<<Carbon material content>>
The carbon material content in the composite sheet is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, and can be 100% by mass or less. If the carbon material content in the composite sheet is equal to or greater than the above lower limit, the resistance of the composite sheet to tearing under pressure can be further increased.

<Other ingredients>
The composite sheet of the present invention may further contain components other than the carbon material described above, such as resins.

<<Resin>>
The resin optionally contained in the composite sheet of the present invention is not particularly limited, but may be, for example, a portion of the resin used to form the primary sheet and secondary sheet, which are precursors of the composite sheet, in the composite sheet manufacturing method described below, which remains unburned during the firing process.
Specific examples of resins that can be contained in the composite sheet include the resins described below in the section "Method for producing composite sheet."

The composite sheet must be resin-free, or if it does contain resin, the resin content must be 15% by volume or less. The upper limit of the resin content in the composite sheet is preferably 10% by volume or less, more preferably 5% by volume or less, and even more preferably 2% by volume or less. The lower the resin content in the composite sheet, the higher the compressibility of the composite sheet can be.

<Thermal conductivity of composite sheet>
The composite sheet must have a thermal conductivity of 10 W/mK or more, preferably 12 W/mK or more. If the thermal conductivity is 10 W/mK or more, the composite sheet has excellent thermal conductivity and can be suitably used as a thermally conductive sheet. The upper limit of the thermal conductivity of the composite sheet is not particularly limited, but can be 30 W/mK or less.

<Density of composite sheet>
The density of the composite sheet is preferably 1.2 g/ ^cm3 or less, and more preferably 1.0 g/cm3 or ^less . If the density of the composite sheet is equal to or less than the above-mentioned predetermined value, the number of voids in the composite sheet increases, thereby further increasing compressibility. The density of the composite sheet is not particularly limited, but is preferably 0.5 g/cm3 or ^more from the viewpoint of improving the sheet strength and thermal conductivity.
The density of the composite sheet can be adjusted, for example, by the steps performed in the composite sheet manufacturing method described below, the types, properties and quantitative ratios of the resin and carbon material used, and the firing conditions (e.g., temperature and time).

<Thickness>
The thickness of the composite sheet is preferably 80 μm or more, more preferably 90 μm or more, and even more preferably 100 μm or more, and is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. If the thickness of the composite sheet is equal to or greater than the lower limit, the strength and thermal conductivity of the composite sheet can be increased. On the other hand, if the thickness of the composite sheet is equal to or less than the upper limit, the thermal resistance value can be appropriately reduced.

(Method for manufacturing composite sheet)
The method for producing a composite sheet of the present invention includes: (A) a primary sheet forming step of pressurizing a composition containing a resin and a carbon material to form it into a sheet to obtain a primary sheet; (B) a laminate forming step of stacking a plurality of primary sheets in the thickness direction or folding or rolling the primary sheet to obtain a laminate; (C) a slicing step of slicing the laminate at an angle of 45° or less with respect to the stacking direction to obtain a secondary sheet; and (D) a firing step of firing the secondary sheet to obtain a composite sheet.
The method for producing the composite sheet of the present invention may optionally include further steps other than the above steps (A) to (D).

The composite sheet manufacturing method of the present invention makes it possible to efficiently produce composite sheets that are resistant to tearing under pressure and have excellent compressibility.

<(A) Primary Sheet Molding Step>
In the primary sheet forming step, a composition containing a resin and a carbon material is pressed and formed into a sheet to obtain a primary sheet.

<<Composition>>
The composition includes a resin and a carbon material. The composition may further include other components in addition to the resin and the carbon material.

-resin-
The resin is not particularly limited, and any resin can be used. For example, either a liquid resin or a solid resin can be used. The resin may be used alone or in combination of two or more types. For example, both a liquid resin and a solid resin can be used. When a liquid resin and a solid resin are used in combination, the mass ratio of the liquid resin to the solid resin can be adjusted within a range that achieves the desired effects of the present invention. Preferably, the resin contains a liquid resin. Furthermore, the higher the proportion of liquid resin in the overall resin, the easier it is to increase the filling rate of the carbon material in the primary sheet.

= Liquid resin =
The liquid resin is not particularly limited as long as it is liquid at room temperature and normal pressure, and for example, a thermoplastic resin that is liquid at room temperature and normal pressure can be used.
In the present invention, "normal temperature" refers to 23° C., and "normal pressure" refers to 1 atm (absolute pressure).

Examples of liquid resins include fluororesin, silicone resin, acrylic resin, epoxy resin, and acrylonitrile-butadiene copolymer (nitrile rubber). These may be used alone or in combination of two or more.

=Solid resin=
The solid resin is not particularly limited as long as it is not a liquid at room temperature and normal pressure, and for example, a thermoplastic resin that is solid at room temperature and normal pressure, or a thermosetting resin that is solid at room temperature and normal pressure can be used.

Examples of thermoplastic resins that are solid at room temperature and normal pressure include acrylic resins such as poly(2-ethylhexyl acrylate), a copolymer of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or its ester, and polyacrylic acid or its ester; silicone resin; fluororesin; polyethylene; polypropylene; ethylene-propylene copolymer; polymethylpentene; polyvinyl chloride; polyvinylidene chloride; polyvinyl acetate; ethylene-vinyl acetate copolymer; polyvinyl alcohol; polyacetal; polyethylene terephthalate; polybutylene terephthalate; polyethylene naphthalate; polystyrene; and polyacrylonitrile. styrene-acrylonitrile copolymer; acrylonitrile-butadiene copolymer (nitrile rubber); acrylonitrile-butadiene-styrene copolymer (ABS resin); styrene-butadiene block copolymer or hydrogenated product thereof; styrene-isoprene block copolymer or hydrogenated product thereof; polyphenylene ether; modified polyphenylene ether; aliphatic polyamides; aromatic polyamides; polyamideimide; polycarbonate; polyphenylene sulfide; polysulfone; polyethersulfone; polyethernitrile; polyetherketone; polyketone; polyurethane; liquid crystal polymer; ionomer; etc. These may be used alone or in combination of two or more.
In the present invention, rubber is included in the "resin".

Examples of thermosetting resins that are solid at room temperature and pressure include natural rubber; butadiene rubber; isoprene rubber; nitrile rubber; hydrogenated nitrile rubber; chloroprene rubber; ethylene propylene rubber; chlorinated polyethylene; chlorosulfonated polyethylene; butyl rubber; halogenated butyl rubber; polyisobutylene rubber; epoxy resin; polyimide resin; bismaleimide resin; benzocyclobutene resin; phenolic resin; unsaturated polyester; diallyl phthalate resin; polyimide silicone resin; polyurethane; thermosetting polyphenylene ether; thermosetting modified polyphenylene ether; etc. These may be used alone or in combination of two or more.

- Carbon materials -
As the carbon material, the carbon materials described above in the section "composite sheet" can be used.

The content of the carbon material in the composition is preferably 50 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 120 parts by mass or more, and preferably 500 parts by mass or less, more preferably 450 parts by mass or less, and even more preferably 400 parts by mass or less, per 100 parts by mass of resin. If the content of the carbon material in the composition is above the above-mentioned lower limit, the density of the composite sheet produced can be appropriately increased, and the resulting composite sheet can be made more resistant to tearing under pressure. Furthermore, if the content of the carbon material in the composition is above the above-mentioned lower limit, the strength and thermal conductivity of the composite sheet can be improved. On the other hand, if the content of the carbon material in the composition is below the above-mentioned upper limit, the compressibility of the resulting composite sheet can be further increased.

The volume of the carbon material in the composition is preferably 31% by volume or more, preferably 40% by volume or more, preferably 75% by volume or less, and more preferably 70% by volume or less, relative to the total volume of the resin and carbon material. If the volume fraction of the carbon material relative to the total volume of the resin and carbon material in the composition is equal to or greater than the above-mentioned lower limit, the resulting composite sheet can be made more resistant to tearing under pressure. If the volume fraction of the carbon material is equal to or greater than the above-mentioned lower limit, the strength and thermal conductivity of the composite sheet can be improved. If the volume fraction of the carbon material is equal to or less than the above-mentioned upper limit, the compressibility of the resulting composite sheet can be made more improved.

-Other ingredients-
The composition may further contain other components in addition to the resin and carbon material described above. Examples of such other components include a dispersant. The dispersant is not particularly limited, and known dispersants can be used. The content of the dispersant in the composition can be adjusted within a range that achieves the desired effects of the present invention.

- Preparation of composition -
The composition is not particularly limited and can be prepared by mixing the above-mentioned components.
The mixing of the above-mentioned components can be carried out using known mixing devices such as kneaders, mixers such as Henschel mixers, Hobart mixers, and high-speed mixers, twin-screw kneaders, and rolls, without any particular limitations. The mixing may also be carried out in the presence of a solvent such as ethyl acetate. The resin may be dissolved or dispersed in a solvent in advance to form a resin solution, which may then be mixed with the carbon material and any other components that may be added. The mixing time may be, for example, 5 minutes to 60 minutes. The mixing temperature may be, for example, 5°C to 160°C.

<<Molding of the composition>>
The composition prepared as described above can be optionally degassed and crushed, and then pressed to form into a sheet. The sheet formed by pressing the composition in this manner can be used as a primary sheet. If a solvent is used during mixing, it is preferable to remove the solvent before forming into a sheet. For example, if degassing is performed using vacuum degassing, the solvent can be removed simultaneously during degassing.

The composition can be formed into a sheet using any known forming method, such as press molding, rolling, or extrusion, as long as the method applies pressure. Among these, the composition is preferably formed into a sheet by rolling (primary processing), and more preferably by passing the composition between rolls while sandwiched between protective films. The protective film is not particularly limited, and may be a sandblasted polyethylene terephthalate (PET) film or the like. The roll temperature can be from 5°C to 150°C, the roll gap can be from 50 μm to 2500 μm, the roll linear pressure can be from 1 kg/cm to 3000 kg/cm, and the roll speed can be from 0.1 m/min to 20 m/min.

<(B) Laminate formation process>
In the laminate formation process, a plurality of primary sheets obtained in the primary sheet molding process are stacked in the thickness direction, or the primary sheets are folded or wound to obtain a laminate in which a plurality of primary sheets containing a resin and a carbon material are formed in the thickness direction. Here, the formation of the laminate by folding the primary sheets is not particularly limited and can be performed by folding the primary sheets at a constant width using a folding machine. Furthermore, the formation of the laminate by winding the primary sheets is not particularly limited and can be performed by winding the primary sheets around an axis parallel to the short or long direction of the primary sheets. Furthermore, the formation of the laminate by stacking the primary sheets can be performed using a lamination device without particular limitations. For example, using a sheet lamination device (manufactured by Nikkiso Co., Ltd., product name "Hi-Stacker") can prevent air from entering between the layers, thereby efficiently obtaining a good laminate.

In the lamination process, it is preferable to apply pressure (secondary pressure) to the resulting laminate in the lamination direction while heating it. By applying secondary pressure to the laminate in the lamination direction while heating it, it is possible to promote fusion between the laminated primary sheets.

Here, the pressure applied to the laminate in the lamination direction can be set to 0.05 MPa or more and 0.50 MPa or less.
The heating temperature of the laminate is not particularly limited, but is preferably 50°C or higher and 170°C or lower.
Furthermore, the heating time for the laminate can be, for example, 10 seconds or more and 30 minutes or less.

In a laminate obtained by stacking, folding, or rolling the primary sheet, the carbon material is presumably oriented in a direction approximately perpendicular to the stacking direction. For example, if the carbon material is a particulate carbon material that is flaky in shape, the direction of the long axis of the main surface of the flaky shape is presumably approximately perpendicular to the stacking direction.

<(C) Slicing step>
In the slicing step, the laminate is sliced at an angle of 45° or less relative to the lamination direction to obtain secondary sheets consisting of slices of the laminate. The method for slicing the laminate is not particularly limited, and examples thereof include the multi-blade method, laser processing, water jet processing, and knife processing. Among these, the knife processing method is preferred because it is easy to achieve a uniform thickness for the secondary sheet. The cutting tool used to slice the laminate is not particularly limited, and a slicing member having a smooth plate surface with a slit and a blade protruding from the slit (e.g., a plane or slicer with a sharp blade) can be used.

The angle at which the laminate is sliced is preferably 30° or less with respect to the stacking direction, more preferably 15° or less with respect to the stacking direction, and preferably approximately 0° with respect to the stacking direction (i.e., in the direction along the stacking direction).
In the secondary sheet obtained in this manner, the carbon material is well oriented in the thickness direction, and the angle between the long axis direction of the carbon material and the sheet surface is 60° to 90°. More specifically, when the carbon material is flaky, the angle between the long axis direction of the main surface of the flaky shape and the sheet surface is 60° to 90°.

Here, the volume fraction of the carbon material in the secondary sheet is preferably the same as the volume fraction of the carbon material in the composition used to form the primary sheet. That is, the volume fraction of the carbon material in the secondary sheet is preferably 31 volume % or more, preferably 40 volume % or more, preferably 75 volume % or less, and more preferably 70 volume % or less, relative to the total volume of the resin and carbon material that make up the secondary sheet. If the volume fraction of the carbon material in the secondary sheet is equal to or greater than the above-mentioned lower limit, the resulting composite sheet can be made more resistant to tearing under pressure. Furthermore, if the volume fraction of the carbon material is equal to or greater than the above-mentioned lower limit, the strength and thermal conductivity of the composite sheet can be improved. If the volume fraction of the carbon material is equal to or less than the above-mentioned upper limit, the compressibility of the resulting composite sheet can be made even greater.

<(D) Firing process>
In the firing step, the secondary sheet is fired to burn off and remove the resin contained in the secondary sheet, thereby obtaining a composite sheet.
The resulting composite sheet is obtained by removing the resin from the secondary sheet. Therefore, in the composite sheet, the carbon material is well oriented in the thickness direction. For example, if the carbon material is a particulate carbon material and has a scale-like shape, the direction of the major axis of the main surface of the scale-like shape is approximately aligned with the thickness direction of the secondary sheet.

The firing in the firing step is preferably carried out under conditions such that the firing state index, which can be calculated according to the following formulas (1) to (3), is 0% or more and 15% or less.
Theoretical firing residual ratio=(amount of carbon material in composition)/(total solid content in composition) (1)
Measured firing residual rate = (mass of composite sheet) ÷ (mass of secondary sheet) (2)
Firing condition index (%) = {1 - (theoretical firing residue rate) ÷ (measured firing residue rate)} × 100 (3)

Here, it is preferable that the value of the firing state index is 1% or more and 14% or less. By ensuring that the value of the firing state index is equal to or greater than the above-mentioned lower limit, the compressibility of the resulting composite sheet can be further increased. Furthermore, by ensuring that the value of the firing state index is equal to or less than the above-mentioned upper limit, the composite sheet can be made more resistant to tearing when pressed. The value of the firing state index can be controlled based on the firing conditions (firing temperature and firing time) in the firing process, the composition of the composition used to form the primary sheet, etc.

Here, the heating temperature when firing the secondary sheet is preferably T - 50°C or higher, more preferably T - 40°C or higher, even more preferably T - 20°C or higher, and preferably T + 2000°C or lower, more preferably T + 1500°C or lower, and even more preferably T + 1000°C or lower, where T°C is the decomposition initiation temperature of the resin contained in the secondary sheet. If the heating temperature when firing the secondary sheet is above the above lower limit, the resin content in the produced composite sheet can be reduced, further increasing the compressibility of the composite sheet. On the other hand, if the heating temperature when firing the secondary sheet is below the above upper limit, damage to the structure of the produced composite sheet due to excessive heating can be suppressed, and the strength of the composite sheet can be ensured to be sufficiently high. Note that when the resin contains multiple types of resin, it is preferable to satisfy the above temperature range based on the resin with the higher decomposition temperature.

The heating temperature when firing the secondary sheet is preferably 300°C or higher, more preferably 350°C or higher, and preferably 2000°C or lower, more preferably 1500°C or lower, and even more preferably 1200°C or lower. If the heating temperature when firing the secondary sheet is above the above lower limit, the resin content in the composite sheet produced can be reduced, further increasing the compressibility of the composite sheet. On the other hand, if the heating temperature when firing the secondary sheet is below the above upper limit, damage to the structure of the composite sheet produced by excessive heating can be prevented, and the strength of the composite sheet can be ensured to be sufficiently high.

The heating time when firing the secondary sheet can be adjusted depending on the heating temperature, but can be, for example, between 30 minutes and 72 hours.

The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. In addition, when calculating volume fractions, the volume of each blended component was calculated by dividing the mass of each blended component by its theoretical specific gravity.
In the examples, various measurements and evaluations were carried out according to the following methods.

<Resin decomposition temperature>
Thermogravimetric analysis (TGA) was performed on each resin used in the examples and comparative examples in an air atmosphere at a temperature range of 30 to 1000° C. at a temperature increase rate of 10° C./min. The temperature at which the weight decreased by 5% was defined as the decomposition starting temperature of the resin.

<Orientation angle of carbon material>
The orientation angle of the carbon material in the composite sheet (or the negative electrode mixture layer of the coated electrode) was determined by observing a cross section of the composite sheet cut into a regular octagon with a scanning electron microscope (SEM, Hitachi High-Technologies Corporation's "SU-3500") at a magnification that fit the entire sheet from top to bottom. The magnification was 700x. Fifty lines were drawn along the long axis of the carbon material in this cross section, and the average angle of the long axis with respect to the surface of the composite sheet was calculated. If the angle was 90° or greater, a supplementary angle was used. This was performed for eight faces, and the largest value among the eight faces was taken as the orientation angle of the carbon material in the composite sheet.
<Resin content>
The composite sheets produced in the examples and comparative examples were subjected to thermogravimetry (TGA) measurement in a nitrogen atmosphere at a temperature range of 30 to 1000°C at a heating rate of 10°C/min. The weight loss rate between 30°C and 1000°C was taken as the resin content rate in the composite sheet.
<Calculation of firing condition index>
The sintered state index for the composite sheet that had undergone the sintering step was calculated according to the following formulas (1) to (3).
Theoretical firing residual ratio=(amount of carbon material in composition)/(total solid content in composition) (1)
Measured firing residual rate = (mass of composite sheet) ÷ (mass of secondary sheet) (2)
Firing condition index (%) = {1 - (theoretical firing residue rate) ÷ (measured firing residue rate)} × 100 (3)

<Thickness>
Using a film thickness meter (manufactured by Mitutoyo, product name "Digimatic Indicator ID-C112XBS"), the thickness of the composite sheet (or the negative electrode mixture layer of the coated electrode) produced in the examples and comparative examples was measured at a total of five points, namely, approximately the center point and the four corners (squares), and the average value (μm) of the measured thicknesses was taken as the thickness of the composite sheet.
<Density>
The mass, area, and thickness of the composite sheets produced in the examples and comparative examples were measured, and the density of the composite sheet (or negative electrode mixture layer) was calculated by dividing the mass by the volume (= area × thickness).
<Thermal conductivity in the thickness direction>
The thermal diffusivity α (m ² /s), specific heat at constant pressure Cp (J/g·K), and specific gravity ρ (g/m ³ ) in the thickness direction of the composite sheets produced in the examples and comparative examples were measured by the following methods.
[Thermal diffusivity α in the thickness direction]
Measurements were made using a thermal diffusion/thermal conductivity measuring device (manufactured by iPhase Corporation, product name "iPhase Mobile 1u") in accordance with the provisions of ISO 22007-3.
[Constant pressure specific heat Cp]
Using a differential scanning calorimeter (manufactured by Rigaku, product name "DSC8230"), the specific heat at 25°C was measured under the condition of a temperature increase of 10°C/min.
[Specific gravity ρ (density)]
The density of the composite sheet was calculated according to the method explained in the section <Density>.
Each measurement value was calculated using the following formula (I):
λ=α×Cp×ρ...(I)
The thermal conductivity λ (W/m·K) in the thickness direction of the composite sheet at 25° C. was calculated.

<Resistance to tearing under pressure>
The composite sheet sized to 10 x 10 mm was placed on a metal plate heated to 120°C.
A smooth metal plate sized to 12 x 12 mm was placed on top of the composite sheet so that its center overlapped the composite sheet, and a pressure of 250 N was applied from above for 10 seconds followed by 10 seconds of release, which constituted one cycle. This cycle was repeated 50 times. After the cycle test, the composite sheet was viewed from directly above and evaluated for protrusion according to the following criteria. Note that when repeated heating and cooling cycles were performed under pressure, composite sheets with poor durability were observed to tear from areas where particularly strong pressure was applied, and protrude outside the electronic device. The smaller the size of the protruding composite sheet, the less likely the composite sheet was to tear under pressure.
A: A composite sheet having at least one side longer than 0 mm and shorter than 1 mm is protruding.
B: A composite sheet having at least one side measuring 1 mm or more and less than 2 mm protrudes.
C: A composite sheet having at least one side measuring 2 mm or more and less than 3 mm protrudes.
D: A composite sheet having at least one side measuring 3 mm or more and less than 6 mm protrudes.
<Thermal resistance value>
The thermal resistance value of the composite sheet was measured using a thermal resistance tester (Hitachi Technology and Services Co., Ltd., product name "Resin Material Thermal Resistance Measuring Device"). A composite sheet cut into a roughly 1 cm square was used as a sample, and the thermal resistance value (°C/W) and sheet thickness (unit: mm) were measured when a pressure of 0.1 MPa and 0.9 MPa was applied at a sample temperature of 50°C. A smaller thermal resistance value indicates better thermal conductivity of the composite sheet, and, for example, better heat dissipation properties when interposed between a heat generating body and a heat sink.
<Compressibility>
The sheet thickness reduction rate was calculated by dividing the sheet thickness when the composite sheets produced in the Examples and Comparative Examples were pressurized at 0.9 MPa by the sheet thickness when pressurized at 0.1 MPa, and subtracting this value from 1. The larger this value, the more easily the sheet is crushed, and the better the sheet is able to conform to the irregularities of the adherend. The sheet thickness at each pressure was measured using the same device as that used to measure the thermal resistance value.
<Strength>
Test specimens were prepared by cutting the composite sheets prepared in the Examples and Comparative Examples to a size of 1 cm x 5 cm. A 6 cm x 6 cm x 2 cm base was also prepared. The right half of the test specimen was placed on the base, with the left half extending beyond the center. A 6 x 6 x 2 mm aluminum plate was then placed on the right half of the test specimen. Weights of 100 mg, 200 mg, and 300 mg were then alternately placed on the portion of the test specimen extending beyond the base until the test specimen broke. The strength of the composite sheet was evaluated according to the following criteria based on the weight of the weight placed when the test specimen broke. Note that when the resin content is low, as in the composite sheets of Examples 1 to 6, strength tends to be low.
A: The test piece broke under the weight of 300 mg.
B: The test piece broke under the weight of 200 mg.
C: The test piece broke under the weight of 100 mg.
D: The test piece broke under the weight of 100 mg.

Example 1
<Preparation of Composition>
As the resin, 70 parts of nitrile rubber (NBR) (manufactured by Nippon Zeon Co., Ltd., trade name "Nipol (registered trademark) 1312", decomposition onset temperature: 336 ° C.) that is liquid at room temperature and normal pressure, 30 parts of nitrile rubber (NBR) (manufactured by Nippon Zeon Co., Ltd., trade name "Nipol (registered trademark) 3350", decomposition onset temperature: 375 ° C.) that is solid at room temperature and normal pressure, and 160 parts of expanded graphite (manufactured by Nippon Graphite Industries Co., Ltd., trade name "EC-300", volume average particle diameter: 50 μm, aspect ratio = 1.5) as a particulate carbon material were mixed and stirred for 20 minutes at a temperature of 150 ° C. using a pressure kneader (manufactured by Nippon Spindle Co., Ltd.). Next, the resulting mixture was placed in a crusher (manufactured by Osaka Chemical Co., Ltd., trade name "Wonder Crush Mill D3V-10") and crushed for 10 seconds to obtain a composition.
<Primary sheet molding process>
Next, 50 g of the obtained composition was sandwiched between sandblasted PET films (protective films) having a thickness of 50 μm, and roll-molded (primary pressing) under the conditions of a roll gap of 1000 μm, a roll temperature of 50° C., a roll linear pressure of 50 kg/cm, and a roll speed of 1 m/min, to obtain a primary sheet having a thickness of 0.8 mm.
<Laminate Forming Process>
Next, the obtained primary sheet was cut into a size of 150 mm length x 150 mm width x 0.8 mm thickness, and 188 sheets were stacked in the thickness direction of the primary sheet.Furthermore, by pressing (secondary pressing) in the stacking direction at a temperature of 120°C and a pressure of 0.1 MPa for 3 minutes, a laminate with a height of approximately 150 mm was obtained.
<Slicing process>
The laminated side of the secondary-pressurized laminate was then pressed with a pressure of 0.3 MPa, and sliced at an angle of 0 degrees to the lamination direction (in other words, in the normal direction to the main surface of the laminated primary sheets) using a woodworking slicer (manufactured by Marunaka Iron Works Co., Ltd., product name "Super Mecha S Super Finishing Planer") to obtain a secondary sheet measuring 150 mm in length, 150 mm in width, and 0.10 mm in thickness.
<Firing process>
Thereafter, the obtained secondary sheet was baked at 380° C. for 8 hours in a nitrogen atmosphere to burn off and remove the resin component, thereby obtaining a composite sheet.
The composite sheet thus obtained was subjected to various measurements and evaluations, and the results are shown in Table 1.

Example 2
In preparing the composition, except that 220 parts of expanded graphite was blended as the particulate carbon material, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

Example 3
In preparing the composition, except that 250 parts of expanded graphite was blended as the particulate carbon material, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

Example 4
The baking conditions in the baking step were changed to 320°C x 4 hours. Except for this, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1. When the composite sheet was observed, residual resin was visually confirmed.

Example 5
In preparing the composition, 110 parts of a fluororubber (Asahi Glass Co., Ltd., "AFLAS (registered trademark)-100S", Mooney viscosity (ML _{1+10, 100°C} ): 160) that is solid at room temperature and normal pressure was blended as the resin instead of nitrile rubber, which is solid at room temperature and normal pressure. The blending amount of expanded graphite as particulate carbon material was changed to 215 parts, and adjustments were made so that the volume fraction of the carbon material in the secondary sheet would be the same as in Example 1. Except for these points, various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

Example 6
In preparing the composition, except that 100 parts of expanded graphite was blended as the particulate carbon material, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 1)
The secondary sheet was subjected to measurement and evaluation of various attributes without carrying out the firing step in Example 1. The results are shown in Table 1.

(Comparative Example 2)
The secondary sheet was subjected to measurement and evaluation of various attributes without carrying out the firing step in Example 5. The results are shown in Table 1.

Table 1 shows that the composite sheets of Examples 1 to 6, which contain carbon material, have a low resin content, have an orientation angle of the carbon material between 60° and 90°, and have a thermal conductivity of 10 W/mK or higher, were resistant to tearing under pressure and had excellent compressibility. On the other hand, in Comparative Examples 1 and 2, in which the baking process was not performed, the volume fraction of resin contained in the secondary sheet exceeded 15%, and these sheets were unable to achieve both resistance to tearing under pressure and compressibility.

The present invention provides a composite sheet and a method for manufacturing the same that is resistant to tearing under pressure and has excellent compressibility.

Claims (9)

A composite sheet containing a carbon material,
the composite sheet is obtained by firing a sheet containing a resin and a carbon material,
The composite sheet does not contain a resin, or if it does contain a resin, the content is 13 % by volume or less;
the angle of the long axis direction of the carbon material with respect to the surface of the composite sheet is 60° or more and 90° or less; and
The thermal conductivity of the composite sheet is 10 W/mK or more.
Composite sheet. The composite sheet according to claim 1, wherein the carbon material is a particulate carbon material. The composite sheet according to claim 2, wherein the particulate carbon material is flake graphite. The composite sheet according to claim 3, wherein the flake graphite is expanded graphite. The composite sheet according to any one of claims 1 to 4, having a density of 1.2 g/cm3 or ^less . A method for producing the composite sheet according to any one of claims 1 to 5,
a primary sheet forming step of pressurizing a composition containing the resin and the carbon material to form it into a sheet to obtain a primary sheet;
a laminate forming step of laminating a plurality of the primary sheets in the thickness direction or folding or rolling the primary sheets to obtain a laminate;
a slicing step of slicing the laminate at an angle of 45° or less with respect to the lamination direction to obtain a secondary sheet;
and a firing step of firing the secondary sheet under conditions of a firing temperature of 300°C or higher and 2000°C or lower for a firing time of 30 minutes to 72 hours so that the resin content in the sheet is 15% by volume or lower, thereby obtaining the composite sheet. The method for manufacturing a composite sheet described in claim 6, wherein the volume fraction of the carbon material in the secondary sheet is 31 volume % or more. The method for producing a composite sheet according to claim 6 or 7, wherein the resin includes a liquid resin. The method for producing a composite sheet according to any one of claims 6 to 8, wherein the value of the fired state index (%) calculated according to the following formulas (1) to (3) is 0% or more and 15% or less.
Theoretical firing residue rate=(amount of the carbon material in the composition)/(total solid content in the composition) (1)
Measured firing residual rate=(mass of the composite sheet)/(mass of the secondary sheet) (2)
Firing condition index (%) = {1 - (theoretical firing residual rate) ÷ (measured firing residual rate)} × 100 (3)