WO2021020922A1 - Method for manufacturing graphite sheet - Google Patents

Abstract

The present invention relates to a method for manufacturing a graphite sheet and, more specifically, can manufacture a graphite sheet with improved thermal conductivity, particularly, improved vertical thermal conductivity by using, as a precursor, a three-dimensional fiber substrate such as paper that comprises a fiber having a carbon fraction of 50% or more, and performing carbonization, graphitization and rolling.

Description

Method for producing graphite sheet

The present invention relates to a method of manufacturing a graphite sheet.

In view of the recent trend of high-functionality, high-integration, light-thin and short-sized electronic devices, it is essential to effectively dissipate heat generated when electronic devices are driven. To this end, various heat dissipation materials are being developed, and are applied in the form of heat dissipation pads, heat dissipation sheets, and heat dissipation paints. Among them, the heat dissipation sheet is manufactured in the form of a graphite sheet, a polymer-ceramic composite sheet, and a multilayer coated metal thin film sheet. In the case of a graphite sheet, it is light weight and slim, yet excellent heat resistance and resistance. In addition to having chemical properties and high thermal conductivity, in particular, it has a very high thermal conductivity than copper and is widely used in various electronic devices.

As an example of a method for producing a graphite sheet, there is a method called "expanded (expanded) graphite method". In this method, artificial graphite is prepared by immersing natural graphite in a mixture of concentrated sulfuric acid and concentrated nitric acid and then rapidly heating. Thereafter, the expanded graphite is cleaned to remove acid, and processed into a film shape by a high pressure press or a roll. However, the graphite sheet manufactured through the above method has problems such as weak strength, not excellent other physical properties, and concern about the influence of residual acids.

In order to solve this problem, a polymer graphitization method has been developed in which a polymer film is directly heat-treated to graphitize it. Specifically, in the polymer graphitization method, a process of carbonizing a polymer film, a precursor for a graphite sheet, at a temperature of 1,200 ℃ to 1,400 ℃, followed by a process of graphitizing a precursor for a graphite sheet carbonized at a temperature of up to 2,800 ℃. This is a method of manufacturing a graphite sheet. Examples of the polymer film used at this time include polyoxadiazole, polyimide, polyphenylene vinylene, polybenzoimidazole, polybenzoxazole, polythiazole, and polyamide films.

This polymer graphitization method is a much simpler method compared to the conventional expanded graphite method, and is a method that essentially does not cause the incorporation of impurities such as acids, and has the characteristics of obtaining excellent thermal or electrical conductivity close to single crystal graphite. .

However, the graphite sheet manufactured by such a polymeric graphitization method has high thermal conductivity (thermal diffusivity) in the horizontal direction, but low thermal conductivity in the vertical direction, and the manufacturing cost is high due to the use of an expensive polyimide film as a polymer film. There are drawbacks.

The low vertical thermal conductivity of the graphite sheet cannot secure sufficient performance when applied as a heat dissipating material, and various techniques have been proposed to improve this.

For example, Korean Patent Application Publication No. 2017-0081874 can improve the thermal conductivity of the graphite sheet in the vertical direction by using a polyimide film containing a thermally conductive material such as carbon nanotubes and boron nitride as a precursor of the graphite sheet. Is being disclosed.

In addition, Korean Patent Registration No. 1885281 is a method of manufacturing a heat-dissipating sheet with excellent thermal conductivity in the horizontal and vertical directions. After perforating a graphite sheet manufactured by heat-treating a substrate coated with a coating liquid containing a polymer, carbonized polymer, or graphite, , Disclosed is a method comprising the step of coating one or both sides thereof with a metal.

These patents have improved the thermal conductivity of the graphite sheet in the vertical direction to some extent by adding or using a material having excellent thermal conductivity properties to form a coating layer, but the effect is not sufficient. In addition, the method proposed in these patents has a problem that the manufacturing cost is high due to the use of an expensive polyimide film or a complicated process. Accordingly, there is a need for further development of a method for manufacturing a graphite sheet capable of economically manufacturing a graphite sheet having excellent thermal conductivity in both horizontal and vertical directions.

Accordingly, the present inventors conducted various studies to solve the above problem, and as a result of using a fiber substrate as a precursor when producing a graphite sheet, impregnating the fiber substrate with a thermally conductive interface material, and rolling it before and after carbonization and graphitization. When performing the process, the present invention was completed by confirming that the thermal conductivity in the vertical direction of the finally produced graphite sheet was improved and the flexibility was improved.

Accordingly, an object of the present invention is to provide a method of manufacturing a graphite sheet capable of simply and economically manufacturing a graphite sheet having improved thermal conductivity, particularly, thermal conductivity in the vertical direction.

In order to achieve the above object, the present invention comprises the steps of (S1) preparing a fibrous substrate using short fibers; (S2) preparing a composite substrate by impregnating the fiber substrate obtained in step (S1) with a thermally conductive interface material; (S3) heat-treating the composite substrate obtained in step (S2) to carbonize and graphitize to produce a graphite sheet; And (S4) rolling to improve the thermal conductivity of the graphite sheet obtained in step (S3).

In the method of manufacturing a graphite sheet according to the present invention, a graphite sheet having excellent vertical and horizontal thermal conductivity can be manufactured by using a high-density fiber substrate as a precursor and performing a rolling process after carbonization and graphitization. In addition, since an expensive polyimide polymer film is not used as a precursor, manufacturing cost is reduced, and thus economic efficiency and productivity of the manufacturing process can be improved.

1 is a scanning electron microscope (SEM) image of the surface of a fiber substrate according to Example 1 of the present invention.

2 is a scanning electron microscope (SEM) image of a cross section of a fiber substrate according to Example 1 of the present invention.

3 is a scanning electron microscope (SEM) image of a cross section of a fiber substrate according to Experimental Example 2 of the present invention before and after rolling.

4 is a scanning electron microscope (SEM) image of a cross section of a composite substrate according to Experimental Example 3 of the present invention.

5 is a scanning electron microscope (SEM) image of a cross section of a composite substrate according to Experimental Example 4 of the present invention after rolling.

6 is a scanning electron microscope (SEM) image on the surface of a graphite sheet according to carbonization and graphitization in Experimental Example 5 of the present invention.

Hereinafter, the present invention will be described in more detail.

The terms or words used in this specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The terms used in the present invention are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present invention, terms such as'include' or'have' are intended to designate the presence of features, numbers, steps, actions, components, parts, or a combination thereof described in the specification, but one or more other features It is to be understood that the presence or addition of elements or numbers, steps, actions, components, parts, or combinations thereof, does not preclude in advance.

The term "thermal conductivity (thermal diffusivity) in the horizontal direction" of the graphite sheet used in the present invention refers to both the thermal conductivity of the graphite sheet in the longitudinal direction and the width direction in the plane.

The term "thermal conductivity (thermal diffusivity)" of the graphite sheet used in the present invention refers to the thermal conduction in the thickness direction (height direction) of the graphite sheet in a plane perpendicular to the longitudinal direction and/or the width direction of the graphite sheet. It refers to both the thermal conductivity of the graphite sheet in the thickness direction (height direction) of the graphite sheet forming a longitudinal direction and an inclination angle in the plane of the graphite sheet.

Recently, electronic devices are required to dissipate heat due to an increase in heat density due to lightness, thinness, high functionality, and high integration, and this heat dissipation characteristic is very important because it is closely related to reliability and lifespan as well as the performance of the electronic device.

Recently, with the spread of electric vehicles and renewable energy vehicles, the use of electronic parts has increased, and since batteries with high heat generation are used as the main power source, measures against heat generation are essential. In particular, self-driving cars are an aggregate of all electric and electronic industries such as mobile communication technology, lighting, cameras, displays, batteries, etc., and there are limitations in development and distribution unless the heat problem is solved.

In addition, as various electronic devices, including portable electronic devices and communication devices, become lighter and slimmer, heat control has been raised as a continuous problem due to an increase in heat density due to high integration of electronic devices. In the electrical and electronic industry, heat generation problems inevitably occur unless superconductors (resistance = 0) are used, and these heating problems are closely related to reliability and lifespan as well as the performance of electronic devices. The demand for next-generation heat dissipating materials that can be efficiently discharged is increasing rapidly.

To this end, in the prior art, a graphite sheet prepared by heat treatment of a polymer film such as polyimide at high temperature is used. The film-based graphite sheet manufactured by the polymer graphitization method has a thermal conductivity of 1,000 W/m·K or more in the horizontal direction due to the structure of the graphene layer arranged in the 2D direction, but up to 20 W in the vertical direction. It has been reported up to /m·K, but generally has a lower thermal conductivity than the horizontal direction of 5 W/m·K or less. This heat conduction property has no problem in dissipating heat generated in a hot spot area where a specific part of the electronic device locally rises to a high temperature. However, as described above, as the use of high-performance and small-sized components integrated in electronic devices in recent years has increased, the low vertical thermal conductivity of the conventional graphite sheet has a limitation in effectively dissipating the heat generated any more.

Accordingly, the present invention provides a method of manufacturing a graphite sheet using a fiber type as a precursor of the graphite sheet in order to improve the thermal conductivity in the vertical direction of the graphite sheet, which has been pointed out as a limitation of the existing heat dissipating material.

Specifically, the method of manufacturing a graphite sheet according to the present invention comprises the steps of (S1) preparing a fibrous substrate using short fibers; (S2) preparing a composite substrate by impregnating the fiber substrate obtained in step (S1) with a thermally conductive interface material; (S3) heat-treating the composite substrate obtained in step (S2) to carbonize and graphitize to produce a graphite sheet; And (S4) rolling the graphite sheet obtained in step (S3).

Step (S1)

In step (S1), a fiber substrate is prepared using short fibers.

In the present invention, the fibrous substrate includes short fibers oriented in a three-dimensional form as a precursor of a graphite sheet. In particular, in the case of the present invention, since a fiber substrate containing short fibers is used as a precursor of the graphite sheet, it has a three-dimensional crystal structure, compared to the conventional graphite sheet using a polymer film having only a crystal structure in the horizontal direction as a precursor. A graphite sheet having improved thermal conductivity in the vertical direction can be manufactured. In addition, by using a fiber substrate containing relatively inexpensive short fibers in place of the polymer film used in the polymer graphitization method, which is a conventional graphite sheet manufacturing method, specifically an expensive polyimide film, the manufacturing cost is lowered to improve economy and productivity. Has an advantage.

The short fiber means that the long fiber is cut to a predetermined length, and the length is not particularly limited, but may be, for example, in the range of 3 mm to 56 mm.

The short fibers include aramids such as meta aramid (m-aramid) and para aramid (p-aramid); Polyimide (PI) such as poly(amideimide), PAI), and polyetherimide (PEI); Polyamide (PA); Polystyrene, PS), polyethylene (PE), polyethylene terephthalate (PET), poly(vinyl chloride), PVC, poly(vinylidene chloride), PVDC ), polypropylene (PP), polysulfone, poly(etheretherketone), poly(phenylene sulfide), polycarbonate (PC), polyaryl ether ketone (poly(aryletherketone)), an acrylonitrile butadiene styrene (ABS) and an acrylate-styrene-acrylonitrile polymer (acrylonitrile styrene acrylate, ASA); (epoxy), phenol (phenol), unsaturated polyester (polyester), polyurethane (polyurethane, PU), melamine (melamine), urea (thermoset polymer), such as; PAN), pitch (pitch), may be one or more selected from the group consisting of carbon fibers derived from cellulose (cellulose) Preferably, the short fiber is 1 selected from the group consisting of aramid, polyimide, and thermosetting polymer It may contain more than one species.

The fibrous substrate is that the aforementioned short fibers are oriented in a three-dimensional form, and when a short fiber is used, the short fiber itself typically takes 1.5 to 20 times of the initial discharge rate, but the draw ratio is less than 1.5 times. It is formed in a higher orientation structure than that of a polyimide polymer film manufactured by non-stretching, so that high thermal conductivity can be realized through a graphitization process. In addition, when the fibrous substrate is used as a raw material for forming a graphite sheet, it has a three-dimensional isotropic structure, and the difference in thermal conductivity in the horizontal direction and the vertical direction is relatively low compared to the polymer film having a two-dimensional structure. It is effective in improving the thermal conductivity in the vertical direction.

The fibrous substrate is a three-dimensional porous structure including voids therein, and may be in the form of a sheet, a fabric, or a web.

Therefore, the fibrous substrate may be paper, nonwoven, woven, knitted, felt, mat, prepreg, or nano web. have.

The manufacturing method of the fiber substrate is not particularly limited, and a known method or various methods of modifying it may be used by a person skilled in the art. For example, methods such as dry laid, wet-laid, spinning, air-laid, melt-blown, and stacked stretching methods can be used. have.

The fibrous substrate may further include a material commonly used in addition to the short fibers. For example, it may further include a binder fiber, a surfactant, a dispersant, a thickener, etc. for bonding the short fibers constituting the fibrous substrate and making the fibrous substrate solid.

Here, in order to increase the strength of the fibrous substrate to facilitate processing in the subsequent step, and to minimize the voids present inside the fibrous substrate, the three-dimensional porous structure includes voids inside, that is, between short fibers, and bulk density In the initial fibrous substrate with a (bulk density) of 10% to 60% of the inherent density, finally, the air contained in the voids present inside the fibrous substrate acts as a resistance in heat conduction, so rolling is performed. By removing this and increasing the bulk density of the fibrous substrate, it is possible to secure an effect of improving the thermal conductivity of the finally produced graphite sheet, especially the thermal conductivity in the vertical direction.

The rolling may be performed according to a conventional method known in the art. The temperature and pressure of rolling may vary depending on the fibrous substrate or the short fibers included therein. For example, the rolling can be performed at a temperature of 80 ℃ to 200 ℃ and a pressure of 30 to 200 kgf/cm in the case of a continuous production method. I can.

The thickness of the fibrous substrate after rolling is preferably 40% to 80% of the thickness of the fibrous substrate before rolling is performed to improve the thermal conductivity and other physical properties of the final product, the graphite sheet. The air permeability of the fibrous substrate after the rolling is preferably 0.1% to 45% of the air permeability of the fibrous substrate before the rolling is performed to improve the thermal conductivity and other physical properties of the final product, the graphite sheet.

Step (S2)

In the step (S2), a composite substrate is prepared by impregnating the fiber substrate obtained in step (S1) with a thermal interface material (TIM).

In the present invention, the composite substrate is a graphite that is finally produced by completely removing air by impregnating the pores of the fibrous substrate prepared in step (S1) through impregnation, and increasing the interfacial contact between short fibers. It is possible to improve the thermal conductivity of the sheet in the horizontal and vertical directions.

The thermally conductive interface material is not limited to the type as long as it is a polymer having a carbon component fraction of 50% or more in molecular structural formula. For example, the thermally conductive interface material is a chemical such as polyimide (PI), lignin, aramid, poly(amideimide), PAI), and polypropylne (PP). It may be one or more selected from the group consisting of cured thermoplastic resins and phenol resins, and preferably, the thermally conductive interface material is a group consisting of resins having a relatively high carbon component fraction and easy control of viscosity It may include one or more selected from.

The composite substrate can be obtained by impregnating the fibrous substrate obtained in step (S1) into a varnish composition containing the thermally conductive interface material.

Examples of the impregnation method include a method of immersing the fiber substrate in a varnish composition containing the thermally conductive interface material, and applying a varnish composition containing the thermally conductive interface material to the fiber substrate by various coating machines. And a method of penetrating the varnish composition containing the thermally conductive interface material into the fiber substrate by spraying, but are not limited thereto. Among them, the method of immersing the fibrous substrate in the varnish composition containing the thermally conductive interface material is preferable because it can improve the impregnation property of the varnish composition with the fibrous substrate.

The content of the thermally conductive interface material in the varnish composition may be 10% to 80% by weight, preferably 30% to 60% by weight, based on 100% by weight of the total varnish composition. If the content of the thermally conductive interfacial material is less than the above range, there is a limit to increase the density due to insufficient filling of the voids in the substrate, which may cause a problem of increasing the resistance.On the contrary, if the content exceeds the above range, the viscosity is too high and impregnation is uniform. Since the property may be deteriorated, it is preferable to determine an appropriate content within the above-described range.

A composite substrate is prepared by curing and drying the fibrous substrate impregnated with the varnish composition to form a composite. At this time, the curing and drying conditions may vary depending on the type of material used.

Accordingly, the content of the thermally conductive interface material in the composite substrate may be 40% to 80% by weight based on 100% by weight of the composite substrate.

The composite substrate may further include a filler to improve thermal conductivity.

The filler may be a ceramic material or a polymer material, and in the case of a polymer material, the filler may be the same material as the short fibers constituting the fiber substrate. In addition, it is preferable that the filler is not in the form of chop or fiber, but in the form of particles or powder in that it is intended to fill the voids inside the fibrous substrate.

The filler is carbon nanotube (CNT), graphene, such as single-well carbon nanotube (SWCNT) and multi-well carbon nanotube (MWCNT), It may include at least one selected from the group consisting of graphite powder, conductive carbon black, and boron nitride nanotube (BNNT).

The carbon nanotubes may have a diameter of 4 nm to 50 nm and a length of 1 nm to 500 μm.

The boron nitride nanotubes may have a diameter of 0.01 µm to 3 µm, preferably 0.1 µm to 1.0 µm.

When the diameter and length of the carbon nanotubes and the diameter of the boron nitride nanotubes are out of the above ranges, the effect of improving the vertical thermal conductivity of the finally manufactured graphite sheet may be weak or a problem may occur in the process.

The filler may be included in the varnish composition as described above and may be impregnated into the fibrous substrate.

In addition, the content of the filler in the composite substrate is preferably 10% by weight or less based on 100% by weight of the composite substrate.

The composite substrate prepared from the step (S2) has a three-dimensional porous structure and includes voids between short fibers, and has a bulk density of 10% to 60% of the inherent density. In the subsequent step, the carbonization graphitization process, since many components other than carbon are thermally volatilized to reduce the weight, the strength of the composite substrate should be increased to facilitate processing, and voids existing inside the fiber substrate should be minimized.

In addition, since the air contained in the voids present inside the composite substrate acts as a resistance in heat conduction, it is removed through rolling to increase the bulk density of the composite substrate, thereby increasing the thermal conductivity properties of the finally produced graphite sheet, especially in the vertical direction. It is possible to secure an effect of improving thermal conductivity.

The temperature and pressure of the rolling may vary depending on the fiber substrate or the varnish composition. For example, the rolling can be carried out at a temperature of 100°C to 350°C and a pressure of 30 kgf/cm to 250 kgf/cm in the case of a continuous production method, and the linear pressure effect of the continuous production method in the case of a sheet rolling method and You can fit it to a similar level.

The thickness of the composite substrate subjected to the rolling is preferably 95% or less of the thickness of the fiber substrate before the rolling is performed to improve the thermal conductivity and other physical properties of the graphite sheet as the final product.

It is preferable that the increase rate of the bulk density by rolling is 40% to 95% of the average density of the fiber substrate and the thermally conductive interface material. If the increase rate of the bulk density is less than the above range, too many pores are formed after the subsequent carbonization and graphitization process, thereby reducing the heat dissipation effect. On the contrary, if it exceeds the above range, the flexibility of the final product, the graphite sheet, decreases and is easily broken. do.

Step (S3)

In the step (S3), the composite substrate obtained in the step (S2) is heat-treated to carbonize and graphite to prepare a graphite sheet.

The heat treatment is for increasing the thermal properties of the composite substrate, and is not particularly limited as long as it is a method for carbonizing and graphitizing polymers and/or fibers, and may be performed according to a conventional method known in the art.

For example, the carbonization temperature may be 800 °C to 1,500 °C, preferably 900 °C to 1,400 °C. In addition, the graphitization may be performed at a temperature of 2,600 °C to 3,000 °C, preferably 2,700 °C to 2,900 °C.

The carbonization and graphitization are preferably performed in an inert atmosphere so that the composite substrate is not oxidized by reacting with air. As the medium used for maintaining the inert atmosphere, nitrogen or argon may be used as long as it is not particularly limited.

In addition, the carbonization and graphitization time may be adjusted according to the material constituting the composite substrate.

Step (S4)

In the step (S4), the graphite sheet obtained in the step (S3) is rolled.

In the present invention, the rolling is to increase the bulk density and flexibility of the expanded graphite sheet due to the gas generated during the heat treatment in the step (S3), and to further improve the thermal conductivity in the horizontal and vertical directions.

The rolling may be performed according to a conventional method known in the art.

The rolling pressure may vary depending on the physical properties of the manufactured graphite sheet. For example, the rolling may be performed at a pressure of 10 kgf/cm to 300 kgf/cm.

According to the method of manufacturing a graphite sheet of the present invention, a fiber substrate is used as a precursor of the graphite sheet, a thermally conductive interface material is impregnated with the fiber substrate, and a rolling process is performed before and after carbonization and graphitization, thereby conducting heat conduction in the horizontal direction. As well as degrees, it is possible to manufacture a graphite sheet having excellent thermal conductivity in the vertical direction. In addition, the method of manufacturing the graphite sheet can economically manufacture a graphite sheet having excellent horizontal and vertical thermal conductivity in a simple process without using an expensive polymer film as a precursor.

The graphite sheet manufactured by the above-described manufacturing method has a thermal conductivity of 1,000 W/m·K to 2,000 W/m·K in a horizontal direction, and a thermal conductivity of 10 W/m·K to 40 in the vertical direction. W/m·K, and may have an average thickness of 10 μm to 200 μm.

In particular, the graphite sheet according to the present invention exhibits thermal conductivity in a vertical direction above the limit of the vertical thermal conductivity of a conventional graphite sheet, and is useful as a heat dissipation sheet for various electronic devices due to improved flexibility.

Hereinafter, a preferred embodiment is presented to aid the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention, It is natural that such modifications and modifications fall within the appended claims.

Examples and Comparative Examples

[Example 1]

A fiber substrate was prepared by using a wet method by mixing short aramid fibers having a length of 6 mm, aramid pulp, and polyvinyl alcohol having a length of 3 mm as a binder fiber at a weight ratio of 25:72:3.

The fibrous substrate was rolled at a temperature of 120° C. and a pressure of 148 kgf/cm.

Subsequently, the rolled fibrous substrate in a padder mangle was impregnated in a varnish composition containing 10% by weight of polyimide, dried at 130° C., and cured at 350° C. to prepare a composite substrate.

The composite substrate was rolled at a temperature of 200° C. and a pressure of 200 kgf/cm.

The rolled composite substrate was carbonized in a nitrogen atmosphere at 1,200°C.

Subsequently, the carbonized composite substrate was graphitized in an argon atmosphere at 2,800° C. and then rolled at a pressure of 30 kgf/cm to prepare a graphite sheet.

[Example 2]

A graphite sheet was prepared in the same manner as in Example 1, except that a varnish composition containing 20% by weight of polyimide was used when preparing the composite substrate.

[Example 3]

A graphite sheet was prepared in the same manner as in Example 1, except that short fibers, pulp, and binder fibers were used in a weight ratio of 50:47:3 when preparing the fiber substrate.

[Example 4]

Example 1 and the same as in Example 1, except that short fibers, pulp, and binder fibers were used in a weight ratio of 50:47:3 when manufacturing a fiber substrate, and a varnish composition containing 20% by weight of polyimide was used when manufacturing a composite substrate. In the same manner, a graphite sheet was prepared.

[Example 5]

A graphite sheet was prepared in the same manner as in Example 1, except that short fibers, pulp, and binder fibers were used in a weight ratio of 60:37:3 when preparing the fiber substrate.

[Example 6]

Example 1 and the same as in Example 1, except that short fibers, pulp, and binder fibers were used in a weight ratio of 60:37:3 when manufacturing a fiber substrate, and a varnish composition containing 20% by weight of polyimide was used when manufacturing a composite substrate. In the same manner, a graphite sheet was prepared.

[Example 7]

A graphite sheet was prepared in the same manner as in Example 1, except that short fibers, pulp, and binder fibers were used in a weight ratio of 70:27:3 when preparing the fiber substrate.

[Example 8]

Example 1 and the same as in Example 1, except that short fibers, pulp, and binder fibers were used in a weight ratio of 70:27:3 when manufacturing a fiber substrate, and a varnish composition containing 20% by weight of polyimide was used when manufacturing a composite substrate. In the same manner, a graphite sheet was prepared.

[Comparative Example 1]

A graphite sheet was prepared by carbonizing and graphitizing a polyimide film having a thickness of 75 μm. At this time, carbonization was performed at 900°C and graphitization was performed at 2,800°C.

Experimental Example 1. Evaluation of physical properties of fiber substrates and analysis of scanning electron microscopy

The weight, thickness, and air permeability per unit area of the fiber substrates prepared in Examples 1, 3, 5, and 7 were measured. At this time, the air permeability was measured through an air permeability tester (FX3300, manufactured by Textest Instruments).

In addition, the surface and cross section of the fiber substrate prepared in Example 1 were observed with a scanning electron microscope (SEM). As a scanning electron microscope, Hitachi's S-4800 was used.

The results obtained at this time are shown in Table 1, FIGS. 1 and 2.

	Weight per unit area (g/m 2 )	Thickness(㎛)	Air permeability
Example 1	30.0	126	5.53
Example 3	34.5	140	22.7
Example 5	32.0	123	17.0
Example 7	42.1	210	63.2

Referring to FIG. 1, it can be seen that a fibrous substrate of a three-dimensional porous structure composed of short aramid fibers and having voids formed therein was manufactured.

Experimental Example 2. Evaluation of physical properties of fiber substrates according to rolling and analysis of scanning electron microscopy

After rolling the fiber substrates prepared in Examples 1, 3, 5, and 7, the weight per unit area, thickness, and air permeability were measured. At this time, the air permeability was measured through an air permeability tester (FX3300, manufactured by Textest Instruments).

In addition, cross-sections of the fiber substrate prepared in Example 1 before and after rolling were observed with a scanning electron microscope (SEM). As a scanning electron microscope, Hitachi's S-4800 was used.

The results obtained at this time are shown in Table 2 and FIG. 3.

	Weight per unit area (g/m 2 )	Thickness(㎛)	Air permeability
Example 1	30.0	52	0.927
Example 3	34.5	79	5.95
Example 5	32.0	69	3.89
Example 7	42.1	84	14.0

Referring to FIG. 3 and Table 2, it can be seen that the thickness and air permeability of the fiber substrate are reduced by rolling.

Experimental Example 3. Evaluation of physical properties of composite substrate and analysis of scanning electron microscopy

The cross section of the composite substrate prepared in Example 1 was observed with a scanning electron microscope (SEM). As a scanning electron microscope, Hitachi's S-4800 was used.

The results obtained at this time are shown in FIG. 4.

Referring to FIG. 4, it can be seen that the varnish composition is uniformly impregnated into the fiber substrate.

Experimental Example 4. Evaluation of physical properties of composite substrate according to rolling and analysis of scanning electron microscopy

For the composite substrates prepared in Examples 1 to 8, the weight after rolling and the thickness before and after rolling were measured.

Further, the cross section of the composite substrate prepared in Example 1 after rolling was observed with a scanning electron microscope (SEM). As a scanning electron microscope, Hitachi's S-4800 was used.

The results obtained at this time are shown in Table 3 and FIG. 5.

	Concentration of thermally conductive interface material (% by weight)	Weight(g)	Before rolling	After rolling
			Thickness(㎛)
Example 1	10	0.398	58	46
Example 2	20	0.471	64	54
Example 3	10	0.629	122	83
Example 4	20	0.778	128	89
Example 5	10	0.392	72	52
Example 6	20	0.460	78	55
Example 7	10	0.586	130	85
Example 8	20	0.799	139	99

Referring to Figure 5 and Table 3, it can be seen that the thickness of the composite substrate is reduced by rolling.

Experimental Example 5. Evaluation of physical properties of graphite sheet and analysis of scanning electron microscopy

For the graphite sheets prepared in Examples 1 to 8, the weight and yield according to carbonization and graphitization were measured.

In addition, the surface of the graphite sheet according to carbonization and graphitization of the composite substrate prepared in Example 1 was observed with a scanning electron microscope (SEM). As a scanning electron microscope, Hitachi's S-4800 was used.

The results obtained at this time are shown in Table 4 and FIG. 6.

	carbonization		Graphitization		Total yield (%)
	Weight(g)	yield(%)	Weight(g)	yield(%)
Example 1	0.183	46.0	0.122	66.7	30.7
Example 2	0.241	51.2	0.183	75.9	38.9
Example 3	0.286	45.5	0.229	80.1	36.4
Example 4	0.385	49.5	0.315	81.8	40.5
Example 5	0.191	48.7	0.139	72.8	35.5
Example 6	0.218	47.4	0.172	78.9	37.4
Example 7	0.283	48.3	0.217	76.7	37.0
Example 8	0.405	50.7	0.320	79.0	40.1

It can be seen from FIG. 6 that the color of the composite substrate changes to black after carbonization and silver after graphitization.

In addition, it can be seen from Table 4 that the carbonization yield of the composite substrate is 45 to 51%, the graphitization yield is 66 to 81%, and the total yield is 30 to 40%.

Experimental Example 6. Measurement of thermal conductivity of graphite sheet

The thermal conductivity of the graphite sheets prepared in Example 1 and Comparative Example 1 in the horizontal and vertical directions was measured. At this time, the thermal conductivity in the horizontal direction and the vertical direction was determined by using the graphite sheet prepared in Example 1 and Comparative Example 1 using a thermal diffusivity measuring device ("LFA447 Nanoflash" manufactured by Netsch) at 25°C It was measured five or more times and expressed as an average value. The results obtained at this time are shown in Table 5.

	Thermal conductivity in horizontal direction (W/mㆍK)	Vertical thermal conductivity (W/mㆍK)
Example 1	800	23
Comparative Example 1	1,200	5

Referring to Table 5, it can be seen that the thermal conductivity of the graphite sheet according to Example 1 is higher than that of Comparative Example 1, and in particular, it can be seen that the thermal conductivity in the vertical direction is significantly improved compared to Comparative Example 1.

The graphite sheet according to the present invention exhibits thermal conductivity in the vertical direction above the limit of the thermal conductivity in the vertical direction of the conventional graphite sheet, and has improved flexibility, and thus is useful as a heat dissipation sheet for various electronic devices.

Claims (10)

1. (S1) preparing a fibrous substrate using short fibers;

   (S2) preparing a composite substrate by impregnating the fiber substrate obtained in step (S1) with a thermally conductive interface material;

   (S3) heat-treating the composite substrate obtained in step (S2) to carbonize and graphitize to produce a graphite sheet; And

   (S4) A method of manufacturing a graphite sheet comprising the step of rolling the graphite sheet obtained in step (S3).
2. The method of claim 1,

   The short fibers include at least one selected from the group consisting of aramid, polyimide, thermoplastic polymer, thermosetting polymer, and carbon fiber.
3. The method of claim 1,

   The fibrous substrate is a three-dimensional porous structure, a method of manufacturing a graphite sheet.
4. The method of claim 1,

   The fibrous substrate is in the form of a sheet, a fabric or a web, a method for producing a graphite sheet.
5. The method of claim 1,

   The thermally conductive interface material includes at least one selected from the group consisting of polyimide, lignin, aramid, polyamideimide, polypropylene, and phenol.
6. The method of claim 1,

   The composite substrate further comprises a filler, a method of manufacturing a graphite sheet.
7. The method of claim 6,

   The filler comprises at least one selected from the group consisting of carbon nanotubes, graphene, graphite powder, conductive carbon black, and boron nitride nanotubes.
8. The method of claim 1,

   The carbonization is carried out at a temperature of 800 ℃ to 1,500 ℃, a method for producing a graphite sheet.
9. The method of claim 1,

   The graphitization is carried out at a temperature of 2,600 ℃ to 3,000 ℃, a method for producing a graphite sheet.
10. The method of claim 1,

    The carbonization and graphitization are performed in an inert atmosphere.

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