METHOD FOR MANUFACTURING HEAT DISSIPATION SHEET USING WASTE GRAPHITE

Abstract

According to the present invention, when manufacturing expandable graphite, it is possible to remarkably reduce the generation of waste acid and waste and economically manufacture expandable graphite having a low content of volatile substances and good appearance, and thus, it is possible to efficiently manufacture a heat dissipation sheet having excellent thermal conductivity.

Claims

1. A method for manufacturing a heat dissipation sheet, comprising the steps of: (1) pulverizing a graphite material comprising waste graphite to produce a powder containing waste graphite; (2) subjecting the powder containing waste graphite to the heat treatment at 900° C. to 1100° C. for 3 to 5 hours; and (3) pressurizing the expandable graphite obtained after heat treatment to produce a sheet.

2. The method for manufacturing the heat dissipation sheet according to claim 1, further comprising an intercalation step of adding and stirring a strong acid and an oxidizing agent to the powder containing waste graphite between steps (1) and (2).

3. The method for manufacturing the heat dissipation sheet according to claim 1, wherein the graphite material comprising waste graphite is a blend of waste graphite and any one or more of artificial graphite, kish graphite, carbon nanotube and carbon fiber in a weight ratio of 90:10 to 60:40.

4. The method for manufacturing the heat dissipation sheet according to claim 1, wherein the graphite material comprising waste graphite consists of waste graphite.

5. The method for manufacturing the heat dissipation sheet according to claim 1, wherein the graphite material comprising waste graphite is pulverized to have a particle diameter of 0.5 .Math.m to 1000 .Math.m.

6. The method for manufacturing the heat dissipation sheet according to claim 2, wherein the strong acid in the intercalation step is any one or more of sulfuric acid and nitric acid, and the oxidizing agent is potassium permanganate.

7. The method for manufacturing the heat dissipation sheet according to claim 6, wherein sulfuric acid, nitric acid and potassium permanganate in the intercalation step are added in an amount of 20% to 40% based on the weight of the powder containing waste graphite.

8. The method for manufacturing the heat dissipation sheet according to claim 1, wherein the heat treatment step further comprises the step of elevating the temperature to 500° C. for 1 hour, maintaining the temperature for 2 hours, and then elevating the temperature to 900° C. for 1 hour before subjecting the powder containing waste graphite to the heat treatment at 900° C. to 1100° C. for 3 to 5 hours.

9. The method for manufacturing the heat dissipation sheet according to claim 1, wherein in the heat treatment step, water is disposed under the loaded powder containing waste graphite to allow water vapor to pass between the powder, and heat treatment is performed.

10. The method for manufacturing the heat dissipation sheet according to claim 1, wherein the heat treatment step is performed in a facility for manufacturing expandable graphite comprising a container in which graphite bricks and refractory bricks in a ratio of 5:5 to 8:2 are bonded with a bonding agent, the graphite bricks are disposed in the center part and/or the lower part of the container, and the outer wall of the container is surrounded by austenitic stainless steel; an airtight furnace for heating the container by electricity or liquefied petroleum gas; and a gas collecting and dust collecting facility disposed at an upper part of the airtight furnace to communicate with the inside of the airtight furnace.

11. The method for manufacturing the heat dissipation sheet according to claim 10, wherein the bonding agent is a mixture of a graphite powder having a particle diameter of 1 .Math.m or less in a refractory mortar capable of withstanding a high temperature of 1000° C. or higher in an amount of 20 to 50% based on the weight of the refractory mortar.

12. The method for manufacturing the heat dissipation sheet according to claim 1, wherein the sheet is manufactured to have a thickness of 5 .Math.m to 2000 .Math.m.

13. A heat dissipation sheet manufactured according to the manufacturing method according to claim 1.

14. An electrical and electronic product comprising the heat dissipation sheet according to claim 13.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0054] FIG. 1 is a photograph of the special container of the facility for manufacturing expandable graphite of the present invention.

[0055] FIG. 2 is a photograph of an electric furnace into which a special container is put in the facility for manufacturing expandable graphite of the present invention.

[0056] FIG. 3 is a photograph of a gas collecting and dust collecting facility connected to the electric furnace of FIG. 2.

[0057] FIG. 4 is a photograph of a transport facility that can be optionally added to the facility for manufacturing expandable graphite of the present invention.

[0058] FIG. 5 is a photograph of a cross section of a commercially available heat dissipation sheet.

[0059] FIG. 6 is a photograph of a cross section of a heat dissipation sheet manufactured according to an embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

[0060] Although described with reference to preferred embodiments of the present invention, the present invention is not limited thereto, and various modifications and changes may be made to the present invention without departing from the spirit and scope of the present invention described in the claims, detailed description and accompanying drawings.

Example 1. Construction of Expandable Graphite Manufacturing System Utilizing Waste Graphite

Example 1-1. Manufacture of Special Container for Manufacturing Expandable Graphite

[0061] A special container for manufacturing expandable graphite was produced to withstand a large amount of corrosive gas generated by the acid component included in the waste graphite and the intercalant included in the pretreatment process, etc. The size of the container was designed so that the powder can be loaded at a level of about 1 ton in consideration of economic feasibility.

[0062] Steel (1st generation), austenitic stainless steel (SUS 304), refractory brick (3rd generation) and heat-resistant treated graphite brick (4th generation) were tested as materials for the container, but all of them failed to manufacture the container because of problems such as oxidation of the container due to corrosive gas and lack of heat-resistant performance. Accordingly, by analyzing the above failure examples, a special container was manufactured using a combination of graphite brick, refractory brick and austenitic stainless steel (SUS).

[0063] First, the SUS plate was made to cover the outermost part of the container so that the structure of the container would not collapse or the block structure would not expand due to heat when heat was applied, and the inside of the container was produced using refractory bricks and graphite bricks.

[0064] In this case, as a result of varying the mixing ratio of graphite bricks and refractory bricks, it was found that the best heat transfer was achieved when the ratio of graphite bricks : refractory bricks was 5:5 to 8:2. In addition, as a result of the experiment, it was found that the positions of the graphite bricks and the refractory bricks are important, and it was found that when the graphite bricks are disposed in the center part or the lower part of the container, the heat is evenly transferred and the air path between the loaded powders is well formed to allow smooth heat treatment.

[0065] On the other hand, in the case of using a general refractory mortar, a brick collapse phenomenon occurred due to volatile substances and heat when subject to heat treatment three or more times. In order to solve this problem, about 20% to 50% of refined graphite powder with a particle diameter of 1 .Math.m or less was mixed in a special refractory mortar (Chosun Refractories, Super 3000) capable of withstanding at 1000° C. or higher, thereby increasing the lifespan of the container by more than 10 times, and thus a container that can be used more than about 30 times could be manufactured. The special container manufactured in this example is as shown in FIG. 1.

[0066] In addition, the lid of the container was produced in an oval shape to facilitate the discharge of gas from the inside of the container, and the lid of the container was made of the lightest refractory bricks, and SUS and a steel plate to withstand high temperature heat and volatile substances.

Example 1-2. Production of Dry Distillation Facility

[0067] First, a conveyor method was introduced as a dry distillation method. A heat source was disposed in the middle of the conveyor, and composite graphite powder was placed in the container manufactured in Example 1-1 and then slowly moved to manufacture expandable graphite. However, there was a problem in that it was not easy to dispose of the gas generated during dry distillation and a lot of cost was consumed.

[0068] Therefore, dry distillation was performed by adopting an electric furnace method as shown in FIG. 2. In this case, the internal space was sealed, and it was easy to construct a ventilation system capable of discharging the gas generated inside at the elevated temperature to the outside.

Example 1-3. Production of Ventilation Facility

[0069] A gas collecting and dust collecting system as shown in FIG. 3 was provided at an upper part of the electric furnace, a special fan was installed to facilitate the discharge of gas and moisture, and an exhaust pipe coated with a special heat-resistant/acid-resistant paint was produced to prevent corrosion due to heat and gas.

Example 1-4. Production of Transport Facility

[0070] Since the heat-treated powder is a fine powder with a low moisture content, a transport facility that can suck the powder and put it in an exclusive loading container was separately manufactured in order to prevent the powder from flying or contaminating the surroundings. As shown in FIG. 4, the produced transport facility is equipped with an intake part, a transfer pipe, a loading part, and an ejection port, and it was designed so that the powder was sucked through the intake part, loaded in the loading part through the transfer pipe and then ejected through the ejection port to be contained in the tone bag.

Example 2. Manufacture of Expandable Graphite

Example 2-1. Preparation of Graphite Material Comprising Waste Graphite

[0071] Waste graphite from a waste lithium secondary cell and a waste fuel cell was collected from recycling companies, and thus waste graphite, artificial graphite and kish graphite were mixed in a weight ratio of 80:10:10.

[0072] The blended composite powder was pulverized by an air jet mill (KMTech, JM-500) for about 2 minutes per kg to have an average particle diameter of about 0.5 mm and a wide particle size distribution.

Example 2-2. Pretreatment Before Expansion (Intercalation)

[0073] Sulfuric acid, nitric acid, and potassium permanganate were added in an amount of about 30% by weight of the powder to the powder containing waste graphite obtained in Example 2-1, and stirred for about 2 hours at 1000 rpm to 2000 rpm in a high viscosity stirrer to produce a lamellar compound.

Example 2-3. Manufacture of Expandable Graphite

[0074] Using the facility for manufacturing expandable graphite constructed in Example 1, the lamella compound powder obtained in Example 2-2 was put into a container, and the container was put into an electric furnace and heated. Heat treatment was carried out under optimal manufacturing conditions confirmed in Experimental Example 1 below (i.e., elevating the temperature to 500° C. over 1 hour, having the maintaining time of 2 hours, elevating the temperature to 900° C. over 1 hour, and then subjecting the powder to the heat treatment for 3 hours).

Example 2-4. Manufacture of Heat Dissipation Sheet

[0075] The heat treated expandable graphite was cooled, and then a plate-shaped heat dissipation sheet was manufactured through a pressurizing process. The pressurizing was carried out by rolling 1 to 5 times with a roller to manufacture a sheet having a thickness of 50 .Math.m and 300 .Math.m.

Experimental Example 1. Evaluation of Manufacturing Efficiency Depending on Temperature and Duration of Heat Treatment

[0076] The content of volatile substances (including moisture) was measured by varying the temperature and duration of heat treatment, and the results are shown in Table 1 below.

TABLE-US-00001 Temperature Content (%) of volatile substances* 7 Hr 10 Hr Room temperature 0.50% or more 0.50% or more 500° C. 0.30% 0.29% 600° C. 0.20% 0.17% 700° C. 0.17% 0.16% 800° C. 0.09% 0.08% 900° C. 0.03% 0.02% 1000° C. 0.02% 0.02% 1100° C. 0.01% or less 0.01% or less * Content (%) of volatile substances = (m - ml) / m * 100 m is the mass before heat treatment, and ml is the mass after heat treatment (measured after 1 hour at 400° C. ± 20° C.)

[0077] As a result, it was confirmed that a significant amount of volatile substances remained regardless of the heating time when the temperature inside the electric furnace ranged from 500° C. to 800° C. It was confirmed that heating should be performed at least 900° C. or higher in order to reduce the volatile substances in the graphite powder to 0.02% or less, which is the target value. In addition, since the oxidation of the graphite powder increases rapidly when the temperature inside the electric furnace becomes 1100° C. or higher, it was confirmed that the optimal temperature for manufacturing expandable graphite in the manufacturing process of the present invention is 900° C. to 1100° C.

[0078] On the other hand, it was found that in the manufacturing process of the present invention, if the heat treatment temperature is elevated vertically at once, the powder on the surface of the electric furnace is quickly oxidized or dried, and the inner powder is relatively less dried, thereby causing problems with the total content and the yield of expandable graphite after dry distillation.

[0079] Accordingly, the remaining amount of the powder was measured by setting 500° C. as the first heating temperature and varying the time taken for the heating, the second heating temperature and the like. The results are shown in Table 2 below. [Table 2]

[0080] From the results of Table 2, it can be seen that elevating the temperature to 500° C. over 1 hour is the most efficient when considering both the economic aspect and the yield of the powder.

[0081] Furthermore, as a result of the experiments where the maintaining time was regulated after elevating the temperature to 500° C. over 1 hour, it was found that, in comprehensive consideration of the removal of volatile substances, the even transfer of heat, the yield of powder, economic feasibility, and the like, the best manufacturing conditions were achieved by having a maintaining time of 2 hours and elevating the temperature for 1 hour to 900° C., which is determined as the optimal temperature for manufacturing expandable graphite in the manufacturing process of the present invention. In addition, it was found that the optimal maintaining time at 900° C. was 3 hours.

Experimental Example 2. Confirmation of Improvement in Manufacturing Efficiency Depending on Moisture Treatment

[0082] Expandable graphite was manufactured according to Example 2 using the manufacturing system of Example 1, and 10 kg (10 L) of water per ton of powder was sprinkled on the lower part of the container, and the intercalated powder containing waste graphite was filled thereon and heated. As shown in Table 3 below, it was found that the volatile substances were discharged much better and the appearance of the obtained powder was excellent compared to the case where the water filling process was not carried out.

TABLE-US-00002 Content of volatile substances Condition of powder Without moisture 0.06% The grain surface is very rough and not neat With moisture 0.03% Appropriate size and good surface condition

[0083] Considering the above results, it is believed that as the moisture escapes as vapor, it forms an air path between the loaded powders, through which heat is more efficiently transferred and volatile substances are better discharged. In particular, it is considered that since graphite bricks having high thermal conductivity were disposed in the lower part of the container according to Example 1, the air path was well formed at a low temperature by speeding up heat transfer.

Experimental Example 3. Evaluation of Thermal Conductivity of Heat Dissipation Sheet

[0084] The sheet produced in Example 2-4 was compared and evaluated for thermal conductivity with a commercially available sheet. The DSN (JIANGXI DASEN TECHNOLOGY CO., LTD) product used as a commercially available sheet is a heat dissipation sheet made of expandable graphite, which is manufactured by extracting graphite from graphite minerals dug up in a mine and subjecting it to intercalation with a strong acid (H.sub.2SO.sub.4, etc.), according to a conventional method for manufacturing a heat dissipation sheet.

TABLE-US-00003 No. Thickness Tested heat dissipation sheet Thermal conductivity Sample 1 50 .Math.m DSN heat dissipation sheet 239 W/mK Sample 2 50 .Math.m DSN heat dissipation sheet 240 W/mK Sample 3 50 .Math.m Sheet produced in Example 2-4 329 W/mK

TABLE-US-00004 No. Thickness Tested heat dissipation sheet Thermal conductivity Sample 4 300 .Math.m DSN heat dissipation sheet 250 W/mK Sample 5 300 .Math.m DSN heat dissipation sheet 250 W/mK Sample 6 300 .Math.m Sheet produced in Example 2-4 390 W/mK

[0085] As a result of the experiments, the heat dissipation sheet manufactured according to the manufacturing process of the present invention showed an improvement in the thermal conductivity by about 37 to 56% compared to the control product. Since waste graphite obtained from a secondary cell, a fuel cell, and the like was used as a raw material in the present invention, the content of impurities is very low, and the content of volatile substances in expandable graphite is significantly reduced according to the manufacturing process of the present invention, and the degree of agglomeration of expandable graphite is increased to make the compression rate higher than that of existing products, and thus the thermal conductivity is greatly improved. Comparing FIGS. 5 and 6, it can be seen that the compression rate of the expandable graphite in the sheet is significantly improved in the heat dissipation sheet manufactured according to the present invention, compared to a commercially available heat dissipation sheet.

[0086] As described above, the present invention has been described with reference to the preferred embodiments and drawings, but those of ordinary skill in the art should understand that the present invention can be variously modified and implemented without departing from the spirit and scope of the present invention described in the claims of the present invention.

[0087] Therefore, the scope of the present invention is not limited to the embodiments of the present invention and should be determined by the contents described in the claims of the present invention.