JP3600640B2 - Heat treatment method of vapor grown carbon fiber - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a method of heat-treating a carbon fiber obtained by a vapor phase method, more specifically, a carbon fiber obtained by a vapor growth method by thermal decomposition of an organic compound.
[0002]
[Prior art]
The method for producing a vapor-grown carbon fiber is an excellent method in which an organic compound is thermally decomposed in a reaction furnace to obtain whisker-like fine carbon fibers in one step. However, there has been a problem in industrial productivity, and various improvements have been made.
For example, at first, a relatively thick and long vapor-grown carbon fiber was produced by depositing ultrafine particles of a transition metal on a ceramic substrate and then supplying and decomposing an organic compound to grow it for a long time. 103528).
[0003]
Although this method can provide carbon fibers having good physical properties, there are many unsatisfactory points such as the fact that the fiber diameter is large and the reaction rate is slow, which is not suitable for industrial production.
In order to improve this, a transition metal such as iron or a compound thereof is used as a catalyst, and the catalyst and a carrier gas and an organic compound such as benzene, toluene and natural gas are introduced into the reaction furnace in a liquid or gaseous state. A method for thermally decomposing organic compounds at about 800 ° C. to 1300 ° C. to produce fine carbon fibers in a short time has been developed, and the productivity has been improved.
[0004]
As a method for producing these vapor grown carbon fibers, (1) a method in which a transition metal compound such as ferrocene is vaporized and introduced into a reaction furnace (pyrolysis furnace) to produce transition metal fine particles and produce them as seeds (Japanese Patent Laid-Open 60-54998).
{Circle around (2)} A method in which a transition metal such as iron is directly vaporized in a pyrolysis furnace to produce a seed and produce it (JP-A-61-291497).
{Circle around (3)} A method in which a transition metal compound such as ferrocene is dispersed or dissolved in a liquid organic compound and sprayed into a reaction furnace to produce a seed as a seed (JP-A-58-180615).
And so on.
The vapor grown carbon fiber obtained by these methods forms a fiber having a fiber diameter of about 0.01 μm to 5 μm and a length of about 1 μm to 1000 μm, and the graphite surface of the graphite structure develops along the fiber axis and is internally formed. The feature is that there is a hollow hole.
[0005]
This carbon fiber usually contains tar content, granular carbon, a catalyst metal or a compound thereof, and the carbon, metal, and the like are attached to the fiber by tar. It is necessary to remove these as a product. Usually, the tar content is carbonized by heating (heat treatment), and the obtained fiber aggregates are crushed or pulverized to remove particulate carbon and metal by airflow classification or the like.
Further, the vapor grown carbon fiber is manufactured at a temperature of at most about 1300 ° C., and the crystal structure of graphite is not sufficiently developed. For this purpose, a method of improving the conductivity of electricity or heat by applying a heat treatment at a higher temperature to develop the crystal structure of graphite depending on the application is adopted.
The conventional heat treatment method employs an external heat method such as passing an aggregate of fibers through a heated tube or placing the aggregate in a container and heating in an electric furnace or the like.
[0006]
[Problems to be solved by the invention]
The vapor grown carbon fiber in the present invention is fine as described above, and forms an aggregate, which is in the form of powder (hereinafter, this aggregate is referred to as fibrous powder).
1) Since the bulk density of the fibrous powder is as small as 0.005 g / cm ³ or less, if the fibrous powder is to be heat-treated, the capacity of the processing equipment including the heating furnace required for the processing becomes large and the processing cost becomes high. .
2) Heat treatment of the fibrous powder usually uses an external heating furnace. However, since the fibrous powder has a low bulk density, it has a large equipment capacity and a small filling rate. Therefore, the thermal conductivity is low and the thermal efficiency is poor, resulting in a high cost.
3) A heating mechanism for the fibrous powder requires a mechanism for transferring the fibrous powder, and a high temperature range of 1500 ° C. or more requires complicated facilities and materials, and is difficult to transfer. And it is difficult to put into practical use due to the problem of adhesion and clogging of the fibrous powder. Therefore, a method is generally used in which a fibrous powder is filled in a container and heat treatment is performed in almost all of the container. However, in this method, in a powder having a low bulk density such as a fibrous powder, the amount of fibers to be filled in the container is extremely small, and heat is mostly spent on heating the container, and as a result, the heat treatment cost increases. .
4) The material of the container is 1400 ° C. or higher, especially at 2000 ° C. or higher. In case of high temperature treatment, there are few materials having this temperature. In consideration of the contamination of the fibrous powder by different elements, a carbon material such as graphite crucible is used. desirable. However, even at high temperatures, graphite is highly corroded by oxygen and nitrogen that are slightly mixed therein, and cannot be used for a long period of time, resulting in consumables. SUMMARY OF THE INVENTION An object of the present invention is to provide a heat treatment method which does not have the above-mentioned problems in the apparatus when heat treating fibrous powder and has good thermal efficiency.
[0007]
[Means for Solving the Problems]
The inventor of the present invention has reported that the fine vapor-grown carbon fiber is a single fiber itself and has an electrical resistivity of 0.0001 to 0.001 Ωcm, and has good conductivity like a normal carbon material. Since the powder has a very low bulk density but contains a tar component and the like, the present invention has been achieved by noting that a compact having a high bulk density can be obtained by compression molding.
That is, in the present invention, fine vapor-grown carbon fibers are compression-molded so that the bulk density of the molded body is 0.03 g / cm ³ or more, electrode terminals are brought into contact with both sides of the molded body, and the molded body is electrically heated. A heat treatment method for vapor grown carbon fiber, characterized in that:
[0008]
The fine vapor-grown carbon fiber used in the present invention is a fibrous powder in which fibers having a diameter of about 0.01 μm to 5 μm and a length of about 1 μm to 1000 μm as described above are collected. Since the as-produced fibrous powder has a very low bulk density of 0.005 g / cm ³ or less, it is first compression-molded. The fibrous powder can be formed into any shape by selecting a mold and a compression method. For example, it has a columnar shape, a cubic shape, a rectangular parallelepiped shape, a prism, a flat shape, and the like, and can be formed into other complicated shapes. However, industrially, it is desirable to use a column or a prism that has a shape as simple as possible and that is easy to conduct electricity from both sides (both ends).
[0009]
As the molding method, a press molding method or an extrusion molding method is easiest. Since the fibrous powder has good entanglement of each fiber and contains a small amount of tar, the shape is maintained to the extent that it does not disturb the electric heating even if compression molding is performed as it is, but if higher strength is desired, the fiber What is necessary is just to add a small amount of a primary binder such as starch, CMC, tar, naphthalene, anthracene to the powdery powder and form the mixture.
[0010]
Since the strength of the compact can be adjusted by the pressurizing pressure during molding and the bulk density of the fibrous powder, optimal pressure conditions are selected according to the strength of the target compact and the target bulk density. Specifically, it may be 0.1 kg / cm ² or more. The higher the pressure, the more stable the molded article can be obtained. However, if the pressure is too high, the fiber collapses and the fiber properties deteriorate.
That is, if the pressure is less than 0.1 kg / cm ² , the strength of the molded product is insufficient, and the probability of collapse and powdering during handling increases. On the other hand, when the pressure is 100 kg / cm ² or more, the fiber is frequently cut or collapsed, and the fiber properties are deteriorated.
In addition, if the pressurizing pressure increases, the equipment cost of the pressurizing system itself including the die increases, and therefore, it is preferable that the pressure is low from the viewpoint of the facility cost. Therefore, the molding pressure is about 0.1 to 100 kg / cm ² , preferably about 1 to 10 kg / cm ² .
[0011]
If the molding pressure is 0.1 kg / cm ² or more, the molded body usually has a bulk density of 0.03 g / cm ³ or more. However, since the fibrous powder compact has the property of being slightly restored when the pressure is released, it is necessary to take this into account when conducting heating. In the present invention, the bulk density of the molded body is set to 0.03 g / cm ³ or more during electric heating. At this density, the electrical resistivity is 100 Ωcm or less.
Electric heating of the molded body is performed, for example, by a method as shown in FIG. In the figure, reference numeral 1 denotes a fibrous powder compact having a bulk density of 0.03 g / cm ³ or more. Reference numeral 2 denotes upper and lower electrodes of the molded body, which are connected to a power supply 4 by a conductor 4 '. As the electrode, a carbon electrode having good conductivity and enduring high temperature is most suitable. Reference numeral 3 denotes an insulator such as ceramics. The energization heating must be performed in a non-oxidizing atmosphere, and since a gas is generated from the molded body, the energization section is surrounded by a housing 5, and the housing 5 has an inlet 6 for nitrogen gas, argon gas, etc. and a gas outlet. 7 is provided.
[0012]
The pressure applied to the compact can be adjusted by a holding device (not shown) attached to the upper electrode. If the contact between the electrode and the molded body is improved and the bulk density of the molded body is 0.03 g / cm ³ , it is possible to conduct electricity even with little pressure, but preferably to 100 g / cm ² or more. Apply electricity by applying pressure. The upper limit is usually about 100 kg / cm ² because the pressure of the molded product collapses when it is too high. The electric heating may be performed while the fibrous powder is formed as it is, or the formed body may be heated once by an external heat method at a possible temperature. It is preferable to heat the as-molded product slowly because rapid generation of gas causes severe gas generation. Electric heating is particularly effective for high-temperature treatment such as graphitization. In the case of a high-temperature treatment at 2500 ° C. or higher, if the voltage between the electrodes is high, there is a risk of thermal discharge. Therefore, it is preferable that the voltage is not too high, for example, about 20 V or less. The cross-sectional area, height, and the like of the molded body are adjusted so that necessary heat is obtained even with this voltage. The heating temperature for the heat treatment is usually selected from the range of about 1300 ° C to about 3000 ° C.
[0013]
FIG. 2 shows a method of electrically heating the molded body in an electrically insulating mold 8. The molded body 1 may be separately molded and placed in the mold 8, or molded with this mold and subsequently energized. In the method shown in FIG. 2, since the molded body does not collapse, it is possible to increase the pressure within the range of the pressure resistance of the mold at the target temperature. However, ceramics such as alumina are used as the insulating mold, but the heating temperature is limited to about 1800 ° C.
[0014]
FIG. 3 shows a configuration in which the molding device and the electric heating device are directly connected so that molding and electric heating can be performed continuously.
The generated fibrous powder is temporarily stored in the hopper 9. A molding device including a cylinder 10 and a plunger 11 is installed below the hopper. 11 'is a driving device for the plunger. A predetermined amount of fibrous powder is dropped from a hopper into a cylinder and compression molded with a plunger. It is preferable that the amount of powder falling is quantitatively adjusted by the feeder 9 'at the bottom of the hopper. Since the volume of the fibrous powder becomes extremely small when compressed, the volume of the part receiving the dropped powder should be sufficiently large to form a predetermined molded body, or the powder should be dropped and pre-compressed repeatedly, and finally the predetermined molded body should be formed. Adopt a compression method. In the former case, a space is provided in the cylinder at right angles (perpendicular to the plane of the paper), and the powder that has fallen there is compressed by a plunger (not shown) operating at right angles to the cylinder to the same position as the cylinder. A method such as compression with the illustrated plunger 11 is possible. The illustrated cylinder has a square cross section, but if the cylinder is a cylinder, the tip of the plunger in the right angle direction is a semicircle with the cylinder fitted.
[0015]
At the bottom of the cylinder, the opening / closing damper 12 slides in the holding device 13 by the driving device 12 '. The holding device has the same hole in the central part as the cylinder. At the time of molding, the damper is closed, and after the molding is completed, the damper is opened, and the molded body is extruded with a plunger.
The molded body is then heated by energization in the same manner as in the case of FIG. The molded body after the heating is accommodated in the receiver 14.
[0016]
FIG. 4 shows a method in which a heating (firing) area is provided by an external heating method before the molded body is heated by electric current. Others are the same as FIG. The molded body coming out of the damper enters a furnace core tube 15 provided with a heating device 16, where it is fired. The furnace core tube is made thicker than the compact so that gas can pass even if the compact is filled.
Gases are generated during the baking by the furnace tube and the electric heating, and these gases pass through the furnace core tube from the electric heating chamber by the introduced nitrogen gas 6 and the like and exit from the outlet 7. It is also possible to recycle it after removing it by methods such as condensation, adsorption and absorption. In this apparatus, the sintering with the furnace core tube is performed at about 1800 ° C. or less, and thereafter, it can be heated to about 3000 ° C. by energizing.
In the above method shown in the drawings, the molded body is vertically energized by sandwiching it between the electrodes from above and below, but it is of course possible to energize horizontally from left and right.
[0017]
【Example】
[Example 1]
As a fibrous powder, most of each fiber had a diameter of 0.05 to 3 μm and a length of 2 to 100 μm, and the molded body was electrically heated by the apparatus shown in FIG. The cylinder is a rectangular cylinder having a cross section of 30 mm × 30 mm. The fibrous powder was supplied from a hopper to a cylinder, and was molded at a pressure of 5 kg / cm ² .
After the molding, the damper 12 was opened, and the molded body was extruded. The molded body had a length of 50 mm, a bulk density of 0.08 g / cm ³ , and an electrical resistivity of 0.4 Ωcm. Both sides of this molded body having a size of 30 × 50 mm were sandwiched between upper and lower graphite electrodes, and electricity was supplied at a pressure of 5 g / cm ² . The initial voltage was 2.5 V and the current was 1 A. The voltage was gradually increased, and finally, the voltage was 15.2 V and the current was 18.5 A. The time from the start was 5 minutes.
The final voltage and current conditions were maintained for 10 minutes. The molded body reached about 1400 ° C. During energization, nitrogen gas was flowed at 0.5 liter / minute. After cooling, the molded body was removed and pulverized to remove a granular carbon or iron compound by an airflow classifier to obtain a product.
The Co of the product was 6.928 ° and the Lc was 50 °.
[0018]
[Example 2]
Heat treatment was performed using the apparatus shown in FIG. The steps up to the molding are the same as in the first embodiment. The molded body was first sent into a furnace core tube and heat-treated. The maximum temperature range of the furnace core tube is 1400 ° C. The residence time in this area is 10 minutes. Next, the molded body was transferred to an electric heating device while being hot, and was sandwiched between graphite electrodes and energized under a pressure of 5 kg / cm ² as in Example 1. The voltage was gradually increased at an initial voltage of 2.5 V and a current of 1 A, and finally was 20.3 V and a current of 61 A, and the time from the start was 10 minutes. The final voltage and current were maintained for 10 minutes. The temperature of the molded body was estimated to be 3014 ° C. by a radiation thermometer.
Pulverized and classified in the same manner as in Example 1 to obtain a product.
The Co of the product was 6.765% and the Lc was 280 ° and was well graphitized.
[0019]
【The invention's effect】
Although the heat treatment of the vapor-grown carbon fiber having a low bulk density was inefficient, the bulk density of the molded product obtained by applying pressure as in the present invention became 0.03 g / cm ³ or more, and Since the electrical resistivity of the body was reduced to 100 Ωcm or less, the vapor-grown carbon fiber could be heated by electricity. As a result, in the present invention, heat treatment can be performed continuously and efficiently in a short time without putting the vapor grown carbon fiber in a container.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing electric heating of a molded article of fine vapor-grown carbon fiber.
FIG. 2 is a cross-sectional view showing a case where the molded body in FIG.
FIG. 3 is a cross-sectional view showing the formation of fine vapor-grown carbon fibers, and then heating by energization.
FIG. 4 is a cross-sectional view showing a case where an external heating type heating device is provided between molding and electric heating in FIG. 3;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Fine vapor-grown carbon fiber compact 2 Electrode 4 Power supply 6 Inert gas inlet 7 Gas outlet 8 Ceramic insulator 9 Hopper 9 'Feeder 10 Cylinder 11 Plunger 12 Damper 14 Receiver 15 Furnace tube 16 Heating apparatus

Claims (7)

The fine vapor grown carbon fiber is compression molded to make the bulk density of the molded body 0.03 g / cm ³ or more, the electric resistivity of the molded body to 100 Ωcm or less, and contact the electrode terminals on both sides of the molded body. And a method for heat-treating a vapor-grown carbon fiber, wherein the molded body is electrically heated. 2. The heat treatment method for vapor-grown carbon fiber according to claim 1, wherein the molded body is electrically heated in the mold. Fine vapor-grown carbon fiber is molded with a cylinder-type molding device equipped with an opening / closing damper at the bottom. A heat treatment method for vapor-grown carbon fiber, comprising contacting and electrically heating a molded body. Fine vapor-grown carbon fiber is molded with a cylinder-type molding device equipped with an opening / closing damper at the bottom. And firing the molded body at a temperature of 1800 ° C. or less, then contacting electrode terminals on both sides of the molded body extruded from the furnace core tube, and electrically heating the fired molded body. Heat treatment method. The heat treatment method for vapor-grown carbon fiber according to claim 3 or 4, wherein the carbon fiber is molded by directly connecting a cylinder-type molding device to a lower portion of a hopper storing fine vapor-grown carbon fiber. The heat treatment method for vapor-grown carbon fiber according to any one of claims 1 to 5, wherein the heating of the formed body is performed under a pressure of 100 g / cm ² or more. Vapor-grown carbon fiber obtained by compression-molding fine vapor-grown carbon fiber to increase the bulk density of the molded body to 0.03 g / cm ³ or more, contacting electrode terminals on both sides of the molded body, and applying electric heating to the molded body A method for producing vapor-grown carbon fiber, wherein the heat treatment is performed by the heat treatment method according to any one of claims 1 to 6.

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