KR20100046445A - Method for purificating carbon nanotube and electromagnetic wave absorption material to include carbon nanotube that fabricated using the same - Google Patents

Abstract

PURPOSE: A purification method of a carbon nano tube and the electromagnetic wave absorber including the carbon nano tube are provided to obtain the carbon nano tube with high purity by removing a metallic catalyst or amorphous carbon. CONSTITUTION: A purification method of a carbon nano tube comprises the following steps: preparing the carbon nano tube in a pristine state; processing a dry purification by blowing oxidative gas at 300~550 deg C to the carbon nano tube for 1~2 hours; processing a wet purification by dipping the carbon nano tube to a purification solution at 60~90 deg C, and stirring for 1~2 hours for removing the solution. The oxidative gas is selected from air, oxygen and carbon dioxide. The purification solution is a mixture of more than substance selected from nitric acid, sulfuric acid, hydrochloric acid, and potassium permanganate.

Description

Carbon nanotube purification method and an electromagnetic wave absorber including carbon nanotubes manufactured using the same.

The present invention relates to a method for purifying carbon nanotubes and an electromagnetic wave absorber including carbon nanotubes prepared using the same, by preparing high purity carbon nanotubes and forming the electromagnetic wave absorbers using the carbon nanotubes prepared as described above. The present invention relates to a technology capable of having excellent electromagnetic wave absorber properties.

Since 1985 the first discovery of Fullerene (a collection of 60 carbon atoms: C60), one of the allotrope of carbon by Croto and Smolley, research on this material has continued.

In 1991, Dr. Ijima (NEC Research Institute) analyzed the mass of carbon formed on the graphite cathode by using the electric discharge method with a transmission electron microscope (TEM). nm and an outer diameter of 2.5 to 30 nm, carbon atoms forming sp2 bonds with three other carbon atoms, and exhibiting hexagonal honeycomb structure, carbon nanotubes (hereinafter referred to as 'carbon nanotubes') were found.

Such carbon nanotubes have excellent mechanical properties, electrical selectivity, excellent field emission characteristics, high efficiency hydrogen storage medium characteristics, and are known as perfect new materials with few defects among existing materials.

In addition, carbon nanotubes, which are attracting attention as next-generation materials, have very small diameters ranging from several nanometers to several tens of nanometers. Nevertheless, it is easy to operate as a conductor or a semiconductor, has a strength about 10 times that of steel, and has a high thermal conductivity.

Such carbon nanotubes are hollow and have a thin fibrous structural characteristic, which makes them infinitely applicable.

Synthesis of the carbon nanotubes as described above may include an electric discharge method, a laser deposition method, a thermal decomposition deposition method, a thermochemical vapor deposition method, a plasma chemical vapor deposition method, and the like.

In addition, the structure of the carbon nanotubes may vary depending on the presence and type of transition metal used as a catalyst.

Therefore, at present, a catalytic chemical vapor deposition method using a metal or metal oxide as a synthesis catalyst and producing carbon nanotubes by thermally decomposing various gases including carbon at high temperature is common.

However, the carbon nanotubes synthesized by the catalytic chemical vapor deposition method include carbon catalysts that are produced by the manufacturing method and carbon particles produced by thermal decomposition under incomplete conditions, that is, amorphous carbon.

These impurities are a factor in reducing the advantages of carbon nanotubes when judged from the aspect of application of carbon nanotubes, and thus, a purification process for removing these impurities is required to obtain high purity carbon nanotubes.

However, since the purification cost is higher than the synthesis cost and the acid is used to remove the metal, there are problems to be solved economically and environmentally.

On the other hand, in the modern society, due to the rapid development of electric and electronic products including computers, mobile phones, etc., miniaturization, thinness, and portability, electromagnetic interference due to radiation noise due to high integration of electronic components and high speed of signal processing speed (EMI, Electromagnetic) Interference (RFI) or Radio Frequency Interference (RFI) is recognized as an important factor that greatly determines the operation and reliability of products.

In addition, the frequency to be used for the next generation multimedia mobile communication is a frequency band of GHz band higher than the frequency band currently used, and when the frequency of the electromagnetic wave rises, it is easy to radiate with noise, Electromagnetic interference is a problem due to the deterioration of the noise environment inside the equipment.

The electromagnetic interference affects various automation equipments and automatic control devices, causing malfunctions, and when infiltrated into the human body, there are various problems such as raising the temperature of biological tissue cells by thermal action and weakening the immune function. For example, various types of electromagnetic compatibility (EMC) countermeasures such as disposing an absorber of electromagnetic waves inside an electronic device and products for the same have been developed.

 The electromagnetic wave absorber developed in the prior art is mainly composed of a composite sheet of an electrically insulating polymer matrix such as rubber or resin, a soft magnetic metal material having a spinel crystal structure as a magnetic loss material, and a carbon material as a conductive loss material. .

However, the specific permeability of the spinel-structured soft magnetic metal material is rapidly reduced in the GHz band according to the Snoke's Limit, and the limit frequency can be raised to GHz by plate-like processing, but the weight of the material There is a disadvantage in that a light electromagnetic wave absorber cannot be manufactured.

Therefore, a composite material in which carbon-based materials such as carbon black and carbon fibers, which are conductive loss materials, is mixed with an electrically insulating polymer matrix such as a polymer resin, has been developed as an electromagnetic wave absorber used in an extreme-high-frequency (EHF) region.

However, due to the impurity problem of the carbon nanotubes described above, the polymer matrix electromagnetic wave absorber including the carbon nanotubes generally exhibits low electromagnetic wave absorption characteristics and thus has poor utility. Therefore, there is a demand for development of an electromagnetic wave absorber that can be used in the EHF region, but there is no specific alternative yet.

The present invention performs a series of purification processes consisting of dry and wet purification to remove amorphous carbon or metal catalysts, and to open the caps of both ends of the carbon nanotubes to produce high purity carbon nanotubes. By adding high-purity carbon nanotubes to the polymer matrix to make it an electromagnetic wave absorber, even if a small amount is added, it does not impair the characteristics of the matrix, has excellent electromagnetic wave absorption characteristics in the GHz band, and absorbs electromagnetic waves of electronic devices The present invention provides a method for refining carbon nanotubes that can dissipate heat generated inside an electronic device to the outside to reduce malfunctions and adverse effects on the human body, and to provide an electromagnetic wave absorber including carbon nanotubes manufactured using the same. The purpose.

Carbon nanotube purification method according to the present invention comprises the steps of performing a dry purification process for blowing the oxidizing gas at a temperature of 300 ~ 550 ℃ for 1 to 2 hours to the carbon nanotubes of the pristine (pristine) and the dry purification process Preparing carbon nanotubes of high purity by performing a wet purification process of removing the purified solution after stirring for 1 to 2 hours while impregnating the performed carbon nanotubes in a purified solution at a temperature of 60 to 90 ° C. do.

The carbon nanotubes may be manufactured by an electric discharge method, a laser deposition method, a gas phase synthesis method, a plasma chemical vapor deposition method, or a thermal chemical vapor deposition method. The oxidizing gas may be selected from air, oxygen, and carbon dioxide. Or a gas in which two or more kinds are mixed, and the purification solution is characterized in that one or two or more kinds of solutions selected from nitric acid, sulfuric acid, hydrochloric acid, and potassium permanganate are mixed. It is characterized in that it further comprises the step of modifying the surface by imparting a functional group mixed with one or two or more selected from carboxyl groups, hydroxyl groups and amine groups on the surface of the carbon nanotubes.

In addition, the electromagnetic wave absorber according to the present invention is characterized in that it is produced in the form of a sheet or film using the composite of the high purity carbon nanotubes and the polymer matrix prepared by the above-described purification method.

Here, the high purity carbon nanotubes are mixed at a ratio of 0.5 to 3.0 phr (per hundred resin) of the polymer matrix, wherein the polymer matrix is a silicone rubber compound, a one-component or two-component thermosetting silicone binder, Characterized in that any one or more selected from the group consisting of acrylic resins, epoxy resins and urethane resins, wherein the electromagnetic wave absorber further comprises a soft magnetic metal powder of 75 to 85% by weight relative to the total weight of the composite Characterized in that, the soft magnetic metal powder is Fe-Si-Al, Sendust, Permalloy (Fe-Ni, Permalloy), Carbonyl Iron, Ni-Zn-based ferrite (Ferrite), Fe- One or two selected from Si and Fe-Si-Cr are mixed, and the electromagnetic wave absorber may be roll-pressed, casted or pressed. The method is characterized in that is made of a sheet or film form.

The present invention provides an effect of obtaining carbon nanotubes of higher purity by removing impurities and metal catalysts such as amorphous carbon contained in carbon nanotubes through a series of processes of dry and wet purification of carbon nanotubes.

Next, the present invention by mixing the high-purity carbon nanotubes prepared as described above with the polymer matrix to produce an electromagnetic wave absorber, by using a high complex dielectric constant, which is a characteristic of high-purity carbon nanotubes, a high electromagnetic wave absorber in the GHz frequency band to be used in the future It provides an effect of obtaining an electromagnetic wave absorber having characteristics.

In addition, the present invention by using the thermal conductivity inherent in the carbon nanotubes to transfer the heat released from the electronic device to the outside more efficiently, it provides an effect that can reduce the malfunction of the machine and adverse effects on the human body .

In the present invention, a carbon nanotube purification method is performed to remove impurities such as amorphous carbon through a dry purification process, and then a metal catalyst is removed through a wet purification process to produce high purity carbon nanotubes.

Next, a composite of carbon nanotubes and a matrix formed by adding high purity carbon nanotubes to a polymer matrix is used as an electromagnetic wave absorber.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, only the present embodiments to make the disclosure of the present invention complete, and common knowledge in the art to which the present invention pertains. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims.

In order to carry out the present invention, first, a carbon nanotube in a pristine state is prepared.

Next, dry and wet purification processes are sequentially performed on the carbon nanotubes. At this time, the first dry purification process is carried out using air, oxygen, carbon dioxide, etc. as the oxidative dry gas, it is performed for 1 to 2 hours in the temperature range of 300 ~ 550 ℃.

In the case of dry refining at less than 300 ℃ temperature of amorphous carbon does not react to the meaning of refining, and if carried out at a temperature above 550 ℃ not only amorphous carbon but also carbon nanotubes are oxidized to high purity carbon nano Problems may arise that the tube cannot be manufactured. In addition, the dry purification execution time may vary depending on the temperature conditions, the type of the reaction gas and the flow rate of the gas. The method used in this process may produce high-purity carbon nanotubes when performed for 1 to 2 hours. Therefore, the scope of protection of the present invention is not limited by the execution time.

In addition, the furnace of the dry tablet used in the present process uses a device that can maintain a constant high temperature and supply the desired gas at a constant flow rate. In general, an ultrasonic vibration device, a microwave, or the like may be used, but the present invention is characterized by using only a heating furnace.

Through the dry purification process described above, impurities such as amorphous carbon contained in the carbon nanotubes are removed. However, since the impurities are not completely removed, and the metal catalyst contained in the carbon nanotubes is not removed through the dry purification process, the following wet purification process is performed.

In the wet purification process according to the present invention, carbon nanotubes are purified by appropriately mixing one or two or more purification solutions selected from nitric acid, sulfuric acid, hydrochloric acid and potassium permanganate. It is characterized by using each purification solution in parallel, it is preferable to perform for 1 to 2 hours at 60 ~ 90 ℃ temperature range when stirring with carbon nanotubes.

When the wet refining is performed at a temperature below 60 ° C., the solubility of the metal catalyst and the excess impurities is lowered, and when the wet treatment is performed at a temperature exceeding 90 ° C., water is vaporized to increase the concentration of the refining solution to produce high purity carbon nanotubes. You can't. In addition, in the case of the purification solution used in this experiment, the most preferable result can be obtained when the processing time is 1 to 2 hours. Since the time for performing the wet purification may vary depending on the temperature conditions, the type of the purification solution, and the concentration of the solution, the scope of protection of the present invention is not limited by the time for the execution.

In addition, in the wet purification process according to the present invention, since both ends of the carbon nanotubes are opened, the purification process can be easily performed.

Through such a series of dry and wet purification processes to produce high-purity carbon nanotubes.

Next, the high purity carbon nanotubes purified through the wet purification process are provided with functional groups (carboxyl groups, hydroxyl groups and amine groups) on the surface of the carbon nanotubes to modify the surface. The surface-modified high-purity carbon nanotubes have improved dispersion in the polymer composite.

In addition, high-purity carbon nanotubes have high conductivity compared to low specific gravity of the electromagnetic loss material, thereby increasing the complex dielectric constant of the composite.

Here, the electrical characteristics of the composite material in which carbon nanomaterials are randomly dispersed in the electrically insulating polymer matrix are explained by the percolation theory.

Carbon nanomaterials three-dimensionally dispersed in the polymer is a part of which is electrically connected to each other to form a network through which current flows, and another part forms an isolated part of the charge. The network inside the complex is involved in the imaginary part of the complex dielectric constant related to the electrical conductivity, and the isolated part of the complex is involved in the complex dielectric constant part by acting as the electric dipole in the electric charge according to the externally applied electric field.

Thus, the high purity carbon nanotubes purified by the present invention have a significantly higher aspect ratio of the fiber form or the curvature of the fiber itself than the carbon black and carbon nanofibers used in the related art.

Therefore, the fibers are tangled with each other very complicated, and this feature can increase the complex dielectric constant in the composite, which can be used to form an electromagnetic wave absorber having high absorption performance in the GHz frequency band.

In addition, the high thermal conductivity, which is inherent to carbon nanotubes, is made of an electromagnetic wave absorber and applied to the inside of an electronic device, so that the heat emitted from the inside of the device can be more efficiently transferred to the outside, and its thermal conductivity is higher than 1.0 W / mK. Becomes

Table 1 is a result of comparing the characteristics of the carbon nanotubes according to the purification method according to the present invention and the purification method according to the prior art described above.

<Example 1>

The carbon nanotubes prepared by plasma chemical vapor deposition were blown with carbon dioxide having a temperature of 300 ° C. for 2 hours to carry out a first dry purification process, and then immediately placed in a sulfuric acid solution at 90 ° C. and stirred for 2 hours. A secondary wet purification process was performed. Next, after removing the sulfuric acid solution was measured the electromagnetic wave absorption rate and thermal conductivity of the purified carbon nanotubes.

Electromagnetic absorptivity was measured according to the standard method proposed by the Ministry of Information and Communication No. 2000-93, and the thermal conductivity was measured using QUICKLINE ^TM -50 equipment of Yeonjin Corporation.

<Example 2>

All the processes of Example 1 were performed in the same manner, but the first dry purification process was performed using carbon dioxide at 550 ° C., and the second wet purification process was performed using a sulfuric acid solution at 90 ° C.

Comparative Example 1

In Example 1, the first dry purification process was omitted, and all remaining processes were performed in the same manner.

Comparative Example 2

Except for the second wet purification process in Example 1, all the other processes were carried out in the same manner.

Comparative Example 3

All the processes of Example 1 were performed in the same manner, but the first dry purification process was performed using carbon dioxide at 280 ° C., and the second wet purification process was performed using a sulfuric acid solution at 55 ° C.

<Comparative Example 4>

All the processes of Example 1 were performed in the same manner, but the first dry purification process was performed using carbon dioxide at 600 ° C., and the second wet purification process was performed using a sulfuric acid solution at 95 ° C.

[Table 1] Comparison of Characteristics of Carbon Nanotubes by Purification Methods

Electromagnetic Wave Absorbance (dB) Thermal Conductivity (W / mK) Example 1 -10 1.0 Example 2 -15 1.4 Comparative Example 1 -5 0.9 Comparative Example 2 -2 0.7 Comparative Example 3 -3 0.5 Comparative Example 4 -One 0.2

Next, in the present invention, the above-described high purity carbon nanotubes are mixed with the polymer matrix to form a composite. High-purity carbon nanotubes form a network within the polymer matrix, resulting in an excellent electromagnetic wave absorber. In this case, the polymer matrix to be used is not particularly limited, and at least one selected from the group consisting of a thermosetting silicone rubber compound, a one-component or two-component thermosetting silicone binder, an acrylic resin, an epoxy resin, and a urethane resin is preferably used. Do.

In addition, in manufacturing the electromagnetic wave absorber according to the present invention, the ratio of the high purity carbon nanotubes mixed in the polymer matrix depends on the kind of the polymer matrix used or the application of the electromagnetic wave absorber, but 0.5 to 3.0 of the polymer matrix to be applied. It is preferable to use the ratio of phr (per hundred resin).

The electromagnetic wave absorber according to the present invention is easy to form a network in the matrix even with a low content of carbon nanotubes as a loss material. In addition, since it has good dispersibility, a high complex dielectric constant value can be obtained. Therefore, the electromagnetic wave absorber according to the present invention can exhibit sufficient electromagnetic wave absorption performance of -10 dB or less in the GHz frequency band.

1 is a graph showing the electromagnetic wave absorption performance in the EHF frequency band when using the carbon nanotubes to prepare an electromagnetic wave absorber having a thickness of 2.0mm.

FIG. 1 is a measurement of reflection loss (S11) at each frequency band by appropriately changing the high purity carbon nanotube content and the soft magnetic metal powder content purified according to the present invention. In this case, the return loss value is measured using a vector network analyzer (VNA).

Here, S11 represents the ratio of the reflected voltage to the input voltage among the S-parameters, which is a measure of the absorbing performance of the absorber. When the high purity carbon nanotubes according to the present invention are added, all of the excellent half-acids are shown. It can be seen that it has a loss value.

The data of -10dB or less at the frequency means absorbing 90% or more of electromagnetic waves, and the data of -20dB or less means absorbing 99% or more of electromagnetic waves. The electromagnetic wave absorbers all have a value of -10 dB or less.

As described above, using the high purity carbon nanotube according to the present invention can produce an electromagnetic wave absorber having a characteristic of -10 dB or less in the target frequency band.

Here, in order to improve the electromagnetic wave absorption rate in the lower frequency band, the soft magnetic metal powder, which is a magnetic loss material, may be plated, mixed with a polymer matrix, and the sheets prepared may be laminated and used in a two-layered structure. At this time, the composition changes depending on the frequency and absorption characteristics to be applied, but the filling rate of the soft magnetic metal powder is preferably filled to 75 to 85% by weight relative to the total weight of the composite.

Soft magnetic metal powder to be applied to the absorber according to the present invention is a sender (Fe-Si-Al, Sendust), permalloy (Fe-Ni, Permalloy), carbonyl iron, Ni-Zn-based ferrite (Ferrite), It is preferable to use one or two selected from Fe-Si and Fe-Si-Cr.

Here, the mixing process for producing the composite is used in a suitable method depending on the polymer matrix used. In addition, the production of sheets and films may be selected depending on the matrix used and the thickness of the electromagnetic wave absorber to be applied, but may be a roll-press, casting or extrusion method.

In either case, high-purity carbon nanotubes are well dispersed in the polymer matrix, and a network can exhibit high electromagnetic wave absorption performance.

Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above embodiments and can be modified in various forms, and having ordinary skill in the art to which the present invention pertains. It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without changing the technical spirit or essential features of the present invention. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

1 is a graph showing the electromagnetic wave absorption performance in the EHF frequency band when the electromagnetic wave absorber having a thickness of 2.0mm using a carbon nanotube.

Claims (10)

Preparing carbon nanotubes in a pristine state; Performing a dry purification process of blowing the oxidizing gas at a temperature of 300 to 550 ° C. for 1 to 2 hours to the carbon nanotubes; And The carbon nanotubes subjected to the dry purification process were stirred for 1 to 2 hours while impregnated in a purification solution at a temperature of 60 to 90 ° C., followed by a wet purification process to remove the acid solution to obtain carbon nanotubes of high purity. Carbon nanotube purification method comprising the step of preparing. The method of claim 1, The oxidizing gas is a carbon nanotube refining method, characterized in that using a mixture of one or two or more selected from air, oxygen and carbon dioxide. The method of claim 1, The purification solution is a carbon nanotube purification method using a mixture of one or two or more selected from nitric acid, sulfuric acid, hydrochloric acid and potassium permanganate. The method of claim 1, The carbon nanotube purification method of claim 1, further comprising the step of modifying the surface by giving a functional group mixed with one or two or more selected from a carboxyl group, a hydroxyl group and an amine group on the surface of the carbon nanotube of high purity. An electromagnetic wave absorber, which is manufactured in the form of a sheet or a film by using the composite of the high purity carbon nanotube and the polymer matrix prepared by the purification method of claim 1. The method of claim 5, The polymer matrix is an electromagnetic wave absorber, characterized in that any one or more selected from the group consisting of a silicone rubber compound, a one-component or two-component thermosetting silicone binder, an acrylic resin, an epoxy resin, and a urethane resin. The method of claim 5, The high purity carbon nanotubes are mixed at a ratio of 0.5 to 3.0 phr (per hundred resin) of the polymer matrix. The method of claim 5, The electromagnetic wave absorber further comprises a 75 to 85% by weight soft magnetic metal powder relative to the total weight of the composite. The method of claim 8, The soft magnetic metal powder may include sendust (Fe-Si-Al, Sendust), permalloy (Fe-Ni, Permalloy), carbonyl iron, Ni-Zn-based ferrite, Fe-Si, and Fe-. Electromagnetic wave absorber characterized in that a mixture of one or two selected from Si-Cr. The method of claim 5, The electromagnetic wave absorber is manufactured in the form of the sheet or film by a roll-press, casting (casting) or extrusion method.

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