KR101758640B1 - Fabrication method of aligned carbon fiber arrays employing metal base - Google Patents

Abstract

The present invention relates to a method of producing carbon fibers oriented in a metal matrix comprising iron or stainless steel and a metal base thereof, and more particularly, to a method of manufacturing a carbon fiber which comprises the steps of: coating an alumina precursor on iron or stainless steel as a metal base; Annealing the coated iron or stainless steel to form an alumina layer from the alumina precursor; And growing a carbon fiber by adding a precursor containing carbon to the alumina layer. The present invention also provides a method of manufacturing a carbon fiber oriented metal matrix and a metal base.
As described above, according to the present invention, since a method of forming an alumina layer on iron or stainless steel is adopted, a functional effect of growing a carbon fiber by performing a simple coating process such as dip coating is expected.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for producing carbon fibers oriented on a metal matrix,

The present invention relates to a method for producing carbon fibers oriented in a metal matrix and a metal matrix thereof, and more particularly, to a method for producing carbon fiber by coating an alumina precursor on a metal matrix; Annealing the coated metal matrix to form an alumina layer from the alumina precursor; And growing a carbon fiber by adding a precursor containing carbon on the alumina layer. The present invention also provides a method of manufacturing carbon fiber oriented in a metal matrix and a metal base thereof.

Numerous studies have been carried out for the development of carbon nanotubes, and application areas utilizing such excellent mechanical, thermal and electrical properties of nanostructured filaments can be created. Although carbon nanotubes show such high application potential in laboratory units, there are many limitations in expanding the unit of production of useful form factors of carbon nanotubes into industrial application units, such as alignment of carbon nanotubes.

Although a number of researchers have focused their efforts on developing suitable methods for mass production of carbon nanotubes, no breakthrough has yet been made. An industrially effective process can be described by (1) a cost-effective carbon nanotube synthesis process, (2) low cost catalysts and substrate starting materials, (3) high yields, and (4) .

Many techniques have been developed for synthesizing carbon nanotubes such as electric arc discharge, laser ablation, pyrolysis and thermal chemical vapor deposition (CVD). Of these, hot CVD is the most promising mass production tool offering the potential for high purity, high yield per unit area, supply elasticity of the carbon source, relatively low process temperature, easy scalability, and inexpensive infrastructure requirements.

In thermal CVD, catalysts such as Fe or oxides such as zirconia, which are currently used, are highly needed to grow carbon nanotubes. As the above catalyst, a conventional catalyst / support placed on a silicon (Si) substrate is used, or a floating catalyst such as ferrocene is pyrolyzed and used. However, since both methods use expensive raw materials, there is a problem in that they are limited in cost-effective mass production. In addition, according to the use of the catalyst as described above, it is not suitable for producing carbon nanotube filaments.

When depositing a catalyst on a substrate prior to CVD, expensive processes operated under high vacuum such as e-beam evaporation are often required, so that a support such as a catalyst and alumina can be stably placed . On the other hand, such a process is required to grow a high yield, but the process cost increases accordingly. In addition, since an expensive Si wafer is mainly used as a substrate material, this also increases the process cost.

In addition, the electron beam apparatus used in the electron beam evaporation method is an expensive vacuum apparatus, and it takes a lot of cost to construct such an apparatus, and there is a problem that the burden is increased in terms of maintenance.

Further, in the case of using the electron beam evaporation method, since the carbon nanotubes can be grown only on the surface exposed by the electron beam, problems such as a decrease in the yield of the carbon nanotubes, an unnecessary repetition of the process, and an increase in the time required for the process occur.

On the other hand, the method of growing the carbon nanotubes after forming the catalyst on the substrate in this manner requires a problem of separately adding a catalyst, a complexity of the catalyst which is exhausted after the catalyst is used, .

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems, and it is an object of the present invention to provide an alumina layer on a metal matrix serving as a catalyst by a simple coating process such as dip coating and spin coating, So that the carbon fibers can be grown in large quantities.

In addition, the present invention forms an alumina layer on a catalytic metal matrix, and during the process of growing carbon fibers, a metal matrix is thermally diffused through the alumina layer to act as a catalyst, The other purpose is to realize a fair economy because there is no need.

In addition, the present invention enables sequential processing of the catalytic metal matrix, the alumina layer thereon, and the carbon fiber grown thereon, so that the base can be reused semi-permanently as long as the thickness of the metal matrix is sufficiently secured .

Another object of the present invention is to reduce the energy required for the process by partially omitting the heating process from the process of forming the alumina layer to the process of growing the carbon fiber.

Another object of the present invention is to maintain the orientation of the carbon fiber properly by controlling the surface roughness of the alumina layer during the formation of the alumina layer or during the growth of the carbon fiber by controlling the annealing or the heat treatment temperature.

In order to achieve the above object, the present invention provides a metal base, An alumina layer formed on the metal matrix; And carbon fibers grown on the alumina layer. ≪ Desc / Clms Page number 2 >

The alumina layer is preferably an amorphous alumina or crystalline alumina thin film or a thick film.

The surface of the alumina layer is preferably formed with a metal element originating from a metal matrix by a thermal diffusion process.

Preferably, the metal matrix is iron or stainless steel, and iron, chromium or vanadium components are formed on the surface of the alumina layer by a thermal diffusion process.

The present invention also relates to a method of manufacturing a metal substrate, comprising: coating an alumina precursor on a metal matrix; Annealing the coated metal matrix to form an alumina layer from the alumina precursor; Adding a precursor containing carbon to the alumina layer to form a metal matrix on the alumina layer by thermal diffusion to grow carbon fibers while acting as a catalyst. A method for producing a fiber is provided.

The step of coating the alumina precursor is preferably performed by a dip coating or a spin coating method.

In the step of forming the alumina layer, it is preferable that the alumina layer is formed by heating the coated alumina precursor to a temperature range of 600 to 800 ° C.

In the step of growing the carbon fiber, it is preferable that the carbon fiber is grown by a thermal CVD method by adding a precursor containing carbon, and then the precursor containing carbon is stopped and the reducing atmosphere is maintained.

The thermal CVD method is preferably performed at a temperature ranging from 450 to 800 ° C.

Preferably, the metal matrix is iron or stainless steel, and iron and vanadium components in stainless steel are formed on the surface of the alumina layer and thermal diffusion to act as a catalyst for the growth of carbon fibers.

According to the present invention, since a method of forming an alumina layer on a metal matrix is adopted, a functional effect of growing a carbon fiber by performing a simple coating process such as dip coating or spin coating is expected.

In addition, according to the present invention, an alumina layer is formed on a catalytic metal matrix, and a metal-based material is thermally diffused through the alumina layer during the process of growing carbon fibers, There is no need to form a film, and an operation effect that realizes a process economy can be expected.

Also, according to the present invention, as long as the thickness of the metal matrix is sufficiently secured through sequential processing of the order of the catalytic metal matrix, the alumina layer thereon, and the carbon fibers grown thereon, So that an action effect is realized.

Further, according to the present invention, since the surface roughness of the alumina layer is controlled, it is expected that the carbon fiber can maintain the orientation in the vertical direction when growing the carbon fiber.

FIG. 1 is an image of carbon nanotubes formed on an alumina layer using an SEM according to an annealing temperature according to an embodiment of the present invention. FIG. 1 (a) (d) about 900 ° C., (e) a photograph of the intermediate process in which the alumina layer is formed, and (f) photographs in which the alumina layer is formed.
FIG. 2 is an image obtained by growing a carbon nanotube array at a temperature of 780.degree. C. using a TEM according to an embodiment of the present invention. FIG.
(B) 700 ° C; (c) 800 ° C; (d) 900 ° C .; FIG. 3 is an image of an alumina layer formed according to an embodiment of the present invention, Lt; 0 > C.
4 is a graph showing XRD analysis of stainless steel coated with an alumina layer according to an embodiment of the present invention, according to an annealing temperature.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings and preferred embodiments.

The present invention relates to a method for orienting a carbon fiber on a metal matrix, and the carbon fiber comprises carbon nanotubes. In the present invention, carbon nanotubes will be described, but it is obvious that other kinds of carbon fibers can be grown by the same process.

According to an embodiment of the present invention, there is provided a method of orienting carbon nanotubes of a multilayer wall having a dense and well-aligned wall structure on a metal matrix, and it is necessary to carry out a process of separately forming a catalyst layer No, and therefore a very cost effective process.

Particularly, by arranging the metal matrix and the alumina layer in sequence, thermal diffusion of the metal matrix material through the alumina layer, and catalyst formation on the alumina layer, carbon nanotubes are formed on the catalyst matrix, It can be used as a base material for the growth of carbon nanotubes.

In the following examples, stainless steel is exemplified as a metal matrix material, and a production example of a carbon nanotube based on stainless steel will be described as follows.

However, the metal matrix can be any metal that acts as a catalyst to enable growth of carbon nanotubes. That is, iron, chromium, vanadium, and the like.

- Formation of alumina layer

First, an alumina layer was formed on the surface of a stainless steel substrate (type 304).

Here, the alumina layer plays two roles. One serves as a catalyst support and promotes the growth of carbon nanotubes during the growth of carbon nanotubes. Also, it serves as a medium to allow the oriented iron particles having suitable size and composition to be leached from stainless steel for orientation growth of carbon nanotubes. Therefore, as long as the alumina layer is formed, the step of separately forming the catalyst and the catalyst precursor on the substrate can be omitted, thereby achieving a process economy. During the annealing process for forming the alumina layer or the heat treatment process for producing the carbon nanotubes, iron atoms are diffused through the alumina layer from the stainless steel, and the catalyst layer is naturally formed on the surface of the alumina layer through the diffusion process.

In the present invention, the dip coating method and the sol-gel method were used for forming the alumina layer in one embodiment. A spin coating method may also be applied. Dip coating or spin coating is advantageous in that an alumina layer can be formed on all surfaces of the metal matrix and it is preferable to employ an aluminum layer because it is an economical process as compared with a CVD method which can be used for forming an alumina layer , But it does not exclude deposition processes such as CVD processes. It is technically possible enough.

To illustrate the more specific method for forming the alumina layer, first 2.5 vol% aluminum tri- sec- butoxide (Aldrich, 97%) was dissolved in 2-methoxyethanol, Aldrich, 99 %) Or ethanol (ethanol, Pharmco-Aaper, 200 proof) to prepare a sol. Subsequently, the sol is dip-coated with stainless steel. Here, as precursors and solvents for alumina, all possible materials other than the above materials can be used.

Then, the dip coated stainless steel base material is heated on a hot plate at 400 DEG C for 10 minutes, and the alumina layer is annealed at a temperature of 600 to 800 DEG C for 30 minutes to be densified.

Thereafter, the carbon nanotubes were charged into a quartz tube (22 mm ID, 25 mm OD, 76 cm length) mounted on an electric furnace (Lindberg / Blue M, 50 at mm diameter, 30 cm heated length) He gas is subjected to heat treatment at a temperature of 780 캜 and a time within 10 minutes at a flow rate of 200 sccm.

The temperature at the above annealing is related to the properties of alumina. Maintaining the above temperature during annealing is to prevent the excessive crystallization of alumina to cause grain growth, thereby increasing the surface roughness of the alumina layer.

Since the surface of the alumina layer should be as flat as possible, the surface roughness is preferably low. If the surface of the alumina layer has a high alumina roughness, the growth direction of the carbon nanotubes grows at random rather than the upward direction, because the upward directivity of the carbon nanotubes growing from the alumina surface is as high as possible. , It is difficult to handle carbon nanotubes because the carbon nanotubes are entangled between the grown carbon nanotubes, and if used properly, it is difficult to manifest proper physical properties by the carbon nanotubes. Accordingly, the above temperature range corresponds to a temperature range having a critical meaning necessary to keep the surface roughness of the alumina layer as smooth as possible.

- Carbon nanotube growing process

After that, C ₂ H ₄ gas was supplied at a flow rate of 100 sccm for 30 minutes, carbon nanotubes were grown on the alumina layer by thermal CVD method. Then, H ₂ gas was supplied at a flow rate of 500 sccm And maintained for 5 minutes. Here, it is a matter of course that other precursors other than the C ₂ H ₄ gas may be used as precursors of the carbon nanotubes.

The heat treatment temperature associated with carbon nanotube growth also remains within the annealing temperature range to prevent excessive crystal growth from growing on the surface of the alumina layer. In the present invention, the temperature range of 450 to 800 DEG C is preferably maintained.

- Evaluation of grown carbon nanotubes

≪ Evaluation of microstructure of carbon nanotube array >

Alumina-coated stainless steel was heat-treated at several temperatures in the temperature range of 450 to 800 ° C as described above to estimate the growth level of the carbon nanotubes.

As expected, carbon nanotubes were grown on both sides and corners of the stainless steel substrate, which can be confirmed as in FIG. 1e.

On the other hand, as can be seen from FIG. 1B, the carbon nanotubes grown on the stainless steel substrate after the heat treatment at a temperature of 700 ° C had the best orientation, were very long, over 200 μm, and had a very high density. Although not shown, it was experimentally possible to grow to a length of 1 mm.

The result of observing the carbon nanotubes grown at the heat treatment temperature of 700 ° C. using a transmission electron microscope (TEM) is shown in FIG.

As shown, the carbon nanotubes are multi-walled and have an outer diameter of about 10-20 nm and an inner diameter of 10 nm. This can be contrasted with carbon nanotubes grown on films formed by e-beam evaporation on Si wafers. Above 700 ° C, it was found that the higher the annealing temperature, the longer the length of carbon nanotubes, the lower the yield of carbon nanotubes (the ratio of carbon nanotubes over a certain length), and the worse the orientation of carbon nanotubes 1d). Therefore, the heat treatment temperature of 700 ° C is the most preferable temperature.

When the temperature is less than 450 ° C., thermal decomposition is insufficient and scattering of volatile components is insufficient. When the temperature exceeds 800 ° C., the surface of the alumina layer becomes rough due to excessive crystal growth. It is not preferable since the vertical orientation is deteriorated when the carbon nanotubes grow. Therefore, the heat treatment temperature has its critical significance in the above range.

≪ XRD analysis for alumina layer >

The surface microstructures required for carbon nanotube growth vary with temperature, particle size and composition. To understand these changes, various analyzes were performed on the alumina layer before carbon nanotube growth.

First, XRD analysis of the annealed substrate was performed based on the temperature parameter. The results are shown in FIG. 4, in which? -Alumina was detected, but the annealing temperature was limited to 800 ° C. or higher. From these results, it can be seen that the coated alumina layer remains amorphous at a temperature of 800 ° C or below, or is a crystalline granule. In other words, the alumina layer is preferably in the form of a thin film or a thick film in a state of being amorphous or having fine crystal grains.

On the other hand, on the X-ray graph, A pair of peaks present in the vicinity are not found in the non-annealed alumina / stainless steel substrate and are only observed when annealed at temperatures above 600 ° C. These peaks are Cr ₂ O ₃ Or Fe ₂ O ₃ . Also, the phase detected at a temperature of 800 ° C is considered to be some spinel structure candidates including Fe ₃ O ₄ , FeAl ₂ O ₄ , and FeCr ₂ O ₄ .

≪ XPS analysis on alumina layer >

In addition, to evaluate the potential presence of iron on the surface of the alumina layer, XPS was used to analyze annealed stainless steel substrates at various temperatures. As a result, small amounts of Mn, V as well as Al, Cr and Fe were detected on the surface of the substrate. Table 1 summarizes the concentrations of these components on substrates annealed at different temperatures before and after carbon nanotube growth.

Surface concentration (at%) Impurities leached from stainless steel Preliminary annealing temperature Al Cr Fe O Mn V Before 700 ° C CVD 12.93 3.77 13.40 66.90 2.88 0.12 After 700 ° C CVD 15.47 3.63 6.72 70.56 2.48 0.00 Before 900 ° C CVD 13.54 13.06 6.00 59.25 5.86 2.29 After 900 ° C CVD 10.38 8.69 2.38 71.52 5.26 0.88

As expected, Fe and Cr were the major components of stainless steel 304. Mn was a well-known component contained in stainless steel 304, but it was not a main component. V was not expected to exist in high concentration in stainless steel 304, but both Mn and V increased with increasing annealing temperature. For reference, V was also detected from stainless steel. Importantly, although Ni is a major component of stainless steel 304, it was not detected by XRD or XPS.

Both Fe and Cr were detected at the surface of alumina at 700 ℃. Two spin of the peak corresponding to the two chemical compositions of Fe-track-pair separation was detected, Fe 2p _3/2 binding energy was measured with each 710.0eV and 712.2eV. This means that Fe ^{2 +} and Fe ^{3 +} are present on the surface in oxide form, respectively.

In addition, four pairs of peaks were observed in the Cr 2p area, the three pairs of 575.3eV, 576.3eV, and is at 577.2eV Cr ^{+ 3} present in the oxide form having Cr2p _3/2 binding energy, the couple 578.8eV Is Cr ^{6 +} in the form of CrO ₃ with binding energy.

At the same time, the relative content of Fe on the surface of stainless steel decreased at 700-900 ° C, but the relative content of Cr increased.

Considering these observations together with the results of XRD, a significant amount of Fe diffused through the aluminum coating layer at annealing or heat treatment temperatures below 800 ° C, forming Fe oxide on the surface. Similarly, Cr also formed oxides on the surface via the alumina coating layer, but the amount of Cr was relatively small.

The two phases were detected in XRD because they had similar crystal structure and small grain size, but the peaks were overlapped in general, and therefore they were not clearly distinguished by XRD. The grain size was measured at 30 to 50 nm as predicted by the Scherrer equation applied to XRD.

Above 800 캜, alumina is crystallized (specified by shifting from Al 2p peak to higher binding energy), so that at the surface of the alumina layer, Cr replaces Fe, and thus contains Fe and / or Al and / or Cr A mixed oxide is formed. Such mixed oxides and spinel structures that can be produced are not clearly distinguished by XRD, and unfortunately, these structures are not clearly distinguished by XPS because the oxidation states of the metals are similar.

These results indicate that the oxidation behavior of stainless steel and the oxidation behavior of Al ₂ O ₃ -Fe ₂ O ₃ and Al ₂ O ₃ -Cr ₂ O ₃ As well as the relationship between the two. Nomuraand Ujihira ^N According to the results of the oxidation of stainless steel 304 at 700 ° C or less, Cr ₂ O ₃ It is reported that Fe ₂ O ₃ is formed on the barrier.

At higher temperatures, a Cr ₂ O ₃ layer was formed on the Fe ₂ O ₃ and mixed Ni / Fe / Cr oxide phase. In addition, according to a report by Karimi et al., In the oxidation mechanism of stainless steel, Cr oxide is first formed on the surface of stainless steel, thereby preventing Ni from diffusing upward from the substrate.

Similarly, when the alumina / stainless steel substrate was annealed, iron oxide containing a small amount of chromium oxide was observed as a main phase at a temperature of 700 ° C, and chromium oxide containing a small amount of iron oxide at a temperature of 900 ° C The oxide was observed as the dominant phase, and Ni was not significantly observed in the observed regions of XRD and XPS.

Since Cr ₂ O ₃ has a large miscibility gap and a high solubility for Al ₂ O ₃ , there may be many intermediate or more homogeneous intermediate phases, and well-known examples of such homogeneous or higher are spinel structures.

≪ Analysis of microstructure for alumina layer >

In FIG. 3, an SEM image of the surface microstructure of the Al ₂ O ₃ / stainless steel substrate is shown as a function of the annealing temperature. Particle formation on the alumina layer was observed. Particularly, as a result of XRD analysis, crystallization of the alumina layer was observed below the annealing temperature. As shown in FIG. 3A, similar granular particles were produced on the surface of the lower stainless steel exposed at the point where the alumina layer was peeled off, which is an oxide of iron or chromium generated from stainless steel during the firing and annealing process . As the annealing temperature is increased, the grain growth in the Al ₂ O ₃ layer is continued, which results in the growth of polyhedral grain clusters with a size in microns. As the grain growth continues and the surface roughness of the alumina layer increases, the orientation of the carbon nanotubes on the alumina layer deteriorates, so that the grain growth of the alumina should be limited.

Comparing the concentrations of the components on the surface of the substrate before and after growing the carbon nanotubes, only the concentrations of Fe and V decrease after carbon nanotube growth and separation of the carbon nanotube array from the surface of the stainless steel substrate.

It is believed that this is due to the fact that Fe and a small amount of V are attached to carbon nanotubes and act as catalysts for carbon nanotube growth. On the other hand, after the annealing, Fe is oxidized but is reduced to metal Fe which can act as a carbon nanotube growth catalyst because it is exposed to H2 during the CVD process. In particular, the concentrations of Cr and Mn are substantially the same before and after growth of the carbon nanotubes, which is thought to be due to the fact that these elements act as nucleation sites of carbon nanotubes.

Therefore, the reason why the growth of carbon nanotubes in stainless steel is inhibited when annealing or heat treatment at 800 ° C or higher is as follows.

(1) the particles containing Fe are gradually coarsened, thereby increasing the size of the particles, while reducing the number of particles (Ostwald ripening) and reducing the influence as a catalyst.

(2) Fe is replaced by Fe as a catalyst from the surface of stainless steel due to the action of Cr ₂ O ₃ produced by the oxidation of the stainless steel base.

(3) As the surface roughness is increased due to excessive crystal growth of alumina, the growth direction of the carbon nanotubes is randomized and the desired yield is not calculated.

Considering that defective or glassy alumina improves the activity of the Fe catalyst to carbon nanotube growth better than stoichiometric Al ₂ O ₃ , the growth of carbon nanotubes may be inhibited due to crystallization in the alumina layer do.

While the present invention has been described in connection with the preferred embodiments thereof, it should be understood that the scope of the present invention should not be construed as limited or limited to the scope of the present invention. It will be self-evident.

Claims (10)

delete delete delete delete Coating an alumina precursor on the metal matrix;
Annealing the coated metal matrix to form an alumina layer from the alumina precursor;
Adding a precursor containing carbon to the alumina layer to form a metal matrix on the alumina layer by a thermal diffusion process to grow carbon fibers while acting as a catalyst;
Wherein the carbon nanotube-oriented carbon fiber comprises a carbon nanotube. 6. The method of claim 5,
Wherein the step of coating the alumina precursor is performed by a dip coating or a spin coating method. 6. The method of claim 5,
Wherein the alumina layer is formed by heating the coated alumina precursor to a temperature in the range of 600 to 800 占 폚 in the step of forming the alumina layer. 6. The method of claim 5,
Wherein the step of growing the carbon fiber comprises growing a carbon fiber by using a thermal CVD method by adding a precursor containing carbon and then stopping the supply of the precursor containing carbon and maintaining a reducing atmosphere, Wherein the oriented carbon fiber is produced by the method. 9. The method of claim 8,
Wherein the thermal CVD method is performed at a temperature ranging from 450 to 800 ° C. 6. The method of claim 5,
Characterized in that the metal matrix is iron or stainless steel and Fe and V components in stainless steel are formed on the surface of the alumina layer by thermal diffusion to act as a catalyst for the growth of carbon fibers. .

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