Method for manufacturing SiC-coated carbon nanotubes

Abstract

This invention provides a method for producing carbon nanotubes coated with SiC. [Solution] The method for producing carbon nanotubes includes a SiC coating step in which SiC is formed on the surface of the carbon nanotubes by thermal CVD, thereby coating the carbon nanotubes with SiC, and vinylsilane is used as the raw material gas in the thermal CVD method.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing SiC-coated carbon nanotubes.

Background Art

[0002] Carbon nanotubes are excellent in electrical conductivity, thermal conductivity, chemical stability, etc., and are expected to be applied to semiconductor devices such as transistors and electrodes. The mechanical strength of a single carbon nanotube itself is strong, tough and flexible, and can be bent. However, due to its high aspect ratio structure, it is difficult to stand independently in a long and vertical state, and it easily bends, and the vertical structure cannot be maintained in applications such as electrode applications.

[0003] On the other hand, SiC has properties such as high strength, wear resistance, acid resistance, and corrosion resistance. Therefore, it is conceivable to coat carbon nanotubes with SiC to supplement mechanical strength.

[0004] Patent Document 1 describes that coating carbon nanotubes with SiC improves oxidation resistance and chemical stability. As a method of coating carbon nanotubes with SiC, Patent Document 1 describes the following method. First, a raw material for generating SiO gas is placed at the lower part of the crucible. Next, carbon nanotubes are placed on the raw material through a carbon felt, and the upper part of the crucible is covered with a carbon sheet and a carbon felt. Then, the crucible is heated to 1150 °C or higher in a vacuum or an inert gas atmosphere. By the above, the carbon nanotubes are coated with SiC.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the method described in Patent Document 1 involves many steps and is complex, so there was a need for a simpler method to coat carbon nanotubes with SiC.

[0007] This invention was made in view of the above background, and aims to provide a method for producing SiC-coated carbon nanotubes. [Means for solving the problem]

[0008] One aspect of the present invention is, The process includes a SiC coating step in which SiC is formed on the surface of carbon nanotubes by thermal CVD, thereby coating the carbon nanotubes with SiC. The present invention relates to a method for producing SiC-coated carbon nanotubes using vinylsilane as a raw material gas in the aforementioned thermal CVD method. [Effects of the Invention]

[0009] In the above embodiment, thermal CVD is used to form SiC, and vinylsilane is used as the raw material gas. Therefore, the raw material gas can easily penetrate the carbon nanotubes. As a result, the carbon nanotubes can be coated with SiC.

[0010] As described above, according to the above embodiment, a method for producing carbon nanotubes coated with SiC can be provided. [Brief explanation of the drawing]

[0011] [Figure 1] A schematic diagram showing the configuration of a thermal CVD apparatus used for manufacturing SiC-coated carbon nanotubes in the embodiment. [Figure 2] A schematic diagram illustrating the manufacturing process of SiC-coated carbon nanotubes in an embodiment. [Figure 3] This graph shows the results of Raman spectroscopy measurements for SiC-coated carbon nanotubes (CNTs) and CNTs before SiC formation. [Figure 4]Graph showing the results of Raman spectroscopy measurement for SiC-coated CNTs. [Figure 5] Figure showing the SEM image of the fabricated SiC-coated CNTs. [Figure 6] Graph showing the relationship between the diameter of SiC-coated CNTs and the distance from the surface of the CNTs. [Figure 7] Graph showing the results of electrochemical measurement for SiC-coated CNT sheets and CNT sheets before SiC formation. [Figure 8] Photos of the CNT sheet before SiC formation taken before and after electrochemical measurement. [Figure 9] Photos of the SiC-coated CNT sheet taken before and after electrochemical measurement. [Figure 10] Graph showing the X-ray diffraction patterns of SiC-coated CNTs and uncoated CNTs. [Figure 11] Photo of the fabricated flaky SiC-coated CNTs. [Figure 12] Photo of an aqueous solution in which SiC-coated CNTs are dispersed. [Figure 13] Photo of an aqueous solution in which CNTs without SiC coating are dispersed. [Figure 14] Photo of the state where water droplets are brought into contact with CNTs not coated with SiC. [Figure 15] Photo of the state where water droplets are brought into contact with SiC-coated CNTs.

Embodiments for Carrying Out the Invention

[0012] The method for manufacturing SiC-coated carbon nanotubes has a SiC coating step of coating carbon nanotubes with SiC by forming SiC on the surface of the carbon nanotubes by thermal CVD method, and uses vinylsilane as a source gas in the thermal CVD method.

[0013] In the method for manufacturing SiC-coated carbon nanotubes, vinyl silane is supplied after being mixed with an inert gas, and the ratio of the flow rate of the inert gas to the flow rate of vinyl silane may be 50 to 500. SiC-coated CNTs can be produced more efficiently.

[0014] In the method for manufacturing SiC-coated carbon nanotubes, the growth temperature of SiC may be 500°C or higher and 1100°C or lower. Within this temperature range, SiC-coated CNTs can be produced more efficiently.

[0015] In the method for manufacturing SiC-coated carbon nanotubes, the carbon nanotubes may be vertically oriented on a substrate. SiC can also be coated on such carbon nanotubes. In this case, the thickness of the layer of carbon nanotubes may be 0.1 to 2 mm. The coating rate of SiC can be increased, and the mechanical strength can be further enhanced.

[0016] In the method for manufacturing SiC-coated carbon nanotubes, by setting the growth temperature of SiC to 800°C or higher, the SiC may be a polycrystal of 3C-SiC. The thermal conductivity of the SiC-coated carbon nanotubes can be increased.

[0017] The SiC-coated carbon nanotubes have carbon nanotubes and SiC that coats the carbon nanotubes, and the SiC is a polycrystal of 3C-SiC.

[0018] The method for manufacturing hydrophilic carbon nanotubes hydrophilizes the carbon nanotubes by coating them with SiC.

[0019] In the method for manufacturing hydrophilic carbon nanotubes, SiC may be formed by thermal CVD using vinyl silane as a source gas.

[0020] The method for producing a carbon nanotube dispersion involves preparing hydrophilic carbon nanotubes using the above-described method for producing hydrophilic carbon nanotubes, and then mixing the hydrophilic carbon nanotubes with a water-containing solvent to produce a carbon nanotube dispersion.

[0021] The carbon nanotube dispersion contains SiC-coated carbon nanotubes and water.

[0022] (Embodiment) 1. Overview of the Embodiment In the SiC-coated carbon nanotube (CNT) manufacturing method of the embodiment, SiC is formed and coated onto the surface of the CNT by thermal CVD.

[0023] The CNTs may be single-walled or multi-walled, and may be semiconducting or metallic. The diameter and length of the CNTs may also be arbitrary. In this embodiment, even a substrate on which vertically oriented CNTs are formed can be coated with SiC.

[0024] Thermal CVD is a method of depositing a film on a substrate by supplying a raw material gas to the substrate and causing the raw material gas to thermally decompose on the substrate. Thermal CVD can be implemented using a cold-wall method, where only the substrate is heated, or a hot-wall method, where the entire reaction chamber is heated.

[0025] 2. Configuration of the Thermal CVD Apparatus Figure 1 shows an example of a thermal CVD apparatus used in the thermal CVD method in the embodiment. The thermal CVD apparatus shown in Figure 1 is a hot-wall type thermal CVD apparatus and has a reaction chamber 10, a stage 11, a heater 12, supply pipes 13 and 14, and an exhaust pipe 15.

[0026] The reaction chamber 10 is a hollow container used for film deposition by thermal CVD. For example, it could be a quartz tube.

[0027] Stage 11 is located inside the reaction chamber 10. Stage 11 is a platform on which the substrate S is placed.

[0028] The heater 12 is located inside the reaction chamber 10. The heater 12 is a heating device that heats the entire reaction chamber 10.

[0029] The supply pipe 13 is a pipe that supplies the raw material gas into the reaction chamber 10. The raw material gas is vinylsilane, as described later. The supply pipe 14 is a pipe that supplies the carrier gas into the reaction chamber 10. The exhaust pipe 15 is a pipe that exhausts the gas from inside the reaction chamber 10. The above is an example of a thermal CVD apparatus. The thermal CVD apparatus may also be a cold wall type.

[0030] 3. Method for producing SiC-coated carbon nanotubes First, a substrate is prepared on which vertically oriented CNT2 are formed (see Figure 2(a)). Substrate 1 is, for example, made of layers of Si and SiO2 stacked in sequence, with the CNT2 formed on the SiO2 layer.

[0031] Note that the CNT2 is not limited to CNT2 formed vertically on the substrate 1, but may be in any state. For example, it may be in the form of powdered CNT2, or CNT2 processed into threads, sheets, etc. However, in order to cover as wide an area of ​​the CNT2 surface with SiC as possible, it is preferable to use CNT2 formed vertically on the substrate 1 as described above.

[0032] Next, the substrate 1 is brought into the reaction chamber 10 of the thermal CVD apparatus and placed on the stage 11. Then, the reaction chamber 10 is sealed.

[0033] Next, the reaction chamber 10 is evacuated, the substrate 1 is heated by the heater 12, and the reaction chamber 10 is purged by supplying an inert gas. Then, the substrate is heated to the growth temperature by the heater 12, and the raw material gas is supplied to the reaction chamber 10 to form a SiC layer 3 on the surface of the CNT 2. This forms a SiC-coated CNT (see Figure 2(b)).

[0034] Vinylsilane (CH2=CH-SiH3) is used as the raw material gas. Although vinylsilane contains both Si-C and Si-H bonds, the Si-H bonds decompose first, so fewer Si-H bonds remain in the film, allowing for the formation of high-quality SiC. Furthermore, vinylsilane serves as both a Si and C raw material, eliminating the need for composition control and simplifying the control of the raw material gas. Also, because only one type of raw material gas is used, the raw material gas circulates well around the CNT2, making it easy to coat the CNT2 with the SiC layer 3. The flow rate of vinylsilane is preferably 1 to 50 sccm. Within this range, high-quality SiC can be formed. More preferably, it is 1 to 20 sccm.

[0035] When supplying the raw material gas, it is preferable to supply it mixed with the carrier gas. The growth rate of the SiC layer 3 can be controlled by the flow rate ratio of the raw material gas to the carrier gas. An inert gas can be used as the carrier gas. For example, nitrogen or noble gases such as argon can be used as carrier gases.

[0036] In this case, the ratio of the carrier gas flow rate to the raw material gas flow rate should be 50 to 500. This reduces the concentration of the raw material gas, suppressing the growth rate of the SiC layer 3, making it easier for the raw material gas to flow into the substrate 1 side, and facilitating the covering of the substrate 1 side of the CNT2 surface with the SiC layer 3. If the SiC layer 3 grows too fast, the SiC layer 3 on the leading edge of the CNT2 may grow first, hindering the inflow of the raw material gas into the substrate 1 side and inhibiting the growth of the SiC layer 3 on the substrate 1 side. A more preferable ratio of the carrier gas flow rate to the raw material gas flow rate is 100 to 400.

[0037] The source gas may be mixed with a dopant gas. The conduction type of SiC can be controlled by introducing impurities into SiC. P can be used as the n-type impurity. When P is used as the n-type impurity, phosphorus trifluoride or diphenylphosphine can be used as the n-type dopant gas.

[0038] Furthermore, Al can be used as the p-type impurity. When Al is used as the p-type impurity, trimethylaluminum (TMA) can be used as the p-type dopant gas. It is preferable to adjust the flow rate of TMA so that the amount of Al relative to SiC is 2 at% or less. If it is 2 at% or less, the reaction between Al and C to form Al4C3 can be suppressed, and low-resistance p-type SiC can be formed. More preferably it is 1 at% or less. The hole concentration of Al-doped p-type SiC is, for example, 1 × 10⁻⁶ 16 ~1 × 10 18 cm -3 It can be done this way.

[0039] The growth temperature is preferably 500 to 1100°C. Within this range, a high-quality SiC layer 3 can be formed on the surface of the CNT2. The higher the growth temperature, the faster the growth rate of the SiC layer 3 tends to be. Therefore, if the CNT2 is long, it is preferable to lower the growth temperature and increase the growth time. A sufficient SiC layer 3 can be formed on the substrate 1 side of the CNT2 surface. For example, if the length of the CNT2 is 0.1 mm or more, it is preferable to set the growth temperature to 800°C or lower, preferably 650°C or lower.

[0040] The SiC layer 3 becomes amorphous if the growth temperature is below 800°C, and polycrystalline (3C-SiC) if the growth temperature is 800°C or higher. The higher the growth temperature, the better the crystallinity of the SiC layer 3 can be improved. When using SiC-coated carbon nanotubes as photoelectrodes, a 3C-SiC crystal structure of SiC is more suitable for photoelectrodes than 4H-SiC or 6H-SiC. This is because the band gap energy of 3C-SiC is smaller than that of 4H-SiC and 6H-SiC, making it more suitable for light absorption. In addition, higher crystallinity suppresses carrier deactivation due to defects, so polycrystalline SiC is more suitable for photoelectrodes than amorphous SiC.

[0041] Furthermore, 3C-SiC is known to have high thermal conductivity. Therefore, if the SiC layer 3 coating the CNT2 is made of polycrystalline 3C-SiC, the thermal conductivity of the SiC-coated CNT can be increased.

[0042] A reduced growth pressure is preferable. This allows the raw material gas to circulate more easily around the CNT2, making it easier to coat the CNT2 with SiC. For example, 1 to 1000 Pa is preferred, and more preferably 1 to 500 Pa.

[0043] It is not necessary for the entire surface of the CNT2 to be covered by the SiC layer 3, but it is preferable that 50% or more of the entire surface is covered. Preferably, it is 70% or more. Furthermore, the coverage rate of the SiC layer 3 may have a distribution along the length of the CNT2. For example, the coverage rate may gradually increase from the substrate 1 side of the CNT2 toward the upper end.

[0044] Furthermore, the average thickness of the SiC layer 3 in the circumferential direction of the CNT2 may increase from one end of the CNT2 to the other. Also, the SiC layer 3 may be in the form of a film or granules. In the early stages of growth, SiC grows in the form of granules on the surface of the CNT, and as growth progresses, the number of granules increases and the granules merge together, and as growth progresses further, the SiC becomes film-like. Therefore, depending on the growth stage, the SiC layer 3 will have different morphologies, such as granular or film-like.

[0045] By covering CNT2 with a film-like SiC layer 3, the mechanical strength of the CNT2 can be further increased. Furthermore, by covering CNT2 with a granular SiC layer 3, crack formation in the SiC layer 3 due to the difference in thermal expansion coefficients between the CNT2 and the SiC layer 3 can be suppressed.

[0046] When CNTs 2 oriented vertically on a substrate 1 are coated with a SiC layer 3, as in the embodiment, the raw material gas is less likely to penetrate towards the substrate 1 side. Therefore, the SiC layer 3 tends to form a film on the upper end side of the CNTs and granules on the substrate 1 side.

[0047] The average thickness of the SiC layer 3 is preferably 10 to 400 nm. Within this range, the mechanical strength of the CNTs can be sufficiently increased. More preferably, it is 20 to 300 nm.

[0048] When covering vertically oriented CNT2 on a substrate 1 with a SiC layer 3, the thickness of the CNT2 layer is preferably 0.1 to 2 mm. Within this range, it is easy to cover the CNT2 with the SiC layer 3.

[0049] In the embodiments described above, thermal CVD is used to form SiC, and vinylsilane is used as the raw material gas. As a result, the raw material gas can easily circulate around the CNTs, enabling the efficient production of SiC-coated CNTs.

[0050] After fabricating SiC-coated CNTs according to the embodiment, a web of SiC-coated CNTs can be formed by pulling the SiC-coated CNTs out from the side end face of the substrate 1. Here, the web of SiC-coated CNTs is formed when the ends of the SiC-coated CNTs, which are oriented vertically on the substrate 1, are grasped and pulled out in a direction horizontal to the main surface of the substrate 1, causing the SiC-coated CNTs to spontaneously bond with each other by van der Waals forces, forming a planar CNT linkage. Even when coated with SiC, a web can be formed in the same way as when not coated with SiC.

[0051] Furthermore, SiC-coated carbon nanotubes (SiC-coated CNTs) can be twisted to produce SiC-coated CNT threads. Additionally, SiC-coated CNT sheets can be formed by weaving and laminating these threads.

[0052] SiC-coated carbon nanotubes (CNTs) are suitable for applications where the mechanical strength of the CNTs needs to be increased, such as electrode materials and field emission sources.

[0053] Furthermore, the inventors' research revealed that coating CNTs with SiC improves their hydrophilicity. Uncoated CNTs have high water repellency and, when mixed with water, aggregate together and do not disperse in the water. On the other hand, SiC-coated CNTs have high hydrophilicity and disperse uniformly in water when mixed. Thus, by coating CNTs with SiC, a CNT dispersion can be prepared, making the handling of CNTs easier.

[0054] By coating carbon nanotubes with SiC to make them hydrophilic, the contact area between the carbon nanotubes and water can be increased. Therefore, by using SiC-coated carbon nanotubes as photoelectrodes, water can be decomposed more efficiently.

[0055] The carbon nanotube dispersion comprises SiC-coated CNTs and a solvent containing water. Preferably, the water content in the solvent is 50 wt% or more; more preferably 90 wt% or more. Distilled water or pure water is preferred. The concentration of SiC-coated CNTs in the carbon nanotube dispersion can be, for example, 0.1 to 10 wt%.

[0056] 4. Various experimental results Next, we will describe the experimental results related to the embodiment.

[0057] Experiment 1 A substrate was prepared in which vertically oriented carbon nanotubes (CNTs) were formed on a Si / SiO2 substrate. The thickness of the CNT layer was 1.0 mm. The CNTs were formed by the CVD method, with hydrocarbons as the raw material gas, a growth temperature of 800°C, and a growth pressure of 10 torr.

[0058] A substrate on which CNTs were formed was introduced into the reaction chamber of a thermal CVD apparatus, and SiC was formed on the surface of the CNTs by thermal CVD. In the thermal CVD method, vinylsilane was used as the raw material gas and argon was used as the carrier gas. The flow rate of vinylsilane was set to 2 sscm and the flow rate of argon gas to 200 sccm. The growth temperature was set to four levels: 600°C, 750°C, 850°C, and 1000°C. The growth pressure was set to 1 torr and the growth time was 1 hour.

[0059] Raman spectroscopy measurements were performed on the fabricated SiC-coated carbon nanotubes (CNTs) and on CNTs before SiC formation. Figure 3(a) is a graph showing the measurement results, and Figure 3(b) is a graph showing the relationship between the growth temperature and the intensity ratio of the SiC peak to the G-band peak in the Raman spectrum. The G-band is due to the graphite structure of carbon.

[0060] As shown in Figure 3(a), SiC peaks were observed in the Raman spectra at all growth temperatures. Furthermore, the G-band peak intensity decreased at all growth temperatures compared to before SiC formation. From this, it was confirmed that SiC was formed on the CNT surface in the growth temperature range of 600 to 1000°C.

[0061] Furthermore, as shown in Figure 3(b), the intensity ratio of the SiC peak to the G-band peak increased with increasing growth temperature. This indicates that higher growth temperatures result in faster SiC growth rates and the formation of thicker SiC layers.

[0062] Experiment 2 Similar to Experiment 1, a substrate was prepared in which vertically oriented CNTs were formed on a Si / SiO2 substrate. The thickness of the CNT layer was 0.3 mm. Then, SiC was formed on the surface of the CNTs in the same manner as in Experiment 1. The Ar gas flow rate was set to three levels: 200 sccm, 570 sccm, and 800 sccm, and the growth temperature was 750°C. Other growth conditions were the same as in Experiment 1.

[0063] Raman spectroscopy measurements were performed on the fabricated SiC-coated carbon nanotubes (CNTs). Figure 4(a) is a graph showing the measurement results, and Figure 4(b) is a graph showing the relationship between the growth temperature and the intensity ratio of the SiC peak to the G-band peak in the Raman spectrum.

[0064] As shown in Figure 4(a), SiC peaks were observed in the Raman spectra at all Ar gas flow rates. Furthermore, the peak intensity in the G-band decreased at all Ar gas flow rates compared to before SiC formation. This confirmed that SiC was formed on the surface of the CNTs at all Ar gas flow rates.

[0065] Furthermore, as shown in Figure 4(b), the intensity ratio of the SiC peak to the G-band peak was found to decrease as the Ar gas flow rate increased. This suggests that the SiC growth rate is faster and thicker SiC layers are formed when the Ar gas flow rate is lower. This is thought to be because the concentration of the raw material gas increases when the Ar gas flow rate is lower.

[0066] Experiment 3 Similar to Experiment 1, a substrate was prepared in which vertically oriented carbon nanotubes (CNTs) were formed on a Si / SiO2 substrate. The thickness of the CNT layer was 0.3 mm. Then, SiC was formed on the surface of the CNTs in the same manner as in Experiment 1. The Ar gas flow rate was set to two levels: 570 sccm and 800 sccm, and the growth temperature was 750°C. Other growth conditions were the same as in Experiment 1.

[0067] Figure 5 shows SEM images of the fabricated SiC-coated carbon nanotubes (SiC) and CNTs before SiC formation. The images were taken from three regions: the top, middle, and bottom of the CNT. The top region is the surface side of the CNT layer, and the bottom region is the substrate side. Figure 6 is a graph showing the relationship between the diameter (μm) of the SiC-coated CNT and the distance (mm) from the surface of the CNT layer. The symbols A to C in the graphs of Figures 5 and 6 correspond to each other, and the SEM images of A to C in Figure 5 were taken at the distances A to C in Figure 6.

[0068] As shown in Figure 5, it was found that the diameter of the CNTs decreased with increasing depth from the surface of the CNT layer. The diameter of the SiC-coated CNTs was greater in the upper region than the diameter of the CNTs before SiC formation, and the diameter was also uniform. Therefore, it was found that in the upper region of the CNTs, the entire surface of the CNTs was covered with SiC, and the thickness of the SiC was uniform. In the middle region of the CNTs, the SiC was granular and partially covered the surface of the CNTs. Granular SiC was also observed in the lower region of the CNTs, but in smaller numbers than in the middle region.

[0069] Furthermore, as shown in Figure 6, it was found that the diameter of the SiC-coated CNTs decreases as the depth from the surface increases, that is, as the depth increases towards the substrate, approaching the diameter of the CNTs before SiC formation. This is thought to be because SiC grows first at the top of the CNT, and this SiC inhibits the entry of the raw material gas into the middle and lower parts of the CNT. The granular formation of SiC in the middle and lower regions of the CNT is thought to be due to the same reason. Also, from Figure 6, it was found that the larger the flow rate of Ar gas, the larger the diameter of the SiC-coated CNTs in the middle and lower regions of the CNT. This is thought to be because the concentration of the raw material gas decreases, making it easier for the raw material gas to enter the substrate.

[0070] Figures 5 and 6 show that very little SiC grows in the lower part of the CNT. This is thought to be because the SiC that grew in the upper part of the CNT made it difficult for the raw material gas to penetrate to the substrate side. Therefore, in order to grow SiC in the lower part of the CNT, the following can be considered. First, increasing the flow rate of Ar can be used to lower the concentration of the raw material gas, making it easier for the gas to penetrate. Second, the growth temperature can be lowered to reduce the growth rate of SiC, so that the raw material gas reaches the lower part of the CNT before the SiC grows significantly in the upper part of the CNT.

[0071] Experiment 4 SiC-coated carbon nanotubes (CNTs) were fabricated in the same manner as in Experiment 1. The thickness of the CNT layer was set to 0.2 mm. The growth temperature was set to two levels, 800°C and 850°C, and all other growth conditions were the same as in Experiment 1. The fabricated SiC-coated CNTs were pulled out from the side edge of substrate 1 and twisted to create SiC-coated CNT threads, and these threads were woven to create a sheet with 480 layers of CNTs. For comparison, a similar 480-layer sheet was fabricated using CNTs before SiC formation.

[0072] Electrochemical measurements were performed using cyclic voltammetry with the fabricated SiC-coated CNT sheet and the CNT sheet before SiC formation as working electrodes. Pt was used as the counter electrode and Ag|AgCl as the reference electrode, and a 0.5 mol / L sodium sulfate aqueous solution was used. The number of cycles was set to 300.

[0073] Figure 7 is a graph showing the results of electrochemical measurements of SiC-coated CNT sheets and CNT sheets before SiC formation. Figure 8 shows photographs of the CNT sheets before and after electrochemical measurements. Figure 9 shows photographs of SiC-coated CNT sheets grown at a temperature of 850°C before and after electrochemical measurements.

[0074] As shown in Figure 7, the SiC-coated CNTs exhibited CV characteristics similar to those of CNTs before SiC formation. This indicates that SiC-coated CNTs can be used as electrodes.

[0075] Furthermore, as shown in Figure 8, the CNT sheet before SiC formation shrank significantly after CV measurement. On the other hand, as shown in Figure 9, the SiC-coated CNT sheet did not shrink as much as the CNT sheet before SiC formation. As a result, it was confirmed that coating CNTs with SiC improves their mechanical strength. In addition, the results in Figures 7-9 show that SiC-coated CNTs are suitable as electrode materials.

[0076] Experiment 5 SiC-coated carbon nanotubes (CNTs) were fabricated in the same manner as in Experiment 1. The growth temperatures were set at three levels: 750°C, 850°C, and 1000°C, while all other growth conditions were the same as in Experiment 1. The fabricated SiC-coated CNTs were then subjected to X-ray diffraction. For comparison, uncoated CNTs were also subjected to X-ray diffraction.

[0077] Figure 10 is a graph showing the X-ray diffraction patterns of SiC-coated and uncoated carbon nanotubes (CNTs). As shown in Figure 10, when SiC-coated CNTs were formed at 850°C and 1000°C, the peaks closely matched those of 3C-SiC. This indicates that the SiC coating the CNTs becomes polycrystalline 3C-SiC at growth temperatures above 850°C. On the other hand, when SiC-coated CNTs were formed at 750°C, the 3C-SiC peak was not clear, suggesting that amorphous SiC was formed.

[0078] Experiment 6 SiC-coated carbon nanotubes (CNTs) were fabricated in the same manner as in Experiment 1, and the SiC-coated CNTs were peeled from substrate 1 to form flakes. The SiC growth temperature was set to 850°C. Figure 11 is a photograph of the fabricated flake-shaped SiC-coated CNTs. These flake-shaped SiC-coated CNTs were mixed and dispersed in water. Figure 12 is a photograph of the aqueous solution in which the SiC-coated CNTs were dispersed. As shown in Figure 12, it was found that the SiC-coated CNTs were uniformly dispersed in the water without agglomerating.

[0079] For comparison, uncoated SiC flakes of CNTs were mixed and dispersed in water. Figure 13 is a photograph of the aqueous solution containing dispersed uncoated SiC CNTs. As shown in Figure 13, the CNTs were aggregated in the water and were not uniformly dispersed.

[0080] Figures 12 and 13 show that coating CNTs with SiC improves their hydrophilicity, making it possible to prepare a dispersion of SiC-coated CNTs.

[0081] When the SiC growth temperature was changed to 1000°C and the experiment was conducted similarly, it showed the same hydrophilicity as when the SiC growth temperature was 850°C.

[0082] Experiment 7 Flake-shaped SiC-coated carbon nanotubes (CNTs) and uncoated SiC-coated CNTs were prepared in the same manner as in Experiment 6. The SiC growth temperature was set to 850°C. Water droplets were then brought into contact with both the SiC-coated and uncoated CNTs.

[0083] Figure 14 is a photograph showing a water droplet in contact with a CNT that is not coated with SiC. As shown in Figure 14, the water droplet maintained its spherical shape even when in contact with the CNT that was not coated with SiC. When the water droplet was then lifted and separated from the CNT, it maintained the same size and shape as before contact with the CNT, indicating that the water droplet was not absorbed by the CNT. Therefore, it was found that CNTs that are not coated with SiC have high water repellency.

[0084] Figure 15 is a photograph showing a water droplet in contact with a SiC-coated carbon nanotube (CNT). As shown in Figure 15, the water droplet was absorbed by the SiC-coated CNT the moment it came into contact with it. Therefore, it was found that SiC-coated CNTs have high hydrophilicity. [Explanation of Symbols]

[0085] 1: Circuit board 2:CNT 3:SiC layer 10: Reaction Chamber 11: Stage 12: Heater 13, 14: Supply pipe 15: Exhaust pipe

Claims

1. The process includes a SiC coating step in which SiC is formed on the surface of carbon nanotubes by thermal CVD, thereby coating the carbon nanotubes with SiC. A method for producing SiC-coated carbon nanotubes, wherein vinylsilane is used as the raw material gas in the thermal CVD method.

2. The method for producing SiC-coated carbon nanotubes according to claim 1, wherein the vinylsilane is supplied mixed with an inert gas, and the ratio of the flow rate of the inert gas to the flow rate of the vinylsilane is 50 to 500.

3. A method for producing SiC-coated carbon nanotubes according to claim 1 or claim 2, wherein the growth temperature of the SiC is 500°C or higher and 1100°C or lower.

4. The method for producing SiC-coated carbon nanotubes according to claim 1, wherein the carbon nanotubes are vertically oriented on a substrate.

5. The method for producing SiC-coated carbon nanotubes according to claim 4, wherein the thickness of the carbon nanotube layer is 0.1 to 2 mm.

6. A method for producing SiC-coated carbon nanotubes according to claim 1, wherein the growth temperature of the SiC is set to 800°C or higher, thereby making the SiC a polycrystalline 3C-SiC.

7. Carbon nanotubes and The carbon nanotube is coated with SiC, The aforementioned SiC is a SiC-coated carbon nanotube, which is a polycrystalline 3C-SiC.

8. A method for producing hydrophilic carbon nanotubes, comprising making carbon nanotubes hydrophilic by coating them with SiC.

9. A method for producing hydrophilic carbon nanotubes according to claim 8, wherein the SiC is formed by a thermal CVD method using vinylsilane as a raw material gas.

10. A method for producing a carbon nanotube dispersion, comprising producing hydrophilic carbon nanotubes by the method for producing hydrophilic carbon nanotubes described in claim 8 or claim 9, and mixing the hydrophilic carbon nanotubes with a water-containing solvent to produce a carbon nanotube dispersion.

11. A carbon nanotube dispersion containing SiC-coated carbon nanotubes and water.

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