Method for producing a forest of ultra-long carbon nanotubes

Abstract

The challenge is to produce forests of ultra-long carbon nanotubes. [Solution] A method for producing an ultra-long carbonanotube forest, characterized in that an electric furnace is placed around the outer circumference of a quartz tube having a reaction substrate coated with an Fe catalyst at its lower part, and a neptilizer device is placed in a vertical thermal CVD apparatus with a Cu coil at the bottom of the electric furnace, a mixture of ferrocene and ethanol is sprayed from the neptilizer device into the quartz tube to synthesize carbonanotubes on the reaction substrate, and then current is passed through the Cu coil to generate magnetic field lines, thereby guiding and growing the synthesized carbonanotubes in the direction of the magnetic field lines.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an ultra-long carbon nanotube (CNTs) forest that can be used for cables of a space elevator.

Background Art

[0002] The principle of a space elevator is relatively simple. One end of a long cable is fixed to the Earth, and the other end is fixed to a counterweight (such as a large space station) on a geostationary orbit. Due to the interaction of gravity and centrifugal force, the cable maintains tension. Along this cable, using the electric power obtained by converting solar energy or laser energy, the space elevator can move up and down.

[0003] Since this cable is subject to both gravity and centrifugal force, its material is the biggest issue. Elevators used in daily life are made using steel cables, or steel cables are also used over long distances in high-rise buildings and bridges. However, when spanning an even longer distance, the cable will break due to its own weight. Therefore, in order to realize a space elevator, a new material that is much stronger and lighter than conventional materials is required.

[0004] The length of this cable is about 100,000 kilometers, and sufficient tensile strength is required to support such a long structure. Carbon nanotubes (CNTs) are one of the toughest materials discovered so far, and their macroscopic strength and toughness are superior to materials such as carbon fibers and aramids that are currently widely used.

[0005] In order to realize a space elevator, the material of the cable needs to have a strength of 7.5 GPa / (g / cm 3 ) or more per unit mass. So far, only CNTs can meet this requirement. The tensile strength of CNTs exceeds 100 GPa. Assuming a density of about 1.6 g / cm 3 its specific strength is 62.5 GPa / (g / cm 3This would result in a specific strength far exceeding that required for a space elevator. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-151515 [Patent Document 2] Special Publication 2019-504838 [Patent Document 3] Retable 2016 / 208558 [Non-patent literature]

[0007] [Non-Patent Document 1] Researcher Koji Sugime and Researcher Tsubasa Inoue, "Topic: Successful Extension of Carbon Nanotubes" Press Release, Research Highlights, Research Activities, Tue, 27, Oct 2020 [Overview of the project] [Problems that the invention aims to solve]

[0008] Carbon nanotube forests are obtained by growing carbon nanotubes (CNTs) at high density. They are essential for maximizing the performance of carbon nanotubes and are expected to have applications in various fields.

[0009] However, high-density growth of carbon nanotubes is difficult, which is a factor hindering their practical application.

[0010] According to Non-Patent Literature 1, a new method was used to successfully grow a forest of carbon nanotubes (CNTs) up to 14 cm. By suppressing the precipitation of amorphous carbon (aC) through the introduction of carbon dioxide, and by suppressing in-plane diffusion with an iron-gadolinium binary catalyst, as well as by continuously supplying small amounts of iron and aluminum, a 14 cm ultra-long carbon nanotube (CNT) forest was achieved in approximately 26 hours. Considering that the longest CNT forest to date was only 2 cm, it is clear how groundbreaking this achievement is.

[0011] According to Non-Patent Document 1, the synthesis time for a 14 cm ultra-long carbon nanotube (CNT) forest was 26 hours. However, considering the costs of future large-scale production, it is necessary to synthesize ultra-long carbon nanotube (CNT) forests of 50-100 cm or more in a shorter time, aiming to reduce costs and improve the practicality of fibrous carbon nanotubes. [Means for solving the problem]

[0012] The present invention proposes a method for producing an ultra-long carbon nanotube forest, which involves placing an electric furnace around the outer circumference of a quartz tube having a reaction substrate coated with an Fe catalyst at its lower part, and a neptilizer device at the bottom of the electric furnace, in a vertical thermal CVD apparatus, spraying a mixture of ferrocene and ethanol from the neptilizer device into the quartz tube, synthesizing carbon nanotubes on the reaction substrate, and then energizing the Cu coil to generate magnetic field lines, thereby guiding and growing the synthesized carbon nanotubes in the direction of the magnetic field lines. [Effects of the Invention]

[0013] Conventionally, in high-density CNT forests where the density reaches more than 100,000 times, the growth time is long (26 hours), and the growth limit is only about 14 cm. However, in this invention, in order to improve the growth rate, a Cu coil that generates magnetic field lines is used, making it possible to grow to about 1 m in a short time (13 hours). [Brief explanation of the drawing]

[0014] [Figure 1] External view image of a vertical thermal CVD apparatus. [Figure 2] Mechanism image diagram of a long CNTs forest. [Figure 3] Access image diagram for fabricating a long CNTs forest. [Figure 4] SEM and TEM diagrams of a long CNTs forest. [Figure 5] Raman diagram of a long CNTs forest. [Figure 6] Image diagram of a space elevator.

Embodiments for Carrying Out the Invention

[0015] A nebulizer device is arranged in a vertical thermal CVD apparatus provided with an electric furnace on the outer periphery of a quartz tube having a reaction substrate coated with an Fe catalyst at its lower part, and a Cu coil is provided at the bottom of the electric furnace. A mixed solution of ferrocene and ethanol is fed into the quartz tube in a spray form from the nebulizer device, carbon nanotubes are synthesized on the reaction substrate, and further, an electric current is passed through the Cu coil to generate magnetic lines of force, and the synthesized carbon nanotubes are induced and grown in the direction of the magnetic lines of force. A method for manufacturing an ultra-long carbon nanotube forest characterized by the above.

Examples

[0016] The present invention will be described based on illustrated embodiments. Figure 1 is an external view of a vertical thermal CVD apparatus, the main components of which are as follows. First, an electric furnace (1) is placed, and a reaction quartz tube (2) is covered by the electric furnace (1). A Cu coil (11) is wound around the bottom of the electric furnace (1), and this coil generates magnetic field lines, thereby effectively controlling the growth direction and growth rate of carbon nanotubes (CNTs). Furthermore, ethanol and ferrocene (15) are converted into a fine mist through an ultrasonic atomizer (4) and introduced into the reaction system. This continuously supplies a carbon source and new catalyst, promoting the growth of CNTs. The flow rate of nitrogen (N2) (6), used as a carrier gas, is precisely controlled by a flow meter 1 (5), and the exhaust gas generated after the reaction passes through a flow meter 2 (12) and is properly discharged through an exhaust quartz tube (13). The Fe catalyst (16) used in the reaction is pre-coated onto the reaction substrate (10) to promote the growth reaction of CNTs on the substrate. The temperature of the reaction substrate (10) is precisely controlled by a temperature controller (3) and a temperature sensor (8) to maintain optimal reaction conditions at all times. Finally, ultra-long carbon nanotubes (CNTs) (9) are grown on the reaction substrate (10), synthesizing the expected high-density CNT forest.

[0017] Figure 2 shows the synthesis mechanism of ultra-long carbon nanotubes (CNTs) (9) according to the present invention. In this method, a metal wire such as a Cu wire is wound spirally, and a magnetic flux is generated by passing an electric current through it. By placing the spirally wound coil (11) at the bottom of the reaction substrate (10), a magnetic field (17) is generated in the same direction as the growth direction of the carbon nanotubes (CNTs) (9). When an electric current is passed through the Cu wire, a magnetic flux is formed according to the direction of the current, which affects the growth of the CNTs. In particular, when the Cu wire is wound in a circular shape, a unidirectional magnetic field is generated inside it.

[0018] Furthermore, by winding the Cu wire in a spiral pattern, the internal magnetic field is strengthened as the number of turns increases. A Cu wire wound multiple times in this way is called a "coil" (11), and the more turns a coil has, the stronger the generated magnetic field (17) becomes, and the amount of magnetic flux in the region covered by the coil (11) also increases. This strong magnetic field (17) not only allows for precise control of the growth direction of CNTs, but is also expected to improve the growth rate. Due to the effect of the magnetic field, the CNTs grow in an orderly manner in the same direction, making it possible to form an ultra-long, high-density CNT forest (9). This technology makes it possible to synthesize high-quality ultra-long CNTs (9) more efficiently.

[0019] Figure 3 shows the process of a simulation for the production of ultra-long carbon nanotubes (CNTs) (9). (a) First, a metal catalyst (16) is coated onto the reaction substrate (10). This catalyst promotes the growth of CNTs. (b) Next, the reaction substrate (10) is heated by the conductor (7), and its temperature is precisely controlled by the temperature sensor (8). (c) The reaction substrate (10) is heated to 1000°C and constantly monitored by a temperature sensor (8) to maintain the optimal temperature for growth. (d) Ethanol (14) is converted into a fine mist using an atomizer and sent together with the carrier gas N2 to the metal catalyst (16) on the reaction substrate (10). There, the decomposition of ethanol is accelerated, and the growth of ultra-long carbon nanotubes (9) begins. (e) To control the growth direction of ultra-long carbon nanotubes (CNTs) (9) in a straight line, an electric current is passed through a Cu coil (11) to generate magnetic field lines. The CNTs grow efficiently along these magnetic field lines, promoting directional growth. (f) Because the "lifetime" of the nano-sized metal catalyst is limited, ethanol containing ferrocene (15) is continuously supplied via a nebulizer system to replenish the catalyst and carbon source. This sustains the growth of CNTs, eventually forming very long CNTs (9), which are expected to reach lengths of 500 cm to 1000 cm.

[0020] Figures 4(a) and (b) show typical SEM and TEM images of synthesized carbon nanotubes (CNTs), where ultra-long carbon nanotubes (CNTs) are observed.

[0021] Figure 5 shows that the G band, one of the characteristic peaks of graphene, originates from the in-plane motion of carbon atoms and is located at 1580–1600 cm⁻¹. -1 It appears in the vicinity. The D band is known as a disorder band, resulting from structural disorder and defects, and is caused by lattice motion away from the center of the Brillouin zone, at 1270–1450 cm. -1 If this peak is present (depending on the excitation wavelength), it indicates a defect or edge in the graphene sample.

[0022] Figure 6 illustrates the concept of a space elevator. A space elevator is a structure that connects the Earth's surface to geostationary orbit (approximately 36,000 km above the Earth), and is expected to be a cheaper and safer way to transport materials to space compared to conventional rocket launches. Because the cable of this elevator will be strongly pulled toward the Earth, a counterweight is required in the opposite direction. The cable requires a material that is lightweight yet high-strength. The "breaking length" (tensile breaking strength divided by density, in km) is used to evaluate these materials. A breaking length of approximately 5,000 km is required for the space elevator cable, but conventional materials do not meet this standard. For example, the breaking length of steel is about 50 km, for Kevlar (registered trademark) it is 250 km, and for carbon fiber it is only about 400 km. On the other hand, the breaking length of ideal carbon nanotubes (CNTs) has been reported to exceed 6,000 km, and if this value is achieved, the construction of a space elevator will become a reality. Due to the properties of CNTs, they are attracting increasing attention as a cable material for space elevators. [Industrial applicability]

[0023] In summary, the method for producing ultra-long carbon nanotube forests according to the present invention shortens the growth time, allowing for more efficient synthesis of ultra-long CNTs and expanding the applications of carbon nanotubes (CNTs) to fields such as electronic devices, composite materials, and the aerospace industry. [Explanation of symbols]

[0024] 1………………Electric furnace 2………………Reaction Quartz Tube 3………………Temperature Controller Manufacturing of Ultra-Long Carbon Nanotube Forests 4………………Nebulizer device 5………………Flowmeter 1 6………………N2 gas 7………………Heating conductor 8………………Temperature sensor 9………………Ultra-long carbon nanotubes (CNTs) ‐ 10………………Reaction substrate 11………………Cu coil 12………………Flowmeter 2 13………………Exhaust quartz tube 14………………Ethanol 15………………Ethanol + Ferrocene 16………………Metal catalyst 17. Magnetic field lines

Claims

[Claim 1] A method for producing an ultra-long carbonanotube forest, characterized by: an electric furnace being placed around the outer circumference of a quartz tube having a reaction substrate coated with an Fe catalyst at its lower part; a neptilizer device being placed in a vertical thermal CVD apparatus with a Cu coil at the bottom of the electric furnace; a mixture of ferrocene and ethanol being sprayed from the neptilizer device into the quartz tube to synthesize carbonanotubes on the reaction substrate; and further energizing the Cu coil to generate magnetic field lines, thereby guiding and growing the synthesized carbonanotubes in the direction of the magnetic field lines.

Images