How to make carbon nanotubes and how to use carbon nanotubes

2021-12-14 23:26:23 By : Ms. Cindy Li

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The term nanotechnology was originally used to define any work done on the molecular scale or one billionth of a meter. This term is now widely (and loosely) used for anything very small (hence nano Ipod). Carbon nanotubes (CNT) have single-wall (SWNT), double-wall (DWNT) and multi-wall (MWNT) types. Carbon nanotubes are best described as graphene sheets rolled into a one-dimensional structure with axial symmetry. Carbon nanotubes are one of the main components crucial to the nanotechnology revolution. Carbon nanotubes have many unique and interesting properties, please visit our application page to learn more about carbon nanotubes.

"There are many ways to make carbon nanotubes and fullerenes. Fullerenes were first observed after a short pulse, high-power laser is used to vaporize graphite, but this is not a practical method for mass production. Carbon nanotubes may already be It has existed for a period of time longer than initially realized, and may have been manufactured in various carbon combustion and vapor deposition processes, but the electron microscopes at the time were not advanced enough to distinguish them from other types of tubes.

The first method to produce reasonable quantities of carbon nanotubes and fullerenes is to apply an electric current to two carbon electrodes in an inert gas (helium or argon) atmosphere. This method is called plasma arc discharge. It involves evaporating one electrode (anode) into cations and then depositing on the other electrode. This plasma-based process is similar to the more familiar electroplating process in liquid media. Fullerenes and carbon nanotubes are formed by a plasma arc of carbonaceous materials, especially graphite. Fullerenes appear in the formed soot, while CNTs are deposited on the opposite electrode (cathode). Another method of nanotube synthesis involves generating a plasma arc at a cobalt concentration of 3% or higher. As mentioned above, the nanotube product is a compact cathode deposit in the form of a rod. However, when cobalt is added as a catalyst, the nature of the product becomes a network, with strands with a thickness of about 1 mm, extending from the cathode to the wall of the reaction vessel. The mechanism by which cobalt changes this process is not yet clear, but one possibility is that this metal will affect the local electric field, thereby affecting the formation of the five-membered ring. "

"The carbon arc discharge method was originally used to produce C60 fullerenes. It is the most common and simplest method to produce carbon nanotubes because it is quite simple. However, this is a technology for producing a complex mixture of components and requires further purification- The carbon nanotubes are separated from the soot and residual catalytic metals present in the crude product.

This method produces carbon nanotubes by arc-evaporating two carbon rods about 1 mm apart from one end to the end in a shell usually filled with low-pressure (between 50 and 700 mbar) inert gases (helium, argon). Recent studies have shown that carbon nanotubes can also be produced by the arc method in liquid nitrogen. A direct current of 50 to 100 A driven by a potential difference of approximately 20 V generates a high-temperature discharge between the two electrodes. The electric discharge evaporates the surface of one carbon electrode and forms a small rod-like deposit on the other electrode. The production of carbon nanotubes at high yields depends on the uniformity of the plasma arc and the temperature of the deposits formed on the carbon electrodes. "

"In 1996, a dual-pulse laser vaporization technology was developed, which produced gram quantities of SWNT with a purity greater than 70%. The samples were prepared by laser vaporization of graphite rods and a 50:50 mixture of Co and Ni catalysts (particles). Size ~1um) in flowing argon at 1200oC, then heat treated in vacuum at 1000°C to remove C60 and other fullerenes. The initial laser vaporization pulse is followed by a second pulse to vaporize the target more uniformly. Both A continuous laser pulse minimizes the amount of carbon deposited as soot. The second laser pulse breaks down the larger particles ablated by the first laser pulse and sends them into the growing nanotube structure. This method The material produced looks like a "rope" with a diameter of 10-20 nanometers and a length of up to 100 microns or more. It is found that each rope is mainly composed of a bundle of SWNT, arranged along a common axis. By changing the growth temperature, catalyst composition and other processes, p parameters, the average nanotube diameter and size distribution can be changed.

Arc discharge and laser vaporization are currently the main methods to obtain a small amount of high-quality carbon nanotubes. However, both of these methods have disadvantages. First of all, both methods involve evaporating carbon sources, so it is not clear how to use these methods to scale up production to an industrial level. The second problem relates to the fact that the vaporization method grows carbon nanotubes in a highly entangled form, mixed with unwanted carbon and/or metallic substances. The carbon nanotubes produced in this way are difficult to purify, manipulate, and assemble to construct a practical nanotube device architecture. "

"Chemical vapor deposition (CVD) of hydrocarbons on metal catalysts is a classic method. It has been used to produce various carbon materials, such as carbon fibers, filaments, etc., for more than 20 years. A large amount of carbon can be formed by catalytic CVD. Acetylene on Co and Fe catalysts supported on nanotube silica or zeolite. Carbon deposition activity seems to be related to the cobalt content of the catalyst, while the selectivity of CNTs seems to be a function of the pH during the preparation of the catalyst. Fullerene and SWNT bundles MWNTs were also found in MWNTs produced on carbon/zeolite catalysts.

Some researchers are experimenting with the use of ethylene to form carbon nanotubes. Supported catalysts (Fe, Co, Ni) containing a single metal or a mixture of metals seem to induce the growth of isolated SWNT or SWNT bundles in an ethylene atmosphere. It also demonstrated the production of SWNT and double-walled CNT (DWNT) on Mo and Mo-Fe alloy catalysts. With or without a nickel catalyst, CVD of carbon in the pores of a thin alumina template (called a membrane) has been achieved. Ethylene is used for nickel-catalyzed CVD, the reaction temperature is 545°C, and the uncatalyzed process is 900°C. The resulting carbon nanostructure has an open end and no cap.

Methane is also used as a carbon source. In particular, it has been used to obtain "nanotube chips" containing isolated SWNTs in controlled locations. High-yield SWNT was obtained by catalytic decomposition of H2/CH4 mixture on well-dispersed metal particles (Co, Ni, Fe) on MgO at 1000°C. According to reports, by selectively reducing oxide solid solutions (between non-reducible oxides such as Al2O3 or MgAl2O4 and one or more transition metal oxides) in an H2/CH4 atmosphere, composites containing well-dispersed CNTs can be synthesized powder. ). At temperatures usually >800°C, the reduction will produce very small transition metal particles. The decomposition of CH4 on the newly formed nanoparticles prevents their further growth, thus resulting in a very high proportion of SWNT and less MWNT. "

"Ball milling and subsequent annealing is a simple method to produce CNTs. Although this type of mechanical abrasion can produce a completely nanoporous microstructure has been recognized, but until a few years ago, carbon and boron nitride CNTs were produced by these The powder is produced by thermal annealing. Basically the method involves putting graphite powder (purity 99.8%) together with four hardened steel balls into a stainless steel container. The container is purged and argon is introduced. The grinding is up to 150 at room temperature Hours. After grinding, the powder is annealed under nitrogen (or argon) gas flow at a temperature of 1400°C for 6 hours. The mechanism of this process is not clear, but it is believed that the ball milling process forms nanotube nuclei and the annealing process activates the nanotubes. Growth. Studies have shown that this method produces more MWNT and less SWNT."

"Carbon nanotubes can also be produced by diffusion flame synthesis, electrolysis, solar energy use, polymer heat treatment, and low-temperature solid pyrolysis. In flame synthesis, the combustion of part of the hydrocarbon gas provides the required high temperature, and the remaining Fuel can be conveniently used as the required hydrocarbon reagent. Therefore, the flame constitutes an effective source of energy and hydrocarbon raw materials. Combustion synthesis has been proven to be used in large-scale commercial production. "

"The purification of carbon nanotubes generally refers to the separation of carbon nanotubes from other entities, such as carbon nanoparticles, amorphous carbon, residual catalysts, and other unwanted species. Classic chemical purification techniques (such as filtration, chromatography, and Centrifugation), but they have not been found to be effective in removing unwanted impurities. Three basic methods have been used with varying degrees of success, namely gas phase, liquid phase and intercalation methods."

NASA's Glenn Research Center has developed a new gas phase method to purify single-walled carbon nanotubes in grams. This method is an improvement on the gas-phase purification technique previously reported by Smalley and others, combining high-temperature oxidation and repeated extraction of nitric acid and hydrochloric acid. This improved procedure significantly reduces the amount of impurities (residual catalyst, and carbon in the form of non-nanotubes) in CNTs, and significantly improves their stability.

The current liquid phase purification procedure follows certain basic steps:

"It is important to keep CNTs well separated in solution, so CNTs are usually dispersed (using surfactants) before the final stage of separation.

Usually, before microfiltration operation, centrifugal separation is required to concentrate SWNT into low-yield soot, because nanoparticles can easily contaminate membrane filters. The advantage of this method is that unwanted nanoparticles and amorphous carbon are removed at the same time, and the carbon nanotubes are not chemically modified. However, 2-3 M nitric acid can be used to chemically remove impurities.

It is now possible to cut CNTs into smaller fragments by prolonged sonication in a concentrated acid mixture. The resulting carbon nanotubes form a colloidal suspension in the solvent. They can be deposited on a substrate or further processed in solution, and many different functional groups can be attached to the ends and sides of the CNT. "

The cheap tube company exclusively uses SONICS VCX750 ultrasonic equipment. We usually use the following process to disperse CNTs. When placing an order through Cheap Tubes Inc, Cheap Tubes Inc customers can enjoy a 5% discount on the ultrasound system Sonics & Materials Line.

The solution is composed of MWNT, PVP and water in a ratio of 10 parts CNT: ~1-2 parts PVP: 2,000 parts water or other solvents. The required dispersion (ultrasonic treatment) time is approximately 2 to 8 minutes, with 10 seconds interruption at full or high amplitude every 30 seconds. If the power of your ultrasonic device is less than that of the SONICS VCX750 device, then you must extend the ultrasonic processing time accordingly. For dispersive SWNT, we recommend continuous sonication at 40% amplitude for 20 minutes. We found that in order to effectively disperse SWNT, we need to set the amplitude at 40% for a longer period of time to break the van der Waals physical bond, so that SWNT can be clustered. Because single-walled carbon nanotubes are such small particles, the aggregated bundles are more difficult to disperse. We also recommend using a magnetic stirrer for mechanical stirring to assist the dispersion process.

Although both probe-type and bath-type ultrasonic systems can be used to disperse CNTs, it is generally believed that probe-type ultrasonic systems are more suitable for dispersing CNTs. As we all know, adding a dispersant (surfactant) to the solution will accelerate the dispersion effect and help maintain the good separation of carbon nanotubes. The reagent polyvinylpyrrolidone (PVP) is a good dispersant for our CNTs. Some people like other surfactants, such as sodium dodecylbenzene sulfonate (SDBS), polyvinyl alcohol (PVA) or Triton X100. We have found that when using different solvents, the dispersants and ratios listed above do indeed change. When trying to disperse CNTs, please note that your chemical reaction will be different. Usually, we disperse COOH-functionalized CNTs in IPA or acetone, and OH in ethanol, then purify our standards into deionized water, and use the ultrasonic process detailed above. According to our experience, the reagents used to disperse in deionized water are much less than other solvents. We believe this is due to the higher polarity of water compared to other solvents. Generally, achieving stable dispersion is a matter of chemistry, proper ratio, and sonication time. Stable dispersion will last for days, weeks or months with almost no settlement.

In some applications, achieving stable dispersion may require other reagents in the solution to prevent CNTs from falling out of the solution over time. The emulsifier T-60 (also known as Tween 60) is usually used with dihydrate or isopropanol. Organic titanates can be used with acetone and xylene. The specific application determines whether these reagents remain in solution during further processing, or whether they need to be removed. By heating the solution to above 2500°C, some organic titanate can be removed. The addition of OH and COOH functional groups facilitates the dispersion of carbon nanotubes and the chemical bonding with other materials during further processing.

Unfortunately, pristine nanotubes are insoluble in many liquids, such as water, polymer resins, and most solvents. Therefore, they are difficult to disperse uniformly in liquid matrices (for example, epoxy resins and other polymers). This complicates the efforts to utilize the excellent physical properties of nanotubes in composite material manufacturing and other practical applications (biology, optics, magnetism, etc.). These applications require the preparation of carbon nanotubes and many different organic, inorganic, and polymers. Material.

In order to make the nanotubes easier to disperse in the liquid, certain molecules (functional groups) must be physically or chemically attached to their smooth sidewalls without significantly changing the desired properties of the nanotubes. This process is called functionalization.

The production of strong composite materials requires a strong covalent chemical bond between the filler particles (CNT) and the polymer matrix, rather than the weak van der Waals physical bond that occurs when the CNT is not properly functionalized.

The functionalization of carbon nanotubes in some polymers, such as chopped, oxidized, and "wrapped", can generate more active binding sites on the surface of the nanotubes.

For biological use, carbon nanotubes can be functionalized by attaching biomolecules (such as lipids, proteins, biotin, etc.) to them. Then they can effectively mimic certain biological functions, such as protein adsorption and binding to DNA and drug molecules. This will enable important medical and commercial applications such as gene therapy and drug delivery.

In biochemical and chemical applications (for example, the development of very specific biosensors), molecules such as carboxylic acid (COOH), poly-m-aminobenzoic acid (PABS), polyimide, and polyvinyl alcohol (PVA) have been used For functionalized carbon nanotubes, amino acid derivatives, halogens and compounds such as potassium permanganate. Poly(acrylic acid) functionalized carbon nanotubes are soluble in water and other highly polar aqueous solvents, such as DMSO

The thermal conductivity of mwnts is about 2000W/m·k, and the thermal conductivity of swnts is about 4000W/m·k.

Use about 150 mg of OH and COOH functionalized carbon nanotubes in about 150-200 ml of deionized water. After stabilization, the pH of OH is 3.8, and the COOH is 4.2.

The theoretical specific surface area of ​​swnts is about 1300m2/g. We tested the SSA of our carbon nanotubes by nitrogen adsorption method. As many of you know, SWNT is control. Therefore, there are many places where nitrogen cannot be adsorbed. Therefore, the test data is usually lower than expected, and we specify the SSA of SWNT as 407 m2/g.

Aspect ratio is a measure of length multiplied by diameter. Compared with traditional additive materials such as carbon black, chopped carbon fiber or stainless steel fiber, the aspect ratio of carbon nanotubes is very high, about 1000:1, which enables carbon nanotubes to impart conductivity under lower loads.

Yes, we can sell a small amount of carbon nanotubes dispersed in deionized water and isopropanol. Please call or email us for more information. Other dispersions will be available soon.

Compared with other conductive or reinforcing materials, one of the many advantages of carbon nanotubes is the low loading percentage required to achieve typical results. Most companies/researchers start with a loading rate of 1-3%, and then adjust it up or down based on the desired result and the actual result.

Currently, Cheap Tubes is finalizing an agreement with a well-known and respected fullerene manufacturer. We can currently supply fullerenes, but there is a delivery date, and they are not shipped from stock.

This information is derived from, reviewed, and adapted from materials provided by cheap management companies.

For more information on this source, please visit Cheap Tube Company.

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