What Are the Different Types of Carbon Nanotubes?

Carbon nanotubes, as one-dimensional nanomaterials, are lightweight and have a hexagonal structure with perfect connections. They have many unusual mechanical, electrical, and chemical properties. In recent years, with the deepening of the research on carbon nanotubes and nanomaterials, its broad application prospects have been continuously shown.

Carbon nanotubes, also known as bucky tubes, are one-dimensional quantum materials with special structures (radial dimensions are on the order of nanometers, axial dimensions are on the order of micrometers, and both ends of the tube are basically sealed). Carbon nanotubes are mainly coaxial tubes composed of carbon atoms arranged in hexagons, ranging from several layers to tens of layers. A fixed distance is maintained between the layers, about 0.34 nm, and the diameter is generally 2-20 nm. And according to the different orientation of the carbon hexagon in the axial direction, it can be divided into three types: zigzag, armchair and spiral. The spiral carbon nanotubes have chirality, while the zigzag and armchair carbon nanotubes have no chirality.

On October 27, 2017, the list of carcinogens published by the International Agency for Research on Cancer of the World Health Organization initially compiled references. Carbon nanotubes and multi-walled MWCNT-7 were on the list of 2B carcinogens. ^[1]

Chinese name: Carbon nanotubes
English name: carbon nanotubes
nickname: Bucky tube
Melting point: Expected 3652-3697
Boiling point: Not determined
Density: 2.1 g / cm3 at 20 ° C
Exterior: powder
Flash point: Not applicable

Application: Composite materials, electronic devices, fluorescent marking
Risk description: There is no danger of explosion from this product
colour: black
Smell: Tasteless
Sublimation temperature: Not determined
Compatible solubility: Have
steam pressure: Not determined
Ignition temperature: Not determined
Decomposition temperature: Not determined

Carbon nanotube discovery history

Carbon nanotubes In 1985, the discovery of the "C ₆₀ " structure of the "football" attracted worldwide attention. Kroto HW, Smalley RE, and Curl RF also won the 1996 Nobel Prize in Chemistry for co-discovering C ₆₀ and confirming and confirming its structure. Driven by fullerene research, a more exotic carbon structure, carbon nanotubes, was discovered by Dr. Iijima of the Japan Electronics Corporation (NEC) in 1991.

Before being officially recognized and named in 1991, carbon nanotubes have been discovered and manufactured in some studies, but it was not recognized at that time as a new and important form of carbon. It was discovered in 1890 that carbon-containing gases could decompose on hot surfaces to form filamentous carbon. In 1953, when CO and Fe ₃ O ₄ reacted at high temperature, a filamentous structure similar to carbon nanotubes was also found. Since the 1950s, the problem of carbon deposition in petrochemical plants and cold nuclear reactors, that is, the accumulation of carbon filaments, has gradually attracted attention. In order to suppress its growth, many studies have been conducted on its growth mechanism. These carbon filaments obtained by catalytic pyrolysis of organic matter have been found to have a structure similar to carbon nanotubes. In the late 1970s, New Zealand scientists discovered that when a spark was generated when two graphite electrodes were energized to generate electric sparks, small fiber clusters were formed on the electrode surface. Electron diffraction measurements revealed that the walls were composed of graphite-like carbon. To multi-walled carbon nanotubes.

Carbon nanotube structure characteristics

Carbon nanotubes

The carbon atoms in carbon nanotubes are mainly sp ² hybrids. At the same time, the hexagonal grid structure is bent to a certain extent to form a spatial topology. Certain sp ³ hybrid bonds can be formed, that is, the chemical bonds formed have sp ² Mixed with sp ³ , and these p orbitals overlap each other outside the carbon nanotube graphene sheet to form highly delocalized large bonds. The large bonds on the outer surface of carbon nanotubes are carbon nanotubes and some have The chemical basis of conjugated macromolecules recombination with non-covalent bonds.

The results of photoelectron spectroscopy studies of multi-walled carbon nanotubes show that no matter whether single-walled carbon nanotubes or multi-walled carbon nanotubes are bound to certain functional groups on the surface, and the carbon nanotubes obtained by different preparation methods are due to preparation methods Different, different post-processing processes have different surface structures. Generally speaking, single-walled carbon nanotubes are relatively chemically inert, and their surfaces are more pure, while multi-walled carbon nanotubes are much more active and have a large number of surface groups, such as carboxyl groups. The surface inspection of carbon nanotubes with variable angle X-ray electron spectroscopy shows that the surface of single-walled carbon nanotubes is chemically inert and the chemical structure is relatively simple. As the number of carbon nanotube wall layers increases, defects and chemical reactivity Reinforced, the surface chemical structure tends to be complicated. The chemical structure of the inner carbon atoms is relatively single, the chemical composition of the outer carbon atoms is more complex, and a large amount of amorphous carbon is often deposited on the outer carbon atoms. Due to the heterogeneity of the physical structure and the chemical structure, a large number of surface carbon atoms in the carbon nanotubes have different surface microenvironments, and therefore also have energy heterogeneity.

Carbon nanotubes are not always straight, but convex and concave phenomena appear in local areas, which are due to the appearance of pentagons and heptagons during the preparation of hexagons. If the pentagon appears exactly at the top of the carbon nanotube, a seal of the carbon nanotube is formed. The nanotubes are recessed when heptagons appear. These topological defects can change the helical structure of the carbon nanotubes, and the electronic band structure near the occurrence of the defects will also change. In addition, two adjacent carbon nanotubes are not directly adhered to each other, but a certain distance is maintained.

Carbon nanotube classification

Carbon nanotubes can be seen as graphene sheet layers rolled, so according to the graphene sheet layer

Defective carbon nanotubes The numbers can be divided into: single-walled carbon nanotubes (or single-walled carbon nanotubes, SWCNTs) and multi-walled carbon nanotubes (or multi-walled carbon nanotubes, MWCNTs), When multi-walled pipes begin to form, layers between layers can easily become trap centers and trap various defects, so the walls of multi-walled pipes are usually covered with small hole-like defects. Compared with multi-walled pipes, the diameter of single-walled pipes has a smaller distribution range, fewer defects, and higher uniformity. The single-walled tube typically has a diameter of 0.6-2nm, the innermost layer of a multi-walled tube can reach 0.4nm, and the thickest can reach hundreds of nanometers, but the typical tube diameter is 2-100nm.

Carbon nanotubes can be divided into three types based on their structural characteristics: armchair-shaped nanotubes (zigzag forms) and chiral nanotubes (chiral forms). The chirality index (n, m) of carbon nanotubes is directly related to their helicity and electrical properties, and n> = m is customary. When n = m, carbon nanotubes are called armchair-shaped nanotubes, and the chiral angle (spiral angle) is 30 ^° ; when n> m = 0, carbon nanotubes are called zigzag nanotubes, and the chiral angle ( Helix angle) is 0 ^o ; when n> m 0, it is called chiral carbon nanotube.

Defective carbon nanotubes Carbon nanotubes can be divided into metal-type carbon nanotubes and semiconductor-type carbon nanotubes according to their conductive properties: when nm = 3k (k is an integer), carbon nanotubes are metal-type; when nm = 3k ± 1, carbon The nanotube is a semiconductor type.

According to whether it contains tube wall defects, it can be divided into: perfect carbon nanotubes and defective carbon nanotubes.

According to the uniformity of the shape and the overall shape, it can be divided into: straight tube type, carbon nanotube bundle, Y type, snake type and so on.

The influence of tube wall defects on the mechanical properties of carbon nanotubes is also worthy of attention, which will also help to further understand carbon nanotubes and their composite materials. Due to the limitations of the carbon nanotube manufacturing process, carbon nanotubes contain a large number of various defects, such as atomic vacancy defects (monoatomic or polyatomic vacancies) and Stone-Thrower-Wales (STW) type defects. See below.

Carbon nanotube properties

Carbon nanotube mechanics

Because carbon atoms in carbon nanotubes adopt SP2 hybridization, compared to SP3 hybridization, the S orbital component in SP2 hybridization is relatively large, which makes carbon nanotubes have high modulus and high strength.

Carbon nanotubes have good mechanical properties. The tensile strength of CNTs reaches 50 to 200 GPa, which is 100 times that of steel, but the density is only 1/6 of steel, at least an order of magnitude higher than conventional graphite fibers. Its elastic modulus can reach 1TPa , Equivalent to the elastic modulus of diamond, about 5 times that of steel. For a single-walled carbon nanotube with an ideal structure, its tensile strength is about 800 GPa. Although the structure of carbon nanotubes is similar to that of polymer materials, its structure is much more stable than that of polymer materials. Carbon nanotubes are currently the materials with the highest specific strength. If other engineering materials are used as the matrix and carbon nanotubes to make composite materials, the composite materials can exhibit good strength, elasticity, fatigue resistance and isotropy, which will greatly improve the performance of composite materials.

Carbon nanotubes have the same hardness as diamond, but they have good flexibility and can be stretched. In reinforced fibers commonly used in the industry, a key factor determining strength is the aspect ratio, which is the ratio of length to diameter. Materials engineers want an aspect ratio of at least 20: 1, and carbon nanotubes generally have an aspect ratio of more than 1000: 1, making them ideal fiber materials with high strength. In October 2000, researchers at Penn State University claimed that the strength of carbon nanotubes was 100 times stronger than that of steel of the same volume, but the weight was only 1/6 to 1/7 of the latter. Carbon nanotubes are therefore called "super fibers".

Researchers at Moscow University once placed carbon nanotubes at a water pressure of 1011 MPa (equivalent to a pressure of 10,000 meters deep), and the carbon nanotubes were squashed due to the huge pressure. After the pressure was removed, the carbon nanotubes immediately regained their shape like a spring and showed good toughness. This suggests that people can use carbon nanotubes to make light and thin springs and use them as shock absorption devices in cars and trains, which can greatly reduce weight.

In addition, the melting point of carbon nanotubes is the highest among known materials.

Carbon nanotubes

Carbon nanotubes The P electrons of carbon atoms on carbon nanotubes form a wide range of delocalized bonds. Due to the significant conjugation effect, carbon nanotubes have some special electrical properties.

Carbon nanotubes have good electrical conductivity. Because the structure of carbon nanotubes is the same as the lamellar structure of graphite, they have good electrical properties. The theoretical prediction of its conductivity depends on its pipe diameter and the helix angle of the pipe wall. When the diameter of CNTs is greater than 6nm, the conductivity decreases. When the diameter of CNTs is less than 6nm, CNTs can be regarded as one-dimensional quantum wires with good conductivity. It is reported that Huang calculated that carbon nanotubes with a diameter of 0.7 nm are superconducting. Although their superconducting transition temperature is only 1.5 × 10-4K, it indicates the application prospect of carbon nanotubes in the superconducting field.

The common vector Ch represents the direction of the arrangement of the atoms on the carbon nanotube, where Ch = na ₁ + ma ₂ , denoted as (n, m). a ₁ and a ₂ represent two basis vectors, respectively. (N, m) is closely related to the conductivity of carbon nanotubes. For a given (n, m) nanotube, if there is 2n + m = 3q (q is an integer), then this direction shows metallicity and is a good conductor, otherwise it behaves as a semiconductor. For the direction of n = m, carbon nanotubes show good electrical conductivity, and the electrical conductivity is usually up to 10,000 times that of copper.

Carbon nanotube heat transfer

Carbon nanotubes have good heat transfer properties, and CNTs have a very large aspect ratio, so their heat exchange performance along the length direction is high, and their heat exchange performance is relatively low in the vertical direction. With proper orientation, carbon Nanotubes can synthesize highly anisotropic thermally conductive materials. In addition, carbon nanotubes have high thermal conductivity. As long as a small amount of carbon nanotubes are doped in the composite material, the thermal conductivity of the composite material may be greatly improved.

Carbon nanotube other

Carbon nanotubes also have other good properties such as optics.

Preparation of carbon nanotubes

Commonly used carbon nanotube preparation methods are: arc discharge method, laser ablation method, chemical vapor deposition method (hydrocarbon gas pyrolysis method), solid phase pyrolysis method, glow discharge method, gas combustion method and polymerization reaction synthesis Law, etc.

Carbon nanotube arc discharge

Preparation of carbon nanotubes The arc discharge method is the main method for producing carbon nanotubes. Japanese physicist Masao Iijima first discovered carbon nanotubes from carbon fibers produced by the arc discharge method in 1991. The specific process of the arc discharge method is: placing a graphite electrode in a reaction container filled with helium or argon, and an arc is excited between the two electrodes, and the temperature can reach about 4000 degrees at this time. Under these conditions, graphite will evaporate to produce products such as fullerene (C60), amorphous carbon, and single-walled or multi-walled carbon nanotubes. By controlling the hydrogen content in the catalyst and vessel, the relative yields of several products can be adjusted. Using this method to prepare carbon nanotubes is relatively simple in technology, but the resulting carbon nanotubes are mixed with products such as C60, and it is difficult to obtain carbon nanotubes with higher purity, and often the multilayer carbon nanotubes are obtained. In actual research, people often need single-walled carbon nanotubes. In addition, the method consumes too much energy. Some researchers have found that if molten lithium chloride is used as the anode, the energy consumed in the reaction can be effectively reduced, and product purification is easier.

The chemical vapor deposition method, or hydrocarbon gas pyrolysis method, was developed to overcome the shortcomings of the arc discharge method to a certain extent. This method allows gaseous hydrocarbons to pass through a template with catalyst particles attached. At 800-1200 degrees, gaseous hydrocarbons can be decomposed to form carbon nanotubes. The outstanding advantage of this method is that the residual reactants are gases and can leave the reaction system to obtain carbon nanotubes with relatively high purity. At the same time, the temperature does not need to be very high, which relatively saves energy. However, the prepared carbon nanotubes have irregular diameters and irregular shapes, and a catalyst must be used in the preparation process. The main research direction of this method is to control the structure of the generated carbon nanotubes by controlling the arrangement of the catalyst on the template, and certain progress has been made.

Carbon nanotube laser ablation

The specific process of the laser ablation method is: a metal catalyst / graphite mixed graphite target is placed in the middle of a long quartz tube, and the tube is placed in a heating furnace. When the furnace temperature rises to a certain temperature, the inert gas is flushed into the tube and a laser beam is focused on the graphite target. Gaseous carbon is generated under laser irradiation. When these gaseous carbon and catalyst particles are brought from a high temperature region to a low temperature region by a gas flow, they grow into CNTs under the action of a catalyst.

Carbon nanotube solid-phase pyrolysis

In addition, there are methods such as solid phase pyrolysis. The solid-phase pyrolysis method is a new method for growing carbon nanotubes by pyrolysis of conventional carbon-containing metastable solids at high temperature. This method is relatively stable, requires no catalyst, and grows in situ. However, due to the limitation of raw materials, production cannot be scaled up and continuous.

Carbon nanotube ion or laser sputtering

There are also ion or laser sputtering methods. Although this method is easy for continuous production, its scale is limited due to equipment reasons.

Synthesis of carbon nanotube polymerization

In the method of preparing carbon nanotubes, the polymerization synthesis method generally refers to a method using template replication and amplification.

The general preparation process of carbon nanotubes is similar to that of organic synthesis, and its side reactions are complex and diverse. It is difficult to ensure that the carbon nanotubes in the same furnace are armchair nanotubes or zigzag nanotubes. Scientists have found that under the action of strong acids and ultrasonic waves, carbon nanotubes can be broken into several segments, and then proliferated and extended under the action of certain nano-scale catalyst particles, and the carbon nanotubes obtained after stretching are rolled in the same manner as the template.

So scientists envisage that if carbon nanotubes are propagated in a manner similar to DNA amplification, only a small number of armchair-type nanotubes or zigzag nanotubes can be found and copied and amplified in a short time Carbon nanotubes of the same type, millions of times the number of templates. This may become a new way to prepare high-purity carbon nanotubes.

Carbon nanotube catalytic cracking

The catalytic cracking method is a method for preparing carbon nanotubes by decomposing carbon-containing gas raw materials (such as carbon monoxide, methane, ethylene, propylene, and benzene, etc.) at a temperature of 600 to 1000 ° C and a catalyst. This method cracks a carbon-containing compound into carbon atoms at a relatively high temperature, and the carbon atoms are attached to the surface of catalyst particles to form carbon nanotubes under the action of a transition metal-catalyst. The active component of the catalyst used in the catalytic cracking method is mostly a Group VIII transition metal or its alloy. A small amount of Cu, Zn, Mg and other additives can adjust the energy state of the active metal and change its ability to chemically adsorb and decompose carbon-containing gases. Catalyst precursors have an effect on the activity of forming metal elements, and metal oxides, sulfides, carbides, and organometallic compounds have also been used.

Health effects of carbon nanotubes

Adverse effects on people

Eye contact: May cause eye discomfort.
Skin contact: In 2012 it was not fully understood whether the penetration of nanoparticles from the skin would adversely affect the human body. However, topical application of single-walled carbon nanotubes to nude mice has been shown to cause skin irritation. Experiments using human skin cells cultured in vitro show that these two single-walled carbon nanotubes and multi-walled carbon nanotubes can enter cells, causing pro-release, inflammatory cytokines, oxidative stress, and reducing cell viability.
Air inhalation: May cause lung cancer formation, pneumoconiosis, granulomas, or mesothelioma.
Ingestion: Intestinal irritation, related experiments are insufficient.

Adverse effects on aquatic life

On August 24, 2012, a study completed by the University of Missouri and the United States Geological Survey showed that carbon nanotubes are toxic to certain aquatic life. Carbon nanotubes are not pure carbon. Nickel, chromium, and other metals used in the production process will remain as impurities. These residual metals and carbon nanotubes can slow the growth rate and even cause death of certain types of aquatic life. Professor Deng Baolin of the University of Missouri said that the future development of carbon nanotubes must be carefully and preparedly weighed. Its impact on the environment and human health is not fully understood, and it should be prevented from entering the environment as a large-scale production material. ^[2]

Application prospects of carbon nanotubes

Carbon nanotubes can be made into transparent and conductive thin films to replace ITO (indium tin oxide) as a material for touch screens. In the previous technology, scientists used powdered carbon nanotubes to prepare a solution and directly coated it on PET or glass substrates, but such technology has not yet entered the mass production stage; currently, successful mass production is by using super-sequence Carbon nanotube technology; this technology is to directly extract a thin film from a super-sequential array of carbon nanotubes and spread it on a substrate to make a transparent conductive film, just like pulling a yarn from a sliver. The core of the technology, the super-sequential carbon nanotube array, is a new material first discovered by Beijing Tsinghua-Foxconn Nano Center in 2002. ^[3]

Carbon nanotube touch screens were successfully developed for the first time between 2007 and 2008, and were industrialized by Tianjin Funa Yuanchuang in 2011. So far, many smart phones have used touch screens made of carbon nanotube materials. The difference from the existing indium tin oxide (ITO) touch screen is that indium tin oxide contains the rare metal "indium", and the raw material of the carbon nanotube touch screen is hydrocarbon gas such as methane, ethylene, acetylene, etc., which is not limited by the rare mineral resources ; Secondly, the carbon nanotube film made by the film-laying method has a conductive anisotropy, just like a natural built-in pattern, and does not require photolithography, etching, and water washing processes, saving a large amount of hydropower, and is more environmentally friendly and energy-saving. Engineers have also developed positioning technology that uses the conductive anisotropy of carbon nanotubes. The X and Y coordinates of the touch point can be determined using only a single layer of carbon nanotube film. The carbon nanotube touch screen is also flexible, anti-interference, waterproof, and resistant. Tapping and scratching characteristics can be used to make curved touch screens with high potential for applications such as wearable devices and smart furniture. ^[4]

According to a report by the physicist organization network and the BBC on September 26, 2013, engineers at Stanford University in the United States have made breakthrough progress in the field of next-generation electronic equipment. Computers are smaller, faster, and more energy efficient.

Professor Giovanni de Mikkeli, director of the School of Electrical Engineering of the Federal Institute of Technology in Lausanne, Switzerland, emphasized two key technical contributions to this worldwide achievement: First, the carbon nanotube circuit-based manufacturing process was put in place. Secondly, a simple and effective circuit was established, showing that calculations using carbon nanotubes are feasible. The next-generation chip design research alliance, Professor Naresh of the University of Illinois at Urbana-Champaign, commented that although carbon nanotube computers may take several years to mature, this breakthrough has highlighted the future of carbon nanotube semiconductors. Possibility of industrial scale production. ^[5]

Hydrogen is considered by many to be the clean energy of the future. However, hydrogen itself has a low density, and it is very inconvenient to compress it into a liquid for storage. Carbon nanotubes are lightweight and have a hollow structure. They can be used as excellent containers for storing hydrogen. The density of stored hydrogen is even higher than the density of liquid or solid hydrogen. With proper heating, hydrogen can be slowly released. Researchers are trying to make lightweight portable hydrogen storage containers from carbon nanotubes.

The carbon nanotubes can be filled with metals, oxides and other materials. In this way, carbon nanotubes can be used as molds. First, carbon nanotubes can be filled with metals and other materials, and then the carbon layer can be etched to produce the finest nanoscale. Wires, or new one-dimensional materials, will be used in future molecular electronics or nanoelectronic devices. Some carbon nanotubes can also be used as nano-scale wires. In this way, micro-wires prepared using carbon nanotubes or related technologies can be placed on silicon chips to produce more complex circuits.

The properties of carbon nanotubes can be used to make many composites with excellent properties. For example, plastics reinforced with carbon nanotube materials have excellent mechanical properties, good electrical conductivity, corrosion resistance, and shielding of radio waves. The carbon nanotube composite material using cement as a matrix has good impact resistance, antistatic, abrasion resistance, high stability, and is not easy to affect the environment. Carbon nanotube reinforced ceramic composites have high strength and good impact resistance. Due to the defect of the five-membered ring on the carbon nanotube, the reactivity is enhanced. Under the conditions of high temperature and the presence of other substances, the carbon nanotube is easy to open at the end face, forming a tube, which is easily wetted by metal and forms metal with metal. Matrix composites. Such materials have high strength, high modulus, high temperature resistance, small thermal expansion coefficient, and strong resistance to thermal change.

Carbon nanotubes also provide physicists with the finest capillaries for studying the mechanism of capillary phenomena, and chemists with the finest test tubes for nanochemical reactions. The extremely small particles on carbon nanotubes can cause the frequency of the carbon nanotubes to change in the current. Using this, in 1999, Brazilian and American scientists invented a "nanoscale" with an accuracy of 10-17kg, which can be weighed The quality of individual viruses. Subsequently, German scientists developed a "nanoscale" capable of weighing individual atoms.

Carbon nanotube dispersant introduction and use recommendations

Taking the carbon nanotubes and carbon nanotube dispersants of Wuxi Juwang Plasticizing Material Co., Ltd. as examples, the research and practical experience are as follows:

1. Three elements of carbon nanotube dispersion technology

Recommended dosage of dispersant

3. Overview of Carbon Nanotube Dispersant (TNWDIS)

Fourth, the use of ultrasonic dispersion equipment recommendations and dispersion examples

V. Suggestions on the use of grinding and dispersing equipment

Three elements of carbon nanotube dispersion technology: dispersion medium, dispersant and dispersion equipment

1.Dispersion medium

(1) According to the viscosity, the dispersion medium is divided into three types of high viscosity, medium viscosity and low viscosity. In low-viscosity media, such as water and organic solvents, carbon nanotubes are easily dispersed. Medium viscosity medium such as liquid epoxy resin, liquid silicone rubber, etc., high viscosity medium such as molten plastic.

(2) The carbon nanotube dispersion technology introduced here is aimed at medium and low viscosity dispersion media.

2.Dispersant

(1) The choice of dispersant is closely related to the structure, polarity, and solubility parameters of the dispersion medium.

(2) The amount of dispersant is related to the specific surface area of carbon nanotubes and functional groups modified by covalent bonds.

(3) In aqueous media, TNWDIS is recommended. In strongly polar organic solvents, such as alcohol, DMF, NMP, TNADIS is recommended. For medium polar organic solvents such as esters, liquid epoxy resins, and liquid silicone rubbers, TNEDIS is recommended.

3. Decentralized equipment

(1) Ultrasonic dispersion equipment: It is very suitable for laboratory-scale, low-viscosity media to disperse carbon nanotubes, and it will be limited when used in medium and high-viscosity media.

(2) Grinding and dispersing equipment: suitable for large-scale dispersion of carbon nanotubes and medium-viscosity media for dispersing carbon nanotubes.

(3) The combined method of "first grinding dispersion, then ultrasonic dispersion" can efficiently and stably disperse carbon nanotubes.

Recommended dosage of dispersant.

1.Specific surface area of carbon nanotubes and the amount of dispersant

Our reagent grade carbon nanotubes are divided into single-walled tubes (outer diameter <2nm) and multi-walled tubes. Multi-wall tubes are divided into TNM1 (outer diameter 50nm) according to their outer diameter. As the outer diameter increases, the specific surface area of carbon nanotubes decreases

The recommended dosage of TNWDIS: 3.5 times the weight of single wall tube, 1.0 times the weight of TNM1, and 0.2 times the weight of TNM8. Reference adjustment for the rest

2. Functionalization of carbon nanotubes and dosage of dispersant

Functionalized carbon nanotubes are easier to disperse in water. Generally, after functionalizing the carboxyl group of carbon nanotubes, the amount of dispersant can be reduced by 50%

Recommended dosage of TNWDIS: 1.5-1.8 times the weight of carboxylated single-wall tube, 0.5 times the weight of carboxylated TNM1, and 0.1 times the weight of carboxylated TNM8

3. For TNADIS, the recommended dosage of TNM8 is 0.2 times the weight. For TNEDIS, the recommended dosage of TNM8 is 0.8 times the weight

The amount of other carbon nanotube dispersants can be adjusted by reference.

Carbon Nanotube Dispersant (TNWDIS) Overview

1. Non-ionic surfactant containing no alkylphenol polyoxyethylene ether (APEO), ecological and environmental protection. European countries have successively formulated regulations restricting production and use of APEO since 1976

2. Contains aromatic groups, which is especially suitable for preparing carbon nanotube aqueous dispersions. Aromatic groups have good affinity with the wall of carbon nanotubes and are easily adsorbed on the wall

3. Performance indicators

Active substance content: 90%

Moisture content: 10%

Cloud point: 68-70 ° C

Carbon Nanotube Dispersant (TNWDIS) Structure

Three kinds of surfactants commonly used in dispersing CNTs are reported in the literature

Recommendations for the use of ultrasonic dispersion equipment

1. Both ultrasonic pulverizer (tip type) and ultrasonic cleaner (bath type) can be used for carbon nanotube dispersion

2. The ultrasonic energy emitted by the ultrasonic pulverizer has a high energy density (the energy is concentrated on the horn rather than a plane) and a low frequency, which is more suitable for the dispersion of carbon nanotubes. According to the amount of carbon nanotube dispersion, select the appropriate ultrasonic crusher power and horn diameter

3. In the aqueous medium, the cavitation of ultrasonic waves will cause a small amount of foam in TNWDIS. The foam will affect the ultrasonic effect. You can choose to stand or add a defoamer to eliminate the foam.

High-viscosity media are not suitable for dispersing with ultrasonic equipment. It is recommended to choose grinding and dispersing equipment.

Example of preparing dispersion by ultrasonic pulverizer

1. Objective: To prepare 100g of multi-walled carbon nanotubes TNM8 aqueous dispersion with a carbon nanotube content of 2%

2. Main equipment

(1) Scientz-D ultrasonic cell pulverizer (made in China). The ultrasonic horn used is 6, the output power is selected as 60%, the ultrasonic on time is 3s, the ultrasonic off time is also 3s, and the total ultrasonic time is set to 5min.

(2) SC-3614 low speed centrifuge (domestic)

(3) HCT-1 microcomputer differential thermal balance (domestic)

Operation steps (1)

1. Dissolve 0.40g of dispersant TNWDIS in 97.60g of deionized water. TNWDIS has low solubility at room temperature. It can be heated with a water bath to aid its dissolution, but the use temperature must not exceed its cloud point temperature.

2.Add 2.00g of carbon nanotubes and stir to make the carbon nanotubes completely wet by the dispersant aqueous solution instead of floating on the water surface.

3. Start ultrasound. During the ultrasonic process, the dispersion will generate heat and foam. Therefore, it is recommended that after 5 minutes of ultrasound, the dispersion can be taken out and left to cool in ice water to defoam, and then continue the ultrasound

4. Observe the degree of dispersion. A small amount of the dispersion liquid was dripped with a glass rod and added to clear water, and the dilution state was observed. Dispersed carbon nanotubes, like a drop of ink falling into water, spread rapidly and uniformly in water, while undispersed carbon nanotubes will have black particles in water. The total cumulative ultrasound time is 30min (ie 5min × 6 times)

5. After the sonication is finished, the dispersion is centrifuged to settle to remove undispersed agglomerated particles. The centrifugation rate is 2000r / min and the centrifugation time is 30min. After centrifugation, the dispersion can be stable for more than half a year

6. After centrifugation, pass the upper layer of liquid through a 300-mesh filter cloth to obtain the final carbon nanotube dispersion. Dry the lower layer to a constant weight and record it as G2. The thermogravimetric analysis was performed on the precipitate. The thermal weight loss rate f (%) at 450 ° C was defined as the dispersant content in the precipitate.

7. The actual content of carbon nanotubes in the dispersion (%) = 2.00- (1-f) × G ₂

Recommendations for the use of grinding and dispersing equipment

1. To prepare 1-2 liters of carbon nanotube water dispersion, laboratory dispersion sand mill can be used, and sand grinding medium can be 1.0-1.2mm zirconium silicate beads or zirconia beads

2. To prepare a 10-20 liter carbon nanotube dispersion, a small basket sand mill can be used. For sanding medium, choose the smaller diameter zirconia beads or zirconia beads allowed by the equipment

3. In the sanding process of aqueous medium, it is necessary to add a defoamer to reduce the effect of foam on the dispersion effect.

4. For medium-viscosity dispersion media, such as liquid epoxy resin, the sand mill cannot drive the media to move effectively. You can choose a cone mill or a three-roller mill to grind and disperse.

Development history of carbon nanotubes

In 1991, Lijima, an electron microscope expert at the Basic Research Laboratory of NEC Corporation of Japan, accidentally discovered tubular carbon nanotubes composed of coaxial coaxial nanotubes when examining spherical carbon molecules produced in graphite arc equipment under a high-resolution transmission electron microscope Carbon molecule, which is now called "Carbon nanotube", that is, carbon nanotube, also known as bucky tube.

In 1993, S. Lijima et al. And DS Bethune et al. Simultaneously reported that by using an arc method, a certain amount of catalyst was added to a graphite electrode to obtain carbon nanotubes with only one layer of tube wall, that is, single-walled carbon nanotube products.

In 1997, AC Dillon and others reported that single-walled carbon nanotube hollow tubes can store and stabilize hydrogen molecules, which has attracted widespread attention. Relevant experimental research and theoretical calculations have also started. It is speculated that the hydrogen storage capacity of single-walled carbon nanotubes can reach 10% (mass ratio). In addition, carbon nanotubes can also be used to store other gases such as methane.

Carbon nanotubes cannot be used for hydrogen storage. There are two main problems: first, if hydrogen storage is used as a container, they cannot be closed and opened in a controlled manner; second, if used for hydrogen adsorption, its adsorption rate Not more than 1% (mass fraction). A 1999 article on "High H ₂ uptake by alkali-doped carbon nanotubes under ambient pressure and moderate temperatures" in Science said that Li-doped CNTs can be used to adsorb up to 20% of hydrogen, which was given by Ralph Yang the following year. Refuting "Hydrogen storage by alkali-doped carbon nanotubes-revised" says that the adsorption is all water. Another article on "Storage of hydrogen in single-walled carbon nanotubes" in "NATURE" in 1997 was even criticized for being incomplete. After more than a decade of research, NSF, DOE, and GM finally concluded that using carbon nanotubes to store hydrogen is ridiculous. It is not used to do this, please everyone to spare it.

Can I control the growth of single-walled carbon nanotubes? For more than two decades, scientists who have been plagued by the research field of carbon nanotubes, whether they can find a control method has also become a bottleneck for the application of carbon nanotubes. Recently, this worldwide problem was overcome by the research team of Professor Li Yan of Peking University. The team proposed the control method of the growth law of single-walled carbon nanotubes for the first time in the world. In the journal Nature ^[6] .

Potential environmental risks of carbon nanotubes

Carbon nanotubes have a strong adsorption capacity for coexisting pollutants, especially organic pollutants, due to their huge surface area and surface hydrophobicity. The adsorption of carbon nanotubes on pollutants will not only change the environmental behavior of the pollutants, but also affect its own environmental behavior. Therefore, due to the large number of applications in the environment, the environmental risks of carbon nanotubes that are widely present in the environment should be paid attention to.