Carbon nanotubes: polymer nanomaterials with stable performance
The most prominent structural feature of carbon nanotubes is that they are formed by curling a single layer or multiple layers of graphite sheets around the same center. Carbon nanotubes, abbreviated as CNT in English, belong to the fuller carbon series, with a length of micrometers and a diameter of nanometers, the most characteristic one-dimensional nanomaterial. On a macroscopic scale, carbon nanotubes are black powder, and on a microscopic scale, carbon nanotubes are carbon molecules composed of coaxial carbon tubes. Each layer of carbon tubes is densely packed with carbon atoms in hexagons, similar to the layered structure of graphene, with a fixed distance of about 0.34nm between layers. Although the structure of carbon nanotubes is similar to that of polymer materials, its structure is much more stable than polymer materials, and it is the material with the highest melting point currently known.
Carbon nanotubes are divided into different categories according to different characteristics, and the commercialization angle is usually classified according to the number of layers of the tube wall and the conductivity. According to the number of layers of carbon tubes, carbon nanotubes can be divided into single-wall carbon nanotubes and multi-wall carbon nanotubes; in terms of their conductivity, carbon nanotubes can be metallic or semiconducting, even in the same Different parts of the carbon nanotubes can also exhibit different conductivity. Therefore, it can be divided into metallic carbon nanotubes and semiconducting carbon nanotubes according to the difference in conductivity.
Vapor deposition (CVD) is the main production process, and catalyst preparation is the core difficulty
The chemical vapor deposition method is commonly used in industrial production to prepare carbon nanotubes. The current preparation methods of carbon nanotubes mainly include chemical vapor deposition, laser evaporation, graphite arc, and hydrothermal methods. However, due to the shortcomings of high cost and difficulty in industrial production of the latter three, carbon nanotube manufacturers use chemical vapor deposition more often.
The CVD process mainly includes four major processes: catalyst preparation, carbon nanotube coarse powder preparation, coarse powder purification and pulverization, of which catalyst preparation is the core link and technical difficulty. The key link in the preparation of carbon nanotubes by chemical vapor deposition is the process of carbon nanotube growth on the catalyst surface. The specific reaction process includes the decomposition of the carbon source compound on the catalyst surface, the carbon atoms enter the catalyst through surface diffusion or bulk diffusion, and finally carbon nanotubes are precipitated from the catalyst particles. Studies have shown that the diameter of the grown carbon nanotubes largely depends on the size of the nanocatalyst particles. Since the ratio of diameter to length of carbon nanotubes (length-to-diameter ratio) largely determines the performance of carbon nanotubes, the preparation of catalysts has become the core link in the CVD process. However, the selection of catalyst materials and the control of particle diameter require repeated experiments to determine. At the same time, the dynamic behavior of carbon nanotube catalysts is very complicated. With the growth of carbon nanotubes, the surface of the catalyst is reconstructed. Therefore, how to control the catalyst to maintain the original state that satisfies the growth of carbon nanotubes will undoubtedly increase the difficulty of catalyst preparation.
Since the ratio of diameter to length of carbon nanotubes (length-to-diameter ratio) largely determines the performance of carbon nanotubes, the preparation of catalysts has become the core link in the CVD process. However, the selection of catalyst materials and the control of particle diameter require repeated experiments to determine. At the same time, the dynamic behavior of carbon nanotube catalysts is very complicated. With the growth of carbon nanotubes, the surface of the catalyst is reconstructed. Therefore, how to control the catalyst to maintain the original state that satisfies the growth of carbon nanotubes will undoubtedly increase the difficulty of catalyst preparation.
From laboratory preparation to large-scale industrial production, it is also necessary to solve the problems of multi-level engineering science coupling and correlation, which has high technical barriers. The growth of carbon nanotubes in the laboratory is carried out on a milligram or gram scale in a smaller reactor. At this time, there is no significant bottleneck in the heat and mass transport in the reactor, and the coupling of transfer and reaction is not significant. The main problems focus on the structure control of carbon nanotubes during the catalytic growth process and the interaction between carbon nanotubes. However, in the process of industrialization, when carbon nanotubes are required to grow on a large scale in a thousand-ton industrial reactor, it is necessary to consider the atomic scale, nanoscale, mesoscale, reactor scale, factory scale, and ecological scale. Coupling and correlation of engineering science. Taking the reactor scale as an example, because carbon nanotube products are not a uniform substance in terms of form, the design of a new type of nanomaterial industrial growth reactor for carbon nanotubes requires an innovative combination of engineering basic research.
Man-made material with the highest specific strength, good electrical and thermal conductivity and hydrogen storage performance
The unique structure and chemical bonds of carbon nanotubes give it unique mechanical, electrical, thermal, and chemical properties, making it widely used in many fields.
1) The highest specific strength: the covalent bond connecting the carbon atoms in the carbon nanotube is the most stable chemical bond in nature. Carbon nanotubes have extremely high tensile strength and elastic modulus. At the same time, the density of carbon nanotubes is only 1/6 that of steel, which is currently the material with the highest specific strength that can be prepared.
2) Strong flexibility: Carbon nanotubes are strong but not brittle. When bending carbon nanotubes or applying pressure in the axial direction, even if the external force exceeds the Euler strength limit or bending strength, the carbon nanotubes will not be broken, but will be bent at a large angle first. When the external force is released, the carbon nanotubes will recover again. Undisturbed.
3) Good conductivity: The structure of carbon nanotubes is the same as that of graphite, and it has good conductivity. The resistance of carbon nanotubes has nothing to do with their length and diameter. When electrons pass through the carbon nanotubes, no heat is generated to heat the carbon nanotubes. The transmission of electrons in carbon nanotubes is just like the transmission of optical signals in optical fiber cables, with little energy loss. It is an excellent battery conductive agent.
4) Excellent thermal conductivity: Carbon nanotubes have extremely high thermal conductivity. The thermal conductivity at room temperature is twice that of diamond, and it is the best thermal conductivity material currently known. In addition, the heat exchange performance of carbon nanotubes in the axial direction is very high, while the heat exchange performance in the radial direction is relatively low. With proper orientation, carbon nanotubes can be synthesized into highly anisotropic heat conductive materials.
5) Good hydrogen storage performance: Carbon nanotubes have a high specific surface area, and after treatment, they have excellent hydrogen storage capacity.
6) Superior lithium insertion characteristics: The hollow cavity of carbon nanotubes, the gap between the tube and the tube, the gap between the middle layer and the layer of the tube wall, and various defects in the tube structure provide abundant storage for lithium ions Space and transportation channels.
7) Chemical stability: Carbon nanotubes are chemically stable, with acid and alkali resistance. Adding carbon nanotubes to polymer composites can improve the acid and oxidation resistance of the material itself.
Large-scale commercial applications are concentrated in the field of lithium batteries and conductive plastics
Carbon nanotubes have been applied to lithium battery conductive agents, conductive plastics, transparent conductive films, supercapacitors, conductive inks, high-strength "super fibers" and other products or materials with a variety of excellent properties. The application areas cover new energy vehicles. , 3C digital, semiconductor, photovoltaic power generation, aerospace, national defense and military industry.
Although the unique properties of carbon nanotubes give them the potential to be used in many fields, the production processes of new products such as transparent conductive films, supercapacitors, and conductive inks are still immature, while materials such as high-strength "super fibers" are mainly used in a small range For high-precision industries such as aerospace and military industry, it will take a long time for the above products or materials to be commercialized in large areas.
At present, commercial and large-scale applications are mainly concentrated in lithium battery conductive agents and conductive plastics. According to estimates, more than 75% of current demand comes from the field of conductive agents for lithium batteries. The upstream chemical industry of the industrial chain is mainly propylene and liquid nitrogen; the downstream is used in lithium batteries to serve the new energy automobile industry and 3C digital industry, and it is used in conductive plastics to serve the power infrastructure and semiconductor industries.
New energy field: carbon nanotubes are excellent conductive agents for lithium batteries
Lithium batteries have excellent performance and have a wide range of applications. Lithium batteries are a kind of chemical batteries that rely on lithium ions to shuttle between the positive and negative electrodes to achieve the purpose of charging and discharging. Because of their advantages of high energy density, high working voltage, long cycle life, and large charge and discharge rates, they have been widely used In the fields of new energy vehicles and 3C products (computers, communications and consumer electronics).
Conductive agent is an important material in lithium batteries, and its main function is to improve the conductivity of the battery. The main materials of lithium-ion batteries include positive electrode, negative electrode, electrolyte and separator. The power supply process of lithium batteries relies on the movement of electrons between the positive and negative electrodes, so the conductivity of the electrodes determines the performance of the battery. As a key auxiliary material for lithium batteries, the conductive agent is used to mix with the positive electrode material and the negative electrode material to make electrode pole pieces to ensure that the positive and negative electrodes of the battery have good conductivity. Lithium-ion battery cathode materials mainly include lithium iron phosphate, ternary materials, lithium cobalt oxide, lithium manganese oxide, etc., which provide lithium source for lithium-ion batteries, which are the key to the energy density, cycle life, safety and other indicators of lithium-ion batteries One of the materials.
The conductivity of lithium ion battery cathode materials is poor, and it is difficult to meet the performance requirements of lithium ion batteries. Therefore, adding a certain proportion of conductive agent to the cathode material can improve the conductivity of the cathode material. The principle is that the conductive material itself has good conductivity. After filling the gap between the active material of the positive electrode material and fully contacting the active material, it can build a bridge for electron flow between the active material of the positive electrode material to form a The conductive network improves the transfer rate of electrons in the lithium battery in the electrode.
The negative electrode of lithium ion battery is usually composed of graphite, which has good conductivity. However, during the process of multiple charge and discharge of graphite, the intercalation and shedding of lithium ions will cause the expansion and contraction of the volume of graphite particles. With the increase of the number of times, the gap between the graphite particles is enlarged, the conductivity is reduced, and some of them will even separate from the electrode, no longer participate in the electrochemical reaction, and reduce the capacity of the lithium ion battery. Therefore, adding a certain proportion of conductive agent to the negative electrode material helps maintain the conductivity of the negative electrode material.
Carbon nanotubes are a new type of conductive agent material, which can improve the conductivity of the positive electrode active material better than traditional conductive agents. It is an excellent lithium battery conductive agent. The current commonly used conductive agents for lithium-ion batteries mainly include carbon black, conductive graphite, carbon nanotubes, carbon nanofibers, and graphene. Carbon black and conductive graphite are traditional conductive agents, which form a point-contact conductive network between active materials; carbon nanotubes, carbon fibers and graphene are new conductive materials, in which carbon nanotubes and carbon fibers are between the active materials A line-contact conductive network is formed, and graphene forms a surface-contact conductive network between active materials. The line contact type and surface contact type can more fully construct a conductive network, and thus can more significantly improve the conductivity of the active material of the positive electrode material, thereby reducing the amount of conductive agent added to the positive electrode material. Generally speaking, the addition amount of carbon black conductive agent in the positive electrode material is usually about 3%, while the addition amount of new conductive agents such as carbon nanotubes and graphene can be reduced to about 0.5%~1.0%, which improves the filling of the positive electrode active material. Quantities help to increase the energy density of lithium-ion batteries.
Conductive plastics: carbon nanotubes have significant advantages as conductive fillers
Conductive plastics are functional polymer materials with conductivity and are widely used. Conductive plastic is an important part of conductive polymer materials. Because of its combination of metal conductivity and plastic characteristics, it has important applications in semiconductors, antistatic materials, integrated circuit packaging, electromagnetic wave shielding and other fields. For example, in the field of integrated circuits, the sensitivity of electronic components to static electricity ranges from 100V to tens of thousands of volts, and they are easily damaged by static electricity. Therefore, anti-static measures are particularly important in the field of integrated circuits. The resistance value of the conductive plastic can be adjusted between 102-109Ω, which can meet the needs of anti-static and anti-static in the field of integrated circuits.
The conductivity of conductive plastic comes from the conductive filler filled in it. The plastic in the filled conductive plastic does not have conductivity by itself and only serves as a structural material. Conductivity is mainly obtained by mixing conductive materials such as carbon materials, metal powders, and antistatic agents. These conductive materials are called conductive fillers, and they play a role in providing carriers and increasing conductivity in the filled conductive plastic. Carbon-based conductive fillers that can be used for conductive plastics include carbon black, acetylene black, carbon nanotubes, graphene, and the like. Conductive fillers are mixed with resin and other base materials to make conductive masterbatches, which are then added to various plastics.
The performance bottleneck of traditional materials in carbon-based fillers has emerged. At present, the carbon-based conductive fillers in conductive plastics are still dominated by traditional materials such as carbon black and acetylene black. With the continuous development of my country's national economy and various industries, the requirements for the performance of basic materials have also continued to increase, and the performance bottleneck of traditional carbon black as a conductive filler has emerged. Take semiconductors as an example. At present, the electrostatic dissipation requirements of high-end semiconductors have been increased from the resistance value of 108-109 to 106-107Ω. In order to achieve the required conductivity, usually domestic carbon black needs to be filled with 15-30% in plastics, imported carbon black The filling amount is about 10%, resulting in a large loss of the mechanical properties of the plastic, and the carbon black particles are easily scattered on the surface, so the carbon black particles have become one of the pollution sources of the semiconductor industry.
The advantages of carbon nanotubes as a new generation of carbon-based conductive fillers have gradually become prominent. As a new generation of carbon-based conductive filler, carbon nanotubes can solve the performance bottleneck problem encountered by traditional carbon black. Because carbon nanotubes have more excellent electrical conductivity, they can achieve the same or even better electrical conductivity. The addition amount is only 1/5-1/15 of traditional carbon black, and it will not cause decarburization pollution due to excessive addition. The problem is that additives developed and used for high-end conductive plastics in recent years. As the production scale of carbon nanotubes is further increased, the cost of using carbon nanotubes is gradually reduced, and its advantages over carbon black-filled conductive plastics will be more obvious.
Semiconductor field: carbon nanotubes are expected to play a huge role
American Nantero company has successfully developed a new type of non-volatile nano-memory (NRAM) based on carbon nanotubes for information storage. In 2019, the British "Nature" magazine published a recent development in computational science: The Massachusetts Institute of Technology team used more than 14,000 carbon nanotube transistors to create a 16-bit microprocessor and successfully executed a simple program. Carbon nanotube transistors are smaller, and their performance and energy consumption are much better than traditional silicon transistors. The research results of the MIT team have taken an important step in realizing the replacement of silicon transistors with carbon nanotubes.
With the wind of new energy and 5G, carbon nanotubes usher in rapid growth
The popularization of new energy vehicles and the wave of 5G replacement have driven the demand for carbon nanotube conductive agents for lithium batteries. In the lithium battery industry, the carbon nanotube conductive agent is finally delivered to lithium battery manufacturers in the form of slurry. With the increase in the penetration rate of new energy vehicles and the promotion of 5G mobile phones triggering a wave of replacement, the lithium battery industry will usher in a booming opportunity, driving the demand for lithium battery conductive paste; and new energy vehicles and 5G mobile phones will have a strong demand for lithium batteries. The continuous improvement of energy density and cycle life requirements will further highlight the advantages of carbon nanotube conductive paste and accelerate the penetration of conductive agents.