Overview of Carbon Nanotubes
Carbon nanotubes (CNTs) are ultrasmall cylindrical nanostructures. They have diameters ranging from 1 to 100 nanometers and lengths ranging from hundreds of nanometers to centimeters. CNTs are simply carbon molecules chemically derived from the graphite and diamond family, but they are stronger than diamond due to increased bond strength as the tube diameter decreases. These carbon nanotubes possess new properties that make them useful in various applications in the field of nanotechnology, electronics, optics, as well as many other material science-related fields and even architectural engineering.
Carbon nanotubes can be divided into single-walled nanotubes (SWCNTs) and multi-walled nanotubes (MWCNTs). Additionally, carbon nanotubes align themselves in the form of tangled ropes held together by van der Waals forces. The following are some types of carbon nanotubes:
- Double-Walled Nanotubes
- Layered Nanotubes
- Toroidal Nanotubes
- Cone-Ended Nanotubes
Types of Carbon Nanotubes
There are two types of carbon nanotubes: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).
Single-Walled Carbon Nanotubes (SWCNTs)
These nanotubes have diameters ranging from 0.7 to 10 nanometers, with an average diameter typically around 2 nanometers. They can reach lengths of several micrometers and have at least one closed end, resembling a cap-like hemispherical structure. The wall of this tube consists of a single carbon atom layer, folded into a cylindrical shape.
Multi-Walled Carbon Nanotubes (MWCNTs)
Multi-walled carbon nanotubes are composed of multiple intertwined single-walled nanotubes with a shared central axis. The diameter of these tubes ranges from 10 to 30 nanometers, and they usually consist of 7 to 20 individual single-walled tubes. Microscopically, MWCNTs have a structural unit composed of six carbon atoms arranged in a hexagonal shape. It’s worth noting that carbon nanotubes can be classified into three types: chiral structures with a 30-degree angle, zigzag structures with a 38-degree angle, and armchair structures with an angle between 0 and 30 degrees.
Properties of Carbon Nanotubes
At the nanoscale, the physical, chemical, and biological properties of carbon nanotubes differ significantly from their macroscopic counterparts due to the quantum mechanical properties that govern atomic interactions. By manufacturing devices based on the nanometer scale, it is possible to control the intrinsic properties of materials, including melting point, magnetic properties, and even color, without altering their chemical composition.
- Mechanical Properties: Carbon nanotubes are among the strongest materials known. Their exceptional strength is due to the strong covalent bonds between carbon atoms. They possess extremely high tensile strength, making them resistant to pressure or stress. They are incredibly rigid and difficult to break, with a high modulus of elasticity. Additionally, they have a low density, ranging from 1.4 to 33 grams per cubic centimeter, making them lightweight compared to materials like aluminum and iron. The high strength-to-weight ratio of carbon nanotubes is one of their remarkable properties.
- Kinetic Properties: Multi-walled carbon nanotubes have a telescopic property, allowing the inner layer to slide effortlessly over the outer layer with minimal friction, either in a linear or rotational manner. This behavior resembles a closing telescope, forming a compact structure.
- Electrical Properties: The arrangement of carbon atoms in carbon nanotubes affects their electrical conductivity. Single-walled carbon nanotubes with a zigzag arrangement exhibit metallic properties, while nanotubes with a helical or chiral arrangement are considered semiconductors. Carbon nanotubes also possess a property called ballistic electron transport, meaning they are excellent conductors along the tube. Metallic carbon nanotubes can carry electrical currents more than 1,000 times greater than good conductors like copper.
- Thermal Properties: Carbon nanotubes exhibit excellent thermal conductivity along the tube and are nearly insulating perpendicular to the tube axis, known as radial thermal conduction. It is expected that carbon nanotubes can conduct heat at approximately 6,000 watts/meter/kelvin at room temperature, compared to copper, which has a thermal conductivity of 385 watts/meter/kelvin. Furthermore, carbon nanotubes have a high thermal stability, with a melting point of approximately 2,800 degrees Celsius in a vacuum and around 750 degrees Celsius in air. This means they can retain their properties and structural integrity at high temperatures.
Manufacturing Carbon Nanotubes
Carbon nanotubes are incredibly strong but challenging to manufacture, requiring specialized techniques and nanotechnology expertise. Despite their difficulty, various manufacturing methods are being tested and utilized to provide cost-effective ways of producing and utilizing carbon nanotubes.
Different techniques for producing carbon nanotubes include:
- Electrochemical analysis using graphite electrodes in molten salts.
- Catalytic thermal decomposition of hydrocarbons.
- Laser vaporization of graphite.
These manufacturing methods allow the production of carbon nanotubes with different electronic properties, with some exhibiting metallic behavior while others act as semiconductors.