Carbon Nanotubes: Types, Properties and Uses

Carbon nanotubes (CNTs) are hollow cylinders made of carbon atoms arranged in a hexagonal lattice, the same arrangement found in graphene. Their diameters are measured in nanometers while their lengths can reach micrometers or more, and that shape gives them unusual strength and electrical and thermal conductivity for their weight.

Types of carbon nanotubes

• Single-walled carbon nanotubes (SWCNTs): one rolled sheet of graphene, typically around 1 nm in diameter. Depending on the angle at which the sheet is rolled, called chirality, an SWCNT can be metallic or semiconducting.
• Double-walled carbon nanotubes (DWCNTs): two concentric tubes. The outer wall can be chemically modified while the inner wall keeps its properties.
• Multi-walled carbon nanotubes (MWCNTs): several concentric tubes, with outer diameters from a few nanometers to tens of nanometers. They are cheaper to produce in bulk and are the most widely used type in industry.

Key properties

• Mechanical: very high tensile strength and stiffness relative to their density, which is why they are used to reinforce polymers and fibers.
• Electrical: metallic tubes carry current with low resistance, and a small amount of CNTs can make an insulating polymer conductive.
• Thermal: high thermal conductivity along the tube axis.
• Surface area: large surface area per gram, useful for adsorption, catalysis and electrodes.

Real-world performance is usually lower than single-tube measurements, because defects, impurities, bundling and poor dispersion all reduce the properties of bulk material.

How carbon nanotubes are made

• Arc discharge: an electric arc between graphite electrodes vaporizes carbon, which forms nanotubes. It was the first method used.
• Laser ablation: a laser vaporizes a graphite target with a metal catalyst. It produces high-quality SWCNTs but at low volume.
• Chemical vapor deposition (CVD): a carbon-containing gas decomposes over metal catalyst particles at high temperature. CVD is the main route for commercial production because it scales and gives control over tube type and alignment.

After synthesis, CNTs are usually purified to remove catalyst metal and amorphous carbon. Purity in wt% is one of the main specifications to compare between products.

Where carbon nanotubes are used

• Battery electrodes: as a conductive additive in lithium-ion cathodes and anodes. A small loading of CNTs can form a conductive network through the electrode with less material than carbon black.
• Composites: in polymers, epoxies and fibers for strength, stiffness, electrical conductivity or electrostatic discharge protection.
• Conductive films and coatings: transparent conductors, heaters and EMI shielding.
• Sensors and electronics: gas sensors, biosensors and field emitters.
• Thermal management: fillers in thermal interface materials.

Dispersion and functionalization

CNTs tend to clump into bundles because of strong van der Waals attraction. Getting them evenly dispersed in a solvent or polymer is often the hardest part of working with them. Common approaches are ultrasonication, high-shear mixing, surfactants and chemical functionalization. Carboxyl (-COOH) functionalized CNTs, for example, disperse more easily in water and polar solvents and bond better to some polymer matrices.

Choosing carbon nanotubes

Match the type and grade to the application:

• For bulk composites and general research, high-purity multi-walled carbon nanotubes or short MWCNTs, which disperse more easily.
• For battery electrodes, battery-grade MWCNTs and carbon nanofibers.
• For aqueous systems and polymer bonding, graphitized carboxylic MWCNTs.
• For work that needs thinner, more uniform tubes, double-walled carbon nanotubes.
• For macroscopic conductors, carbon nanotube fibers.

Browse the full range in carbon-based nanomaterials.

Safety

Treat CNT powders as a respiratory hazard. Handle dry powder in a fume hood or ventilated enclosure, wear gloves and suitable respiratory protection, and clean up with wet wiping or a HEPA vacuum rather than sweeping. Check the safety data sheet for the specific product.