Carbon nanotubes (CNTs) represent nanoscale tubular structures made entirely of carbon atoms. Carbon nanotubes stand out as one-dimensional nanomaterials because they exhibit exceptional physical and chemical properties like high strength and excellent thermal and electrical conductivity. Research into carbon nanotubes developed through investigations about fullerene (C60). Harold Kroto from Britain along with American scientists Richard Smalley and Robert Curl identified fullerene (C60) in 1985 which initiated a new era in carbon material research.

Carbon nanotubes are mainly divided into two categories: Carbon nanotubes consist of two fundamental types which include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).

A single graphene layer coils to create single walled carbon nanotubes which form a continuous hollow cylindrical shape. Single walled carbon nanotubes possess superior strength and conductivity capabilities yet they present challenges in preparation and remain expensive. The applications of single walled carbon nanotubes span multiple fields including electronic devices while also being used in biomedicine and sensors.

Multiple layers of graphene create concentric coil structures in multi walled carbon nanotubes. Multi-walled carbon nanotubes possess strong toughness characteristics while being simple to manufacture with economical production costs. The main applications of this technology include composite material reinforcement and energy storage.

Structure and Properties of Carbon Nanotubes
Structure of carbon nanotubes

Carbon nanotubes (CNTs) have unique one-dimensional nanostructures. The structure of carbon nanotubes appears as a cylinder created through the rolling of graphene which consists of a single atom-thick layer of carbon atoms. Carbon nanotubes derive their exceptional strength and stability from the hexagonal arrangement of their carbon atoms which forms sp² hybrid covalent bonds. Carbon nanotubes categorize into multiple types based on their curling methods including armchair type, serrated type, and spiral type.

Their unique structure gives carbon nanotubes exceptional physical and chemical properties leading to extensive application possibilities across various fields.

Mechanical strength

The mechanical strength and toughness of carbon nanotubes are exceptional as their tensile strength is 100 times greater than steel's and their Young's modulus is almost six times that of steel. Carbon nanotubes weigh one-sixth of steel yet hold the record as the strongest material by specific strength in the natural world. Carbon nanotubes maintain their flexibility under extreme forces beyond their elastic boundary without breaking and return to their original condition once the force is removed.

Conductivity

The conductivity of carbon nanotubes reaches up to 10⁸ S · m⁻¹ which equals ten thousand times the conductivity of copper metal. The sp² hybrid structure of carbon nanotubes enables electrons to be transmitted with minimal energy loss much like optical signals travel through optical fibers. Carbon nanotubes' conductivity changes based on their curling techniques which makes them function as either metallic conductors or semiconductors.

Thermal conductivity

Carbon nanotubes demonstrate extremely high thermal conductivity values at room temperature which is much greater than other metals. The axial thermal conductivity of carbon nanotubes excels but radial thermal conductivity remains weak which makes them ideal for producing anisotropic thermal conductive materials. Cross-linked networks created through chemical vapor deposition can improve horizontal thermal conductivity of carbon nanotube arrays.

Chemical stability

Carbon nanotubes maintain good chemical stability alongside strong resistance to both acidic and alkaline conditions. When polymer composite materials are mixed with carbon nanotubes they show enhanced resistance to acids and oxidation. The ultra clean and chemically stable surface of carbon nanotubes makes them ideal for use in high-precision electronic devices.