PROPERTIES OF CARBON NANOTUBES C SIDDARTH 60011115007

introduction Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Carbon nanotubes are unique "one-dimensional systems" which can be envisioned as rolled single sheets of graphite. This rolling can be done at different angles and curvatures resulting in different nanotube properties.  The length-to-diameter ratio, can be as high as 132,000,000:1, which is unequalled by any other material. 

In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials.

Types 2 types:- Single Walled Nanotubes (SWNT) These can be imagined as a single atom thick graphite sheet, i.e. graphene, rolled into a cylinder and capped by fullerene hemisphere. Multiwalled Nanotubes (MWNT) These can be considered as Nanotubes within Nanotubes.

stRUCTURE Zigzag Armchair Chiral

properties

STrength The strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively.  Covalent sp2 bonds formed between the individual carbon atoms. Studies revealed that individual CNT shells have strengths between 63 to ~100 Gpa.  Low density for a solid of 1.3 to 1.4 g/cm3. Its specific strength of up to 48,000 kN·m·kg−1 is the best of known materials, compared to high-carbon steel's 154 kN·m·kg−1.

Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multi-walled carbon nanotubes. This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes,

Simple geometrical considerations suggest that carbon nanotubes should be much softer in the radial direction than along the tube axis. Indeed,  observation of radial elasticity suggested that even the van der Waals forces can deform two adjacent nanotubes.

hardness Standard single-walled carbon nanotubes can withstand a pressure up to 25 GPa without deformation. Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. 

Electrical properties The electrical properties of carbon nanotubes depends upon their diameter and chirality. They show electrical properties ranging from semiconductors to those of good conductors. The energy gap decreases as the diameter of the CNT is increased. Due to very low resistivity, the heat dissipation in the CNT is very small and hence they can carry much larger currents than the metals.

The conductivity of a CNT is maximum along its axis and very low in a perpendicular direction. Hence they are equivalent to one dimensional conductors. At low temperatures, the resistance decreases with increasing magnetic field applied across the CNT. This effect is known as magneto resistance.

Thermal properties All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as ballistic conduction. Good insulators laterally to the tube axis.  Measurements show that a SWNT has a room-temperature thermal conductivity along its axis of about 3500 W·m−1·K−1; compare this to copper, which transmits 385 W·m−1·K−1. 

A SWNT has a room-temperature thermal conductivity across its axis (in the radial direction) of about 1.52 W·m−1·K−1,which is about as thermally conductive as soil.

defects The existence of a crystallographic defect affects the material properties. Defects can occur in the form of atomic vacancies.  High levels of such defects can lower the tensile strength by up to 85%. Crystallographic defects also affect the tube's electrical properties. A common result is lowered conductivity through the defective region of the tube. Crystallographic defects strongly affect the tube's thermal properties; reduces the thermal conductivity of nanotube structures.

Chemical Properties Optical Properties CNTs are chemically more inert compared to other forms of carbon. Optical Properties  the practical use of optical properties is yet unclear.

toxicity  Preliminary results highlight the difficulties in evaluating the toxicity of this heterogeneous material. Available data clearly show that, under some conditions, nanotubes can cross membrane barriers, which suggests that, if raw materials reach the organs, they can induce harmful effects such as inflammatory and fibrotic reactions.

applications Fuel cells Microscopic probes Flat panel displays Chemical sensors etc.

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