Carbon nanotube

Carbon nanotube. A carbon nanotube is a tube-shaped allotrope of carbon with a diameter on the nanometer scale, forming a cylindrical nanostructure. These structures exhibit extraordinary mechanical strength, unique electrical properties, and efficient thermal conductivity. Their discovery has significantly advanced fields ranging from materials science to nanotechnology and molecular electronics.

Structure and properties

The structure is based on a two-dimensional hexagonal lattice of carbon atoms, akin to a rolled-up sheet of graphene. The specific rolling direction, defined by the chirality vector, determines whether the nanotube behaves as a metallic conductor or a semiconductor. This atomic arrangement results in a tensile strength exceeding that of high-strength steel and a Young's modulus rivaling that of diamond. Their thermal conductivity is comparable to that of pure diamond or in-plane graphite, while their electrical properties can be tuned from metallic to semiconducting based on their structure. Research at institutions like the Massachusetts Institute of Technology and IBM has extensively characterized these properties.

Synthesis methods

Primary production techniques include arc discharge, laser ablation, and chemical vapor deposition. The arc discharge method, similar to that used for producing fullerenes, involves vaporizing graphite electrodes in an inert atmosphere like helium. Laser ablation uses a high-power laser to vaporize a graphite target in a heated furnace. The most scalable method for device integration is chemical vapor deposition, where a carbon-containing gas, such as acetylene or ethylene, decomposes on a metal catalyst like iron, cobalt, or nickel nanoparticles supported on a substrate such as silicon dioxide or alumina. Work by teams at Rice University and Stanford University has been instrumental in refining these synthesis routes.

Applications

These nanostructures are integrated into a wide array of advanced technologies. In composite materials, they reinforce polymers for use in aerospace components, a field advanced by organizations like NASA and Boeing. In electronics, they serve as channels in field-effect transistors, interconnects, and transparent conductors for flexible displays, with significant development from Samsung and Intel. Their high surface area makes them excellent for electrodes in supercapacitors and lithium-ion batteries. Other uses include drug delivery vehicles in nanomedicine, probes for atomic force microscopy, and sensors for detecting gases like nitrogen dioxide or ammonia.

Toxicity and environmental impact

The toxicology of these materials is an active area of research due to their fiber-like shape and biopersistence, drawing comparisons to asbestos. Studies, including those by the National Institute for Occupational Safety and Health, indicate that exposure to certain forms can induce inflammatory response and fibrosis in the lungs of animal models. Their environmental fate is also under investigation, with research examining their interaction with soil microorganisms, potential for bioaccumulation, and lifecycle in wastewater treatment plants. Regulatory bodies like the Environmental Protection Agency are assessing frameworks for their safe handling and disposal.

History and discovery

The modern discovery is widely credited to Sumio Iijima of NEC in 1991, who published high-resolution transmission electron microscopy images of multi-walled tubes in the journal Nature. Earlier observations, however, were documented in the scientific literature. In 1952, L. V. Radushkevich and V. M. Lukyanovich published images of hollow graphite fibers in the Soviet Journal of Physical Chemistry. The 1970s and 1980s saw related work on filamentous carbon and the invention of the Krätschmer-Huffman generator for fullerene production, which also created nanotubes. Iijima's 1991 paper, followed by his and Donald Bethune's independent synthesis of single-walled tubes in 1993, ignited global research efforts at institutions like IBM and Rice University.

Category:Allotropes of carbon Category:Nanomaterials Category:Nanotechnology