CNT Technology Overview
The discovery of carbon nanotubes (CNT) in 1991 opened up a new era in materials science. These incredible structures have an array of fascinating electronic, magnetic and mechanical properties. CNT are at least 100 times stronger than steel, but only one-sixth as heavy, so nanotube fibers could strengthen almost any material. Nanotubes can conduct heat and electricity far better than copper. CNT are already being used in polymers to control or enhance conductivity and are added to anti-static packaging.
A carbon nanotube is a tube-shaped material, made of carbon, having a diameter measuring on the nanometer scale. A nanometer is one-billionth of a meter, or about 10,000 times smaller than a human hair. CNT are unique because the bonding between the atoms is very strong and the tubes can have extreme aspect ratios. A carbon nanotube can be as thin as a few nanometers yet be as long as hundreds of microns. To put this into perspective, if your hair had the same aspect ratio, a single strand would be over 40 meters long.
Carbon nanotubes have many structures, differing in length, thickness, and number of layers. The characteristics of nanotubes can be different depending on how the graphene sheet has rolled up to form the tube causing it to act either metallic or as a semiconductor. The graphite layer that makes up the nanotube looks like rolled-up chicken wire with a continuous unbroken hexagonal mesh and carbon molecules at the apexes of the hexagons.
Schematic of carbon nanotube made from rolled graphene sheet
There are many different types of carbon nanotubes, but they are normally categorized as either single-walled (SWNT) or multi-walled nanotubes (MWNT). A single-walled carbon nanotube is just like a regular straw. It has only one layer, or wall. Multi-walled carbon nanotubes are a collection of nested tubes of continuously increasing diameters. They can range from one outer and one inner tube (a double-walled nanotube) to as many as 100 tubes (walls) or more. Each tube is held at a certain distance from either of its neighboring tubes by interatomic forces.
Single vs multi-walled CNT
Currently, the most popular use for carbon nanotubes is in structural reinforcement. The high strength and low weight of CNT combined with their flexibility allow for them to be added to other materials like rebar to concrete. Advances are being made using carbon nanotubes to extract power from sunlight and even as a heat source. Carbon nanotubes are unique in that they are thermally conductive along their length but not across the tube itself. This lets carbon nanotubes play a role on both sides of thermal insulation. CNT are also highly electrically conductive, which could potentially make for an extremely cost effective replacement for metal wires. The semiconducting properties of CNT make them candidates for the next generation of computer chips. Other applications under investigation are their use as chemical carriers for pharmaceutical applications. Specific drugs can be attached to CNT that can target and attack only certain types of cells, including cancer cells, for example.
It is important to note that growth of carbon nanotubes in general always requires the presence of a catalyst. The catalyst is usually predeposited on the surface on which growth will take place. The catalyst can also be suspended in a hot gas stream and nanotube formation will occur that way as well, though the latter is a very inefficient way to do so. It also should be mentioned that it is possible to inject both a hydrocarbon compound and a catalyst-bearing compound simultaneously into a CVD system and get growth without prepatterning the substrate. An example would be injecting toluene and ferrocene simultaneously, in which case the growth is characterized as a two-source process and would require two injection mechanisms operating simultaneously as well as a carrier gas injection assembly. The single source precursor that comes with the SSP354 is much simpler, less expensive and more reliable to use.
Basics of how to synthesize carbon nanotubes
Scanning Electron Microscopes
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Properties and Applications of Carbon Nanotubes
Physical properties and applications of CNT