Carbon nanotubes are molecular-scale tubes of graphitic carbon with outstanding components. They’re among the stiffest and strongest fibres acknowledged, and have remarkable electronic properties and several other distinctive characteristics. For these reasons they have attracted massive academic and industrial interest, with thousands of papers on nanotubes being published each and every year. Commercial applications are already rather slow to develop, on the other hand, primarily simply because in the large production costs from the ideal high quality nanotubes.
Theory suggests that carbon nanotubes have a variety of useful properties, and experiments to test these redictions are just becoming achievable.
The present large interest in carbon nanotubes is really a direct consequence on the synthesis of buckminsterfullerene, C60 , and other fullerenes, in 1985. The discovery that carbon could form stable, ordered structures other than graphite and diamond stimulated researchers worldwide to search for other new forms of carbon. The search was given new impetus when it absolutely was proven in 1990 that C60 could be produced in a straightforward arc-evaporation apparatus readily available in all laboratories. It absolutely was employing such an evaporator that the Japanese scientist Sumio Iijima discovered fullerene-related carbon nanotubes in 1991. The tubes contained at least two layers, generally many additional, and ranged in outer diameter from about three nm to 30 nm. They had been invariably closed at both ends.
A transmission electron micrograph of some multiwalled nanotubes is shown inside the figure (left). In 1993, a new class of carbon nanotube was discovered, with just a single layer. These single-walled nanotubes are commonly narrower than the multiwalled tubes, with diameters usually in the array 1-2 nm, and tend to be curved rather than straight. The image around the right shows some typical single-walled tubes. It was soon established that these new fibres had a range of exceptional attributes (see below), and this sparked off an explosion of research into carbon nanotubes. It truly is critical to note, nevertheless, that nanoscale tubes of carbon, generated catalytically, had been acknowledged for several years prior to Iijima’s discovery. The main reason why these early tubes did not excite wide interest is that they were structurally rather imperfect, so did not have particularly interesting attributes. Recent exploration has focused on improving the good quality of catalytically-produced nanotubes.
The bonding in carbon nanotubes is sp², with each atom joined to 3 neighbours, as in graphite. The tubes can as a result be considered as rolled-up graphene sheets (graphene is an individual graphite layer).<br> There are 3 distinct ways in which a graphene sheet might be rolled into a tube, as revealed inside the diagram below.
The first two of these, known as “armchair” (top left) and “zig-zag” (middle left) have a large degree of symmetry. The terms “armchair” and “zig-zag” refer for the arrangement of hexagons around the circumference. The third class of tube, which in practice will be the most typical, is known as chiral, meaning that it can exist in two mirror-related forms. An example of a chiral nanotube is demonstrated at the bottom left.
The structure of a nanotube is usually specified by a vector, (n,m), which defines how the graphene sheet is rolled up. This may be understood with reference to figure for the proper. To generate a nanotube with the indices (6,three), say, the sheet is rolled up to ensure that the atom labelled (,) is superimposed about the one labelled (6,3). It may be seen from the figure that m = for all zig-zag tubes, whilst n = m for all armchair tubes.
The arc-evaporation process, which produces the ideal excellent nanotubes, entails passing a current of about 50 amps between two graphite electrodes in an atmosphere of helium. This causes the graphite to vaporise, some of it condensing for the walls on the reaction vessel and some of fat burning furnace it for the cathode. It could be the deposit on the cathode which contains the carbon nanotubes. Single-walled nanotubes are generated when Co and Ni or some other metal is added towards the anode. It has been acknowledged since the 1950s, if not earlier, that carbon nanotubes may also be made by passing a carbon-containing gas, such as a hydrocarbon, more than a catalyst. The catalyst consists of nano-sized particles of metal, usually Fe, Co or Ni. These particles catalyse the breakdown in the gaseous molecules into carbon, and a tube then begins to grow with a metal particle at the tip. It had been shown in 1996 that single-walled nanotubes can also be made catalytically. The perfection of carbon nanotubes developed in this way has typically been poorer than those manufactured by arc-evaporation, but wonderful improvements within the technique have been created in recent years. The large advantage of catalytic synthesis more than arc-evaporation is that it may be scaled up for volume production. The third critical strategy for producing carbon nanotubes involves employing a powerful laser to vaporise a metal-graphite target. This could be used to create single-walled tubes with high yield.