Properties of Carbon Nanotubes
1. Strength: These are the strongest and stiffest materials known. This strength results from the covalent bonds formed between the carbon atoms. In 2000, a multi walled carbon nanotube was tested to have a tensile strength of 63 GPa and this is equivalent to the ability to withstand tension due to 6422 kg on a cable with cross section of 1 mm2. Its specific strength of up to 48,000 kN m/kg is the best of materials known when compared to high-carbon steel value which is 154 kN m/kg.
2. Hardness: A super-hard material has been synthesised by compressing SWNTs to above 24 GPa at room temperature. The hardness of this material was measured to be 52 GPa, whereas the hardness of reference diamond and boron nitride samples is 150 GPa and 62 GPa, respectively. The bulk modulus of compressed SWNTs was 462 - 546 GPa, a value higher than that of 420 GPa for diamond.
3. Kinetic: In multiple concentric nanotubes, an inner nanotube core may slide within its outer nanotube shell almost without friction, hence creating an atomically perfect linear or rotational bearing. This is one of the first valuable examples of molecular nanotechnology.
4. Thermal: AII nanotubes exhibit good thermal conducting properties along the tube, but they are good insulators laterally to the tube axis. Measurements show that a SWNTs have a room temperature thermal conductivity along its axis of about 3500 W/mK (compared to copper, which has 385 W/mK) and thermal conductivity across the axis of about 1.52 W/mK (comparable to that of sand).
Applications of Carbon Nanotubes
1. In electrical circuits: Carbon nanotubes find application in nano electronics. The nanotube integrated memory circuit was first made in 2004. CNT-based transistors can operate at room temperature and are capable of digital switching using a single electron. Depending on the surface features, a carbon nanotube may act as a conductor or as a semiconductor. Structures of carbon nanotubes can be used for thermal management of electronic circuits. An approximately I mm thick carbon nanotube layer was used to fabricate coolers.
2. As paper batteries: A paper battery is made up of a paper - thin sheet of cellulose infused with aligned carbon nanotubes. The nanotubes act as electrodes to conduct electricity. These batteries provide along and steady power output comparable to a conventional battery (lithium battery), as well as quick release of high energy (as a super capacitor). A conventional battery contains a number of components while the paper battery integrates all the components in a single Structure, making it more energy efficient.
3. Solar Cells: These cells have been developed at the new jersey institute of technology using a carbon nanotube complex, formed by a mixture of carbon nanotubes and carbon buckyballs (fullerenes) to form snake like structure. Carbon buckyballs trap electrons, but they cannot facilitate electron flow. Sunlight excites the polymers while the buckyballs capture electrons. Nanotubes, thus act like metallic wires making the electrons or current flow.
a. Ultra capacitors: Carbon nanotubes may be used to improve the efficiency of ultra capacitor. The activated charcoal used in conventional ultra capacitors has many small hollow spaces of various sizes, which create a Large surface to store electric charge. But,the hollow spaces are not compatible with the electron charge requirements. However, the sizes of the spaces within a carbon nanotube electrode may be tailored to meet the requirement of holding the required charge and consequently the capacity can be increased significantly.
b. They have been introduced in nano-electromechanical Systems, including mechanical memory elements and nanoscale electric motors.
c. CNTs find applications in composite polymer materials where they are used to further add to the strength of the polymers.
d. As CNTs have high mechanical strength, they are being explored for fabricating clothes stab-proof and bullet-proof properties.
e. A flywheel fabricated using CNTs, when spun as a very high velocity on a floating magnetic axis in vacuum, can store energy at a density approaching that of fossil fuels.
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