Saturday, April 17, 2010

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Thoughts - The Wonder Material : Carbon Nanotubes

Recently while researching for one of my projects, I stumbled upon some documents highlighting the promise of nanotechnology in changing of our lives. It was then that I remembered of some research I did earlier on one of the marvels of nanotechnology – Carbon Nanotubes. So here I am summarizing my research.* With increased emphasis on miniaturization and advent of nanotechnology these molecular tubes have gained quick popularity.

The limitation of various materials like piezoelectric ceramics, electro active polymers etc. in applications involving extremes of mechanical and electrical conditions led the scientific community to look out for new materials that could be used in conditions of high voltage, current or strain. Their search of the required material ended with Iijima’s discovery of carbon nanotubes in 1991. It was then that because if its mechanical and electrical properties Carbon Nanotubes gained popularity in no time. Carbon nanotubes are molecular scale tubes, which could be thought as atomic layers of carbon rolled in form of cylinders.

Advanced mechanical, electrical and thermal properties possessed by CNTs have earned them great attention from the scientific community. Nanotubes are stiffest known fibers with a very high tensile strength. Apart from this, these tubes have very high thermal conductivity. Electronically these nanotubes can be conducting or semiconductor, which depends on the chirality of composition. These tubes have been known to conduct current without dissipating much heat. Current density possible in these nanotubes is highest among all known materials. Magneto resistance and transport properties of Carbon nanotubes is spin dependent. Further slightest of strain can cause a change in tube’s conductance by orders of magnitude. The thermoelectric power of CNTs is positive in whole temperature range and increases with increasing temperature. All these properties, unique in themselves, provide these nanotubes a special place in research activities.

Various methods have been explored to produce carbon nanotubes. Nanotube structures formed are essentially in the same way but the process that makes them is different. Some of the employed methods are arc discharge, condensation vaporization (CVD) etc. While arc discharge is the oldest known method for CNT production, CVD is most attractive means commercially. Laser Ablation is yet another method to produce nanotubes of high quality and purity. Various chemical methods have also been developed for CNT synthesis. Some of them are catalytic decomposition of hydrocarbons, electrolysis and low temperature solid pyrolysis. Two important and widely used methods for carbon nanotube synthesis have been discussed below:

1.) Arc discharge: This was the first method developed for CNT production. Iijima,in 1991, extracted his first nanotube by this method. This method is undertaken in neutral environment like that of low pressure Helium, which is provided using Seales reaction chamber and vacuum equipment. The most widely used process under this method is electric discharge where an electric discharge creates very high temperature, which causes sublimation of carbon. Nanotubes so formed are then purified by gasification. Major problem in this method is hat the growth process needs to be interrupted at regular intervals to remove product from the chamber.

2.) CVD Process: CVD is a short name for condensation vaporization method. Under this process, catalyst materials are heated to high temperature using hydrocarbon gas through tube reactors for long stretch of time. During the process first the transition metal catalyzes the decomposition of hydrocarbon and then the carbon atom are saturated in the metal nano particles. Various catalysts are used additionally to improve the quality and lower production cost. Concluding purification is achieved by oxidative or thermal treatment, micro filtration and ultrasound methods. Finally the sample is characterized using techniques like Raman scattering, thermal gravimetric analysis (TGA) and atomic source microscopy (AFM).

Carbon nanotubes have drawn wide scale attention of the world because of their unique properties with aspect to utilization in many promising applications. The areas where these have already been employed are chemical sensors, electronic devices nanotubes, super capacitors, hydrogen storage etc. Applications in diamond industry are most likely to be explored soon. Some of the fields where these CNTs can be utilized leading to far reaching improvement in efficiencies are NRAMs, interconnects and cooling aids.

a.) NRAMs: Many nano-electro mechanical systems, which are based on CNTs, have already become popular like sensors, nano-relays, and oscillators. The latest breakthrough in the field is development of NRAM i.e. nano random access memory. An NRAM consists of a drain, a source and a gate. Also we have a suspended bridge of nanotubes between source and drain electrode. Various studies reveal that memory arrays based on NEMS are non-volatile. All this has been possible because of mechanical properties of the CNT Bridge and Vander Walls interaction of the bridge with substrate. Electromechanical modeling of a nanotube based NRAM is achieved through model shown in fig1, which is basically a qualitative illustration of how a NRAM works. When sufficiently high voltage, called turn on voltage, is applied to the gate electronic charges are induced in the carbon nanotube. These charges deflect the suspended nanotube because of electrostatic forces. The resulting capacitive action leads to atomic contact of CNT and top of the gate. With gate voltage less than turn on voltage, elastic forces are dominant and keep CNT away from the gate. Whenever CNT and gate are in contact we call it a on state (i.e. binary 1) and otherwise off state (i.e. binary 0)

b.) Interconnects: The ever-decreasing size of microchips called for new material for interconnection within ICs. Ballistic electron transport and huge current carrying capacity of CNT made them obvious alternative. Multi walled carbon nanotubes are particularly appropriate fro wiring and interconnections. Replacing the copper wires from use in vertical wires called vias, in on-chip interconnects has witnessed high impetus because of high failure rate of copper wires due to stress and electro migration. The high thermal conductivity of CNT, around 15 times that of copper, further promotes its use as interconnects.
c.) Cooling aids: The most recent use of carbon nanotubes is as heat sinks and cooling material. CNTs can be used in place of classical thermal pastes in use, because of their mechanical and high thermal conductivities. Another way of use can be as nanotube electrodes to generate ions. Ionization of air causes electric discharge, which generates tiny breezes. These breezes could be used for cooling purposes. With shrinking sizes these cooling procedure would soon replace classical practices.

Although conceptualized no more than a decade ago, carbon nanotubes have caught the attention of researchers the word over. They have amazing electronic and mechanical properties, which lead to incredible form of strength and conductivity. Being nano-structures with such unique properties, their applicability in newer fields is constantly being explored. Latest field of applications of CNTs is gene therapy, where these are being used as transporters. Considering the quick popularity that these molecular tubes have gained it would not be an exaggeration to comment that the time is not far when CNTs would be providing solutions to large number of real world problems.

*PS: I in no way claim to have discovered any of these properties or researched on the topic scientifically. It is based on my understanding from secondary research of documents available in public domain. If you believe that this is in infringement to your copyright privileges, please do let me know and I would be happy to provide the relevant references. 


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