05-04-2012, 03:02 PM
carbon nanotubes
Carbon Nanotube.docx (Size: 3.14 MB / Downloads: 46)
Introduction
Carbon nanotubes have attracted lots of attention of scientist from various fields because they exhibit extraordinary physical, chemical and mechanical properties due to their intrinsic nano-sized one-dimensional nature. Thus, their intrinsic characteristics will revolutionize everything from computer, bio- and medical-devices, batteries, nanocomposite-based vehicles to electronic nano-device in the near future.
1.1. Origin of carbon nanotube
Carbon is an element with unique properties and is the lightest among all the elements of group IV of the periodic table. It differs in many ways from other elements like Si, Ge, Sn and Pb of the same group, which all have sp3 bonding in their cubic solid ground states, while carbon in the condensed phase has a hexagonal ground state graphite with sp2 bonding and is highly anisotropic, nearly two dimensional semimetal. Lying close in energy to graphite is diamond, a three-dimensional material with nearly isotropic properties.
1.2. Structure of carbon nanotubes
A carbon nanotube is a graphene sheet appropriately rolled into a cylinder of nanometre size diameter as shown in fig 1.2. The curvature of the nanotubes admix a small amount of sp3 bonding so that the force constant in the circumferential direction is slightly weaker than along the nanotube axis[1]. CNTs are hexagonal networks of carbon atoms of approximately 1 nm diameter and 1 to 100 microns of length. They can essentially be thought of as a layer of graphite rolled-up into a cylinder.
1.3. Types of carbon nanotubes
Depending on the arrangement of the grapheme cylinders, there are two types of nanotubes: single walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs)[3].
A SWNT is a molecular scale wire that has two key structural parameters. By folding a graphene sheet into a cylinder so that the beginning and end of a (m, n) lattice vector in the graphene plane joined together, one obtains an (m, n) nanotube. The (m, n) indices determine the diameter of the nanotube, and also the so-called chirality. (m, m) tubes are arm chair tubes, since the atoms around the circumference are in an arm-chair pattern (Figure 1.4). (m, 0) nanotubes are termed as zigzag in view of the atomic configuration along the circumference (Figure 1.4).
Production techniques
For application of carbon nanotubes, large quantities of nanotubes are required, and scale-up limitations of the arc discharge and laser ablation techniques would make the cost of nanotubes based devices prohibitive. During nanotube synthesis, impurities in the form of catalyst particles, amorphous carbon and non-tubular fullerenes are also produced. Thus, subsequent purification steps are needed to separate the carbon nanotubes. The gas phase processes tend to produce nanotubes with fewer impurities and are more amenable to large scale processing. It is believed that the gas phase techniques, such as CVD, for nanotube growth offer greater potential for the scaling up of nanotubes production for applications. The three techniques of carbon nanotube production are discussed in detail below:
Carbon Arc-Discharge Technique
Two carbon electrodes are used in the carbon arc-discharge technique to generate an arc by DC current. The electrodes are kept in a vacuum chamber and an inert gas is supplied to the chamber. The purpose of the inert gas is to increase the speed of carbon deposition. Initially, the two electrodes are kept independent. Once the pressure is stabilized, the power supply is turned on (about 20 V) and the positive electrode is then gradually brought closer to the negative electrode to strike the electric arc.
Applications of Nanotubes
Carbon nanotubes are the smart materials recently being developed. It has got some unique and technologically promising properties which leads to its possible applications in following fields.
3.1. Hydrogen storage in CNTs
The use of hydrogen as fuel has some great advantages, however it is a tough job to store it and release when needed. Carbon nanotubes show very surprising hydrogen storage capacity, in spite of their relatively small surface area and pore volume. The values published for the quantity of hydrogen absorbed in nano-structured carbon materials varies between 0.4 and 67 mass%[2]. For nanotubes, one important issue currently being debated is whether hydrogen adsorption also occurs in the interstitial channels between adjacent nanotubes in a rope of SWNTs.
3.2. As sensors
Sensors are devices that detect or measure physical and chemical quantities such as temperature, pressure, sound, and concentration. The measurands are converted into an electrical signal. The main requirements of a good sensor are high sensitivity, fast response, low cost, high volume production, and high reliability.
Conclusion
Carbon nanotubes have been utilized either individually or as an ensemble to build functional device prototypes. Ensembles of nanotubes have been used for field emission based flat-panel displays, composite materials with improved mechanical properties and electromechanical actuators. Bulk quantities of nanotubes have also been suggested as high capacity hydrogen storage media. Individual nanotubes have been used for field emission sources, tips for scanning probe microscopy, nanotweezers and chemical sensors. Nanotubes are also promising as the central elements for future miniaturized electronic devices. The success in nanotube growth has led to the wide availability of nanotube materials, which is a main catalyst behind recent leaps and bounds in basic physics studies and applications of nanotubes. The full potential of nanotubes for applications will not be realized until the growth of nanotubes can be further optimized and controlled
Carbon Nanotube.docx (Size: 3.14 MB / Downloads: 46)
Introduction
Carbon nanotubes have attracted lots of attention of scientist from various fields because they exhibit extraordinary physical, chemical and mechanical properties due to their intrinsic nano-sized one-dimensional nature. Thus, their intrinsic characteristics will revolutionize everything from computer, bio- and medical-devices, batteries, nanocomposite-based vehicles to electronic nano-device in the near future.
1.1. Origin of carbon nanotube
Carbon is an element with unique properties and is the lightest among all the elements of group IV of the periodic table. It differs in many ways from other elements like Si, Ge, Sn and Pb of the same group, which all have sp3 bonding in their cubic solid ground states, while carbon in the condensed phase has a hexagonal ground state graphite with sp2 bonding and is highly anisotropic, nearly two dimensional semimetal. Lying close in energy to graphite is diamond, a three-dimensional material with nearly isotropic properties.
1.2. Structure of carbon nanotubes
A carbon nanotube is a graphene sheet appropriately rolled into a cylinder of nanometre size diameter as shown in fig 1.2. The curvature of the nanotubes admix a small amount of sp3 bonding so that the force constant in the circumferential direction is slightly weaker than along the nanotube axis[1]. CNTs are hexagonal networks of carbon atoms of approximately 1 nm diameter and 1 to 100 microns of length. They can essentially be thought of as a layer of graphite rolled-up into a cylinder.
1.3. Types of carbon nanotubes
Depending on the arrangement of the grapheme cylinders, there are two types of nanotubes: single walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs)[3].
A SWNT is a molecular scale wire that has two key structural parameters. By folding a graphene sheet into a cylinder so that the beginning and end of a (m, n) lattice vector in the graphene plane joined together, one obtains an (m, n) nanotube. The (m, n) indices determine the diameter of the nanotube, and also the so-called chirality. (m, m) tubes are arm chair tubes, since the atoms around the circumference are in an arm-chair pattern (Figure 1.4). (m, 0) nanotubes are termed as zigzag in view of the atomic configuration along the circumference (Figure 1.4).
Production techniques
For application of carbon nanotubes, large quantities of nanotubes are required, and scale-up limitations of the arc discharge and laser ablation techniques would make the cost of nanotubes based devices prohibitive. During nanotube synthesis, impurities in the form of catalyst particles, amorphous carbon and non-tubular fullerenes are also produced. Thus, subsequent purification steps are needed to separate the carbon nanotubes. The gas phase processes tend to produce nanotubes with fewer impurities and are more amenable to large scale processing. It is believed that the gas phase techniques, such as CVD, for nanotube growth offer greater potential for the scaling up of nanotubes production for applications. The three techniques of carbon nanotube production are discussed in detail below:
Carbon Arc-Discharge Technique
Two carbon electrodes are used in the carbon arc-discharge technique to generate an arc by DC current. The electrodes are kept in a vacuum chamber and an inert gas is supplied to the chamber. The purpose of the inert gas is to increase the speed of carbon deposition. Initially, the two electrodes are kept independent. Once the pressure is stabilized, the power supply is turned on (about 20 V) and the positive electrode is then gradually brought closer to the negative electrode to strike the electric arc.
Applications of Nanotubes
Carbon nanotubes are the smart materials recently being developed. It has got some unique and technologically promising properties which leads to its possible applications in following fields.
3.1. Hydrogen storage in CNTs
The use of hydrogen as fuel has some great advantages, however it is a tough job to store it and release when needed. Carbon nanotubes show very surprising hydrogen storage capacity, in spite of their relatively small surface area and pore volume. The values published for the quantity of hydrogen absorbed in nano-structured carbon materials varies between 0.4 and 67 mass%[2]. For nanotubes, one important issue currently being debated is whether hydrogen adsorption also occurs in the interstitial channels between adjacent nanotubes in a rope of SWNTs.
3.2. As sensors
Sensors are devices that detect or measure physical and chemical quantities such as temperature, pressure, sound, and concentration. The measurands are converted into an electrical signal. The main requirements of a good sensor are high sensitivity, fast response, low cost, high volume production, and high reliability.
Conclusion
Carbon nanotubes have been utilized either individually or as an ensemble to build functional device prototypes. Ensembles of nanotubes have been used for field emission based flat-panel displays, composite materials with improved mechanical properties and electromechanical actuators. Bulk quantities of nanotubes have also been suggested as high capacity hydrogen storage media. Individual nanotubes have been used for field emission sources, tips for scanning probe microscopy, nanotweezers and chemical sensors. Nanotubes are also promising as the central elements for future miniaturized electronic devices. The success in nanotube growth has led to the wide availability of nanotube materials, which is a main catalyst behind recent leaps and bounds in basic physics studies and applications of nanotubes. The full potential of nanotubes for applications will not be realized until the growth of nanotubes can be further optimized and controlled