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Carbon nanotubes (CNTs; also known as buckytubes) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been synthesised with length-to-diameter ratio of up to 132,000,000:1, which is significantly larger than any other materials known. They possess extraordinary strength and electrical and thermal properties. These novel properties make them useful in many nano electronics, optics and other areas of materials science.
1. Single-walled nanotubes (SWNTs) have a diameter of approx 1nm, with a tube length many million times longer. The structure of a SWNT can be imagined to be a wrapping of a one-atom-thick layer of graphite (graphene) into a seamless cylinder. The way the graphene sheet is wrapped is denoted by (n,m) called the chiral vector. If m=0, the nanotubes are called zigzag. If n=m, the nanotubes are called armchair.
2. Multi walled nanotubes (MWNTs) consist of multiple rolled layers or otherwise concentric tubes of graphite. There are two models that describe the structures of multiwalled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, that is, single-walled nanotube with in a larger single walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a rolled newspaper. The interlayer distance in multiwalled nanotubes is approximately equal to the distance between graphene layers in graphite which is 3.4 Ao.
1. Various physical properties such as thermal, optical and magnetic properties are dependent on the electronic state of the material. Some characteristics of electronic structure of solids are:
2. In an isolated solid, the electrons move in quantified energy levels. When the distance between atoms is less, electron orbitals interact with each other, which leads to broadening of energy levels to form energy bands.
The inner-shell electrons form narrow bands called internal bands and electrons in external shell form valence bands. The electrons in excited state form conduction band.
The difference of energy between valence band and conduction band is called energy gap. In metals (or conductors) this energy gap is zero; it is small in case of semiconductors and large in case of insulators.
The maximum energy for electrons at absolute zero (0 K) is called Fermi level or Fermi energy. The physical properties of materials are mainly governed by electrons that have energy larger than Fermi energy.
Nanocrystalline materials have an average crystallite size in the range 1 to 100 nm and are characterized by numerous grain boundaries due to the small size of the grain. The mechanical properties of nanocrystalline materials are determined by their small grain size and the grain boundary. These properties are enhanced by reducing the grain size, as grains of nano-size have no defects inside, unlike micro-grains of relatively larger size.
For example, nanocrystalline copper is found to be three times more resistant to applied stress than normal copper crystals and deformed homogeneously. The crystallinity of the grain structure is maintained right up to the grain boundary. The fraction of atoms out of the total volume present at the grain boundaries is large for small size grains and decreases with increase in their size.
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Shared publicly - 2019-08-23 00:00:00
Don’t want your columns to simply stack in some grid tiers? Use a combination of different classes for each tier as needed. See the example below for a better idea of how it all works.
Shared publicly - 2019-08-24 00:00:00
For grids that are the same from the smallest of devices to the largest, use the .col and .col-* classes. Specify a numbered class when you need a particularly sized column; otherwise, feel free to stick to
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