Custom Search

Thursday, July 10, 2008


Nanotechnology deals with nanostructures having dimensions of the order on nanometer. Size of nanomaterials/nanostructures varies from a few nanometers to a few hundred nanometers. Both surface effects and size effects plays crucial role in the properties of nanomaterials. Nanomaterials exhibit properties strikingly different from those of bulk materials.
Nanomaterials are classified as quantum wells, quantum wires and quantum dots. In a three dimensional structure, if one dimension, say thickness, is of nanosize, then the structure is called quantum well. If two dimensions are of nanosize, then it is called a quantum wire and if all the three dimensions are of nanosize, then it is called quantum dot. The word quantum is associated with the structures because the properties exhibited by them are described by quantum mechanics.
Fullerenes, carbon nanotubes and nanowires are examples of inorganic nanomaterials. Each fullerene molecule comprises of sixty carbon atoms arranged in globular shape. Nanowires are arrangements in one direction made of silicon, zinc oxide and various metals. Artificially synthesized DNA strands are examples of organic nanomaterials.
Nanomechanical bearings
Nanomechanical bearing is a simplest nanomechanical system. Here molecules are polycyclic in nature resembling a conventional bearing. For nanomechanical bearing, static friction is low and there is no need of any lubrication. As the atoms are granular, they will not provide a smooth surface in the molecular bearing because of which it results in friction. But by choosing stiff materials, low friction bearing can be constructed. A type of nanostructure named ‘nested structure’ formulated using carbon nanotubes is showing promise for building low-friction rotational bearings.
Fabrication technology
Mainly there are two approaches in the fabrication of nanomaterials: Top down approach and bottom up approach.
In top down process, conventional mechanical methods are employed to reduce the size of the particles from micron level to nanolevel. One example is high energy mechanical milling or ball milling. During milling, balls in the vial impart energy to powder material splitting them further to nanoscale.
In bottom up process, wet chemical methods, plasma chemical methods, sol gel methods etc. are used. Self assembly method also used. Here under specific conditions, atoms and molecules arrange themselves spontaneously to form the final product. This bottom up approach is termed as molecular manufacturing which requires new tools and techniques.
Carbon nanotubes
Carbon has two well known natural forms namely, diamond and graphite. In 1985, carbon nanoclusters (fullerene or buckyball) were discovered. It contains sixty carbon atoms with 20 hexagonal and 12 pentagonal faces symmetrically arranged to form a molecular ball of carbon atoms.
Carbon nanotubes are cylindrical fullerenes with diameter a few nanometer and length a few millimeter. By virtue of their unique molecular structure, they are characterized by high tensile strength, high ductility, high resistance to heat and chemically inert. They are metallic or semiconducting in nature depending on the diameter of the tube.
1. In the construction of electronic devices like transistors, logic gates etc.
2. In the construction of nanowires.
3. In flat panel displays.
4. In scanning probe microscopes.
5. In fuel cells/batteries.
6. As catalysts in chemical reactions.
7. As chemical sensors.
Scaling laws
Physical quantities of nanoscale systems differ enormously from those familiar in macroscale systems. Some of these quantities (magnitudes) can, however, be estimated by applying scaling laws to the values for macroscale systems. Thus scaling laws are size dependent laws which are used to compute the magnitudes of physical parameters for nanoscale systems from their macroscale counterpart.
Ultrasonics are sound waves whose frequencies are greater than 20 kHz.
Properties of ultrasonics
1. Attenuation: The amplitude of the ultrasonic waves diminishes as it propagates through a medium.
2. Undergo diffraction just like light and sound waves.
3. Require a material medium for propagation.
4. Obey law of reflection and law of refraction just like light waves.
5. Carry much more energy than audible sound waves.
6. Can penetrate large distances through matter.
7. Produce heat while passing through a medium.
Applications of ultrasonics
1. In the determination of elastic constants of solids and liquids.
2. In ultrasonic signaling.
3. In the production of emulsions.
4. To destroy microbiological organisms.
5. To clean delicate material surfaces.
6. In non-destructive testing.
Non-destructive testing of materials
Non-destructive testing is the testing of a material without impairing its future usefulness.
Ultrasonic non-destructive testing of materials
The method is based on the principle that, when a beam of ultrasonics passes from one medium to another, at the boundary of separation between the two media, part of the wave is reflected back into the medium in which it is incident at the boundary. If there is any flaw/defect present in the sample, the reflected wave will also contain the echo pulse corresponding to the flaw/defect.
An arrangement for nondestructive testing by using ultrasonics is shown below: Pulse generator-transmitter system produce ultrasonics that is allowed to pass through the specimen material. The receiver receives the reflected signal and amplified echo signal is displayed on a CRO. In CRO, ‘P’ is the source signal and ‘R’ is the signal reflected at B. ‘Q’ corresponds to the signal reflected by the flaw A. ‘x’ and ‘y’ represent the distances of the echoes ‘Q’ and ‘R’ from source pulse ‘P’. The size of the flaw and its location in the specimen are determined using the values ‘x’ and ‘y’.

Determination of velocity of ultrasonics in solids
The velocity of ultrasonics in a solid can be measured by pulse-echo method. The experimental setup is the same as the one used for non destructing testing of materials. The amplitude of the echo pulses decays exponentially with time.

The distance between two echo pulses gives the time ‘t’ required for ultrasonic pulse to travel through the specimen. The distance traveled by the ultrasonic pulse is twice the thickness of the sample. I.e., 2d.
\Velocity of ultrasonic waves (v) = 2d/t
Determination of elastic constants in solids
In solids, ultrasonics can travel in the form of longitudinal and transverse wave.
VL be the velocity of longitudinal wave and VT be the velocity of transverse wave.
Rigidity modulus h = rVT2
Youngs modulus E = 2r VT2 (1+s)
Where r is the density of the solid and
is the Poisson’s ratio.
Determination of velocity of ultrasonics in liquids
The given liquid is taken in a small transparent rectangular cell. Quartz crystal produces ultrasonic waves. These waves travel through the liquid column and get reflected at the reflector located at the top.

No comments: