JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year PROPERTIES OF NANOMATERIALS Previous Question Paper (February/March - 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the physical concepts and mechanism for electrical conductivity and electrical resistance of nanomaterials.
Answer
- Quantum Confinement and Density of States: In bulk materials, energy bands are continuous. As dimensions shrink to the nanometer scale, the Density of States (DOS) becomes discrete. When the size of the material is comparable to the de Broglie wavelength of electrons, quantum confinement effects dominate, leading to a significant modification of the electronic band structure.
- Surface Scattering: Electrical resistance in nanomaterials is heavily influenced by surface scattering. As the surface-to-volume ratio increases, electrons frequently collide with grain boundaries and surface defects. This reduces the mean free path of charge carriers, thereby increasing the electrical resistivity compared to bulk counterparts.
- Quantum Tunneling: In nanostructures, electrons can exhibit quantum tunneling through thin insulating barriers or potential energy gaps. This mechanism allows for electrical conduction even when classical physics would predict zero current, significantly altering the macroscopic conductivity of nanocomposites.
- Ballistic Transport: When the characteristic dimension of the nanostructure is smaller than the mean free path of the electron, transport becomes ballistic. In this regime, electrons travel through the material without scattering, resulting in very low resistance and high conductivity, often observed in carbon nanotubes or metallic nanowires.
Differentiate between specular surfaces and diffuse surfaces.
Answer
| Feature | Specular Surface | Diffuse Surface |
|---|---|---|
| Definition | A surface where incident waves or particles reflect at an angle equal to the angle of incidence. | A surface where incident waves or particles are scattered in many directions due to surface roughness. |
| Surface Nature | Atomically smooth or perfect crystalline planes. | Rough, irregular, or polycrystalline surfaces. |
| Momentum Conservation | Parallel momentum of the charge carrier is conserved during reflection. | Parallel momentum is not conserved; it is randomized upon collision. |
| Effect on Resistance | Minimal impact on electrical resistance as the carrier velocity vector is preserved. | Significant increase in electrical resistance due to momentum relaxation and scattering. |
Explain the mechanism for reducing sound intensity and methods for sound management.
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Highlight the role of nanomaterials in cultural heritage with examples.
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Explain the calculation of surface-to-volume ratio for spherical and cylindrical shaped nanoparticles.
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What are the properties of 1-D nanomaterials?
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Discuss the effects of surface curvature on systems pressure and equilibrium number of vacancies.
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Explain the calculation of strain energy of an infinite crystal and of a perfect edge dislocation loop contained in a nanoparticle.
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Discuss the calculations for melting point of nanomaterials using free energy of bulk materials and change in surface energy.
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What are nanocomposites? List the properties and applications.
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Discuss the calculation for the magnetic properties of nanomaterials.
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Discuss the calculation for elasticity of discrete nanomaterials and elasticity of bulk nanomaterials.
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Differentiate between crystal plasticity and polycrystal plasticity.
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Write notes on physical basis of yield strength.
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