Size effect of nanomaterials part1
- 1. SIZE EFFECT OF NANOPARTICLES Course Instructor : Dr. A.Subramania NAST 613 : ELEMENTS OF MATERIALS SCIENCE AND PHYSICAL PROPERTIES OF NANOSTRUCTURED MATERIALS SUBMITTED BY, MUGILANE.N M.TECH 1ST YEAR NANOSCIENCE & TECHNOLOGY
- 2. SIZE Nanoparticles are the simplest form of structures with sizes in the nm range. The physical properties of materials are dependent on the dimensions of the material – its properties (e.g. conductivity, elasticity, etc.) are scalable with respect to the amount of atoms in the material. There are basically two types of size-dependent effects: Smoothly scalable ones which are related to the fraction of atoms at the surface. Quantum effects which show discontinuous behaviour due to completion of shells in systems with delocalised electrons.
- 3. PROPERTY APPLICATION Single magnetic domain Small mean free path of electrons in a solid Size smaller than wavelength High & selective optical absorption of metal particles Formation of ultra fine pores due to superfine agglomeration of particles Uniform mixture of different kinds of superfine particles Grain size too small for stable dislocation Magnetic recording Special conductors Light or heat absorption, Scattering Colours, filters, solar absorbers, photovoltaics, photographic material, phototropic material Molecular Filters R&D of New Materials High strength and hardness of metallic materials
- 4. PROPERTY APPLICATION Large specific surface area Large surface area, small heat capacity Lower sintering temperature Specific interface area, large boundary area Superplastic behaviour of ceramics Cluster coating and metallization Multi-shell particles Catalysis, sensors Heat-exchange materials Combustion Catalysts Sintering accelerators Nano-structured materials Ductile ceramics Special resistors, temperature sensors Chemical activity of catalysts, Tailored Optical elements Surface/ Interface
- 5. SHAPE Small structures or Nanoparticles are not just the fragments of bulk materials. There can be entirely different structures as well as bond and bond strength in Nanomaterial. Temperature and pressure also have profound effect on the crystal structure.
- 6. EXAMPLE : Silicon crystal Experiments suggests that the shape of small size clusters are quite different
- 7. Even though some may acquire bulk crystalline structure, lattice parameters may not be the same as in the bulk material. For Example, X-Ray diffraction patterns of ZnS that as small as 1.4 nm particles had liquid like disorder. However larger nanocrystals of ZnS indeed show same sphalerite structure (cubic structure) as in the bulk. It has been observed that there is a lattice contraction of nearly 1% for 1.4 nm ZnS Nanoparticles.
- 8. With increase in temperature the disordered structure of small particles of ZnS were found to transform to wurtzite (hexagonal) structure. The chemical capping often used in the synthesis of nanoparticles, gets removed and the particles tend to agglomerate or coalesce forming larger particles. For structural transformation the nanoparticles require larger pressure and depends upon the particle size.
- 10. EXAMPLE : CdSe Nanocrystals CdSe nanoparticles of 2 to 4 nm size required 4.9Gpa to 3GPa pressure to transform them from wurtzite to rock salt structure. Bulk CdSe needs just 2.0Gpa for the same transformation
- 11. EQUILIBRIUM SHAPE The equilibrium crystal shape is the shape obtained by minimizing the total surface free energy for a fixed crystal volume. The key factor for calculating the equilibrium shape of a cluster is the cohesive energy of the atoms in a given geometry WULFF POLYHEDRON: Cluster is made by assembling atoms, treated as spheres with varying levels of order. This assembly of spheres cannot give rise to another sphere. The shape and nature of polyhedron depend on the binding energy of the atoms.
- 12. Wulff’s Plot -A Wulff Plot is a polar plot of the surface free energy as a function of orientation and fundamentally, the shape of a nanocrystal in equilibrium.
- 13. The construction criterion satisfies the following rule: If a face is characterised by Miller indices hkl and has area S, then Ƴhkl/Rhkl = constant If there is no anisotropy, as in the drop model where we have ƳhklShkl= ƳS, we simply obtain Ƴ/R = constant This is the equation for a sphere because R must be constant. If the need for faces is taken into account, the construction become much more difficult
- 14. MELTING POINT A decrease in the bonding energy would result in a lower melting temperature. Melting starts at the surface of a material. Surface atoms contribute to a lowering of the melting temperature of the particle.
- 15. Melting Point The melting point decreases dramatically as the particle size decreases below 100 nm
- 16. The change in melting temp. dependence thus as 1/R The melting temperature decreases rapidly for clusters with diameter below 5nm. According to this model a cluster with radius 2nm has melting temperature of 880K
- 17. SPECIFIC SURFACE AREA Specific surface area is measure applied to granular or granulate solids. It is the surface area per unit mass. It is important because many physical and chemical process takes place at the surface of solids. Unit : square meters per gram. Denoted by the symbol S. The general expression for this specific surface area per gram S is
- 18. Surface area increases with decrease in the particle size
- 19. Specific surface area depends on the shape... Scub = 1.24Ssph . So a cube has 24% more specific surface than a sphere with the same volume. General expression for the shape dependence of the area: volume ratio,
- 20. Dependence of the surface area S(L/D) of a cylinder on its length :diameter ratio L/D Specific surface areas of GaAs spheres, long cylinders (wires) and thin disks as a function of their size.
- 21. DENSITY Density can be generally varied by changing the pressure or the temperature. It has been observed that density changes with the change in the thickness of the layer in nm range. Mass density of Cu, Cr, TiN film on MgO was found to be lower than the corresponding bulk value. SiO2, SiC on stainless steel showed increase in density. Cu, Ag, Au showed no significant change.
- 22. Density varies with the size… The density decreases with the reduction in size but not in quantitative agreement with the results reported.
- 23. Optical Properties The band gap increases as the particle size decreases.
- 24. Image of Don Quixote become invisible at temperature < 341K
- 25. THERMAL PROPERTIES Nanocrystalline materials expected to have lower thermal conductivity compared to conventional material. In nanocrystalline materials, size become comparable to mean free path of phonons. Phonon scattering Phonon confinement and Quantization effects of phonon. • The use of nanofluids to enhance the thermal transport is another promising application the thermal properties of nanomaterials. •
- 26. ELASTIC PROPERTIES Elastic modulus is of material is proportional to the bond strength between the atoms or molecules. • Structure independent and dependent on temperature and defect concentration. • A large increase in vacancy and other defect concentrations can be treated as equivalent to higher apparent temperature. • If the temperature is increased, the mean separation between the atoms increase and modulus decreases. • Thus, the nanomaterials by virtue of their high defect concentration, may have considerably lower elastic properties in comparison to bulk materials.
- 27. E.O.Hall and N.J.Petch have derived the following relation, famously known as Hall-Petch relation between yield strength (σy) and grain size (d): Hall-Petch relation where σi is the ‘friction stress’, representing the overall resistance of the crystal lattice to dislocation movement, k is the ‘locking parameter’ that measures the relative hardening contribution of the grain boundaries and d is the average grain diameter
- 28. MORE ON TEMPERATURE DEPENDENT MICROSTRUCTURAL PROPERTIES
- 29. Fracture Mechanisms At higher temperatures the yield strength is lowered and the fracture is more ductile in nature At lower temperatures the yield strength is greater and the fracture is more brittle in nature This relationship with temperature has to do with atom vibrations. As temperature increases, the atoms in the material vibrate with greater frequency and amplitude. This increased vibration allows the atoms under stress to slip to new places in the material ( i.e. break bonds and form new ones with other atoms in the material). This slippage of atoms is seen on the outside of the material as plastic deformation, a common feature of ductile fracture
- 30. When temperature decreases however, the exact opposite is true. Atom vibration decreases, and the atoms do not want to slip to new locations in the material. So when the stress on the material becomes high enough, the atoms just break their bonds and do not form new ones. This decrease in slippage causes little plastic deformation before fracture. Thus, we have a brittle type fracture So, temperature determines the amount of brittle or ductile fracture that can occur in a material.
- 31. DISLOCATION DENSITY Another factor that determines the amount of brittle or ductile fracture that occurs in a material is dislocation density. The higher the dislocation density, the more brittle the fracture will be in the material. The idea behind this theory is that plastic deformation comes from the movement of dislocations. As dislocations increase in a material due to stresses above the materials yield point, it becomes increasingly difficult for the dislocations to move because they pile into each other. So a material that already has a high dislocation density can only deform but so much before it fractures in a brittle manner
- 32. Grain size As grains get smaller in a material, the fracture becomes more brittle. This phenomena is do to the fact that in smaller grains, dislocations have less space to move before they hit a grain boundary. When dislocations can not move very far before fracture, then plastic deformation decreases. Thus, the material's fracture is more brittle. Dislocation movement is temperature dependent. Their motion (slip) occurs by sequential bond breaking and bond reforming . The number of dislocations per unit volume is the dislocation density, in a plane they are measured per unit area. The growth of grain size with temperature can occur in all polycrystalline materials. It occurs by migration of atoms at grain boundaries by diffusion, thus grain growth is faster at higher temperatures
- 34. creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near melting point. Creep always increases with temperature. The effects of creep deformation generally become noticeable at approximately 30% of the melting point for metals and 40–50% of melting point for ceramics. Small grain size lowers creep resistance and Large grain size increases creep resistance. CREEP
- 35. SINTERING It is based on atomic diffusion. Diffusion occurs in any material above absolute zero but it occurs much faster at higher temperatures Sintering in practice is the control of both densification and grain growth. Densification is the act of reducing porosity in a sample thereby making it more dense. Grain growth is the process of grain boundary motion and Ostwald ripening to increase the average grain size. Many properties (mechanical strength, electrical breakdown strength, etc.) benefit from both a high relative density and a small grain size
- 36. Sintering occurs by diffusion of atoms through the microstructure. The different paths the atoms take to get from one spot to another are the sintering mechanisms. The six common mechanisms are: Surface diffusion – Diffusion of atoms along the surface of a particle Vapor transport – Evaporation of atoms which condense on a different surface Lattice diffusion from surface – atoms from surface diffuse through lattice Lattice diffusion from grain boundary – atom from grain boundary diffuses through lattice Grain boundary diffusion – atoms diffuse along ground boundary Plastic deformation – dislocation motion causes flow of matter