nanomaterials

1. NAST 613 : ELEMENTS OF MATERIALS SCIENCE AND
PHYSICAL PROPERTIES OF NANOSTRUCTURED
MATERIALS
SIZE EFFECT OF NANOPARTICLES
Course Instructor : Dr. A.Subramania
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.
have
⚫Temperature and pressure also
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.

9. EXAMPLE : NaCl
Room
Temperature
High
temperature
(upto melting)

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
applied to
⚫Specific surface area is measure
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 σ
i 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
above
It is based on atomic diffusion. Diffusion occurs in any material
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