Nanoscience and Nanotechnology with Dr. Abeer Alshammari

DR.ABEER ALSHAMMARI 2021

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PHYS 476: 

Introduction to Nanoscience and Nanotechnology

(Elective course) - 

2(2+0+0)

Lecturer: 

dr. Abeer Alshammari

Room no. 213- building 5

a

alshammri@ksu.edu.sa

Unique Properties of Nanomaterials

2.1 Microstructure and defects in nanocrystalline materials

2.2 Effect of nano-dimensions on materials behaviour

2.1.1 Dislocations

2.1.2 Twins, stacking faults and voids

2.1.3 Grain boundaries, triple junctions and disclinations

2.2.1 Elastic properties

2.2.2 Melting point

2.2.3 Diffusivity

2.2.4 Grain growth characteristics

2.2.5 Enhanced solid solubility

2.2.6 Magnetic properties

2.2.7 Electrical properties

2.2.8 Optical properties

2.2.9 Thermal properties

2.2.10 Mechanical properties

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2.1 MICROSTRUCTURE AND DEFECTS IN NANOCRYSTALLINE MATERIALS

The microstructural features of importance in nanomaterials include:

• Grain size, distribution and morphology

• The nature of grain boundaries and interphase interfaces

• Nature of intragrain defects

• Composition profiles across grains and interfaces

• Residual impurities from processing



Crystal lattice imperfections, such as point, linear, planar and volume defects, lead to the structure-sensitive properties of

materials.

In order to understand the novel properties of nanostructured

materials, we need to understand the structure and its inter -

relationship with properties.



Crystals are three-dimensional, periodic arrangements of

atoms/molecules in space.



Any imperfection leading to disruption of periodicity is referred to as

a ‘crystalline defect’. These defects are usually classified based on

their dimensionality, namely, point defects (0D), line defects (1D),

surface defects (2D) and volume defects (3D).

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2.1.1 Dislocations

•

Missing rows of atoms in a crystal are regions of high energy and stress due to disruption of the atomic bonds in the plane.

This provides a driving force for dislocations to be annihilated at surfaces or grain boundaries to minimize the strain energy

of the crystal.

•

Line defects (one dimensional defects), or dislocations, are

lines along which whole rows of atoms in a solid are arranged

anomalously. The resulting irregularity in spacing is most

severe along a line called the line of dislocation. Dislocation is

perhaps the most important crystalline defect , they can

weaken or strengthen solids.



W

ith decreasing grain size of nanograined materials, dislocation stability is reduced, due to the large grain boundary area.

•

The typical dislocation density in annealed crystalline materials is about 10

10

/cm. As the grain size is reduced to about 10

nm, the dislocation density can reduce by 2–3 orders or more and finally, below a critical grain size, dislocations are no

longer stable, i.e., there will be no dislocations in the nanocrystalline materials below the critical grain size.



The behaviour of nanocrystalline materials is decided to a large extent by these defects, and as such, nanomaterials exhibit

vastly different properties compared to bulk materials.

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2.1.2 Twins, stacking faults and voids

•

planar defects are often observed, even after annealing, in many faceted nanomaterials, including nano rods and

nanowires. These planar defects include twins and stacking faults (intrinsic or extrinsic), and are usually neglected by most

analytical models.

For example, many bulk metals have the face-centred cubic structure, but nanocrystals

and nanorods of the same material often exhibit various structural modifications such

as single or multiple symmetric twinning, as well as five-fold cyclic twinning, resulting

in decahedral and truncated decahedral nanostructures below critical sides.

•

Twins are generally observed in crystals subjected to deformation under high strain

rate or at low temperatures. During crystallization of liquid metal, it is expected that

volume misfit strains can be easily accommodated in the liquid phase, and hence one

does not expect the formation of twins in the nucleating crystals.

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Voids in nanocrystallites may be located at either

Triple junctions

Large porosities

Due to insufficient compaction and sintering of nanocrystallites

synthesized from the powder method. Although both types of voids

influence the behaviour of the nanocrystallites, the first is structurally

more important. It is suggested that triple junction voids arise as a result of

relaxation of nanocrystalline grain boundaries.

2.1.3 Grain boundaries, triple junctions and disclinations

A schematic representation of a hard-sphere model of an equiaxed nanocrystalline

metal as shown in Figure.

Two types of atoms can be distinguished:

Crystal atoms with nearest-neighbour configurations corresponding to the lattice

(black circles) .

Boundary atoms with a variety of interatomic spacings (white circles).

Nanocrystalline materials typically contain a high number of interfaces with

random orientation relationships, so, a substantial fraction of atoms lies in the

interfaces.

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Assuming that the grains have the shape of spheres or cubes, the volume fraction of nanocrystalline materials

associated with the boundaries (Vi) is estimated to be:

where δ is the average interface thickness and d is the average grain diameter. Thus, the volume fraction of

interfaces can be as much as 60% for 5 nm grains, 30% for 10 nm grains, and about 3% for 100 nm grains, for a

grain boundary thickness of 1 nm.

When the grain size is smaller than about 20 nm, the total volume of the intercrystalline region

(grain boundary and triple junctions) becomes significant. The density of grain boundaries in

nanocrystals is very large (~ 1019 cm–3) and there is wide distribution of interatomic spacing at

these grain boundaries.

It has been suggested that the triple junctions can be described based on the disclination defect

model. Disclinations (as Figure) are line defects  characterised by a rotation vector ω in contrast to

the translational vector b for dislocations.

The triple junctions may be considered to form a network of disclinations. Theoretical calculations

have shown that triple junction energies are comparable to dislocation energies, and that

compensating disclinations play a significant role in the properties of nanocrystalline metals with

grain size less than about 10 nm. These triple junctions are linear defects that play a significant role

in the mechanical, thermodynamic and kinetic properties of polycrystals.

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2.2 EFFECT OF NANO-DIMENSIONS ON MATERIALS BEHAVIOUR

2.2.1 Elastic properties

•

The elastic modulus of a material is proportional to the bond strength between atoms or

molecules. The higher the bond strength, the higher will be the melting point and elastic

modulus

•

The elastic properties of crystalline materials are usually considered to be structure

(microstructure) independent.

•

If the temperature is increased, the mean separation between atoms increases and

the elastic modulus decreases. A large increase in vacancy and other defect concentrations can

be treated as equivalent to higher apparent temperature. Increasing the defect concentration is

thus expected to decrease the elastic modulus

•

Nanomaterials, by virtue of their very high defect concentration, may have considerably lower

elastic properties in comparison to bulk materials. The elastic modulus of nanocrystalline

compacts was found to be 30%–50% lower than bulk values.

•

T

he porosities in the compact due to inadequate sintering can also influence the measured elastic properties. In contrast to

nanograined materials, it has been suggested that the elastic modulus of carbon nanotubes increases with decrease in tube

diameter. The increase of apparent elastic modulus for smaller diameters is attributed to the surface tension effects.

•

The effect of porosities on the elastic modulus is particularly predominant in materials

with grain size below ~ 20 nm. The presence of porosities can result in a decrease in

modulus of up to 20%–25%

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2.2.2 Melting point



T

he decrease in bonding energy due to enhanced surface and grain boundary area in nanocrystalline materials can

reduce the enthalpy of fusion and the melting temperature. At the melting point of a material, the solid and liquid

phases are in equilibrium.



T

he change in melting temperature is inversely proportional to the radius of the sphere. In other words, as the grain

size is reduced, the melting point is lowered.



It has been observed that nano-CdS of diameter ~ 2.5 nm melts at 600 K, much lower than the bulk melting point

(1675 K). The single-walled carbon nanotube melts at ~ 1600 K, 0.42 times its bulk melting point (3800 K).



It is known that atoms in a solid vibrate about their mean position. The amplitude of the vibrations increases with

increasing temperature. When the vibration amplitude exceeds a certain percentage of the bond length, melting

begins at the surface and propagates through the solid. Atoms at the surface and grain boundary are less constrained

to vibrate compared to atoms inside the crystal lattice. As the grain size decreases, the percentage of atoms residing

at surfaces and grain boundaries increases significantly. Hence, freestanding nanoparticles may show a lower melting

point compared to bulk.

     A similar effect has been reported on zinc nanowires embedded in holes in an anodic alumina membrane.

     The melting point of zinc nanowires was found to decrease with decreasing diameter of the nanowire.

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

It is fascinating to observe that the melting temperature does not continuously decrease with decreasing grain size in nano-

dimensions. In fact, as the cluster size is reduced below a critical limit, the melting point of clusters is seen to increase above

the bulk melting temperature of the material, at least in some cases.

It has been found that a solid containing about 10 atoms of Ga or IV A elements (C, Si, Ge, Sn and Pb) melts at temperatures

that are higher than the melting point of the corresponding bulk solid.



In contrast to nanoclusters and nano-agglomerates,

nanoparticles within a matrix may, in fact, experience an

enhancement in the melting temperature.

T

he melting point of nanoparticles embedded in a bulk matrix

increases with decreasing size of particulate as the pressure increases

with decrease in particle size. It was found that Ge nanocrystals

embedded in silica glass do not melt until temperatures are almost

200°C above the melting point of bulk Ge, and resolidify only when

the temperature is more than 200°C below its bulk melting point.