Module 5 functional materials

Module:5 Functional materials 7 hours
❖ Oxides of AB, AB2, ABO3 type (specific examples)
❖ Composites - types and properties
❖ Polymers - thermosetting and thermoplastic polymers –
synthesis and application (TEFLON, BAKELITE)
❖ Conducting polymers- polyacetylene and effect of doping
❖ Chemistry of display devices specific to OLEDs
❖ Nanomaterials – introduction, bulk vs. nano (quantum dots)
❖ Top-down and bottom-up approaches for synthesis, and
❖ Properties of nano Au.
1

Properties of different type of Solids

Octahedral holes Tetrahedral holes
✔ Ionic solids are considered as close packing of anions and the cations, occupying
the tetrahedral or octahedral sites.
✔ Solids can be categorized as mono component (one element A e.g, Cu), binary
[two elements AB (e.g, NaCl), or AB2 (e.g, TiO2)] and ternary, three elements
ABC (e.g, ABO3)
✔ The structure types are classified as A, AB, AB2 and ABO3.
Take three balls and arrange them in a triangle. Take
another three balls and arrange them in a triangle. Place
the two triangles on top of each other, twisted against
each other by 60 degrees. This is a octahedron on its side,
and in the center is the octahdral hole.
Place thumb, pointer and middle finger together so that the
finger tips form a triangle. Point at the center of the
triangle with the pointer of the other hand. That's the
tetrahedral hole. Now, form a triangle with the finger tips
of your other hand

• NaCl type
• ZnS type
• CsCl type

AB type Cesium Chloride (CsCl) Structure

✔ General chemical formula is ABO3, where 'A‘
and 'B' are two cations of very different sizes,
and O is an anion.
Ionic Compound of ABO3 type

❑ A composite material is a combination of two
materials with different physical and chemical
properties.
❑ The materials combined to produce a new material
which is specific to a definite work, for instance, to
become stronger, lighter, or resistant to electricity and
also improve strength and stiffness.
❑ The components maintain their identity within the
composite, i.e. they do not dissolve or completely
merge into one another, though they act in concert.
What is a Composite Material?

Example for composites showing superior properties

Constituents of composites
❑ Matrix (Continuous phase) : Continuous or bulk material
❑ Reinforcement (Dispersed Phase) : Added primarily to increase the strength
and stiffness of matrix
❑ The reinforcement is generally can be in the form of fibres, particles,
whiskers or flakes
Reinforcement
Composite
Interface
Re-
inforcement
(Dispersed
Phase)
Matr
ix
Composi
te

Functions (requirements) of the
matrix:
1. Keep the fibers in place in the structure;
2. Help to distribute or transfer loads;
3. Protect the fillers, both in the structure and before
and during fabrication;
4. Control the electrical and chemical properties of the
composite;
5. Carry interlaminar shear.
20

Types of composites
Natural Composites
(Exist in animal and
plants)
Wood = Cellulose +
Lignin
Bone= Hydroxyapatite
+ Collagen
Synthetic composites
(Man-Made)
Mud Bricks = Mud+
Straw
Reinforced Concrete
Fibre glass etc.

Classification of
composite
materials
Based on the matrix
materials
Based on the
geometry of
reinforcement
⮚ Polymer matrix composite
(PMCs)
⮚ Metal Matrix composite (MMCs)
⮚ Ceramic matrix composite (CMCs)
⮚ Carbon/carbon composites (c/Cs)
⮚ Particulate reinforced composites
⮚ Whisker /Flakes reinforced
composites
⮚ Fibre reinforced composites

23
Polymer matrix Composites
o Polymers constitute the most important matrix
materials and are used in more than 95% of the
composite products in use today.

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Polymer matrix composites
⮚ Thermoplastic polymer matrices
• Thermoplastics are incorporated in the
composite system by melting and
solidifying by cooling.
• The physical reaction being reversible in
nature.
• Thermoplastics have low creep resistance
and low thermal stability compared to
thermosetting resins.
⮚ Thermoset polymer matrices
▪ Thermosetting resins are more common
for the development of composite systems.
▪ Solidification from the liquid phase takes
place by the action of an irreversible
chemical cross- linking reaction, generally
in the presence of heat and pressure.
⮚ Elastomer based composites
- The greater extensibility and
high-energy storing capacity
make them a suitable
continuous phase for
composite materials.
- Unlike plastics, a wide variety
of flexible products can be
made using elastomers as the
matrix phase.
- They offer elastic strain higher
than that of metals and can
be stretched rapidly, even
under small loads.

Applications of Polymer matrix
composites (PMC)
The Fiber-line Bridges,Denmark

Metal- matrix composites (MMC)
❖ Metal Matrix Composites are composed of a metallic matrix (Al,Mg,Fe,Cu
etc) and a dispersed ceramic (oxide, carbides) or metallic phase( Pb,Mo,W
etc).
❖ Ceramic reinforcement may be silicon carbide, boron, alumina, silicon
nitride, boron carbide, boron nitride etc.,whereas metallic reinforcement
may be tungsten, beryllium etc.
❖ From a material point of view, when compared to polymer matrix
composites, the advantages of MMCs lie in their retention of strength and
stiffness at elevated temperature, good abrasion and creep resistance
properties.
❖ Most MMCs are still in the development stage or the early stages of
production and are not so widely established as polymer matrix composites.
The biggest disadvantages of MMCs are their high costs of fabrication.
❖ There are also advantages in some of the physical attributes of MMCs such
as no significant moisture absorption properties, non-inflammability and
resistance to most radiations.
26

Metal- matrix composites (MMC)
❖ Only light metals are responsive, with their low density proving an advantage.
Titanium, aluminium and magnesium are the popular matrix metals currently
in vogue, which are particularly useful for aircraft applications.
❖ If metallic matrix materials have to offer high strength, they require high
modulus reinforcements.
❖ The strength-to-weight ratios of resulting composites can be higher than most
alloys.
❖ The melting point, physical and mechanical properties of the composite at
various temperatures determine the service temperature of composites.
❖ They can withstand elevated temperature in corrosive environment than
polymer composites. However, practically, the choices for low temperature
applications are not many.
❖ The choice of reinforcements becomes more stunted with increase in the
melting temperature of matrix materials.
27

o Carbide drills are often made from a tough cobalt
matrix with hard tungsten carbide particles inside.
o Modern high-performance sport cars, such as those
built by Porsche, use rotors made of carbon fiber
within a silicon carbide matrix.
o Metal Matrix Composite (MMC) are used in Diesel
Piston, disk break
o The F-16 Fighting Falcon uses monofilament silicon
carbide fibres in a titanium matrix for a structural
component of the jet's landing gear.
o Today these applications are found most often in
aircraft components, space systems and high-end
or "boutique" sports equipment.
o MMCs are used for Space Shuttle, commercial
airliners, electronic substrates, bicycles,
automobiles, golf clubs and a variety of other
applications.
Applications of Metal matrix composites (MMC)
28
Diesel Piston
Cast Brake
Disk

Ceramic Matrix composite (CMC)
o A given ceramic matrix can be reinforced with either
discontinuous reinforcements, such as particles,
whiskers or chopped fibres, particulates having
compositions of Si3N4, SiC, AlN, titanium diboride,
boron carbide, and boron nitride or with continuous
fibres.
o Characteristics of CMC
❖ High-temperature stability
❖ High thermal shock resistance
❖ High hardness
❖ High corrosion resistance
❖ Light weight
❖ Nonmagnetic and nonconductive properties
❖ Versatility in providing unique engineering solutions
29

30
Ceramic matrix composites
(CMC)
❖ Ceramic-matrix composite development has continued to focus on
achieving useful structural and environmental properties at the
highest operating temperatures. The high risk associated with this
task foreshadows the relatively small number of commercial
products.
❖ However, development of CMCs for other uses has also been
pursued, and significant commercial products now exist.
❖ Ceramics can be described as solid materials which exhibit very
strong ionic bonding in general and in few cases covalent bonding.
❖ High melting points, good corrosion resistance, stability at
elevated temperatures and high compressive strength, render
ceramic-based matrix materials a favourite for applications
requiring a structural material that doesn’t give way at
temperatures above 1500ºC.
❖ Naturally, ceramic matrices are the obvious choice for high
temperature applications.

Applications:
❖ CMCs find promising applications in the area of cutting tools and
in heat engines where the components should withstand aggressive
environments.
❖ In Aircraft engines - use of stator vanes formed of CMC in the hot
section of the F136 turbofan engine is under consideration.
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32
Applications of CMC
• Exhaust nozzle flaps and seals in the F414 engine of the Navy F-18 E/F.
The exhaust temperature of the F414 is over 800C (1450F) higher than the F404
engine used in the previous version of the F-18. As a result, the metal flaps and
seals were failing in tens of hours. The CMC parts consist of a Nicalon (Dow
Corning Corp.) fiber with an inhibited carbon matrix. A thick SiC overcoat and
glaze provide protection from oxidation.
Insertion of the CMC flaps and seals has produced a weight savings of nearly 1
kg (2 lb) per engine relative to the metal parts. Because this mass is at the very
back of the aircraft, additional weight savings can be obtained by removing
ballast to shift the center of gravity of the aircraft. The CMC flaps have a useful
life that is at least double the design requirement of 500 hours.

Applications of CMC
• Ceramic-matrix composites are now also commercially available as brake rotors
for automobiles.
✔ The new Porsche braking system uses an MMC brake pad.
✔ Ceramic-matrix composite brake rotors have also been demonstrated for the
Inter-City Express high-speed trains in Germany, where a total weight savings of
5.5 metric tons is obtained per train set.

Plastics are classified into two types………..
1. Thermoplastic
2. Thermosetting resins
Thermoplastics are the plastics that do not undergo chemical change in
their composition when heated and can be molded again and again.
✔ They are prepared by addition polymerisation.
✔ They are straight chain (or) slightly branched polymers and
✔ Various chains are held together by weak vanderwaal’s forces of attraction.
✔ It can be softened on heating and hardened on cooling reversibly.
✔ They are generally soluble in organic solvents
Examples: Polyethylene, Polyvinylchloride
✔ Common thermoplastics range from 20,000 to 500,000 amu
✔ Each polymer chain will have several thousand repeating units.
They can be recycled and reused many times by heating and cooling
process.
Types of Plastics
1. Thermoplastic

✔ Thermosetting resins can melt and take shape once; after they have
solidified, they stay solid.
✔ They are prepared by condensation polymerisation.
✔ Various polymer chains are held together by strong covalent bonds
(cross links)
✔ These plastics get harden on heating and once harden, they cannot be
softened again.
✔ They are almost insoluble in organic solvents.
Examples: Bakelite, Polyester
Thermoset Polymers whose individual chains have been chemically cross
linked by covalent bonds and form a 3-D cross linked structure.
⮚ Therefore, they resist heat softening and solvent attack.
⮚ These are hardened during the molding process and once they are cured,
they cannot be softened and they cannot be recycled and reused
Eg. Phenol-formaldehyde resins, urea-formaldehyde paints.
2. Thermosetting resins or Thermosets

Thermoplastic polymers Thermosetting polymers
Consists of long-chain linear polymers
with negligible cross-links.
Have 3-Dimensional network structures
joined by strong covalent bonds.
Soften on heating readily because
secondary forces between the individual
chain can break easily by heat or pressure.
Do not soften on heating; On prolonged
heating, they are charred.
By re-heating to a suitable temperature,
they can be softened, reshaped and thus
reused.
Retain their shape and structure even on
heating. Hence, cannot be reshaped.
Usually soft, weak and less brittle. Usually, hard, strong and brittle.
Can be reclaimed from wastes. Cannot be reclaimed from wastes.
Usually soluble in some organic solvents. Due to strong bonds and cross-linking,
they are insoluble in almost all organic
solvents.
Difference between Thermoplastic and Thermosetting Polymers………

Properties and engineering
applications
Types of Thermoplastic resins:
Vinyl resins
(i) PVC
(ii) TEFLON or FLUON
(iii) ABS (Acrylonitrile Butadiene Styrene)
Types of Thermosetting resins:
Phenolic resins or phenoplasts
(i) Bakelite
(ii) Novolac

(ii) TEFLON or FLUON (polytetrafuloroethylene) PTFE
✔ Teflon is obtained by polymerization of tetrafluoro ethylene under pressure in the
presence of benzoyl peroxide as catalyst.
Properties:
✔ This polymer is a hard, strong, extreme toughness, High chemical
resistance with a high m.pt. and very low surface friction.
✔ High softening point (350 C ), waxy touch, good mechanical and
electrical properties.
Due to all these qualities, the polymeric material can be machined.
✔ Its softens at about (350 C ), hence at this high temperature it can be
moulded applying high pressure.

Thermosetting Plastics:
(a) Phenolic resins or Phenoplasts : Novoloc and Bakelite
Phenolic resins are condensation polymerization products of
phenol derivatives and aldehydes.
Novolac
At first, Phenol reacts with
Formaldehyde in presence of
acidic / alkaline catalyst to form
Monomethylnol phenol.
Monomethylol phenol further
reacts with Phenol to form a linear
polymer “Novolac”.
Water is removed as the by-
product.

Leo Baekeland
✔ bakelite is a crosslinked polymer of phenol and formaldehyde
Bakelite

Bakelite
✔ Further addition of HCHO at high
temperature and pressure converts
Novolac (soft and soluble) into cross-
linked “Bakelite” (hard and insoluble).
Thermosetting Plastics: Bakelite

Properties:
✔ Bakelite is resistant to acids, salts and most organic solvents, but it is
attacked by alkalis because of the presence of –OH groups.
✔ It possesses excellent electrical insulating property.
✔ As thermoset it is difficult to recycle.
Uses:
✔ Bakelite is used as an adhesive in plywood laminations & grinding
wheels, etc
✔ It is also widely used in paints, varnishes,
✔ It is used for making electrical insulator parts like plugs, switches, heater
handles, paper laminated products, thermally insulation foams etc.
Bakelite

Conducting Polymers
• Polymers, particularly those with a conjugated p-bond structure often
show higher conductivity when doped with conductive materials.
• But the use of conductive polymers is limited since they have poor
mechanical strength.
• Hence, a combination of mechanical and electrical properties can only
find good applications in conductive polymers area.
• Sometimes, in a polymer blend, a bifunctional linker is doped to
increase the conductivity of Polyaniline (PANI) (having conductivity) and
polycaprolactum (PCL) (having mechanical strength) blend.
• Conductive polymers can be made using simple procedures like melt
blending, solution blending etc., and can be used for antistatic and
electromagnetic shielding applications.
43

44
Some Examples of Conductive Polymers

Conjugation of π-electrons
+
A- Dopant anion for charge neutrality
Conducting
Insulating
Undoped
Doped
45
Mechanism of Conduction in Polymers

Different Types of Conducting Polymers:
1. Intrinsically conducting polymers (ICP)
2. Doped Conducting polymers
3. Extrinsically conducting polymers (ECP)
Factors that affect the conductivity:
1. Density of charge carriers
2. Their mobility
3. The direction
4. Presence of doping materials (additives that
facilitate the polymer conductivity in a better way)
5. Temperature
46

1. Intrinsically Conducting Polymers (ICPs)
⮚ Polymer consisting of alternating single and double bonds
is called conjugated double bonds.
⮚ In conjugation, the bonds between the carbon atoms are
alternately single and double. Every bond contains a
localised “sigma” (σ) bond which forms a strong chemical
bond.
⮚ In addition, every double bond also contains a less
strongly localised “pi” (π) bond which is weaker.
⮚ Conjugation of sigma and pi-electrons over the entire
backbone, forms valence bands and conduction bands.
Eg: Poly-acetylene, poly-p-phenylene, polyaniline, polypyrrole
polymers
47
Polyacetylene

2. Doped Conducting Polymers
It is obtained by exposing a polymer to a charge transfer agent
either in gas or solution phase. ICPs possess low conductivity (10-10
S (SIEMEN) or Mho/cm), but they possess low ionisation potential
and high electron affinity. So they can be easily oxidised or reduced.
DOPING:
The conductivity of ICPs can be increased by creating positive
charges (oxidation) or by negative charges (reduction) on the
polymer backbone. This technique is called DOPING .
In otherwords….
The polymer structure has to be disturbed - either by removing
electrons (oxidation) from or inserting them (reduction) into the
materials. The process is known as Doping.
There are two types of doping:
1. Oxidation with halogen (p-doping).
2. Reduction with alkali metal (n-doping). 48

(i) p-Doping:
⮚ It involves treating an ICP with a Lewis acid which leads to
oxidation process and positive charges on the polymer
backbone are created.
⮚ Some of the p-dopants are I2, Br2, AsF5, PF5 etc.
2(C2H2)n + 3I2 2[(C2H2)n
+ I3
-
polyacetylene Lewis acid
(ii) n-Doping:
⮚ It involves treating an ICP with a Lewis base which leads to
reduction process and negative charges on the polymer
backbone are created.
⮚ Some of the n-dopants are Li, Na, Ca, FeCl3, naphthylamine etc.
_
…-CH=CH-CH=CH-… + C10H7NH2 …-CH=CH-CH=CH- + C10H8
I
+NH
49

• In p-type doping, the dopant (Iodine, I2) attracts an
electron from the polyacetylene chain to form (I3
-) leaving
a positive soliton (carbenium ion) in the polymer chain
that can move along its length.
• The lonely electron of the double bond, from which an
electron was removed, can move easily.
• As a consequence, the double bond successively moves
along the molecule, and the polymer is stabilized by
having the charge spread over the polymer chain.
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Doping in Trans-Polyacetylene

51
51
Mechanism of Conduction in Polyacetylene

52
52
Conductivity Mechanism in Polyacetylene:
• The mechanism followed by polyacetylene for the transfer of charge from one
chain to another is called intersoliton hopping.
• What is a soliton? The soliton is a charged or a neutral defect in the
polyacetylene chain that propagates down the chain, thereby reducing the
barrier for interconversion.
• In n-type doping (This can be
done by dipping the film in
THF solution of an alkali
metal) soliton is a resonance-
stabilized polyenyl anion of
approximately 29-31 CH
units in length, with highest
amplitude at the centre of the
defect.
• The solitons (anions)
transfer electrons to a neutral
soliton (radical) in a
neighboring chain through an
isoenergetic process.
• The charged solitons are
responsible for making
polyacetylene a conductor.

3. Extrinsically Conducting Polymers
These are those polymers whose conductivity is due to the presence
of externally added ingredients in them.
Two types:
(1)Conductive element filled polymer:
▪ It is a resin/polymer filled with carbon black, metallic fibres, metal
oxides etc. Polymer acts as a binder to those elements.
▪ These have good bulk conductivity and are low in cost, light
weight, strong and durable. They can be in different forms,
shapes and sizes.
(2) Blended Conducting Polymers:
▪ It is the product obtained by blending a conventional polymer
with a conducting polymer either by physical or chemical change.
▪ Such polymers can be processed and possess better physical,
chemical and mechanical strength. 53

54
54
Conductivity of Polymers and Metals

55
55
55
Conductivity Comparison between Conductive Polymers
and Other Materials

56
Chemistry of display devices specific to OLEDs
An organic light-emitting diode (OLED or organic LED), also known
as organic electroluminescent (organic EL) diode, is a light-emitting diode
(LED) in which the emissive electroluminescent (EL) layer is a film of organic
compound that emits light in response to an electric current.

Working principle of OLEDs
⮚ The holes lie in the valence band, while the free electrons are in the conduction
band of material.
⮚ When there is a forward bias in the p-n junction, the electron which is a part of the
n-type semiconductor material would overrun the p-n junction and join with the
holes in the p-type semiconductor material. Therefore, regarding the holes, the free
electrons would be at the higher energy bands.
⮚ When this movement of free electron and hole takes place, there is a change in the
energy level as the voltage drops from the conduction band to the valance band.
⮚ There is a release of energy due to the motion of the electron.
⮚ In standard diodes, the release of energy in the manner of heat. But in LED the
release of energy in the form of photons would emit light energy.
⮚ This entire process is known as electroluminescence, and the diodes are known
as a light-emitting diode.

§ Introduction to Nanomaterials
•A nanoparticle is an entity with a width of a few nanometers to a few
hundred, containing tens to thousands of atoms. Their defining
characteristic is a very small feature size in the range of 1-100 (nm).
•Nano size:
One nanometre is a millionth part of the size of the tip of a needle.
1 nm = 10-6 mm = 10-9 m
Table 1. Some examples of size from macro to molecular
Size
(nm)
Examples Terminology
0.1-0.5 Individual chemical bonds Molecular/atomic
0.5-1.0 Small molecules, pores in
zeolites
Molecular
1-1000 Proteins, DNA, inorganic
nanoparticles
Nano
103-104 living cells, human hair Micro
>104 Normal bulk matter Macro
From: USDA’s
roadmap of
nanotechnology.
59
Bulk Gold

60
Size and shape dependent colors of Au and Ag nanoparticles
25 nm
Sphere
reflected
50 nm
Sphere
reflected
100 nm
Sphere
reflected
100 nm
Sphere
reflected
40 nm
Sphere
reflected
100 nm
prism
reflected
Gold NPs in Glass Silver NPs in Glass
60

61
Size and shape dependent colors of Au and Ag nanoparticles
Note: nanomaterials scatter visible light rather than absorb
⮚ Distance between particles also effects colour
Surface plasmon resonance: Excitation of surface plasmons by light (visible
or infra red) is denoted as a surface plasmon resonance
Localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures
61

62
Categories of Nanomaterials

Properties of Semiconductor Nanomaterials
⮚ The properties of nanosized semiconductor particles
depend very sensitively on the particle size .
⮚ The physical properties of semiconductor nanocrystals
are dominated by the spatial confinement of excitations
(electronic and vibrational).
⮚ Quantum confinement that manifests itself in widening
of HOMO-LUMO gap or the bandgap increase with
decreasing crystallite size and its implications on the
electronic structure and photophysics of the crystallites
has generated considerable interest. 63

Quantum Confinement
❑ In the nanoparticles with some of its dimensions smaller
than 10nm, show new effects, since the laws of classical
physics no longer valid and we need quantum physics to
explain them.
❑ For example, the minimum potential energy of an electron
confined in a nanoparticle is higher than expected in
classical physics and energy levels of different electronic
states are discrete.
❑ Due to quantum confinement, the particle size has a
drastic effect on the density of electronic states and thus
on the optical response 64

65
Quantum dots (QDs) are semiconductor particles of few nanometers (<10
nm) in size, having optical and electronic properties that differ from larger
particles.
Quantum Dots and applications
5 nm
Cancer cell imaging Metal ions sensing Light emitting diode

66
66
Emission properties of Quantum Dots
1. Band gap decreases as the QDs size increases
2. Emission wavelength becomes red shifted with increase in size of QDs

67
Synthetic Procedures of Quantum Dots
1. High temperature synthesis
2. Hydrothermal synthesis
3. Microwave synthesis
4. Ultrasonication synthesis

•Nanomaterials show unusual
⮚ Mechanical
⮚ Electrical
⮚ Optical
⮚ Magnetic properties.
Hence it has potential applications in
⮚ Different industries
⮚ Biomedical
⮚ Electronic applications.
⮚ For example, long lasting medical implants of
biocompatible nanostructured ceramic and
carbides,
⮚ Biocompatible coating
⮚ Drug delivery
⮚ Protection coatings
⮚ Composite materials
⮚ Anti fogging coatings for spectacles and car
windows etc. 68

⮚ At the nanomaterial level, some material properties are
affected by the laws of atomic physics, rather than behaving
as traditional bulk materials.
⮚ Nanomaterials can be metals, ceramics, polymeric materials,
or composite materials.
⮚ The beautiful ruby red color of some glass is due to gold
nanoparticles (Au NPs) trapped in the glass matrix.
⮚ The decorative glaze known as luster, found on some
medieval pottery, contains metallic spherical nanoparticles
dispersed in a complex way in the glaze.
69
Spherical
AuNPs
Rod shaped
nano Au

⮚ Deviation from the reduced size and dimensionality of the nanometer-sized
building blocks (crystallites), the numerous interfaces between adjacent
crystallites, grain boundaries and surfaces
⮚ These building blocks have different crystallographic orientation that may
lead to incoherent or coherent interfaces between them
⮚ Lead to inherent heterogeneous structure on a nanometer scale.
⮚ Grain boundaries make up a major portion of the material at nanoscales,
and strongly affect properties and processing.
⮚ Surfaces and interfaces- half or more than half atoms near to interfaces
⮚ Hence, surface properties such as energy levels, electronic structure,
and reactivity are different from bulk materials
What makes these nanomaterials so different and so intriguing?
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71
Gold : Bulk vs. Nano
Bulk Gold Nano Gold
1 Lustrous–they have a shiny surface
when polished
Are never gold in colour but found in a
range of colours
2 Malleable–they can be hammered, bent
or rolled into any desired shape
Size & Shape of the nanoparticles
determines the colour
3 Ductile–they can be drawn out into wires For example, Gold particles in glass
25 nm > red reflected
50 nm > green reflected
100 nm > orange reflected
4 Is metallic, with a yellow colour when in
a mass
Are not “metals” but are semiconductors
(Band gap energy = 3.4 eV)
5 Good conductors of heat and electricity Are very good catalysts
6 Generally have high densities
7 Have high melting point (~1080oC) Melts at relatively low temp (~940º C)
8 Are often hard and tough with high
tensile strength
9 Having high resistance to the stresses
of being stretched or drawn out
10 Not easily breakable
11 Inert-unaffected by air & most reagents

⮚ Band Gap
BAND GAP IN NANO AND BULK MATERIALS
73

Schematic representation of the ‘bottom up’ and top down’
synthesis processes of nanomaterials
High-energy
wet ball milling
By
atom-by-atom
molecule-by-molecule
cluster-by-cluster
Less waste
More economical
74

Any fabrication technique should provide the followings:
⮚Identical size of all particles (also called mono sized or with
uniform size distribution
⮚Identical shape or morphology
⮚Identical chemical composition and crystal structure
⮚Individually dispersed or mono dispersed i.e., no
agglomeration
75

❖ Top-down approaches
⮚ High-energy ball milling/Machining
⮚ Chemical Oxidation Process (CNTs to QDs)
⮚ Electrochemical Oxidation Process (Graphite rod to QDs)
⮚ Lithography (photo- and electrochemical)
⮚ Etching/Cutting
⮚ Coating
⮚ Atomization
❖Bottom-up approaches
⮚Gas Condensation Processing (GCP)/Aerosol Based Processes
⮚ Chemical Vapour Condensation (CVC)
⮚ Atomic or Molecular Condensation
⮚ Laser ablation
⮚ Supercritical Fluid Synthesis
⮚Wet Chemical Synthesis of nanomaterials (Sol-gel process)
⮚ Precipitation method
⮚ Spinning
⮚ Self-Assembly
⮚ DNA Origami
Nanoparticles preparation:
76

Schematic representation of the principle of mechanical milling
50 µm powder
WC coated
ball
77

❖ Procedure of milling process
⮚ Particle size reduction, solid-state alloying, mixing or blending, and
particle shape changes
⮚ Restricted to relatively hard, brittle materials which fracture and/or
deform and cold weld during the milling operation
⮚ To produce nonequilibrium structures including nanocrystalline,
amorphous and quasicrystalline materials
⮚ Users are tumbler mills, attrition mills, shaker mills, vibratory mills,
planetary mills etc
⮚ Powders diameters of about 50 µm with a number of hardened steel or
tungsten carbide (WC) coated balls in a sealed container which is
shaken or violently agitated. The most effective ratio for the ball to
powder mass is 5 : 10.
Mineral, ceramic processing, and powder metallurgy industry
78

• Shaker mills (e.g. SPEX model 8000) uses small batches of powder
(approximately 10 cm3 is sufficient
❖ Advantage: High production rates
❖ Limitation
⮚ Severe plastic deformation associated with mechanical attrition due to
generation of high temp in the interphase, 100 to 200 oC.
⮚ Difficulty in broken down to the required particle size
⮚ Contamination by the milling tools (Fe) and atmosphere (trace
elements of O2, N2 in rare gases) can be a problem (inert condition
necessary like Glove Box)(Fe <1-2% and Trace elements<300 ppm)
⮚ Protective coating to reduce milling tools contamination (MTC)
increases cost of the process
⮚ Working duration (>30 h) increases MTC (>10%)
79

Wet Chemical Synthesis of nanomaterials (Sol-gel process)
Schematic representation of sol-gel process of synthesis of nanomaterials.
Thermal
evaporation
800oC
SOL - nanoparticle dispersion
GEL - crosslinked network
Calcine
80

• Overall Steps:
• Step 1: Formation of different stable solutions of the alkoxide (the sol).
• Step 2: Gelation resulting from the formation of an oxide- or alcohol-bridged
network (the gel) by a polycondensation or polyesterification reaction
• Step 3: Aging of the gel, during which the polycondensation reactions
continue until the gel transforms into a solid mass, accompanied by
contraction of the gel network and expulsion of solvent from gel pores.
• Step 4: Drying of the gel, when water and other volatile liquids are removed
from the gel network.
– If isolated by thermal evaporation, the resulting monolith is termed a
xerogel.
– If the solvent (such as water) is extracted under supercritical or near
super critical conditions, the product is an aerogel.
• Step 5: Dehydration, during which surface- bound M-OH groups are removed,
thereby stabilizing the gel against rehydration. This is normally achieved by
calcining the monolith at temperatures up to 8000C.
• Step 6: Densification and decomposition of the gels at high temperatures
(T>8000C). The pores of the gel network are collapsed, and remaining organic
species are volatilized. The typical steps that are involved in sol-gel
processing are shown in the schematic diagram above. 83

• Sol/gel transition controls the particle size and shape. Calcination of
the gel produces the product (eg. Oxide).
• Sol-gel processing > hydrolysis and condensation of alkoxide-based
precursors such as Si(OEt)4 (tetraethyl orthosilicate, or TEOS).
• The reactions are as follows:
MOR + H2O → MOH + ROH (hydrolysis)
MOH+ROM→M-O-M+ROH (condensation)
• If the aging process of gels exceeds 7 days it is critical to prevent the
cracks in gels that have been cast
• Steps are:
Sol Gel Ageing Drying Dehydration
Densification & Decomposition Product
84

❖ Advantages
• Synthesizing nonmetallic inorganic materials like glasses, glass
ceramics or ceramic materials at very low temperatures compared to
melting glass or firing ceramics
• Monosized nanoparticles possible by this bottom up approach.
❖ Disadvantages
• Controlling the growth of the particles and then stopping the newly
formed particles from agglomerating.
• Difficult to ensure complete reaction so that no
unwanted reactant is left on the product
• Completely removal of any growth aids
• Also production rates of nanopowders are very slow by this process
85

Module 5 functional materials

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Module 5 functional materials