Unit‐IV

UNIT – IV: ADVANCED MATERIALS

Composites –Introduction, Constitution-Matrix phase, Dispersed phase.
Characteristic properties of composite materials. Classification- (A)Particle
- reinforced composites- i) Large –particle reinforced composites ii)
Dispersion – strengthened composites. (B) Fiber – reinforced composites-
i) Continuous – aligned ii) Discontinuous – aligned (short)- (a) aligned (b)
randomly oriented (C) Structural Composites- i) Laminates (ii) Sandwich
Panels.

Nanomaterials - Classification. Synthesis – Top down method (Ball

milling), Bottom up methods - wet chemical, physical and chemical vapor
deposition, Sol-Gel method, Self-assembly (DNA directed self-assembly of
AuNPs), Applications of nanomaterials - AuNPs based anti-cancer agents,
ZnO and Fe3O4 based memristors, Water purification using AgNPs, TiO2
based self cleaning glass.
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 Composites

 Composite: any multiphase material which consists of two or more

physically and/or chemically distinct phases with an interface
separating them
 obtained from conventional materials like metals, ceramics and
polymers by adding fibers, particles etc.
 two or more materials with significantly different physical and/or
chemical properties, when combined yields a material with
properties completely different from the individual components
 individual components remain distinct within the final structure
 E.g.: Wood (composite of cellulose fibers and lignin cementing
materials), bone (protein collagen (a soft but strong) with apatite
(hard but brittle)), insulating tape; reinforcing concrete
 in general are composed of two phases namely, the matrix phase and
the dispersed phase; surface forming the common boundary
between these two phases is known as interphase
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 Matrix phase (MP)

 continuous body constituent that encloses the composite and

gives it its bulk form
 MP may be a metal (metal matrix composites (MMC)), ceramic
(ceramic matrix composites (CMC)) or polymer (polymer matrix
composites (PMC))
 Functions of MP: (i) binds the dispersed phase together through
its cohesive and adhesive forces; (ii) acts as a medium for transmit
and distribute when an externally applied load to the dispersed
phase; (iii) maintains the dispersed phase in proper position and
orientation so as to carry the intended load and also protects from
chemical action; (iv) prevents propagation of brittle cracks by
virtue of its plasticity and softness

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 Dispersed phase (DP)

 structural constituent that determines the internal structure of a

composite
 DP may be fibers, particles, whiskers, and flakes
 Fibers: long and thin filament of any polymer, metal or ceramic
having high aspect ratio (i.e., the length-to-diameter ratio nears
the crystal-sized diameter); have high strength and stiffness;
orientation of the fiber is very important as far as the properties
of fibrous composites are concerned; E.g.: glass fibers, carbon
fibers, aramid fibers, etc.

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 Glass fibers

 obtained by extruding glass melt through small orifices, rapid

pulling, followed by cooling
 Individual fiber is called monofilament which has a diameter of ~
10 µm
 Glass fibers are used as reinforcing material for the following
reasons: (i) readily available in low cost; (ii) easy to draw from
molten melt; (iii) can be used to prepare glass-reinforced polymer
composites; (iv) can be coupled with polymer matrix to develop
chemical inertness

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 Carbon fibers

 obtained by the pyrolysis of an organic carbon rich filamentary

material such as cellulose and polyacrylonitrile in inert
atmosphere
 monofilaments have diameters of about 5 – 10 µm
 used as reinforcing material with epoxy or polyester resins to
from composites, which have higher specific strength than metals
 high performance fibers but quite costly
 Carbon fibers are used as reinforcing material for the following
reasons: (i) have high modulus, specific strength, and stiffness
even at elevated temperatures; (ii) resistance to moisture, acids,
bases and a number of solvents

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 Aramid fibers

 aromatic polyamides such as Kevlar and Nomex

 prepared from nematic liquid crystalline solution using a dry jet
wet spinning technique or using H2SO4 as solvent and cold water
as coagulant
 weak in compression and susceptible to degradation by acids and
strong bases
 Aramid fibers are important for the following reasons: (i) high
modulus and tensile strength, (ii) stability at temperatures and
retention of mechanical properties in wide range of temperatures
(–200 to 200 oC), (iii) resistant to creep and fatigue failure, (iv)
excellent toughness and impact resistance

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 Particulates

 small pieces of hard solid metallic or non-metallic materials

 distribution of particles in a given matrix is mostly random -
resulting composites are isotropic
 Addition of particles bring following changes in matrix: (i)
improvisation of performance at elevated temperatures, (ii)
increase in surface hardness, (iii) improvisation of wear and
abrasion resistance, (iv) changes in the thermal and electrical
conductivities

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 Flakes

 thin solids having a two-dimensional geometry, e.g., mica flakes

 impart equal strength in a plane compared to fibers, which
reinforcement is unidirectional
 packing of flakes is more efficient than those of fibers and
spherical particles

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 Whiskers

 thin strong filaments or fibers made by growing a crystal with

high aspect ratio
 E.g.: made up of graphite, silicon carbide, sapphire, silicon nitride
and aluminium oxide
 possess high elastic modulus, high degree of crystalline perfection
and exceptionally high strength
 However, their usage in composites is limited, due to high cost and
difficulty in incorporation in matrix

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 Characteristics of composites

 higher specific strength

 lower specific gravity
 higher specific stiffness
 maintain strength even up to high temperatures
 better toughness, impact and thermal shock-resistance
 cheaply and easily fabricable
 better creep and fatigue strength
 lower electrical conductivity
 lower thermal expansion
 better corrosion and oxidation-resistances

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 Types of composites

 classified on the basis of type of reinforcement used in the matrix,

type of matrix, and number of layers
 On the basis of type of reinforcement used in the matrix: (i)
particle-reinforced composites; (ii) fiber-reinforced composites
and (iii) structural composites

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 Particle ‐ reinforced composites

 made by dispersion of particulates of different shape and size in a

matrix of another material - matrix as well as particulates shares
the load-bearing function
 behavior is govern by: (i) nature of distribution of particles, i.e.,
uniform/non-uniform, (ii) relative volume fraction of
constituents, (iii) strength of the force at the interface of particles
and (iv) matrix
 sub-classifications: large-particle composites and the dispersion-
strengthened composites - difference depends on the
reinforcement or strengthening mechanisms

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Large – particle composites:
 particle size is 1-50 µm; harder and stiffer than the matrix -
interaction between particle and matrix phases is not on the
atomic or molecular level
 Particles provide strength to the composite by preventing the
movement of the matrix
 Particles can have varied geometries, but they should have
approximately the same dimension in all directions (equi-axed
particles)
 For effective reinforcement, the particles should be small in size
with uniform distribution throughout the matrix
 volume fraction of the two phases influences their properties in
the final structure
 manufactured with all three material types such as metals,
polymers, and ceramics 14
 Cermets are examples of ceramic–metal composites; cemented
carbide is the most common cermet, and is composed of hard
particles of tungsten carbide (WC) or titanium carbide (TiC)
embedded in a metal matrix like cobalt or nickel - used
extensively as cutting tools for steel
 Concrete is composed of coarse rock or gravel is embedded in a
matrix of cement. The coarse rock or gravel provides strength and
stiffness while the cement acts as a binder to hold the structure
together

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Dispersion – strengthened composites:
contains uniformly dispersed, fine, hard and inert particulates as
reinforcement
size of the particulates is less than 0.1μm
interaction between particulate and matrix phases occurs at the
atomic or molecular level
Metallic or non-metallic and metal oxide materials are as
dispersion phase
addition of 3 vol% of thoria (ThO2) particles, the high-temperature
strength of nickel alloys can be enhanced significantly -> thoria-
dispersed nickel

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 Fibre ‐ reinforced composites

 fibers are used as dispersed phase for reinforcement of matrices

 strength and other mechanical properties of fiber-reinforced
composites are influenced by: (i) orientation of fibers relative to
one another, (ii) fiber concentration, and (iii) distribution of fibers
 With respect to the orientation, two cases are possible: (a)
parallel alignment of fibers in a single direction, and (b) random
alignment

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 Continuous fibers are usually aligned (Figure a), whereas
discontinuous fibers can be aligned (Figure b), and randomly
oriented (Figure c). But, the overall composite properties are
better when the fiber distribution is uniform

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Continuous and aligned fiber composites:
 reinforcement efficiency is maximum when continuous fibers are
used
 Fibers should be longer than a critical length (minimum length
required to transmit entire load from the matrix to the fibers)
 If fibers are shorter than the critical length, only part of the load is
transmitted to the matrix
 Aligned and continuous fibers give the efficient and effective
strength to fiber composites

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Discontinuous & Aligned fiber composites:
 Fibers which are shorter than the critical length are used
 discontinuous fibers are less effective in strengthening compared
to continuous fibers. However, their resultant modulus and tensile
strengths can be made up to 55-90% of their continuous and
aligned counterparts
 cheaper, faster and easier than the continuous fibers and can be
made into complicated shapes
 Chopped glass fibers, carbon and aramid discontinuous fibers are
usually employed in these composites

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Random fiber composites:
 also called as chopped (or discrete) fibers
 strength of this kind of fibers is not high as with aligned fibers,
however, the advantage is that the composites will be cheaper and
isotropic in nature

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Some important types of fiber-reinforced composites:

Glass fiber‐reinforced polymer composites:

 glass fibers (long as well as short) are employed for enhancing the
properties of polymeric matrices containing nylons, polyester, etc.
 These composites possess lower density, higher tensile strength
and impact resistance and resistance towards chemicals and
corrosion
 These fiber-reinforced polymer composites have applications in
making of automobile parts, storage tanks, floorings, plastic pipes,
etc.

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Carbon fiber‐reinforced polymer composites:
 composites are useful in situations which require (i) excellent
corrosion resistance, (ii) low density, and (iii) retaining of
properties even at high temperatures – hence, these composites
are known as advanced polymer composites or high performance
composites
 find applications in structural components (wings, body) of
aircrafts and helicopters, sports materials (golf balls), etc.

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Aramid fiber‐reinforced polymer composites:
 Short fiber-reinforced composites: Due to the high aspect ratio,
high surface area, toughness, strength, heat stability and wear
resistance, short fibers provide effective reinforcement. These
find applications in the making of automobile brakes and clutches

 Long fiber-reinforced composites: These composites are ductile

and respond to compressive stresses, because aramid fibers are
capable of absorbing energy. These composites are excellent
structural and engineering materials and applications in the
manufacturing of aircrafts, helicopter parts, protective materials,
etc.

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 Structural composites

 usually composed of both homogeneous and composite materials

 properties of these composites depend not only on the properties
of the constituent materials but also on their geometrical design
 Two sub-classes: laminar composites and sandwich panels

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Laminates:
 consist of a number of two-dimensional layers/sheets that are
stacked and glued together so that the orientation of
reinforcement varies with each successive layer

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properties of such composites depend upon the properties of the
constituents and their geometrical design. E.g.: plywood, which is a
laminated composite of thin layers of wood with alternate layers
cemented together so that the grain of each layer is at right angles of
its neighbor
Can also be constructed using fabric material such as cotton, paper,
or woven glass fibers embedded in a plastic matrix

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Sandwich Panels:
 consists of two strong outer sheets known as faces and an
intervening layer of comparatively less-dense core material. These
three layers are joined by using adhesive
 The function of core is to reduce the deformation perpendicular to
the face plane by providing shear rigidity

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In these composites, faces bear most of the in-plane loading as well
as transverse bending stresses
When the thickness of core panel is increased, its stiffness also
increases considerably
Most popular core material is of ‘honey comb’ consisting of thin
foils forming interlocked hexagonal cells with their axes oriented at
right angles to the direction of face sheets - such sandwich panels
find application in the fabrication of wings of aircrafts, boat hulls,
ship structure parts, building structures (roofs, floors, walls, etc.)

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 Applications of composites

 In space craft applications like antenna structures, solar

reflectors, satellite structures, radar, rocket engines, etc.
 Aeronautical applications like components of rockets, aircrafts,
helicopters, missiles, etc.
 Marine applications like shafts, hulls, propellers, and ship parts.
 In automobile industries, bearing materials, turbine engines,
pressure vessels, abrasive materials, electrical machinery,
fabrication of floors, furniture, cutting tools, electrical brushes,
sports goods, high speed machinery, etc.

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 Nanomaterials

 In 1959 Richard P. Feynman with his famous talk "There is plenty of

room at the bottom" predicted that one day we will be making things
at the atomic level
 Before 640 AD: The “Lycurgus Cup” is a Roman artifact. It is dichroic,
changing colour when illuminated from inside. This effect is caused
by gold and silver nanoparticles, and was likely produced by accident

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 Nanomaterials

 show abrupt change of materials properties such as optical,

electrical, mechanical, thermal and catalytic properties when size
enters into the nanoscale domain
 strong enhancement of surface to volume ratio and quantum
confinement effects is two main factors behind materials to exhibit
different properties at nanoscale
 E.g.: (i) inert, insoluble and opaque gold at bulk becomes
catalytically active, soluble and transparent at nanoscale; (ii) golden
yellow colour of the gold and silvery white colour of silver converted
to appealing beautiful colours by reducing to nanoscale

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 materials whose size range from 1 to 100 nm in any one of the
dimension are known as nanomaterials

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 Nanoscience: study on fundamental relation between physical
properties and materials or molecules or structures with at least one
dimension at nanoscale
 Nanotechnology: make use these nanostructures and principle
behind them to fabricate useful nanoscale devices and produce new
materials
 nanotechnology has been involved in almost all fields, including
electronics, magnetics, optics, optoelectronics, information
technology, therapeutic and biomedicine

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 Classification

 nanoscale length of nanomaterials can be in one-dimension (thin

film), two-dimensions (rods or wires), or three-dimensions (dots or
particles)
 Based on the growth dimensionality, nanomaterials are classified as
zero-dimensional, one-dimensional, two-dimensional and three-
dimensional nanomaterials

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 Zero-dimensional: When all three dimensions are measured within
the nanoscale - growth of particles is restricted in all three
dimensions. E.g.: Nanoparticles
 One-dimensional: When two dimensions are in the nanoscale and
one-dimension is allowed to grow bigger than nanoscale, it leads to
needle or wire shaped nanostructures. E.g.: nanowire and
nanotubes, nanorods - nanowires and nanorods are differentiated
based on the aspect ratio (length/diameter). If the aspect ratio of the
one-dimensional materials is greater than 20, they are nanowires
otherwise they called as nanorods
 Two-dimensional: When one-dimension is confined to nanoscale and
other two dimensions are allowed to grow they exhibit thin film or
sheet like nanostructures. E.g.: nano-films, nano-sheets

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 Three-dimensional: aggregation of smaller nanostructured materials
that includes nanoparticles, nanowires, nanotubes, and nanospheres
into three-dimensional architectures. dimensions are not restricted
to the nanoscale rather the aggregated materials would have any one
of the dimension in the nanoscale

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 Types of nanomaterials

 classified based on the materials used to fabricate nanomaterials

(metals, semiconductors, carbon materials, polymer,dendrimers, bio-
macromolecules, and composite nanomaterials)
 Metals based nanostructured materials: based on pure metals such
as gold, silver, copper and platinum etc.
 Semiconductor nanostructured materials: include metal oxides
(TiO2, ZnO, CuO, Cu2O and Fe3O4), metal sulphides (ZnS, CdS, CuS,
Cu2S an PbS), Gallium arsenide (GaAs) and Indium arsenide (InAs)
 Carbon nanostructured materials: includes fullerenes, carbon
nanotubes, graphene
 Polymers and bio-macromolecules self-assemble into an organized
structure via weak intermolecular interactions such as hydrogen
bonding, electrostatic interactions and hydrophobic interactions

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 Types of nanomaterials

 Dendrimers are nano-sized polymers constructed using branched

oligomeric units. The surface functionality of the dendrimers could
be tailored to control the assemblies and properties
 Composite nanomaterials are inclusion of metallic or semiconductor
nanostructured materials into bulk materials in order to achieve
enhanced optical, electrical, mechanical and chemical properties

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 Effect of nanoscale on properties

 show drastically different optical, mechanical, catalytic, magnetic,

and thermal properties - enhanced surface area and quantum
confinement effect plays significant role on the change of properties
 nanoscale of the materials renders them to have large fraction of
surface atoms, high surface energy, spatial confinement and reduced
imperfections
 increase of surface atoms leads to high surface energy and acts as
good catalyst for chemical reactions
 The coinage metals (Ag, Au and Cu) and semiconductor (ZnO, CdS,
CdSe, etc) exhibit size and shape dependent unique colour and light
emitting properties due to quantum confinement effect. The
quantum confinement effect is realized when the size of the
materials is too small compare to the wavelength of the electron

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 confinement means to limit the motion of randomly moving electron
to restrict its motion in specific energy levels (discreteness) and
quantum reflects the atomic realm of particles. The consequence of
this confinement in space is the quantization of their energy and
momentum

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 Preparation of nanomaterials

 Top-down approach and bottom-up approach

 top-down approach is a process of breaking down of bulk materials
to nanoscale using various physical forces

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 nanomaterials are generated from atomic scale by
assembling/stacking atoms/molecules in the bottom-up approach
 bottom up approach has more advantages over top down approach
such as producing nanostructures with less defects, more
homogenous chemical composition, and better short- and long-range
ordering. However, large scale production with controlled size and
shape is the main advantage of top down approach

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 Top‐down approach

 milling/grinding method are to reduce the particle size, making

solid-state alloys, mixing or blending of different components, and
modulating shape of the particles
 Different types of ball mills have been developed over the years for
different purposes that include tumbler mills, attrition mills, shaker
mills, vibratory mills, and planetary mills

44
 Powdered particles with spherical or different shape in the size
range of 50-100 µm are placed together with a number of hard steel
or tungsten carbide (WC) coated balls in a sealed container that is
shaken or vibrated violently
 Mostly inert atmosphere is preferred while shaking or agitating.
Since mass and velocity controls the kinetic energy of the balls,
dense materials (steel or tungsten carbide) are preferred over
ceramic balls
 possible to create high-energy grinding forces by using high
frequencies and small amplitudes of vibration. The high energy and
continuous strong plastic deformation associated with mechanical
grinding leads to a continuous fine-tuning of the size and
morphology of the powder materials to nanoscale materials. During
the mechanical ball milling, the temperature rise to 100 to 200°C

45
 The contamination of milling tools (Fe) and trace amount of
atmospheric gases such as nitrogen and oxygen is a serious problem
in the ball milling process
 The use of milling tool that is pre-coated as thin film with the
respective powder materials and minimizing the grinding time could
avoid iron contamination
 Atmospheric contamination can be prevented or minimized by
sealing the mill air-tightly after loading the powder materials in inert
gas environment
 The other disadvantages of ball milling process are long processing
time, no control on particle morphology, aggregates, and crystal
phase

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 Bottom‐up approach

Wet chemical method: synthetic approach by building atom by atom

or molecule by molecule via chemical bonding – involves chemical
reaction (oxidation or reduction) – often produces colloidal solution

Advantages of wet chemical methods are:

Simple techniques
Cost-effective and easy to handle
Relatively low temperature (<350°C) synthesis
Doping, hybrid or core@shell nanostructure fabrication
Various size and shape of nanoparticles
Preparation colloidal solution but can also easily convert to powders
Self-assembly or patterning

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Wet chemical method:
 Colloidal metal nanoparticles (Ag, Au, Pd, Pt, Cu, etc.) are prepared
by reducing metal ions in presence of capping molecules
 capping molecules are surfactants that interact on the surface of
growing nanoparticles and prevent aggregation of nanoparticles
 Reducing agent used: sodium borohydride or hydrazine or ascorbic
acid
 Other stabilizing agents: polyvinyl alcohol, proteins, and fatty acids

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Wet chemical method: synthesis of semiconductor nanoparticles
 Sulphide semiconductor nanoparticles are prepared by co-
precipitation

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Sol‐gel method:
 mostly employed for the synthesis of metal oxide nanoparticles
 involves the preparation of colloidal suspension using desired
precursors that produces inorganic network in the solution (sol).
The sol undergoes subsequent gelation to form a network in
continuous liquid phase (gel). This process is called as sol-gel
process

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Important steps involved in sol‐gel method:

 Formation of stable alkoxide or metal precursor solution (sol)

 Gelation via formation of oxo or hydroxo bridged network (gel)
 Aging of the gel, during which the condensation reaction continues
and transform the gel into a solid mass. The aging process is
important for avoiding cracks in gel thin film casting
 Drying of the gel. Depending upon the method used for removing
water or solvent from the gel network, it produces xerogel or
aerogel. The removal of solvent by thermal evaporation produces
monolith which is termed as xerogel. In contrast, extraction of
solvent under supercritical or near supercritical conditions produces
aerogel

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 Calcination of monolith at temperature up to 800°C in which surface
bound M-OH groups are removed. This produces stable gel against
rehydration
 Densification and decomposition of the gels at above 800°C. The gel
network is collapsed and organic molecules are volatilized

Advantages: cheap and tailorable, easy to prepare monodispersed

nanoparticles and various non-metallic inorganic nanoparticles such
as glasses, glass ceramic and ceramic materials can be prepared
relatively at low temperature compared to other traditional method

Limitations: difficulty of nanoparticles growth process and new

nanoparticles formation, unable to confirm completion of the
reaction and low rates of nano powder productions are the
limitation of sol-gel method
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• Involves the uses of metal alkoxides or metal halides
• Steps in sol‐gel
• Hydrolysis of M(OR)n to M‐OH
• Condensation: M(OR)n + M‐OH gives M‐O‐M & R‐OH
• Condensation: M‐OH + M‐OH gives M‐O‐M & H2O
• Reaction rate influenced by
• Choice of solvent, alkoxide group, system pH, ratio of water to
precursor
• Size and morphology depend on kind of M‐OR, pH,
water:M‐OR ratio, temperature, solvent
• Use electrostatic stabilization for aqueous solutions & steric
stabilization for organic solutions

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Physical vapour deposition (PVD):
 method of fabricating metals, metal oxides, and metallic alloys
nanostructured thin-film
 involves physically depositing atoms, ions or molecules without
undergoing any change chemically on to a substrate under
controlled atmosphere at reduced pressure

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 comprises three steps such as evaporation of coating materials,
vapour transport and condensation of gases onto the substrate
surface
 vaporisation can be accomplished by different heating methods
including electric resistance heating or ion bombardment
 Since the process is carried-out under reduced pressure, the
temperature required for converting the solid precursor to vapour
state will be significantly lower than the corresponding melting
temperature of precursor at atmospheric pressure
 vaporisation at relatively lower temperature and absence of air
prevents decomposition & oxidation of precursor
 evaporated atoms/molecules travel cone-shaped path before striking
a solid substrate. Since the process is carried-out in a vacuum
chamber, it practically eliminates interruption of other gas molecules

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Chemical vapour deposition (PVD):
 hybrid technique used to produce high purity thin films of inorganic
or organic materials
 volatile chemicals are used as precursors and vapours of the
precursors are carried to the substrate surface by a carrier gas that is
kept at high temperature compared to the sample source and
produce nanostructured thin film coating of the required materials

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 Various types of reaction including reduction of gas precursor,
chemical reaction between different source gases, oxidation or
disproportionate reaction occurs on the substrate surface
 volatile by-products formed are removed by gas flow through the
reaction chamber
 first step involves the activation of precursor and substrate surface –
achieved by either thermal or using plasma
 widely used by the semiconductor industry particularly for micro or
nano-fabricating into various forms including amorphous,
polycrystalline and single crystalline state. These materials include
silicon, carbon nanofibres, carbon nanotubes, silicon carbide and
other high dielectric material
 Advantages: relatively simple instrumentation, ease of processing,
high purity deposition and economical viability; film quality,
thickness and crystallinity can be controlled by growth rate and
substrate temperature 57
 one of the highly used technique for synthesizing carbon nanotubes
 To synthesize CNT, substrate is prepared with a layer of metal catalyst
particles (commonly nickel, cobalt, iron, or a combination) that controls
the diameter of the nanotubes
 Carbon sources: acetylene and ethylene or ethanol and methane
blended and carried to the substrate surface by carrier gas (ammonia,
nitrogen, hydrogen)
 Substrate is heated >700°C to initiate the growth of CNT
 carbon source gas is decomposed on the catalysts surface and carbon is
transported to the edge of the particle, where it grows as nanotubes
 Depending on the metal catalysts interaction with the substrate surface,
metal catalyst nanoparticles either can stay at the tip of the growing
carbon nanotubes or remain at the nanotubes base
 Disadvantage: contamination of metal nanoparticles catalyst in the
carbon nanotubes that has to be removed by acid washing that could
sometime affect the structure or property of the nanotubes
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Important steps in synthesizing CNT:
 CNT synthesis is achieved by taking a carbon species in the gas phase
and using an energy source to impart energy to a gaseous carbon
molecule
 Commonly used gaseous carbon sources include methane, carbon
monoxide, and acetylene
 The energy source is used to "crack" the molecule into a reactive
radical species
 These reactive species then diffuse down to the substrate, which is
heated and coated in a catalyst (usually a first row transition metal
such as Ni, Fe, or, Co) where it will bond
 CNT forms on the catalyst surface

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 Excellent alignment as well as positional control on the nanometer
scale, can be achieved by the use of CVD
 Control over the diameter, as well as the growth rate of the nanotube
can also be maintained
 The appropriate metal catalyst can preferentially grow single rather
than multi-walled nanotubes

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Self‐assembly (DNA‐directed self‐assembly of AuNPs):
Self-assembly is a process by which molecules adopt a well-defined
arrangement without any external driving force by make use of non-
covalent interaction such as van der Waals, hydrogen bonding and π-π
interactions etc
functional molecules with well-defined interacting groups can drive
the molecules to adopt unique structural organization and properties

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 hydrophobic interactions hydrophobic tails of membrane fatty
acid produces bilayer structure, complementary hydrogen
bonding in DNA resulted in double-helical structure and balancing
of hydrophobic/hydrophilic interactions in surfactants produced
micelle structure
 metal interaction functionality of DNA biomolecule can be used for
incorporating metal nanoparticles and self-assembling of
nanoparticles into an ordered structure via self-assembly properties
of DNA

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 single DNA strand stabilized gold nanoparticles can be self-
assembled into a ordered structure by the addition of
complementary DNA strand that drive into self-assembled structure
via hydrogen bonding interactions

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 Applications

 Health care – therapeutic and diagnosis

 Water – purification (disinfect – Ag nanoparticles)
 Energy – harvesting, storage and utilization
 Catalyst – accelerate the rate of reaction
 Electronic industry
 Environmental applications, etc.

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AuNPs based anti‐cancer agents:
 Pt-based anti-cancer drugs cisplatin, carboplatin and oxaliplatin are an
important component of chemotherapy but are limited by severe dose-
limiting side effects and ability of tumors to develop resistance rapidly.
Also affect rapidly-developing healthy cells as well.

 Can be improved through the use of drug delivery vehicles (AuNPs, 20 –

100 nm) that are able to target cancer cells only (not the healthy cells)

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ZnO and Fe3O4 based memristors:
 Memristor is an electrical component – known as memory resistor used
for regulating flow of current through the circuit
 Unlike resistor, resistance of a memristor is not a fixed value but
changes according to the current passes
 Remembers the amount of charge that had earlier flowed through it –
retains memory even without power – implies that in the case of power
shut-down, all the applications that were open prior to the power
outage would be available on the computer screen when it restarted
 TiO2, ZnO and Fe3O4 are extensively used as resistive memories of
different thickness
 Memristor is constructed by sandwiching a metal oxide between two
metal electrodes – use ferroelectric metal oxide results in ferroelectric
memristor

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 On application of a + or – voltage, polarization of ferroelectric material
switches resulting in a two order change in the magnitude of resistance
between on and off states
 When the voltage is cycled, it is possible to regulate the resistance
values of the ferroelectric domain
 Tunable ferroelectric domain dynamics enables to engineer memristor
response

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Water purification using AgNPs:
 Antimicrobial properties of Ag known for many centuries – (i) Ag
dollars used in the milk bottles to keep milk fresh, (ii) water tanks of
ships and aero planes are silvered to render water potable for months
and (iii) even water bottles were coated with Ag to discourage
contamination of water by microbes; (iv) new born children were
administered drops of aqueous solution of AgNO3 to eyes to prevent
transmission of Neisseriagonorrhoea from infected mother during 1884
 Discovery of antibiotics decreased the use of Ag. However increasing
antibiotic resistance renewing the interest in using Ag as an
antibacterial agent
 Owing to increased specific surface area, AgNPs are used in water
purification – porous polymer membrances are embedded with AgNPs
are used to filter raw water. Any bacteria that passes through
membranes are destroyed

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 It has been reported that slowly Ag NPs ionize into Ag+ and Ag+ enters
water and then enters cells of bacteria and disrupts the hydrogen bonds
in DNA and destruct microbes
 Excess Ag+-ions present in water is found to be safe to drink and do not
cause any damage to the human body cells

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Environmental applications of TiO2 :
 absorption of light by semiconductor nanoparticles such as TiO2
undergoes electronic transition and produces electron (e-) in the excited
state and hole (h+) in the ground state
 excited state electron interacts with oxygen (O2) and produce
superoxide anion (O2-) where interaction of hole with water resulted in
the formation of hydroxyl radicals
 superoxide anion and hydroxyl radical reacts with harmful organic
pollutants and breaks the organic structure and finally convert to non-
toxic H2O and CO2

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TiO2 based self cleaning glass:
 TiO2 nanomaterial is integrated
with solid matrices such as glass
 Oily components from smog
adhere on the surface of TiO2
glass followed by dust particles
into oily drops
 absorption of sun light produces
superoxide anion and hydroxyl
radical that decomposes organic
dust particles and converts into
water soluble non-toxic
components and water and
carbon dioxide gases
 broken organic components
finally washed away by rain or
water 71