4. POLYMERS
• Non-metallic materials are broadly of two kinds – Polymers
and Ceramics.
• The term "polymer" comes from the Greek words; poly
(meaning "many") and mers (meaning "units").
• At the molecular level polymers consist of extremely long,
chain-like molecules.
• Polymer molecules are typically made up of thousands of
repeating chemical units
• A single mer is called a monomer
• Polymers may be natural, such as cellulose or DNA, or synthetic,
such as nylon or polyethylene.
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5. HYDROCARBON MOLECULES
• Most polymers are organic, and formed from hydrocarbon molecules
• Each C atom has four e- that participate in bonds, each H atom has one
bonding e-
• Examples of saturated (all bonds are single ones) hydrocarbon
molecules include Methane, Ethane, Propane etc
• Double and triple bonds can exist between C atoms (sharing of two or
three electron pairs). These bonds are called unsaturated bonds.
Unsaturated molecules are more reactive
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Saturated hydrocarbons
Unsaturated hydrocarbons
6. POLYMERISATION
• Ethylene (C2H4) is a gas at room temp and pressure. It transforms to
polyethylene (solid) by forming active mers through reactions with an
initiator or catalytic radical (R.)
• (.) denotes unpaired electron (active site)
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7. HOMOPOLYMER AND COPOLYMERS
• Homopolymers: Polymer chain is made up of single repeating units.
Example: AAAAAAAA
• Copolymers: Polymer chains made up of two or more repeating
units.
i. Random copolymers: Different monomers randomly arranged
in chains. Eg:- ABBABABBAAAAABA
ii. Alternating copolymers: Definite ordered alterations of
monomers. Eg:- ABABABABABAB
iii. Block copolymers: Different monomers arranged in long
blocks. Eg:- AAAAA…….BBBBBBBB……
iv. Graft copolymers: One type of monomer grafted to long chain
of another. Eg: AAAAAAAAAAAAAAAAAAA
• Mer units that have 2 active bonds to connect with other mers are
called bifunctional.
• Mer units that have 3 active bonds to connect with other mers are
called trifunctional. They form 3-d molecular network structures.
B
B
B
B
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8. MECULAR WEIGHT
• Final molecular weight (chain length) is controlled by relative rates
of initiation, propagation, termination steps of polymerization
• Formation of macromolecules during polymerization results in
distribution of chain lengths and molecular weights
• The number-average molecular weight, Mn is obtained by dividing
the chains into a series of size ranges and then determining the
number fraction of chains within each size:
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M X Mn i i
wM Mw ii
Where Mi is mean molecular weight of range i, wi is the weight fraction of
chains of length i & Xi is the number of fractions of chains of length i
Alternative way to express average polymer chain size is degree of polymerization
- the average number of mer units in a chain
9. MECULAR SHAPE
• The angle between the singly bonded carbon atoms is 109o -
carbon atoms form a zigzag pattern in a polymer molecule.
• Random kinks and coils lead to entanglement, like in the spaghetti
structure: Moreover, while maintaining the 109o angle between
bonds polymer chains can rotate around single C-C bonds (double
and triple bonds are very rigid).
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For (a), the rightmost atom may lie anywhere on
the dashed circle and still subtend a 109 degree
angle with the bond between the other two
atoms. Straight and twisted chain segments are
generated when the backbone atoms are
situated as in (b) and (c), respectively
single polymer chain molecule that
has numerous random kinks &
coils produced by chain bond
rotations.
10. POLYMER MOLECULAR
STRUCTURE
• The molecular structure of a fully polymerized polymer can
be classified according to one of three major types:-
I. linear Polymers
II. branched, or
III.cross-linked
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11. LINEAR POLYMER
• are those in which the repeat units
are joined together end to end in
single chains. These long chains
are flexible and may be thought of
as a mass of “spaghetti”.
• there may be extensive van der
Waals and hydrogen bonding
between the
chains.
• Some of the common polymers
that form with linear structures are
polyethylene (PE), polyvinyl
chloride (PVC), polystyrene (PS),
polymethyl methacrylate, nylon, &
the fluorocarbons
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• in which side-branch chains are
connected to the main ones
• The branches, considered to be part of
the main-chain molecule, may result from
side reactions that occur during the
synthesis of the polymer.
• The chain packing efficiency is reduced
with the formation of side branches, which
results in a lowering of the polymer
density.
• Polymers that form linear structures may
also be branched. For
example, high-density polyethylene
(HDPE) is primarily a linear polymer,
where as low density polyethylene
(LDPE) contains short-chain branches
BRANCHED
POLYMER
12. CROSSED-LINKED POLYMER
• In cross-linked polymers, adjacent
linear chains are joined one to
another at various positions by
covalent bonds.
• The process of crosslinking is
achieved either during synthesis or by
a nonreversible chemical reaction.
• Often, this crosslinking is
accomplished by additive atoms or
molecules that are covalently bonded
to the chains.
• Many of the rubber elastic materials
are cross-linked; in rubbers, this is
called vulcanization
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• Multifunctional monomers forming
three or more active covalent bonds
make 3-d networks and are termed
network polymers.
• Actually, a polymer that is highly
cross-linked may also be classified as
a network polymer.
• These materials have distinctive
mechanical and thermal properties;
the epoxies, polyurethanes, and
phenol-formaldehyde belong to this
group
NETWOK POLYMER
13. HYDROCARBON MOLECULES
• Isomers are molecules that contain the same atoms but in a
different arrangement. An example is butane and isobutane:
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butane
Isobutan
e
14. • Two types of isomerism are possible:-Stereoisomerism &
geometrical isomerism
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ISOMERISATION
• atoms are linked together in the same
order, but can have different spatial
arrangement
i. Isotactic configuration: all side groups
R are on the same side of the chain.
ii. Syndiotactic configuration: side
groups R alternate sides of the chain.
iii. Atactic configuration: random
orientations of groups R along the
chain.
STEREOISOMERISM
GEOMETRICAL ISOMERISM
• are possible within repeat units
having a double bond between chain
carbon atoms.
• Can take two forms:-
i. CIS Structure: R (CH3)& H
atoms are positioned on the
same side of the double bond
ii. TRANS Structure:- R (CH3)& H
atoms reside on opposite sides
of the double bond
15. POLYMER CRYSTALLINITY
• Atomic arrangement in polymer crystals is more complex than in metals
or ceramics (unit cells are typically large & complex).
• Polymer molecules are often partially crystalline (semicrystalline), with
crystalline regions dispersed within amorphous material.
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16. POLYMER CRYSTALLINITY
• Degree of crystallinity is determined
by:-
i. Rate of cooling during solidification:
time is necessary for chains to move
and align into a crystal structure
ii. Mer complexity: crystallization less
likely in complex structures, simple
polymers, such as polyethylene,
crystallize relatively easily
iii. Chain configuration: linear polymers
crystallize relatively easily, branches
inhibit crystallization, network
polymers almost completely
amorphous, cross-linked polymers
can be both crystalline and
amorphous
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iv. Isomerism: isotactic,
syndiotactic polymers
crystallize relatively easily -
geometrical regularity allows
chains to fit together, atactic
difficult to crystallize
v. Copolymerism: easier to
crystallize if mer arrangements
are more regular - alternating,
block can
crystallize more easily as
compared to random and graft
More crystallinity: higher density,
more strength, higher resistance to
dissolution and softening by
heating
17. POLYMER CRYSTALLINITY
• Crystalline polymers are denser than
amorphous polymers, so the degree of
crystallinity can be obtained from the
measurement of density
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a) Thin crystalline platelets
grown from solution - chains
fold back and forth: chain-
folded model
a)
b)
b) Spherulites: Aggregates of
lamellar crystallites ~ 10 nm
thick, separated by
amorphous material.
Aggregates approximately
where
ρc: Density of perfect crystalline polymer
ρa: Density of completely amorphous
polymer
ρs: Density of partially crystalline polymer
that we are analyzing
19. CLASSIFICATION OF POLYMERS
• There are many different polymeric materials
• one way of classifying them is according to their end use. Within this
scheme the various polymer types include plastics, elastomers (or
rubbers), fibers, coatings, adhesives, foams, and films.
• Depending on its properties, a particular polymer may be used in two or
more of these application categories. For example, a plastic, if cross-
linked and used above its glass transition temperature, may make a
satisfactory elastomer, or a fiber material may be used as a plastic if it is
not drawn into filaments.
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20. CLASSIFICATION OF POLYMERS
• Depending on the response to temperature increase, two types of
polymers can be distinguished:
i. Thermoplastic polymers: soften and liquefy when heated, harden when
cooled (reversible).
– Molecular structure: linear or branched polymers, with secondary bonding holding the
molecules together.
– Easy to fabricate/reshape by application of heat and pressure
– Examples: polyethylene, polystyrene, poly(vinylchlodide).
ii. Thermosetting polymers: become permanently hard during their
formation, do not soften upon heating.
– Molecular structure: network polymers with a large density of covalent crosslinks
between molecular chains (typically, 10-50% of repeat units are crosslinked).
– Harder and stronger than thermoplastics, have better dimensional and thermal stability.
– Examples: vulcanized rubber, epoxies, phenolics, polyester resins.
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21. CLASSIFICATION OF POLYMERS
• Polymers can also be categorised as either Commodity Polymers or Engineering
Polymers
i. Commodity polymers:- are basically those polymers which are found in our daily
life usage from low value items such as plastic bags to high value items which
doesn’t require precise and high mechanical properties.
– Commodity polymers are utilized for bulk & high-volume ends (like containers and
packaging).
– Such polymers exhibit relatively low mechanical properties & are of low cost.
– Most of the commodity polymers are made by addition polymerization & those
commodity polymers are thermoplastic polymers.
– Examples of commodity polymers are Polyethylene (PE), Polypropylene (PP),
Polystyrene (PS), Poly(vinyl chloride) (PVC), Polytetrafluoroethylene (PTFE),
Poly(methyl methacrylate) (PMMA), Poly(ethylene terephthalate) (PET), and more
– The range of products includes Plates, Cups, Carrying Trays, Medical Trays,
Containers, Seeding Trays, Printed Material and other disposable items.
– applications such as photographic and magnetic tape, clothing, beverage, trash
containers, film for packaging, and a variety of household products where mechanical
properties & service environments are not critical
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22. CLASSIFICATION OF POLYMERS
ii. Engineering polymers:- are a group of polymers that have better mechanical
and/or thermal properties than the more widely used commodity polymers.
These materials have exceptional mechanical properties such as stiffness,
toughness, and low creep that make them valuable in the manufacture of
structural products like gears, bearings, electronic devices, and auto parts.
– Being more expensive, engineering polymers are produced in lower quantities and tend
to be used for smaller objects or low-volume applications (such as mechanical parts),
rather than for bulk and high-volume ends (like containers and packaging).
– These plastics normally are not available to the public and frequently are available only
to manufacturers in raw material form in order to be melted and molded into end
products.
– The term usually refers to thermoplastic materials rather than thermosetting ones.
– Examples of engineering plastics include acrylonitrile butadiene styrene (ABS), used
for car bumpers, dashboard trim and Lego bricks; polycarbonates, used in motorcycle
helmets; and polyamides (nylons), used for skis and ski boots.
– Engineering polymers have gradually replaced traditional engineering materials such
as wood or metal in many applications. Besides equaling or surpassing them in
weight/strength and other properties, engineering polymers are much easier to
manufacture, especially in complicated shapes.
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23. PLASTICS
• Plastics are materials that have some structural rigidity under load & are
used in general-purpose applications.
• Polyethylene, polypropylene, poly(vinyl chloride), polystyrene, and the
fluorocarbons, epoxies, phenolics, and polyesters may all be classified as
plastics. They have a wide variety of combinations of properties.
• Some plastics are very rigid and brittle. Others are flexible, exhibiting
both elastic and plastic deformations when stressed and sometimes
experiencing considerable deformation before fracture
• Polymers falling within this classification may have any degree of
crystallinity, and all molecular structures and configurations (linear,
branched, isotactic, etc.) are possible.
• Plastic materials may be either thermoplastic or thermosetting
• linear or branched plastic polymers must be used below their glass
transition temperatures (if amorphous) or below their melting
temperatures (if semi-crystalline), or they must be cross-linked enough to
maintain their shape
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26. ELASTOMERS
• The typical properties of elastomers depend on the degree of
vulcanization & on whether any reinforcement is used.
• Natural rubber is still used to a large degree because it has an
outstanding combination of desirable properties. However, the most
important synthetic elastomer is SBR, which is used predominantly in
automobile tires, reinforced with carbon black.
• NBR, which is highly resistant to degradation and swelling, is another
common synthetic elastomer.
• For many applications (e.g., automobile tires), the mechanical properties
of even vulcanized rubbers are not satisfactory in terms of tensile
strength, abrasion and tear resistance, and stiffness. These
characteristics may be further improved by additives such as carbon
black
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28. FIBERS
• Fiber polymers are capable of being drawn into long filaments having at least a
100:1length-to-diameter ratio.
• Most commercial fiber polymers are used in the textile industry, being woven or
knit into cloth or fabric.
• In addition, the aramid fibers are employed in composite materials
• To be useful as a textile material, a fiber polymer must have a host of rather
restrictive physical and chemical properties.
• While in use, fibers may be subjected to a variety of mechanical deformations—
stretching, twisting, shearing, and abrasion. Consequently, they must have a high
tensile strength (over a relatively wide temperature range) and a high modulus of
elasticity, as well as abrasion resistance.
• Convenience in washing and maintaining clothing depends primarily on the
thermal properties of the fiber polymer, that is, its melting and glass transition
temperatures.
• Furthermore, fiber polymers must exhibit chemical stability to a rather extensive
variety of environments, including acids, bases, bleaches, dry-cleaning solvents,
and sunlight.
• In addition, they must be relatively nonflammable and amenable to drying
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29. COATINGS
• Coatings are frequently applied to the surface of materials to serve one
or more of the following functions: (1) to protect the item from the
environment, which may produce corrosive or deteriorative reactions; (2)
to improve the item’s appearance; and (3) to provide electrical insulation.
• Many of the ingredients in coating materials are polymers, most of which
are organic in origin. These organic coatings fall into several different
classifications: paint, varnish, enamel, lacquer, & shellac.
• Many common coatings are latexes. A latex is a stable suspension of
small, insoluble polymer particles dispersed in water.
• they have low volatile organic compound (VOC) emissions
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30. ADHESIVES
• An adhesive is a substance used to bond together the surfaces of two solid
materials (termed adherends).
• There are two types of bonding mechanisms: mechanical & chemical.
• In mechanical bonding there is actual penetration of the adhesive into surface
pores and crevices.
• Chemical bonding involves intermolecular forces between the adhesive and
adherend, which forces may be covalent and/or van der Waals; the degree of van
der Waals bonding is enhanced when the adhesive material contains polar
groups
• Can be natural (animal glue, casein, starch, and rosin) or synthetic
(polyurethanes, polysiloxanes (silicones), epoxies, polyimides, acrylics, and
rubber materials.)
• the choice of which adhesive to use will depend on such factors as
i. the materials to be bonded & their porosities;
ii. the required adhesive properties (i.e., whether the bond is to be temporary
or permanent);
iii. maximum/minimum exposure temperatures;
iv. processing conditions.
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31. FILMS
• Polymers used in the form of thin films.
• Films having thicknesses between 0.025 and 0.125 mm are fabricated and used
extensively as bags for packaging food products and other merchandise, as
textile products
• Important characteristics of the materials produced & used as films
include low density, a high degree of flexibility, high tensile & tear
strengths, resistance to attack by moisture & other chemicals, & low
permeability to some gases, especially water vapor
• Some of the polymers that meet these criteria & are manufactured in film
form are polyethylene, polypropylene, cellophane, & cellulose acetate.
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32. FOAMS
• Foams are plastic materials that contain a relatively high volume percentage of
small pores and trapped gas bubbles.
• Both thermoplastic & thermosetting materials are used as foams; these include
polyurethane, rubber, polystyrene, and poly(vinyl chloride).
• Foams are commonly used as cushions in automobiles and furniture, as well as
in packaging and thermal insulation.
• The foaming process is often carried out by incorporating into the batch of
material a blowing agent that, upon heating, decomposes with the liberation of a
gas. Gas bubbles are generated throughout the now-fluid mass, which remain in
the solid upon cooling and give rise to a sponge-like structure. The same effect is
produced by dissolving an inert gas into a molten polymer under high pressure.
When the pressure is rapidly reduced, the gas comes out of solution and forms
bubbles & pores that remain in the solid as it cools.
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33. ADVANCED POLYMERIC
MATERIALS
• new polymers having unique and desirable combinations of
properties have been developed over the past several years
• Some of these include:-
i. ultra-high-molecular-weight polyethylene,
ii. liquid crystal polymers, &
iii. thermoplastic elastomers
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34. ULTRA-HIGH-MOLECULAR-WEIGHT
POLYETHYLENE (UHMWPE)
• is a linear polyethylene that has an extremely high molecular weight of
approximately 4x106 g/mol,
• In fiber form, UHMWPE is highly aligned and has the trade name Spectra. Some of
the extraordinary characteristics of this material are as follows:
i. An extremely high impact resistance
ii. Outstanding resistance to wear and abrasion
iii. A very low coefficient of friction
iv. A self-lubricating and nonstick surface
v. Very good chemical resistance to normally encountered solvents
vi. Excellent low-temperature properties
vii. Outstanding sound damping and energy absorption characteristics
viii. Electrically insulating and excellent dielectric properties
• However, because this material has a relatively low melting temperature, its
mechanical properties deteriorate rapidly with increasing temperature.
• This unusual combination of properties leads to numerous and diverse applications
including bulletproof vests, composite military helmets, fishing line, ski-bottom
surfaces, golf-ball cores, bowling alley and ice-skating rink surfaces, biomedical
prostheses, blood filters, marking-pen nibs, bulk material handling equipment (for
coal, grain, cement, gravel, etc.), bushings, pump impellers, and valve gaskets.
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35. LIQUID CRYSTAL POLYMERS (LCPs)
• LCPs are a group of chemically complex and structurally distinct materials that have unique
properties and are used in diverse applications.
• Are composed of extended, rod-shaped, and rigid molecules.
• In terms of molecular arrangement, these materials do not fall within any of conventional
liquid, amorphous, crystalline, or semicrystalline classifications but may be considered a
new state of matter—the liquid crystalline state, being neither crystalline nor liquid.
• In the melt (or liquid) condition, whereas other polymer molecules are randomly oriented,
LCP molecules can become aligned in highly ordered configurations. As solids, this
molecular alignment remains, &, in addition, the molecules form in domain structures having
characteristic intermolecular spacings.
• Three types of liquid crystals, based on orientation and positional ordering are smectic,
nematic, & cholesteric;
• The principal use of liquid crystal polymers is in liquid crystal displays (LCDs) on digital
watches, flat-panel computer monitors and televisions, and other digital displays.
• Also used extensively by the electronics industry (in interconnect devices, relay and
capacitor housings, brackets, etc.), by the medical equipment industry (in components that
are sterilized repeatedly), and in photocopiers and fiber-optic components.
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36. LIQUID CRYSTAL POLYMERS (LCPs)
• these materials exhibit the following behaviors:
i. Excellent thermal stability; they may be used to temperatures as high
as 2300C
ii. Stiffness and strength; their tensile moduli range between 10 and 24 GPa &
their tensile strengths are from 125 to 255 MPa
iii. High impact strengths, which are retained upon cooling to relatively low
temperatures.
iv. Chemical inertness to a wide variety of acids, solvents, bleaches, and so on.
v. Inherent flame resistance and combustion products that are relatively nontoxic.
• The following may are their processing and fabrication characteristics:
i. All conventional processing techniques available for thermoplastic materials may be used.
ii. Extremely low shrinkage and warpage take place during molding.
iii. There is exceptional dimensional repeatability from part to part.
iv. Melt viscosity is low, which permits molding of thin sections and/or complex shapes.
v. Heats of fusion are low; this results in rapid melting and subsequent cooling, which shortens
molding cycle times.
vi. They have anisotropic finished-part properties; molecular orientation effects are produced from
melt flow during molding
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37. POLYMER ADDITIVES
• Foreign substances called additives are intentionally introduced to enhance or
modify many of these properties and thus render a polymer more serviceable.
• Typical additives include filler materials, plasticizers, stabilizers, colorants, and
flame retardants.
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• are most often added to polymers to improve tensile and compressive strengths, abrasion
resistance, toughness, dimensional and thermal stability, and other properties.
• Materials used as particulate fillers include wood flour (finely powdered sawdust), silica flour
and sand, glass, clay, talc, limestone, and even some synthetic polymers.
• Polymers that contain fillers may also be classified as composite materials,
• Often the fillers are inexpensive materials that replace some volume of the more expensive
polymer, reducing the cost of the final product.
Fillers
PLASTICISERS
• The flexibility, ductility, and toughness of polymers may be improved with the aid of
additives called plasticizers. Their presence also produces reductions in hardness &
stiffness.
• Plasticizers are generally liquids with low vapor pressures and low molecular weights.
• The small plasticizer molecules occupy positions between the large polymer chains,
effectively increasing the interchain distance with a reduction in the secondary
intermolecular bonding.
• Plasticizers are commonly used in polymers that are intrinsically brittle at room
temperature, such as poly(vinyl chloride) and some of the acetate copolymers.
• The plasticizer lowers the glass transition temperature, so that at ambient conditions the
polymers may be used in applications requiring some degree of pliability & ductility. These
38. POLYMER ADDITIVES
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• Additives that counteract deteriorative processes are called stabilizers.
• One common form of deterioration results from exposure to light. There are two primary
approaches to UV stabilization. The first is to add a UV absorbent material, often as a thin
layer at the surface. This essentially acts as a sunscreen & blocks out the UV radiation
before it can penetrate into & damage the polymer. The second approach is to add
materials that react with the bonds broken by UV radiation before they can participate in
other reactions that lead to additional polymer damage.
• Another important type of deterioration is oxidation It is a consequence of the chemical
interaction between oxygen [as either diatomic oxygen (O2) or ozone (O3)] & the polymer
molecules. Stabilizers that protect against oxidation consume oxygen before it reaches the
polymer &/or prevent the occurrence of oxidation reactions that would further damage the
material.
STABLISERS
FLAME
RETARDANTS
• The flammability resistance of the remaining combustible polymers may be enhanced by
additives called flame retardants
• Most polymers are flammable in their pure form; exceptions include those containing
significant contents of chlorine and/or fluorine, such as PVC & PTFE
• The retardants may function by interfering with the combustion process through the gas
phase or by initiating a different combustion reaction that generates less heat, thereby
reducing the temperature; this causes a slowing or cessation of burning
39. POLYMER ADDITIVES
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• Colorants impart a specific color to a polymer
• They may be added in the form of dyes or pigments.
• The molecules in a dye actually dissolve in the polymer.
Pigments are filler materials that do not dissolve but remain
as a separate phase; normally, they have a small particle
size and a refractive index near that of the parent polymer.
• Others may impart opacity as well as color to the polymer
COLORANTS
40. CERAMICS
• The term ceramic comes from the Greek word keramikos, which means
“burnt stuff,” indicating that desirable properties of these materials are
normally achieved through a high-temperature heat treatment process
called firing
• Usually a compound between metallic and nonmetallic elements
• Bonds are partially or totally ionic, and can have combination of ionic and
covalent bonding
• Always composed of more than one element (e.g.,Al2O3, NaCl, SiC, SiO2)
• Generally hard and brittle
• Generally electrical and thermal insulators
• Can be optically opaque, semi-transparent, or transparent
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41. CERAMIC BONDING
• The atomic bonding in these materials ranges from purely ionic to totally
covalent; many ceramics exhibit a combination of these two bonding
types.
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NaCl
MgO
CaF2
CsCl
This type of bonding gives the following properties:-
Brittle
High Tm
Poor conductor of heat & electricity
42. CERAMIC STRUCTURES
• Crystal structures for ceramics are generally more complex than those for
metals.
• Ceramics that are predominantly ionic in nature have crystal structures
comprised of charged ions, where positively-charged (metal) ions are
called cations, and negatively-charged (non-metal) ions are called anions
• The crystal structure for a given ceramic depends upon two
characteristics:-
i. The magnitude of electrical charge on each component ion,
recognizing that the overall structure must be electrically neutral
ii. The relative size of the cation(s) and anion(s),which determines the
type of interstitial site(s) for the cation(s) in an anion lattice
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CaF2
Ca2+
cation
F-
F-
anions+
• Charge Neutrality:
--Net charge in the
structure should
be zero.
SiO2, MgO, SiC, Al2O3
(i)
(ii)
43. CN IN CERAMICS
• Coordination Number is the number of adjacent atoms (ions) surrounding
a reference atom (ion) without overlap of electron orbitals. Also called
ligancy
• Calculated by considering the greatest number of larger ions (radius R)
that can be in contact with the smaller one (radius r).
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44. AX-TYPE CRYSTAL STRUCTURES IN
CERAMICS
• are those in which there are equal numbers of cations and anions. These
are often referred to as AX compounds, where A denotes the cation and
X the anion.
• There are several different crystal structures for AX compounds; each is
typically named after a common material that assumes the particular
structure
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A unit cell for the cesium
chloride (CsCl) crystal structure.
A unit cell for the rock salt,
or sodium chloride (NaCl),
crystal structure. (Two
interpenetrating FCC lattices
NaCl, MgO, LiF, FeO have
this crystal structure)
A unit cell for the zinc blende
(ZnS) crystal structure.
45. AmXp & AmBnXp -TYPE CRYSTAL STRUCTURES IN CERAMICS
• If the charges on the cations and anions are not the same, a compound
can exist with the chemical formula AmXp, where m and/or p ≠1. An
example is AX2, for which a common crystal structure is found in fluorite
(CaF2). The ionic radii ratio rC/rA for CaF2 is about 0.8, which gives a
coordination number of 8
• It is also possible for ceramic compounds to have more than one type of
cation; for two types of cations (represented by A and B), their chemical
formula may be designated as AmBnXp. Barium titanate (BaTiO3), having
both Ba2+ & Ti4+cations, falls into this classification.
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A unit cell for the perovskite
crystal structure..
A unit cell for the fluorite
(CaF2) crystal structure
46. DENSITY COMPUTATIONS IN
CERAMICS
• For a crystalline ceramic material theoretical density can be computed
from unit cell data in a manner similar to that for metals.
• In this case the density, ρ may be determined using the equation as
follows
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47. AMORPHOUS SILICA
• Silica gels - amorphous SiO2
– Si4+ and O2- not in well-ordered
lattice
– Charge balanced by H+ (to form
OH-) at “dangling” bonds
– SiO2 is quite stable, therefore
un-reactive to makes good
catalyst support
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SILICA GLASS
• Dense form of amorphous
silica
– Charge imbalance
corrected with “counter
cations” such as Na+
– Borosilicate glass is the
pyrex glass used in labs
• better temperature
stability & less brittle
than sodium glass
Si, B - Network former
Other Cations - Network modifier
48. SILICATE
CERAMICS
Most common elements on earth are Si
& O
• SiO2 (silica) structures are quartz,
crystobalite, & tridymite
• The strong Si-O bond leads to a
strong, high melting material
(1710ºC)
Si4+ O2-
crystobalite
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• Combine SiO4
4- tetrahedra by
having them share corners,
edges, or faces
• Cations such as Ca2+, Mg2+, &
Al3+ act to neutralize & provide
ionic bonding
Mg2SiO4 Ca2MgSi2O7
SILICATE
ELEMENTS
49. LAYERED
SILICATES• Layered silicates (clay
silicates)
– SiO4 tetrahedra
connected together to
form 2-D plane
• (Si2O5)2-
• So need cations to balance
charge
=
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• Kaolinite clay
alternates (Si2O5)2-
layer with Al2(OH)4
2+
layer
LAYERED
SILICATES
Note: these sheets loosely
bound by van der Waal’s forces
50. • Frenkel Defect
--a cation is out of place.
• Shottky Defect
--a paired set of cation and anion vacancies.
DEFECTS IN CERAMIC STRUCTURES
Shottky
Defect:
Frenkel
Defect
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56. Ceramics Vs Metals
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Property Ceramic Metal Polymer
Hardness Very High Low Very Low
Elastic modulus Very High High Low
Thermal expansion High Low Very Low
Wear resistance High Low Low
Corrosion resistance High Low Low
Ductility Low High High
Density Low High Very Low
Electrical conductivity Depends High Low
on material
Thermal conductivity Depends High Low
on material
Magnetic Depends High Very Low
on material
57. CLASSIFICATION OF CERAMICS
• Based on their engineering
applications, ceramics are
classified into two groups
as:-
i. traditional &
ii. engineering ceramics.
• Based on their specific
applications, ceramics are
classified as:-
i. Glasses
ii. Clay products
iii. Refractories
iv. Abrasives
v. Cements
vi. Carbons
vii. Advanced ceramics
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58. CLASSIFICATION OF CERAMICS
• Based on their composition, ceramics are:
i. Oxides
ii. Carbides
iii. Nitrides
iv. Sulfides
v. Fluorides
• Traditional ceramics – the older and more generally known types
(porcelain, brick, earthenware, etc.). Based primarily on natural raw
materials of clay and silicates
• Engineering ceramics – Include artificial ceramic raw materials, exhibit
specialized properties, require more sophisticated processing . Applied
as thermal barrier coatings to protect metal structures, wearing surfaces.
Engine applications (silicon nitride (Si3N4), silicon carbide (SiC), Zirconia
(ZrO2), Alumina (Al2O3)
•
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60. GLASS
• These are noncrystalline silicates containing other oxides, notably CaO,
Na2O, K2O, and Al2O3,which influence the glass properties.
• A typical soda-lime glass consists of approximately 70 wt% SiO2, the
balance being mainly Na2O (soda) and CaO (lime).
• Possibly the two prime attractive properties of these materials are their
optical transparency and the relative ease of fabrication.
• Typical applications include containers, lenses, and fiberglass
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61. GLASS-CERAMICS
• Inorganic glasses are transformed from a noncrystalline state into a
crystalline state by proper high-temperature heat treatment called
crystallization, & the product is a fine-grained polycrystalline
material, often called a glass–ceramic
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Continuous-cooling transformation diagram for
the crystallization of a lunar glass (35.5 wt%
SiO2, 14.3 wt% TiO2, 3.7 wt% Al2O3, 23.5
wt% FeO, 11.6 wt% MgO, 11.1 wt% CaO, and
0.2 wt% Na2O).
Superimposed on this plot are two cooling
curves, labelled 1 and 2
• Glass–ceramic materials have been designed
to have the following characteristics:
relatively high mechanical strengths;
low coefficients of thermal expansion (to avoid
thermal shock);
good high-temperature capabilities;
good dielectric properties (for electronic
packaging applications);
good biological compatibility.
• Some glass–ceramics may be made optically
transparent; others are opaque.
• Possibly the most attractive attribute of this
class of materials is the ease with which they
may be fabricated; conventional glass-
forming techniques may be used conveniently
in the mass production of nearly pore-free
ware.
62. GLASS-CERAMICS
• Glass-ceramics are manufactured commercially under the trade names of
Pyroceram, CorningWare, Cercor, and Vision.
• Most common uses for glass-ceramics are as ovenware, tableware, oven
windows, and range tops—primarily because of their strength and
excellent resistance to thermal shock.
• They also serve as electrical insulators & as substrates for printed circuit
boards
• They are also used for architectural cladding & for heat exchangers &
regenerators
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63. CLAY PRODUCTS
• Clay is the most widely used ceramic raw material.
• Is an inexpensive ingredient, found naturally in great abundance & can be used as
mined
• Another reason for its popularity lies in the ease with which clay products may be
formed
• When mixed in the proper proportions, clay and water form a plastic mass that is very
amenable to shaping. The formed piece is dried to remove some of the moisture, after
which it is fired at an elevated temperature to improve its mechanical strength
• Most clay-based products fall within two broad classifications:
i. the structural clay products - include building bricks, tiles, & sewer pipes
(applications in which structural integrity is important).
ii.Whiteware ceramics - become white after high-temperature firing - includes
porcelain, pottery, tableware, china, and plumbing fixtures (sanitary ware). In
addition to clay,
• These products may contain nonplastic ingredients, which influence the
changes that take place during the drying and firing processes and the
characteristics of the finished piece
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64. REFRACTORY
CERAMICS
• They have capacity to withstand high temperatures without melting or
decomposing & the capacity to remain unreactive and inert when exposed
to severe environments.
• These provide thermal insulation which is often an important
consideration.
• Refractory materials are marketed in a variety of forms, but bricks are the
most common.
• Typical applications include furnace linings for metal refining, glass
manufacturing, metallurgical heat treatment, & power generation
• Their depends to a large degree on its composition & based on their
compositions, they are classified as
i. fireclay,
ii. silica,
iii. basic, &
iv. special refractories.
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Compositions of Five Common Ceramic Refractory
Materials
65. ABRASIVE CERAMICS
• These are used to wear, grind, or cut away other material. Thus, the prime requisite for this
group of materials is hardness or wear resistance;
• In addition, a high degree of toughness is essential to ensure that the abrasive particles do not
easily fracture.
• Some refractoriness is also desirable ass high temperatures may be produced from abrasive
frictional forces
• Diamonds, both natural and synthetic, are used as abrasives; however, they are relatively
expensive.
• The more common ceramic abrasives include silicon carbide, tungsten carbide (WC), aluminum
oxide (or corundum), & silica sand.
• Abrasives are used in several forms:
i. bonded to grinding wheels - the abrasive particles are bonded to a wheel by means of a
glassy ceramic or an organic resin. The surface structure should contain some porosity; a
continual flow of air currents or liquid coolants within the pores that surround the refractory
grains prevents excessive heating.
ii. as coated abrasives - those in which an abrasive powder is coated on some type of paper
or cloth material; sandpaper is probably the most familiar example. Wood, metals,
ceramics, and plastics are all frequently ground and polished using this form of abrasive.
iii. as loose grains - are delivered in some type of oil- or water-based vehicle. Grinding,
lapping, and polishing wheels often employ loose abrasive grains. Diamonds, corundum,
silicon carbide, & rouge (an iron oxide) are used in loose form over a variety of grain size
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66. CEMENTS
• The inorganic ceramic material cement, plaster of Paris, & lime, as a group,
are produced in extremely large quantities.
• The characteristic feature of these materials is that when mixed with water,
they form a paste that subsequently sets and hardens.
• Some of these materials act as a bonding phase that chemically binds
particulate aggregates into a single cohesive structure
• The role of the cement is similar to that of the glassy bonding phase that
forms when clay products and some refractory bricks are fired. One important
difference, however, is that the cementitious bond develops at room
temperature.
• Of this group of materials, Portland cement is consumed in the largest
tonnages.
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67. CEMENTS – PORTLAND CEMENT
• is produced by grinding & intimately mixing clay & lime-bearing minerals
in the proper proportions & then heating the mixture to about 1400oC
(2550oF) in a rotary kiln; this process, sometimes called calcination, produces
physical & chemical changes in the raw materials. The resulting “clinker”
product is then ground into a very fine powder, to which is added a small
amount of gypsum (CaSO4–2H2O) to retard the setting process (producing a
product known as Portland cement).
• The properties of Portland cement, including setting time & final strength, to a
large degree depend on its composition.
• Several different constituents are found in Portland cement, the principal
ones
being tricalcium silicate (3CaO–SiO2) and dicalcium silicate (2CaO–SiO2).
• The setting & hardening of this material result from relatively complicated
hydration reactions that occur among the various cement constituents and
the water that is added. For example, one hydration reaction involving
dicalcium silicate is as follows:
2CaO-SiO2 + xH2O 2CaO-SiO2-xH2O
where x is variable that depends on how much water is available.
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68. CARBONS• In terms of crystal structures there are two polymorphic forms of carbon:-
i. diamond &
ii. graphite.
• Furthermore, fibers are made of carbon materials that have other
structures.
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DIAMOND
• Have extra ordinary physical properties
• Chemically, it is very inert & resistant to attack.
• Of all known bulk materials, diamond is the hardest as a result of its extremely strong
interatomic sp3 bonds.
• Of all solids, it has the lowest sliding coefficient of friction.
• Extremely high thermal conductivity ,
• optically, it is transparent in the visible & infrared regions of the electromagnetic spectrum - has
the widest spectral transmission range of all materials. The high index of refraction and optical
brilliance of single crystals makes diamond a most highly valued gemstone.
• High-pressure high-temperature (HPHT) techniques to produce synthetic diamonds were
developed in mid-1950s. These have been refined that today a large proportion of industrial-
quality diamonds are synthetic, as are some of those of gem quality.
• Industrial-grade diamonds are used for diamond-tipped drill bits & saws, dies for wire drawing, &
as abrasives used in cutting, grinding, & polishing equipment
69. GRAPHITE
• Is highly anisotropic (property values depend on crystallographic direction along
which they are measured)
• Have weak interplanar van der Waals bonds, relatively easy for planes to slide past
one another hence have excellent lubricative properties
• When compared to diamond, graphite is very soft & flaky & has a significantly smaller
modulus of elasticity; Its in-plane electrical conductivity is higher than that of diamond;
thermal conductivities are approximately the same; coefficient of thermal expansion
for diamond is relatively small and positive while graphite’s in-plane value is small &
negative, & the plane-perpendicular coefficient is positive and relatively large.
• Graphite is optically opaque with a black–silver color.
• Other desirable properties of graphite include good chemical stability at elevated
temperatures & in nonoxidizing atmospheres, high resistance to thermal shock, high
adsorption of gases, & good machinability
• Applications for graphite include lubricants, pencils, battery electrodes,
friction materials (e.g., brake shoes), heating elements for electric furnaces,
welding electrodes, metallurgical crucibles, high-temperature refractories and
insulations, rocket nozzles, chemical reactor vessels, electrical contacts (e.g.,
brushes), & air purification devices.
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70. CARBON FIBERS
• These are small-diameter, high-strength, & high-modulus fibers composed of carbon
• Used as reinforcements in polymer-matrix composites
• Carbon is in the form of graphene layers. However, depending on precursor (i.e.,
material from which the fibers are made) and heat treatment, different structural
arrangements of these graphene layers exist. These include:-
i. graphitic carbon fibers - the graphene layers assume the ordered structure of
graphite & planes are parallel to one another having relatively weak van der Waals
interplanar bonds.
ii. turbostratic carbon - a more disordered structure results when, during fabrication,
graphene sheets become randomly folded, tilted, and crumpled
iii. Hybrid graphitic-turbostratic fibers - composed of regions of both structure types,
may also be synthesized
• Because most of these fibers are composed of both graphitic and turbostratic forms,
the term carbon rather than graphite is used to denote these fibers
• Of the three most common reinforcing fiber types used for polymer-reinforced
composites (carbon, glass, and aramid), carbon fibers have the highest modulus of
elasticity & strength & are the most expensive.
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73. ADVANCED CERAMICS
• These are unique ceramics having superlative combination of properties
such as electrical, magnetic, & optical exploited in a host of new products
• Advanced ceramics include materials used in microelectromechanical
systems (MEMS) as well as the nanocarbons (fullerenes, carbon
nanotubes, and graphene)
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MICROELECTROMECHANICAL SYSTEMS (MEMS)
• are miniature “smart” systems consisting of a multitude of mechanical devices that
are integrated with large numbers of electrical elements on a substrate of silicon.
• The mechanical components are microsensors & microactuators.
• Microsensors collect environmental information by measuring mechanical, thermal,
chemical, optical, and/or magnetic phenomena. The microelectronic components
then process this sensory input and subsequently render decisions that direct
responses from the microactuator devices - devices that perform such responses as
positioning, moving, pumping, regulating, and filtering.
• These actuating devices include beams, gears, motors, membranes, etc. which are
of microscopic dimensions, on the order of microns in size.
• The processing of MEMS is virtually the same as that used for the production of
silicon-based integrated circuits; this includes photolithographic, ion implantation,
etching, and deposition technologies. In addition, some mechanical components are
fabricated using micromachining techniques.
•
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MICROELECTROMECHANICAL SYSTEMS
(MEMS)
• MEMS components are very sophisticated, reliable, and minuscule in size.
Furthermore, because the preceding fabrication techniques involve batch
operations, the MEMS technology is very economical and cost effective
• There are some limitations to the use of silicon in MEMS. Silicon has a low fracture
toughness (~0.90 MPam1/2) & a relatively low softening temperature (600oC) & is
highly active to the presence of water & oxygen. Hence research is being conducted
into using ceramic materials which are tougher, more refractory, & more inert—for
some MEMS components, especially high-speed devices and nanoturbines. The
ceramic materials being considered are amorphous silicon carbonitrides (silicon
carbide–silicon nitride alloys).
• One example of a practical MEMS application is an accelerometer
(accelerator/decelerator sensor) that is used in the deployment of air-bag systems in
automobile crashes
75. MICROELECTROMECHANICAL SYSTEMS
(MEMS)
• Potential MEMS applications include electronic displays, data storage units,
energy conversion devices, chemical detectors (for hazardous chemical and
biological agents & drug screening), & microsystems for DNA amplification
and identification
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Scanning electron micrograph showing
a linear rack gear reduction drive MEMS. This
gear chain converts rotational motion from the
top-left gear to linear motion to drive the linear
track (lower right). Approximately 100X.
76. NANOCARBONS
• The “nano” prefix denotes that the particle size is less than about 100
nanometers. In addition, the carbon atoms in each nanoparticle are bonded
to one another through hybrid sp2 orbitals.
• They have novel and exceptional properties & are currently being used in
some cutting-edge technologies
• Three nanocarbons that belong to this class are:-
i. fullerenes,
ii. carbon nanotubes, &
iii. graphene.
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77. NANOTUBES
• Consists of a single sheet of graphite (i.e., graphene)
that is rolled into a tube
• The term single-walled carbon nanotube (abbreviated
SWCNT) is used to denote this structure.
• Each nanotube is a single molecule composed of
millions of atoms; the length of this molecule is much
greater (on the order of thousands of times greater)
than its diameter.
• Multiple-walled carbon nanotubes (MWCNTs)
consisting of concentric cylinders also exist
• Nanotubes are extremely strong, stiff & relatively
ductile
• Carbon nanotubes have the potential to be used in
structural applications. Most current applications,
however, are limited to the use of bulk nanotubes -
collections of unorganized tube segments
• Bulk nanotubes are currently being used as
reinforcements in polymer-matrix nanocomposites to
improve not only mechanical strength, but also
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The structure of a
single-walled carbon
nanotube (schematic).
78. NANOTUBES
• Carbon nanotubes also have unique & structure-sensitive electrical
characteristics.
• the nanotube may behave electrically as either a metal or a semiconductor. As a
metal, they have the potential for use as wiring for small-scale circuits. In the
semiconducting state they may be used for transistors & diodes
• nanotubes are excellent electric field emitters. As such, they can be used for flat-
screen displays (e.g., television screens and computer monitors).
• Other potential applications are varied and numerous, and include the following:-
More efficient solar cells
Better capacitors to replace batteries
Heat removal applications
Cancer treatments (target and destroy cancer cells)
Biomaterial applications (e.g., artificial skin, monitor and evaluate
engineered tissues)
Body armor
Municipal water-treatment plants (more efficient removal of pollutants &
contaminants)
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79. GRAPHENE
• is a single-atomic-layer of graphite, composed of
hexagonally sp2 bonded carbon atoms
• These bonds are extremely strong, yet flexible, which
allows the sheets to bend.
• Two characteristics of graphene make it an exceptional
material:-
i. The perfect order found in its sheets: no atomic
defects such as vacancies exist; also these sheets
are extremely pure—only carbon atoms are
present.
ii. The second characteristic relates to the nature of
the unbonded electrons: at room temperature, they
move much faster than conducting electrons in
ordinary metals & semiconducting materials.
• It is the strongest known material (~130 GPa), the best
thermal conductor (~5000 W/m.K), & has the lowest
electrical resistivity (is the best electrical conductor). It is
transparent, chemically inert, & has a modulus of elasticity
comparable to the other nanocarbons (~1 TPa).
• Economical & reliable methods of mass production not yet
revolutionised
• Has potential applications in electronics, energy,
transportation, medicine/biotechnology, & aeronautics.
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The structure of a
graphene layer
(schematic).
