(GS MATERIAL SCIENCE)
Structure of atom and interatomic Bonding:
electron
proton
neutron
positron
neutrino and antineutrino
Stable
particle
Composed of
matter
(Physical substance)
Tiny particles
(atom)
unstable
particle
mesons
-particle
unstable
particle
Deutron
(i) Stable particle:
Charge
Mass
0.00054u 1
1.6 10 C
uncharged
1.67 10 kg
~ proton mass
1.007274u 1
(a) electron –1.6 10 19 C 9.11 10 31 kg or
1
times mass
1836
of proton
(b)
Proton
(c) Neutron
19
27
amu or u
1.00867u 1
Note:
1. Isotopes: Same atomic number, different mass number.
2. Isobars: Same mass number, different atomic number
3. Isotones: Same number of neutrons but different number of protons (or) mas number.
4. isoelectric: Atoms/molecules/ions having same number of .
(ii) Unstable particles:
ii) Unstable particles
a) Positron: +1e
b) Neutrono
Antineutrino
c) Mesons
iii) Composite particles
a) -particles: doubly
charged He
: uncharged
- meson ; heavy
µ-meson; Light
eithel + ve/-vely
charged
Note:
z
En = – 13.6 ev
n
2
1. Energy of nth orbit
:
2. Radius of nth orbit
:
n2
rn 0.529 A
z
3. Orbital velocity is inversely proportional to n.
4. Orbital frequency is inversely proportional to n3.
b) Deutron: + vely charged
heavy hydrogen
Periodic Table:
Group number
atomic number
Element symbol
Element name
atomic weight (average)
Electronegativity
ses
ionization
Energy
Electron gain Enthalpy
atomic radius ses
llic
eta
m
n
No
ses
ses
Atomic Radius
Electronegativity
Ionization Enthalpy
Electron Gain Enthalpy
Period
ter
rac
a
h
c
•
Note: Ist transition series starts from the atomic number 21 [Sc]
•
Chemical Bonds:
Bonds
Primary bonds Interatomic bonds
stable
Strong
Bond energy : 100 – 1000 kJ/mol
Secondary Bonds
Ionic Bond
Covalent Bond
Metallic Bond
Intermolecular bonds
unstable
weak
Bond energy
: 10 KJ/mol
Note:
Of all the primary bonds, ionic is strongest and metallic weakest
Ionic solids are water soluble but covalent bonds are soluble in non-polar solvent.
Of all the secondary bonds, Hydrogen bond is strongest
Bonds and their properties:
Properties
Ionic bond
Covalent bond
Metallic bond
Secondary bond
Mechanical Hardness
increases with
ionic charge;
break by cleavage
fracture; it is nondirectional bond.
Very hard and
brittle; break by
cleavage
fracture, it is a
directional
bond
Tough and ductile,
non-directional;
can change shape
permanently without
breakking weaker
than ionic or covalent
bonds
Soft; can be
plastically
deformed
Electrical
Insulators
Semi conductors
Electronic conductors
Insulators
Thermal
Fairly high
melting point,
thermal insulators
Very high melting
point, good
thermal insulators
Modernately high
melting po int,
good heat conductors
Low melting
po int
Optical
Transparent
coloured by
absorption of
ions
Transparent or
opaque,high
refractive index
Opaque and reflecting Transparent
Examples
Magnesia, calcite,
NaCl, CaCl 2
Diamond, carborundum Sodium, iron copper
Argon, paraffins
ice
Crystal Structure:
Solid
1 Crystalline solids
periodically arranged atoms
Definite geometric shape
have sharp melting point
anisotropic in nature
Ex: NaCl
7. Crystal system and 14 bravais lattices
Crystal
Bravcis
system
Lattice
1. Cubic
P, I, F
2. Non-crystalline camorphous) solid
Non-periodically arranged atoms
No definite geometric shape
Melt over wide range of temp.
Isotropic in nature
Ex.: Glass
abc
90
2. Rhom sohedral
(Trigonal)
P
abc
3. Tetragonal
P, I
abc
90
4. Hexagonal
P
5. Othor hom sic
P, I, F, E
abc
90
abc
90
6. Monoelinic
P, E
7. Triclinic
P
abc
abc
90
90, 120
90
P Pr imitive
I Bodycentre
F Face centre
E End Centre
atomic radius
()
N
CN
APF
Example
1. Simple cubic
a/2
1
6
0.52
P0
2. BCC [ABAB]
a 3 /4
2
8
0.68
Cr, -fe, K, MO, W
3. FCC [ABCABC]
4
12
0.74
Al, Cu, Ag, Au, Ni, Pt, Pb
4. HCP [ABAB]
a 2/4
a/2
6
12
0.74
Graphite, Zn, Mg
5. DC
a 3 /8
8
4
0.34
Si, Ge, Sn
Structure
Crystal imperfection:
1. O-dim ensional (point) defects :
associated with
Vaccancies
interstitial atom
impurities
substitutional atoms
frenkel defect
Schottky defect
2. 1- dim ensional (line) defects :
associated with dislocations
Edge dislocation
Screw dislocation
Mixed dislocation
Note:
1. For edge dislocation, Burger, vector is perpendicular to the dislocation.
2. For screw dislocation, Burger vector is parallel to the dislocation.
3. Burger vector described the magnitude and direction of slip.
4. Slip is defined as the movement of block of atoms along certain crystallographic plane and
direction.
3. 2-dimensioal (area) defects:
– associated with faults
– stacking faults
– external faults
Grain boundaries
Tilt boundary
Twin boundary
Note:
1. A grain boundary is the imperfections that separate crystal or grains of different orientation in
polycrystalline aggregate during crystallization
2. Average number of grains per inch at magnification of 100X is N = 2n–1;
n = grain size number
[Higher n indicates fine grain]
3. When the mismatch in orientation between adjacent boundaries is small (<10°), then it is
called as tilt boundaries.
4. If the atoms on one side of the boundary is mirror image of atoms on the other side of the
boundary then it is called as Twin boundary.
5. 3-dimensional (volume) defects:
– associated with
– Cracks
– Pores
– Foreign inclusions
6. Phase diagram:
Gibbs phase rule:
F = degree of freedom (it is the number of independent variables {P, T, composition}
which can be varied independently without changing the number of phase in equilibrium.
C = Number of components forming the system.
P = Number of phases that coexist in a chosen system at equilibrium.
2 = Number of external factor (P, T).
Note:
1. If phase diagram is plotted between T and percentage composition keeping P = const. then,
F = C – P + 1.
2.
F 0 P c 1.
3. at triple point F 0
Types of phase diagram:
(i) Unary phase diagram
(ii) Binary phase diagram
Type-I: Soluble in liquid as well as solid state.
Note:
1. Solid solution structure of solvent does not change
Substitutional solid solution: difference in atomic radii should be <15%
Interstitial solid solution: difference in atomic radii should be < 59%
2. Lever rule: It is used to find
– Amount of each phase in the two region.
Type-II Binary phase diagram
– Soluble in liquid but partially soluble in solid state.
Various types of phase diagram reaction:
1. Monotetic Reaction:
L 2 solid
L1
2. Eutectic Reaction:
solid1 solid2
L
3. Eutectoid reaction:
solid2 solid3
solid1
4. Perictectic reaction:
solid2
L solid1
5. Peritectoid reaction:
New solid
solid1 solid2
Type-III Binary phase diagram
– soluble in liquid but completely insoluble in solid state.
Fe-Fe3 C phase diagram
1. Fe – Fe3 C has four invariant reaction
– Monotectic
– Eutectic
– Eutectoid
– Peritectic
2. Fe – Fe3 C has 5 individual phases.
(a) –ferrite:
– BCC structure
– soft and ductile
– Chromium is used as stabilized
– Carbon content is 0.025% @ 723°C.
(b) –ferrite:
– BCC
– soft and ductile
– carbon content is 0.09% @ 1493°C
(c) Austenite ():
– FCC
– Soft, ductile, malleable, non magnetic
– Nickel is used as stabilizer
– Carbon content is 2% @1147°C and 0.8% @ 727°C
(d) Comentite (Fe3C):
– Complex orthorhombic crystal structure having 12 iron atoms and 4 carbon atoms per
unit
– Hard and brittle
– Carbon content 6.67%
(e) Lade burite [ Fe 3 C] :
– Eutectic mixture of austenite and cementite
– Observed in cast iron containing 4.3%C.
(f) Pearlite [ Fe3 C] :
– It is a mechanical mixture of 87% ferrite and 13% cementite.
– Pearlite containing 0.8% C is known as eutectoid steel which makes steel more ductile.
A0 : This temperature line lies in the bottom section of iron carbide diagram that is the 210°C
temperature line. Here cementite changes from ferromagnetic to paramagnetic in nature.
A1: It is the lower critical temperature. Here at 727°C temp erature austentine to pearlite Eutectoid
transformation takes place.
A2 : This indicates curie temp. that is 768°C below whi ch a ferrite is ferromagnetic in nature.
A3 : This indicates upper critical temp. below which ferrites start to form from austenite.
Acm: This indicates upper critical temperature below which cementite starts to form from austentine.
Heat Treatment
Objectives of heat treatment:
1. To remove internal stresses due to unequal contraction of castings.
2. To improve machinability
3. To improve electrical and magnetic properties
4. To improve mechanical properties like tensile strength, ductility, hardness, shock resistance etc.
5. To improve corrosion resistance.
Heat treatment process:
(I) Annealing:
Purpose:
1000
900
A3
Acm
0.8
1. Reduce residual stress, hardness
2. Increase ductility and toughness
3. To refine grain size i.e. uniform grain structure results.
(II) Normalizing:
Purpose:
(i) Increase the strength of medium carbon steel.
(ii) Improve machinability of low carbon steel.
(iii) It is less ductile than annealed steel.
(iv) Improve mechanical and electrical properties
(III) Hardening:
Purpose:
(i) Produces extreme hardness
(ii) Maximum tensile strength
(iii) Minimum ductility
(iv) Materials are too brittle
Note:
(1)
g
o lin
t co
Fas
Moderate
Austenite
(2) Martensite =
=
Cooling
Slo
w co
ol in
g
Martensite
@lo
Bainite
[C in ferrite]
Heat
Treatment
Pearlite
[87% ferrite
13% cementite]
Structure is body centred tetragonal
Very strong and brittle.
mp
w te
Martensite
@ high
temp
Spheroidite
(3) During heat treatment of steel, the hardness of various structures in increasing orders is
spherodite < pearlite < martensite
(4) Spheroidizing:
– Used for high carbon steel (C > 0.6%)
– This type of heat treatment produces comentite in the form of globulus particles (Spheroids)
from pearlite.
Purpose:
1. Good machnability
2. High ductility
3. Maximum softness
Quenching media for hardening:
Quenching media
1. Mineral Oils
Application
–
used in hardening alloy steels
–
Very fine pearlite structure is produced
2. Water (or) aqueous
solution of NaOH or
NaCl (Brine)
–
used for quenching carbon and low allow
steel
3. Air
–
Martensite structure is produced
–
Used for rails, pipes etc.
–
fine pearlite structure is produced
Note:
(1) Brine solution provides fastest cooling amongst all.
(2) Martempering: [stepped quenching]
→ Austenite is converted to martensite
(3) Austempering: Austenite is converted to Bainite.
IV. Tempering:
Purpose:
1. Residual stresses are relieved
2. Ductility is improved
3. Toughness is increases
4. Decreased hardness.
5. Improve strength
6. Reduce brittleness
7. Increase wear resistence
900
A
A3
B
723°
C
D
0.8
% wt of c
A = Normalising
B = Annealing (or) Hardening
C = Spheroidising (or) process annealing
D = Tempering
Case hardening
(1)
Hard surface = case provides hard wear resistance
Soft core provides toughness
(2)
Mild steels cannot be hardened by quenching, so their strength is increased by case hardening.
(I)
Chemical heat treatment of steel
(a) Carburising:
(a)
Pack carburising:
–
–
(b)
BaCO
Carburising mixture: 50% charcoal + 20%
1444442
4444433 + 5% CaCO3 + 5 – 12% Na2CO3.
Heating time 6 – 8 hours @ 950°C
Energizer
Gas carburising:
–
heated in the medium of gases containing CO + hydrocarbon such as CH4, propane,
butane @950°C for 3–12 hrs.
(c)
Liquid carburising:
950C
dipped in molten state bath containing 20% NaCN.
Work piece
(d)
Nitriding:
(500 600)C
(1) Work piece
hardest case on steel.
NH atmosphere
3
(2) Cooled in the spring of ammonia.
(e)
Cyaniding:
Work piece immersed in molten cyanide bath containing
20 – 30% NaCN, 25 – 50% NaCl,
25 – 50% Na2CO3 for 30 – 90 minutes
Note: Hardness of steel greatly improves with cyaniding.
(f)
Carbonitriding:
Work piece is heated in the mixture of ammonia and hydrocarbon to a temp. 850°C for
2–10 hrs.
(II)
Induction Hardening:
(III)
austentic range.
Quenched immediately to form martensite.
Flame hardening:
(I)
The heating effect is due to eddy currents flowing in the surface of work piece up to
Oxyacetylene torch is used to produce a temperature of 2400–3500°C
Austenite is converted to martensite.
Testing of materials.
Ductility testing:
(a) Close bend test
(b) Angle bend test
(c) 180° bend test
(II)
Impact testing (Toughness test)
(a) Izod test → cantilever type
(b) Charpy test → beam type
(III)
Hardness test:
(a)
Brinell hardness test:
Specification:
Load (P)
D
d
D = 10 mm ± 0.0045 mm
3000 kg hard material
P = 1500 kg int ermediate hardness
500 kg soft material
15 sec for ferrous metals
T =
30 sec for softer metals.
BHN =
(b)
P
D
D D2 d2
2
Vicker’s hardness test:
It uses a diamond square based pyramid indenter with 136° angle between opposite faces.
Load varies from 5kg – 120kg in increment of 5kg.
VHN = 1.854
(c)
P
d2
Rockwell hardness test:
Dial A to H and K with 9 scales.
B & C scales are mostly used
B scale
→ for soft metal
→ steel ball indenter
C scale
→ for hard metal
→ diamond cone as indenter
Note:
It is a non-destructive testing.
Non-destructructive testing [NDT]:
(i)
(ii)
(iii)
(iv)
(v)
Visual inspection
Magnetic particle inspection
Ultrasonic testing
Liquid - penetration test
X-ray / -ray testing.
Magnetic properties of materials:
(i)
Pm = Pm(orbit) Pm(spin) Pm(nuclear )
Pm(spin)
(ii)
=
M
= r 1
H
Magnetic material:
(a)
For diamagnetic material:
H M
m 1
r 0
Ex : Si, Ge, Au etc
(b)
Paramagnetic material:
(i)
at
H = 0; M = 0
at
H 0; M 0
m = +ve
(ii)
(c)
r 1
(iii)
It follows curie law: m =
(iv)
Above curie temperature it’s M = 0.
Ferromagnetic material:
(i)
Mainly d-block elements exhibit ferro-magnetism.
(ii)
Pm = Pm(spin)
(iii)
at H = 0; Bresidual = 0M
(iv)
It follows curie-weiss law;
m =
(d)
C
T
(v)
Above curie temp.; it becomes paramagnetic
(vi)
Ex: Fe, Co, Ni
(vii)
Exhibit hysteris.
Anti ferromagnetic:
(i)
(e)
C
T
m = 0
(ii)
0 = 1
(iii)
Follows neel’s law, m =
(iv)
T > N Paramagnetic.
C
T N
Ferrimagnetic material:
→ Antiparallel and equal dipole moments
→ Above curie temp., converts to paramagnetic.
Ex: Ferrites
→ Suitable for high frequency application.
Electric and magnetic characteristics of ferrites:
(i)
High resistivity
(ii)
Low dielectric loss
(iii)
High permeability
(iv)
High curie temp.
(v)
Mechanically hard and brittle.
(v)
Low eddy current losses.
Types of ferrites:
(I)
Hard ferrites:
Used to make permanent magnet
These are having
(i) Higher curie temp.
(ii) High coercive force
(iii) High residual magnetism i.e., wide B-H curve.
(iv) High permeability.
(II)
Sr ferrites
14243
Ex: Ba and
exhibit semiconductor
behaviour
Application: Small motors, TV tube focusing magnet.
Soft Ferrites:
→ These are having
Ex:
High permeability
Low coercive force
Narrow B-H loop.
Mn – Zn ferrites
Ni – Zn ferrites
Application: Core of inductors and transformers.
(III)
Rectangular loop ferrites:
→
Ex: Mg – Mn ferrites
Cu – Mn ferrites
→
(IV)
Application: For magnetic memory storage in HDD, floppy etc.
-wave ferrites/Garnets:
Ex: Mn – ferrites
Ni – ferrites
Appl: For Gyrator, circulater, isolater etc.
Magnetostriction:
→
Change in the dimension of a magnetial material when it is magnetized.
Longitudinal magnetastriction
–
Transverse magnetostriction
–
Note:
Change in the dimension is in the direction of
Change in the dimension is in the perpendicular direction of
Volume magnetostriction
Humming sound in transformer is due to magnetostriction.
Soft / Hard magnetic materials:
(a)
Soft magnetic material:
Characteristics:
(1)
Low retentivity, coercivity and hystrisis loss.
(2)
High permeability and magnetic saturation
(3)
Hysterisis curve is tall and thin.
Ex:
Silicon steel, soft iron, Fe-Si alloy.
Note:
Si ses DC resistivity, ses area of hysterisis.
Appl.:
(1)
Used in mfd. power transformer @ 50 – 60 Hz.
(2)
Fe – Ni alloy
→ 36% Ni used for high frequency application.
→ 50% Ni used for magnetic memory.
→ 77% Ni used for current transformer
45% Ni Permalloy
79% Ni Superalloy
Loss
79% Ni Mu metal
(b)
Hard magnetic material: [Permanent magnet]
Charactristics:
(1)
High , coercive force, curie temp, hysterisis loss
(2)
Ramanent flux density
Application: Permanent magnet, focusing magnets in TV tubes.
Hard magnetic materials are:
(4)
(1)
Carbon steel – 98% Fe, 10% Mn, 0.37°C ; Compass needle
(2)
Tungsten steel – Fe, W, C, Cr. ; DC – Motor.
(3)
AlNiCO – Al, Ni, CO → Permanent magnet
Platinum cobalt –
77% Pt
23% Co
(5)
Aloemax
–
55% Fe
11% Ni
22% Co
8% Al
4% Cu
(6)
Hycomax
–
50% Fe
21% Ni
20% Co
9% Al
Magnetic Anisotropy:
If material is having direction dependent magnetic properties then material is said to be magnetically
anisotropy and this property of material is called anisotropy.
There are 3 methods to include magnetic anisotropy in a materials:
(I)
Cold working: In this method, material is cooled down using cold rollers. This method induces
magnetic anisotropic in direction of rolling.
(II)
Magnetic annealing:
It is the process of heating and slow cooling in the presence of magnetic
field.
(III)
Magnetic quenching: It is the process of heating and test cooling in the presence of magnetic
field. In this method, material is cooled down to curie temp. in order to induce anisotropy.
Conducting materials:
(1)
J = E ; ohm’s law
=
ne2
; conductivity
m
ne e ; e mobility of [cm² / v–s]
; as T ; . [Metals shows PTC]
(2)
(3)
Mattheisein’s Rule:
1
T
=
ralloy
ar
Lin e
Constant
rr
T
QD
Debye temperature
(4)
Thermoelectric effect:
Seeback effect
Peltier effect
Thomson effect
Fe
Zn
Cu
Au
Ag
Pb
Al
Hg
Pt
Constantan
(i)
@ Cold junction
(ii)
Super conductivity:
– R O at T TC
– Diamagnetic Ex : Hg @ 4.2 K
–
H HC
Commercial superconductors:
Tc
V3Ga → 16.8 K
Nb3Sn → 18.5 K
Nb3Ge → 23.2 K
Meissner’s effect:
The repulsion of magnetic flux from the interior of a piece of super conducting material, as the
material under goes to the transition to the super conducting phase is known as meissner’s effect.
Critical magnetic field:
HC
H0
Super
-conductor
Normal
TC
T
T 2
HC H0 1
TC
Silsbee’s rule:
The maximum current allowed is IC to retain super conductivity.
Types of super conductor:
Type - I
Type - II
(a)
Ideal super conductor. and exhibits
diamagnetic nature at super
conductivity
Non-ideal/ hard super conductor
(b)
Critical field and transition temp. are
low
Critical field and transition temp. are
high
(c)
Exhibit complete meissner's effect
and follows silsbee rule.
Exhibit incomplete meissner's effect
and shows some deviaton from
silsbee's rule.
(d)
Transition state is sharp.
Transition state is gradual.
–M
–M
Vortex region
Super
conducting
Normal state
H
HC
Super conducting
state
r
Normal
state
HC1
H
HC2
r
Vortex
Super
conducting
Super
conducting
Normal state
H
H
HC
Ex: Pb, n, Zn
Normal
state
Ex: Nb3Al, NbTi, Nb3Sn
Semiconductors:
Eq.:
(1) For semiconductor.
Si
T = 0°K
T = 30°K
1.21 eV
1.1 eV
Semiconductor exhibits
NTC.
Ge
0.78 eV
0.72 eV
Note:
(2)
(i)
Width of energy band depends on Temp.
(ii)
Si is more sensitive to temp. variation as compared to Ge.
For metals:
–
(3)
Both CB and VB overlaps.
For insulater
–
EG 5eV
@ T = 0°K
Insulator
Note: (1) Semiconductor
@ T = 300°K
Conductor
Extrinsic semiconductor:
Do
na
to
ra
A
P,
n = p = ni
m
s,
)
(N D
,
Sb
n ta
Pe
Bi
le n
va
n > ni
P < ni
@ equilibrium: n p = n i
n < ni
P > ni
@ equilibrium: np = n i
2
tom
ta
Intrinsic
simiconductor
2
(B, Al, Ga, In, Tl)
Trivalent atom
Note:
(1)
(2)
N-type (or) P-type are always neutral.
ni2 A0T3 eEGO /KT
ni Ge ni si
N-type semiconductor
C.B.
P-type semiconductor.
C.B.
EC
ED
EC
EA
EV
EV
V.B.
V.B.
@ T = 0°K
(Insulator)
@ T = 0°K
(Insulator)
N-type semiconductor
P-type semiconductor.
EC
ED
EC
EA
EV
EV
@ T = 300°K
(conductor)
@ T = 300°K
(conductor)
Note:
(1)
Temperature coefficient of resistance of doped semiconductor may be +ve/–ve depending
upon level of doping.
(2)
When alloying is done for group III–V elements, the crystalline structure is formed of the
type ZnS (Zinc blende).
Hall effect:
–
Used to find:
n-type / P-type
Conductivity
Carrier concentration
Mobility
2
2
2
+
VH
–
–
d
w
Note:
B
W
N-type/metal
Voltage mv
1
v
(1)
VH = RH
(2)
ve ; P type
RH =
ve ; N type, int rinsic semiconductor
(1)
1 e
1
The fermi level represents the energy state at which 50% probability of finding an electron
EF n2/3 ; n = number of free es/volume.
(2)
EC
ED
EA
EF = N-type
EF = intrinsic
EF = P-type
EV
P–type
E EF /KT
exist:
Note:
1
Fermi-dirac probability function f(E):
f(E) =
→
VH
+
1
Formulated (compound and alloyed) semi conducting materials.
(I)
Gallium Arsenide (GaAs).
Uses: (a) Switching and parametric diodes
(b) Semiconducto lasers
(c) Tunnel diodes
(d) Hot electron diodes
(II)
Indium Antimonide [InSb]
Uses: (a) Infrared detectors
(b) Laser diodes
(c) Infrared filter material
(d) Hall effect devices
(e) Tunnel diodes
(f) Transistors.
(III)
Cadmium sulphide [Cds]
Uses: (a) As constituent of cathode ray.
(b) To record modulating light intensity on sound track.
(c) As on-off light relay in digital and control circuits.
(IV)
Silicon carbide [SiC]
Uses: (a) High temp. rectifiers
(b) High temp. transistors.
Applications of few semiconductor device.
(1)
Junction diodes: Rectifier, clipper, clamper
(2)
Zener diodes: Voltage regulator
(3)
Transient condition diode:
These are: (i) Schottky diode
(ii) Varactor diode
(4)
Bipole Junction transitor (BJT) : Amplifier
(5)
Field Effect Transistor (FET) : Amplifier
(6)
Metal oxide semiconductor (FET) : Digital circuits.
(6)
Opto electronic devices:
These are:
– Photodiodes
– Photocells
– LED
– Optical fibres
– Solar cells
(7)
Negative conductance microwave devices:
These are:
– Tunnel diode
– Gunn diode
– Impatt diode
(8)
Power devices:
These are:
– P–n–p–n diode
– Silicon controlled rectifier (SCR)
Polymers:
Polymers are gigantic molecules (or) macromolecules which is formed by repeated linking of the
small molecules called monomers that are covalenthy bonded together. The average molecular
weight of polymer varies from 10,000 to more than 10,00,000 gm/mole.
Polymerisation: The process by which a monomer is converted into polymer under specific
condition. The reaction may be initiated by initiater i.e., a catalyst.
Note:
(1)
Ex:
Homopolymer : When the repeating units along a chain are of some type,
A
A
A
A
A
(2)
Copolymer : Two or more different monomer subunits linked to create a polymer chain.
Ex:
(1) Block copolymer :
A
(2) Random copolymer :
(3) Alternate copolymer :
(4) Graft copolymer :
A
A
A
A
B
B
Classification of polymer:
(I)
Based on source:
(a) Natural polymers
(b) Semi-synthetic polymers
(c) Synthetic polymers
(II)
Based on structure:
(a) Linear polymer
(b) Branched polymer
A
B
B
A
B
A
A
A
B
B
A
B
A
B
B
A
B
A
A
A
B
A
B
A
B
A
A
A
A
B
B
B
B
A
A
A
(c) Cross-linked polymer
(d) Network polymer
(III)
Based on polymerisation:
(a) Additional polymers
(b) Condensation polymers
Characteristics of polymers:
(1)
Low density
(2)
Good corrosion resistance
(3)
Good mould ability
(4)
Economical
(5)
Low mechanical properties
(6)
Can be produced transparent or in different colours.
(7)
Low coefficient of friction
(8)
Poor tensile strength.
Classification of plastics:
Thermoplastics
Thermosetting plastics
(1)
They have long chains of polymers
which are not cross linked.
They have long chains of polymers
which are cross linked.
(2)
They become soft on heating.
Hence they can be moulded on
heating and under pressure.
Once set or mould it does not
become soft on further heating
i.e., the cannot be remoulded
on heating
(3)
They are used for making:
(i) Buckets
(ii) Pipes
(iii) Carry bags
(iv) Toilet goods
(v) toys
They are used for hot temperature
application such as:
(i) Utensil handle
(ii) Body of socket and plug
(iii) Furniture
(iv) Telephone body
(v) Automobile parts.
(4)
Ex: Polyethlene, Polyestyrene,
Nylon etc.
Ex: Bakelite, malamine
(5)
Formed by addition polymerisation.
Formed by condensation
polymerisation