4-
Contents
CLASSIFICATION OF
MATERIALS
;PageNo.
Ckapl(r •
c1assification of Materials
:tt 1'lof~-
CHAPTER
1-9
t '-«'
Conducting Materials-#- l\!ol-e.lS(For this chapter also see Appendix page no.119-121)
10-3!
Semiconducting Materials:#= No~
32-40
Insulating Materials. ;ff:
Magnetic Materials
.boo'\<
I 1.1. lntroductron
The main aim of any engineering works is the practical application of right material to
/ manufacture the electrical machines and appliances. While selecting the material for the required
application one should keep in ~iew the h'.gest degree of ~con~my and to meet the requirements
perfectly. Before selecting tt1e nght matenal for the requ1red JOb deep and thorough knowledg~
about the material should be attained. If there is lack of knowledge about the material then it will
be a big cause of br~kdown in the appliances and durability of the appliances also get affected.
There are many factors that o~e should ~onside~ before the selection o~ material. Physical,
mechanical, chemical and electncal properties are important factors but besides these followmg
are the main factors that must be kept in mind :
j
I
80-89
,pecial Pwpose Materials #book
For this chapter also see Appendix page no.122-124)
I
90-110
!
/
errous and Non-ferrous Materials #-
opendix
amioatioo Paper
ko h
ii 2
ttt
•
j
41-79
# back
et
I
lll-118
119-124
(i)
Cost
(ii) Availability
.
(iii) Ease of fabrication
(i) Cost : Cost of a material is a very important factor while selecting an engineering
material. Material should be chosen in such a way that highest degree of economy is achieved. For
/ example silver is the best conductor of electricity but it can not be used as conductor of electricity
j because of its high cost. And on the other hand, copper is not as good conductor as silver. But it is
/ widely used iis conductor in many engineering applications. Because copper is cheaper than silver.
/ Hence by using copper as a conductor in the required application overall cost is reduced.
I
/
(ii) Availability : Availability of materials and their continuous supply are also important
/ factors. If the material is easily available then the manufacturing process will be continuous. For
I example, many a time aluminium is preferred over copper, although it is an inferior material than
I copper. It has _lower c~nd~~tivity than_~at o~ copper. Because in India a!uminium is available in
, large amount 1.e, availab1hty of alummmm 1s more than copper, so due to many considerations
aluminium is used as a conductor in many applications like transformer windings, inductive
chokes for fluorescent tube lights and in overhead transmission lines.
I
I
Electrical and Electronics Engineering Materials
. -~ ~
c~fication
~----------...:..) ;
of Materials
(i v) It should have high mechanical strength
Anothe, example is that in earlier days mica is used as an insulating material worldwid~.
On the b i
(v) Free from atmospheric and chemical effects
But after the development of PVC and plastic materials, mica is replaced by PVC a nd plasttc
_
as follows :
(vi) It should have good electrical properties as per the requirement.
materials because these are the easily available materials having low cost. So in _present time P~C
th
@iMaterii
·
t rials for the plug top. 1n e same ·
and other plastic materials are used as an insulating material in many apphcauons like ~Ire
The above was a particular example of selecting
ma e
purpos,
b
t d
,
.
.
•
·
g
products
can
I
e
se
ec
e
.
insulation, switch board, switch outlets, panels, casing and to make the outer body of electncal , anner materials for the manufactunng
of vanous engmeenn
.
known
.th .
jl11
·
·
·
·
L
mber
appliances.
of
matenals
along WI their
.
The field of electrical engineenng 1s very wide. arge nu
.
f
t . fi th
&ateri:
Ease of fabrication : Last but not the least, ease of fabrication is also an important fact~r l · 0 us types are being used in the electrical engineering field. Right sel~cllon _ma enf al1 . otnr. el
extem2
jva0
.
·
·
ca
while selecting right material for the required application. In pres~nt a~e, s~eed of p~oductton is 'required
application is a tough task. Before selectmg
engm~enng m atenals funcllon o e ec
·
@,tateri,
very important. Many materials have restricted electrical applicattons, insp1te of having smta~le !engineering material should be known. These functions are hsted below.
anothe
properties because of having low fabrication speed. For example, ceramic is used to make h~e
(i) To carry the electric current from one place to another place.
insulators over a long period of time. But in present time ceramic is replaced by ~o~y resm.
@Mate~
(ii) To prevent the flow of electric current.
Although ceramic has favourable electrical and mechanical properties _for being a_ line insulator
@Maten
(iii) To control the flow of electric current.
.
·
but it has low fabrication speed. On the other side, ·epoxy resin has high fabncatton speed that
station
(iv) To allow the flow of electric current in one direction and obstruct the flow m another
tends high production speed.
To understand the above factors let us take an example : Consider a two pin or three pin direction.
1.2. Atomic
(v) To store electrical energy.
plug which is used to connect the many electrical appliances (like electric iron, i_table fan ,
,
Atomic
television) to the· supply mains. To manufacture the plug we need conducting matenal so that
(vi) To provide the easy path to the magnetic flux .
A ; ! ma1
appl i"! nces·can draw the electric supply from the supply mains and insulating material to prot_e ct
(vii) To convert the electrical energy into neat and light energy.
small pa ·lei
the h• ,nan beings from accidents and to avoid short circuit. Pins of.plug is made of conductmg
The above functions are performed by means of following, written in the same order as the
as elemen ai
mater, 11, conducting material of pin should be of such type that it has good electrical properties
functions written above.
An ·o
and h,gh mechanical strength as well. Because electric plugs are used in variety of applications
~opper and aluminium used to carry electric current from one place to another plac~.
where plugs have to bear different mechanical stress. So pins are made of brass to work as
types of parti•
Alumiruum is used as a conductor in overhead transmission line. Copper is used as a conductor m
conductors. Because brass has good mechanical strength, suitable electrical properties, high
charged parti
domestic wiring.
resistance against chemicals and atmospheric effects. These pins are ·embedded in an insulating
The neutron i
0.Plastic
materials,
ceramic,
~or~elain
obstruct
the
flo"'. of_electric curre~t. In overhead
body. It is required that there should not be any flow of current between the pins to avoid short
Nuclei
transnussion
lines
ceramic
and porcelam 1s used to manufacutre lme msul~tors o~ different shapes. \
circuit and to protect human being from electric shocks. Insulating material of plug top should
spinning aro,
have suitable electrical properties and good mechanical strength. The plug should have sufficient Plastic materials are used to manufacture the external body of electnc appliances and many
different orb
·mechanical strength to withstand stress during thousands of insertions and it should be moisture electrical products.
and chemical resistant because plugs are handled by wet and greasy hands and are used in open
Carbon, mang~ and constantan are used to manufacture different types of wrre
the order of
atmosphere where it is affected by moisture, humidity and gases present in the atmosphere. wound resistors of different shapes and rheostat which are used to control flow of electric current
diameter of :
Ceramic materials meet these requirements but its fabrication process takes too much time. Ease in electrical and electronic circuits.
space betwe
of fabrication of a material is an_ essential factor. It is difficult to give appropriate shape of plug to
Semiconducting materials such as silicon and germanium are used to convert I\
ceramic material in less time. So plastic material is mostly used as insulating material in place of jaltemating current (A.C.)
order i.e, h
into direct current (D.C.).
ceramic material because plastic materials meet the requirements of ease of production and also
valence ort
Mica,
paper,
polythene
etc.
are
used
as
dielectric
material
ii;i.
the
manufacturing
of
has low cost.
electrons o 1
capacitors which are used to store electrical energy.
From the above example we can understand the importance of various factors to be
negative ch
@steel and silicon alloys are used to manufacture magnetic core for transformers, electric ,
;onsidered while selecting right materials for the required job. These factors may be summarized
nucleus hai
/g~nerators
and
electric
motors.
These
magnetic
core materials provide easy path to the magnetic \
s listed below :
!flux.
has low attJ
(i) It should be of low cost
°
@
,
@
'
I
'fp
I
i
(ii) It should be easily available from local source
(iii) It should be easily moulded in different shapes
@Tungsten and nichrome are used to make electric bulb/lamp filaments and heating
1element of electric irons, heaters, electric ovens etc. These materials convert electrical energy into
lheat energy and light energy.
I
electrons c
Now this e
Electrical and Electronics Engineering Materials
Classification of Materials
According to modem electron theory. each element has different a1< 111i c lllllll hC! .-\n
analysis· we can functionally classify electrical engineering materials atomic number of element represents the number of electrons present m the atom. 11 enc.: 11 1,
above
f
.
Onthe basis o
clear that all elements have different number of electrons and valency i.e. number of valence
as follows ·
t in a particular atom are different. Each material has different properties has
~Materials which are used to allow _the electnc current to pass through them for vanous ' I t
d I 1ng wire in domestic wmng filaments of electric lamps are I e ec rans presen
~
, different ro erties on the basis of number of electrons. Atomic structure of aluminium and
'
'
purpose such as ~on uc •
.
p p .
'
known as conducting matenals.
copper has been 11lustrated m fig. 1.1
. .
.
fl ow of electric current used m lme insulators, !
the
obstruct
to
used
are
which
Cu(29) = 2, 8, 18, 1
1
. ,,
.
...
kn
atena1s
,
external body of electric apphances are own as msulatmg matenals.
0
®'1
.
t rials used to allow the flow of electric current in one direction and prevent the flow in
rr::...,
.
. d .
~nae
A1(13) = 2, 8, 3
another direction are known as "semi con uctmg materials."
@Materials which are used to store electrical energy are classified as •:dielectric materials."
@Materials which are used to provide easy path to magneltc flux m many rotatmg and
stationary electrical devices are known as "magnetic materials."
1.2. Atomic Theory and Classification of Materials on the Basis of
Atomic Structure
AII matters consist of minute particles called molecules which are further made by very
small pa •Jes known as atoms. Those substances whose molecules have similar atoms are known
Fig. 1.1 : Atomk structure of aluminium and copper
as elemen and substances whose molecules have dissimilar atoms are known as compounds.
An ·om consists a hard central core which is known as nucleus. A nucleus cons\sts two
Atomic number of aluminium is 13. It has three orbits namely K ,L,M. Electrons in the
types of particles, one is known as proton and another is known as neutron. Proton is a positively
2
) where n = No. of orbits from centre of atom.
charged particle whereas neutron does not consist any charge i.e., neutron is electrically neutral. orbit can ~e found out by applying the formula (2n
The neutron is as heavy as proton. Weight of both particles are equal to weight of hydrogen atom. So accordi.ng to this formula, no. of electrons in aluminium's three different orbits will be 2, 8 and
Nucleus is surrounded by a number of tiny particles called ~lectrons. The electrons are 3 in the successive orbits. It has three electrons in the valence orbit i.e. aluminium's valency is
spinning around themselves and also revolving around the nucleus. These electrons are placed in three. When external energy is applied to aluminium then all the atoms of aluminium relase their
different orbits (K ,L, M,N) having different energy levels. The effective diameter of an atom is of electrons and get three positive charges.
In same manner copper has 29 electrons. It has four orbits K ,L,M ,N (2, 8, 18, )) and in
the order of 10-8 cm and diameter of nucleus is of the order of 10- 13 cm. Hence we can·say that
diameter of an atom is approximately I05 times greater than that of nucleus. It means there is large valence orbit it has one electron i.e. valency of copper is one.
In the case of co nducting materials, there are l, 2, or 3
space between electron orbits and nucleus. The energy levels of different orbits ai:e in increasing
order i.e, higher order orbit has higher energy level. The outermost orbit of an atom i s, ~alled electrons present in the v"i<!nce orbit i.e.materials having l, 2 or ,
valence orbit and electrons present in the valence orbit are called valence electrons..All the 3 velency are categorized as conducting material and in the case
electrons of an atom are lightly held by nucleus. Nucleus has positive charge and electro~ has ! of insulating materials all atoms are fully stable. All the
negative charge so an attraction .force acts between the nucleus and electrons. Electrons nearer the electrons present in the atom are tightly held with nucleus. And
nucleus has highest attraction force and having low energy whereas electrons far from the nucleus in the case of semiconducting materials there are 4 electrons
has low attraction force and high energy. When external energy like, heat is given to the atom then present in the valence orbit i. e. materials having 4 valency are
electrons of valence orbit break the attraction force of nucleus and atom releases the electrons. categorized as semiconducting material. Atomic structure of
silicon semiconductor has been shown in the fig. 1.2.
Now this electron moves all around the matter. This electron is called free electron.
1
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Electrical and Electronics Engineering Materials
~C~la~ss~i~fic~a~l•~·o~n~o:!._f~M!..'.!a~lc:'._!:r:!_:ia~ls~--- - - - -- -- - - - - : - - - - -- - -- - -
Sil i.:un has 1-1 ekctrons and its electron distribution is 2. 8, 4 in K, L, M orbits. II has 4
ekctrons in the outern1ost orbit i.e. its value of valency is 4. According to no. of electrons present,
in the atomic structure each material shows different properties. For example, in conducting)
material it is easy to loose its valence e- electrons so they are good conductor of electricity and!'
insulating material resists the flow of current because number of free electrons are almost zero.
And in the case of semiconductors valency is four. So in ordirary condition they behave like anj
insulator and when temperature gets increased they behave like a conductor.
I
j
Inter Atomic Bonds
The bond between atoms gives the solid shape to the metal. These bonds are of three types~
as given below :
(i) Metallic bond
(ii) Covalent bond
Fig, 1.3 : Covalent Bond of Germanium Atoms
(iii) Ionic bond
I/iii) Ionic bond : An ionic bond is formed between the atoms of two different eleme,
Bond 'tietween sodium and chlorine atom is an ideal example of ionic bond. Here the sodium at,
has one valence electron, due to this'sodium atom is more unstable. It has tendency to release 1
electron to get stable state. On the other side, chlorine has seven valence electrons, due to·u
chl~rine atom is. more st~ble. It has t~ndency to accept t_he electron to get stable .stat§ Wh
sodium reacts with chlonne then sodmm atom releases its electron and gets positive char§
whereas chlorine atom accepts this electron and gets negative charge. This kind of bond is cal11
@> Metallic bond : Elements which have one, two or three valence electrons are very ionic
bond and hence the atoms formed are called "donor ion" and "acceptor ion " respective!}
unstable because as we know that an atom requires eight electrons to get stable. In such case
r11•
.~X-ERCISE .
atoms of these elements give up their electrons to another atom to complete its octajHence an
electron cloud is formed throughout the space occupied by the atoms.
1. Explain the calssification of materials in three parts based on their atomic structure. ·- ✓
After releasing the electrons the atoms get positive charge. An electrostatic force acts
(BTE1005
2. Write short note on covalent bond.
(BTE1005
between the electron cloud and positive charge due to this the metal crystals remain in solid
3. List the name of groups in which the electrical engg. materials can be categorized. Ex~lai;
shapes] This type of bond is characteristic of elements having small number of valence electrons
how conduction of electricity takes place in conductor and semiconductor.
i e. metals, hence this kind of bond is called metallic bond. Due to metallic bond metals remain in
(BTE 2006, 2009)
4. On the basis of atomic structure explain how metals are good conductors of electricity.
solid st.ate and because of having less valence electrons these materials show the property o
The attraction force in all the three types of bonds are due to valence electrons present in th
outennost orbit of an atom. Unstable atom wants to complete 8 electrons in the valence orbit to get
slabilized. To complete its octal, atom can acquire more electrons or lose all its electrons to
another atom.
(BTE 2007)
5. On the basis of atomic structure explain the conductor, insulator and semiconductor.
(BTE 2008)
e-0nductivity, ductility, positive temperature coefficient etc.
g,ovalent bond : Elements which have four electrons in the outermost orbits have atoms
neither e-0mpletly st.able nor astable. In such cases atoms do sharing of electrons with the nearby
atom
to get
st.able state.]
.
Referring to figure J .3 which shows the e-0valent bonding between the germanium atom. I
,erma.nium has four valence electrons in the outermost orbit. Germanium atoms have no power of
•leasing of electrons. So each atom of germanium share electron with the neighbouring atom. ;
iis kind of sharing is called "e-0valency" and bond is called covalent bond.
6. Describe classification of materials on the basis of atomic structure- Li 5t th e names of any
two materials and their application from each group.
.
rare 20J2J
. wh.1ch matena
. 1s can be class1.fi ed• Explam how rnsuJ .
7. List the name of groups m
atmg
· ·
materials are bad conductors of eIectnc1ty.
b d (BT£ 20J3)
.1
. f perg)' an s. (BT£ 20
8. Discuss classification of electrical engg. matena
s on the basrs o e . mic structu
14)
.1
h
. f th elf ato
re
9. Explain electrical and electronics engg. matena
s on t e basis o
(BT. ·
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Electrical and Electronics Engineering Matcri~
l)
Classification of Materials
♦ Multiple Choice Questions -
. .1
- .
•
([)n the manufacniririg of line insulators epoxy resm is preferred over ceramic materials\ ♦ Fill in the Blanks
. 1s 1.s base d upon _c_J_.,> r:•) il S: } ; 1~ ( J VS\ (
. matena
.
I. Classification of electrical engmeenng
-.
because of :
_
~ .i.ly_~;n is best known conductor.
~
(b) availability
~
(a ) cost
Q) Because of low cost Cap.p 0ns used over silver. .
(d) none of these
.
~ase of fabrication
_
4 _ Ionic bond is formed between tw~1fferent tle~;J-~
s. Semiconducting materials have . .-:-r. .... electrons m the outennost orbi t.
2. According to atomic theory. the maximum number of electrons can be there in outermostn
orbit is :
6. Germanium semiconductor has atomic number of
~8
7. Because of its .. . . . . .. . epoxy resin is used over ceramics.
(a) I
(d) any number
@ \?,.n~~- is used to make pins of two pin plug. .
(c) 18
9. Insulating materials are used to prevent . (.\~c;.,b.1, c. S-f..o c k1;
3. An atomic number of an element represents the number of:
To store electrical energy . . . ... . . . materials are used.
10.
protons
(b)
W,Clectrons
11. Tungsten is used to make .. ... . . . . of electric bulb.
(d) electrons and protons
(c} neutrons
12. Materials which provide easy path to magnetic flux are called .
@Anion is :
13 . . . .. . . . .. is formed between the atoms of semiconductors.
(b) a free neutron
(a) a free electron
Answers
~ari atom with unbalanced electric chp.rge
(c) a free proton.
1. Atomic structure, 2. Silver, 3. Copper, 4. elements 5. 4 , 6. 32, 7 . ease of fabrication ,
@Bond fonped between the atoms of semiconducting material is called :
8. Brass, 9. electric shocks, 10. dielectric materi.als, 11. filament, 12. magnetic material ,
®,.covalent bond
(a) metallic bond
13. covalent bond.
(d) none of these
(c) ionic bond
@aond'formed between the atoms of two different elements is called:
(b) covalent bond
(a) metallic bond
(d) none ofthese(8,ionic bond
@Materials used to store the electric energy are called :
~Dielectric materials
(a) Insulating materials
(d) Magnetic materials
(c) Conducting materials
Materials which provide path to the magnetic flux are classified ·a s :
(b) Semiconducting materials
(a) Insulating materials
(d) Dielectric materials
(s),Magnetic materials
I
.32. ....
•
@
@To convert A.C. into D.C. which kind of materials are used?
(&Semiconducting materials
(c) Magnetic materials
(b) Conducting materials
(d) Insulating materials
@ In the manufacturing of electric bulb filament material used is :
(b) Copper
(d) Mica
~Tungsten
(c) Aluminium
1. (c)
2. (b)
9. (a)
10. (a)
3.(d)
Answers
S. (b)
4. (d)
6. (c)
7. (b)
8. (c)
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CON DUC TING MAT ERI ALS
I V=IR I
V = Applied voltage across th_e terminals (in volts)
I= Current-flowing through the conductor (in amperes)
where
Thus
p = resistivity of a material in ohm-metre
R =resistance of material in ohm
p=T
R oc!_
a
l
If
a can be written as
.
t
.
Assume that at 09 C temperature resistance of a conductor be Ro
and this conductor is
heated up to t°C temperature then expression for resistance at t°C tempera
ture can be written as :
R1 =R 0 (l+a. t)
... (3)
0
"\
Temperature is themost important tad& which affects the value ofresis
tivity. The value
of resistance increases by increasing the value of temperature. If
we draw a' graph between I
temperature and value of resistance for a given matetrial, then we get
a straight line. Increase .in
resistance depends on the following factors :
(i) Initial resistance of a material
(ii) Increase in temperature of a material
(iii) Nature of a material
Above relation can be written as :
I
R1 -R 0 ocR0 t
R, - Ro =a. Rot
... (I)
where a is a constant which is kn~as
own
temperature coefficient of resistance.
2.3. (i) Effect of Temperature on Resistivity
• Resistivity of a material does noi;emain constant. Value of resistiv
ity of material gets j
aff~c~e~ by the various factors. Following are the three main factors
that affect the value of
res1st1v1ty :
(a) Temperature
(b) Alloying
(c) Mechanical deformation.
2.3. Factors AffectinQ Resistivity
\
2.4 .
valu
d
t
a
if va
coeffi
coeffi
1
II
If
then
R =p
So resistivity can be difined as at 0°C temperature the reistance
of a material per unit
volume is known as resistivity or specific resistance.
1
I= length of a conducting material in metre
a =I metre 2 and I =I metre
a= cross-sectional area of a material in metre
2
t = 1°c, Ro = 1n
a.=R 1 -Ro
With the help of above expression a can be defined as : by increasi
ng the temperature of
R = pl
any conducting material from 0°C to 1°C change in resistance is known
as
temperature coefficient
a
j of that material at 0°C.
where 'p' is the coefficient of proportionality and it is called the
resistivity or specific
Equation (1) can be written as R = R (1 + a. t)
resistance of the material in ohm-metre.
1
0
0
... (2)
Ra
2.3. (ii) Value of Temperature Coefficient (a) at Different Temperatures
Hence
and
R = Resistance of the conductor (in ohms)
The resistance R of any material is directly proportional to its lenght
'/' and inversely
proportional to its cross-sectional area 'a'.
where
In simple words, resistivity of a material can be defined as "proper
ty of a material which
offers resistance in the path of electricity is called resistivity".
We all know the Ohm's law which can be written as :
2.2. Resistivity
Materials which are used for conduciing electricity in the field of electric
al engineering are
known as conducting materials and materials which are used for prevent
ing the flow of electricity
are known as insulating or non-conducting materials.
Conducting materials have very low value of resistivity and on the
other hand, insulating
materials have very high value .of resistivity. To determiue which materia
l is conducting material
and which material is insulating material we· should understand the
term resistivity and should
know the value of their resistivity. On the basis of resistivity, conduc
ting materials are further
subclassified into low resistivity materials and high resistivity materia
ls.
2.1. Introduction
etr:w5:er:1:ittS
:1 •., :
~'-~~
~ .~ -~....-
CHAPTER
- ~~' ,
r
,conduc ting Materia ls
1~
t
~
--
:70·
,. , --
65
60
55
50
45
40
35
-• --
.
I
I
We know the relation,
Ithe ~ is ·40Mm.
~!
coefficient of'copper af different value o temperature is shtJWn in the table :
1•
.
For any mate~~!-value ?f t~~pera~~~ coeffic'.ent can be _cal~ulat~d, various temp!ratw;es
tf value of temperature cdeffictent at zero degree celcius 1s- known'.' Value of temperature
1
coefficient o coppei'(Cu) at zero degree cefoius (cx 0 ) is 0.00427 per°C. ' he value of temperature
0.00328
R6o
R 60
0,0038 X 40
1
=4S6.SlQ
=400[ 1+ -l+-0.-00_3_8_x_2_0
·
R 60 =R 20 [1+
t)1
a
(60-20)1
l+ a. x 20
= R6o• R, = R20
t 1 =6D°C,
R,I
R 11 =R 1 [1+-a- (t 1 l+a t
t=2D°C
Putting all the values in the above equation, we get
temperatures
-
0.00333
0.00339
0.00339
0.00352
0.00358
0.00364
0.00371
E ~ A coil is made of sopper wire. .t\t a temperature of 20°C the resis.t ance of
Calculate the resistance of the _coil at a temperature of 60°C.
Temperature coefficient of copper at 0°C is 0.0038 ohm per degree C.
Solution : Given R 20 = 400 ohm, a O = 0.0038
I
I
.
.
0.00385
25
0.00378
0.00393
20
30
0,00401
0,00409
10
15
0,004 I 8
5
2.4. Value of Temperature Coefficient (a) of copper (Cu) at different
...(7)
--
~ --
lJ,UU427
0
Here
or
_
~
13
Value of temperature coefficient of copper at various temperatures
. ature in °c
Value of temperature coefficient (a) in /°C
Temper
-- -
Hence value of temperature coefficient can be calculated at various values of temperature if
value oftemper~ture coefficient at 0°C, ~ .O· is kn,oVl'.fl_.
Clo
at 1= - I+a.ot,
1+~ 0 t 2 -l-a_0 t 1
O+,a 0 f 1)(t2 -t 1)
at1
[ 1+aot1
I+aot2]-1=a t 1 U2 -t,)
or
or
l+a.ot2] =R [l+at,(t2-t1)]
R 1 ----=--=1
! [ l+a.ot,
i
Comparing Eq. (6) with Eq. (7)
R 12 =R1I [l+a.t1(t2 -t 1)]
R -R [ l+a.ot2]
'2 '1 l+a.ot1
a 0 (t2 -t 1 )
(l+a 0t 1)(t 2 -t 1)
. .. (5)
... (6)
... (4)
R11
Equation (5) divided by equation (4)
Rt2 can be written as :
I RaO'C
Fig. 2.1
~
;I
li§ i
,-
R, t' C
=Ro(l+aot1)
Ar
,§1
8
-
Rt2 =R 0(1+a 0t 2 )
Now assume that a conductor is heated up from 0°e to t°C and its
in'.tial temperature at 0°e be point A and temperature at t 0 e be point B . Now
this conductor is again cooled down from t0 e to ooe. Now it is clear that
when this condudor is heated up from 0°e to t0 e then its initial temperature
at 0°e be point A . But when this conductor is cooled down from t 0 e to ooe
then its initial temperature at 1°C be point B which has been shown in the
diagram_ According to equation (3),
8
Electrical and Electronics Engineering Materials1 Conducting Materials
Here a O is temperature coefficient at 0°e. Here assume that a
conductor has resistance R I at 1°C. Now this conductor is cooled down from
.
t0e to 0°C. (However practically it is not possible).
12
Elertrkal and Electronics Engineering Materials
C
(,
~~
(.~ '-
d--. 6
=0.2s[1+
R40 =0.301.Q
R40
0.004x20 ]
I+ (1.004) x 20
-,,(of\.--
(40-20)]
ao
I+(a 0 x20)
=0.28.Q
R20
R40 =R20[l+
=2.8 X 10-8 X IO 7
6
CXo =0.004
20xl0-
=2.8 X 10-8 X ~
R20
R20
R20 =P20 a
a = 20mm 2
20°C = 2.8 ~ I o-8 0-m,
.
We.know
l2
1+ 0.0042 X 12
l+0.084
= 71.61°C
1.2
60 l+0.0042x t2
-=-----=50 1+ 0.0042 X 20
R 2 l+a 0 12
-=---'-~
R 1 l+a 0 11
a 0 =0.0042/"C
Change in temp.= t2 - t1 = 71.61- 20
= 51.61"C
New resistance=R2 =60.Q
Intial resistance R 1 = 50 .Q
C,
g
R9.)
-
,,-
S'o
ld
(
z_(J ;>}
/4-6"<1L
~ ?,,o
\ \
6.o(l4
- ~ 2-3. At 20°C resistance of a machine's coil is SO ohm. After working few
h o u ~r~istance becomes 60 ohm. If temperature coefficient at 0°C oJ.qiif material is
0.0042 per degree C, then find the increase in temperature.
Sol: Given, Initial temperature 11 =20"C
~
\2- ~~-
,.,,d,oi-;\
X. / \>(
(Ay..\')
of.-1/
~
Solution : Given / = 200 m
\2- ,.
Q/Oc.
~ l e 2 ·2 · Calculate the resistance of a wire at 40°C which is ZOO m long a nd its
cross-sectional area is 20 mm 2. The wire is made of aluminium. Resistivity of aluminium at
zooc is 2· 8 x I o-8 0-m and its value of temperature coefficient of resistance at ooc is O.oo4
14
--!i
= 2D°C,
for copper a
~ I .0 /\
R1
R2
-::
= 230 = 230'2
I
1.2
l+Uo/ 2
l+u 0 i 1
/2
= Vz
I
= 0.00427
230
V
R = = - : : l9166H
Rz
O
I2
1.0854
J+0.00427
X
11
After substituting the values
R =40
I = 400metre
(i) For copper, p = 1.7 x I0- 8 0-m
4=
a
1.7 X IO-S X 400
rJ
As we know > R = pl
Example 2.5. Consider a copper wire having length of 400 metre. This "in 1s 11-sed·•
the manufacturi';)g of D.C. machine coil. The res.i stance of this tota l wir, is 40 .
(i) Find the diameter of the wire.
(ii) Also find the diameter of wire if it Is made of aluminium
'1 = 70. 25°C
1.30- I = 0.00427 11
0.03
'1 = - - - = 70 25°C
0 .00427
l.2x 1.0854= l+0.00427 11
1.2
_2_7_x_1-=2
1. = _I+_0_._004
2
l+0.0854
After putting the values, we get
I+ 0.00427 x 12
230
I 91.66 = I+ 0 .00427 x 20
t1
temperature of the field coil.
Sol. V = 230 V, t 1 = 1.2 A
Example 2.4. A shunt field coil of a I).<.. . 111 11cbinc i3 ta kin~ l . l a rnpt'.rt! ln1111 2 \IJ , ~
D.C. line. The temperature of 11,i~ coil Is 20"('. Afte r workln~ fur ~•11nc hour\ a1 full 1,~
current reduced upto 1.0 amr,crc. If coil b miadc of copper, !he n find t he in c r ea,i~ al
c·unducling Marerlul•
rtd:
. 1=-4
d=
. -a
2.8 X 10- ~ X 400
2 .8 X 10- S X 400
4= - - - - - -
3.142
I
75 to 98
10 to 14
Carbon steel
(upto 4.% carbon)
Iron (cast)
15 to 45
7
50 to 60
1.60
Brass
Carbon steel (high carbon)
1.77
Silver
Silicon steel
1.72
Copper (Hard drawn)
= 1.89 mm
Alloying is another ·factor which affects the resistivity of a material. By mixing some
impurities (some other material in small extent) to a metal its resistivity can be increased. Value of
rcistii,ity of an alloy is always higher than the value of resistivity of a pure base m~tal. The
structure of an alloy is not homogeneous compare to pure metal. Alloy metal acquires properties
like higher mechanical strength which are needed for the required applications. For example, by
adding ,:inc into pure copper we get an alloy called brass (60% coppei:, .40% zinc). The resistivity
of alloy metal brass is higher (about 4 times) than the value of resistivity of pure copper. And the
1cn, ile strength of brass is much more than that of copper. Because of'these properties brass is
" "'ti in such as : plug points, socket outlets, knife switches, heavy plates, shafts, rods etc. where
11 1gh tensile strength and hardness is required.
d
2.4.1. Effect of alloyinli on resistivity
2 .8
20°cx10:-
8
Resistivity
. (oh_m-µi) at
Aluminium (Cast soft)
. Material
2.9
d=~=.j3.56
1t
d= ✓ax4 = ✓2: 8x4
a = 2.8 mm 2
a=2.8x10-6 m
a = -----4
⇒
(,,,2
=v2.16=1.47mm
I
I
17
I
-
40 to 50
2 to 40
-
15 to 20
40
39
39
35
35
Temperature
coefficient per <\egree
cat 20°c xfo:-4
I
780
780
-
7.70
8.40 to
8.70
10.50
8.89
8.89
2.71
2.68
Density
1350
I 1500 (O 1530
I
I
-
-
960
1084
1084
630
655
C)
I Melting
point
(degree
, .4.2 . Effect ~s.!J,rutic~}..Deformation on Re~ vity .
. . .
2
:
Ir;;chanical force is applied on the conductor, then its _v alue of res1 s1Jv1ty gets changed
i. T
f
1· d mechaincal force influences the change in res1s11v1ty.
ype o app 1e
If tensile force is applied, then
(i) Length of conductor increases
~
(ii) Cross-sectional area decreases
Hence resistivity of conductor increases.
If in place of tensile force compressive force is applied on the conductor, then effects will
; be reverse as compared to above effects, that is
~
(i) Length of conductor decreases
(ii) Cross-sectional area increases
Hence resistivity of conductor decreases.
Now it is clear that value of resistivity changes due to changes in length and cross-section
area of the conductor. (Because of applying mechanical force/deformation).
Values of resistivity, temperature coefficient, density and melting point for different
materials.
•
•
Following table gives the comparative study of the value of resistivity, temperature
coefficient, density and melting point of different materials by which right material for the
required application can be selected.
Table2.1
Conducting Materials
Aluminium (hard drawn)
=
R= pl
a
2
~
l
a x 4 - ~ -x7
d= - - -4
l rt
3.142
p=2.8x10- 8 and / =400m, R=4 O
(ii) For aluminium
1 Jl'(';J
4
1. 7 x IO - x x 400
·-
=1.7 x 10- 6 m = 1.7 mm 2
a=
Elccl.-icul and Electrunic~ ~. ngineering Materiah
Copper (Annealed)
16
I:
f:
\·\ , ~~- ; rnt~C\ ( . \\\•1\ '\
in d ·, ,.,;"l
~ !~~.~Ltt..~~!l~ls and Thei r t\ReJicagont :
Cond uctin g Materials
.
~
-
At prese nt abou t 30 super cond uctor meta
ls (soft superconductors) and more than 600
supe rcond uctor alloy s (hard super cond uctor
s) are already know n.
Many meta ls and comp ound s show the
property of superconductivity at very low
temp eratu re. Good conductors of electricity
like copper, silver and aluminium do not get the
state
of super cond uctiv ity at lowe r temperature,
it shows that it is not necessary that only
good
cond uctor s of elect ricity can be supercondu
ctors. Superconductivity has been observed to
occur in
poor er meta llic cond uctor s such as tin, lead
and tenta\um rather than good conductors. It
is clear
that super -cond uctor s may not only be pure
metals but various alloys and chemical comp
ound
s as
well.
8
t
2 5 2 Aluminium :
.cal
• • • Alumm1
. •um is widely available in India and
used
exten sivelgy in the field of elect n
.
.
.
.
1
. eenn
.
. g. t -is n ext best to copper. Value ofres1shv1 . ..,,_
ty 1s__
2.8 x__
10-_ ohm- m whic h is 1.6 times
engmt r as compared to copper. Its density is
2.68 that 1s 1/3
.
grea1 _e g point is....:6::.:5::.:S=-=d-=::egr~e~e....centigrade. Like- - - - 1 . . time less than that of copp er.. Its
~oppehr, a urruftnium can be easil y draw n into
wtres,
me tm
.
d rods. Its resistivity and density c ange a
er the meta l is mech ainca lly drawn.
sheets, SlnP
an·
·um
is ·a soft metal
h alumm1
. but: when ith is . alloy
I ed With some other mate rials like
Althou~ um rro
. n or silico
n, it acquires highe
rhigh
mec an1ca· strength_ Alum inium fonn s an
·
h
oxide
magnesi '. 1 is exposed to atmosphere. It a~
corrosion resistance like copp er. ·It is diffic
ult
\aye~'lw
dA~
~re
a:ih
~,nu ~niu
.
tos~
This is the great disadvantage of aluminium again
st its use in dome stic
wiring.
.
. . .
.
Low resis tivity meta ls are follo wing whic
f
.incre ase m
t ls increases with
.
h are main ly used m elt:ct n cal engineerin
lt has been studied earlier thal the res1s11v1ty O
g field •
me• a compounds h
.
. .
l, Copper ,\ ;) &'le\
w ose res1stw1ty
temperature and vice-versa. There are some metal
1
4. Merc ury 35 1s and chemica
·
O
oo kelvin (- 273 C). At this• stage
2. Alum inium b '.).;
becomes zero when their temperature is decreased
~- Plati num i -:/I: • ;
h
(brou
g
t
near
·
·
d · ·ty for example, mercury beco mes ·.
3.Ste el
1~ 3L
such metals or chemical compounds attain
s the supercon uct1v1 •
__
super-conducting at approximately 4.5 kelvin
2.5.1. Cop per
(-268.5°C). Hence super-conductwity can be
defined as : "Stag e at which the value of resist
ivity of a metal becomes zero, is calle d supe
Amo ngst al\ the cond uctor s copp er is most
r- ·
wide ly used in the fi~ld o f elect rical engin
conductivity ." Superconducting material was
eering
due to its high cond uctiv ity and low resist ivity.
disconverd by Heike Kamerlingh Onne s at
Silve
r has lowe st res1sliv1ty but beca use of its high
the
_
University of Leiden in the Netherlands in 1911.
lt is an important research for the mode rn
~ cost it is not used as a cond uctin g material.
science. Heike Karner\ingh Onnes found that at
Copp er is reddi sh in colour. It has fine phys
4.5°K resistivity of mercury becomes zero. In
ical, chem ical and elect ric al prope rties. It
this
condition when electric current was flowing in
is a
non-magnetic material whic h is found in natur
the mercury ring, then it continuously was flow
e in the form of ores (mai nly copp er pyrit
ing
e
(Cu
without any power losses. lt shows the state of
'i"nd
cupri
2S)
te (Cu 2 C)). It can be avail able in hard drow
super-conductivity.
n or anne aled form . Mech anica l properti~
i
are different for hard draw n copp er and anne
(!be transition from normal conductivity to super
aled copp er. Anne aled copp er is soft, flexib
-conductivity takes place almost sudd enly.
le and has
less tensile strength than hard draw n copp er.
It occurs over a very narow range of temperature
Hard draw n copp er is obtai ned by drawing
about 0.05°K. The temperature at whic h the
copper
bars in cold condition. Anne aling proce ss invol
transition takes place from normal conductivit
ves heali ng at a speci fic temp erature than
y to super-conductivity is called "tran sition
coolin
g.
Resistivity of copp er is l.72xlO-s ohrn-m
temperature'
at 20 degre e C. Copp er can be draw n into
very thin
wires, sheets and bars of various thicknesS'
can be made . Its corro sion resistance is
2.5. Typ es of Superconductors
very high.
When it is exposed in atmosphere copp er oxide
layer form ed on it. Dens ity & melti ng point
There are two types of superconductors. First
of.
copper is 8.93 and 1084 degree centigrade. Tens
is soft super-conductors which are usual ly
ile stren gth varie s from 8.15 to 4 .72 tonne
made up of pure metals their range of transi
s/cm
2.
tion temperature is 0.01°K to 9.15°K. It has
Copp er can be easily soldered & weld ed. Copp
less
er joint s offer low conta ct resis tance. Coppe
technical uses.
r
has following applications :
On the either hand, another type is known as
I . In domestic wirin g
hard super-conductors they are usually made
up of alloy metals with high value of resist
2.
Transformer winding and moto r wind ing
ivity in normal state. Their value of transistion
temp eratu re is comparatively large. Their
_
value of transition temperature is high, it mean
3 Overhead transmission inductor
s as
comp ared to soft super-conductor, they get the
4 Bus-bars
state of super-conductivity in much time. Thes
e are
very usefu l as compared to type l supercondu
ctors.
5·_High voltage underground cables
6 _ Contact material for control relays
2.5.1. Superconducting Materials
Supe r-con ducti vit)'
18
Electrical and Electronics Engineering Mate rials
a
/\ lu,11i11rum is extensively used
.1,
a conduclPr \i•irl, in overhead lransmission lines, bus bars.
Ell'l'lrirnl 1111<1 Ell'l'tronks Engineering M1lll•rinls
-~
12.5 g/cm 3 . In the field of electrical engineering it has some specific applications such as mercury
switches, mercury vapour lamps, fluorescent tube lights, mercury arc rectifiers and in the
manufacturing of many electronic equipments.
2.5.4. Mercury
It is an important metal for electrical engineering field. It is the only metal which remains in
liquid form at -38.87"C temperature. It is a heavy metal. It is white and bright like silver so it is
6
also known as 'liquid silver'. Resistivity of mercury is 0.95 x 10- !2-m and value of temperature
coefficient of resistance is 2. 7 x 10-6 per°C. It ha5Imelhng poiniJ~i51°~and having density o(
copper. Its melting point is l 530°C because of high resistivity steel is not used as a conductor. But
it is used in the overhead transmission lines to reinforce the aluminium conductor. Zinc coated
steel wire is used as earth wire in overhead lines and as an ann-our wire around the conductors of
power cable and underground cables. The silicon steel alloy is used as magnetic core in electrical
machines such as AC/DC generators, motors and transformers.
4·
____
/
_
9 x J0 - 6
125-240
105
71.6
19 77
2 1 45
3li2'\
17(,li l
Value/Rang~
- ---- -
_ _ _ _ _ Z_I
2.6. Properties of Conductjng Materials
Platinum has various applications in the field of electrical engineering as follows :
J. In electrical electrodes
2. Elecrrical contacts
3•. Platinum resistance thermometers
4. As ignitor in mercury arc rectifier
5. For rust resistant surface
6. Used as a catalyst in chemical reactions
Tensile strength (MPa)
Thermal coefficient of expansion (per°C)
Resistivity at 2ff'C (in !2-m)
Thermal conductivity W/(m-k)
Density (g/cm )
3
Density when liquid al melting point (g/cm )
3
l3oiling poinl ( °C)
Melling roin' ( °C )
-- ~ e ! t_t .
('ondncling 1\1:tlcrials
(iii) Ductility : It is a property of metal due to which it can be drawn into wires and rolled
into sheets. bepenctmg upon the required application conducting material is drawn into wires of
various size and sheets of different width. So the ductility of metal should b,e high so 11 can be
easily drawn into wires and sheets.
Mechanical strength : In overhead transmission lines, line conductors have to bear \vind
pressure, stress due to ice and their own weight too. So the mechamcal strength of !me conductor
should be high so that it e_asily face these stress and strokes.
(v) Flexibility : In many applications conductor wire is used m the shape oi cod I.Ike in the
motor wmdmg and m the same way in the transmission distribution lines at the termmatmg end
ti!lliperature
(i) Resistivity : Resistivity of the conducting material should be low. We know that rn
electrical transmission and distribution lines voltage drop= IR and power losses = J 2 R.
Hence to minimize voltage drop and to keep power losses low resisti vity of the conducting
material should be low.
(ii) T.emperature coefficeint of resistance : The change in resistance with respect to
coefllc1ent of resistance for the given conducting material.
temperature 1s detmeu §§
of resistance should be low. Because if temperature of
coefficient
The value of temperature
transmission line changes there should not be any change in the resistivity of the conducting
material so that parameters of the transmission line remain constant at all temperature.
Steel is an alloy of iron. Steel is obtained by adding small percentage of carbon into iron
Iron is a soft metal when carbon is added into it acquires good mechanical properties and fi
increased tensile strength but at the same time its ductility decreases. If the percentage of carbon is f
high, then it becomes brittle. On the basis of carbon percentage, steel can be classified into !
following catogories :
~ Mild steel containing carbon about 0. 15% to 0.25%
~i) Medium steel containing carbon about 0.45% to 0.70%
Jiii) High carbon steel containing carbon about 0. 70% and above
8
The resistivity of steel is 9.8 xl0- .Q.m (at 20°C) i.e. 8 to 9 times higher than that of
2.5.3. Steel
squirrel cage inductor, motor rotor bars and in many othcr application. However the important
application ufaluminium is in winding of electrical 111achincs and large rating transfonner because
of bei ng low density material. Since resistivity of aluminium is much higher than that of copper.
So to keep / 2 R losses low cross-section has to have thicker. The total winding occupies more
space and the size of machine also increases. But because of lower density of aluminium,
aluminium winding has less weight as compare;:u to copper.
Zfl
2.5.5. Platinum
Platinum is an element represented by the symbol Pt and it has atomic number 78. It is a
dense, malleable, ductile, highly non-reactive, precious, gray white metal.
Platinum is one of the least reactive metals. It has remarkable resistance to corrosion, even
at high temperatures. It has great physical an_d-chemical properties which make it more suitable for
the industrial applications.
Other properties of platinum are given in the table.
er
2
in
h.
of
per
ng.
has
2S)
ies
IS
ring .
high
t'ld :
~
v
H1~P., CD'lc!.ul.J-iv~
l
--·•- -
l.\
- - -- - ---- - ·-
-
.
PP
lications
_
•
2.8.1. Carbon
Carbon is available i~ nature in different forms such as : graphite, diamond, charcoal.
Carbon materials used in the field of electrical engineering are manufactured from graphite and
other forms of carbon. These carbon graphite and other form of carbon ma terial powder are mixed
up with binding material (e. g . coal-tar, liquid, gases etc.) and this mixture is moulded in different
shapes then baked in electric ovens. To increase the conductivity of carbon material copper or
bronze powder_are mixed into the carbon mixture before the moulding process.
resistance.
of
coefficient
temperature
negative
Carbon is a high resistivity material. It has
Its melting point is-3'S'50°C. It is sensitive against pressure.
Carbon is used m making carbon brushes fo~-DC machines, as electrodes in arc lamps, in
dry cell, non-wire resistance, sliding contact in rheostafefo-:-·-· -· -
2.8. High Resistivity
atena
pre-he~ters at nuclear af!d .fossil fuel power plants.
. Is And Their A
l!)LD 4J() clyc.}-iu,~
m akL· <;on1rol ,pn ng'> fi,r c k<:1ri, al
.
•
•
.
•
.
In the fie ld ol electrical engmecnng 11 1s u,ct1 1O
vane! ~ ol
. . . gaug e · bean nl!- plate and wide
.
.
.
F"
.
. struments electrical contacts, diaphragm for pressure.:
· . th. use of mon: expensive c,ery 111um
.
.
'
1n
wire fo nns where the dcs1red properties do not require.: e ·
r.
II
.
.
.
, copper.
· ke I (a Iso known as .copper-nickel) 1s an a oy o copper
(iv) Cugronickel Bronze: Cupromc
;
_
~ that contains nickel and other strengthening elemerits, such as iron _a nd manganese. .
Cupronickel is a metal having silver color. It is high ly resistant to corroswn m sea-water
. b cause its electrode potential is adjusted to be neutral with regard to sea-water. It has good tensile
good
. . when annealed, therrna I cond uc t.ivi·1Y and them1al expansion.
.
.
te ength excellent ducllhty
11
y
easil_
b~
c~n
alloy
this
in
::ennal 'conductivity and ductility. By mixing 7.5% aluminium
verted into thin sheets by cold and hot rolling process. In the fie ld of_electn cal engmeenng it is
cond to make condenser tubes, heat exchanger tubes. In power generation cuprornc~el alloys are
~:: d in steam turbine condensers, oil coders, auxiliary cooling systems and high pressure
conducting Materia ls
Phosphor Bronze : Phosphor bronze is an alloy of copper with 3.5-10% of tin and
upto I% phosphorus. The phosphorus is added as deoxidizing agent dtlritig melting.
(ill)
Brgnze :
excellent formability and solderability.
4--
q...
drawn
1
Manganin is an alloy of copper. Wh£P 86% cop pfu, ' 12% maganese and 2% nickel are
into wires Its
added together then we get an alloy called manganin. It can be coldiy
working temperature is 60°C to 10°C and melting poi~t is 120°C. At higher value of temperature
its value of temperature coefficie nt of resistance increases. Due to this at higher temperature its
5
resistance varies. So it is not used at higher temperature. Manganin has 4 .55 xl0- ohm-cm
2.8.3. Manganin
When 60% of copper is mixed with 40% of zinc, we get an alloy called brass. It
(i)
has higher value of resistivity than copper but has higfl tensile strength. It can be easily converted
into rods, wire, sheets and tube. It can be easily soldered and welded. It has high resistance against
corrosion. Brass is used in making knife switches, plug tops, ceiling rose, lamp holder, contactors,
brush holders for DC machines and sliprings for AC generators and motors.
·.'
When copper (82-90%) and tin (8-16%) are mixed together with third element 2.s.2. Tungsten
(ii)
like cadmium, phosphorus, beryllium or silicon etc., then we get an alloy called Bronze-Bronzes
It is a very hard metal. Its resistivity is approximately twice that of aluminium i.e. 5.65
are given their name based on the third element which is added. For example, when the third
8
Q-m. It has great tensile strength. Its melting point is the highest of all metals i.e. 3422°C. It
oi
x
element is phosphorus, the alloy is called phosphor bronze. If the third element is silicon or
ps,
e.asily .drawn .into thin wires required for making filaments used in elec~
be
can
cadmium, the alloy is called silicon bronze or cadmium bronze respectively. All bronzes have high
tubes and electronic valves etc. It can work up to 2000°C when used in presence of
fluorescent
mechaincal strength and highly resistant to corrosion. Cadmium bronze is used for making
inert gares like: nitrogen, argon etc. or in vacuum. Even at few hundred degree temperature it can
commutator sagments and contacting conductors: Other applications of bronze are in si iding
easily ovidizes in the presence of oxygen.
contacts, knife switch blades and in current carrying springs.
Brass :
2.7. Low Resistivity Copper Alloys
and weather,,; effects :
These alloys have properties of toughness, strength, low coefficient of friction and fine
grain. It is a hard metal having good tolerance. It is a non-ferrous metal having high electrical
conductivity, high resistance to chemical and ·corrosion, high wear resistance, non-magnetism,
.
(mm
Electrical and Electronics Engineering Materials
•
f . .
·
• th d .
w here c irect10n o 1me 1s to be changed, conductor has to bend. If the conducting material -is
. ..
.
.
.
.
.
b
·
h d t h en II
can e broken at the ttme ofbendmg. So conducung matenal should contam flex1b1hty
ar
.
.
property sothat 11 can easily bend and turned into shape of coil.
gxida$iM : Many metals form oxide layer when placed into an open
(vi) .fEGG
· open atmosphere, there
·
·
· ·
m
where conductor exists
Ime
a Imosph ere. In overh ead transm1ss10n
aryg_c~n be oxidized
moisture
and
)
(0
con_ductor can be easily affected by the atmospheric gases 2
oxidation shoul be
against
resistance
its
e.
i.
oxidation
from
free
be
should
e~sily. So the conductor
high.
•
·
· placed mto
· : When conductor 1s
·
·
··) Res1stance
( vn
an open atmosphere 11
corrosion
agamst
should ~ot fiave the cor:roMon . p~o~erty. Corrosion degrades the mechanical efficiency of the
conductmg metal and its res1stiv1ty also gets affected. Hence the overall efficiency and
performance of the transmission line or electrical machine decreases. So the resistance against
corrosion should be high for the conducting material.
and weldin : In many applications where length of conductor
(viii) Easiness i
needs to be extended, soldering an we mg 1s to be done on the conductor to join the one
conductor to the another. So the conducting should be of such type that it can be soldered and
'
.
welded easily.
The conductor used in
(ix) Resistance against chemical
transmission line and in electrical machme are placed into open atmosphere where it has to face
many weather conditions and also affected by the chemicals. So the conducting material should
have high resistance ag_ainst chemicals and atmospheric effects.
22
rn-an
ure its
:-rature
e~ - Its
el are
.
5.65
zoc. It
.ps,
nee of
it can
nps, in
stance.
1arcoaL"
ite and
mixed
ifferent
pper or
ea-wate r
d tensile
. good
e easily
ring it is
Joys are
ressure
f copper
elec1nc<1 I
,a, iety of
ery ll1um
ZJ
't->"·-
1
3
,
0.39
455-860
19.5
8
5x10-7
1210
500
8.9 x10 3
·Value/Range
~
.
Nichrmna,
·•;:- r ;,
3
Melting point("C)
Density (lcg/m )
l40Q
g apparatu s li ke hea ter,
(i) Resistivity : High resistivity materials are used in heat producin
2
So to produce large
Rt.
1
=
H
etc. We know that for specific length produced heat
2.9. Pro erties of Hi h Res1stivit Materials
27
Applications
wi1.k , an ~1 y "' tk , 1ct·,
In the field of electrical engineering nichrome is used in a , 1.cry
rn it ,, 11 sed 10 mak e:
wire,
thin
into
drawn
easily
be
can
It
.
where electric healing is required
and electric "'•<.:n l"lc.
elements of heating devices such as kitc~en heater, geaser, elcctnc iron
C
'1
t1(lll0-l
CC
low
Conducting Matef'ial s
-- --
_1O. ..G6r
.
to a more stable form, such as
~ o s i o n is a natural process, which converts a pure metal
material s (usually metals) by
of
tion
~ oxide, hydroxid e or sulphide . It is the gradual deteriora
.
medium
ing
surround
their
chemical or electroch emi~al reaction with
with an oxidant such as
reaction
in
metal
of
n
oxidatio
Corrosio n means electroch emical
-known example of
well
a
is
oxides,
oxygen or sulphur. Rusting, the formation of iron
salts of the original
or
oxides
s
produce
electroch emical corrosio n. This type of damage ·typically
· d. · • orange co1ouration .
ta! d
me an resu 1ts m a 1stJnctJve
as ceramic s o r po lymers,
Corrisio n can also occur in material s other than metals, such
of corrosio n process in
Theory
.
common
more
is
tion"
"degrada
term
the
although in this context,
do not oc-:ur in the
metals
,
metals can be understo od as; apart from gold, platinum and few others
2
n of Metals
~~
ffim
electric iron
amount of heat resistivity should be high .
temperat ure. It should be
(ii) Melting point : High resistivity material has to bear high
high melting point.
have
should
It
capable to work at high temperat ure and not to be melt. So
are used generall y in
(iii) Low Temperature Coeffici ent : High resistivity materials
material s should be of
these
electrical riteasuring instruments, resistance box and rheostat etc. So
stance should not be
resi
of
value
such type that if value of their temperat ure changes, then their
low.
be
should
;°t
changed much. It means their value of temperat ure coefficie
g devices in the
(iv) Ductili : High resistivity material s are used in many heat producin
co.nverti ng th e
For
metal.
y
form of eating element. Heating element is a coil ofthin high resistivit
that metal has
essential
is
it
material
e
thick diameter wire in thin diamete r wire of high resistanc
wires.
thin
into
property of ductility so that metal can be easily drawn
applicat ions such as : in
(v). Free from oxidatio n : High resistivit y materials have many
in the element is not
used
electric ovens, kitchen heater, hot plate or room heater etc. If material
the material also
of
strength
and
free from oxidatio n then there should be less heat transfer
y.
efficienc
better
for
degrades . Hence material should be free from oxidatio n
resisti vity material s are
(vi) High mechan ical strength : In variety of applicati ons high
diameter for shunt resistanc e of measuri ng instrume nts
used to make the resistanc e wrre of
tensile strength so that they
and resistance boxes. So it required that material should have high
subsequ ent operatio n
and
y
may not break during the drawing of the wire or during the assembl
l
75~78% and chro~um is
"iHs an alloy of nickel and chromiu m in which ratio of_nick~I is
O
mto this alloy. Properties
29-30%,. ~n. and manganese in small quantity is often ID1Xed
nichrome 1s given below in the table.
Table : Propert ies of Nichrom e
s--~~ ~f?~ .v;iue/R iinge ,-:,,8 · ..,~•.. •. ,
.
Brope _· t · ,·
"·
--6
--6
·
,.
/
·'·
rty
. . .
10
5
10
x
- 1.
1.0 x
,
ResJStJvity at room temp. (ohm-m)
11.3
"C)
Thermal conductivity (W/m
13 4
.
.
Thermal ex
·
pans1on coefficient
-'
"C
(at 20°C to JOO'C) per
S400
.
2.8.5.
turing ofrheost at fo '
Applications : It is used to measure tempera ture. It is used in manufac
the formation o
for
used
also
is
It
meters.
ampere
of
e
resistanc
laboratories and shunt
Its value o
chrome.
and
iron
copper,
thennocouple, along with the wires of other metals such as
is used fo
it
this
to
due
ture,
tempera
its
in
change
the
with
much
resistance does not change
for this
resistant
of
nt
coefficie
ture
resistance purpose. It is used for DC shunts value of tempera
nts.
instrume
n
precisio
more
in
used
is
it
reason
this
to
due
n
metal is higher than rnangani
Tensile strength (MPa)
Specific heat capacity J/(g.K)
Temperature coeffi~ient at 20°C (in 1°C)
Thermal conductivity (W/m-K ) at23°C
Resistivity (at room temp. in .Q-m)
Melting temperature (°C)
Density (kg/m )
0
Highest working temperat ure ( C)
Property
r:;;;;,
·e box and slandard resistanc
_
·siivity. !Is melting pomt
.
N
~
C.....
eter shunts. res1slam
res1
~
~
resistors, amm
t
consistin g of 55¾_copper and'
Eureka. It is a coppe r-nic~ y us~a
2.8.4. ConS antgp
constant over a wide range o~
1s
which
ty,
res1st1v1
its
is
metal
s
i
h
t
~
as
known
It is also
45
coefficient, easily ductile.a
ture
tempera
le
% nickel. Important feae~r;co heat resistanc e, negligib
properties of constanto ' i
other
moulded
and
soldered
easily
be
has highncsp
effects, can
Properties. Ittmosphe
table.
the
.
resistant to a
·
Table : Propert ies of Constan ton
has been shown m
4
-~
. - 1.-5~96~0=o=c~_-:;E~le:c~tr~ic:a~l-:a:p:p~li:ca~t~io:n:s~o~f~m::a:ng:a:n~i:n~ar:e~;~w~i~re~~~
2 - - - -- -- - e coil etc.
Electrica l and Electron ics Engineer ing MateriaJ
··..
,
·
t .
proce ss
'1f::::::t
'Blast furnac e
Energ y
Level
y-
Fig. 1.2 : Energy slate of Metal in various forms
◄----
temperature
High pressu rn
and
/Energy
Output)
Corrosion
/~
~
~...,......,..,.,...,=-- -
('5 '--st~f ~
9.
8·
7.
5.
6.
Co
er
As cableJungs
winding
In
transformer from
kVA to 1000 kVA
In power wiring
In domesting wiring
Underground cable
Mostly used
Aluminium
Steei
_
in
Used
ACSR
conductor
_as centre
core for tncreas'
the tensile stren ~ng
aluminium · g of
conductor.
of / Less used
25
Mostly used
Not used
Not used
Not unsed
Not used
Not used
Rarely used
Not used
Not used
Rarely used
Mostl y used
Used as under- Mostl y used as Not used as cable
conductor but used as
ground telephone underground
distribution
armour for cab!
cable
e
safety
cable
Not used
Used
Used
Not used
Not used
Used
In winding of trans- / Mostl y used
former more than 1000
kVA
. _
s
3
/ AinCs~tdor ~mdm g of ~ I Mo tly used
m uction motor
· m armature and field I Mostl y used
DC
of
winding
1
lication
In overhead electrical Rarely used
and
transmission
distribution lines
A
ti
, --'-=-'=::..::..:==--- --- -....I...-- ___ __
machines
· commonly known as
· number 80. It 1s
. an eIement w1'th sym boI Hg and atomic
/ Rarely used
Mercu ry 1s
Not used
Mostl y used
industries' offices and
In
0.
I
2
.
h
.
.
h
heavy,
A
.
g/cm
13.55
1s
t
eart
.
weigh
c
.
as
specifi
Its
.
ouses
gyrurn
quick silver and was formerly named hydrar
_
electrode
is liquid at standard conditions for
.J__ _ _ _ ___J_ __ _ _ __
silver y d-bloc k element, mercury is the only metal that
good
a
i~
ury
]Merc
-39°C°
is
point
g
meltin
and
357°C
is
tempe rature and-pressure.ffi_s boiling point
3
6
Used in Manufacturing of Electric Lamp
Its electrical resistivity is 0 · 96 x 10- · •1 · Material
condu ctor of electricity but bad conductor of heat.
electric lamp :
Following materials are used in the manufacturing of
Its thermal
C.
degree
per
7
0.0002
is
nce
resista
of
ient
coeffic
ohm-m etre and temperature
ng of filaments of electric lamps
(i) Carbon, tantalum and tungsten : Used for manuf acturi
K)
µml(m
60.4
is
ion
expans
l
thenna
and
)
Wl(mK
8.30
is
condu ctivity
This meltin g points are 3900, 2800 and 3400° respec tively.
es, relays, fluorescent tubes,
In the field of electrical engineering mercury is used in switch
coveri ng.
(ii) Quartz Glass : Used for manuf acturi ng of outer glass
ents, liquid electrolytes,
instrum
and
ents
equipm
nic
electro
rs,
rectifie
arc
y
alarm circuits, mercur
caps.
(iii) Aluminium and brass : Used for manuf acturi ng of bulb
of mercury being used for making and
Hg-oxide batteries, alkaline batteries etc. An important use
als.
(iv) Copper : Used as a conne cting wire betwe en termin
breakins contact in Buchholz relay used for transfonner protection.
in the glass envelope to preven
(v) Nitrogen and Argon : These are inert gases, filled
oxidat ion ·of filament.
2.11. Uses of Mercury as Conducting Material
I
3.
2.
J.
4.
'
•
_s. No.
for Vario us~
i~ ~ a r i s o n of Copp er, Alum inium and Steel
uctor
!0A ~at ion s as Electrical Cond
,Condu cting Materi als
Electri cal and Electronics Engine ering Materi als
bound to other substances in ores, such as
nature in their pure form. They are normally chemically
a blast furnace) to extract the metals from
sulphi des, oxide s etc. Energ y must be exllended (e.g., in
the sulphi des, oxides etc. to obtain pure metals.
higher energy state than that found
Pure metals contai n more bound energy, representing a
in the nature as oxides or sulphides.
energy state, pure metals also
As all materi als in the universe strive to return to its lowest
as sulphides or oxides. Energy state of
strive to revert to their lowest energy state which they had
low
in which metals can revert to their
..
metal in variou s fonns has been shown in Fig. 2.2. The way
.
des.
sulphi
or
oxides
often
metals are
energ y level is by corrosion. The products of corrosion of
24
--
I
.
ICII lllld Flt•ctr
·11111cs
-
Ekctr·
me llng pom1
. is a
Elec1ro-graphi1e is used as brush contact material. Its · I .
- - - -
.
7
ni.: M:it(•ri~\
n
(ii) Two diffrrnet semi-conducting materials such as :
6 9
For P-type semi-conductor lead telluride (Pb T
and 38 1% (Te) and fc
(e) 21.), %
manufacturing ofN-type semi-cond uctor di- iod"d
bismuth te;itell un·ct·e (Ta
I
Pb
e
t
Tel
etc.
(BTTE 2010, 12, I
4. What . do you understand by supercond uctivity? List the names of supercond uctin
(BTE 2010, 1~, ~3,_ 15, I
matenals and their possible applicatio ns.
1ty.
5. What do you understand by resistivity ? Discuss the effect of temperatu re on·res1sttv
(2011,.13, 14, 1
3. Define the term hard and soft solder and also write the names of soldering materials.
(BTE 201
materia
2. List the names of high resistivity materials and write any two applicatio ns of each
(BTE201
resistivity.
1. What do you understand by the term resistivity ? Explain ilie various factors which affe
--E}(-ER_<:;I_SE ·
(i) Soft solder : It is used for joining the copper or brass wires together. It is an alloy off
and lead.in which the ratio oflead is 50% each. Its melding point is 400°C.
60% an
(ii) Hard solder : It is an alloy of copper and zinc and ratio of copper and zinc is
bra
and
steel
mild
of
sheets
thin
and
wires
steel
70%. It is used for joining the brass, copper and
etc. Its melting point is higher than that of soft solder.
V" Soldering materials are classified into two categorie s :
~Sold ering Materials
(vi) Tin 232°C
(vii) Indium 1564°C
(i) Copper l 884°C
(ii) Aluminium 659°C
(iii) Lead 327°C
(iv) Cadmium 321°C
(v) Bismuth 271 °C
:
Following are the materials used as fuse element and their respective melting point
2.16. Materials Used in Manufacturing of Fuse Elemen t
....,,_
6. Write ihc general properties o l low 1c~1st1 v11y malcria l.
7. Write the properties of" high n.:sistiv1ty 111ateriab: L>i~rn~~ 1hc rncia l u~cd
·
111
II , I 5 J
hcalt:r and
(lf/1 I J
29
(201 3)
1
(d) 4 .9 K
(b) 4.3 K
(d) 8.33
{l>r8.93
(9}-coppe r
(d) steel
6. Copper is widely used as conducto r over silver because of :
(b) high conducllvi.ty
W,ow cost
(d) easy fabricatio n
(c) availabili ty
S. Density of copper is :
(a) 8.83
(c) 8.63
(a) aluminium
(c) brass
@ l .72 x 1o-8 ohm-m is the resistivity of :
3. At present how many supercond uctor metals are known?
~ 30
(a) 20
(d) SO
(c) 40
0)-4.5 K
(c) 4 .6 K
@Mercu ry becomes supercond uctor at :
1. If length of conductin g wire increases, then its resistivity :
"8,-increa ses
(a) decreases
(d) none of these
(c) remains constant
Multiple Choice Questio ns
16. Write the names and properties, of materials used in(ii) Soldering
(i) Brush contact
17. Why tungston is used as filament in bulb.
(2015)
(2014)
(2014)
(2013)
13. List the names of any two low resistivity materials and also wri te their applicatio n.
(2013)
brass.
of
14. Discuss the properties and applicatio ns
is
wire
a
of
length
the
If
n.
ex.pressio
its
Write
15. On what factors resistance of a wire depends?
?
resistance
its
on
effect
the
e
b
will
what
doubled
area
ional
reduced to half and its cross-sect
12. Discuss the properties of mercury to use it as a conductor .
.(2013)
1 t. List the names of any two high resistivity materials. Also write down the ir properties
(2013. 15)
10. Discuss properties of copper and aluminium for the applicatio n as elcctncal conducto r
(ZO i l . /4)
t 2fJ
rheostat.
(WJI , JlJ
n
applicatio
their
and
alloys
copper
two
any
of
8. List the names
1.:opper.
of
9. What do you understand by low resistivity materials·! (j ive 1.:ornpan~on
machme~
tncal
ckc
111
and
aluminium and steel for the applicatio n in di stribution lin1.:~
:._---- - - - -- -- - -- ---~:.:.:;::.
Mntcrinls
Conducting:.:.:..:.:;c.;
f
.-
PP oxmiaicly 39oooc:
r
l•.ni.:im•l•ri
used in the manufact ur·mg o f thenn
.
. 1hc .materials
.Following arr
ocouplc :
such as steel and c
metals
(1) Couple of I\\ O d11lcrent
a
.
opper, copper and constanto n, steel
constantnn , plutinum and iridium etc.
2.15. Materials Used in Thermocouple
2.14. Brush Contact Materials
ZII
--
(d) silver
fb) platinum
Jl>t0.45-0.70%
(d) None of these
lltt<riul ,od El-";" E,g;,..,;,g ""•ri•\
!.9}355(1'(
(d) 366CY'C
(b) Low
(d) None of these
(a).High
(e) Constant
1. (b)
9. (b)
17. (a)
2. (a)
10. (b)
3. (b)
11. {b)
4. (b)
12. (d)
13. (c)
s. (b)
Answers
6. (a)
14. (b)
~ For conducting materials resistance to chemical should be :
(b) Alloys
(d) None of these
(b) Aluminium
(d) Silver
(a) Metals
(9.Ceramics
@ Degradation is more common term used for :
(£),Gold
(a) Copper
@ Which metal occurs in the nature in its pure form ?
@Which metal is used as armouring material for cable safety?
(a) Aluminium
(b) Copper
(s)-Stecl
(d) Iron
14. Electro-graphite is used in :
(a) Thermocouple
(b}Brush contact
(c) Fuse element
(d) Lamp
~
(b) Tungsten
ui}-Constantan
7. (b)
15. (c)
8. (c)
16. (c)
I 1. ln the presence of inert gases like nitrogen and argon tungsten filament can work upto
(a) I SOO'C
<.bf100<1'C
(c) 2S00'
(d) 3000"C
(a) 345<J'C
(c) 365<J'C
10. Melting point of carbon is :
9. To make control springs for electrical instrument, metal used is :
(a) cupronickel bronze
J})rphosphor bronze
(c)bronzc
(d)noneofthese
~ercury
8. Which metal is also lcnov,'n as quick silver?
(a) aluminium
' ..?_:.,Medium steel contains carbon about (a) 0. 1S-0.2S¾
(c) above 0.70%
@Which metal is also known as Eureka?
(a) Manganin
JO
~
1. Copper, 2. electrochemical, 3. length, 4. increased, 5. 0.00427 per°C, 6. Superconducti
7. 1.78 x 10-8 n-m, 8. 1S30, 9. ductility, 10. copper, 11. Eureka, 12. Copper, 13. quartz glass
Answers
@ in cable lungs l:'..a ~.f~is used.
13 .. ... ... .. is used to manufacture outer covering of electric bulb.
7. Resistivity of copper is .. ..... .. .
8. Melting point of steel is .. .... ... °C.
9. ......... is a property by which a metal can be drawn into wires.
l O. Phsophor bronze is an alloy of ..... ... . .
11. Constantan is also known as .... .... . .
t. Brass is an alloy of .. ....... .
2. Corrosion is an ......... process.
3. Resistivity is dirc:ctly proportional to the ..... .... of conductor.
4. By increase in temperature resistivity of conducting material gets ...... ... .
s. Value of temperature coefficient of copper at (J'C is .... ..... .
6. Stage at which the value of resistivity of a metal becomes zero, is called .. ....... .
♦ Fill in the Blanks
( ,odo<<iog M,.,ri,ls
-
~ .::,.A;:e
--
Ge(32)
-<.:.,;. •• - - ---- -:...,.~
, * ..,.,...--.~~--------·~·-·. _,__ ~~.-....J.....- .•.
SEMICONDUCTING MATERIALS
Ge(32) = 2,8,18,4
An electron revolving around the nucleus of an at<''11
and
has potential energy, centrifugal energy, rotational energy
as .
As we know that an atom has different orbit around the
c
nucleus in various levels or shells. Fig. 3.1 shows the atomi
32.
structure of germanium. Atomic number of gennanium is
is
It has four orbits. Its electron distribution in these four shells
3.1. Electron Energy and Energy Band Theory
Fig. 3.1 : Atomic Structure of Ge
ics engineering. Semiconducting
Semiconductors played a vital ro!e in the field_ of e_lectron
stud1 ~d _h~le about semiconductors in chapter-I.
materials have been known for a long time. We
1st
th
of conductors and insulators i.e.
Resistivity of semiconductors lies between e res lVlty
letely conductors nor the insulators. On
approximately Io-2 Q-m. These materials are neither com.12
temperature gets increased or some
room temperature they behave like an insulator a~d if their
impurity is added , they behave like conductor.
ial, then we get that it has
If we discuss about the atomic structure of semiconducting mater
their valency is 4. Their atoms are neither
4 electrons in the outermost orbit or we can say that
g of electrons in their internal crystal
completely stable nor unstable. So their atoms do sharin
and hence the bond formed is called
structure. This kind of sharing of electrons is called covalency
are as below:
co-valent bond. Other features of semiconducting material
22
18
per unit volume is 10 to 10 .
I. Number of free electrons in semiconducting material in its
rature coefficient of resistance
2. Semiconducting materials have negative value of tempe
of resistivity decreases.
because when temperature of semiconductor rises their value
le pair generates in their
3. When some kind of impurity is added into it, new electron-ho
crystal structure hence their conductivity increases.
= 2, 8, 18, 4
3.0. Introduction
CHAPTER.....
~f:,·~· "
, _, 3
1
J .1
1
atom then it requires addition
To move an electron further away from the nucleus of an
n is called ''excitation of atom··.
:nergy. This process of giving external energy to the electro
energy, heat energy , magnetic
The additional energy can be given by providing light
nal energy is given to 1he elecrro n, II
nergy, kinetic energy and electrostatic energy. When additio
to be an exctted arom
ill jump into the higher energy level, in this stage atom is said
energy level to highc::r enc::rgy
Amount of energy required to move an electron from one
required varies from materi al tu
level is measured in electron-volt . This amount of energy
I eV and in insulating materi al th,~
aterial. For example, in semiconductor this energy value is
·alue is 5-10 eV.
.2. Excitation of Atoms
c11crgy or the crrerl.! ~ In cl .,f rli,
• magnetic energy. all of which togc.:ther determine the total
expressed as cV.
,: electron. This value of energy is mi:asurcd in electron volts,
to
bound
The electron nearer to the nucleus is more tightly
;s and
i the nucleus. This electron has higher allrQ.tion force of nucleu
t the M 1 - - - -- - - - - - 1
agains
work
more
do
to
;, has lower energy because it has
L 1-- --- - - - ~
nucleus
\ attraction force. On the other hand electron far from the
electron has least
~ (valuce ;) is very less bound to the nucleus. This
e it doesn ' t K ~ - -- - - - - i
becaus
high
is
~attraction force on it and its energy
Atoms
force.
ion
attract
~require to do much work against the nucleus
n
contai
j have different shells namely K ,L, M , N. These shells
, the Fig. 3.2: Simpli ~td Energ_v le-el
) electrons of different energy levels. K sh_ell has least e~erg~
Representatw n or Shell,
ns higher
Iorbit closest to the nucleus. Each succeeding shell contai
to the respective shell is shown in the
energy levels. These increasing levels of energy according
_
fig. 3.2.
e.
e
valenc
called
the electron is
The outermost orbit of an atom is called valence orbit and
an atom gets excited by giving
When
band.
energy
Ast we discussed earlier that atom has different
level. All shells of an· a1om i~
;external en~gy, electrons move towards the higher energy
only
has
completely filled bµt the outermost orbit is incomplete. It
And
band.
e
valanc
called
is
band
energy
of
kind
four electron. This
the
in
come
ns
electro
e
valenc
its
state,
d
'hen atom gets an excite
ion
attract
the
from
free
and
s
nucleu
,and which is very far from the
band
orce of atom, this band is known as conduction band. In this
lectron is in free state.
Energy required to excite the valence electron so that it can
n
ome in free state. It means energy required to excite an electro
as
to
d
referre
lly
genera
rom valence band to conduction band is
band
and gap energy and denoted by Eg· Generally the value of
Fig. 3.3 : Energy Band Diagram
for
m
diagra
band
energy
The
eV.
for Semico nducto n
1.12
to
ev_
0.72
is
ap energy Eg
3.3.
fig.
the
in
shown
:emiconductors has been
Semiconducting Materials
J4
E ll'ctrka l and EIN·lron il·, Eni:iner rini: \l a 1ai ah
I
(b) Semi-con ductor
(c) Conductor
' Scmicon ducllni: M nh· r iuh
L
l1
mostly b) dop.q
In N-type materials conduct ion takes place through free electron s crea1cd
of holes created by thermai
and a small number created by thcmial generation. The small number
free electron s 1s large they an
generation move in the opposite d irection. Since the number of
mmomy earners
called majority carriers. The holes being small in number are called
ly b;r doping are ..:alleG
Conversely in P-type materia ls, number of ho les cre.-11ed mo~t
thermal genl!fllt1 00 an- call.;
majority carriers and a small number of electrons created b)
3.5. Majori ty and Minori ty Carrie rs
h,. ,r h 11;h cllUf.(/ I,· . , pla.-."
ideal conduclor. elcclrnn~ :11 c.: ve, y I" " " ' y 1).,,,11.J 1,, the ""<le ,, , I
\'.' he n n:qu m.:d amount o f energy '1,1 absorbed by a
ut• 111n h,md Ah.,,irpt1•,n of
· w ill leave va lence band and may move
ihcrn very close to lhe io111za11on kvc: I ;imJ tl, u, v~, y d o.,~ to tlw cond
va lence C Icc t ron, II
t Conducllon
l:, lhruugh(Jtlt
,,11,dom
mo,l
~ and
small amounls of energy scls rnany va lc11cc clccln >11, lrcc: lliey then
up to ionizati on le vel. In this stage it will break the
Ionization
g tht:11 atquired
n:lt:a,,n
om
Level
the material and arc drawn back 11110 lhc valence: n n~ u l au 10111 /.cd at
attrac tion force of nucleus. No w this electron is free to
goe~ ,m i\ ltho ug.li
pr<1lC\~
energy as heat. The released heat then free, other c lcL·tm m and tile
move around the atoms and to conduct . An electron above >
Valence
m~ neu tral bcLau<,e •t ,;-till
there are free electrons and positive ions, the malerial, ,n tlm l.J'>C. ,cnr.a
} Band
the ionizati on level is said to be in conduction band as .,
has an equal number of protons and clectrom .
shown in fig. 3.4. This electron is now called as free
electron . The word ionizati on level is used because when
3.4. Covalent Bonds
an elec tron leaves the valence band, the atom gets positive
atum.. 111 order to fil
As we have studied in chapter I that in scmicon ducllng rrum:n ab i:-aLh
charge and is called a "positive ion". The atom is said to be
ul ~hai mg electron , 15 called
its valence orbit with free electron, shares electron s. Thi~ tendenc y
ionized .
c {_;,~
Fig. 3.4 : Energy Band Represent ation of
covalency and hence the bond is called covalent bond.
Ionization
'J,.)
The atom that has been ionized by the loss of an
( c, , "'
Fig. 3.6 shows covalent bonding between the
electron does not remain for long. Its positive charge will
- {; ; ,
atom of silicon semiconductor. Each silicon atom has
Thus there is a constanr
attract a nearby free electron, which will give up its3cqui red energy.
~~ ·
·
four electrons in the valence orbit and hence forms
intercha nge of electron s being given up and retrieved.
covalent bond with the neighb_ouring atom. When atoms
ctors
enter into covalent bonding, each atom in effect has
3.3. ~nergy Band of lns4'a tors, Semic onduc tors and Condu
t, semiconductor and
eight electrons and this, it would appear, would result in
Fig. 3.5 shows the energy levels and ionization levels of insulato
band the conduct ion band is
making such a material a good insulator. However, this
conduct or. Fig. (a) shows the forbidden zone between the valence
band require large amount of
structure is not a good insulator for several reasons.
quite large in insulato r. This indicates that electrons in the valence
s are unable to move up to the
First, a good insulator must have a perfect crys tal
energy to move up and get free state. As long as the valence electron
in the case of semiconductors
structure. Second, due to added impurities there may be
conduct ion band there can be no electron flow. Fig. (b) shows that
to free themselves from
extra electrons which can not lock into the covalen t
the forbidde n zone is reduced . Thus valence electrons require .Jess energy
bond structure . Some impurities can a lso cause missing
the attraction of the nucleus.
of electron from the structure. Third, energy in the form
=n1.10on of Co.... 1ea1 Bou Ill
Fig. _J.6 ~
of heat, light can cause structure disorder.
~ Silic-nn St-mko-n.doctor
.
As a result of the above reasons, the matenal does
r r>ul a poor 111sulato r
not have a perfect insulator structure and is, therefore, nol a good 111s-ulato
band
Valence
(a) Insulator
Fig. 3.5 : Energy Levels and lonlzallon Levels
minority carriers.
conduction
Fig. (c) shows that in a conduct or there is no gap between the valence band and
als
ors. Electron s. 3.6. Classification of Semiconducting Materi
band. The valence band and conduction band overlap each other in good conduct
of energy.
Semicon ducting materials arc classtficd mamly mto t" o .::.1te~o n i",
from the valence band may move upto the conduct ion band by a small amount
the atom. Th
I. Intrinsic semicon ducting materia l/semiconductor
In an insulator the valence electron s are tightly bound to the nucleus of
ion band. In
2. Extrinsic semicon ducting matena l/semtco nductor
electrons of valence band need large amount of energy to move up to the conduct
-
onductor
------
electrons as shown below :
l'tor
.
.
" -~ ~
.
l"l1Ttril-11I anc' Fkctr onl cs. ,,.
1:.ni:1111•1-r/nn ••
-
. . , .· .
Ge (32) =
K
L
2, 8.
18, 4
MN
·
uctor s. We shoul d know about the internal
Befure studying the µ-type and 11-type semi cond
pure
uctor. Here we will discu ss about the
crystal structure or pure (instrinsic) semi cond
gemtanium semiconductor.
ned from earth upto 0.0007%. Its atomic
Germanium is white colour, brittle meta l. It is obtai
. Its
the outer most orbit. Its atom ic wigh t is 72.59
number is 32 i.e, it has 4 valen ce electr ons in
.
37.4°C. Its work ing temp eratu re is 85 to 100°C
specific density is 5.35 and melti ng point is 9
4
s aroun d the nucle us. Outer most orbit has
According to its atomic structure, it has 4 orbit
. .
extnns1c s 11
(i) P-type
(ii) N-type
icon duc tor
3.7. Crystal Structure of Germanium Sem
en ·conductors arc furthc:r classified mto two types as follow
. .
J.6.2. Extrinsic Semicondu
of an ins u I a tor. C onducttv1ty
of
chara ctl c1, •shes to . that
. . ductor show~ the
.
. .
.
Intnns1c scnucon
al
extern
~he cond u_cttv1ty of pure semic ondu ctor
ure semiconductor is low. Hence ~o mcrease _
y other
cally possi ble. ~o incre ase the condu ctivit
~eat or impurity is to be needed. It is not pract1
called
c~nd uctor . Tlus kind of semic ondu ctor is
material in small amount is added to t_he pure sem1
tor,
of impu rity adde d into the pure semiconduc
extrinsic semiconductor. On the basis o'. typ~
ing .
and Silicon.
·I sk semic
. t . .
e arc know n ·is
j
. .-. • ,htaim:d from natur
sen ·
' · Ill nns1c
3.6.1. Intl n
.
· , nute1 "' 1~ '
of t · • i,conc uctors. It
type
.
1
one
only
ms
·
. . ,.ns i.e. if it conta
a oms (only sc nucon
Se1111conductmg
.
.
ductor
:
y 1mpunty •'11 · through it.
.
.
l: noug h exter nal is
I
·11 flew,
red to r·us, ti
ll nn
requi
•
docs not contalntle
.
current w1
\ . c le va cncc
carrie r E
d t'on hand w make them tree as a curre nt
atoms). Very •
. xamp c : Genn anium
electrons into the con uc '
Jb
~--a,v. -~-
s
f
s
.t
J
,l
h
d
II
I•'
:,
i (:
.ii
·d
·,l
__ ,.. ~, •~mkonductor
- - "--
~mico nduct ing Mate rials
17
t 1l h":·:I I'
ruur cku, on, ul va knu urh.: 11 ,.11-.c u • -.1k
fourt h orbil is i11..:0111pklc I 11..:,c
u11J11 ct11 1
Al rou111 lcrnp.:ratur..: pure· [! Cl ina111 u111 ,c:llltL
germanium with ncaresl g<.:rmani um alum .
clecr mn,
d up. the~c covak nt horHJ, bn:ak am.I trcc
does not have fre electr on. When il is heale
ondu c,1nr
Intern al cryst al ,truc rurc o l 1•c•n11arnu1r1 , cn11c
generate in the germ anium semic onductor.
has been show n in the fig. 3.7.
duct or
3.8. Effect of Tem pera ture on Sem icon
rcon or
temp eratu re) pure semrL ontlu ctor li ke sll
As we know that at 0°K (abso lute zero
are
s
atom
intern al cryst al struc ture all ~em1 condu ctor
germanium behav es like an insula tor. In the
re:
pu
th<::
. Free dectr ons arc 1101 presc: nt in
joined toget her with the coval ent bond
and
break
s
to the semiconductor. the coval ent bond
semiconductor.When heat energy is given
act as
h
e the numb er of free elect rons incre ase~ whic
electron-ho le pairs have been produ ced. Henc
can say
we
s
semic ondu ctor incre ased or in other word
cu1Tent carrier. Henc e the cond uctiv ity of
cond uctor s
of resist ivity gets decre ased . It mean s semi
that by heal ing the semic ondu ctor, value
ance.
have negative temp eratu re coeff icien t of resist
3.9. f'-Type Sem icon duc tor
/
to.r
\tl\
up
in the form of impu rity has been m ixed
If eleme nt of 3rd group like indium, boron etc.
as P-typ e
then we get an semic ondu ctor know n
in least amou nt with pure semic ondu ctor,
semiconductor.
e that
of ?-typ e semic ondu ctor. let us assum
To under stand the intern al cryst al struc ture
n
lndiw
.
exten t with pure germ anium semi cond uctor
trivalent eleme nt indiu m is adde d ins small
orbit .
most
germ anium has four elect rons in the outer
has ~ee electr ons in the outer most orbit and
m atom
from all sides. All three elect rons of indiu
Germ anium atom s surro und the Indiu m atom
\a)
~ -m k 11od11c
Fig. 3.8 : intern al C!1'stai stn~t u.t t of l'-ty\W
JH
1':lcctl'lrnl llllll Ek,·tronks En11hwcrln11 M11h'rl11ls
n,akc ,·,wuknt h 11 11d wuh th,· 11,·11rer germ 11 nium :lloms. But •Ith atnm of f\<'rt11:111iu111 is 1111ahlc ll>
make <:ovakt1t h,,nd with the indium utom because it hus 110 fourth clcctm;, 111 the valence orbit. It
ht1s v:1cn111 scat i. ,·. called hole.
It has b,·cn illustrated in fig . 3.~ ta).
lllh:nrnl Crystal structure <>f P-typc semiconductor.
Hole has positive charge, it meuns hole has tendency to ullrnct clcctrnn from the nearest
gcrmuniu1n 11tom, due to which deficiency of one electron hus been crcntcd in the nearest covalent
bond and indium atom completes its ull (four) covalent bond. Due to one extra electron on indium
r
I
atom it gets negntive ion which is immobile in nature.
Hence we can suy that in />-type semiconductor when one hole is generated, one negative
ion is creutcd. It meuns in P-typc semiconductor.
No. of holes = No. of negative ions. Churgc distribution of P-typc semiconductor has been
shown in the lig. 3.9.
0
0
0
positive ion on it. So it is clear that when one free electron generates then one positive iron is
created which is immobile in nature. I lcncc in N-type semiconductor;
No. of free ~= No. of positive ions
Charge distribution of N-type semiconductor has been shown in lig. 3. 11 .
3.11. Application of Semiconduc ting Materials
Semiconducting materials are used in the following applications :
I . Used in manufacturing semiconductor diodes, BJT and and FET.
2. Used in manufacturing of solar cells.
3. Used in manufacturing of computer memory.
4. Used in manufacturing IC chips for microprocessor and micro controller
5. Used to manufacture power electronic device like : power diodes, SCR, TRIAC, DIAC,
power transistors etc.
Fig. 3.9 : Cbargc Distribution or P-typc semiconductor
3.1 o. N-type semiconductor
When element of fifth group like arsenic, antimony or phosphorus has been mixed in small
extent with the pure semiconducting material, this semiconductor is called N-type
semiconductor.
To understand the N-type internal crystal
structure let us assume that pentavalent impurity
arsenic is added in small amount with germanium
semiconductor. Arsenic has five electrons in the
outermost orbit and gennaniu~ has four electrons
in the outermost orbit. Arsenic atom is surrounded
by gennanium atoms. Arsenic atom make covalent
bonds with the nearer germanium atom. A!l the
covalent bonds of arsenic atom with the germanium
atom get completed but there is an extra electron (5
of As). Arsenic atom releases this extra
th
electron which is called free elctron. Because
arsenic atom releases an electron hence it gets
e
Fig. 3.10: Internal structure of N-type semiconductor
0 d 0
0 0 0
0 0 0
Fli:. 3. 11 : Chur~c dblrlbullon of N -ty11c scmkomlucto n
0 0 0 0
0 0 0 ·0
0 0 0 0
I.
.w
St•nikondut.·tln~ l\<l11t4.•ri11l!<i
EXERCISE
1. Discuss the properties of semiconducting materials. Give comparison between germanium
(2011)
,
and silicon
(2012, 2006)
2. What do you understand by p-type and n-type semiconductors ?
3. What do you understand by semiconducting materials? How do they differ from conducting
(2013, IS)
materials? Why is silicon mostly used as a semiconductin g material?
(2005)
4. Write the difference between intrinsic and extrinsic semiconductor s.
(2005)
5. Write a short note on germanium.
(2006)
6. Write short notes on :
(i) Intrinsic semiconductor
(ii) Silicon
7. Write the applications of semiconducting materials.
8. Write the effect of temperature on semiconductor.
9. What are semiconductor s? Explain their types. Discuss their applications.
+
,M_ultiple Choice Questions
(l}Resistivity of semiconductor is approximately :
2
~ 0 - il-m
(b) 10-3 il-rn
(c) 10--4 U-m
(d) 10- 5 il-m
(2001)
(201J)
(2016/
40
2. N=be,
e l fr<< clcroro,,_ '" scm, rn od,c,oc
,::::•;::•:::•v::::::k•
Eogiamia,
:t:i::::i::
::;:i::::i::
3. Atomic no. of germanium is
2. (b)
3. (b)
...,.
4
1
f
(d) 4
INSULATING MATERIALS
~
t CHAPTER
(b) giving heat
(d) none of these
is
(b) I.J eV
~
t
f
Answers
5. (b)
4. (a)
Fill in the blanks
atoms.
The main aim of an ii:isJJlating material -is to separate electrical conduc tors w11 hout passing
current from one to the other and to safeguard. individuals from electrically energized wires and
parts. An insulating material used in bulk to wrap electrical cables or other equaipmenl!> 1s called
insulation. Insulating materials _are nsed to separate electrical conductors witho ut passing current
through H. Many' riiateri~ls li'fce· PVC, glass, asbestos, varnish, resin. paper and teflon are good
electrical insulators.
I
I
l
7. (b)
6. (a)
__
~
4.0. Introduction
A material that.responds with very high resistance to th e fl ow of electric: current o r tota lly
' resists electric current is called an insulating material. In insulating matcnaJ number of fre-e
_ electrons is almost zero. In their atomic structure valence electrons are tJ ght ly bonded to !heJT
(d) 2.2 eV
(c) 2.1 eV
6. _;::;nperature of a semiconductor gets increase(bd), ~hen its resistivity :
mcreases
....,.,,- uecreases
(d) none of these
(c) remains unchanged
7. In p-type semiconductor holes are the :
-----(brmajority charge carriers
(a) Minbrity charge carriers
(d) none of these
(c) immobile ions
8. A pure semiconductor under ordinary conditions behaves like
.(.b)-an insulator
(a) a conduf=tOr
(d) a ferroelectric material
(c) a magnetic material
I. (a)
-
!_ ~
,_
(.bf-32
(a)l6
(c) 14
4. Jn semiconductors doping is a process of :
(ef'adding impurity
(c) giving temperature
5. Value of band gap energy for semiconductors
...(efl.2 eV
J/1'
M""'"•,
8 · (b)
' , ·,, ·; :,, ;;,_ · f:~~?=!?(:f.~J,ff{_';Jj
,
J. Silicon has .. . .. ... ...... . . . . ... in the outermost orbit.
Before selecting an insulating "niat'erial for tlie required apphcauon, a complete knowledge
of insulating materials.and standards for safe working practice 1s required Kn owledge of vanous
prog~rt~es o_f insulati_ng ~ateria]~ _is. the_most important tool while select mg an msulatmg matenal
,
for the requtred app!JcatJOn.
The different properties.of insulating materials-are classified as
,(i) Electrical properties ' ] '
"(ii) Mechanical properties
i
2. Bond gap energy Ill semiconductors is·· ·· ···· ··· ----··· eV.
3. Semiconducting materials found in the nature are called ... . .. . .. . .... . semiconductors.
4 . ... ... .... .. ... are the majority charge carriers in the p-type semiconductor.
5. In the crystal structure of semiconductor, atoms are connected by .. ..... .. .. : .. . bond.
6. Holes are the ... ... .. .. . .. .. . ... . . charge carriers in then-type semiconductors.
4.1. General Properties of Insulating Materials
•
(iii) Physical properties
(iv) Thermal properties .
_.
(vj·Chemical properties
.
.
. 1 . l ~ J properties
4c:....-.,.~"'-----The primary function of an jnsulating material is elecrn cal The ,mporunce of these
properties are therefore obvio~. The various electrical prope rti es are gr.en belo,\
4.1.1.1. Insulation llesistance: It is a property of an msul ating ma1enal wtnch resi:,ts fl o\\
of electri"c current. Irisulation resistance of an insulating materia l should be very high Ideall y 1t
should be infinite but practically it is not possible. There will alwa ys be an atremclv small flo"'
o( curre~t. An lllSUl,!tOt to which a voltage V is applied and t.hc:re ,~ small curre~I ' !lo ¼ mg
through it. The insulation resistance is then given by :
l:. ll- r rriral a11d E lectronics 1-'n gin ct'rin g \late r ia ls
I
~
Insulation resistance is of two types :
(i) Volume Resistarn.:e
(ii) Surface Resistance
~ l u m e Resistance: The resistance offered to current i.e, Flowing through the material
is called volume resistance.
where
ti
[nsulMing
i\.1atcrials
~
I
I
-
.
f I ·· k J '"' c urrent ,n rl ic !f' r l l('•I " r 1, i " • ' · •
I11 most o f insul ators th e !eJ d111,: ;i 11 g k o ~a :1 g
.
, Th
... .. I.
always less th an lJ() · I hL· , u 111 pl L1 n,·11 1.,r,
11as~ ang l: 15
90°
as
shown
in the Il g. 4 .....
"
p
•
.
_ .
_ . .
exacr Iy
.
d. I
. I
I F an insulator hav in l!. " ca p.i( itanc~ < ,11,t1
t"k Ii = 90"-0 1s called 1e ectn c ass ang e. or
.
_ .- _ _ _
aI \ ·d vo ltacre 'V' at a frequency '/ ' the dielectri c loss ca n be calculated a s ·
apple
C,
P= Vlcose
0
•
I
R = -I
Resistance of a cube of unit dimensions is
called volume resistivity.This resistivi ty is the
resistance offered to the flow of current through the
body of the material and is expressed in the same way
as the resistivity of a conducting material. Fig. 4.1
shows a piece of insulating material in cross-section.
If potential difference is applied across A and B then a
current takes one path i. e, straight through the
material which is denoted by Iv · Unit of vofume
resistivity is ohm-m. Thus the volume resi!;tance of an
insulating material may be experssed as :
d
R v =pv -
f
=
vx
~cos 0 (where X
X
•
= capacit ivt: n.:act ance )]
= V 2 x 2rc/C sin 8
Is
Jn most insulators the angle 8 is negligibly small
P = V2 2rcfC tanl:i
I
A
tan 8 is known as power factor of insulating material.
~ Dielectric constant : When voltage is applied to the insulating materi als then it stores
~ - This accumulated charge is directly proportional to the apphed voltage 1.e, Q oc V
Q=CV
or
Fig. 4.1 : Volume resistance and surface
resistivity in insulating material
a
p v= volume resistivity (ohm-m)
d =length of the current path (metre)
a= cross-sectional area of the current path (metere 2)
Where V =Applied voltage, Q
=Charge, C = Capacitance
C is the property of storing charge when voltage is applied
across _the insulating material, called "capacitance" Value of
capacitance is different for different insulating material. Every
insulating material behaves like a capacitor but only· some are used
as practical capacitor. For same physical dimension value of
\..(j.i)-Sii'rlace resistance : The resistance offered to current which flows over the surface of _capacitance of various insulating materials is different, this property
the insulating material is called surface resistance. The moisture, soil dust, metallic dust · etc.
deposited on the surface of insulating material are the reason of flow of ieakage current on the
surface of material. This resistance for a cube of unit dimensions is known as surface resistivity.
Value of surface resistance is affected by humidity. In fig 4.1, Is the current which flows over the
surface.
!JII
Dielectric Loss : When alternating voltage is ap~d
across an jnsvlating material it is like applying alternating voltage
to a perfect capacitor. Charging current of capacitor remams 90°
leading from applied voltage. In this case there is no consumption
of power. This ideal condition can be achieved by vacuum or
purified gas only. But it is not possible in most of insulating
material. There is some definite amount of energy dissipation in the
insulating materials when alternating voltage is applied on it. This
energy dissipation is called dielectric loss.
'---
Coe~
d
or
__ ______ I (leakage Current)
n
= distance between two faces
E = dielectric constant or permittivity
d
E =EoE,
where
E,. = dielectric constant o f the material
v (Applied Voltage)
Fig. 4.2 : Phase relation between
leakage current llod applied
voltage in dielectrics
---o V
t>-- - -- '
is called dielectric constant or permittivity. According to fi g. 4.3. Fig. 4.3 : Ao insulating mattrlal
across which voltage V is applied.
capacitance can be expressed as
E o = dielectric constant of vacuum.
value of E O =8 .8S4 x 10- 12 farad perm.
_.u_ _ _ _ - - - --··- - - -
Elrl'tril-al and Ekdroni cs Laginel'rin~ ~latcrials
._
,
.
f\ 1 c;! :dec1nca
l apparc11us IS des1g111.:J lu 11-ork \l\•,1 a Jd111~J range
Ir
, insulating :vJatt•rial"'
I l'!;_~'.'..'..'.!i,..:.:'.~~:_:_- - -- - -- - - - - -be degraded hy rnoi-1111,· ab" " I''"''• r Ii, ,·I k ci ,,I , 11 1c11 sc an d
n occurs. !"his will spoil the durability . mechainc,il ,trength ma,
of oprnning , oltagc . Ir this operating voltage ~xceeds then breakdow
can cause long las ting ~urfacc J1"l1JJ g,, '" op,-ra1111~ , .,!,age or
s such type of breakdown is continuous moisture absorption
insula11on pennanently or temporarily. The property which attribute
for short time.
di fined as th, ;naxinrnm vol~c can be reason for naslwvcrs
called diekctric ~trenglh. Or dielectric strength simply can be
sive strengt h: During the app l,ca 11011 an rnsula tur has tu
compres
and
Tensile
(ii)
·
or
volts
in
d
n. This is expresse
which can be applied on 111sulating material with~ut its breakdow
force On" i~ comprc, sivc force
withstand variety of mechanical forces. There arc two types of
kilovolts per unit thicknes, of the insulating material.
d from , omrres, ing an object
generate
force
and another is tensile force. compressive force is the
~rha nica l prop erties
defined
str
surfaces. Compressjye ength of an insulatin g material can be
great mechanical strength ·as it has to or substance on opposite
will
• (i) Mrcha~ical strength : An insulator shoi1ld have
material
g
insula1m
an
load
st
applied
ress that under gradually
the. operation -as well. For the as, the maximum compressive
ends
both
a1
bear various load during the manufacn1ring process and· during
pulling
(tension)
force
g
stretchin
the
tensile force -is
ial strength 10 bear the load. In suslaiii without facture.Whereas
required application insulator should have enough mechanc
strength of an insulating material ca n be defined as. the
Tensile
length:
its
along
e
substanc
a
of
,J!2_
example
For
.
de is required
different applications mechanical strength of different magnitu
_it can take before failure. Both strength s (tensile and
that
t types ofrriechani~~l.!o ~ maximum a~ount of tenspe stress
overhead transmission line different rypes ofinsu\ators'face ·differen
be of high value. so that it can bear maximum
shou.\11
material
g
icalstrength:of-an insulator may c~mpressiv~) ~fan insulatin
and in rotating machines,-windings face centrifugaHorce. Mechan
value of compressive strength is
mechanical load w_ithout any breakdown or failure. Ocnerally.the
be affected by temperature rise and climatic effects._·
for a .given material. For example . the porcelain insulator have
on the basis of their volume in higher .than tensile strength
•(ii) Density : Insulating materials have not been .used
kg/cm ;i. tensile strength 500 kg/cm 2.
g waterials of low d_ensi\Y have Compressive strpnoth ?MOO
• port11ple ,quipments and aircraft equipme11t~. Mostly insulatin ·eredncal machines insulating
•ey-=-c
s to withstand the
been used . and in high voltage eqtiipnients ancr"ii,gh 'capacity
(iii) Abias~·tresistaii21i: Abrasive rbsistance is the ability of material
.
'nibbing, scrapping etc. ll is a property that
materials ofhighc{!!!nsity have been used,\ ·•
.· aj:,Ie : repeated \V~aring,
effects ofabras1bn,' for ~xm
.
. •.
.
.
-liquid dielectric materials .-only.- If allows a material to resist · d'ainage. The abrasive · resistance of a material helps 10 w11nstand
(iii) Viscosity .: The importance of viscocity is:. in
the
,
·
its wholematerial then its thermal mechanical -action arid tends to pro.tect the removal of materials from its surface .. This allows
dielectric property of liquid insulating material.will-be same in
that it can
so
•
resistant
abrasion
be
should
material
g
insulatin
Ari
.
) L~-~· -· material 'io retain its· integrity
_..__e·. .~ -. , - _
and electrical proporties will alsQ_ .be sam
dal ~eari~g arid dain~ge can occur.
rna.ieri_al protect_tlheir ,~ithsfand tlie situations in which niechani
g
insulatin
~f
~ardness
q~ce
u
~{s,
PJ9pe~
:
.
Surface
,
of
ss
.·
·
(iv) Hardri~
..
.
,·
1,0 s.!l- .
;
l~Y,t:rs
_
~l
fall upon the application of a
uneqµ,
d?~}() softm~ss
chmg :,yh~n on other, side,
. (iv)' Britdeneii : Tendency or ·m:ateriaf tofr'iicture or ss. It is opposite oftoughnm.
f d · th
surface from damage ·by·friction anas~at
·
·
th
..
b
·
·
··
·
·
,
·
·
brittlene
·
is'call~d
orkhock
·
iiripact
,
otforce'
11
a'mou'nf
~inall
'
th
relatively
e
except is It Wl . e e cause? pr? ucmg arid
.. . .... , .. break suddenly. Brittle material inc\~Jes
. ,
.
o(moisture deposited on th e.surface of material
. ..,, . . . .
to
mateiial '§liould be smooth
Brittle rriaierials -'do' i-1ot"defo'rm tndet loac(bht
·
corona and surface deterioration effect so' surface of'.fusulating
. -·· i; ,. ,,, . . . ·I
cerainic; glas~~\\.d m\mypolyiners..
.
.
·•
..
material.
g
insulatin
for
harmful
is
ss
roughne
~uch
~o
hard
tly
s~fficien
.... • ,. . ,.. : _ , . ::,·,, ., ., " ,
should be low because
(v) Surface Tension: Surface tension ofliquid:welectric material
T~i:r!)lal .P,,;m1~rHes,
tL~.
well
very
parts
machine
wet the
d hi gh temperature without
due to less surface tension Ii uid dielectric material is ca able to
, ,.(i). ije_aJResistani:~,.:- lt i& ran abil_ity ofan ins_ulator to withstan
.should
material
of.liquid
ty
uniformi
;we!~
vety
done
being
is
it, all prnparies unchanged
retain
by whic 1 cooing and impregnation
should
insulator
id~L
An
.
rties_
l stress may be produced equally any.d3\D,~ge .an9.Ios.i,l),g·i1$ propi:_
much su ,tabk for the n:quired
be equal so that dielectric loss may be minimum and electrica
i,
insulator
an
If
limits,
Q\:§irijqle
w,i{hin_.
ure
t
.
.te1wiern
at,high,er
;
ure th~n 1t is nor a good
under high voltage difference.
applic_ation,-but ;i_ts ,p~opi:,:tjes d~ not-resist At higb.er working temperat
ab ility 10 withstand high
their
for
n
_ insulator. For example : Conventional ceramics are we ll kno"
_~'.:'l , .. · _. . : · .··. _ . ~._ _ ,
) are 1110re heat
4~ic al properties
c~ramics
advanced
as
known
(also
ceramics
an open atmosphere, 1t lempera~res. None\heless, fµie
(i) Hygroscopicitv : Sometimes when an msularor 1s placed mto
t.
s 10 m~lt at approxim arely 660°('
ni"begin
~h1ri1ini~
While'
f~r.
l:iy
i~J~is
i~e~X~~
tha,r
resisiah't
of absorbing moisturn from
absorbs 1he moisture contents present in the atmospherd.-The property
begin to melt or decompose a l temperature 3bove 2,OOO"C'
should be of'si.ich type alumina fine cermics only
insulator
ideal
An
.
it1>ulator
an
of
picity
hygrosco
it
called
is
the atmosphere
ivity : Therinal conductivity is a property of an insulator b} wi11ch
-·may affect the properties of • (ii) Therinal coriduct
moisfute
as·
e'te,
atmosph
the
from
moisture
absorb
not
does
it
2
iliat
t~-1 R loss~s and dielectri c losses
th
is absorbed by e surface_ of dissipai~ heat p~oduced due
I.he
insulating material. Moisture absorption can be oftwo ·types. First
5 5
can be defined as a measure 0f the ability of material 10 a/lo\\
vity
conducti
Thermal
dielectric strength, re i tlvtty,
the
as
ed
determm
surface.
colder
its
10
insulator and another is absorbed by .the insulator .- Properties like
al
ten
ma
the
flow of heat from its warmer surface through
.
.
l' strrn~th : i:lcty
e D1electn
a
tena
\
Electrical :ind Electroriics Engineering 1\1:nn ial,
.
.
.
.
. .d d b the tcmpcr:1tu1 e
hc:al energy translerred per u111t of 11111e
and per unit of surlace area divi
c Y
f .
.
gradient. which is the temperature diffcren1:e divided by
the d1stance betwc,;·n the twofliwr. a1:cs.
.
.
lthc thickness of the material, expre~scd in watts per kelvin
SI
o1
per metre ( syS\em)1 · Coe f1c1ent
h
. . .
.
.
thermal conductivity
Id
h
riate
1s K or)._ _An .msu\atmg
value
o t enna1
material shou ave approp .
.
h
· · ty because
con duct1v1
· then 1t· can
·1y
when heat is produced ·m 1t
dessip
eas1
· ate heat mto t e
atmosphere and will not allow temperature rise.
~ m i c a l Properties
. .
(i) Solubility : For certain application it is required to
dissolve insulating material mto
solvent. In such cases insulating material should be of such
type that it can be easily soluble 111 the
~ri ate solvent. For exmaple, before wrapping varnish
around the conductor it is dipped into
suitable solvent which is thinner than varnish e.g, aceton
e and toluene etc. so that varnish can
enter into the depth of coil. And if the insulating material
used in the electrical machine•is soluble
in water then moisture present in the atmosphere will
damage the insulation and cause break
down. So insulating material should have higher solubility
for the appropriate solvent.
(ii) Chemical resistance : Many electrical machi
nes and appliances are place in open
atmosphere. Insulating material used in such applications
should be of such type that it does not
affect by chemical reaction with gases, moisture, acid, alkalin
e material salt etc. which are present·
in atmosphere. For example, insulating materials used in overhe
ad transmission lirte near chemical
foactory or industrial area face the many chemical gases
present in the atmosphere. Chemical and
moisture effects decrease the dielectric strength of the insulat
ing material.
(iii) Weat her ability : Mostly electrical transmission
lines exist in open atmosphere and
due to chaiiges in weatheric conditions insulatiµg materi
als et affected and their dielectric
streng a so decreased. So insu a mg m ena s ould be
of such type that there is no effect of
weatheric changes on it and it should be able to bear
all weatheric conditions. In summer
temperature gets higher and in winter value of temperature
is low. Operating range ·of temperature
should be wide enough so that insulating material can work
without losing its dielectric properties.
(iv) Effect of contact with conducting materials : Insula
ting materials remain in contact
with that metal of conductor (except atmosphere of gases,
air and moisture) to which it has to
insulate, these metals of conductors also affect the insulat
ing material. For example, rubber creates
the chemical action when it remains in contact with copper
conductor hence to protect the copper
conductor from chemical action of rubber insulation and
other chemical action copper conductor
surface has been created by tin (metal). In same manner as
in the capacitor in which synthetic oil is
filled and due to reaction of synthetic liquid with interna
l walls of the capacitor, dielectric strength
of synthetic oil reduces slowly.
~ f i c a t i o n of Insulating Mater ial According
to High Permissible Temperature
Rise
Accordin to ndian standard Institute LS.I.) of electrical
equipments and machines on ~he ·
.
.
.
.
basis o recommended operating
temperature msulatmg
matenals are c ass1 ie into followmg
categories :
~c,
- - - -- - -
I.
I:
-------
a
Insulating materials used in electrical Egineering are divide
d·
. 11, c-.
.
.
~
, u,helic
resms
Into two groups •
.
t'>, ,._,
·
~
~ a t ural resins
~ (a) Thermosetting materials
(b) Thermoplastic materials
l'h,,
~
iecau
.peci<
n\·:in
·bjcc
· 111a1cna,1 ul tn1~
Class 'i' : 111,111,,1 1111!
• · cla,,
:· , jll l' ' ) ( J ( : ;1rou· i:, . ,,: 1 ~
can bcar H:111r ~''f. ·illlll
.
' .....- --:
·
,
I ·
·
__ ___
b -:-·
msulat111g- -proper
ties
1·.x:imp
e ol matcrnll\ or coin
"'~
1n;it1on '' , 11r11
· - ', ,,ii
· ,,t 1 hi- ela,, ,r.. .:..;:,
paper. cotton, press hoard. woo<l. PVC with or witho ut plast1
c1L.:!· riJhh,·i . .:il
, u, ,
Class A: Insulating material of this class ca n ~ tcmpc 1
rn uan: ur 10 ~
b amrlt ,,h:,i
cotton·, silk and papcr when impregnated or immersed in
a li4uid_~~c_l_::w_~ ~uch a~ oil.
~~
~
: Insulating materi al of this class can bear temperature
up 10 i ::o~r. Examp le, ia ktn
Polyviny] fom1al. polyurethane or epoxy resins
, mouldin ~ r ow<ier plaSltc, on rope
Phenol-formaldehyde laminated plastic s on paper. triacetate
cellulo~t: III111
.
·
~ : Insulating material of this class can bear ten1pc
11L'l l l
ratU fl' upto ~ hamplc · , a t,·:
Mica, glass, fibre, asbestos etc., with suitable bonding substa
nce~.
Class F : Insulating materi al of this class can bear tempe
ratu1T t: ;;i, • ~ hmarilc, ·
Mica, ~ r e, asbestos etc. with suitable bonding substa
nces :1s well a,; other material
Class H : Insulating material of this class can bear tempe
rature upto I 80°C. Example, .
materials such as silicon elastomer and combinations
of matcr iah. such as mica. glass fibre.
.asbestos etc. with suitable bonding substances such as appro
pria te silicon resins .
~ Insulating material of this class can bear teyt1p
erarurc above I 80°C. Example;, ·
Mica, porcelain, glass and quartz with or without an inorga
nic binde r.
4.1.7. Effect of Overloading on the Life of an Electrical
Appl janc ~
~El~clrical appliances like washing machine, table fan, mixtu
re-grinder etc. are when used ~
on overload they draw excess current and their rotating speed
also decreased. Due to increasing of nakin
current, electric power loss is also increased. Due to power
loss heat is produced into these 'heno
appliances. Insulating material used in the winding of
appliances become heated hence the ,repo
insulation become weak. Due to this coverall efficiency of
appliances decreased and life of these henr
appliances will become shQ!!:
nd th
4.1.8; Increase in Ratin g with the Use oflnsulating Mate
rial Havin g High er Therm al
Stability
ataly
·h
·
. •
.
On t e bas1s
of expen•ments 1t
,xcep
1s found that when .msulatmg
materials are u~ed at 8-1o°C f
higher than their rated temperature than its life reduced
50%. Thermal stab·i·n., f . _ - 0 tc~
· 1 d•
.
.
.
.
matena use m the machme 1s low. But when msulat
mg1y "rt
mg material of hioh thenn1 1. _, ob"l·m~u· 1a1-ed.m h
.
"'
.
t e same machm
:,
e, its rated current can be mcrea
a1 ~ 1a 1 1ry 1s u~ ihrc
sed.
Hence
th~
·
•bl
k.
.
.
f
.
temperature o electric machme can be mcreased by usmg
.
• , pcm11ss1 e wor mg , , · n:,,
high
thenn
al
..
b.
.
.
.
"~ 1 ·
· Ialion
· can be used on higher
mach.me msu
1 msu latwn And 1~~ta 11ty
temperature than its rat d
·
<:i.;orr
e
temperature.
4.2. Clas sific a~ of Insulating Materials
ln~ulatinl,!. Matcr iab
- -- -"-'- ----- - ·-- -- -- ·- -
t-:kctrirnl and Ell·rtrunic, Engineci-ing 'Vlater ia ls
(a) Thcrmo-sl'ltin g ma teria ls: Thermo-setting material inc ludes _larmaldehytk_resi ns such
u, eca formalde hyde a ~ 1 i n c fo1malg£l!Y..J£.. ..!!nd _gio_-;_Y- ~~ii~- Thennosets contain
y,ners that cross-link t~gcther during the curing process lo form an irreversible chemical bond.
is applied,
c cross-linking process elim inates the risk of the product remelting when heat
king thern1osets ideal for high heat app lic;tions. The1mosets improve the material's mechanical
iperties. prnvidi_ng enhanced chemical ·rbistance, heat resistance and structural integrity.
ennoset polymer resin is used in electrical engin_eering works as structural and insulating
.te11al. lt has been used in the manufacturing of outer covering for electrical equipments.
operties of Ther~oset Resins
I. High mechanical strength .
2 . High working temperature
3. Can_not be dissoved iucommeq:;_ial S()~_vents
. 4. Die)ectria,los.s es ofnew type,thermoset resins are very less ·
·
5. They are hard.
d then ,heating,tlie, assembly underrnressurerto farm :a,pl:)<'W00d.
The other method involves reacting formaldehyde with an excess ;iJf'.phenD1-, using,irn:y!3cid,:
taly~tr'. ,T);J.\s; P:-?R5~Wl;{R~PHS '~:i~9½?.J?1;,rl?PtY,~f; f!l},le,g a JW?R!\iC, ,yrl}\fi~; 'hf .,f)p~\ P.P}~JP~i;,.
cept that it is of much lower moleculai weight and is still thermoplastic (that j ~t) f ;9,1n be
fte~ed _b,y reheating ';"ith<>11~ ~,n~er~~!~g,..c~1e~i~~] ~~~~~fo~it~<> ?\f~~i~g ?,~~ ~fa~~RI!JpJished
.gn_~dmg ~e n~~9lac to a ,P.9\:Vde,rr: !.11!.~!~-~ :)t. w1~~ fi,l}ers -~uch_~~ ,~oC>cl f\o~r,.,m1n<rra~s. ?,f
,res, ·and then heating the mjxture iri a pressiitized·mo uld:-rn,order to'be cured tq th~rmo~e.ftili& .
~;~i;:~t;t;r;: !~~~:hya::i~ it. ~tt:t:·ro~at~ enyde o;itre
Formaiion of pheno l formaldehyde resin is shown in the fig 4 .2
a
H
H
H
~
L
L__.....-___________,
1-1
I
I
I
I
CH
2
+HCHO--~
~
~
CH
2
A
¥
CH -
"'~ter
2
H
H
I
I
OH
(i) Phenol 'formaldehyde resin : Phenol formJtdehyde resins are also krrown as"ph~nolic
,insTese are obtained by the readiorr-ofphe nol oi"substituted phenol with formaidehyde'. It~·t
·lier name was bake lite a :first ,cqmpletely ,syi:ithetic polymer. There. iire .two basic methods .' for '·
t.king .the pozymer U1tQ .u~ful resin~, •!n oniunethod, an -exc,e ss of form!!,ld$yde is reacted with
enol ..in the pres,ence of a. base ,ca~aly1>~ . jp. water, ,s9hHio1J: ·to yield. a lowcmole.c ular w.eight .
!polymer called, a resole. T<he resole, ~quendyin liquid form or-solutiort, can b~ cured .to a solid .
:nnosettlng .network:polym er. .by,: ,for instap.ce, ,sandwiching' it .betweendayers of wood .vene~,·
49
Insu lating Materia ls
CH -6-CH - & L,CH,v
'~ ·'U ' _! .
u
.cgr,r-©t·, -cH,;"1Qr ~-~ -~
H,~
~ :~ '~C
~,:~;J. ' .. -· - '.
H
·~b,
0
Fig. 4.2: Form~\~~+f.~Hf~},C+~1f1,~hy:~e ':;'in
,
i!,~~~,
~?.~~ :~~~t'.:.
comi#dhI\·~.:c9~pf
Phenol-form~l,d,e.~yde res\~, plalf.,e ~?'~eJ1.e.,n,; ,w9od,,ad4e~i~es fqr plyY;ood ancj. p~niqle l?,o,arg,.
cause they form chemical bonds with the p~eno_~~l_i~~ l!g~i_n _~oi:np._o~~n~ _<>f,, w.?g?-. !h,e~ a e,,_
ecially desirable for exterior plywood, owing°fo tfreifg'odd·ml:i'tsfute ·resistari.cE: Phenolic re'sins,'·
also mdldb-d:info ·irtsulating and hea( fe'sistant
, 1 ariably reinforced ;with fibres° 'Or ;flak:es/ ~re
0
.
j ects such as appliance hanclles,'tltstri.butor caps and brake linings.
Phenonl formaldehyde is normally added with sdri:ie filit:r to' obtain th~;req~ir_e d properties.
~
'
1e properties without fillers and with fillers are givenih t~ble ',d: ' '
spbcitie?gravity ·;,.
Tensile strength
(kgfcm4x-10~3-)- -- ..
·:t) qc-=1 1.1 :: ·c1 :; ;,- '"). ,
· 15~'f<.0
,. ,, - ; .Q11f.,., 0-Q~'. , 0.3_5,:-0.64. . , ~.7,4-
·l :5-2.0 '
~64 Jn5- 0.5
10.7
0 .3- 0 .7
... _,'
·.: , i,i] -- -. .
Comp.res~iv.E&ti:iri'gth
(kg/cm 2 x10- 3 ) , , .
(
-, .
.1.0-2.2
1.0- 2.5
. . i..D-1.75
l.0-1.8
Water absorption··(¾)
0.1-2.2
0.3-1.0
0.1-0.5
0.01-d .!
0 l - 0 .2
Dielectric constmt ·
5-6
4 .5-9
fo-is
4-7 .5
7 .0