FIELD EFFECT TRANSISTOR (FET):

The field effect transistor is a three terminal uni-polar semiconductor device,
in which current is controlled by an electric field. Current conduction is only by
majority carriers.

Based on the construction, the FET can be classified into two types as

1.Junction FET and

2.Metal Oxide Semiconductor FET

Advantage of FET over the conventional transistor:
The  FET  is  a  unipolar  device  depending  only  upon  majority  carriers.  The
conventional transistor is a bipolar device.

FET has high input resistance, in the order of 100M  for JFET and 1010 to 1015
for MOSFET. Thus FET is a voltage controlled device.

FET is less noisy than a tube or bipolar transistor.

Disadvantages of FET over conventional transistor:

Transconductance  is  low  and  hence  the  voltage  gain  is  low.  In  case  of
transistor, transconductance is high, so the voltage gain is high.

They are more costly than junction transistors.

FET has relatively small gain band width product.


OPERATION OF N-CHANNEL FET:



The operation of N channel FET can be understood with the help of the

figure shown below:

Let us first suppose that the gate has been reverse-biased by a gate battery
VGG and the drain battery VDD is not connected.

We know that there exists a space charge region on either side of a reverse
biased  P-N  junction.  Now  space  charge  region  or  depletion  layers  located
symmetrically about the gates are formed.

Further consider the  effect  of drain  battery  VDD while VGG is  removed.  The
voltage VDD is dropped across the N channel resistance giving rise to a drain
current         ID = VDD / RDS.

Due to this current flow there will be a uniform voltage drop while going from
drain to source.  Consider two points A and B in N channel.

Let VA and VB be potential drop at these points. Certainly VA > VB. so due to
the  progressive  voltage  drop  along  the  length  of  the  channel,  the  reverse
biasing effect of PN junction is stronger near drain than near source.

Due to this reason, the penetration of depletion region A is more than at B.
this explains that is why depletion region extend farther into the channel at A
than at point B when both VDD and VGG are applied.


Let  no  potential  be  applied  between  gate  and  source  (VGS  =  0)  and  a potential VDD is applied between drain and source.

Now a current ID flows from drain to source, which is maximum because the
channel is widest. Let the gate be reverse biased by applying a voltage VGG
between gate and source.

This gate bias increases the depletion region and thereby decreases the cross
section  of  N  channel.  Since  there  is no  current carrier  available  in  depletion
region,  its conductivity is zero.

Due  to  the  decrease  of  cross  sectional  of  N  channel,  the  drain  current
decreases. When gate bias is increased further, a stage is reached when two
depletion regions touch each other and the ID becomes zero.

So  according  to  a  fixed  drain  to  source  voltage,  the  ID  is  a  function  of
reverse  bias  voltage  at  gate. Since  negative  gate  voltage  controls  the  drain
current, FET is called a voltage-controlled device.

When a varying signal voltage is applied in series with the gate voltage, the
resulting variation in drain current causes a similar though amplified voltage
variation, across the load resistance connected in drain circuit.
Static characteristics of FET:

DIODE SWITCHING TIMES

To understand the various switching times, consider simple diode circuit and an input waveform as shown in the Fig.

                                                           Simple diode circuit      

                                              
The following events will take place due to the nature of the applied voltage  

Event  1:  Till  time t the forward voltage  applied  is VF  and  diode  is  forward  biased. The value of R is  large enough such that drop across forward biased diode is very     small  compared to  drop across  R. The forward current  is then
neglecting
forward resistance of diode.


Event 2: At time t the applied voltage is suddenly reversed and reverse voltage of -
VR  is  applied  to  the  circuit.  Ideally  diode  also  must  become  OFF  from  ON  state instantly. But this does not happen instantly.


The number of minority carriers take time to reduce from p no to zero at the junction Due to this, at t1 current just reverses and remains at that reversed value IR till the minority carrier concentration reduces to zero. This current is given by -IR = -VR / R. This continues to flow till time t


Event3: From t onwards, the diode voltage starts to reverse and the diode current
starts  decreasing  as  shown  in  the  Fig.  At  t  =  t;  the  diode  state  completely  gets reversed and attains steady state in reverse biased condition.

The total time required by the diode which is the sum of storage time and transition     time, to recover completely from the change of state is called reverse recovery time
of  the  diode  and  denoted  as  trr.  This  is  an  important  consideration  in  high  speed switching applications.

The reverse recovery time depends on the RC time constant where C is a transition capacitance of a diode. Thus the transition capacitance plays an important role in the switching  circuits  using  diodes.The  total  switching  time  trr  puts  the  limit  on  the maximum  operating  frequency  of  the  diode.  Hence  trr  is  an  important  datasheet specification.  To  minimize  the  effect  of the reverse  current,  the  time  period  of  the operating frequency must be at least ten times trr.

where fmax is the maximum operating frequency.

SWITCHING CHARACTERISTICS OF DIODE

• When  diode  is  switched  from  forward  biased  to  the  reverse  biased  state  or viceversa, it takes finite time to attain a steady state.

• This  time  consists  of  a  transient  and  an  interval  of  time  before  the  diode attains  a  steady  state.  The  behavior  of  the  diode  during  this  time  is  called switching characteristics of the diode.

• In  the forward-bias state, there are a large number of electrons from the n

• side  diffusing  into  p  side  and  a  large  number  of  holes  diffusing  into  n  side from  p  side.  This  diffusion  process  establishes  a  large  number  of  minority carriers in each material.

• When forward biased, let n is concentration of electrons on p side at thermal equilibrium and p is concentration of holes on n side thermal equilibrium. This is concentration level far away from the junction.

• It  increases  towards  the  junction  and  becomes  n  and  Pn  on  p  and  n  side respectively in steady state. These minority charge carriers are supplied from  other side of the junction, where those carriers are majority in number.

• When  the  diode  is  reverse  biased,  again  far  from  the  junction  the  minority charge concentration is n on p side and Pno on n side.

• In reverse biased condition, as they approach the junction, they quickly cross     the  junction.  Hence  minority  carrier  concentration  decreases  to  zero  at  the junction in steady state. This is .shown in the Fig.

• Now when  a  forward  biased  diode is  suddenly reverse  biased, it  takes  finite time  to  change  the  minority  charge  carrier  concentration  and  to  attain  new steady state value.

• The   diode   can   not   attain   steady   state   till   the   minority   charge   carrier concentration changes from that corresponding to the forward biased to that corresponding to the reverse biased.

• Till the excess charge carrier concentration pn-pno and np-npo reduces to zero, the diode continues to conduct. This current is decided by the current limiting external resistance connected in the circuit.

• Hence in switching applications, the time required by the diode to attain new steady state, plays an important role.

DIFFUSION CAPACITANCE

During  forward  biased  condition,  another  capacitance  comes  into  existence called diffusion capacitance or storage capacitance, denoted as CD.

In forward biased condition, the width of the depletion region decreases and holes from p side get diffused in n side while electrons from n side move into the p-side. As the applied voltage increases, concentration of injected charged particles increases.

This rate of change of the injected charge with applied voltage is defined as a capacitance called diffusion capacitance.

The diffusion capacitance can be determined by the expression

where r = mean life time for holes.

So  diffusion  capacitance  is  proportional  to  the  current.  For  forward  biased condition, the value of diffusion capacitance is of the order of nano farads to  micro farads while transition capacitance is of the order of pico farads. So CD
is much larger than CT.
Diffusion capacitance versus applied forward biased voltage

However in forward biased condition, CD appears in parallel with the forward resistance  which  is  very  small  hence  the  time  constant  which  is  function  of product  of  the  forward  resistance  and  CD  is  also  very  small  for  ordinary signals.

Hence for normal signals CD has no practical significance but for fast signals CD must be considered.The graph of CD against the applied forward voltage is shown in the Fig

TRANSITION CAPACITANCE (CT)

When a diode is reverse biased, the width of the depletion region increases.
So  there  are  more  positive  and  negative  charges  present  in  the  depletion region.

Due  to  this, the  p-region  and  n-region  act like  the  plates  of  capacitor while the depletion region acts like dielectric.

Thus there exists a capacitance at the p-n junction called transition capacitance, junction capacitance, space charge capacitance, barrier capacitance or depletion region capacitance. It is denoted as CT.

Mathematically it is given by the expression,

where
= permittivity of semiconductor =   o
r
o= 1/(36  ×10  9)8.849x10-12 F/m
Cr = relative permittivity of semiconductor = 16 for Ge, 12 for Si
A = area of cross section
W = width of depletion region

As   the   reverse   biased   applied  to  the  diode  increases,   the   width   of   the depletion region (W) increases. Thus the transition capacitance CT decreases.


In  short,  the  capacitance  can  be  controlled  by  the  applied  voltage.  The variation of CT with respect to the applied reverse bias voltage is shown in the Fig.

As reverse voltage is negative, graph is shown in the second quadrant. For a particular diode shown, CT varies from 80 pF to less than 5 pF as VR changes from 2V to 15 V.

COMPLETE V-I CHARACTERISTICS OF A DIODE

 The complete V-I characteristics of a diode is the combination of its forward as well as reverse characteristics.

In forward characteristics, it is seen that initially forward current is small as long as the bias voltage is less than the barrier potential.


At  a certain  voltage close to  barrier  potential, current increases rapidly.  The voltage  at  which  diode  current  starts  increasing  rapidly  is  called  as  cut  in voltage. It is denoted by V

Below this voltage, current is less than 1% of maximum rated value of diode current. The cut-in voltage for germanium is about 0.2V while for silicon it is 0.6 V.

The  voltage  at  which  breakdown occurs  is  called  reverse  breakdown  voltage denoted as VBR

REVERSE BIASED DIODE

The Fig shows the reverse bias diode. The reverse voltage across the diode VR    while  the  current  flowing  is  reverse  current  ‘R  flowing  due  to  minority  char carriers. The graph of ‘R against VR is called reverse characteristics of a diode.

As reverse voltage is an increased, reverse current increase initially but after
a  certain  voltage,  the  current  remains  constant  equal  to  reverse  saturation current 10 though reverse voltage is increased.


The point A where breakdown occurs and reverse current increase rapidly is called knee of the reverse characteristics.

FORWARD RESISTANCE OF A DIODE

The resistance offered by the p-n junction diode in forward biased condition is called forward resistance. The forward resistance is defined in two ways

1) Static Forward Resistance

This is the forward resistance of p-n junction diode when p-n junction is used in d.c. circuit and the applied forward voltage is d.c. This resistance is denoted as RF and is calculated at a particular point on the forward characteristics.

Thus at a point E shown in the forward characteristics, the static resistance RF is defined as the ratio of the d.c. voltage applied across the p-n junction to the d.c. current flowing through the p-n junction.
RF = Forward d.c. voltage/Forward d.c. current  RF =OA/OC    at point E

2) Dynamic forward resistance


The resistance offered by the p-n junction under a.c. conditions is called dynamic resistance denoted as rf.
Consider the change in applied voltage from point A to B shown in the Fig. 1.18. This  s denoted as AV. The corresponding change in the forward current is from point C to D. I is denoted as J. Thus the slope of the characteristics is t\l/AV. The reciprocal of the e is dynamic resistance rf.

rf = V/ I

=1/slope of forward characteristics

THE VOLT-AMPERE (VI CHARACTERISTICS OF A DIODE)

The response of a diode when connected in an electrical circuit, can be judged   from its characteristics known as Volt-Ampere commonly called V-I    characteristics.  The  V-I  characteristics  in  the  forward  biased  and  reverse biased  condition  is  the  graph  of  voltage  across  the  diode  against  the  diode current.

The   response   of   p-n   junction   can   be   easily   indicated   with   the   help   of characteristics  called  V-I  characteristics  of  p-n  junction.  It  is  the  graph  of voltage applied across the p-n junction and the current flowing through the p-
n junction.

The applied voltage is V while the voltage across the diode is Vf. The current flowing in the circuit is the forward current I. The graph of forward current If against the forward voltage Vf across the diode is called forward characteristics of a diode.

Basically forward characteristics can be divided into two regions

1. Region 0 to P As long as Vf is less than cut-in voltage (V , the current flowing is very small. Practically this current is assumed to be zero.

2. Region P to Q and onwards: As V increases towards V the width of depletion  region goes on reducing. When Vf exceeds V i.e. cut-in voltage, the depletion region becomes very thin and current I increases suddenly. This increase in the current is exponential as shown in the Fig. by the region P to Q.

                                                                Forward biased diode

The point P, after which the forward current starts increasing exponentially is called knee of the curve.

THE P-N JUNCTION DIODE

• The PN junction is formed of the P type and N type semiconductor material.

• In P type, the holes are the majority charge carriers

• In N type material, the electrons are the majority charge carriers.

• Therefore at the junction there is a tendency for the free electrons to diffuse over to the P-side and holes to the N-side. This process is called diffusion.

• The diffusion of holes and free electrons   is due   to the difference   in concentration of the two regions.

• This  difference  in  concentration  creates  a  concentration  gradient  across  the junction.

• Due to the process of diffusion the negative acceptor ions in the P region and positive  donor  ions  in  the  N region  are  left  uncovered  in  the  vicinity  of  the junction as seen in

• The  additional  holes,  trying  to  diffuse  to  the  N-region,  are  repelled  by  the uncovered positive charge of the donor ions.

• Similarly, the electrons trying to diffuse into the P-region are repelled by the uncovered negative charge of the acceptor ions.

• Thus   a   barrier  is   set   up   near  the   junction,  which   prevents   the   further movement  of  charge  carriers.  This  is  called  as  potential  barrier  or  junction barrier V0.

• As a result, further diffusion of free electrons and holes across the junction is stopped.

• The region containing the uncovered acceptor and donor ions, in the vicinity of the junction is called depletion region.

• Since  this  region  has  immobile  ions,  which  are  electrically  charged,  the depletion region is also known as space-charge region.

• The  width  of  the  depletion  layer  depends  upon  the  doping  level  of  the impurity in N-type or P-type semiconductor.

• The higher the doping level, the thinner will b e the depletion layer and vice versa.

• The  depletion  layer  consists  of  fixed  rows  of  oppositely  charged  ions  on  its two sides.

• Because  of this  charge separation,  an electric potential is  established  across the junction, even when no external voltage being applied.

• This electric potential is called junction or potential barrier.

WORKING AND VI CHARACTERISTICS: UNDER FORWARD BIAS CONDITION:

When  positive  terminal  of  the  battery  is  connected  to  P-type  and  negative terminal  to  N-type  of  the  PN  junction  diode,  the  bias  applied  is  known  as  forward bias.

• Under  forward bias,  the  applied  positive  potential  repels  the holes  in  P-type region so that the holes move towards the junction.

• The  applied  negative  potential  repels  the  electrons  in the N-type  region  and the electrons also move towards the junction.

• Eventually, when the potential applied exceeds the internal barrier potential, the depletion region and internal potential barrier disappear.

                                               V/I characteristics under forward bias condition

As  forward  voltage  increases,  for  VF  <  V0  (potential  barrier),  the  forward current  I  is  almost  zero,  because  the  potential  barrier  the  potential  barrier prevents the flow of electrons and holes across the depletion region from N

and P regions respectively.

For VF >V0, the potential barrier is overcome and current increases rapidly.
Cut-in or threshold voltage: Below which the current is very small and at the cut-
in  voltage  the  potential  barrier  is  overcome  and  the  current  through  the  junction starts to increase rapidly




UNDER REVERSE BIAS CONDITION:

When  the  negative  terminal  of  the  battery  is  connected  to  the  P-type  and positive terminal of the battery is connected to N-type of the PN junction, the    bias applied is known as reverse bias.

                                                            Under reverse bias condition


                          

• Under  reverse  bias  condition, holes  from  P  side  move  towards the  negative terminal of the battery.

• The electrons  from N  side  are  attracted towards  the  positive  terminal of the battery.

• Hence the width of the depletion region increases.

• The  resultant  potential  barrier  also  increases,  which  prevents  the  flow  of majority charge carriers in both directions.

• Theoretically, no current should flow in the external circuit.

• But in practice, a very small current flows under reverse bias condition.

• Due to the  absorption of energy by the electrons cause the breaking of the covalent bonds.

• This results in the generation of electron-hole pairs.

• The thermally generated charge carriers cross the junction and giving rise to what is known as reverse saturation current.

• For large  applied  reverse  bias,  avalanche  effect  takes place, leading  to  very large reverse current.

• This leads to the breakdown of the junction.

• Breakdown voltage: The reverse voltage at which the junction breakdown occurs is called breakdown voltage.

                                            V/I characteristics under reverse bias condition



BREAKDOWN IN REVERSE BIASED

Though   the   reverse   saturation   current   is   not   dependent   on   the   applied reverse voltage, if reverse voltage is increased beyond particular value, large reverse    current  can  flow  damaging  the  diode.  This  is  called  reverse  breakdown  of  a diode. Such a reverse breakdown of a diode can take place due to the following two effects,

1. Avalanche effect and
2. Zener effect


BREAKDOWN DUE TO THE AVALANCHE EFFECT

Though  reverse  current  is  not  dependent  on  reverse  voltage,  if  reverse voltage is increased, at a particular value, velocity of minority carriers increases. Due
to the kinetic energy associated with the minority carriers, more minority carriers are generated  when  there  is  collision  of  minority  carriers  with  the  atoms.  The  collision makes  the  electrons  to  break  the  covalent  bonds.  These  electrons  are  available  as minority carriers and get accelerated due to high reverse voltage. They again collide

with another atom to generate more minority carriers. This is called caner
multiplication.  Finally  large  number  of  minority  carriers  move  across  the  junction, breaking the p-n junction. These large number of minority carriers give rise to a very     high  reverse  current.  This  effect  is  called  avalanche  effect  and  the  mechanism  of destroying the junction is called reverse breakdown of a p-n junction. The voltage at    which the  breakdown  of  a p-n  junction  occurs is  called reverse  breakdown  voltage. The  series  resistance  must  be  used  to  avoid  breakdown  condition,  limiting  the reverse current.



BREAKDOWN DUE TO THE ZENER EFFECT

The breakdown of a p-n junction may occur because of one more effect called zener  effect.  When  a  p-n  junction  is  heavily  doped  the  depletion  region  is  very narrow. So under reverse bias conditions, the electric field across the depletion layer
is  very  intense.  Electric  field  is  voltage  per  distance  and  due  to  narrow  depletion region and high reverse voltage, it is intense. Such an intense field is enough to pull
the electrons out of the valence bands of the stable atoms. So this is not due to the collision of carriers with atoms. Such a creation of free electrons is called zener effect which is different than the avalanche effect. These minority carriers constitute very     large current and mechanism is called zener breakdown.


The  breakdown  effects  are  not  required  to  be  considered  for  p-n  junction diode. These effects  are  required to be  considered  for special  diodes such as  zener diode as such diodes are always operated in reverse breakdown condition.

Non-Linear Materials:

We know that a light wave is electro magnetic in nature. When it propagates through a material, it changes the properties of the medium, such as the refractive index. It depends on the electric and magnetic fields associated with the light beam. For example the non-linear properties of the material will be absent if the incident light beam is of low intensity, since the electricfields associated with the light beam is very weak. On the other hand, for a high intensity light beam such as laser, the non-linear effect will be more strong and interesting.

Classifications:  

    The materials which are used to produce the non-linear optical effects are classified into two categories, namely, passive and active.

Passive materials:

    The materials which are simply used as catalyst without imposing their characteristic internal resonance frequencies on to the incident beam of light are known as passive materials and the effect is known as passive optical effect.


Active materials:

    The materials which impose their characteristic resonance frequencies onto an incident beam of light are known as active materials. The corresponding effect known as active non-linear effect.

Properties:

Polarization:

    When a light beam is incident on a non-linear materials. The electric field interacts with an atom in the material. As a result, electric dipoles are created and hence, an induced charge polarization is produced in the material.
    The magnitude of the polarization depends on the applied electric field(E), the polarization is given as

This is the experimental arrangement used for the production of second harmonic generation. The light radiation from the ruby laser with a wave length 6943Ao gets converted into two wave lengths of 3472 Ao and 6943Ao.

    Similarly, based on the intensity of the laser beam one can find the third, fourth, etc…, harmonics.

Applications:

    The non-linear optical materials are very important for application such as frequency doubling (or) tripling of laser light(harmonic generation), optical mixing, telecommunications (such as parametric amplifications), and information processing and computing (such as image processing etc)

    Non-linear materials have many potentials applications in optical communications systems. The radio frequency techniques like mixing, heterodynamics and modulation can be done at optical frequencies. Due to these reason non-linear materials are finding an increasing role in laser applications.

Non-linear optical phenomena:

•     Optical mixing.

•     Optical phase conjugation.

•     Optical rectification.

•     Phase matching.

•     Frequency doubling (or) tripling.

Bio-Materials:

“Bio-materials are defined as synthetic materials that can be implanted in the body to provide special prosthetic functions or used in diagnostic, surgical and therapeutic applications without causing adverse effect on blood and other tissues”

    Bio-materials have been developed from among metals, ceramics and polymers.

Metallic Bio-materials:

    Metals are used as bio-materials due to their excellent electrical and thermal conductivity and mechanical properties. Since some electrons are independent in metal, they can quickly transfer an electric charge and thermal energy. The mobile free electrons act as the binding force to hold the positive metal ions together, this attraction is strong, as evidenced by the closely packed atomic arrangement resulting in high specify gravity and high melting points of most metals.

    Since the metallic bond is essentially non-directional, the positions of the metal ions can be altered without destroying the crystal structure resulting in a plastically deformable solid.

Uses:

    Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixations devices, and dental implants because of their excellent mechanical properties and corrosion resistance. Some metallic alloys are used for more active roles in devices such as vascular stents, catheter guide wires, orthodontic arch wires and cochlea implants.

Ceramic Bio-materials:

    Ceramic which are used as bio-materials are classified as bioceramics, the relative inertness to the body fluids, high compressive strength, and aesthetically pleasing appearance led to the use of ceramics in density as dental crown. Some carbons have found use as implants especially for blood interfacing applications such as heart valves.

    Due to high specific strength as fibers and their biocompatibility, ceramics are also being uses as reinforcing components of composite implant materials and for tensile loading applications such as artificial tendon and ligaments.

Medical applications of polymeric biomaterials:

Polyvinyl chloride:

    Blood and solution bag, surgical packaging, IV sets, dialysis devices, catheter bottles, connectors, and cannulae.

Polyethylene:

    Pharmaceutical bottle, nonwoven fabric, catheter, pouch, flexible container, and orthopaedic implants.

Poly propylene:

    Disposable syringes and reservoirs, membrane for blood dialyzer, implantable ocular lens and bone cement.

Polysterene:

    Tissue culture flasks, roller bottles, and filter waves.

Polyethylenter phthalate:

        Implantable suture, mesh, artificial vascular grafts and heat valve.

Polytetra fluoroethylene:

    Catheter and artificial vascular grafts.

Polyurethane:

    Packaging film, catheters, sutures and mould parts.

FIBERS :

A fiber is a composite material that has been drawn into a long and thin filament.

Classifications:

•     Glass fibers.

•     Armid fibers.

•     Carbon fibers.

Glass fibers:

•     E- glass fiber (Electrical)

•     S-glass fiber (High strength)

E-glass fiber:

The composition of E-glass ranges from 8-13%  , 12-16%  , 16-25% CaO,  .

1.    Its tensile strength is about 3.44GPa.

2.    It is used for continuous fibers.

S-glass fiber:

        Composition are 10% Mgo, 25%   , 65% .

1.    Its tensile strength is over 650 Ksi.
2.    Used in military applications.



Armid fibers:

1.    It is characterized by low density and high strength.
2.    It is used as ballistic protection, ropes and cables.

Carbon fibers:


    Composite materials made by during carbon fibers for reinforcing plastic resin matrices such as epoxy are characterized by having a combination of light weight, high stiffness and high strength.

Elastomers: (Rubber)

     Rubbers are high polymers, which have elastic properties in excess of 300 percent. Thus, a rubber-band can be stretched to 4 to 10 times its original length and as soon as the stretching force is released, it returns to its original length, the coiled Elastomers chain of natural rubber (Poly isoprene).

PLASTICS :

Organic materials of high molecular weight and it can be moulded into any desired form when it is subjected to heat and pressure in the presence of a catalyst are called plastics.

    Resins are the basic binding materials which form a major part of the plastics and which actually has undergone polymerizations and condensation reactions during their preparation.

Plastics divided into two groups.

1.     Thermo plastics.
2.    Thermo setting plastics.

Thermo plastics:

    Plastics which can be softened on heating and hardness on cooling are called thermo plastics. The chemical nature of thermo plastics are un altered by repeated heating and cooling.

Eg:

    PVC, Polyethylene, Polystyrene.

Thermo setting plastics:

    Plastics which get hardened during heating (mounting process) and then cannot be softened ate called as thermo setting plastics. Since they acquire three dimensional cross-linked structures with predominantly strong covalent bonds, these bonds retain the strength on reheating.

Eg:

    Bakelite, polysters, silicons.

POLYMERS :

Polymers are macro molecules built-up by the linking together of large number of small molecules called manomers. The number of repeating units in a polymer is known as the degree of polymerization.



In general, most of the polymers fall into the 5000-200,000 molecular mass range.

Properties:

Physical:   

    Polymers are in the form of crystal or amorphous or crystallites embedded with an amorphous matrix.

Thermal:

    Below glass transition temperature, they are in hard condition and above the glass transition temperature they become soft nature.

Optical:

    The appearance of a transparent plastics and opaque plastics are characterized by its transmittance or reflectance properties respectively.

Chemical:

    Chemically resistant.

Eg:

1.    Polymers having polar groups are chemically resistant to non-polar solvents.

2.    Similarly, polymers having non-polar groups are chemically resistant to polar solvents.

Electrical:

    In general, Polymers are insulators due to their wide band gap. Now a days, we can manufacture conducting polymers by synthetic methods.

Classifications:

     On the basis of manomer unit.

1.     Homo polymer
2.     Co-polymer.

Homo polymer:

    The are made up of identical manomer unit.

           -M-M-M-M-M-

Co-polymer:

    They are having different kind of manomer units.

Important classification:

    Polymers can be classified into following three types.

    Plastics (resins)

    Elastomers (rubber)

    Fibers (nylon).

CERAMICS :

Ceramic are inorganic materials consisting of metallic and non-metallic elements bonded together mainly by ionic or covalent bonds. They are in the forms of crystalline, non-crystalline or mixtures of both.

Properties:

•     High hardness.

•     High temperature strength.

•     Good chemical resistance.

•     They tend to be brittle.

•     Low thermal and electrical conductivities.

The fundamental basis for its characteristics lies within the electronic behaviour of constituent atoms. The metallic elements release their outermost electron and give there electron to non metallic atoms which retain them. The result is that these electrons are immobilized and this situation indicates the absence of conduction electrons. Hence a typical ceramic materials act as a good insulators.

    In our case, the atoms which lost outermost electrons are called positive metallic ions. Similarly, the atoms which gain electrons are called negative metallic ions. The positive metallic ions and negative metallic ions develop strong attractions for each other. Each cation surrounds itself with anions. To separate the two ceramic materials are mechanically resistance (hard), thermally resistant (refractory) and chemically inert.

Classifications:

•     Traditional ceramics.

•     Advanced ceramics.

Traditional ceramics:

    The important characteristics of traditional ceramic are that all traditional ceramics use materials or minerals occurring state.

Eg:

        China ware, sanitary ware, etc…..





Advanced ceramics:

    Advanced ceramics refers pure or nearly pure ceramic components alone (or) in combination; they are manufactured by using highly refined raw materials by using several chemical techniques. Therefore the starting materials for advanced ceramics have already undergone chemical transformation and refinement.


Eg:
 
 
    

Applications:

1.    It is used in capacitors, electronic circuits, electronic sensors, integrated components, etc..

2.    Piezo electric ceramics are used in phonograph pickups, microphones, gas lighters, quartz watches, SONAR devices.

3.    Ferroelectric ceramics are used for the manufacture of capacitors.

4.    Ceramic chip capacitors are used in ceramic-based thick film hybrid electronic circuits.

5.    Ceramic semiconductors are used in some electrical device  eg: thermister.

Shape Memory Alloys:

   Shape memory alloys (SMA) are special alloys which, after being deformed at some relatively low temperature and will return to their original shape when it is subjected to the appropriate thermal procedure.
    Shape memory alloys can be plastically deformed at some relatively low temperature and will return to their original shape prior to the deformation at some higher. This amazing behaviour is called shape memory effect.

Types of shape memory effect:

•  One way shape memory.

•   Two way shape memory.

Certain materials exhibit shape memory effect only upon heating and they are referred to as a one way shape memory. Materials which also exhibit shape memory effect upon recooling are referred to as having two-way shape memory.
 
Structure of SMA:

    The shape memory alloy is easily deformed into any shape and easily remembers the shape before it was deformed respectively at low and high temperatures. It may display two distinct crystal structures or phases.

•     Martensite

•     Austenite.

At low temperatures, the crystal structure of SMA is called Martensite. At higher temperatures, the crystal structure of SMA is called Austenite.

    The shape memory alloy is easily deformed into any shape and easily remembers the shape before it was deformed respectively at low temperatures.

Characteristics of SMA:

    The properties of SMA depend upon the amount of each crystal phase present. SMA, can easily be deformed when it is in Martensite phase. Similarly, it can recover its shape when return to Martensite transformation, SMA yields a thermo elastic Martensite and develops from a high-temperatures austenite phase with long range order. Such transformation occurs over a range of temperature and not a single temperature. The range of temperature varies with each alloy system.


    Even though the transformation during heating and on cooling actually extends over much larger temperature range, most of the transformation occurs over a relatively narrow temperature only. The transformation also exhibits hysteresis in that the transformations on heating and on cooling do not overlap.

T1 – transformations by hysteresis.
Ms- Martensite start.
Mf- Martensite finish.
As- Austenite start
Af- Austenite finish

Nano Phase Materials:

Nano phase materials are recently developed new materials with the grain size in the range 1 to 100nm. The particles size in nano materials is about 1nm.

Preparation of Nano materials:

    Various methods are employed to produce nano-structured materials. Depending upon the desired properties or applications, each method will have some advantages and disadvantages.

Vapours condensations Methods:

    Cluster of atom are typically synthesized via vapour condensation which is essentially the evaporation of a solid metal followed by rapid condensation to form nano sized clusters. The resulting powder can be used as filters for composite materials (or) consolidated into bulk materials. Hence this method can be used to produce ceramic or metal nano structured powders.


Chemical synthesis:

    This method can be used to produce both metals and ceramics by using a variety of chemical approaches such as sol-gel (or) thermal decomposition. These methods provide large quantities of nano-sized materials at low cost.

Mechanical deformation:

    It is a common method to producing nano structured powders and it is through mechanical deformation by deformation by milling of mechanical deformation by means of milling or shock deformation. The nanometer sized grains nucleate with in the disassociation of cell structures located in the shear bands. The resultant grain size function of

•     Amount of energy input during milling.

•    Amount of time

•     Temperature during milling and

•     Milling atmosphere.

Thermal crystallization:

    This method is also used to synthesize three dimensional nanostructured materials. By controlling the nucleation and growth, during annealing of an amorphous material one can produce bulk material with an average grain size of less than 20 nm without the need for consolidation and sintering steps. 


 Properties:

1.    With in a diameter of only a few micrometers, a cluster of particles contains less than 104 atoms or molecules.

2.    The chemical, mechanical, electrical, electronic, optical and magnetic properties of nanophase materials are different from the bulk materials.

3.    Since the size of nanophase materials is inbetween the molecular and bulk solid structures, these have hybrid properties.

They have non-linear optical and magnetic properties.

Applications:   

•   Nano phase materials are used to produce very tiny permanent magnets with high energy products.

            Hence they are used in high density magnetic recording.

•    Nano phase materials have a large volume fraction of grain boundaries between surface area and volume. To improve the mechanical behaviour like higher hardness in ceramics, this property is used.

Structural applications:

    Functional applications are based on the transformation of external signals such as filtering of the incident light, the change of electrical resistance in different gas concentration and luminescent behaviour when the materials are electrically activated.

METALLIC GLASSES :

A metallic glasses is a solid resulting from non-crystallization during cooling from liquid state. The properties of the metallic glasses are a combination of both metals and alloys. 

Preparation:

    The metallic glasses are prepared by several methods employing special techniques, which involve rapid solidification of the melt. Melt spinning is one such technique used to prepare metallic glasses.

The molten alloy flow through the outlet of the quartz tube and it is cooled at a ultrafast rate with the help of a rotating cooled copper cylinder. On impact with the rotating drum, the melt is frozen with in a few milliseconds producing a long ribbon of metallic glasses.

Properties:

1.    Metallic glasses are non-crystalline, they are ferromagnetic. The lack of long range ordering results zero bulk magnetic crystal anisotropy on an average. Due to that, they posses low magnetic losses, high permeability and saturation magnetization with low coercivity. Thus they resemble the very soft magnetic alloys.

2.    Metallic glasses posses high strength and tensile strength, it is around 3.6 GPa. Thus, metallic glasses are superior than common steels. This is based on their structure since the random ordering does not have any lattice defects like dislocation and grain boundaries.

3.    Metallic glasses have higher workability. Thus they can be cold worked upto half their thickness without cracking.

4.    Metallic glasses have high electrical resistance with nearly zero temperature coefficient of resistance. Only at very low temperature there is a sharp variation in resistance.

5.    Metallic glasses are not affected by irradiation, high corrosion resistance.

Applications:

1.    Metallic glasses are suitable for applications in electronic circuits because of their insensitivity to temperature variation.

2.    They are widely used as resistance elements in electric circuits due to their high electrical resistivity.

3.    Metallic glasses posses high Vicker’s hardness and corrosion resistance, they have found applications as materials for magnetic tape recording heads.

4.    The use of metallic glasses in motors can reduce core loss by as much as 90% as compared with conventional crystalline magnets.

5.    Possible applications of metallic glasses include sensitive and quick response magnetic sensors or transducers. Security systems and power transformer cores.

Metallic glasses as transformer core material:   

    The ferromagnetic properties of metallic glasses have received a great deal of attention, probably because of the possibility that these materials can be used as transformer cores. Because some metallic glasses have excellent magnetic properties, there is a great incentive for developing advanced techniques for producing large sheets of these materials to be used as transformer cores.

    These large sheets of metallic glasses are widely used in power distribution transformers which convert high voltage electricity in power lines to 240 V for domestic use. Power transformers made of metallic glass are smaller in size and efficient in their performance as compared to the conventional transformers which are very large in size.

Photo Conducting Materials:

    Cadmium sulphide and cadmium selenide:

       It is used to make photo conductors with high dissipation capability and excellent sensitivity in visible spectrum.
  
  Lead sulphide:

        It is used to make commercial photo conductive cells and it is used in the infra red detection.

   Indium antimonide:

        It is used as infra red detector which detects waves upto 7 . It has melting point and can be produced as a single crystal.
   
Mercury cadmium telluride:

        The resistance of these devices is very low.

    Doped semiconductors:

        Zinc and boron doped germanium detectors has detecting range from 20 to 100 , but they require cooling upto 4K.

Photo conductive and photoconductive detectors:

The phenomenon of increasing in electrical conductivity of the crystal with respect to the incident light radiation onto the crystal is called photoconductivity.

Principle:

    When photons of energy equal to   is incident on the crystal, then the crystal absorbs the energy and creates an electron hole pair, (ie) electrons from valence band goes to conduction band, thereby creating a hole in valence band. In this case both electrons and holes will contribute electrical conductivity and these detectors are called photoconductive detector.

Photoconductive Gain:

An important advantage of the photoconductive detector is the gain of the device (ie) it can produce more number of electron hole pairs for a single incident photon on it.

    Let us consider a Photoconducting material in the form of slab of length ’L’ and area ‘A’ biased with the help of external circuit. The load resistance RL is used to control the sensitivity and the blocking capacitor ‘C’ is used to remove the dc component when light is falling on the detector.


In the absence of light signal:

    In the absence of a light signal, the current will flow through the circuit due to bias voltage. Let ne and nh be the electron and hole densities without light respectively, then

In the presence of light signal:

    Now when the light is allowed to fall on the detector, electron hole pairs are generated equally. These excess carriers increase the conductivity. If  and  are the excess carrier densities of electrons and holes respectively, then the electrical conductivity due to addition of these charge carriers is

Applications of Thermography:

Electrical engineering applications:

• We know that the electrical energy can be dissipated in the form of heat. Therefore the heat emitted can be detected using thermography technique.

• Some connections (or) joints produce heat; therefore thermography camera can be used to focus the hot spots in electrical panels and wirings. Here the heated components appear as bright spots on Thermograms.

• In the case of 3-phase connections, sometimes the current may not have equal magnitude, then there exists a temperature difference between the phases. This can be detected using thermographic cameras.

• It is very useful in finding the faults in transformers, overhead electrical connections, etc from the ground itself.

Medical applications:

• It is used to find the heat emitted from human body for diagnostic purposes.

• DITI technique is used in screening breast cancer, vascular disease etc., by detecting abnormal heat emitted from the particular part of the body.

General applications:

• It is used to find the heat loss around windows, door frames, buildings etc.,

• It is used to detect the roof problems (or) defects (due to rain water trapping)

• It is used for detecting the poor bonding, cracking, voids etc.,

• It is used in refineries, power plant boilers, gas leakage detection, furnace refratories.

• It is used to find the faults in IC’s.

2D-THERMOGRAPH :

Principle:

The IR radiations from the object are detected and are converted into shades of gray. There shades of gray represent the temperature levels of the surface of the object over CRT screen.

Construction:

The IR camera consists of a plane mirror which is oscillating about a horizontal axis and an eight sided prism. Both can be rotated using motors.

 

 

 

 

The detector system is made of Indium antimonide which converts heat radiation into electrical signal. A separate cooling arrangement is made to cool the detector by placing liquid N2 into a dewar flask.

Working:

The IR radiation from the object is focused onto the oscillating plane mirror. This provides the vertical scanning of the object and the synchronized signal can be sent to display unit.(signal 1).

The image from the oscillating mirror is focused onto a rotating eight sided prism. This provides the horizontal scanning and the synchronized signals (signals 2) are sent to display unit.

These two signals (signal 1 and signal 2) produces an image of the object in the CRT display unit. The radiation from the eight sided prism is allowed to fall on the detector, after passing through the lens system.

Now the detector converts the IR radiation into electrical signals and is fed to the display unit after suitable amplification. The electrical signals produced depends on the energy of the incident radiation over the detector. Thus the signals modulate the beams intensity in the CRT display.

Thus the shades of gray of various intensity level is displayed in CRT. Each portion of gray (black and white) represents the various temperature of the object.

A digital display unit can be fixed below the CRT so that the temperature corresponding to any particular point of the object can be viewed directly.

Advantages:

1.  It can be used to measure the temperature differences between any points of the object even upto 0.1oC.

2. Using colour display units we can find the temperature, with respect to the colours obtained on CRT.

Spot radiometer:

Here the thermograph receives the IR radiations from a particular point and the temperature of that point can be displayed with the help of a digital meter. Since the thermograph is used to measure the temperature of a spot, it is called spot radiometer.