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MATERIAL TECHNOLOGY II
P. VENU GOPAL, M. Tech, PGDMM
Copyright © P. Venu Gopal, M. Tech
All Rights Reserved.
This
book has been self-published with all reasonable efforts taken to make the
material error-free by the author. No part of this book shall be used,
reproduced in any manner whatsoever without written permission from the author,
except in the case of brief quotations embodied in critical articles and
reviews.
The
Author of this book is solely responsible and liable for its content including
but not limited to the views, representations, descriptions, statements,
information, opinions and references [“Content”]. The Content of this book
shall not constitute or be construed or deemed to reflect the opinion or
expression of the Publisher or Editor. Neither the Publisher nor Editor endorse
or approve the Content of this book or guarantee the reliability, accuracy or
completeness of the Content published herein and do not make any
representations or warranties of any kind, express or implied, including but
not limited to the implied warranties of merchantability, fitness for a
particular purpose. The Publisher and Editor shall not be liable whatsoever for
any errors, omissions, whether such errors or omissions result from negligence,
accident, or any other cause or claims for loss or damages of any kind,
including without limitation, indirect or consequential loss or damage arising
out of use, inability to use, or about the reliability, accuracy or sufficiency
of the information contained in this book.
Dedication
I dedicate this work to my parents,
my wife, whose love, guidance, and unwavering support have shaped who I am
today; to my brothers, for their encouragement and companionship throughout
life; and to my wife, for her patience, inspiration, and constant belief in me.
This work is a tribute to all of you, whose presence has made every achievement
possible.
Contents
1.1.1 Classification of
Plastics:
1.2.1 General
properties of Thermoplastics:
1.2.4
Polytetrafluoroethylene (PTFE):
1.2.5 PVC (Poly Vinyl
Chloride)
1.4 Materials for
processing plastics
2.2 Types of Composite
Materials
2.3.1 Important Terms
in Agglomerated Materials
2.5.1 Reinforced Cement
Concrete
2.5.3 Glass-Fibre
Reinforced Plastic
3.2 Classification of
corrosion
3.2.1 Direct chemical
corrosion (Dry corrosion)
3.2.2 Electrochemical
corrosion (Wet corrosion)
3.3 Important Types of
Corrosion
3.4 Prevention and
Control of Corrosion
3.4.1 Suitable design
& Fabrication procedure
3.4.3 Modification of
the corrosive environment
3.4.4 Use of protective
coating
3.4.6 Use of cathodic
protection
3.4.7 Heat treatment of
metals
4.3 Manufacturing of
Metal Powders
4.5 Products of Powder
Metallurgy
4.6 Advantages of the
powder metallurgy process
4.7 Disadvantages and
Limitations of the process
5. Semiconductors
& Insulators
5.3.1 Classification of
Semiconductors
5.3.2 Intrinsic
semiconductors:
5.3.3 Extrinsic
semiconductors:
5.4.1 Applications of
Insulating materials:
6.2 Functions of Waste
Disposal Department:
6.3 Steps in Disposal
of Waste materials
6.4.1 Functions of
Recycling Department:
6.4.2 Steps in
Recycling process:
Foreword
It gives me great pleasure to write a few words
about this book Material Technology – II, authored by my esteemed colleague and
academician. Having spent several decades in the field of engineering education
and research, I can say with confidence that the subject of Material Technology
remains one of the most vital foundations for students aspiring to build
careers in manufacturing, design, and applied sciences.
Unfortunately, the vastness and complexity of
this subject often overwhelm students, particularly when they are first
introduced to it. Standard textbooks, though comprehensive, tend to be bulky
and highly technical, making it difficult for learners to identify and retain
the essentials. In such a context, this book emerges as a much-needed
contribution.
Drawing from his experience as Academic Advisor
at the Central Institute of Tool Design (CITD), the author has distilled years
of teaching, observation, and student interaction into this work. He has made a
deliberate effort to simplify complex concepts, explain principles with
clarity, and connect theory with practical application. The language is
student-friendly, the structure is logical, and the explanations are supported
by examples that make the subject both accessible and engaging.
This book does more than prepare students for
examinations—it builds their conceptual foundation, encourages analytical
thinking, and nurtures confidence to engage with advanced materials and literature
in the future. For teachers, it serves as a reliable companion to classroom
instruction; for students, it is both a guide and a source of inspiration.
I commend the author for this valuable academic
effort and strongly recommend this book to all students of engineering who wish
to strengthen their understanding of Material Technology. It is a welcome
addition to the teaching-learning ecosystem and will surely make a lasting
impact.
Dr. V. G.
Naidu, PhD
(Scholar & Academic Mentor)
Date: 13th
Oct. 2025
During my tenure as Academic Advisor at the
Central Institute of Tool Design (CITD), I had the opportunity to teach the
subject Material Technology – II to engineering students. While teaching, I
observed that most reference books available in this field were either too
detailed or highly technical, often making it difficult for students to grasp
the core concepts with clarity.
This experience inspired me to prepare a book
that presents the subject in a simplified and structured manner. The aim has
been to explain complex topics in a concise way, supported by clear
explanations, illustrations, and practical insights. I have tried to bridge the
gap between voluminous textbooks and the essential requirements of students,
making the learning process easier and more effective.
It is my hope that this book will serve as a
useful guide not only for students preparing for examinations but also for
those who wish to strengthen their understanding of material technology
concepts for practical applications in the field of engineering and
manufacturing.
I welcome constructive suggestions and feedback
from readers, which will help me further improve this work in future editions.
Pala Venu
Gopal
Date: 20th Sept. 2025
I would like to express my heartfelt gratitude
to all my teachers and mentors for their guidance, encouragement, and
invaluable support throughout my academic and professional journey. Their
wisdom, patience, and inspiration have been instrumental in shaping my
knowledge, skills, and outlook, and this work would not have been possible
without their mentorship.
Material Technology is a cornerstone subject
for engineering students—particularly in disciplines that deal with manufacturing,
design, and applied mechanics. Yet, when I began teaching Material Technology –
II at the Central Institute of Tool Design (CITD) as Academic Advisor, I soon
realized that many students struggled—not for lack of effort, but because the
available textbooks, though rich in information, were often overwhelming. The
concepts were buried under layers of theory, the illustrations too generic, and
the connections to practical applications too few.
This book is born of those real classroom
moments: the questions raised in labs, the discussions during tutorials, and
the struggle to apply theoretical knowledge to real-world problems. My goal has
been to distil the essential ideas, to present them in a way that is both
rigorous and accessible, and to show not just what Material Technology is, but
why it matters in the daily work of engineers, designers, craftsmen, and
technologists.
You will find that this book emphasizes
clarity. Complex topics are introduced with motivation—why they are needed,
where they are used—followed by simplified explanations, helpful diagrams, and
examples drawn from actual practice. The intention is to build confidence, so
that once you have mastered the basics, you can engage with more advanced
texts, research articles, and industrial challenges with ease.
As you read through, I hope you gain more than
just knowledge—you will also develop a sense of curiosity: What are various
materials useful in Mechanical Engineering? What are their properties and
applications? And how can you, as a future designer, engineer, or technologist,
make informed material selections that balance cost, strength, durability, and
sustainability?
This book is for you who want to understand—not
simply memorize; to apply—not just pass; to explore—not just be told. Let’s
begin the journey through the world of materials: their structure, their
behavior, and their endless possibilities.
1
Plastics
1.1
Introduction
Plastics
Plastics are a group of materials which, when
heated, can be formed into a variety of useful articles by molding, casting or
extrusion. E.g. chair, domestic appliances etc.
Monomers and Polymers
The basic structural units of plastics are
referred to as monomers. These are the molecules consisting of carbon atoms
with attached ribs of other atoms. Such as hydrogen, florins, chlorine. These
monomers are joined end to end to produce long chains of molecules known as
polymers
1.1.1 Classification of Plastics:
By virtue of thermal characteristics, the plastics
are divided into two types.
These are: 1) Thermo plastics 2) Thermo
settings
Figure: Classification
of Plastics
Thermoplastics: These soften on heating and can be
reshaped repeatedly.
Some examples are: Polyamides, Polypropylenes,
Polystyrene, PVC.
Thermosetting Plastics: These harden permanently on heating
and cannot be remolded.
Some examples are: Epoxy Resins, Amino Resins, Phenolics, Silicones.
Advantages
of plastics:
1.
Light weight
2.
Low cost
3.
Strength
4.
Moisture resistance
5.
chemical resistance
6.
Toughness
7.
Appearance
8.
Thermal &
Electric insulators
9.
Formability
10.
Machinability
1.2 Thermo plastics
Thermoplastics are linear polymers, the
molecules of which are synthesized in the shape of long threads. They undergo
no chemical change in the mould operation and they soften with the application
of heat and harden upon cooling.
1.2.1 General properties of Thermoplastics:
1.
They can be
reshaped while in softened state and they will reharden so we can use any scrap
material to make thermoplastics
2.
They may become too
soft to use at temperatures from 60 C to 315 C.
3.
As the temperature
increases, they become softer. Hence, they are liable to permanent deformation
(Under mechanical strain at low temperatures)
4. They may flow to an appreciate extent
under load at room temperature
1.2.2 Nylon (Polyamides)
Nylon is a class of synthetic thermoplastic
polymers known as polyamides, characterized by the presence of recurring amide
(–CONH–) linkages in the molecular backbone. It is produced through
condensation polymerization of diamines and dicarboxylic acids (or lactams).
Properties of Nylon:
Nylons are strong, lightweight,
abrasion-resistant materials with good toughness, chemical resistance, and low
friction properties.
Applications: Nylons are widely used in textiles, ropes,
engineering plastics, gears, bearings, and various industrial applications.
1.2.3
Polyethylene
Polyethylene is the simplest and most widely
used synthetic thermoplastic polymer, consisting of long chains of repeating
–CH₂– units. It is produced by the polymerization of ethylene (C₂H₄) gas.
Polyethylene is lightweight, chemically resistant, flexible, and an excellent
electrical insulator.
Types of Polyethylene:
Depending on its density and branching, it is
classified into types such as:
1. Low-Density Polyethylene (LDPE)
2. High-Density Polyethylene (HDPE)
3. Low-Density Polyethylene (LLDPE).
Applications:
Common applications of Polyethylene include
plastic bags, bottles, containers, packaging films, pipes, and household goods.
By doing blow moldings (or) injection moldings and extruding polyethylene can
be used to make bowls, plates, dishpans, paint brush handles, Flexible tubing,
bags for packaging bottles, tubes etc Polyethylene is the highest in the volume
of resin used each year
1.2.4
Polytetrafluoroethylene (PTFE):
PTFE is a synthetic fluoropolymer of
tetrafluoroethylene, consisting of a carbon backbone fully surrounded by
fluorine atoms with the repeating unit –CF₂–CF₂–
Properties of PTFE:
●
Toughness
●
Exceptional
chemical resistance
●
High thermal
stability,
●
Low coefficient of
friction, and non-stick
●
Excellent
di-electric properties
●
Good colourability
●
Very low moisture
absorption and relatively low cost
Applications:
PTFE is widely used in non-stick cookware
coatings, gaskets, seals, bearings, electrical insulation, and
chemical-resistant linings.
Polypropylene is a thermoplastic polymer made
by the polymerization of propylene (C₃H₆) monomers. It consists of repeating
units of –CH₂–CH(CH₃)– in its backbone.
Properties:
●
Polypropylene is
lightweight, tough, chemically resistant, and has a high melting point compared
to many other plastics.
●
It can be easily
molded into different shapes
●
It is an excellent
insulator
●
It is stiffer than
polyethylene
●
It can be moulded
or extruded into sheet, film
●
Unlimited
colourability
Applications:
●
Packaging
materials, ropes, containers. and textiles
●
Automobile
accelerator pedals
●
Medical devices
●
Luggage and non-woven
fabrics
●
Hospital equipment
1.2.4 Polystyrene:
Polystyrene is a synthetic thermoplastic
polymer made by the polymerization of styrene (C₆H₅–CH=CH₂) monomers. It
consists of long chains with repeating units of –CH₂–CH(C₆H₅)–, where each unit
carries a benzene ring as a side group. It is produced in different forms such
as solid plastic, expanded polystyrene (foam), and extruded sheets.
Properties:
●
Lightweight, rigid,
transparent in its pure form,
●
It is a crystal
clear colorless, tasteless plastic with a high gloss.
●
Good electrical
insulator, but it is brittle and not resistant to many solvents.
●
Unless modified it
will hold static electricity
●
It has excellent
tensile strength, but it can be used only up to temperatures of 66 °C to 90 °C
●
It is easily
produced in any form and it can be easily joined by cementing
Applications:
●
Common applications
of Polystyrene include disposable cutlery,
●
Packaging
materials, insulation boards
●
CD/DVD cases, and
laboratory ware.
●
It is used for
bottles, low-cost picnic utensils, model kits, sign, toys etc in wide choice of
colours
●
Expanded form in
slabs or beads is leading use of polystyrene
1.2.5
PVC (Poly Vinyl Chloride)
Polyvinyl chloride is a widely used synthetic
thermoplastic polymer made by the polymerization of vinyl chloride (CH₂=CHCl)
monomers. Its repeating unit is –CH₂–CHCl–, which contains a chlorine atom
attached to every alternate carbon in the backbone.
Properties
●
The presence of
chlorine gives PVC high strength, chemical resistance, flame retardancy, and
durability.
●
This is very clear
transparent plastic and can be easily colored
●
Resistant to most
chemicals
●
Very water
repellent
●
It can stand cut
door exposure
●
It is quite
abrasion resistant
●
It has low tensile
strength
●
It can be made
rigid or flexible by adding plasticizers.
Applications:
●
It can be extruded
as wire insulation, Chemical tubing and refrigerator door gaskets
●
As a coating and
fabrics of all kinds for outdoor uses (such as tents, tarpaulin type
covers)
1.3 Thermo setting plastics
Thermosetting plastics are made from chains
which have been linked together, referred to as cross-linked. These have a three-dimensional
network of molecules and will not soften when heated. They are practically
induced fireproof and usually hard and brittle, these plastics cannot be reused
1.3.1 Types of Thermosetting
1. Epoxy resins
2. Amino resins
3. Phenolics
4. Silicones
1.3.2 Epoxy Resins:
Epoxy resins are a class of thermosetting
polymers widely used in engineering and industry. They are formed by the
reaction of epichlorohydrin with bisphenol-A (or similar polyols) and then
cured (hardened) with a hardener such as amines, anhydrides, or phenols.
Properties:
1. They have
excellent resistance
2. Good electrical insulating properties
3. Their working temperature ranges from 150 C to 160 C with fillers and
additions
4. Coatings made from these resins combine the properties of toughness,
flexibility, adhesion and chemical resistance to a degree not found in other
coating materials
Applications:
1. Aircrafts,
Automobiles and Home appliances
2. These are also used as castings for pipe fill electrical and other equipments
3. Used in laminations such as printed circuit board, boat bodies etc
1.3.3 Amino Resins:
Amino resins are a class of thermosetting
polymers formed by the reaction of ammonia or amines with aldehydes (commonly
formaldehyde). They are widely used in coatings, adhesives, and molding
compounds.
Properties:
1. These are divided into two types. They are urea and melamine
formaldehyde resins
2. Less water resistant but better electrical insulator
Applications:
1. Both of
these are used as adhesives in making plywood
2. Melamine is laminated with cloth to make and counter taps
3. Melamine can be moulded into very hard scratch resistant dinnerware,
business m/c housing, electrical switch Cover plates, radio cabinets etc.
1.3.4 Phenolics:
Phenolic resins are thermosetting polymers
formed by the reaction of phenol with formaldehyde. They were the first
synthetic polymers to be widely used and are known for their heat resistance,
strength, and chemical stability.
Properties:
1. The
phenolics resins are made by combining phenol and formal dehyde in the presence
of catalysts
2. These are hard, brittle and heat resistant, thermally known as Bakelite in
the market
3. These are inexpensive
4. Excellent insulators
5. Their heat distortion temperatures are up to 180 °C
6. Working temperatures up to 160 °C
Applications:
1. They may be
molded or casted into any shape
2. These are used as coatings and in adhesive applications
1.3.5 Silicones:
Silicones, also called polysiloxanes, are a
class of synthetic polymers with a backbone of alternating silicon and oxygen
atoms (Si–O–Si), often with organic side groups attached to the silicon. They
are distinct from traditional carbon-based polymers.
Properties:
1.
These are chemical
Hybrid of Organic and inorganic materials
2.
They have
combination of many properties such as strength, toughness and electrical
resistance etc
3.
These can be
processed by extrusion, transfer molding etc
Applications:
1. Used in
large group of industrial products like greases, oils, adhesives, resins and
runner components
2. Special silicones fluids are used as an ingredients in cosmetics
3. These are also used to make furniture, auto, shoe, glass and silver polishes
1.3.6 Elastomers
The elastomers are polymers which are less
tightly bonded together. They have the unique property of high elasticity. They
can be stretched to 10 times their original length on loading in tension and
reverting back to their original dimensions on relaxing of load
Properties of Elastomers
1).
They are non-crystalline polymers at room temperature
2) They are intermediate between long chain of molecules and
three-dimensional networks
Examples of Elastomers:
1. Natural rubber which
is made of latex which is viscous milky fluid which contains a linear polymer
of polyisoprene
2. Other examples are silicon, urethane and chlorinated polyethylene
Applications:
Gaskets, moulds, foam, mattresses and
insulations
1.4 Materials for processing plastics
Most plastic resins have to be combined
which are otherwise chemically treated with processing material before they are
ready for processing. It is important to mix the ingredients thoroughly for all
classes of polymers and special equipments are developed for the purpose
1.4.1 Plasticizers
There are substances which act as internal
lubricants and improve the flow of material. They give toughness and
flexibility to the material. Organic solvents, resins and even water are used
as plasticizers. Plasticizers are also used to prevent crystal by keeping
chains separated from another,
For example: Vinyles are generally hard and
brittle materials. By adding a plasticizer, they can be soft and flexible
1.4.2 Fillers
These are the materials which are added in high
proportions to many plastics. Essentially to get strength, dimensional
stability and heat resistance. Examples of fillers which include wood, flour,
asbestos fibre, glass fibre, mica, slate powders etc
1.4.3 Catalyst
These are usually added to promote faster and
more complete polymerization. These Catalysts are also called accelerators or
hardeners
1.4.4 Initiators
As the name indicates the initiators are used
to initiate the reaction to allow polymerization to begin. They stabilize the
ends or reaction sites of the molecular chains. H2D2 is a common initiator
1.4.5 Dyes and Pigments
Pigments and dyes are added to plastics to give
brilliant colours. The coloring is important as it provides sales appeal. The
colorants must be able to disperse evenly throughout the molten plastics and
must have heat stability.
1.4.6 Blowing Agents
A plastic resin such as polystyrene is formed
by injecting an inert gas (Nitrogen/ Argon) before the molten material is
forced into the mould. The process creates porous interiors. The familiar
disposable cups are examples of blowing agents
1.4.7 Modifiers
The modifiers are added to improve mechanical
properties / characteristics of the base resin
1.4.8 Antioxidants
Antioxidants are added to plastics to provide
resistance to ultraviolet rays. They also impart melt-flow retention for easy
molding
2. Composite Materials
2.1 Introduction
Sometimes two or more materials are combined to
produce a new material that possesses superior properties compared to any one
of the constituent materials. Such a material is known as a composite material.
A common example of a natural composite material is wood, which consists of
long cellulose fibres held together by amorphous lignin. Some of the artificial
(synthetic) composite materials include cement concrete, glass-reinforced
plastic (GRP), and plywood.
2.2 Types of Composite Materials
The composite materials, which are important
from the subject point of view. following three types:
1. Agglomerated materials 2. Laminated
materials 3. Reinforced materials.
Now we shall discuss all the above-mentioned
types of composite materials in detail
2.3 Agglomerated Materials
The materials, in which the particles are
condensed together to form an integral known as agglomerated materials. A
common example of agglomerated materials is concrete, which is formed by mixing
coarse aggregate, fine aggregate, cement and different proportions. Another
useful agglomerated material is grinding wheel (abrasive which is formed by
mixing asphalt, stone and resin of different sizes. Some more agglomerated
materials are cemented carbide tools and ceramics. The cemented carbide is
manufactured by agglomerating small particles of hard tungsten carbide with
alloys of nickel. These alloys act as a binder between the tungsten carbide
particles. The ice is produced by combining a ceramic material and metal
through various processes. In this area of ceramic material and metal is taken
in the form of powder, which is mixed, pressed and sintered to produce a
composite material. In cermets, the metal provides high s and thermal shock
resistance. The ceramic materials provide high refractoriness and resistance.
They have high abrasion resistance, excellent chemical stability, good
mechanical strength, low porosity, and high thermal stability
Figure: Agglomerated Materials
2.3.1 Important Terms in Agglomerated Materials
Below are the important terms, which are
frequently used in the study of agglomerated
1. Particle size 2. Packing factor 3. Density
and porosity.
2.3.2 Particle Size
It has been observed that most of the particles
used in agglomerated materials are not perfectly spherical. As a result, it is
usually difficult to measure their exact size. In most cases, variations from a
true spherical shape are quite common. To determine the particle size of the
constituent materials, the powders are passed through standard sieves (screens),
which have a fixed number of openings per square centimeter. The mesh size of
the screen thus provides a practical measure of the particle size distribution.
It should be noted, from a practical point of
view, that it is not sufficient to depend upon the average size of an
aggregate, since the aggregate mixture will be composed of a varying range of
particle sizes
2.3.3 Packing Factor
The term packing factor may be defined as the
ratio of true volume of the total constituent to the bulk volume of the
agglomerated materials. It has been observed that whenever rv sized material is
packed into a large mass, a considerable amount of porosity exists in the body,
whose value depends upon the size and shape of the packing material. Locally
packing factor,
2.3.4 Density and Porosity
We have already discussed in the previous
article that a considerable amount of porosity exists in the body of an
agglomerated material. It has been observed that this porosity is mainly due to
open pores, which allow the penetration of fluids and gases into the material
Figure: Laminated Materials
Laminated materials are produced by bonding two
or more layers of different materials together, and are commonly referred to as
laminates. The constituent layers may be metallic or non-metallic, depending on
the type of laminate being produced.
Common examples of laminated materials include
Plywood, Tufnol, Sunmica, and Linoleum. In such materials, the top layer
provides the desired appearance and workability, while the lower layers
contribute primarily to strength and stability.
At present, a variety of techniques are
employed to manufacture laminated composite materials. The most common methods
include rolling, co-extrusion, explosive welding, and brazing.
Though there are a number of laminated materials
these days, yet the foil important from the subject point of view:
1. Plywood 2. Tufnol.
2.4.1 Plywood
Plywood is produced by binding together an odd
number of thin layers (called plies) with a resin under pressure. These plies
are arranged so that the grain structure of alternate layers is oriented at
right angles to each other. The number of plies usually ranges from three to
nine, though in special cases it may be higher. Plywood has several advantages
over solid wood: it is less liable to warp and is equally strong in all
directions due to its cross-laminated structure
2.4.2 Tufnol
This type of material is produced by combining
a laminated structure consisting of layers of woven textile impregnated with a
thermosetting resin. In the composite material, the resin provides the required
appearance, rigidity, and chemical resistance, while the woven textile imparts
the necessary strength and toughness.
2.5 Reinforced Materials
Reinforced materials are those which are
produced by combining a suitable reinforcing medium with a base material in
order to impart additional strength and properties that do not exist in the
individual material alone. A common example is reinforced cement concrete
(R.C.C.), in which steel bars are embedded in concrete to improve its tensile
strength. Other examples include glass-reinforced plastics (GRP) and
fiber-reinforced plastics (FRP).
The materials which are produced by combining a
suitable reinforcing medium with a base material, in order to impart additional
strength and properties not present in a single material alone, are known as
reinforced materials.
Common examples include reinforced cement
concrete (R.C.C.), reinforced plastics, and other fiber-reinforced composites.
Important Reinforced Materials
Though there are a number of reinforced
materials these days, yet the foI1 important from the subject point of view:
1. Reinforced cement concrete.
2. Nylon reinforced rubber.
3. Glass-fibre reinforced plastic
2.5.1 Reinforced Cement Concrete
R.C.C. is produced by placing steel rods
(reinforcement bars) within a cement concrete mixture. The resulting composite
provides excellent resistance to both compressive and tensile stresses.
It is interesting to note that the
reinforcement steel, being strong in tension, is placed in the tensile zone,
while the concrete, being strong in compression, is utilized in the compression
zone. In cases where the bending moment is very high, reinforcement is provided
in both the tensile and compression zones to improve strength and durability.
These days, torsteel (i.e., twisted steel rods)
is commonly used as reinforcement in all types of R.C.C. structures. In
addition, thick steel wires are employed as high-tensile steel in the
construction of long-span bridges.
2.5.2 Nylon Reinforced Rubber
These composites are commonly used in the
manufacture of automobile and bicycle tyres. In such materials, nylon threads
(and in some cases, steel wires or glass fibres) provide the necessary strength
and reinforcement, while the rubber serves as the outer surface material,
offering flexibility, grip, and wear resistance.
2.5.3 Glass-Fibre Reinforced Plastic
Glass Fibre Reinforced Plastic is produced by
combining glass fibre with plastic. In this composite material, the glass
provides the necessary strength and the plastic reduces brittleness. The
commonly used polyester resin, phenolics, silicons etc. The fibres can be
employed either in the form of lengths, staples or whiskers. Complete material
with desired yield strength and modulus of elasticity can be obtained by the
maximum number of fibres per unit volume. This will allow each fibre to take
its load. The fibre-reinforced composites are generally anisotropic (i.e., the
material has properties in different directions). Moreover, the maximum
strength of the composite is in the direction of alignment of fibres.
Figure: Glass Fibre Reinforced Plastic
Following three conditions are essential to prepare a glass-fibre
reinforced plastic:
1. The coefficient of expansion of the
fibre should match closely to that of the plastic material.
2. The fibre and the plastic material should be chemically compatible with each
other to avoid undesirable reactions.
3. The fibre should be stable at room temperature and it should retain its
strength at high temperatures.
3. Corrosion
of Metals
3.1
Introduction
Corrosion can be defined as the process of
deterioration of a metal at it’s surface by its surrounding environment
containing liquids and gases etc. This type of deterioration may be due to
direct chemical attack or electrochemical attack. Common examples of corrosion
are:
i) Rusting of
iron and the formation of green film
ii) The formation of green film on the surface of copper when it is exposed to
moist air containing carbon dioxide
3.2
Classification of corrosion
The corrosion may be classified into the
following two categories.
1) Direct chemical corrosion (Dry corrosion)
2) Electrochemical corrosion (Wet corrosion)
3.2.1 Direct chemical
corrosion (Dry corrosion)
The corrosion which involves direct combination
between metals and dry gases which results in the formation of scale. Chemical
reactions of dry chlorine, Hydrogen sulphide, oxygen etc. with dry metal are
few examples. Generally, oxygen is the most commonly encountered reacting gas
due to which direct oxidation takes place at high temperature. However, in case
of alkaline earth metals, the oxidation takes place at low temperature.
The dry corrosion may also take place due to
the chemical action of flowing liquid metal at high temperatures. On the solid
metal or alloy this corrosion is known as liquid-metal corrosion. E.g. devices
used in nuclear power plants. The direct chemical corrosion of a metal surface
may also take place in the presence of liquids instead of gases. These liquids
may be acidic or alkaline.
See below figure for constituents and process
of Dry Corrosion.
Figure: Dry corrosion
3.2.2 Electrochemical corrosion (Wet corrosion)
This type of corrosion is caused due to the
flow of electric current between two dissimilar metals. This type of corrosion
takes place at or near the room temperature because of the reaction of metals
with water or aqueous solution of acids and bases.
In this type of corrosion two principal types
of reactions take place one at the cathode and other at anode. These are called
oxidation reactions and cathodic reactions respectively. The electrons which
are produced by the anodic reaction flow through the metal are used up in the
cathodic reaction.
Figure: Wet Corrosion
3.3
Important Types of Corrosion
3.3.1 Uniform Corrosion
It is a type of corrosion which occurs where
metal or alloy is completely homogeneous of the same nature both chemically as
well as mechanically). As a result of this, the gap is established between any
two points (on the metal surface) in the presence of acids. It may be noted
that it is a type of corrosion (as the name indicates) in which corrosion is
‘uniform’.
Figure: Uniform
Corrosion
 |
3.3.2 Intergranular Corrosion
It is a type of corrosion that occurs along the grain boundaries of a metal.
Intergranular corrosion generally takes place in the grain boundary regions,
which exhibit a solution potential more anodic than that of the grain centers
in the corroding medium. This potential difference may arise either due to
differences in crystallographic orientations or because of the precipitation of
phases at the grain boundaries.
 |
Figure:
Intergranular Corrosion
It has been observed that intergranular
corrosion commonly begins at the grain boundaries, then rapidly progresses
inward, causing significant damage to the internal structure of the material.
3.3.3 Pitting corrosion
Pitting corrosion is a type of localized corrosion
that occurs at a particular point, such as a pinhole, forming a small cavity on
the metal surface. The pit penetrates deeply into the metal, while the
corrosion products are often expelled to the surface, as illustrated in the
figure. It has been observed that pitting corrosion occurs more commonly in
tubes, pipes, and vessels due to the breakdown or cracking of the protective
film on the metal surface. This typically happens as a result of mechanical
factors arising from the turbulent flow of the solution over the metal surface.
Various Mechanical factors causing pitting
corrosion include:
1. Surface
roughness or non-uniform finish
2. Scratches or cut edges
3. Stress corrosion cracking
4. Alternating stresses
5. Sliding under load
It has also been observed that stainless steel
and aluminum exhibit characteristic pitting in halide solutions (such as
chloride or bromide solutions). Oxygen concentration cells can initiate
pitting. Additionally, plating out of a noble metal (a metal that does not react
easily with others, such as gold or platinum) from a salt solution may result
in pitting due to the formation of localized galvanic cells.
 |
Figure: Pitting
corrosion
Strictly speaking, pitting corrosion results in
the breakdown of the metal at specific localized points, giving rise to small
anodic areas and large cathodic areas. In such environments, this produces a
high corrosion current, leading to a rapid rate of corrosion.
Generally, pitting corrosion does not
significantly affect the mechanical properties of the metal, although it may
slightly increase its apparent strength due to localized work hardening.
3.3.4 Stress corrosion
This type of corrosion occurs in engineering
components exposed to corrosive environments due to internally developed stresses.
Stress corrosion failures typically involve high residual stresses approaching
the yield strength of the material. These stresses, often tensile in nature,
may result from precipitation and phase transformations, uneven cooling, cold
working, or welding.
The magnitude of stress required to cause the
Stress Corrosion Cracking (SCC), depends upon the nature of corrosive
environment, microstructure and geometry of the metal specimen. Pure metals do
not get stress corrosion cracking easily. The prevention of stress corrosion
cracking is generally done by the removal of tensile stresses or removal of
corrosive medium.
3.3.5 Season corrosion
This type of corrosion occurs in brass
especially in the presence of moisture and traces of ammonia. Small amounts of
alloying elements such as zinc, aluminum, silicon, antimony, arsenic and
phosphorus result in the possibility of Intergranular cracks. Examples of
season corrosion are 1) The zinc and copper in ammonia solutions form stable
complex ions that result in the formation of cracks when exposed to high
tensile stresses. Another example of season cracking is caustic embrittlement
of steel due to corrosion caused by exposing the metal to a solution containing
sodium hydroxide as the Intergranular cracks formed from one rivet to another.
3.3.6 Crevice corrosion:
The Crevice corrosion is formed due to the
different geometry of the component with openings or cracks called crevices.
This type of corrosion is localized in the crevices and shielded areas on the
metal surfaces. Some examples of crevice corrosion include small volumes of
stagnant corrosion caused by holed gaskets, surface deposits, Lap joints etc.
3.3.7 Fatigue corrosions
Fatigue corrosion is corrosion caused by
fatigue of materials. It refers to the fatigue fracture of a metal aggravated
by a corrosive environment or stress corrosion cracking (SCC) of a metal
aggravated by cyclic stress. Fatigue corrosion is similar to SCC, except that
the stresses are cyclic and it can occur in any environment. Due to fatigue
corrosion the ability of a metal is reduced to withstand repeated stress when
exposed to the combined action of stress and a corrosive environment as
compared to the effects of stress alone. Fatigue corrosion is caused by crack
development under the simultaneous action of corrosion and cyclic stress. It
causes a fracture surface similar to ordinary fatigue except that in some cases
corrosion products are present in the outer sections of the cracks.
Some examples of Fatigue corrosion are heavy equipment;
the metal panels used in construction and shipbuilding. Fatigue corrosion is
dependent on factors such as Loading, Environmental, Metallurgical; Fatigue
corrosion can be prevented through reducing/ minimizing vibration and pressure
fluctuation and by using high-performance alloys resistant to corrosion fatigue
and by using coatings and inhibitors to delay the initiation of corrosion fatigue
cracks. In fatigue corrosion, low-cycle stresses are more damaging than
high-frequency stresses.
3.3.8 Erosion corrosion
This type of corrosion is caused by the
combined effect of erosion by the turbulent flow of gases or liquids and the
rubbing of solids over metal surfaces. This type of corrosion occurs due to the
breakdown of a protective film at the place of impingement and it’s subsequent
and loses it’s ability to repair itself under the abrasion conditions. Due to
these galvanic cells are formed in such areas and leads to corrosion. This type
of corrosion occurs generally in piping agitators, condenser tubes and in such
vessels as the steam of liquid or gases emerges from an opening and strikes the
side wall at very high velocities.
Figure: Erosion corrosion
 |
3.3.9 Atmospheric corrosion
Atmospheric corrosion is the deterioration and
destruction of a material and its important properties due to electrochemical
as well as the other reactions of its surface with the constituents of the
atmosphere surrounding the material. Different atmospheric substances cause
corrosion and erosion of metals and nonmetals. Earth’s natural environment of
oxygen and condensed water vapor is enough to start gradual corrosion of iron
and steel surfaces, producing iron oxide, which is known as rust. Corrosion
alters the micro structure and drastically reduces the mechanical strength and
useful life of the metals. Salts of sulfur and chlorine can aggravate corrosion
by forming electrolytes in industrial atmospheres. Ambient temperature and air
pressure also affect corrosion. Some electrolytes become highly reactive at
higher temperatures. Every metal has a specific humidity limit (Critical
humidity) beyond which the corrosion becomes unavoidable.
3.3.10 Fretting corrosion
This type of corrosion occurs in situations where
there is slight relative movement of contacting surfaces due to the action of
an alternating load, the fretting corrosion occurs more often in bolted joins
and other assemblies. Due to localized flow of metal cold working, the welded
joints get ruptured and loose metal particles are formed subsequently
oxidization occurs in presence of oxygen due to braking of protective Filenes
leading to more oxidation. Due to this localized pitting occurs which becomes
the source of fatigue cracks.
Localized plastic flow + Cold working below
effects are found.
1.
Rupture of welded
joints
2.
Oxidation
3.
Localized pitting
4.
Fatigue cracks
3.3.11 Selective corrosion
In this type of corrosion, it involves the
selective dissolution of one of the elements in a single-phase alloy or one of
the phases in a multiphase alloy. This type of corrosion is also called
"dealloying" or "selective leaching", The most well-known
example is the dezincification of brass (e.g. 70Cu – 30Zn). In this case, as
the zinc is removed a red coppery type of traces are formed on the brass. It
also becomes porous and very brittle, without modification to the overall
dimensions of the part. This problem can be overcome by using an alloy such as
a copper-rich cupro-nickel. Brasses with lower zinc contents or containing
elements such as tin (1%), small quantities of arsenic, antimony, or phosphorus
have much greater resistance.
Some common examples of selective corrosion
include de-nickelization, which occurs in Cu–Ni alloys; de-aluminization,
observed in aluminum bronzes; and the graphitization phenomenon in grey cast
irons.
3.4 Prevention and Control of Corrosion
Below are ways by which we can prevent
corrosion and control its growth
3.4.1 Suitable design & Fabrication procedure
The corrosion may be prevented or minimized by
adopting a suitable design and manufacturing procedure for a particular shape
of the component while selecting the material below points must be kept in
view, such as, use of dissimilar metal contacts to prevent galvanic corrosion,
following a design procedure to avoid the formation of stagnant areas using
welded joints instead of the riveted joints to prevent corrosion.
3.4.2 Use of inhibitors
An inhibitor is a material which, when added to
an electrolyte in small quantities, reduces the rate of corrosion. Both organic
and inorganic inhibitors can be used. However, an effective inhibitor should be
soluble in the corroding medium and capable of forming a protective layer
at either the anodic or cathodic areas of the metal surface.
Some examples of inhibitors include:
●
Anodic inhibitors:
chromates, phosphates
●
Cathodic
inhibitors: magnesium salts, calcium salts
Inhibitors are commonly used to prevent
corrosion in radiators, steam boilers, and other containers exposed to
corrosive fluids.
3.4.3 Modification of the corrosive environment
The rate of corrosion can be reduced to a large
extent by making small changes in the environment which is causing corrosion. A
small decrease in temperature causes an appreciable decrease in the rate of
corrosion. Also, by removing dissolved gases the rate of corrosion can be
reduced.
3.4.4 Use of protective coating
By applying a protective coating on the base
metal, the corrosion can be prevented or reduced. Both metallic and
non-metallic coatings can be applied. Metallic coatings can be applied by
processes such as electroplating, Dipping, Spraying, Cladding and cementation
Non-Metallic coatings such as paints and lacquering plastic coatings, vitreous
coatings, oxide coatings, chemical dip coatings.
3.4.4.1 Electroplating:
It is a process of depositing a very thin layer
of metal coating by passing DC current through an electrolyte solution
containing some salt of coating metal. It is one of the best methods for the
commercial production of metallic coating. In this process the component of
base metal is made to act as cathode whereas the coating metal as anode in a
solution of electrolyte. When a direct current is passed for a known time to
obtain the coating of desired thickness, the electroplating happens. The
commonly used metals for protective coating of desired thickness, the
electroplating happens. The commonly used metals for protective coating are
nickel, zinc, silver, copper, chromium, cadmium, gold, tungsten etc.,
 |
Figure: Electroplating
In the dipping process the metal piece which needs to
be protected from corrosion is dipped in a liquid solution of a corrosion
resistant substance. Galvanization is a widely used industrial procedure for
rust removal. The first step is to dip the steel in molten zinc, which protects
it from corrosion (The corrosion resistance properties of zinc are greater than
that of iron or steel). Zinc reacts with oxygen to form zinc oxide, which again
reacts with water molecules in air to form zinc hydroxide. Zinc hydroxide, in
turn, reacts with carbon dioxide to form an impermeable, insoluble layer of
zinc carbonate, which adheres well to the underlying zinc thus protecting it
from further corrosion. In this process, zinc acts as the sacrificial anode and
it cathodically protects the exposed steel. Hence, even if the coating is
scratched or worn out, the exposed steel will be protected from corrosion by
the remaining zinc. This is the advantage that galvanizing has over other
methods like enamel, powder coating or paint. Also, galvanizing is cost
effective and has a long low maintenance service life. So, it is regarded as
one of the most efficient methods to stop rust on metal. The resulting coating
is durable, tough, abrasion resistant, and provides cathodic (sacrificial)
protection to any small damaged areas where the steel substrate is exposed.
3.4.4.3 Spraying
In the spraying process the coating material is
fed in the hot gas stream as powder or wire or rod. Coatings are built by the
flattening and solidification of droplets impacting onto the part to be
covered. Thermal spraying involves the projection of small molten particles
onto a prepared surface where they adhere and form a continuous coating. To
create the molten particles, a heat source, a spray material and an
atomization/projection method are required. Upon contact, the particles flatten
onto the surface, freeze and mechanically bond, firstly onto the roughened
substrate and then onto each other as the coating thickness is increased.
Thermal spray coatings are widely used in preventing corrosion of many
materials, along with additional benefits of properties such as wear resistance
etc. due to the very wide selection of coatings that can be sprayed. Broadly,
thermal spray coatings fall into three main groups:
●
Anodic Coatings
●
Cathodic Coatings
●
Neutral Coatings
3.4.5 Cladding
The process of cladding involves fusing of
corrosion-resistant metal alloys with the base metal (ferrous metal or alloy)
to the facade of walls and roofing systems. This process aims at improving the
look and feel value and the durability of the underlying metal that is bound to
get corrosion. Material used for making the cladding alloys come from the
following metals such as Aluminum and its alloys, Steel and its alloys, Zinc
and its alloys, Lead and its alloys and Copper and its alloys The cladding material
acts as a barrier between the substrate (metal to be protected) and the
corrosive environment thereby improves durability and protects the appearance.
Hence the cladding acts as a sacrificial protection against galvanic corrosion
that occurs on unprotected points on a metal surface. The final appearance may
vary in color and finish by coating of the suitable cladding materials.
3.4.6 Use of cathodic protection
In this technique the corrosion is controlled
by making the surface of the metal as cathodic with respect to some other metal
which acts as anode. In this technique there are two methods to control the
corrosion. These are 1) Sacrificial anode method 2) External voltage method
Alloying of metals: The corrosion resistance of some metals may be increased
by alloying them with suitable constituents. These alloying constituents are
capable of thoroughly mixing up with the parent metal. The resulting alloys
will have adequate mechanical properties and corrosion resistance
3.4.7 Heat treatment of metals
Heat treatment is a process of combination of
heating and cooling of a metal or alloy in the solid state for the purpose of
obtaining desired properties. The corrosion resistance of the metal increases
by heat treatment processes. Some examples of heat treatment processes are:
Annealing, Normalizing, Hardening, Tempering, Case hardening, Surface
hardening, Diffusion coatings etc.
4. Powder Metallurgy
4.1 Introduction
The powder metallurgy is a process of
preparation and processing of powdered iron and nonferrous metals. The powder
metallurgy is being used to produce refractory metals, cutting tools of high
hardness, porous self-lubricating bearing etc.
4.2 Process description
Figure: Powder Metallurgy Process
4.3 Manufacturing of Metal Powders
The powders are made of any metal and alloys.
These powders are generally copper base and iron base materials, including
metal powders of stainless steel, titanium, nickel, Chromium etc. General
properties of powders are composition, size, form and structure of particle,
specific surface, porosity, fluidity, strength, hardness, permeability,
regarding liquids & gases, electric conductivity, compressibility and
sinterability. The particle size of powders is in the range of 0.1Mue to
several millimeters (1Mue =10-6 mm)
Following methods are used to produce metal powders
4.3.1 Mechanical
In this method the required size of powders is
made by disintegrating the metal by crushing, rolling and milling. In the end
the crushed metal is ground in ball mill in which many steel balls impinge upon
the powder to grind it to the required size
4.3.2 Atomization
In the atomization process the metal in its
molten state is forced through a nozzle into a stream of water or air with a
pressure of about 2 to 3 x 10 -2 kgf/mm2.
Once the molten metal comes in contact with a
stream, it gets solidified in particles due to air pressure, nozzle size and
metal flow rate.
4.3.3 Reduction
In this process the pulverized metal oxide e.g.
Tungsten oxide is heated in a current of hydrogen to produce a fine powder
…i.e. tungsten powder. Similarly Iron chloride is reduced in hydrogen to get
Iron powder
4.3.4 Electrolysis
In this process two metal plates one acting as
anode and another as Cathode are placed into the tank which contains
electrolyte which is an acid solution. A powdery like deposit is produced on
the cathode due to high electric current. These cathodes are removed from the
tank, then rinsed to remove the electrolyte solution and then dried. The
deposit is scrapped off and pulverized to produce the powder of the desired
size.
4.3.5 Shotting
In this method the molten metal is made to pass
through as sieve and then the particles are dropped in a liquid (Water or
Kerosene)
4.3.6 Blending of powders
The blending or mixing of powders is the first
step in the forming of powder metal parts. The blending of many different
metals and of metals with ceramics or other materials gives the characteristics
of heat resistance, frictional properties, heavy weight and hardness that are
not possible to get by other methods. The mechanical mixer is used to blend the
powder and the mixer type will depend on the amount of powder handled and the
type of powder.
4.3.7 Compacting
Compacting is the process of converting loose
powder into a "green compact" of accurately defined size and shape.
 |
Figure:
Compacting
The compacting stage consists of two sub-stages. These are (i)
Filling (ii) Pressing
4.3.8 Filling
This process consists of pouring the metal powder in the cavity of
a die setup press. The die consists of a cavity (The shape of the desired part
which is tow to ten times deeper, based on the type of material to be handled.
The metal powder is poured in the cavity and the excess metal powder is removed
from the top of the die to complete the filling process.
4.3.9 Pressing
In order to distribute uniformly, the pressure
applied there are two dies used in the pressing process i.e. top and bottom
dies. These two dies are forced together under pressure into the die cavity and
the powder is compressed to the desired shape. The final volume after
compression is about 30% of the original volume.
The process used for compacting may be either
Mechanical or Hydraulic or the combination of both. The pressure used ranges
from 100 to 1000 MN/m2
4.3.10 Pre-sintering
Once the pressing is completed, the component
is ejected out of the press. It is then subjected to pre-sintering.
Pre-sintering is the process of heating the green compact to a temperature
below the sintering temperature.
Pre-sintering is done to increase the strength
of the compact by removing the lubricants and binders added during blending.
Pre-sintering is usually done to metals which become hard after they are
sintered. Hence any machinery can be finished after pre-sintering and before
sintering.
4.3.11 Sintering
Sintering is a process of heating the compacted
finished work piece in a furnace to a temperature close to the melting point of
the basic metal of the mixture. The process of sintering is carried out in a
controlled atmosphere furnaces or it may also be carried out under protective
gas normally hydrogen or in a vacuum if the material tends to react with the
protective gas. Due to the heating a proportion of metal particles partly melts
and cements the remaining particles together in a cellular structure.
Figure: Sintering
Sintering time should be such that the work
piece obtains the required properties. Sintering is performed to get all
possible final strength and hardness needed in the finished product by
controlling three most important variables that governing the sintering process
which are temperature, time and sintering environment. Generally bronze tends
to expand and iron and brass tend to contract.
4.4
Secondary operations
There are many secondary operations possible
after the product is successfully made using the powder metallurgical process.
Below are some of these secondary operations and their purpose.
4.4.1 Sizing: This operation is done to make the dimensions
of the sintered product accurate, improve surface finish, and ensure better
interchangeability of parts. The sizing operation is done by using i) Punch +
Die setup ii) Coining iii) Broaching
4.4.2 Machining
4.4.3 Impregnated or saturated oil
4.4.4 Infiltration: This process is done to increase the
Strength, Hardness and density which cannot be obtained by straight sintering
4.4.5 Plating: Plating is done to give a pleasing appearance
for the component and give the necessary protection from corrosion
4.4.6 Heat Treatment
4.5
Products of Powder Metallurgy
Powder
metallurgy can be used in manufacturing of the following.
● High production parts such
as gears, Cam, pawls activating levers
Parts of Cars, Aircraft, Gas turbines
● Home appliances parts like
Clocks, Sewing machine refrigerators and vacuum cleaners
● Parts of guns
● Porous metal bearings and
porous metal appliances of filters
● Gas diffusers, Welding rod containing
a powdered flux
● Diamond impregnated wheels,
communicator segments, contacts
● Electrical engineering
articles filament of electric bulbs radio valves, X-ray tubes etc
● Magnetic materials and
articles
● Bi-metallic strips
● Manufacturing of hard carbide
alloys for cutting tools and drawing dies and various composites
4.6
Advantages of the powder metallurgy process
1.
No loss of
material, clean and bright products which are ready for use
2.
Composition of the
product can be controlled. No risk of contamination with any other substance
3.
With help of sizing
operation, close dimensional tolerances can be maintained
4.
Non-metallic
substances can be added as required in any proportion to get the properties
needed
5.
The properties like
density, porosity, particle size etc. can be obtained/ controlled, with
variations for particular applications
6.
Material can be
combined if alloying is not possible
7.
The Powder
metallurgy process can be used for production of magnetic cores having special
desirable properties
8.
Reduction in
production time and High production rate can be achieved by adoption of the
process as it is an automated process
9.
Highly skilled
labour is not required
10. Savings in materials as there is no wastage
during fabrication
11.
Composition,
structure and properties can be controlled more easily and closely than any
other fabricating process
4.7 Disadvantages and Limitations of the
process
1.
Pure metal powders
are very expensive
2.
The size of parts
produced is limited due to increased tool and press costs.
3.
The lack of simple
methods of obtaining alloy powders of steels, bronzes, brasses etc.
4.
Strength products
produced is lower when compared with products produced by conventional means
5.
Increased chances
of oxidation throughout the whole body as a result of their porosity
6.
Powder metallurgy
products have poor plastic properties (Impact strength, elongation)
7.
Type of shapes
produced are very limited
Not practically economical if very few products
are made unless at least few thousand pieces are manufactured
5. Semiconductors & Insulators
5.1
Introduction
Based on Band Theory, solids can be broadly
classified into three categories depending on the arrangement of their valence
band and conduction band:
1.
Conductors
2.
Semiconductors
3.
Insulators
5.2 Conductors
In conductors, the valence
band and conduction band either overlap or the conduction band is partially
filled. This allows electrons to move freely, resulting in high electrical
conductivity. Metals such as copper, silver, and aluminum fall into this category.
5.3
Semiconductors
Semiconductors are those materials whose
electrical conductivity is greater than insulators and lesser than conductors.
At normal temperatures there is no electron in the conduction band of a
semiconductor material whereas the valence band is completely filled. However,
when the temperature is increased some of the covalent bonds break. Due to
this, some electrons jump from the valence band into the conduction band. This
results in the flow of current (conductivity) through the semiconductor. Hence
the electrical conductivity of a semiconductor increases with the increase in
temperature. The electrical conductivity in semiconductor material lies between
conductors and insulators. Some common examples of semiconductors are
germanium, silicon, indium, antimonite, gallium, arsenide etc. These materials
are widely used in the fabrication of electronic devices such as diodes,
transistors, silicon-controlled rectifiers, photocells etc.
 |
5.3.1 Classification of
Semiconductors
The semiconductors are mainly classified as:
i)
Intrinsic
Semiconductors
ii)
Extrinsic
semiconductors
5.3.2 Intrinsic
semiconductors:
A semiconductor which is made of extremely pure
form is known as intrinsic semiconductors. Some examples of the intrinsic
semiconductors are Germanium and Silicon
5.3.3 Extrinsic
semiconductors:
An intrinsic semiconductor, having an extremely small amount of (1
part in 108 parts) of suitable impurity is known as extrinsic
semiconductors. Some examples of such impurities are: Gallium, Boron, Indium,
Arsenic, Antimony, Phosphorous etc. The process of adding impurities is known
as ‘doping’ and the impurities as doping agents.
Based on the type of impurity of the doping agent used, the extrinsic
semiconductors are further divided into the following two types. These are
i) N-Type semiconductors and ii) P-Type semiconductors
5.3.3.1 N-Type
semiconductors
Pentavalent elements such as Phosphorus(P),
Arsenic (As) or Antimony (Sb) have five electrons in their outermost orbits.
When any one such impurity like P is added to the intrinsic semiconductors like
Germanium in small quantities all the four electrons form a Covalent bonding
with four neighboring pure semiconductor atoms. With a small rise in
temperature the fifth electron is released from the parent atom leaving it
positively ionized. Since these pentavalent elements donate negative charges
(electrons) they are called N-type impurities and the semiconductors doped with
pentavalent impurities are called N-type semiconductors. In addition, if the
temperature is sufficiently high electron-hole pairs are generated due to
braking of covalent bonds. Hence electrons are more in number than holes and
hence in N-Type semiconductors the electrons are majority carriers and holes
are minority carriers in N-Type semiconductors.
Figure: N-Type semiconductors
5.3.3.2 P- Type
semiconductors
Trivalent elements such as Aluminum (Al),
Gallium (Ga), Indium (In), or Boron (B) have three electrons in their outermost
shell. When a small quantity of such an impurity (for example, Indium) is added
to an intrinsic semiconductor like Silicon, three of the impurity’s electrons
form covalent bonds with three neighboring Si atoms.
However, the impurity atom requires one more
electron to complete its fourth bond. This missing electron creates a vacant
electron position, called a hole, on the semiconductor atom. This hole can be
thought of as a positive charge carrier, giving rise to the term P-Type
semiconductor.
Figure: P-Type semiconductors
 |
A large number of holes are created because the
acceptor atoms (such as In) become ionized, taking electrons from the valence
band and thus generating holes for conduction. This process is known as
ionization.
Additionally, at sufficiently high
temperatures, electron-hole pairs are generated due to the breaking of covalent
bonds. Hence holes are more in number than electrons, therefore in P-Type
semiconductors, holes are the majority carriers, while electrons are the
minority carriers.
5.3.4 Applications of Semiconductors
The following are some important semiconductor
devices:
- Semiconductor diodes: These are basically a P-N
Junction. They are used to convert alternating current (AC) into direct
current (DC) - Zener
diodes: These are
used to break the circuits and also the voltage stabilizers etc - Transistors:
These are used in
amplifiers, oscillators, voltage stabilizers, circuits etc) - Silicon
controlled rectifiers:
These are used in switching and control units e.g DC shunt motors,
Detectors etc) - Thermistors: These are used to measure
temperatures - Varistors:
These are used to
measure temperatures - Integrated
Circuits: These
are used for electronic circuits - Photo-Cells:
These are used to
convert the light energy into electrical energy - Lasers: These are used in communication
engineering, fabrication of electronic devices, High accuracy distance
measurement in metal cutting machine tools etc.
5.4 Insulators
Insulators are such materials in which the
electrical conduction cannot occur. These materials are also known as
dielectric materials or dielectrics which are used to store electricity and
release the same under controlled conditions. But when dielectric materials are
used to prevent the flow of electricity through them, on the application of
potential difference, they are called insulators.
 |
Figure: Insulators
5.4.1 Applications of
Insulating materials:
1. Electrical
Wiring & Cables: Used as insulation around copper/aluminum conductors
(e.g., PVC, rubber, XLPE). Prevents leakage currents and short circuits.
2. Transformers & Motors: Insulating oils, varnishes, and solid materials
(mica, paper, epoxy) are used to insulate windings and cores. Ensures safe
operation under high voltages.
3. Capacitors: Dielectric materials (mica, ceramic, plastic films) act as
insulators between capacitor plates. Stores electrical energy effectively.
4. Electronic Devices: Insulating substrates (like silicon dioxide in ICs,
epoxy resins in PCBs) separate conducting paths. Prevents unwanted current
flow.
5. Household Applications: Thermal and electrical insulation in appliances
(refrigerators, ovens, heaters). Rubber, ceramics, and plastics used for safe
handling.
6. High-Voltage Transmission Lines: Porcelain, glass, and polymer insulators
support overhead power lines. Prevent current leakage to towers and
ground.
7. Building Construction: Glass
wool, asbestos, and thermocol used for thermal insulation. Reduces heat
transfer and improves energy efficiency.
8. Cryogenic and Space Applications: Special insulating foams and ceramics
withstand extreme conditions. Used in spacecraft, satellites, and cryogenic
equipment.
Some Important Insulating
Materials are:
1.Glass
2.Mica
3.Ceramics or Refractories
4.Asbestos
5.Resins
6.Rubber
6.
Material Recycling
6.1 Disposal of Materials
It is very important to dispose the waste
material carefully so that it will not create any environmental hazards like
Air/Water pollution, Hygiene problems causing health issues for the neighborhood
etc.
6.2 Functions of Waste Disposal
Department:
●
Collection of Gas
waste, Wet Waste, Dry waste.
●
Collection of
Drainage etc.
●
Treatment of Gas,
Wet and Dry wastes and disposal
●
Treatment of
Drainage and disposal
●
Treatment of
Wastewater and disposal
6.3 Steps in Disposal of Waste materials
In general, the Waste Disposal process consists
of the following steps.
1) Fill up all the waste materials into
individual containers
2) Carry them to far off places from the
society like low areas, designated places like dumping yards etc and dump it in
the dumping yard.
3) Destroy the waste materials completely by
burning etc.
Proper care must be taken to dispose of the
waste material such that it will not create any further problems to the
environment. Also, it should be carried in sealed containers so that there is
no spillage of waste on the roads etc. The waste materials should never be
disposed of into rivers, reservoirs etc because it will pollute the water which
is being supplied for drinking and agricultural purposes.
6.4 Recycling of Materials
Recycling is a process by which the material in
one shape or form is converted to another for a useful purpose. Generally, the
recycling is carried out on the objects or components which have been used for a
certain period after which their usefulness or efficiency is lost due to the
expiry of lifespan or wearing out, damage etc. For example, used plastic
containers, worn-out gears, damaged bearings, turnout cardboard containers,
spoiled electrical windings, broken circuit boards etc. can be recycled to make
some useful components and articles from these materials.
6.4.1 Functions of Recycling
Department:
●
To encourage
households and businesses to recycle more;
●
Start new recycling
schemes and implement;
●
Monitor, Improve
and expand existing schemes and facilities, e.g. recycling banks, collections
and composting;
●
Plan for the
management and development of recycling and find new ways to meet local and
national targets;
●
Implement ‘best
practices’ in order to maximize resources and reduce costs;
●
Collect data,
compile statistics and draft reports;
●
Manage budgets,
assess tenders and prepare funding bids;
●
Advise and assist
local community groups in carrying out their recycling;
●
Prepare, manage and
monitor recycling contracts;
●
Manage and promote
initiatives through advertising and publicity campaigns;
6.4.2 Steps in Recycling process:
In general, the recycling process consists of
the following steps:
1.Separation: Separate materials into different groups such
as plastics, non-plastics, metals, etc.
2.Sorting and Screening: Within each group, identify useful
and non-useful items based on their condition, structure, and usability.
Eliminate the non-useful items and send them for proper disposal.
3.Processing: The useful plastics, metals, and non-metals
are sent for further processing, such as melting, casting, pressing, or
molding, to produce new, useful components.
Some examples of recycled materials are:
Plastic mugs, pens from recycled plastics, Metal needles, etc. from used
bearings etc., Card board containers from paper
