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Introduction to Materials
1.1 INTRODUCTION
The importance of the study of solid state physics and materials science for engineers needs
no emphasis. The growth of science and technology is intimately linked with the availability
of newer and better materials. The social scientists even employ the extent of usage of materials
as an index to characterise human societies and civilisations. The use of terms such as the
stone age, bronze age and iron age used to describe ancient societies illustrates this idea.
Every engineer ought to have an intimate knowledge of the materials he uses in his work. The
kind of material he deals with depends on his work and specialisation. A civil engineer would
be interested in materials that can be used in the construction of a building or a bridge. An
architectural engineer would have to know materials with special reference to their sound
absorption to design an auditorium with good acoustics. An aerospace engineer would look for
an ideal material for designing an aeroplane or a spacecraft. A mechanical engineer would
deal with materials that can be used in designing a variety of machines, (both simple and
complex). An electrical engineer has to know the electrical properties of materials which would
help to design a circuit or a circuit element. Needless to say, the ongoing revolution in electronics,
computer science and information technology has been made possible because of the progress
in the preparation and processing of semiconducting materials. It is thus essential for an
engineer to have a firm grasp of the underlying principles that govern the different properties
of various materials.
1.2 ENGINEERING MATERIALS
It is conventional to classify the engineering materials into several broad classes. They are
metals, semiconductors, polymers, elastomers, ceramics, glasses, liquid crystals, composites
and nanomaterials. The members of each class have similar properties, similar processing
routes and often similar applications. There could be exceptions to this general categorisation.
Metals have relatively high elastic moduli and are highly ductile and malleable. They
are very good thermal and electrical conductors. Some of them are superconductors at very
low temperatures. They show interesting magnetic properties. They can function at high
temperature. Their main disadvantage is their susceptibility to corrosion. They possess shock
resistance and are particularly useful for structural and load bearing applications. Pure metals
are occasionally used. Usually a combination of metals called alloys serves the purpose better.
Metals are opaque to visible light. They can be polished to high lustre.
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APPLIED SOLID STATE PHYSICS
Ceramics and glasses, also, have high elastic moduli. But unlike metals they are brittle
and do not exhibit ductility or malleability. Their attractive features are their stiffness, hardness
and abrasion as well as corrosion resistance. Usually they are insulators or semiconductors.
They possess poor thermal conductivity. They can also be used in high temperature
environment. A number of ceramic materials exhibit superconductivity, ferroelectricity,
piezoelectricity, pyroelectricity and magnetostriction. These properties are useful in many
device applications. They have very high melting point.
Polymers and elastomers have low elastic moduli. Their elastic moduli are roughly about
50 times less than those of metals. But they can be strong. They are easy to shape and are
corrosion resistant. Their main disadvantage is that they cannot be used at high temperatures.
In recent years, polymers showing conducting property have been synthesised. Some of the
polymers exhibit piezoelectricity, pyroelectricity and ferroelectricity. The subject of organic
electronics is an upcoming field. Polymers are made up of long chains of organic molecules.
They include rubber, plastics and many types of adhesives. They have low electrical and thermal
conductivity. Thermoplastic polymers in which the long molecular chains are not rigidly
connected have good formability. Thermosetting polymers are stronger but more brittle because
the molecular chains are rigidly linked.
A broad comparison of the properties of these classes of materials is shown in Table 1.1.
Table 1.1 Comparisons of properties of metals, ceramics and polymers
Property
Metals
Ceramics
Polymers
Structure
Mostly Crystalline
Cubic Systems
(FCC, BCC, HCP)
Complex
Crystalline
Structure
Amorphous and
Semicrystalline
Bonding
Metallic
Predominantly
Predominantly
Ionic and Covalent
Covalent,
van der Waal and
Hydrogen bonds
Density
Medium to high
Medium
Low
(g/cc)
2–16
2–17
1–2
Low to high
High
Low
Pb: 32°C, Sn: 232°C,
2000 to 4000°C
70 to 200°C
Melting point
W : 3400°C
Specific heat
Low
High
Medium
Hardness
Medium
High
Low
Machineability
Good
Poor
Good
Tensile strength
Medium to high
Low
Low
(MPa)
100 to 2500
10 to 400
30 to 120
Compressive strength
Medium to high
High
Low
(MPa)
Up to 2500
Up to 5000
Up to 350
Young’s modulus
Medium to high
High
Low
(GPa)
40–400
150–450
0.001–3.5
Hight temperature
Poor
Excellent
–
creep resistance
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INTRODUCTION TO MATERIALS
Toughness
Good
some good
and some poor
and some poor
Poor
Thermal expansion
Medium to high
Low to medium
Very high
Thermal conductivity
High
Medium but often
Low
decreases rapidly with
temperature
Thermal shock
Good
Generally poor
–
Wear resistance
Medium
High
Low to moderate
Electrical properties
Conductors
Majority of them are
Majority of them are
insulators
insulators
Chemical resistance
Low to medium
Excellent
Generally good
Oxidation resistance
Poor, except for rare Oxides excellent;
at high temperatures
metals
resistance
–
SiC and Si3N4 are
good
Optical properties
Opaque to visible
Some transparent,
Some transparent,
light
Some opaque
Some opaque
Semiconductors have interesting electrical and optical properties. Semiconductors are a group
of materials having electrical conductivities intermediate between metals and insulators. Their
conductivities can be varied by orders of magnitude through variation of temperature and
doping. Hence, they are used in electronic devices such as transistors, diodes and integrated
circuits. They are the basic materials for microelectronic and optoelectronic devices. Silicon,
germanium and a number of compounds such as gallium arsenide are semiconductors.
Semiconductors are made use of in photodiodes, light emitting diodes and laser diodes which
are routinely used in optical communication.
Liquid crystals are materials which have properties in between those of crystals and
liquids. In liquid crystals the molecules have an orientational order but are devoid of any
translational order. This configuration of molecules enables liquid crystals to possess unique
optical and electrical properties. These have been exploited in display devices, which are an
important component of computer and information technology.
Composite materials: There are many situations in engineering where no single material
will be suitable to meet a particular design requirement. However, two materials in combination
may posses the desired properties and provide a feasible solution. Composite materials are
formed from two or more materials. They have properties that are not found in the materials
of which they are composed. Concrete, plywood and fibreglass are typical examples of composite
materials. With composites, one can produce light weight, very strong, very stiff, high
temperature resistant material or produce hard yet shock resistant cutting tools that would
otherwise shatter. Advanced aircraft and aerospace vehicles rely heavily on composites such
as carbon reinforced polymers and Kevlar reinforced polymers.
Nanomaterials: These materials are made up of grains whose dimensions are less than
100 nm. Nanomaterials could be metals, semiconductors, ceramics or polymers. Their properties
are considerably different from those of the bulk and might exhibit novel effects. Their properties
ought to be understood on the basis of quantum mechanics.
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APPLIED SOLID STATE PHYSICS
Table 1.2 shows the use of various types of engineering materials and their applications.
Table 1.2 Important engineering materials and their applications
Materials
Applications
Properties
Metals and Alloys
Copper
Gray cast iron
Conducting wire
Automobile engine blocks
Electrical conductivity
Malleable, castable,
machineable, vibration damping
Significantly strengthened by heat
treatment
Alloy steels
Wrenches, automobile chassis
Titanium alloys, steels
Aircraft structures, bridges
Load bearing
Nb3Sn
Superconducting magnets
Superconductivity
Ceramics and Glasses
SiO2–Na2O–CaO
Window glass
Optically transparent, thermally
insulating
Al2O3, MgO,SiO2
Refractories (heat resistant
lining of furnaces)
Thermal insulation, low thermal
conductivity
BaTiO3
Capacitors for microelectronic
Ferroelectric
transducer
PZT(PbZrxTi1–xO3)
Transducer
Piezoelectric
Silica
Optical fibers for
communication
Refractive index, low optical
losses
Polymers (Thermoplastics)
Polyethylene(PE)
Food packaging, flexible bottles,
toys, tumblers, battery parts, ice
trays, film wrapping materials,
wire insulation, household items,
tubing
Easily formed into thin flexible
air tight film, chemically
resistant, and electrically
insulating; tough and relatively
low coefficient of friction; low
strength and poor resistance to
weathering
Polystyrene(PS)
Video cassette cases, CD jackets,
coffee cups, knives, spoons,
cafeteria trays, wall tile, battery
cases, toys, indoor lighting panels,
housing appliances, packing,
insulation foams
Excellent electrical properties and
optical clarity; good thermal and
dimensional stability; relatively
inexpensive
Acrylonitrilebutadiene
styrene (ABS)
Refrigerator linings, lawn and
garden equipment, toys, highway
safety devices, textile fibres
Outstanding strength and
toughness, resistant to heat
distortion; good electrical
properties; flammable and
soluble in some organic solvents
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INTRODUCTION TO MATERIALS
Materials
Applications
Properties
Acrylics (polymethyl
methacrylate; PMMA)
Lenses, transparent aircraft
enclosures, drafting equipment,
outdoor signs, auto rear light
covers
Outstanding light transmission
and resistance to weathering;
only fair mechanical properties
Fluorocarbons
(PTFE or TFE)
Polytetrafluoroethylene
or Teflon
Anticorrosive seals, chemical
pipes and valves, bearing,
anti-adhesive coatings, high
temperature electronic parts
Chemically inert in almost all
environments, excellent
electrical insulating properties;
good thermal conductivity, low
coefficient of friction; may
be used up to 260° C (500°F);
relatively weak and poor
cold-flow properties
Polyamides (PA6)
(nylons)
Bearings, gears, cams,
bushings, handles, and
jacketing for wires and cables
automotive components, ropes
Good mechanical strength,
abrasion resistance, and
toughness; low coefficient of
friction; absorbs water and
some other liquids
Polycarbonates (PC)
Safety helmets, lenses, light
globes, base for photographic
film, housing and electrical
appliances, car board lamp
mouldings
Dimensionally stable; low
water absorption; transparent;
very good impact resistance
and ductility; chemical
resistance not outstanding
Polypropylene (PP)
Sterilizable bottles, packaging
film, TV cabinets, luggage,
tanks, carpet fibres, ropes and
packing, semi-rigid moulded
products, car interior
components
Resistant to heat distortion;
excellent electrical properties
and fatigue strength;
chemically inert; relatively
inexpensive; poor resistance to
UV light. This is usually
copolymerised with PE
Poly vinyl chloride
(PVC)
Floor coverings, pipe, electrical
wire insulation, garden hose,
phonograph records
Good low cost, general purpose
materials; ordinarily rigid, but
may be made flexible with
plasticisers; often copolymerised;
susceptible to distortion by heat
Polyethylene
tetrapthalate
(PET or PETE)
Magnetic recording tapes,
clothing, automotive tire
cords, beverage containers,
boil in bag containers
One of the toughest of plastic
films; excellent fatigue and
tear strength, and resistance to
humidity, acids, greases, oils
and solvents
Polyoxy methylene
(POM)
Engineering mouldings
Good mechanical properties,
high elastic moduli
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APPLIED SOLID STATE PHYSICS
Materials
Applications
Properties
Polymers (Thermosets)
Phenolics
Motor housings, telephones, auto
distributors, electrical fixtures,
Adhesives, coating laminates
Excellent thermal stability to
over 150°C (300°F); may be
compounded with a large
number of resins, fillers, etc;
inexpensive
Unsaturated polyesters
Helmets, fibreglass boats,
autobody components, chairs,
fans, Electrical mouldings,
decorative laminates,
polyester matrix in fibre glass
Excellent electrical properties
and low cost; can be formulated
for room or high temperature use;
often fibre reinforced
Epoxies
Encapsulation of integrated
circuits, sinks, adhesives,
protective coatings, used with
fibreglass laminates
Excellent combination of
mechanical properties and
corrosion resistance;
dimensionally stable; good
adhesion; relatively inexpensive;
good electrical properties
Silicone
Adhesives, gaskets, sealants
Good adhesive properties
Urethanes
Fibre coatings, foams,
insulation
Good thermal properties
Semiconductors
Silicon
Transistors and integrated
Unique electrical behaviour
circuits
GaAs, GaP
Optoelectronic systems
Lasing properties
Composites
Graphite-epoxy
WCCo
Aircraft components
High strength to weight ratio
(Tungsten-carbidecobalt)
Cutting tools
High hardness, good shock
resistance
Titanium clad vessel
Reactor vessel
High strength of steel and anticorrosive properties
1.3 PROPERTIES OF MATERIALS
The properties of materials can be broadly classified as physical properties and chemical
properties.
1.3.1 Physical Properties
Physical properties describe the response of materials to mechanical force, heat, light, sound,
electric fields and magnetic fields. Depending on the nature of stimulus, the physical properties
are classified as mechanical, thermal, optical, acoustical, electrical and magnetic (Table 1.3).
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INTRODUCTION TO MATERIALS
Table 1.3 Physical properties of materials
Property
Stimulus
Material parameters for characterisation
Mechanical
properties
Mechanical force
Young’s modulus, Bulk modulus, Rigidity modulus,
Poisson ratio, Hardness, Yield strength, Fracture
strength, Fatigue strength, Creep rate
Thermal
properties
Heat
Specific heat, Thermal conductivity, Emissivity
and absorbptivity (of surfaces), Thermal
expansion coefficient
Electrical
properties
Electric field
Resistivity, Temperature, Coefficient of resistivity,
Dielectric constant, Dielectric loss, Dielectric
breakdown
Magnetic
properties
Magnetic field
Magnetic susceptibility, Temperature dependence
of magnetic susceptibility, Hysteresis loss
(magnetic materials)
Acoustical
properties
Sound
Sound absorption and sound transmission
coefficients
Optical
properties
Light
Reflectivity, Transmittivity, Absorptivity,
Refractive index
1.3.2 Mechanical Properties
These describe the characteristics of the material when subjected to a mechanical force. They
relate to the elastic or plastic behaviour of the material. The parameters characterising the
mechanical properties are the following:
Moduli of elasticity: This is the ratio of stress to strain. The strain could be linear,
volume or shear. The corresponding moduli are denoted as Young’s modulus, Bulk modulus
and Rigidity modulus.
Yield strength: This is the stress at which the material exhibits a specified deviation
from the proportionality of stress and strain.
Compressive yield strength: The stress in compression at which a material exhibits a
specified deviation from the proportionality of stress and strain.
Tensile strength: This is the maximum tensile stress the material is capable of
withstanding.
Compressive strength: This is the maximum compressive stress that a material is capable
of withstanding.
Poisson ratio: This is the ratio of the lateral strain to longitudinal strain.
Hardness: It is the resistance of the material to plastic deformation. The deformation
could be due to indentation (i.e., making a mark on the surface), abrasion, scratching and
machining.
Impact strength: The amount of energy required to fracture a given volume of the
material.
Endurance limit: The maximum stress below which a material can theoretically endure
an infinite number of stress cycles.
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APPLIED SOLID STATE PHYSICS
Creep: This is the time-dependent permanent strain under stress.
Creep strength: This is the constant nominal stress that will cause a specified measure
of creep in a given time at constant temperature.
Fracture: One of the important effects on applying the load to a material beyoud a limit,
is fracture. Fracture is concerned with the initiation and propagation of a crack or cracks in
the material until the extent of cracking is such that the applied loading can no longer be
sustained by the material. Two basic types of fracture are observed in tension, depending on
the material, temperature, strain rate etc.
These are termed as ‘brittle’ and ‘ductile’. The main features of the former are that
there is little or no plastic deformation, the plane of the fracture is normal to the tensile stress
and separation of crystal structure occurs. In the ductile type, the fracture is preceded by a
considerable amount of plastic deformation and the fracture is by shear or sliding of the crystal
structure on microscopic planes at about 45° to the tensile stress.
Thermal properties describe the response of materials to heat. Following are the
important thermal parameters for the material.
Specific heat or heat capacity: If heat Q is supplied to an object of mass m, the
temperature change (Tf – Ti) is related to Q by the relation
Q = C (Tf – Ti)
where C is the heat capacity of the object. The specific heat ‘c’ of the material (the heat capacity
per unit mass) is defined as
Q = cm (Tf – Ti)
The molar specific heat (the heat capacity per mole of a substance) shows an interesting
regularity that helps us to understand the mechanisms in heat absorption. The specific heat
depends on the conditions of measurement and is a function of temperature. In the case of
gases the specific heat at constant volume or constant pressure has to be specified.
Heat of transformation: Heat supplied to a material may change the material’s physical
state, for example from solid to liquid (melting) or from liquid to gas (vaporisation) or from
solid to gas (sublimation). The amount of heat (Q) required per unit mass for a particular
phase change is the heat of transformation and is called the latent heat since the phase change
occurs at constant temperature.
Q = Lm
where L is the heat of transformation and m is the mass. The heat of vaporisation Lv is the
amount of heat per unit mass that must be supplied to vaporise a liquid or that must be
removed to condense a gas into liquid. The heat of fusion Lf is the amount of heat per unit
mass that must be supplied to melt a solid or that must be removed from a liquid to freeze it
into a solid. The heat of sublimation Ls is the amount of heat that must be supplied to directly
convert the solid into its vapour or removed from a vapour to condense it into a solid.
Thermal conductivity: The heat transfer through a solid is characterised by the parameter
called the thermal conductivity. The rate H at which heat is conducted through a slab whose
faces are maintained at Th and Tc is
H = Q/t = kA (Th – Tc)/L
in which A and L are the face area and length of the slab. The thermal conductivity ‘k’ is
defined as the rate of heat flow through a slab of unit cross-section and whose faces are
maintained at unit temperature gradient.
INTRODUCTION TO MATERIALS
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Thermal expansion: All objects change size with changes in temperature. The change ∆l
in any linear dimension L for a temperature change ∆T is given by
∆l = Lα∆T
in which α is the coefficient of linear expansion. In the case of a planar object the change in
area ∆A is given by
∆A = Aβ∆T
in which β is the coefficient of a real expansion. The change in volume ∆V of a solid or liquid is
given by
∆V = Vγ∆T
where γ is the coefficient of volume expansion. It can be shown that β = 2α and γ = 3α.
Since, strain is defined as the change in dimension to original dimension, α, β and γ can
be defined as the linear, areal and volume strain resulting from a unit change in temperature.
Thermal emissivity: All objects radiate heat at any temperature. The rate of energy
radiated per unit area depends on the nature of the surface. It is given by
E = εσT4
where E is the rate of energy radiated per unit area of cross-section, T is the temperature in
Kelvin at which the surface is maintained, ‘σ’ is the Stefen-Boltzmann’s constant and ε is the
emissitivity. ε = 1 for a perfect blackbody.
Electrical properties describe the response of materials to electric fields (both dc and
ac):
Resistivity: In a conductor the application of an electric field leads to a flow of charge,
which constitutes the current. If an application of an electric field E, leads to a current density
j then
j=σE
where σ is called the conductivity. The resistivity ρ is defined as the reciprocal of σ.
ρ = 1/σ
The temperature variation of resistivity is also an important parameter. In the case of
metals and insulators the resistivity increases with temperature while in the case of
semiconductors it decreases with temperature.
Electric susceptibility: In an insulator the application of an electric field leads to
polarisation. Polarisation is the induced electric dipole moment per unit volume. The
polarisation can be classified as ionic, electronic, orientational or space charge. Polarisation
depends on the nature of the material and the frequency of the electric field. If the application
of an electric field E leads to a polarisation P, then
P = χel E
where χel is called the electric susceptibility. The dielectric constant ε is related to the electric
susceptibility χ by the relation
ε = 1 + 4πχel
Insulators are used as materials for capacitors. The dielectric constant ε is equal to the
ratio of the capacitance of the capacitor with the material and without the material i.e.,
ε = C/C0
where C is the capacitance of the capacitor with the material and C0 is the capacitance of the
capacitor without the material.
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APPLIED SOLID STATE PHYSICS
However, no material is a perfect dielectric. Hence, on the application of an electric
field a minute current will flow. This is taken care of by treating ε as a complex quantity. i.e.,
ε = ε 1 + i ε2
where ε2 is a measure of its conductivity.
The insulators are also characterised by another parameter called the dielectric strength.
This is the electric field at which there is a sudden increase of current in the material. The
phenomenon is known as the dielectric breakdown.
Magnetic properties describe the response of material to magnetic field. Following
are the important material parameters:
Magnetic susceptibility: In the case of magnetic materials the application of a magnetic
field H, leads to magnetisation M. Magnetisation is defined as the induced magnetic moment
per unit volume. Analogous to electrical susceptibility, magnetic susceptibility χm is defined
as
H = χm M
Magnetic permeability µ is related to magnetic susceptibility by the equation
µ = 1 + 4πχm
Magnetic materials are classified as diamagnetic, paramagnetic and ferromagnetic
depending on whether χ is < 0, χ is > 1 or χ is >>> 1. The ferromagnetic materials exhibit
magnetisation even in the absence of magnetic field and exhibit hysteresis loop. On this account
the ferromagnetic materials are used as memory devices. The material parameters for
ferromagnetic materials include remnant magnetisation, hysteresis loss and coercive field
strength. Remnant magnetisation is the magnetisation present in the material when the applied
magnetic field is removed. Hysteresis loss is the energy dissipated in moving the domain
boundaries during a single magnetisation cycle. Coercive field strength is the magnetic field
needed to reverse the magnetisation in the material. The ferromagnetic materials are also
characterised by Eddy current loss. This is the power loss in ferromagnetic materials in
alternating fields due to induced surface currents in the materials.
Optical properties describe the response of materials to light. Optical properties are
described by the following parameters.
Reflectivity (r): This is a surface property and is defined as the ratio of the intensity of
reflected light to that of incident light.
Transmittivity (t): This is defined as the ratio of the intensity of light transmitted through
the material to that of the incident light.
Absorptivity (a): This is defined as the ratio of the intensity of light absorbed by the
material to that of incident light.
Refractive index (n): The refractive index of a material is a complex quantity (n1 + in2).
n1 = c/ν where c is the velocity of light in vacuum and ν is the velocity of light in the material.
The imaginary part n2 characterises the optical absorption in the material. The optical
properties (r, t, a and n) are functions of wavelength and is usually specified from infrared to
the ultraviolet region of the electromagnetic spectrum.
Acoustical properties describe the response of the material to sound waves. The
acoustical properties of materials are of interest especially to architectural engineers involved
in the design of auditoria or noise control. The material is used to maximise sound absorption
by promoting frictional processes. The most commonly used materials are porous, such as
mineral fibre materials or certain types of open-cell foam polymer materials. The material