PPT 2.pdf

MATERIALS SELECTION
Materials selection process involve deciding on the constraints of the problem,
and, from these, establishing criteria that can be used in materials selection to
maximize performance.
Many of the selection criteria relate to material properties or property
combinations—mechanical, electrical, thermal, corrosion, etc.
The performance of some component will depend on the properties of the material
from which it is made.
Processability or ease of fabrication of the component may also play a role in
the selection process.

ECONOMIC CONSIDERATIONS
Critical driving force in engineering practice is that of economics.
The company or institution must realize a profit from the products that it
manufactures and sells.
The engineer might design the perfect component, however, as manufactured, it
must be offered for sale at a price that is attractive to the consumer, and return a
suitable profit to the company.
There are three factors over which the materials engineer has control and which
affect the cost of a product.
They are (1) component design (2) Material(s) used and (3) The manufacturing
technique(s).

1)COMPONENT DESIGN :
Component design is the specification of size, shape, and configuration, which will
affect in-service component performance.
For example, if mechanical forces are present, then stress analyses may be
required.
Detailed drawings of the component must be prepared.
Computers are normally employed, using software that has been generated for
this specific function.
A system consisting of a large number of components (e.g., television,
automobile, etc.).

2) MATERIALS :
In terms of economics, we want to select the material or materials having the
appropriate Combination(s) of properties which are the least expensive.
Once a family of materials has been selected that satisfy the design constraints,
cost comparisons of the various candidate materials may be made on the basis of
cost per part.
Material price is usually quoted per unit mass. The part volume may be
determined from its dimensions and geometry, which is then converted into mass
using the density of the material.
During manufacturing there is some unavoidable material waste, which should
also be taken into account in these computations.

3) MANUFACTURING TECHNIQUES :
The choice of manufacturing process will be influenced by both the material
selected and part design.
The entire manufacturing process will normally consist of primary and secondary
operations.
Primary operations are those that convert the raw material into a recognizable part
(e.g., casting, plastic forming, powder compaction, molding, etc.).
secondary ones are those subsequently employed to produce the finished part
(e.g., heat treatments, welding, grinding, drilling, painting, decorating).
The major cost considerations for these processes include capital equipment,
tooling, labor, repairs, machine downtime, and waste.
There are also other factors not directly related to design, these factors include
labor fringe benefits, supervisory and management labor, research and
development, property atnd rent, insurance, profit, taxes, and so on.

ENVIRONMENTAL AND SOCIETAL CONSIDERATIONS
Our modern technologies and the manufacturing of their associated products impact
our societies in a variety of ways.
Some are positive, others are adverse. Furthermore, these impacts are economic
and environmental in type.
Materials play a crucial role in this technology-economy-environment scheme.
A material that is utilized in some end product and then discarded passes through
several stages or phases.
These stages are shown in Figure. Which is termed as ‘‘total materials cycle’’ or just
‘‘materials cycle’’.

Beginning on the far left side of Figure, raw materials are extracted from their
natural earthly habitats by mining, drilling, harvesting, etc.
These raw materials are then purified, refined, and converted into bulk forms such
as metals, cements, petroleum, rubber, fibers, etc.
Further synthesis and processing results in products that are what may be termed
‘‘engineered materials’’; examples include metal alloys, ceramic powders, glass,
plastics, composites, semiconductors, elastomers.
Next, these engineered materials are further shaped, treated, and assembled into
products, devices, and appliances that are ready for the consumer—this constitutes
the ‘‘product design, manufacture, assembly’’ stage as shown in Figure.
The consumer purchases these products and uses them (the ‘‘applications’’ stage)
until they wear out or become obsolete and are discarded.
At this time the product constituents may either be recycled/reused or disposed of
waste, normally being either incinerated (to burn) or dumped as solid waste in
municipal land-fills—as such, they return to the earth and complete the materials
cycle.

It has been estimated that worldwide, on the order of 15 billion tons of raw materials
are extracted from the earth every year, some of these are renewable and some are
not.
As our societies mature (development) and populations increase, the available
resources become scarcer, and greater attention must be paid to more effective
utilization of these resources relative to this materials cycle.
Energy is a resource that, to some degree, is limited in supply and measures must
be taken to conserve and more effectively utilize it in the production, application,
and disposal of materials.
And, finally, there are interactions (to burn) with and impacts on the natural
environment at all stages of the materials cycle.
The condition of the earth’s atmosphere, water, and land depends to a large extent
on how carefully we traverse this materials cycle.
Some ecological damage and landscape spoilage undoubtedly result during the
extraction of raw materials phase.
Pollutants may be generated that are expelled into the air and water during the
synthesis and processing stage; in addition, any toxic chemicals that are produced
need to be disposed of or discarded.

The final product, device, or appliance should be designed such that during its
lifetime, any impact on the environment is minimal; furthermore, at the end of its life
that, at best, provision be made for recycling of its component materials, or at least
for their disposal with little ecological degradation (i.e., it should be biodegradable).
Recycling of used products rather than disposing of them as waste is a desirable
approach for several reasons.
First of all, using recycled material obviates (eliminate) the need to extract raw
materials from the earth, and thus conserves natural resources and eliminates any
associated ecological impact from the extraction phase.
Second, energy requirements for the refinement and processing of recycled
materials are normally less than for their natural counterparts; for example,
approximately 28 times as much energy is required to refine natural aluminum ores
than to recycle aluminum beverage can scrap.
And, finally, there is no need to dispose of recycled materials.
Thus, this materials cycle is really a system that involves interactions and
exchanges among materials, energy, and the environment.

Correcting any environmental problems associated with manufacturing will
influence product price. That is, manufacturing cost is normally greater for a
‘‘green’’ (or ‘‘environmentally friendly’’) product than for its equivalent that is
produced under conditions wherein environmental issues are minimized.
Thus, a company must confront the dilemma of this potential
economic-environmental trade-off and then decide the relative importance of
economics and of environmental impact.
One approach that is being implemented by industry to improve the environmental
performance of products is termed life cycle analysis/assessment.
With this approach to product design, consideration is given to the cradle-to-grave
environmental assessment of the product, from material extraction to product
manufacture to product use, and, finally, to recycling and disposal; sometimes this
approach is also labeled as ‘‘green design.’’
One important phase of this approach is to quantify the various inputs (e.g.,
materials and energy) and outputs (e.g., wastes) for each phase of the life cycle; this
is represented schematically in Figure.

Fig. Schematic representation of an input/output inventory for the life-cycle assessment of
a product.

RECYCLING ISSUES IN MATERIALS SCIENCE
Important stages in the materials cycle where materials science and engineering
plays a significant role are recycling and disposal.
The issues of recyclability and disposability are important when new materials are
being designed and synthesized.
During the materials selection process, the ultimate disposition of the materials
employed should be an important criterion.
From an environmental perspective, the ideal material should be either totally
recyclable or completely biodegradable.
Recyclable means that a material, after having completed its life cycle in one
component, could be reprocessed, could reenter the materials cycle, and could be
reused in another component—a process that could be repeated an indefinite
number of times.
Engineering materials exhibit varying degrees of recyclability and biodegradability.

METALS
Most metal alloys (e.g., Fe, Cu), to one degree or another experience corrosion
and are also biodegradable.
Some metals (e.g., Hg, Pb) are toxic and, when land-filled, may present health
hazards.
Alloys of most metals are recyclable; on the other hand it is not feasible to recycle
all alloys of every metal.
The problems of recycling involve separation of various alloys types (e.g.,
aluminum from ferrous alloys) after dismantling and shredding.
Joining of dissimilar alloys presents contamination problems; for example, if two
similar alloys are to be joined, welding is preferred over bolting or riveting.
Coatings (paints, anodized layers, claddings, etc.) may also act as contaminants,
and render the material non recyclable.
Aluminum alloys are very corrosion resistant, and, therefore, non biodegradable.
Aluminum is the most important recyclable nonferrous metal.
Since aluminum is not easily corroded, it may be totally reclaimed.
The primary sources of recycled aluminum are used beverage cans and scrapped
automobiles.

GLASS
The one ceramic material that is consumed by the general public in the greatest
quantities is glass, in the form of containers.
Glass is a relatively inert material, and, as such, it does not decompose; thus, it is
not biodegradable.
Advantages of utilizing recycled glass include more rapid and increased production
rates and a reduction in pollutant emissions.
COMPOSITE MATERIALS
Composites are inherently difficult to recycle because they are multiphase in
nature.
Complete phase/material separation is virtually impossible, and recycling
procedures that require material separation are impractical.

PLASTICS AND RUBBER
Synthetic polymers (including rubber) are so popular as engineering materials lies
with their chemical and biological inertness. This characteristic is really a liability
when it comes to waste disposal.
Polymers are not biodegradable, and, as such, they constitute a significant land-fill
component; major sources of waste are from packaging, junk automobiles,
automobile tires, and domestic durables.
Biodegradable polymers have been synthesized, but they are relatively expensive
to produce.
Since some polymers are combustible and do not yield appreciable toxic or
polluting emissions, they may be disposed of by incineration(to burn).
Thermoplastic polymers, specifically polyethylene and polypropylene, are those
most amenable to reclamation and recycling, since they may be reformed upon
heating.
The recycled plastic is less costly than the original material, and quality and
appearance are generally degraded with each recycle.
Typical applications for recycled plastics include shoe soles, tool handles etc.

The recycling of thermoset resins is much more difficult since these materials are
not easily remolded or reshaped due to their cross linked or network structures.
Rubber materials present some disposal and recycling challenges.
When vulcanized, they are thermoset materials, which makes chemical recycling
difficult.
Scrap tires have been utilized as a fuel for some industrial applications (e.g.,
cement plants), but yield dirty emissions.
Recycled rubber tires that have been split and reshaped are used in a variety of
applications such as automotive bumper guards, mud flaps, door mats, and
conveyor rollers; and, of course, used tires may also be recapped.

Nano materials
Nano materials can be defined as those materials which have structured components
with size less than 100nm at least in one dimension.
Nanotechnology is an advanced technology, which deals with the synthesis of
nano-particles, processing of the nano materials and their applications.
The Nano size range is 1- 100 nm.
Classification of nano materials
Depending on the number of dimensions in the nanorange, materials can be
classified as
1) Zero- dimensional (nano particle)
2) One-dimensional
3) Two-dimensional
4) Three-dimensional

1) Zero- dimensional (nano particle) :
Where all three dimensions of the particle are in the nanorange.
d < 100 nm.
Examples : spheres and clusters.
2) One- dimensional :
Where two dimensions of the particle are in the nanorange and third dimension is
much larger.
d < 100 nm.
Examples : nanorods, nanowires ,nanotubes and carbon nanotube.

3) two- dimensional :
Where one dimension in the nanorange and the other two
are much larger.
t < 100 nm.
Examples : nanofilms, nanosheets and nanocoatings.
Where all three dimensions of a particle are much larger
than the nanorange.
L > > 100 nm.
Examples : nanosized crystals in a bulk polycrystalline
-material.
4) three- dimensional :

processing of nano materials
There are two approaches for synthesizing or processing
of nanostructural materials.
1) Top Down approach
2) Bottom up approach
1) Top Down approach :
It involves breaking down the bulk material into nano
sized particles.
eg : Mechanical-ball milling , extrusion, grinding, etc.
In this approach individual atoms or molecules are put
together to form the desired nano particle.
eg : Inert-gas condensation, physical / chemical vapour
deposition, sputtering, electrodeposition etc.
2) Bottom up approach :

BALL MILLING METHOD
It is a top-down method.
Ball milling is a method of production of nano materials.
This process is used in producing metallic and ceramic nano materials.
Milling of elemental or compound powder is termed as mechanical milling.
Milling of different powders together is termed as mechanical alloying ,as the powders
react during the process.

HIn this method , small hard balls made of hardened steel are placed along with the
material to be crushed in a rotating drum.
During rotation , the collision with the balls break up the material in to fine particles
and also causes heavy plastic deformation.
A continuous refinement of the internal structure occurs.
The average grain size can be reduce to a few nm after extended milling.
Ball milling can be used to make carbon nanotubes and boron nitride nanotubes.

SOL – GEL METHOD
It is a bottom – up method.
Sol – gel processing is a wet chemical method for self-assembling a nanomaterial in
the shape of either particles , nano film or bulk solid.

The process consists of forming a colloidal suspension (called sol) and gelation of
the sol to form a continuous liquid phase (gel) .
The starting material is a metal organic compound , e.g., Ti(OC4
H9
)4.
(Tetrabutyl
titanate).
It is dissolved in water or alcohol and is subjected to a polymerization reaction to form
a sol of discrete.
Finely dispersed particles kept in suspension by adding a surfactant.
The surfactant is then removed to yield a gel.
Calcination of the gel is done to evaporate solvent and produce the nanomaterial.
The sol – gel method is the basis for producing a wide variety of materials in the
nanosize range including ceramics, paints, cosmetics and detergents.
Composites can also be made by mixing a monomer with a nanocrystalline ceramic
powder, followed by polymerization.

INERT GAS CONDENSATION METHOD (IGC)
It produces particles of well-defined grains , with a
narrow size distribution.
It is a bottom – up method , with two basic steps.
The first step is the evaporation of the material and
the controlled condensation of the evaporated atoms
(or molecules) to produce the desired particle size.
Evaporation is done inside a chamber filled with an
inert gas like helium of partial pressure equal to a few
hundred pascals.
The evaporated atoms collide with the helium atoms ,
lose their energy and condense as discrete crystals
of loose nanosized powder.
The powder particles condense on the surfaces of a
rod cooled by liquid nitrogen, are scraped off and
sent for compaction (second step).
This technique is suffers from the common problem
of two-step methods in that the subsequent step of
compaction may not produce a fully-dense or
fully-equipped
bonded material.

PROPERTIES OF NANOMATERIALS
1) MECHANICAL PROPERTIES:
Mechanical properties like physical properties like strength, melting point etc. also
shows drastic change at nano scale level. Let us discuss the example of this
Tougher and Harder Cutting Tools
Cutting tools made of nano materials, such as tungsten carbide, tatalum carbide, and
titanium carbide, are much harder, much more wear-resistant, erosion-resistant, and
last longer than their conventional (large grained) counterparts.
For the miniaturization of microelectronics circuits, the industry needs micro drills
(drill bits with diameter less than the thickness of an average human hair or 100 nm)
with enhanced edge retention and far better wear resistance. As nano-crystalline
carbides are much stronger, harder, and wear-resistant, they are currently being used
in these micro drills.

Automobile with Great Fuel Efficiency
As nanomaterials are stronger, harder, and much more wear-resistant and
erosion-resistant, they are can be used in spark plugs.
Automobiles waste significant amounts of energy by losing the thermal energy
generated by the engine. Thus, the engine cylinders are envisioned to be coated with
nanocrystalline ceramics, such as zirconia and alumina, which retain heat much more
efficiently that result in complete and efficient combustion of the fuel.
Aerospace Components with Enhanced Performance Characteristics
The fatigue strength increases with a reduction in the grain size of the material.
Nanomaterials provide such a significant reduction in the grain size over conventional
materials that the fatigue life is increased by an average of 200-300%.
In spacecrafts, elevated-temperature strength of the material is crucial because the
components (such as rocket engines, thrusters, and vectoring nozzles) operate at
much higher temperatures than aircrafts and higher speeds.

Ductile Ceramics
Ceramics are very hard, brittle, and hard to machine even at high temperatures. But,
with a reduction in grain size, their properties change drastically.
Nanocrystalline ceramics can be pressed and sintered into various shapes at
significantly lower temperatures. For example, Zirconia is a hard, brittle ceramics, has
even been rendered superplastic, i.e., it can deformed to great lengths (up to three
times of its original length). However, these ceramics must possess nanocrystalline
grains to be super-plastic.
Ceramics based on silicon nitride(Si3N4) and silicon carbide (Sic), have been used
in automative applications as high-strength springs, ball bearings, and valve filters,
and because they possess good formability and machinability combined with
excellent physical, chemical, and mechanical properties.
They are also used as components in high temperature furnaces.

2) ELECTRICAL PROPERTIES:
Electrical Properties of Nanoparticles discuss about fundamentals of electrical
conductivity in nanotubes and nanorods, carbon nanotubes, photoconductivity of
nanorods, electrical conductivity of nanocomposites.
One method which can be used to demonstrate the steps in conductance is the
mechanical thinning of a nanowire and measurement of the electrical current at a
constant applied voltage.
The important point here is that, with decreasing diameter of the wire, the number of
electron wave modes contributing to the electrical conductivity is becoming
increasingly smaller by well-defined quantized steps.

In electrically conducting carbon nanotubes, only one electron wave mode is
observed which transport the electrical current.
As the lengths and orientations of the carbon nanotubes are different, they touch the
surface of the mercury at different times, which provides two sets of information:
(i) the influence of carbon nanotube length on the resistance; and
(ii) the resistances of the different nanotubes.
As the nanotubes have different lengths, then with increasing protrusion of the fiber
bundle an increasing number of carbon nanotubes will touch the surface of the
mercury droplet and contribute to the electrical current transport.
Fig. Electrical behavior of nanotubes

3) MAGNETIC PROPERTIES:
Bulk gold and Pt are non-magnetic, but at the nano size they are magnetic.
Surface atoms are not only different to bulk atoms, but they can also be modified by
interaction with other chemical species, that is, by capping the nanoparticles.
This phenomenon opens the possibility to modify the physical properties of the
nanoparticles by capping them with appropriate molecules.
It should be possible that non-ferromagnetic bulk materials exhibit ferromagnetic-like
behavior when prepared in nano range.
One can obtain magnetic nanoparticles of Pd, Pt and the surprising case of Au (that
is diamagnetic in bulk) from non-magnetic bulk materials.
In the case of Pt and Pd, the ferromagnetism arises from the structural changes
associated with size effects.
gold nanoparticles become ferromagnetic when they are capped with appropriate
molecules: the charge localized at the particle surface gives rise to
ferromagnetic-like behavior.

Surface and the core of Au nanoparticles with 2 nm in diameter show ferromagnetic
and paramagnetic character, respectively.
The large spin-orbit coupling of these noble metals can yield to a large anisotropy
and therefore exhibit high ordering temperatures.
More surprisingly, permanent magnetism was observed up to room temperature for
thiol-capped Au nanoparticles.
For nanoparticles with sizes below 2 nm the localized carriers are in the 5d band..
Fig. Magnetic properties of nanostrucutred
materials

Bulk Au has an extremely low density of states and becomes diamagnetic, as is also
the case for bare Au nanoparticles.
This observation suggested that modification of the d band structure by chemical
bonding can induce ferromagnetic like character in metallic clusters.
4) OPTICAL PROPERTIES:
One of the most fascinating and useful aspects of nanomaterials is their optical
properties.
Applications based on optical properties of nanomaterials include optical detector,
laser,sensor,imaging,phosphor,display,solarcell,photocatalysis,photoelectrochemistry
and biomedicine.
The optical properties of nanomaterials depend on parameters such as feature size,
shape, surface characteristics, and other variables including doping and interaction
with the surrounding environment or other nanostructures.
shape can have dramatic influence on optical properties of metal nanostructures.
Fig. Exemplifies the difference in the optical properties of metal and semiconductor
nanoparticles.

Fig. Fluorescence emission of (CdSe) ZnS quantum dots of various sizes
and absorption spectra of various sizes and shapes of gold nanoparticles.
With the CdSe semiconductor nanoparticles, a simple change in size alters the
optical properties of the nanoparticles.
When metal nanoparticles are enlarged, their optical properties change only slightly
as observed for the different samples of gold nanospheres.
However, when an anisotropy is added to the nanoparticle, such as growth of
nanorods,the optical properties of the nanoparticles change dramatically.

Solar PV Materials (SPV MATERIALS)
1) Amorphous silicon (multijunction)
1) Cadmium Telluride (CdTe) thin film
1) Copper-Indium-Gallium-Selenium (CIGS)
4) Gallium Arsenide (GaAs)
5) Multi crystalline or polycrystalline silicon
6) Mono crystalline silicon
7) High performance mono-crystalline silicon
8) Triple-junction (GaInP/GaAs/Ge) cell

MONO CRYSTALLINE MATERIALS
Monocrystalline silicon solar cells are probably the oldest type of solar cells.
They are made from pure silicon crystal, which has continuous lattice and almost no
defects.
Its properties provide for high efficiency of light conversion (typical ~15%; recent
developments by SunPower boast improved efficiencies up to 22-24% ).
Manufacturing of the Si crystals is rather complicated, which is responsible for high
cost of this type of photovoltaics.
Recent developments have decreased the total thickness of Si material used in
monocrystalline cells to reduce cost.
The monocrystalline silicon cells have a typical black or iridescent blue color.
The monocrystalline silicon cells are believed to be very durable and last over 25
years.
However, their efficiency will gradually decrease (about 0.5% per year), so
replacement of operating modules might be needed sooner.
The main disadvantages of the monocrystalline silicon panels are high initial cost
and mechanical vulnerability (brittle).

POLYCRYSTALLINE MATERIALS
Polycrystalline cells are made by assembling multiple grains and plates of silicon
crystals into thin wafers.
Smaller pieces of silicon are easier and cheaper to produce, so the manufacturing
cost of this type of PV is less than that of monocrystalline silicon cells.
The polycrystalline cells are slightly less efficient (~12%).
These cells can be recognized by their mosaic-like appearance.
Polycrystalline cells are also very durable and may have a service life of more than
25 years.
The cons of this type of PV technology are mechanical brittleness and not very high
efficiency of conversion.

AMORPHOUS MATERIALS
Amorphous silicon (a-Si) is the non-crystalline form of silicon.
It is the most well developed of the thin film technologies having been on the market
for more than 15 years.
Amorphous silicon panels are formed by vapor-depositing a thin layer of silicon
material – about 1 micrometer thick – on a substrate material such as glass or metal.
Amorphous silicon can also be deposited at very low temperatures, as low as 75
degrees Celsius, which allows for deposition on plastic as well.
The principal advantage of amorphous silicon solar cells is their lower manufacturing
costs, which makes these cells very cost competitive.
Amporphous silicon can be produced in a variety of shapes and sizes (e.g., round,
square, hexagonal, or any other complex shape).
This makes it an ideal technology to use in a variety of applications such as powering
electronic calculators, solar wristwatches, garden lights, and to power car
accessories.
Small solar cells used in pocket calculators have been made with a-Si for many
years.

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