UNIT VI CHEMISTRY OF ADVENCED MATERIALS 2015-16 Engineering Chemistry Page 143 CHEMISTRY OF ADVANCED MATERIALS Syllabus: 1. Nanomaterials - structure, synthesis, properties and applications of carbon nanotubes (fullerenes, SWNT, MWNT) 2. Green chemistry - Methods for green synthesis (at least three) and their applications. 3. Solar cells- construction and working -Solar heaters – Photo voltaic cells – Solar reflectors 4. Cement – types of cement, Manufacture of Portland cement – Reactions involved- setting and hardening – decay of cement. 5. Lubricants- Definition–Mechanisms of lubrication- importance of lubrication. 6. Introduction to liquid crystals Objectives: Prospective engineers are expected to know about some of the advanced materials that are becoming available. Hence some of them are introduced here. Outcomes: Students gain knowledge on advanced materials like carbon nano tubes and fullerenes, their properties and applications, manufacturing of cement, need for green chemistry, principles of green chemistry solar cells and greenhouse effect and their importance OUTLINES Nanomaterials Solar Cells Green Chemistry Cement Lubricants Liquid Crystals
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UNIT VI CHEMISTRY OF ADVENCED MATERIALS 2015-16
Engineering Chemistry Page 143
CHEMISTRY OF ADVANCED MATERIALS
Syllabus:
1. Nanomaterials - structure, synthesis, properties and applications of carbon nanotubes (fullerenes,
SWNT, MWNT)
2. Green chemistry - Methods for green synthesis (at least three) and their applications.
3. Solar cells- construction and working -Solar heaters – Photo voltaic cells – Solar reflectors
4. Cement – types of cement, Manufacture of Portland cement – Reactions involved- setting and
hardening – decay of cement.
5. Lubricants- Definition–Mechanisms of lubrication- importance of lubrication.
6. Introduction to liquid crystals
Objectives: Prospective engineers are expected to know about some of the advanced materials that are
becoming available. Hence some of them are introduced here.
Outcomes: Students gain knowledge on advanced materials like carbon nano tubes and fullerenes, their
properties and applications, manufacturing of cement, need for green chemistry, principles of green
chemistry solar cells and greenhouse effect and their importance
OUTLINES
Nanomaterials
Solar Cells
Green Chemistry
Cement
Lubricants
Liquid Crystals
UNIT VI CHEMISTRY OF ADVENCED MATERIALS 2015-16
Engineering Chemistry Page 144
1. Nano-materials
Nano-materials are nano powders or nano-crystalline materials or nano particles are novel
materials, whose molecular structures have been engineered at the nanometer scale in the order of 1-100
nm. A nano-meter is one billionth (10-9
) of a meter. The significance of nano-materials is due to their
small size. They exhibit unique properties different from their bulk materials like melting point, electrical
conductivity, transparency, etc. They have increased surface area and quantum effects. These have
components of at least in one dimension. This changes the properties such as reactivity, strength, optical,
electrical and magnetic behaviour of metals. Nano materials are strong, hard, and ductile at high
temperatures, wear resistant, erosion and corrosion resistant.
A nano-particle is defined as a small object that behaves as a whole unit in terms of its transport
and other properties and exhibits a number of special properties relative to its bulk material. It is an
object with all the three dimensions on a nano-scale. Nano-materials can be biological, inorganic or
organic by their origin. Volcanic ash, carbon soot and incidental by products of welding and internal
combustion engines are examples of natural nano-particles.
Nanomaterials in one dimension are layers like thin films or surface coatings
Nanomaterials in two dimensions are tubes like nanotubes, fibres and nano wires
Nano particles in three dimensions are particles like precipitates, colloids and quantum dots.
1.1. Nanowires: One dimensional nano structures can control the density of states in a semiconductor,
which in turn control their electronic and optical properties. Hence nano-wires are employed in next
generation electronics, photonics, sensors and energy application. They allow the growth of an axial
hetero structure and provide the flexibility to create hetero structures, which allow integration of
compound semiconductor based opto-electronic devices with silicon based micro-electronics.
1.2. Quantum dots: A quantum dot is a particle having an approximate size of 1mm and has the
properties of a semiconductor. Silicon is the most popular material used in the creation of a quantum dot.
Quantum dots exhibit unusual properties, which are not present in usual semi conducting materials.
Electrons in general occupy one of the two bands in a crystal (valence band VB and conduction band,
CB). By proper excitation, the electron moves from VB to CB creating a hole in VB. The distance
between the electron and hole is called Excitation Bohr Radius. This gap can be reduced, if the size of
crystal is reduced. This increases the absorption of energy by crystal and crowds the gap. Hence these
have a unique application in various fields. Multiple quantum dots are used as LEDs in sign board
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displays and cell staining for life science observations, as luminescent dust to track trespassers in
restricted areas and they are also used to transmit data, similar to fibre optics.
1.3.Carbon NanoTubes (CNT): Carbon nanotubes are allotropes of carbon with a nanostructure having
length to diameter ratio greater than 100,000. They are also called as Bucky tubes. These are long, thin
cylinders of carbon, discovered by S. Iijima in 1991.They are considered as a sheet of graphite rolled into
a cylinder. These have a broad range of electronic, thermal and structural properties depending on the
length, diameter, chirality or twist of nanotube.
1.4 Types of carbon nanotubes: Depending on the arrangement of atoms in carbon nanotubes, there are
two types of carbon nanotubes.
i) Single walled nanotubes (SWNT) ii) Multi walled nanotubes (MWNT)
Single Walled Nano Tubes (SWNT)
Single walled nanotubes have a diameter close to 1nm and run into million times longer than its
diameter.
They are obtained by wrapping a sheet of graphene ( a single layer of graphite) into seamless
sheets.
There are three types of single walled nanotubes base on the way the graphene sheet is wrapped.
Graphene sheet is represented by a pair of indices (n, m) called the chiral vector. The integers n
and m denote the number of unit vectors along two directions in the honey comb crystal of
graphene.
If m=0, the nanotubes are zig-zig. The lines of the carbon bonds are down the centre.
If n=m, the nanotubes are called arm- chair. The lines of hexagons are parallel to the axis of the
nanotubes.
Otherwise, they are called ‘chiral’. They have a twist or spiral around the nanotubes.
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Multi Walled Nano Tubes (MWNT):
Multi- walled nanotubes consist of multiple rolled concentric tubes of graphite.
The interlayer distance in multi-walled nanotubes is close to the distance between graphene
layers. In graphite it is approximately 3.3A.U.
There are two models which can be used to describe the structures of multi-walled nanotubes
a) In the Russian Doll model, sheets of graphite are arranged in concentric cylinders
E.g:- A (0,8) single walled nanotube within a larger (0,10) Single – walled nanotube.
b) In the Parchment model, a single sheet of graphite is rolled around itself, resembling a rolled
newspaper.
1.5. Synthesis of carbon nanotubes: Carbon nanotubes are generally prepared by three main techniques.
a) Arc discharge method b) Laser ablation method c) Chemical vapour deposition method
a) Arc discharge method:
This method, initially used for producing C60 fullerenes, is the most common and perhaps easiest
way to produce carbon nanotubes. This produces a mixture of components and requires separation
of nanotubes from the soot.
Nanotubes are produced through arc- vaporization of two carbon rods placed end to end separated by
1mm, in an enclosure filled by a mixture of inert gases (He & Ar) at low pressure (50-700m bar). A
direct current of 50 to 100A driven by 20V battery is applied, which produces a high temperature
arc-discharge between the two electrodes. The discharge vaporises one of the carbon rods and forms a
deposit nano products on the other rod. Measurements have shown that different diameter
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distributions are formed depending on the mixture of He and Ar, as they have different diffusion
coefficients and thermal conductivities.
These properties affect the speed with which carbon and metal catalyst diffuse and cool, thereby
affecting the diameter of the nanotube.
Depending on the exact technique, it is possible to selectively grow SWNTs or MWNTs but they
have few structural defects.
b) Laser ablation method:
In 1995, Smalley’s group reported the synthesis of carbon nanotubes by laser vaporization
A pulsed or continuous laser is used to vaporise a graphite target in an oven at 1200°C. The oven
is filled with helium or argon gas in order to keep the pressure at 500 torr. A very hot vapour
forms, expands and cools rapidly. On cooling, small carbon molecules and atoms quickly
condense to from large clusters, possibly including fullerenes.
The catalysts also begin to condense, attach itself to carbon clusters, prevents their closing into
cage structures or even open cage structures.
From these clusters, tubular molecules grow into single walled carbon nanotube, until the catalyst
particles becomes too large or until the condition, where the carbon no larger diffuses from the
surface. The yield is up to 70% but the method is expensive compared to other methods.
C) Chemical vapour deposition (CVD) method:
Chemical vapour deposition is achieved by putting a carbon source in the gas phase in an energy
source such as plasma or resistively heated coil, to transfer energy to gaseous carbon molecule.
The energy source ‘crack’ the molecules into reactive atomic carbon, which get settled on the
surface of the catalyst (viz, Ni, Fe or Co)
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Excellent alignment, as well as positional control on nanometer scale, can be achieved by using
CVD. It is the most promisable method for industrial production of CNT, because of low cost
and direct growth of desired material on the catalyst surface.
The catalyst is generally prepared by sputtering a transition metal on to the substrate and etched
thermally or chemically to induce catalyst particle nucleation. Ammonia is used for etching.
The temperature for the synthesis of nanotubes are generally from 650-900°C.The nanotubes
produced have yield up to 30%.
1.6. Properties of Carbon Nano Tubes
CNTs have several unique chemical, optical, electrical and structural properties that make them
attractive. The nano-materials possess very good catalytic activity due to increased area of contact. Due to
edges and points, their catalytic activity is maximum. They exhibit good ability for easy dispersion. But
CNTs possess toxicity and can cause harmful effects to vital organs with cell decay.
I. Mechanical properties
a) Strength
The strength of sp² C-C bonds of carbon nanotubes gives amazing mechanical strength. They are
the strongest and stiffest materials in terms of tensile strength and elastic modulus respectively.
They have a low density for a solid of 1.3 to 1.4 g / cm³ with specific strengths up to 48,000 KN
m / Kg.
Because of their hollow structure and high aspect ratio, they tend to undergo bucking when
placed under compressive, torsional or bending stress.
These properties, give them great potential in aerospace applications
b) Hardness: Super hard material was prepared by compressing single walled nanotube to above 25
GPa at room temperature. The measured hardness was 62-152 GPa. The bulk modulus was about 462-
546 GPa.
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II. Electrical properties: Because of the symmetry and unique electronic structure of graphene, CNT
is semi conducting with a very small band gap between its valence band and conduction band. Since
electrons propagate only along the tube axis and involve quantum effects, CNT is regarded as a one
dimensional conductor.
III. Vibrational properties: Atoms in CNT are continuously vibrating back and forth. They have two
modes of vibration which are Raman active.
IV. Optical properties: The optical properties of CNT are due to the absorption of photoluminescence
and Raman effect, which allows the quick and reliable characterization of nano tube quality in terms
of non tubular carbon content. CNT possess microwave absorption characteristics which are useful in
military radar systems.
V. Thermal properties: CNT are very good thermal conductors and exhibit a property called ballistic
condition.
VI. Functionalization: Grafting of chemical function at the surface of the nanotubes is called
functionalization. Functionalization gives scope for the addition of new properties to carbon
nanotubes.
1.7. Engineering applications of Carbon Nanotubes (CNT)
The small dimensions, strength and the remarkable physical properties of these structures make
CNTs a very unique material with a whole range of promising applications. They are used in energy
storage, energy conversion devices, sensors, field emission displays and radiation sources, hydrogen
storage media and nanometer- sized semi conductor devices. They are used as nanometers in metrology,
biological and chemical investigations. They have emerged as a new alternative and efficient tool for
transporting and translocation therapeutic molecules. CNT can be functionalized with bioactive peptides,
proteins, nucleic acids and drugs and can be used to deliver their cargos to cells and organs.
Functionalized CNT display low toxicity and are not immunogenic and hence used in the field of nano
biotechnology and nano medicine.
Applications in industry and research:
CNTs are used to make space elevators, stab proof and bullet proof clothing due to their
superior mechanical properties.
CNT – polymer composites are used for making electrical cables and wires due to their
superior conductivity.
CNT infused with cellulose is used to make paper thin batteries. Here CNT acts as electrodes
allowing storage devices to conduct electricity, which can provide steady output comparable to
a conventional battery.
CNTs are used in solar panels due to their strong UV/Vis-Near IR absorption characteristics
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CNTs are used for coating textile fibres which is anti-bacterial, electrically conductive, and
flame retardant with electro-magnetic absorption properties used in special equipment.
A spray-on mixture of CNT and ceramic coating gives unprecedented ability to resist damage
while absorbing LASER.
Hydrogen can be stored in the carbon nanotube, which can be used in the fuel cells. The
SWNTs are effective as hydrogen storage material for fuel cell driven electric vehicles. A
group of scientists has created a new, improved fuel-cell electrode that is very light in its
weight and thin. Composed of a network of single-walled carbon nanotubes, the electrode
functions nearly as conventional electrodes and renders the entire fuel cell much lighter weight
with greater efficiency.
Carbon nanotubes can replace platinum as a catalyst in fuel cells, which could significantly
reduce the overall cost. Carbon nanotube has advantage over platinum, since they are resistant
to corrosion.
The nanotube network from the fuel cell’s gas diffusion electrode is a layer of a porous
material that allows gas and water vapour to pass through to the catalyst layer. In the catalyst
layer, which typically consists of platinum particles, the protons and electrons of the gaseous
reactant material i.e., the fuel of the cell are separated and the electrons cause flow of
electricity.
The electric power densities produced using the Pt / CNT electrodes are larger than that of the
Pt/CB (carbon black) by a factor of two to four on the basis of the Pt load per power. CNTs are
thus found to be a good support of Pt particles for PEFC electrodes.
A catalyst having CNTs makes a reaction milder, safer and more selective.
CNTs are increasingly recognised as materials for catalysis, either as catalyst themselves or as
catalyst additives or as catalyst supportive materials.
The tightly packed, vertically aligned carbon nanotubes doped with nitrogen, are used as
cathodes in highly alkaline solution, to catalyze the reduction of oxygen more efficiently than
platinum.
Researchers have developed a novel catalyst using CNTs for the electrochemical reduction of
oxygen.
Oxidized CNTs with phosphorus added are a selective catalyst for the oxidative
dehydrogenation of butane to butadiene.
[O]
P-CNT +CO2+H2O+CO
+ other butanes
Catalyst
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CNTs along with ruthenium (Ru) metal are used as catalyst in the hydrogenation reaction
of cinnamaldehyde.
Some chemical reaction that are carried out inside the nanotubes:
i) Reduction of nickel oxide (NiO) to Ni.
NiO Ni
ii) Reduction of AlCl3 to its base metal.
AlCl3 Al
iii) Cadmiun sulphide (CdS) crystals have been formed inside the carbon nanotubes by
reacting cadmium oxide (CdO) crystals with hydrogen sulphide gas (H2S) at 400°C.
CdO + H2S CdS + H2O
Applications in medicine:
Carbons nanotubes (CNTs) are being highly used in the fields of efficient drug delivery and bio-
sensing methods for disease treatment and health monitoring.
Functionalization of SWNTs enhances solubility of drugs and allow for efficient tumor
targeting/drug delivery systems. It prevents SWNTs from being cytotoxic and altering the
function of immune cells.
Researches show that functionalized carbon nanotubes are non- cytotoxic and preserve the
functionality of primary immune cells. Certain types of CNTs functionalized with lipids are
highly water soluble, which would make their movement through the human body easier and
would also reduce the risk of blockage of vital body organ pathways, thus making them more
useful as drug delivery vehicles.
CNTs as drug delivery vehicles have shown potential in targeting specific cancer cells with a
dosage lower than conventional dosage of drugs used and do not harm healthy cells and
significantly reduce the side effects.
Due to high electrochemically accessible surface area, high electrical conductivity and useful
structural properties, single walled nanotubes (SWNT) and multi-walled nanotubes (MWNT)in
highly sensitive non-invasive glucose detectors.
Carbon nanotubes can be used as multifunctional biological transporters and near- infrared agents
for selective cancer cell destruction.
An aligned carbon nanotube ultra sensitive biosensor for DNA detection was developed. The
design and fabrication of the biosensor was based on aligned single wall carbon nanotubes
(SWCNT) with integrated single- strand DNAs (ssDNA).
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1.8. Fullerenes
The third newly discovered allotrope of carbon is Buck minister’s fullerene during laser
spectroscopy experiments. The structure of C60 resembles the geodesic dome (foot ball type) and named
after its architect Buck minister Fuller. In 1996, Prof. Robert. F. Curl Jr, Richard .E. Smalley and Sir
Harold .W. Kroto were awarded Nobel Prize for their discovery.
A fullerene is any molecule entirely composed of carbon, in the form of hollow sphere, ellipsoid, tube
or plane. Thus fullerenes are of the following types:
1. Spherical fullerenes: They look like soccer (foot ball) ball and are often called bucky balls. Fullerenes
are similar in structure to graphite composed of stacked graphene sheets, linked mostly of hexagonal or
sometimes pentagonal / heptagonal rings. Buck minister’s fullerene C60 is the simplest of all.
2. Cylindrical fullerenes: These are called carbon nanotubes or bucky tubes
3. Planar fullerenes: Graphene is an example of planar fullerene sheet.
1.9. Preparation of fullerenes
Fullerenes are prepared by vaporizing a graphite rod in He atmosphere when mixture of fullerenes
formed are separated by multi step solvent extraction methods. C60 is isolated by column chromatography
using alumina/hexane solvent system.
1.10. Properties of fullerenes
The bucky ball has cage like structure with certain unique properties. It is stable, denoted as C60 and has
sp2 hybridized carbon atoms, whose reactivity is increased by attaching active groups on the surface. It
exists as a discrete molecule. C60 is a mustard coloured solid . When its thickness increases, it appears
brown and then black. It is moderately soluble in the common organic solvents, especially aromatic
hydrocarbons like toluene. It dissolves in benzene forming a deep magenta solution. It has a high tensile
strength of any known 2D structure or element and has a high packing density. It can be compressed to
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30% of its original volume, without destroying its cage structure. It is stable up to 600 °C and undergoes
sublimation under vacuum at 600°C. it undergoes electrophillic addition at 6-6 double bonds. Other atoms
can be tapped inside to form inclusion compounds. When metal atoms are tapped inside, it is called
metallo fullerene, the best example being steel.
1.11. Engineering applications
Fullerenes have amazing conducting, magnetic, optical and mechanical properties.
1. They can easily accept electrons, therefore, they may be used as charge carries in batteries.
2. They can be used as organic photo voltaic cells as they have optical absorption properties.
3. Alkali metal fullerides are super conductors.
4. They can be used as soft ferro-magnets.
5. Its spherical structure makes it suitable to be used as a lubricant.
6. Because of their extreme resilience and sturdy nature, fullerenes are used in manufacture of
armor.
7. The water soluble derivatives inhibit the HIV-1 protease enzyme. Hence they are useful in the
treatment of HIV.
8. These are used as powerful anti-oxidants.
9. The fullerenes and fullerene black are chemically reactive and are added in the manufacture of
copolymers with specific physical and mechanical properties.
10. They are used as catalysts as they have the ability to accept and transfer hydrogen atoms. They
are highly effective in converting methane to higher hydrocarbons.
-oOo-
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2. SOLAR CELLS
Solar energy is through the sun’s rays that reach the earth. Solar energy originates from the
thermonuclear fusion reactions taking place in the sun. Only 0.2 to 0.5% of the solar energy reaching the
earth is trapped in photosynthesis. Thus only a tiny fraction of the solar energy reaching the earth drives
all our ecosystems. Solar energy is a renewable eco-friendly, perennial source of energy.
In 1830’s the British astronomer John Herschel used a solar thermal collector box to cook food
during an expedition to Africa.
Solar energy can be converted into Electricity by the following two ways.
1. Photo voltaic cells or Solar cells 2. Solar power plants
2.1. Photovoltaic cells: (PV cell (or) Solar cell (or) Solar Battery)
The basic unit of a photovoltaic system is the solar cell. The most common solar cells are made up of
highly refined silicon. These solar cells can change the sunlight directly into electricity.
Working principle of a solar cell: (Photo voltaic cell):
Solar cell constitutes a p-type semiconductor in
contact with a n-type semiconductor. Due to close
contact, the migration of holes or electrons is limited.
The outer layer of p-type semiconductor is struck by a
beam of light from the sun. When enough sunlight is
absorbed by the semiconductor, electrons are
dislodged from the atoms, and migrate to the surface
leaving positive holes. So a potential difference arises
between the p-type and n-type semiconductors. When
the terminals are connected to an external circuit,
electrons flows from n-layer to p-layer, which converts
directly the solar energy into electrical energy. This
device is called as a photo voltaic cell. The
photovoltaic individual cells can vary in size from
about 0.5 inches to about 4 inches. However one
single cell produces 1 or 2 watts. To increase the
power put cells are electrically connected out into a
packaged module.
The modules can be further connected to form an array. It refers to an entire photovoltaic power plant.
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Advantages of Photovoltaic power plants
1. The conversion of sunlight directly to electricity does not need any bulky mechanical generators.
2. PV arrays can be installed quickly in any size.
3. The environmental impact is minimal, requiring no water for system cooling and generating no
by products.
4. These will be producing DC (Direct current) which is used for small loads.
Disadvantages
1. The photovoltaic array is dependent on sunlight which is not constant but depends on location,
time of the day, time of year and weather conditions.
2. The Photovoltaic cells used for commercial applications must have an arrangement to convert the
resultant DC power into AC power.
2.2. Solar power plant
Solar thermal power plants generate electricity by using the heat from solar thermal collectors. The sun
rays are used to heat a fluid to very high temperatures. The fluid is then circulated through pipes and
transfers its heat to water to produce steam. The steam drives the turbine to produce mechanical energy
and into electricity by using a conventional generator. The heat required is produced by the solar
collectors. Solar thermal technologies use concentrator systems to achieve the high temperatures needed
to heat the fluid.
There are three main types of solar thermal power systems.
1. Solar parabolic trough
2. Solar dish
3. Solar power tower.
1. Parabolic trough: A long parabolic shaped reflector that focuses the sun’s rays on to a receiver
pipe. The collector tilts with the sun as the sun moves from east to west during the day to ensure that
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the sun is continuously focused on the receiver. Because of this parabolic shape of a trough it can
focus the sun light 30 to 100 times compare to the normal intensity. The receiver pipe located at the
focal line of the trough to achieve over 750 oF.
2. Solar Dish
The “Solar field” has many parallel rows of parabolic trough collectors aligned on a north-south
horizontal axis. The receiver fluid gets heated and runs to the series of “heat exchangers”. Here it can
transfer the heat to water to generate high pressure super heated steam. The hot fluid passes through the
heat exchangers cools down, and then re-circulated through the solar field to get heated up again.
Parabolic trough power plants can use fossil fuel combustion to supplement the solar out put during the
cloudy days.
A solar dish system uses concentrating solar collector that track the sun. So the concentrated solar energy
is collected at the focal point of the solar dish. The concentration ratio is much higher than the solar
trough typically over 2000 with temperature over 1380 oF. The engine in a solar dish system converts
heat to mechanical power by compressing the working fluid when it is cold, heating the compressed
working fluid, and then expanding the fluid through a turbine (or) with a piston to produce work, then it is
converted into electric power.
2.3. Solar power tower
A solar power tower or a central receiver generates electricity from sunlight by focusing concentrated
solar energy on a tower mounted heat exchanger. This uses the system of hundreds to thousands of flat-
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tracking mirrors called heliostats to reflect and concentrate the Sun’s energy to a central receiver tower.
The energy can be concentrated as much as 1500 times.
The energy losses are minimized as solar energy is being directly transferred by reflection from the
heliostats to a single receiver, rather than being moved through a transfer medium. Power towers must be
large to be economical. This is a promising technology for large scale grid-connected power plants.
2.4. Solar collectors
1. Non concentrating collectors
The collector area is same as the absorber area. Flat plate collectors are the non-concentrating
collectors, and are used when temperatures are below 200 oF.
It consists:
a) Flat plate: It absorbs the solar energy.
b) Transparent cover: It allows the solar energy to pass through and to reduce the loss of heat.
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c) Heat transport fluid: It is flowing through tubes to remove heat from the absorber.
2. Concentrating collectors
The area intercepting the solar radiation is more than the absorber area. Concentrating solar
systems require water for regular cleaning and for cooling the turbine generator.
Advantages
a) Very high temperatures are reached. High temperatures are suitable for electricity generation.
b) Good efficiency by concentrating sunlight hence current systems can get better efficiency.
c) A large amount of energy can be produced by using inexpensive mirrors.
d) Concentrated light can be redirected to a suitable location for illumination.
e) Heat can be stored by using molten salts in underground tank. This energy is used to be
converted into electricity during cloudy days and overnight conditions.
Disadvantages
a) These systems require sun tracking to collect the focused sun light.
b) In concentrating systems electricity drops drastically in cloudy condition.
-oOo-
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3. GREEN CHEMISTRY
Introduction: Green chemistry is also called as sustainable chemistry. It is a philosophy of chemical
research and engineering which encourages the design of products and processes that minimize the use
and generation of hazardous substances potentially dangerous to life on earth. The concept of green
chemistry (Environmentally benign synthesis) was coined by Paul Anastas of America. He enunciated 12
principles of Green chemistry in 1994 towards ideal synthetic methods to save natural resources.
“Green chemistry is the use of chemistry for pollution prevention by environmentally – conscious design
of chemical products and processes that reduce or eliminate the use or generation of hazardous
substances”.
3.1. Need for Green Chemistry
The 20th century brought the highest scientific development with respect to various benefits to the
mankind, but in turn has been responsible for a number of environmental problems at local and global
level. Our environment is to be protected from increasing chemical pollution associated with
contemporary life styles and emerging technologies. This is essential for survival of life systems.
Green chemistry is an essential piece of comprehensive program to protect human health and
environment. Green chemistry includes chemical process or technology that improves the environment
and thus our quality of life. Green chemistry applies across life cycle of a chemical product including
design, manufacture and use.
3.2. The Principles of Green Chemistry
Green chemistry is considered as a science based non-regulatory, economically driven approach and
essential piece of a comprehensive system to achieve the goals of environmental protection, human
health, sustainable development and eco-efficiency. Paul. T. Anastas and John Warner proposed twelve
Green Chemistry
Non toxic
Simple
Economical Safe
Avoid Waste
Sustainable
Environment friendly
Atom efficient
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principles of green chemistry. These are the guidelines for the development of next generation products,
processes and design of more efficient synthesis.
1. It is better to prevent waste than to treat or cleanup waste after it is formed.
2. Synthetic materials should be designed to maximize the incorporation of all materials used in the
process into the final product.
3. Wherever practicable, synthetic methodologies should be designed to use and generate substances
that possess little or no toxicity to human health and environment.
4. Chemical products should be designed to preserve efficacy of function while reducing their
toxicity.
5. The use of auxiliary substances (such as solvent, separation agents etc) should be made
unnecessary wherever possible.
6. Energy requirements should be recognized for their environmental and economic impacts and
they should be minimized. Synthetic methods should be conducted at ambient temperature and
pressure.
7. A raw material or feed stock should be renewable rather than depleting, wherever technically and
economically practicable.
8. Unnecessary derivations (blocking groups, protection/deprotection, temporary modification of
physical /chemical processes) should be avoided wherever or whenever possible.
9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
10. Chemical products should be designed so that at the end of their function, they do not persist in
the environment and break down into innocuous degradation products.
11. Analytical methodologies need to be further developed to allow for a real time, in-process
monitoring and control prior to the formation of hazardous substances.
12. Substance and the forms of a substance used in a chemical process should be chosen so as to
minimize the potential for chemical accidents including releases, explosions and fires etc.
3.3. Methods for Green synthesis (or) Green reactions
Chemistry plays an important role to develop the quality of our life and achieving a sustainable
civilization on earth. Synthetic methodologies are adopted which require the use of volatile solvents, dry
conditions, using of some hazardous chemicals and produce number of by-products which may be
harmful to the environment and human health.
Following are few of the methods of examples for greener synthesis.
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1) Aqueous phase method for green synthesis
An ideal solvent should solve solubility issues, inertness to the relevant chemistry, cost, safety of
handling, solvent recycling and environmental preferability. The role of solvent is very crucial in green
synthesis. In view of the environmental concerns caused by pollution of organic solvents, chemists all
over the world have been trying to carryout organic reactions in aqueous phase. The advantages of using
water as a solvent are its eco-friendly nature, low cost, non-inflammable nature, devoid of any toxicity or
carcinogenic effects, high specific heat resistance, unique enthalpic and entropic properties and easy
handling.
Ex: Knoevenagel Reaction:
The condensation of carbonyl compounds (mostly aromatic) with active methylene compounds in the
presence of weak base like ammonia, amine or pyridine is known as Knoevenagel reaction. If the
reaction is carried in presence of pyridine as a base, decarboxylation usually occurs
It is found that the aqueous phase reaction gives a better yield.
2) Phase transfer catalyst for green synthesis
Phase transfer catalyst is a heterogeneous catalyst, which is used to dissolve all salts which are
insoluble in organic phase solvent. It facilitates the migration of a reactant from one phase into another
where a reaction occurs. It transfers the anions from reagent (in aqueous phase) to substrate (organic
phase) to make the reaction occur faster. By using PTC, one can achieve faster reactions and higher
yields are obtained. The normal PTCs are quaternary ammonium salts like benzyl trimethyl-ammonium
chloride, phosphonium salts like hexadecyl tributyl phosphonium bromide and crown ethers.
R Y
Organic Phase
+ X
Q
PTC
R X + Y
Aqueous Phase
For example
H3CH2C
H2C Br
6+ NaCN
R4P+Br
PTCH3C
H2C
H2C CN
6R4P+Br+
1-bormooctane Sodium cyanide Nonyl nitrile
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The reaction between 1 – bromo octane and NaCN will not readily occur as it is poorly soluble in
water. The same reaction is carried out in presence of hexadecyl tributyl phosphonium bromide (PTC)
which yields nonyl nitrite.
3) Biocatalyst for Green synthesis
Biocatalysis is defined as a traditional chemical catalysis and the use of natural substances, which can
be one or more enzymes or cells, living or dormant, to catalyze a chemical reaction or series of chemical
reactions. It has many advantages in relevance to green chemistry. They are
1. Most of the reactions are performed in aqueous medium at ambient temperature.
2. They normally involve one-step processes.
3. Protection and deportation of functional groups is not necessary.
4. Reactions are faster.
5. These reactions are highly enantiomeric excess.
6. These show high chemo selectivity, enantio selectivity and region selectivity.
There are major six classes of enzymes.
Oxido reductase: These enzymes catalyze oxidation and reduction reactions.
Transferases: They catalyze the transfer of various functional groups.
Lyases: These are two types, one which catalyses addition to double bond and other catalyses
removal of groups and leaves double bond.
Hydrolases: These enzymes catalyze hydrolytic reactions.
Isomerases: These catalyze various types of isomerizations.
Ligases: These catalyze the formation of cleavage of Sp3 hybrid carbon.
4) Microwave assisted method for green synthesis
Microwaves have the wavelength ranging from 1cm to 1m. This is too low to induce a chemical
reaction. The exposure of heat under microwaves is microwave interactions. It is brought about by the
transformation of energy into the form of heat. Polar molecules absorb microwaves where as non-polar
molecules are inert to microwave radiations. In absence of electric field, dipoles are randomly oriented
and are under Brownian movement. In the presence of electric field, all the dipoles are lined up together
and this rapid re-orientation produce homogeneous heating.
Example:
Microwave – assisted reaction in organic solvents
Microwaves have been used for synthesis of chalcones and related enones in presence of organic
solvents. The reaction time decreases and the yield increases.
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C
O
CH3+
CHO
EtOH
Catalytic NaOH C
OKetoneChalcone90 - 100%
5) Ultrasound assisted method for Green synthesis
Ultrasound frequencies used for chemical reactions are in the range of 20 KHz–100 KHz.
Ultrasound is generated by an instrument having ultrasonic transducer, which converts electrical
or mechanical energy to sound energy. The commonly used transducer is made of quartz and it
works on Piezo electric effect.
Sono Chemistry is the branch of chemistry that is used to describe the effect of ultrasound waves
on chemical reactivity. This depends upon phenomena of ‘sonic cavitation or acoustic
cavitation’.
Advantages
It enhances chemical reactivity in a number of systems by as much as a million fold.
It effectively activates the catalyst by excitation of the atomic and molecular modes of the system.
It can increase solid surface area of the system through cavitation; it increases the observed rate
of reaction.
Example( Esterification)
Esterification carried out in the presence of acid catalyst like H2SO4, gives low yield and takes
longer time. In the presence of ultrasound, less time and more yields at ambient temperature are reported.
RCOOH + R'OHH2SO4 / RT
UltrasoundRCOOR'
RT = room temperature
3.4. Green synthetic methods should have :
1. High efficiency
2. Low waste
3. Low energy requirements
4. Environmentally benign reagents, catalysts, by-products and solvent systems
5. High atom efficiency to give high yields
6. High quality with no contaminations
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3.5. Engineering applications of Green synthesis:
Enormous growth in chemical and allied industries in last few decades has resulted in extensive
pollution of all segments of environment. Some of achieved goals and applications green synthesis are
given below.
1. Processes have been developed to remove oxides of sulfur and nitrogen, volatile organic compounds,
reactive organic gases and other air pollutants from fuel gases and other sources prior to allow their
emissions into the atmosphere.
2. Improving industrial processes to eliminate waste and reduce consumption of organic solvents
3. A new commercialized and greenery processes in biomass conversion used in synthetic fabric in
furniture and water based paint in the auto industry.
4. The main aim of nano technology is to minimize the environmental effects on human health, risk
associated with existing manufacture products and replacement of them.
5. By using biotechnology processes, the biomass is converted to fermentable sugars in presence of
enzymes.
6. The biggest success of green fuel technology is the replacement of gasoline with biodiesel.
7. The catalytic efficiency of the engineered micro-organism allows replacement of petroleum feed
stocks, reducing the amount of energy required and improving the process safety. The microbial
processes are less expensive, environmentally green and more productive.
8. In chemical production processes, the consumption of energy and water is reduced. Noise and even
by products are also reduced.
-oOo-
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4. CEMENT
Introduction: Cement is a composite building material possessing adhesive and cohesive properties and
capable of bonding materials like stones, bricks, building blocks etc. It is most widely used non-metallic
material in construction purposes. The principal constituents of cement used for constructional purpose
are compounds of Ca (Calcareous) and Al & Si (Argillaceous). Cement has a characteristic property to
form a paste with water and set into a hard solid mass having varying degree of strength and bonding
properties.(hydraulic in nature).
4.1. Portland Cement: It is an extremely finely ground product obtained by calcinations of an intimate
and properly proportioned mixture of argillaceous (clay-containing) and calcareous (lime-containing) raw
materials and gypsum together at a temperature of 1500 0C. At first it was made by Joseph Aspidin in
1824. It was so-named because a paste of cement with water on setting and hardening resembled in
colour and hardness to a “Portland stone”, a lime-stone quarried in Dorset, England.
Manufacture of Portland cement
Raw materials
1. Calcareous materials, CaO [such as limestone, chalk, marble etc.].
2. Argillaceous materials, Al2O3 and Sio2 [such as clay, slate, shale etc.].
3. Powdered Coal or fuel oil and
4. Gypsum (CaSO4. 2H2O).
Composition and functions of ingredients of Portland cement
1. Lime :( 61-67%) It is the principal constituent of the cement. Excess of lime reduces the strength
of the cement.
2. Silica :( 19-23%) It imparts strength to the cement.
3. Alumina :( 2.5-6%) It makes the cement to undergo quick-setting.
4. Gypsum :( 1.5-4.5%) It helps to retard the setting action of cement. It actually enhances the
initial setting time of the cement.
5. Iron Oxide :( 0-6%) It provides color, strength and hardness to the cement.
6. Sulphur trioxide :( 1-3%) It imparts soundness to cement and in excess reduces the soundness of
the cement. It is formed in the process.
7. Alkalis :( 0.3-1.5%) They make the cement efflorescent.
8. Magnesium oxide :( 1-5%) It provides hardness and color to the cement.
Manufacture of Portland cement involves the following processes.
1. Mixing of raw materials
It can be done either by a) Dry process or b) Wet process.
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a) Dry Process: The raw materials viz. limestone or chalk and clay are crushed into roughly 2-5cm pieces
and are ground into a fine powder using ball mills. In dry process the material is well mixed and directly
used.
b) Wet Process: The calcareous raw materials are crushed, powdered and stored in storage tanks called
silos. The argillaceous material is thoroughly mixed with water and washed in wash mills to remove any
adhering organic matter etc. and is stored in basins. Now powdered limestone and washed wet clay are
allowed in proper proportions into grinding mills, where they are mixed intimately to form a paste
called ‘slurry’. The slurry is led to a correcting basin, where its chemical composition is adjusted. The
final slurry contains about 38 to 40% of water. The slurry is dropped at a slow rate through a hopper at
the top of a rotary kiln, where calcinations takes place.
2. Burning: Burning or calcination is usually done in a rotary kiln, which is a steel tube, about 2.5 to
3.0 m in diameter and 90 to 120m in length, lined inside with refractory bricks. This is slightly inclined at
a gradient of 5o to 6
o.The kiln is rested on roller bearings and is rotated at 1 r.p.m. about its longitudinal
axis. The fuel for burning is usually powdered coal or vaporized burning oil and air which is injected at
the lower end of the kiln. A long hot flame is produced which heats the kiln up to a maximum
temperature of 17500C, with top middle and bottom zones having different temperatures to cause
sequential chemical reactions while the slurry slowly comes down the kiln on rotation.
Process: The ‘raw-mix’ or ‘corrected slurry’ is injected into the kiln at the upper end; while a hot flame is
forced into the kiln from the lower end. Due to the slope and slow rotation of the kiln, the material fed
move continuously towards the hottest-end at a speed of about 15m per hour.
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As the slurry gradually descends down the kiln due to rotation, the temperature rises. The chemical
reactions that take place in the kiln are divided into the following zones.
a. Drying Zone:
It is the upper part of the kiln, the temperature of which is around 4000C. Here most of the water in slurry
gets evaporated.
Slurry elimination of moisture
b. Calcination Zone:
It is the central part of the kiln, where the temperature is around 10000C. Here the limestone of dry mix
or slurry undergoes decomposition to form quicklime and carbon dioxide. The material forms small
lumps called nodules.
CaCO3 CaO + CO2
Limestone Quick lime
c. Clinkering Zone:
It is the lower part of the kiln, where the temperature is 15000C to 1700
0C. Here lime and clay undergo
chemical interaction yielding calcium aluminates and silicates.
2 CaO + SiO2 Ca2SiO4(C2S)
Di Calcium Silicate
3 CaO + SiO2 Ca3SiO5 (C3S)
Tri Calcium Silicate
3 CaO + Al2O3 Ca3Al2O6 (C3A)
Tri Calcium aluminate
4 CaO + Al2O3 + Fe2O3 Ca4Al2Fe2O10 (C4AF)
Tri Calcium Alumino Ferrite
The aluminates and silicates of calcium fuse together to form small, hard greyish stones known as
clinkers. These are very hot and are at about 1000 oC. The rotary kiln is supported with small kiln,
used to cool the clinkers by air-counter blast. This hot-air is further used for burning powdered coal/oil
left if any.
3. Grinding: The cooled clinkers are collected from cooling cylinders at the bottom of the kiln and are
ground to a fine powder in ball mills. During final grounding, a small quantity (2.3%) of powdered
gypsum is added, so that the resulting cement does not set quickly, when it comes into contact with water
400 0C
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or moisture. Gypsum retards the early setting of cement. After the initial set, the cement water paste
become stiff, but the gypsum retards the dissolution of C3A by forming tricalcium sulphoaluminate