unit vi chemistry of advenced materials -16 · 2015. 12. 3. · unit vi chemistry of advenced...

UNIT VI CHEMISTRY OF ADVENCED MATERIALS 2015-16

Engineering Chemistry 43

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


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



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.



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



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)



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.



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



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



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).



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



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-



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.



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



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-


Engineering Chemistry 7

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.



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-



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



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.



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



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.



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



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-



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.



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.



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



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

(3CaO.Al2O3. x CaSO4. 7H2O).

3 CaO.Al2O3 + x. CaSO4 . 7H2O 3 CaO.Al2O3.x. CaSO4 . 7H2O

Tri Calcium Sulpho Aluminate

(Insoluble)

The insoluble tricalcium sulphoaluminate prevents too early further reactions of setting and hardening.

4. Packing: The ground cement is stored in silos, from which it is fed to automatic packing machines.

Each bag contains 50Kg of cement.

4.2. Setting and hardening of cement

When cement is mixed with water, called ‘cement paste’, hydration reaction takes place, resulting in the

formation of gel and crystalline products with plasticity. The interlocking of crystals finally binds the

inert particles of the aggregates into a compact rock-like material. Slowly they lose their plasticity and

become stiff and hard in the process called setting, while the anhydrous compounds become hydrated to

give a rocky mass.

The process of solidification comprises of

(i) Setting (ii) Hardening

1. Setting is defined as stiffening of original plastic mass, due to the initial gel formation.

2. Hardening is development of strength, due to crystallization.

The strength developed by cement paste at any time, depends upon the amount of gel formed and the

extent of crystallization. Initial setting of cement – paste is mainly due to the hydration of tri calcium

aluminate (C3A) and gel formation of tetra calcium aluminoferrite.

3 CaO. Al2O3 + 3 CaO. Al2O3.6H2O + 880 KJ / Kg

C3A + C3A. 6 H2O + 880 KJ / Kg

Hydrated Tri CalciumAluminate (Crystalline)

Tri CalciumAluminate

or6 H2O

6 H2O

4. CaO. Al2O3. Fe2O3 + + CaO. Fe2O3. H2O3 CaO. Al2O3. 6 H2O

Crystal Gel

C4AF + +

(or)

7 H2O

7 H2O C3A. 6H2O CF. H2O

The di calcium silicate starts hydrolyzing to tobermonite gel, which also contributes to initial setting.



2 [ 2CaO.SiO2] + 3 CaO. 2 SiO2. 3 H2O + Ca(OH)2 + 60 Cal / g

+ C3S2. 3 H2O + Ca(OH)2 + 60 Cal / g

or4 H2O

2 C2S 4 H2O

Final setting and hardening of cement paste is due to the formation of tobermonite gel, crystallization of

calcium hydroxide and hydrated tricalcium aluminate.

2 [3CaO.SiO2] + 3 CaO. 2 SiO2. 3 H2O + 3 Ca(OH)2 + 500 Cal/g

+ C3S2. 3 H2O + 3Ca(OH)2 + 500 Cal/g

or6H2O

2 C3S 6 H2O

The setting and hardening may be depicted as

Sequence of chemical reactions during setting and hardening of cement

Specifications for cement as per Indian Standards:

The lime saturation factor shall be between 0.6 to 1.22. The ratio of aluminium and iron oxide shall not be

less than 0.66. The insoluble residue should not exceed 2%. The weight of magnesia should not exceed

6%. Total sulphur content should not be more than 2.75%. Loss on ignition should not exceed 4%. The

initial setting time shall be 30minutes and final shall be 10 hours.

4.3. Decay of cement

Although cement concrete is strong, yet it is highly susceptible to chemical attack, because concrete

contains some free lime. Salty water and other acidic solutions attack cement and concrete. In acidic

water, lime in concrete dissolves making it weak. As the pH of most of the natural waters is slightly

greater than 7, such water does not have any effect on the strength of the concrete. However, as the

acidity increases (or pH decreases), the deterioration of concrete enhances.

Unhydrate Cement

Hydration

Metastable gel Crystalline hydration products

Stable gel Crystalline products

Cement +

Water (Paste)

Hydration

of C3A

and C4 AF

1 day Gelation

Of C3S

Gelation of

C2S And C3S 7 day 28 day



It may be pointed, that lime is more soluble in soft water than hard water. The deterioration of concrete is

quicker, when in contact with soft water.

The presence of sulphates and chlorides in water also removes lime. The sulphates combine with

tricalcium aluminate to form sulphoaluminate, which occupy more volume. This causes expansion;

thereby the life of concrete is greatly reduced. If the concrete is soaked in mineral oil for some time, its

resistance to abrasion decreases. Sugar also causes concrete failure. If even as low as 0.1% sugar is added

to cement, the setting time is delayed and its strength is greatly reduced.

4.4. Protection of concrete: A surface coating of bituminous materials prevents direct contact between

concrete and water. By coating the surface with silicon fluoride in a soluble form, together with

oxides of Zn, Mg or Al the deterioration of cement can be prevented to some extent. The precipitate

of CaF2 so formed in capillaries prevents the dissolution of lime.

Environmental and social impacts

1. Air Pollution: Cement manufacture causes environmental impacts at all stages of the process. These

include emissions of airborne pollutents in the form of dust, gases, noise and vibration. Reductions of

these emissions is highly essential.

2. Impact on climate: Cement manufacture contributes gases both directly and indirectly. The fine dust

particles in colloidal form cause silicosis or fibrosis and respiratory problems for work men as well as

inhabitants in the vicinity of the industry. The CO2 associated with Portland cement manufacture fall

into three categories.

1. CO2 derived from calcinations of limestone

2. CO2 from kiln fuel combustion

3. CO2 produced by vehicles in cement plants and distribution.

It is producing 5% of global manmade CO2, of which 50% is from chemical process and 40%

from burning fuel. One alternative in reducing CO2 emission is lime mortar, which reabsorbs CO2 during

manufacture and has lower energy requirements.

-oOo-



5. Lubricant

Lubricant: The substance introduced between two moving/sliding surfaces to reduce the frictional forces

between them is known as a lubricant.

5.1. Lubrication: The process of reducing frictional resistance between moving/sliding surfaces, by the

introduction of lubricants in between them, is known as lubrication.

Functions of Lubricant:

1. It reduces surface deformation, wear and tear, because the direct contact between rubbing

surfaces is avoided.

2. It reduces the loss of energy in the form of heat (or) it acts as a coolant.

3. It reduces the energy loss, so that machine efficiency is enhanced.

4. It reduces the expansion of metal by local frictional heat.

5. It avoids seizure of moving parts.

6. It avoids the unsmooth relative motion of the moving parts.

7. It reduces the maintenance and running cost of the machine.

8. Sometimes lubricant acts as a seal. E.g.: Lubricant used between piston and the cylinder wall of

an internal combustion engine acts as a seal, thereby preventing the leakage of gases.

5.2. Mechanism of Lubrication:

The lubrication is affected mainly by three types of mechanisms.

1. Fluid film or thick film or hydrodynamic lubrication:

In this mechanism the moving surfaces are separated from each other by a fluid thick film of at

least 1000Å thick, so that this thick film avoids the surface to surface contact. The lubricant film covers

the surfaces and forms a thick layer in between them. This consequently reduces wear. The resistance to

movement of sliding is only due to the internal resistance between the particles of the lubricant moving

over each other. Hence lubricant should have minimum viscosity under working conditions and should

remain in between the two surfaces. This Hydrodynamic lubrication occurs in the case of a shaft running

at a fair speed with low load.

Delicate instruments, light machines like watches, clocks, guns, sewing machines, specific

instruments etc. are provided with this type of lubrication.

Hydrocarbon oils are considered to be satisfactory lubricants for fluid film lubrication.

2. Boundary Lubrication or thin film lubrication:

This is at high load and low speed. This is happens when



A shaft starts moving from rest. (or)

The speed is very low (or)

The load is very high and

Viscosity of the oil is too low under such conditions; the clearance space between the moving

surfaces is lubricated with an oil lubricant. Under high load conditions, the friction is reduced with a

suitable lubricant which forms only one or two molecules thick protection layer to prevent the metal-

metal contact. This type of boundary protection is known as boundary lubrication. The coefficient of

friction in such cases is 0.05 to 0.15.

Vegetable and animal oils are the glycerides of higher fatty acids. These are either physically

absorbed to metal surfaces (or) chemically reacts to metal surfaces.

The load is carried by the two layers of adsorbed lubricant. The fatty oils possess a greater

adhesion property than mineral oils, but they tend to break down at higher temperatures and to improve

the oiliness of mineral oils, small amounts of fatty oils are added. Graphite & Molybdenum disulphide

either alone or as stable suspension in oil are also used for boundary lubrication, which possess low

internal friction and can bear compression and high temperatures.

For boundary lubrication, the lubricant should have:

Long hydro carbon chains

Polar groups to promote spreading and orientation over the metallic surface.

Active groups or atoms, which can form chemical linkages with the metals or other surfaces.

High viscosity index, resistance to heat and oxidation, good oiliness and low pour point.

3. Extreme pressure lubrication:

Under heavy load and high speed operating conditions, the liquid lubricants cannot service as

lubricants at that high pressure and high temperatures. They may decompose or even vaporize. In order

to with stand the high temperatures produced due to the frictional heat

Some extreme pressure additives are added to the lubricant. The chlorinated esters, sulphurized oils

and tricresyl phosphate are the examples of extreme pressure additives. These additives react with

metallic surfaces, at high temperatures to form metallic chlorides, sulphides or phosphides in the form of

durable films. These films can with stand very high loads and high temperatures.

5.3. Classification of Lubricants:

The lubricants are classified into 4 types based on their physical state.

1. Liquid Lubricants, (or) Lubricating oils

2. Semi-solid, Lubricants (or) Greases



3. Solid Lubricants

4. Emulsions.

1. Liquid Lubricants:

These lubricating oils provide a continuous fluid film over the metallic surfaces. A good liquid

lubricant must have the following characteristics.

These lubricating oils are sub classified into 4 types.

i. Animal & Vegetable oils: The animal and vegetable oils are the esters of higher fatty acids.

ii. Mineral Oils: These are obtained by the distillation of petroleum.

iii. Blended oils: To improve the typical properties of oils, additives are added. These are called

as “Blended oils”

iv. Synthetic lubricants: Synthetic lubricants have been developed, to meet the requirements at

operating conditions; these can serve as lubricants in the temperature range -50 to 250 oC.

They have high viscosity index.

E.g.: Polyglycol ethers, fluoro and chloro hydro carbons, organo phosphates and silicones

2. Semisolid Lubricants: (Greases)

Lubricating grease is a semi-solid, consisting of a soap through out a liquid lubricating oil. The

Greases are to 6 types.

i. Calcium based grease: (or) Cup –greases.

These are the emulsions of oils with calcium soaps

ii. Soda Base greases:

These are the petroleum oils mixed with sodium soaps.

iii. Lithium base greases:

These are petroleum oils these are thickened by mixing lithium soaps.

iv. Aluminium Soap Grease:

Aluminium – soap greases are special purpose lubricants.

v. Barium – Soap Grease:

These Barium – soap greases are buttery (or) fibrous texture and are reddish yellow in color.

vi. Rosin – soap Grease: (Axle greases)

These are very cheap resin greases, prepared by adding lime to resin & fatty oils.

3. Solid Lubricants:

Solid lubricants are used, where

The liquid lubricating oils and grease cannot be used as lubricants.

Combustible lubricants must be avoided.



Contamination of grease or lubricating oil is unaccepted.

The operating temperatures (or) load is too high.

i. Graphite:

Graphite consists of a multitude of flat plates held together by only weak bonds. So that the force to

shear the crystals is very low, consequently the layers can slide one over another. Due to this property the

graphite can be used as a lubricant. Graphite is soapy to touch and non-inflammable and non-oxidized in

air below 375 °C. It is used in powdered form (or) as suspension. When graphite is colloidal dispersed in

oil, it is called “Oil dag” and when graphite is dispersed in water, it is called “aqua dag”. It is ineffective

above 370 °C.

Uses: Used for lubricating air-compressors, general machine open gears, chains, cast iron bearings,

internal combustion engine.

ii. Molybdenum disulphide: (MoS2)

This is stable up to 400 °C. It has a sandwich like structure. It has poor inter laminar attraction is

responsible for low shear strength. This property serves the MoS2 as a lubricant. It’s fine powder

sprinkled on surfaces sliding at high velocities. It also used along with solvents and in greases. The other

substances like soapstone, talk, mica are also used as solid lubricants.

4. Lubricating Emulsions:

An emulsion is a two phase system, one being dispersed as fine droplets in the other. In order to

prevent over heating of the tool, efficient cooling and lubrication is required. This is done by emulsions.

Two types of emulsions are used for lubricants:

i) Oil in water type emulsions (or) cutting emulsions:

ii) Water in oil type emulsions (or) cooling liquids:

5.4. Applications of Lubricants:

Lubricants are mainly used for reducing the frictional force between surfaces. They have

following uses.

Anti wear, anti oxidants, and antifoaming agents.

As demulsifying and emulsifying agents.

As rust and corrosion inhibitors.

In machinery as engine oil, compressor oils gear oils, and piston oils.



As hydraulic, brake and gear box fluids.

Used in the soap and paint industries.

Synthetic lubricants are used in turbines, vacuum pumps and semiconductor devices.

Liquid lubricants are used in medicines.

Some lubricants are used as cutting fluids in many industries. Oil, water and oil emulsions are used

as a cutting fluids. These are also used as cutting fluids in cutting, grinding, trading and drilling of

metals.

The cutting fluids are used both as lubricant and cooling agent in the machines where the friction is

very high and generates a large amount of local heat. As a result the tool is over heated and may even

lose its temper and hardness.



6. LIQUID CRYSTALS

6.1. Liquid crystals (LCs) are a state of matter that has properties between those of a conventional

liquid and a solid crystal. In 1888, Austrian botany physiologist Friedrich Reinitzer, examined that

cholesteryl benzoate had two distinct melting points. At 145.5 °C it melts into a cloudy liquid and at

178.5 °C it melts again and the cloudy liquid becomes clear, which now belongs to cholesteric liquid

crystals. Liquid crystals may flow like a liquid, but their molecules may be oriented in crystal like

organized way. Liquid crystals have generally a rod like molecular structure, rigidness of the long axis

and easily polarizable substituents. The distinguishing characteristic of the liquid crystalline state is the

tendency of the molecules (mesogens) to point along a common axis, called the director. The tendency of

the liquid phase molecules are in contrast to this, which have no intrinsic order and hence show isotropic

behaviour. In the solid state the molecules are highly ordered and have little translational freedom and

thus are anisotropic. The tendency of the liquid crystal molecules to point along the director leads to a

condition known as anisotropy comparable to solid crystals.

6.2. Characteristics of Liquid cyrstals:

The following parameters describe the liquid crystalline structure.

1. Positional order refers to the extent to which an average molecule or group of molecules shows

translational symmetry.

2. Orientational represents a measure of the tendency of the molecules to align along the director on a

long-range basis.

3. Bond orientational order describes a line joining the centers of nearest – neighbour molecules without

requiring a regular spacing along that line.

i. Structure: Long, narrow elongated molecules having sufficient molecular interactions: Long and

narrow molecules does not satisfy the mesomorphic structure. Eg: n-paraffins and homologues of

acetic acid. The forces of attraction between these molecules are not sufficiently strong for an ordered,

parallel arrangement to be retained after the melting of the solid. The particular mesomorphic

structure not only depends on molecular shape but also depends on the position of polar groups with in

the molecule. Molecular interactions lead to dipole-dipole interactions, dipole-induced dipole

interactions, dispersion forces and hydrogen bonding. These are necessary to achieve the degree of

molecular order, required to liquid crystalinity.

Eg: XR1 R1

http://en.wikipedia.org/wiki/State_of_matter

http://en.wikipedia.org/wiki/Crystal

http://en.wikipedia.org/wiki/Friedrich_Reinitzer

http://en.wikipedia.org/wiki/Friedrich_Reinitzer

http://en.wikipedia.org/wiki/Cholesteryl_benzoate



A. Central core of benzene rings is linked by a functional group “G”. Some Central Linkages:

B. Presence of Unstauration: The unsaturated compounds can form mesophase, where as n-aliphatic

compounds cannot form and the double bonds enhance the polarizability of the molecules. The overall

linearity of the molecules must not be sacrificed in liquid crystals.

Eg: a)

RO

COOH

ROCOOH

Trans-m-alkoxy cinnamic acid. It shows mesomorphic behaviour

cis-m-alkoxy cinnamic acid. Does not shows mesomorphibehaviour

CH2 H2C CO2CH3H3CO2C (b)

The freedom of rotation about the double methylene bridge in compound (c) destroys the rod shape of the

molecules.

CHH3CO2C CH

CO2CH3

(c)

X Series name

- CH = N - Schiff base

-N=N- Diazo compounds

- N = N - O - Azoxy compounds

- CH = N - O - Nitrones

- CH = CH - Stilbenes

- C = C - Tolans

- O - |

C = O Esters



These (c) Stilbens derivatives show no rotation and are linear and mesomorphic.

C. Absence of bulky functional groups: Bulky, even it highly polarizable, functional groups or atoms that

are anywhere, but on the and of a rod shaped molecule. Destroy liquid crystal capabilities. Enhanced

interactions in the molecule deviates the linearity.

Eg:

COOHC10H20

Br

M-bromo 4-deceloxy – benzoic acid.

D. Presence of carboxylic group at the end of side chain:

In this type of situation in the molecules leads to hydrogen bonding, that can induce mesomorphic

behavior by lengthening the molecular unit through dimerisation

O

O H O

OH

In some cases the hydrogen bonding may lead to non-linear molecular associations, which can destroy

parallelism

6.1.Types of Liquid Crystals

This classification is based on breaking order of the solid state and has two types:

1. Thermotropic liquid crystals 2. Lyotropic liquid crystals

6.1.1 Thermotropic liquid crystals

Thermotropic phases are those that occur in a certain temperature range. If temperature is too high,

thermal motion may destroy the ordering in the LC phase and isotropic liquid phase will occur. Ex:

Cholesteryl benzoate, p-azoxy anisole etc. These have been classified into following types.

a) Smectic liquid crystals

b) Nematic liquid crystals

c) Cholesteric liquid crystals

a) Smectic (or) soap-like liquid crystals: Smectic is the name given by G. Friedel for certain

mesophase with mechanical properties similar to soaps.



.

Smectic phases

All smectic LCs have layered structure, with definite interlayer spacing. This can be measured by X-ray

diffraction. Smectic liquid crystals on heating retain long-range order, yielding smectic phase. They lose

the periodicity with in the planes, but retain the orientation and arrangement in equi-spaced planes.

b) Nematic or thread like liquid crystals: These are less ordered. These on heating lose their planar

structure, but retain a parallel alignment. Thus they retain orientation, but lose periodicity. The

molecules tie parallel to each other but can move up or down or sideways or can rotate along their axes.

N-paramethoxy benzylidene – p – butyl aniline changes to nematic liquid at 240C and this state persists

up to 430C, after which it melts in to an isotropic liquid. Nematic liquid crystals do not conduct

electricity when they are in pure form. They flow like liquids, but their mechanical (like viscosity,

elasticity) electrical (like dielectric constant) , optical properties and diamagnetism etc., depend upon the

direction along which they are measured.

Nematic phase

c) Cholesteric liquid crystals: These are optically active and possess the arrangement of molecules

similar to those in nematic type. Such liquid crystals are characterized by very high optical rotation,

probably a thousand times greater than that of their crystalline variety. Moreover, on raising the

temperature, the pitch decreases. This results in corresponding change in the wavelength of reflection.

They are named so because the skeleton of these substances pass through a state similar to that of

cholesterol, a steroid present in blood.



6.1.2. Lyotropic liquid crystals

Some compounds are transformed to a LC phase, when mixed with other substance (solvent) or when the

concentration of one of the component is increased. Such compounds are called lyotropic LCs. A

lyotropic liquid crystal exhibits liquid-crystalline properties in certain concentration ranges. Many

amphiphillic molecules show lyotropic liquid-crystalline phase. Examples are: Sodium laureate in water,

Dhosphatidly choline in water.

6.2. General & engineering applications of LCs

1) LCs find wide use in LC displays, which rely on optical properties of LCs in the presence and absence

of an electrical field. The orientation of molecules in a thin film of nematic liquid crystal is easily

altered by pressure and by an electric field. The altered orientations affect the optical properties of the

film, such as causing the film to become opaque. If we arrange certain patterns when an external

magnetic field is imposed on these electrodes on to a thin film of liquid crystal the patterns of the

electrodes become visible. This is the principle used in LCD watches and in digital calculators and

such other LCD based instruments. They consume little electric power.

2) When a beam of light strikes a film of a smectic liquid crystal, the properties of the reflected light

depend on this characteristic distance. Since this distance is temperature sensitive, the reflected light

changes with changing temperature. This phenomenon is the basis of liquid crystal temperature

sensing devices, which can detect temperature changes as small as 0.01oC with ordinary light.

http://en.wikipedia.org/wiki/Lyotropic_liquid_crystal



3) Liquid crystals are used in gas liquid chromatography, because their mechanical and electrical

properties lie in between crystalline solids and isotropic liquids.

4) Liquid crystals are employed as solvents during the spectroscopic study of structure of anisotropic

molecules,.

5) Cholesteric liquid crystals are used in thermograph a method employed for detecting tumors in body.

-oOo-



Exercise Questions

1. Define nano materials and how are they classified based on the dimension?

2. Explain in brief carbon nano tubes. What are the various types of carbon nano tubes? Discuss the

synthesis and properties of carbon nano tubes?

3. What are carbon nano tubes? Discuss the applications of these materials in medicine, fuel cells and

catalysis?

4. What are fullerenes? Explain the different types of fullerenes?

5. Discuss the preparation, properties and applications of fullerenes.

6. Write short note on following

a. Nano wires b. Quantum dots

7. Properties and engineering applications of carbon nano tubes.

8. Write the advantages and disadvantages of solar energy.

9. Discuss in brief about (i) Photo – voltaic cell (ii) Solar Thermal power

10. Explain how solar energy is converted to electricity by different methods.

11. What do you mean by Green –House effect? How the green house effect is useful to mankind.

12. Discuss the need and the principles involved in green chemistry?

13. Explain in brief various methods employed for green synthesis?

14. Write short notes on the following

a. Ultrasonic method

b. Biocatalyst & Phase transfer catalyst methods.

c. Aqueous phase method.

15. Give the engineering applications of Green chemistry.

16. Define nano materials and how are they classified based on the dimension?

17. What are fullerenes? Explain the different types of fullerenes?

18. Properties and engineering applications of carbon nano tubes.

19. What are liquid crystals? How do they differ from crystalline state and liquid state?

20. Write the advantages of solar energy.

21. Discuss the need and the principlesinvolved in green chemistry?

22. Give the engineering applications of Green chemistry.

23. Explain in brief the manufacturing of Ordinary Portland Cement with a neat flow diagram? Write the

chemical reactions involved in manufacturing of OPC.

24. Explain in brief carbon nano tubes. What are the various types of carbon nano tubes? Discuss the

synthesis and properties of carbon nano tubes?

25. What are carbon nano tubes? Discuss the applications of these materials in medicine, fuel cells and

catalysis?

26. Discuss the preparation, properties and applications of fullerenes.



27. Give in brief the classification of liquid crystals along with their characteristics and give few

applications of liquid crystals.

28. Discuss in brief about (i) Photo – voltaic cell (ii) Solar collectors

29. Give a detailed account of setting and hardening of cement.

30. Write about the classification of lubricants with examples.

31. Write short note on following

a. Nano wires b. Quantum dots

32. Explain how solar energy is converted to electrical energy.

33. Explain in brief any two methods employed for green synthesis?


a. Ultrasonic method. b. Biocatalyst & Phase transfer catalyst methods. c.Aqueous phase method.


a. Effect of CO2, SO2 and chlorides on cement concrete

b. Decay of cement

c. Soundness of cement

d. Hydration in hydrolysis of cement

36. Define a lubricant. Why is lubricant needed?

35. Write about the classification of lubricants with examples.

36. Explain the mechanism of lubrication.

37. Write a short note on any three properties of lubricants

a) Acid number

b) Viscosity index

c) Aniline point

d) Saponification number

e) Flash point and fire point

38. Explain the method of lubrication employed under the following conditions

a) Low load and high speed

b) Extreme pressure

39. Explain the following terms

a) Absolute and kinematic viscosity

b) Types of friction involved in the mechanism of machines

c) Give some standard automobile engine oils used for lubrication

unit vi chemistry of advenced materials -16 · 2015. 12. 3. · unit vi chemistry of advenced...

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