CHAPTER I: GENERAL INTRODUCTION AND LITERATURE...

CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY

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CHAPTER-I

GENERAL INTRODUCTION AND LITERATURE SURVEY

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SSeeccttiioonn –– AA :: General Introduction

1.1 Introduction 22

1.1.1 Sensor 22

1.1.2 Classification of Sensors 33

1.1.3 Gas Sensors 44

1.1.4 Types of Gas Sensors 55

1.1.5 Heterojunction Gas sensors 88

11..22 Principle of Operation of Heterojunction Gas Sensors 1111

1.2.1 Characteristics of Heterojunction Gas Sensing

Performance

1111

11..33 Need for Gas Sensors 1133

11..44 Liquefied Petroleum Gas (LPG) 1144

1.4.1 Properties of LPG 1155

1.4.2 Hazards of Liquefied Petroleum Gas (LPG) 1177

SSeeccttiioonn –– BB :: Literature Survey

1.5 NiFe2O4 Thin Films 1188

11..66 Polyaniline Thin Films 2200

11..77 Orientation and Purpose of the Dissertation 2255

References 2288


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SSeeccttiioonn –– AA :: General Introduction

1.1. Introduction

“Reliable detection of hazardous, harmful or toxic gases has become a

major issue due to more stringent environment or safety regulations worldwide.”

Research and development of gas sensing devices is in the focus of

activity of scientists and engineers in many countries during last three decades.

Such detectors can be used for different applications viz continuous monitoring

of the concentration of gases in the environment and premises, detection of toxic

gases, drugs, smoke, fire, energy saving, anti-terrorist defense, health, amenity,

control of automotive and industrial emissions as well as various technological

processes in industry. Sensors have certainly poised on the brick of revolution

similar to that experienced in microcomputers in 1980’s. Tremendous advances

have been made in sensor technology and many more on horizon. The detection

of combustible gases like coal gas and liquefied petroleum gas (LPG) has

become very important because explosion accidents might be caused when they

leak out accidentally or by mistake. In spite of considerable effort, good sensors

for LPG have not been found hitherto, the problem being of vital significance to

industry as well as general public.

1.1.1 Sensor

The word “sensor” is derived from the Latin sentire which means “to

preserve”. A sensor therefore suggests some connection with human senses. It

may provide us information about physical and chemical signals which could not

be otherwise directly provided by our senses. A direct definition of sensor is a

device that responds to a physical (or chemical) stimulus (such as heat, light,

sound, pressure etc.) and transmits a resulting impulse (as for measurement or

operating a control). Thus sensor can detect an input signal (or energy) and

convert it into an appropriate output signal (or energy). Generally, sensors

provide an interface between the electronic equipments and the physical world


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typically by converting nonelectrical, physical or chemical quantities into

electrical signals.

1.1.2 Classification of Sensors

Sensors can be classified according to their principle of conversion (the

physical or chemical effects on the basis of which they operate) as physical

sensors and chemical sensors (Chart. 1). Physical sensors employ physical effects

such as piezoelectric, magnetostriction, ionization, thermoelectric, photoelectric

and magnetoelectric etc.

Chart 1: Classification of sensors according to principle of operation

Chemical sensors include gas, humidity, ionic and biochemical sensors.

Detection of toxic and flammable gases is one of the hot topic in both domestic

and industrial environments. The atmospheric pollution has lead more focused

research and development for a variety of sensors using different innovative

materials and technologies for low cost and lower operating temperatures.

Olfaction may recognize a harmful substance, but it is more sensitive to natural

and harmless compounds resulting from different biological processes. Gas


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chromatography combined with mass spectrometry is widely used for flame

ionization, photo-ionization and electron-capture gas chromatography detectors.

It shows a very limited time and space resolution. To meet this demand,

considerable research for new sensors is underway, including efforts to enhance

the performance of traditional devices, such as resistive metal oxide sensors,

through nano-engineering.

1.1.3 Gas Sensors

The increasing demand of gas sensors for environmental monitoring, gas

sensors can be manufactured using different materials, technologies and

phenomena. Tremendous advances have been made in sensor technology and

many more on horizon. The semiconductor gas sensors have got remarkable

position in science and technology, as they allow fast producing, reliable, non-

expensive and low-maintenance devices using modern technologies. Initially

ceramic gas sensors were employed to detect the presence of the gas. These are

of two types based on the chemical properties of gas to which the sensor is

sensitive as oxidizing gas sensors ( O2 and Cl2 ) and reducing gas sensors ( H2,

CO, NH3 and CH4 ). The oxidizing gas sensors cause the formation of acceptor

surface states in the semiconductive sensors and the reducing one causes the

formation of donor surface states. The most semiconductive gas sensors are

surface sensors. All semiconductor sensors are electronic and the semiconductor

sensors made of solid electrolyte are ionic. As per the measured quantity to

which sensor is sensitive to the influence of gases there are resistive,

potentiometric and amperometric sensors. The resistive sensors are mainly

semiconductive sensors and some sensors are made of solid electrolyte. The

resistance of these sensors is a function of gas atmosphere. The potentiometric

and amperometric include predominantly sensors made of solid electrolytes. The

electromotive force and the current strength are the parameters that changed with

gas.


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1.1.4 Types of Gas Sensors

There are three types of solid state gas sensors, currently in large scale

use. They are based on

1. Solid electrolytes

2. Catalytic combustion

3. Resistance modulation of semiconducting oxides.

1.1.4.1 Solid Electrolyte Sensors

Solid electrolytes are the materials that allow the conduction of ions but

not the conduction of electrons. Similar to liquid electrolytes, they support the

function of electrochemical cells, in which chemical reactions are allowed to

proceed to completion only if separate paths are provided for the flow of ions

(through the electrolyte) and electrons (through an electronic conductor). The use

of solid electrolytes in chemical sensing has a considerable history stemming

from the work of Nernst at the turn of the century. The essential function of the

solid electrolyte is to separate two regions of distinct activity of the species to be

monitored and to allow high mobility of an ion of that species between the two

regions. The function of the sensor is addressed through measurements of

potential (potentiometric), current (amperometric) or resistivity or conductivity

(conductometric). These sensors are only suitable for low concentration ppm

ranges. However, for portable applications, the electrochemical sensor life

expectancy is two year or it can be much shorter depending on the application

and the cost of sensors is high.

1.1.4.2 Catalytic Sensors

Potentially explosive mixtures of methane or other flammable gases, with

air can be monitored by means of a catalytically active solid state sensor [1]. The

device, often referred to as the ‘pellistor’ is essentially a catalytic

microcalorimeter. It consists of a catalytic surface constructed around a

temperature sensor and a heater which maintains the catalyst at a sufficiently


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high temperature to ensure rapid combustion of any flammable gas molecules

present. However, these sensors suffered from several problems, including loss

of sensitivity with time due to poisoning and burning out when exposed to high

gas concentrations.

1.1.4.3 Semiconducting Oxide Gas Sensors

The third type of gas sensor used widely and effectively is a gas sensitive

resistor. A sensing element normally comprising a semiconducting material

having a high surface to bulk ratio is deployed on a heated insulating substrate

between two metallic electrodes. The reactions of gas molecules can take place at

the semiconductor surface to change the density of charge carriers available on

the surface. Hence, the conductance of the device changes progressively with

changing atmospheric composition. Semiconductor gas sensors were originally

commercialized employing tin oxide as the sole sensitive component, but due to

lack of specificity between gases they were limited in application [2]. A range of

more specifically reacting materials is now employed and all above three of the

categories of gas can now be monitored. This is the fastest growing of the three

main types of solid state gas sensor at the present time. The materials most

widely preferred for the sensor application are metal oxides. They present an

opportunity for reactions that involve molecular chemistry, that is confined to the

surface layer of atoms but the electrical consequences of which are manifest

through a considerable volume of the solid. In the cases, where the surface

reactions are followed by a bulk change in stoichiometry, the resistance of the

entire artifact becomes modified. In other instances, surface reactions may

modify the conductance only to a depth of the order of a micron or so, but,

provided the oxide is presented in a high surface to bulk form (e.g. in a porous

thick film). In either case, semiconductor gas sensors function as gas sensitive

resistors and thus represent a simple, low cost and rugged means of atmospheric

monitoring. The effectiveness of gas sensors prepared from semiconducting

oxides depends on several factors including the nature of the reaction taking


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place at the oxide surface, temperature, catalytic properties of the surface,

electronic properties of the bulk oxide and the microstructure.

(A) Oxygen Detection with Semiconducting Thin Films

In recent years, a strong research effort has been made for developing

oxygen sensors that work at high temperatures. Generally, semiconducting

oxides that have been explored as oxygen sensors respond to changes in oxygen

partial pressure at high temperatures (973 K and above) by exploiting the

equilibrium between the composition of the atmosphere and the bulk

stoichiometry. At working temperatures, there is equilibrium between the

atmosphere surrounding of the sensor and the bulk stoichiometry [3].

Semiconducting thin films with a conductance variation due to surface effects

were studied for detecting low oxygen pressure (1-1000 mbar) at medium

temperatures (673-873 K). In that case, there is no oxygen diffusion in the bulk.

The oxygen chemisorption on the surface of polycrystalline thin films causes the

formation of Schottky barriers among the grains; when the condition Ld >Lc/2 is

satisfied (Ld is the Debye length and Lc is the crystalline size), complete

depletion of grains is obtained and the mobility is not thermally activated [4].

(B) Minority Gases Detection in Air

The second major category of gas detection for which semiconducting

oxides are employed, involves their use in an atmosphere of fixed oxygen partial

pressure (air) to detect minor concentrations of potentially hazardous gases. Bulk

changes in oxygen stoichiometry are not relevant to this type of application and

the materials are normally held at temperatures in the range 573–773 K, where

useful surface reactions proceed at a sufficient rate. The central surface reaction

of semiconducting oxides operating in air at temperatures in the range 573–773

K involves changes in the concentration of surface oxygen species such as O2.

The formation of such ions by oxygen adsorbed at the gas/solid interface

separates electrons from the bulk of the solid; thus the oxygen can be thought of

as a trap for electrons from the bulk. In the case of n-type semiconducting oxides,


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since the electrons are drawn from ionized donors via the conduction band, the

charge carrier density at the interface is reduced and a potential barrier to charge

transport is developed. When the semiconductor is exposed to a reducing gas, it

reacts with adsorbed oxygen and looses the charge carriers and decrease the

resistance of the semiconducting oxide. Thus, at temperatures in the range 573–

773 K, semiconducting oxides can be used as gas sensitive resistors to monitor

impurities in air that can react in such a way that the quantity of charge trapped at

the surface is altered. In the case of a p-type oxide, adsorbed oxygen acts as a

surface acceptor state, separating electrons from the valence band and hence,

giving rise to an increase in the charge-carrier (hole) concentration at the

interface. Any decrease in the surface coverage of oxygen ions, leads to a

decrease in charge-carrier concentration and hence, to an increase in the

resistance of the material. According to the above discussion, materials can be

classified as n-type or p-type, according to whether they show a decrease or an

increase an resistance when they are exposed to a reducing gas in an atmosphere

of fixed oxygen partial pressure (air). The resistance of some semiconducting

oxides is also found to respond to the introduction of oxidizing gases, such as

chlorine and nitrogen dioxide in air. It is very unlikely that these gases will react

with surface oxygen ions. The response of oxide semiconductor to oxidizing gas

takes place in the opposite sense to those with reducing gases.

1.1.5 Heterojunction Gas sensors

Heterojunctions are p-n junctions formed between dissimilar materials. A

heterojunction is defined as the interface formed between two dissimilar

materials usually two semiconductor having different band gaps. The interface

between two semiconductors possesses interesting properties. The nature of

interface formed helps to classify junction as an abrupt or graded. The junction

shows rectifying properties in that a current in one direction can flow quite easily

whereas in the other direction it is limited by a leakage current which is generally


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very small. The electronic behavior of heterojunctions is similar to that of

homojunctions except for new boundary conditions at the interface. The

transition between two materials at the interface causes a discontinuity in the

conduction band edges and valence band edges. These discontinuities originate

from band offsets, carrier effective mass, carrier mobility and the dielectric

constant difference between the two materials [5, 6]. Literature survey of

heterojunction during last 30 years revealed that, mostly heterojunctions are

fabricated for photovoltaic applications. Recently, heterojunctions have been

receiving great attention for considerably long time because of their several

useful applications such as sensors, switching devices, solar cells and in junction

field effects transistor [7]. Use of heterojunction for sensing gas is in practice

since last three decades. Usually a heterojunction gas sensor consists of two

semiconducting oxides in contact, with enhanced sensing behavior occurring at

the interface between the two materials. Large amount of research and

investigations are tried to obtain the gas sensor operating at room temperature

with miniaturizing sensor size. The semiconductor heterojunction gas sensor

consists of p-type and n-type material in contact forms interface. The external

effect of any gas influences the conductivity. The heterojunction sensors work on

the principle of barrier mechanism, which needs no adsorption and desorption of

oxygen for the detection of gases. The use of heterojunction as gas sensors was

first proposed by Nakamura et al. in 1986 [8]. The sensor was simply made by

mechanically contacting a p-type CuO ceramic and an n-type ZnO ceramic

pellet. The heterocontact showed a high sensitivity and selectivity to CO gas over

H2. The working mechanism suggested by Nakamura et al. was that the adsorbed

molecules of reducing gases form interface states that can change the potential

barrier height and consequently the current across the junction. Several systems

have been proposed for heterojunction sensors, mostly based on rectifying

junctions formed between p-type and n-type semiconducting ceramics, including

CuO(p)/ZnO(n), La2CuO4(p)/ZnO(n) and SmCoO3(p)/MOx(n), where M=Fe, Zn,


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In and Sn. For example a gas sensor based on a heterojunction formed between

the n-type ceramics SiC and ZnO has been investigated. The interface was found

to be rectifying and the forward bias current (defined by connecting the positive

terminal to SiC) was decreased on introduction of NO2 gas. Since the resistance

of the SiC component of the heterojunction was unaffected by the introduction of

NO2 and that of the ZnO component was only slightly affected, it was concluded

that the increase in electrical resistance associated with the introduction of the

NO2 was due to the behaviour of the n–n heterojunction interface. The gas

responses of several heterojunction sensor systems to target gases including CO,

H2, H2O, NO2 and C2H5OH have been investigated [9-17]. Also several

heterojunction sensor systems have been reported, that are mostly based on

interfaces between p-type (p) and n-type (n) semiconducting ceramics [14-16],

although heterojunction sensors formed between two n-type semiconducting

ceramics, giving rise to an n–n interface have also been described [16, 17]. Ling

et al. [18] have studied the NO2 and CO2 sensing properties of a heterojunction

gas sensor formed between n-type ZnO and p-type composite based on a mixture

of BaTiO3/CuO/La2O3 have been evaluated and compared with the performance

of its component p and n-type materials. It was found that the individual ZnO

and BaTiO3/CuO/La2O3 sensors showed resistance increase when exposed to

NO2. The resistance also decreased when exposed to low levels of NO2,

indicating that a different detection mechanism was operative at the

heterocontact compared with the single-phase materials. The sensing properties

of CuO/ZnO heterojunction gas sensors have been studied by Hu et al. [19] for

H2S and alcohol vapors operated at 381 K. Also the sensing properties of

heterojunction SnO2/La0.8Sr0.2CO0.5Ni0.5O3 thin film CO sensor have been

investigated by Chen et al. [20]. The SrCu2O2–ZnO heterojunction sensitive

transparent gas sensors have been designed by Nakamura et al. [21]. In this

study, transparent oxide p–n heterojunction diodes based on SrCu2O2 were

fabricated by RF magnetron sputtering. Gas sensitive properties of thin film


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heterojunction structures based on Fe2O3–In2O3 nanocomposites was investigated

by Ivanovskaya et al. [22] for C2H5OH, CH4, CO, NH3, NO2 and O3 gases at an

operating temperature 523–573K. The infrared photoconductive heterojunction

sensor between InAlAs/InGaAs has studied by Zhang et al. [23]. Electrical

properties of doped polypyrrole/silicon heterojunction diodes and their response

to NOx gas have been studied by Tuyen et al. [24]. Polyaniline–titanium dioxide

nanocomposite thin film gas sensor fabricated by Tai et al. [25] was reported for

NH3 and CO sensing. Laranjeira et al. [26] have fabricated Si-polyaniline

heterostructure for sensing radiation (γ) and ammonia gas.

1.2. Principle of Operation of Heterojunction Gas Sensors

The semiconductor heterojunction gas sensor consists of p-type and n-

type materials in contact that forms interface. The external effect of any gas

influences the conductivity of gas sensor. The heterojunction sensors work on the

principle of barrier mechanism, which does not adsorption and desorption of

oxygen for the detection of gases.

1.2.1 Characteristics of Heterojunction Gas Sensing Performance

The selectivity, sensitivity, stability, response and recovery time are the

important measures to decide the merits of gas sensor.

1.2.1.1 Sensor Current to Gas Concentration Characteristics

This sensor characteristic illustrates the influence of gas concentration on

the sensor current or conductivity. The concentration of gas is usually measured

in parts per millions (ppm) values or volume percent using the following relation

[27].

(v)chamber of Volume

gas of ppmKnown (ppm)ion concentrat Gas = (1.1)


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1.2.1.2 Sensitivity (Sg)

The sensitivity is defined as the degree of influence of a certain gas on the

conductivity or current of the sensor at a particular biasing voltage. Generally, it

is determined by the ratio of the current, (Ia) in the air to the current, (Ig) in a gas

with given concentration of the gas. The electrical currents of a heterojunction in

air (Ia) and in the presence of gas (Ig) are measured and using the following

relation the gas response is calculated.

(1.2) 100x aI

∆I 100x

aI

gIaI (%)

gS =

−

=

The sensitivity (Sg) is therefore a function of the concentration of the gas.

However, this definition of sensitivity is not only disadvantageous, since it is

difficult to compare the sensor parameters when the current changes within the

different boundaries, but also inconvenient. Since it is a function of concentration

of gas and not a universal parameter.

1.2.1.3 Response Characteristics

It is a current to time relation, taken down at sharp changes of the gas

concentration. Generally, a transition is made from clean air to an atmosphere

with a fixed concentration of some gas and vice-versa. The response is measured

at definite gas concentration. The response time is defined as the time at which

the current of the heterojunction reaches to 90 % of saturation value, as a result

of exposure of test gas and the recovery time is defined as the time required for

recovering the 90 % of the original current of the heterojunction. Fast response

and recovery times are essential for designing good sensor elements.

1.2.1.4 Selectivity

It is the ability of a sensor to respond to a certain gas in presence of other

gases is known as selectivity [28]. A good sensor will discern a particular signal


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by allowing adsorption of the desired gas while remaining insensitive to others.

The selectivity coefficient/factor (K) of ‘target gas’ to another gas is defined as:

B

A

S

SK = (1.3)

where, SA and SB are the sensitivities of sensor in “target gas” (A) and B gas,

respectively.

1.3 Need for Gas Sensors

The technology necessary for the monitoring of gases has developed in

parallel with the progressive industrialization of society through the course of the

twentieth century. Even though the olfactory system of humans is excellent for

the detection and identification of many odours, most hazardous gases or vapours

can be recorded only at too high concentrations or cannot be detected at all [29].

Since, the organic fuels and other chemicals have become an essential part of

domestic as well as industrial life and the awareness of the need to environment

protection has grown, the specific needs for gas detection and monitoring have

emerged. Currently, there are needs of gas monitoring which are divided in the

following three broad categories [30].

(i) For oxygen, in connection with the monitoring of breathable atmospheres

and for the control of combustion of processes i.e. in the boilers and

internal combustion engines where, the oxygen concentration of 20 % and

0-5 %, respectively is required.

(ii) For the toxic gases in air, for the protection of human health, where the

need is to monitor concentrations around the exposure limits which range

from less than 1 parts per million (ppm) to several hundreds ppm.

(iii) For the flammable gases in air to protect against the unwanted occurrence

of fire or explosion. In this case, concentrations to be measured are in the

range up to lower explosive level (LEL) which, for most of the gases, is

up to few percent.


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In other words, for toxic gas applications, a sensor must be able to detect

gases at low concentrations and the detection ranges are in ppm concentrations

while, for combustible gas monitoring, a sensor must detect high gas

concentrations, in the range of several percent.

1.4 Liquefied Petroleum Gas (LPG)

Liquefied petroleum gas (LPG) is a colourless odourless liquid which readily

evaporates into a gas. The group of products includes saturated hydrocarbons-

propane (C3H8) and butane (C4H10), which can be stored/transported separately

or as a mixture. They exist in the form of gases, as gases at normal temperature

and atmospheric pressure. This is because these gases liquefy under moderate

pressure readily vaporizing upon release of pressure. It is due to the property that

permits transportation and storage of LPG in concentrated liquid form. Generally

LPG comes from two ways; it can be obtained from the refining of crude oil, in

this way it is in pressurized form. LPG can be extracted from natural gas or crude

oil streams coming from underground reservoirs. The 60% of LPG in the world

today is produced this way whereas 40 % of LPG is extracted from refining of

crude oil. Ideally products referred to as "propane" and "butane" consist very

largely of these saturated hydrocarbons; but during the process of

extraction/production certain allowable unsaturated hydrocarbons like ethylene,

propylene, butylenes etc. may be included in the mixture along with pure

propane and butane. The presence of unsaturated hydrocarbons in moderate

amounts would not affect LPG in terms of combustion, but affects other

properties slightly (such as corrosiveness or gum formation). LPG is odourless

that can be odorized by adding an domestic as well as industrial supply to user,

so as to aid the detection of any leaks. It is slightly heavier than air and hence if

there is a leak it flows to lower lying areas. In a liquid form, its density is half

that of water and hence it floats initially before it is vaporize. It is non-toxic but

can cause asphyxiation in very high concentrations in air. LPG expands upon


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release and 1 liter of liquid will form approximately 250 liters of vapour. LPG is

used to provide lighting through the use of pressure lanterns. While butane and

propane are different chemical compounds, their properties are similar enough to

be useful in mixtures. Butane and Propane are both saturated hydrocarbons, they

do not react with other. Butane is less volatile and boils at 272.4 K. Propane is

more volatile and boils at 231 K. These products are liquids at atmospheric

pressure when cooled to temperatures lower than their boiling points.

Vaporization is rapid at temperatures above the boiling points. The calorific

(heat) values of both are almost equal. Both are thus mixed together to attain the

vapor pressure that is required by the end user and depending on the ambient

conditions. If the ambient temperature is very low propane is preferred to achieve

higher vapor pressure at the given temperature [31].

LPG can also be used as a fuel for domestic, industrial, horticultural,

agricultural, heating and drying processes. LPG can be used as an automotive

fuel or as a propellant for aerosols, in addition to other specialist applications.

1.4.1 Properties of LPG

A) Density:

LPG at atmospheric pressure and temperature is a gas which is 1.5 to 2.0

times heavier than air. It is readily liquefied under moderate pressures. The

density of the liquid is approximately half that of water and ranges from 0.525 to

0.580 @ 150c. Since LPG vapour is heavier than air, it would normally settle

down at ground level/ low lying places and accumulate in depressions.

B) Vapour Pressure:

The pressure inside a LPG storage vessel/cylinder will be equal to the

vapour pressure corresponding to the temperature of LPG in the storage vessel.

The vapour pressure is dependent on temperature as well as on the ratio of

mixture of hydrocarbons. At liquid full condition any further expansion of the

liquid, the cylinder pressure will rise by approx. 14 to 15 kg./sq.cm for each


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degree centigrade. This clearly explains the hazardous situation that could arise

due to overfilling of cylinders.

C) Flammability:

LPG has an explosive range of 1.8% to 9.5% volume of gas in air. This is

considerably narrower than other common gaseous fuels. This gives an

indication of hazard of LPG vapour accumulated in low lying area in the

eventuality of the leakage or spillage. The auto-ignition temperature of LPG is

around 410-5800c and hence it will not ignite on its own at normal temperature.

Entrapped air in the vapour is hazardous in an unpurged vessel/cylinder during

pumping/filling in operation. In view of this it is not advisable to use air pressure

to unload LPG cargoes or tankers.

D) Combustion:

The combustion reaction of LPG increases the volume of products in

addition to the generation of heat. LPG requires upto 50 times its own volume of

air for complete combustion. Thus it is essential that adequate ventilation is

provided when LPG is burnt in enclosed spaces otherwise asphyxiation due to

depletion of oxygen apart from the formation of carbon dioxide can occur.

E) Odour:

LPG has only a very faint smell and consequently, it is necessary to add

some odourant, so that any escaping gas can easily be detected. Ethyl mercaptan

is normally used as stenching agent for this purpose. The amount to be added

should be sufficient to allow detection in atmosphere 1/5 of lower limit of

flammability or odour level 2 as per IS: 4576.

F) Colour:

LPG is colourless both in liquid and vapour phase. During leakage the

vapourisation of liquid cools the atmosphere and condenses the water vapour

contained in them to form a whitish fog which may make it possible to see an

escape of LPG.


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G) Toxicity:

LPG even though slightly toxic, is not poisonous in vapour phase, but can,

however, suffocate when in large concentrations due to the fact that it displaces

oxygen. In view of this the vapour posses mild anaesthetic properties.

1.4.2 Hazards of Liquified Petroleum Gas (LPG)

LPG is a flammable gas which has the potential to create a hazard.

Therefore it is important that the properties and safe handling of LPG are

understood and applied in the domestic and commercial/industrial situations.

LPG in liquid form can cause severe cold burns to the skin owing to its rapid

vapourisation. LPG forms a flammable mixture with air in concentrations range

2% to 10%. If LPG stored in correctly can cause fire and explosion hazard.

Vapour/air mixtures arising from leakages may be ignited some distance from

the point of escape and the flame can travel back to the source of the leak. Since,

an empty LPG cylinder must handle carefully because they may contain small

amount of LPG vapour that can cause hazard [32].


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SSeeccttiioonn –– BB :: Literature Survey

1.5 NiFe2O4 Thin Films

Spinel metal iron oxides, MFe2O4 (M is divalent metal ion) have wide

range of applications in various technical fields due to their interesting electrical

and magnetic properties [33]. Among them, NiFe2O4 is a ferromagnetic material

having potential applications in magnetic cores, optomagnetic devices, memory

devices and vertical recording magnetic materials in thin film form [34]. The

NiFe2O4 is also one of the most versatile and technologically important ferrite

material, because of its typical ferromagnetic properties, low conductivity, low

eddy current losses, high electrochemical stability, catalytic behaviour and

abundance in nature. It crystallizes in a spinel structure and exhibit tuneable

conducting behaviour [35]. Stoichiometric NiFe2O4 films exhibit n-type electrical

conductivity. NiFe2O4 is the most suitable material for device applications in the

upper microwave and lower millimetre wave ranges. Many reports are available

on the preparation of NiFe2O4 thin films by chemical as well as physical methods

and application of NiFe2O4 films as a gas sensor for detection of various gases.

Before commencing the studies on chemical synthesis of NiFe2O4 and its

application as a gas sensor, it is necessary to survey the literature on synthesis of

NiFe2O4 films and their use as sensors. NiFe2O4 is distinguished ferrite material

due to its high Neel temperature, low microwave loss, low magnetic anisotropy

and low magnetostriction [36]. NiFe2O4 doped with zinc or manganese and some

other additives can be used in electronic devices such as filters, high-frequency

transformers or inductors, magnetic recording heads or antennas, requiring quite

high permeability, saturation magnetization and electrical resistivity [37-39].

NiFe2O4 has been used as a highly reproducible gas and humidity sensor

material. A variety of physical as well as chemical methods have been used for

the deposition of NiFe2O4 thin films including pulsed laser deposition (PLD) [40,

41], sputtering [42] ferrite plating [43], dip coating process [44], spray pyrolysis


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[45], electrodeposition [46] etc. The PLD has been used to produce NiFe2O4 by

three different methods. These methods include direct deposition of the

stoichiometric NiFe2O4 spinel layer and in other two are solid-state reaction

between NiO and Fe2O3 was utilized to produce the spinel [47]. Negulescu et al.

[48] prepared NiO/ NiFe2O4 bilayer using pulsed laser deposition and studied

exchange biasing in NiFe2O4 in bilayers. Venzke et al. [49] have prepared

epitaxial NiFe2O4 thin films by sputtering onto SrTiO3 (STO) and Yittria

Stabilized Zirconia (YSZ) substrates. Magneto-optical Kerr effect in NiFe2O4

thin films doped with zinc was observed by Calle et al. [50] using sputtering

technique. Kitada [51] found the formation of NiFe2O4 thin films during

exposion of sputtered permalloy films into dry air. Hsiao and Mauri [52] have

studied formation of NiFe2O4 from plasma induced surface segregation of NiFe

and its oxidation during plasma oxidation. Preferred oxidation of Fe and NiFe

films has been observed by Valetta et al. [53]. Literature adopted sol–gel method

for fabrication of NiFe2O4 substituted with Cu and Zn, found that the values of

saturation magnetization Ms and coercivity Hc of the films as a function of the

film composition [54]. Abe and Tamaura invented ferrite plating in 1983

facilitates the formation of crystalline spinel ferrite films with various

compositions, (Fe, M)3O4 (M, Fe, Co, Ni, Zn, Al, Cr, Ti etc.) directly from an

aqueous solution in the temperature range at 333–373K [55]. Mathe and Bhosale

studied the annealing effect on spray deposited Ni-Zn ferrite [56]. Bruckner et al.

have prepared NiFe2O4 thin films by oxidation of NiFe (20 wt %) thin films

deposited by magnetron sputtering at temperature 673 K in air [57]. The physical

methods have many drawbacks such as small area of deposition, requirement of

sophisticated instruments, high working cost of system, wastage of depositing

material, cleaning after each deposition etc. The stoichiometric problem

encounters in producing ternary compounds (oxide) thin films because of

creation of point defects and secondary phases. However, requirements of high

deposition temperature, heat treatment, stiochiometry and control over film


20

thickness make deposition of ferrite thin films a critical job, which has limited

the development of ferrite technology in microelectronics for their use in

information storage devices [58]. Sartale et al. [59, 60] have introduced a novel

two step electrochemical route for the deposition of NiFe2O4 thin films at room

temperature. They have prepared NiFe2O4 thin film by electrochemical oxidation

of pre-electrodeposited stoichiometric NiFe2 alloys. Kulkarni et al. [61]

deposited NiFe2O4 thin films by electrodeposition using both aqueous and non

aqueous bath. Recently, Gunjakar et al. reported nanocrystalline NiFe2O4 thin

films obtained from CBD method [62].

1.6 Polyaniline Thin Films

The polyaniline is a conducting polymer of the semi flexible polymer

family. Recently polyaniline has captured the attention of the scientific

community due to the discovery of its high electrical conductivity. Amongst the

family of conduting polymers, polyaniline is unique due to its ease of synthesis,

environmental stability and simple doping/dedoping chemistry. Although the

synthetic methods to produce polyaniline are quite simple, its mechanism of

polymerization and the exact nature of its oxidation chemistry are quite complex.

Because of its rich chemistry, polyaniline has been one of the most studied

conducting polymers in the past 20 years. Polyaniline was synthesized either

chemically or electrochemically. Chemically synthesized polymer is either

deposited from dispersion [63] or solution [64]. Since electrochemical synthesis

takes place directly on the metal surface, it is expected to have better adherence

than in the case of chemically synthesized polyaniline. The main problems of the

electrochemical synthesis are essentially related to the nature of the metal, since

each metal needs specific conditions to deposit the conducting polymer. An

important criterion for the success of the electrochemical polymerization reaction

is the choice of proper solvent which should have a high dielectric constant, low

viscosity and a low freezing point.


21

The first report on the production of “aniline black” dated back to 1862

when Letheby used a platinum electrode during the anodic oxidation of aniline in

a solution containing sulphuric acid and obtained a dark green precipitate [65].

This green powdery material soon became known as ‘aniline black’. The interest

in this material retained almost academic for more than a century since “aniline

black” was a powdering and intractable material. Green and Wood head

performed the first organic synthesis and classification of intermediate products

in the “aniline black” formation and three different aniline octamers were

identified and named as leucoemeraldine, pernigraniline and emeraldine base.

These names are still used, indicating various oxidation states of polyaniline

(Fig. 1.1).

Fig. 1.1 Various states of oxidation and protonation of polyaniline.

Polyaniline is a typical phenylene based polymer having a chemically

flexible —NH— group in a polymer chain flanked either side by a phenylene

ring. It is a unique polymer because it can exist in a variety of structures

depending on the value of (1–y) in the general formula of the polymer shown in

Fig. 1.1 (a) [66, 67]. The electronic properties of polyaniline can be reversibly


22

controlled by protonation as well as by redox doping. Therefore, polyaniline

could be visualized as a mixed oxidation state polymer composed of reduced {–

NH–B–NH–} and oxidized {–N=Q=N–} repeat units where –B– and =Q= denote

a benzenoid and a quinoid unit, respectively forming the polymer chain (Fig. 1.1

(a)), the average oxidation state is given by 1–y. Depending upon the oxidation

state of nitrogen atoms which exist as amine or imine configuration, polyaniline

can adopt various structures in several oxidation states, ranging from the

completely reduced leucoemeraldine base state (LEB) (Fig. 1.1 (b)), y–1 = 0, to

the fully oxidized pernigraniline base state (PNB) (Fig. 1.1 (c)), where 1–y = 1.

The “half” oxidized (1–y = 0.5) emeraldine base state (EB) (Fig. 1.1 (d)) is a

semiconductor and is composed of an alternating sequence of two benzenoid

units and a quinoid unit. The protonated form is the conducting emeraldine salt

(ES). The electronic structure and excitations of these three insulating forms

(LEB, PNB and EB) are contrasted. However, the LEB form can be p-doped

(oxidatively doped), the EB form can be protonic acid doped and the PNB form

can be n-doped (reductively doped) to form conducting ES systems. The EB,

intermediate forms of polyaniline can be non-redox when doped with acids to

yield the conductive emeraldine salt state of polyaniline as demonstrated in Fig.

1.2. It can be rendered conductive by protonating (proton doping) the imine

nitrogen, formally creating radical cations on these sites. This doping introduces

a counterion (e.g. Cl─ if HCl was used as the dopant) and counterion was affixed

to the parent polymer by partially sulfonating the benzene rings in the polymer,

resulting in a so-called “self-doped” polymer. Both organic acids such as HCSA

(camphor sulfonic acid) and inorganic acids, such as HCl are effective with the

organic sulfonic acids leading to solubility in a wide variety of organic solvents,

such as chloroform and m-cresol. The protonic acid may also be covalently

bound to the 16 polyaniline backbone has been achieved in the water soluble

sulfonated polyaniline (Fig. 1.2). Similar electronic behavior has been observed

for the other nondegenerate. Polyaniline is unique among intrinsically conducting


23

polymers in that it can be doped with a proton donor, usually by adding an

organic acid. This “protonic acid doping” gives a positive charge to the structure

and results in a manyfold increase in the conductivity of polyaniline compared

with the undoped form. Polyaniline is stable in air and its electronic properties

are readily customized [68-73]. Each oxidation state has a specific colour and

specific electrical properties. The interest in polyaniline-based junction devices is

because of the processing ease and stability of polyaniline and the extensive

knowledge base existing for this material. Conducting polyaniline has been used

as sensing material for different vapors like methanol, ethanol, acetone and

benzene and for various gases like NH3 and hydrogen [74, 75].

Fig. 1.2 Protonic acid doping of polyaniline (emeraldine base) to polyaniline

(emeraldine salt).

Electrochemically polyaniline can be synthesized either by potentiostatic,

galvanostatic or cyclic voltammetric techniques. The electro-oxidation of aniline

by continuous cycling between the predetermined potentials produces an even

polymeric film to the electrode surface [76]. Polyaniline can be prepared in

several oxidation states, with electrical conductivity varying progressively from

10-11

to 10 S cm-1

.


24

The research in electrosynthesized polyaniline thin films during last two

decades showed that intensively films are synthesized as doped polyaniline (by

adding protonic acids such as HCl, H2SO4, HNO3 and HClO4) on various

substrates. The bronsted acids are very common dopant [77], or by adding

inorganic dopants. As far as doping in polyaniline is considered, many studies

[78] have established that the macromolecular structural, morphological and

electrical properties of polyaniline are found to be dependent on the nature of

dopant ion inserted into polymer. Recently heteropolyanions have been used as

dopant in polyaniline in its electrochemical studies [79]. Chloride doped

polyaniline films have been prepared in HCl acid using potentiodynamic method

by Hussain and Kumar [80]. The results showed better adhesion, less porosity

and uniform deposition. Cyclic voltammetry result showed that oxidation of

polyaniline occurs in the potential range of +1 to +1.2 V and reduction occurs in

the range + 0.2 to + 0.8 V. The studies on effect of concentration and substrate

resistance on redox properties of polyaniline prepared using HCl acid is

investigated by Bedekar et al. [81]. Thin polymeric films were deposited by

cyclic voltammetry, potentiostatic or galvanostatic techniques on the Fe-disc

electrode from aqueous oxalic acid solutions [82]. Also the films deposited on

iron substrate electrode were studied as enzyme modified electrode [83].

Electropolymerization of polyaniline has been carried out on different high

surface area carbon substrates: carbon fiber and reticulated vitreous carbon

(RVCe). The electrical and morphological characteristics of the polyaniline films

were strongly affected by the chosen substrate [84]. Polyaniline films prepared

by electrochemical polymerization in sulfuric acid showed stability for doping

(energy density 140-160 Ah/kg) and undoping in non-aqueous electrolyte by

choosing suitable conditions. According to cyclic voltammetry with lower

potential sweep rates (< 5 mV/s), oxidation curves of polyaniline films exhibit

three peaks in non-aqueous solution instead of two in aqueous solution. These

oxidation curves can be related to radical cation formation and polaronic


25

transition by in-situ measurements of UV-visible spectra, ESR spectra and

impedance combined with cyclic voltammetry [85]. Synthesis of polyaniline with

doped copper ion was carried out by Chakane et al. [86]. Polyaniline films have

been electrodeposited on aluminum electrode surfaces in acidic electrolytes as

anti-corrosion coatings [87]. Electrochemical polymerization of aniline (ANI)

was performed in aqueous 0.5 M H2SO4 using pulse potentiostatic method

(PPSM). The polymer was also characterized in various acidity conditions. The

electroactivity of the film was found to be retained in highly acidic solution and

showed decreasing activity with increasing pH [88]. The synthesis of polyaniline

thin films was made onto commercially available polyethylene terephthalate

(PET)/indium tin oxide (ITO) substrates. Current research is directed to

polyaniline films that find applicability as sensors and biosensors [89-92], solar

cells [93], batteries [94], electrochromic devices [95] and antistatic coatings [96].

The polyaniline is also one of the most promising materials for electrochemical

supercapacitors [97, 98]. Recently, Dhawale et. al [99-101] have reported

electrosynthesized polyaniline thin films for supercapacitor application, because

of polyaniline films shows high specific capacitance [102].

1.7 Orientation and Purpose of the Dissertation

The fabrication of cost effective heterojunction is an industrial demand of

device technology. Method of fabrication and materials used decides the

industrial importance of device in the fields such as opto-electronic device,

storage device, microelectronic device, sensors etc. The detection of combustible

gases like coal gas and liquefied petroleum gas (LPG) has become very

important because explosion accidents might be caused when they leak out

accidentally or by mistake. In spite of considerable effort, good sensors for LPG

have not been found hitherto, the problem being of vital significance to industry

as well as general public. LPG sensor must be able to measure gases at low

concentrations and the detection ranges are in ppm concentrations while, for


26

combustible gas monitoring, a sensor must measure high gas concentrations, in

the range of several percent. Hazardous gases specifically, liquefied petroleum

gas (LPG) has been widely used for several industrial and domestic applications.

Being highly explosive, leakage of LPG is a serious problem. Consequently,

there is a need for development of cost effective sensors to monitor LPG of the

lowest possible concentration at room temperature (300 K). Especially LPG

sensor application, requires low density and high surface area materials

(nanocrystalline). A large number of metal oxides and ferrites have shown

sensitivity to certain gas species. Spinel compounds, with a general formula of

AB2O4, have also been proved as important oxides in gas sensors and have been

investigated for the detection of both oxidizing and reducing gases. But, at

certain low concentration of the gases, these metal oxide sensors show poor

performance with respect to the sensitivity, long term stability, selectivity at high

temperature etc. However, thin films prepared by high temperature physical

methods are very expensive, in which there is no control on grain size and

uniformity. Present the electrodeposition and chemical bath deposition CBD

methods are simple, easy, reasonable and working at low temperature methods.

Due to the low temperature, we can better control on grain size, which results in

to nanostructured NiFe2O4 and polyaniline thin films. Specifically,

nanocrystalline NiFe2O4 thin films can be deposited on cost effective stainless

steel substrates by chemical and electrochemical deposition methods and

followed by deposition of polyaniline film by an electrodeposition method. The

structural, surface morphological, elemental, optical and electrical

characterization of these films will be carried out prior to the fabrication of

NiFe2O4/polyaniline based heterojunction. Current research is directed towards

the fabrication of polyaniline based heterojunctions for detection of LPG at room

temperature. Recently, n-CdS/p-polyaniline and n-CdTe/p-polyaniline

heterojunctions investigated at room temperature LPG sensors. The maximum

response were achieved an up to 70% upon exposure of 0.08 vol% or (1040 ppm)


27

LPG [103, 104]. The stability is essential factor in gas sensor. However metal

chalcogenides was used in the heterojuction devices which are less stable as

compared to the metal oxides. Therefore, TiO2/polyaniline based heterojuction

devices were utilized by Dhawale et al. [105] in gas sensor application which

showed 63% maximum sensitivity at 0.1 vol% (1300 ppm).

The main goal of present work is the fabrication of n-NiFe2O4/p-

polyaniline based heterojunction using cost effective chemical and

electrochemical methods for the detection of LPG at room temperature with low

gas concentrations. The attempt is made to improve the sensing performance and

stability of the sensor.

The X-ray diffraction (XRD) technique will be used for the structure

identification. The surface morphology of the films will be studied using scanning

electron microscopy (SEM). The crystallite size of these films will be determined using

transmission electron microscopy (TEM). Elemental bonds of material confirmed by

fourier transform infrared (FTIR) spectroscopy. The optical properties will be studied in

the visible range of spectrum using UV-VIS spectrophotometer. The electrical

properties will be studied by dc two-point probe method. Surface wettability of these

films will be studied using contact angle meter. The current-voltage (J-V)

characteristics will be studied in order to evaluate various electrical parameters to test

the diode quality. The sensing performance at different concentrations of LPG will be

studied at room temperature (300 K) by current-voltage (J-V) characteristics under the

forward biased condition. Lastly, the performance evaluation of n-NiFe2O4/p-

polyaniline heterojunction LPG sensor will be evaluated in terms of their selectivity,

sensitivity and stability.


28

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CHAPTER I: GENERAL INTRODUCTION AND LITERATURE...

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