CHAPTER -V SENSORS

208

CHAPTER -V

SENSORS

5.1. INTRODUCTION:

Life and environment are interdependent. The plant and animal life is affected

by various environment factors; in turn these lives modify their environment in many

ways. Man himself is no exception to it and is involved in a tremendous struggle

against the pollution of the environment. The pollution from various sources has

gone to such an extent that the human beings are unable to breathe a fresh air. The

environmental pollution is unfavorable alteration of our surroundings, wholly or

largely as a byproduct of our actions through direct or indirect effects of changes in

air quality around us. The development of efficient devices for detection of toxic

gases in the environment is an important challenge in the area sensor development.

The recent development of polymeric materials for fabrication of sensors, with

every imaginable combination of physical and chemical characteristics for fabrication

of sensors has led to the fabrication of efficient gas sensors. These sensors find

applications in industrial, technological, medical, civilian and strategic sector.

Polymeric thin-film sensors are the latest devices to be used for the above

mentioned applications and have been tested for their efficiency, reliability, cost

effectiveness and performance. The most important advantage of these polymeric

sensors is their room temperature operation and high sensitivity.

Sensors are key elements in this rapidly moving evolution, so the demand for

sensors has soared in the last decade. Solid-state sensors that combine integration

circuits and micro machining technologies as well as new materials open an avenue

that can lead to many families of sensors to meet the new demands in performance,

size and cost. Sensor research and development has flourished during the last

decade and a wealth of knowledge has accumulated.

Gas sensor resistor array research and development is conducted towards

three directions in order to get a wide variety of behavior and also a number of

209

possible gas compounds to be monitored. The main sensor element types should be

based on the following technologies and materials:

Thin film SnO2 resistor elements sputtered on Mica substrates,

operated at high temperatures.

Thick film SnO2 resistor elements prepared from Organo-metallic

pastes on Allumina substrates, operated at high temperatures.

Electro-active conducting polymers (ECPs) deposited electro-

chemically on thick film substrates operated from room temperature to

high temperatures.

A common property of all the three types is that the sensor elements are

resistors. They can be used to sense gases and vapors while monitoring the

changes in conductance on exposure of the films to the gas sample. Electro-

conducting polymers (ECPs) can be used to sense gases and vapors while

monitoring the changes in conductance on exposure of the polymer to the sample

gas. This type of gas sensors should operate at room or closed room temperatures.

These Electroactive-conducting polymers (ECPs) are still under development for

appropriate applications such as rechargeable batteries, capacitors, field-effect

transistors, enzymatic sensors and gas sensors [1]. Their behavior differ

considerably from that found with inorganic SnO2 based gas sensor resistors and

gives a good chance to improve the selectivity and combining organic and inorganic

based sensors elements with in an intelligent monitoring unit.

Wilthelm Von Siemens, for example, built one of the first sensors in 1860. He

made use of the temperature dependence of a resistance of copper wire, for

temperature measurement. The development of semiconductor technology had its

beginnings in the 1950s, since then the opportunities for electronic signal processing

and control techniques have improved enormously. The first stage of the

development of gas sensors has been attributed to the period 1960-1980 when

various types of gas sensors were first introduced. Development of new gas sensors

210

in terms of sensing materials, fabrication techniques, application and understanding

of their sensing mechanism accelerated the development of new sensor techniques

over the last decade.

The first gas sensor using oxide semiconductors, as a gas sensitivity layer

was proposed in 1962 [2]. It was based on the fact that gas exposure causes a

change in the electrical conductivity of Zinc Oxide (ZnO) thin films. Since then many

other metal-oxide semiconductors, such as tin dioxide (SnO2) [3-12], titanium

dioxide, gallium oxide and ferric oxide have been researched for their gas sensing

applications. Thick and thin film technology have been the most frequently used

method in manufacturing actual gas sensitivity layers for gas sensors. Recent

development in semiconductor thin-film gas sensors is given by Sbseveglieri [13],

material selection for semiconductor gas sensors by Moseley [14], and new

materials and transducers for chemical sensors by Gopel [15], describe well the

recent research of numerous gas sensitivity materials for the detection of various

gases.

Using conducting polymers in impedance type sensors gives a possibility to

build up low cost, highly sensitive and selective room temperature gas sensors [16-

20]. They may offer a number of advantages over the conventional inorganic based

sensors. Because of their very unique and specific behavior, they are considered

intelligent material systems. ECPs can easily be synthesized and deposited onto

conducting surfaces by a simple electrochemical polymerization method. The

electrical conductivity related to the doping level of an ECP may also modulated by

the interactions with various substrates and analytes.The incorporated doping ions or

other species transmit the environmental effects into the film [1].

Conducting polymers can also be used to sense gases and vapours by

monitoring the change in conductance on exposure of the polymer to the gas

sample. Preliminary studies on these materials have shown that they exhibit

fastened reversible response even at room temperature, which is generally not

211

expected with inorganic films [19-20]. These polymers have a number of distinct

advantages in the point of view of gas sensing.

A wide variety of polymer materials are available.

They can be formed by electrochemical polymerization of the monomer

under Coloumbmetrically controlled conditions.

A number of doping materials can be incorporated.

The thickness of the film is variable by changing the polymerization time.

The gas sensors can be operated at room or close to room temperature.

They are cheap enough to provide disposable sensor elements.

Chemical sensors provide direct, real-time information on the contents of

certain chemical substance(s) present in their environment [21]. Their practical

importance continuously increases as they are not only offer an advantageous

alternative to time and cost demanding laboratory analysis but, primarily need

necessary input information to a great variety of automatic devices, regulating

mechanisms and robots; therefore, humans often cease to be direct users of the

information provided and can concentrate on the designing and operating larger

scientific or technological systems.

Vacuum deposited polyanniline film sensors have been prepared for detection

of toxic gases and microbiological species in environment, coal mines, semiconductor

industry and medical applications and food processing industry. The behavioural

acceptance tests have shown that the sensors are highly specific, selective and cost

effective. The sensitivity of the sensors is high and detection limit is low. The stability,

reproducibility and self-life are excellent. A model has been proposed for the sensing

mechanism in polyaniline thin films [22]. Ellipsometric sensitivity to halothane vapors

of hexgmethydisiloxgne plasma polymer films has been studied by Guo etal.[23].

Multicomponent polymer electrolyte new extremely versatile receptor materials of gas

212

sensors (VOC monitoring) and electric noses (odour identification discrimination)

have been studied by Cammann et al. [24].

Amrani et al. [25] studied multi frequency interrogation technique applied to

conducting polymer gas and odour sensors. Sensing behaviours of the

electrochemically co-deposited polypyrrole (Vinyl alcohol) thin film exposed to

ammonia gas has been studied by Lin et al. [26]. Another interesting application was

proposed by Bing Joettwang et al. [27], a microscopic gas sensing model for ethanol

sensors based on conductive polymer compositor from polypyrrole and poly

(ethylene Oxide).

The above review on polymer electrolytes sensors clearly indicated that the

research work done so far is mainly concerned to the polymer sensors having

photon conducting polymers. No effort has been made for the preparation of sensors

using potassium ionic conductors. In this direction, an attempt has been made in the

present studies to prepare a polymer sensor using potassium ionic conducting

polymers.

5.2. CLASSIFICATION OF SENSORS: -

Sensor is common technical term that has been in frequent use for only about

a decade, instruments working just like sensors have been in use ever since man

first attempted to gather reliable information concerning his physical, chemical and

biological environment. Wolber and wise [28] defined a sensor as a “single-

parameter measuring instrument which transduces a physical parameter into a

corresponding electrical signal with significant fidelity”. Midelhock and Noorlag [29]

defined a sensor as an “input transducer of an information system”. Recalling the

definition we adopted in this section, sensor is supposed to supply a “usable output

in response to a specified measurand”. Interms of today‟s analog or digital electronic

world, a usable output can only be some sort of electrical signal which leads itself to

signal processing, the establishment of control loops, etc. On the other hand, the

213

most interesting measurands are temperature, geometrical quantities and fluid

mechanical quantities. The very general definition of a sensor means that an

enormous variety of instruments will be included in this concept. Therefore some

classification of sensor is an essential as it has been for the physical and chemical

transduction principles. It appears that a number of different criteria are in use for

classifying sensors. Some of which are listed below:

physical or chemical effect / transduction principle;

Measurand (primary input variable);

Technology and material;

Application;

Cost;

Accuracy.

A classification by sensor materials and technology is also of great relevance

because the availability of materials and technologies governs the availability of

sensors. In fact, strong efforts are being made to nature technologies, which will in

particular the possibility of producing inexpensive sensors. Gas sensors can be

classified according to their sensing mechanism. They are further classified

depending on their applications as follows.

Mechanical sensors

Thermal sensors

Magnetic sensors

Optical sensors

Chemical and Bio chemical sensors

Most of the research and development work on sensors is concerned with

single parameter sensing and measurement, in particular the sensitivity, selectivity,

reliability and stability of device. In recent years, however there is an increasing

interest on the development of multi sensor system or arrays including data

214

processing technique to better resolve and identify the response signals for the

sensing of different parameters simultaneously.

Polymer electrolyte based chemical sensors are the simplest and most

popular among the different types of sensors. The characteristics and the details of

the operation of these sensors are elaborated in the following text.

5.3. GAS SENSORS: -

Gas sensors for automotive use are classified into two types: One is used for

the measurement of oxygen concentration inside the automobile. The former is

installed in the emission control system to reduce the amount of gas and at the same

time to improve fuel consumption. The zirconia and titania sensors have already

been used in automobiles to measure the stoichiometric air-to-fuel ratio. A niobium

oxide sensor is under development recently; lead air-to-fuel ratio sensors have also

been attracting attention for lean fuel combustion control. Some of these are already

in use. Other gas sensors such as smoke, humidity, and odour sensors are required

for the detection of the atmosphere inside automobile [30]. A sensor is a device in

which a reversible reaction takes place at the sample surface, which results in a

change of one of its electronic properties usually, conductance.

The chemical sensor acts as a transducer for detecting elements and

provides vital information about the specific chemical constituents in the

environment. These sensors generally contain a physical transducer and a

chemically selective layer. The transudation modes employed are thermal, mass,

electrical, electrochemical and optical. Most gas sensors give an electrical output,

measuring the change in a property such as resistance or capacitance. Sensors that

are capable of giving outputs directly as electrical signals are suitable for the

detection of gases and vapours. Such sensors are classified as semiconductor type

and contact combustion type. The semiconductor type sensors operate in the

following way. If a semiconductor heated to high temperature comes in contact with

a combustible gas or vapour, its electrical resistance changes. This property is

utilized for gas sensing. In case of contact combustion type sensors, the combustible

215

gas or vapour reacts with the catalyst and burns on heated platinum wire and thus is

detected as the electrical resistance of the platinum wire. Between the two types of

sensors, the semiconductor type is highly sensitive to low concentration of gases

and vapours, while the contact combustion type is sensitive to higher concentration

[31].

It must be started at the outset that operating principles of semiconductor gas

sensors are not well understood and comparatively little work has been carried out in

this particular area. However, it is generally agreed that the absorption and

subsequent reaction of gases with already absorbed atmospheric oxygen can

markedly change the surface conductivity of semiconductors. This implies that, for

an n-type material can change the concentration of electronics available for

conduction processes.

5.4. APPLICATIONS OF GAS SENSORS:-

Gas sensors have a wide range of applications and these are constantly

being extended to new areas. Presently six major areas of applications can be

identified, namely environmental monitoring and control of combustion process,

hazards analysis, and in the medical applications. The sensors mostly operate in an

amperometric (three-electrode) mode with the indicator electrode potential being

maintained constant by using a potentiostat. Solid-state sensors are sometimes used

as galvanic cells with the potentials of the indicator and counter electrode controlled

by appropriate electrochemical reactions. In both these cases, the current flowing

between the indicator and counter electrode corresponds to the analyte

concentration. Examples of typical gaseous analytes and their electrochemical

reactions are given in the Table#1. During the oxidation or reduction of analyte, the

electrons flow through an external conductor with a meter and the proton required are

transported by the solid polymer electrolyte. The electrochemical reaction at the

counter electrode maintains overall electroneurality of the system [32].

216

Table#1:- Gaseous analysis that are most often detected by solid polymer

electrolyte- based sensors

Analyte Indicator electrode reaction Example of counter electrode

H2 H2 2H1 +2e- 2H1+2e- +1/2O2 2H2O

CO CO+ H2O CO2+2H1+2e- 2H1+2e-+1/2 O2 H2O

O2 O2+4 H1+4 e- 2H2O H2 2H-+2e-

Among a number of electronic components, the capabilities of sensors are a

decisive factor in determining whether a system is of practical use or not. Therefore.

Much attention has been directed towards the improvement of their durability and

reliability since cooperation between the fields of electronic and automobile

mechanisms has been promoted and strengthened. Sensor technology p-lays a

very important role whenever electronics and automobile technology interact.

However there is still incompatibility among the electronics, sensor and automobile

technologies, probably owing to the differences in the environment in which each

technology has been developed [33].

In view of the demand for a safe and smooth flow of traffic on our roads in the

face of ever-increasing traffic volumes and a limited number of available roads, it is

clear that measures taken effective traffic management are indispensable. The most

important prerequisite for traffic, management is information about the traffic flow

and one way to supply the pertinent data is through the use of vehicle detectors.

Requirements for home appliances are comfort and convenience, high

performance (automatic), energy and resource savings and safety [34] various kinds

or sensors and microcomputers have been introduced into home appliances in order

to meet these requirements.

217

5.5. FABRICATION OF POLYMERIC FILM CARBON MONOXIDE GAS

SENSORS: -

Polymer-based solid electrolytes are of growing importance in solid-state

electrochemistry in view of their applications. Especially, poly ethylene oxide (PEO)

has been studied as an ion conductive matrix which is useful in a lot of applications

such as solid state batteries, electro chromic display devices, sensors [35-37].

The polymer is the host and inorganic salt is dissolved in adequate reciprocal

compositions in suitable solvents such as methanol. Appropriate amounts of the salt

mixture in the chosen stoichiometry and PEO were separately dissolved in methanol

and the two solutions were then stirred together for approximately 24 hours. An

alumina tube was dipped in the solution container for 10 minutes. Then it is removed

from that solution. Solvent was allowed for evaporation at room temperature. When

this procedure is done three to four times, then the aluminium tube substrate carries

a layer of polymer electrolyte. These aluminium tube substrates are provided with

two silver electrodes. The sensor element was subjected to measurements of the

electrical resistance in the presence and absence of carbon monoxide gas in air. The

operating temperature and concentrations of various carbon monoxide gases were

varied in order to study the sensitivity of gas sensors.

For the resistance measurements the sensor element was placed on

temperature-controlled tungsten coil heater inside a glass enclosure. A load

resistance RL was connected in series with the sensor element RS. The input circuit

voltage was applied across RS and RL. Test gases were passed into the enclosure

through the inlet. The resistance of the sensor was obtained by measuring the

voltage drop (VS) across the sensor element [4, 5, 38]. A chromentalumel

thermocouple (TC) was placed on the device to indicate the operating temperature.

This temperature measurement is with in an accuracy of 20C, but we have found

that there is no significant change in the sensitivity. The schematic of the

measurement setup is shown in Fig.5.1. The sensor sensitivity (S) is defined as the

ratio of the change in electrical resistance in the presence of gas Ra – Rg = R, to

that in air, Ra.

218

S = (Ra – Rg) / Ra = R / Ra ----- (1)

To calculate the sensitivity the electrical resistance of the electrolyte was

measured in the presence and absence of gas.

5.6. RESULTS AND DISCUSSION: -

The sensor characteristics of the polymeric film doped with particular dopants

were obtained without and with exposed to carbon monoxide gas. The operating

voltage for sensor was 10V. The sensitivity (S) of a sensor is described as the ratio

of change in electrical resistance in the presence of carbon monoxide gas to that in

the presence of air.

S = R / Ra ------ (2)

R = Resistance in presence of CO gas

Ra = Resistance in presence of air.

Using the polymer electrolyte films based on poly (ethylene oxide) (PEO)

complexed with Potassium per Chlorate (KClO4) and Potassium Nitrate (KNO3),

(PEO+PEG) complexed with KClO4, (PEO+PEG) complexed with KNO3, gas sensors

have been fabricated. The effect of the addition of nano filler (Al2O3) to the polymer

electrolyte on the sensor performance has been studied for various compositions

weight percentage and various concentrations (PPM) of Carbon monoxide gas.

219

Fig.5.1. Schematic of measurement setup used for the gas sensitivity studies

220

5.6.1 [PEO+KClO4] ELECTROLYTE BASED CARBON MONOXIDE GAS

SENSOR: -

Polymer electrolytes have been synthesized by using Poly (ethylene oxide)

(PEO) complexed with KClO4 salt for various composition ratios [(90:10), (80:20) and

(70:30)]. Using these polymer electrolytes, the gas sensors have been designed and

their characteristics were studied for carbon monoxide gas. The sensor resistance

changed when carbon monoxide gas was exposed to the polymer electrolyte film.

The sensor returns to its original state as soon as the carbon monoxide gas is

removed. The output values change to its original value within 8-10 seconds time.

Fig.5.2, Fig.5.3, and Fig 5.4 shows the variation of the sensitivity with carbon

monoxide gas concentration for different operating temperatures. From the figure, it

is observed that the gas sensitivity increases with increase in gas concentration and

with increase in operating temperature.

The sensitivity (S) have been measured as a function of composition (Wt%) of

(PEO+KClO4) polymer electrolyte with carbon monoxide gas concentration for

various temperatures is shown in Fig. 5.5. From the figure, it is clear that the gas

sensitivity also increase with increase in the composition of the polymer electrolyte.

The variation of the sensitivity of carbon monoxide gas sensor with

temperature for various gas concentrations is shown in Fig. 5.6,5.7, and 5.8. From

the above figures, it is clear that the gas sensitivity also increases with increase in

the composition of polymer electrolyte.

The values of sensitivity obtained for various compositions are given in

Table.5.1. From these figures and Table, the following observations have been

made.

221

Fig.5.2. (PEO+KClO4) (90:10) based gas sensor sensitivity as a function of

temperature for different gas concentrations (ppm).

222



223



224

Fig.5.5. (PEO+KClO4) based gas sensor sensitivity as a function of composition

(Wt%) at different temperatures.

225

Fig.5.6. (PEO+KClO4) (90:10) based gas sensor sensitivity as a function of gas

concentrations (ppm) for different temperatures.

226

Fig.5.7. (PEO+KClO4) (80:20) based gas sensor sensitivity as a function of gas


227

Fig.5.8. (PEO+KClO4) based gas sensor sensitivity as a function of gas

concentration at different temperatures.

228

a) The sensitivity of the gas sensor is found to increase with an increase

in the composition of the salt in the polymer PEO.

b) The sensitivity of the gas sensor increases with an increase in the

temperature.

c) The sensitivity of the gas sensor showed an increase with in increase

in carbon monoxide gas concentration.

5.6.2. [PEO+ KClO4+Nano filler (Al2O3)] POLYMER ELECTROLYTE CARBON

MONOXIDE GAS SENSORS: -

Polymer electrolytes have been synthesized by using

Poly (ethylene oxide) (PEO) complexed with (KClO4+Nano filler (Al2O3)) for various

composition ratios [(90:10), (80:20) and (70:30)]. Using these polymer electrolytes,

the gas sensors have been designed and their characteristics were studied for

carbon monoxide gas. The sensor resistance changed when carbon monoxide gas

was exposed to the polymer electrolyte film. The sensor returns to its original state

as soon as the carbon monoxide gas is removed. The output values change to its

original value within 8-10 seconds time.

Fig:5.9, Fig:5.10 and Fig:5.11 shows the variation of the sensitivity with

carbon monoxide gas concentration for different operating temperatures. From the

figures, it is observed that the gas sensitivity (∆R / Ra) increases with increase in gas

concentration and with increase in operating temperature.

The sensor sensitivity (S) has been measured as a function of composition

(Wt%) of (PEO+KClO4+Nano filler (Al2O3)) polymer electrolyte with Carbon

monoxide gas concentration for various temperatures as shown in Fig.5.12. From

the figure, it is clear that the gas sensitivity also increases with an increase in the

composition of the polymer electrolyte.

The variation of the sensitivity of carbon monoxide gas with temperature for

different gas concentrations is shown in Figs.5.13, 5.14, and 5.15. From the above

fig‟s, it is clear that the sensitivity also increases with increase in the composition of

the polymer electrolyte.

229

Table # 5.1

The values of sensitivity obtained for (PEO+KClO4) and (PEO+KClO4+nano

filler) polymer electrolytes for different gas concentrations

S.NO

Polymer Electrolyte Gas

Sensor

Sensitivity at 50° C

200PPM 400PPM 600PPM 800PPM 1000PPM

01. PEO+ KClO4 (90:10) 0.057 0.077 0.085 0.099 0.119

02. PEO+ KClO4 (80:20) 0.144 0.174 0.187 0.192 0.194

03. PEO+ KClO4 (70:30) 0.115 0.150 0.161 0.175 0.190

04. PEO+ KClO4+nano filler 0.116 0.196 0.272 0.335 0.399

(90:10)


(80:20)


(70:30)

230

Fig.5.9. (PEO+KClO4+nano filler) (90:10) based gas sensor sensitivity as a

function of temperature for different gas concentrations (ppm).

231



232



233

Fig.5.12. (PEO+KClO4+nano filler) based gas sensor sensitivity as a function of

composition (Wt %) at different temperatures.

234


function of gas concentrations (ppm) for different temperatures.

235



236



237



made.

a) The sensitivity of the gas sensor is found to increase with an increase in

the composition of the salt in the polymer PEO.


temperature.



The sensor sensitivity is found to be better in nano filler added polymer

electrolyte complexed sensors. The nano filler added polymer electrolyte sensors

have shown better sensor performance than pure polymer electrolyte sensors. It

may be occurring because of the nano filler is a inorganic, ceramic, non-volatile

substance, which when added to a polymer, improves its flexibility, possibility and

hence utility. The nano filler substantially reduces the brittleness of many polymers

because its addition even in small quantity markedly reduces the Tg of the polymer.

The effect is due to a reduction in the cohesive chains. Nano filler molecule

penetrates into the polymer matrix and establishes attractive force between nano

filler molecules and the change segments. These attractive forces reduce the

cohesive force between the polymer chains and increase the segmental mobility, this

enhances the sensitivity. The sensitivity of the polymer electrolyte gas sensor

therefore increases due to the addition of the nano filler to the polymer electrolyte.

5.6.3. [PEO+KNO3] ELECTROLYTE CARBON MONOXIDE GAS SENSORS: -

Polymer electrolytes have been synthesized by using Poly (ethylene oxide)

(PEO) complexed with KNO3 salt for various composition ratios [(90:10), (80:20) and

(70:30)]. Using these polymer electrolytes, the gas sensors have been designed and

their characteristics were studied for carbon monoxide gas. The sensor resistance

changed when carbon monoxide gas was exposed to the polymer electrolyte film.

238

The sensor returns to its original state as soon as the carbon monoxide gas is

removed. The output values change to its original value within 8-10 seconds time.

Fig.5.16, Fig.5.17 and Fig.5.18 shows the variation of the sensitivity with


figure, it is observed that the gas sensitivity increases with increase in gas


The sensitivity as a function of polymer electrolyte composition for various

temperatures is shown in Fig.5.19. From the figure, it is clear that the gas sensitivity

(∆R / Ra) increases with increase in the composition of the polymer electrolyte.


temperature for various gas concentrations is shown in Figs.5.20,5.21, and 5.22.

From the above figure‟s, it is clear that the gas sensitivity also increases with

increase in the composition of polymer electrolyte.



made.




temperature.



239

Fig.5.16. (PEO+KNO3) (90:10) based gas sensor sensitivity as a function of


240



241



242

Fig.5.19. (PEO+KNO3) based gas sensor sensitivity as a function of composition

(Wt%) at different temperatures.

243

Fig.5.20. (PEO+KNO3) (90:10) based gas sensor sensitivity as a function of gas


244



245



246

Table # 5.2

The values of sensitivity obtained for (PEO+KNO3) and PEO+ KNO3+nano filler)

Polymer electrolytes for different gas concentrations

S.NO

Polymer Electrolyte Gas Sensor

Sensitivity at 50° C

200PPM

400PPM

600PPM

800PPM

1000PPM

01. PEO+KNO3 (90:10) 0.170 0.214 0.267 0.312 0.339

02. PEO+KNO3 (80:20) 0.190 0.244 0.281 0.308 0.361

03. PEO+KNO3 (70:30) 0.137 0.245 0.302 0.363 0.405

04. PEO+KNO3+nano filler

(90:10)

0.090 0.169 0.263 0,301 0.327


(80:20)

0.117 0.218 0.273 0.371 0.366


(70:30)

0.164 0.258 0.342 0.388 0.412

247

5.6.4. [PEO+KNO3+ Nano filler (Al2O3)] POLYMER ELECTROLYTE CARBON

MONOXIDE GAS SENSOR: -


Poly (ethylene oxide) (PEO) complexed with (KNO3+Nano filler (Al2O3)) for various

composition ratios [(90:10), (80:20) and (70:30)]. Using these polymer electrolytes,

the gas sensors have been designed and their characteristics were studied for

carbon monoxide gas. The sensor resistance changed when carbon monoxide gas

was exposed to the polymer electrolyte film. The sensor returns to its original state

as soon as the carbon monoxide gas is removed. The output values change to its

original value within 8-10 seconds time.

Fig: 5.23, Fig: 5.24 and Fig: 5.25 show the variation of the sensitivity with


figures, it is observed that the gas sensitivity (∆R / Ra) increases with increase in gas


The sensor sensitivity (S) has been measured as a function of composition

(Wt %) of (PEO+KNO3+Nano filler (Al2O3)) polymer electrolyte with Carbon

monoxide gas concentration for various temperatures as shown in Fig.5.26. From

the figure, it is clear that the gas sensitivity also increases with an increase in the

composition of the polymer electrolyte.


different gas concentrations is shown in Figs.5.27, 5.28, and 5.29. From the above

fig‟s, it is clear that the sensitivity also increases with increase in the composition of

the polymer electrolyte.



made.

a) The sensitivity of the gas sensor is found to increase with an increase in the

composition of the salt in the polymer PEO.


temperature.

248

Fig.5.23. (PEO+KNO3+nano filler) (90:10) based gas sensor sensitivity as a


249



250



251

Fig.5.26. (PEO+KNO3+nano filler) based gas sensor sensitivity as a function of

composition (Wt%) at different temperatures.

252



253



254



255

c) The sensitivity of the gas sensor showed an increase with in increase in

carbon monoxide gas concentration.

The sensor sensitivity is found to be better in nano filler added polymer

electrolyte complexed sensors. The nano filler added polymer electrolyte sensors

have shown better sensor performance than pure polymer electrolyte sensors. It may

be occurring because of the nano filler is an inorganic, ceramic, non-volatile

substance, which when added to a polymer, improves its flexibility, possibility and

hence utility. The nano filler substantially reduces the brittleness of many polymers

because its addition even in small quantity markedly reduces the Tg of the polymer.

The effect is due to a reduction in the cohesive chains. Nano filler molecule

penetrates into the polymer matrix and establishes attractive force between nano

filler molecules and the change segments. These attractive forces reduce the




5.6.5. [PEO+PEG+KCIO4] ELECTROLYTE CARBON MONOXIDE GAS SENSOR:


(PEO+PEG) complexed with KCIO4 salt for various composition ratios [(50:50:10),

(50:50:20) and (50:50:30)]. Using these polymer electrolytes, the gas sensors have

been designed and their characteristics were studied for carbon monoxide gas. The

sensor resistance changed when carbon monoxide gas was exposed to the polymer

electrolyte film. The sensor returns to its original state as soon as the carbon

monoxide gas is removed. The output values change to its original value within 8-10

seconds time.

Fig.5.30 shows the variation of the sensitivity with carbon monoxide gas

concentration for different operating temperatures. From the figure, it is observed

256

Fig.5.30. (PEO+PEG+KClO4) based gas sensor sensitivity as a function of

temperature for different gas concentrations(ppm).

257

that the gas sensitivity increases with increase in gas concentration and with

increase in operating temperature. The sensitivity as a function of polymer electrolyte

composition for various temperatures is shown in Fig.5.31. From the figure, it is clear

that the gas sensitivity also increases with increase in the composition of the polymer

electrolyte.


temperature for various gas concentrations is shown in Fig.5.32. The values of

sensitivity obtained for various compositions are given in Table.5.3. From these

figures and Table, the following observations have been made.




temperature.



The sensitivity of the polymer electrolyte gas sensor increases due to the

addition of PEG with (PEO+KClO4).

5.6.6. [PEO+PEG+KNO3] POLYMER ELECTROLYTE CARBON MONOXIDE GAS

SENSOR: -


(PEO+PEG) complexed with KNO3 for various composition ratios [(50:50:10),

(50:50:20) and (50:50:30)]. Using these polymer electrolytes, the gas sensors have

been designed and their characteristics were studied for carbon monoxide gas. The

sensor resistance changed when carbon monoxide gas was exposed to the polymer

electrolyte film. The sensor returns to its original state as soon as the carbon

monoxide gas is removed. The output values change to its original value within 8-10

seconds time.

Fig.5.33 shows the variation of the sensitivity with carbon monoxide gas

concentration for different operating temperatures. From the figure, it is observed

258


composition (Wt%) for different temperatures.

259



260

Table # 5.3

The values of sensitivity obtained for (PEO+PEG+KClO4) and

(PEO+PEG+KNO3) polymer electrolytes for different gas concentrations

S.NO Polymer Electrolyte

Gas Sensor Sensitivity at 50° C

200PPM

400PPM

600PPM

800PPM

1000PPM

01. PEO+PEG+ KClO4

(50:50:10)

0.075 0.095 0.099 0.105 0.120

02. PEO+PEG+ KClO4

(50:50:20)

0.148 0.179 0.193 0.200 0.216

03. PEO+PEG+ KClO4

(50:50:30)

0.140 0.215 0.231 0.239 0.248

04. PEO+PEG+ KNO3

(50:50:10)

0.158 0.217 0.278 0.319 0.350

05. PEO+PEG+ KNO3

(50:50:20)

0.163 0.216 0.270 0.332 0.405

06. PEO+PEG+ KNO3

(50:50:30)

0.179 0.237 0.311 0.376 0.445

261

Fig.5.33. (PEO+PEG+KNO3) based gas sensor sensitivity as a function of


262

that the gas sensitivity increases with increase in gas concentration and with

increase in operating temperature. The sensitivity as a function of polymer electrolyte

composition at various temperatures is shown in Fig.5.34. From the figure, it is clear

that the gas sensitivity also increases with an increase in the composition of the

polymer electrolyte.


different gas concentrations is shown in Fig.5.35. The values of sensitivity obtained

for various compositions are given in Table.5.3. From these figures and Table, the

following observations have been made.




temperature.



The sensitivity of the polymer electrolyte gas sensor increases due to the

addition of the nano filler.

5.6.7. EFFECT OF NANO FILLER ON THE SENSOR SENSITIVITY: -

Figs. 5.36 and 5.37 show the sensor sensitivity of polymer electrolytes without

the addition of nano filler(Al2O3). From the figures, it is found that the sensor

sensitivity is more in nano filler added polymer electrolyte sensors when compared

with pure polymer electrolytes.

The sensor sensitivity is found to be better in nano composite filler added

polymer electrolyte complexed sensors. The nano filler added polymer electrolyte

sensors have shown better sensor performance than pure polymer electrolyte

sensors. It may be occurring because of the nano filler is an inorganic, ceramic, non-

volatile substance, which when added to a polymer, improves its flexibility, possibility

and hence utility. The nano filler substantially reduces the brittleness of –

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composition (Wt%) of the electrolyte for different temperatures.

264



265

Fig.5.36. (PEO+KClO4) based gas sensor sensitivity with and without nano filler

as a function of temperature for different gas concentrations (ppm).

266

Fig.5.37. (PEO+KNO3) based gas sensor sensitivity with and without nano filler

as a function of temperature for different gas concentrations (ppm).

267

many polymers because its addition even in small quantity markedly reduces the Tg

of the polymer. The effect is due to a reduction in the cohesive chains. Nano filler

molecule penetrates into the polymer matrix and establishes attractive force between

nano filler molecules and the change segments. These attractive forces reduce the




5.6.8 EFFECT OF ADDITION OF (PEG) TO (PEO) POLYMER ELECTROLYTES:-

Fig.5.38 shows the sensor sensitivity of PEG added polymer electrolytes.

From the figure, it is clear that the sensitivity is more in PEG added polymer

electrolytes compared with pure polymer electrolytes. The addition of PEG to

PEO drastically reduces the Tg of the polymer electrolyte and the sensitivity of

the sensor therefore increases.

5.7. CONCUSIONS: -

i) Polymer electrolytes have been synthesized by using

Poly(ethylene oxide) (PEO) complexed with (KClO4), (KClO4+nano filler),

(KNO3), (KNO3+nano filler), (PEG+KClO4), and (PEG+KNO3) for various

composition ratios [(90:10), (80:20) and (70:30)]. Using these polymer

electrolytes, the gas sensors have been designed and their characteristics

were studied for carbon monoxide gas.

ii) The sensitivity of the sensor is found to increase with an increase in the

composition of salt in PEO.

iii) Carbon monoxide gas sensor sensitivity increases with an increase in the

Carbon monoxide gas concentration (PPM) in air.

268

Fig.5.38. Gas Sensor sensitivity with gas concentration at RT (a) PEO+KNO3 (b) PEO+PEG+ KNO3 (c) PEO+KClO4 and (d) PEO+PEG+ KClO4

269

iv) The sensitivity of gas sensor showed an increase with increase of

operating temperature.

v) The sensor sensitivity is found to be better in nano filler added polymer

electrolyte complexed sensors. Thus nano filler added polymer electrolyte

sensors have shown better performance than pure polymer electrolyte

sensors.

vi) The polymer electrolyte films studied are found to be very good sensing

elements for carbon monoxide gas in air.

270

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CHAPTER -V SENSORS

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