chapter -v sensors
TRANSCRIPT
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
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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
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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
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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].
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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.
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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.
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
Fig.5.3. (PEO+KClO4) (80:20) based gas sensor sensitivity as a function of
temperature for different gas concentrations (ppm).
223
Fig.5.4. (PEO+KClO4) (70:30) based gas sensor sensitivity as a function of
temperature for different gas concentrations (ppm).
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
concentrations (ppm) for different temperatures.
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)
05. PEO+ KClO4+nano filler 0.184 0.184 0.273 0.364 0.406
(80:20)
06. PEO+ KClO4+nano filler 0.191 0.203 0.315 0.370 0.408
(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
Fig.5.10. (PEO+KClO4+nano filler) (80:20) based gas sensor sensitivity as a
function of temperature for different gas concentrations (ppm).
232
Fig.5.11. (PEO+KClO4+nano filler) (70:30) based gas sensor sensitivity as a
function of temperature for different gas concentrations (ppm).
233
Fig.5.12. (PEO+KClO4+nano filler) based gas sensor sensitivity as a function of
composition (Wt %) at different temperatures.
234
Fig.5.13. (PEO+KClO4+nano filler) (90:10) based gas sensor sensitivity as a
function of gas concentrations (ppm) for different temperatures.
235
Fig.5.14. (PEO+KClO4+nano filler) (80:20) based gas sensor sensitivity as a
function of gas concentrations (ppm) for different temperatures.
236
Fig.5.15. (PEO+KClO4+nano filler) (70:30) based gas sensor sensitivity as a
function of gas concentrations (ppm) for different temperatures.
237
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.
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.
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
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 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.
The variation of the sensitivity of carbon monoxide gas sensor with
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.
The values of sensitivity obtained for various compositions are given in
Table.5.2. From these figures and Table, the following observations have been
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.
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.
239
Fig.5.16. (PEO+KNO3) (90:10) based gas sensor sensitivity as a function of
temperature for different gas concentrations (ppm).
240
Fig.5.17. (PEO+KNO3) (80:20) based gas sensor sensitivity as a function of
temperature for different gas concentrations (ppm).
241
Fig.5.18. (PEO+KNO3) (70:30) based gas sensor sensitivity as a function of
temperature for different gas concentrations (ppm).
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
concentrations (ppm) for different temperatures.
244
Fig.5.21. (PEO+KNO3) (80:20) based gas sensor sensitivity as a function of gas
concentrations (ppm) for different temperatures.
245
Fig.5.22. (PEO+KNO3) (70:30) based gas sensor sensitivity as a function of gas
concentrations (ppm) for different temperatures.
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
05. PEO+KNO3+nano filler
(80:20)
0.117 0.218 0.273 0.371 0.366
06. PEO+KNO3+nano filler
(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: -
Polymer electrolytes have been synthesized by using
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
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+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.
The variation of the sensitivity of carbon monoxide gas with temperature for
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.
The values of sensitivity obtained for various compositions are given in
Table.5.2. From these figures and Table, the following observations have been
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.
b) The sensitivity of the gas sensor increases with an increase in the
temperature.
248
Fig.5.23. (PEO+KNO3+nano filler) (90:10) based gas sensor sensitivity as a
function of temperature for different gas concentrations (ppm).
249
Fig.5.24. (PEO+KNO3+nano filler) (80:20) based gas sensor sensitivity as a
function of temperature for different gas concentrations (ppm).
250
Fig.5.25. (PEO+KNO3+nano filler) (70:30) based gas sensor sensitivity as a
function of temperature for different gas concentrations (ppm).
251
Fig.5.26. (PEO+KNO3+nano filler) based gas sensor sensitivity as a function of
composition (Wt%) at different temperatures.
252
Fig.5.27. (PEO+KNO3+nano filler) (90:10) based gas sensor sensitivity as a
function of gas concentrations (ppm) for different temperatures.
253
Fig.5.28. (PEO+KNO3+nano filler) (80:20) based gas sensor sensitivity as a
function of gas concentrations (ppm) for different temperatures.
254
Fig.5.29. (PEO+KNO3+nano filler) (70:30) based gas sensor sensitivity as a
function of gas concentrations (ppm) for different temperatures.
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
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.
5.6.5. [PEO+PEG+KCIO4] ELECTROLYTE CARBON MONOXIDE GAS SENSOR:
Polymer electrolytes have been synthesized by using
(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.
The variation of the sensitivity of carbon monoxide gas sensor with
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.
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.
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: -
Polymer electrolytes have been synthesized by using
(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
Fig.5.31. (PEO+PEG+KClO4) based gas sensor sensitivity as a function of
composition (Wt%) for different temperatures.
259
Fig.5.32. (PEO+PEG+KClO4) based gas sensor sensitivity as a function of
temperature for different gas concentrations (ppm).
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
temperature for different gas concentrations (ppm).
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.
The variation of the sensitivity of carbon monoxide gas with temperature for
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.
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.
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 –
263
Fig.5.34. (PEO+PEG+KNO3) based gas sensor sensitivity as a function of
composition (Wt%) of the electrolyte for different temperatures.
264
Fig.5.35. (PEO+PEG+KNO3) based gas sensor sensitivity as a function of
temperature for different gas concentrations (ppm).
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
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.
5.6.8 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
REFERENCES
[1] Bidan,G.(1992)Electroconducting conjugated polymers:new sensitive matrices
to build up chemical or electrochemical sensors. A review., Sensors and
Actuators B,6,pp.45-46.
[2]. Seiyama,T., kato.A., Fujishi.K and Nagatani.M., Analytical chemistry 334
(1962) 1502.
[3]. Taguchi. N., Gas detecting element and method of making it, U.S. patent
No.3, 644 (1972) 795.
[4]. Manorama, S.V., Sarala Devi G. and Rao, V.J. Applied Physics Letters,
64(1994)3163.
[5]. Sarala Devi.G., Manorama.S.V. and Rao.V.J. Sensors and Actuators B,
28(1995)31.
[6]. Sarala Devi.G., Manorama.S.V. and Rao.V.J., J. of Electrochemical Society,
142(1995)2754.
[7]. Ratna Phani.A., Manorama.S.V. and Rao.V.J., Applied Physics Leters,
66(1995)3489
[8]. Phani.A.R., Manorama.S.V. and Rao.V.J., Applied Physics Letters
71(1997)2358.
[9]. Sarala Devi.G., Manorama.S.V., and Rao.V.J., J. of Electrochemical Society,
145(1998) 1039
[10]. Phani.A.R., Manorama.S.V. and Rao.V.J., Materials Chemistry and Physics,
58(1999)101.
[11]. Gopal Reddy.C.V., Manorma.S.V. and Rao.V.J., J. of Marerial science letters,
18(1999) 673
[12]. Sarala Devi.G., Manorama.S.V. and Rao.V.J., Sensors and Actuators B,
56(1999) 98.
[13]. Sbseveglieri.G., Sensors and Actuators B, 23(1995) 103.
[14]. Moseley.P.T., Sensors and Actuators B, 149(1992).
[15]. Gopel.W., Sensors and Actuators B, 18(1994) 191.
[16]. Chares worth.J.M., Partridge.A.C., Garrarad.N., Mechanistic studies in the
interactions between poly (pyrrole) and organic vapors, J.Phys.Chem.
97(1993)5418.
271
[17]. Bartett.P.N., Archer.P.B.M., Ling-Chung,S.K. Conducting polymer gas
sensors part I : Fabrication and characterization, Sensors and Actuators B,
19(1989)125.
[18]. Bartett.P.N., Archer.P.B.M., Ling-Chung,S.K., Conducting polymer gas
sensors part II: Response of polypyrrole to methanol vapor, Sensors and
Actuators B, 19(1989)141.
[19] Bartett.P.N., Archer.P.B.M., Ling-Chung.S.K., Conducting polymer gas
sensors part III: Results for four different polymers and five different vapors,
Sensors and Actuators B, Vol. 20(1989) 287.
[20]. Krutovertsev.S.A., Sorokin.S.I., Zorin.A.V.,Letuchy.Ya.A., Antonova.Yu.O.,
Polymer film-based sensors for smmonia detection, Sensors and Actuators B,
7 (1992)492.
[21]. Gopel.W., J.Hesse and J.N.Zemel (eds)., Sensors: A comprehensive survey,
Vol.2 and 3,VCH, Weinheim,1989.
[22]. Misra.S.C.K., Archna Suri, Sudhas chandra & R Bhatta charya. Inddian
Journal of Pure & Applied Physics. 38(2000) 545.
[23]. Guo.S., Rochotzki.R., Lundstrom.I., Arwin.H., Sensors and Actuators B, 44, ,
(1997)243.
[24]. Cammann.K., Buhlmann.K., Schlatt.B., Muller.H., Choulga.A., 1997
International conference on solid state sensors and Actuators. 2 (1997) 1395.
[25]. Amrani.M.E.H., Payna.P.A., IEEE Proc.Sci.Meas.Technol.(U.K). 146(1999)95.
[26]. Lin.C.W., Hwang.B.J., Lee.C.R. Mater. Chem. Phys. Vol.58(1999) 114.
[27]. Bing Joettwang. Jin-Yi Yang, Chi-Wenlin. J. Elecrochem. Soc. (USA). 146,
(1999)1231.
[28]. Wolber.W.G., and Wise K.D., "Sensor development in the microcomputer
age" IEEE Transactions ED-26 (1979) 1864-1874.
[29]. Middelhoek.S., and Noorlog.D.J.W., Sensors and Actuators 2 (1981/82) 29-
41.
[30]. Gopel.W., Jones.T.A., Zemel.J.N., and Seiyama.T., Sensors: A
comprehensive survey, Vol.2 W.Gopel, J.Hesse and J.N.Zemel (eds) 1990.
[31]. Ichinose.N., Ceram.AM., Soc. Bull(1985)64.
[32]. Frantisek Opekar, Karel Stulik, Electrochemical sensors with solid polymer
electrolytes, Analytica Chemica Acta (1999) 151-162.
272
[33]. Gopel.W., J.Hesse and J.N.Zemel (eds)., Sensors: A comprehensive survey,
Vol.1,VCH, Weinheim.1989.
[34]. Kobayashi.T., Sensors and Actuators 9 (1986) 235-248.
[35]. S.Take Oka, H.ohno, E.Tsuchida, Polym.Adv.Tech.4 (1993) 53.
[36]. J.R.Maccallum, C.A.Vincent (Eds), Polymer electrolyte Reviews-1,
Elsevies, London, 1987.
[37]. F.M.Gray Solid polymer electrolytes, VCH.Weinheim, 1991.
[38]. Satyanarayana.L, Gopal Reddy.C.V., Manorama.S.V., Rao.V.J., Sensors and
Actuators B, 46(1998) 1.