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5.1 Introduction

According to Jacob Fraden [1], a sensor is a device that receives a signal or

stimulus and responds with an electrical signal. The reason for the output of a sensor to

be limited to electrical signals is related to the present development of signal processing,

that is almost exclusively performed using electronic devices. Hence, a sensor should be

a device that receives a physical, chemical or biological signal and converts it into an

electric signal that should be compatible with electronic circuits. Sensor seems to have

come from the word „sense‟ given that usually sensor devices try to mimic human senses

characteristics. In the biological sense, the output is also an electrical signal that is

transmitted to the nervous system. Usually sensors are part of larger complex systems,

made by many other transducers, signal conditioners, signal processors, memory devices

and actuators.

5.2 Types of Sensors

Generally, sensors can be classified into many types based upon the applications,

input signal, and conversion mechanism, material used in sensor, production technologies

or sensor characteristics such as cost, accuracy or range.

Table 5.1: Classification of sensors

Type Detection Properties

Thermal Sensor Temperature, Specific heat, Heat flow, etc.

Electrical Sensor Charge, Current, Voltage, Resistance, Inductance, etc.

Magnetic Sensor Magnetic flux density, Magnetic moment, etc.

Optical Sensor Light intensity, Wavelength, Polarization, etc.

Mechanical Sensor Length, Acceleration, Flow, Force, Pressure, etc.

Chemical Sensor Composition, Concentration, pH, etc.

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Based on the input signal given to the sensor, the sensors can be classified into six

different types, which are mentioned in the Table 5.1.

5.3 Gas Sensors

Gas sensor is a device that can change the concentration of an analyte gas into an

electronic [2] or electrical signal [3]. A gas sensor is a chemical sensor that is operated in

the gas phase. It is an important component of devices commonly known as „electric

nose‟ [4].

A gas sensor must possess at least two functions: (i) to recognize a particular gas

and (ii) convert the output into measurable sensing signals. The gas recognition is carried

out through the surface chemical processes due to gas-solid interactions. These

interactions may be in the form of adsorption, or chemical reactions. Most of the gas

sensors give an electrical output, measuring the change of current or resistance or

capacitance.

The given signal can be related to the chemical environment it is exposed to. The

response of a gas sensor to a single gas can be described as:

x = fgas (cgas)

where fgas is a function (usually non-linear) and cgas, the concentration of the gas. The

response is in most cases defined as the difference or ratio between the steady-state

sensor response when exposed to the sample gas and the sensor response when exposed

to a reference atmosphere (not sample gas). The concentration-response relationship for

most gas sensors approximately exhibits either saturated linear behavior, i.e. linear for

low concentrations and saturated for higher concentrations, or logarithmic behavior.

Three important parameters when describing the response of a sensor are sensitivity;

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selectivity and stability. The sensitivity of the sensor towards a specific gas is, thus,

defined as

gas

gas

x

c

The selectivity (E) of a single sensor is usually defined as the ratio of the

sensitivity related to the gas concentration to be monitored in the linear region and the

maximal sensitivity to all other interfering components. It is given by

max

gas

allothergases

E

The stability of the sensor response is defined as the reproducibility of the

sensitivity and selectivity as a function of time.

Most of the drawbacks of the commonly used gas sensing technologies come

from their lack of stability. There are other demands to be met when producing gas

sensors, such as short response time, good reversibility, low cost, small size and low

power consumptions.

When a sensing material is exposed to gas, then it interacts with the gas, this

interaction may be by adsorption or desorption, chemical reactions on the surface or the

bulk of the material. The interaction changes some physical properties of the sensing

material, such as the electrical conductivity or the mass. The change in conductivity

is detected by the voltage drop over a series resistor or a change in mass is detected by

the shift in frequency of a resonator. A schematic description of the working principles of

solid-state gas sensors is depicted in Figure 5.1 [5].

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Figure 5.1: Principle of solid-state gas sensors

5.3.1 Need for Gas Sensors

A numerous luxury items have been brought by today‟s society but with them

series of problems like air pollution and emission of toxic gases have also been

introduced to our society. The necessity to constantly monitor and control the gases

emitted, sprouted the need for gas sensors. The various uses of gas sensors vary across a

wide range from industrial to domestic applications; for example monitoring air

pollution, chemical processes and exhaust from combustion engines. In recent years,

several types of gases have been used in different areas of industries as raw materials. It

becomes really important to control and monitor these gases, as there is a huge risk of

damage to property and human lives if a leak occurs. Certain gases are corrosive,

explosive or can be toxic for human beings.

Currently, there are three categories in which gas monitoring is needed:

• For oxygen (O2), in connection with the monitoring of breathable atmospheres

and for the control of combustion.

• For flammable gases in air, in order to protect against the unwanted occurrences

of fire or explosive limit, which is up to a few percent for most gases.

• For toxic gases in air, where the need is to monitor concentrations around the

exposure limits which range from less than 1 ppm to several hundreds of ppm.

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Therefore, we require gas sensors that can detect these gases continuously and

effectively to avoid most of the problems.

The increasing demand is for better gas sensors with higher sensitivity and greater

selectivity. Intense efforts are being made to find more suitable materials with the

required surface and bulk properties for the use as gas sensors. Among the gaseous

species to be observed are carbon monoxide (CO), carbon dioxide (CO2), nitric oxide

(NO), nitrogen dioxide (NO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3),

ammonia (NH3), and organic gases such as methane (CH4), propane (C3H8), liquid

petroleum gas (LPG), and many others.

5.3.2 Sensing Materials

A large number of different materials have their physical properties modified after

interaction with a chemical environment such as ionic compounds (metal oxides), metals,

polymers and supramolecular structures [6]. Properties of the sensing materials, such

as molecular size, polarity, polarisability, and affinity, along with the matching

characteristics of the sensing material, govern the interaction.

Two main types of interaction between the presence of gas and the sensing

material are:

Lock-and-key-type interaction, which usually consists of organic materials. They

can be arranged either as a monolayer of the recognition molecules or as specific

recognition sites in a polymeric matrix [7]. Typical materials employed are cage-

like molecules such as calixarenes [8].

Chemical sensing with inorganic materials. Reactions at the surface and/or in the

sensing material may lead to chemisorptions or catalytic reactions between the

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molecules present. Thus, the charge distribution, the carrier concentration or

mobility in the sensing material might change [9].

Hydrocarbon gases are being used as fuel for domestic and industrial purposes.

Domestic liquefied petroleum gas (LPG) and CO are combustible gases. They are

potentially hazardous; increasing usage of liquefied petroleum gas (LPG) has increased

the frequency of accidental explosions due to leakage. The toxic hazardous gases

combine with hemoglobin very quickly and results in human death. People have been

trying to detect them in its early stages to give alarm and perform effective suppression.

Thus the requirements for reliable and sensitive gas detecting instruments have increased

for safety at home and industry. Most of previous researches were focused on propane or

butane gas sensor but little work has been done on LPG sensor. From the literature, it is

inferred that presently available sensors have two major shortcomings, one, low

sensitivity and two, its operation at a high temperature. One has to compromise with

either the sensitivity or the operating temperature. A highly sensitive sensor mostly works

at a very high operating temperature thus increasing the power consumption. On the other

hand, other sensors which operate at low temperature are not sensitive enough for trace

level detection of LPG [10-14]. To improve this problem, some researchers paid

attention to the study of signal analysis. Many reports showed that it was possible to

discriminate gases by evaluating the sensor response features [15].

5.4 Liquefied Petroleum Gas (LPG)

LPG is the abbreviation of Liquefied Petroleum Gas. Like all fossil fuels, it is a

non-renewable source of energy. The main components of LPG are Propane (C3H8) and

Butane (C4H10), which can be stored separately or as a mixture. LPG may contain

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components other than hydrocarbons depending upon the source of LPG and its method

of production. It may contain components other than hydrocarbons. LPG is a gas at

atmospheric pressure and normal ambient temperatures, but it can be liquefied when

moderate pressure is applied or when temperature is sufficiently reduced. Due to these

properties, LPG can be easily condensed, packaged, stored and utilized, which makes it

an ideal energy source for a wide range of applications.

There are two main sources of LPG, namely:

(i) Wet natural gas

(ii) Refinery operations

LPG prepared from wet natural gas consists entirely of saturated hydrocarbons,

i.e. propane and butane. LPG produced by both cracking and reforming processes will

have, in addition to saturated hydrocarbons, some quantities of unsaturated hydrocarbons

also (i.e. propylene and butylene). LPG produced will have impurities like moisture and

sulphur compounds like hydrogen sulphide and mercaptans. Moisture may lead to

clogging of regulators; valves etc. and sulphur compounds cause corrosion. Moisture and

sulphur compounds are, therefore removed by suitable treatment at the refinery.

LPG is widely used in the food industry for domestic (cooking) purpose. It can be

used in glass and ceramic industry as fuel like LPG can enhances the product quality

thereby reducing the technical problems in manufacturing. LPG is used in metal

industries basically for cutting, heating and melting processes also in agro industries for

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.

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5.5 Metal Oxide for Sensor Application

Since Seiyama and Taguchi used the dependence of the conductivity of ZnO on

the gas present on the atmosphere for sensing applications, many different metal oxides

have been proposed for humidity and gas sensing detection. Generally speaking, these

oxides can be divided into binary oxides and more complex oxides, the former being

much more common in gas sensing applications. Among binary metal oxides, SnO2 is the

one that has received by far more attention since Taguchi built the first tin oxide sensor

for Figaro Sensors in 1970. This is probably due to its high reactivity to many gaseous

species. However, this characteristic has also revealed as a lack of selectivity and thus

investigation on other metal oxides has been considered necessary. Besides, developers

of electronic noses have experimented with arrays of different sizes that may include

around ten metal oxide sensors, apart from other types of chemical sensors. The use of

different metal oxide sensors is highly recommended in order to increase the amount of

information [16].

5.6 Design of Gas sensor setup

Gas Sensor chamber consists of side glass plates of 360 mm x 300 mm dimension

and 6 mm thickness, provided with top & bottom plates made of fiberglass. The chamber

is made airtight by rubber beading. This chamber house has sample holder and dc fan in

order to distribute the gas molecules uniformly throughout the chamber. Gas from the

cylinder is made to flow through a flow meter to the chamber through the gas valves

provided at the top of the chamber. The electrometers used are digital meters of high

accuracy. The block diagram of the sensor setup is shown in Figure 5.2(a), along with the

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external circuit for the measurement of electrical resistance. The complete photograph of

the setup used for the measurement is shown in Figure 5.2(b).

Figure 5.2(a): Block diagram of gas sensor set-up

Figure 5.2(b): Photograph of gas sensor set-up

The present investigation describes the detection of LPG at room temperature

using polypyrrole and metal oxide composites. In the present work, LPG sensing and

successful utilization of Polypyrrole-NiO, Polypyrrole-Nb2O5 and Polypyrrole-CeO2

composites as a LPG sensor has been reported.

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5.6.1 Experimental Technique

The samples in the pellet form were used for LPG sensing behavior of

Polypyrrole and Polypyrrole - oxide composites. The planar resistance of the sensor was

recorded versus time by controlling LPG in a closed glass chamber at room temperature.

Controlled LPG at room temperature was introduced steadily into the chamber to increase

the gas concentration inside the chamber. The flow rate of 20 cc / min was adjusted by

regulator and gas control valve, the flow rate is monitored by a flow meter. The planar

resistance of the Polypyrrole and its composites was recorded versus gas concentration.

The block diagram of the gas sensor setup is shown in Figure 5.2 (a) and complete

photograph of the setup used for the measurement of electrical resistance versus LPG

concentration is shown in Figure 5.2(b).

5.6.2 Results and Discussion

5.6.2.1 Polypyrrole

Figure 5.3(a) shows the change in electrical resistance with concentration of LPG

in parts per million at constant volume (PPM) for pure Polypyrrole at room temperature.

It is observed that when the pellets were exposed to a gas, the polymer matrix swells due

to absorption of gas. The increase in volume causes an increase in resistance which

causes the disruption of conducting pathways through the composites. When the gas is

released, the polymer returns to its original size, restoring the conducting pathways. Thus,

the electrical resistance increases with increase in the concentration of the gas.

Figure 5.3(b) shows the variation of sensitivity with time for pure Polypyrrole. It

is observed that, the sensitivity is up to 27% for pure PPy.

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Figure 5.3(a): Change in Resistance versus Gas Concentration (PPM) of pure PPy

Figure 5.3(b): Variation of sensitivity versus time for pure PPy

5.6.2.2 Polypyrrole-NiO composite

Figure 5.4(a) shows the change in electrical resistance with concentration of LPG

in parts per million at constant volume (PPM) for Polypyrrole-NiO composites at room

temperature. It is observed that when the pellets were exposed to a gas, the polymer

matrix swells due to absorption of gas. The increase in volume causes an increase in

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resistance which causes the disruption of conducting pathways through the composites.

When the gas is released, the polymer returns to its original size, restoring the conducting

pathways. Thus, the electrical resistance increases with increase in the concentration of

the gas.

Figure 5.4(a): Change in Resistance versus Gas Concentration (PPM) of PPy-NiO

It is also observed that among all the PPy-NiO composites, 30 and 40 wt%

composites shows maximum change in resistance when compared to other composites of

different wt% of NiO in PPy (10, 20 and 50 wt%). This might be due to two reasons:

reaction between the metal oxide and gas

swelling and curling of the polymer.

The gas-sensing mechanism of NiO based sensors belongs to the surface

controlled type and the resistance change is controlled by the species and amount of

chemisorbed oxygen on the surface. However, for LPG, the reaction mechanism is quite

complex and proceeds through several intermediate steps, which are not yet fully

understood [17]. At an operating temperature (750C), in the absence of a target gas,

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oxygen gets adsorbed on the surface of the sensor and it extracts electrons from the

conduction band of the sensor material, which can be explained by the following

reactions (1)–(3):

O2(gas)↔O2(ads) (1)

O2(ads)+e− ↔O2(ads) (2)

O2(ads)−+e

− ↔2O(ads) (3)

Thus, the equilibration of the chemisorption process results in stabilization of

surface resistance. Any process that disturbs this equilibrium gives rise to change in the

conductance of the semiconductors. It is well known that LPG consists of CH4, C3H8,

C4H10, etc. and in these molecules the reducing hydrogen species are bound to be carbon

atoms. Therefore, LPG dissociates less easily into reactive reducing components on the

Nickel oxide surface. The overall reaction of LPG molecules with adsorbed oxygen can

be explained as follows [18]

CnH2n+2 + 2O−→ H2O + CnH2n – O + e (4)

where CnH2n+2 represent the CH4, C3H8 and C4H10. On exposure of the PPy/NiO pellets to

the LPG, the LPG molecules react with adsorbed oxygen in the same manner as

described in Equation (4).

Figure 5.4(b) shows the sensitivity against time for Polypyrrole - NiO composites.

It is observed that, the sensitivity lies between 55 to 77% for 20, 30 and 40 wt% of PPy-

NiO composites, whereas for 10 and 50 wt% composites sensitivity is less than 25%.

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Figure 5.4(b): Variation of sensitivity versus time for PPy-NiO

5.6.2.3 Polypyrrole-Nb2O5 composite

Figure 5.5(a) shows the change in resistance with concentration of LPG in parts

per million at constant volume (PPM) of Polypyrrole-Nb2O5 composites at room

temperature. It is observed that when the pellets were exposed to a gas, the polymer

matrix swells. The increase in volume causes an increase in resistance due to this the

conductive pathways through the material are disrupted. When the gas releases the

polymer returns to its original size, restoring the conductive pathways. Thus the

sensitivity increases with increase in the concentration of the gas.

Among all the PPy-Nb2O5 composites, 40 and 50 wt% show maximum change in

resistance when compared to the other composites of different wt% of Nb2O5 in PPy ( 10,

20 and 30 wt%). It is found that as PPM increases the change in resistance increases

almost linearly and saturates around 10000 and 15000 PPM for 40 and 50 wt%

respectively. For rest of the composites the variation is almost linear.

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Figure 5.5(b) shows the sensitivity against time for PPy-Nb2O5 composites. It is

observed that, the sensitivity lies between 45 to 65% for 40 and 50 wt% of PPy-Nb2O5

composites, whereas for 10, 20 and 30 wt% of the composites sensitivity is less than 40

wt%.

Figure 5.5(a): Change in Resistance versus Gas concentration (PPM) of PPy-Nb2O5

Figure 5.5(b): Variation of sensitivity versus time for PPy-Nb2O5

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5.6.2.4 Polypyrrole-CeO2 composite

Figure 5.6(a) shows the change in resistance with concentration of LPG in parts

per million at constant volume (PPM) of Polypyrrole-CeO2 composites at room

temperature. It is observed that when the pellets were exposed to a gas, the polymer

matrix swells. The increase in volume causes an increase in resistance due to this the

conductive pathways through the material are disrupted. When the gas releases, the

polymer returns to its original size, restoring the conductive pathways. Thus the

sensitivity increases with increase in the concentration of the gas.

Among all the PPy-CeO2 composites, 40 wt% show maximum change in

resistance when compared to the other composites of different wt% of CeO2 in PPy (10,

20, 30 and 50 wt%).

Figure 5.6(b) shows the sensitivity against time for PPy-CeO2 composites. It is

observed that, the sensitivity lies between 60 to 85% for 20 and 40 wt% of the

composites, whereas for and 10, 30, and 50 wt% composites sensitivity is less than 30%.

Figure 5.6(a) Change in Resistance versus Gas Concentration (PPM) of PPy-CeO2

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Figure 5.6(b): Variation of sensitivity against time for PPy-CeO2

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