PROS Slamet Widodo Fabrication process fulltext

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Fabrication process of CO gas sensor devices based tin oxide (SnO2)

by Thick Film Technology

Slamet Widodo

Research Center for Electronic & Telecommunications-LIPI Jl. Cisitu No.21/154D, Komplex LIPI Sangkuriang,

Phone: +62-22-2504660, Fax: +62-22-2504659, Bandung 40135, Indonesia E-mail: [email protected]; [email protected]

Abstract

In this paper is described the design and fabrication of tin oxide (SnO2) based gas sensors by thick-film technology. Sensors are designed consisting of constituent components as such as: heater, electrodes (interdigital fingers), and the sensitive layer of the Tin Oxide (SnO2) material. These sensors are made multilayers, heater and two components have been designed in a single electrode surface by considering the aspects of miniaturization, heat distribution, and power consumption of sensor devices. While the process of heater and the electrode on a substrate made of alumina (Al2O3) with silver paste (Ag). In this study there are differences in the thickness of the sensitive layer affects the atomic bonding materials that affect the change in resistance . The results indicate a sensitive coating material change in value of the linear resistance to temperature changes is given. Results expected maximum is the lowest resistance for sensors that work can be accomplished at a temperature that is not too high, so the sensors will work faster and more sensitive . The test results given gas flow sensor with CO showed good results that CO gas sensors can work at temperatures of 165 °C -sensitive layer which is used to detect CO gas sensor is SnO2 with a sensitivity of 4 %. Keywords Al2O3 substrates, electrodes, gas sensor, heater, SnO2-sensitive layer, thick films

1. Introduction

Air pollution is a global problem faced by almost all major cities in the world. The air around us is composed of a mixture of various gases, which can be caused by natural processes or air pollution from human activities, such as in the fields of transport, industry or forest fires and others. Gases which are harmful to human health, among others, SOx, NOx, NH3, H2S, CO and many others (Barsan, 2008).

The impact of air pollution is accumulative from day to day. Although the effect is different for each individual, but in the long term exposure will result in health problems such as bronchitis, emphysema, and lung cancer. The impact will be felt more vulnerable for toddlers and elderly individuals. Terrible is the impact of lead for children because it can affect brain function and intelligence, as well as damaging the various organs such as the kidneys, nervous system, and reproductive. Table 1 presents types of air contaminants, the main source of emissions, as well as standards for health according to the rules of the WHO (Cirera, 2000).

Judging from the condition of the environment and air pollution as above, it is essential to the control of the intensity of the discharge or at least a reduction in the concentration of the exhaust gases. This requires a detector in the form of electrochemical sensors to

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determine the concentration of the exhaust gases. However, accurate instrumentation sensors that already exist tend to be relatively expensive, on the other hand in the field of microelectronics technology is undergoing rapid changes that offers development and production at relatively low cost, including for gas sensor technology. Successful gas sensor microelectronics made are based metal oxide (MOX) including ZnO, Fe2O3, and SnO2.

Table 1. Substances air pollutants, sources of emissions, and health standards according to WHO.

Pollutants Sources Health Standards

Carbon Monoxide (CO)

Exiles motor vehicle 10 mg / m3_{(9 ppm)} Sulfur Dioxide

(SO2)

Power generation facilities

80 ug / m3_{(ppm 0.03)} Particulate

Matter (SPM)

Motor vehicle Exiles 50 ug / m3_{for 1 year} Nitrogen Dioxide

(NO2)

Discharge of motor vehicles

100 pg / m3_{(ppm 0:05)} for 1 hour

Ozone (O3) Formed in the

atmosphere

235 ug / m3_{(ppm 0:12)} for 1 hour

Furthermore, the most popular and widely developed are SnO2-based gas sensors for a variety of advantages compared to other materials. And development are considered the most successful, conducted by a Japanese scientist Prof. Naayoshi Taguchi. Taguchi successfully developed a ceramic gas sensors named Taguchi Gas Sensors (TGS), and to this day TGS already mass produced by the company.

The advantages of SnO2 gas sensor that is not owned by the sensors are made conventionally, among others:

# Early use of long and relatively stable.

# Has excellent resistance to corrosive gases.

# Having strong construction and good mechanical resistance.

# The cost of production is relatively cheap.

# small dimensions and is easy to use and maintenance (Hann, 2002: 3).

However, despite the SnO2-based sensors are the most widely understood and developed, but until now have not been able to SnO2-based sensors produce high sensitivity and selectivity.

From the explanation and the reasons above, this activity will be the design and fabrication of a gas sensor with thick film technology, which is based metal oxide SnO2.

In designing the system up and characterize the metal oxide based gas sensors as will be done in this study, the problems encountered can be formulated as follows:

a. Sensitivity

The sensor was developed to be able to detect gases with low concentration, in the order of ppm.

Sensitivity stated:

• For n-type sensor material and gases such as reducing gas:

g o

R

S

=

g o

R

S

−

=

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o g

R

S

=

o o g

R

S

=

−

, (2)

where S is sensitivity, Ro is sensor resistance at normal air (no gas), and Rg is the resistance of the sensor when there is a gas.

As for the p-type material sensor, sensitivity above definition be changed (Cirera, 2000) for the CO sensor range 9 ppm.

b. Selectivity

The existence of two or more of the gas mixture should be discriminated against by the sensor system developed without any interference from each other.

c. Power consumption

Option fabrication technology used must consider the power consumption of sensor devices produced. For power consumption used in gas sensors are: Thick Film Technology ranged from 200 mW–1 W.

d. Originality

Choice of materials, modifications, and methods used must pay attention to the process of "novelty", so it can produce devices that have high commercial potential.

This study aims to apply a thick film technology in the manufacture of gas sensor CO of tin-dioxide SnO2 materials. In this study will be the design, fabrication and characterization system based metal oxide gas sensor, gas sensor device which includes a single-use technology thick films with sensitive materials such as SnO2. Various additives such as Pt, Pd, and Ag will be used as a dopant and a catalyst to increase the sensitivity and selectivity of the sensor, but it also will be made of a digital display system hardware to display the measurement results of the gas in the environment.

Sensitive layer

Sensitive layer or layers of the sensor material is the part that is directly related to gas, where the electrochemical reaction occurs at the surface of this layer. This layer is made of SnO2, the metal oxide n-type which has a relatively wide energy gap (3.6 eV).

The dimensions of this layer (which represents the concentration of SnO2) will determine the measurement range of the sensor. The theory of the determination of the dimensions of the sensor layer is as follows. The first thing is to determine the maximum measurement range of the sensor in units of ppm. Because this process is happening is the reaction gas, ppm converted into mol/L. Assuming the gas is a gas under ideal conditions, the equation used is as follows.

L mol ppm

L mol

15 , 24

1 x

= , (3)

with reference to the equation equilibrium reaction between carbon dioxide and SnO2, namely XO2 + 2X + ↔ 2YO 2YO2 + 2e, it is known molarity ratio between SnO2 gas and reducing gas. With reference to Eq. (3), the mole SnO2 will be obtained. Furthermore, the mass of SnO2 obtained by

M

m

n

=

, (4)

with n is molality (mol), m is mass (g), and M is molarity (g/mol). Furthermore, by looking at the density (ρ) of the material data obtained dimensions (volume) of the sensor layer, using the following equation:

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V

m

=

ρ

, (5)

in which ρ is density (g/volume), m is mass of material (grams), and V is dimensions/volume (unit volume). By determining the layer thickness, the width of the sensor layer will be obtained.

Shape sensor response

Basically, the gas sensor response thick film technology is the change in the value of konduktans sensor to change the gas concentration. Generally expressed as:

G

S

=

, (6)

with S is sensitivity, G is conductance sensor when a reducing gas, G0 is conductance sensor

when no reducing gas. The above equation is identical to the equation proposed by Cirera sensitivity. Maxwell-Boltzmann statistics appropriate, conductivity G is formulated by

[ ]

eVs

e

env

R

G

=

1 =

.

, (7)

with v is the bulk mobility and n is the electron concentration. Medium voltage Vs is the Schottky barrier, is defined as:

2 0 1       Π + =

ρ

Vs . (8)

V0 is the barrier height in the absence of a reducing gas, is defined as:

D s

N

V

. . e . 2 2 0

ε

=

, (9)

while the parameters of the gas conditions in concentration, pressure and temperature. By

Ns is the sensing surface density (m-2_),_ND_{is the donor concentration (oxygen vacancies) (m} -3_{), e is the electron charge (eV),}_ε_{is the dielectric constant of the semiconductor material,}_ρ

is the density of the gas (kgm-3_),_p_{is the partial pressure of the gas (Nm}-2_{), and П being defined}

as:

kT h

mkT 32

2 2       =

π

, (10)

where m is time reducing gas (in this case CO), h is planck constant (4,134.10-5 eV), and T = absolute temperature (o_{K). Using the above equation, and by defining G0 as a conductivity}

sensor in free air, then

kT eV vne e G 0

0 = . (11)

From the above equations, the equation for the relationship obtained sensor sensitivity (G/G0) as follows:









































Π

+

−

=

p

kT

eV

G

ρ

1 exp

0 0 (12)

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At the moment there is no gas (p = 0), the equation of sensitivity being worth 1 (G/G0 = 1). At high gas concentrations (high pressure), ie when ρ (p/ П) >> 1, then the sensitivity reached saturation point:

kT eV

sat

e

G

.

=

. (13)

From Eqs. (9) and (10), we obtain:

     

− −













=

sat

G

1 β

1 1

0 0

, (14)

where c is the concentration of the gas (in ppm), (G/G0) sat and β are parameters obtained by combining equations with experimental data (Barsan, 2008).

Broadly speaking, the gas sensor technology thick film is composed of a pair of electrodes, heater and sensitive layer sensitive to stimuli gas, all of which are printed on the substrate strip of material alumina (Al2O3) 96%.

Our expectations are

1. Modification of metal oxide material to improve the sensitivity of the sensor. 2. The use of an array of sensors to increase the selectivity of the sensor.

3. The use of thick films technology and Sputtering techniques to produce devices with low power consumption.

4. Selection of materials and methods that process has not been much explored its use in the design of the gas sensor will give originality aspect.

2. Methodology

In carrying out this research, several research methods in the preparation of the report, namely: literature review, a process of testing, data collection, processing experimental data, to conclusions. Systematically can be seen in Figure 1.

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The data is used as a parameter of this sensor is a data change heater temperature to the direct current input data and sensor resistance value changes to changes in gas concentration. The data is taken by measuring the change in resistance is directly change the value of the resistance of the sensor by using an ohmmeter. From the data obtained, analyzed and ultimately drawn conclusions about the characteristics of the sensor, whether the results achieved as planned.

3. Results and discussion

Among the various types of chemical sensors, gas sensors carbon monoxide (CO) is needed to monitor carbon dioxide and ammonia pollution resulting from the disposal of car/motorcycle and natural gas due to incomplete combustion. CO gas sensor consists of a heater (heater), a pair of electrodes (interdigital electrodes) and the sensor layer (sensitive layer) of material SnO2 (tin dioxide). To avoid short-contact heating coated with an insulating

layer (coating). Basically thick films gas sensor technology is a gas sensor that works using the principle chemoresistor, conductivity sensors will change with the presence of chemical elements (of gas) which acts on the surface layer of the sensor (in this case SnO2). The

conductivity changes due to a change or transfer electrons valence electrons in the atoms of the sensor layer due to the reaction with gaseous reactant gas (reducing gas). Oxidation reaction occurs on the surface of SnO2, the working temperature between 300 °C–450 °C in

pure SnO2 and 200 ° C - 250 ° C on SnO2 + dopant) in the absence of reducing gas. To solve

the above problems and reach the right target, then the phases of achieving the targets that have been implemented in this study are as follows.

1. Stage design

To achieve the results as expected, the sensor manufacturing process was carried out in several stages. These stages can be seen in Figure 2.

Figure 2. Stages of process design and fabrication of gas sensors.

a. Sensor specifications

In the process of designing the devices, as the initial step is to determine the specifications of the devices to be created. The researchers expected the general specifications of the sensor is as follows:

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# Dimensions: ≤10 mm x 10 mm # Operating temperature: 25 °C–300 °C # Work heater Power: 3W

# Measurement Range: 0 ~ 1000 ppm b. Designing and making lay out sensor

In general, changes in the resistivity of the material when it reacts with the gas sensor is influenced by the reaction of oxygen atoms in the air with oxygen atoms in the surface layer of the sensor. This reaction change potential barrier between the bonding atoms. Change the sensor response signal is determined by the type of material and gas sensors are censored. Dimensions of the sensor layer (SnO2) will determine the measurement range of the sensor. The initial phase of the sensor layer design is to determine the maximum measurement range of the sensor in ppm, in this draft is expected is 1000 ppm. With reference to the equation equilibrium reaction between carbon dioxide and SnO2, namely:

SnO2+2CO ↔ 2Sn+2CO2+2e (15)

and

V

m

=

ρ

(16)

It is known molarity ratio between the reducing gases and SnO2, which is 2 times the molarity SnO2 molarity CO. Furthermore, determine the mass and volume of SnO2, from calculations with the above equation is obtained V = 0.001726 cm 3_, so that the layer thickness determines the width of the sensor layer will be obtained as in Figure 3.

Figure 3. Construction layer sensor.

Electrode design

Electrodes used in the design of the gas sensor is a pair of interdigital electrodes shaped fingers of nobel metal material is Ag or Au. Determination of the electrode resistance value is equal to the determination of the resistance value of the thick-film resistors. To perform the calculation of the resistance value of the electrodes in this design, dimensions can be divided into small parts. For the design that will be processed on alumina substrate was like in Figure 4.

Design heater

Temperature is an important thing to consider in the design of a gas sensor. The speed of the temperature distribution will affect the level of selectivity and sensitivity of the sensor. To minimize space and so fast heat distribution is reached, the heater is designed with the same dimensions and placed on top of the insulator layer that has been superimposed over the first layer of the electrode fingers. Calculations for Heaters layer can be seen in Figure 5.

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Figure 4. Interdigital fingers.

Figure 5. Design calculation heater.

To determine the characterization of the heater, the parameters that must be considered are: the desired temperature, the power required and the extent of the area being heated, as well as the character of the material heater itself (TCR, the maximum current dissipation is able to pass through and others).

Characteristics heater:

Th : working temperature (300 ° C)

Tc : initial temperature (25 ° C)

P : Power on the working temperature (3W)

TCR : Temperature Coefficient resistance (3900)

Then determine the value of the resistance heater at the working temperature (RH), using the source voltage of 3 V and a desired power 3 W, the work flow heater are:

I = P/V = 3W/3V = 1A.

In order to get the heater resistance at operating temperature (RH). After the results obtained by the design of the sensor fabrication process is carried out, using thick film technology.

c. Fabrication process sensor with thick film technology

Thick Film Technology (TFT) is one part of a technological process for the fabrication of microelectronics electronic components in screen-printing. Since billowy 1960, process technology has been used for thick films miniaturized an electronic circuit into a chip substrate, because it produces a very small conductor lines (fine line). Thick film technology has been widely used in industry microelectronics hybrid components and applied in various fields, such as automotive, telecommunications, medical and development of sensors and actuators. The main materials used in thick-film technology is the substrate and pasta. The substrate is a medium thick film components are implemented, while the paste is thick film forming material component, which is formulated such that it can be formed by molding. Thick film process (thick films process) consists of several stages which include the manufacture of the screen, printing, drying (drying), burning (firing), trimming and a number of additional processes such as the installation of the foot (lead frame) and packaging (encapsulation).

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Until this time the activities carried out until the stage of the manufacture of the sensor fabrication electrode layer, insulating layers and layers of heaters. These three processes above goes well, the results of the sensor fabricated in accordance with the design have made.

Figure 6. Fabrication sensor with Thick Film Technology.

d. Coating process sensitive materials

Sensitive material coating process performed after fabrication phase sensor made with thick film technology. While the technology used for coating sensitive material is sputtering technology.

Materials used were the target of SnO2 materials. Samples in the form of sensors and heater electrode layers are coated upper surface of sensitive materials, coating performed on some samples to distinguish the length of time the coating. It is intended to get the best resistance value, because it will affect the operating temperature of the sensor.

SnO2 coating With the RF-sputtering method with a time of 60 minutes, the

power of 200 watts, the gas flow meter (Ar) at 15 mTorr obtained as shown in Figure 7.

e. Testing sensor characteristics

Sensors that have been coated SnO2 sensitive material with characteristics measured sputtering technique, so it can be seen in the performance of the sensitive layer to temperature changes. Tests on the surface of the coating was conducted to determine the morphology of the lining thickness and flatness.

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Testing resistance sensor

Testing purposes

• Knowing the sensor resistance value and changes to temperature changes.

• Knowing the sensor resistance changes in response to changes in temperature.

Equipment and materials

• Sources of direct current (Kenwood Regulated Power Supply Type PD18-30AD).

• Digital Multimeter (Sanwa Digital Multimeter PC 100).

• Digital Thermometer (Lutren TM-914C).

Table 1. Changes in resistance to temperature sensors for sensitive SnO2 coating materials 1 hour measurements the first without heating beginning.

Voltage (V) Resistance (Ω) Temperature (°C)

0 125 25

1 122 27

2 79 52

3 14 96

4 13 150

5 21 206

6 14 264

Table 2. Changes in resistance to temperature sensors for sensitive SnO2 coating materials 1 hour measurements the second with heating beginning.

Voltage (V) Resistance (Ω) Temperature (°C)

0 28 25

0.5 24 30

1 22 34

1.5 21.5 42

2 20 53

2.5 18.5 70

3 17.5 96

3.5 16.5 116

4 15.7 153

4.5 15 185

5 16.5 204

5.5 18.5 240

In this there is a difference chip measurement results on the measurement of the 1st and 2nd measurement. This happens because at the first measurement of the chip directly heated and measured at the foot of the electrodes while the measurements to -2 chips preheated at 200 °C and then measured after cooling. This difference occurs because the moisture-sensitive layer at the 1st measurement less intact so that a change in resistance that is less stable.

In this there is a difference chip measurement results on the measurement of the 1st and 2nd measurement. This happens because at the first measurement indirectly heated chips and are measured at the foot of the electrodes while the measurements to -2 chips preheated at 200 °C and then measured after cooling. This difference occurs because the moisture-sensitive layer at the 1st measurement less intact so that a change in resistance that is less stable. However, this chip experiencing physical kerusan occur because of lack of prudence in storage.

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Figure 8. Diagram box testing feedback sensor.

Figure 9. Test circuit to the flow of CO gas sensor.

Figure 10. Schematic tool gas sensor characterization.

Testing of gas flow sensor

# testing circuit arranged as in Figure 10. Sources of direct current supplied to the heater generates heat which in turn alter the resistance of the sensor.

# Put the heater in the form of a direct current of 1.3 is the operating current # according to previous testing.

# gas used is 35% CO gas.

# given concentration of CO gas was 10 ml.

# Value initial resistance (R0) was measured at room temperature during testing, and the clean air (without gas CO).

# Tests conducted on two different sensors

The test results of the change in temperature sensor resistance changes and changes in the concentration of CO gas, shown in Figure (11).

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Figure 11. Graph of change resistance to change in volume of CO gas flow at room temperature, R = 91.7 K Ohm with SnO2 sensitive layer.

Designing and testing sensitive layer

From the shape of the gas sensor design made with dimensions of 10mm x 10mm, shows that this is done to minimize the dimensions so that the shape can be smaller. Also in terms of warming will be faster evenly as smaller dimensions and also the nature of the substrate that is resistant to high heat, so it takes a shorter time to achieve a uniform and stable temperature, so that the resulting sensor is more sensitive and reaction to the gas will faster.

From the calculations have been done to get the shape and dimensions of the corresponding sensor. Calculations dimension sensor sensitive areas undertaken with a view to obtain the optimal size, as adapted to sensitive materials used and the type of gas to be detected, so that the reaction gas sensors to be quickly detected.

Lay out the design and calculations are done on the design dimensions of the electrodes will affect the value of the expected resistance, by adjusting the paste material to be used. The width of the fingers and the distance between the fingers magnitude must follow the rules of design in thick film technology. Expected results of the resistance value is not much different from the calculation in the draft.

Design and calculations performed on the heater, which is located above the insulator and electrodes, it is entered to get results fast heating and stable. Furthermore paste material used must have appropriate characteristics to conditions designed sensor.

As in the design of the electrodes, heater design must follow the rules for the design of the resistor in thick film technology. Sensor fabrication process using thick-film technology has been made with ingredients that are still there, and the result of the resistance measurement in accordance with the results of the design and in accordance with the desired specifications are 1.5Ω. While the electrode layers and layers of heaters are not in contact, so that no voltage is showing any thing good isolator layer.

Of the overall data shows the results where as the temperature rises the resistance produced will decrease. If the test to temperature changes continue, it will obtain the corresponding value of the desired resistance as the value of the design. With the results of the resistance temperature sensor work will be obtained.

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Sensitive layer testing determination operation temperature sensor and sensor

To determine the characteristics of the sensitive layer which has disputtering then measured changes in resistance to a given temperature at the sensor. Of the overall data shows the results where as the temperature rises the resistance produced will decrease. If the test to temperature changes continue, it will obtain the value of the desired resistance value as the result of design. The result of the resistance of the sensor operating temperature to be obtained, which is where the resistance of the sensitive layer of the sensor starts approximately constant. Only resistance change caused by the presence of gases surrounding because the sensitive layer is a semiconductor material, then the provision of thermal energy (heating) can alter the electrical properties of the sensitive layer, so that the function of determining the operating temperature sensor is so sensitive layer can function as a gas sensor. In SnO2 sensitive layer that began at a temperature of 165 ° C drop in the resistance

began to decrease, and began mencai stable condition in the range 165 °C to 200 °C. Sensors for gas flow testing

In the graph drawing 9, and 10 is the measurement of the change of resistance to changes in the concentration of CO gas stream. From these data indicate that the influence of the change in gas concentration sensor resistance value changes, while the sensitivity indicates how far the sensitivity of the sensor to the quantity being measured, and expressed with numbers that indicate changes compared to changes in the input or output of the sensor's ability to respond to the presence of gas associated with the concentration of the gas . The concentration of gas is increasingly concentrated contain more CO molecules that can bind oxygen on the surface of the sensitive layer of SnO2, resulting vacancy bound oxygen

gas. The plenty of oxygen atoms of CO to CO2 equivalent to the formation of free electrons,

which results in reduced material resistance, so that the difference in the resistance of the sensitive layer before and after the given gas increases. The sensitivity is proportional to the difference in the resistance of the material before and after gassed.

S = [| Rn - Rg |/Rn] x 100%. (17) From the above formula, it can be obtained value sensitivity of the sensor with the sensitive layer SnO2 by 4%. With this data, it can be easily made gas detection system as data acquisition, the system will detect in real time, ie made amplifier and signal conditioner.

4. Conclusions

It has done the design and fabrication of gas sensors with thick film technology consists of three basic components, namely: a heater (heater), electrodes and sensors sensitive layer. Design of Gas Sensors with thick film technology, the shape and dimensions must follow the rules of designing thick-film resistors. Sensors sensitive coating material to be used must be in accordance with the plan of the gas to be detected. Resulting from the design dimensions of the sensor corresponding to the value of the resistance calculation using thick film resistor technology.

The difference in the thickness of the sensitive layer affects the atomic bonding materials that affect the change in resistance. Sensitive material coating results indicate a linear change in resistance value of the temperature change is given. Maximum expected results is the lowest resistance for sensor work that can be achieved is at a temperature that is not too high, so that the sensor will work faster and more sensitive. The test results are given gas flow sensor with CO showed good results that CO gas sensor can work at

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temperatures of 165 ° C sensitive layer is used to detect the CO gas sensor is SnO2 with a sensitivity of 4%.

References

Barsan, N. (2008). Gas sensing mechanisms in thick and porous SnO2 layers. Institute for Interface

Analysis and Sensor, Tuebingen.

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Figure 6. Fabrication sensor with Thick Film Technology. d. Coating process sensitive materials

Sensitive material coating process performed after fabrication phase sensor made with thick film technology. While the technology used for coating sensitive material is sputtering technology.

SnO2 coating With the RF-sputtering method with a time of 60 minutes, the power of 200 watts, the gas flow meter (Ar) at 15 mTorr obtained as shown in Figure 7.

e. Testing sensor characteristics

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Testing resistance sensor

Testing purposes

• Knowing the sensor resistance value and changes to temperature changes. • Knowing the sensor resistance changes in response to changes in temperature. Equipment and materials

• Sources of direct current (Kenwood Regulated Power Supply Type PD18-30AD). • Digital Multimeter (Sanwa Digital Multimeter PC 100).

• Digital Thermometer (Lutren TM-914C).

Table 1. Changes in resistance to temperature sensors for sensitive SnO2 coating

materials 1 hour measurements the first without heating beginning.

Voltage (V) Resistance (Ω) Temperature (°C)

0 125 25

1 122 27

2 79 52

3 14 96

4 13 150

5 21 206

6 14 264

Table 2. Changes in resistance to temperature sensors for sensitive SnO2 coating materials 1 hour measurements the second with heating beginning.

Voltage (V) Resistance (Ω) Temperature (°C)

0 28 25

0.5 24 30

1 22 34

1.5 21.5 42

2 20 53

2.5 18.5 70

3 17.5 96

3.5 16.5 116

4 15.7 153

4.5 15 185

5 16.5 204

5.5 18.5 240

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Figure 8. Diagram box testing feedback sensor.

Figure 9. Test circuit to the flow of CO gas sensor.

Figure 10. Schematic tool gas sensor characterization.

Testing of gas flow sensor

# testing circuit arranged as in Figure 10. Sources of direct current supplied to the heater generates heat which in turn alter the resistance of the sensor.

# Put the heater in the form of a direct current of 1.3 is the operating current

# according to previous testing.

# gas used is 35% CO gas.

# given concentration of CO gas was 10 ml.

# Value initial resistance (R0) was measured at room temperature during testing, and the clean air (without gas CO).

# Tests conducted on two different sensors

The test results of the change in temperature sensor resistance changes and changes in the concentration of CO gas, shown in Figure (11).

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SWUP Figure 11. Graph of change resistance to change in volume of CO gas flow at room temperature, R = 91.7 K Ohm with SnO2 sensitive layer.

Designing and testing sensitive layer

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Sensitive layer testing determination operation temperature sensor and sensor

sensor. In SnO2 sensitive layer that began at a temperature of 165 ° C drop in the resistance

began to decrease, and began mencai stable condition in the range 165 °C to 200 °C.

Sensors for gas flow testing

can bind oxygen on the surface of the sensitive layer of SnO2, resulting vacancy bound oxygen

gas. The plenty of oxygen atoms of CO to CO2 equivalent to the formation of free electrons,

S = [| Rn - Rg |/Rn] x 100%. (17)

From the above formula, it can be obtained value sensitivity of the sensor with the sensitive layer SnO2 by 4%. With this data, it can be easily made gas detection system as data acquisition, the system will detect in real time, ie made amplifier and signal conditioner.

4. Conclusions

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SWUP temperatures of 165 ° C sensitive layer is used to detect the CO gas sensor is SnO2 with a sensitivity of 4%.

References

Barsan, N. (2008). Gas sensing mechanisms in thick and porous SnO2 layers. Institute for Interface

Analysis and Sensor, Tuebingen.