SEMICONDUCTOR GAS SENSOR FOR DETECTING AIR POLLUTION
SEMICONDUCTOR GAS SENSOR FOR DETECTING AIR POLLUTION
Nicolae-Marius BÂRLEA, Sînziana Iulia BÎRLEA
Technical University of Cluj-Napoca,
Physics Department, Str. C. Daicoviciu 15, ROMANIA,
E-mail:
ABSTRACT
Over 50 million tin dioxide gas sensors have been used in domestic gas alarms since 1968 to 1990 only in Japan. Many other oxides are gas sensitive as LICKE iron oxides, chromium oxides, zinc oxide, etc. In this paper we characterize gas sensitive sensors made in our laboratory. A hot filament heats the oxide material of the gas sensor (deposited on a ceramic cylinder) to the functioning temperature (100-400°C). When it is exposed to an atmosphere containing a reducing gas (a gas which can interact with the oxygen from the air), the electric resistance of the semiconducting material is dramatically modified, even at very low gas concentrations. We present the influence of the supplied electric current to the sensor sensitivity and the response time and recovery time. The advantages for using semiconducting gas sensors for detection of the air pollution (toxic and/or flammable gases) are emphasized.
INTRODUCTION
Air pollution by noxious exhaust gases from autovehicles’ engines and from burning methane gas and oil is becoming a serious danger. For effective prevention of air pollution a stable and inexpensive sensor for detecting dangerous gases is needed.
In the scientific literature, various oxides were proposedas gas-sensor elements, like iron oxides, chromium oxides, zinc oxide, etc. Tin dioxide (SnO2) is by far the most popular semiconductor oxide used in semiconductor gas sensors, mainly for its ability to sense hydrocarbures and carbon monoxide. The tin dioxide is used as a thin film or as a thick film sintered powder. Naoyoshi Taguchi designed the semiconductor gas sensors based on tin dioxide in 1962 [1]. Only in Japan there have been used over 50 million Taguchi sensors in domestic gas alarms since 1968 to 1990 [3].
The main mechanism of operation is through surface electrical conductivity changes of the semiconductor induced by chemical reactions on the surface [4]. Atmospheric oxygen is chemisorbed on the surface primarily as O–, and it ties the electronic carriers, decreasing the electrical conductivity of the N-type semiconductor sensor. Any reducing gases that may be present in the atmosphere will remove the chemisorbed oxygen, liberating electronic carriers into the conduction band of the semiconductor and enhancing its the electrical conductivity. Therefore any given mixture of atmospheric oxygen and a reducing gas will produce a unique sensor conductance for that gas concentration.
THE DEVICE
As it is show in figure 1, the semiconductor sensor for gas detection made in our laboratory contains:
- a cylindrical ceramic body (Al2O3)
- a coiled filament for heating (stainless steel AISI 304, 50 m diameter),
- a pair of platinum contacts,
- the semiconductor material, SnO2, deposited over the contacts,
- a porous case for protection, made from sintered bronze balls, gas permissive,
- a mounting base which connect the sensor’s inner wires with external wires.
The inner hot filament heats the semiconducting material of the gas sensor (placed on the ceramic cylinder) to the functioning temperature (100-400°C). The platinum wires make a nonrectifying contact with the semiconductor (tin dioxide).
Figure 1. Sensor's structure (protected by a porous bronze case and closed by a mounting base with the terminal leads): Al2O3 cylinder with inner heater filament coil and external SnO2 semiconductor sensing material, which have two platinum contacts.
THE EXPERIMENT
Our sensors were tested with carbon monoxide (CO), liquid petroleum gas (LPG, mainly butane) and methane gas. Known quantities of test gas were injected in the test chamber which volume was 20 ml. The electrical resistance of the sensor was recorded. After each test the chamber was washed with clean air and we waited that electrical resistance of the sensor to return at the clean air value. This protocol was repeated for each gas concentration and each supply voltage.
The heating current for the filament is provided by a monolithic adjustable three terminals voltage regulator (LM317). The output voltage is set by the adjustment potentiometer and measured with a digital voltmeter (3 ½ digits). The electrical resistance of the SnO2 thick film is measured by an ohmmeter (3 ½ digits) as shown in figure 2.
Figure 2. Measurement set-up: the heater is biased by the stabilized tension from a monolithic adjustable three terminal voltage regulator (LM317) and the resistance is measured by an ohmmeter (3 ½ digits).
Sensor’s sensitivity (Rair/Rgas) for methane grows from 2,8V to 3,6V of the supply voltage, at 4V being slightly lower. For LPG the sensitivity grows from 2,2V to 3,1V and the sensitivity for CO grows from 2V to 3,5V. In table 1 we present a set of measurements for methane in air at a supply voltage of 3,6V.
TABLE 1.
The values of the electrical resistance of the gas sensor versus methane concentration
c (%CH4) / R(k) / ln (c) / lnR0 (clean air) / 124 / - / 4,82
0,4 / 21 / -0,916 / 3,04
0,8 / 14 / -0,223 / 2,64
1,2 / 9,8 / 0,182 / 2,28
1,6 / 8,3 / 0,470 / 2,12
2 / 6,0 / 0,693 / 1,79
4 / 3,8 / 1,386 / 1,33
6 / 2,4 / 1,792 / 0,87
8 / 1,8 / 2,079 / 0,59
Response time was determined with the aid of a chart recorder. As a rule, the recovery time (time needed toreturn at the clean air sensor’s resistance) was longer than the response time (time needed for stabilizing the resistance's value in air with gas). Response time was longer for lower supply voltage (many seconds, even minutes) and became very short (under 1 second) at the greatest voltages.
CONCLUSION
SnO2 electrical conductance depends on reducing gas concentration [5]:
G = G0 + ·p S(1)
where:
"S" – an exponent (value close on 0,5),
"" – a constant,
"p" – partial pressure of the reducing gas,
"G0" – clean air conductance (very low in general).
If we neglect the clean air conductance (G0) then the sensor’s resistance (1/conductance) is:
R p S (gas concentration) S(2)
a useful relation, because the exponent "S" is an indicator of the quality of the gas sensitivity of the semiconductor material (greater "S", greater sensitivity). For the data in table 1, presented in a double logarithmic scale in figure 3, the parameter "S" is 0,83, a good figure of merit.
Figure 4. Logarithm of sensor's resistance versus logarithm of gas concentration.
The advantages for using semiconducting gas sensors for detection of the flammable and/or toxic gases are: high sensitivity, resistance to poisoning, robust construction, long life (over 10 years), no need for recalibration (low maintenance cost), low device’s price (~ 20 $).
REFERENCES
- Naoyoshi Taguchi, US patent 3,644,795 (1972);
- T. Seiyama et al. Anal. Chem. (1962) 34, 1502-1503.
- FIGARO products catalog, pag. 2, (1990).
- S. Roy Morrison, "Selectivity in semiconductor gas sensors", Sensors and Actuators (1987) 12, 425-440.
- N. M. Bârlea "Fizica senzorilor", pag. 85, Editura Albastra, Cluj-Napoca, 2000.