Sensors and Actwxors B, 18-19 (1994) 155-157 155

Selective petrol vapour sensor based on an Fe,O, thin film

AS. Poghossian, H.V. Abovian and V.M. Aroutiounian State Engineeting University of Armenia, Terian 105, Ymn 375&W (L4mwuh)

AbStIlNt

The gas-sensing characteristics of F%OS thin tlms prepared by the electron-beam evaporation method are presented. The F@03 films show a high selectivity and sensitivity to petrol vapours. The response and recovery times and reproducibility of the gas-sensitive elements are studied. The obtained results shows the possibility of using FezOs thin tihns as material for semiconductor thin-film petrol vapour sensors.

1. 1ntmduction

The principle of semiconductor gas sensors is based on the conduction change phenomenon of the metal oxide semiconductor by reversible chemisorption, as well as on the catalytic oxidation of active gases on its surface. To improve the sensor characteristics the ma- terial of the gas-sensitive elements usually contains catalysts made of noble metals. In recent years Japanese specialists have used the ceramics o-Fe,O, [l] and ‘y- Fe,O, [2,3] as the sensitive material to reducing gases, which has reduced the use of catalysts made of noble metals. From the point of view of lowering the cost and power consumption, decrease of weight and overall dimensions, possibility of mass production with the use of well developed microelectronic methods, as well as because of the developed surface and as a consequence of it the advantageous relation between surface and volume, the thin-film gas-sensitive sensors are more perspective. In this paper the results of experimental investigations of the gas-sensing characteristics of FeO, thin films are presented.

2. Experimental

Fe,O, iihns with thickness of about 0.3 w were deposited by the electron-beam evaporation method. As source material for the evaporation pressed ceramic pellets of FeZO, with the addition of 2 wt.% Mg were used. The fihns were prepared in a vacuum at lo-’ Pa on a glaze ceramic substrate which was heated up to a temperature of 300 “C. ‘Ihe deposition rate at 1 kW electron-beam power was 20 &s.

The Fe,O,-based thin-film sensors consist of a glaze ceramics substrate with comb-shaped metallic elec-

09254005/94/$07.00 6 1994 Elsevier Sequoia. All rights reserved SSDI 0925-4005(93)00938-U

trodes, on which the gas-sensitive fihn is deposited. Leads of gold wire are then attached to the sample’s bonding pads by means of thermo-compression bonding. The Ta-based thin&n heater is fabricated on the backside of the substrate. For stabilization of the pa- rameters the F&O, fihnswere reduced by heat treatment in air at 350 “C for 1 h. After the thermal treatment the square resistivity of the films was about 5X10” G./U at room temperature. X-ray diffraction measure- ments show that the FeZO, thin films made by the electron-beam evaporation method are amorphous bod- ies.

Gas-sensing characteristics of the FQO, frhns were measured in a gas-flow system within a test chamber, which allowed the measurement of the sensitivity to various gases and vapours. The vapours-gas mixture passed from the mixer to the test chamber, containing a quartz tube with a heater in which samples of the sensitive elements were placed and fized by a quartz holder. Five samples were mounted simultaneously on the holder. The desired temperature in the chamber was maintained by a temperature regulator with an accuracy of f3 “C. A chromel/alumel thermocouple located close to the gas-sensitive element indicated the temperature inside the test chamber. The gas (vapour) concentration in the chamber was periodically controlled by LCM4iM and Gasochrom 3101 gas chromatographs. The flow rate of the vapour-gas mixture in the test chamber was 0.1-0.3 l/s.

D.c. electrical resistance measurements of the gas- sensitive elements were carried out under various gas concentrations in the temperature range 150-330 “C. The gas sensitivity was defined as the ratio (R.-R&/ R,, where R, and R, are the sample resistances measured in pure air and in air containing the test gas, respectively.

156

3. Results

The gas sensitivity as a function of working tem- perature for Fe,O,-based thin-film sensors is shown in Fig. 1. The gas percentage in air was equal to 0.1 vol.% for petrol and ethanol vapours, and 2.4vol.% for natural gas. The,resistance of the F%O, fihns decreased when they were exposed to a reducing gas atmosphere, which is characteristic for n-type semiconductors. According to the type of gas present in the air the gas-sensitivity curves as a function of temperature show a maximum at different temperatures. The temperature at which the maximum gas sensitivity is observed is 230 “C for ethanol vapours, 250 “C for natural gas and 280 “C for petrol vapours. By choosing the required operation temperature, one may obtain a certain selectivity of the gas-sensitive element. All samples were practically insensitive to the applied gases at temperature below 150 “C.

The gas sensitivity as a function of the concentration of analysed gas in the air present in the test chamber is shown in Fig. 2 at the mean temperature of maximum gas sensitivity. The percentage of petrol, ethanol and natural gas in the air changed over the range 0.05-0.3, 0.05-0.4 and 0.6-4.8 vol.%, respectively. It can be seen that with the increase of concentration of the petrol and ethanol vapours resistance of the Fe,O, films

0 (30 200 250 3~0 T, *C

Fig. 1. Gas sensitivity vs. temperature for Fe,03-based thin-film sensor: 0, petrol 0.1 vol.%; X, ethanol 0.1 vol.%; 0, natural gas 2.4 vol.%.

Fig. 2. Gas sensitivity of Fe203-bastd thin-tilm sensor as a function of the concentration of various gas in air: 0, petrol at 280 “C, X, ethanol at 230 “C; 0, natural gas at 250 T.

decreased at first, then at comparatively high concen- trations reached saturation. For natural gas sensitivity saturation was not observed over the investigated range of concentration. Analogous effects were observed in the case of y-Fe,O, [2] ceramics sensitivity to methane - the main component of natural gas. The spread in gas-sensitivity values from sample to sample was no higher than 20%.

The important parameters of gas sensors are their fast action (response and recovery times). Response and recovery times depend on the nature and con- centration of the active gases, as well as on the sensitive material and the operation temperature. We have mea- sured the response and recovery times of Fe,O, films for all analysed gases at the temperature of maximum gas sensitivity, when the gas-sensitive elements are exposed to step changes from air to a gas mixture. The 90% response and recovery times (defined as the time taken to reach 90% of the final signal) of Fe,O, samples at 280 “C for 0.05 vol.% of petrol vapour were equal to 60 and 80 s, respectively.

4. Discussion

4.1. Mechankm of sensor The revealed gas sensitivity of the tested Fe203

samples may be explained by the continuous reversible redox processes taking place on the sample surface of the Ghns (as well as on the surface of the pores and on the grain boundaries). When the Fe,O, films are exposed to the active gas atmosphere, as a result of the reduction reaction a change of concentration of oxygen vacancies in the oxide takes place, resulting in the change of iron valency (the concentration of the two-charged ions Fe’+ increases) and consequently in the conductance modulation. It is known [2] that for the solid solution y-Fe,O,-Fe,O, the decrease of re- sistance as the Fe*+ ion concentration increases is typical. This was also observed here, in our experiments.

4.2. Selectiviry In practice it is often necessary for the sensor to

possess selectivity, i.e. the ability to detect a definite component in the presence of other gases. As shown in Fig. 1, Fe,O, Ghns possess selectivity to petrol vapours. In the temperature range 270-300°C and for the realized gas concentrations in air, the sensitivity of Fe203 films to petrol vapours was more than 20 times higher than that to ethanolvapour and naturalgas (the concentration of natural gas being an order of magnitude higher than the petrol vapour concentration), which allows us to recommend Fe,O, films as the sensitive material for semiconductor thin-film petrol vapour sensors.

157

References

1 Y. Nakatani, M. Sakai and M. Matsuoka, Microstructure of a-FelOa ceramics as a combustible gas sensor, Pnx hr. Meer. Chemical Sensors, Fukuoku, Japaq 1983, pp. 147-152.

2 Y. Nakatani and M. Matsuoka, Some electrical properties of yFe,O, ceramics, Jpn. J. Appl. Phys., 22 (1983) 233-239.

3 J. Shin and S. Park, Some charactexistics of yFe20, ceramic gas sensor, Pmt. 2nd Meet. Chemical Sensor, Bordeaw France, 2986, pp. 123-126.