DEVELOPMENT AND CHARACTERISATION OF TIN OXIDE BASED GAS SENSORS

Abstract

Industrialization has brought in its wake serious impairment of the atmospheric environment. Many a gases, hazardous in nature i.e. such gases which are present in the atmosphere in such concentration, that it can produce undesirable effect on life, health, property and environment are being released from industrial activity. For example, hydrocarbons which are widely used as fuel and raw materials for the production ofvarious products by the industries are potentially hazardous, if not used with safety precautions. A sight leak (either accidental or intentional) can result in explosion and fire. As a result, the protection ofenvironment from the emissions of hazardous gases has become imperative. This can be accomplished only when these gases can be detected accurately and with instantaneous swiftness with very little lag time. This necessitates the development of sensing and monitoring devices for such hazardous gases as hydrocarbons, H,, CO, NO,, H2S, CI,, SO,, HCI etc. The instruments which are currently in use are chromatographs, mass spectrometer, calorimeter and electrochemical sensors, for monitoring on-line analysis of the gases. In-situ operation of these instruments results in bulky structure, complexity and high cost. This has drawn the attention of scientists and engineers, all over the world to develop sensors which are miniaturized, economical, I.C. compatible and are capable to detect and discriminate between the hazardous gases. Significant efforts have been made to realize gas sensing by using different technologies, such as MOS, thin and thick-film. Thick-film technology has an advantage over other technologies in terms of cost, design flexibility, ruggedness, high operating temperature and hybridization of signal processing circuit along with the sensory elements. In the present investigation, thick-film technology has been used for the fabrication of the gas sensor. Various oxides have been used as sensing material in these gas sensors. However, tin oxide has been found to possess a better response for oxidizing as well as reducing gases at a relatively lower operating temperature. Therefore, tin oxide has been chosen as the base sensing material. Undoped and doped tin oxide thick-film gas sensors have been used for detection of hydrogen, carbon monoxide, methane and LPG. Palladium has been used as main dopant for the fabrication of sensors but other dopants like platinum and gold have also been tried to achieve the different sensitivities of sensors for the development of sensor array. For the characterisation of the sensors, the required test chamber and measurement set-up were developed for detection of test gases in low concentration range (0-1000 ppm). The resistance of sensor (Rg) with varying concentration (0-1000 ppm) of test gas was recorded. The sensitivity 'S' of the sensor defined by [(R„ - RK)/R0] is plotted against the concentration of test gas (where R0 is the resistance of the sensor in clean air). The response and recovery time of the sensors were recorded at fixed concentration of the test gases.

Hydrogen gas is widely used as a fuel in industry, as a reducing agent in epitaxial chemical reactions and in various other commercial and industrial applications. Leakage of hydrogen can result in undesirable and disastrous consequences. Therefore, monitoring of traces of hydrogen has become extremely important. A number of gas sensors based on hydrogen induced change in the electrical conductivity of the gas sensitive semiconductor metal oxides such as Sn02, ZnO, Fe203 and the photopyroelectric (P2E) effect have been developed. But the concentration of hydrogen that these sensors could detect was reported to be 0.075%. However, the developed Pd-doped tin oxide sensor in the present study, is capable to detect hydrogen at still lower concentrations. Also, a better reaction scheme has been proposed to explain the sensing meechanism of Pd-doped tin oxide thick-film sensor for the detection of hydrogen. The same sensor has also been characterised for CO. The detection of CO is very effective in monitoring of flaming and smoldering fires and hence can be usefully incorporated in domestic security and alarm system. It has been found that the fabricated Pd-doped tin oxide thick-film sensor is quite suitable for the detection of hydrogen and carbon monoxide well below 0.01% concentrations. The sensitivity towards these gases can be changed by varying the amount of palladium catalyst in the Sn02 layer. The optimum palladium content in Sn02 is found to be 0.25 wt% for the detection of hydrogen well below 0.01%concentration while 1 wt% for detection of carbon monoxide in the concentration range of 0-1000 ppm. In addition, the sensitivity for hydrogen and carbon monoxide was found to be function of operating temperature. The sensitivity for these gases was very poor for operating temperature below 250"C. The optimum temperature for maximum sensitivity was observed at 350°C. It was further observed that at 350°C operating temperature, the effect iii of humidity can be neglected. This was verified by testing sensors in different humidity conditions. No appreciable change in results were observed at the selected operating temperature. Though, sensors are very sensitive in the lower concentration range they are also suitable for detection at higher concentrations. The proposed reaction scheme for hydrogen on Sn02 surface in air implies tliat hydrogen reacts with adsorbed oxygen atoms in either molecular or atomic forms, resulting in an accumulation of electrons near the surface and thus increase the conductance. The proposed reaction scheme for CO on Sn02 surface shows that CO may react directly with adsorbed oxygen or adsorbed CO ion on palladium may react with adsorbed oxygen in second step, both resulting in an increased conductivity of Sn02 sensors. Earlier studies made on thick-film sensor reveals that electrode material do not influence the properties of the sensors, though some elaborations have been made that electrode materials significantly influence the response of the sensor. To study the effect of electrode materials on the response of sensors, four sensors were fabricated. The selected material for electrodes were gold and silver. In one case, two undoped tin oxide sensors were fabricated having gold and silver electrodes and in another case two silver doped tin oxide sensors have gold and silver electrode. The response of all the sensors was noted for H2, CO and CH4. It was found that an undoped and silver doped SnO, sensor with gold electrode possesses the similar responses for H2 and CO. However, undoped tin oxide sensor with gold electrode has very poor sensitivity for all the test gases. iv The enhanced sensitivity of undoped tin oxide sensor with silver electrode for CO and H2 is due to the catalytic action of the silver electrode itself. However, this effect was not observed for CH4. It is, thus, concluded that even undoped Sn02 sensors having a silver electrode will be potential gas sensors for H2 and CO detection. Such a structure eliminates the need for intentional doping of the tin oxide with silver. The effect of firing and operating temperatures has a strong influence on the sensitivity of Pd-doped Sn02 sensors to H2, LPG, CO and CH4. The samples fired at 550"C show maximum sensitivity at an operating temperature of 350°C for all gases but at higher firing temperatures, the sensitivity of the sensors shows a reducing trend for hydrogen, methane and carbon monoxide. However, in case of LPG, sensitivity drops at 650°C of firing temperature and shows a increasing trend beyond 650°C. Thus, the firing temperature of 550°C was found to be suitable for doped Sn02 paste for development of sensor to detect H2, CO, CH4 and LPG. This is the reason that during the course of investigations, all the sensors were fired at 550°C. At this firing temperature, the sensitivity of LPG and hydrogen is very close. In order to achieve better separation between the sensitivity for LPG and hydrogen, firing tempeature of 850°C is suitable. These results reveal that the idea of achieving different sensitivity of sensors in a sensor array for discrimination of gases could also be obtained if sensors are fired at different temperatures having same dopant with same composition of gas sensitive layer. The common problem associated with gas sensors is the lack of selectivity. The simplest way to detect, identify and display the presence of gases/odours and their concentration is IGS (Integrated gas sensor). Thick-film technology has been found to be quite attractive for the realisation of an IGS. In the past, various approaches, like different sensing material, operating temperature and frequencies for measurements have been reported for the realisation of gas sensor array. But the most common approach for the realisation of sensor array is to develop sensors with different dopants to a sensing material (Sn02). The available literature in the area of gas sensors towards the development of an IGS present more emphasis for the development of proper sensor array using tin oxide in association with different dopants to have different sensitivities of the sensors in an array. The response of such an array has widely been studied by several workers. However, very scanty information is available on the response and the recovery times of the sensor. In the present study, an array of four sensors is fabricated with doped (Pd, Pt,and Au) and undoped SnO,. The sensors are characterised for H2, CO, CH4 and LPG. Further, the transient response of palladium doped sensor is studied in detail. The available literature presents a higher order of response and recovery time, but the fabricated sensors in the present investigation show a relatively lower response and recovery time. The fabricated sensor array which consists of undoped and doped (palladium, platinum and gold) SnO, have different sensitivities to the test gases (H2, CO, CH4 and LPG) and can serve the purpose of IGS. The undoped Sn02 sensor is sensitive to all the test gases, while doped SnO, significantly improves the sensitivity of the sensors. However, doping with Au,

improves the sensitivity of the sensor by a very small amount. The platinum doped sensor possesses the highest sensitivity for CO, whereas palladium doped sensor posseses highest sensitivity for H2, LPG and CH4. Though platinum doped sensor has higher sensitivity for CO, but not too much as compared to Pd-doped sensor. The transient response ofthe sensor tor H,, CO and LPG implies that the response of the sensor is quite reproducible with very short response and recovery time. The experimental results of the sensor array (sensitivity versus concentration and sensitivity versus response time) for all the test gases at an operating temperature of 350°C have been validated and the observed experimental results are in good agreement to that predicted by the theoretical models.