Effect of Noble Metals on Selective Detection of LPG Based on SnO 2
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EXPERIMENT The schematic diagram of the measurement set up shown in the Figure 1. The sensor elements have been prepared by co-precipitation method using the following procedure. 10 ml of SnCI4 are transferred into a 50 ml capacity two necked round bottomed flask connected with a dropping funnel placed in an ice trough. Aqueous solution of ammonia is added drop by drop to SnCI4 till the pH is slightly more than 7.5, so as to obtain a complete precipitation. The flask is kept in ice cold temperature to avoid spillage because the reaction is vigorous. At the same time 0.2235 gm of PdCI2 is dissolved in ethanol and added to the above solution in order to get 1.5 wt% of Pd in Sn0 2. The contents are stirred by using a magnetic stirrer. After stirring for about 30 min the contents are filtered. In the present case, Pt ( in the form of H 2PtCl6 , 61120 ) and Rh ( in the form of RhCl 3) are added to SnO 2 in place of Pd to see the effect of gas sensitive properties. The resulting precipitate is filtered and washed thoroughly with hot deionized water to remove CI ions and dried at 120"C for 24h in an electrical oven. The dried calcined powder is ground in an agate mortar and pestle and calcined at different temperatures ranging from 500-1000'C for 5 h. The calcined powder is sieved in 170 mesh to obtain fine particles. In order to improve the mechanical properties, the powder is mixed with few drops of binder Tetraethylorthosilicate (TEOS) and the resulting paste is applied on alumina tube ( 3mm diameter x 10mm length) attached with two platinum electrodes 8mm apart from each other. The elements are again sintered in the range of 500-1 000°C for 5h. For gas sensing experiments the element is kept in a flow apparatus through which air or sample gas is let to flow. The concentration of the gases injected into the system is 1000 ppm. The temperature of the element is controlled with a tungsten heating coil provided inside the alumina tube.
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Figure 1 Schematic diagram of the measurement set up of the sensor element
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After heat treatment in dry air at 400'C, the element is cooled to a prescribed temperature in the range of 50-400'C at which the electrical resistance of the element in dry air (R.) is measured by means of conventional circuit in which the element is connected with an external resistor in series. The out put voltage across the external resistor at a circuit voltage of 1OV is used to evaluate the electrical resistance of the element.The values of the device resistance are obtained by monitoring the output voltage across the load resistor. A chromel-alumel thermocouple placed on the element indicated the operating temperature.The gas sensitivity (S) is defined as the resistance ratio (Rl-Rg / RF). Where R. and Rg are the resistance's of the sensor element before exposed to the gas and after respectively. RESULTS AND DISCUSSION Microstructure X-ray diffractometer (
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