WO_3 thin films have been prepared by Sol-Gel (SG), Radio Frequency Sputtering (RFS) and Vacuum Thermal Evaporation (VTE) techniques on alumina substrates and annealed at 600°C for 24 h in air. The morphology, crystalline phase and chemical composition have been characterized using SEM, XRD and XPS techniques. The electrical response has been measured by exposing the films to O_3 (10 to 180 ppb), NO_2 (0.2 to 1 ppm) and Cl_2 (0.1 to 1 ppm) and operating temperatures between 150°C and 450°C and 50% humid air. The response to O_3 has been found to be at maximum at 400°C. At this temperature that of NO_2 and Cl_2 is negligible. Improvements on the O_3 gas sensitivity and selectivity can be achieved by fixing the operating temperature at 400°C. The most recommended temperature for NO_2 and Cl_2 detection has resulted to be 200°C.
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