Bilayer Structures of NiOx and Pd in Surface Acoustic Wave and Electrical Gas Sensor Systems

A bilayer sensor structure of nickel oxide NiOx (approximate to 60 nm) with a very thin film of palladium (Pd approximate to 18 nm) on the top, has been studied for gas-sensing application at relatively low temperatures of about 30 degrees C and 60 degrees C. The bilayer structure was obtained by rf sputtering and by vacuum deposition (first the NiOx and then the Pd film) onto a LiNbO3 Y-cut Z-propagating substrate, making use of the surface acoustic wave method, and additionally (in the same technological processes) onto a glass substrate with a planar microelectrode array for simultaneous monitoring of the planar resistance of the layered structure. Such a bilayer structure was investigated in a low concentration range (from 50 ppm to 400 ppm in air) of nitrogen dioxide (NO2), carbon monoxide (CO) and ammonia (NH3) in a dry and wet air atmosphere and in a medium hydrogen concentration (1-2.5%) in dry air. The NiOx and Pd bilayer structure interact rather weakly with NO2 molecules but with CO and NH3 this interaction is much greater, especially at higher temperature (approximate to 60 degrees C). The hydrogen sensitivity is on the medium level, not exceeding 600 Hz (relative change in the differential frequency of approximate to 2.3%) at interaction temperature of 35 degrees C.