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

A bilayer sensor structure of nickel oxide NiOx (≈ 60 nm) with a very thin film of palladium (Pd ≈ 18 nm) on the top, has been studied for gas-sensing application at relatively low temperatures of about 30◦C and 60◦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 (≈ 60◦C). The hydrogen sensitivity is on the medium level, not exceeding 600 Hz (relative change in the differential frequency of ≈ 2.3%) at interaction temperature of 35◦C.