New properties of SAW gas sensing.

Novel attractive properties of SAW gas sensing are theoretically predicted and experimentally verified. The response upon gas exposure of SAW-based gas sensors can be increased, decreased, reversed, cancelled, speeded up, aged down, and selected for a given sensitive layer, simply by changing the substrate material and orientation. When utilized as a tool for analytical chemistry, the steady-state and kinetic properties of adsorption, desorption, and diffusion, together with other related processes, can be simply deduced from pure acoustic measurements. These new properties are shown to be produced by the change of the components of the elliptic polarization of the wave, varying with the propagation direction and the substrate material. Experimental results, obtained for quartz substrates coated with polycrystalline palladium and palladium-nickel films exposed to H(2), CO, N(2)O, and different concentrations of relative humidity in air are presented as an example.