Effect of humidity on SAW devices

Adsorption of water vapors on single crystal and polycrystalline substrates is analyzed using the propagation of surface acoustic waves as an experimental tool. The perturbation in the SAW velocity (Δv/v₀), the number N of adsorbed species as well as the changes in the density Δρ/ρ and in the elastic moduli Δc_ij/c_ij of the sorbent materials, are deduced in terms of the normalized SAW displacements A_x, A _y, A_z and of the unperturbed SAW velocity v₀ . Vapour adsorption on polished plates of single crystal quartz and polycrystalline films of Pd and Pd:Ni is experimentally investigated. Results of the measurements clearly show that humidity adsorption gives rise to effects whose amplitude is larger and whose time duration is longer in the films than in the plates. These effects are also dependent on the orientation of the quartz, allowing to monitor the value and the rate of the effect by proper selection of a substrate material