Humidity sensing properties of uncoated and coated SAW delay lines

The relative changes in the SAW propagation velocity ?v/v were measured as a function of the relative humidity (RH) at the normal conditions. The results of the measurements were compared for different bar substrates (AT,x-, ST,x-, yx-SiO2, yz-, 128° y,x-LiNbO3) and different coatings deposited on these substrates (polyvinyl alcohol, Pd, Pd0.99Ni0.01, Pd0.97Ni0.03). In accordance with theoretical predictions [V. I. Anisimkin and E. Verona, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45(5), 1-8 (1998)], the behavior of the ?v/v(RH) curves for a given coating was found to be dependent on the material of substrate and its crystallographic orientation. The magnitudes ?v/v for uncoated SiO2 were two to ten times larger than those were for uncoated SiO2. The coating of the substrates by polyvinyl alcohol and Pd increased sensitivity of SAW towards humidity by 30 to 100 times, providing large response ?v/v?100 ppm even at small RH?1%. The threshold concentration for the coated SAW devices was estimated as RH?0.01%.