SOL-GEL PROCESSED TIO2-BASED THIN-FILMS AS INNOVATIVE HUMIDITY SENSORS

Active elements for humidity sensors based on alkali-doped and undoped TiO2 films have been prepared by the sol-gel technique, TiO2-based fi lms are deposited onto Al2O3 substrates with comb-type Au electrodes b y dip-coating, from the appropriate solutions, to obtain four differen t compositions of 1, 3, 6, and 10 at.% Li and K, with respect to the a lkali metal/Ti system. The humidity-sensing electrical properties of t he films are evaluated using d.c. and a.c. measurements. The electrica l characterization of TiO2 films doped with 10 at.% K shows that this material has some unique properties. Resistance versus relative humidi ty (rh) values show variations as high as seven orders of magnitude in the humidity range tested. Very high humidity sensitivity is also obs erved at the lowest rh values (4-10% rh), at 40 degrees C, when the re sistance is measured at frequencies lower than 1 Hz. The results also show great stability over time. The possibility of modifying the rh se nsitivity of TiO2 films doped with 10 at.% K by varying the frequency at which the resistance is measured is a characteristic that allows th is material to be described as smart.