CERAMIC THIN-FILMS BY SOL-GEL PROCESSING AS NOVEL MATERIALS FOR INTEGRATED HUMIDITY SENSORS

Thin films based on 10 at% alkali-doped TiO2 and on iron oxide with ve ry large humidity sensitivity were prepared using a sol-gel technique. Prototype humidity sensors were prepared by depositing metal alkoxide solutions onto Al2O3 substrates with comb-type Au electrodes, by dip- coating and heating in air. Heating to 300 degrees C produces continuo us films, free of porosity, which appear as layers covering the substr ates, with no detectable grains. The humidity sensing electrical prope rties of the films were evaluated using electrochemical impedance spec troscopy (EIS) measurements. Resistance changes, although the films ar e dense, even larger than those reported for porous ceramics are recor ded, as high as 9 and 8 orders of magnitude in the relative humidity r ange tested (4-87%), for the iron oxide and 10 at% K-doped titania fil ms heated to 300 degrees C, respectively. These outstanding results ar e interpreted in terms of conduction mechanisms different from the mec hanism usually accepted for porous ceramics. EIS results confirmed tha t, for 10 at% alkali-doped TiO2 films, charge carriers are alkali ions and not surface protons.