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 very 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°C produces continuous films, free of porosity, which appear as layers covering the substrates, with no detectable grains. The humidity sensing electrical properties of the films were evaluated using electrochemical impedance spectroscopy (EIS) measurements. Resistance changes, although the films are dense, even larger than those reported for porous ceramics are recorded, as high as 9 and 8 orders of magnitude in the relative humidity range tested (4-87%), for the iron oxide and 10 at% K-doped titania films heated to 300°C, respectively. These outstanding results are interpreted in terms of conduction mechanisms different from the mechanism usually accepted for porous ceramics. EIS results confirmed that, for 10 at% alkali-doped TiO₂ films, charge carriers are alkali ions and not surface protons.