PREPARATION OF NIOBIUM OXIDE-FILMS AS A HUMIDITY SENSOR

Niobium oxide films were prepared over a quartz plate by withdrawing i t from a solution of niobium penta-isopropoxide (NIP) dissolved in sec -propyl alcohol. The films calcined at 673 K were well controlled in t he film thickness either by NIP concentration in the solution or by th e withdrawing rate of the quartz plate from the solution. BET surface area of the calcined films was as huge as around 400 m2g-1, and was no t reduced by cyclic adsorption/desorption of water vapor. A rapid decr ease and increase in the electrical resistivity of the calcined films was well associated with the cyclic adsorption/desorption of water vap or. The decrease in the electrical resistivity of the films by water v apor adsorption was more than 10 times sensitive than the decrease cau sed by the adsorption of ethanol, hydrocarbons, carbon monoxide and ca rbon dioxide. These results suggest an application of the niobium oxid e films as an element of a humidity sensor. The calcined films were pr oved by SEM observation to consist of tiny particles possessing a lot of micropores sized less than 20 A. The decrease in the electrical res istivity of the films, or the increase in the electrical conductivity, was attributed to the water vapor adsorbed in these micropores. In or der to identify the charge carriers during water vapor adsorption on t he films, changes in the impedance and the phase shift caused by water vapor adsorption were measured using an LCR meter in AC frequency ran ge of 10 Hz to 100 kHz. From a complex impedance plotting, single semi circule was obtained for water vapor adsorption onto the films, sugges ting single adsorbed species as a charge carrier. Assuming an equivale nt electric circuit for the films adsorbing water vapor, a constant ca pacitance was calculated under various partial pressures of water vapo r, probably suggesting that the charge carriers will be H3O+ on the fi lms.