Ac response to humidity and propane of sprayed Fe-Zn oxide films

Abstract

Iron-zinc oxide films with different Zn contents were ultrasonically sprayed on glass substrates and inter-digital gold electrodes were evaporated on them. Films were deposited from solutions containing 2, 10 and 30 at.% Zn. Hematite, amorphous and Franklinite structures resulted, respectively. They were assessed as humidity and propane detectors under alternating-current conditions for frequencies from 1 to 10$^{5}$ Hz and two temperatures: 30 and 250$^{\circ}$C. Their impedances in dry air, humid air and humid air plus propane were determined from voltage measurements with a Lock-In amplifier. Sensitivity values to both humidity (53% RH.) and 189, 500 and 786 ppm of propane from the response of the real and imaginary parts of the impedance and also the total impedance were determined as functions of frequency. Maximum sensitivity for each case and the corresponding frequency are reported. The maximum sensitivity to humidity ranges from 24% up to 308%. In the case of propane, the maximum sensitivity ranges from 45% up to 711%. The largest sensitivity values correspond in all cases to the imaginary part of the impedance. From the dynamical response, the response and recovery times are determined, along with the concentration-dependence of the sensitivity. The sensing mechanisms are also discussed.