Study of xα-Fe2O3-(1-x)ZrO2 solid solution for low-temperature resistive oxygen gas sensors

A noble type of oxygen-sensitive and electrical-conductive material, ZrO2-based with alpha - Fe2O3 thick-film gas sensor, was investigated for low operating temperature. Amorphous-like solid solutions of xalpha - Fe2O3 - (1 - x)ZrO2 powders were derived using the high-energy ball milling technique, and their physical and microstructural properties were characterized from DTA, XRD, TEM, and XPS. The oxygen gas-sensing properties of the screen-printed thick-film gas sensors fabricated from such mechanically-alloyed materials were characterized systematically. Very good sensing properties were obtained with a relative resistance value of 82 in 20% oxygen, and at a low operating temperature of 320 degreesC. AC impedance spectra and thermally stimulated current were characterized to investigate the conduction properties of the solid solution, 0.2alpha - Fe2O3 - 0.8ZrO(2), in air and nitrogen (carrier gas), respectively. It was found that the Arrhenius plots of sigmaT versus 1000/T have two distinct gradients corresponding to two activation energies in the high and low temperature regions. The transition temperature occurs at about 320 C that corresponds to an optimal operating temperature of the gas sensor. It is believed that the high oxygen vacancy concentration present in the solid solution, 0.2alpha - Fe2O3 - 0.8ZrO(2), and the dissociation of the associated oxygen vacancy defect complexes at 320 degreesC are the critical factors for the high relative resistance to oxygen gas at low operating temperature.