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 α-Fe₂O₃ thick-film gas sensor, was investigated for low operating temperature. Amorphous-like solid solutions of xα-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°C. AC impedance spectra and thermally stimulated current were characterized to investigate the conduction properties of the solid solution, 0.2α-Fe₂O₃-0.8ZrO₂, in air and nitrogen (carrier gas), respectively. It was found that the Arrhenius plots of σT 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, 0.2α-Fe₂O₃-0.8ZrO₂, and the dissociation of the associated oxygen vacancy defect complexes at 320°C are the critical factors for the high relative resistance to oxygen gas at low operating temperature.