Oxygen sensing properties of strontium titanate ferrite composite films at low temperature

Strontium titanate ferrite solid solutions are reported as potential resistive oxygen sensors for automobile application due to its zero temperature coefficient of resistance between 700 to 900 °C and strong sensitivity to oxygen partial pressure. In this contribution, the oxygen sensing properties of strontium titanate ferrite at lower temperature range (<450 °C) were characterized. SrTi_0.8Fe_0.2O_3-s (STF20) were synthesized and fabricated into gas sensing devices using modified sol gel composite film method. Two different synthesis methods, namely high-energy ball milling and conventional high-temperature solid state reaction, were investigated and compared. The crystal structure, microstructural properties and oxygen sensing properties of the synthesized STF20-based sensing devices were characterized using X-ray diffraction, scanning electron microscope, and gas sensing measurements. High-energy ball milled STF20 was found to be milled down to around 14 nm, which is much smaller than powders prepared by solid state reaction. High-energy ball milled STF20 device showed better sensing response to 20% oxygen with optimal relative resistance of 159 at around 300 °C. The oxygen partial pressure dependency of the conductivity was also investigated. Improved 1/m value confirmed its excellent performance as oxygen sensors due to extra oxygen defects induced by high-energy ball milling process.