Barium hexaferrite thick-films for ozone detection at low temperature

Crystalline BaFe12O19 powder was synthetized by auto-combustion sol gel method, in which citric acid was used both as a fuel and chelating agent. The pre-combusted precursor was calcined at 900 °C for 1 h and characterized by laser granulometry, X-ray diffraction, scanning electron microscopy, thermal analysis and transmission electron microscopy. Sensors were prepared by screen-printing the sensing material onto α-alumina substrates with platinum interdigitated electrodes. After drying overnight, sensors were fired at 600 °C in air. The sensor response was measured in the range 150 °C–350 °C performing all measurements in a flow chamber at a constant flow rate of 1 L/min of dry air in the range 200–500 ppb O3. Response time and recovery time (e.g., the times taken for the sensor to attain 90% of total impedance change from its initial impedance value) were determined, together with cross-sensitivity tests towards NH3, CH4, humidity, CO2, N2O and NO2. Sensor's maximum responses were observed at 250 °C, with SR equal to 18% under 500 ppb O3, and SR of 10% under 200 ppb O3. Such results are extremely encouraging and support the exploitation of barium hexaferrite as relatively low-temperature ozone sensors.