Poisoning of Temperature Independent Resistive Oxygen Sensors by Sulfur Dioxide

Lanthanum-doped strontium titanium ferrite (SrTi0.65Fe0.35O3) powders were prepared by the mixed oxide technique, screen printed onto alumina substrates and fired at 1100 C, to obtain very porous sensitive film. Platinum thick film electrodes were applied. Oxygen sensitivity of the films was assessed by measuring the electrical resistivity in atmospheres of different pO2 over the temperature range 700-900 C. The film sensors were deemed to be temperature-independent over this temperature range. The influences of sulphur dioxide concentration, pO2, temperature and sensor morphology on the sulphur dioxide-poisoning of the sensors was then investigated. At low temperatures, poisoning occurred by sulphur dioxide absorption and sulphate ion formation at grain surfaces. At higher temperatures the sensor film decomposed into strontium sulphate, iron-depleted strontium titanium ferrite, and Fe2TiO5. It was concluded that temperature-independent resistive oxygen sensitive sensors with long-term stability should be fabricated using alkaline earth-free compositions. 18 refs.