Sulfur poisoning of powder aerosol deposited films of BaFe₀.₇₄Al₀.₀₁Ta₀.₂₅O₃−δ : A material for resistive temperature independent oxygen sensors

Barium-iron-aluminum-tantalate (BFAT) is a promising candidate for resistive oxygen sensors with temperature independent sensor characteristics for exhaust gas purposes. To evaluate the long-term stability of the dense sensor films that were prepared by powder aerosol deposition (PAD), this study investigates sulfur dioxide (SO₂) poisoning and its effect on the sensor characteristics. The results show that exposure to SO₂ significantly affects the electrical properties of the film material. After contact to SO₂, the resistance of BFAT heavily increases by about one decade and the oxygen sensitivity of the sensor element decreases. In addition, the selectivity is also negatively affected. Despite fresh sensors are nearly unaffected by interfering compounds, cross-sensitivities for poisoned sensors, appear, primarily towards nitrogen dioxide (NO₂), ammonia (NH₃), and to a lesser extent also towards other gases, that may be present in typical exhausts. X-ray diffraction (XRD) patterns of the poisoned powder and X-ray photoelectron spectroscopy (XPS) of the BFAT film confirmed the formation of BaSO₄, which also suggests that other reaction products may be generated during poisoning. Based on the experimental results, some ideas on the poisoning mechanism are discussed. The regeneration of a sulfur poisoned sensor element at high temperatures is only partially possible under oxidizing conditions, but it can provide a limited recovery of the BFAT resistance, sensitivity, and selectivity. In agreement with literature, these conditions will probably allow the removal of surface adsorbed sulfates only, while crystalline BaSO₄ is hardly removed. Overall, the material's resistance to sulfur dioxide poisoning remains a challenge that needs to be solved.