Sulfur adsorber for thick-film exhaust gas sensors

Due to increasing environmental requirements the automotive industry may need novel gas sensors for exhaust gas applications in the future. Employed directly in the tailpipe, these sensors have to withstand the harsh and aggressive conditions that are present in engine exhausts. Being one of the most critical components, sulfur oxides may be a reason for reduced long-term stability. This paper deals with the realization of porous sulfur oxide adsorbers that are applied onto the sensitive film of such sensors. Deduced from diffusion calculations an optimal range for pore diameters and adsorber thickness was found and successfully implemented utilizing carbonates as adsorbing materials. The tests were performed using earth alkaline containing and temperature-independent resistive titanate thick-film oxygen sensors as an example for sulfur instable gas sensor materials. It was found that porous carbonate adsorbers could protect these materials in such a way that sensors can be operated for more than 450 h in a sulfur oxide enriched atmosphere without deterioration. The influence of adsorbers on the sensor response time was estimated and validated to be negligible in the applied pore diameter and adsorber thickness range.