The influence of SO 2 on the output of resistive oxygen sensors using CeO 2 and Ce 0.8 Zr 0.2 O 2 thick films was investigated. At 873 and 1073 K, the resistance of the sensors using CeO 2 film was strongly influenced by 1, 500 ppm SO 2 , and SO 2 caused a decrease of resistance. On the contrary, the resistance of the sensors using Ce 0.8 Zr 0.2 O 2 was hardly influenced. The resistivity of the reaction product, which was Ce 2 ͑SO 4 ͒ 3 , was 100 times greater than that of CeO 2 at 873 K. Thermodynamic calculation revealed that CeO 2 was more stable than Ce 2 ͑SO 4 ͒ 3 at 1073 K in 500 ppm SO 2 . These results showed that the resistance decrease of the sensor using CeO 2 film, caused by the introduction of SO 2 , was unrelated to the formation of the reaction product. It was believed that the resistance decrease of the sensor using CeO 2 film was due to the adsorption of electrically conductive substances on the CeO 2 surface. It was considered that the reason the influence of SO 2 on Ce 0.8 Zr 0.2 O 2 thick film was less than that on CeO 2 thick film was that the resistivity of Ce 0.8 Zr 0.2 O 2 was 1/10 times that of CeO 2 .The exhaust emission regulations for two-wheeled vehicles are becoming strict globally. In Japan, the amounts of HC and NO x will be restricted to 0.5 and 0.15 g/km, respectively. In order to reduce exhaust emissions, an oxygen gas sensor is indispensable. 1,2 A small sensor is required for two-wheeled vehicles because they do not have sufficient space. Resistive oxygen gas sensors have therefore become the focus of much attention, because they have a simple structure and it is easy to downsize them. 3-5 We have researched and developed a resistive oxygen sensor using CeO 2 or CeO 2 -ZrO 2 . 6-9 Especially, we decreased the response time to several tens of milliseconds at 1073 K by decreasing the particle size of CeO 2 thick film. 10 It is important to investigate the influence of corrosion gas in exhaust gas on a resistive oxygen sensor for practical use. 11 The influence of SO 2 on an oxygen sensor using CeO 2 thin film has been reported. 12 However, there is no report regarding the influence of SO 2 on oxygen gas sensors using CeO 2 thick film, and there is no report regarding the influence on oxygen sensors using CeO 2 -ZrO 2 thick or thin film.In this work, the influence of SO 2 on oxygen sensors using CeO 2 or Ce 0.8 Zr 0.2 O 2 thick films was investigated. Results showed that the influence of SO 2 was larger in the case of CeO 2 thick film compared with Ce 0.8 Zr 0.2 O 2 thick film. Furthermore, the reason the influence was larger in the case of CeO 2 thick film was also investigated.
ExperimentalSensor fabrication.-The powder of oxide including cerium and zirconium ions was prepared as follows. The aqueous solutions of cerium nitrite and zirconium oxynitrate were mixed using a predetermined ratio ͑x Zr /͑x Zr + x Ce ͒: 0, 20 mol %͒, and the cation concentration of the resulting mixed aqueous solution was 0.010 mol/dm 3 . Next, mist of the mixed solution was introduced into a furnace ...