Capacitive-type gas sensors combining silicon semiconductor and NaNO2-based solid electrolyte for NO2 detection

A capacitive-type NO2 gas sensor, developed by replacing metal in metal-insulator-semiconductor structure with solid electrolyte as sensing material, was investigated to improve the response and recovery rates and the operating temperature by modifying NaNO2-sensing material. Binary sensing materials of NaNO2-Ca3(PO4)2 and NaNO2-WO3 were tested for auxiliary phase of the device. The Ca3(PO4)2-containing phase showed a response closer to ideal Nernstian behavior at 140 °C as compared to WO3-containing phase. On the other hand, the WO3-containing phase showed faster response and recovery rates at 160 °C, while the response deviated from Nernstian correlation. In order to obtain optimum sensor response, a ternary phase of NaNO2-Ca3(PO4)2-WO3 was used as sensing phase. The device with ternary auxiliary phase was found to show more stable and faster response and recovery rates at 130 °C, as compared to previous devices attached with binary phases, corresponding Nernstian correlation in the concentration range of 20-500 ppb NO2. Furthermore, the effects of modification process on the auxiliary phase were investigated through SEM observation of the surface structures and measurement of ionic conductivity.