Capacitive-type gas sensors combining silicon semiconductor and NaNO ₂-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(PO ₄)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.