Application of a Mixed Oxide Capacitor to the Selective Carbon Dioxide Sensor II. CO₂ Sensing Characteristics of a Cuo-Based Oxide Capacitor

Characteristics for CO₂ sensing of a CuO-based oxide capacitor were studied. The capacitances of CuO mixed with BaSnO₃ SrTiO₃, CaTiO₃, and ZnO are dependent on the CO₂ concentration, similar to CuO-BaTiO₂. In particular, the mixed oxide, CuO-SrTiO₃, is more sensitive to CO₂ than CuO-BaTiO₃, but requires a rather long time to recover. On the other hand, the mixed oxide capacitor, CuO-BaSnO₃, exhibits high CO₂ sensitivity, and a monotonic and approximately logarithmic relationship exists between the sensitivity and the concentration of CO₂ from 100 ppm to 50%. Furthermore, the capacitance of CuO-BaSnO₃ increased rapidly within 30s after exposure to 2% CO₂, and returned rapidly to the original level after reexposure to air. Therefore, the system, CuO-BaSnO₃ is promising for sensing CO₂ over a wide range of concentrations. From a complex impedance measurement, the equivalent circuit of the sensor element can be expressed by a series connection of three parallel resistors and capacitors which correspond to the intragrain region, grain interface between CuO and BaSnO₃, and electrode interface. Since exposure of the element to 2% CO₂ varied the capacitance of the grain interface, the grain interface between CuO and BaSnO_a has an important role for CO₂ detection.