APPLICATION OF A MIXED-OXIDE CAPACITOR TO THE SELECTIVE CARBON-DIOXIDE SENSOR .2. 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 30 s 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 BaSnO3 has an important role for CO2 detection.