Oxide capacitors consisting of BaTiO3 and an oxide are studied as a sensor for CO2 detection. Although the capacitance change of PbO on exposure to CO2 was small, it was greatly enhanced by the combination with BaTiO3. Sensitivity to CO2 and optimum operating temperature were strongly dependent on the oxide combined with BaTiO3. The element which contains basic oxides such as MgO and CaO, is highly sensitive to CO2, but the operating temperature exceeded 1073 K. Among the elements investigated in this study, CuO-BaTiO3 exhibited the highest sensitivity to CO2. CuO-BaTiO3 selectively responded to CO2, and this element could distinguish the CO2 concentrations up to 6%. On the other hand, the capacitance of NiO-BaTiO3 decreased linearly with increasing CO2 concentration up to 20%. Mixed oxide capacitors of CuO-BaTiO3 and NiO-BaTiO3, therefore, are suitable for sensing CO2 at the concentration range of hundreds of ppm, and at a percentage level, respectively.
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