Synthesis and characterization of calcium double perovskites for the potential application of semiconducting CO₂ sensors

A conventional solid-state sintering method was used to prepare double perovskite structured compounds BCN (Ba₂Ca_0.67Nb_1.33O₆), BCNCo (Ba₂Ca_0.67Nb_1.33-xCo_xO_6-δ) and BCNCoFe (Ba₂Ca_0.67Nb_0.67Co_0.66-yFe_yO_6-δ), which exhibit significant chemical stability in nitrogen, air, and 2 % CO₂ (balanced by nitrogen). SEM images show that the Co dopant causes a dense microstructure for BCNCo compounds. In contrast, the introduction of Fe tends to produce a porous and web-like microstructure for the BCNCoFe series. Ba₂Ca_0.67Nb_0.67Co_0.66O_6-δ has a reasonable response (recovery) time at 750 °C and it seems suitable for CO₂ detection at elevated temperatures. Among the BCNCoFe compounds, Ba₂Ca_0.67Nb_0.67Co_0.33Fe_0.33O_6-δ has the largest capacitance as the CO₂ concentration changes from 0 to 2000 ppm, and it exhibits satisfactory sensitivity and repeatability in the temperature range of 450–700 °C with an extra voltage of 0.1 V. To further consider the Ba₂Ca_0.67Nb_0.67Co_0.33Fe_0.33O_6-δ compound for CO₂ sensing, a sol-gel method was utilized. The sol-gel-prepared Ba₂Ca_0.67Nb_0.67Co_0.33Fe_0.33O_6-δ senses CO₂ well, and its concentration changes from 0 to 2000 ppm (or from 0 to 300 ppm) in the temperature range of 400–600 °C. The different CO₂ sensing properties of all prepared double perovskite compounds can be interpreted by the different surface areas that play critical roles in the chemical adsorption of related gaseous species.