Combined resistive and thermoelectric oxygen sensor with almost temperature-independent characteristics

Abstract. The present study is focused in two directions. In the first part, BaFe ( 1 - x ) - 0.01 Al 0.01 Ta x O 3− δ (BFAT x ) thick films with a Ta content between 0.1 and 0.4 were manufactured using the novel room temperature coating method “aerosol deposition” (ADM), and its material properties were characterized to find the best composition of BFAT x for temperature-independent oxygen sensors. The material properties “Seebeck coefficient” and “conductivity” were determined between 600 and 800 ∘ C at different oxygen partial pressures. BaFe 0.69 Al 0.01 Ta 0.3 O 3− δ (BFAT30) was found out to be very promising due to the almost temperature-independent behavior of both the conductivity and the Seebeck coefficient. In the second part of this study, films of BFAT30 were prepared on a special transducer that includes a heater, equipotential layers, and special electrode structures so that a combined direct thermoelectric/resistive oxygen sensor of BFAT30 with almost temperature-independent characteristics of both measurands, Seebeck coefficient and conductance could be realized. At high oxygen partial pressures ( p O 2 > 10 −5 bar), the electrical conductance of the sensor shows an oxygen sensitivity of m = 0.24 (with m being the slope in the log σ vs. log p O 2 representation according to the behavior of σ α p O 2 m ), while the Seebeck coefficient changes with a slope of − 38 µ V K −1 per decade of p O 2 at 700 ∘ C. However, at low p O 2 ( p O 2 10 −14 bar) the conductance and the Seebeck coefficient change with p O 2 , with a slope of m = − 0.23 and − 21.2 µ V K −1 per decade p O 2 , respectively.