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) − 0.01Al0.01TaxO3 − δ (BFATx) 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 BFATx for temperature-independent oxygen sensors. The material properties Seebeck coefficient and conductivity were determined between 600 and 800 °C at different oxygen partial pressures. BaFe0.69Al0.01Ta0.3O3 − 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 (pO2 > 10−5 bar), the electrical conductance of the sensor shows an oxygen sensitivity of m = 0.24 (with m being the slope in the logσ vs. logpO2 representation according to the behavior of σαpO2m), while the Seebeck coefficient changes with a slope of −38 µV K−1 per decade of pO2 at 700 °C. However, at low pO2 (pO2 < 10−14 bar) the conductance and the Seebeck coefficient change with pO2, with a slope of m = −0.23 and −21.2 µV K−1 per decade pO2, respectively.