α-Iron oxide: An intrinsically semiconducting oxide material for direct thermoelectric oxygen sensors

Intrinsically semiconducting oxide materials offer the possibility for highly sensitive direct thermoelectric gas sensors. Intrinsic α-Fe2O3 has been chosen as a well suited candidate for direct thermoelectric gas sensors. The band gap of α-Fe2O3 is relatively low for an oxide material (2.2 eV). The intrinsic behavior is present for temperatures above 400 °C. The tested direct thermoelectric oxygen sensors were actively heated to the operating temperature at 580 °C. The achieved sensitivity (85 μV K−1 per decade oxygen partial pressure) is about 3 times higher compared to typical conductometric oxygen sensitive materials like SrTiO3/SrTi1−xFexO3−δ. The used temperature modulation technique combined with a regression analysis allowed a determination of the measurand thermopower within 6.4 s and the possibility for self-diagnostics. These presented results show a possible realization of fast, accurate, highly sensitive direct thermoelectric gas sensors.