Merits of Bi₃TiNbO₉ for Humidity Sensors

Abstract

The fast and reproducible electrical response of the new, metastable fluorite phase of Bi₃TiNbO₉ to ambient humidity is reported. Pellets were prepared by high energy mechanochemical activation of the precursor powders, pellet pressing, sintering, and sputter deposition of Pt electrodes. Thermal analysis shows exothermic peaks at 465 and 594 °C. X ray diffraction analysis identifies a metastable fluorite like phase, isomorphous to the δ phase of Bi₂O₃, between the first and the second peaks, and the stable Aurivillius phase after the second peak.

Over a wide (-40 to 20 °C) dew point (DP) range, at 23 °C ambient temperature, the conductivity increases close to exponentially, from nearly 10⁻³ to 10² pS/cm. Upon removal of the bias, and under DP cycling, the relaxation current responds to humidity steps, on top of the long term current decay, in a manner similar as under bias. In addition, upon a step into humidity, after a dry spell, the current overshoots (before proceeding to the long term decay) to a maximum value (I_m) which depends on the duration of the previous dry spell. If this is short, I_m remains below the current value (I_ph) at the end of the previous humidity stage. If the dry spell is long, I_m overshoots above I_ph. This behavior is unlike that of any discharge current through a leaky capacitor. A preliminary model for understanding it proposes the surface transport of a charge carrier, assisted by diffusion of some species from the interior of the ceramic grains.