Abstract
Abstract. We present an equivalent circuit model for a titanium dioxide-based humidity sensor which enables discrimination of three separate contributions to the sensor impedance. The first contribution, the electronic conductance, consists of a temperature-dependent ohmic resistance. The second contribution arises from the ionic pathway, which forms depending on the relative humidity on the sensor surface. It is modeled by a constant-phase element (CPE) in parallel with an ohmic resistance. The third contribution is the capacitance of the double layer which forms at the blocking electrodes and is modeled by a second CPE in series to the first CPE. This model was fitted to experimental data between 1 mHz and 1 MHz recorded at different sensor temperatures (between room temperature and 320 ∘C) and different humidity levels. The electronic conductance becomes negligible at low sensor temperatures, whereas the double-layer capacitance becomes negligible at high sensor temperatures in the investigated frequency range. Both the contribution from the ionic pathway and from the double-layer capacitance strongly depend on the relative humidity and are, therefore, suitable sensor signals. The findings define the parameters for the development of a dedicated Fourier-based impedance spectroscope with much faster acquisition times, paving a way for impedance-based high-temperature humidity sensor systems.
Highlights
Detection and quantification of humidity as a process parameter are important for many different branches of industry like food processing, medicine, climate control, or industrial drying (Blank et al, 2016)
We measured the contribution of electronic conductance at 0 %RH from room temperature (Fig. 2) up to 250 ◦C (Fig. 3)
20 %RH, CPEdl is not visible in the Bode diagram and, cannot be fitted properly. This effect is never visible at high temperatures and, the element is shorted in the model, leaving CPEionic to be the only element in the pathway
Summary
Detection and quantification of humidity as a process parameter are important for many different branches of industry like food processing, medicine, climate control, or industrial drying (Blank et al, 2016). An alternative read-out method of TiO2-based sensors measures the humidity-dependent conduction, providing better sensitivity at low temperatures. Conduction in TiO2 has been well described by several groups covering several conducting mechanisms (Traversa, 1995; Wang et al, 2011; Wu et al, 2017) These different mechanisms can be identified by impedance spectroscopy, which is a powerful tool to characterize electrochemical systems and a promising method to gain many different, humidity-dependent signals from one physical sensor. A FoBIS, must always be carefully designed with respect to the dynamic frequency and impedance range to obtain the desired resolution and accuracy To this end, the impedance characteristic of a newly developed TiO2 sensor has been carefully characterized using a highly versatile laboratory impedance spectroscope of the frequency response analyzer type. Interesting parts of the impedance spectrogram were identified and will be used in the future to tailor the newly developed FoBIS towards quick measurements which can be fitted with this physical model
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