Sensors and Actuators B 140 (2009) 128–133 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Establishing and interpreting an electrical circuit representing a TiO 2 –WO 3 series of humidity thick film sensors P.M. Faia ∗ , A.J. Ferreira, C.S. Furtado Laboratory of Electronic Materials and Ultrasounds, CEMUC, Portugal article info Article history: Received 28 November 2008 Received in revised form 9 March 2009 Accepted 9 April 2009 Available online 24 April 2009 Keywords: Humidity sensor TiO2–WO3 thick film Impedance spectroscopy Electrical circuit modelling abstract This paper reports the establishment and interpreting of an equivalent electrical circuit representing the humidity-sensitive electrical properties of TiO 2 –WO 3 thick films. Prototype sensors have been prepared by depositing an emulsion of TiO 2 and WO 3 powders in acetone with cellulous glue onto an alumina substrate, by a spin coating technique using a low spreading speed (250rpm). Films were prepared with five different atom proportions of Ti and W: 100:0, 100:1, 100:6, 100:18 and 100:36. The oxides thick films were successively fired at the temperatures of 600, 700, 900, 1100 and 1300 ◦ C, for 2 h at each of these temperatures. The variation of the electrical signal with humidity in ceramic sensors is originated by the chemical and physical sorptions of water molecules existing in the atmosphere. For all the sensors, the same type of circuit was established based both on circuit theory and on the phys- ical mechanisms underlying conduction and polarization. The best fitting for the Nyquist plots obtained at the temperature of 25 ◦ C and various relative humidities (RHs), in the frequency range 0.1 Hz to 40 MHz, was achieved with two R, C parallel circuits in series with two parallels of constant-phase elements (CPEs) and capacitances. As examples, for two of the sensors, TiW2 600 and TiW2 700, the values of the electrical components are presented in tables and graphs. The evaluated values of the components are interpreted and compared, and conclusions could be drawn about the mechanisms of conduction and polarization taking place in the adsorbed water. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The physical phenomena of electrical conduction and polar- ization that occur in a ceramic humidity sensor are not only represented by resistances and capacitances [1–4], but also by the use of constant-phase elements (CPEs) to take into account diffu- sion and interfaces phenomena [5–7]. The electrical behaviour of our samples is understood by mea- suring their complex impedances when subjected to an external sinusoidal electric field. In this paper, Nyquist plots (-Z ′′ = f(Z ′ ), in Z = Z ′ + jZ ′′ ), i.e., minus the imaginary part vs. the real one of the total impedance of the sensor were registered. Conduction mechanisms are dependent on charge carriers, while the dielectric behaviour has to do with polarization processes. The electrical variations observed in the humidity measure- ments in ceramic sensors are originated by the chemical and physical sorptions of water molecules existing in the atmosphere ∗ Corresponding author at: Electrical and Computers Engineering Department, Faculty of Sciences and Technology, University of Coimbra, Pólo 2, 3030-290, Coim- bra, Portugal. Tel.: +351 239 796200. E-mail address: faia@deec.uc.pt (P.M. Faia). [8–11]. The electrical response of a porous material is related to the surface moisture affinity, and the surface microstructure, mainly in relation to the dimensions and interconnections of the capillary micropores. With the study of composite materials based on the TiO 2 –WO 3 pair, we look for a better sensitivity and selectivity to the gases, than those of the sensors made out of only one of those metal oxides. This would be due to the fact that some of the interstitial positions that were initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if the single covalent/ionic adsorption can be important in the observed changes in the mate- rials conductivity specially at low RH, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity and selectivity. Several oxide sensors with different proportions of Ti and W atoms were fabricated and measured. For all our sensors, the same equivalent circuit within the frequency range 0.01 Hz to 40 MHz was established based on the chemical and physical phenomena that preside to the electrical conduction and polarization in humidity ceramic sensors. For the purpose of this work, the values of just two of our sam- ples, TiW2 600 and TiW2 700, (with the Ti atoms to W atoms proportion equal to 100/36, and fired at 600 and 700 ◦ C, respec- 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.04.016