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. The aim of the work described in the present paper is to establish an equivalent electrical circuit for the case of two titania thick-film samples. It is shown, at least for the temperature of 23 °C, that the same type of circuit represents adequately these two samples for various relative humidities. Chemisorption and physisorption are responsible for the different charge transport mechanisms - ion hopping, ion diffusion and electrolytic conduction. Complex impedance data were obtained at the temperature of 23 °C and various relative humidities, in the frequency range 0.1 Hz-40 MHz. The best and simpler circuit representation we found, which gives the best fitting for the Cole-Cole and Bode plots, consists of two RC parallel circuits in series with two constant-phase elements (CPEs). The values of the electrical components are tabled and, as an example, the Cole-Cole and Bode plots fitting obtained for one of our samples, the sample B, for 87.5% RH, in the frequency range 0.1 Hz-40 MHz is shown.

A ceramic thick film humidity sensor was produced from an emulsion of titania powders by a spin coating technique using a low speed. Electrical measurements were taken between interdigital electrodes obtained by depositing silver paste on the oxide, then cured at 500 °C for 15 min. Different relative humidities of a dynamic atmosphere were obtained by mixing dry and 23 °C saturated synthetic air in convenient proportions. Complex impedance spectra of the titania sensor at various relative humidities (RH) and different temperatures were measured and compared. The humidity sensing behaviour is due to surface water molecules adsorption and capillary condensation. The proposed sensing mechanisms, explaining the registered impedance spectra, are a combination of proton hopping, hydronium electrical drift and diffusion, and electrolytic conduction. In the frequency range 1-400 Hz, resistance and capacitive reactance show variations of three to four-order magnitude over the RH range 10-100%. The curves representing the variations of resistance and capacitive reactance versus RH show clearly the existence of two dominant electrical charge transport mechanisms. A parameter called characteristic humidity is defined to represent the sensitive response of the sensor. It was found that the sensitivity was highly dependent on the frequency. This work also shows that, for the same RH, both resistance and capacitive reactance vary with the atmosphere temperature.

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films incorporating 40 %wt. of different types of imidazolium ionic liquids (ILs) comprising different magnetic anions (tetrachloroferrate [FeCl4] - , tetrathiocyanatocobaltate [(SCN)4Co]2 - and tetrachlorocobaltate [CoCl4] - ) and imidazolium cation chain lengths 1-butyl-3-methyl imidazolium and 1-ethyl-3-methyl imidazolium, have been developed for humidity sensing applications. The influence of the different ILs into the morphology of the IL/PVDF-HFP composites has been evaluated, being observed a porous structure independently of the IL anion type and cation chain length. The inclusion of the ILs into PVDF-HFP induces the polymer crystallization into the electroactive β phase, being the increase more noticeable for PVDF-HFP/[Bmim][FeCl4], which reaches a β phase content of 85%. The degree of crystallinity slightly decreases with the incorporation of the filler, the incorporation of [Bmim]2[CoCl4], [Bmim]2[Co(SCN)4], and [Emim]2[Co(SCN)4] leading to a degree of crystallinity between 11 and 13%. A decrease in the thermal stability and the yield strength is observed in the hybrid samples with respect to neat PVDF-HFP, being this decrease more noticeable for the PVDF-HFP/[Bmim]2[Co(SCN)4] samples. Further, the highest magnetization has been obtained for the PVDF-HFP/[Bmim]2[Co(SCN)4] and PVDF-HFP/[Bmim][FeCl4] composites with similar filler concentrations. The suitability of the developed materials for humidity sensing has been evaluated by analysing the impedance variation with varying relative humidity from 30% to 85%. [Bmim]2[CoCl4]/PVDF-HFP displays the highest relative humidity (RH) sensing response of -2245 ±240 Ω/RH. Finally, the applicability of the developed hybrid IL/polymer composites has been demonstrated by the development of a real time breathing monitoring device.

For impedance-type sensors based on semiconducting metal oxides, the overall conduction mechanisms strongly influence the magnitude and the direction of the sensor signal variation. For humidity, in particular, the electronic/ionic charge-transfer reactions that take place at the semiconductor surface can be used to monitor and control it. In recent years, various mechanisms have been proposed to explain the variations of electrical response to humidity. With the study of composite materials, we expect to obtain a better sensitivity of these sensors, when compared with the ones made out of only one metal oxide. This could be due to the fact that some of the positions initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if a single covalent/ionic adsorption is decisive in the observed changes in the material's conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity. In this paper, TiO2:WO3 composite oxide bulk sensors, using 48.92 and 51.08% (w/w) of titanium and of tungsten atoms, respectively, doped with the same proportions of cooper and zinc oxides (7%), were prepared by a conventional sintering method, and their dependence of their complex impedance spectra, measured in the range 100 Hz–10 MHz, on the relative humidity (RH), operating temperature and on the measuring frequency is shown and explained.

Photocurable thermochromic and humidity responsive materials based on polyurethane acrylated (PUA) and bis(1-butyl-3-methylimidazolium) tetrachloronickelate ([Bmim]2[NiCl4]) ionic liquid (IL) have been prepared with varying IL content up to 40 wt% within the polymer matrix. The influence of IL content on the photopolymerization process, morphology, Young modulus and electrical conductivity of the materials was evaluated. All materials present a total polymer conversion of at least 88%. The presence of the IL influences the morphology of PUA, being observed a porous network structure upon IL incorporation into the PUA matrix. All samples show a Tg around room temperature, which is independent of the IL content and a Young modulus that decreases with the IL content. All composites exhibit humidity dependent thermochromism from colourless to blue. Further, the influence of the relative humidity in the colour change was accessed for the sample containing 20 wt% [Bmim]2[NiCl4], being observed that the colour variation is thermally activated and humidity governed, being 55% the minimum value of the relative humidity for colour change to occur.

This paper presents the design of low-cost, conformal UHF antennas and RFID tags on two types of cork substrates: 1) natural cork and 2) agglomerate cork. Such RFID tags find an application in wine bottle and barrel identification, and in addition, they are suitable for numerous antenna-based sensing applications. This paper includes the high-frequency characterization of the selected cork substrates considering the anisotropic behavior of such materials. In addition, the variation of their permittivity values as a function of the humidity is also verified. As a proof-of-concept demonstration, three conformal RFID tags have been implemented on cork, and their performance has been evaluated using both a commercial Alien ALR8800 reader and an in-house measurement setup. The reading of all tags has been checked, and a satisfactory performance has been verified, with reading ranges spanning from 0.3 to 6 m. In addition, this paper discusses how inkjet printing can be applied to cork surfaces, and an RFID tag printed on cork is used as a humidity sensor. Its performance is tested under different humidity conditions, and a good range in excess of 3 m has been achieved, allied to a good sensitivity obtained with a shift of >5 dB in threshold power of the tag for different humid conditions.

In this paper we show the design of passive UHF RFID tag antenna on cork substrate. Due to the cork sensitivity to humidity changes, we can use the developed sensor to sense changes in the relative humidity of the environment, without the need for batteries. The antenna is built using inkjet printing technology, which allows a good accuracy of the design manufacturing. The sensor proved usable for humidity changes detection with a variation of threshold power from 11 to 15 dB between 60 and near 100% humidity levels. Presenting, therefore, reading ranges between 3 to 5 meters. © 2015 EurAAP.

The Internet of Things (IoT) has revolutionized the requirements for sensors and smart devices, where high performance, power efficiency and low manufacturing costs are mandatory. As a consequence, the research for better performing materials and more cost-effective manufacturing techniques is being performed. This work reports on ink formulations based on nitrogen-doped graphene and environmentally friendly polymers (Polyvinylpyrrolidone (PVP) and Carboxymethyl cellulose (CMC), and their use in fully screen-printed transistors and humidity sensors. The ink formulations were developed using environmentally friendly solvents and showed non-Newtonian behaviour. The N-rGO composites showed a high electrical conductivity of 1.9 ± 0.5 S∙cm−1, 100 times higher than the reduced graphene oxide (rGO) composites. Fully printed Graphene field effect transistors (GFET) were developed with low operating voltage of < 2 V to be implemented as humidity sensors with the fully a linear response and high sensitivity (4.25 Ω∙%RH−1 as resistive sensor), allowing integration into low-power microcontrollers for sensing applications.