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The results of an investigation of humidity sensors based on ceramic material are discussed. It is widely approved that the ceramics based humidity sensor is superior to the polymer one due to their better thermal stability over a variety of chemical species, wide range of operating temperature and rapid response to change of humidity. Properties of humidity sensor based on ceramic system Ca_1-xY_xTi_1-xCo_xO₃ are presented. The role of each part of the sensitive material in the electrical conduction process is determined based on measurements and calculated equivalent circuits. The proposed model describes the water absorption on grain and grain boundaries surfaces with very good fit to the experimental data. A new approach to the modeling of the impedance frequency dependence by means of an equivalent circuit yields very promising results for sensors.

This paper reports the enhancement in the sensing properties of organic dye methyl orange (MO) by introducing TiO₂ nanoparticles. For this purpose, two surface type Ag/MO/Ag and Ag/MO:TiO₂/Ag multifunctional sensors were fabricated by spin coating a 3.0 wt.% solution of MO and 3.0:0.3 wt.% of MO:TiO₂ composite on pre-patterned silver (Ag) electrodes. The gap between Ag electrodes was 40 μm. The Ag/MO/Ag and Ag/MO:TiO₂/Ag structures were characterized to investigate their response towards humidity and temperature variations. The Ag/MO:TiO₂/Ag sensor exhibited better sensitivity and response time than Ag/MO/Ag sensor. The large surface to volume ratio of TiO₂ nanoparticles is the primary reason for the higher sensitivity of Ag/MO:TiO₂/Ag sensor. The sensors can be used to detect humidity variations from 30% to 95% RH and temperature variation from 30°C to 200°C with good stability. Surface morphologies of the film were investigated by scanning electron microscope (SEM).

The humidity sensor based on silicon substrate is presented in this paper, which consists of anodic aluminum oxide (AAO) film and interdigitated electrodes. By using electro-chemical oxidizing technique, AAO film with high porosity is fabricated on the silicon substrate. Under optimal oxidization condition, pore diameter of 37–79 nm and depth about $7\mu$m is achieved. Interdigitated electrodes are fabricated on the top of AAO film by vacuum evaporation deposition method. The results show that the sensor has different nonlinear response in whole range of relative humidity (RH). Moreover, it has almost linear relationship between the capacitance and RH at high RH from 75% to 95%. The highest sensitivity is obtained 613 pF/%RH at 1 kHz, which is much higher than other frequencies.

In this study, the conductive polyaniline doped by hydrochloric acid (PANIHCl)/zinc oxide (PANIHCl/ZnO) conductive composite was synthesized by in-situ polymerization. The conductive PANIHCl solution or PANIHCl/ZnO conductive composite solution was coated on an interdigital electrode (IDE) by drop coating to make a humidity sensor. The synthesis process, chemical structure, surface morphology, and humidity sensing characteristics of PANIHCl and PANIHCl/ZnO conductive composite were investigated. Field emission scanning electron microscope (FE-SEM) images suggested that the surface morphology of the PANIHCl/ZnO conductive composite is fibrous or rod-shaped, with a diameter of about 100 nm. The sensing responses of PANIHCl and PANIHCl/ZnO were 40% and 87%, respectively. The humidity sensing response of PANIHCl/ZnO as the sensing material was more than twice that of PANIHCl as the sensing material. The response time (Tres) is 13 s, and the recovery time (Trec) is 171 s. Research results showed that adding ZnO to PANIHCl can effectively improve the response of the sensor to humidity detection. Therefore, using PANIHCl/ZnO conductive composite as a sensing material will have great potential for application in humidity or other gas sensing devices.

In this paper, we have studied the effects of changing relative humidity on the electrical parameters and their multi frequency response of the surface type Ag/formyl-TIPPCu(II)/Ag humidity sensors. The silver electrodes of thickness 100 nm were primarily deposited on cleaned glass substrates by thermal evaporator. A gap of 40 μm was created between the electrodes of each device by using mask during the evaporation process. Thin films of formyl-TIPPCu(II) of 140 nm thickness were grown on silver electrodes by thermal sublimation technique. The values of capacitance and resistance of the sensors were found at different humidity levels at frequency of 1, 10 and 100 kHz of AC input signal. A remarkable increase in capacitance and decrease in resistance were observed during the rise of relative humidity from 45% to 95% RH. The hysteresis response of these humidity sensors was also studied at 1 kHz AC signal.

Poly (N-ethyl aniline)/Ag organic–inorganic composite has been synthesized by a single step in situ chemical oxidative polymerization method. The synthesis of Poly (N-ethyl aniline)/Ag nanocomposite has been confirmed by X-ray diffraction (XRD), Ultraviolet-Vis Spectroscopy (UV-visible), Fourier transform infrared analysis (FTIR) and FE-SEM investigations. XRD spectral study exhibited major diffraction in the range 20–80${}^{\circ }$ (2$\theta$) and indicated the semicrystalline nature of poly (N-ethyl aniline)/Ag nanocomposite. Characteristic peaks in UV-visible and FTIR spectra of poly (N-ethyl aniline) switched to higher wave numbers in poly (N-ethyl aniline)/Ag nanocomposite. Peaks at 1789cm${}^{-1}$, 1595cm${}^{-1}$, 667cm${}^{-1}$ and 501cm${}^{-1}$ in FTIR spectrum confirmed the formation of poly (N-ethyl aniline)/Ag nanocomposite. FE-SEM photographs reported agglomerated granular particulate nature of poly (N-ethyl aniline)/Ag nanocomposite. Synthesized poly (N-ethyl aniline)/Ag nanocomposite exhibited a high response to humidity, good reproducibility and stability at room temperature. An appreciable response was shown in the presence of 40% humid atmosphere for up to successive four cycles. Composite sensitivity has been found to increase with the increasing concentration of humidity, at room temperature.

A quantum chemical investigation has been performed to spotlight the structure–property relationship among methoxybenzeylidene-based humidity sensor and water molecules. The chemical interactions among (E)-2-(4-(2-(3,4-dimethoxybenzeylidene)hydrazinyl)phenyl) ethane-1,1,2-tricarbonitrile (DMBHPET) sensor and water molecules have been studied using density functional theory (DFT) methods. The molecular structural parameters, binding energies and Infrared (IR) spectroscopic analyses have been performed to assess the nature of intermolecular interactions. Three different positions have been identified for possible attachments of H₂O molecules through hydrogen bonding interactions. These positions include NH (complex 1a), p-OCH₃ (complex 1b) and N=N (complex 1c) group in sensor molecule (1) for the chemical adsorption of water molecules. While, the complex 1abc includes all three sites with simultaneously three H₂O molecules attached to it through hydrogen bonding. The binding energies calculated for complex 1a(NH…H₂O), complex 1b(CH₃O…H₂O), complex 1c(N=N…H₂O) and complex 1abc are -30.97, -18.41, -13.80 and -65.36 kcal/mol, respectively. The counterpoise (CP) scheme has been used to correct the basis set superposition error (BSSE) in calculation of binding energies of sensor and H₂O complexes. The higher binding energy of -65.36 kcal/mol for complex 1abc represents that the present methoxybenzeylidene-based sensor has significant potential through hydrogen bonding formation for sensing humidity as indicated in our previous experimental investigation. The evidence of hydrogen bonding interactions between sensor 1 and H₂O molecules has been traced through structural parameters, red shift in IR spectra as well as molecular electrostatic maps. Thus the present investigation highlights the first computational framework for a molecular level structure-binding activity of a methoxybenzeylidene-based sensor and water molecules.

In last few decades, great deal of research efforts were directed towards the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer wide spectrum of materials with their high sensitivities for the detection of different gaseous species. In this paper, we report the humidity sensing behavior of cobalt oxide (Co₃O₄) pellets. The Co₃O₄ powder was prepared from the precipitation of CoSO₄ using NaOH solution. This as-prepared powder was heated at 600 °C for 2hr. The as-prepared powder was characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The humidity sensing behavior of Co₃O₄ pellets (wt. ~ 0.5 gm, diameter ~ 10 mm) was noted by using home-built static gas sensing system. The characterization studies showed the high material purity and single phase Co₃O₄ with cubic spinel symmetry in the as-prepared particles of resultant powders. The humidity sensing characteristics of Co₃O₄ pellets were found to be good. The recovery time (~ 20 sec) was found to be small as compared to its response time (~ 60 sec).

In this study, humidity-sensitive polymer layer containing phenol-formaldehyde resin prepared by spinning coating on the comb-shape type electrode followed by UV-graft polymerization of soluble monomer was performed to improve the sensitivity. In this investigation, grafting of P-styrenesulfonic acid sodium salt (NaSS) treated with hexamethyldisilazan (HMDSZ) plasma and acrylic acid (AAc) followed by isopropanol (IPA) plasma were used as monomer for UV surface graft polymerization. Results show the first one adding different ratio of phenol-formaldehyde resin. This type of sensor has linear response with %RH, and high sensitivity (impedance from 10⁶ to 10³). The impedance of second type of sensor was linear response with %RH, impedance varies from 10⁷ to 10⁵.

Air humidity is a basic element of ground and upper air meteorological observation. Measurement of air humidity in the whole range of air temperatures and humidity has been a worldwide problem in the field of atmospheric sounding. By relying on the principle of the condensation method, as well as the rapid developments in microscopic imaging, image recognition and embedded system technology, this article solves key problems such as the technology of condensation mirror design, mirror temperature control and condensate identification. A prototype condensation-type humidity sensor is also successfully developed. Synchronous observations with a humidity standard device and a number of preliminary tests are carried out in different conditions of environment temperature and humidity. The test results show that the deviation of relative humidity between the proposed sensor and the standard device falls within the range of [-2.86, -0.95] %RH, which meets the requirements of business applications for ground meteorological observation.