Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

No abstract

This paper shows a correlation between surface effective porosity due to various sintering regimes and humidity sensitive electrical properties of the perovskite-based bulk type humidity sensors, at room temperature. Furthermore, room temperature humidity transduction mechanism of the thick film type humidity sensors was studied in detail through electrochemical impedance spectroscopy (EIS) and major transmissive components were detected by the fitting of the Bode diagrams and Nyquist complexes to the equivalent circuits. The microstructural, morphological and elemental characterizations were carried on using XRD, EFTEM, FESEM, and EDX. Physical properties including open porosity/bulk density were investigated through American Standard Test Method (ASTM). An innovative selfdesigned material test fixture with ceramic supports was fabricated for a high S/N ratio electrical measurement of the bulk samples. All the sensors were set up at 20-95% RH. The morphological, physical, and electrical results of the bulk pellets indicate a direct correlation of the open cavities and AC conduction. The higher the open porosity is, the greater is the conduction and vice versa. Presence of the ionic transport is clearly observed from the frequency-conductance spectra at room temperature. Noise-free detected behavior via EIS proves that the proton transfer mechanism is a dominant responsible, and executed by both charge transfer resistance and kinetically controlled charge transfer (diffusive species) at low and middle to high RH. Next to the Warburg effect (at 80% RH), for the first time, a Gerischer impedance was found as a dominant agent of transduction at 85% RH to above.

Investigation of the effect of addition of external positive ions on the humidity-sensing properties of perovskite-based oxides is an interesting topic. Humidity-sensitive properties of barium strontium titanate nanocomposite films have been investigated in the range of 20–95% relative humidity (RH) at room temperature in the pure form (Ba0.5,Sr0.5)­TiO3 (BST) and doped with 1, 3, and 5 mol % magnesium oxide (MgO) nanoparticles. Microstructural characterization of nanopowders and films was performed by X-ray diffraction (XRD), surface area and pore size analyzer, field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDX) exhibiting an ordered crystallite size, surface area, average pore volume, and open porosity reduction as a function of doping. X-ray photoelectron spectroscopy (XPS) was applied as a complementary characterization technique. None of the (1 – x)­(Ba0.5,Sr0.5)­TiO3 – (x)­MgO (with x = 0.01, 0.03, 0.05) nanocomposites shows a remarkable improvement in the performance of humidity-sensing devices such as intrinsic impedance and hysteresis. BST as a humidity-sensing material shows the lowest and suitable intrinsic impedance, an excellent linearity, an acceptable sensor repeatability with a total impedance/bulk resistance change of 3 orders of magnitude for the entire RH range at 10 kHz, and a small hysteresis of about 4.9% RH. The BST sensor also exhibits the desired response time of about 34 s and adequate stability. Possible sensing mechanisms for different humidity levels based on the experimental results were discussed and further confirmed by impedance spectroscopy of the materials. For the first time, a Gerischer element was detected as an interfacial electrical phenomenon for mass-transfer control at high RH levels. This phenomenon arises from the contribution of a kinetic process and a diffusion process associated with a chemical reaction(s) over the surface.

No abstract