Development Of Humidity Sensors Research Articles

Design and Characterization of Printed Flexible Humidity Sensor

The development of humidity sensors is essential for applications in the environmental, agriculture, medical and semiconductor industries [1-2]. The basic mechanism of humidity sensors is explained by a Grotthuss chain reaction, which is usually simplified as a proton-hopping process [3]. This refers to a dynamic charge transfer process at a certain relative humidity (RH). The humidity sensors can be of different types such as capacitive type, resistive, colorimetric and surface acoustic wave [4-7]. Capacitive type sensors are linear, require less complex circuitry and can operate for a wide range of humidity [8]. A typical capacitive type humidity sensor uses hydrophilic films as sensing layers which are stable at higher humidity levels [9]. Polymers that are being used for the fabrication of capacitive type humidity sensors include poly (2-hydroxyethyl methacrylate) (pHEMA) [9], cellulose acetate butyrate (CAB) [10], polyimide [11] and polymethyl methacrylate (PMMA) [12]. In this work, a humidity sensor has been fabricated on glass and flexible substrates.The design of humidity sensor consists of printed pattern of a conductor such as silver and a spin coated dielectric such as polymer poly (2-hydroxyethyl methacrylate) (p-HEMA) and polyimide. The printing of conductor was done in an interdigitated pattern as shown in figure 1. The thickness of sensitive materials was controlled by varying the spin time from 10 to 30 seconds. A thick solution of sensitive materials was prepared by adding a 3 gm of polymer in 50 mL ethanol in the case of p-HEMA. Currently we are preparing one more set of samples using polyimide and we will present a comparison of two humidity sensors- one with p-HEMA and another with polyimide. A humidity sensor that employs interdigitated capacitors (IDC) printed with silver on a glass and flexible substrate was successfully fabricated using p-HEMA and polyimide at spin speed of 2000 rpm for 20 seconds. Our goal is to integrate the humidity sensor into a robotic arm to perform real-time object characterization in agricultural applications via comprehensive understanding of the operations of harvesting robots in various crop types and environmental conditions.

Improving humidity sensor performance through phenylphosphonic acid-intercalated layered double hydroxide

This work delineates the development of humidity sensors utilizing layered double hydroxides (LDH) intercalated with phenylphosphonic acid (PPA), characterized by strong coordination and hydrophilicity. Following the meticulous synthesis of pristine LDH and PPA-LDH, sensor fabrication and comprehensive performance evaluation were conducted. Morphological analysis through SEM and TEM revealed a layered, thin PPA-LDH structure, augmenting the surface area for interaction with water. Density functional theory (DFT) computations elucidated the enhanced sensing mechanism, demonstrating that PPA intercalation significantly amplifies the adsorption energy of water molecules at metal sites, thereby facilitating proton and charge transfer. At 85 % relative humidity, the PPA-LDH sensor manifested an astonishing response of 4392.3 %, showcasing exceptional cyclic stability and rapid response/recovery times of 23 and 10 s, respectively. Comparative analysis with existing technologies underscored the superior sensitivity and response characteristics of the PPA-LDH sensor. The study concludes that PPA-intercalated LDH notably enhances humidity sensing capabilities, furnishing a material with heightened sensitivity and stability for the forthcoming generation of humidity sensors.

Design and Characterization of Printed Flexible Humidity Sensor

The development of humidity sensors is essential for applications in the environmental, agriculture, medical and semiconductor industries. This research focused on using advanced printed board circuit (PCB) printing technology to fabricate a humidity moisture sensor. The fabrication of humidity sensor was done using two different polymers: p-HEMA and polyimide, and the performance was compared. Humidity sensors with p-HEMA showed higher capacitance value and more sensitivity than the humidity sensors with polyimide. The variation in sensitivity was higher between the relative humidity from 60% to 90% than between 45% to 60%.

Surface Acoustic Wave Humidity Sensor: A Review.

The Growing demands for humidity detection in commercial and industrial applications led to the rapid development of humidity sensors based on different techniques. Surface acoustic wave (SAW) technology is one of these methods that has been found to provide a powerful platform for humidity sensing owing to its intrinsic features, including small size, high sensitivity, and simple operational mechanism. Similar to other techniques, the principle of humidity sensing in SAW devices is also realized by an overlaid sensitive film, which serves as the core element whose interaction with water molecules is responsible for overall performance. Therefore, most researchers are focused on exploring different sensing materials to achieve optimum performance characteristics. This article reviews sensing materials used to develop SAW humidity sensors and their responses based on theoretical aspects and experimental outcomes. Herein the influence of overlaid sensing film on the performance parameters of the SAW device, such as quality factor, signal amplitude, insertion loss, etc., is also highlighted. Lastly, a recommendation to minimize the significant change in device characteristics is presented, which we believe will be a good step for the future development of SAW humidity sensors.

Humidity Sensing with Supramolecular Nanostructures.

Precise monitoring of the humidity level is important for the living comfort and for many applications in various industrial sectors. Humidity sensors have thus become one among the most extensively studied and used chemical sensors by targeting a maximal device performance through the optimization of the components and working mechanism. Among different moisture-sensitive systems, supramolecular nanostructures are ideal active materials for the next generation of highly efficient humidity sensors. Their noncovalent nature guarantees fast response, high reversibility, and fast recovery time in the sensing event. Herein, the most enlightening recent strategies on the use of supramolecular nanostructures for humidity sensing are showcased. The key performance indicators in humidity sensing, including operation range, sensitivity, selectivity, response, and recovery speed are discussed as milestones for true practical applications. Some of the most remarkable examples of supramolecular-based humidity sensors are presented, by describing the finest sensing materials, the operating principles, and sensing mechanisms, the latter being based on the structural or charge-transport changes triggered by the interaction of the supramolecular nanostructures with the ambient humidity. Finally, the future directions, challenges, and opportunities for the development of humidity sensors with performance beyond the state of the art are discussed.

A self-powered PVA-based flexible humidity sensor with humidity-related voltage output for multifunctional applications

In the context of the industry that supports green energy and electronics, the development of a humidity sensor with self-powered characteristics and the ability to be worn by humans is crucial for expanding its practical applications. Here, we report a self-powered flexible humidity sensor based on a sandwich structure of polyvinyl alcohol (PVA)/nanocarbon powder (NCP)/magnesium chloride (MgCl2) moisture-sensitive composite film (PCMF) and Cu and Al conductive adhesive tape. This unique sandwich structure allows it to readily adsorb and diffuse water molecules in humid environments, greatly enhancing its responsiveness to humidity changes. The Cu/PCMF/Al sensor generates voltage corresponding to different relative humidity (RH) levels and has a unique RH-Voltage linear response characteristic (R2 = 0.9978), notable for its fast response/recovery (6/11 s), wide detection range (RH 11%−98%), long-term stability (over 30 days), and high voltage (∼0.6 V) and current (∼2.3 µA) output at RH 98%. More importantly, due to the good water solubility of PVA and MgCl2 salts, the designed sensor exhibits recyclability and reusability properties (90.31% of the original response voltage value), which can significantly minimize material waste and potentially reduce manufacturing costs. In addition, this sensor can be successfully applied to domains involving human wearables, such as being integrated into a mask for monitoring human respiration, and has excellent identification capabilities even for simple coughs and pronunciations. The demonstration of non-contact humidity sensing validates the prospective application of this sensor in non-contact switching devices. This work presents a novel design idea for the development of wearable humidity sensors with low cost, simple preparation, and high responsiveness, as well as self-powered characteristics.

Magnesium‐Substituted Zinc Ferrite as a Promising Nanomaterial for the Development of Humidity Sensors

Herein, the synthesis of zinc ferrite (ZnFe2O4) nanomaterials, substituted with magnesium (Mg), using the sol–gel autocombustion method, is reported. Powder X‐ray diffraction (PXRD), Fourier‐transform infrared (FTIR), field‐emission scanning electron microscopy (FESEM), and energy‐dispersive X‐ray spectroscopy (EDX) are used to characterize the synthesized Mg‐substituted ZnFe2O4. PXRD results show that unit cell volume and crystallite size decrease when the concentration of Mg ions rises. PXRD confirms that the nanomaterial has a single phase with the Fd3m space group. EDX confirms that the material is composed of Mg, Zn, Fe, and O elements. The FTIR spectra show a clear indication of the spinel ferrite structure, and the bands in the high‐frequency region reveal the hygroscopic nature of the prepared materials. The synthesized nanomaterial is used as a sensing element for the development of a humidity sensor. The average sensitivity (2.93 MΩ/%RH) is found to be highest for the composition Mg0.1Zn0.9Fe2O4. The response and recovery times of the fabricated thin‐film humidity sensor are 31 and 53 s, respectively. The developed sensing material has good repeatability (≈98%) and excellent stability.

Studying the humidity sensing behavior of MWCNTs boosted with Co3O4 nanorods

For the development of humidity sensors, multiwall carbon nanotubes boosted with cobalt oxides nanorods (MWCNTs/Co3O4) has been prepared by precipitation method to be utilized in humidity sensing application. To evaluate the prepared materials quality, both of functionalized MWCNTs and MWCNTs boosted with Co3O4 nano rods are characterized by High Resolution Transmission Electron Microscope (HRTEM), X- ray powder diffraction (XRD), thermal analysis (DTA and TGA), Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy. Meanwhile the humidity sensing behavior was also investigated. The proposed composite was tested in the wide range in relative humidity and testing frequency. The obtained results confirmed that the optimum testing frequency is 50 Hz. The MWCNTs/Co3O4 nanocomposite exhibited a good sensitivity toward humidity from 11% up to 97% RH with reasonable response and recovery time. Also, the sensor revealed a low hysteresis and good repeatability with increasing and decreasing of humidity levels.

Facile and Low Cost Temperature Compensated Humidity Sensor and Signal Conditioning System

Most relative humidity sensors are resistive or capacitive sensors. It’s a fact that the resistance and capacitance of a vast majority of such sensors also depend on the surrounding temperature and a change of temperature can cause significant errors in the final reading of relative humidity. This fact of temperature dependence is scarcely investigated in development of humidity sensors. In this work, a very simple temperature compensation method has been developed that automatically eliminates the effect of change in temperature from the final output without requiring separate measurement of temperature or any data processing. Amorphous polyethylene oxide (PEO) was used as the active material for humidity sensor owing to its high sensitivity and almost linear response. Interdigitated type sensor electrodes (IDE) were fabricated using reverse offset printing on glass substrate and the active layer was deposited through spin coating. An identical twin sensor was fabricated with the same parameters and was additionally encapsulated form the open environment. The dummy sensor was used as opposing pair with the active sensor in a bridge configuration to achieve temperature compensation. The bridge sensitivity was recorded to be ~2.9 mV/%RH that was further improved to 100 mV/%RH using a differential instrumentation amplifier based signal conditioning circuit. The sensor system was deployed in a real-life relative humidity measurement with changing temperature that returned a remarkable accuracy of 98.76%. The error due to temperature dependence reduced from 22.4% to just 1.24%. The transient response time of the sensor was found to be ~2.4 s.

Design and Development of Humidity Sensor Based on Alumina and Zirconium Dioxide

Humidity calculation is one of the most significant issues in different applications such as automated system, irrigation, instrumentation, climatology and GPS. The design and fabrication of Humidity sensor are mostly used for industries and laboratory applications. This work highlights the fabrication of humidity sensor based on alumina (Al2O3) used as substrate and Zirconium Dioxide (ZrO2) as active or sensing material to detect both moisture and air temperature in the environment. The objective was to fabricate a humidity sensor of compact size with better sensitivity and spatial resolution at low cost. In this project, AUTOCAD software was used for designing and was developed a miniaturized thin film sensor over a ceramic substrate and deposition of active sensing material was done through RF sputtering process. The fabricated humidity sensor has been characterized under different humidity conditions and the results showed a faster response.

Generation of Lossy Mode Resonances in Planar Waveguides Toward Development of Humidity Sensors

Lossy mode resonances (LMRs) are typically obtained with optical fibre. The Kretschmann configuration is an alternative but LMRs are generated with angles approaching grazing incidence. In this work, a new setup is explored, based on the lateral incidence of light on conventional planar waveguides such as glass slides or coverslips. Indium tin oxide was deposited onto both types of waveguides generating LMRs. The results of the simulations carried out agree well with the experimental results. As an example of the potential of this new and simple optical configuration, a humidity sensor with a sensitivity of 0.212 nm/% relative humidity (RH) in the range from 65% to 90% of RH was developed, which expedites the development of other types of sensors already explored with LMR-based optical fibre sensors.

Development of humidity sensor using modified curved MWCNT based thin film with DFT calculations

One-dimensional carbon nanostructures e.g. Carbon nanotubes (CNTs) possess outstanding physical properties owing to their unique structure and atomic arrangement. High electrical conductivity, highly exposed surface area and stability of these carbon nanostructures introduce them as the leading choice of nanomaterials for a number of electrical and industrial applications like humidity and gas sensing. The conductance or capacitance of CNTs varies greatly with the adsorption-desorption of molecules such as hydroxyl (OH−) ions. In this paper, we report the preparation of carbon nanotubes based thin film using a CVD technique which has been characterized by using Scanning Electron Microscopy (SEM), UV–vis microscopy and X-ray diffraction technique. The characterized film was investigated as a humidity sensor. In the experimental part, the variations in the impedance of film were observed by Impedance Analyzer 6440 B on varying the humidity levels. In the theoretical part, we have simulated the CNTs using Ab initio density functional theory (DFT) calculations to investigate the formation of endohedral complexes among CNTs and OH− groups. The binding energy, dipole moment, electronegativity and HOMO-LUMO gaps were monitored by increasing the hydroxyl group levels, which results in its better use in developing a robust and cost-effective humidity sensor.

Black Phosphorus-New Nanostructured Material for Humidity Sensors: Achievements and Limitations.

The prospects of using nanostructured black phosphorus for the development of humidity sensors are considered. It was shown that black phosphorus has a set of parameters that distinguish it from other two-dimensional (2D) materials such as graphene, silicone, and dichalcogenides. At the same time, an analysis of shortcomings, limiting the use of black phosphorus as a humidity sensitive material in devices aimed for market of humidity sensors, was also conducted.

Improving emerging European NMIs’ capabilities in humidity measurement

The control and measurement of humidity is important for many industrial applications and to ensure the appropriate storage of materials and products. Humidity measurement techniques are diverse and each presents different challenges for use and calibration for a range of pressures and gases. Over the past few years, the development of humidity sensors and apparatus has matured to a level where traceable calibration is beneficial to all industries in which humidity and moisture measurement and control are important. This paper deals with a European project in which the overall objective is to develop or extend the measurement and research capabilities of the participating emerging NMI/DIs’ countries in the field of humidity measurements, where access to these types of facilities is currently limited.

Polyimide-Based Capacitive Humidity Sensor.

The development of humidity sensors with simple transduction principles attracts considerable interest by both scientific researchers and industrial companies. Capacitive humidity sensors, based on polyimide sensing material with different thickness and surface morphologies, are prepared. The surface morphology of the sensing layer is varied from flat to rough and then to nanostructure called nanograss by using an oxygen plasma etch process. The relative humidity (RH) sensor selectively responds to the presence of water vapor by a capacitance change. The interaction between polyimide and water molecules is studied by FTIR spectroscopy. The complete characterization of the prepared capacitive humidity sensor performance is realized using a gas mixing setup and an evaluation kit. A linear correlation is found between the measured capacitance and the RH level in the range of 5 to 85%. The morphology of the humidity sensing layer is revealed as an important parameter influencing the sensor performance. It is proved that a nanograss-like structure is the most effective for detecting RH, due to its rapid response and recovery times, which are comparable to or even better than the ones of commercial polymer-based sensors. This work demonstrates the readiness of the developed RH sensor technology for industrialization.

Humidity-Sensing Performance of 3DOM WO3 with Controllable Structural Modification.

The development of humidity sensors with excellent sensing performance is a great challenge in the field of material chemistry. Here, we synthesized 3DOM WO3 nanomaterials through a poly(methyl methacrylate) template method, and first, we applied it to humidity measurement. For the goal of better sensing performance, the structural modification of Li/K-codoping was adopted, and the test results showed that Li/K-codoped 3DOM WO3 possessed highly improved humidity sensing performances, such as high response, low-humidity hysteresis, good long-term stability, great repeatability, decent response, and recovery properties. To deeply understand the great effect of Li/K-codoping on sensing performance, the pure, Li-monodoped, and Li/K-codoped 3DOM WO3-based humidity sensors were compared, and we found that the structure defects and adsorbed oxygen as well as the co-effect of Li/K dopants were key factors for the improved sensing performance. Additionally, a possible humidity sensitive mechanism was proposed to further study the promotion effect of Li/K-codoping on humidity sensing process.

Deposition and Characterization of Various Types of Coatings Development of Humidity Sensor

Deposition and Characterization of Various Types of Coatings Development of Humidity Sensor

Development of Humidity Sensor Using Nanoporous Polycarbonate Membranes

In the present work, the effect of temperature and moisture has been studied on nano-porous polycarbonate membranes of size 15, 50, and 80 nm, respectively to formulate the calibration curves for the future development of humidity sensors. These studies were conducted for virgin as well as humid samples while the temperature range was kept from room temperature to 160°C. Interesting variations in the capacitance with pore size and temperature have been observed. Calibration curves demonstrate that nanoporous polycarbonate samples are one of the important candidates for developing humidity sensors. In addition surface behavior has also been studied for the selective samples to better understand the dielectric behavior for nascent as well as moisturized samples.

Correlation between the sensitivity and the hysteresis of humidity sensors based on graphene oxides

Abstract We report on the correlation between the sensitivity and the sorption/desorption hysteresis of thin-film humidity sensors based on graphene oxides (GO). Both properties are systematically investigated by the conductance change of thin-film humidity sensors as well as the resonance frequency shift of quartz crystals coated with GO with varied pH. GO-based humidity sensors made at pH 3.3 present a lower level of sensitivity (2.1 ± 0.4 μS/%RH) and hysteresis-induced error (2.8 ± 0.1%RH) whereas those made at pH 9.5 present an increased level in both sensitivity (12.3 ± 2.2 μS/%RH) and hysteresis-induced error (3.7 ± 0.6%RH) by conductance measurements in the humidity range from 10%RH to 90%RH. Such correlation between two properties is also observed when the thickness and the functionality of GO films are varied. The correlation in GO-based humidity sensors is consistent with the correlation determined by a quartz crystal microbalance, indicating that the actual change of the water content in GO films underlies such behaviour. We discuss a possible mechanism for the observed correlation between the sensitivity and the hysteresis based on charged states on GO surface and their interactions with water molecules by using Fourier transform infrared (FT-IR) spectroscopy, water contact angle measurements, and response/recovery time measurement. Our findings would provide a new insight for the development of humidity sensors based on GO and its derivatives.

Growth and characterization of sol–gel processed rectangular shaped nanostructured ferric oxide thin film followed by humidity and gas sensing

In the present work nanostructured ferric oxide has been synthesized using sol–gel method. Thin films of ferric oxide were fabricated via spin coating process. The surface of the thin film was scanned by scanning electron microscope that exhibited the surface morphology of ferric oxide nanostructures. The material was also characterized by XRD, Acoustic particle sizer and FTIR. All the particles distributed on the surface have some spaces among them known as pores. These pores serve as adsorption sites for moisture and other gases. EDX confirmed the elements forming the ferric oxide in pure form. The particle size of the ferric oxide was estimated as ~12.2 nm. The pore size of the film was ~50 nm i.e., nature is mesoporous. Annealing effect on the surface morphology was also observed. Humidity sensing, electrical as well as optical of the prepared film was carried out. The results showed the suitability of material for the development of humidity sensors. Variations in resistance with the exposure of LPG were recorded and found that resistance of film increases with the increasing exposure time and concentration of gas. The maximum sensing response of the sensor was recorded as 3.26 for 1000 ppm at room temperature. The response and recovery times of the sensor were found to be ~12 and 9 min, respectively.