Capacitive Humidity Sensor Research Articles

Flexible capacitive humidity sensor based on potassium ion-doped PVA/CAB double-layer sensing film

Flexible capacitive humidity sensor based on potassium ion-doped PVA/CAB double-layer sensing film

Low humidity hysteresis and fast response silicon based capacitive humidity sensor based on SiO2 supported GO film

Low humidity hysteresis and fast response are important parameters for evaluating sensor performances. However, it is difficult for a sensor to simultaneously obtain a low humidity hysteresis and fast response time. In this work, a low humidity hysteresis and fast response silicon based capacitive humidity sensor was fabricated based on a two-step deposition strategy using SiO2 and GO. The scanning electron microscope (SEM) was used to characterize the morphologies for the fabricated sensor, demonstrating the successful synthesis of sensitive films. The experimental results suggested that, compared with the pure GO and GO/SiO2 composite film sensors, the two-step deposition strategy fabricated layered SiO2 supported GO film humidity sensor exhibited the lowest humidity hysteresis and the fastest response speed. The humidity hysteresis and response/recovery time are 3.5 % and 3 s/2 s, respectively. The humidity sensing mechanism for the observed low humidity hysteresis and fast response was analyzed via morphologic structures and Young-Laplace equation. This work demonstrates that the morphological structures modulation is an important and effective method for improving sensor performance, providing a valuable reference for high performance sensor fabrication.

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.

Ultra-sensitive humidity sensing based on Coherent perfect absorber-lasing system

Abstract As a singularity in non-Hermitian physics, coherent perfect absorber-lasing (CPAL) point theoretically has an infinite quality factor and can detect small perturbations, so it has potential applications in ultra-sensitive sensors. However, in the radio frequency field, due to deviations in actual components, the system will deviate from the CPAL point and the performance will degrade significantly (the output coefficient amplitude is only 30 dB). At the same time, actual sensing research is also very lacking. Based on these, we demonstrate a CPAL-based sensor system that increases its output coefficient amplitude to 60dB. When using the lasing state as the sensing mode, it has a capacitance detection accuracy of 10 fF. Then a high-frequency capacitive humidity sensor based on polyaniline/graphene oxide/titanium dioxide film is designed. The heterojunction formed between these materials increases the number of active sites for adsorbing water molecules. With the ultra-high capacitance detection accuracy of the CPAL sensor system, it is finally able to distinguish a 1% relative humidity change at 20%RH (corresponding to a 20 fF capacitance change, or a 1.63 dB output coefficient change), showing potential sensing application prospects.

High-Sensitivity Fully Printed Flexible BaTiO3- Based Capacitive Humidity Sensor for In-Space Manufacturing by Electrohydrodynamic Inkjet Printing

High-Sensitivity Fully Printed Flexible BaTiO₃- Based Capacitive Humidity Sensor for In-Space Manufacturing by Electrohydrodynamic Inkjet Printing

Impact of Compositional Engineering on PTB7-Th:PC71BM Capacitive Humidity Sensor Performance

Impact of Compositional Engineering on PTB7-Th:PC71BM Capacitive Humidity Sensor Performance

Unveiling capacitive humidity characteristic of CdSe quantum dots synthesized by facile route

Improving the practical uses of a multifunctional humidity sensor requires developing an easy, economical, and environmentally friendly synthesis process. Unfortunately, most humidity sensors have a complicated fabrication process, which drives up their price and restricts their range of applications. In this present work, quantum dots have prevailed as a potential sensing material owing to their small size and large surface area. Herein, we reported the three different colored (green, yellow, and red) based cadmium selenide (CdSe) quantum dots (QDs) using a solution-processed method. Physical characterization of as synthesized CdSe QDs is confirmed using photoluminescence (PL), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–visible analysis, diffused light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR). TEM analysis of CdSe QDs revealed the average particle size of 6 nm. These CdSe QDs were further employed as capacitive humidity sensors. Among the investigated samples, Cd-1 (prepared by 225℃) exhibited the highest sensitivity 93.842 pF/% RH with a rapid response and recovery time of 10 s and 13 s, respectively at 20 Hz. The excellent sensitivity of the Cd-1 is accredited to its least particle size and wider energy band gap as compared to Cd-2 and Cd-3 (prepared by 235 and 245℃) samples. Overall, this work opens an avenue for high performance CdSe QDs based humidity sensors.

Monoolein-Based Wireless Capacitive Sensor for Probing Skin Hydration.

Capacitive humidity sensors typically consist of interdigitated electrodes coated with a dielectric layer sensitive to varying relative humidity levels. Previous studies have investigated different polymeric materials that exhibit changes in conductivity in response to water vapor to design capacitive humidity sensors. However, lipid films like monoolein have not yet been integrated with humidity sensors, nor has the potential use of capacitive sensors for skin hydration measurements been fully explored. This study explores the application of monoolein-coated wireless capacitive sensors for assessing relative humidity and skin hydration, utilizing the sensitive dielectric properties of the monoolein-water system. This sensitivity hinges on the water absorption and release from the surrounding environment. Tested across various humidity levels and temperatures, these novel double functional sensors feature interdigitated electrodes covered with monoolein and show promising potential for wireless detection of skin hydration. The water uptake and rheological behavior of monoolein in response to humidity were evaluated using a quartz crystal microbalance with dissipation monitoring. The findings from these experiments suggest that the capacitance of the system is primarily influenced by the amount of water in the monoolein system, with the lyotropic or physical state of monoolein playing a secondary role. A proof-of-principle demonstration compared the sensor's performance under varying conditions to that of other commercially available skin hydration meters, affirming its effectiveness, reliability, and commercial viability.

A high-sensitive capacitive humidity sensor based on chitosan-sodium chloride composite material

Chitosan, as a hydrophilic, renewable, biodegradable, and cost-effective natural polymer, holds great promise in humidity sensor applications. This study proposes a green high-performance capacitive humidity sensor based on chitosan (CS)-sodium chloride (NaCl) composites. The CS-NaCl films were fabricated through a simple drip coating method, and their surface morphology and structure were characterized using SEM, XRD, FTIR and XPS techniques. The pure chitosan humidity sensor demonstrates an impressive response of 1180.17 % with excellent linearity. The introduction of NaCl markedly boosted the sensor response, with CS-50 wt% NaCl composites exhibiting a 72-fold increase (86, 492.84 %) in capacitance response compared to pure CS films. The contact angle test demonstrated that the chitosan film exhibited good hydrophilicity, which was further enhanced by the addition of NaCl. XRD and FTIR characterization confirmed that NaCl altered the structural properties of chitosan, disrupted hydrogen bonds, and consequently increased the mobility of polarizing groups and the dielectric constant. A water adsorption model was developed to elucidate the humidity sensing mechanism of the CS-NaCl sensor. The CS-NaCl composite material presents advantages such as easy preparation, high sensitivity, eco-friendliness, and promising practical applications in humidity sensing.

A Micro Capacitive Humidity Sensor Based on Al-Mo Electrodes and Polyimide Film.

Quickly sensing humidity changes is required in some fields, such as in fuel cell vehicles. The micro humidity sensor used for the relative humidity (RH) measurement with fast response characteristics, and its numerical model and method are rare. This paper firstly presents a numerical model and method for a parallel plate capacitor and a numerical analysis of its dynamic characteristics. The fabrication of this sensor was carried out based on the numerical results, and, the main characteristics of its moisture-sensitive element are shown. This parallel plate capacitor is made using complementary metal-oxide semiconductor (CMOS)-compatible technology, with a P-type monocrystalline silicon wafer used as the substrate, a thin polyimide film (PI) between the upper grid electrode and the lower parallel plate electrode, and electrodes with a molybdenum-aluminum bilayer structure. The shape of the micro sensor is square with 3 mm on the side of the source field. The humidity sensor has a linearity of 0.9965, hysteresis at 7.408% RH, and a sensitivity of 0.4264 pF/%RH. The sensor displays an average adsorption time of 1 s and a minimum adsorption time of 850 ms when the relative humidity increases from 33.2% RH to 75.8% RH. The sensor demonstrates very good stability during a 240 h test in a 25 °C environment. The numerical model and method provided by this study are very useful for predicting the performance of a parallel plate capacitor.

A capacitive humidity sensor based on nanodiamond/silver nanoparticles (ND/Ag) nanocomposite with high stability and rapid response for respiratory monitoring

In this work, a high-performance capacitive humidity sensor based on nanodiamond/silver nanoparticles (ND/Ag) nanocomposite is proposed. The influence of silver nanoparticles with different content in nanodiamond is investigated, and the optimum ratio of 5 wt% Ag in the composite shows the maximum humidity sensing performance, which is three times than that of the pure nanodiamond based humidity sensor. Moreover, the proposed sensor exhibits a superior repeatability, ultrafast response time (<2 s) and recovery time (<1 s). More importantly, the sensor can maintain a good repeatability and rapid response speed when exposure to high humidity for a long time, demonstrating the excellent stability of the proposed sensor. Furthermore, complex impedance spectroscopy was applied to account for the underlying sensing mechanisms of the ND/Ag composite film humidity sensor. Finally, we design a circuit to realize non-contact breathing control by using the fabricated humidity sensor, which demonstrate that the fabricated humidity sensor could be applied to some auto-control systems by human-breath. This work demonstrated that ND/Ag composite film is an ideal candidate material for constructing high-performance humidity sensors for respiratory applications.

Humidity Diode Sensors Based on 1D Nanosized Silicon Structures

Introduction. Humidity measurement is essential in microelectronics, aerospace, biomedical, and food industries, as well as in households for climate control. Currently, various types of devices have been used as humidity sensors: capacitive, resistive, diode, gravimetric, optical structures, field-effect transistors and devices based on surface acoustic waves.Problem Statement. Today, there is a need to develop IC-compatible humidity sensors that have high sensitivityand low cost. To this end, silicon nanowires have been successfully used in resistive and capacitive humidity sensors. However, there is a lack of research on the nanowire effect on device parameters of diode-type humidity sensors.Purpose. To develop diode sensors based on silicon nanowires and to determine the effect of process parameters of synthesis and structural features of nanowires on the performance of humidity sensors.Materials and Methods. The process of sensor fabrication includes several steps: chemical cleaning of silicon wafer, synthesis of silicon nanowires using standard or modified metal-assisted chemical etching, phosphorus diffusion to create a p-n junction, front and back metallization. The surface morphology of the nanostructures has been studied by scanning electron microscopy. The humidity-sensitive characteristics have been studied with theuse of salt hygrostats.Results. It has been shown that the addition of one-dimensional silicon nanostructures to the diode-type sensor signifi cantly improves its characteristics. The rectification ratio increases from 161 to 1807, the response ups from 4.5 to 25, the sensitivity grows from 1.6 to 4.02 (%RH)–1, while the response time and recovery time are reduced from 85/90 to 25/30 s, the hysteresis value goes down from 75 to 16%, the signal deviation after cycling drops from 15to 3%, and the signal fluctuation during continuous device operation decreases from 17 to 15%.Conclusions. The results have shown that the use of a simple and cheap nanowire synthesis technology is effective to produce humidity sensors.

Mems capacitive humidity sensors based on different structures for low humidity applications

Mems capacitive humidity sensors based on different structures for low humidity applications

DHT11 Sensor: A Comprehensive Study on Temperature and Humidity Sensor

The DHT11 sensor is a fundamental component in the realm of sensor technology, offering a simple yet effective solution for measuring temperature and humidity. This paper presentation aims to provide a comprehensive overview of the DHT11 sensor, highlighting its key attributes, operating principles, and practical applications. It is a compact and affordable sensor that utilizes a capacitive humidity sensor and a thermistor to accurately measure both temperature and humidity. Its digital output makes it easy to interface with microcontrollers and data acquisition systems, making it a popular choice for DIY enthusiasts, hobbyists, and professionals alike. Keywords: 1.Arduino IDE program software. 2.Development board ESP8266. 3.DHT 11

An Ultrafast Capacitive Humidity Sensor With Excellent Static and Dynamic Characteristics

An Ultrafast Capacitive Humidity Sensor With Excellent Static and Dynamic Characteristics

Influence of Anodic Aluminum Oxide Nanostructures on Resistive Humidity Sensing

Humidity nanosensors play a vital role in modern technology industries, including weather forecasts, industrial manufacturing, agriculture, food and chemistry storage. In recent years, research on humidity sensors has focused on different materials such as ceramics, polymers, carbon-based materials, semiconductors, MXenes or triboelectric nanogenerators, each with their own advantages and disadvantages. Among them, anodic aluminum oxide (AAO) is a well-known ceramic humidity sensor material with a long history of research and development. AAO humidity sensors offer advantages such as simple manufacturing processes, controllable nanostructures, high thermal stability and biocompatibility. However, traditional AAO fabrication still has disadvantages like high costs and longer process times. Hence, finding a low-cost and efficient method to fabricate AAO for controlling different nanostructures to meet the requirements is consistently a major research topic. From our previous studies, we have studied the relationship between the AAO capacitive humidity sensor and its nanostructures. In this paper, we explore the effect of an AAO nanoporous structure controlled by an anodization voltage of 20–40 V on the resistive-type humidity sensor performance instead of a capacitive one. We efficiently apply one-step hybrid pulse anodization at 25 °C to significantly reduce the processing time compared to the traditional two-step process under 0–10 °C. The AAO nanostructures and their impact on sensor measurements of humidity at 20–80 RH% will be discussed in detail. An electrical resistive sensing mechanism is established for further performance improvement by controlling anodization voltage.

PRACTICAL APPLICATION OF A FRACTIONAL FACTOR EXPERIMENT FOR ASSESSING THE INFLUENCE OF FACTORS ON THE SENSITIVITY OF A CAPACITIVE HUMIDITY SENSOR OF A TWO-LAYER STRUCTURE

The work uses a fractional factorial experiment in order to reduce the number of experiments. Capacitive humidity sensors of a two-layer structure made on a sitall substrate served as experimental samples. Hygroscopic salt NaCl was used to create a moisture-sensitive layer that would perform the function of a dielectric. Solutions of this salt with concentrations of 0,89 mol/l and 5,33 mol/l were applied by spraying on the surface of capacitive humidity sensors with thicknesses of 5,0 μm and 10.0 μm. In order to prevent the drop of the dew point, a protective layer was created − it is a moisture-absorbing polymethyl methacrylate film with thicknesses of 40 μm and 80 μm. According to the Cochrane’s test, it is proved that the fractional factorial experiment is reproducible. Using regression analysis, a quantitative assessment of the influence of factors on the sensitivity of the capacitive humidity sensor of the two-layer structure was carried out. Based on the developed regression equation, it was established that the concentration of the NaCl salt solution significantly affects the sensitivity of the capacitive humidity sensor of the two-layer structure. Significant coefficients of the regression equation were determined using the Student's test. Adequacy of the adjusted regression equation to the results of the factorial experiment was proved using Fisher's test. Based on the regression equation on the scale of real factors, which contributes to the optimization of the humidity sensor manufacturing parameters, it was established that the sensitivity of the capacitive humidity sensor of the two-layer structure depends on such factors as the thickness of the protective layer and the concentration of the NaCl salt solution. However, the sensitivity of the capacitive humidity sensor of the two-layer structure is significantly affected by the concentration of the hygroscopic NaCl salt solution. The highest sensitivity of 2,46 nF/% is provided with the following optimal parameters of the manufacturing process of the capacitive humidity sensor of the two-layer structure: the concentration of the hygroscopic NaCl salt solution is 5,33 mol/l, the thickness of the protective layer is 80 μm. Considering that the thickness of the moisture-sensitive layer does not significantly affect the sensitivity of the capacitive humidity sensor, the thickness of this layer can be taken in the range from 5,0 μm to 10,0 μm.

Flexible Chitosan-Based Capacitive Humidity Sensors for Respiratory Monitoring.

As one of the most important human health indicators, respiratory status is an important basis for the diagnosis of many diseases. However, the high cost of respiratory monitoring makes its use uncommon. This study introduces a low-cost, wearable, flexible humidity sensor for respiratory monitoring. Solution-processed chitosan (CS) placed on a polyethylene terephthalate substrate was used as the sensing layer. An Arduino circuit board was used to read humidity-sensitive voltage changes. The CS-based sensor demonstrated capacitive humidity sensitivity, whereby the capacitance instantly increased from 10-2 to 30 nF when the environmental humidity changed from 43% to 97%. The capacitance logarithm sensitivity and response voltage change was 35.9 pF/%RH and 0.8 V in the RH range from 56% to 97%. And the voltage variation between inhalation and exhalation was ~0.5 V during normal breathing. A rapid response time of ~0.7 s and a recovery time of ~2 s were achieved during respiration testing. Breathing modes (i.e., normal breathing, rest breathing, deep breathing, and fast breathing) and tonal changes during speech could be clearly distinguished. Therefore, such sensors provide a means for economical and convenient wearable respiratory monitoring, and they have the potential to be used for daily health examinations and professional medical diagnoses.

Investigation on acceptor-donor co-doped SnO2 nanoparticles enriched with oxygen vacancies: a capacitive humidity sensor for respiration detection.

In this work, we develop a novel capacitive humidity sensor based on Al-Si acceptor-donor co-doped SnO2 for real-time monitoring of ambient humidity and human respiration. XRD measurements reveal that all samples exhibit a tetragonal rutile phase and the crystallite size of SnO2 decreases with increasing Al-Si content. The high intensity of the Raman peak at 762 cm-1 confirms the presence of bridging mode oxygen vacancies in (Al + Si)0.02Sn0.98O2. The EPR results show that the amount of singly ionized oxygen vacancies increases after the introduction of Al-Si. Both types and amounts of oxygen vacancy defects are particularly sensitive to the adsorption of water molecules. Moreover, according to DFT calculations, the contribution of the Si 3s orbital and Al 3s orbital to the band edge verifies the formation of acceptor-donor complexes in Al-Si co-doped SnO2. The humidity sensing results reveal that the (Al + Si)0.02Sn0.98O2 humidity sensor shows high sensitivity (S = 839), low hysteresis (1.94%) and fast response/recovery times (25 s/5 s). The respiratory intervals during shallow, medium and deep breathing states of (Al + Si)0.02Sn0.98O2 were measured at 2.8 s, 3.8 s and 4.5 s, respectively. The chemical mechanism for the enhancement of humidity sensing performance corresponding to the oxygen vacancy defects induced by Al-Si interplay is proposed.

Fabrication and performance of capacitive humidity and strain sensors that incorporate 3D-printed nanocomposite electrodes

This work reports on advances in capacitive humidity and strain sensor technologies through the development of state-of-the-art 3D-printed Interdigitated Electrodes (IDEs) coated with a unique GO/ PVA coating. These IDEs are constructed using a novel composite filament of MWCNTs and polylactic acid (PLA) that offer superior flexural strength and electrical properties compared to conventional polymer matrices.We show that the GO/ PVA coating appears to be sensitive over the full range of relative humidity (RH) from 0% to 100%, with a remarkable capacitance change of 300 pF/%RH. Fast response and recovery times of 60 and 42 s, respectively, have been measured outperforming existing works that utilize metal electrodes. Regarding temperature dependence, the coatings endure conditions ranging from ambient to −25 °C, even in the presence of ice. Furthermore, at 50% RH, the sensors are shown to achieve a maximum sensitivity of 34.2 within a strain range of up to 2%.In conclusion, this work represents a significant advancement in cutting-edge sensor technologies, offering unprecedented capabilities for humidity and strain sensing performance for a wide range of applications.