Type Humidity Sensor Research Articles

Synthesis-in-place of V2O5 nanobelts for wide range humidity detection

Resistive type humidity sensors offer advantages in simplicity, low cost, compact size, and remote operation. Due to the benefits, they have great potential for use in a variety of environments and future applications, including the Internet of Everything (IoE). However, resistive humidity sensors have limitations in detecting extreme humidity levels below 10 % or above 90 % relative humidity (RH) and are also vulnerable to high temperatures, which hinders their use in harsh environments applications. For broader applications, humidity sensors with reliable performance across the full range of humidity levels and high stability at elevated temperatures need to be developed. Here, we report high performance resistive humidity sensors based on V2O5 nanobelts operating at high temperatures. The V2O5 nanobelts are directly deposited between Pt electrodes by O2-assisted physical vapor transport (PVT) method. The V2O5 nanobelt humidity sensor exhibits reliable humidity sensing properties over a wide range from 1 % to 100 % RH. Furthermore, long-term stability and reproducibility of V2O5 nanobelts are demonstrated through 25 consecutive humid air pulses and measurements obtained after 4 months of storage. The high performance of the V2O5 nanobelt humidity sensor is advantageous for practical use in applications ranging from IoE environments to harsh industrial conditions.

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.

A comparative study on humidity response of graphene oxide with reduced graphene oxide and its multifunctional applications

In this paper, we present a comparative study of Graphene Oxide (GO) and chemically reduced Graphene Oxide (rGO) based resistive type humidity sensors fabricated on Ti/Au Inter-Digital Electrodes (IDEs) coated SiO2 substrates. The sensing performance, including sensitivity, response/recovery time, and hysteresis has been evaluated for both sensors. The GO film exhibited quick response (0.3 s) and high sensitivity (104.7 kΩ/%RH) with remarkable repeatability and negligible hysteresis as compared to rGO film sensor. The excellent humidity sensing properties of GO are attributed to the presence of abundant oxygen-containing polar functional groups on GO relative to rGO. To demonstrate the multifunctional applications of developed GO based sensor, we have also demonstrated its use as respiratory monitoring, non-contact proximity detector, and speech event detector through the detection of humidity perturbations. The work suggests GO sensitive material has high potential for the development of wearable and integrated health monitoring platforms and non-contact sensing devices.

Biosynthesis, structural characterization and humidity sensing properties of cellulose/ZnO nanocomposite

The present work demonstrates the applications of a nanocomposite of the cellulose polymer and ZnO nanoparticles with 1:1 weight ratio, prepared by a green assisted precipitation method for a high-performance resistive type humidity sensor. The morphology and nanostructure of prepared cellulose/ZnO composite were characterization by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and transmission electron microscopy are confirmed the decoration of ZnO nanoparticles on cellulose polymer matrix surface. The humidity sensing martials was coated onto the interdigital electrodes. The sensitivity, response/recover and stability studies performance of fabricated humidity sensors have been monitored in 5 to 98% relative humidity (%RH) at room temperature (37 °C). The response and recovery times of the fabricated sensors are observed as ≈ 26 s and ≈ 53 s respectively, and the sensitivity factor (Sf) is 2656 ± 103 Ω. Possible mechanisms of the humidity sensor based on water-induced conductivity increase are discussed. Also, the energy band of cellulose/ZnO nanocomposite was simulated by density functional theory (DFT) studies. The cellulose/ZnO nanocomposite humidity sensor has great potential for practical field applications.

Graphene oxide boosted high surface area CeO2 for humidity sensing

Precise measurement of humidity is of high significance in diverse fields like instrumentation, automated systems, agriculture and climatology. Herein as a first ever attempt, we report ceria-graphene oxide (CeO2-GO) composites as resistive type humidity sensor, that shows attractive response to moisture. High surface area ceria is synthesised via ammonia mediated precipitation from salt precursor, and CeO2-GO composites are achieved via hydrothermal route. The composites are well characterized using various analytical tools. Experiments using different mass ratios of GO in the composite show that there is an optimum amount of GO to realise maximum efficiency in the test parameters. It is further noted that the performance of the sensor is controlled by the active adsorption sites, which is proportional to surface area of the composite. An optimized CeO2-GO matrix yielded a response and recovery time of 19 s and 10 s respectively. The narrow hysteresis in moisture adsorption-desorption study and excellent stability for about 60 days are also observed for the system, warranting the successful utility of the system for practical applications.

SnO2 nanostructured thin film as humidity sensor and its application in breath monitoring

In the present paper, we studied SnO2 nanostructured thin film-based resistive type humidity sensor fabricated by spin coating method on alumina substrate and showed its application in real time respiratory monitoring. Experiments show the growth of crystalline SnO2 nanoparticles with a tetragonal phase, with an average particle size of ∼15 nm and a high surface roughness of the order of 50-60 nm. The gold-interdigitated electrodes were patterned on the SnO2 film surface by DC sputtering technique to study the sensing response of the film. The sensor shows a high sensitivity of 1.34 kΩ/%RH, a minimal hysteresis of 0.94%, and good repeatability. The developed sensor is tested for monitoring human breath patterns in different physical conditions, and apnea like situations, which suggests its potential for clinical applications. Overall, the present work proposes an environmental friendly, easy-to-synthesize, highly stable, and simple facile SnO2 based deployable humidity sensor to diagnose human health patterns in a non-contact manner.

Positive impedance and low hysteresis MOS type humidity sensors via SRCBD poly crystalline SnxOy thin films

Resistivity-type humidity sensors have undergone extensive research as a result of the increasing demands in industry, agriculture, and daily life. Only a few carbon composites have been reported to exhibit positive humidity impedance, and the vast majority of humidity sensors currently in use are based on negative humidity impedance, in which electrical resistance increases as humidity levels increases. However, in this case, we only have fabricated positive impedance MOS type humidity sensors using SRCBD polycrystalline SnxOy thin films. By changing the composition of x and y due to air annealing, the resistance of SnxOy film sensors changes in response to relative humidity from a negative to a positive value. It was found that oxygen vacancy defects were the root cause of the positive humidity impedance. Positive humidity impedance sensors are more energy-efficient, simpler to miniaturize, and electrically safer than their negative counterparts as they operate at lower voltages, expanding the range of applications for humidity sensors. We also believe that by making vacancies in semiconducting materials, positive impedance humidity sensors can be made universally. A MOS device with low hysteresis was developed using SnxOy films that were air annealed at 450o C and can be used as a reliable humidity sensor in a variety of applications.

Geometric design optimization of polyaniline/graphite nanocomposite based flexible humidity sensor for contactless sensing and breath monitoring

• Polyaniline-graphite nanocomposite based 2D humidity sensor was screen-printed. • Adding graphite leads to faster response, improved thermal stability and hysteresis. • Improved long term stability with high performance was demonstrated over 180 days. • Sensor geometry (inter-electrode distance and number of electrodes) was optimized. • Contactless sensing and respiration monitoring applications were demonstrated. We report a low cost flexible polyaniline-graphite nanocomposite based resistive type humidity sensor, screen printed on a PET substrate with interdigitated silver electrodes. The composite with a 3:1 ratio of polyaniline and graphite was found to be ideal with a sensing response of 93.4%, took 24 s to reach 54% RH, a maximum response/recovery time of 35/39 s and low hysteresis (1.12%). Graphite acting as the support material increased the sensor's thermal stability, allowing it to be operated at temperatures as high as 55 ⁰C. To further improve sensor performance and find an optimum device architecture, geometric parameters such as inter-electrode distance and number of electrodes were studied and optimised. Based on this, a relative humidity contactless finger sensor (1.5 cm range) and a human breath monitoring sensor to map different breathing rates were demonstrated.

Composite Effect on Zinc Oxide based Resistive Type Humidity Sensor Performance: A Preliminary Assessment

Composite Effect on Zinc Oxide based Resistive Type Humidity Sensor Performance: A Preliminary Assessment

Study of humidity sensing properties of TiO2/Ethyl cellulose (EC) composite

Monitoring environment parameters such as humidity have received wide attention from the past up until now. One of the main factors to ensure the accuracy and response speed of the humidity sensor is the selection of appropriate materials and its fabrication process. The selection of suitable materials should be based on materials that exhibit good sensitivity over the entire humidity range, low hysteresis, and are resistant to various chemicals vapor. In this study, we report a capacitive type humidity sensor has been fabricated by combining titanium dioxide (TiO2) with Ethyl Cellulose (EC) polymer. The analysis of the structure of the material was characterized by XRD. Humidity sensing performance studies on TiO2/EC sensor sample were measured at various humidity levels from 11% to 96% relative humidity (RH) at a frequency of 1 kHz. The measurement results and analysis show that the dynamic response of the TiO2/EC sensor capacitance with time tends to increase at higher RH, the linearity of the increase in capacitance response is relatively good, the sensor hysteresis is less than 4% from the adsorption–desorption process of water molecules, and the sensitivity of the sensor to RH is good compared to some previously reported studies, reaching up to 600% at 96% RH. Based on the results obtained, it can be concluded that TiO2 mixed with EC is very suitable to be applied to humidity sensors.

Capacitive and Conductometric Type Dual-Mode Relative Humidity Sensor Based on 5,10,15,20-tetra Phenyl Porphyrinato Nickel (II) (TPPNi).

(1) Background: A quest for a highly sensitive and reliable humidity monitoring system for a diverse variety of applications is quite vital. Specifically, the ever-increasing demand of humidity sensors in applications ranging from agriculture to healthcare equipment (to cater the current demand of COVID-19 ventilation systems), calls for a selection of suitable humidity sensing material. (2) Methods: In the present study, the TPPNi macromolecule has been synthesized by using a microwave-assisted synthesis process. The layer structure of the fabricated humidity sensor (Al/TPPNi/Al) consists of pair of planar 120 nm thin aluminum (Al) electrodes (deposited by thermal evaporation) and ~160 nm facile spin-coated solution-processable organic TPPNi as an active layer between the ~40 µm electrode gap. (3) Results: Electrical properties (capacitance and impedance) of sensors were found to be substantially sensitive not only on relative humidity but also on the frequency of the input bias signal. The proposed sensor exhibits multimode (capacitive and conductometric) operation with significantly higher sensitivity ~146.17 pF/%RH at 500 Hz and 48.23 kΩ/%RH at 1 kHz. (4) Conclusions: The developed Al/TPPNi/Al surface type humidity sensor’s much-improved detecting properties along with reasonable dynamic range and response time suggest that it could be effective for continuous humidity monitoring in multi environmental applications.

Enhanced sensitivity of zinc phthalocyanine-based microporous humidity sensors by varying size of electrode gaps

The use of organic materials has become an increasingly important issue in sensing devices in recent times. Phthalocyanine is among the most promising materials in this undertaking. Zinc phthalocyanine (ZnPc) based microporous device was fabricated and its capacitance was utilized as the sensing mechanism for a humidity sensor. The effect of the electrode gap of the device on the electrical properties was investigated along with the correlation between the device’s performances and the morphology of the sensing film. Using the solution-processed spin coating method, the capacitive type humidity sensor devices have been fabricated in a planar geometry of Al/ZnPc/Al with the presence of a microporous template. The size of electrode gaps measured with a surface profiler was 53.00 ± 0.06 μm, 119.00 ± 0.03 μm and 286.00 ± 0.01 μm. The surface morphology was characterized by using transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). Analysis of the experimental results showed that the device with the shortest electrode gap (53.00 μm) produced the best sensitivity of 1.03 ± 0.04 pF/%RH than that of the longer gaps. Additionally, hysteresis as well as response and recovery performances have also been investigated.

High-Sensitivity Whispering Gallery Mode Humidity Sensor Based on Glycerol Microdroplet Volumetric Expansion.

We demonstrate a highly sensitive whispering gallery mode (WGM) relative humidity (RH) sensor based on a glycerol microdroplet. WGMs were excited using a 760 nm tunable semiconductor laser. We used free space coupling, which is effective when using a liquid resonator. A detailed analysis of different parameters influencing the sensor’s characteristics (sensitivity, hysteresis, resolution, stability, and temperature) is presented. The sensitivity of the sensor is one of the highest reported (2.85 nm/% RH in the range 50–70% RH with the resolution 1 × 10−4% RH). This type of humidity sensor has several advantages, such as high sensitivity, extended lifetime, good repeatability, and low cost, as well as the use of a non-toxic and environmentally friendly liquid.

Capacitive humidity sensing performance of naphthalene diimide derivatives at ambient temperature

We report for the first-time the development of capacitive type humidity sensors employing naphthalene diimide derivatives (NDI) as sensing layer. Three different naphthalene diimide derivatives bearing imide side chains of different hydrophilicity were designed, synthesised and characterised. X-ray diffraction and thermogravimetric analyses gave useful information about structural and thermal behaviour of the newly developed materials, indicating their crystallinity and stability. Atomic force microscopy analysis revealed a variety of morphologies in thin films as a result of the structural properties of the NDIs. Devices bearing NDI layers were fabricated on ceramic substrates with gold interdigitated electrodes spaced 200 µm apart. Humidity sensing performance, as a change in capacitance, was studied upon exposure to a wide range of relative humidity levels (0–95%) at ambient temperature. Importantly, an increase in the capacitance of the sensors was recorded with an increase in relative humidity. The developed sensors exhibited high sensitivity, good long-term stability, excellent reproducibility, and low hysteresis. The sensor performance was also tested against different operating frequencies (250 Hz–2 kHz) to improve linearity, illustrating directions for optimised performance. These results confirm that sensors based on NDIs possess better sensing performance to other types of reported capacitive humidity sensors.

Synthesis and characterization of natural zeolite-clay as resistive humidity detection

Zeolite-clay was successfully employed in as resistance type humidity sensor. Zeolite-clay humidity sensor was synthesized by crushing into powder and soaked into 6% H2SO4while stirred for two hours using a magnetic stirrer. The solution was filtered, washed using aquadest and dried at 100°C for 5 hours with the mixture process of zeolite and clay were varied into 10%:0%, 9.5%:0.5%, 9%:1%, 8.5%:1.5%, and 8%:2%. The result pressed with a size of 3x3x1 cm3 using a hydraulic under 3 tons pressure for 10 minutes and heated at temperatures of 800°C.Zeolite-clay was placed in a testing chamber equipped with the electrode that connected to an electrical supply of 3 Volt. The electrical resistance was recorded using Multimeter Hydra Series III Fluke when exposed with humidity in range of 18.5-99% RH. Zeolite 8%-clay 2% sensor exhibits the highest sensitivity properties, which indicate saturated resistance value at 99% RH.Meanwhile, Zeolite 8.5 gr-clay 1.5 gr revealed the highest response value at 54.3 Mega Ohm. Based on SEM and EDX characterization, the ratio of Si/Al and microstructure surface have an effect towards humidity detection process. The results indicate enhancement than previous research on zeolite as a sensor, and therefore Zeolite-Clay has proved the capability to detect humidity.

SENSOR DE HUMEDAD USANDO PELÍCULAS DELGADAS DE NANOARROCES DE CuO

In this paper, a humidity sensor based on CuO has been successfully fabricated. Thin-film of CuO nanorices were synthesized by Successive Ionic Layer Adsorption and Reaction (SILAR) method on silicon wafer with Si3N4thinlayer as a substrate. The microstructure and morphology of the samples were investigated using X-ray diffraction(XRD) and scanning electron microscopy (SEM). Humidity sensing properties of the thin films have been studied. The sensing response has been measured in the relative humidity (RH) range from 20 up to 80 % at room temperature. It was found that impedance of the system decreases as the RH was increased. The prepared sensor revealed good reversibility with response and recovery time of 130 s and 320 s respectively. The complex impedance spectra were analyzed in the range of 0.1 to 1 kHz. This type of humidity sensor can be used as a new generation of ecological sensors with low cost and good stability. It makes the sensor a candidate for practical applications.

Fabrication of a surface type humidity sensor based on methyl green thin film, with the analysis of capacitance and resistance through neutrosophic statistics.

In this work, we propose a humidity sensor based on methyl green thin film. A surface-type humidity sensor Al/MG/Al was fabricated by depositing methyl green thin film between aluminum electrodes. The structural, optical and surface morphological properties of the thin film were characterized by XRD, UV Vis and FESEM. The sensing properties with high sensitivity as well as response time 200 s and recovery 60 s of the humidity sensor were investigated by measuring the capacitance and resistance at 1, 10 and 100 kHz with the humidity varying range from 32 to 85% RH. These measured values were analyzed by classical statistics and neutrosophic statistics. As a result, it was observed that neutrosophic statistics were more informative, flexible and adequate than classical statistics for analyzing the measured values of capacitance and resistance.

High-sensitivity fiber-optic humidity sensor based on microfiber overlaid with niobium disulfide

Humidity sensors by exploiting transition metal dichalcogenides have attracted enormous attention in various applications. Here, we demonstrate a high-sensitivity fiber-optic humidity sensor based on microfibers overlaid with niobium disulfide (NbS2). Our experimental results show that this type of humidity sensor has high sensitivity, and good repeatability and reversibility. In the RH range of 72–97%, a sensitivity of up to 1.05 dB/RH% and an excellent linearity correlation of 99.5% were achieved. The transmission spectrum of the humidity sensor indicates that this sensor has a wide wavelength detection range from 1530 to 1565 nm. We applied the sensor to monitor human breath with a fast response. We anticipate that the humidity sensor will be useful for the environmental monitoring, food production, and semiconductor industries.

Aerosol deposited BaTiO3 film based interdigital capacitor and squared spiral capacitor for humidity sensing application

In this article, ultra-high sensitive aerosol deposited BaTiO3 film based inter-digital capacitors (IDCs), and a squared spiral capacitor (SSC) have been investigated for humidity sensing application. BaTiO3 based sensing film is prepared at room temperature, then post-annealed at 200 °C and 400 °C to enhance material's sensing properties. Thermal exposure of BaTiO3 sensing film increases electron density, improves material defects, enhances grain to grain connectivity, and escalates hydrophilicity. The sensitivity of all fabricated devices is evaluated by comparing capacitive variation at different relative humidity (RH) levels. The electric field distribution and open area ratio (OAR) are scrutinized to understand the effect of these parameters on the BaTiO3 integrated capacitive type humidity sensor. The obtained results indicate that the IDC-1 with large OAR reveals high sensitivity due to the availability of the wider interaction area between fringing electric field intensity and water vapors adsorbed on sensing material surface. Similarly, the SSC structure reveals high sensitivity over IDC-2 and IDC-3 due to the existence of concentrated electric field intensity and large OAR. The intensified electric field polarizes the water vapor molecules and sensing material ions that upsurge the device capacitance at various RH levels, which enhances sensitivity. This study reveals that the design structure, electric field intensity, and sensing material property are equally responsible for high sensitivity in capacitive humidity sensors.

Investigation of Room Temperature Protonic Conduction of Perovskite Humidity Sensors

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 self-designed 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.