Characterization Of Humidity Sensor Research Articles

Novel highly sensitive QCM humidity sensor with low hysteresis based on graphene oxide (GO)/poly(ethyleneimine) layered film

In this work, a novel graphene oxide (GO)/Poly(ethyleneimine) (PEI) layered film was designed and prepared on quartz crystal microbalance (QCM) for humidity detection through a feasible spray process at room temperature. The morphological and chemical properties of GO/PEI layered film were examined by means of field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR), and Raman spectroscopy. The effect of GO solution concentration on humidity-sensing characteristics was investigated and an optimized QCM humidity sensor was obtained. An intriguing phenomenon was observed that a three-dimension structure of GO sheets was formed due to the redissolution effect at the interface between PEI and GO layers. Results showed that a layered film QCM sensor exhibited superior humidity sensing properties than a pure GO film or a PEI one including high response, short response/recovery time, small humidity hysteresis (less than 1%RH), excellent repeatability, selectivity and long-term stability. Moreover, the humidity features of sensors were also researched via the method of impedance analysis. Lastly, a convincing sensing mechanism model was established to interpret the enhanced humidity-sensing performance of layered film sensors. The results demonstrated the potential application of GO/PEI layered films to humidity sensors.

Humidity sensor characteristics and electrical properties of Ni–Zn–Dy ferrite material prepared using different chelating-fuel agents

Humidity sensor characteristics and electrical properties of Ni–Zn ferrite material doped with Dy were studied after synthesis via sol–gel self-combustion method. In order to obtain different micro configurations of the material reflected in different porosities, four different fuel agents were involved: citric acid, tartaric acid, cellulose and urea. XRD and XPS techniques were used to certify the composition. The influence of fuel agent and structural features on the electrical properties of ferrite sintered pellets was investigated at room temperature as a function of frequency and humidity. Because the electrical characteristics of a porous material can significantly affect by the humidity, the electrical measurements were carried out in controlled humidity environments. Conduction mechanism is explained in terms of fuel agent influence on phase composition, grain size value, cation distribution and induced strain within spinel lattice by dissolution of voluminous Dy3+ ions. The humidity sensors applicative characteristics of the material are highlight by analyzing the sensitivity, response and recovery times of nickel–zinc–dysprosium ferrite used as active material.

A plausible ISFET drift-like aging mechanism for Al2O3 humidity sensor

An early paper [1] proposed the virtual pore widening model to explain the temporal drift in the characteristics of porous aluminum oxide humidity sensor. It is shown here that this model does not provide a plausible mechanism for the decrease in sensitivity caused by aging. Instead, it predicts the contradictory behavior of an increase in sensitivity with aging, which is not true. Alternatively, hydration of the aluminum oxide on exposure to moisture provides a more cogent explanation of the aging process, as observed in an alumina-gate ion-sensitive field-effect transistor (ISFET). In this paper, the monotonic decrease in capacitance of Al2O3 humidity sensor with the extent of hydration has been established, both by equivalent circuit approach and by modeling the sensor as a heterogeneous dielectric. Capacitance calibration characteristics of the humidity sensor have been simulated by employing the theory of alumina humidity sensor. Similarities and dissimilarities of aging of this sensor with ISFET drift have been elucidated. Whereas in ISFET, a thin hydrated layer is formed only at the surface of the gate dielectric, in Al2O3 sensor, hydration of inner layers in the porous film is highly likely. Unification of aging mechanisms in sensors is briefly discussed.

Humidity sensor characteristics based on ZnO nanostructure grown by sol-gel method

This work deals with ZnO thin films, electrical and structural properties characterisation and relative humidity sensing test. Films were deposited on glass substrate by sol-gel method. The sols were prepared using zinc acetate mixed with 2-methoxyethanol and monoethanolamine (MEA) as solvent and stabiliser, respectively. Obtained films were post annealed at 500°C in ambient atmosphere for 2 h. The films' crystalline structure was investigated by means of X-ray diffraction (XRD) analysis. XRD patterns of ZnO thin films exhibit three strong peaks (101), (002) and (100) planes assigned to ZnO wurzite structure. ZnO films morphology was investigated by scanning electron microscope (SEM). The SEM images reveal that the synthesised ZnO films are composed of irregular fibre-like stripes and wrinkle network. Resistivity and mobility of films were characterised by means of four probe resistivity and Hall effect measurement. The thin films humidity sensing properties have been analysed through electrical resistance measurements at various temperatures and humidity levels. It was observed that the sensor resistance decreases from 22 MΩ to 0.40 MΩ accordingly with increasing relative humidity from 15% to 95% at 25°C. Sensor sensitivity has been explained on the basis of variation of electrical conductivity of water molecules adsorption on film surface owing to wrinkle network morphology. At relative humidity of 95%, we recorded a response time of 7 s at 100°C which reveals that temperature has strong influence on the response and recovery times. Obtained results suggest that the prepared ZnO film has interesting potential in humidity sensing.

Fabrication of silver interdigitated electrodes on polyimide films via surface modification and ion-exchange technique and its flexible humidity sensor application

In this paper, the flexible capacitive humidity sensors using the polyimide foils as the substrate together with the moisture sensitive materials had been investigated. A simple and low cost inkjet-printing method had been developed to form silver interdigitated electrodes on PI foils. Two kinds of capacitive humidity sensors with Ag IDEs were constructed. And the humidity sensor characteristics such as sensitivity and hysteresis were analyzed. The results indicated that the metallization of Ag on PI is potential for various future flexible applications.

Role of morphological structure, doping, and coating of different materials in the sensing characteristics of humidity sensors.

The humidity sensing characteristics of different sensing materials are important properties in order to monitor different products or events in a wide range of industrial sectors, research and development laboratories as well as daily life. The primary aim of this study is to compare the sensing characteristics, including impedance or resistance, capacitance, hysteresis, recovery and response times, and stability with respect to relative humidity, frequency, and temperature, of different materials. Various materials, including ceramics, semiconductors, and polymers, used for sensing relative humidity have been reviewed. Correlations of the different electrical characteristics of different doped sensor materials as the most unique feature of a material have been noted. The electrical properties of different sensor materials are found to change significantly with the morphological changes, doping concentration of different materials and film thickness of the substrate. Various applications and scopes are pointed out in the review article. We extensively reviewed almost all main kinds of relative humidity sensors and how their electrical characteristics vary with different doping concentrations, film thickness and basic sensing materials. Based on statistical tests, the zinc oxide-based sensing material is best for humidity sensor design since it shows extremely low hysteresis loss, minimum response and recovery times and excellent stability.

Research of radiosonde humidity sensor with temperature compensation function and experimental verification

In order to avoid radiosonde humidity sensor condensation and eliminate the humidity value influence due to board capacitance distribution in high-altitude environment, a sandwich-plate capacitive humidity sensor, integrated with a snake-like platinum-resistance heater, was designed. Finite-element analysis method was used to obtain the temperature distribution of sensor surface when the sensor is heated. Then, the function relationship between environment temperature and heating power was obtained. A measuring circuit was designed based on the humidity sensor characteristics and ground experiment results show that the humidity sensitivity is about 2.3RH%/pF, hysteresis is about 0.7%F·S, repeatability is about 1.9%, measurement uncertainty is about 4%, and time constant is about 0.5s. Atmosphere aloft experiment results show that the humidity sensor which is described in the paper has good response time and desirable repeatability. Moreover, the condensation problem can be avoided effectively. The research results provide theoretical basis for carrying out comparison experiments.

Electrical properties and humidity sensor characteristics of lead hydroxyapatite material

Crystalline lead hydroxyapatite is obtained by direct precipitation from aqueous solution, the method being easy, fast and reproducible. The synthesis route which does not involve sodium salts or sulfates makes the material suitable for applications in catalysis. The electrical characteristics of lead hydroxyapatite material treated at different temperatures, made us focus on the analysis of the influence of water vapors upon the electrical characteristics. Thus, the electrical response to humidity adsorptive processes of lead hydroxyapatite material suggested us to analyze the material characteristics in terms of its use as a humidity sensor. The hydrophilic properties of lead hydroxyapatite material are reflected especially in high sensitivity and reduced time of response of the humidity sensors but also in long time of recovery, which suggests that Pb10(PO4)6(OH)2 material can be used for humidity sensors specialized in monitoring fluctuating humidity environments.

Humidity-sensing characteristics of multi-walled carbon nanotube sheet

The properties of humidity sensors made of spin-capable multi-walled carbon nanotubes (MWCNTs) were characterized and their sensing mechanisms analyzed. The results showed that their resistance increased linearly upon exposure to water molecules. Within the range of 10–90% relative humidity, the resistance of the MWCNT humidity sensor increased due to charge transfer. We found that the sensitivity and response time of the sensor could be improved by acid treatment. In addition, the surface area and contact number of MWCNTs played an important role in the improvement of sensor performance.

Preparation and characterization of humidity sensor based on poly(3-alkyl)pyrrole containing phosphonic acid groups

Poly(3-alkyl)pyrroles containing phosphonic acid groups with different alkyl segment lengths were chemical synthesized and the properties were measured by FTIR and UV-Vis spectroscopy. FTIR and UV-Vis results showed that the synthesized polymers were in a low doping level through chemical polymerization. By spin-coating on the surface of substrates, the polymer can be used as a humidity sensor. The change of DC electric current of the polypyrroles varies with the chain length of the alkyl substituents. The capacitance increases with the increase of the humidity and resistance decreases with the increase of humidity. This result is different from that of polypyrrole without alkyl substituents due to the influence of the phosphonic acid group. The sensor showed the resistive-type at high relative humidity, and the capacitivetype at the low relative humidity. The sensor exhibited very fast response to the change of environment humidity.

A resistive-type sensor based on flexible multi-walled carbon nanotubes and polyacrylic acid composite films

A humidity sensor film was fabricated by loading high numbers of multi-wall carbon nanotubes (MWCNTs) in a poly(acrylic acid) (PAA) polymer matrix containing poly(4-styrenesulfonic acid) (PSS) to enhance the MWCNT dispersion. Cross-section images demonstrate that the MWCNTs distribute evenly throughout the matrix. The fabrication processes and sensing mechanisms of the film are explained to investigate the flexible properties and humidity-sensing characteristics of the film. The film loaded with 33wt% MWCNTs is much more flexible than an overhead projector (OHP) film and shows similar electrical resistance to pure CNT Bucky paper. The sensor film composed of 1:2 MWCNTs:PAA is highly sensitive to humidity (0.069/%RH) and displays good linearity (0.99).

Preparation and Characteristic of Relative-Humidity Sensors Based on Laterally Grown ZnO Nanowires

The use of laterally oriented zinc oxide nanowires (ZnO NWs) grown by a hydrothermal growth (HTG) method for relative-humidity (RH) sensing devices at room temperature (RT) is demonstrated. Sensing response under various RH conditions (12–96%) at RT is presented and discussed. A humidity sensor based on laterally oriented ZnO NWs with a sensing response (R12%/R96%) as high as 2.2 was obtained at RT. The RT-operable humidity-sensing characteristics with comparably good sensitivity of the proposed humidity sensors is attributed to the full utilization of the entire NW surface, because the current path is aligned with the orientation of the bridged lateral ZnO NWs during the humidity sensing application, thus making possible.

Influence of Ce(III)/Ce(IV) - supplements on the Characteristics of Humidity Sensors with TiO<sub>2</sub> Films Prepared via a Sol-gel Method

espanolLos sensores de humedad han sido desarrollados mediante el metodo sol-gel para depositar peliculas superficiales basadas en TiO2 con adiciones de compuestos de cerio sobre sustratos de corindon y electrodos de aleacion de plata y paladio. Se han realizado observaciones mediante el Microscopio Electronico de Barrido (MEB) para determinar la morfologia superficial de las capas respectivas. Las caracterizaciones de la estructura y composicion han sido realizadas mediante Difraccion de Rayos X (DRX) y espectroscopia de dispersion energetica de rayos X (EDERX), con el fin de investigar la relacion entre la estructura de las capas y los parametros de los sensores respectivos. La influencia de los compuestos de cerio sobre las caracteristicas electricas de los sensores de humedad obtenidos se ha evaluado mediante el analisis de impedancia electrica. EnglishHumidity sensors have been prepared via a sol-gel method to deposit TiO2 films with additions of Ce-compounds on alumina substrates, with interdigitated silver palladium electrodes. Observations by scanning electron microscopy (SEM) were performed in order to determine the surface morphology of the respective layers. Structural and compositional characterization was done by X-ray diffraction analysis (XRD), and energy dispersive X-ray spectroscopy (EDX) for investigation of the relation between the film structures and the parameters of the respective sensors. The influence of Ce-compounds on the electrical characteristics of the samples as humidity sensing elements has been evaluated by an impedance analyzer.

Preparation of flexible resistive micro-humidity sensors and their humidity-sensing properties

A novel resistive-type micro-humidity sensor was constructed from an interdigitated micro-electrode on a flexible polyimide substrate of size 2.0mm×3.0mm. The humidity-sensing film was composed of a thermal curable epoxy resin and photocurable polyelectrolyte and fabricated by screen printing process, followed by thermal and photocuring reaction. The humidity sensing characteristics of the flexible micro-humidity sensor were comparable with a sensor fabricated on a commercial alumina substrate. The micro-humidity sensor was found to have a sensitivity of 0.046logΩ/%RH, a linearity of R2=0.990, a hysteresis of 1.48%RH, a response time of 65s, and a temperature coefficient of 0.50%RH/°C. Bode diagrams and Nyquist plots for two samples (the micro-humidity sensor and the typical alumina-based sensor) were investigated to demonstrate the effect of sensor size and different substrates on humidity-sensing characteristics.

Characteristics of Humidity Sensor Fabricated Using Nanostructured ZnO Thin Film by Sol-Gel Method

We have investigated the morphology and sensitivity characteristics of nanostructured ZnO thin film prepared by sol-gel method from different molarities of zinc nitrate hexahydrate and hexamethylenetetramine (HMT) solutions. ZnO thin film coated glass substrates were used to growth the ZnO structures and have been annealed at different temperatures. The morphology of the samples was observed by scanning electron microscopy (SEM) and their sensitivity characteristics were measured by using the I-V measurement. The SEM studies shows that the nanostructured ZnO was denser at higher molarities and I-V measurement demonstrates that the nanostructured ZnO having higher sensitivity at higher molarities and higher annealing temperatures.

Morphology-dependent humidity adsorption kinetics of ZnO nanostructures

The humidity-sensing characteristics of ZnO nanostructures are investigated using a quartz crystal microbalance (QCM) measurement. ZnO nanostructures are synthesized via sol–gel route in nanoparticle (ZnO-NP) and nanowire (ZnO-NW) morphologies with diameter about 20–30nm. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods are used to determine the morphology and crystal structure of ZnO nanostructures. Humidity sensing capabilities are discussed in terms of the different morphologies. The results show that ZnO-NP is more sensitive to humidity changes than ZnO-NW. QCM results are analyzed using Langmuir adsorption model to determine adsorption rates, Gibbs free energy of adsorption (ΔG), and adsorbed mass amount by the synthesized ZnO nanostructures. Negative value of ΔG for humidity adsorption on ZnO nanostructures indicates that the process is spontaneous and adsorption capacity increases with size reduction. Gibbs free energy of the ZnO-NP is found to be more negative, indicating that the ZnO-NP has more favorable adsorption sites compared to the ZnO-NW. Experimental and theoretical results exhibit that humidity-sensing properties of ZnO nanostructures are morphology-dependent.

Influence of Cubic Structured-ZnSnO3 Immersion Time to the Performance of Humidity Sensor

ZnSnO3 thin film was deposited at different deposition time (0.5 h, 2 h, 4 h and 6 h) using sol-gel immersion method and the electrical, optical and structural properties of this film was investigated. This research involved the preparation of nanostructured ZnO thin film by using RF magnetron sputtering, preparation of ZnSnO3 sol-gel solution, metal contact deposition and characterization of humidity sensor. The thin film was characterized using current-voltage (I-V) measurement (Keithley 2400) and field emission scanning electron microscopy (FESEM) (JEOL JSM 6701F) for electrical and structural properties respectively. The sensor was characterized using I-V measurement in a humidity chamber (ESPEC SH-261) and the chamber has been set at room temperature with varied relative humidity (% RH), in the range of 40-90% RH. The film prepared with a deposition time of 2 h shows better sensitivity for humidity sensor. The FESEM investigation shows that crystal size increases with the increasing deposition time.

인쇄공정으로 제조된 저항형 습도센서의 감습특성에 대한 전극패턴의 영향 연구

저항형 습도센서의 개발을 위하여 스크린 인쇄방법으로 glass epoxy(GE) 기재 위에 핑거의 수와 그 사이의 간격이 다른 여러 가지 바이트형의 금 전극을 제조하였다. 전극의 기본적인 구조는 핑거 수가 3, 4 및 5를 가지며 간격은 각각 310과 460 <TEX>${\mu}m$</TEX>이다. 전극은 처음에 은 나노 페이스트로 전극 패턴을 인쇄하고 연속적으로 Cu, Ni 및 Au를 무전해 도금을 진행하여 제조하였다. 감습성 고분자 전해질은 [2-(methacryloyloxy)ethyl] dimethyl benzyl ammonium chloride(MDBAC)과 methyl methacrylate(MMA)의 공중합체를 사용하였으며 스크린 인쇄 방법으로 Au 전극/GE 기재 위에 인쇄하였다. 이렇게 제조된 습도 센서의 20-95%RH 영역의 상대습도 대 로그 임피던스 그래프에서 좋은 직선성을 보여주었으며 히스테리시스는 1.5%RH 이하 및 75초의 응답 및 회복 속도를 보여주었다. 전극의 구조는 고분자 저항형 습도센서의 감습특성에 큰 영향을 미치며 전해질 고분자와 전극 사이의 활성화 에너지, 온도의존성, 감도, 직선성 및 히스테리시스와 같은 감습특성 등의 차이를 비교하였고 특히 이온의 이동에 관여하는 활성화 에너지 및 온도 의존성에 큰 영향을 미쳤다. Based on our experience in developing resistive humidity sensor, interdigital gold electrodes with different fingers and gaps have been fabricated on a glass epoxy (GE) substrate using screen printing techniques. The basic structure of the electrode consisted of a 3-, 4- and 5-fingers with gaps of 310 and 460 <TEX>${\mu}m$</TEX>. Gold electrode/GE was prepared by first printing silver nanopaste, followed by consecutive electroless plating of Cu, Ni and then Au. Copolymer of [2-(methacryloyloxy)ethyl] dimethyl benzyl ammonium chloride (MDBAC) and methyl methacrylate (MMA) was used as a humidity-sensing polyelectrolyte, which was fabricated by a screen printing method on the Au electrode/GE substrate. The flexible humidity sensor showed acceptable linearity between logarithmic impedance and relative humidity in the range of 20-95%RH, low hysteresis of 1.5%RH, good response and recovery time of 75 sec at 1 V, 1 kHz, and <TEX>$25^{\circ}C$</TEX>. Electrode construction had a significant influence on the humidity-sensing characteristics of polymeric humidity sensors. The activation energy between electrode and ion conducting polyelectrolyte plays an important role in explaining the differences of humidity sensing characteristics such as temperature dependence, sensitivity, linearity and hysteresis.

Preparation of flexible resistive humidity sensors with different electrode gaps by screen printing and their humidity-sensing properties

A novel, flexible resistive-type humidity sensor was fabricated through silk-screen printing of a humiditysensitive copolymer of [2-(methacryloyloxy)ethyl] dimethyl benzyl ammonium chloride and methyl methacrylate on a glass epoxy (GE) substrate. The basic structure of the sensor consisted of a 4-fingered interdigital electrode with various gaps (310, 360, 410, and 460 μm). Gold electrodes GE substrate were prepared by first printing silver nanopaste (thickness 6–7 μm), followed by consecutive electroless plating of Cu (5 μm), Ni (2 μm), and then Au (80 nm). The activation energy for ion conduction and the copolymer/substrate interface were used to explain the differences of humidity-sensing characteristics of sensors fabricated on a GE substrate. Electrode construction had a significant influence on the humidity-sensing characteristics of polymeric relative humidity sensors. Details regarding humidity sensor characteristics, including sensitivity, linearity, temperature dependence, hysteresis, and response time depending on electrode construction were compared. The flexible humidity sensor showed acceptable linearity between logarithmic impedance and relative humidity in the range of 20–95%RH, low hysteresis (within 1.5%RH), good response (75 s) and recovery time (95 s), and long-term stability (225 days at least), measured at 1 V, 1 kHz, and 25 °C. Open image in new window

Humidity-sensing properties of conducting polypyrrole-silver nanocomposites

We report here the humidity-sensing characteristics of conducting polypyrrole (PPY)-silver nanocomposites prepared by wet-chemical technique. Addition of silver into the conducting polymer network shows remarkable change in resistance with relative humidity (RH). The resistance of PPY-silver nanocomposites is found to increase with RH till a threshold value, above which the resistance tends to decrease. The threshold RH value is significantly reduced with the increase in silver concentration into the conducting polymer network. The modified metal–polymer interface and the associated interfacial water molecules play the critical role in determining the electrical transport and hence the humidity-response characteristics of the nanocomposites.