A Novel Highly Sensitive Humidity Sensor Based on ZnO/SBA‐15 Hybrid Nanocomposite

This work deals with the study of hydrothermally synthesized zinc oxide (ZnO) loaded mesoporous SBA‐15 hybrid nanocomposite for relative humidity sensing (RH) at room temperature. The sensor exhibits an excellent ~5 orders impedance change along with excellent linearity, quick response time (17 s), rapid recovery time (18 s), negligible hysteresis (1.2%), good repeatability, and stability (1.8%) in 11%–98% RH range. In addition, complex impedance spectra of the sensor at different RHs were analyzed to understand the humidity sensing mechanism. Our study can open a new way for realizing ZnO/SBA‐15 hybrid nanocomposite for fabrication of high‐performance RH sensors.     

Dielectric and AC conductivity studies of novel porous armalcolite nanocomposite‐based humidity sensor

Armalcolite, a current motivated rare earth ceramic usually available in the moon, had been used for the first time, as dielectric‐type humidity sensors. The armalcolite nanocomposite was prepared using multistep solid‐state sintering under high pressure and a high‐sensitive dielectric sensor was developed for humidity controlling applications. Different concerning phases developed by the proper sintering were analyzed precisely by X‐ray diffraction (XRD) as well as scanning electron microscopy (SEM). At 100 Hz frequency, the obtained dielectric constant was 24 times greater at 95% relative humidity (RH) as compared to 33% RH. The armalcolite‐based sensor showed lower hysteresis (<3.5%), good stability, and faster response (~18 seconds) and recovery (~35 seconds) times compared to conventional humidity sensors. The sensing mechanism of the nanocomposite was categorically determined by the analyzed characteristics parameters such as dielectric constants, normalized loss tangent, and alternating current conductivity properties. This study also confirmed that the whole conduction mechanism was accomplished by electrons or ions and dipoles in the entire RH range. Therefore, the present armalcolite‐based porous nanocomposite would be a potential sensing material for novel humidity sensors.     

Hyperbranched polycarboxylates and their nanocomposites with ZnO: Investigations on the humidity‐sensitive properties

Hyperbranched polycarboxylates (HBPC) with different alkali metal cations (Li⁺, Na⁺, and K⁺) were prepared and characterized by ¹H‐NMR and thermal gravimetric analysis. Thin film humidity sensors based on HBPC and its composite with ZnO nanorods were fabricated. The morphologies of films of HBPC and the nanocomposite were investigated by atomic force microscopy, which revealed uniform distribution of ZnO nanorods in HBPC. The humidity‐sensitive characteristics of HBPC and the nanocomposite were investigated at room temperature. It was found that the type of cations significantly affected the humidity‐sensing behaviors of HBPC. In addition, the nanocomposite exhibited better humidity‐sensitive properties than HBPC alone. Its impedance decreased for about three orders of magnitude over the range 19–97% RH, showing high sensitivity. Moreover, the nanocomposite exhibited fast response (～ 9 and 10 s for response and recovery time between 97% RH and 33% RH, respectively) and small hysteresis (～ 1.4% RH). The improved humidity‐sensing behaviors of the nanocomposite over HPBC alone is explained by taking into account the hyperbranched structure of the polymer and the special interactions of the polymer and ZnO with water molecules. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and humidity sensitive properties of pyrrole‐based polyelectrolytes

Pyrrole (Py)‐based polyelectrolytes (Py‐PE): P(Py‐COOLi), P(Py‐COONa), and P(Py‐COOK) was synthesized, characterized, and used to prepare thin film resistive humidity sensors. Their humidity sensitive properties have been investigated, and sensing mechanism was presented. The Py‐PE contains PPy as backbone and the side chain bearing carboxylic salt group, which made its sensor exhibited a very wide humidity sensing range of 0–97% relative humidity (RH), high conductivity even at very low humidity, and both ionic and electronic conduction contributed to its conductivity. Among all the Py‐PE, P(Py‐COOK) showed high sensitivity, with the impedance changing of about three orders of magnitude (10³–10⁶ Ω) from 97 to 0% RH, whereas P(Py‐COONa) showed quick response for both absorption (12.5 s) and desorption (15.2 s). Py‐PE prepared is promising for preparation of thin film resistive humidity sensors capable of detecting low humidity. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

UV‐Printable and Flexible Humidity Sensors Based on Conducting/Insulating Semi‐Interpenetrated Polymer Networks

Humidity sensors are of great interest in many fields because humidity plays a crucial role in several processes. Nevertheless, their application is often limited by the expensive fabrication and the stiffness of the substrates usually employed. In this work, novel UV‐curable and flexible humidity sensors based on semi‐interpenetrated polymer networks are fabricated. They can be prepared either as self‐standing sensors or applied on different bendable substrates. The fabrication consists of a simultaneous UV‐curing of an insulating network (acrylic or epoxy) and photopolymerization of conducting polypyrrole (PPy). The detection mechanism involves proton transfer on the PPy chains that can be macroscopically observed by electrical impedance variations. These devices show promising humidity‐sensing properties from 20 to 97% of relative humidity with a maximum response of about 180%. The dynamic sensing investigation proves that the recovery process can be tailored playing on the glass transition temperature and wettability of the films. The remarkable sensing capabilities of these sensors make them a valid alternative in many applications where printability and flexibility are required along with simple fabrication method consisting of one‐step synthesis.     

Effect of BaTiO3 on the sensing properties of PVDF composite-based capacitive humidity sensors

Capacitive humidity sensors consisting of materials such as polymers, ceramics, and piezoelectrics are widely used to monitor relative humidity levels. The effect of barium titanate (BaTiO₃) nanoparticles on the humidity sensing properties, dielectric response, thermal stability, and hydrophilicity of the polyvinylidene fluoride (PVDF)-BaTiO₃ composite films is investigated. Hydrophilicity and surface morphology of the PVDF-BaTiO₃ composite films are modified for the development of a good humidity sensor. The nanocomposite solutions are prepared by mixing an optimized concentration (2.5 wt%) of PVDF with different concentrations (0.5, 1, and 2 wt%) of BaTiO₃ nanoparticles. X-ray diffraction, thermogravimetric analysis, field emission scanning electron microscopy, and contact angle measurements are used to characterize the structure, morphology, thermal stability, and hydrophilicity of the spin-coated sensing films. The dielectric study of PVDF-BaTiO₃ composite film shows that as the concentration of BaTiO₃ particles increase, the dielectric constant of the composite films increases as well. PVDF-BaTiO₃ (2.5 wt%-1 wt%) based capacitive sensors show stable capacitive response and low hysteresis as compared to the other concentrations of the PVDF-BaTiO₃ composites. The maximum hysteresis of the capacitive PVDF-BaTiO₃ (2.5 wt%- 1 wt%) humidity sensor is found to be ~2.5%. The response and recovery times of the PVDF-BaTiO₃ (2.5 wt%-1 wt%) based capacitive sensors are determined as 40 s and 25 s, respectively, which are significantly lower than those reported for the other PVDF composite based sensors.     

Humidity Sensing Property of NaCl‐Added Mesoporous Silica Synthesized by a Facile Way with Low Energy Cost

NaCl‐added mesoporous silica SBA‐15 was prepared by a simple grind method which needs low energy cost. Humidity sensing property was studied. Study results indicated that NaCl‐added material possesses higher humidity sensitivity. Its impedance changed more than four orders of magnitude when the relative humidity changed from 11% RH to 95% RH. From the viewpoint of economy, NaCl‐added SBA‐15 material which possesses high sensitivity and needs low energy cost is more suitable to be a promising humidity sensing material. Complex impedance spectra, the corresponding equivalent circuit, and bode diagrams were carefully analyzed to explore sensing mechanism.     

基于电纺NiI2/TPU纳米纤维膜的湿敏变色传感器的制备及性能

以热塑性聚氨酯(TPU)母粒、碘化镍为原料,通过静电纺丝法制备了基于碘化镍/热塑性聚氨酯(NiI2/TPU)纳米纤维膜,将NiI2/TPU纳米纤维膜贴合在聚酰亚胺(PI)基叉指电极上制得湿度传感器.对纳米纤维膜的表面形貌及微观结构进行了表征分析,并研究了该传感器基于颜色变化和电阻电容响应的湿度敏感特性.结果表明,由于碘化镍的颜色变化特性,随相对湿度(RH)从0增加到97％,NiI2/TPU纳米纤维膜显示了从橘红色到黄绿色的颜色转变.此外,该湿度传感器表现出快速的响应/回复时间(0.9 s/9.9 s)、较宽的湿度监测区间(0～97％RH)、较小的洄滞度(0.4％RH)以及优异的稳定性能(>30 d).     

Functionalized multi‐wall carbon nanotubes/silicone rubber composite as capacitive humidity sensor

Functionalized multi‐wall carbon nanotubes (MWCNTs) treated by mixed acids have been used to develop a capacitive humidity sensor based on MWCNTs/silicone rubber (SR) composite film. The MWCNTs/SR composites were prepared through conventional solution processed method. The micrographs of MWCNTs/SR composites were observed by transmission electron microscopy (TEM) and scanning electron microscope. The FT‐IR spectra demonstrated the successfully grafting of ? OH groups on the treated MWCNTs. The sensing properties of the composite at different relative humidity (RH) and frequency were characterized and linear sensing responses of the MWCNTs/SR composites to RH were observed. The treated MWCNTs/SR composite film (Tr‐film) had higher sensitivity than that of the untreated MWCNTs/SR composite film (Un‐film). Experimental data indicate that the Tr‐film exhibits an excellent long‐term stability, small hysteresis, and fine reproducibility. The response and recovery time of the Tr‐film were 30 and 27 s, respectively. Thereby, such Tr‐film had potential applications as humidity sensors. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40342.     

Wide range humidity sensors printed on biocomposite films of cellulose nanofibril and poly(ethylene glycol)

Cellulose nanofibril (CNF) films were prepared from side streams generated by the sugarcane industry, that is, bagasse. Two fractionation processes were utilized for comparison purposes: (1) soda and (2) hot water and soda pretreatments. 2,2,6,6-Tetramethylpiperidinyl-1-oxyl-mediated oxidation was applied to facilitate the nanofibrillation of the bagasse fibers. Poly(ethylene glycol) (PEG) was chosen as plasticizer to improve the ductility of CNF films. The neat CNF and biocomposite films (CNF and 40% PEG) were used for fabrication of self-standing humidity sensors. CNF-based humidity sensors exhibited high change of impedance, within four orders of magnitude, in response to relative humidity (RH) from 20 to 90%. The use of plasticizer had an impact on sensor kinetics. While the biocomposite film sensors showed slightly longer response time, the recovery time of these plasticized sensors was two times shorter in comparison to sensors without PEG. This study demonstrated that agroindustrial side streams can form the basis for high-end applications such as humidity sensors, with potential for, for example, packaging and wound dressing applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47920.     

Effect of m‐nitroaniline doping on the optical humidity‐sensing characteristics of cobalt/poly(vinyl alcohol) nanocomposites

Cobalt (Co) nanoparticles (with different loadings, 1 and 2 wt %, of Co) were synthesized in situ in a poly(vinyl alcohol) (PVA) matrix with and without meta‐nitroaniline (m‐NA) as a dopant (2.5 wt %). The obtained nanocomposite films were characterized with various physicochemical techniques, including ultraviolet–visible spectrophotometry, X‐ray diffraction analysis, scanning electron microscopy, and Fourier transform infrared analysis. To study the effect of the humidity, the nanocomposite solutions were coated on planar glass substrates. The beam of an He–Ne laser was incident normal to the film surface and was subjected to variable relative humidities (RHs; 4–93%); the transmitted intensity was measured on a photovoltaic diode. Variations in the intensity of light caused by the changes in RH within the range 3–93% were recorded. We optimized the response by varying the film thickness by coating the solution layer by layer. We generated the RH (4–100%) by passing wet water vapors. The neat PVA film of similar thickness gave humidity sensing in the range 78–93% RH. The sensors with m‐NA‐doped Co/PVA gave better sensitivity (6.4 mV/% RH) than the undoped samples (1.78–2.45 mV/% RH), exhibiting a fast response of 3 s (2–93% RH) and a recovery of 10 s (93 to 2% RH). These samples also showed reversible behavior and long‐term stability (for nearly a year) with a good sensitivity and a large dynamic range (2–95% RH). An attempt was made to explain the results on the basis of a bulk mechanism. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

HDPE-AA-SSS预辐照接枝膜的表征及其湿敏性能的研究

采用预辐照法将亲水性单体丙烯酸(AA)和对苯乙烯磺酸钠(SSS)接枝到疏水性高密度聚乙烯(HDPE)薄膜上,制备出新型的接枝膜湿敏元件.通过扫描电镜观测了辐照接枝前和接枝后HDPE膜的表面形貌,并通过红外光谱表征了膜的结构,同时测定了接枝膜湿敏元件的湿敏性能.实验结果表明,制备的接枝膜湿敏元件具有良好的湿敏特性,响应和恢复时间短.它具有较好的稳定性,能在高湿、高温环境下使用.实验结果也表明了接枝HDPE膜具有良好的湿敏性能.     

A novel resistive‐type humidity sensor based on poly(p‐diethynylbenzene)

Poly(p‐diethynylbenzene) (PDEB) synthesized with nickel catalyst Ni(CC ○ CCH)₂(PPh₃)₂ (Ni C) in dioxane–toluene mixed‐solvent system at 25°C shows a rich trans structure with pendant‐group ( ○ CCH) content of about 35% having higher molecular weight and good solubility. A novel resistive‐type humidity sensor based on PDEB is presented. Its humi‐sensing characteristics are described and discussed. The impedance of the sensor changed from ∼ 10³–10⁷ Ω in almost the whole humidity range [∼ 15–92% relative humidity (RH)], which is low compared with sensors based on other humi‐sensitive conjugate polymers, and hysteresis of no more than 3% RH was observed. The sensor prepared by Langmuir–Blodgett (LB) deposition method shows the best humidity response. An explanation of humi‐sensing behavior of PDEB is attempted by taking into account the interaction between hydrogen protons and super π‐conjugate orbits in PDEB. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2010–2015, 1999     

Mn‐Loaded Mesoporous Silica Nanocomposite: A Highly Efficient Humidity Sensor

We report a relative humidity sensor based on manganese‐nanoparticle‐loaded mesoporous silica SBA‐15 using a facile hydrothermal route. The as‐developed nanocomposite material (Mn/SBA‐15) possesses a high surface area and a high pore volume. The obtained samples were characterized by using low‐angle X‐ray Diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR), N₂ adsorption–desorption, high‐resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and energy‐dispersive X‐ray (EDX) spectroscopy techniques. The Mn/SBA‐15 exhibited, improved humidity response and recovery time as compared to pure SBA‐15 in the 11%–92% RH range. Optimal results were obtained for the 5 wt% Mn‐loaded SBA‐15 sample, which displayed excellent linearity, low hysteresis, and high humidity response. A change in ~5 orders of magnitude in resistance was observed over 11%–92% RH range. The investigation of humidity sensing properties of Mn/SBA‐15 nanocomposite shows that this material has good prospects as humidity sensor.     

Humidity sensing properties of the vinylpyridine–butyl acrylate–styrene copolymers

In this article, the humidity sensors based on the vinylpyridine (VP)–butyl acrylate (BA)–styrene (St) copolymers are developed. The influencing factors of the copolymer's humidity sensing properties, such as the mol percentage of the fed monomers and the quaternization reagent ratio (namely, dibromobutane : butyl bromide ratio), are studied, and the long‐term stability of the copolymers is investigated as well. The results show that as the content of BA increases and the content of St decreases, the copolymer's hysteresis and response time decreases, and with the increasing of the quaternization reagent ratio, the copolymer's hysteresis and response time decreases. Also, the sensors based on the copolymers show 2–3% RH reproducibility under various long‐term test conditions. These results demonstrate an overall excellent performance in the reproducibility and long‐term stability. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1992–1996, 1999     

Humidity‐sensitive properties of new polyelectrolytes based on the copolymers containing phosphonium salt and phosphine function

A simple strategy was developed based on a new reactive function‐ and a salt‐containing new monomer, 4‐vinylbenzyl dimethyl 2‐(dimethylphosphino)ethyl phosphonium chloride (VDEPC), to obtain stable humidity‐sensitive membranes. The major ingredient of a humid membrane is crosslinked polyelectrolytes obtained from copolymers of VDEPC/2‐ethylhexyl acrylate (2‐EHA) = 1/0, 4/1, and 2/1. Isothermal humidity absorption experiments were performed for the estimation of humidity‐sensing materials. The crosslinked copolymers prepared from the reaction of VDEPC/2‐EHA = 4/1 with 1,4‐dichlorobutane showed an average impedance of 595, 39.1, and 3.9 KΩ at 30, 60, and 90% RH, respectively. Their hysteresis, temperature dependence, frequency dependence, and response time were measured. The reliability including water resistance and a long‐term stability were estimated for the application of the common humidity sensor. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1062–1070, 2003     

Selective detection of propanol vapour at low operating temperature utilizing ZnO nanostructures

We report on the propanol vapour (C₃H₈O) gas sensing characteristics of ZnO nanostructures prepared via hydrothermal assisted method. The ZnO-4h sensor showed a high response (i.e. resistance ratio), sensitivity and selectivity toward C₃H₈O gas at low operating temperature of 125 °C. A response and recovery times of approximately 190 and 200 s were recorded. The response of ZnO-4h based sensor to 40 ppm C₃H₈O was approximately 2 times higher than that of other sensing materials in dry air, while in the presence of 40% RH the response was 5 times higher. The exceptional C₃H₈O-sensing performance of ZnO-4h is related to more C₃H₈O adsorption sites provided by V_O. The ZnO-04h based sensor showed a clear repeatability towards 40 ppm C₃H₈O for four successive cycles in the presence of various RH of 40 and 60% at 125 °C. The sensor response improved in the presence of RH humidity showing that the water vapour was not competing with the C₃H₈O for the pre-adsorbed oxygen ions, thus its interfering effect in the C₃H₈O sensing was considerably minimized. The ZnO-4h based sensor was further tested for long-term stability and the sensor was very stable after 45 days. The fundamental sensing mechanism towards C₃H₈O vapour is also discussed.     

Synthesis and structural,microstructural and humidity sensing behavior of (x)rGo+(1-x)CoCr2O4 composite for humidity sensor applications

Immediate research is needed to determine why monolayer carbon atom (rGO) composites function so poorly as humidity sensors. Here, an attempt is made on (x)rGo+(1-x)CoCr₂O₄(x = 0,0.1,0.2 and 0.3) composite for humidity sensor. The method is straightforward, cheap, and basic, making it ideal for mass-producing (x)rGo+(1-x)CoCr₂O₄ composite. In this research, we investigate whether or not the (x)rGo+(1-x)CoCr₂O₄ composite can be used to improve the responsiveness of humidity sensors at ambient temperature. The use of X-ray diffraction allows for the investigation of crystallinity, phase, and structure (XRD). Crystallite size were estimated and found 9–11 nm. Morphology of the samples were seen ultrathin, wrinkled, paper-like, spherical type image. Samples were subjected to study the humidity sensing behavior. Within the range of 11%–97% RH, CCR-most rGO's impressive sensing response was 92%. The compound's stability was evaluated over the course of three months, and its response time was found to be between 32 and 36 s. To illustrate the mechanism of humidity sensing, we will use the stages of an adsorption process. For x = 0.3 concentration we observed high sensing response.     

Ag‐nanocomposite based on carboxymethylcellulose for humidity detection: Green synthesis and sensing performances

A novel highly sensitive Ag‐nanocomposite for humidity detection has been successfully prepared. Initially, cellulose isolated from Tunisian palm date petiole was converted to carboxymethyl cellulose (CMC) as biomatrix under heterogeneous conditions. The synthesized product was thoroughly characterized by means of FT‐IR spectroscopy, viscosity analysis, and high performance size exclusion chromatography multiangle laser light scattering. CMC was used as reducing and stabilizing agent to prepare CMC‐stabilized silver nanoparticles via a rapid green method. The bioreduction of silver ions under different experimental conditions, including Ag⁺ concentration and pH, was investigated. Optimal experimental conditions provided a long‐term stable colloidal suspension and well‐dispersed spherical shape Ag NPs with a size ranging from 13 to 28 nm. Ag‐nanocomposite coated quartz microbalance crystal was used as sensitive layer for humidity detection. A comparative study showed that the immobilized metallic nanostructures greatly reduced changes in visco‐elastic properties, increased surface area as well as surface local charge density of the CMC. Consequently, sensor performances were greatly enhanced: better stability even at higher relative humidity (RH), good reproducibility and linearity (11–98% RH), low hysteresis characteristics, and rapid response and recovery times (14 and 6 s, respectively) were obtained. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43686.     

Ultra-low power hydrogen sensing based on a palladium-coated nanomechanical beam resonator

Hydrogen sensing is essential to ensure safety in near-future zero-emission fuel cell powered vehicles. Here, we present a novel hydrogen sensor based on the resonant frequency change of a nanoelectromechanical clamped-clamped beam. The beam is coated with a Pd layer, which expands in the presence of H(2), therefore generating a stress build-up that causes the frequency of the device to drop. The devices are able to detect H(2) concentrations below 0.5% within 1 s of the onset of the exposure using only a few hundreds of pW of power, matching the industry requirements for H(2) safety sensors. In addition, we investigate the strongly detrimental effect that relative humidity (RH) has on the Pd responsivity to H(2), showing that the response is almost nullified at about 70% RH. As a remedy for this intrinsic limitation, we applied a mild heating current through the beam, generating a few μW of power, whereby the responsivity of the sensors is fully restored and the chemo-mechanical process is accelerated, significantly decreasing response times. The sensors are fabricated using standard processes, facilitating their eventual mass-production.