Resistive Humidity Sensor Research Articles

Uncertainly Analysis of Two Types of Humidity Sensors by a Humidity Generator with a Divided-Flow System.

Humidity measurement is an important technique for the agricultural, foods, pharmaceuticals, and chemical industries. For the sake of convenience, electrical relative humidity (RH) sensors have been widely used. These sensors need to be calibrated to ensure their accuracy and the uncertainty measurement of these sensors has become a major concern. In this study, a self-made divided-flow generator was established to calibrate two types of electrical humidity sensors. The standard reference humidity was calculated from dew-point temperature and air dry-bulb temperature measured by a chilled mirror monitor. This divided-flow generator could produce consistent result of RH measurement results. The uncertainty of the reference standard increased with the increase of RH values. The combined uncertainty with the adequate calibration equations were ranged from 0.82% to 1.45% RH for resistive humidity sensors and 0.63% to 1.4% for capacitive humidity sensors, respectively. This self-made, divided-flow generator, and calibration method are cheap, time-saving, and easy to be used. Thus, the proposed approach can easily be applied in research laboratories.

Microstructure, electrical and humidity sensing properties of TiO2/polyaniline nanocomposite films prepared by sol–gel spin coating technique

High sensitive resistive type humidity sensor based titanium oxide/polyaniline (TiO2/PANI) nanocomposite thin films prepared by a sol–gel spin coating technique on an alumina substrate. The resultant nanocomposites were characterized by using X-ray diffraction (XRD), Field emission electron microscopy, Fourier transform infrared spectroscopy (FTIR), UV–Vis absorbance and energy dispersive spectra analysis. In the XRD patterns of both pure and TiO2/PANI composite confirms the deposition of PANI on TiO2 and the average size of the composite particle was found to be 32 nm. Large number of nano grain surface being covered by PANI, which agrees very well with the results obtained by XRD studies. FTIR and UV–Vis spectra reveal that the PANI component undergoes an electronic structure modification as a result of the TiO2 and PANI interaction. The room temperature resistivity was found to be for TiO2 and TiO2/PANI nanocomposite films 1.42 × 106 and 2.56 × 103 Ω cm respectively. The obtained TiO2/PANI nanocomposites sensor exhibited higher humidity sensing performance such as high sensitivity, fast response (20 s) and recovery time (15 s) and high stability.

Pt decorated MoS2 nanoflakes for ultrasensitive resistive humidity sensor

In this work, we report the fabrication of a low power, humidity sensor where platinum nanoparticles (NPs) decorated few-layered molybdenum disulphide (MoS2) nanoflakes have been used as the sensing layer. A mixed solvent was used to exfoliate the nanoflakes from the bulk powder. Then the Pt/MoS2 composites were prepared by reducing Pt NPs from chloroplatinic acid hexahydrate using a novel reduction technique using sulphide salt. The successful reduction and composite preparation were confirmed using various material characterization tools like scanning electron microscopy, atomic force microscopy, transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy and UV–visible spectroscopy. The humidity sensors were prepared by drop-coating the Pt-decorated MoS2 on gold interdigitated electrodes and then exposed to various levels of relative humidity (RH). Composites with different weight ratios of Pt were tested and the best response was shown by the Pt/MoS2 (0.25:1) sample with a record high response of ∼4000 times at 85% RH. The response and recovery times were ∼92 s and ∼154 s respectively with repeatable behaviour. The sensor performance was found to be stable when tested over a few months. The underlying sensing mechanisms along with detailed characterization of the various composites have been discussed.

Non-covalent functionalization of CNTs with polycarbazole: a chemiresistive humidity sensor with tunable chemo-electric attributes at room temperature

This article describes the non-covalent functionalization of multiwall carbon nanotubes (MWCNTs) with polycarbazole (PCz) via an in situ chemical oxidative polymerization method and subsequent fabrication of resistive humidity sensors.

Self‐Assembly of Functionalized Oligothiophene into Hygroscopic Fibers: Fabrication of Highly Sensitive and Fast Humidity Sensors

AbstractA new symmetric oligothiophene exposing tetraethylene glycol (TEG)‐based side‐chains is designed and synthesized. This molecule is found to self‐assemble in solution forming supramolecular fibers, via π–π stacking between the conjugated oligothiophene backbones, which are phase segregated on the sub‐nanometer scale from the TEG side‐groups. The delocalization of the charges through the oligothiophene π–π stack ensures efficient charge transport while the hygroscopic shell, decorating the surface of the fibrillar structures, determines a certain affinity for polar molecules. Upon exposure to humidity, under environmental conditions, such supramolecular architectures are capable of reversibly absorbing and desorbing water molecules. Absorption of water molecules, due to increased environmental humidity, causes a fast and reproducible increase of the electrical current through the fibers by a factor 100 from 15% to 90% relative humidity, as measured in 2‐terminal devices. Such process is extremely fast, taking place in less than 45 ms. The humidity‐responsive characteristics of the presented oligothiophene‐based fibers can be exploited for the facile fabrication of high‐performances and solution‐processable electrical resistive humidity sensors.

Sequential Elemental Dealloying Approach for the Fabrication of Porous Metal Oxides and Chemiresistive Sensors Thereof for Electronic Listening.

Highly porous materials, with large surface area and accessible space, variable chemical compositions, and porosity at different length scales, have captivated the attention of researchers in recent years as an important family of functional materials. Here, we report a novel approach to grow porous metal oxides (PMOs) by sequential elemental dealloying in which a highly mobile element gets dealloyed first under the thermal treatment (annealing) and facilitates the formation of PMOs. Subsequently, a chemiresistive sensor based on porous SnO2 was fabricated for humidity sensing at room temperature which shows a high sensitivity of 348 in a fully humid [>99% relative humidity (RH)] atmosphere with an accuracy of 1% RH change. In addition, the sensor is highly durable and reproducible. Eventually, the chemiresistive sensor has been exploited for electronic listening toward speaking, whistling, and breath monitoring. Overall, the results advocate the fabrication of PMOs and the development of resistive humidity sensors for electronic listening as well as for biomedical applications.

Flexible room-temperature resistive humidity sensor based on silver nanoparticles

In this work, a low-cost and flexible room-temperature humidity sensor was developed from pure resistive silver nanoparticles (AgNPs) synthesized through a simple green route using Pistia stratiotes extract as a reducing agent for AgNO3 under light illuminations. UV–Vis spectroscopic results showed that various synthesis parameters including AgNO3 concentration, reaction time, pH value and light irradiation strongly affected on the formation of AgNPs. AgNPs were also confirmed to exhibit spherical shapes with different sizes depending on pH by transmission electron microscopy. To fabricate the sensor, AgNPs were deposited on a transparent polyethylene substrate with pre-patterned Ag interdigitated electrodes via a drop coating method. From humidity-sensing results, the flexible pure AgNPs sensor exhibited high sensitivity to relative humidity (RH) with high repeatability and stability at room temperature. Moreover, the sensor electrical resistance and sensor response showed linear relationships to RH in the range of 20–85% with short response and recovery times of 10 s and 11 s, respectively.

Self-Powered Multi-Port UHF RFID Tag-Based-Sensor

In this paper, multi-port UHF RFID tag-based sensor for wireless identification and sensing applications is presented. Two RFID chips, one with attached sensor and the other without, are incorporated in a single tag antenna with two excitation ports. The chip with the integrated sensor (sensor port) transmits a signal impacted by the sensed temperature or humidity, while the other RFID chip serves as the reference signal (reference port) transmitter in the sensing process. The proposed tag-based sensor is fabricated and experimentally evaluated. The measured results demonstrate that the sensed data can be extracted using a commercial RFID reader by recording and comparing the difference in the reader output power required to power up the reference port and the power required to power the sensor ports. To improve the reading range of the proposed sensor, a dual-port solar powered RFID sensor is also presented. The reading range of the sensor is increased by two times compared to a similar prototype without solar energy harvesting. The experimental evaluation demonstrates that the proposed tag-based sensor can be easily integrated with a resistive humidity or temperature sensor for a low-cost solution to detect the heat or humidity exposure of sensitive items for several applications such as supply chains and construction structures.

Fabrication and characterization of NiPcTs organic semiconductors based surface type capacitive–resistive humidity sensors

This work describes the fabrication of surface type capacitive–resistive humidity sensors based on nickel(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (NiPcTs) organic semiconductors as an active materials. The surface morphology of the NiPcTs thin film deposited on indium-tin-oxide (ITO) glass substrate at room temperatures is studied using field emission scanning electron micrographs (FE-SEM), shows that the film has high porosity and there is no particular crystal orientations in the NiPcTs films exist. A change in capacitance and electrical resistance of the fabricated devices are observed with the increase of relative humidity, for different applied frequencies. It is observed that the capacitance increases by 7.8 times with the increase in relative humidity (RH) from 60 to 95%, whereas the resistance is changed 22.8 times with the increase in relative humidity (RH) from 5 to 40%. The small values of hysteresis have been attained over a wide range of relative humidity (5–95%), indicating a good response/recovery of this device.

Novel high-performance self-powered humidity detection enabled by triboelectric effect

In this work, a fundamental new approach for humidity detection was proposed. By coupling triboelectric nanogenerator (TENG) with resistive humidity sensor, the degree of ambient moisture can be actively detected at room temperature, where the voltage drops across the interdigital electrodes (IDEs) has a reversely proportional relationship with relative humidity. The prepared self-powered triboelectric humidity sensor (THS) holds high humidity response of ∼7, fast response/recovery time of 2.8s and 3.5s, stable repeatability and excellent long term stability. With respect to selectivity, the response of THS to gaseous water was at least 2-fold higher than those for other gases. Furthermore, through a signal processing circuit, a self-powered humidity alert system was developed for real-time monitoring the ambient humidity. Finally, the underlying working principle of the proposed sensor was discussed in detail. This research not only provides a simple, low-cost and efficient means in humidity sensing but also greatly broadens the applicability of TENGs as self-powered active sensors.

Nano-enabled paper humidity sensor for mobile based point-of-care lung function monitoring

The frequency of breathing and peak flow rate of exhaled air are necessary parameters to detect chronic obstructive pulmonary diseases (COPDs) such as asthma, bronchitis, or pneumonia. We developed a lung function monitoring point-of-care-testing device (LFM-POCT) consisting of mouthpiece, paper-based humidity sensor, micro-heater, and real-time monitoring unit. Fabrication of a mouthpiece of optimal length ensured that the exhaled air was focused on the humidity-sensor. The resistive relative humidity sensor was developed using a filter paper coated with nanoparticles, which could easily follow the frequency and peak flow rate of the human breathing. Adsorption followed by condensation of the water molecules of the humid air on the paper-sensor during the forced exhalation reduced the electrical resistance of the sensor, which was converted to an electrical signal for sensing. A micro-heater composed of a copper-coil embedded in a polymer matrix helped in maintaining an optimal temperature on the sensor surface. Thus, water condensed on the sensor surface only during forcible breathing and the sensor recovered rapidly after the exhalation was complete by rapid desorption of water molecules from the sensor surface. Two types of real-time monitoring units were integrated into the device based on light emitting diodes (LEDs) and smart phones. The LED based unit displayed the diseased, critical, and fit conditions of the lungs by flashing LEDs of different colors. In comparison, for the mobile based monitoring unit, an application was developed employing an open source software, which established a wireless connectivity with the LFM-POCT device to perform the tests.

A water durable resistive humidity sensor based on rigid sulfonated polybenzimidazole and their properties

A water-durable, polymeric resistive humidity sensor was designed using rigid sulfonated polybenzimidazole (SPBI) with tetramethylammonium (Me4N+) as the counter cation. The Me4N–SPBI structure based on the imidazole group was used to obtain a cross-linkable moiety to attach the humidity-sensitive polyelectrolyte film to an alumina substrate pretreated with an aminosilane coupling agent using an epoxy cross-linker. The SPBI in Me4N+ structure is derived from 3,3′-diaminobenzidine (DAB), sodium 6,6′-oxybis(3-carboxybenzenesulfonate) (SOCBS) and 4,4′-oxybisbenzoic acid (OBBA), followed by quaternization, which allows for easy fabrication, higher sensitivity and better adhesion between the film and the substrate. The effect that cross-linking and anchoring the Me4N–SPBI films had on the humidity-sensing properties and water durability of sensors were investigated. The SPBI films with Me4N–DAB/SOCBS/OBBA=100/100/0 and 100/70/30 exhibited a linear response with a resistivity of 6.11–1.19kΩ and 7.60MΩ–2.51kΩ between 20 and 95% RH, respectively. Consequently, cross-linking and anchoring the film to the substrates with a coupling reagent significantly improves the water durability and stability at a high humidity and high temperature.

Electrometric monitoring of water adsorption by porous glass modified with copper(II) oxide

Modification of porous glass with copper(II) oxide with planar structure does not change the run of the water-adsorption isotherm, whereas in the case when nanoparticles are formed from the supported oxide, a cluster-type adsorption mechanism is operative when a relative pressure p/p 0 > 0.5 is reached. These specific features are reflected in the different dependences of the electrical resistance of the samples on humidity. It was shown that a porous glass with distributed copper(II)oxide planar layer can be used as a sensitive element of a resistive humidity sensor.

Effect of thickness on humidity sensing properties of RF magnetron sputtered CaCu3Ti4O12 thin films on alumina substrate

Resistive type relative humidity (RH) sensors, which were based on CaCu3 Ti4O12 (CCTO) nanostructure, were deposited on interdigitated silver electrode-patterned alumina substrates at temperatures 300 °C by radio frequency magnetron sputtering. The CCTO thin films thickness effect on morphological, structural, and humidity sensing properties were investigated by X-ray diffraction, field emission scanning electron microscopy-energy dispersive spectroscopy (EDS), and $I$ – $V$ characterization. The grain size increased from 32 to 37 nm with increasing film thickness. Intergranular porosity and microstructure of the CCTO thin film were observed by FESEM. Film thicknesses of 200, 400, and 600 nm were observed from the cross-sectional image of samples. The EDS was used to estimate chemical composition of the films. Hygroscopicity of Ag–CCTO/Al2O3 sensor was studied to observe the relationships between the RH and the resistance recorded in different thicknesses. The measurement of the resistance of the CCTO thin films was done under various RHs (% RH) 30%–90% at room temperature by using a direct current analysis method. The humidity sensing behavior was strongly dependent on % RH and applied voltage. A good humidity response, negligible humidity hysteresis, high stability, and response/recovery time were obtained. Response times were 10, 24, and 30 s while the recovery times were 450, 510, and 630 s for 200-, 400-, and 600-nm CCTO thin films, respectively. The results illustrated that CCTO thin film with 200 nm thickness had the best humidity sensing property. Therefore, CCTO thin film could be described as a promising humidity sensor with wide range of application.

Evaluation of DC Resistive Humidity Sensors Based on Conductive Carbon Ink

Evaluation of DC Resistive Humidity Sensors Based on Conductive Carbon Ink

Giant Humidity Response Using a Chitosan-Based Protonic Conductive Sensor

As a natural biopolymer, chitosan has a very unique protonic conductivity that is highly sensitive to water vapor. However, there has been no report on pure chitosan-based resistive humidity sensors, probably due to the difficulty of direct-current measurement of protonic current in chitosan. Here, a simple and cost-effective chitosan-based protonic conductive humidity sensor is presented. The sensor exhibits a large conductance response of four orders of magnitude when relative humidity is increased from 10% up to 90%. The effects of adsorbed water on proton density and dielectric constant change of chitosan could be responsible for the giant humidity response. More importantly, the sensor shows high stability, repeatability, recoverability, and selectivity to water vapor over ethanol, acetone, and toluene vapors.

Uniformly Porous Nanocrystalline CaMgFe1.33Ti₃O12 Ceramic Derived Electro-Ceramic Nanocomposite for Impedance Type Humidity Sensor.

Since humidity sensors have been widely used in many sectors, a suitable humidity sensing material with improved sensitivity, faster response and recovery times, better stability and low hysteresis is necessary to be developed. Here, we fabricate a uniformly porous humidity sensor using Ca, Ti substituted Mg ferrites with chemical formula of CaMgFe1.33Ti3O12 as humidity sensing materials by solid-sate step-sintering technique. This synthesis technique is useful to control the grain size with increased porosity to enhance the hydrophilic characteristics of the CaMgFe1.33Ti3O12 nanoceramic based sintered electro-ceramic nanocomposites. The highest porosity, lowest density and excellent surface-hydrophilicity properties were obtained at 1050 °C sintered ceramic. The performance of this impedance type humidity sensor was evaluated by electrical characterizations using alternating current (AC) in the 33%–95% relative humidity (RH) range at 25 °C. Compared with existing conventional resistive humidity sensors, the present sintered electro-ceramic nanocomposite based humidity sensor showed faster response time (20 s) and recovery time (40 s). This newly developed sensor showed extremely high sensitivity (%S) and small hysteresis of <3.4%. Long-term stability of the sensor had been determined by testing for 30 consecutive days. Therefore, the high performance sensing behavior of the present electro-ceramic nanocomposites would be suitable for a potential use in advanced humidity sensors.

High performance humidity sensor based on metal organic framework MIL-101(Cr) nanoparticles

Metal organic framework MIL-101(Cr) nanoparticles were successfully HF- free synthesized via hydrothermal method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Nitrogen adsorption-desorption technology. Then resistive humidity sensor was fabricated to investigate humidity sensing properties. The results indicate that the sensor shows high sensitivity and rapid response-recovery time. The impedance changes more than three orders of magnitude in the range of 33–95% RH at 100 Hz. The response time and recovery time are 17 s and 90 s respectively. Finally, the humidity sensing mechanism was discussed through complex impedance analysis. The results illustrate that porous MIL-101(Cr) nanoparticles have great potential to be used as humidity sensing materials.

Synthesis and Fabrication of Effective In-Situ Zinc Oxide-Graphene Nanocomposite for Swift Response Resistive Humidity Sensor

Synthesis and Fabrication of Effective <i>In-Situ</i> Zinc Oxide-Graphene Nanocomposite for Swift Response Resistive Humidity Sensor

Effective Enhancement of Humidity Sensing Characteristics of Novel Thermally Treated MWCNTs/Polyvinylpyrrolidone Film Caused by Interfacial Effect

Multi‐walled carbon nanotubes (MWCNTs)/hydrophilic poly(vinylpyrrolidone) (PVP) films are prepared by a simple spin‐coating method and the resistive humidity sensing properties of the films are studied in detail. It is demonstrated that the thermally treated MWCNTs/PVP film exhibits a reproducible and great response over a wide relative humidity range from 11% to 94%, fast response time (less than 15 s) and ultrafast recovery time (less than 1.8 s), which are superior to that of pristine PVP films. The effective enhancement of humidity sensing characteristics of MWCNTs/PVP films can be explained by the MWCNTs/PVP interfacial effect. The thermally treated MWCNTs/PVP films can be used as robust, cost‐effective, and easy‐to‐use resistive humidity sensors. And the design concept used in this work can be extended to other carbon composites to fabricate excellent sensing materials for sensors.