Composite Humidity Sensors Research Articles

Surface Defects of Electron Irradiation Engineering for Graphene/Polymer Composite-Based Flexible Humidity Sensors

A facile approach for surface defect engineering based on high-energy electron irradiation is proposed here to produce a graphene/polymer composite flexible humidity sensor with high humidity resistance and responsiveness. The responsiveness of the graphene/polymer composite humidity sensor following electron irradiation was ∼22.4-fold greater than the unirradiated humidity sensor. When the relative humidity was 97.3%, the responsiveness of the sensor was 1.2 × 105. In addition, the irradiated humidity sensor maintained an excellent response after 10 water resistance tests. Furthermore, material characterization techniques demonstrated that graphene/polymer composite materials with low crosslinking density could be produced by electron irradiation at the appropriate fluence. Concurrently, the specific surface area of the graphene/polymer composite material was increased, and several oxygen-containing functional groups were generated, providing more active sites for water molecules to enhance the stability and responsiveness of the sensor. Thus, the irradiated graphene/polymer composite humidity sensor has potential applications in human respiratory monitoring and noncontact switching. Moreover, the surface defect engineering of electron irradiation provides a facile strategy for obtaining graphene/polymer composite flexible humidity sensors with high comprehensive properties.

Highly Sensitive Interdigitated Capacitive Humidity Sensors Based on Sponge-Like Nanoporous PVDF/LiCl Composite for Real-Time Monitoring

In this study, a sponge-like poly(vinylidene fluoride) (PVDF)/lithium chloride (LiCl) nanocomposite-entrenched interdigitated capacitive (IDC) sensor was developed for real-time humidity-sensing applications. Here, we demonstrated a sponge-like nanoporous structure ranging from 200 nm to 2 μm size holes, the PVDF/LiCl structure fabricated on an interdigitated capacitor (IDC) electrode functioning as a high-performance sensor because of the presence of ionized LiCl. The nanoporous PVDF/LiCl composite-based humidity sensor exhibited a high sensitivity of 12.6 nF/% relative humidity (RH), a linearity of 0.990, and a low hysteresis of 2.6% in the range of 25-95% RH. The composite film exhibited a response time of 17.7 s, a recovery time of 21 s, and an intensified increase of 8.02 nF/s (a decrease of 6.7 nF/s). The sensor designed demonstrates ultra-high sensing characteristics with 10 times higher sensitivity, i.e., 12.678.96 pF/%RH as compared to other polymer-based composite humidity sensors. Owing to the sensing performance and portability, the proposed nanoporous PVDF/LiCl composite-based IDC sensor is expected to be a promising platform for a wide range of humidity-sensing applications, including real-time breath monitoring and non-contact sensing.

Study on the design of ZnO/PANI composites and the mechanism of enhanced humidity sensing properties

ZnO/PANI composite humidity sensor was prepared by hydrothermal method. The first principles of density functional theory study the sensing mechanism. The calculation shows that the oxygen vacancy on ZnO surface is beneficial to the adsorption of water molecules. The {0 0 0‾1} crystal plane with the largest lattice oxygen number in ZnO has a strong adsorption capacity for water molecules, which is also conducive to improving the humidity sensitivity. PANI is easy to be combined on {0 1‾1 0} plane of ZnO, and it indirectly promotes the growth of {0 0 0‾1} plane, increasing the adsorption of water molecules and the proportion of H+ and H3O+ ions. In addition, the N–H group in ZnO/PANI enhances the H+ conduction, which further improves the performance of the sensor. The results concluded that the proportion of lattice oxygen in humidity sensor is an important factor of humidity sensor sensitive detection.

Investigation of humidity effects on electrical properties of PTB7-Th and PCBM sensor

This paper reports the fabrication of semitransparent semisurface-type samples of an indium tin oxide (ITO)/poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b 0] dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl}: (PTB7-Th):[6,6]-Phenyl C61 butyric acid methyl ester (PCBM)/graphene composite humidity sensor. The transparency of the sensor was 58–60%. The dependences of the resistance, impedance and capacitance at 100 Hz, 1 kHz, 10 kHz, 100 kHz and 200 kHz of the ITO/PTB7-Th:PCBM/graphene composite samples on relative humidity in the range 50–93% were investigated. It was observed that as humidity increased from 50 to 93%, the resistance and impedances (at 1 kHz) of the samples decreased on average by factors of 7·48 and 58·75, respectively. Under the same experimental conditions (1 kHz), the capacitances of the samples increased by a factor of 42. As the frequency was increased from 100 Hz to 200 kHz, the impedance decreased by factors of 20 and 7 at relative humidity values of 50 and 62%, respectively. The corresponding capacitance decreased by factors of 33 and 178, respectively. The semitransparent PTB7- and -PCBM-based humidity sensors can be used for measurement of humidity, as well as a teaching aid where control of illumination or light intensity is desirable.

Graphene-PEDOT: PSS Humidity Sensors for High Sensitive, Low-Cost, Highly-Reliable, Flexible, and Printed Electronics.

A comparison of the structure and sensitivity of humidity sensors prepared from graphene (G)-PEDOT: PSS (poly (3,4-ethylenedioxythiophene)) composite material on flexible and solid substrates is performed. Upon an increase in humidity, the G: PEDOT: PSS composite films ensure a response (a linear increase in resistance versus humidity) up to 220% without restrictions typical of sensors fabricated from PEDOT: PSS. It was found that the response of the examined sensors depends not only on the composition of the layer and on its thickness but, also, on the substrate used. The capability of flexible substrates to absorb the liquid component of the ink used to print the sensors markedly alters the structure of the film, making it more porous; as a result, the response to moisture increases. However, in the case of using paper, a hysteresis of resistance occurs during an increase or decrease of humidity; that hysteresis is associated with the capability of such substrates to absorb moisture and transfer it to the sensing layer of the sensor. A study of the properties of G: PEDOT: PSS films and test device structures under deformation showed that when the G: PEDOT: PSS films or structures are bent to a bending radius of 3 mm (1.5% strain), the properties of those films and structures remain unchanged. This result makes the composite humidity sensors based on G: PEDOT: PSS films promising devices for use in flexible and printed electronics.

The preparation of graphene/polyethylene oxide/sodium dodecyl sulfate composite humidity sensor via electrohydrodynamic direct-writing

Continuous accurate deposition of composites can be achieved by electrohydrodynamic direct-writing, but the eddy flow with the Weissenberg effect may affect stable jet, which is important in electrohydrodynamic direct-writing. So electrohydrodynamic direct-writing of micro-scale graphene/polyethylene oxide/sodium dodecyl sulfate composite (the pumping and preparation of graphene composites based on the Weissenberg effect) is studied for the first time. This paper explores the effects on the morphology of graphene composite films such as voltage and collection distance. In addition, the composite is firstly used in a resistance-based humidity sensor. It is found that the composite films are very sensitive to humidity when its width is less than 200 µm due to its porous structure and high specific surface area. When the width of the composite films is about 100 µm, its resistance decreases rapidly as the humidity increases from 63% RH to 100% RH (the response time is less than 2.5 s).

Effective enhancement on humidity sensing characteristics of sulfonated poly(ether ether ketone) via incorporating a novel bifunctional metal–organic–framework

Recently, sulfonated poly(ether ether ketone) (SPEEK) has been used as the humidity sensing material due to its simple processing, environmental stability, and large humidity responding range. However, intrinsic shortcomings such as large hysteresis and lags in response restrict its practical application. Thus, a novel bifunctional metal–organic–framework, MNS (abbreviation for MIL-101-NH2-SO3H) was prepared as an inorganic nano-filler to improve sensing properties of humidity sensor based on SPEEK, especially small hysteresis. Humidity sensitive properties of sensors were investigated by measuring the complex impedance spectra of sensors at different relative humidity (RH). The organic-inorganic humidity sensor based on the composite material (SPEEK/MNS-30%) showed the best sensing properties, such as quick response (absorption: 9s) and small hysteresis (<3% RH, almost eighth of SPEEK sensor). Besides, the mechanism of the enhancement caused by MNS was proposed and proved by investigating the surface structure and microstructure of humidity sensing film via scanning electron microscopy and transmission electron microscopy, respectively. In addition, the equivalent circuit was built up by complex impedance spectra to study the sensing behavior of the composite humidity sensor. The good reproducibility and stability of the humidity sensor fabricated by SPEEK/MNS-30% indicated that it had the potential to be applied for practical humidity detection.

Material-integrated composite humidity sensors for condition monitoring of fiber-reinforced plastics

Penetrating water in fiber-reinforced plastics can alter the mechanical properties considerably. To avoid potential resulting failure of the component, we propose continuous monitoring of the humidity inside the material by highly-sensitive humidity sensors based on nano- or microcomposites. Here we report on the inline-capable fabrication and integration of humidity sensors in glass fiber-reinforced polyamide (GF-PA6). Mean water concentrations of less than 0.5 wt. % have been clearly determined inside the laminate.

Modified graphene oxide/Nafion composite humidity sensor and its linear response to the relative humidity

Graphene oxide (GO) and its derivatives have been applied to the humidity sensing area in recent years due to its abundant hydrophilic functional groups, and proved to be ultrasensitive to the humidity changing. But the stability of the sensors is always an obstacle to their application. Besides, the long-term linearity is seldom studied. In this work, diamine modified GO (MGO) has been synthesized and mixed with Nafion polymer to form a hybrid film, and applied to the humidity sensors. We have applied two measurement methods by either fitting of the impedance spectra to extract charge transfer resistance (Rct) or measurement of the impedance under a certain frequency, to evaluate the response to the relative humidity (RH). Both MGO and its composite show good sensitivity to the RH under the two testing methods The Rct shows good linearity at the full testing RH range even after two months, while the impedance under a certain frequency can’t keep the linear response to the RH after certain period of time. The addition of Nafion decreases the Rct and increases the sensitivity of MGO based sensor. Linearity of the sensor is improved by the combination of MGO and Nafion. Long-term testing results show the linearity of the composite sensors changes over time but always keeps a better linearity than the pure MGO sensor.

Effect of V2O5 doping on p- to n-conduction type transition of TiO2:WO3 composite humidity sensors

Actually, humidity sensors have got multiplying applications from the industrial processing to the environmental control, mainly those based on semiconducting metal oxides. Certain ceramic oxides or composite oxides are wide-bandgap semiconductors and the overall conduction mechanisms are highly influenced by the sensing material physical/chemical properties. For the humidity measurement (relative humidity (RH)), the electronic/ionic charge transfer reactions that take place at the semiconductor surface can be used for this purpose.It is well recognized that water molecules are observed to increase the conductivity of n-type ceramics and to decrease the conductivity of p-type ceramics. Nevertheless several mechanisms have been proposed to explain electrical response variations of oxide metals of with humidity.The present paper describes part of the work that is being carried out to investigate the effect of doping metals oxides on TiO2:WO3 bulk sensors humidity response.For that the V2O5 ceramic was selected. The influence of different contents, 3, 5 and 7 percentage in weight, were investigated concerning microstructural characterization and their electrical response was measured in the range 400Hz–40MHz, at operating temperatures of 20 and 30°C and on the relative humidity (RH) range between 10 and 100%. For all doped sensors a p- to n-conduction transition was found, and the best humidity response was observed for the sensor containing 3 percentage in weight of oxide doping. Besides a new non-classical model for the doped sensors electrical behaviour with moisture is depicted, which allow to better comprehend the diverse phenomena present in our sensors contributing to the overall electrical response.

Bilayer-structured composite sensor based on polyaniline and polyelectrolyte for sensitive detection of low humidity

A bilayer-structured composite humidity sensor based on quaternized and crosslinked poly(4-vinylpyridine) (QC-P4VP) and polyaniline (PANI) was fabricated by depositing thin films of QC-P4VP and PANI onto interdigitated gold electrode in sequence. The composite was characterized by Fourier transform infrared spectroscopy, Ultraviolet–visible spectroscopy, scanning electron microscopy, and atomic force microscopy. The composite sensor could sensitively detect very low humidity (down to ∼1%RH) (impedance increase of ∼860% from 15% to 1%RH). Furthermore, the sensor exhibited relatively fast response (t90% of 24s and 35s for adsorption and desorption processes, respectively), small hysteresis (∼3%RH) and good repeatability. In addition, the composite sensor revealed impedance change close to 103 from 1% to 98%RH, suggesting its capability of detecting full-range humidity with high sensitivity. The effect of the concentration of poly(4-vinylpyridine) and PANI, deposition sequence of the sensitive layers on the humidity sensing characteristics of the composite has been examined. The humidity sensing mechanism of the composite sensor was proposed by considering the electrical properties of QC-P4VP and PANI at different humidity levels and the special bilayer structure.

TiO2:WO3 composite humidity sensors doped with ZnO and CuO investigated by impedance spectroscopy

For impedance-type sensors based on semiconducting metal oxides, the overall conduction mechanisms strongly influence the magnitude and the direction of the sensor signal variation. For humidity, in particular, the electronic/ionic charge-transfer reactions that take place at the semiconductor surface can be used to monitor and control it. In recent years, various mechanisms have been proposed to explain the variations of electrical response to humidity. With the study of composite materials, we expect to obtain a better sensitivity of these sensors, when compared with the ones made out of only one metal oxide. This could be due to the fact that some of the positions initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if a single covalent/ionic adsorption is decisive in the observed changes in the material's conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity. In this paper, TiO2:WO3 composite oxide bulk sensors, using 48.92 and 51.08% (w/w) of titanium and of tungsten atoms, respectively, doped with the same proportions of cooper and zinc oxides (7%), were prepared by a conventional sintering method, and their dependence of their complex impedance spectra, measured in the range 100Hz–10MHz, on the relative humidity (RH), operating temperature and on the measuring frequency is shown and explained.

Facilely prepared composites of polyelectrolytes and graphene as the sensing materials for the detection of very low humidity

Two polyelectrolytes, i.e., cationic poly(diallydimethylammonium chloride) (PDDA) and anionic sodium poly(4-styrenesulfonate) (PSSNa), were separately mixed with graphene oxide (GO) and hydrazine in solution, and heat-treated to result in composites of PDDA/graphene (GN) and PSSNa/GN, respectively. The composites were characterized by scanning electron microscopy, transmittance electron microscopy and Raman spectroscopy. Impedance-type humidity sensors were constructed by depositing the composite films onto interdigitated gold electrodes via dip-coating. The electrical response of the composite humidity sensors toward relative humidity (RH) was investigated at room temperature. It was found that the composites with higher content of GN exhibited lower impedance at low humidity levels. Both composite sensors could detect very low humidity (down to 0.2%RH), and showed good response magnitude in the low humidity range (relative impedance change of 300% and 1000% from 0.2 to 30%RH for PSSNa/GN and PDDA/GN, respectively). Moreover, the composites exhibited slight hysteresis similar to that of the polyelectrolytes alone, while their response time for desorption was even reduced. The sensing mechanism of the composites was explored by complex impedance spectra analysis, and the good sensing behavior at low humidity was proposed to the special electrical property and unique two-dimensional nanosheet structure of GN.

Structure Analysis of Humidity Sensing Composite of BaTiO3 and Polymer QAR

The sensitivity of humidity sensing composite material made of nanometer BaTiO3 and polymer QAR is higher than that of either BaTiO3 or QAR sensors. The XRD and XPS spectra were used to analyze the structure of the materials. The positions and strengthes of the binding energies of O, Ba, Ti, C, N and Cl in the composite change compared with those in BaTiO3, which may be part of the reasons of the high sensitivity of the composite humidity sensor.

Mechanism analysis of BaTiO 3 and polymer QAR composite humidity sensor

The humidity sensing mechanism of the composite material of nanometer barium titanate and polymer quaternary acrylic resin (QAR) is discussed in terms of d.c. property, dielectric loss and thermally stimulated depolarization current (TSDC). Various conducting carriers in different humidity ranges are identified by the direct current curves. The dielectric loss may be caused by the polarizations of dipole orientation and interface charge. The TSDC patterns show that the active energy and relaxation time of the dipoles in the composite materials take continuous, rather than discrete, values.

Zinc(II) Oxide-Yttrium(III) Oxide Composite Humidity Sensor

A dc mode compatible ZnO–Y2O3 composite humidity sensor operating at room temperature was fabricated by the conventional solid state route. The base matrix was doped with Li+ to enhance the sensitivity. The activation energy for dc conductance was found to be 0.75 eV. Thermoelectric power measurements revealed the nature of the materials to be of n-type. Resistance measurements were made on the composites at different relative humidities, generated by using different water buffers. The firing temperature was varied and the ZnO–Y2O3 composite sintered at 1273 K showed maximum sensitivity, which is attributed to an increase in porosity.