Humidity-sensitive Characteristics Research Articles

Humidity sensor based on LiCl-doped ZnO electrospun nanofibers

The humidity sensitive characteristics of sensors fabricated from pure ZnO nanofibers and LiCl-doped ZnO composite fibers by screen-printing on ceramic substrates with carbon interdigital electrodes have been investigated. The best result is obtained for 1.2 wt% LiCl-doped sample, which exhibits high humidity sensitivity, rapid response and recovery, small hysteresis, excellent linearity, and good reproducibility. The impedance of the sensor varies more than four orders of magnitude during the whole relative humidity (RH) from 11 to 95%. The response time and recovery time of the sensor is about 3 and 6 s, respectively. These results make our product a good candidate in fabricating high performance humidity sensors.

Humidity-sensitive properties and conductance mechanisms of SnO 2–K 2O–LiZnVO 4

The SnO 2–K 2O–LiZnVO 4 materials were prepared using the coprecipitation process. We examined the effects of liquid-phase doping of potassium (K +) on the humidity-sensitive characteristics of the nano-materials. Direct current (DC) measurements were used to investigate the mechanisms of electrical conductance of the SnO 2–K 2O–LiZnVO 4 materials. The results demonstrated that by appropriately doping of K + ions, a humidity-sensitive material with low resistance and moderate sensitivity could be obtained. The analysis on DC characteristics revealed a mixed electronic–ionic mechanism of electrical conductance. It was suggested that the liquid phase potassium doping at the [K] concentration of 2.5 mol% could improve the humidity-sensing performance of the nano-materials.

Properties of humidity sensing ZnO nanorods-base sensor fabricated by screen-printing

The humidity sensitive characteristics of a sensor fabricated from flower-like ZnO nanorods by screen-printing on a ceramic substrate with Ag–Pd interdigital electrodes have been investigated. The sensor shows high humidity sensitivity, rapid response and recovery, small hysteresis, and good stability. It is found that the impedance of the sensor decreases by about five orders of magnitude with increasing relative humidity (RH) from 11 to 95%. The response and recovery time of the sensor is about 5 and 10 s, respectively. These results indicate that the flower-like ZnO nanorods can be used in fabricating high-performance humidity sensors.

Humidity sensors fabricated with photo-curable electrolyte inks using an ink-jet printing technique and their properties

Abstract Photo-curable humidity-sensitive electrolyte inks were prepared by dissolving the following in methanol/2-methoxyethanol: [2-[(methacryloyloxy)ethyl]dimethyl]propyl ammonium bromide (MEPAB), 1,5-[bis(2-methacryloxyethyl)dimethylammono]pentane dibromide (MDAPD), a multifunctional reactive oligomer (SP1013), trimethylolpropane triacrylate (TMPTA), and photo-initiators. The inks were composed of mixtures with the following contents: MEPAB/MDAPD/SP1013/TMPTA = 6/24/17.2/12.8, 10/20/17.2/12.8, 15/15/17.2/12.8, 20/10/17.2/12.8, and 24/6/17.2/12.8. The humidity-sensitive membranes were fabricated on an electrode using a wide format drop-on-demand ink-jet printer equipped with a UV lamp system, and the printed inks were UV-cured simultaneously. The humidity-sensing characteristics including water durability and high humidity–temperature cycle were measured and estimated. The humidity sensors obtained using the ink-jet print technique showed similar humidity-sensitive characteristics within ± 1%RH.

A highly water-resistive humidity sensor based on silicon-containing polyelectrolytes prepared by one-pot method

3-Aminopropyltriethoxysilane (APTS) was quaternized with n-butyl bromide (BB) followed by hydrolysis via a simple one-pot method to obtain an electrolyte with crosslinkable silicol groups. This sample was dip-coated on interdigitated gold electrodes and formed a crosslinking structure by heat treatment to prepare a thin film resistive humidity sensor. It was found that the sensor showed an impedance change from ∼10 7 to 10 3 Ω in the humidity range of 11–97% RH, exhibiting a high sensitivity and moderate linearity on a semi-logarithmic scale. In addition, the impedance of the sensor changed little even after being soaked in water for 2 h, indicating a high water-resistance and good stability. This is proposed to result from the formation of a crosslinking structure and better adhesion with the electrode substrate. The sensor exhibited a highly reversible response characterized by a very small hysteresis of ∼1% RH. The response time and the effect of concentration of dip-coating solution on the humidity sensitive characteristics were investigated.

Diffusion of I Group Metals in Porous Silicon

Porous silicon (PS) layers with porosity of 60% on n-type (111) Si substrates were prepared by anodic etching under the white illumination. Metal (Cu, Ag or Au)/PS/Si and metal (Cu, Ag or Au)/Si structures have been fabricated by evaporation of thin metal film onto the PS or Si surface, respectively. The diffusion annealing of structures was carried out in air at 100-250oC. Examination of Cu, Ag and Au concentration distribution in PS layer and monocrystalline Si substrate was performed by successive removal of thin layers from sample and measuring the energy dispersive X-ray fluorescence (EDXRF) intensity of CuKα1, AgKα1 and AuLβ1 peak. The effective diffusion coefficients for investigated metals along PS surfaces decrease in series Cu, Ag and Au and temperature dependences are described as D(Cu)=7.8 exp(-0.62eV/kT), D(Ag)=4.2x10 exp(-0.72eV/kT) and D(Au)=1.2x102 exp(-0.81eV/kT). Diffusion coefficients of Cu, Ag and Au along PS surfaces are larger (by a factor of 104-105) than those into monocrystalline Si. The diffusion mechanism of Cu, Ag and Au along PS surfaces is discussed and data on influence of diffusion of these metals on humidity-sensitive characteristics of metal(Cu, Ag or Au)/PS Schottky type gas sensors is also presented. Diffusion of metals of I group in PS is accompanied by increase of humidity-sensitivity of metal/PS structures by a factor of 1.2-1.4.

Humidity sensors fabricated with polyelectrolyte membrane using an ink-jet printing technique and their electrical properties

Humidity-sensitive membranes made of ionene oligomer have been fabricated on the electrode substrate using a commercial drop-on-demand ink-jet printing system using a 15 μm diameter nozzle driven by a piezoelectric device. After deposition and evaporation of the solvent, the humidity sensors have been cross-linked by heating. Their humidity-sensitive properties have been determined by using a thermostatic humidity chamber. Electrical measurements under various relative humidity were performed. The humidity-sensitive characteristics of sensors obtained by ink-jet printing technique were compared with that of dip-coating method. Humidity sensors showed a decrease in impedance as an increase of relative humidity and their impedance characteristics are in a close agreement.

Humidity-induced magnetization and magnetic pole inversion in a cyano-bridged metal assembly

In general, magnetic properties of bulk magnetic materials are independent of the humidity of the environment. To obtain a magnetic material that has humidity-sensitive characteristics, water vapour must penetrate the lattice and act on spin sites. Nanoporous materials composed of metal-assembled complexes may be expected to display some humidity response because materials in this category can show functionalities such as gas storage and molecular recognition. Here, we demonstrate humidity-induced reversible variations in the magnetic properties of cyano-bridged cobalt(II)-manganese(II)-chromium(III) metal assemblies. The observed magnetic humidity response is due to adsorption and desorption of a ligand water molecule on the cobalt ion, which changes cobalt (II) between a 6- and 4-fold coordination geometry and switches the magnetic interaction between ferromagnetic coupling and antiferromagnetic coupling.

Zinc oxide nanorod and nanowire for humidity sensor

ZnO nanorod and nanowire films were fabricated on the Si substrates with comb type Pt electrodes by the vapor-phase transport method, and their humidity sensitive characteristics have been investigated. These nanomaterial films show high-humidity sensitivity, good long-term stability and fast response time. It was found that the resistance of the films decreases with increasing relative humidity (RH). At room temperature (RT), resistance changes of more than four and two orders of magnitude were observed when ZnO nanowire and nanorod devices were exposed, respectively, to a moisture pulse of 97% relative humidity. It appears that the ZnO nanomaterial films can be used as efficient humidity sensors.

Preparation structure and Humidity-sensitive Characteristics of the Lead Chromate Thick Film Elements

Preparation structure and Humidity-sensitive Characteristics of the Lead Chromate Thick Film Elements

Sol–gel BaTiO 3 thin film for humidity sensors

BaTiO 3 ceramic thin films were prepared by sol–gel processing and their humidity sensitive characteristics have been investigated. The films were formed on Al 2O 3 substrates with comb type Au electrodes by spin coating. It was found that the impedance of the thin films decreases with increasing relative humidity (RH). The humidity sensitivity of the sensor depends on the applied frequency of the electric signal and the activation energy for conduction is reduced with water adsorption. The BaTiO 3 thin films show a negligible hysteresis under cyclic humidity changes with a good long-term stability and fast response time. It appears that the sol–gel process produces films that can be used as efficient humidity sensors.

Crystalline Lead Sulfate Films Grown by Electron Beam Epitaxy and its Humidity-Sensitive Characteristics.

Crystalline thin films of lead sulfate, Pb1+ySO4+z(y=0 or 1, 0≤z≤1) could be prepared by an electron beam method with using a source of PbSO4+xPbO (0≤x≤1), under the following conditions: substrate temperature; 300°C, acceleration voltage; 20 kV and emission current; 58 mA. The sulfate films formed were composed of PbSO4 or Pb2SO5, and their compositions of the films changed with changing source composition. Either single phase of PbSO4 (y=0, z=0), or Pb2SO5, (y=1, z=1), or a mixture of these phases was formed with the source composition of x=0, x=1, or approximately x=0.5, respectively. Evaluation of the humidity sensitivity for these films revealed that crystalline thin films of PbSO4 and Pb2SO5 had the hydrophobic and hydrophilic properties, respectively, being one of the most promising humidity sensors because of quick response.

Polymeric humidity sensor using phosphonium salt-containing polymers

New phosphonium salt-containing monomer, (vinylbenzyl)tributyl-phosphonium chloride (VTBPC) was prepared and polymerized for humidity-sensitive polyelectrolyte. The humidity-sensitive membranes were composed of cross-linked copolymers with different contents of VTBPC, n-butyl acrylate ( n-BA) and 4-vinylpyridine (VP): VTBPC/ n-BA/VP=5/0/0, 5/1/0, 5/1/5 and 4/1/6. When the resistance dependencies on the relative humidity of the copolymers were measured, it was found that the impedance was decreased with an increase in the content of VTBPC. The impedance ranged from 10 7 to 10 3 Ω between 20 and 95% RH, which was required for a humidity sensor operating at ambient humidity. Temperature dependence, hysteresis and a response time were also measured. The humidity-sensitive characteristics of the cross-linked copolymer of VTBPC, n-BA and VP changed little even after soaking in water for 120 min.

Humidity sensitive characteristics of Zn 2SnO 4–LiZnVO 4 thick films prepared by the sol–gel method

Thick film ceramic humidity sensitive layers composed of the Zn 2SnO 4–LiZnVO 4 system were sintered using nanometer powders. The humidity sensitive powders prepared by the sol–gel route have the advantages of lower resistance in the low relative humidity (RH) range, better resistance-RH linearity and long-term stability compared with layers sintered from conventional solid state reaction powders. The impedance measurements at different operation frequencies indicate that the sensors sintered at 900 °C for 1 h have an optimum resistance-RH linearity at 1 kHz. scanning electron microscopy (SEM) micrographs of the grain structures and complex impedance analysis confirm that high performance of the sensitive layer is associated with a fine microstructure morphology, which is characterized by suitable rod shaped grains forming mesopores and by well distributed LiZnVO 4 additives, forming a thin glassy surface covering.

Humidity sensor using epoxy resin containing quaternary ammonium salts

Humidity-sensitive epoxy monomer, glycidyl trimethyl ammonium chloride (GTMAC) was selected as the humidity-sensing resin. Polypropylene glycol diglycidyl ether (PPGDGE) and methyl tetrahydrophthalic anhydride (MTPHA) were used as a comonomer and a curing agent, respectively. The humidity-sensitive membranes were composed of GTMAC, PPGDGE and MTPHA. When impedance characteristics of the epoxy resins containing quaternary ammonium salts were measured, the impedance decreased linearly with an increase in the content of GTMAC in its semi-logarithmic graph. The impedance changed from 10 7 to 10 3 Ω between 30 and 90%RH, which was required for a common humidity sensor. Temperature dependence, frequency dependence and response time were also measured. The humidity-sensitive characteristics of the sensor did not change even after soaking in water.

Humidity sensor using cross-linked copolymers containing viologen moiety

1-[3-(2-Methacryloxyethyl dimethyl ammono)propyl] 1′-propyl-4,4′-bipyridinium tribromide (MDAPBT) was selected as a humidity-sensing monomer. n-Butyl acrylate ( n-BA) and 2-( N, N′-dimethylaminoethyl)methacrylate (DAEMA) were used as a comonomer. The major ingredient of the humid membrane is the copolymers of MDAPBT with n-BA and DAEMA. The cross-linked copolymer of MDAPBT/ n-BA/DAEMA=40/30/30 showed average resistance of 1000, 52 and 3.1 KΩ at 30, 60 and 90% relative humidity (RH), respectively. Temperature dependence, frequency dependence and response time were also measured. The temperature dependence at 15–40°C is −0.42 to −0.45% RH °C −1 and the hysteresis falls on the range of ±2% RH. The humidity-sensitive characteristics of the sensor did not change even after 20 min when the sensor was soaked in water.

Sensing Properties of Ba1划xLaxNbyTi1划yO3 (x=0.25%, y=0.25%) Thin-Film on SiO2/Si Substrate

AbstractBarium lanthanum titanate-niobate (Ba1划xLaxNbyTi1划yO3) film deposited on a SiO2/Si substrate by the argon ion-beam sputtering technique has been used to fabricate a thin-film resistor and a metal-insulator-semiconductor (MIS) capacitor by standard integrated-circuit technology. Measurements show that the film resistor has superior sensitivity for visible light and a thermal sensitivity within the range 28∼400 °C, while the MIS capacitor is highly sensitive to relative humidity. The optical absorption spectrum of the film has been measured and the bandgap of the film determined. The effects of test frequency on the impedance of the film resistor at various temperatures and on the humidity-sensitive characteristics of the MIS capacitor have been investigated.

Humidity sensitive properties of copolymers containing phosphonium salts

(Vinylbenzyl)triphenylphosphonium chloride (VTPC) was prepared for a humidity-sensitive polyelectrolyte. The humidity-sensitive membranes were composed of cross-linked copolymers with different contents of VTPC, n-butyl acrylate ( n-BA) and 2-( N, N-dimethylaminoethyl)methacrylate (VTPC/ n-BA/DMAMA=10/35/0, 10/45/0, 10/35/4.5 and 10/35/9). When impedance characteristics of the copolymers were measured, the impedance was decreased with an increase in the content of VTPC. The impedance ranged from 10 7 Ω to 10 3 Ω between 50% RH and 95% RH, which was required for a humidity sensor operating at high humidity or a dew point. Temperature dependence, hysteresis and a response time were also measured. The humidity-sensitive characteristics of the cross-linked copolymer of VTPC, n-BA and DAEMA changed little even after soaking in water for 120 min.

Photo-, thermal and humidity sensitivity characteristics of Sr 1− xLa xTiO 3 film on SiO 2/Si substrate

Strontium lanthanum titanate (Sr 1− x La x TiO 3) film deposited on a SiO 2/Si substrate by the argon ion-beam sputtering technique has been used to fabricate a thin-film resistor and a metal-insulator-semiconductor (MIS) capacitor by standard integrated-circuit technology. Measurements show that the film resistor has superior sensitivity for visible light and a thermal sensitivity within the range 28–200°C, while the MIS capacitor is highly sensitive to relative humidity. The optical absorption spectrum of the film has been measured and the bandgap of the film determined. The effects of test frequency on the impedance of the film resistor at various temperatures and on the humidity-sensitive characteristics of the MIS capacitor have been investigated.

Sensors based on the dissipation characteristics of n-BaTiO 3 ceramics and its solid solutions

The humidity, heat flux and mass flow sensing capability of n-BaTiO 3 and its solid solutions were evaluated based on their dissipation characteristics. The cubic/tetragonal phase content of the ceramics seem to play an important role in their sensitivity towards the measurand. The humidity-sensitive characteristics of these perovskites were studied with respect to different moisture sensitive coating materials. The sensor was also used to determine the heat of hydration during the curing process of cements and the mass flow rate of the gases. For all these applications, suitable operating points have been fixed from the highly non-linear I-V characteristics with the retention of good stability and high sensitivity.