Resistive Humidity Sensor Research Articles

Synthesis and Characterizations of Novel Ca-Mg-Ti-Fe-Oxides Based Ceramic Nanocrystals and Flexible Film of Polydimethylsiloxane Composite with Improved Mechanical and Dielectric Properties for Sensors.

Armalcolite, a rare ceramic mineral and normally found in the lunar earth, was synthesized by solid-state step-sintering. The in situ phase-changed novel ceramic nanocrystals of Ca-Mg-Ti-Fe based oxide (CMTFOx), their chemical reactions and bonding with polydimethylsiloxane (PDMS) were determined by X-ray diffraction, infrared spectroscopy, and microscopy. Water absorption of all the CMTFOx was high. The lower dielectric loss tangent value (0.155 at 1 MHz) was obtained for the ceramic sintered at 1050 °C (S1050) and it became lowest for the S1050/PDMS nanocomposite (0.002 at 1 MHz) film, which was made by spin coating at 3000 rpm. The excellent flexibility (static modulus ≈ 0.27 MPa and elongation > 90%), viscoelastic property (tanδ = E″/E′: 0.225) and glass transition temperature (Tg: −58.5 °C) were obtained for S1050/PDMS film. Parallel-plate capacitive and flexible resistive humidity sensors have been developed successfully. The best sensing performance of the present S1050 (3000%) and its flexible S1050/PDMS composite film (306%) based humidity sensors was found to be at 100 Hz, better than conventional materials.

TiO2-TiO2 composite resistive humidity sensor: ethanol crosssensitivity

The fabrication method and characterization results of a TiO2-TiO2 composite bead used for humidity sensing along with its negative cross-sensitivity to ethanol vapor are reported. The bead shaped resistive sample sensors are fabricated by the drop-casting of a TiO2 slurry on two Pt wire segments. The dried bead is pre-fired at 750°C and subsequently impregnated with a Ti-based sol. The sample is ready for characterization after a thermal annealing at 600°C in air. Structurally, the bead is a composite of the micron-sized TiO2 crystallites embedded in a matrix of nanometric TiO2 particle aggregates. The performance of the beads as resistive humidity sensors is recorded at room temperature in standard humidity level chambers. Results evince the wide dynamic range of the sensors fabricated in the low relative humidity range. While the sensor conductance is not sensitive to ethanol vapor in dry air, in humid air, sensor's responses are negatively affected by the contaminant.

Temperature-modulated graphene oxide resistive humidity sensor for indoor air quality monitoring.

In this paper we present a temperature-modulated graphene oxide (GO) resistive humidity sensor that employs complementary-metal-oxide-semiconductor (CMOS) micro-electro-mechanical-system (MEMS) micro-hotplate technology for the monitoring and control of indoor air quality (IAQ). GO powder is obtained by chemical exfoliation, dispersed in water and deposited via ink-jet printing onto a low power micro-hotplate. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) show the typical layered and wrinkled morphology of the GO. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infra-red (FTIR) spectroscopy indicate that the GO flakes possess a significant number of oxygen containing functional groups (epoxy, carbonyl, hydroxyl) extremely attractive for humidity detection. Electro-thermal characterisation of the micro-hotplates shows a thermal efficiency of 0.11 mW per °C, resulting in a sensor DC power consumption of only 2.75 mW at 50 °C. When operated in an isothermal mode, the sensor response is detrimentally affected by significant drift, hysteretic behaviour, slow response/recovery times and hence poor RH level discrimination. Conversely, a temperature modulation technique coupled with a differential readout methodology results in a significant reduction of the sensor drift, improved linear response with a sensitivity of 0.14 mV per %, resolution below 5%, and a maximum hysteresis of ±5%; response and recovery times equal to 189 ± 49 s and 89 ± 5 s, respectively. These performance parameters satisfy current IAQ monitoring requirements. We have thus demonstrated the effectiveness of integrating GO on a micro-hotplate CMOS-compatible platform enabling temperature modulation schemes to be easily applied in order to achieve compact, low power, low cost humidity IAQ monitoring.

Highly proton conducting MoS2/graphene oxide nanocomposite based chemoresistive humidity sensor

This paper reports the development of MoS2/GO nanocomposite based sensing layers for resistive humidity sensors.

A resistive type humidity sensor based on crystalline tin oxide nanoparticles encapsulated in polyaniline matrix

We have synthesized a nanocomposite consisting of crystalline tin oxide (SnO2) nanoparticles and polyaniline (PANI) by in-situ polymerization and composite formation (IPCF). The structure and morphology was characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The nanocomposite is shown to represent a viable material for electrical resistivity based sensing of humidity in the 5 to 90 % relative humidity (RH) range. The electrical resistance of the composite linearly decreases from 127.5 to 11.5 kΩ with humidity from 5 to 95 %. The sensitivity is 0.22 % RH‾1, the response time is 26 s, and the recovery time is 30 s. The fabrication of SnO2/PANI composite combines the high sensitivity of SnO2 towards moisture with good electrical conductivity of PANI, which influences the electronic properties of the material and enables the design of more efficient humidity sensors. The water vapor layering growth kinetics on the composite was investigated by isothermal thermogravimetric analysis and an interaction with limited diffusion aggregate type kinetics has been proposed.

Highly sensitive humidity sensors made from composites of HEC filled by carbon nanofillers

Resistive type humidity sensors made from the composite films of HEC (hydroxyethyl cellulose) and MWCNTs (multiwalled carbon nanotubes) were developed. Three kinds of HEC/MWCNT composite films were fabricated, which contained 7.0, 8.0 and 9.0 wt-% MWCNTs. Rapid response capability and high sensitivities of the composite sensors were confirmed. For the convenience of sensor’s calibration, the non-linear resistance change ratios can be linearised by performing natural logarithm operation on them. A new sensitivity factor was defined based on this linear relationship between the logarithm of the resistance change ratio and relative humidity. For comparison, a reference humidity sensor was also made from the composite film of HEC filled by 50.0 wt-% CBs (carbon blacks). Compared with the HEC/MWCNT sensors, the HEC/CB sensor was of much higher sensitivity, but lower repeatability and stability under cyclic humidity changes. Possible sensing mechanisms were discussed in detail.

How Does Moisture Affect the Physical Property of Memristance for Anionic–Electronic Resistive Switching Memories?

Memristors based on anionic–electronic resistive switches represent a promising alternative to transistor‐based memories because of their scalability and low power consumption. To date, studies on resistive switching have focused on oxygen anionic or electronic defects leaving protonic charge‐carrier contributions out of the picture despite the fact that many resistive switching oxides are well‐established materials in resistive humidity sensors. Here, the way memristance is affected by moisture for the model material strontium titanate is studied. First, characterize own‐processed Pt|SrTiO3‐δ|Pt bits via cyclic voltammetry under ambient conditions are thoroughly characterized. Based on the high stability of a non‐volatile device structures the impact of relative humidity to the current–voltage profiles is then investigated. It is found that Pt|SrTiO3‐δ|Pt strongly modifies the resistance states by up to 4 orders of magnitude as well as the device's current–voltage profile shape, number of crossings, and switching capability with the level of moisture exposure. Furthermore, a reversible transition from classic memristive behavior at ambient humidity to a capacitively dominated one in dry atmosphere for which the resistive switching completely vanishes is demonstrated for the first time. The results are discussed in relation to the changed Schottky barrier by adsorbed surface water molecules and its interplay with the charge transfer in the oxide.

Humidity sensing properties of reduced humic acid

Humidity sensors are utilized as part of the control system in all air conditioning equipment in the world. These important sensors can be divided into three main types including capacitive, resistance, and thermal conductivity. Of these the resistive type are the most cost effective, smallest in size, best interchangeability in the field, and long term stability. For the first time, we report the use of reduced humic acid as a resistive humidity sensor (RH 12.4% up to 97.1%). This material is promising due to the low cost, high sensitivity and environmentally friendliness. Moreover, this material has a fast response and recovery time compared to the graphene oxide. In contrast to graphene oxide it was found that the resistance of the sensor decreases as the relative humidity increases. Humidity sensing mechanism of the reduced humic acid sensor has been discussed.

Graphene quantum dots as a novel sensing material for low-cost resistive and fast-response humidity sensors

A room temperature graphene quantum dots (GQDs) based sensing material for relative humidity monitoring is presented. GQDs are synthesized by pyrolysis of citric acid and are deposited on a metallic interdigitated microelectrodes by drop-casting technology. GQDs are nanosheets around 20nm in average lateral size and an average height of 2.7±0.9nm. The response time is around few seconds and the sensitivity shows an exponential relation between 15 and 80% of RH, in resistive configuration device working at room temperature. The electrical changes as a function of the RH are related to the capillary condensation produced on the surface of the GQDs. Due to this physical behavior, a resistive path is formed between the microelectrodes and the condensed water vapor, and the resistivity of the system diminishes as the RH increases. A comparison on real time between the sensor developed and a commercial device corroborates the potential application of the novel sensor presented in this work.

Self-Assembly of an Amphiphilic π-Conjugated Dyad into Fibers: Ultrafast and Ultrasensitive Humidity Sensor.

The self-assembly of an amphiphilic monomolecular electron acceptor-donor dyad into electroactive π-π stacked fibrillar structures can be triggered by irradiation with visible light. These fibers, exposing hydrophilic ethylene glycol in their external shell, show unique characteristics as resistive humidity sensors that exhibit high sensitivity and ultrafast response.

Highly sensitive surface acoustic wave (SAW) humidity sensors based on sol–gel SiO2 films: Investigations on the sensing property and mechanism

The sensing property and mechanism of surface acoustic wave (SAW) humidity sensors based on sol–gel SiO2 films were investigated. The SiO2 films were fabricated with a sol–gel method. The SEM, AFM, and porosity investigations showed that the SiO2 films had a porous structure with a porosity of 53.4%. Through a comparative study between SAW type and resistive type humidity sensors based on semi-conducting oxide (ZnO, Co3O4, CuO, TiO2) and SiO2 films, it was found that not only the change of film conductivity (electric loading effect) but also the change of mass loading on the films (mass loading effect) contributed to the response of SAW humidity sensors when the RH (relative humidity) value changed. Moreover, the mass loading effect made the major contribution to the responses of SAW humidity sensors based on SiO2 films due to the films’ porous structure. Consequently, the sensors based on SiO2 films showed much higher responses, and the responses increased with the increase of the thickness of SiO2 films. The sensor with the thickest SiO2 film (∼230nm) showed a response of −520kHz when the RH value increased from 30% to 93%.

Resistive humidity sensors based on proton-conducting organic–inorganic silicophosphates doped by polyionenes

Silicophosphate organic–inorganic nanocomposites synthesized by sol–gel method were studied. Fractal characteristics were investigated by small-angle X-ray scattering and transmission electronic microscopy. A correlation between proton conductivity, composition and structure of system, as well as synthesis conditions is revealed. The dependence of nanocomposite conductivity on alternating current and relative media humidity from 23.5 to 96.5 % was stated, that is evidence of their availability as active media for humidity sensors.

Impact of Pulse Thermal Processing on the Properties of Inkjet Printed Metal and Flexible Sensors

We report on the low temperature processing of environmental sensors employing pulse thermal processing (PTP) technique to define a path toward flexible sensor technology on plastic, paper, and fabric substrates. Inkjet printing and pulse thermal processing technique were used to realize mask-less, additive integration of low-cost sensors on polymeric substrates with specific focus on temperature, humidity, and strain sensors. The printed metal line performance was evaluated in terms of the electrical conductivity characteristics as a function of post-deposition thermal processing conditions. The PTP processed Ag metal lines exhibited high conductivity with metal sheet resistance values below 100 mΩ/□ using a pulse width as short as 250 μs. The flexible temperature and relative humidity sensors were defined on flexible polyimide substrates by direct printing of Ag metal structures. The printed resistive temperature sensor and capacitive humidity sensor were characterized for their sensitivity with focus on future smart-building applications. Strain gauges were printed on polyimide substrate to determine the mechanical properties of the silver nanoparticle films. The observed electrical properties of the printed metal lines and the sensitivity of the flexible sensors show promise for the realization of a high performance print-on-demand technology exploiting low thermal-budget PTP technique.

Resistive graphene humidity sensors with rapid and direct electrical readout.

We demonstrate humidity sensing using a change of the electrical resistance of single-layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N2), oxygen (O2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, the devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. The interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO2 substrate leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical readout and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.

Resistive humidity sensor based on vanadium complex films

A resistive-type relative humidity (RH) sensor based on vanadium complex (VO2(3-fl)) film is reported in this study. Gold electrodes were deposited on the glass substrates in a co-planar structure. A thin film of vanadium complex was coated as a humidity-sensing material on the top of the pre-patterned electrodes. The humidity-sensing principle of the sensor was based on the conductivity change of coated sensing element upon adsorption/desorption of water vapor. The resistance of the humidity sensor measured at 1 kHz decreased linearly with increasing the humidity in the range of 35%–70% RH. The overall resistance of the sensor decreases 11 times. An equivalent circuit for the VO2(3-fl) based resistive-type humidity sensor was developed. The properties of the sensor studied in this work make it beneficial for use in the instruments for environmental monitoring of humidity.

A gold/organic semiconductor diode for ppm-level humidity sensing

The cooperative electric field of surface-adsorbed polar organic molecules has been shown to control charge transport in adjacent solid layers or interfaces. While the adsorption of water monomers on different metal surfaces has also been extensively investigated, the electronic implications of the mechanism have not yet been explored. Here, we show that H2O molecules, selectively adsorbed on a ∼10nm-thick gold layer deposited on a hydrophobic semiconductor, substantially change the electron distribution in the gold layer, affecting the electron energy profile at the gold/semiconductor interface and altering the current–voltage characteristics of the structure. This concept is used for the fabrication of a resistive humidity sensor for ppm-level hygrometry. Made by depositing gold nanolayers on an air-stable hydrophobic organic semiconductor, oxidized poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV), the device demonstrates high sensitivity at H2O concentrations as low as ∼1ppm in air, vacuum and inert backgrounds. The presence of gases such as CO2 and H2 in substantial concentrations and oxygen partial pressure variations in air do not interfere with the sensing process. Unlike common Kelvin condensation-based resistive humidity sensors, the electrical resistance of the presented device increases upon exposure to humid atmospheres.

Effects of Partial Replacement of Iron with Tungsten on Microstructure, Electrical, Magnetic and Humidity Properties of Copper-Zinc Ferrite Material

This paper presents the analysis of the influence of partial replacement of iron with tungsten on the properties of copper-zinc spinel ferrite material. The samples of Cu0.5Zn0.5WxFe2−xO4 spinel powder ferrites were prepared by using a sol–gel self-combustion technology. The ferrite samples were treated for 30 min at 1000°C. The x-ray diffraction was used in order to establish the differences between the phase compositions of ferrites with different tungsten content. Scanning electron microscopy was used to highlight the influence of the tungsten content on the crystallites. All the samples of Cu0.5Zn0.5WxFe2−xO4 were subject to investigation of the influence of the substitution of tungsten upon their electrical and magnetic properties. The measurements of the electrical properties were performed in different humidity conditions, in order to highlight the effect of moisture conditions on the electrical properties of the material and to analyze the applicability of Cu0.5Zn0.5WxFe2−xO4 ferrite for resistive or capacitive humidity sensors.

Influence of partial substitution of Fe3+ with W3+ on the microstructure, humidity sensitivity, magnetic and electrical properties of barium hexaferrite

This paper investigates the influence of partial substitution of iron with tungsten on the properties of barium hexaferrite. The BaWxFe12−xO19 hexaferrite samples were prepared by using a sol–gel autocombustion technology. The powder ferrites were heat treated for 30min at 1000°C. The samples were investigated by using X-ray diffraction in order to highlight the differences between the phase compositions of comparative samples. The influence of the substitution of tungsten ions on the size and shape of the crystallites was revealed by the scanning electron microscopy. The investigations of the substitution of tungsten were focused mainly on magnetic and electrical properties of the BaWxFe12−xO19 ferrites; the measurements of electrical properties were performed in different humidity conditions, aimed at obtaining a ferrite useful as a material support for capacitive or resistive humidity sensors. In this respect, the partial substitution of iron with tungsten contributes to an increasing of permittivity and to a reduced electrical resistivity of the material, without significantly lowering its sensitivity to humidity.

Humidity Sensing Properties of Screen-printed Carbon-black an Fe(II) Spin Crossover Compound Hybrid Films

We report on the humidity sensing properties of a screen-printed polymeric film loaded with carbon-black and Fe[(Htrz)2(trz)]BF4 nanoparticles. The new film shows excellent humidity sensitivity in a w de relative humidity range and the processing conditions make it suitable for a new eneration of inexpensive, resistive humidity sensors on flexible substrates tags

Optimization and Simulation of humidity sensor readout circuitry using two stage op amp

In this paper a humidity sensor readout circuitry using two stage opamp is designed. This circuitry is realized in TSMC 0.18μm CMOS Technology. Designing of two-stage op-amps is a multi-dimensional- optimization problem where optimization of one or more parameters may easily result into degradation of others. The OPAMP circuit is designed to exhibit a unity gain frequency of 2.02GHz and exhibits a gain of 49.02dB with a 60.50 phase margin and CMRR 39dB, Slew Rate 154dB, UGB (Unity Gain Bandwidth) are in a higher unity gain frequency under the same load condition. Also presented humidity sensor can achieve good linearity over the relative humidity range from 20% to 80%. The total power dissipation of readout circuitry is 21.8 pW. Terms: CMOS humidity sensor, humidity sensor, Op-Amp. I. Introduction Humidity sensors have a widely used in application areas, including agriculture, climate control, food storage, and domestic appliances. In order to satisfy the requirements of these applications, humidity sensors should provide high sensitivity over a wide range of humidity and temperature and linear response. Other important parameters of the humidity sensors can be listed as long-term stability, response time, and power consumption. A desirable feature of the humidity sensors is their compatibility with standard IC fabrication technologies. There are various types of humidity sensors based on the sensing principle they use, such as resistive, mechanical, gravimetric, capacitive, and thermal humidity sensors. This approach provides a number of advantages. The diodes can be heated up with a low power, and they provide high sensitivity. As the diodes are heated up, no water condensation can occur on the sensing elements. In most of the electronics circuits the Operational Amplifiers is the most common building blocks. So as the transistor channel length and power supply is reduced then the design of Op amps face continuous challenge. Due to different aspect ratio (W/L), there is a tradeoff among speed, gain, power and the other parameters. The implementation of a CMOS OPAMPs that combines a considerable dc gain with higher unity gain frequency has been a most difficult problem. There has been several circuits design to evaluate this problem. The purpose of the design methodology in this paper is to design accurate equations for the design of high- gain 2 staged CMOS op-amp. Outline of paper