Fabrication and characterization of nano-sized SrTiO/sub 3/-based oxygen sensor for near room-temperature operation

Nano-sized SrTiO/sub 3/-based oxygen sensors were fabricated from synthesized SrTiO/sub 3/ and commercial SrTiO/sub 3/ using the high-energy ball milling and the thick-film screen-printing techniques. The particle sizes, microstructural properties, oxygen-sensing properties, and humidity effects of the synthesized nano-sized SrTiO/sub 3/-based oxygen sensors were characterized using X-ray diffraction (XRD), transmission electron microscope, scanning electron microscope (SEM), and gas sensing measurements. Experimental results showed that the particle size of the powders was milled down to be around 27 nm. The effect of different annealing temperatures (400/spl deg/C, 500/spl deg/C, 600/spl deg/C, 700/spl deg/C, and 800/spl deg/C) on the gas sensing properties of the synthesized SrTiO/sub 3/ sensor from nitrogen to 20% oxygen was characterized. The commercial SrTiO/sub 3/ devices annealed at 400/spl deg/C, both with 0-h and 120-h milling time, were used for comparison. The optimal relative resistance (R/sub nitrogen//R/sub 20%oxygen/) value of 6.35 is obtained for the synthesized SrTiO/sub 3/ sample annealed at 400/spl deg/C and operating at 40/spl deg/C. This operating temperature is much lower than that of conventional metal oxide semiconducting oxygen gas sensors (300/spl deg/C-500/spl deg/C) and SrTiO/sub 3/ oxygen gas sensors (>700/spl deg/C). The response and recovery times are 1.6 and 5 min, respectively. The detected range is 1-20% oxygen. The impedance of the synthesized SrTiO/sub 3/ sensor with annealing at 400/spl deg/C and operating at 40/spl deg/C (from 1 mHz to 10 MHz) in 20% oxygen ambient was found to be independent of the relative humidity (dry, 20% RH, 80% RH, near 100% RH).     

Resistive and capacitive response of nitrogen-doped TiO2 nanotubes film humidity sensor

Dielectric oxides are traditionally used to fabricate resistive surface humidity-sensing devices, as well as capacitive sandwich-structured sensors. In the present work, relative humidity (RH) sensors were fabricated by employing vertically aligned TiO(2) nanotubes array (TNA) film produced using electro-chemical anodization of Ti foil followed by a nitrogen-doping process, simultaneously showing resistive and capacitive humidity-sensing properties in the range of 11.3-93.6%. For the sample formed at optimized experimental conditions, the capacitance (C(S)) and resistance (R(S)) of the as-fabricated RH sensors made from nitrogen-doped TiO(2) nanotubes film could be simultaneously obtained. Both the resistive and capacitive sensitivity (K(R) and K(C)) of the as-fabricated TiO(2) nanotube RH sensors show distinct dependence on the frequency of alternating current (AC) voltage signal and RH. At higher water coverage, water-water interaction will result in lowering of the water dissociation barrier, leading to an increase of conductance. With the increase of RH, the polarization of as-adsorbed water molecules will also occur, causing a sharp increase of capacitance. For an explanation of the frequency response of both C(S) and R(S), ionic transport, as well as the polarization effect, should be comprehensively considered. The changes of capacitance and resistance at different temperatures are plausibly caused by thermal expansion and surface state modification by adsorption and desorption of oxygen and water.     

Humidity sensor structures with thin film porous alumina for on-chip integration

Our aim was to produce a cheap, reliable, low-power and CMOS-MEMS process compatible relative humidity (RH) capacitive sensor that can be incorporated into a state-of-the art wireless sensor network. Porous alumina, produced by electrochemical oxidation of aluminum thin film under anodic bias (AAO, Anodic Aluminum Oxide) was used for the purpose of the sensing layer. We prepared two different capacitive sensor structures on silicon substrate using semiconductor processing steps and anodic oxidation in addition. The first sensor has an ultra-thin, vapor-permeable palladium upper electrode, while in the second case, an electroplated gold-grid is used for the same purpose. The highest achieved average sensitivity is approx. 15 pF/RH%, which is much higher than the values found in product catalogues of discrete, off-the-shelf capacitive humidity sensors (0.2-0.5 pF/RH%).     

Surface-type multifunctional sensor based on 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin

In this study, an organic semiconductor 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin (TIPP) was synthesized and investigated as an active material in surface-type multifunctional sensor. As electrodes, 100 nm thick Ag films were deposited on 25 mm × 25 mm glass substrate with 40 μm gap between them. Thin film of TIPP of area 15 mm × 15 mm was thermally sublimed to cover the gap between the silver electrodes. Thickness of TIPP film was 100 nm. A change in electrical resistance and capacitance of the fabricated device was observed with the increase in relative humidity (RH), temperature, and illumination. Hysteresis, response, and recovery times were investigated over a wide range of RH (0–94%). Activation energy of the TIPP was estimated. An equivalent circuit of the Ag/TIPP/Ag humidity, temperature, and illumination sensor was developed. Humidity, temperature, illumination dependent capacitive, and resistive properties of this sensor make it promising for use in a humidity, temperature, and lux multi-meters.     

水热合成一维α-MoO3纳米棒及其湿敏性能研究

采用水热法,以钼酸铵和硝酸为原料合成了一维纳米α-MoO_3棒状材料,并利用X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)对其物相及形貌进行了表征。一维α-MoO_3纳米棒直径为200~300nm,长度为5~10μm。一维α-MoO_3纳米棒表现出良好的湿敏性能,所制得的传感器在100Hz、11%~95%湿度范围内,其复阻抗-相对湿度关系在半对数坐标下有5个数量级的变化,线性度好。元件的恢复和响应时间较短,分别为3s和35s,元件的湿滞约为4%RH。利用器件在不同湿度下的阻抗图谱建立了相应的等效电路,分析其电导过程。结果表明,在低湿度范围内,器件传导主要依靠一维α-MoO_3材料内少量的自由电子传导以及材料本身束缚电荷的极化;在高湿度范围内,吸附水分子的分解和极化所引起的离子导电占主导地位。     

A solid-state relative humidity measurement system

The implementation of a solid-state capacitive relative humidity (RH) sensing system which employs a polymer as the moisture-sensitive material is discussed. The sensing mechanism is based on the fact that the dielectric permittivity of the insulating polymer, polyimide (PI), is linearly related to the ambient relative humidity. Therefore, the capacitance of a parallel-plate device using PI as the dielectric is a linear function of ambient RH (because PI does not swell during moisture absorption). Standard IC processing techniques are used to fabricate the PI-based integrated sensor capacitor. In order for such as sensor to be useful in the field, a voltage, current, or frequency output is desirable. A discrete capacitance-to-voltage converter circuit using this sensor capacitor has been constructed. The performance of this system is reported     

Nitrogen‐Doped Single Graphene Fiber with Platinum Water Dissociation Catalyst for Wearable Humidity Sensor

Humidity sensors are essential components in wearable electronics for monitoring of environmental condition and physical state. In this work, a unique humidity sensing layer composed of nitrogen‐doped reduced graphene oxide (nRGO) fiber on colorless polyimide film is proposed. Ultralong graphene oxide (GO) fibers are synthesized by solution assembly of large GO sheets assisted by lyotropic liquid crystal behavior. Chemical modification by nitrogen‐doping is carried out under thermal annealing in H₂(4%)/N₂(96%) ambient to obtain highly conductive nRGO fiber. Very small (≈2 nm) Pt nanoparticles are tightly anchored on the surface of the nRGO fiber as water dissociation catalysts by an optical sintering process. As a result, nRGO fiber can effectively detect wide humidity levels in the range of 6.1–66.4% relative humidity (RH). Furthermore, a 1.36‐fold higher sensitivity (4.51%) at 66.4% RH is achieved using a Pt functionalized nRGO fiber (i.e., Pt‐nRGO fiber) compared with the sensitivity (3.53% at 66.4% RH) of pure nRGO fiber. Real‐time and portable humidity sensing characteristics are successfully demonstrated toward exhaled breath using Pt‐nRGO fiber integrated on a portable sensing module. The Pt‐nRGO fiber with high sensitivity and wide range of humidity detection levels offers a new sensing platform for wearable humidity sensors.     

Surface-Mediated Interconnections of Nanoparticles in Cellulosic Fibrous Materials toward 3D Sensors

Fibrous materials serve as an intriguing class of 3D materials to meet the growing demands for flexible, foldable, biocompatible, biodegradable, disposable, inexpensive, and wearable sensors and the rising desires for higher sensitivity, greater miniaturization, lower cost, and better wearability. The use of such materials for the creation of a fibrous sensor substrate that interfaces with a sensing film in 3D with the transducing electronics is however difficult by conventional photolithographic methods. Here, a highly effective pathway featuring surface-mediated interconnection (SMI) of metal nanoclusters (NCs) and nanoparticles (NPs) in fibrous materials at ambient conditions is demonstrated for fabricating fibrous sensor substrates or platforms. Bimodally distributed gold–copper alloy NCs and NPs are used as a model system to demonstrate the semiconductive-to-metallic conductivity transition, quantized capacitive charging, and anisotropic conductivity characteristics. Upon coupling SMI of NCs/NPs as electrically conductive microelectrodes and surface-mediated assembly (SMA) of the NCs/NPs as chemically sensitive interfaces, the resulting fibrous chemiresistors function as sensitive and selective sensors for gaseous and vaporous analytes. This new SMI–SMA strategy has significant implications for manufacturing high-performance fibrous platforms to meet the growing demands of the advanced multifunctional sensors and biosensors.     

A surface acoustic wave humidity sensor with high sensitivity based on electrospun MWCNT/Nafion nanofiber films

Humidity detection has been widely used in a variety of fields. A humidity sensor with high sensitivity is reported in this paper. A surface acoustic wave resonator (SAWR) with high resonance frequency was fabricated as a basic sensitive component. Various nanotechnologies were used to improve the sensor's performance. A multi-walled carbon nanotube/Nafion (MWCNT/Nafion) composite material was prepared as humidity-sensitive films, deposited on the surface of an SAWR by the electrospinning method. The electrospun MWCNT/Nafion nanofiber films showed a three-dimensional (3D) porous structure, which was profitable for improving the sensor's performance. The new nano-water-channel model of Nafion was also applied in the humidity sensing process. Compared to other research, the present sensor showed excellent sensitivity (above 400 kHz/% relative humidity (RH) in the range from 10% RH to 80% RH), good linearity (R(2) > 0.98) and a short response time (～3 s@63%).     

Design and Characterisation of a Temperature Compensated Relative Humidity Measurement System with On Line Data Logging Feature

A method of relative humidity (RH) measurement with on line temperature correction and data logging feature is described here. The RH sensor is a thermoset polymer capacitive type with on chip conditioning. Output voltage of RH to voltage converter is digitised using a 12-bit A/D converter and it is interfaced with an 8-bit microcontroller. Temperature correction of RH in real time is done using a temperature to digital converter interfaced with the microcontroller. The corrected data is displayed in a liquid crystal display and transmitted for on line monitoring and logging via RS232C. Calibration of the system is performed using standard saturated binary salt solutions. Accuracy and precision are found out. Hysteresis and ageing effect is discussed. The system performance is also compared with a dry/wet bulb psychrometer at different ambient conditions. The performance of the system in an industrial environment is presented.     

Flexible Sensing Electronics for Wearable/Attachable Health Monitoring

Wearable or attachable health monitoring smart systems are considered to be the next generation of personal portable devices for remote medicine practices. Smart flexible sensing electronics are components crucial in endowing health monitoring systems with the capability of real‐time tracking of physiological signals. These signals are closely associated with body conditions, such as heart rate, wrist pulse, body temperature, blood/intraocular pressure and blood/sweat bio‐information. Monitoring such physiological signals provides a convenient and non‐invasive way for disease diagnoses and health assessments. This Review summarizes the recent progress of flexible sensing electronics for their use in wearable/attachable health monitoring systems. Meanwhile, we present an overview of different materials and configurations for flexible sensors, including piezo‐resistive, piezo‐electrical, capacitive, and field effect transistor based devices, and analyze the working principles in monitoring physiological signals. In addition, the future perspectives of wearable healthcare systems and the technical demands on their commercialization are briefly discussed.     

A flexible and high temperature tolerant strain sensor of La_(0.7)Sr_(0.3)MnO_3/Mica

Flexible sensors have been widely investigated due to their broad application prospects in various flexible electronics. However, most of the presently studied flexible sensors are only suitable for working at room temperature, and their applications at high or low temperatures are still a big challenge. In this work, we present a multimodal flexible sensor based on functional oxide La_(0.7)Sr_(0.3)MnO_3(LSMO) thin film deposited on mica substrate. As a strain sensor, it shows excellent sensitivity to mechanical bending and high bending durability(up to 3600 cycles). Moreover, the LSMO/Mica sensor also shows a sensitive response to the magnetic field, implying its multimodal sensing ability. Most importantly, it can work in a wide temperature range from extreme low temperature down to 20 K to high temperature up to 773 K.The flexible sensor based on the flexible LSMO/mica hetero-structure shows great potential applications for flexible electronics using at extreme temperature environment in the future.     

A hygrometer comprising a porous silicon humidity sensor with phase-detection electronics

A novel hygrometer is presented, comprising a capacitive humidity sensor with a porous silicon (PS) dielectric and electronics. The adsorption of water vapor by the PS layer leading to change of its effective dielectric constant is modeled with an effective medium approximation (EMA). A simple, but precise, phase-sensitive electronic circuit has been developed. This detects any change of phase of a sinusoidal signal transmitted through the PS dielectric and correlates to ambient humidity. It is outlined how the nonlinear response of the sensor is compensated through piecewise linearization. The sensor is tested in combination with the phase detection circuitry. Excellent linearity over the entire range of relative humidity is achieved. Experimental results show a resolution better than 0.1% and an accuracy of 2% (near the transition region) and better than 0.1% (otherwise). The response time is less than 10 s with good stability.     

Nanoporous polymeric transmission gratings for high-speed humidity sensing

Nanoporous polymeric transmission gratings are demonstrated to be an excellent platform for high-speed optical humidity sensing. The grating structures were fabricated with a modified holographic, polymer-dispersed liquid crystal (H-PDLC) system. The sensing mechanism was based on changes in the relative transmission associated with the adsorption and desorption of water vapour by nanopores. The spectral changes due to varying humidity levels were measured by a spectrometer and compared with the calculated results based on the coupled wave theory. When the relative humidity (RH) changed from 40% to 95%, the relative transmission at 475?nm increased from 6.3% to 46.6% and that at 702?nm increased from 4%?to 64%; these results indicate the sensor's high sensitivity. In addition, the sensor demonstrated excellent reversibility and reproducibility over a large RH range (from 20% to 100% RH). Moreover, the response time of the sensor was measured to be less than 350?ms, making it suitable for many high-speed humidity-sensing applications.     

Humidity sensors using KH2PO4-doped porous (Pb,La)(Zr,Ti)O3

Humidity sensing devices were prepared by using fine porous (Pb, La)(Zr, Ti)O₃ (PLZT) particles with ferroelectricity instead of insulating metal oxides such as alumina and zircon. The impedance of PLZT with 1 wt %. KH₂PO₄ was 10⁶ and lower than that of zircon with 3.8 wt % KH₂PO₄ by a factor of 10² in a dry atmosphere. In addition, the impedance in a humid atmosphere was controlled by the adding of potassium dihydrogen phosphate and changed by about four orders of magnitude in the humidity region 0 to 90% relative humidity at 1 kHz for samples burnt at 700° C. The humidity dependence of impedance is mainly governed by the change of coverage of adsorbed water. The hydrophilicity is affected by the burning temperature and lanthanum content of PLZT used as starting particle for porous PLZT ceramics burnt with KH₂PO₄. From complex impedance analysis it is confirmed that the resistive component inserted in parallel with the capacitive component decreases steeply with an increase in humidity, while the capacitive component is poorly dependent on the humidity.     

Highly conformal SiO2/Al2O3 nanolaminate gas-diffusion barriers for large-area flexible electronics applications

The present study demonstrates a flexible gas-diffusion barrier film, containing an SiO(2)/Al(2)O(3) nanolaminate on a plastic substrate. Highly uniform and conformal coatings can be made by alternating the exposure of a flexible polyethersulfone surface to vapors of SiO(2) and Al(2)O(3), at nanoscale thickness cycles via RF-magnetron sputtering deposition. The calcium degradation test indicates that 24 cycles of a 10/10 nm inorganic bilayer, top-coated by UV-cured resin, greatly enhance the barrier performance, with a permeation rate of 3.79 × 10(-5) g m(-2) day(-1) based on the change in the ohmic behavior of the calcium sensor at 20?°C and 50% relative humidity. Also, the permeation rate for 30 cycles of an 8/8 nm inorganic bilayer coated with UV resin was beyond the limited measurable range of the Ca test at 60?°C and 95% relative humidity. It has been found that such laminate films can effectively suppress the void defects of a single inorganic layer, and are significantly less sensitive against moisture permeation. This nanostructure, fabricated by an RF-sputtering process at room temperature, is verified as being useful for highly water-sensitive organic electronics fabricated on plastic substrates.     

Adhesion characteristics of silver tracks screen-printed on polyimide with an environmental reliability test

Printable and flexible electronics are increasingly being used in numerous applications that are miniaturized, multi-functional and lightweight. Simultaneously, reliability issues of the printed and flexible electronic devices are getting more attention. The adhesion of screen-printed silver (Ag) tracks on a polyimide (PI) film was investigated after two kinds of the environmental reliability test: a constant-temperature storage test, and a steady-state temperature and humidity storage test. Atmospheric-pressure plasma (APP) was adopted on the PI film surface to improve the poor adhesion derived from the inherent hydrophobicity. The Ag tracks constructed via screen printing were sintered at 250 degrees C for 30 min in air using a box-type muffle furnace. Some samples were exposed under 85 degrees C and 85% relative humidity (RH) for various durations (24, 72, 168 and 500 h), and others were aged at 85 degrees C with same durations to compare the influence of moisture on the adhesion. The adhesion of the screen-printed Ag tracks was evaluated by a roll-type 90 degrees peel test. The peel strength of the screen-printed Ag tracks decreased by 76.74% and 69.88% after 500 h run of the 85 degrees C/85% RH test, and the aging test, respectively. The weakest adhesion was 4.98 gf/mm after the 500 h run of the 85 degrees C/85% RH test. To demonstrate these experimental results, the microstructural evolution and chemical bonding states of the interfacial surfaces were characterized using a field emission scanning electron microscope (FE-SEM), and X-ray photoelectron spectroscope (XPS), respectively.     

Synthesis,characterization and weight percent effect on humidity sensing properties of Polypyrrole/AlCeO3 (PPy/ACO) nanocomposites

Polypyrrole (PPy)/aluminum cerium oxide (ACO) nanocomposites with different weight percentages of the ACO nanoparticles in the matrix of PPy have been synthesized successfully. The addition of fillers in the conducting polymer makes significant changes in their electrical, chemical, mechanical and gas sensing properties. The exact amount of filler is one of the essential parameters to get the desired properties in the host polymer. So, the optimization of the percent weight of filler in the host conducting polymer is very much crucial. The structural and morphological studies were done by XRD, FTIR, SEM, and TEM. These experimental studies reveal that nanoparticles of Aluminum cerium oxide are dispersed uniformly in the PPy chains. The humidity sensing properties for different weight percent of nanocomposites were studied by using impedance analyzer in the frequency range from 100?Hz to 5?MHz and at relative humidity (% RH) variation from 30% RH to 90% RH. The graphs of impedance versus frequency for all nanocomposites showed a slightly linear dependence at all the frequencies over the entire humidity range. The complex impedance (cole-cole) graphs and the corresponding equivalent circuits were studied to understand the conduction phenomenon in nanocomposites. The conduction in pure PPy shows the contribution from grain and grain boundaries while all the nanocomposites show contribution only from grains. The lower weight percent nanocomposites exhibit better response compared to higher weight percent.     

Splashing-Assisted Femtosecond Laser-Activated Metal Deposition for Mold- and Mask-Free Fabrication of Robust Microstructured Electrodes for Flexible Pressure Sensors (Small 24/2023)

Flexible pressure sensors play an indispensable role in flexible electronics. Microstructures on flexible electrodes have been proven to be effective in improving the sensitivity of pressure sensors. However, it remains a challenge to develop such microstructured flexible electrodes in a convenient way. Inspired by splashed particles from laser processing, herein, a method for customizing microstructured flexible electrodes by femtosecond laser-activated metal deposition is proposed. It takes advantage of the catalyzing particles scattered during femtosecond laser ablation and is particularly suitable for moldless, maskless, and low-cost fabrication of microstructured metal layers on polydimethylsiloxane (PDMS). Robust bonding at the PDMS/Cu interface is evidenced by the scotch tape test and the duration test over 10 000 bending cycles. Benefiting from the firm interface, the developed flexible capacitive pressure sensor with microstructured electrodes presents several conspicuous features, including a sensitivity (0.22 kPa⁻¹) 73 times higher than the one using flat Cu electrodes, ultralow detection limit (<1 Pa), rapid response/recovery time (4.2/5.3 ms), and excellent stability. Moreover, the proposed method, inheriting the merits of laser direct writing, is capable of fabricating a pressure sensor array in a maskless manner for spatial pressure mapping.     

Optical Fiber Humidity Sensors Using Nanostructured Coatings of SiO$_{2}$ Nanoparticles

In this paper, a new optical fiber humidity sensor based on superhydrophilic coating is proposed. The electrostatic self-assembly technique has been used to create a nanometric scale surface on the tip of a standard single-mode pigtail. The fabricated sensor has demonstrated a good linearity in the range from 40% to 98% of relative humidity (RH). A variation of 10 dB in reflected optical power is achieved with a response time of only 150 ms. Among other applications, this sensor is intended to be used for monitoring the human breathing, so high dynamic performances are required, specially in the higher RH ranges.