Effects of Ag nanoparticles decorated on ZnO nanorods under visible light illumination on flexible acetylene gas sensing properties

In this work, piezoelectric and plasmonic effects on a flexible acetylene (C₂H₂) gas sensor based on silver (Ag) nanparticles (NPs)-coated ZnO nanorods (Ag-ZnO) were realized. Using visible light illumination, the sensing properties can be modulated and the power consumption can be reduced significantly. Upon exposure to 1000 ppm C₂H₂ under 8.36 mW cm^?2 light illumination, the power consumption of the sensor noticeably reduced from 3.48 W (in dark) to 1.64 W. A large number of light-induced chemisorbed oxygen ions were generated in the Ag-ZnO forest due to the strong coupling effect between the plasmonic Ag NPs and the ZnO NRs. This resulted in increased surface charge densities, which facilitated the sensor to react with the C₂H₂ molecules at lower operating temperature, hence reduced the power requirement. Moreover, the sensor exhibited reliable detection of C₂H₂ gas within the concentration of 3–1000 ppm including a maximum sensor response of 26.2, response-recovery time of 66/68 s, the excellent mechanical stability at a bending angle up to 90^o, and 10⁴ cycles of repeated deformation processes. These results might facilitate research in developing a low power C₂H₂ gas sensor and will open up new approaches for future light modulated gas sensors.     

WO3/BTO heterostructures based NO2 sensor with enhanced response characteristics

Abstract

In the present work, an efficient NO₂ gas sensor has been realised using single phase Barium titanate, BaTiO₃, (BTO) thin film, grown by chemical solution deposition technique (CSD). The gas sensing characteristics of BTO thin film were enhanced by integrating WO₃ modifier in the form of uniformly distributed circular nano-clusters and continuous overlayer. The WO₃ nanoclusters/BTO sensing element exhibited enhanced sensor response (～156) with fast response speed (16?s) at a relatively low operating temperature (140?°C) towards 50?ppm NO₂ gas. An attempt has been made to explain the sensing mechanism involving the twin effect of “Fermi-level exchange mechanism” and “spill over mechanism” upon interaction with target NO₂ gas. The obtained results in the present work are encouraging for the realization of hand-held NO₂ gas sensor.     

Characteristics of fabricated catalytic combustible micro gas sensor with low power consumption for detecting methane leakage of compressed natural gas bus

For keeping the safety of compressed natural gas-powered buses (CNG, CH₄), a small sized and low power consumption gas sensor is needed for the fuel leakage alarm system. So catalytic combustible micro gas sensors were designed and fabricated by microelectromechanical systems (MEMS) technology and the sensors were measured output voltage potential of the sensor circuit as sensitivities to methane. The sensor was consisted of a sensing platform and a compensation platform, and the length and width of the fabricated platform were 3 mm by 3 mm. The output voltage of a fabricated micro sensor were 0.727 mV, 0.548 mV, 0.45 mV, and 0.29 mV at methane concentrations of 4,630 ppm, 3,473 ppm, 2,315 ppm, and 1,158 ppm respectively, at the input power of 104 mW. Fabricated micro gas sensors could be operated at low power consumption and showed good selectivity. For further study, the long-term stability of micro sensors will be done before applying to CNG powered buses.     

Fabrication and characterization of co-planar type MEMS structures on SiO2 /Si 3 N 4 membrane for gas sensors with dispensing method guided by micromachined wells

MEMS structures for micro gas sensors had advantage for lower power consumption, reducing size, and easily making cavity structures. Also, co-planar type MEMS structures (CPMS) for gas sensors with low power consumption heater and dispensed sensing materials were newly proposed and investigated. CPMS, which were formed with micro heater and sensing electrodes at the same layer, to reduce process steps, diffusions between upper layer and lower layer, and thermal differences between the center and the periphery of the sensing layer compared with stacked structure. Dispensing method guided by back-side etched well was good for forming sensing material on sensing electrode and had advantage that various sensing materials could be applied for array type sensors. CPMS were fabricated on four-inch diameter and double side polished (100) silicon wafers and using anisotropic bulk silicon micromachining for membrane formation and etched well. A size of chips with 1.15 mm × 1.15 mm membrane was 4.8 mm × 4.8 mm. And co-planar type sensing electrodes were located in the middle of low stress SiO₂/Si₃N₄ (400 nm /1 μm) membranes. Membranes are thermally isolated from the chip frame because they have low thermal conductivity, generally. Temperatures were measured using IR thermometer with linearly increasing applied power. Power consumption at 400^∘C was 150 mW. Membranes of CPMS were withstood up to 730^∘C at the power of 350 mW. Characteristics of micro heaters for various heater widths of 50 μm, 75 μm, 100 μm and ratios of membrane dimension to heater dimension were measured. Sensing materials guided by micromachined well were dispensed on sensing electrodes. CPMS were mounted on a TO-8 package. From these results, fabricated and characterized CPMS could be used for applications in portable gas sensors for detection of CO, NO_x, CH_x, H₂S, and so on.     

NO2 adsorption behaviour on LaFeO3 electrodes of YSZ-based non-nernstian electrochemical sensors

X-ray photoelectron spectroscopy (XPS) was used to examine the NO₂ adsorption behaviour on the LaFeO₃ and Pt electrodes of planar yttria stabilized zirconia non-Nernstian gas sensors. The electrochemical sensors were exposed to the same gas atmosphere containing 1000 ppm NO₂ at 650°C. XPS of the as-prepared sensors and sensors after exposure to NO₂ revealed bonded nitrogen peaks on the surface of the semiconducting oxide but no nitrogen peaks on the Pt electrode. Therefore, NO₂ adsorption on a LaFeO₃ electrode plays an important role in the NO₂ detection mechanism.     

Novel SnO2@open microcell-liked graphene network as efficient detection for NO2

Abstract

The design of reduced graphene oxide (RGO) with novel porous structure has attracted tremendous attention owing to their larger specific surface area. Herein, three-dimensional open microcells, bowl-shaped RGO were fabricated through spray drying method which employed polystyrene spheres as a sacrificial template. The bowl-shaped, open microcell-liked pores observed in the RGO network had an average diameter of ≈1?μm. Subsequently, the catalytic SnO₂ nanoparticles were loaded on RGO network via a simple solvothermal method (SnO₂@RGO), and their gas sensing properties were investigated at room temperature (RT). In a comparison with pristine RGO network, the SnO₂@RGO composite exhibited almost 4 times higher response to 400?ppm NO₂ at RT and rapid recovery time. The extraordinary sensing performance can be attributed to the novel open microcell-liked porous microstructure with the SnO₂ catalyst nanoparticles.     

Exhaled VOCs sensing properties of WO3 nanofibers functionalized by Pt and IrO2 nanoparticles for diagnosis of diabetes and halitosis

This work presents a simple synthetic route to produce WO₃ nanofibers functionalized by catalytic Pt and IrO₂ nanoparticles and their superior acetone and H₂S sensing characteristics, demonstrating the potential use of Pt and IrO₂ nanoparticles in applications as sensors of biomarkers of diabetes and halitosis, respectively, in exhaled breath. The individual WO₃ fiber, calcined at 500 °C, was composed of small nanoparticles with a size distribution in the range of 30?C100?nm. Networks of WO₃ fibers exhibited a high surface-to-volume ratio and unique morphologies, thus facilitating efficient gas transport into the entire fiber layers. Pt (4?C7?nm) and Ir (4?C8?nm) nanoparticles were synthesized by polyol methods and were used as additives to decorate the surface of the WO₃ fibers. After a heat treatment, those catalyst particles were partially or fully oxidized to Pt/PtO_x and IrO₂, respectively. To investigate the advantages of Pt-decorated WO₃ fibers (Pt-WO₃) and IrO₂-decorated WO₃ (IrO₂-WO₃) fibers as acetone (CH₃COCH₃) and H₂S sensing materials, respectively, we carried out gas-sensing measurements in a highly humid atmosphere (RH 75?%) similar to that of an oral cavity. The Pt-WO₃ fibers showed a high acetone response (R_air/R_gas?=?8.7 at 5?ppm) at 350?°C and a superior H₂S response (R_air/R_gas?=?166.8 at 5?ppm) at 350?°C. Interestingly, IrO₂-WO₃ fibers showed no response to acetone, while the gas response to H₂S exhibited temperature-insensitivity, which has never been reported in any other work. Thus, the highly selective cross-response between H₂S and acetone was successfully achieved via the combination of IrO₂ particles on WO₃ fibers. This work demonstrates that accurate diagnosis of diabetes and halitosis by sensing exhaled breath can be realized through the use of electrospun WO₃ fibers decorated with Pt and IrO₂ catalysts.     

Nanostructured TiO2-based mixed metal oxides prepared using microemulsions for carbon monoxide detection

Nanostructured powders of Nb-doped TiO₂ (TN) and SnO₂ mixed with Nb-doped TiO₂ in two different atomic ratios—10 to 1 (TSN 101) and 1 to 1 (TSN 11)—were synthesized using the reverse micelle microemulsion of a nonionic surfactant (brine solution/1-hexanol/Triton X-100/cyclohexane). The powders were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Thick films were fabricated for gas sensors and characterized by XRD analysis and field emission scanning electron microscopy (FE-SEM). The effects of the film morphology and firing temperature in the range 650–850 °C on CO sensitivity were studied. The best gas response, expressed as the ratio between the resistance in air and the resistance under gas exposure (R _air/R _gas), was measured for TSN 11 at 11 for 1,000 ppm CO exposure. All types of sensors showed good thermal stability. The electrochemical impedance spectroscopy (EIS) measurements were performed in different gas atmospheres (air, O₂, CO and NO₂) to better understand the electrical properties of the nanostructured mixed metal oxides.     

Electrospun SnO2 nanofiber mats with thermo-compression step for gas sensing applications

SnO₂ nanofiber mats fabricated through electrospinning followed by thermo-compression and subsequent calcination steps exhibited unique morphologies facilitating efficient gas transport into the layers combined with high surface area (～73.5 m²/g, measured by BET) and small grain size (～5–15 nm), which are well suited for ultrasensitive gas detection. Single SnO₂ nanofibers were found to have a belt-like structure of closely packed nanocrystallites, facilitating excellent adhesion to the substrate and good electrical contact to the electrodes. I–V measurements of single SnO₂ nanofibers displayed ohmic behavior with electrical conductivity of 1.5 S/cm. Gas sensor prototypes comprising a random network of SnO₂ fibers exhibited high sensitivity when exposed to NO₂ at 225°C and CO at 300°C. A detection limit of 150 ppb NO₂ at 185°C was estimated by extrapolating the sensitivity results obtained on exposure to higher gas concentrations, demonstrating potential of achieving ultra-sensitive gas detection at low operating temperatures enabled by the present synthesis method.     

Tailoring the pore structure of cathode supports for improving the electrochemical performance of solid oxide fuel cells

Two types of lanthanum doped strontium manganite (LSM)-yttria-stabilized zirconia (YSZ) composite cathodes were prepared, one with the finger-like straight open pores by the phase inversion tape casting, and the other with the randomly distributed tortuous pores by the conventional tape casting. A gas permeation flux of 42.5?×?10⁵ Lm^?2 h^?1 was measured under a trans-membrane pressure of 0.6 bar for the former while only 10.6?×?10⁵ Lm^?2 h^?1 for the latter. Fuel cells supported on the as-prepared LSM-YSZ composite cathodes were fabricated, comprising a 15 μm thick YSZ electrolyte layer and a 20 μm thick NiO-YSZ anode. The electrochemical performance of the fuel cells was measured using H₂ as fuels and air as oxidants. The cell supported on the phase-inversion derived cathode showed a maximum power density of 362 mWcm^?2 at 850 °C, while only 149 mWcm^?2 for the cell supported on the cathode formed by the conventional method. The difference in the electrochemical performance between the two cells can be attributed to the pore structure of the cathode supports. It is concluded that the phase inversion tape casting provides a simple and effective approach for tailoring the pore structure of the cathode support and thus enhancing the electrochemical performance.     

Energy harvesting with a cymbal type piezoelectric transducer from low frequency compression

In this paper a piezoelectric energy harvester based on a Cymbal type structure is presented. A piezoelectric disc ?35?mm was confined between two convex steel discs ?35?mm acting as a force amplifier delivering stress to the PZT and protecting the harvester. Optimization was performed and generated voltage and power of the harvester were measured as functions of resistive load and applied force. At 1.19?Hz compression frequency with 24.8?N force a Cymbal type harvester with 250?μm thick steel discs delivered an average power of 0.66?mW. Maximum power densities of 1.37?mW/cm³ and 0.31?mW/cm³ were measured for the piezo element and the whole component, respectively. The measured power levels reported in this article are able to satisfy the demands of some monitoring electronics or extend the battery life of a portable device.     

Selective CO Gas Detection of Zn2SnO4 Gas Sensor

The ability of semiconductor gas sensors to differentiate between gases is essential but difficult to obtain. In this study, Zn₂SnO₄ was made to be CO selective and the possible mechanism for the selectivity was studied.The electrical and the gas-sensing properties of uncoated and CuO-coated Zn₂SnO₄ were investigated. In order to obtain an ohmic contact to Zn₂SnO₄, a ZnO layer was stacked on top of Zn₂SnO₄ and co-fired. CuO was coated by immersing the sintered sample in Cu-containing solution. Both uncoated and CuO-coated samples showed the higher sensitivity to 200 ppm CO gas than to 200 ppm H₂ gas. However, the CuO-coated Zn₂SnO₄ showed much enhanced sensitivity and thus good selectivity for CO gas (S _CO/S _{H 2} 6) compared to the uncoated sample. The excellent selectivity of Zn₂SnO₄-based materials for CO gas was explained by the difference in the mechanisms of CO and H₂ oxidation.     

Effects of different morphologies of ZnO films on hydrogen sensing properties

This paper describes the characteristics of chemiresistor hydrogen (H₂) sensors with different ZnO film structures in which ZnO dense films, nanoparticles (NPs), and nanorods (NRs) were prepared by RF magnetron sputtering, the sol–gel method, and the hydrothermal method, respectively. These were decorated with a Pt NP catalyst to investigate the performance of devices comprised of these structures. The effects of the ZnO morphology and operating temperature on the gas sensing behavior of the sensor are reported in detail. The various ZnO film morphologies, which contributed significantly to differences between sensors, play a very important role in enhancement of the supported Pt catalyst area and initial oxygen absorption on the ZnO surface. ZnO dense films prepared by sputtering showed the fastest response with a 13.5 % resistance variation at 1,000 ppm H₂ because gas adsorption occurred only on the film surface. The sensor with ZnO NRs showed a slower response, but the highest change in resistance of 65.5 % occurred at 1,000 ppm H₂ at room temperature. H₂ sensing performance of the chemiresistor sensors was improved due to the Pt catalyst, which was more efficient in dissociating H₂ gas molecules even at low temperature. The best chemiresistor sensor was fabricated using ZnO NRs and had a response time of approximately 10 s, a 27 s recovery time, and an 81.5 % change in resistance at 200 °C.     

RF transmitter using the dual‐pulse position modulation method for low‐power smart micro‐sensing chip

This paper presents a low‐power radio frequency (RF) transmitter using dual‐pulse position modulation (DPPM) for a smart micro‐sensing chip (SMSC) with sensors and large scale integrated circuit (LSI) on the same chip. The DPPM method is presented by a fixed pulse and a variable pulse within the same time frame. The distance between the fixed pulse and the variable pulse describes the amplitude of the input signal. A modulator and a ring oscillator were designed for the RF transmitter using the DPPM method. In the modulator, the pulse width modulation (PWM) signal is generated by the intersective method, and narrow pulses are extracted at the rising and falling positions of the generated PWM signal. The designed oscillator has the function of an oscillation controller. The RF transmitter was fabricated with sensors for an SMSC by complementary metal–oxide–semiconductor (CMOS) technology. The power consumption of the fabricated modulator was 4.5 mW. The power consumption of the proposed RF transmitter was measured as 7.0–7.3 mW at an input signal of 0.8–2.5 V. The RF transmitter using the DPPM method was able to reduce the power consumption by a maximum of 50.3% compared to a transmitter using the PWM method, because in the latter the dissipated power was 8.4–14.5 mW at the same input signal. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Gas Sensors Based on Piezoelectric Micro-Diaphragm Transducer

ABSTRACT

We have fabricated high sensitive gas sensor based on piezoelectrically driven micro-diaphragm transducers. The micro-diaphragm transducer was fabricated using micro-electro-mechanical-system (MEMS) technique. The diol based sol-gel derived Pb(Zr_0.52,Ti_0.48)O₃(PZT) film was used as a piezoelectric actuating layer. We have used the resonant frequency change of micro-diaphragm transducer upon mass increase as a sensing signal. The resonant frequency values were measured by analysis of electrical signals from the micro-diaphragm transducer. The fundamental resonant frequency of the micro-diaphragm was in the range of 250 to 360 kHz, depending on their physical boundary conditions. The mass sensitivity of bare micro-diaphragm transducer was 66.5 Hz/ng. Two polymer sensing layers such as the polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS) films were used to estimate the gas sensing behavior of microtransducers for various vapors of organic compounds. PMMA was used to detect primary alcohols while PDMS was used for toluene and benzene. The resonant frequency of micro-diaphragm transducer was shifted toward lower frequency range as the vapor concentration increased. With PMMA gas sensing layer, the micro-diaphragm showed a gas sensitivity of 0.456 Hz/ppm for ethanol vapor. When the PDMS gas sensing layer was used, the micro-diaphragm showed a gas sensitivity of 0.143 Hz/ppm for toluene vapor. When the test vapors were removed from the reaction chamber, the resonant frequencies of micro-diaphragm sensors were completely recovered to their initial state.     

NO2 sensing characteristics of ZnO nanorods prepared by hydrothermal method

ZnO nanorods were prepared via a hydrothermal reaction in a solution containing Zn(NO₃)₂⋅6H₂O, NaOH, cyclohexylamine, ethanol and water, and their NO₂ and CO sensing behaviors were investigated. The morphology and agglomeration of ZnO nanorods could be manipulated by controlling the amount of water in the solution, which was explained by the variation in the [OH⁻] due to an interaction between the water and cyclohexylamine. Sea-urchin-like and well-dispersed ZnO nanorods were prepared at low and high water content, respectively. Well-dispersed ZnO nanorods showed 1.8 fold change in resistance at 1 ppm NO₂ while there was no significant change in resistance at 50 ppm CO. The present ZnO nanorods can be used in automated car ventilation systems to detect NO₂ in the presence of CO. Presenting author in conference: PYEONG-SEOK CHO     

Treatment of exhaust gas from vehicles by discharge plasma reactors

The exhaust gas from diesel engines is one of the major causes of air pollution. As one method for pollution control, the authors have been developing plasma reactors. The authors' reactor is a concentric cylinder type which has a glass tube barrier between electrodes. Connecting two of these reactors in series, the NO_x (NO + NO₂) component from diesel engine exhaust gas is decomposed up to 93%. This reactor was mounted on a 3-liter diesel engine and received six-mode load testing at an authorized public facility (JARI, Tsukuba City) and cleared the legal critical value of 350 ppm of NO_x. By this success, the effectiveness of discharge plasma treatment of diesel exhaust gas and its practical evaluation have been confirmed. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(2): 1–7, 1997     

Modelling of chemical surface acoustic wave sensors and comparative analysis of new sensing materials

Comparative analysis of different, new gas sensing materials in surface acoustic wave chemical sensors is presented. Different gas sensing materials as polyaniline (PANI), Teflon AF 2400, polyisobutylene (PIB), polyepichlorohydrin (PECH) are considered. They are chosen according to the type of gas to be detected and the desired accuracy: Teflon AF 2400 thin film for the detection of CO₂, PANI nanocomposites film that belongs to the group of conductive polymers for the detection of CO, NO₂ and phosgene (COCl₂), and PECH and PIB for the detection of dichloromethane (CH₂Cl₂, DCM). In the analysis, the simple and useful method of the complete analyses of gas chemical sensors is used. The method is based on the electrical equivalent circuit of the surface acoustic wave sensor. The method is very efficient and can be used for the optimal design of CO₂ sensors. The results are compared with those presented in public literature and good agreement is obtained, demonstrating the validity of modelling. Copyright © 2012 John Wiley & Sons, Ltd.     

Potentiometric Gas Sensors for Oxidic Gases

Solid electrolyte-based electrochemical devices combined with an auxiliary phase of oxyacid salt have, in this decade, emerged as new attractive sensors to detect oxidic gases of CO₂, NO, NO₂ and SO₂. Various combinations of solid electrolytes and auxiliary phases as well as various new single or multi-component auxiliary phases have been exploited to improve the gas sensing properties and stability of these devices. Some of the potentiometric sensors developed e.g., CO₂ sensors using NASICON and Li₂CO₃-CaCO₃, NO₂ sensors using NASICON and NaNO₂-Li₂CO₃ and SO₂ sensors using MgO-stabilized zirconia and Li₂SO₄-CaSO₄-SiO₂, exhibit excellent gas sensing performances in laboratory tests and appear to be promising for monitoring the respective gases in ambient environments and/or combustion exhausts. This paper aims at describing our exploratory works on and the state of the art of these potentiometric gas-sensing devices.     

Studies about Microstructures and Mechanical Properties of Ti(C,N) Ceramic Material Using Different Solvents

Abstract

Ti(C,N) ceramic materail plays an important role in the field of material processing due to their good mechanical properties and thermal stability. In this experiment, Ti(C,N) powders were successfully prepared by solvothermal and high-temperature calcining method, using TiOSO₄ and C₃H₆N₆ as raw material, and n-propanol and ethylene glycol as solvents. The microstructure of Ti(C,N) powders were characterized by X-ray diffraction and field emission scanning electron microscope, and their hardness were tested by vickers microhardness tester. The microstructure and mechanical properties of Ti(C,N) powders using two different solvents were investigated comparatively. Ti(C,N) powders prepared using n-propanol with a size of about 20–30?μm can reach the maximum hardness of 660 HV after sintering. Ti(C,N) powders prepared using ethylene glycol with size range of 3?μm to 5?μm come up to the maximum hardness of 889 HV. The different mechanism of solvents in preparation of Ti(C,N) ceramic material was discussed.