Gas Sensitive Layer Research Articles

Construction of Fully Integrated and Energy Self-Sufficient NO2 Gas Sensors Utilizing Zinc-Air Batteries.

Exploration of novel self-powered gas sensors free of external energy supply restrictions, such as light illumination and mechanical vibration, for flexible and wearable applications is in urgent need. Herein, this work constructs a flexible and self-powered NO2 gas sensor based on zinc-air batteries (ZABs) with the cathode of the ZABs also acting as the gas-sensitive layer. Furthermore, the SiO2 coating film, serving as a hydrophobic layer, increases the three-phase interfaces for the NO2 reduction reaction. The constructed sensors exhibit a high sensing response (0.3 V @ 5 ppm), an ultralow detection limit (61 ppb), a fast sensing process (129 and 103 s), and excellent selectivity. Moreover, the sensors also possess a wide working temperature range and a low working temperature tolerance (0.34 V at -15 °C). Simulations indicate that the hydrophobic surface at the cathode-hydrogel interface will accommodate more NO2 gas molecules at the reaction sites and prevent the influence of inner water evaporation and direct dissolution of NO2 in the electrolyte, which is beneficial to the enhanced gas sensing abilities. Finally, the self-powered sensing device is incorporated into a smart sensing system for practical applications. This work will pave a new insight into the construction of integrated and energy self-sufficient smart gas sensing systems.

Conductivity and response formation of semiconductor gas sensors based on tin dioxide nanomaterials

Highly sensitive to methane semiconductor sensors were obtained with the use of nanosized tin dioxide synthesized via sol-gel method. Sensitization of the sensors was achieved by introduction of Pt additives into semiconductor gas sensitive materials. Experimental data of dependences of conductivities of the created sensors on methane concentrations were used for approximation by a mathematical model of the sensor conductivity and response formation. The model is based on features of a methane oxidation reaction on the surface of the gas sensitive material and allows to connect conductivity of the sensor with surface processes occurred on its gas sensitive layer.

Tin Oxide Modification of Indium Oxide Gas Sensitive Layers to Increase Efficiency of Gas Sensors

Monitoring of air pollutions is one of actual trends in the development of industrial and domestic instrumentation. There are sets of tasks for improving gas analytical instruments because of increasing demand for control of a concentration of explosive and toxic gases on a level with maximum allowable concentration. The aim of the paper was to investigate the methods of formation and elemental composition of indium oxide films modified with tin oxide on the surface of gas sensor elements as one of the promising compounds for improving the detection efficiency of explosive and toxic gases in the environment. The processes of formation of gas-sensitive films deposited on the surface of nichrome alloy information electrodes were studied in this article. Substrates of anodic aluminum oxide with area of 10 × 10 mm2 and a thickness of 45 ± 0,5 μm were chosen for research. Two layers on the surface of the samples were formed. The first layer was formed from NiCr alloy (Ni – 80 %, Cr – 20 %) with a thickness of ≈ 0.3 μm by ion-plasma sputtering. The second layer was based on indium oxide with addition of tin oxide with thicknesses from ≈ 0.3 μm to ≈ 1.0 µm and coated with sol-gel technology. Five samples of gas-sensitive films were formed with different methods of deposition and heat treatment. Scanning electron microscopy was used for study of films’ morphology and elemental compositions of samples. The most perfect continuous semiconductor films were obtained by multilayer applying of a sol-gel paste. When semiconductor films were processed at annealing temperatures of 700 °C and higher in vacuum so there was observed cracking of semiconductor films up to a layer of NiCr alloy. The developed surface of gas-sensitive films allows to reach high sensitivity and affectivity of semiconductor sensors for control of air gas composition.

Gas Sensing Properties of CuWO4@WO3 n-n Heterojunction Prepared by Direct Hydrolysis of Mesitylcopper (I) on WO3·2H2O Nanoleaves

The nanometer size Cu2O@WO3·H2O composite material has been prepared by the direct hydrolysis of mesitylcopper (I) on WO3·2H2O nanoleaves. The synthesis has been performed in toluene without the addition of any ancillary ligands. The prepared nanocomposite has been deposited as a gas-sensitive layer on miniaturized silicon devices and heated up gradually to 500 °C in the ambient air. During the heating, the CuWO4 phase is formed upon the reaction of Cu2O with the WO3 support as revealed by the XRD analyses. The as-prepared CuWO4@WO3 sensors have been exposed to 10 ppm of CO or 0.4 ppm of NO2 (RH = 50%). At the operating temperature of 445 °C, a normalized response of 620% towards NO2 is obtained whereas the response to CO is significantly lower (S = 30%). Under these conditions, the sensors prepared either with pristine CuO or WO3 nanostructures are sensitive to only one of the two investigated gases, i.e., CO and NO2, respectively. Interestingly, when the CuWO4@WO3 sensitive layer is exposed to UV light emitted from a 365 nm Schottky diode, its sensitivity towards CO vanishes whereas the response towards NO2 remains high. Thus, the application of UV illumination allowed us to modify the selectivity of the device. This new nanocomposite sensor is a versatile sensitive layer that will be integrated into a gas sensor array dedicated to electronic nose platforms.

Physical properties of thermal annealing induced In2O3 thin films for sensing layer applications

Detection and monitoring of flammable, toxic and explosive gases are crucial, where active research on nanostructured metal oxide materials and thin films is enthusiastically performed to explore new paths for improving the characteristic parameters of sensors. The higher stability and appropriate electrical properties along with wide band gap of In2O3 material make it suitable for active layer to gas sensors where properties of In2O3 films could be engineered by post treatments. The present work demonstrates an influence of thermal annealing on structural, optical, topographical, morphological, elemental and electrical properties of In2O3 films for gas sensor applications. The In2O3 films of thickness 500 nm are grown onto soda lime glass and conducting ITO substrates employing physical vapor deposition based thermal evaporation technique followed by thermal annealing at 200 °C, 300 °C and 400 °C for one hour in air atmosphere. The XRD analysis indicated phase transformation from metastable rhombohedral to stable cubic phase where crystallite size corresponding to associated preferred (110) and (222) peaks is tuned in range of 32–51 nm with annealing. Optical analysis unveiled higher transmittance in ultraviolet and visible regions whereas absorbance of films is fluctuated with thermal annealing. All the In2O3 films indicated the Ohmic nature where the resistivity is detected to be enhanced with annealing. EDS patterns confirmed deposition of the films. The SEM analysis showed nanospheres like grain morphology and such well connected nanospheric features offer large surface area and stimulate adsorption of gas effectively. Hence structural, optical, electrical and morphological properties of gas sensitive In2O3 layers are greatly stimulated by thermal annealing.

Highly Sensitive Isopropanol Gas Sensor based on SnO2 Nano-Flowers on Gold, Silver, and Aluminum Interdigitated Electrodes

In this research we have proposed a high selectivity Isopropanol gas sensor. The sensor shows significant resistance change only to Isopropanol gas. The synthesis method of flower-like SnO2 nanostructures, the electrode material and design, and the optimized working temperature provide the high selectivity and high response of the sensor. The SnO2 nanoflowers (NFs) have been synthesized in a two-step process as the gas sensitive layer. The sensor shows its best performance on Au interdigitated electrodes. The optimized working temperature is obtained at 150 °C. The proposed sensor has a high sensitivity, good repeatability, long-term stability and remarkable selectivity. The responses of the sensor to 100 ppm of isopropanol at 150 °C is 71 and the sensor is capable of keeping almost 96% of the initial response in a 40 d period.

Thermo-cyclically operated metal oxide gas sensor arrays for analysis of dissolved volatile organic compounds in fermentation processes: Part I – Morphology aspects of the sensing behavior

For a comparative study of the gas sensing performance to dissolved volatile organic compounds (VOCs), the SnO2-powders have been prepared using two different fabrication routes; the flame-spray-pyrolysis (FSP) and the sol-gel (SG) route; and were admixed with the same additive-powders (alumina, YSZ, and NASICON). The morphology of the two different SnO2/additive families were investigated by ESEM analysis and Energy-dispersive X-Ray Spectroscopy (EDS). Both SnO2/additive material families were separately deposited as thick-film-layers on two four-fold sensor-chips which were simultaneously thermo-cyclically operated in a measurement cell combined with a carrier gas probe, which enables sensing tests with evaporated VOCs (acetic acid, propionic acid, ethanol, acetone) dissolved in water. The resulting Conductance-over-Time-Profiles (CTPs) highlight better sensitivities of most of the FSP-layers to all the analytes compared to the SG-prepared layers. Furthermore, the CTP shapes of the FSP layers show clearly enhanced specificity representing the individual analyte components. This was interpreted to be the consequence of the extremely fine, scarcely agglomerated grain morphology of FSP-prepared powders and their very narrow grain size distribution which provide better conditions for enhanced gas specific surface reactions. Results promise a better chemical analysis capability of dissolved VOCs by numerical analysis of the CTP of FSP-prepared gas sensitive layers.

A novel room temperature SO2 gas sensor based on TiO2/rGO buried-gate FET

Gas sensors are able to detect the type and concentration of flammable, explosive, toxic gases (e.g. NO2, SO2, NH3), showing wide applications in petroleum, chemical, medical, transportation and safety protections in the domestic and industrial environment. Currently, the SO2 gas sensors are mainly based on metal oxide semiconductors. However, its operating temperature is generally much higher than room temperature, and the operation at higher temperature often requires complex circuits and high power consumption. A novel room temperature (RT) SO2 gas sensor based on buried-gate field effect transistor (FET) and interdigitated electrode structure with titanium dioxide (TiO2) nanoparticles and reduced graphene oxide (rGO) composite materials as gas-sensitive layer, has been fabricated and demonstrated in this paper. The TiO2/rGO FET sensor shows a relative high responsivity (3.46%) to 20 part per million (ppm) SO2 gas, under a source-drain voltage of 2 V and a zero gate voltage. Experimental results have shown that the responsivity and response-recovery time of the sensor could be controlled by the source-drain voltage. This paper demonstrates the feasibility of TiO2/rGO nanocomposite as a gas-sensing material for high-performance SO2 gas sensors. In addition, this approach would provide a feasible way for the realization of miniaturized, integrated and high-performance SO2 gas sensors.

A Micropowered Chemoresistive Sensor Based on a Thin Alumina Nanoporous Membrane and SnxBikMoyOz Nanocomposite.

This work presents and discusses the design of an efficient gas sensor, as well as the technological process of its fabrication. The optimal dimensions of the different sensor elements including their deformation were determined considering the geometric modeling and the calculated moduli of the elasticity and thermal conductivity coefficients. Multicomponent SnxBikMoyOz thin films were prepared by ionic layering on an anodic alumina membrane and were used as gas-sensitive layers in the sensor design. The resistance of the SnxBikMoyOz nanostructured film at temperatures up to 150 °C exceeded 106 Ohm but decreased to 104 Ohm at 550 °C in air. The sensitivity of the SnxBikMoyOz composite to concentrations of 5 and 40 ppm H2 at 250 °C (10 mW) was determined to be 0.22 and 0.40, respectively.

Chromatographic Detector Based on Adsorption-Semiconductor Sensor for Detection of Reducing Gases in Air

A semiconductor sensor with a gas sensitive layer based on SnO2 and Sb2O5 with Pd additive synthesized by co-precipitation was studied as a chromatographic detector. It was found that the subject detector, using air as a carrier gas, can selectively detect each of the reducing gases (H2, CO, CH4, C2H2, C2H4, C2H6) which can leak into transformer oil if defects emerge during high-voltage transformers operation. It was established that for the fabricated detector the dependences of the signals on concentrations of the analyzed gases are linear in the range of 0–50 ppm for H2, CO, CH4 and 0–100 ppm for C2H2, C2H4, C2H6. The detector can be on-stream as combined with a chromatograph instrumentality which is rational for a wide practical application. The usage of the semiconductor detector based on the adsorption-semiconductor sensor is promising to significantly reduce the detection cost of the gases and to simplify diagnostics of the high-voltage transformer conditions.

Multilayer sensor structure based on porous silicon

The work is devoted to the creation of a sensor structure based on a porous silicon membrane. The structure under study integrates a porous layer used as a gas transport layer and a gas sensitive layer of non-stoichiometric tin oxide. The paper investigates the morphology of the structure and shows the gas permeability of the membrane on porous silicon. The gas sensitivity of the test structure obtained by passing a gas-air mixture containing NO2 has been studied. Keywords: porous silicon, membrane, non-stoichiometric tin oxide, thin films, resistive gas sensor.

Многослойная сенсорная структура на основе пористого кремния

Abstract The work is devoted to the creation of a sensor structure based on a porous silicon membrane. The structure under study integrates a porous layer used as a gas transport layer and a gas sensitive layer of non-stoichiometric tin oxide. The paper investigates the morphology of the structure and shows the gas permeability of the membrane on porous silicon. The gas sensitivity of the test structure obtained by passing a gas- air mixture containing NO2 has been studied.

Semiconductor metal oxide sensor for hydrogen sulphide operating under non-stationary temperature conditions

The aim of the work was to create a selective gas sensor for hydrogen sulphide. As a result of adding ammonia to the zinc acetate solution, centrifuging the obtained zinc hydroxide and subsequent calcination, a polydisperse zinc oxide powder with a grain size of 5–50 nm was obtained. The material was characterized using X-ray phase analysis and transmission electron microscopy. Subsequently, silver nitrate and terpeniol were added to the zinc oxide nanopowder to form a paste. The gas-sensitive material was obtained by applying the resulting paste on a special dielectric substrate and subsequent calcination, as a result of which the terpeniol burned out, and the silver nitrate turned into an oxide (the mass fraction of the silver was 3%). A non-stationary temperature mode for the operation of the sensor was selected, in which, after rapidheating of the sensor to 450 °C (2 seconds), slow (13 seconds) cooling to 100 °C occurred. Each subsequent heating-cooling cycle with a total period of 15 seconds began immediately after the end of the previous cycle. The use of an unsteady temperature mode in combination with the selection of the composition of the gas-sensitive layer made it possible to obtain a response of 200 for a hydrogen sulphide concentration of 1 ppm. Along with an increase in sensitivity, a significant increase in selectivity was also observed. The cross-sensitivity for the determination of hydrogen sulphide and other reducing gases (CO, NH3, H2) was more than three orders of magnitude. Thus, this sensor can be used to detect hydrogen sulphide even in the presence of interfering components. The use of highly selective sensors in the tasks of qualitative andquantitative analysis can significantly simplify the calibration in comparison with “electronic nose” devices. Devices based on highly selective sensors do not require the use of mathematical methods for processing multidimensional data arrays.

Research of Low-Power MEMS-Based Micro Hotplates Gas Sensor: A Review

In this paper, the research status of low power micro hot plate gas sensor is reviewed. This type of sensor is characterized by the combination of MEMS technology and related insulation measures. Compared with the traditional ceramic tubular sensor, the power consumption of the sensor is greatly reduced, which provides the possibility of remote sensor arrangement and long term stable operation. This paper firstly collects the current sensor-related heat conduction, heat convection and heat radiation theories, and summarizes the calculation model of heat loss in different regions of the micro hot plate gas sensor. Then, the application examples of the finite element method in the analysis of thermal characteristics of the sensor and the common preparation process of the substrate and gas sensitive layer of the micro hot plate are listed. Finally, the main test methods of the stability and gas sensitivity of the micro hot plate gas sensor are summarized.

Accurate Quantification of Formaldehyde at ppb Level for Indoor Air Quality Monitoring

Accurate Quantification of Formaldehyde at ppb Level for Indoor Air Quality Monitoring

(IMCS Third Place Best Paper Award) Experimental Verification of the Temperature Homogeneity of Heated Gas Sensor Transducers inside a Protection Cap

Introduction High temperature gas sensors play an important role in monitoring or controlling energy conversion processes. Sensors containing functional materials often need to be heated to a certain operating temperature to achieve functionality. This can be realized using planar thick-film heaters, which are screen-printed on the sensor substrate. When sensors are installed in the exhaust gas flow in order to achieve a fast response behavior, high volume flow has cooling effects on the sensor and leads to an inhomogeneous temperature distribution. This might cause a changed sensor function and cross-sensitivities [1]. For this reason, protective caps are used to reduce the flow influence on the temperature distribution with sufficient response time [2]. Resulting temperature distribution can be simulated using FEM models but cannot be verified directly inside protective caps [3, 4]. The present work solves this uncertainty in two ways to directly measure the temperature homogeneity within a cap. Methods and Setup Sensors are built up on alumina substrates (Figure 1a). On the reverse side, a meander-shaped platinum structure acts as heating element. Two methods are used to determine the temperature distribution on the front side.Firstly, the temperature distribution is recorded by an IR camera (Figure 1c). For optically access, an IR-transparent glass replaces one part of the cap. The requirements for this glass are the transmission of the IR radiation (here attention must be paid to the appropriate wavelength of the camera) and a certain temperature resistance, since the cap and the glass are also heated by the heat radiation of the sensor.Secondly, the temperature distribution is directly measured by means of thermocouples (Figure 1b). Instead of a sensor structure, a matrix of planar screen-printed thermocouples (Figure 1d) is applied onto the front side of the transducer. For this purpose, Pt and Au feedlines are arranged so that five different measuring points are realized at the sensor tip, i.e., that area where a gas sensitive layer would be located. Temperature values are derived from the measured thermvoltage on basis of Seebeck coefficients for Pt/Au given in the literature. Results and Discussion In order to verify the function of the printed thermocouples, the sensor temperature was increased to 600 °C by means of the heater on the reverse side. The heat distribution was recorded by the thermocouples without the protective cap. Thermal images were taken simultaneously with the IR camera. The measured temperature of the thermocouples corresponds well to the thermal images.Afterwards the sensor was placed inside a protective cap containing the described IR-permeable glass window. After increasing the sensor temperature to 600 °C, thermal images of the sensor tip could be taken again. The transmission coefficient of the glass is required for this purpose. To check the accuracy of the transmission coefficient, the temperatures of the thermocouples were recorded simultaneously. The temperature measured by the IR camera again corresponds to the thermocouples values. Thus, the application of this method was also checked and confirmed.Both methods were used to investigate the influence of a protective cap on the temperature distribution on the front side of the sensor at 600 °C operating temperature. Figure 1e) and f) show the temperature distribution of a sensor in a horizontal mounting position recorded by an IR camera (temperature gradients in general come from the individual heater structure and several heat losses, especially heat flow along the substrate material). Figure 1e) shows the measurement without a protection cap. The thermocouple measurement shows a temperature gradient of 6.78 °C/mm between point 1 and 2. The temperature gradient derived from the thermal image is 5.88 °C/mm. Both data (thermocouple and thermal image) agree very well. Then the same measurement was carried out with a protective cap (Figure 1f). The temperature gradient, measured with the thermocouples, is 4.97 °C/mm. The thermal image shows a value of 4.43 °C/mm. The protective cap thus has a positive influence on the temperature homogeneity of a sensor even without a flow. Outlook In the next steps, the temperature distribution in an exhaust flow is investigated by means of the thermocouples. Variations in sensor orientation or mounting positions is possible. Here as well it is possible to investigate the influence of a protection cap. A high temperature setup might be realized by use of screen-printed Pt/PtRh thermopiles [5].

Field Study of Metal Oxide Semiconductor Gas Sensors in Temperature Cycled Operation for Selective VOC Monitoring in Indoor Air

More and more metal oxide semiconductor (MOS) gas sensors with digital interfaces are entering the market for indoor air quality (IAQ) monitoring. These sensors are intended to measure volatile organic compounds (VOCs) in indoor air, an important air quality factor. However, their standard operating mode often does not make full use of their true capabilities. More sophisticated operation modes, extensive calibration and advanced data evaluation can significantly improve VOC measurements and, furthermore, achieve selective measurements of single gases or at least types of VOCs. This study provides an overview of the potential and limits of MOS gas sensors for IAQ monitoring using temperature cycled operation (TCO), calibration with randomized exposure and data-based models trained with advanced machine learning. After lab calibration, a commercial digital gas sensor with four different gas-sensitive layers was tested in the field over several weeks. In addition to monitoring normal ambient air, release tests were performed with compounds that were included in the lab calibration, but also with additional VOCs. The tests were accompanied by different analytical systems (GC-MS with Tenax sampling, mobile GC-PID and GC-RCP). The results show quantitative agreement between analytical systems and the MOS gas sensor system. The study shows that MOS sensors are highly suitable for determining the overall VOC concentrations with high temporal resolution and, with some restrictions, also for selective measurements of individual components.

Nanomaterial based on SnO2 with Co and Sb additives – perspective material for H2 semiconductor sensors

Semiconductor microcrystalline and nanosized materials based on SnO2/Sb2O5 and Со/SnO2/Sb2O5 were obtained using a coprecipitation method and sol-gel technology. The morphology and phase composition of the obtained materials were studied. It was found that the obtained Co-containing sensors based on gas sensitive layers created with nanosized material SnO2/Sb2O5 (0.15% Sb) show higher sensitivities to microconcentration of H2 (40 ppm) compared to the sensors based on gas sensitive layers with microcrystalline material. Studies of dynamic parameters of the sensors with similar chemical composition have shown that the sensors based on the nanomaterial have almost twice better response time and shorter recovery time compared to the sensors based on the microcrystalline material. The obtained results are explained by energy inhomogeneity of surfaces of gas sensitive layers of the sensors based on the nanosized and microcrystalline materials.

Radiowave technology of resonant gas-sensor microwave telemetry

The possibility of using microstrip reflector microwave resonators in solving problems of resonant gas-sensor telemetry on layered dielectric substrates with gas-sensitive sputtering was investigated. It is noted that the use of chemically active sputtering, for example, on the basis of zeolites having a high selective gas adsorbent kinetics in terms of speed, makes it possible to create radiosensor materials capable of changing the dielectric constant in the process of absorbing gases, as well as of sublimated vapors of solid and liquid phases of various compounds. As an alternative approach in the field of dosimetric gas monitoring, a modification of radiosensor applications based on microwave sensors is proposed, which allows using microwave solutions based on microstrip microwave resonators with active gas-sensitive sorption zeolite sputtering on a dielectric substrate to conduct gas analysis in real time. The radio-wave principle of the microstrip gas sensor analyzer was formulated. An electrodynamic model of a microstrip gas sensor analyzer in the Altair Feko environment was developed. An experiment was planned, and gas-sensor telemetry tests of ammonia vapors dissolved in water were carried out. It was established that the amount of sorbed water and ammonia in the zeolite unambiguously conforms both to the absolute value of the reflection coefficient at resonance and to the resonant frequency itself. Using the example of recording hydrogen nitride vapors it was shown that the reflection coefficient and frequency shift in the resonator, which depend on the concentration of the adsorbed gas, correspond to the saturation characteristics of the gas sensor and make it possible to repeatedly measure small concentrations of a gas that can be absorbed by zeolite at a temperature corresponding to the condition of rapid evaporation of controlled gas from the active dielectric layer, which guarantees desorption of the sensor. It was established that in order to increase the speed of the gas sensor response it is advisable to create a microstrip resonator for the resonance region of 8...10 GHz and to use a microstrip sensor substrate material with a high dielectric constant. This is due to the fact that the transition to the upper microwave frequencies will allow reducing the size of the topology of the microstrip resonator and reducing the effective area of the zeolite deposition, and, consequently, increasing the adsorption rate of the gas-sensitive layer of the active dielectric.

Perspectives for usage of adsorption semiconductor sensors based on Pd/SnO2 in environmental monitoring of carbon monoxide and methane emission

Nanosized semiconductor sensor materials based on SnO2 with different palladium contents were obtained via zol-gel technology with the use of ethylene glycol and hydrate of tin (VI) chloride as precursors. Morphology and phase composition of nanosized sensor materials were studied by X-ray diffraction and TEM methods. Catalytic activities of the Pd/SnO2 nanomaterials in the reaction of H2 and CO oxidation were investigated. Adsorption semiconductor sensors based on Pd/SnO2 nanomaterials were made by their calcination up to 620 0C in air and the sensors were found to be highly sensitive to presence of CO and CH4 in air ambient. Higher responses to CO of Pd-containing sensors in comparison with their responses to CH4 were confirmed by higher reaction activity of CO in catalytic oxidation reaction. Differences in sensitive properties of the sensors to methane and carbon monoxide were explained by features of the catalytic reactions of methane and carbon monoxide oxidation occurring on surfaces of the gas sensitive layers of the sensors.