A simple and high effective tellurium based sensor for No/sub 2/ detection

CdO:ZnO nanocomposite thin films for oxygen gas sensing at low temperature

This review contains comprehensive study of semiconductor metal oxide films gas sensors. SMO are smaller in size, simple fabrication technique, sensitivity and selectivity is good. Review study paper offers understanding about the dopants impurity made the deviations in the SMO materials used to fabrication of gas sensor. Reviews study shows SnO₂-WO₃, ZnO, Au composites good for H2S and methane (CH4) gases it has sensitivity in the range 75 % to 90% at operating temperature between 200 0C to 350 0C. Composites of ZnO-Au, SnO₂, TiO₂ shows decent sensitivity in the range between 72 % to 95% at operating temperature between 200 0C to 350 0C towards H2S and methane (CH4) gases. Pure and composite of different SMO films prepared using Sol-Gel, Sputtering, CVD and Screen printing method result shows Screen printing technique is most preferred and it shows increase in sensitivity at lower operating temperature. This review provides summary of the advance and performance of pure and composites of ZnO and SnO2 based films sensors for H₂S and CH₄ detection, focusing on key parameters such as response time, operating temperature, sensitivity and fabrication technologies. It provides detail analysis of the pure and composite of SnO2 &ZnO towards H₂S and CH₄ and shows future perspective towards increase in sensitivity, lower operating temperature, response time and stability of gas sensor.

Cocoon-like ZnO decorated graphitic carbon nitride nanocomposite: Hydrothermal synthesis and ethanol gas sensing application

Room‐temperature detection of volatile organic compounds in particle‐per‐billion concentrations is critical for the development of wearable and distributed sensor networks. However, sensitivity and selectivity are limited at low operating temperatures. Here, a strategy is proposed to substantially improve the performance of semiconductor sensors. Tunable oxygen vacancies in thick 3D networks of metal oxide nanoparticles are engineered using deep ultraviolet photoactivation. High selectivity and sensitivity are achieved by optimizing the electronic structure and surface activity while preserving the 3D morphology. Cross‐sectional depth analysis reveals oxygen vacancies present at various depths (≈24% at a depth of 1.13 µm), with a uniform distribution throughout the thick films. This results in ≈58% increase in the sensitivity of ZnO to 20‐ppb ethanol at room temperature while ≈51% and 64% decrease in the response and recovery times, respectively. At an operating temperature of 150 °C, oxygen‐vacant nanostructures achieve a lower limit of detection of 2 ppb. Density functional theory analysis shows that inducing oxygen vacancies reduces activation energy for ethanol adsorption and dissociation, leading to improved sensing performance. This scalable approach has the potential for designing low‐power wearable chemical and bio‐sensors and tuning the activity and band structure of porous, thick oxide films for multiple applications.

The response time of fire sprinklers is essential for their performance, especially in applications where life safety protection is desired. The earlier the sprinkler activates, the smaller the size of the fire. Most commercial residential sprinklers are fitted with 3 mm, 68°C glass bulbs. However, thinner sprinkler glass bulbs with lower operating temperatures are available. The aim of this study was to determine the response time—and the corresponding heat release rate—of different glass bulbs in a residential room fire scenario. A series of tests were conducted inside a compartment measuring 3.66 m by 3.66 m having a ceiling height of 2.5 m. The compartment was either enclosed or had two walls removed to provide a more ventilated scenario. A propane gas burner was positioned at one of the corners. The mass flow rate of the gas was controlled such that either ‘slow’, ‘medium’ or ‘fast’ fire growth rate scenarios were simulated. In each test, nine Response Time Index (RTI) and operating temperature combinations were tested. Each test was replicated three times. In addition, two commercial fire detectors were tested. The results show that the fire is considerably smaller upon activation with a combination of a low RTI and a low operating temperature, as compared to the 3 mm, 68°C glass bulb typically used for residential sprinklers. The operating temperature proved to have a larger impact on the results than the RTI. The heat from the fire was typically detected by the fire detectors prior to the activation of the sprinkler glass bulbs, especially for the ‘slow’ and ‘medium’ fire growth rate scenarios.

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Low operating temperature micro solid oxide fuel cell (micro-SOFC) systems which utilize liquid fuels are attractive for high energy efficiency and density. Long time operation is possible for micro-SOFCs by refueling and it is not restricted by battery charge. Hence, it is expected to alternative power source for compact mobile electronic devices which consume a large amount of power. In previous, diverse types of micro-SOFCs focused on its performance was reported and a maximum power density of 1 W/cm2 at 500oC was achieved. However, they have many problems of durability and repeatability for practical use. Additionally, it is necessary to consider how to heat up micro-SOFC cells to operating temperature, compatibility with other electronic circuits and thermally self-sustained condition. In this study, micro-SOFC system was proposed in order to adopt the micro-SOFCs into a power source of compact mobile electric devices. We designed the micro-SOFC package with thermal insulation and heat exchanger. Its insulation performance was experimentally confirmed and power generation test with micro heater in the micro-SOFC package was performed. Micro-SOFC packages require thermal insulation to keep the surface temperature below about 85oC, which is the maximum operating temperature of other electronic circuits. We applied vacuum insulation widely used for thermal insulation in MEMS devices to the package. The heat of the exhaust gas was recovered by the liquid fuel through heat exchanger. From the calculation of temperature distribution by the finite element method, surface temperature of the package was kept from 57 to 88oC when operating temperature is 500oC. As long as the liquid is evaporated, temperature of the heat exchanger is kept constant. Therefore, heat conduction from high temperature glass flow channels to vacuum insulator was prevented and low surface temperature was achieved. Applying micro-SOFC system as independent power sources, thermally self-sustained condition is required, i.e. micro-SOFC packages should keep operating temperature more than 300oC only by the generated heat from reaction of fuel cells during power generation. For thermally self-sustained condition, the amount of heat generated during power generation needs to exceed that of heat loss from surface of the package and transferred heat by exhaust gas. We calculated thermally self-sustained condition of a micro-SOFC system with 4 W output fueled by methanol and water that has reformer and heat exchanger from exhaust gas. From the result, generated heat from reaction of fuel cells was 4.8W, heat loss from surface was 1.5 W and transferred heat by exhaust gas was 4.6 W at 500oC. The result confirmed that transferred heat by exhaust gas was dominated in this system. Hence, recovering the heat of exhaust gas with the gas-liquid heat exchanger was essential to achieve thermally self-sustained condition. Micro-SOFC cells and micro heater were fabricated in general MEMS technology. Power generation test and electrochemical impedance spectroscopy of micro-SOFC (Pt-Pd | 15 mol% Y-doped BaZrO3 | Pt-Pd) by micro heater were measured in the temperature range of 200-330oC. A maximum power density of 1.12 mW/cm2 and open circuit voltage of 1.09 V at 330oC were obtained using wet H2 as fuel and air as the oxidant. As a result of impedance measurement, electrode reaction resistance was very high in cell total resistance. Therefore, the improvement of electrode will reduce cell resistance to a level for actual utility. We actually fabricated the micro-SOFC package installed vacuum structure and heat exchanger to evaluate cooling performance of the insulation package and performed power generation test in the package. In the case of heat exchanger filled with 50 vol% ethanol solution, surface temperature was 89.6oC when micro heater temperature was 500oC. In accordance with the result, it was possible to keep surface temperature of the package sufficiently low with heat exchanger. As a result of power generation test of micro-SOFC inside the package, a maximum power density of 1.27 mW/cm2 and open circuit voltage of 0.94 V at 300oC were obtained using wet 25% or 50% H2 as fuel and air as the oxidant. Additionally, surface temperature of the package was kept under 80oC. Therefore I achieved power generation in compact micro-SOFC system with thermal insulation. The micro-SOFC package was designed for applying micro-SOFCs to power source of mobile electronic devices in this study. As a result of calculation, the package installed vacuum insulation and heat exchanger could achieve thermally self-sustained condition and low surface temperature. Power generation test of micro-SOFC inside the prototype package was achieved and the package kept low surface temperature. Hence, this study presented that applicability of micro-SOFC system to power source for compact mobile electronic devices.

Metal oxide semiconductors materials such as zinc oxide (ZnO) are often used in the fabrication of chemoresistive gas sensors, but ZnO materials require high operating temperatures to operate. In another side, carbon nanotubes (CNT) have many distinct properties and recently have been exploited as the next generation of sensors, including chemoresisitive type gas sensors. This study was aimed to investigate the performance of MWNT-ZnO composites as SO2 gas sensitive layer. By fabricate composites of MWNT and ZnO, have been obtained a sensitive layer that can be utilized for application as gas sensitive layer with relatively lower operating temperature. A sensitive layer of MWNT-ZnO based composites have been successfully fabricated on a alumina substrate and several characterization techniques has been performed, i.e. XRD, SEM and EDS to study the formed crystalline phase, the morphology of the nanostructures, and the elemental composition of synthesized composites. MWNT-ZnO sensitive layer was tested by exposure to SO2 gas at various operating temperatures and gas concentration. From the performance testing results, it could be found that the composite materials have a prospective as a gas sensor at lower operating temperature with short response time and good sensitivity.

CO gas sensing performance of electrospun Co3O4 nanostructures at low operating temperature

Low temperature gas sensors for monitoring low ppm volatile organic compounds (VOCs) in indoor climate have critical advantages in designing portable sensing devices. Herein, a highly sensitive as well as fast response In(III)-SnO2 loaded cubic mesoporous mpg-CN (commonly known as g-C3N4) based toluene gas sensor has been fabricated through template inversion of KIT-6. When exposed to a variety of test VOCs at 200 °C, the In(III)-SnO2/mpg-CN (1% In) showed a highly selective response (Ra/Rg = 61.4) and fast reversible response/recovery (4/2 s) to 50 ppm toluene gas. The sensor was also able to selectively detect 50 ppm (R = 2.4) and 100 ppm (R = 3.6) toluene gas at 90 °C. Compared to mesoporous In(III)-SnO2, the mpg-CN supported In(III)-SnO2 nanocomposite shows ∼2 fold increase in response to toluene gas while reducing the optimized operating temperature by 50 °C. The significant enhancement in sensitivity and lowering down of optimized operating temperature is attributed to the higher surface area, easily accessible mesopore channels of 3D mesoporous cubic structure and suitable heterojunction formation by the semiconducting In(III)-SnO2/mpg-CN nanocomposite. The findings reported in this study shows promising glimpse for designing a novel strategy to the development of ultrasensitive VOCs sensors working at low operating temperature.

CuO modified hollow SnO 2 nanofibers with large specific surface area were successfully synthesized via a simple two-step process, using the electrospinning method and hydrothermal technique. Gas-sensing experiments on hierarchically CuO modified SnO 2 hollow nanofibers displayed higher sensitivity and selectivity to H 2 S gas at lower operating temperatures compared with unmodified hollow SnO 2 nanofibers. At the optimal operating temperature of 125 °C, the highest response value to 10 ppm H 2 S gas reached 410. Response time decreased when the concentration of H 2 S increased. It is almost no response to other gases when the operating temperature falls below 200 °C. The specific interaction between H 2 S gas and the CuO nanoparticles, as well as the amplifier effect on the gas response by the p–n junctions, strongly enhances the selectivity to H 2 S gas at a lower operating temperature.

ZnO:Cd nanorods hydrogen sensor with low operating temperature

<div class="htmlview paragraph">The major targets of transmission design today are higher efficiency, higher torque capacity and reduced size. Increasingly smaller transmissions with higher torque lead to increasing operating temperatures. This trend is further intensified by the use of noise abatement devices and improved aerodynamic body styling that reduces the airflow around transmissions. The friction in the transmission is responsible for temperature increase and efficiency losses, and thus the reduction of friction is the main measure in order to improve the efficiency and to keep the operating temperature low.</div> <div class="htmlview paragraph">The lubricant influences and is subjected to all changes of operating conditions. Higher operating temperatures result in a higher consumption of friction modifiers, extreme pressure and anti wear additives, higher corrosion and oxidation rates, and a thinner oil film separating the various components. If the lubricant is not formulated to withstand these more severe conditions the transmission may be damaged. On the other hand, the torque transmission losses and, as a consequence, the operating temperature can be reduced by the right choice of base oil and additives.</div> <div class="htmlview paragraph">In vehicle application today, most OEMs start the lubricant development in an early phase of the development of a new transmission in order to get the optimum contribution of the lubricant. These OEMs see the lubricant as a construction element needed for the development of performance optimized transmissions. In order to select the most effective combination of additives and base stocks offered by the lubricant industry, the OEMs have developed screening tests with good correlation to field performance.</div> <div class="htmlview paragraph">Such tests range from a simple bearing test to OEM-specific transmission or axle tests. We have used two tests developed by VW, that correlate well with a car installed on a chassis-dynamometer and a transmission rig test, in order to identify and quantify the influences of fluid viscosity, base stock types, and VI Improver chemistry on efficiency and operating temperature.</div> <div class="htmlview paragraph">Finally, examples of improved semi- and fully synthetic formulations with significantly reduced operating temperatures and optimum efficiency are presented.</div>

Co3O4 hollow nanotubes: Facile synthesis and gas sensing properties

Metal oxide chemical sensors based on nanomaterials are gaining popularity and finding extensive use in automotive industries, process control and environmental monitoring. ZnO, a semiconducting metal oxide has attracted great interest over the years for its sensitivity to a variety of gases. Nanostructured sensing materials, such as thin films, nanowires, tetrapods, nanoflackes offer an inherently high surface area, reducing operating temperatures and increasing sensitivity to low concentrations of analytes. In this thesis, ZnO nanostructures have been tested as chemical sensors and a detailed study on the effect of different process parameters such as grain size, roughness, surface-to-volume ratio, depletion layer, temperature, gas concentration and material properties on gas sensitivity is presented. Initially, ZnO nanodevices were prepared with a variety of techniques, such as RF sputtering, electrodeposition, hydrothermal growth, chemical vapour deposition, thermal evaporation and controlled oxidation. The structural characterization of the nanodevices has been done by a FEI QUANTA 3D dual beam SEM/FIB machine and by a Dimension 3100 Atomic Force Microscope (AFM) (Digital Instruments) in tapping mode. X-ray diffraction (XRD) spectra were recorded on an AXS D8 diffractometer (Bruker) with a Cu Kα X-ray tube. The gas sensor substrate based on alumina consisted of Pt grid of 50nm thickness and golden contacts of 200nm thickness creating an alumina patterned substrate. The sensor deposition area was coated with ZnO nanostructures to form the sensing material. Sensing measurements are performed in a closed steel chamber where air and tested gases have been inserted. ZnO based nanostructures' response was measured in different concentrations of Ethanol, CO and NO2. Initially the role of grain size and roughness has been investigated in several thin film based nanodevices. Grain size is decreasing with increasing RF sputtering power and increasing by post-annealing treatment. Roughness instead is increasing with both the increasing of RF sputtering power and post-annealing treatment. High response was observed for those films with smaller grain size, while the roughness seems to influence very little the response of the sensor. For all thin films, the response is increasing with ii temperature and gas concentration. Recovery time and response time seem to follow a non-linear behavior with the above parameters. Extended studies have investigated the role of surface-to-volume ratio and depletion layer in the sensing performance. It has been observed that the increase of surface-to volume ratio has an important effect on the sensitivity, increasing, more than twice the response of such a device in respect to another that is based on a ZnO thin film. On the other hand, the dimensions of a nanostructure play the most crucial role in the depletion layer width in respect to the sensing properties. The diameter of a nanowire should be comparable with its depletion layer width. In this case the depletion layer has strong effect, which makes the sensor's response depend also on it. The sensing properties of all fabricated structures have been compared to find the optimum sensor that could face the demands of automotive industries. All fabricated structures have been compared in different configurations to find out which one presents the best sensing performance. To that direction sensors based on thin film, tetrapods, nanowires, nanoflackes have been tested in same environmental conditions. Advanced nanostructures present better sensing properties. Sensing response of every advanced nanostructure presents more than double sensing response than every thin film-based nanostructure. Comparing the advanced nanostructures with each other, tetrapods based sensor has higher response and recovery time, while the sensitivity is slightly higher for the nanowires-based sensor. Theoretical studies have been performed by ab-initio simulations in NO2 environment. They have revealed that the sensing mechanism is driven almost exclusively by competitive adsorption between NO2 and atmospheric oxygen mediated by temperature change. The influence of the NO2 on the electronic properties of ZnO has been assessed and it is in accordance with the experiments. Our future work is the investigation of other materials for the development of sensing nanodevices targeting to develop more sensitive nanosensors in the same or lower cost. Additionally, the investigation of other growth techniques that could develop more complicated structures in low cost is another point of interest for the future