Gas Sensing by Microwave Transduction: Review of Progress and Challenges

Microwave transduction is a novel research field in gas sensing owing to its simplicity, low cost, passive, and non-contact operations that indicate a huge potential in applications for gas sensing. At present, the study of microwave gas sensors is limited to testing the macroscopic performance of materials. Although the permittivity change of materials is the reason for microwave transduction, the knowledge of the fundamental mechanism is an urgent requirement for boosting the development of microwave gas sensors. In this review, we summarized the presented progresses of microwave gas sensors, including the performance of the different materials and propagative structures, as well as their sensitive signal. We have attempted to explain the sensitive mechanism of these materials, discuss the function of the propagative structure, and analyze the sensitive signal extracted from the parameters of electromagnetic waves. Finally, the challenges in the study of sensitive materials and propagative structures used in microwave gas sensors are analyzed. It is clear from the published literatures that microwave transduction provide a new route for gas detection, and it is expected that commercial manifestation will occur. The future research on microwave gas sensors will continue to exploit their sensitive materials and propagative structure for different applications. The understanding of the microscopic polarization interactions with gases will assist in and guide the fabrication of high-performance microwave gas sensors in the future

Exploitation of Unique Properties of Zeolites in the Development of Gas Sensors

The unique properties of microporous zeolites, including ion-exchange properties, adsorption, molecular sieving, catalysis, conductivity have been exploited in improving the performance of gas sensors. Zeolites have been employed as physical and chemical filters to improve the sensitivity and selectivity of gas sensors. In addition, direct interaction of gas molecules with the extraframework cations in the nanoconfined space of zeolites has been explored as a basis for developing new impedance-type gas/vapor sensors. In this review, we summarize how these properties of zeolites have been used to develop new sensing paradigms. There is a considerable breadth of transduction processes that have been used for zeolite incorporated sensors, including frequency measurements, optical and the entire gamut of electrochemical measurements. It is clear from the published literature that zeolites provide a route to enhance sensor performance, and it is expected that commercial manifestation of some of the approaches discussed here will take place. The future of zeolite-based sensors will continue to exploit its unique properties and use of other microporous frameworks, including metal organic frameworks. Zeolite composites with electronic materials, including metals will lead to new paradigms in sensing. Use of nano-sized zeolite crystals and zeolite membranes will enhance sensor properties and make possible new routes of miniaturized sensors

Wearable electronic nose for human skin odor identification: A preliminary study

Wearable electronic nose for human skin odor identification: A preliminary stud

Ammonia adsorption-induced change in permittivity of zeolite Y in millimeter-wave band

Millimeter wave is a prominent topic for next-generation communication system. To develop functional materials to be used in sensors operated in the millimeter-wave, it is necessary to obtain the variation in the permittivity of the materials. In this study, the effect of ammonia adsorption on the permittivity of zeolite Y in the millimeter-wave of 60-90 GHz is investigated. The results indicate that the ammonia adsorption increases the permittivity of zeolite Y. Specifically, the permittivity change of the gas concentration resulted in 0.025/100 ppm. Based on the characterizations and calculation, ammonia adsorption in zeolite Y stretches the bond between mobile cations and zeolite framework, which results in an increase in the ionic dipole moment of the zeolite. This study provides a fundamental insight on gas sensing materials for millimeter-wave sensors. (C) 2020 Elsevier B.V. All rights reserved

The Squeeze Film Effect with a High-Pressure Boundary in Aerostatic Bearings

The squeeze film effect was discussed in several fields, but mostly under the same pressure boundary conditions. However, pressures at the inlet and outlet are different for aerostatic bearings. In this paper, the dynamic Reynolds equation group, with the stiffness and damping pressure written separately, is deducted and numerically solved with a high-pressure boundary for a parallel flat and circular thin film. The circular thin film considers the two results of the supply pressure boundary inside and outside. All dynamic pressure distribution and stiffness curves are given in a dimensionless form, and a comparative analysis of squeeze film characteristics with and without external pressure is conducted. From the calculation results, it can be concluded that the squeeze effect shows damping for zero-frequency and stiffness for infinite-frequency for compressible lubricants. The dynamic pressure in the static high pressure region is also high at high frequencies affected by gas compressibility. Based on these analytical results, the transfer functions of the thin film are given to further analyze the dynamic performance of aerostatic bearings, and the shape of the response curve approximates an exponential decay form, even when the amplitude increases to 10% of the gas film thickness

Evaluation and Application of an Engineering Calculation Method of the Static Performance of an Aerostatic Journal Bearing with Multiple Orifice-Type Restrictors

A simplified calculation method is evaluated to calculate the static performance of an aerostatic journal bearing with multiple orifice-type restrictors. This method adopts a one-dimension flow assumption and is a fast calculation method to design journal bearings in engineering by directly linking the structural parameters and performance parameters affecting radial bearings with nonlinear equations. In addition, this method is verified with computational fluid dynamics by two actual case studies, and it is found that the LCC difference between those two methods is less than 5% for a 200 mm diameter spindle, and less than 10% for a 100 mm diameter spindle. Subsequently, the influence of a key parameter ζi on the static performance of journal bearings is explained theoretically. This method is much easier and more intuitive compared with numerical computational methods. Furthermore, it promotes the application of aerostatic journal bearings

The Squeeze Film Effect with a High-Pressure Boundary in Aerostatic Bearings

The squeeze film effect was discussed in several fields, but mostly under the same pressure boundary conditions. However, pressures at the inlet and outlet are different for aerostatic bearings. In this paper, the dynamic Reynolds equation group, with the stiffness and damping pressure written separately, is deducted and numerically solved with a high-pressure boundary for a parallel flat and circular thin film. The circular thin film considers the two results of the supply pressure boundary inside and outside. All dynamic pressure distribution and stiffness curves are given in a dimensionless form, and a comparative analysis of squeeze film characteristics with and without external pressure is conducted. From the calculation results, it can be concluded that the squeeze effect shows damping for zero-frequency and stiffness for infinite-frequency for compressible lubricants. The dynamic pressure in the static high pressure region is also high at high frequencies affected by gas compressibility. Based on these analytical results, the transfer functions of the thin film are given to further analyze the dynamic performance of aerostatic bearings, and the shape of the response curve approximates an exponential decay form, even when the amplitude increases to 10% of the gas film thickness

Potentiometric NO2 Sensors Based on Thin Stabilized Zirconia Electrolytes and Asymmetric (La0.8Sr0.2)0.95MnO3 Electrodes

Here we report on a new architecture for potentiometric NO2 sensors that features thin 8YSZ electrolytes sandwiched between two porous (La0.8Sr0.2)0.95MnO3 (LSM95) layers—one thick and the other thin—fabricated by the tape casting and co-firing techniques. Measurements of their sensing characteristics show that reducing the porosity of the supporting LSM95 reference electrodes can increase the response voltages. In the meanwhile, thin LSM95 layers perform better than Pt as the sensing electrode since the former can provide higher response voltages and better linear relationship between the sensitivities and the NO2 concentrations over 40–1000 ppm. The best linear coefficient can be as high as 0.99 with a sensitivity value of 52 mV/decade as obtained at 500 °C. Analysis of the sensing mechanism suggests that the gas phase reactions within the porous LSM95 layers are critically important in determining the response voltages

In-Situ Measurement and Slow-Tool-Servo Compensation Method of Roundness Error of a Precision Mandrel

This paper describes a method for measuring and compensating the roundness error of a larger mandrel manufactured by an ultra-precision diamond-turning lathe aimed to obtain a calibration cylinder with a roundness of less than 0.1 μm. The diamond-turning machine has a cross-stacked hydrostatic guideway, produces a cutting depth and feed movement direction, and a dual-spindle system that is firmly connected to the bed. Due to the good repeatability of aerostatic spindles, only in situ rather than online real-time measurements are required. To this end, three high-precision capacitance displacement sensors were utilized to detect the cross-section of the workpiece and the time domain via the three-point error separation technique to separate the roundness error from the rotation motion error. The slow tool servo (STS) cutting technique was employed to compensate for the roundness error, which did not require extra axes, only the excellent dynamic response of the feed axis; hence, the servo control parameters could be suitably adjusted. The experimental results reveal that the low-order harmonic error, often caused by aerostatic spindles, is almost removed completely. For this particular lathe, the experiments indicate that about 60% of the rotational error motion is compensated, and the roundness error is reduced to less than 0.1 μm, which is evaluated by the least-squares circle method