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12 pages, 6811 KiB

Open AccessArticle

The Fabrication and Characterization of Surface-Acoustic-Wave and Resistive Types of Ozone Sensors Based on Zinc Oxide: A Comparative Study

by Sheng-Hua Yan and Chia-Yen Lee

Sensors 2025, 25(9), 2723; https://doi.org/10.3390/s25092723 - 25 Apr 2025

Viewed by 83

Abstract

Micro-Electro-Mechanical System (MEMS) technology is employed to fabricate surface acoustic wave (SAW)-type and resistive-type ozone sensors on quartz glass (SiO₂) substrates. The fabrication process commences by using a photolithography technique to define interdigitated electrodes (IDEs) on the substrates. Electron-beam evaporation (EBE) [...] Read more.

Micro-Electro-Mechanical System (MEMS) technology is employed to fabricate surface acoustic wave (SAW)-type and resistive-type ozone sensors on quartz glass (SiO₂) substrates. The fabrication process commences by using a photolithography technique to define interdigitated electrodes (IDEs) on the substrates. Electron-beam evaporation (EBE) followed by radio frequency (RF) magnetron sputtering is then used to deposit platinum (Pt) and chromium (Cr) electrode layers as well as a zinc oxide (ZnO) sensing layer, respectively. Finally, annealing is performed to improve the crystallinity and sensing performance of the ZnO films. The experimental results reveal that the ZnO thin films provide an excellent ozone-concentration sensing capability in both sensors. The SAW-type sensor demonstrates a peak sensitivity at a frequency of 200 kHz, with a rapid response time of just 35 s. Thus, it is suitable for applications requiring a quick response and high sensitivity, such as real-time monitoring and high-precision environmental detection. The resistive-type sensor shows optimal sensitivity at a relatively low operating temperature of 180 °C, but has a longer response time of approximately 103 s. Therefore, it is better suited for low-cost and large-scale applications such as industrial-gas-concentration monitoring. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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21 pages, 8728 KiB

Open AccessArticle

CH₄, C₂H₆, and C₂H₄ Multi-Gas Sensing Based on Mid-Infrared Spectroscopy and SVM Algorithm

by Wenyuan Shao, Yunjiang Jia, Xilian Su, Benlei Zhao, Jiachen Jiang, Limei Gao, Xiaosong Zhu and Yiwei Shi

Sensors 2025, 25(5), 1427; https://doi.org/10.3390/s25051427 - 26 Feb 2025

Viewed by 292

Abstract

A multi-gas sensing system based on mid-infrared spectral absorption was developed for the detection of CH₄, C₂H₆, and C₂H₄. The system utilized a broadband infrared source, a hollow waveguide (HWG) absorption cell, and [...] Read more.

A multi-gas sensing system based on mid-infrared spectral absorption was developed for the detection of CH₄, C₂H₆, and C₂H₄. The system utilized a broadband infrared source, a hollow waveguide (HWG) absorption cell, and a tunable Fabry–Pérot (FP) detector. The limits of detection (LODs) of CH₄, C₂H₆, and C₂H₄ were 7.33 ppm, 2.13 ppm, and 8.09 ppm, respectively. For multi-gas measurements, the support vector machine (SVM) algorithm model was employed to calculate the concentration of each component. The root mean square error of prediction (RMSEP) values for CH₄, C₂H₆, and C₂H₄ were 15.91 ppm (1.26%), 1.64 ppm (0.57%), and 6.95 ppm (0.55%), respectively. The generation of stimulated absorption spectra of mixed gases was realized, and the sample selection of measurement for accurate concentration calculation of each gas was optimized. The system proposed in this work provides a simple, miniaturized, and cost-effective solution for multi-gas sensing. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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21 pages, 6239 KiB

Open AccessArticle

Electrochemical Sensor for Hydrogen Leakage Detection at Room Temperature

by Gimi Aurelian Rîmbu, Lucian Pîslaru-Dănescu, George-Claudiu Zărnescu, Carmen Alina Ștefănescu, Mihai Iordoc, Aristofan Alexandru Teișanu and Gabriela Telipan

Sensors 2025, 25(1), 264; https://doi.org/10.3390/s25010264 - 5 Jan 2025

Viewed by 1522

Abstract

The use of hydrogen as fuel presents many safety challenges due to its flammability and explosive nature, combined with its lack of color, taste, and odor. The purpose of this paper is to present an electrochemical sensor that can achieve rapid and accurate [...] Read more.

The use of hydrogen as fuel presents many safety challenges due to its flammability and explosive nature, combined with its lack of color, taste, and odor. The purpose of this paper is to present an electrochemical sensor that can achieve rapid and accurate detection of hydrogen leakage. This paper presents both the component elements of the sensor, like sensing material, sensing element, and signal conditioning, as well as the electronic protection and signaling module of the critical concentrations of H₂. The sensing material consists of a catalyst type Vulcan XC72 40% Pt, from FuelCellStore, (Bryan, TX, USA). The sensing element is based on a membrane electrode assembly (MEA) system that includes a cathode electrode, an ion-conducting membrane type Nafion 117, from FuelCellStore, (Bryan, TX, USA). and an anode electrode mounted in a coin cell type CR2016, from Xiamen Tob New Energy Technology Co., Ltd, (Xiamen City, Fujian Province, China). The electronic block for electrical signal conditioning, which is delivered by the sensing element, uses an INA111, from Burr-Brown by Texas Instruments Corporation, (Dallas, TX, USA). instrumentation operational amplifier. The main characteristics of the electrochemical sensor for hydrogen leakage detection are operation at room temperature so it does not require a heater, maximum amperometric response time of 1 s, fast recovery time of maximum 1 s, and extended range of hydrogen concentrations detection in a range of up to 20%. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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27 pages, 17715 KiB

Open AccessArticle

Calibration and Inter-Unit Consistency Assessment of an Electrochemical Sensor System Using Machine Learning

by Ioannis D. Apostolopoulos, Silas Androulakis, Panayiotis Kalkavouras, George Fouskas and Spyros N. Pandis

Sensors 2024, 24(13), 4110; https://doi.org/10.3390/s24134110 - 25 Jun 2024

Cited by 2 | Viewed by 1318

Abstract

This paper addresses the challenges of calibrating low-cost electrochemical sensor systems for air quality monitoring. The proliferation of pollutants in the atmosphere necessitates efficient monitoring systems, and low-cost sensors offer a promising solution. However, issues such as drift, cross-sensitivity, and inter-unit consistency have [...] Read more.

This paper addresses the challenges of calibrating low-cost electrochemical sensor systems for air quality monitoring. The proliferation of pollutants in the atmosphere necessitates efficient monitoring systems, and low-cost sensors offer a promising solution. However, issues such as drift, cross-sensitivity, and inter-unit consistency have raised concerns about their accuracy and reliability. The study explores the following three calibration methods for converting sensor signals to concentration measurements: utilizing manufacturer-provided equations, incorporating machine learning (ML) algorithms, and directly applying ML to voltage signals. Experiments were performed in three urban sites in Greece. High-end instrumentation provided the reference concentrations for training and evaluation of the model. The results reveal that utilizing voltage signals instead of the manufacturer’s calibration equations diminishes variability among identical sensors. Moreover, the latter approach enhances calibration efficiency for CO, NO, NO₂, and O₃ sensors while incorporating voltage signals from all sensors in the ML algorithm, taking advantage of cross-sensitivity to improve calibration performance. The Random Forest ML algorithm is a promising solution for calibrating similar devices for use in urban areas. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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26 pages, 8206 KiB

Open AccessArticle

Mathematical Physics Analysis of Nozzle Shaping at the Gas Outlet from the Aperture to the Differentially Pumped Chamber in Environmental Scanning Electron Microscopy (ESEM)

by Jiří Maxa, Vilém Neděla, Pavla Šabacká and Tomáš Binar

Sensors 2024, 24(10), 3243; https://doi.org/10.3390/s24103243 - 20 May 2024

Viewed by 1032

Abstract

A combination of experimental measurement preparations using pressure and temperature sensors in conjunction with the theory of one-dimensional isentropic flow and mathematical physics analyses is presented as a tool for analysis in this paper. Furthermore, the subsequent development of a nozzle for use [...] Read more.

A combination of experimental measurement preparations using pressure and temperature sensors in conjunction with the theory of one-dimensional isentropic flow and mathematical physics analyses is presented as a tool for analysis in this paper. Furthermore, the subsequent development of a nozzle for use in environmental electron microscopy between the specimen chamber and the differentially pumped chamber is described. Based on experimental measurements, an analysis of the impact of the nozzle shaping located behind the aperture on the character of the supersonic flow and the resulting dispersion of the electron beam passing through the differential pumped chamber is carried out on the determined pressure ratio using a combination of theory and mathematical physics analyses. The results show that nozzle shapes causing under-expanded gas outflow from the aperture to the nozzle have a worse impact on the dispersion of the primary electron beam. This is due to the flow velocity control. The controlled reduction in the static pressure curve on the primary electron beam path thus causes a significantly higher course of electron dispersion values than variants with shapes causing over-expanded gas outflow. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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13 pages, 3643 KiB

Open AccessArticle

Evaluation of Low-Cost CO₂ Sensors Using Reference Instruments and Standard Gases for Indoor Use

by Qixiang Cai, Pengfei Han, Guang Pan, Chi Xu, Xiaoyu Yang, Honghui Xu, Dongde Ruan and Ning Zeng

Sensors 2024, 24(9), 2680; https://doi.org/10.3390/s24092680 - 23 Apr 2024

Cited by 3 | Viewed by 1927

Abstract

CO₂ monitoring is important for carbon emission evaluation. Low-cost and medium-precision sensors (LCSs) have become an exploratory direction for CO₂ observation under complex emission conditions in cities. Here, we used a calibration method that improved the accuracy of SenseAir K30 CO [...] Read more.

CO₂ monitoring is important for carbon emission evaluation. Low-cost and medium-precision sensors (LCSs) have become an exploratory direction for CO₂ observation under complex emission conditions in cities. Here, we used a calibration method that improved the accuracy of SenseAir K30 CO₂ sensors from ±30 ppm to 0.7–4.0 ppm for a CO₂-monitoring instrument named the SENSE-IAP, which has been used in several cities, such as in Beijing, Jinan, Fuzhou, Hangzhou, and Wuhan, in China since 2017. We conducted monthly to yearly synchronous observations using the SENSE-IAP along with reference instruments (Picarro) and standard gas to evaluate the performance of the LCSs for indoor use with relatively stable environments. The results show that the precision and accuracy of the SENSE-IAP compared to the standard gases were rather good in relatively stable indoor environments, with the short-term (daily scale) biases ranging from −0.9 to 0.2 ppm, the root mean square errors (RMSE) ranging from 0.7 to 1.6 ppm, the long-term (monthly scale) bias ranging from −1.6 to 0.5 ppm, and the RMSE ranging from 1.3 to 3.2 ppm. The accuracy of the synchronous observations with Picarro was in the same magnitude, with an RMSE of 2.0–3.0 ppm. According to our evaluation, standard instruments or reliable standard gases can be used as a reference to improve the accuracy of the SENSE-IAP. If calibrated daily using standard gases, the bias of the SENSE-IAP can be maintained within 1.0 ppm. If the standard gases are hard to access frequently, we recommend a calibration frequency of at least three months to maintain an accuracy within 3 ppm. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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18 pages, 10886 KiB

Open AccessArticle

Exhaust Emissions from Gasoline Vehicles with Different Fuel Detergency and the Prediction Model Using Deep Learning

by Rongshuo Zhang, Hongfei Chen, Peiyuan Xie, Lei Zu, Yangbing Wei, Menglei Wang, Yunjing Wang and Rencheng Zhu

Sensors 2023, 23(17), 7655; https://doi.org/10.3390/s23177655 - 4 Sep 2023

Cited by 2 | Viewed by 2832

Abstract

Enhancing gasoline detergency is pivotal for enhancing fuel efficiency and mitigating exhaust emissions in gasoline vehicles. This study investigated gasoline vehicle emission characteristics with different gasoline detergency, explored synergistic emission reduction potentials, and developed versatile emission prediction models. The results indicate that improved [...] Read more.

Enhancing gasoline detergency is pivotal for enhancing fuel efficiency and mitigating exhaust emissions in gasoline vehicles. This study investigated gasoline vehicle emission characteristics with different gasoline detergency, explored synergistic emission reduction potentials, and developed versatile emission prediction models. The results indicate that improved fuel detergency leads to a reduction of 5.1% in fuel consumption, along with decreases of 3.2% in total CO₂, 55.4% in CO, and 15.4% in HC emissions. However, during low-speed driving, CO₂ and CO emissions reductions are limited, and HC emissions worsen. A synergistic emission reduction was observed, particularly with CO exhibiting a pronounced reduction compared to HC. The developed deep-learning-based vehicle emission model for different gasoline detergency (DPVEM-DGD) enables accurate emission predictions under various fuel detergency conditions. The Pearson correlation coefficients (Pearson’s r) between predicted and measured values of CO₂, CO, and HC emissions before and after adding detergency agents are 0.913 and 0.934, 0.895 and 0.915, and 0.931 and 0.969, respectively. The predictive performance improves due to reduced peak emissions resulting from improved fuel detergency. Elevated gasoline detergency not only reduces exhaust emissions but also facilitates more refined emission management to a certain extent. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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12 pages, 3213 KiB

Open AccessArticle

Low-Power AlGaN/GaN Triangular Microcantilever for Air Flow Detection

by Balaadithya Uppalapati, Durga Gajula, Manav Bava, Lavanya Muthusamy and Goutam Koley

Sensors 2023, 23(17), 7465; https://doi.org/10.3390/s23177465 - 28 Aug 2023

Cited by 1 | Viewed by 1269

Abstract

This paper investigates an AlGaN/GaN triangular microcantilever with a heated apex for airflow detection utilizing a very simple two-terminal sensor configuration. Thermal microscope images were used to verify that the apex region of the microcantilever reached significantly higher temperatures than other parts under [...] Read more.

This paper investigates an AlGaN/GaN triangular microcantilever with a heated apex for airflow detection utilizing a very simple two-terminal sensor configuration. Thermal microscope images were used to verify that the apex region of the microcantilever reached significantly higher temperatures than other parts under applied voltage bias. The sensor response was found to vary linearly with airflow rate when tested over a range of airflow varying from 16 to 2000 sccm. The noise-limited flow volume measurement yielded ~4 sccm resolution, while the velocity resolution was found to be 0.241 cm/s, which is one of the best reported so far for thermal sensors. The sensor was able to operate at a very low power consumption level of ~5 mW, which is one of the lowest reported for these types of sensors. The intrinsic response time of the sensor was estimated to be on the order of a few ms, limited by its thermal properties. Overall, the microcantilever sensor, with its simple geometry and measurement configurations, was found to exhibit attractive performance metrics useful for various sensing applications. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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11 pages, 4974 KiB

Open AccessCommunication

High Selectivity Hydrogen Gas Sensor Based on WO₃/Pd-AlGaN/GaN HEMTs

by Van Cuong Nguyen, Ho-Young Cha and Hyungtak Kim

Sensors 2023, 23(7), 3465; https://doi.org/10.3390/s23073465 - 26 Mar 2023

Cited by 7 | Viewed by 3482

Abstract

We investigated the hydrogen gas sensors based on AlGaN/GaN high electron mobility transistors (HEMTs) for high temperature sensing operation. The gate area of the sensor was functionalized using a 10 nm Pd catalyst layer for hydrogen gas sensing. A thin WO₃ layer [...] Read more.

We investigated the hydrogen gas sensors based on AlGaN/GaN high electron mobility transistors (HEMTs) for high temperature sensing operation. The gate area of the sensor was functionalized using a 10 nm Pd catalyst layer for hydrogen gas sensing. A thin WO₃ layer was deposited on top of the Pd layer to enhance the sensor selectivity toward hydrogen gas. At 200 °C, the sensor exhibited high sensitivity of 658% toward 4%-H₂, while exhibiting only a little interaction with NO₂, CH₄, CO₂, NH₃, and H₂S. From 150 °C to 250 °C, the 10 ppm hydrogen response of the sensor was at least eight times larger than other target gases. These results showed that this sensor is suitable for H₂ detection in a complex gas environment at a high temperature. Full article

(This article belongs to the Special Issue Advanced Sensors for Gas Monitoring)

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