Metal oxide nanostructures are the most promising materials for the fabrication of advanced gas sensors. However, the main challenge of these gas sensors is humidity interference and issues related to the selectivity and high operating temperature, which limits their response in real-time applications. In this study, we proposed nanohybrids of Pt-functionalized Al2O3/ZnOcore-shell nanorods (NRs) for a real-time humidity-independent acetylene gas sensor. The core ZnO NRs have been fabricated on microelectromechanical system (MEMS) microheater, followed by a coating of a thin nanoscale moisture-blocking conformal Al2O3 shell by atomic layer deposition (ALD) and decoration of Pt NPs using photochemical deposition and e-beam evaporation. Prior to the fabrication, a COMSOL simulation was performed to optimize the microheater design and moisture-blocking layer thickness. A comparative study of the decoration of Pt NPs on the ZnO surface by photochemical (s-Pt/ZnO) and e-beam evaporation (e-Pt/ZnO) and a Al2O3 thin moisture-blocking shell layer (Pt/Al2O3/ZnO) in sensor response has been conducted. The fabricated sensors (s-Pt/ZnO) and (e-Pt/ZnO) showed a high response ΔR/R (%) of 96.46% and 68.15% to 200 ppm acetylene at 120 °C and detect trace concentrations of acetylene down to 1 ppm, but the response is influenced by humidity. Moreover, the sensor (Pt/Al2O3/ZnO) exhibited nearly the same sensing characteristics and high acetylene selectivity despite the wide range of humidity variation from 20% RH to 70% RH. The Pt-functionalized Al2O3/ZnOcore-shell NR-based sensor showed better sensing and stable performance than other sensors (s-Pt/ZnO and e-Pt/ZnO) under humidity conditions.

Nanohybrids of Pt-Functionalized Al2O3/ZnO Core–Shell Nanorods for High-Performance MEMS-Based Acetylene Gas Sensor

In the past decade, the global industry and research attentions on intelligent skin-like electronics have boosted their applications in diverse fields including human healthcare, Internet of Things, human-machine interfaces, artificial intelligence and soft robotics. Among them, flexible humidity sensors play a vital role in noncontact measurements relying on the unique property of rapid response to humidity change. This work presents an overview of recent advances in flexible humidity sensors using various active functional materials for contactless monitoring. Four categories of humidity sensors are highlighted based on resistive, capacitive, impedance-type and voltage-type working mechanisms. Furthermore, typical strategies including chemical doping, structural design and Joule heating are introduced to enhance the performance of humidity sensors. Drawing on the noncontact perception capability, human/plant healthcare management, human-machine interactions as well as integrated humidity sensor-based feedback systems are presented. The burgeoning innovations in this research field will benefit human society, especially during the COVID-19 epidemic, where cross-infection should be averted and contactless sensation is highly desired. 

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Multifunctional Flexible Humidity Sensor Systems Towards Noncontact Wearable Electronics

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The monitoring of moisture inside a transformer is necessary for its continuous reliable operation. It is monitored by the utility as a part of its periodic maintenance activities. Online monitor permits the timely control of moisture buildup inside the transformer. In this work, the development of a capacitive humidity sensor using porous alumina prepared through the anodization of 99.9 % pure aluminium (Al) sheet for online moisture monitoring of transformers has been reported. The capacitive responses of the sensor are studied with varying %RH and ppm moisture. The sensitivity of the sensor is optimized by controlling the pore morphology of the nano structured thin film of Al2O3. The responses of the three sensors developed through the different anodization conditions are compared. The sensor is found to be sensitive for detection of moisture from 180 ppm to 1000 ppm. The designed sensor is used to monitor the moisture generated in highly contaminated transformer oil at different temperatures. A model for detecting the moisture in oil based on the response of the designed sensor is developed. Corrective action based on the response of the humidity sensor will improve the condition of the transformer in the long run.

Anodic aluminium oxide based humidity sensor for online moisture monitoring of power transformer

Dissolved hydrogen gas analysis (DHGA) is a key aspect that defines the operational status of a transformer and is vital to the maintenance of safety standards in the power grid. In this report, we proposed, MEMS gas sensor arrays for DHGA with a specially designed system having a Wheatstone bridge circuitry with two pairs of resistors. The optimal temperature of the sensor was controlled using a monolithically integrated in-plane microheater and a temperature sensor to monitor the temperature change in the oil and to compensate for the temperature variations in resultant hydrogen sensor signals. The fabricated sensor in a transformer oil showed excellent H2 response (ΔVout = 62μV to 10 ppm and 4.71 mV to 2000 ppm). In addition, long-term stability of sensor performance was observed, confirming that the Al2O3 passivation layer and packaging were effective to enhance the robustness of the sensor in an oil environment. A customized, compact, and portable sensor interface electronics was developed and applied to the sensor system. The results obtained with the developed sensor interface were very similar to those measured with a digital multimeter. Our results demonstrate that the developed sensor system and packaging are very promising for long term monitoring of the operational status of transformers.

MEMS hydrogen gas sensor for in-situ monitoring of hydrogen gas in transformer oil

A high-performance planar metal-polyimide-interdigital transducer (IDT) metal-type miniaturized capacitive humidity sensor was developed for real-time detection of moisture dissolved in transformer-insulating oil. The sensor with the sensitive film having a concave surface and a porous structure was fabricated through using a micromachined process. Additionally, the sensor interface electronics were implemented for the measurement system of the fabricated sensor and immediate use of the sensor in oil. COMSOL simulation was performed to determine the top metal IDT configuration and polyimide thickness prior to sensor fabrication. The performance of the developed sensor was tested in air and transformer oil. In the air test, the sensor showed a high sensitivity of 1.60 pF/% relative humidity (RH) and a large capacitance variation from 204 to 374 pF with a humidity change from 20% to 90% RH at a frequency of 3 kHz; a low hysteresis of 2.07% at 40% RH and a stable capacitance value with a maximum error rate of 0.15% over 450 h of continuous testing. In the transformer oil, the sensor exhibited a sensitivity of 0.90 pF/%RH and an extremely small long-term drift of the capacitance corresponding to humidity variation of ±2.0%RH. Overall, the modification of the polyimide film enhanced the performance of our sensor, such as high sensitivity, fast response, and long-term stability in the transformer–oil environment.

Development of highly sensitive and stable humidity sensor for real-time monitoring of dissolved moisture in transformer-insulating oil

Reduced graphene oxide and molybdenum disulfide (rGO:MoS2) are the most representative two-dimensional materials, which are promising for a humidity sensor owing to its high surface area, a large number of active sites, and excellent mechanical flexibility. Herein, we introduced a highly sensitive and stable rGO:MoS2-based humidity sensor integrated with a low-power in-plane microheater and a temperature sensor, directly insertable to transformer insulating oil, and analyzed by a newly developed customized sensor interface electronics to monitor the sensor's output variations in terms of relative humidity (RH) concentration. rGO:MoS2 sensing materials were synthesized by simple ultrasonication without using any additives or additional heating and selectively deposited on titanium/platinum (Ti/Pt) interdigitated electrodes on a SiO2 substrate using the drop-casting method. The significant sensing capability of p-n heterojunction formation between rGO and MoS2 was observed both in the air and transformer insulating oil environment. In air testing, the sensor exhibited an immense sensitivity of 0.973 kΩ/%RH and excellent linearity of ∼0.98 with a change of humidity from 30 to 73 %RH, and a constant resistance deviation with an inaccuracy rate of 0.13% over 400 h of continual measurements. In oil, the sensor showed a high sensitivity of 1.596 kΩ/%RH and stable repeatability for an RH concentration range between 34 and 63 %RH. The obtained results via the sensor interface were very similar to those measured with a digital multimeter, denoting that our developed total sensor system is a very promising candidate for real-time monitoring of the operational status of power transformers.

A Highly Sensitive and Stable rGO:MoS2-Based Chemiresistive Humidity Sensor Directly Insertable to Transformer Insulating Oil Analyzed by Customized Electronic Sensor Interface.

Yongbum Lee

Papers

Development of highly sensitive and stable humidity sensor for real-time monitoring of dissolved moisture in transformer-insulating oil

A Highly Sensitive and Stable rGO:MoS2-Based Chemiresistive Humidity Sensor Directly Insertable to Transformer Insulating Oil Analyzed by Customized Electronic Sensor Interface.