Wide-range and high-sensitivity hydrogen sensors are critically important for hydrogen safety in aerospace and advanced transportation sectors. This work demonstrates a thermal-conductivity surface acoustic wave (SAW) based sensor to achieve high sensitivity hydrogen sensing. By integrating thermal balance and acoustic wave equations, a precise mechanistic model elucidating the structure-activity relationships among gas flow rate, operating temperature, and MEMS architecture in determining sensing sensitivity is constructed. Guided by this model, the SAW hydrogen sensor with on-chip microheater integration was developed. Furthermore, a highly integrated SAW hydrogen sensing system with ultra-low baseline noise (<30 µV) was constructed for performance evaluation. Leveraging the exceptional thermal sensitivity of the SAW device and system stability, the optimized sensor achieves wide detection range (up to 100% vol), low detection limit (~6 ppm), rapid response and recovery time (T90/T10: ~15 s), excellent repeatability (error<2.4%) at a relatively low operating temperature (120 °C). The prepared SAW sensor provides an effective solution for hydrogen leakage monitoring across unprecedented concentrations (ppm-100% vol), establishing a new paradigm for hydrogen safety applications.

Surface acoustic wave (SAW) gas-based sensors have attracted significant attention as an emerging sensing technology due to their unique micro/nano-scale acoustic sensing structures and multi-physical field coupling mechanisms, which feature high sensitivity, rapid response, wide detection range, and lightweight. This paper systematically reviews the SAW gas sensing effect and mechanisms, sensing device design and fabrication, signal acquisition and processing circuits. Potential applications in fields such as renewable energy, aerospace, defense, industrial control, and intelligent manufacturing are also discussed, followed by an outlook on future development trends.

To address the challenge of accurately simulating the temperature sensitivity of LGS-based surface acoustic wave (SAW) sensors in high-temperature environments, where conventional models often neglect the geometric effects of thermal expansion, this paper proposes an optimized sensitivity simulation model that systematically incorporates these effects. By developing temperature dependence models for both wavelength and electrode thickness, our approach describes the evolution of key geometric parameters at high temperatures and establishes a more comprehensive sensitivity simulation framework. For experimental validation, an LGS-based SAW high-temperature sensor with Pt electrodes was fabricated, based on which experimental data from room temperature (25°C) to 1000°C was collected as a benchmark. The results demonstrate that our method reduces the average simulation error throughout the entire temperature range to 2.30%, achieving a 57.73% improvement compared with conventional simulation approaches. Furthermore, the error is reduced by an average of 21.61% across different material parameter sets, with the improvement showing no clear correlation to the accuracy of the material parameters. This refined modeling of thermal expansion mechanisms for temperature sensitivity simulation provides a reliable methodology for the precise design and performance prediction of high-temperature SAW sensors.

This work illustrates a new pressure sensing method by measuring the surface acoustic wave (SAW) propagation attenuation. The quality value (Q value) of the sensing chip decreases with the external pressure because of the energy radiation of the Rayleigh wave. This method abandons the traditional pressure-sensing structure, which can expand the measurement range and eliminate the negative effects, such as hysteresis and creep arising from the sealant layer. The sensing chip is developed by the standard photolithographic technique. A high Q value of over 9000 is achieved from the developed sensing chip operating at approximately 430 MHz. Then, the pressure sensing experiment is conducted using the developed sensing chip, which demonstrates excellent performance in the 0.1-4-MPa range, with low hysteresis of -2.02%, excellent nonlinearity of -0.44%, low-temperature drift coefficient of 0.01068 ppm/degrees C, and the maximum relative error of -2.453%. Excellent stability and consistency in several rounds of experiments are also obtained.

Acoustic impedance enables many interesting acoustic applications. However, acoustic impedance for gas sensing is rare and difficult. Here we introduce a micro-nano surface acoustic wave (SAW) chip based on the acoustic impedance effect to achieve ultra-fast and wide-range gas sensing. We theoretically established the relationship between surface load acoustic impedance and SAW attenuation, and analyzed the influence of acoustic impedance on acoustic propagation loss under different gas/humidity media. Experimental measurements reveal that the differences in acoustic impedance generated by different gases trigger different acoustic attenuation, and can achieve wide-range (0–100 v/v

Hydrogen sulfide (H2S) sensing is utilized for monitoring the concentration in environment protection and industrial process control, of which high selectivity and ppb detection limit are required. Here, a ppb-concentration and highly selective surface acoustic wave (SAW) H2S sensing has been developed with the Cu-3(HHTP)(2) MOFs conductive metal-organic frameworks (MOFs), which have been synthesized by connecting Cu2+ with the organic ligands via hydrothermal process. Typically, as-prepared Cu-3(HHTP)(2) MOFs take on shapes including nanoparticles and nanorods. Beneficially, the SAW H2S sensing presents excellent selectivity to H2S and enables to detection of H2S as low as 6 ppb at room temperature (similar to 26 degrees C). Remarkably, the SAW sensor prototypes exhibit high sensitivity (0.02 mV/ppb) to 50-2000 ppb H2S, fast response (T-90: 281 s) and 46 day-long stability at room temperature. Theoretically, such excellent H2S SAW sensing performance might be attributed to the interaction between Cu ions and H2S molecules and the strong acoustoelectric effect of SAW. Practically, our ppb-concentration and highly selective H2S SAW sensing have the potential in the real-time detection of H2S.

The wireless and passive surface acoustic wave (SAW) sensing system comprises SAW sensors and a SAW reader. In dynamic application scenarios, such as strain, torque, and vibration measurements, there is a critical need for high sensing speeds. However, the existing high-speed readers encounter challenges related to limited sensing range and reduced demodulation accuracy, which impede the broader adoption of SAW wireless passive sensing technology. This article presents a time-domain simulation model for SAW echo signals and explores the amplitude-frequency response characteristics of the reader to optimize its excitation time. Additionally, a noise model is developed based on actual data to assess the sensor bandwidth coverage provided by a single reader excitation, thereby optimizing the reader's interrogation frequency. Experimental platforms for wireless temperature and strain sensing are also constructed. Experimental results indicate that the developed high-speed reader achieves a measurement speed of 13 kHz while maintaining practical measurement accuracy and distance for the SAW sensor.

PdNi alloy thin films demonstrate exceptional hydrogen sensing performance and exhibit significant potential for application in surface acoustic wave (SAW) hydrogen sensors. However, the long-term stability of SAW H2 sensors utilizing PdNi films as catalysts experiences a substantial decrease during operation. In this paper, X-ray photoelectron spectroscopy (XPS) is employed to investigate the failure mechanisms of PdNi thin films under operational conditions. The XPS analysis reveals that the formation of PdO species on PdNi thin films plays a crucial role in the failure of hydrogen sensing. Additionally, density functional theory (DFT) calculations indicate that hydrogen atoms encounter a diffusion energy barrier during the penetration process from the PdNiOx surface to the subsurface region. The identification of PdNi film failure mechanisms through XPS and DFT offers valuable insights into the development of gas sensors with enhanced long-term stability. Guided by these mechanisms, we propose a method to restore the hydrogen sensing response time and magnitude to a certain extent by reducing the partially oxidized surface of the PdNi alloy under a hydrogen atmosphere at 70 °C, thereby restoring Pd to its metallic state with zero valence.

The transformation of acoustic impedance (AI) in different media regulates the transmission and reflection characteristics of acoustic waves between media, thus enabling many interesting acoustic applications. Here we introduce for the first time a surface acoustic wave (SAW) chip based on the AI effect, which utilizes its piezoelectric effect and unique interdigital transducer to excite a mechanical wave (SAW) propagating along the surface of the piezoelectric crystal, and it features high sensitivity to surface-loaded media such as gases and humidity changes due to the SAW energy being localized on the surface. On this basis, we theoretically established the relationship between surface load AI and SAW propagation loss, and analyzed the influence of AI on acoustic propagation loss under different gas/humidity media using mass conservation and ideal gas state equations. Experimental measurements using SAW chips reveal that the differences in AI generated by different gases trigger different acoustic propagation loss signals, and can achieve wide-range (1-100 v/v%) gas monitoring, with fast response and recovery speeds reaching sub-second levels (t90<1 s, t10<0.5 s). This capability can also be perfectly utilized for human respiratory monitoring, accurately reflecting respiratory status, frequency, and intensity. Therefore, the SAW sensing method and chip based on the AI effect proposed in this work provide a new solution for in-situ detection of gas leaks and precise monitoring of human respiration.

In this paper, the Zeolitic imidazole framework (ZIF) is designed and synthesized as the anti-interference layer of the Palladium-Nickel (PdNi) alloy thin-film coated surface acoustic wave (SAW) hydrogen (H-2) sensing device. Dimension regulation of the ZIF is conducted to obtain superior interference immunity. The two-dimensional (2D) and three-dimensional (3D) ZIF are prepared on top of PdNi thin-films by drop-coating. The gas experimental results indicate that 2D ZIF@PdNi sensors are more sensitive than 3D ZIF@PdNi and pure PdNi. At ambient temperature (25 degrees C), a fast response/recovery time of 12.03/14.62 s towards 8% H-2 gas, an ultralow limit of detection of 5 ppm, and a significantly increased sensitivity of 16.77 mV/% are achieved. Additionally, the 2D ZIF@PdNi sensors feature outstanding selectivity for H-2 gas against other gases (CO/NH3/H2S), good reversibility, and long-term stability for 30 days. The improved H-2 sensing performance can be attributed to that the two-dimensional anti-interference layer (2D ZIF) is more conducive to the diffusion of gas molecules, which in turn increases the interaction between PdNi and the target gas.

Multiphysics coupled sensing mechanism of palladium/nickel (Pd/Ni) alloy thin-film coated surface acoustic wave (SAW) hydrogen (H2) sensor is demonstrated theoretically and experimentally to allow the optimization of the sensing device in this work. The resistor-capacitance circuit model is used to describe the interaction between Pd/Ni film and H2. Referring to the perturbation theory, the relationship between the changes in SAW velocity/phase and the multi-physical field quantities of the Pd/Ni film are analyzed. To verify the theoretical model, the Pd/Ni film is sputtered on the Y35 degrees X quartz substrate to build the delay-line patterned SAW H2 sensor. Experimental results have well verified the theoretical predictions. That is, the main response mechanism is the mass loading effect, and the contribution of the acoustoelectric effect can be neglected. The expansion effect induced by hydrogen adsorption is completely different from the mass loading effect, which causes the sensing response failure, but it can be effectively improved by increasing the working temperature or decreasing the thickness of the Pd/Ni thin-film. Wide detection range (100 ppm similar to 38 v/v %), rapid response (t 90 similar to 7 s), and good humidity stability are achieved from the optimized SAW H2 sensor.

Hydrogen (H2) sensors with excellent performance play an important role in the safety application of H2 energy. Among current H2 detection technologies, surface acoustic wave (SAW) based H2 sensors attract more attention due to their features of high sensitivity, fast response, mico-nano scale and excellent stability, which was composed of a SAW sensing chip and sensing materials depositing along the acoustic wave propagation path, and utilizing the coupling mechanism of force-acoustic-electric multiple physical fields. Here we review the SAW H2 sensor, focusing on the H2 sensing materials, SAW sensing mechanisms, sensing devices, sensing circuits, and development status. And prospects were made for its development trends and challenges.

Palladium-nickel (Pd/Ni) thin-film-coated surface acoustic wave (SAW) sensor exhibits excellent performance for sensing hydrogen (H2), but it still suffers from the response drift induced by environment temperature/humidity in practical applications. To enhance the stability and accuracy of SAW sensors, this work demonstrates the effects of temperature and humidity on Pd/Ni alloy thin-film-coated SAW sensors and proposes a temperature/humidity compensation method. Pd/Ni-loaded SAW sensor integrated microheater is fabricated on Y35 degrees X quartz crystals by a photolithographic technique and a magnetron sputtering method, and the hydrogen-sensing experiments are conducted at different temperatures (-25 degrees C to 55 degrees C) and humidity (0%-60%). The results indicate that the baseline and response sensitivity of the sensor are significantly affected by temperature. The interference of humidity is relatively small due to the microheater, which reduces the adsorption of water molecules on the Pd/Ni film. Further analysis shows that the effect of temperature on the system components is cross-coupled, so an in situ temperature/humidity compensation method is proposed from the perspective of the system as a whole. By applying this compensation method, the baseline drift induced by temperature/humidity of the system is reduced by 97.86%, and the concentration prediction error is reduced from 22.58% to 4.83%. This demonstrates the effectiveness of the compensation method and provides technical support for the reliable application of SAW sensors in complex environments.

Abstract This study focuses on determining the optimized design parameters of palladium/nickel (Pd/Ni) thin film coated surface acoustic wave (SAW) hydrogen (H2) sensors with high sensitivity and fast response by modulating the ratio and thickness of Pd/Ni thin film. The Pd/Ni thin films deposited on the wave propagation path of delay-line patterned SAW sensors had Ni content ranging from 0 to 30 at% and thickness ranging from 20 to 400 nm. A phase discrimination circuit was used to collect the signal of SAW H2 sensor. The experimental results indicated that the response sensitivity of the sensor decreased with the increase of Ni content, while the response time gradually accelerated. Besides, the stability of the sensor deteriorated with increasing thickness, and there existed a non-linear relationship between sensitivity and thickness, and the response time slowed down as the thickness increased. To analyze the SAW sensing mechanism, a theoretical simulation model of the Pd/Ni thin film coated SAW H2 sensor was established using perturbation theory, which was well-validated by experimental results. Considering the requirements of fast response, high sensitivity, and stability of the H2 sensor, the optimal thin film structure parameters were determined as Pd9Ni1 thin film with the thickness of 40 nm.

A high-sensitive wireless and passive surface acoustic wave (SAW) humidity sensor operating at 433 MHz was designed and developed. SiO2 thin film with large specific surface was employed as the humidity-sensitive interface and deposited along the sensing area of the SAW device patterned by a reflective delay line by using plasma-enhanced chemical vapor deposition (PECVD) to build the sensing chip. Coupling of modes (COMs) model was utilized to simulate the SAW device prior to device fabrication to achieve low time-domain insertion and high signal-to-noise ratio (SNR). The transceiver based on frequency-stepped continuous-wave (FSCW) and planar dipole antennas was developed to construct successfully the wireless measurement system. In wireless humidity sensing experiments, high humidity sensitivity of 11.8°/% RH in the measurement range of 45%–90% RH was achieved at the wireless distance of 0.5 m. The corresponding response and recovery times were evaluated as 60 and 90 s, respectively. Also, excellent repeatability and stability in humidity sensing were obtained.

A Love wave-based sensing chip incorporating a polydimethylsiloxane (PDMS) micro-tank was proposed for ice sensing. The waveguide effect in the SU-8/ST-90°X quartz structure confines the acoustic energy in the SU-8 thin film, which is beneficial to improving the mass loading sensitivity and reducing longitudinal coupling attenuation in liquid. Self-compensation in temperature effect can be achieved by using the reverse polarity of the temperature coefficient (TCF) of SU-8 and ST-90°X. The micro-tank made of PMDS material was bonded on the acoustic wave propagation path to protect the interdigital electrodes and provide a sensitive area. Coupling of models (COM) and finite-element method (FEM) were employed to extract the optimal design parameters allowing low insertion loss of Love wave device, and the influence of PDMS toward Love wave propagation was also analyzed theoretically to determine the PDMS design. Using the standard photolithographic technique and molding method, a Love wave-based ice sensing chip was prepared to operate at 200 MHz and characterized by being connected to the amplitude detection circuit, and the corresponding icing sensing characteristics were studied. High sensitivity (11.894 mV/mm) and fast response (170 ms) were successfully achieved.

In this work, we propose a precise sensitivity calculation method for a surface acoustic wave (SAW)-based pressure sensor. The Tiersten–Sinha perturbation theory and finite-element method (FEM) were used to analyze the mechanical properties of a diaphragm based on piezoelectric material and achieved the pressure sensitivity distribution. Many factors affecting the pressure sensitivity could be taken into consideration by this method, including the material, the dimension of the diaphragm, as well as the position of electrodes. To prove the theoretical prediction, SAW pressure-sensing devices in ST and AT quartz diaphragm at two different thicknesses are prepared, and electrodes are patterned at different positions on the surface defined as type I and type II. Measured results confirmed the excellent consistency between theoretical and experimental results. For the group of 0.45-mm diaphragm, comparing the theoretical and the experimental results, errors of the type I and type II ST quartz sensors are 3.92% and 2.63%, and the error of the type I AT quartz sensor is only 0.96%. For the group of 0.3-mm diaphragm, errors of type I ST and AT quartz are 6.3% and 1.6%, respectively.

To feature fast hydrogen detection, a new design of surface acoustic wave (SAW) based hydrogen sensor integrated with a micro-heater is proposed in this paper. A 200 MHz delay-line patterned SAW sensing chip coated Pd/Ni alloy hydrogen sensitive film and a micro-heater are prepared on a Y35oX quartz wafer. The hydrogen gas adsorption in Pd/Ni thin-film modulates the SAW propagation, and against the temperature interference, the corresponding changes in acoustic attenuation are collected as the sensor signal. The Micro-heater is designed to catalysis the gas-sensitive effect by regulating the working temperature, and the influence law of heating temperature towards response speed is revealed theoretically, allowing determination of the optimal working temperature. The experimental results verify the theoretical prediction. Fast response (t90 < 2 s), lower power consumption (<1.35 W), and the low detection limit (<15 ppm) are achieved at low working temperature of 75 degrees C. Furthermore, very low crossed humidity sensitivity and excellent long-term stability are observed. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

A Pd/Ni nanowires (NWs) film-coated surface acoustic wave (SAW) device is proposed for sensing hydrogen with fast response. The gas adsorption in Pd/Ni NW film modulates the conductivity, and perturbs the SAW velocity, and the corresponding changes in phase can be collected as the sensing signal. 200 MHz SAW device with delay line pattern is prepared by using the lithographic technique on Y35oX quartz substrate. The larger specific surface of Pd/Ni NW contributes well to improvement of the response time. By using anode-aluminum-oxide (AAO) template-confined electrodeposition, the Pd/Ni NWs have been prepared, then dissolved in ethanol and finally deposited on the surface of the SAW device by drop coating method to build the SAW hydrogen sensor. The developed SAW sensor is integrated in a phase discriminating circuit, and experimentally characterized. Fast response less than 2 s, high sensitivity of 1.65 mV/% (0.3-3.5 v/v % in nitrogen gas), and low detection limit of 7 ppm have been achieved at room temperature (25 celcius). Further, the developed sensors show excellent selectivity and stability. Also, the film thickness - and humidity - dependent sensing performance was studied.

A new AlN film-based surface acoustic wave (SAW) device was explored for sensing strain at high tem-perature. AlN/metal/Si multilayer composite structure was proposed to construct the strain sensing chip, and the corresponding theoretical analysis on SAW propagation were performed by using finite element method (FEM). High-quality AlN piezoelectric thin-film was prepared by using magnetron sputtering on Si substrate, and Pt electrodes was then lithographically prepared to build the sensing chips with one-port resonator pattern operating at 607 MHz and 620 MHz. To compensate the temperature cross sensitivity and enhance the strain sensitivity, two sensing chips were positioned perpendicular on a ceramic package, then the differential frequency signal was extracted to evaluate the applied strain. A high-temperature strain testing platform was constructed to characterize the prepared strain sensor with orthometric structure, larger strain sensitivity of 0.84 ppm/mu epsilon at 550 degrees C in the range 0 - 500 mu epsilon and excellent temperature stability of 0.624 ppm/degrees C at the temperature range of 20-600 degrees C were achieved successfully. (C) 2021 Elsevier B.V. All rights reserved.