Surface Acoustic Wave Hydrogen Sensor Research Articles

Multiphysics coupled sensing mechanism of Pd/Ni alloy thin-film coated SAW hydrogen sensor

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°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 ∼ 38 v/v %), rapid response (t 90 ∼ 7 s), and good humidity stability are achieved from the optimized SAW H2 sensor.

SAW Hydrogen Sensors with Pd/SnO2 Layers.

Pd/SnO2 bilayers for surface acoustic wave (SAW) sensors were obtained using pulsed laser deposition (PLD). Bilayers were made at several deposition pressures in order to observe the influence of the morphology of the sensitive films on the response of the sensors. The morphological properties were analyzed by scanning electron microscopy (SEM). The SnO2 monolayers were initially deposited on quartz substrates at 100, 400 and 700 mTorr, to observe their morphology at these pressures. The Pd/SnO2 bilayer depositions were made at 100 and 700 mTorr. The sensors realized with these sensitive films were tested at different hydrogen concentrations, in the range of 0.2-2%, at room temperature. In order to establish selectivity, tests for hydrogen, nitrogen, oxygen and carbon dioxide were carried out with SnO2-700, Pd-100/SnO2-700 and Pd-700/SnO2-700 sensors. The sensor with the most porous sensitive film (both films deposited at 700 mTorr) had the best results: a sensitivity of 0.21 Hz/ppm and a limit of detection (LOD) of 142 ppm. The morphology of the SnO2 film is the one that has the major influence on the sensor results, to the detriment of the Pd morphology. The use of Pd as a catalyst for hydrogen improved the sensitivity of the film considerably and the selectivity of the sensors for hydrogen.

SAW sensor employing Pd/Ni nanowire for super-fast hydrogen detection at room temperature

A Pd/Ni nanowire (NW) film coated surface acoustic wave (SAW) sensor exhibited super-fast response as 2 s to 1 ∼ 3 v/v % hydrogen at room temperature (25 ℃). By using anode aluminum oxide (AAO) template-confined electrodeposition, the Pd/Ni NWs were prepared, and then deposited on the surface of the SAW device to build the SAW hydrogen sensor. The developed SAW sensor was integrated in a phase discriminating circuit, and experimentally characterized. The larger specific surface of Pd/Ni NW contributes well to improvement of the response time.

A surface acoustic wave hydrogen sensor with tin doped indium oxide layers for intermediate temperatures

High temperature surface acoustic wave (SAW) gas sensors with conducting sensing layers require tuning of the sheet conductivity for optimal response. Conducting metal oxides are attractive sensing materials for their tunable electronic properties and high thermal stability, amongst others. Here, we have investigated the application of indium oxide (IO) and indium tin oxide (ITO) films on langasite (LGS)-based SAW reflective delay line sensor devices for monitoring hydrogen at 350 °C. Specifically, we modeled the effect of the IO and ITO sensing layer thickness on the wave velocity, attenuation, and effective electromechanical coefficient. This was followed by an experimental demonstration of tuning of the ITO film sheet conductivity by controlling the dopant concentration and yielding an improvement in the sensor sensitivity. The current study provides a pathway towards the development of conductivity-based sensing layers for high temperature SAW gas sensors with improved sensitivity.

Pd/Ni nanowire film coated SAW hydrogen sensor with fast response

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 ℃). Further, the developed sensors show excellent selectivity and stability. Also, the film thickness - and humidity - dependent sensing performance was studied.

FEM simulation of palladium thin film coated surface acoustic wave hydrogen sensor for high frequency applications

A higher operating frequency is desirable for Surface Acoustic Wave (SAW) based sensors as they become more sensitive at high frequencies. The acoustic wave gets more confined near the surface at high frequencies and become more sensitive to the external stimulations. This makes SAW devices a suitable device for sensing gaseous state chemicals. SAW devices have become the basic building block of wireless sensor networks with its advantages enabling remote sensing. In this paper, a SAW based Hydrogen sensor is realized through the Finite Element Analysis tool ANSYS. Hydrogen even though has a significant role in many industries, its explosive nature demands constant monitoring. SAW delay line made up of XY-LiNbO3 as substrate with a thin layer of Palladium coated along the delay length as the sensing element is modeled. Palladium with its high affinity for Hydrogen absorbs the same and undergoes changes in properties like density and stiffness. This disturbs the surface wave propagation and in turn, affects the operating frequency which is the sensor response parameter. The frequency shift of 1.91 MHz for Hydrogen concentration of 0.3 a.f. as compared to 0.49 MHz with YZ- LiNbO3. The operating frequency also shifts to a higher range as the acoustic velocity of the substrate increases.

A SAW hydrogen sensor based on decoration of graphene oxide by palladium nanoparticles on AIN/Si layered structure

This work proposes a surface acoustic wave (SAW) hydrogen sensor based on the decoration of graphene oxide (GO) by palladium nanoparticles on AIN/Si layered structure. A thin decoration of GO by palladium nanoparticles layer is directly coated on the sensor surface as sensing layer. The SAW hydrogen sensor shows higher sensitivity (~9.5 times) and lower cross-sensitivity towards NO2 compared to those based on GO only. The hydrogen sensor has good stability and fast response. The response time and recovery time are both 7 s. The frequency shift of the sensor with hydrogen concentration change is described by a fitting formula. Response mechanism is investigated based on complex impedance spectroscopy at various hydrogen concentration levels.

Development of a Pd/Cu nanowires coated SAW hydrogen gas sensor with fast response and recovery

Employing palladium and copper nanowires (Pd/Cu NWs) as the sensitive interface, a new surface acoustic wave (SAW) sensor with fast response and recovery was developed successfully for hydrogen gas sensing. The adsorption in Pd/Cu NWs towards hydrogen molecules modulates the SAW propagation by the induced acoustoelectric coupling effect from the changes in the resistivity of Pd, and the corresponding shifts in oscillation frequency proportional to gas concentration was collected as the sensor signal. Larger surface-to-volume of the Pd/Cu NWs contributes to improve the response speed and sensitivity. A 150 MHz SAW device with delay line pattern was fabricated photolithographically on 128° YX LiNbO3 substrate, and the Pd/Cu NWs that synthesized by optimized electrochemical deposition were dissolved in ethanol and deposited onto the SAW device surface by dropping method to build the SAW hydrogen sensors. The prepared SAW sensors were characterized by using the differential oscillation loop experimentally, excellent repeatability, fast response and recovery within 4s, and a sensitivity of 1.5 kHz/%were achieved at room temperature (25°C). Additionally, the response mechanism in hydrogen sensing was investigated experimentally.

Surface Acoustic Wave Hydrogen Sensors Based on Nanostructured Pd/WO₃ Bilayers.

The effect of nanostructure of PLD (Pulsed Laser Deposition)-deposited Pd/WO3 sensing films on room temperature (RT) hydrogen sensing properties of SAW (Surface Acoustic Wave) sensors was studied. WO3 thin films with different morphologies and crystalline structures were obtained for different substrate temperatures and oxygen deposition pressures. Nanoporous films are obtained at high deposition pressures regardless of the substrate temperature. At lower pressures, high temperatures lead to WO3 c-axis nanocolumnar growth, which promotes the diffusion of hydrogen but only once H2 has been dissociated in the nanoporous Pd layer. XRD (X-ray Diffraction) analysis indicates texturing of the WO3 layer not only in the case of columnar growth but for other deposition conditions as well. However, it is only the predominantly c-axis growth that influences film sensing properties. Bilayers consisting of nanoporous Pd layers deposited on top of such WO3 layers lead to good sensing results at RT. RT sensitivities of 0.12–0.13 Hz/ppm to hydrogen are attained for nanoporous bilayer Pd/WO3 films and of 0.1 Hz/ppm for bilayer films with a nanocolumnar WO3 structure. SAW sensors based on such layers compare favorably with WO3-based hydrogen detectors, which use other sensing methods, and with SAW sensors with dense Pd/WO3 bilayers.

Highly Sensitive Ball Surface Acoustic Wave Hydrogen Sensor with Porous Pd-Alloy Film

In order to commonly use explosive H2 gas as an energy source, fast, sensitive, and low-power consumption sensor is required. We developed a ball surface acoustic wave (SAW) sensor with porous Pd­Pt alloy film to realize such a sensor. The sensor with 20% Pt alloying film was useful for the suppression of a phase transition of Pd and the detection of ppm order concentration at 35°C. The amplitude response was proportional to the square root of the concentration, which was demonstrated for the first time in the field of SAW sensor, resulting in the detection limit of 3.7ppm at signal to noise ratio of 3. The response time decreased to 1/5, compared with those of pioneering H2 sensors working at room temperature. From these results, it was shown that the fastest and most sensitive hydrogen sensor working at room temperature could be realized using the ball SAW sensor with porous Pd alloy film. [doi:10.2320/matertrans.I-M2014816]

Surface acoustic wave hydrogen sensors based on ZnO nanoparticles incorporated with a Pt catalyst

This paper describes the fabrication and characterization of surface acoustic wave (SAW) hydrogen sensors using ZnO nanoparticles incorporated with a Pt catalyst for hydrogen detection at room temperature. The Pt/ZnO nanoparticle sensing layer was deposited onto SAW delay line sensor-based interdigitated electrodes (IDTs)/aluminum nitride (AlN)/silicon (Si) structure. The average ZnO crystallite sizes varied from 30 nm to 40 nm and were synthesized by a sol–gel method and deposited onto SAW sensors by spin coating. The Pt-like catalyst was deposited onto ZnO nanoparticles by RF sputtering for 2 s. The effect of ZnO crystalline quality, crystallite size and humidity on hydrogen sensing performance was evaluated. Moreover, the sensitivity of SAW delay line sensors as they depend on conventional structures and layered structures were also characterized in this work. The SAW delay line sensors using AlN/Si structures have fundamental frequency of 129.28 MHz for uncoated, 128.85 MHz and 126.93 MHz for coated Pt/ZnO sensing layer corresponding to the conventional and layered structures, respectively. The layered SAW structure using Pt/ZnO nanoparticles as sensing layer show frequency shift of 55 kHz in 1% H 2 concentration at room temperature with good repeatability and stability.

Transmission Line Matrix Modeling for Analysis of Surface Acoustic Wave Hydrogen Sensor

An analysis of a surface acoustic wave (SAW) sensor is carried out by transmission line matrix (TLM) modeling, which provides time domain characteristics. In the present approach, the equivalent circuits of interdigital transducers (IDTs), the free space, and the Palladium (Pd) film for the transmission are replaced by a TLM model. The circuit now consists of the connection of transmission lines, in which the circuit response is traced as the sequences of impulses in a time domain. The frequency response Fourier-transformed is compared with that obtained for the equivalent circuit model (ECM). From the time domain results obtained on the basis of Pd thickness, we found the possibility of the analysis and estimation of the characteristics of the SAW hydrogen sensor by TLM modeling.

Layered Surface Acoustic Wave Hydrogen Sensor with Nanoporous Polyaniline as the Active Layer

In recent years, a small but critically informed literature has emerged which points to the link between opportunities to engage in generative acts and desistance from crime. This paper outlines the nature and limits of generative moments (conceived as the philosophy or practice of caring in non-violent and durable ways for self, other and future) with regard to the incarceration of a group of young males interviewed since 2003. Specifically, it poses the question of who or what it has been possible for these young men to care about within and beyond custody and highlights some of the factors which undermine generative concerns and actions. In concluding, the paper offers several ideas for enhancing generative opportunities within custodial environments.

Finite Element Method Simulation of a Surface Acoustic Wave Hydrogen Sensor with Palladium Nano-Pillars as Sensing Medium

Finite Element Method Simulation of a Surface Acoustic Wave Hydrogen Sensor with Palladium Nano-Pillars as Sensing Medium

A Finite Element Model of a MEMS-based Surface Acoustic Wave Hydrogen Sensor

Hydrogen plays a significant role in various industrial applications, but careful handling and continuous monitoring are crucial since it is explosive when mixed with air. Surface Acoustic Wave (SAW) sensors provide desirable characteristics for hydrogen detection due to their small size, low fabrication cost, ease of integration and high sensitivity. In this paper a finite element model of a Surface Acoustic Wave sensor is developed using ANSYS12© and tested for hydrogen detection. The sensor consists of a YZ-lithium niobate substrate with interdigital electrodes (IDT) patterned on the surface. A thin palladium (Pd) film is added on the surface of the sensor due to its high affinity for hydrogen. With increased hydrogen absorption the palladium hydride structure undergoes a phase change due to the formation of the β-phase, which deteriorates the crystal structure. Therefore with increasing hydrogen concentration the stiffness and the density are significantly reduced. The values of the modulus of elasticity and the density at different hydrogen concentrations in palladium are utilized in the finite element model to determine the corresponding SAW sensor response. Results indicate that with increasing the hydrogen concentration the wave velocity decreases and the attenuation of the wave is reduced.

Layered Surface Acoustic Wave Hydrogen Sensor Based on Polyethylaniline Nanofibers

Here we report on a layered Surface Acoustic Wave (SAW) gas sensor featuring polyethylaniline nanofibers as the active layer. A rapidly-mixed reaction was employed to synthesize polyethylaniline nanofibers. The product was deposited on a microfabricated ZnO/36° YX LiTaO3 SAW transducer. Scanning Electron Microscopy (SEM) and Ultraviolet-visible spectroscopy (UV-vis) were utilized to characterize the nanomaterials. The novel sensor was tested towards different concentrations of hydrogen at room temperature. It was observed that the sensor maintained a stable baseline with good repeatability. Hence, the sensor is expected to be potentially attractive for industrial applications.

A room temperature surface acoustic wave hydrogen sensor with Pt coated ZnOnanorods

A surface acoustic wave (SAW) sensor with Pt coated ZnO nanorods as the selective layerhas been investigated for hydrogen detection. The SAW sensor was fabricated based on a128°YX-LiNbO3 substrate with a operating frequency of 145 MHz. A dual delay line configuration wasadopted to eliminate external environmental fluctuations. The Pt coated ZnO nanorodswere chosen as a selective layer due to their high surface-to-volume ratio, large penetrationdepth, and fast charge diffusion rate. The ZnO nanorods were synthesized byan aqueous solution method and coated with the noble metal Pt as a catalyst.Finally, the SAW sensor responses to humidity and hydrogen were tested. Resultsshow that the sensor is not sensitive to humidity; moreover, the frequency shiftfor a hydrogen concentration variation of 6000 ppm is 26 kHz while operating atroom temperature. It can be concluded that the Pt coated ZnO nanorod basedSAW hydrogen sensor exhibits fast response, good sensitivity and short-termrepeatability. It is worth noting that not only is the sensor sensitive enough tooperate at room temperature, but also it can avoid the influence of humidity.

Evaluation of Response Time in Ball Surface-Acoustic-Wave Hydrogen Sensor using Digital Quadrature Detector

Hydrogen leak detection sensors must have high sensitivity and a short response time of 1 s or less. A ball surface-acoustic-wave (SAW) hydrogen sensor has a high sensitivity and can detect hydrogen in a very wide concentration range of 10 ppm to 100%. Moreover, a fast response can be expected because of the very thin sensitive film used. In this study, we developed a digital quadrature detector (DQD) to measure responses of less than 1 s, and measure phases in 1 ms intervals with excellent sensitivity. We evaluated the response time of the ball SAW hydrogen sensor where the signal was averaged 256 times in 0.256 s using the DQD. As a result, the response time was found to be 1 s or less for 3.0 vol % hydrogen gas in nitrogen.

P1-29 Evaluation of response time in ball surface acoustic wave hydrogen sensor(Poster session 1)

P1-29 Evaluation of response time in ball surface acoustic wave hydrogen sensor(Poster session 1)