Surface Acoustic Wave Gas Sensor Research Articles

SAW Gas Sensors with Titania Nanotubes Layers

A surface acoustic wave (SAW) sensor with titania nanotubular layer as the gas sensitive material has been developed. This SAW 433 MHz two-port resonator has been fabricated on ST-cut quartz substrate and functionally characterized as an oscillator in dual differential mode. Solvent-casting coatings of anodized TiO2 nanotubes have been used for coating the devices for room-temperature detection of Volatile Organic Compounds (VOCs) including methanol, ethanol, acetone and m-xylene as well as hydrogen gas. The results demonstrate linear responses and high sensitivity of these sensors with the largest sensitivity achieved for ethanol to be 156 kHz/ppm at room temperature.

New sensitive layers for surface acoustic wave gas sensors based on polymer and carbon nanotube composites

Surface acoustic wave (SAW) gas sensors based on carbon nanotube polymer composites as sensitive layers were investigated for the detection of low concentrations of volatile organic compounds as octane and toluene. Several nanocomposites based on polyepichlorohydrin (PECH) and polyetherurethane (PEUT) with different percentage of multiwalled carbon nanotubes (MWCNT) were tested to study the effect of MWCNTs in the response of sensors.

Analytical Model of Semiconductor Sensor Layers in SAW Gas Sensors

In the paper a new theoretical model for analyzing a surface acoustic wave gas sensor is presented. Basing on the electric load of the piezoelectric acoustic line the effect of surface acoustic wave velocity changes vs. surface conductivity is predicted which depends on the profile concentration of gas molecules diffused into the porous film. Inside the sensor layer Knudsen’s model of gas diffusion was used.

A Novel Dual Track SAW Gas Sensor Using Three-IDT and Two-MSC

In this paper, a sensor architecture of a dual track surface acoustic wave (SAW) gas sensor using three-interdigitated transducers (IDTs) and two-multistrip couplers (MSCs) is proposed. The dual track SAW gas sensor consists of three IDTs and two MSCs fabricated on 128degY-X LiNbO3 piezoelectric substrate. Dual track structure and MSCs are incorporated into the sensor, thus the perturbations of the measurement signal from external environment are eliminated, and spurious responses caused by bulk acoustic waves (BAWs) are suppressed. Furthermore, the side lobe rejection is better than that of SAW sensors with uniform IDTs due to input IDT apodized by Morlet wavelet function. An experiment of dual track SAW gas sensor coated with polyaniline film sensitive to SO2 gas is presented. Experiment results confirm that the sensor has good reproducibility and response properties to SO2 gas at different concentrations. In the measurement ranging from 312 ppb to 20 ppm, the sensor has good linearity, and its sensitivity is approximately 6.8 KHz/ppm.

Comparison study of conductometric, optical and SAW gas sensors based on porous sol–gel silica films doped with NiO and Au nanocrystals

Conductometric, optical and surface acoustic wave gas sensors based on sol–gel silica porous films doped with NiO and Au nanoparticles are studied. The nanocomposite films are characterized by optical absorption in the visible and near-infrared region, spectroscopic ellipsometry, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) analysis and X-ray photoemission spectroscopy (XPS). They show poor conductometric gas sensing response toward CO and H 2. However, both optical and surface acoustic wave (SAW) responses are reversible and stable to H 2. Moreover, exploiting the wavelength dependence of the optical sensor response it is possible to selective recognition of H 2. Choosing the appropriate wavelength, almost no variation in absorption is observed when introducing different gases into the testing chamber.

Experiments for self-temperature-compensated characteristics of SAW gas sensor at fundamental and third-harmonic frequencies

A novel surface acoustic wave (SAW) gas sensor with self-temperature-compensated characteristics is proposed. Any type of piezoelectric crystal substrate irrelevant to their temperature coefficients of SAW velocities, can be adopted in this gas sensor. The sensing dynamic range can be extended utilising both the fundamental and third-harmonic frequency signals. Experimental results at the 75 MHz fundamental frequency and the 225 MHz third-harmonic frequency showed validity of the proposed sensor structure.

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.

Wireless surface acoustic wave chemical sensor for simultaneous measurement of <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">C</mi><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></mrow></math></inline-formula> and humidity

A 440-MHz wireless and passive surface acoustic wave (SAW) microsensor was developed for simultaneous measurement of CO2 and relative humidity (RH). The developed SAW microsensor was composed of SAW reflective delay lines structured by an interdigital transducer (IDT) and several shorted grating reflectors, CO2, and water vapor-sensitive films. A theoretical model was established to predict the response mechanism of the polymer-coated SAW gas sensor. Infusion of CO2 into the chamber and humidity changes induced large phase shifts in the reflection peaks. Good linearity and repeatability were observed in the CO2 concentration of 0~400 ppm and 20~80% RH. The obtained sensitivity was ~2° ppm−1 for CO2 detection and 7.45°/%RH for humidity sensing. Temperature and humidity effects were also investigated during the sensitivity evaluation process.

Layered SAW gas sensor based on CSA synthesized polyaniline nanofiber on AlN on 64° YX LiNbO 3 for H 2 sensing

A 64° YX LiNbO 3 surface acoustic wave (SAW) transducer was fabricated and then aluminium nitride (AlN) layer was deposited on the active area of the device as acoustic wave guiding layer. Structural studies revealed that the deposited AlN thin films have strong preferential c-axis orientation. Morphological analysis showed that the AlN layer is compact with grain dimensions of less than 80 nm. Polyaniline nanofibers were polymerized from the reactions in which an oxidant was rapidly mixed with the aniline monomer in an acidic environment. The polymer was synthesized in camphor sulfonic acid (CSA) to obtain 50 nm average diameter polyaniline nanofibers. These nanofibers were deposited on the layered SAW device and were tested towards hydrogen (H 2) gas while operating at room temperature. The device demonstrated a large and reproducible response to different concentrations of the H 2 gas making it an ideal candidate for H 2 sensing at room temperature.

High-frequency stability oscillator for surface acoustic wave gas sensor

We present a 300 MHz surface acoustic wave (SAW) oscillator for gas sensor application. As the oscillator element, SAW delay lines on an ST-X quartz substrate with low insertion loss and single-mode-selection capability were developed. And they were structured by Electrode Width Control Single Phase Unidirectional Transducer (EWC/SPUDT) configuration and comb transducer. The coupling of modes (COM) model was used to predict device performance prior to fabrication. The measured frequency response S12 showed a good agreement with simulated results; a low insertion loss of less than 9 dB and a linear phase in the 3 dB bandwidth were observed. The experimental results show that the baseline noise of the fabricated oscillators was typically up to ∼0.7×10−7 in a laboratory environment with temperature control. The oscillator was successfully applied to a gas sensor coated with fluoroalcoholpolysiloxane (SXFA) as the sensor material for O-isopropyl methyphosphonofluoridate (GB) detection, and a superior threshold detection limit was obtained (<<0.4 mg/m3).

Graphene-like nano-sheets for surface acoustic wave gas sensor applications

The gas sensing properties of graphene-like nano-sheets deposited on 36° YX lithium tantalate (LiTaO 3) surface acoustic wave (SAW) transducers are reported. The thin graphene-like nano-sheets were produced via the reduction of graphite oxide which was deposited on SAW interdigitated transducers (IDTs). Their sensing performance was assessed towards hydrogen (H 2) and carbon monoxide (CO) in a synthetic air carrier gas at room temperature (25 °C) and 40 °C. Raman and X-ray photoelectron spectroscopy (XPS) revealed that the deposited graphite oxide (GO) was not completely reduced creating small, graphitic nanocrystals ∼2.7 nm in size.

Polypyrrole nanofiber surface acoustic wave gas sensors

Abstract We present for the first time, to the best of authors’ knowledge, surface acoustic wave (SAW) gas sensors featuring polypyrrole nanofibers as the active component for hydrogen (H2) and nitrogen dioxide (NO2) detection at room temperature. Polypyrrole nanofibers were synthesized through a template-free chemical route by introducing bipyrrole as an initiator to speed up the polymerization of pyrrole in the presence of iron (III) chloride (FeCl3) as the oxidizing agent. Polypyrrole nanofibers were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopic (FT-IR) techniques which indicated that the average diameter of the nanofibers was 18 nm with lengths in the order of several microns. The polypyrrole nanofibers were dispersed onto the surface of a ZnO/36°YX LiTaO3 SAW transducer. Gas testing towards H2 and NO2 was conducted in an enclosed environmental cell at room temperature. Measured frequency shifts due to sensor responses were 20 kHz towards 1% of H2 and 4.5 kHz towards 2.1 ppm NO2, respectively. The sensor performance was assessed during a five-day period and repeatable results were obtained.

Study on the c-axis preferred orientation of ZnO film on various metal electrodes

ZnO thin film was deposited on various metal electrodes by reactive sputtering, and c-axis preferred orientation of the film has been studied. ZnO, which has high piezoelectricity, is promising for oscillators or filter devices such as surface acoustic wave (SAW) device, gas sensor, and film bulk acoustic resonator (FBAR). But, for the application of ZnO film for these devices, the film should be grown with c-axis normal to the electrode. In this study, Pt, Al, and Au were deposited on Si wafer, and the surface roughness and crystal structure of the ZnO film on the electrode were investigated using AFM, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Columnar structures of ZnO films were grown with c-axis normal to all electrodes, and among them Pt electrode showed the highest preferred orientation of ZnO film.

One-Millimeter Diameter Harmonic Ball Surface Acoustic Wave Gas Sensor with Temperature Compensation by Itself

A newly designed 1-mm diameter harmonic ball surface acoustic wave (SAW) gas sensor has a double-electrode interdigital transducer (IDT) for driving multiple frequencies. The fundamental frequency is 80 MHz. The difference in delay time between a fundamental frequency component and a high harmonics was used to compensate temperature drift. A temperature coefficient of about 0.1 ppm/°C was obtained after compensation.

Titanium Dioxide‐Based 64∘ YX LiNbO3 Surface Acoustic Wave Hydrogen Gas Sensors

Amorphous titanium dioxide (TiO2) and gold (Au) doped TiO2‐based surface acoustic wave (SAW) sensors have been investigated as hydrogen gas detectors. The nanocrystal‐doped TiO2 films were synthesized through a sol‐gel route, mixing a Ti‐butoxide‐based solution with diluted colloidal gold nanoparticles. The films were deposited via spin coating onto 64∘ YX LiNbO3 SAW transducers in a helium atmosphere. The SAW gas sensors were operated at various temperatures between 150 and 310∘C. It was found that gold doping on TiO2 increased the device sensitivity and reduced the optimum operating temperature.

A polyaniline/WO 3 nanofiber composite-based ZnO/64° YX LiNbO 3 SAW hydrogen gas sensor

Polyaniline/WO 3 nanofiber composite-based surface acoustic wave (SAW) gas sensor has been investigated towards hydrogen (H 2). Chemical oxidative polymerization of aniline was employed to synthesize polyaniline nanofibers with WO 3 nanoparticles. The nanocomposite was deposited onto a layered ZnO/64 ° YX LiNbO 3 SAW transducer. The sensor was exposed to various concentrations of H 2 gas and operated at room temperature. The sensor response was found to be 7 kHz towards 1% of H 2 in synthetic air. A fast response and recovery with good repeatability in a stable baseline condition were observed at room temperature.

폴리우레탄 감지막에 의한 표면탄성파 가스 센서의 감지능 향상

This paper presents characteristics of surface acoustic wave (SAW) gas sensor for detecting volatile gases such as ethanol gas by measuring the phase shift of output signal. A delay-line with a center frequency of 400 MHz was fabricated on <TEX>$128^{\circ}$</TEX> Y-Z <TEX>$LiNbO_{3}$</TEX> substrates. Experimental results, which showed the phase change of the output signal under the absorption of volatile gas on sensor surface, were presented. The sensitivities of SAW delay lines coated with polyurethane films were greatly increased compared to those for uncoated devices. This SAW gas sensor system may be well suited for a high sensitivity electronic nose system.

A novel wireless, passive CO2 sensor incorporating a surface acoustic wave reflective delay line

A 440 MHz wireless and passive surface acoustic wave (SAW) chemical sensor was developed forCO2 detection. The developed SAW gas sensor is composed of single phaseunidirectional transducers (SPUDTs), three shorted grating reflectors, andCO2-sensitivepolymer film on 41° YX LiNbO3 substrate. Coupling of modes (COM) modeling was used to find optimal designparameters. Using the extracted design parameters, the SAW device wasfabricated. Teflon AF 2400 was used as the sensitive film because it provides highCO2 solubility, permeability and selectivity. In wireless device testing using anetwork analyzer, four sharp reflection peaks with high signal-to-noise(S/N) ratio, small signal attenuation, and few spurious peaks were observedin the time domain. The time positions of the reflection peaks werewell matched with the predicted values from the simulation. Infusion ofCO2 into thechamber induced large phase shifts of the reflection peaks. Good linearity and repeatability were observedfor a CO2 concentration of 0–450 ppm. The obtained sensitivity was1.98° ppm−1. Temperature and humidity effects were also investigated during the sensitivity evaluationprocess.

Metal-free phthalocyanine and palladium sensor structure with a polyethylene membrane for hydrogen detection in SAW systems

Abstract The results concerning bilayer sensor structures (metal-free phthalocyanine (H2Pc) + palladium (Pd)) with various thicknesses of metal-free phthalocyanine (∼120 nm, ∼160 nm and ∼200 nm), but the same thickness of palladium (∼20 nm), have been studied with respect to the application of hydrogen gas-sensing at a relatively low interaction temperature ∼30–38 °C, employing a method based on the surface acoustic wave (SAW) system. All the films are produced by physical vapour deposition (PVD) in a SAW gas sensor system with a dual-delay line configuration. In the case of the best structure, H2Pc ∼ 160 nm and Pd ∼ 20 nm (the greatest change in frequency due to hydrogen gas interaction at the same temperature), single and double novel polyethylene (PE) membranes are placed on the top of the bilayer structure to reduce the influence of humidity. These PE membranes are made in a special way also by means of the PVD technology (at a temperature of ∼190–250 °C). The thickness of a single PE film amounts to about several score nanometers (60–80 nm). These membranes can also reduce the influence of other gases, like CO, NO2 and ammonia.

Layered SAW gas sensor with single-walled carbon nanotube-based nanocomposite coating

A Surface acoustic wave (SAW) sensor using a ZnO guiding layer on 36° Y-cut X-propagation LiTaO 3 piezoelectric substrate has been designed and fabricated. Single-walled carbon nanotube (SWCNT)-based nanocomposite coatings have been deposited using the Langmuir–Blodgett technique. The sensor was tested towards H 2 , NH 3 , NO 2 gases, in the range of 0.030–1%, 30–1000 ppm, 1–10 ppm, respectively. The output signal was the phase of the SAW device, which was monitored using a network analyzer. The developed sensor has a high sensitivity, good repeatability and low detection limit at sub-ppm levels, at room temperature. The experimental results obtained are compared to a theoretical model and the SAW gas sensing mechanisms are discussed with the acoustoelectric effect that appears to be dominant in the SAW response.