Surface Acoustic Wave Gas Research Articles

Simulation and Fabrication of SAW-Based Gas Sensor with Modified Surface State of Active Layer and Electrode Orientation for Enhanced H2 Gas Sensing

The design, analysis, optimization, and fabrication of layered and nanostructure-based surface acoustic wave (SAW) gas sensors are presented. A lithium niobate and zinc oxide (ZnO) nano multilayer structure is proposed to enhance the sensitivity of the SAW-based gas sensor. Different materials are considered for the intermediate layer in the design for optimization purposes. The sensitivity of the sensor could be improved due to increased active surface area obtained by varying the aspect ratio of the nanorods, the thickness of the intermediate layer, and the gap between the electrodes. The total displacement and frequency shift of the device were significantly improved. Overall, the mechanically engineered surface-based (nanorod) SAW gas sensor offered better sensing response than the layered SAW gas sensor in terms of sensitivity performance.

Detection of third-hand smoke on clothing fibers with a surface acoustic wave gas sensor.

Third-hand smoke (THS) is a new cigarette-related issue defined as the residual contamination from cigarette smoke after a cigarette is extinguished. To detect THS on three commonly used clothing fibers-wool, cotton, and polyester, we applied two methods to measure the adsorption of THS: one was the gain of mass with an analytical balance after exposure to cigarette smoke; and the other was to detect the THS chemical compounds such as nicotine and 3-ethenylpyridine with a surface acoustic wave (SAW) sensor composed of coated oxidized hollow mesoporous carbon nanospheres. In the mass measurement, the gain of mass decreased in the order wool, cotton, and polyester; the latter gain was about one tenth that of wool. In the SAW detection, the frequency shift decreased in the same order-wool, cotton, and polyester. The residence period of THS on natural fiber (wool and cotton) is greater than on synthetic polyester fiber. These two tests provide quantitative results of THS on varied clothing fibers, to assess their risk after exposure to cigarette smoke.

Sniffing lung cancer related biomarkers using an oxidized graphene SAW sensor

Decane is one of the volatile organic compounds (VOCs) in human breath. Successful detection of decane in human breath has vast prospects for early lung cancer diagnosis. In this paper, a novel detecting device based on a filter surface acoustic wave (SAW) gas sensor is presented. SAW sensors coated with a thin oxidized graphene film were used to detect decane in parts per million (ppm) concentrations. Control and signal detection circuits were designed using a vector network analyzer with a detection resolution of insertion loss down to 0.0001 dB. The results showed that the SAW sensor could respond quickly with great sensitivity when exposed to 0.2 ppm decane. This device shows tremendous potential in medical diagnosis and environmental assessment.

Optimal Design of SAW Gas Sensing Device by Using Improved Adaptive Neuro-Fuzzy Inference System

A Taguchi-based-genetic algorithm (TBGA) is used in an adaptive neuro-fuzzy inference system (ANFIS) to optimize design parameters for surface acoustic wave (SAW) gas sensors. The Taguchi method is used to reduce the number of experiments and collect performance data for an SAW gas sensor. The TBGA has two optimization roles. In the ANFIS, the TBGA selects appropriate membership functions and optimizes both the premise and the consequent parameters by minimizing the performance criterion of the root mean squared error. Another role of the TBGA is optimizing design parameters for an SAW gas sensor. Simulated experimental application of the proposed TBGA-based ANFIS approach showed that, in terms of both resonant frequency shift and precision performance, this systematic design approach obtains far superior results compared with the conventional trial-and-error design methods and other Taguchi-based design methods.

Performance Prediction and Sensitivity Analysis of SAW Gas Sensors

An improved adaptive neuro-fuzzy inference system (IANFIS) is proposed to build a model to predict the resonant frequency shift performance of surface acoustic wave (SAW) gas sensors. In the proposed IANFIS, by directly minimizing the root-mean-squared-error performance criterion, the Taguchi-genetic learning algorithm is used in the ANFIS to find both the optimal premise and consequent parameters and to simultaneously determine the most suitable membership functions. The five design parameters of SAW gas sensors are considered to be the input variables of the IANFIS model. The input variables include the number of electrode finger pairs, the electrode overlap, the separation distance of two interdigital transducers on the substrate, the dimensions of the stable temperature-cut (ST-cut) quartz substrate, and the electrode thickness. The output variable of the IANFIS model is composed of the resonant frequency shift performance. The results predicted by the proposed IANFIS are compared with those obtained by the back-propagation neural network. The comparison has shown that the performance prediction of resonant frequency shift using the proposed IANFIS is effective. In addition, the sensitivity analyses of the five design parameters have also shown that both the electrode overlap and the dimensions of the ST-cut quartz substrate have the most influence on the resonant frequency shift performance.

Nanocrystalline SnO2 film prepared by the aqueous sol–gel method and its application as sensing films of the resistance and SAW H2S sensor

Tin oxide (SnO2) has been employed as the sensitive film of surface acoustic wave (SAW) gas sensor, which enhanced the sensor performance toward H2S gas. However, the conventional sol–gel method is unfit for depositing SnO2 films on SAW devices as the sol can corrode the electrodes made of conventional metals due to its strong acidity. In this paper, a new aqueous and non-corrosive sol–gel method was adopted to deposit SnO2 films on SAW device substrate with little corrosion of electrodes. The prepared sol is weak alkaline and noncorrosive, which prevent the contact between the electrodes and the air during the sintering process. The surface morphology and microstructure of the prepared films were investigated and the H2S sensing properties of the films were studied. The gas sensing results showed that the prepared SnO2 films have the highest resistance response (the ratio of film resistance in air versus in H2S) 58,057 toward 13.7ppm H2S at 60°C and good selectivity for H2S detection. The SAW gas sensors had a good performance for detecting H2S at room temperature and at low relative humidity. This method can also be applied to prepare the sensing membrane of the other acoustic wave sensors.

The Designation and Simulation of Amplifying and Shaping Circuit of SAW Gas Sensor Signal

In this paper, a kind of amplifying and shaping circuit was designed according to the characteristics of high frequency and small amplitude of surface acoustic wave (SAW) gas sensor signal. The circuit is applied to two-channel mixing detection method to solve the problem that backend devices cannot identify sensor signal. The whole circuit is simple in structure and has very good shaping effect on voltages signal of millivolt level, meeting the need of the sensor of gas detection.

Elemental theory of a SAW gas sensor based on electrical conductivity changes in bi-layer nanostructures

An elemental theory for a surface acoustic wave (SAW) gas sensor is reported within this work. The sensor mechanism is generally based on changes in electrical conductivity of thin bi-layer sensor nanostructures under the influence of gas molecules. In turn, the interaction between the electrical surface potential (associated with SAW propagating on a piezoelectric substrate) and the mobile electric charges of bi-layer sensor nanostructures (characterized by different electrical conductivities σ1, σ2 and thicknesses h1, h2), cause an acoustoelectric perturbation of the SAW propagation. Both films are very thin in comparison to the SAW wavelength and the Debye screening length. This allows for a one-dimensional treatment of the problem. As a result the relative changes in SAW velocity and attenuation is highly dependent on the electrical surface conductivities ratio of the thin film bi-layer nanostructures (x=σs2/σs1). A proper fitting of the construction parameter x and the initial “work point” (σs1 or σs2 vs. voCs) for such structures allows for obtaining considerable sensitivities in SAW gas sensors.

Response mechanism for surface acoustic wave gas sensors based on surface-adsorption.

A theoretical model is established to describe the response mechanism of surface acoustic wave (SAW) gas sensors based on physical adsorption on the detector surface. Wohljent's method is utilized to describe the relationship of sensor output (frequency shift of SAW oscillator) and the mass loaded on the detector surface. The Brunauer-Emmett-Teller (BET) formula and its improved form are introduced to depict the adsorption behavior of gas on the detector surface. By combining the two methods, we obtain a theoretical model for the response mechanism of SAW gas sensors. By using a commercial SAW gas chromatography (GC) analyzer, an experiment is performed to measure the frequency shifts caused by different concentration of dimethyl methylphosphonate (DMMP). The parameters in the model are given by fitting the experimental results and the theoretical curve agrees well with the experimental data.

A New System for Acoustoelectronic Gas Sensors Analysis

Typical approach to the surface acoustic waves sensors response analysis is based on the use of self-oscillating circuits with surface acoustic wave device working inside positive feedback loop of an ampli er. Such kind of parametric measurement allows to track the center frequency of the sensor changes in particular. The method is widely used mainly due to their relative simplicity. Unfortunately, it has many disadvantages like frequency (phase) instability, sensitivity to unwanted factors, surface acoustic wave substrate mass-load limit etc. A new system to the analysis of surface acoustic wave gas sensors response as well as an exemplary measurement results are described in the paper. The presented system make the rst step to the more complex conception realization.

Functional poly(urethane-imide)s containing Lewis bases for SO2 detection by Love surface acoustic wave gas micro-sensors

Functional polymers containing Lewis bases are known as promising materials for the elaboration of highly sensitive micro-sensors for acid gas detection. In this work, poly(urethane-imide)s (PUIs) with Lewis bases were synthesized in two steps only from various functional diols (N-methyldiethanolamine, N-tert-butyldiethanolamine, N-phenyldiethanolamine and 1,4-diethanolpiperazine) containing one or two tertiary amine groups with different chemical structures. The synthesis led to high yields, high molecular weights and a good control of the PUI amine content. These good film-forming polymers were then used as SO2 sensitive coatings on Love surface acoustic wave (L-SAW) micro-sensors. The physical and mechanical properties required for an optimal micro-sensor design were determined by High Resolution Brillouin Spectroscopy of PUI thin layers. Three-layer micro-sensors were developed by respecting the conditions for the L-SAW generation. The three-layer structure included Quartz ST-90 as the piezoelectric substrate, a ZnO guiding layer and a PUI sensitive layer. These micro-sensors were weakly sensitive to temperature and thus compatible with the targeted application. The experimental results for SO2 detection showed that all the PUI coatings greatly improved the sensitivity compared to a micro-sensor without polymer coating. The basicity of the amine groups was not determining for the micro-sensor sensitivity to the SO2 acid gas and the key factor was their steric hindrance. Finally, the micro-sensor sensitivity increased with the accessibility of the amine groups in the order Piperazine-diol<tBu-DEA≅Ph-DEA≪MDEA.

Langasite surface acoustic wave gas sensors: modeling and verification

We report finite element simulations of the effect of conductive sensing layers on the surface wave velocity of langasite substrates. The simulations include both the mechanical and electrical influences of the conducting sensing layer. We show that three-dimensional simulations are necessary because of the out-of-plane displacements of the commonly used (0, 138.5, 26.7) Euler angle. Measurements of the transducer input admittance in reflective delay-line devices yield a value for the electromechanical coupling coefficient that is in good agreement with the three-dimensional simulations on bare langasite substrate. The input admittance measurements also show evidence of excitation of an additional wave mode and excess loss resulting from the finger resistance. The results of these simulations and measurements will be useful in the design of surface acoustic wave gas sensors.

Design of SAW Specific Gas Sensor with High Sensitivity

To improve the temperature stability, response speed and sensitivity of surface acoustic wave (SAW) gas sensor, the relationship between the sensing region of the resonator for SAW gas sensor and the sensitivity of sensor is studied, a specific resonator with big space topology structure is proposed. A SAW resonator with high temperature stability is investigated from the viewpoint of piezoelectric material, cut type and fabrication process. A nano-wire bundle based SAW gas sensor with big specific-surface-area is proposed.

Realization of Surface Acoustic Wave (SAW) and Semiconductor Gas Sensors for Room Temperature Detection of NO2 Gas

SnO2 thin films deposited using RF sputtering technique have been exploited to detect NO2 gas using two different gas sensing techniques namely: (i) conductometeric sensing and (ii) surface acoustic wave (SAW) sensing. The surface morphology and structural property of the sensing SnO2 thin film were studied using, Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) techniques respectively. The SnO2 thin film based conductometric sensor is found to exhibit higher sensing response (1.6 × 104) as compared to the one obtained using SAW gas sensor (-25 MHz). However, the conductometric sensor demands a high operating temperature of 100 oC and exhibits relatively poor response time and recovery time of about 2 min and 22 min respectively. Whereas, the SAW sensor shows the possible room temperature sensing of NO2 gas with faster response and recovery speeds of 8 s and 40 s respectively.

ZnO Nanomaterials Based Surface Acoustic Wave Ethanol Gas Sensor

ZnO nanomaterials based surface acoustic wave (SAW) gas sensor has been investigated in ethanol environment at room temperature. The ZnO nanomaterials have been prepared through thermal evaporation of high-purity zinc powder. The as-prepared ZnO nanomaterials have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray Diffraction (XRD) techniques. The results indicate that the obtained ZnO nanomaterials, including many types of nanostructures such as nanobelts, nanorods, nanowires as well as nanosheets, are wurtzite with hexagonal structure and well-crystallized. The SAW sensor coated with the nanostructured ZnO materials has been tested in ethanol gas of various concentrations at room temperature. A network analyzer is used to monitor the change of the insertion loss of the SAW sensor when exposed to ethanol gas. The insertion loss of the SAW sensor varies significantly with the change of ethanol concentration. The experimental results manifest that the ZnO nanomaterials based SAW ethanol gas sensor exhibits excellent sensitivity and good short-term reproducibility at room temperature.

Continuous Measurement of Multiple Gases Using Ball Surface Acoustic Wave Gas Chromatograph

Although portable gas chromatographs (GCs) have been developed for the monitoring of volatile organic compounds (VOCs) in working environments, they still need high power consumption for the heating column. Thus, we previously developed a portable surface acoustic wave (SAW) GC equipped with a ball SAW sensor and a micro-electromechanical-system column (ball SAW GC) and proved the usefulness of the forward flush (FF) method for realizing the fast analysis of multiple gases without a heater. However, its ability to measure ten kinds of VOCs at ppm order and automatic continuous measurement were not demonstrated. In this study, a ball SAW GC employing the FF method and equipped with a gas sampler for continuous injection was developed. Then, the performance of monitoring multiple gases in working environments was verified by measuring ten kinds of VOCs with maximum acceptable concentrations. Moreover, real-time monitoring of seven kinds of VOCs with a linear change in the response value to concentration changes was demonstrated.

Continuous Measurement of Multiple Gases Using Ball Surface Acoustic Wave Gas Chromatograph

Continuous Measurement of Multiple Gases Using Ball Surface Acoustic Wave Gas Chromatograph

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

Abstract Surface acoustic wave (SAW) gas sensors based on polymers and carbon nanotube 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. The sensors exhibited at room temperature high response to volatile gases (toluene and octane) but did not detect other gases tested as H2, NH3, NO2 and CO.

Theory of SAW Gas Sensor based on Bi-layer Conductivity Changes

The elemental theory of surface acoustic wave (SAW) gas sensor principles is reported within this work. The sensor mechanism is generally based on changes in conductivity of a bi-layer thin sensor structures. In turn, the interaction between electrical surface potential is associated with SAW propagating on piezoelectric and mobile electric charges in bi-layer sensor structures of different electrical conductivities and thicknesses. Both films are very thin in comparison to SAW wavelength and Debye screening length. This allows on one-dimensional treatment of the problem by perturbation theory.

Advances in SAW Gas Sensors Based on the Condensate-Adsorption Effect

A surface-acoustic-wave (SAW) gas sensor with a low detection limit and fast response for volatile organic compounds (VOCs) based on the condensate-adsorption effect detection is developed. In this sensor a gas chromatography (GC) column acts as the separator element and a dual-resonator oscillator acts as the detector element. Regarding the surface effective permittivity method, the response mechanism analysis, which relates the condensate-adsorption effect, is performed, leading to the sensor performance prediction prior to fabrication. New designs of SAW resonators, which act as feedback of the oscillator, are devised in order to decrease the insertion loss and to achieve single-mode control, resulting in superior frequency stability of the oscillator. Based on the new phase modulation approach, excellent short-term frequency stability (±3 Hz/s) is achieved with the SAW oscillator by using the 500 MHz dual-port resonator as feedback element. In a sensor experiment investigating formaldehyde detection, the implemented SAW gas sensor exhibits an excellent threshold detection limit as low as 0.38 pg.