Shitang He

Advances in SXFA-Coated SAW Chemical Sensors for Organophosphorous Compound Detection

A polymer-coated surface acoustic wave (SAW)-based chemical sensor for organophosphorous compound sensing at extremely low concentrations was developed, in which a dual-delay-line oscillator coated with fluoroalcoholpolysiloxane (SXFA) acted as the sensor element. Response mechanism analysis was performed on the SXFA-coated chemical sensor, resulting in the optimal design parameters. The shear modulus of the SXFA, which is the key parameter for theoretical simulation, was extracted experimentally. New designs were done on the SAW devices to decrease the insertion loss. Referring to the new phase modulation approach, superior short-term frequency stability (±2 Hz in seconds) was achieved from the SAW oscillator using the fabricated 300 MHz delay line as the feedback element. In the sensor experiment on dimethylmethylphosphonate (DMMP) detection, the fabricated SXFA-coated chemical sensor exhibited an excellent threshold detection limit up to 0.004 mg/m3 (0.7 ppb) and good sensitivity (∼485 Hz/mg/m3 for a DMMP concentration of 2∼14 mg/m3).

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.

High frequency stability oscillator for surface acoustic wave-based gas sensor

This paper presents a 158 MHz surface acoustic wave (SAW) oscillator used for a gas sensor. As the oscillator element, a SAW delay line on ST-X quartz substrate with low insertion loss (<8 dB) and single mode selection capability was developed. Low insertion loss was achieved by an electrode width control single phase unidirectional transducer (EWC/SPUDT) configuration. Single mode selection was simply accomplished by a comb transducer which is a means of combining the frequency selectivity of two interdigital transducers (IDTs). Coupling of modes (COM) simulation was performed to predict device performance prior to fabrication. The measured frequency response S12 showed a good agreement with simulated results. The effect of the oscillator circuit system temperature shift upon frequency stability was observed in detail. The experimental results showed that the baseline noise 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 self-assembled composite monolayer as a sensor material for dimethyl-methyl-phosphonate (DMMP). The sensitivity for low DMMP concentration detection was evaluated as ~25 Hz mg−1 m−3, and the threshold detection limit was up to 0.5 mg m−3.

Theoretical analysis on Love waves in a layered structure with a piezoelectric substrate and multiple elastic layers

A method is developed to analyze the existence and behavior of piezoelectric Love waves in a multilayered structure consisting of a piezoelectric substrate and multiple elastic layers which are isotropic, nonpiezoelectric materials. The acoustic waves and electric fields in the substrate and the layers are investigated. A general dispersion equation is derived to describe the existence of Love surface waves with respect to phase velocity as a function of normalized layer thickness. An iteration formula for XN is introduced to describe the mechanical action between the layers and the substrate at the interface. Another formula for e¯LN, the equivalent permittivity of the wave-guide layers, is produced to describe the electric fields in the layers. The dispersion equation including a mass loading on the surface of the top layer is deduced, and a formula for calculating the mass sensitivity of the phase velocity is presented. We also find the dispersion equation with an electric shorted interface and introdu...

Properties of Love waves in a piezoelectric layered structure with a viscoelastic guiding layer

A theoretical method is developed for analyzing Love waves in a structure with a viscoelastic guiding layer bounded on a piezoelectric substrate. The dispersion equation previously derived for piezoelectric Love waves propagating in the layered structure with an elastic layer is adopted for analyzing a structure with a viscoelastic layer. A Maxwell–Weichert model is introduced to describe the shear stiffness of a polymeric material. Newton’s method is employed for the numerical calculation. The dispersion equation for piezoelectric–elastic Love waves is proved suitable for solving a structure with a viscoelastic layer on a piezoelectric substrate. The theoretical results indicate that the propagation velocity of the Love wave is mainly decided by the shear stiffness of the guiding layer, whereas the propagation loss is approximately proportional to its viscosity. A detailed experimental study was conducted on a Love wave delay line fabricated on an ST-90° X quartz substrate and overlaid with various thicknesses of SU-8 guiding layers. A tail-raising caused by the viscosity of the guiding layer existed in both the calculated and the measured propagation velocities. The calculated insertion loss of the Love wave delay lines was in good agreement with the measured results. The method and the results presented in this paper are beneficial to the design of Love wave sensors with a viscoelastic guiding layer.

A New Micro-rate Sensor Based on Shear Horizontal Surface Acoustic Wave Gyroscopic Effect

In this paper, we present a new micro-rate sensor based on the shear horizontal surface acoustic wave (SH-SAW) gyroscopic effect. The new SH-SAW propagating along the ST-90°X quartz substrate with heavy metallization exhibits excellent temperature stability, large acoustic velocity, high electromechanical coupling factors, and very small propagation attenuation. The response mechanism of such an SH-SAW micro-rate sensor was established using partial-wave analysis methods. The angular detection sensitivity in the propagation path of SH-SAW was evaluated and the effect of the metal interdigital transducer (IDT) electrode thickness on the sensor performance was also studied, resulting in the realization of the optimized design parameters prior to fabrication. Two SH-SAW delay lines with a reverse direction and an operation of 80 MHz on the same chip are fabricated as the feedback of SAW oscillators. The single-phase unidirectional transducer (SPUDT) was used to structure the delay lines to decrease the insertion loss. The Coriolis force from the external rotation acts on the particles along the SAW propagation path, then a pseudo SAW was induced, and couples with the initial SH-SAW; thus, the SAW velocity was deviated. Meanwhile, the differential oscillation frequency was changed linearly and used to characterize the input angular rate. Then, using the precise rate table, the performance of the fabricated SH-SAW rate sensor was evaluated experimentally. A sensitivity of 1.268 Hz deg-1 s-1 at angular rates of up to 2000 deg s-1, good linearity, and excellent temperature stability are observed.

Development of a Room Temperature SAW Methane Gas Sensor Incorporating a Supramolecular Cryptophane A Coating.

A new room temperature supra-molecular cryptophane A (CrypA)-coated surface acoustic wave (SAW) sensor for sensing methane gas is presented. The sensor is composed of differential resonator-oscillators, a supra-molecular CrypA coated along the acoustic propagation path, and a frequency signal acquisition module (FSAM). A two-port SAW resonator configuration with low insertion loss, single resonation mode, and high quality factor was designed on a temperature-compensated ST-X quartz substrate, and as the feedback of the differntial oscillators. Prior to development, the coupling of modes (COM) simulation was conducted to predict the device performance. The supramolecular CrypA was synthesized from vanillyl alcohol using a double trimerisation method and deposited onto the SAW propagation path of the sensing resonators via different film deposition methods. Experiential results indicate the CrypA-coated sensor made using a dropping method exhibits higher sensor response compared to the unit prepared by the spinning approach because of the obviously larger surface roughness. Fast response and excellent repeatability were observed in gas sensing experiments, and the estimated detection limit and measured sensitivity are ~0.05% and ~204 Hz/%, respectively.

P6G-3 Switchable SAW Filter Bank with Both Narrow & Wide Channel Bandwidth and 10 Channels SAW Filter Bank

The design and testing of 8 channels and 10 channels low loss switchable SAW filter bank is described. There are both narrow & wide channel bandwidths in 8 channels filter bank. The 1 dB bandwidth of narrow channel is about 0.3% and that of wide channel more than 3.2 MHz. Cascade-connected two stages are constructed to achieve higher stop-band rejection. Every narrow bandwidth channel is constructed with two same SPUDT SAW filter on ST-X quartz with insertion loss less than 6 dB and stop-band rejection more than 40 dB. Each wide bandwidth channel includes of a fan-type SPUDT filter on X-112degY LiTaO3 with insertion loss less than 10 dB and a longitudinally-coupled DMS filter on 36degY-X LiTaO3 less than 2 dB. The experimental 8 channels SAW filter bank results in insertion loss less than 12.5 dB and stop-band rejection more than 70 dB. In 10 channels filter bank, the individual filter consists of fan-type SPUDT on X-112degY LiTaO3 with insertion loss of less than 12 dB, stop-band rejection more than 40 dB, magnitude ripples in pass-band less than 0.5 dB and nonlinear phase less than 2deg per 1 MHz. The experimental 10 channels filter bank results in insertion loss of 14.6 dB, deviation of insertion loss among different channels less than 0.5 dB, and stop-band rejection more than 40 dB.

Optimal Design of a Polyaniline-Coated Surface Acoustic Wave Based Humidity Sensor

This paper presents an optimal design for a new humidity sensor composed of a dual-resonator oscillator configuration with an operation frequency of 300 MHz, and a polyaniline (PANI) coating deposited along the resonation cavity of the sensing device. To improve the corrosion resistance of the sensor chip, Al/Au electrodes were used to form the SAW resonator. Prior to device fabrication, the coupling of modes (COM) model was used for the performance prediction and optimal design parameters determination. Two SAW resonators with Al/Au electrodes were fabricated on an ST-X quartz substrate, and used as the frequency control element in the feedback path of an oscillator circuit. A PANI thin coating was deposited onto the resonator cavity of the sensing device by a spinning approach as the sensor material for relative humidity (RH) detection. High detection sensitivity, quick response, good repeatability and stability were observed from the sensor experiments at room temperature.

The Development of Love Wave-Based Humidity Sensors Incorporating Multiple Layers

A Love wave humidity sensor is developed by using a multilayer structure consisting of PVA/SiO2 layers on an ST-90°X quartz substrate. The theoretical result shows that the sensor with such a two-layer structure can achieve a higher sensitivity and a smaller loss than the structures with a single polymer layer. Comparative experiments are performed for the sensor incorporating PVA/SiO2 layers and the sensor incorporating a PVA layer. The experimental results agree well with the theoretical predication.