A surface acoustic wave (SAW) sensor with Pt coated ZnO nanorods as the selective layer has been investigated for hydrogen detection. The SAW sensor was fabricated based on a 128 degrees YX-LiNbO(3) substrate with a operating frequency of 145 MHz. A dual delay line configuration was adopted to eliminate external environmental fluctuations. The Pt coated ZnO nanorods were chosen as a selective layer due to their high surface-to-volume ratio, large penetration depth, and fast charge diffusion rate. The ZnO nanorods were synthesized by an aqueous solution method and coated with the noble metal Pt as a catalyst. Finally, the SAW sensor responses to humidity and hydrogen were tested. Results show that the sensor is not sensitive to humidity; moreover, the frequency shift for a hydrogen concentration variation of 6000 ppm is 26 kHz while operating at room temperature. It can be concluded that the Pt coated ZnO nanorod based SAW hydrogen sensor exhibits fast response, good sensitivity and short-term repeatability. It is worth noting that not only is the sensor sensitive enough to operate at room temperature, but also it can avoid the influence of humidity.
A high-precision ultraviolet (UV) detector combining ZnO nanostructure and a dual delay line surface acoustic wave (SAW) oscillator system is presented. The UV detector is made of ZnO nanorods on a 128 degrees YX-LiNbO(3)-based two-port SAW oscillator. The ZnO nanorod synthesized by chemical solution method is used as a UV sensing material. The center frequency of the SAW device is at 145 MHz. A dual delay line SAW oscillator system was constructed to eliminate external environmental fluctuations. Under illumination of a UV source consisting of an Xe lamp and a monochromator, frequency shifts of the UV detector were measured. A maximum frequency shift of over 40 kHz was observed under 365 nm illumination for several on-off cycles, indicating the ZnO nanorod-based detector was sensitive to UV light and with good repeatability. Moreover, frequency shifts reached a value of 19 kHz after 365 nm was turned on for 10 s, which implies a real-time high-sensitivity UV sensor was successfully fabricated. Results show a ZnO nanostructure-based SAW oscillator system is a promising candidate for a real-time, fast-response, high-precision UV detector.
In this paper, a highly sensitive humidity sensor is reported. The humidity sensor is configured by a 128 • YX-LiNbO 3 based surface acoustic wave (SAW) resonator whose operating frequency is at 145 MHz. A dual delay line configuration is realized to eliminate external temperature fluctuations. Moreover, for nanostructured materials possessing high surface-to-volume ratio, large penetration depth and fast charge diffusion rate, camphor sulfonic acid doped polyaniline (PANI) nanofibres are synthesized by the interfacial polymerization method and further deposited on the SAW resonator as selective coating to enhance sensitivity. The humidity sensor is used to measure various relative humidities in the range 5-90% at room temperature. Results show that the PANI nanofibre based SAW humidity sensor exhibits excellent sensitivity and short-term repeatability.
In this paper, a design that combines two-port surface acoustic wave (SAW) devices and phononic crystals (PCs) acting as reflected gratings is demonstrated. Finite-difference time-domain method is used to analyze SAWs encountering the PC and optimize the design. A layered ZnO/Si SAW device and a square lattice PC composing of cylindrical holes on silicon were fabricated. With the PC of 15-layer cylinders, experimental insertion loss shows a 7 dB improvement at 212 MHz at central frequency. In addition, the size of gratings is reduced significantly as compared to the traditional gratings with hundreds of metal strips.
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