This work presents the single-chip integration of a multi-frequency surface acoustic wave resonator (SAWR) based humidity sensor. Graphene oxide (GO), a humidity-sensing material, is integrated onto a confined sensing area of SAWR via electrospray deposition (ESD). The ESD method allows ng-resolution deposition of GO, optimizing the amount of sensing material. The proposed sensor consists of SWARs at three different resonant frequencies (180, 200 and 250 MHz) with a shared common sensing region, thus allowing direct analysis of sensor performances at different operating frequencies. Our findings reveal that the resonant frequency of the sensor impacts both measurement sensitivity and stability. A higher operating frequency ensures better sensitivity but suffers from a larger damping effect from absorbed water molecules. The maximum measurement sensitivity of 17.4 ppm/RH% is achieved with low drift. In addition, the developed sensor exhibits improved stability and sensitivity by as much as 150% and 75% in frequency shift and Quality factor (Q), respectively, by carefully selecting the operating frequencies at a given RH% range. Finally, the sensors are used for various hygienic applications, such as non-contact proximity detection and face mask inspection.
This study presents the effects of bottom electrode designs on the operation of laterally vibrating aluminum nitride (AlN) contour-mode resonators (CMRs). A total of 160 CMRs were analyzed with varying bottom electrode areas at two resonant frequencies (f0) of about 230 MHz and 1.1 GHz. Specifically, we analyzed the impact of bottom electrode coverage rates on the resonator quality factor (Q) and electromechanical coupling (k2), which are important parameters for Radio Frequency (RF) and sensing applications. From our experiments, Q exhibited different trends to electrode coverage rates depending on the device resonant frequencies, while k2 increased with the coverage rate regardless of f0. Along with experimental measurements, our finite element analysis (FEA) revealed that the bottom electrode coverage rate determines the active (or vibrating) region of the resonator and, thus, directly impacts Q. Additionally, to alleviate thermoelastic damping (TED) and focus on mechanical damping effects, we analyzed the device performance at 10 K. Our findings indicated that a careful design of bottom electrodes could further improve both Q and k2 of AlN CMRs, which ultimately determines the power budget and noise level of the resonator in integrated oscillators and sensor systems.
Accurate monitoring is needed for pulsating flow in many healthcare and bio applications. Specifically, real-time monitoring of pulsating blood flow provides rich information regarding a patient’s health conditions. This paper proposes a flexible strain sensor capable of detecting the pulsating fluid flow by directly measuring the circumferential strain induced by a rapid change in the flow rate. The thin and flexible strain sensor consists of a polydimethylsiloxane (PDMS) with a Triton-X treatment to enhance the adhesive property and multi-walled carbon nanotubes (MWCNT) as the piezoresistive sensing layer. MWCNT integration implements a simple spray-coating method. The adhesive PDMS/CNT strain sensor exhibits a highly adhesive nature and can be attached to a silicone tube’s curved surface. By analyzing the theoretical modeling based on fluid energy equation and solid mechanics, strains induced on the soft tube by the change in flow rate, viscosity, and fluid density can be predicted. We performed the flow rate measurement at varying fluid-flow rates and liquid viscosities, and the results match our prediction. The sensitivity and limit of detection of the presented strain sensor are about 0.55 %min/L and 0.4 L/min, respectively. Both the calculation and experiment confirm that the sensor resistance is most sensitive to the fluid-flow rate, thus, enabling the accurate tracking of pulsating fluids’ flow rate, regardless of the viscosity or density.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.