Sensors surface acoustic wave oscillator

Chemical sensors have been reported that are based on quartz micro balances or surface acoustic wave oscillators coated with the trimethylsilyl ethers of and 6 " and that are claimed to detect various solvent vapors in ppm amounts. ... 

Penza, M., Antolini,F. and Vittori-Antisari, M. (2005), Carbon nanotubes-based surface acoustic waves oscillating sensor for vapour detection Thin Solid Films, 472,246-52. 

Fig. 21. A surface acoustic wave dual-delay line oscillator. The sensitise layer is placed in the propagation path of one of the two SAW devices. The differenee in Ireqnency (At) between the two channels provides a dtrecl result of the mass loading and electric field effects associated w ith the sensor layer...

The addition of mass provides the means of transduction for many chemical sensors, including surface acoustic wave (SAW) devices, quartz crystal microbalances (QCM), and microcantilevers. In all these devices, the mass addition either perturbs the vibration, oscillations, or deflection within the transducer. The mode of transduction in an optical interferometer can also be linked to mass addition the sensor s response is altered by refractive index changes in the material being monitored. It is possible that this change can be elicited solely from refractive index changes without the addition of mass, although in sensing a particular... 

The most commonly known oscillator sensors are bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices. The BAW devices operate according to the Sauerbrey principle that very thin films on AT-cut crystals can be treated as equivalent mass changes of the crystal. The SAW devices can operate either on the Rayleigh wave propagation principle at solid thin-film boundaries [3] or as bulk wave devices [4]. 

Microcantilever sensors offer many orders of magnitude better sensitivity compared to other sensors such as quartz crystal microbalances (QCM), flexural plate wave oscillators (FPW), and surface acoustic wave devices (SAW). There are several distinct advantages of the microcantilever sensors compared to the above mentioned and other MEMS sensors ... 

Figure 2. Surface acoustic wave gas sensor consisting of dual delay line oscillator[4].

Perturbations of the medium adjacent to the device surface result in variations in the phase, amplitude, and velocity of the surface acoustic wave. Specifically, these properties will be affected by changes in the density, viscosity, or elastic properties of the medium in contact with the surface. Since the acoustic wave has an electric potential wave associated with it as well, the SAW can also be used to probe the dielectric and conductive properties of this surface medium. By far, the largest number of chemical sensor applications of SAW devices take advantage of the mass sensitivity of SAW oscillators. 

H.C. Hao, et al., Development of a portable electronic nose based on chemical surface acoustic wave array with multiplexed oscillator and readout electronics. Sensors and Actuators B Chemical 146(2) 545-553 (2010)... 

For both types of SAW oscillators, the resonant frequency is dependent upon the phase velocity of the surface acoustic wave. In addition, the phase shift across an SAW delay is also dependent upon phase velocity. Any surface perturbation which causes an alteration of the SAW velocity will be detectable either as a frequency or a phase delay shift. The deposition of a foreign mass on an SAW device surface will cause such a velocity change and is the basis for the development of chemical sensors from SAW devices. 

The surface acoustic wave device (SAW) is an example of a transducer that is batch fabricated using IC technology and provides improved performance. SAW devices operate at much higher frequencies than the quartz crystal oscillator (or microbalance, QCM), and this results in improved detection limitsl, 29, 30 This can make measurements of absorption into films coating the SAW device possible, under circumstances where the QCM is insufficiently sensitive. On the other hand the ( M can be used in aqueous systems, while the SAW device is essentially restricted to gas phase measurements. Here too, IC techniques have provided means to fabricate thin membranes that can be made to oscillate at frequencies similar to the SAW device, but in a mode that is not over-damped in aqueous solutions. Nevertheless, regardless of the specific oscillator involved, it is the coating films and interfaces that provide the chemical specificity required of the sensor. 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Slip is not always a purely dissipative process, and some energy can be stored at the solid-liquid interface. In the case that storage and dissipation at the interface are independent processes, a two-parameter slip model can be used. This can occur for a surface oscillating in the shear direction. Such a situation involves bulk-mode acoustic wave devices operating in liquid, which is where our interest in hydrodynamic couphng effects stems from. This type of sensor, an example of which is the transverse-shear mode acoustic wave device, the oft-quoted quartz crystal microbalance (QCM), measures changes in acoustic properties, such as resonant frequency and dissipation, in response to perturbations at the surface-liquid interface of the device. 

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