Acoustic Plate Mode APM Devices

The particle displacement associated with the order SH plate mode (propagating in the z-direction) has only an jc-component, given by [54] 

Modeling the quartz plate as an isotropic medium, the /V order SH plate mode will be generated most efficiently by a transducer of period d at a frequency approximated by [54] 

Equation 3.63 gives an approximation for the SH plate-mode spectrum found in an unperturbed quartz plate. The presence of surface features, including transducers, perturbs the wave velocity, and hence the excitation frequency, of each mode. 

In order to avoid problems of mode interference, it is desirable when using APM devices as sensors to excite only a single mode. This can be accomplished by using a transducer whose bandwidth is less than the frequency separation between modes. 

A number of interactions can affect plate-mode propagation characteristics, particularly in a liquid environment. In the following sections, models of several of the important modes of interaction will be outlined and compared to experimental results. These include (1) mass accumulation on the device surface, (2) viscous entrainment of the contacting liquid medium by the oscillating device surface, and (3) acoustoelectric coupling between evanescent plate mode electric fields and the liquid. 

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Devices based on piezoelectric crystals, which allow transduction between electrical and acoustic energies, have been constructed in a number of conrigurations for sensor applications and materials characterization. This cluqtter examines those devices most commonly utilized for sensing a( licatithickness-shear mode (TSM) resonator, the surface acoustic wave (SAW) device, the acoustic plate mode (APM) device, and the flexural plate wave (FPW) device. Each of these devices, shown schematically in Figure 3.1, uses a unique acoustic mode. 

Figure 3.33 Schematic of an acoustic plate mode (APM) device showing the shear horizontal (SH) displacement of the mode as it propagates between input and output transducers. (Reprinted with permission. See Ref. (54). 1989 Elsevier Publishers.)...

An example of a two-port device is the surface acoustic-wave (SAW) delay line shown in Figure 6.3. Acoustic plate mode (APM) devices utilize a two-port configuration that is conceptually identical to that of the SAW for the flexural plate wave (FPW), there is typically a third connection to its ground plane (see Section 6.2.3). In principle, the ground plane connection is unnecessary, but in practice more stable operation results when this connection is made. Notice that there... 

The response of piezoelectric devices propagating shear horizontal acoustic plate modes (SH-APMs) has been modeled and experimentally characterized for variations in surface mass, liquid rheological properties, and solution dielectric coefficient and electrical conductivity. The nature of the SH-APM and its propagation characteristics are outlined and used to describe a range of Interactions at the solid/liquid interface. Sensitivity to sub-monolayer mass changes is demonstrated and a Cu sensor is described. The APM device is compared to the surface acoustic wave device and the quartz crystal microbalance for liquid sensing applications. 

Chemical sensors based on acoustic wave (AW) devices have been studied for a number of sensing applications, the majority of which fall in the category of gas and vapor detection (1-8). Recently, the use of these sensors in liquid environments has been explored (9-13). AW sensors utilize various types of acoustic waves, including the surface acoustic wave (SAW), the shear-horizontal acoustic plate mode (SH-APM) (10-13), and the Lamb wave (also a plate mode) (3.14). Even though most studies of these piezoelectric sensors have centered on SAW devices (1.2.4-8), differences in the propagation characteristics of the various acoustic modes make some better suited than others for a given sensing application. 

BAW Device Shear Horizontal Acoustic Plate Mode (SH-APM) Sensor... 

If the wave propagates through the substrate, the wave is called a bulk wave. The most commonly used BAW devices are the thickness shear mode (TSM) resonator and the shear-horizontal acoustic plate mode (SH-APM) sensor. The TSM, also widely referred to as a quartz crystal microbalance (QCM), is the best-known and simplest... 

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