Bulk acoustic wave transducers

Bulk and surface imprinting strategies are straightforward tools to generate artificial antibodies. Combined with transducers such as QCM (quartz crystal microbalance), SAW (surface acoustic wave resonator), IDC (interdigital capacitor) or SPR (surface plasmon resonator) they yield powerful chemical sensors for a very broad range of analytes. 

Laser ultrasonic transducers are truly non-contact devices which effectively avoid acoustic coupling problems (e.g. damping in the transducer and couplant reflection and transmission losses at the interface). Most laser ultrasonic devices have been used for excitation and detection of bulk elastic waves in point source or planar geometry, but also surface acoustic (Rayleigh or Brillouin) waves. Unlike the bulk wave regime, only one sample side is needed for excitation and detection when surface waves are used. This not only renders the measurements easier, but also avoids the need for an accurate knowledge and uniformity of the sample thickness. In addition, the excitation laser can be focused using cylindrical lenses in order to obtain an excitation line. 

Piezoelectric phenomena are related to the reversible electric polarization generated by mechanical strain in crystals that do not display a centre of S5mimetry. The signal produced by acoustic wave devices is generated by bulk or surface acoustic waves launched by metal transducers at ultrasonic fi-equencies. Such waves are propagated through piezoelectric materials. 

On the transducer side, there have been more recent developments like the Lamb oscillator or the so-called film bulk acoustic resonators (FEAR). Lamb wave devices are related to SAW in terms of using interdigitated structures for transduction. The difference, however, is that in a Lamb wave resonator not only the surface, but the entire bulk of the device oscillates. This makes it much less sensitive against viscous damping. FEAR on the other hand consist of a metal/aluminium nitride/metal sandwich, where bulk waves (thickness oscillations) are induced in the AIN material. Eoth these devices have in com-... 

In these relatively new sensors, an acoustic wave is propagated by an externally applied alternative current between two electrodes or interdigited electrodes deposited on a piezoelectric substrate such as quartz. There are several subclasses of piezoelectric transducers (bulk wave, surface acoustic wave, or... 

Elastic Coefficients The elastic stiffness coefficients Cy can be calculated from the measured velocity of propagation of bulk acoustic ultrasonic waves, according to the Papadakis method (quartz transducer with center frequency of 20MHz) (Papadakis, 1967), on differently oriented bar-shaped samples using the equations given by Truell et al. (1969) and corrected for the piezoelectric contributions (Ljamov, 1983 Ikeda, 1990). The samples were oriented in axial directions XYZ, and 45° rotated against the X- and Y-axes, respectively. In order to obtain optimized values for the elastic materials parameters, the elastic stiffness coefficients Cy were used to calculate and critically compare the results of surface acoustic wave (SAW) measurements. 

The use of air-bome ultrasound for the excitation and reception of surface or bulk waves introduces a number of problems. The acoustic impedance mismatch which exists at the transducer/air and the air/sample interfaces is the dominant factor to be overcome in this system. Typical values for these three media are about 35 MRayls for a piezo-ceramic (PZT) element and 45 MRayls for steel, compared with just 0.0004 MRayls for air. The transmission coefficient T for energy from a medium 1 into a medium 2 is given by... 

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