Piezoelectric Wave-Propagation Transducers

Piezoelectricity derives its name from the Greek word “piezein,” to press. When a piezoelectric crystal is strained by an applied stress, an electric polarization is produced within the material which is proportional to the magnitude and sign of the strain—this is the direct piezoelectric effect. The converse effect takes place when a polarizing electric field produces an elastic strain in the same material.

Typical materials used in transducers are crystalline quartz, lithium niobate, several compositions of ferroelectric ceramics, ferroelectric polymers, and evaporated or sputtered films of cadmium sulfide and zinc oxide. The ferroelectric ceramics commonly used are PZT4, PZT5A, PZT5H, and PZT7. The elastic, dielectric, and piezoelectric properties of these materials can be found in Refs.

In addition, using these PZT materials, new and more efficient piezocomposites have been developed. Ferroelectric ceramics are not piezoelectric when they are manufactured and therefore have to be poled. Poling is done by applying a high dc voltage to the electrode faces normal to the thickness direction, while maintaining the ceramicm.

In a high-temperature environment.38 Ceramic transducers are generally rectangular, circular with parallel faces, or have a spherical curvature.

The conversion efficiency of a piezoelectric transducer is defined by the electromechanical coupling factor K.

The unit of acoustic impedance is kg/m2 • s, and it is expressed in Rayls. A mega Rayl (MRayl) is equal to 106 Rayls. Typical values of Z are  MRayl for quartz, 40.6 MRayl for brass, 1.5 MRayl for water, and 0.00043 MRayl for air.

Figure 5.18 Longitudinal and shear waves.

The piezoelectric transducers that are most commonly used generate longitudinal and shear waves which propagate with velocities VL and Vs, respectively. Figure 5.18 shows characteristics of these waves. The longitudinal mode is a compression wave, and its particle motion is in the direction of wave propagation. On the other hand, the particle motion in a shear wave is normal to the direction of wave propagation.

In some chemical, environmental, and physical transducers, surface acoustic waves (SAWs) are used. These waves travel on the surface, where mowhich they propagate is piezoelectric, then the electric field associated with the SAW can be used to dest of their energy is confined. SAWs propagate with a velocity VR that is less than VL and Vs. If the medium on tect the wave with interdigital transducers (IDTs).41

In wave-propagation transducers, maximum energy is transfered to another medium, when its impedance ZM is equal to the impedance Z0 of the transducer.

For example, the energy could be efficiently transferred from a PZT5A longitudinal mode transducer into a brass rod, whereas the transfer into water would be quite inefficient.  

To optimize that transfer, a quarter-wave matching layer is commonly used. The impedance ZML of the matching layer should be approximately and its thickness should be a quarter wavelength, evaluated using VL for that material.

Figure 5.19 shows a piezoelectric transducer with two electroded faces normal

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