Piezoelectric ceramics acoustic impedance

CH2—CI2—) —(—CF2— CFH—) (39). Ceramic crystals have a higher piezoelectric efficiency. Their high acoustic impedance compared to body tissues necessitates impedance matching layers between the piezoelectric and the tissue. These layers are similar in function to the antireflective coatings on a lens. Polymer piezoelectric materials possess a more favorable impedance relative to body tissues but have poorer performance characteristics. Newer transducer materials are piezoelectric composites containing ceramic crystals embedded in a polymer matrix (see Composite materials, polymer-MATRIX Piezoelectrics). 

With its low acoustic impedance, extreme bad width, high piezoelectric coefficient, and low density (only one-quarter the density of ceramic materials), PVDF is ideally suited as a transducer for hroad hand rmdenvater receivers in hghtweight hydrophones. The softness and flexibiHty of PVDF give it a comphance 30 times greater than ceramic. PVDF can thus he utilized in a hydrophone structure using various device configurations, such as compliant tubes, rolled cylinders, discs, and planar stacks of laminated material. 

Shown in Fig. 3.16 is a 1-3 piezoelectric composite with PZT ceramic rods embedded in a polymer resin. This structure is now widely used in medical ultrasonic transducers because the polymer helps reducing the acoustic impedance mismatch between human body and the PZT so that energy transmission becomes more efHcient. The load on the polymer phase can be transferred to the ceramic so that the effective load on the ceramic is enhanced, which produces higher electric signal when it is used as stress sensor. This composite structure also gives a much higher figure of merit for hydrophone applications [18],... 

The most interesting properties of polymers are their high mechanical and electrical strength and low electrical conductivity and acoustic impedance, whereas the ferroelectric ceramics exhibit good dielectric, pyroelectric, and piezoelectric properties (4,8,67]. 

Theoretical study of the propagation of the elastic wave in 1-3 composites [8334.147-149] enable one to indicate how electromechanical properties oi piezoelectric composites depend on the properties of the component phases and the volume fraction of tte piezoelectric ceramics. The dependence between electromechanical coupling foctor k) and the acoustic impedance (Z) can be determined theoretically for composites of d erent ceramic contents and to find a compromise between increasing k and Z while increasing ceramic contents. High values of coefficient k and small acoustic impedance Z are required in applications for ultrasonic transducers. 

The electrical and mechanical properties trf piezoelectric polymers make them a possible alternative to ferroelectric ceramics such as lead zirconate titanate. For several reasons, they are attractive for transducer design. The mechanical flexibility and conformability of thin-film PVDF means that it can be configured into a wide range of transducer products. The low acoustic impedance of PVDF is companrf>le to body tissues, which makes it useful for acoustic imaging applications. Short impulse response and high axial resolution in acoustic imaging systems arc possible with PVDF-featured devices because of the robustness and broadband characteristics of the polymer. 

A 9< un film piezoelectric polymer was used to a transducer, as shown to Figure 16, which resulted to a peak to the transmitttog voltage response of 52-62 dB re 1 Pa/V at a distance of SO cm over a frequency range of 1 to 40 MHz [17. This compares to a similar piezoelectric ceramic hydrophone backed with a material of an acoustic impedance of 18 MRayl with a pe response of 62 dB re 1 Pa/V but over a miK more limited frequency range. The directivity patterns diqrlayed to Figure 17 show a 20-dB decrease for 1 off the acoustic axis at IS MHz. 

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