Piezoelectric ceramics hydrophones

The initial efforts in PVDF hydrophone development in the 1970s were motivated by the lack of a broadband sensor to measure the pulsed pressure fields produced by diagnostic ultrasound devices. The piezoelectric ceramic hydrophones available at the time were suitable for characterizing the continuous-wave and narrow band tone-burst pressures... 

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. 

Ferroelectric—polymer composite devices have been developed for large-area transducers, active noise control, and medical imaging appHcations. North American Philips, Hewlett-Packard, and Toshiba make composite medical imaging probes for in-house use. Krautkramer Branson Co. produces the same purpose composite transducer for the open market. NTK Technical Ceramics and Mitsubishi Petrochemical market ferroelectric—polymer composite materials (108) for various device appHcations, such as a towed array hydrophone and robotic use. Whereas the composite market is growing with the invention of new devices, total unit volume and doUar amounts are small compared to the ferroelectric capacitor and ferroelectric—piezoelectric ceramic markets (see Medical imaging technology). 

Quartz and piezoelectric ceramic crystals have more temperature independent constants than PVDF, so they are used for force and acceleration transducers. However, PVDF films can be used for large area flexible transducers. Their sensitivity to stress or strain allows the construction of pressure sensors (using the J33 coefficient), and accelerometers by mounting a seismic mass on the film. PVDF electrets are particularly suited for large area hydrophones (Fig. 12.21) that detect underwater signals. Their... 

The analysis presented above was made by Hilezer and Mailecki [4,119], and it h dear from this that single-phase piezoelectric systems, both ceramics and piezopolyinets. do not fulfill all the requirements to be applied in hydrophones and ultrasonic transducers for medical diagnosis. They can be fulfilled by multiphase system composite materials consisting of piezoelectric ceramics and a polymer. Properties of the composites depend on the properties of particular phases, the volume fractim of the phases, and the means of their connectivity. 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

R. Y. Ting, "Evaluation of New Piezoelectric Composite Materials for Hydrophone AppUcations," presented at the Bernard Jaffe Memorial Colloquium, American Ceramics Society, 86 Meeting, Pittsburgh, 1984. 

For low frequency electromechanical applications in which the acoustic wavelength is much larger than the scale of component phases, some of the ceramic-polymer composites have piezoelectric voltage coefficients orders of magnitude larger than solid PZT. Such materials have obvious applications in hydrophones and other listening devices. 

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],... 

There is considerable practical interest in developing low-density, compliant, flexible piezoelectric transducers. A low-density piezoelectric would have better coupling to water and have a more easily adjusted buoyancy than the higher-density ceramics used for hydrophones. A complaint material would have a better resistance to mechanical shock than a conventional ceramic transducer, and a large compliance would also mean high damping, which is desirable in a passive device. A flexible material could also be formed to any desired profile. 

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