Hydrophones, piezoelectric applications

PbTi03, for pyroelectric detectors and hydrophones LiTa03, for pyroelectric sensors Pb(Sci/2Tai/2)03, (Bao.6oSro.4o)Ti03, for pyroelectrics Pb(Zri Ti )03, for piezoelectric applications of all types... 

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. 

A large number of apphcations have been proposed for piezoelectric polymers. The types of applications can be grouped into live major categories sonar hydrophones, ultrasonic transducers, audio-frequency transducers, pyroelectric sensors, and electromechanical devices. The principal polymers of interest in these applications are PVDF and copolymers of vinylidene fluoride and trifluoroethylene. 

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],... 

Currently, quartz is often utilized in accelerometers. Due to their high piezoelectric voltage coefficient lithium sulfate and tourmaline are often applied in commercial hydrophones especially to measure shock and pressure waves. Rochelle salt can be found in acoustic pickups and special devices to measure acoustic pressure. Due to their long-term stable piezoelectric properties natural crystals are in particular perfect for sensor applications where the monitoring of a quantity has to be made over long periods [85]. 

Target applications of piezoelectric films include switches, computer graphics, robotics tactile sensors, infrared detectors, medical sensors, musical instrument pickup, and hydrophones. 

When piezoelectrics are used in hydrostatic pressure conditions, such as hydrophone applications, the hydrostatic piezoelectric coefficient determines performance. The hydrostatic coefficients take into account the transverse piezoelectric effect from directions orthogonal to the poling axis. They are the hydrostatic strain coefficient... 

Zhang et al. [83,84] have shown theoretically that the basic elastic coupling mechanism between the two componeata b the 2>2 piezoelectric composite is similar to that of the 1-3 composite, although the 2-2 structure b not widely us There ate two major areas where 1-3 composites have been widely used underwater hydrophone applications and ultrasonic actuators and sensors for medical dUgnostk devices [15,16,24,8334]. 

Such polymers as PVDF, in particular, have wide applications [3] sometimes its properties are advantageous for some reasons, e.g., low electric permittivity and small thickness, but limit ite application in other devices. When, for example, hydrophones, which are electroacoustic transducers used in a water environment, b use of the low transducer capacity made of PVDF, amplifiers should be placed very near. Moreover, vduge sensitivity in the open system given in dB in the relation 1/piPa determined by a product giz, (where g indicates hydrostatic piezoelectric voltage coefficient and Xj is transducer thickness), is low for transducers m of PVDF films. 

Perhaps the simplest cylindrical hydrophone is a cylinder-shaped piezoelectric polymer. Such a product is available as a piezoelectric copolymer hydrophone cylinder for hydrostatic-mode operation [7], The cylinders are available with nominal outside diameters ranging from 0.64 to 2.S4 cm and lengths up to 30 cm. With nominal wall thicknesses of 0 J to U mm available, hydrostatic free-field voltage sensitivities (FFVS) of 202 to -194 dB re 1 V/pPa may obtained. This type of performance and me-riiankal characteristics are for the active material. For many hydrophone uses, the cylinder must be placed into a housing or an encapsulani for underwater application. 

The results discussed in this chapter can promote the selection of the proper SC and FC components for novel composites with high piezoelectric sensitivity, large piezoelectric anisotropy, considerable hydrostatic response, and other remarkable effective parameters. It is believed that the development of high-performance composites will be attraetive for modem piezotechnical applications (sensors, hydrophones, transdueers, etc.). 

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