Hydrophones, piezoelectric

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. 

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). 

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. 

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

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. 

There are many different kinds of acoustical probes including microphones [57-62], hydrophones, radiometers, and piezoelectric devices (most often small barium titanate transducers) [63-68], and the hot wire microphone (based on acousto-resistive effect) [63], Their resonance frequency is generally very different from that of the ultrasonic field under study. 

Although these acoustical probes can be made very small they will always slightly disturb the ultrasonic field. Just as in the case of coated thermal probes, the response signal depends on the nature and size of the probe, thus it is important that the microphones are carefully calibrated. They are however widely used, especially to calibrate medical ultrasonic equipment. Recently, very small and sensitive devices using PVDF membranes [68,69] or fiber optics [70] have been described. PVDF has piezoelectric properties and miniature membrane hydrophones (about 0.5 mm in diameter) are available. Fiber optic probes can even be smaller and a spatial resolution of 0.1 mm has been claimed [70],... 

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. 

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. 

FIGURE 5.35 Cross-sections of cylindrical piezoelectric polymer receivers, (a) Membrane microphone formed by fixing a taut film over an air-filled cylindrical chamber, (b) Membrane hydrophone with an oil-filled chamber containing plastic compliant tubes to provide compliance while withstanding hydrostatic pressure, (c) Flexural disk hydrophone backed by air. The disk both excites the polymer film and provides strength against hydrostatic pressure. 

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... 

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... 

Aim of this paragraph is a short description of the very wide material gronp -piezoelectric composites - with the special attention to the hydrophone apphcations. Non-homogeneous stmctures are often called piezoelectric composites if at least one of the phases is piezoelectric. Two or even more phases joined together are involved in the composite structure. Quality of the mutual joints between phases involved is the most important material issue today (chemistry, physics, material research etc.). 

Smith WA (1990) Calculating the hydrophone response of piezoceramic-rod/piezopolymer-matrix composites. Proceedings of IEEE ultrasonics symposium, pp 757-761 Smith WA, Auld BA (1991) Modelling 1 -3 Composite piezoelectrics Thickness-mode oscillations. IEEE Trans UFFC 38 40-47... 

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]. 

The cavitation pressure has been calibrated by three methods. The flrst uses a commercial piezoelectric needle hydrophone placed at the focus. This method has a limited accuracy (13% uncertainty in the hydrophone gain), and moreover, as the needle is fragile, it can be used only at low amplitude, so that an extrapolation to... 

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... 

Figure 19.2 In this example of a spot poled membrane hydrophone, unpoled PVDF film 25 pm in thickness is mounted on a metal annular frame. Electrical leads on the upper and lower film surfaces adjoin overlapping 0.3 mm diameter electrodes. An electrical poling field applied to the leads renders the region of the film between the electrodes piezoelectrically active. An encapsulated preamplifier is located within the frame between the two coaxial connectors, one for preamp power in and one for signal out. The labeled scale dimensions are in centimeters.

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