Piezoelectric ultrasonic transducer

Figure 7. Schematic representation of a piezoelectric ultrasonic transducer.

The piezoelectric ultrasonic transducer is more common. When a voltage is applied to a piezoelectric material, it will compress or expand. If the voltage is alternating at ultrasonic frequency, the piezoelectric will compress and expand at the same frequency. The mechanical vibrations can be transferred to a diaphragm to produce ultrasonic sound waves. 

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. 

Another important class of titanates that can be produced by hydrothermal synthesis processes are those in the lead zirconate—lead titanate (PZT) family. These piezoelectric materials are widely used in manufacture of ultrasonic transducers, sensors, and minia ture actuators. The electrical properties of these materials are derived from the formation of a homogeneous soHd solution of the oxide end members. The process consists of preparing a coprecipitated titanium—zirconium hydroxide gel. The gel reacts with lead oxide in water to form crystalline PZT particles having an average size of about 1 ]lni (Eig. 3b). A process has been developed at BatteUe (Columbus, Ohio) to the pilot-scale level (5-kg/h). 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

Bowen, L.J., et al. (1993) Injection moulded fine-scale piezoelectric composite transducer. 1993 IEEE Ultrasonics Symposium, pp. 499-503. 

Park, S.-E. and Shrout, T.R. (1997) Characteristics of relaxor-based piezoelectric single crystals for ultrasonic transducers, IEEE Trans. Ultrasound, Ferroelectrics and Frequency Control, 44, 1140-7. 

Mixing can be achieved by means of acoustic stirring created by ultrasonic waves [42, 44, 176-181]. Ultrasounds are introduced into the channel by integrated piezoelectric ceramic transducers [42, 44, 176, 177]. The ultrasonic action causes an acoustic stirring of the fluid perpendicular to the flow direction and leads to an enhancement of the mixing inside the microfluidic channel [42] or chamber [44, 180]. A turbulent-like mixing was achieved at Re < 1. 

Chemical and physical processing techniques for ferroelectric thin films have undergone explosive advancement in the past few years (see Ref. 1, for example). The use of PZT (PbZri- cTi c03) family ferroelectrics in the nonvolatile and dynamic random access memory applications present potentially large markets [2]. Other thin-film devices based on a wide variety of ferroelectrics have also been explored. These include multilayer thin-film capacitors [3], piezoelectric or electroacoustic transducer and piezoelectric actuators [4-6], piezoelectric ultrasonic micromotors [7], high-frequency surface acoustic devices [8,9], pyroelectric intrared (IR) detectors [10-12], ferroelectric/photoconduc-tive displays [13], electrooptic waveguide devices or optical modulators [14], and ferroelectric gate and metal/insulator/semiconductor transistor (MIST) devices [15,16]. 

The ultrasonic transducer was a PZ26 piezoelectric ceramic disc (Ferroperm Piezoceramics A/S, Kvistgard, Denmark). The tranducer should be designed to operate in the region of the resonance frequency of the resonator channel. 

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. 

Berlincourt D. Ultrasonic transducer materials piezoelectric crystals and ceramics. In MattiatOE, editor. London Plenum 1971. 

The contact method can be used to obtain the complete set of stiffness constants for samples with much smaller dimensions. In this method [3,5], a piezoelectric ceramic transducer, bonded to one surface of the sample, generated a beam of pulsed 10 MHz elastic waves that was subsequently received by another transducer bonded to the opposite surface. The wave velocity was calculated from the transit time of the ultrasonic pulse measured on a gated time interval counter. Longitudinal and transverse waves were generated using two different types of transducers. 

An ultrasonic transducer is an integrated component of acoustic pumps. The transducers that generate ultrasonic energy with megahertz frequency for ultrasonic pumps make use of the piezoelectric effect. A piezoelectric layer is the vital component of the ultrasonic transducer and provides the oscillatory motion that ultimately produces the surface acoustic waves (a good review on IDT and SAW can be found elsewhere [3]). 

The acoustic matching layer, next to the piezoelectric element, is called the first matching layer, and the other layer is called the second matching layer. Figure 33.9 shows the results of the computer simulations of the two-acoustic matching layer type of ultrasonic transducer. 

Relaxor-type electrostrictive materials, such as those from the lead magnesium niobate-lead titanate, Pb(Mgp 3Nb2/3)03-PbTi03 (or PMN-PT), solid solution are highly suitable for actuator applications. This relaxor ferroelectric also exhibits an induced piezoelectric effect. That is, the electromechanical coupling factor kt varies with the applied DC bias field. As the DC bias field increases, the coupling increases and saturates. Since this behavior is reproducible, these materials can be applied as ultrasonic transducers which are tunable by the bias field [12]. 

---