Piezoelectric ceramics ultrasonic transducers

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. 

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. 

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. 

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. 

Li Z, Grimsditch M, Xu X, Chan SK (1993) The elastic, piezoelectric and dielectric constants of tetragonal PbTiOs single crystals. Ferroelectrics 141 313-325 MarraSP, Ramesh KT, Douglas AS (1999) The Mechanical properties of lead-titanate/polymer 0-3 composites. Compos Sci Technol 59 2163-2173 Materials Data Sheets of APC International, Tokin, Ferroperm, Morgan Matroc, Siemens Mattiat OE (1971) Ultrasonic transducer materials. Plenum Press, Tokyo McLachlan DS, Blaszkiewicz M, Newnham RE (1990) Electrical resistivity of composites. J Am Ceram Soc 73 2187-2203... 

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],... 

H.L.W. Chan, J. Unsworth, Simple model for piezoelectric Ceramic/Polymer 1-3 composites used in ultrasonic transducer appHcations. IEEE Trans. Ultra-son., Ferroelectr. Preq. Control 36(4), 434 441 (1989)... 

Ultrasonic transducers for medical imaging utilize piezoelectric ceramics and composites to generate an ultrasonic beam that can penetrate soft tissue. The same transducer then picks up the reflections off internal structure, organs, fetus, etc. These transducers most often consist of square-cross-section piezoelectric rods in a polymer matrix. These are formed by a dice-and-fill process. Figure 3.47 shows various piezoelectric-polymer composites used to optimize the performance for various applications. The diced 1-3 composite is typically used in ultrasonic transducer applications. 

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. 

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 of piezoelectric polymers make them interesting also in the development of electroacustk or ultrasonic transducers for medical applications. Comparison of tbe representative PVDF material characteristics with a convenlional ferroelectric ceramic as PZT shows several features of piezoelectric polymers which make them attractive in transducer design (TU>le 1). 

Electroactive polymers have piezoelectric and pyroelectric properties that are lower than those of ceramics materials. However, they have low permittivity as well as other advantages that enable their use in applications (Lang, 2008) such as acmators, vibrational control, ultrasonic transducers, and others such as shock sensors, health monitoring, tactile sensors, and energy conversion devices. 

Transducer. A transducer or a probe is a device that emits a beam of ultrasonic waves when bursts of alternating voltage are applied to it. An ultrasonic transducer is comprised of piezoelectric material. Piezoelectric material is material that vibrates mechanically under a varying electric potential and develops electrical potentials under mechanical strain, thus transforming electrical energy into mechanical energy and vice versa (2). As the name implies, an electrical charge is developed by a piezoelectric crystal when pressure is applied to it and reverse is also true. The most commonly encountered piezoelectric materials are quartz, lithium sulfate, and artificial ceramic materials such as barium titanate. 

Piezoelectric materials may be used as transducers between electrical and mechanical energies. One of the early uses of piezoelectric ceramics was in sonar systems, in which underwater objects (e.g., submarines) are detected and their positions determined using an ultrasonic emitting and receiving system. A piezoelectric crystal is caused to oscillate... 

The practical application of ultrasonics requires effective transducers to change electrical energy into mechanical vibrations and vice versa. Transducers are usually piezoelectric, ferroelectric, or magnetostrictive. The application of a voltage across a piezoelectric crystal causes it to deform with an amplitude of deformation proportional to the voltage. Reversal of the voltage causes reversal of the mechanical strain. Quartz and synthetic ceramic materials are used. 

The compression of a powder is a complex process that is usually affected by different kinds of problems. These problems have been widely investigated and mainly concern the volume reduction and the development of a strength between the particles of the powder sufficient to ensure tablet integrity [82], The application of ultrasonic energy shows a great ability to reduce and even avoid these problems [83], Ultrasound refers to mechanical waves with a frequency above 18 kHz (the approximate limit of the human ear). In an ultrasound compression machine, this vibration is obtained by means of a piezoelectric material (typically ceramics) that acts as a transducer of alternate electric energy of different frequencies in mechanical energy. An acoustic coupler, or booster, in contact with the transducer increases the amplitude of the vibration before it is transmitted (usually in combination with mechanical pressure) to the material to be compressed. 

The most commonly employed US transducers are thin platelets of piezoelectric (mainly ceramic) materials with metallic electrodes on both surfaces (see Section 1.4). Such transducers have resonance frequencies determined by the interference of the ultrasonic signals created at both surfaces and internal reflections. The interferences cause distortions (broadening) in the echo pattern in pulse-echo measurements. The resonances also introduce phase shifts in the signals and lead to restrictions in the bandwidth. The frequency range can be expanded by using concave, cylindrical or spherical piezotransducers. 

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