Combining a piezoelectric ceramic and a passive polymer to form a piezocomposite allows the transducer engineer to design new piezoelectrics that offer substantial advantages over the conventional piezoelectric ceramics and polymers. The rod composite geometry provides materials with enhanced electromechanical coupling and with acoustic impedance close to that of tissue; these factors yield transducers for medical ultrasonic imaging with high sensitivity and compact impulse response. The dice-and-fill technique produces piezocomposites that can be readily formed into complex shapes to facilitate focusing the ultrasonic beam. Proper design of the rod spacing yields materials that exhibit low crosstalk between array elements formed by patterning the electrode alone, without cutting between the elements. In this way, curved annular arrays have been made that provide high-quality clinical images of substantial diagnostic value to physicians. Included is an extensive bibliography of papers documenting the role of piezocomposites in ultrasonic imaging transducers. >

The role of piezocomposites in ultrasonic transducers

A simple physical model of 1-3 composite piezoelectrics that was advanced for the material properties relevant to thickness-mode oscillations is extended to address the hydrostatic response. The model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions are derived for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer. The results are similar to those derived by Haun and Newnham (1983, 1986) using a parallel-series connectivity model. The model is illustrated by analyzing composites made from conventional PZT5 and anisotropic modified lead titanate piezoelectric ceramics. For PZT5, the composite structure enhances its hydrostatic charge coefficient, hydrostatic voltage coefficient, hydrophone figure of merit, and hydrostatic coupling coefficient, while three of these quantities fall short of their pure ceramic values in the modified lead titanate composites. The shortfall is due to an enhanced composite that arises from lateral stress on the polymer being transferred to a longitudinal stress along the ceramic rods by the Poisson effect in the polymer, thus producing a charge through the ceramic's d/sub 33/. >

Modeling 1-3 composite piezoelectrics: hydrostatic response

A new type of piezoelectric PZT-polymer 1-3-0 composite was fabricated with a single large void present in the center of the composite. Experimentally and theoretically the addition of this type of void to a 1–3 composite significantly enhances the hydrostatic piezoelectric [dbar] h and [gbar] h coefficients. A comparison of the experimental results and theoretical predictions was made. The pressure and frequency dependences of the properties of the 1-3 and 1-3-0 composites were measured, along with the effect of hydrostatic pressure cycling. The 1-3-0 composites partially depoled from pressure cycling to 600 psi (4.1 MPa), indicating that the limit of hydrostatic sensitivity, physically possible for the soft PZT used, was exceeded.

An experimental and theoretical study of 1–3 AND 1-3-0 piezoelectric PZT-Polymer composites for hydrophone applications

A hydrophone is an underwater microphone or transducer used to detect underwater sound. The sensitivity of a hydrophone is determined by the voltage that is produced by a hydrostatic pressure wave. The hydrostatic voltage coefficient, gh, relates the electric field appearing across a transducer to the applied hydrostatic stress, and is therefore a useful parameter for evaluating piezoelectric materials for use in hydrophones. Another piezoelectric coefficient frequently used is the hydrostatic strain coefficient, dh, which describes the polarization resulting from a change in hydrostatic stress. The gh coefficient is related to the dh coefficient by the relative permittivity (K): gh = dh/ɛoK, where ɛo is the permittivity of free space.

Composite piezoelectric sensors

The piezoelectric properties of lead zirconate titanate (PZT)–polymer composites were studied as a function of composition and phase connectivity. PZT skeletal structures were fabricated by robotic deposition, densified at 1275 °C, and subsequently infiltrated with epoxy to produce the desired PZT–polymer composites. These 3-X structures consisted of a three-dimensional lattice of PZT rods (3–3) embedded in a polymer matrix, a PZT lattice/polymer matrix capped with PZT face plates (3–2), or PZT lattice/polymer matrix capped with PZT face plates and encircled by a solid PZT ring (3–1). The PZT:polymer ratio was varied systematically by changing the lattice (rod) spacing in each composite architecture. The concentration of PZT pillars, which formed along the poling direction at the intersections between PZT rods, varied as the PZT volume fraction squared. These 3-X composites displayed enhanced hydrostatic figures of merit relative to monolithic PZT due to stress concentration in the PZT pillars and their d...

Piezoelectric properties of 3-X periodic Pb(ZrxTi1−x)O3–polymer composites

Piezoelectric PZT-polymer 1-3 and 1-3-0 composites were transversely reinforced with glass fibers to increase the hydrostatic piezoelectric coefficients for possible use in hydrophone applications Modeling of these composites theoretically showed that the dhgh figure of merit is a function of the volume fractions of PZT rods, glass fibers, and polymer porosity, and of the Poisson's ratio and compliance of the polymer matrix Experimental results showed significant enhancements of the dhgh figure of merit with the addition of glass fibers Comparisons of the theoretical predictions and the experimental results were made

Transversely reinforced 1-3 and 1-3-0 piezoelectric composites

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Transversely reinforced 1-3 and 1-3-0 piezoelectric composites