W. A. Schulze

Piezoelectric Composite Materials for Ultrasonic Transducer Applications. Part I: Resonant Modes of Vibration of PZT Rod-Polymer Composites

Abstmcr-The objective of the present work was to gain a deeper mensions of the transducer are much smaller than the understanding of the behavior of lead zirconate titanate (PZT) polymer composites for applications such as ultrasonic medical diagnosis in the megahertz frequency range. These composites were originally developed for low-frequency hydrophone applications. The PZT rod-polymer composites have been prepared with five to 30 volume percent PZT using 0.28 mm and 0.45 mm rods. In a disc of PZT rod-polymer composite material, there are three principal types of resonance: the planar mode, the thickness mode, and various lateral modes caused by the regular periodicity of the PZT rod in the composite. These resonance modes have been studied with the following techniques: 1) electrical impedance measurement as a function of frequency and 2) laser probe dilatometry of the dynamic displacement as a function of frequency and position in the composite lattice. The observed resonance behavior is found to be a result of lateral interactions in the composite through the epoxy medium. The effect of temperature on the electromechanical properties of the composite has also been investigated. Implications of these results for optimizing the design of ultrasonic transducers are discussed.

Piezoelectric Composite Materials for Ultrasonic Transducer Applications. Part II: Evaluation of Ultrasonic Medical Applications

Abstmct-The electro-acoustic properties of Lead zirconate titanate (PZT) rod-polymer composites relevant for ultrasonic transducer applications are reported. Acoustic impedance of the composite materials was measured by three different techniques in the frequency range 0.33.5 MHz. Dependence of the acoustic impedance as a function of volume fraction of PZT and frequency was also modeled theoretically. Time-delay spectrometry was employed to calibrate the free-field transmitting and receiving voltage responses of the composite materials. The acoustic impedance of the composite materials was in the range of 3-10 M rayl. The figure of merit in the receiving mode of composite materials was three times that of PZT. The figure of merit for a 20percent PZT composite (2 = 7.3 M rayl) was further enhanced by 50 percent using a single-layer impedance transformer of lucite (2 =3.3 M rayl). These composite materials were molded into curved shapes by simple thermal process to fabricate focused transducers. The axial and lateral beam profiles of focused composite transducers are presented.

Piezoelectric 3–3 composites

Abstract Piezoelectric composites of PZT and polymers were prepared by mixing tiny plastic spheres with PZT powder in an organic binder and firing the mixture to give a ceramic skeleton. After cooling, the skeleton was back-filled with polymer and poled. Dielectric and piezoelectric properties were measured on samples ranging from 30 to 70 volume % PZT, and compared with a rectangular skeleton model for 3–3 composites. Composites containing 50% PZT-50% silicone rubber appear especially useful for hydrophone applications with dhgh products a hundred times larger than PZT.

Simplified fabrication of PZT/polymer composites

Abstract Skinner, et al. prepared PZT/polymer composites using the coral replamine process. The composites were found to have, for certain applications, greatly improved electromechanical and physical properties over those of conventional piezoelectric materials. However, the coral replamine process requires natural coral from the ocean as the starting replica material, making this process industrially unfavorable. This paper discussed PZT/polymer composites electromechanically and physically comparable to the Skinner replamine composites. Fabrication is simplified by the use of a compacted mixture of volatilizable plastic spheres and PZT powder, which when sintered yields a piezoelectric with a density of less than 2.9 g/cm 3 , a permittivity of ∼120 and a d 33 of 180×10 −12 C/N. The improved connectivity increases d h (hydrostatic) from ∼35×10 −12 C/N (solid PZT) to 100×10 −12 C/N.

Perforated pzt-polymer composites for piezoelectric transducer applications

Abstract Composites of PZT and polymer with 3-1 and 3-2 connectivity patterns have been fabricated by drilling holes in sintered PZT blocks and filling the holes with epoxy. The influence of hole size and volume fraction PZT on the hydrostatic properties of the composite was evaluated. By decoupling the piezoelectric [dbar]33 and [dbar]31 coefficients in the composite, the hydrostatic coefficients are greatly enhanced. On samples optimized for hydrophone performance, the dielectric constants of 3-1 and 3-2 composites are 600 and 300 respectively. The piezoelectric coefficients [dbar]h, [gbar]h, and [dbar]hdh for 3-1 composites are 230 (pCN−1), 34 (x10−3 VmN−1), and 7800 (10−15 m2N−1) respectively, and the corresponding values for 3-2 composites are 372 (pCN−1), 123 (10−3 VmN−1), and 45000 (10−15 m2N−1).

Fabrication and electrical properties of grain oriented Bi4Ti3O12 ceramics

Abstract A fabrication process has been developed by which Bi4Ti3O12 can be made in ceramic form with high densities (92% theoretical) and substantial grain orientations (>95%). The process consists of molten salt synthesis of platelets of Bi4Ti3O12, tape casting, and conventional sintering. Electrical measurements have verified the high degree of grain orientation obtained in these ceramics. Permittivities in the a-b oriented direction (along the plane of the tape) and the c oriented direction (perpendicular to the plane of the tape) at 100 KHz are 153 ± 5 and 118 ± 5, respectively. Conductivities at 1 KHz and 500°C in the a-b and c oriented directions are 6.3 × 10−2 and 4.2 × 10−3 (ohm-m)−1, respectively. The c oriented direction was poled to a d33 of 5 × 10−12 C/N with the application of a DC field. The a-b oriented direction was poled to a d33 of 10 × 10−12 C/N with the application of a pulsed field on cooling through the transition.

High frequency applications of PZT/polymer composite materials

Abstract PZT/polymer composite hydrophone materials of four connectivity patterns have been evaluated for high frequency applications such as ultrasonic medical diagnosis and nondestructive testing. The amplitude and bandwidth of the pulse echo test are used as the principal figure of merit. Transducers made with thin PZT rods cast in an epoxy matrix have pulse echo sensitivity comparable to unmatched commercial transducers.

Piezoelectric properties of internally electroded PZT multilayers

Abstract Platinum internal electrodes have been introduced into PZT compositions in a multilayer configuration by a conventional tape casting process. Electrical property data for the devices suggest that the piezoelectric coefficients of the multilayer configuration are approximately 20% lower than those of similar plain PZT devices. However, the applied voltage/displacement ratio is much lower in the multilayer compared to the plain devices. Resonant properties are almost unaffected by the presence of internal electrodes for hard PZT compositions. Piezoelectric transformers with multilayer primaries show improved transformer ratios compared to conventional devices.

Extruded PZT/polymer composites for electromechanical transducer applications

PZT/polymer composites having a 1–3 parallel connectivity were fabricated by impregnating a sintered, extruded honeycomb configuration of PZT with various polymers. The resultant composites were found to have densities less than 2900 kg/m3, a dielectric constant of ∼500 and a piezoelectric d33 of ∼300×10−12 C/N. The 1–3 connectivity increases the piezoelectric voltage coefficient (g33) from 22×10−3 Vm/N (solid PZT) to ∼70×10−3 Vm/N. The composites have thickness mode electromechanical coupling coefficients (kt) which are ∼25% greater than that of homogeneous PZT, and are readily adaptable for broad bandwidth operation. This combination of electromechanical properties makes these composites ideal for low voltage displacement and pulse echo applications.

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

Abstract 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 Poissons 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.