Roger H. Tancrell

Properties of PVDF Polymer for Sonar

Polyvinylidene fluoride (PVDF) polymer is evaluated for application in large sonar arrays. The piezoelectric coefficients and elastic constants of thick (550 urn) PVDF produced at Raytheon are measured, and the apparatus for making the measurements is described. It is shown, both theoretically and experimentally, that electrodes deposited on the surface can stiffen the polymer and significantly alter the observed properties. The free-f ield frequency response for an experimental hydrophone is shown over a broad frequency range, f ee of spurious resonances. Stability to relatively high temperature (90°C) and to high static pressure (6.9 MPa = 1000 ps 1 are shown from experimental data.

PVF 2 Polymer Microprobe

This paper describes the design and performance of a PVF2 microprobe for use in mapping the pressure field from arrays. For medical applications, the probe must have short pulse response (i.e., broad, flat bandwidth) in addition to high sensitivity, low noise, and a wide acceptance angle. The factors that influence critical parameters, such as ringing and delayed reverberations, are specifically discussed. The probe has a center frequency of about 3 MHz. It consists of a 1.5 mm diameter disc of PVFZ polymer (30 pm thick) operated well below its resonance to obtain very broad frequency response. The PVFz is completely encased in a metal cylinder to shield it from spurious electromagnetic interference and to protect the polymer from possible corrosion by external fluids. An FET amplifier is integral to the probe housing. The experimental performance of the probe is compared with theoretical predictions, acoustically and electrically. It is also compared with the performance of other probes, including a laser pellicle system, for pulse fidelity and signal-to-noise ratio.

Transmitters and Receivers for Medical Ultrasonics

Wide bandwidth transducers are essential for good range resolution in pulse-echo medical ultrasound. Within this bandwidth objective, however, the criteria that determine whether a given piezoelectric material is a good transmitter are, in general, different from those that determine a good receiver. estimating the merits of a particular piezoelectric as a transmitter or receiver, and the advantages that may be gained by separating these roles in a medical ultrasound system are also discussed. Although some materials are better suited for receivers than for transmitters, this is shown to be consistent with the reciprocity principle.

Approaches for determining the properties of materials by FEM

Finite element modeling (FEM) is a very powerful tool for designing transducers, but precise values of material properties are essential. By the traditional experimental method, a ceramic should be made into a special shape so that only one mode dominates and is treated as a one dimensional problem. Because it is difficult to isolate multi-mode interference, traditional experiment methods yield significant errors. This paper outlines approaches for determining properties of materials by choosing a round disk of ceramic, measuring a set of resonant frequencies and impedances and using finite element modeling. The usual difficulty with this approach is that resonance frequencies are affected by almost all the elastic constants. With the large variety of possible combinations, it is difficult to arrive at a unique solution. We first investigate how the individual elastic constants affect basic eigenmodes differently. We then proceed to determine elastic and piezoelectric constants in an orderly fashion. Practical examples are given step by step.