Apparatus for Quasihydrostatic Measurement of Piezoelectric dhCoefficient

Abstract

Measurement of piezoelectric response of materials (dh) by a pressure pulse technique is described in this article. The sample is placed in a fluid-filled chamber fitted with a piston at one end. Impact on the piston creates a pressure pulse in the chamber, the duration and shape of which can be controlled. This method is effective with composite samples as well as polymer film samples. Introduction This paper describes a convenient and reliable method for measuring the hydrostatic piezoelectric coefficient of materials, dh. In this technique the sample is placed in a fluidfilled chamber and a hydrostatic pulse is applied by impacting a piston in the chamber. Pulse height, duration, and shape can be controlled readily, and in principle, information about frequency response can also be obtained. Automated data acquisition makes possible accurate calculation of response over the entire pressure pulse. Results on samples of various PbTiOj-based composites and poly(viny1idene fluoride)(PVF2) will be discussed. Other transient and ramp methods for measuring dh are described in the literature.[ll Experimental A . Pressure Generation The apparatus required for measurement is shown in Figure 1, and consists of a fluid-filled cell (Fiugre 2) containing the sample and a calibrated pressure transducer. The apparatus is an improved version of apparatus described previously, with digital processing added.[2,3] The cell was made by cutting a cylindrical chamber 3 inches long and 2 inches in diameter into a 4inch diameter 303 stainless steel rod. A calibrated pressure transducer was placed with the sensing diaphragm at the bottom of the chamber, and provision was made to connect a sample electrically to the outside of the chamber via a BNC connector. A piston of armoloy-plated brass was made to fit in the chamber for applying the pressure pulse. This sample cell containing the sample and a suitable fluid is placed in a drop tester, used to generate pulses reproducibly. Fig. 1-System block diagram Figure 2. Pressure Pulse Measurement Cell Pulses can be shaped by judicious selection of a material to be placed on top of the piston. The compliance of this coupling slug will determine the pulse length and shape. For example, a relatively soft material like a rubber vacuum hose produces a relatively long pulse of 10-15 msec duration. Harder materials produce shorter pulses of 1-5 msec duration. Some unusual pulse shapes can also be generated, depending on the resilience of the intermediate material. Pulse shapes for some typical materials are shown in Figure 3.