A8.4 - Modelling and Characterization of Piezoelectric 1-3 Fiber Composites

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
A8 - Ultrasonic Sensors III
R. Steinhausen, C. Pientschke, S. Kern, H. Beige - Martin-Luther-Universität Halle-Wittenberg, Halle (Germany)
199 - 204


Piezoelectric composite materials have a high potential for ultrasonic transducers due to their high coupling coefficients and a low acoustic impedance. In dependence on the ratio of piezoelectric active material in the polymer matrix the acoustic properties of the composite can be varied in a wide range.
Commercial composites were usually prepared by the dice-and-fill-technology. Thin slices were sawn into a bulk ceramic plate. The diced structure is filled by an epoxy resin. After curing the bottom plate is removed and the composite is polished and electroded. Regular structures with typical width of the rods down to 50 μm can be produced. Such piezoelectric composites have coupling coefficients of the thickness resonance more than kt=0,60 at frequencies of 2 to 20 MHz.
In the last years technologies for preparing piezoelectric ceramic fibers with diameters from 100 to 500 μm were developed. The preparation of fiber composites is cost-efficient due to lesser preparation steps and a lower material usage compared to the loss of material in the sawing process. Additionally, any high aspect ratios (length to thickness relation of rods or fibers) can be realized by fibers. An other advantage of fiber composites is the lack of spurious lateral resonances because of the non-regular distribution of the fibers in the polymer matrix.
Unfortunately, the measured piezoelectric coefficient of the most fiber composites is mostly lower than expected. Either the non-regular distribution of the fibers or the piezoelectric properties of the fibers themselves yield to a worse performance of the composite. On one hand, deviations of the local volume content of fibers were obtained due to their random distribution in the polymer matrix. The influence of this effect on the measured properties is discussed.
On the other hand, the preparation technology of single ceramic fibers is difficult to handle. The ratio of volume and surface of single fibers is low and the surface properties play an important role compared to bulk ceramics. Thus, fiber with the same chemical composition can be different from the bulk ceramics. We developed a experimental method to measure the piezoelectric coefficient d33 of single ceramic fiber with diameters from 200 μm. Both the linear properties at low electric field strength and the high-field piezoelectric strain can be measured. This allows to compare the experimental values of the fibers with the values calculated from the fiber composite. The experimental method and the modeling of the effective composite properties by analytic models and Finite Element Method (FEM) will be discussed in this work.