An experimental and theoretical study of electric field effects on cracked ceramics

Abstract

Increasing uses of perovskite-type ferroelectric ceramics in smart structures, such as capacitors, sensors, actuators and memories, raise the important issue of their reliability. These materials are brittle and susceptible to cracking. Understanding of their degradation mechanisms is essential to design devices and materials for better performance. To this end, investigations of electric ®eld effects on cracked ceramics are necessary. However, the current status in this ®eld is preliminary in nature. Real cracks usually have permeable non-conducting interiors; yet most existing theories in this ®eld assume that the medium inside a crack is either conducting [1, 2] or impermeable [3, 4]. An impermeable crack intensi®es a ®eld applied perpendicular to the crack but does not perturb a ®eld parallel to the crack. Conversely, a conducting crack intensi®es a ®eld applied parallel to the crack but not a ®eld perpendicular to it. Also, the energy release rate or crack driving force is different for the two types of crack, positive for a conducting crack and negative for an impermeable crack [2]. Consequently, these theories predict that an impermeable crack will not grow but a conducting crack might. Real defects are different from the idealized cracks in both their geometry and the permittivity and dielectric strength of air inside cracks. These differences may result in different responses from those predicted by the existing theories. In this work, direct observations of the electric ®eld effects on permeable cracks are conducted. The material used for the experiments is lead lanthanum zirconate titanate (PLZT). PLZT is a relaxor ferroelectric whose properties can be tailored by changing compositions. In this work, specimens with a composition of 9.4 at % La±65 at % PbZrO3 (PZ)±35 at % PbTiO3 (PT) are adopted. The Curie transition temperature of this material is 25 8C. At this temperature, 9.4 at % La±65 at % PZ±35 at % PT is at its dielectric maximum in the transition zone and is quadratic electrostrictive. This material has a rhombohedral crystal structure and an average 5 im grain size. The properties of this composition are given in Table I. Specimens were cut to the dimensions shown in Fig. 1. Ceramic specimens with the composition 9.4 at % La±65 at % PZ±35 at % PT are transparent and display electro-optical and piezo-optical coupling. A standared photostress arrangement gives a direct view of the electric ®eld and stress concentrations. The specimens are placed in servohydraulic test frames between crossed polarizers and quarterwave plates (Fig. 2). In this arrangement, light is passed through the ®rst polarizer resulting in plane polarization, through the quarter-wave plate resulting