GRAIN-SIZE DEPENDENCE OF ELECTRIC FATIGUE BEHAVIOR OF HOT-PRESSED PLZT FERROELECTRIC CERAMICS

The decrease in remnant polarization, P(r), and increase in coercive field, E(c), of ferroelectric ceramics subjected to a large a.c. electric field is termed electric fatigue. In transparent hot-pressed lanthanum doped lead zirconate titanate (PLZT) ceramics, electric fatigue was not detected after 10(9) cycles when grain size is small (approximately 5 mum) but became increasingly severe as the grain size increases to 10 mum and beyond. The electric field causes large strains (greater-than-or-equal-to 10(-3)) due to the anisotropic piezoelectric and electro-strictive coefficients of the material, which results in intergranular microcracking due to incompatible deformations. The effective field on a grain in a cracked sample is less than the applied field, limiting domain switching and polarization. The microcracking occurs only in the electroded, fatigued region of the sample and is initiated on the high voltage terminal side. The microscopic and acoustic emission data support the proposed mechanism of electric fatigue. An estimate of the critical grain size for microcracking due to electric stress, based on energy criterion, is in good agreement with experiment. The process of crack growth during electric fatigue is also elucidated.