A STUDY OF CRACK CLOSURE IN ELECTRIC-FIELD-INDUCED FATIGUE IN FERROELECTRIC CERAMICS

Considering the influence of the domain switching near the tip of a crack andapplying the idea of multiscale singularity fields in piezoelectric fracture, we have obtained anempirical criterion for the crack closure. Based on the domain switching in the electric yieldregion, referring to Yang's results on the small scale yield model for the electrical fatigue crack,a model of the crack closure during electric-field-induced fatigue is developed to analyze thecrack growth. In terms of the model we have obtained the formula of the rate of the crack growthunder cyclic electric loading. Finally we compare the theoretical predictions with the results givenby Cao and Evans experimentally. It should be pointed out that the model proposed is empirical andneeds to be verified by more experimental results.

ELECTROMECHANICAL DEFORMATION AND FRACTURE OF PIEZOELECTRIC/FERROELECTRIC MATERIALS

This review presents the progress and current status of the investigation on electromechanical deformation and fracture of piezo electric/ferroelectric materials. An attempt is made to summarize a few fundamental aspects, which include electromechanical constitutive relations, piezoelectric micromechanics and electric fracture and fatigue, instead of describing all technological backgrounds, basic physics, experimental findings, and theoretical developments. A number of open questions and future prospective are presented. It is hoped that this review will encourage people to join the exploration of this important and interesting field.

Phase fragility and mechatronic reliability for Pb(Mg_(1/3)Nb_(2/3))O_(3)-PbTiO_(3) ferroelectric single crystals-A review

Single crystals of(1x) Pb(Mg_(1/3)Nb_(2/3))O_(3)-PbTiO_(3)(PMN-xPT)near their morphotropic phase boundaries(MPBs)are under extensive investigations for their extraordinary high dielectric and piezoelectric behavior.Applications of those single crystals facilitated the breakthrough in ultrasonic transducer materials and devices.Ferroelectric materials are known to be fragile which often leads to various reliability failures in applications involving electric loadings.In a mechanical sense,the failure modes concern the fracture under an intensive electric field,and the fatigue crack propagation under an alternating electric field.In an electrical sense,the failure is exhibited by degenerated hysteresis loop by shrinking the remnant polarization and expanding the coercive field.All these modes degrade the performance for ferroelectric devices.As a departure from the tetragonal TÞferroelectric materials,exemplified by BaTiO_(3) and Pb(ZrTi)O_(3),the domain structures of PMN-PT around the MPB are versatile and intricate,depending sensitively on the composition variation,orientation and previous loading history.In this review,the attention is mainly focused on three aspects.First,the phase fragility and multiphase coexistence are presented for both[100]-and[101]-oriented PMN-PT single crystals.Second,investigations on electric field-induced fatigue crack propagation are described,along with the orientation effect on the crack propagation behavior.Third,the inverse effects of the phase transition and fatigue crack growth on the polarization behavior,or the interaction between the mechanical and electrical degradations will be elucidated.The review aims for better understanding the underlying mechanism for the ultrahigh performance of the PMN-PT single crystals,to bridge the studies of ferroelectric materials from the mechanical and electrical senses,as well as to evaluate the reliability of PMN-PT single crystals under device applications.

Stable dielectric response in lead-free relaxor K_(0.5)Na_(0.5)NbO_(3)-SrTiO_(3) thin films

K_(0.5)Na_(0.5)NbO_(3)-SrTiO_(3)(KNN-STO)thin films of different compositions were prepared by the chemical solution deposition method.While structural investigations confirmed the formation of perovskite solid solution in all developed films,dieletric experiments revealed a relaxor broad dispersive maximum in the sample with 15 mol%of STO,exhibiting for a thin film high ε′~330 at~210K.The history-dependent effects of this relaxor sample were compared to those of KNN STO ceramics and,furthermore,shown to be much weaker than in widely used lead-based ferroelectric and relaxor(Pb,L a)(Zr,Ti)O_(3) ceramics:While fatigue endurance results revealed a slight drop in polarization after 3 × 10^(5) switching cycles,the results of aging of the dielectric constant revealed no notable decrease in its values after 10^(6)s.