Experimental Investigation of Strain Distributions and Polarization in Lead-based and Lead-free Ferroelectrics

Abstract

Experimental investigation of strain and polarization induced by uniaxial loads in lead-based and lead-free ferroelectrics was performed in this work. The commercial soft PZT (PIC 151) and (Na1/2Bi1/2)TiO3-based lead-free piezoceramics are the main objects of this study. The lead-free specimens include pure 0.93(Na1/2Bi1/2)TiO3-0.07BaTiO3 (NBT-7BT) ceramics, pure 0.91(Na1/2Bi1/2)TiO3-0.06BaTiO3-0.03K0.5Na0.5NbO3 (NBT-6BT-3KNN) ceramics, bilayer composites and trilayer “sandwich” composites based on these two pure lead-free materials. The electromechanical properties of these materials were measured by using two setups. One is designed for checking the electric field-induced strain and polarization, the other is assembled for monitoring the deformation of piezoceramics under uniaxial compression. For the PZT samples, the deformation induced by a pure bipolar electric field and a pure compressive stress were monitored, respectively. Simultaneously, the polarization can be obtained during the electric field and compressive loadings. The digital image correlation (DIC) method was employed to monitor the deformation under larger signal electric field and stress. Both the longitudinal and transverse strains were calculated from imaging a bulk sample under a ±2 kV/mm electric field. Compared with linear variable displacement transducer (LVDT) data, the results from this correlation method were validated. The checking of strain distribution on the surface of the sample reveals that electric field-induced deformation of bulk ceramic without any mechanical constraints is highly homogenous at the macroscopic scale. A mechanical compressive was loaded at the PZT bulk specimens with an aspect ratio of 3:1. Because of the friction at both the top and bottom surfaces of the PZT ceramic specimen, the deformation of the sample under a large uniaxial compressive stress usually shows a barrel shape which was proved by the results of the strain distribution. This mechanical compressive loading test provides the experimental justifications for the selection of an aspect ratio of 3:1 for the specimen, where only the central cubic region of a specimen represents the desired purely uniaxial stress state. Both the NBT-7BT and NBT-6BT-3KNN ceramics were loaded by the cyclic electric fields and constant electric fields. Due to the properties of nonergodic relaxor and ergodic relaxor in NBT-7BT and NBT-6BT-3KNN, respectively, the polarization-electric field (P-E) loop and strain-electric field (S-E) loop measured from the two ceramics show obvious differences. With the purpose of obtaining a better understanding of the nonergodic and ergodic behaviors in the two relaxors, the creep in these two ceramics induced by a constant electric field was measured. A so-called “two-stage” cascade phenomenon can be obtained from an unpoled NBT-7BT specimen when the electric field is higher than a certain value and held there for a long time. However, this effect cannot be observed on the NBT-6BT-3KNN specimens. The time-dependent electromechanical behavior shows that creep reduces gradually and reaches saturation eventually. The DIC method was also used for monitoring the electric field-induced deformations of NBT-6BT-3KNN/NBT-7BT bilayer and NBT-6BT-3KNN/NBT-7BT/NBT-6BT-3KNN trilayer composites. By means of selecting the regions of interest (ROIs) located on each layer and calculating the strains there, the strain distribution can be achieved. All these results were listed in a comparison of bipolar longitudinal strain and polarization loops among the two pure components, bilayer composites and trilayer composites. Although the internal stresses induced by the variation in sintering rates between the layers are missing, the stress states in each layer were estimated roughly by using the strain data. Especially, these data of strain distributions provide a bridge to build a model considering both the polarization coupling and strain coupling to obtain a better understanding of this electromechanical behavior.