Core–Shell Lead–Free Piezoelectric Ceramics: Current Status and Advanced Characterization of the Bi_1/2Na_1/2TiO₃–SrTiO₃ System

Abstract

The design of core-shell materials affords additional degrees of freedom to tailor functional properties as compared to solid solution counterparts. Although to date most of the work in core-shell materials has focused on dielectrics, piezoelectric coreshell ceramics may gain similar interest. Generalities of core-shell functional ceramics features are addressed in this work. A model system, Bi 1/2 Na 1/2 TiO 3 -SrTiO 3 , is introduced to discuss structure-property relationships. We demonstrate that this system features a core-shell microstructure for the composition corresponding to 25 at.% Sr. The material is studied by means of macroscopic functional properties and in situ structural characterization techniques at different length scales, such as X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The evolution of the core-shell with field and temperature determines its functional properties. The high strain of the system, 0.3% at 4 kV/mm, is due to an electric-field-induced phase transition of the core and shell. Upon field removal the core remains in a poled state, whereas the shell is characterized by a reversible transformation. The reversibility of the phase transition of shells and associated switching are key features in the observed giant strain. Dielectric anomalies are found to be related to changes in oxygen octahedral tilting angles within the core and shell.

Piezoelectricity and rotostriction through polar and non-polar coupled instabilities in bismuth-based piezoceramics

“…Thereafter, for higher electric field the irreversible transformation of cores leads to a higher remanent strain. The remanent strain observed after 20 s of electric field removal is below 10% of the values of the strain under electric field, indicating reversibility of field induced processes in the shell regions22.…”

“…Further enhancements of the probing electric field lead to much more gradual increments in strain below 15% for each step. The considerably high change in strain from 1.5 kV/mm to 2 kV/mm suggests an electric field induced phase transformation into a ferroelectric state that should be followed by domain switching222526. The microstructure of BNT-25ST consists of cores that transform irreversibly from a non-ergodic relaxor state into a ferroelectric state, whereas the surrounding shells transform reversibly from an ergodic relaxor state into a ferroelectric state providing the bulk strain response22.…”

“…The reversible phase transformation is mostly attributed to the response of the shell22 and allows exploitation of the FE and AFD instabilities at each field cycle. The core-shell structure of BNT-25ST appears pertinent to the strain mechanisms since, apart from a redistribution of the local electric field, it may impart an initial state with two different phases that feature different polarization and oxygen octahedral tilts22. This promotes a strong competition between FE and AFD instabilities and, together with the different grain orientations with respect to the electric field, favors the nucleation of either the average rhombohedral or tetragonal phases.…”

“…Bi 1/2 Na 1/2 TiO 3 (BNT)-based materials are prime candidates in the quest for lead-free environmentally friendly alternatives to PZT due to their giant strain response212223. These materials feature phases with FE and AFD instabilities, attributed to a frustrated ferroelectric state inherent to their local disorder24.…”

“…These materials feature phases with FE and AFD instabilities, attributed to a frustrated ferroelectric state inherent to their local disorder24. For this study, we selected the (1−x)Bi 1/2 Na 1/2 TiO 3 -xSrTiO 3 system with 25 mol % SrTiO 3 (BNT-25ST) since it features a core-shell structure with local polar and non-polar phases representing different octahedral tilts22. This intricate microstructure entails a strong competition between FE and AFD instabilities and is a prominent candidate to investigate FE-AFD couplings.…”