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Tailoring the Electromechanical Behavior of Lead-Free Ceramic/Ceramic Composite Ferroelectrics

Subject Area Synthesis and Properties of Functional Materials
Term from 2016 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 320512068
 
Final Report Year 2022

Final Report Abstract

The focus of this project was to understand the influence of electrical conductivity, internal mechanical fields, and interface regions on the large unipolar strain behavior of lead-free composite ferroelectrics. In order to accomplish this, the project was organized into an experimental and a simulations part. The experimental part was focused on investigating the role of stress on the small and large signal electromechanical response of the bulk and composite lead-free ferroelectrics at the macroscopic and microscopic length scales. In order to more fully understand the composite structures and the local fields, the processing induced effects, such as the sintering and thermal expansion strains, local diffusion across the interface, variations in microstructure and elastic properties, and changes in crystal structure were investigated by a variety of experimental techniques. These measurements revealed significant changes into the microstructure, local composition, and crystal structure that are responsible for the observed bulk electromechanical properties. An important scientific question addressed within the framework of this project was the role of the interface on composite ferroelectrics, in particular related to strain- and polarization-coupling. Here, a novel method was developed consisting of attaching pre-sintered discus together with silver paste. By investigating composites with and without the adhesive, it was possible to directly observe the effect of the strain coupling of the transverse piezoelectric effect and remanent strain. It was found that these effects significantly increased the large signal strain response in comparison to composites with only polarization coupling. Finally, ex situ piezoresponse force microscopy as well as in situ Raman and Brillouin spectroscopy and synchrotron diffraction were employed to directly investigate the local phenomena during the application of a mechanical field. For the first time the formation of long-range ferroelectric order and the corresponding domain structure were observed with PFM, in addition to local stress concentrations near defects, such as pores, that displayed a transition at applied global stresses below the coercive stress. Interestingly, the in situ Brillouin spectra were found to have the appearance of a transversal acoustic wave mode at approximately 32 GHz and longitudinal acoustic wave mode peak splitting due to stress. This unique method is promising for characterization of other functional ceramic systems, such as others relaxor and antiferroelectrics. For the simulation part, the first major result is the development of a phenomenological constitutive model for lead-free ferroelectric ceramics, where it is a key feature that the model can flexibly be adjusted to materials with stable and metastable relaxor to ferroelectric phase transition. Based on the experimental results obtained by the project partners in Erlangen, the parameters of this constitutive model could be identified to represent NBT-6BT showing a long-range ferroelectric order and metastable NBT-6BT-4KNN. Sacrificing perfect accuracy in representing all details of the hysteresis curves, a limited number of clearly defined parameters is enough to model the relevant features of the electromechanical large signal hysteresis behavior. We consider this an advantage when discussing which end member property has which effect on the composite response. Altogether, a finite element simulation tool was established for the coupled non-linear electromechanical response and non-ohmic electric transport of leads-free ferroelectrics. Concerning space charge transport, the results showed that there is an impact on the composite response due to relaxation of polarization coupling at the interface, but it turned out to be not as pronounced as expected. Based on the constitutive model for the two end member materials, the bilayer response for polarization and longitudinal and transverse strain hystereses could be well predicted. Concerning the local distribution of the transverse strains at the specimen surface reasonable agreement between simulation and DIC results was found. As a contribution to virtual materials development, an extensive parameter study of more than 1000 composite simulations has been executed. Parameter combinations could be found for which the large signal longitudinal piezoelectric coefficient is higher than that of the end member materials. For optimization, as a qualitative statement, quenching of the polarization hysteresis of the metastable partner should rather be weak, the coercive field strengths should be similar, as well as saturation polarization. Furthermore, roughly equal volume fractions are required, as well as, last not least, moderate field amplitudes below the saturation fields for the occurrence of pronounced minor hysteresis. In summary, despite some delays due to Corona related restrictions, the project met all of the primary research goals related to investigating the influence of stress on the large unipolar strain behavior of lead-free composite ferroelectrics. This work was successfully done collaboratively, resulting in 15 published papers, including 6 papers through collaborations that were made possible through this project on various related topics, such as nuclear magnetic resonance of BNKT-BA, phase field modeling of ferroelectric composites, and temperature-dependent functional properties of various other mateirals, such as ZnO and (B1/2K1/2)TiO3. In addition to advancing the state-of-the-art in understanding stress-dependent electromechanical response of NBT-based materials, a novel experimental arrangement was developed, that is highly useful for future studies of functional ceramics.

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