Designing properties of (Na1/2Bix) TiO3-based materials through A-site non-stoichiometry

Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on the impact of cation vacancies on electroceramics, especially in (Na1/2Bi1/2)TiO3 (NBT)-based materials. Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and functional properties in the representative ferroelectric (Na1/2Bi1/2)TiO3–SrTiO3 (NBT–ST). Experiments on NBT/ST bilayers and NBT–ST with Bi non-stoichiometry reveal that Sr2+-diffusion is enhanced by up to six orders of magnitude along the grain boundaries in Bi-deficient material as compared to Bi-excess material with values of grain boundary diffusion B108 cm2 s 1 and B1013 cm2 s 1 in the bulk. This also means a nine orders of magnitude higher diffusion coefficient compared to reports from other Sr-diffusion coefficients in ceramics. Bi-excess leads to the formation of a material with a core–shell microstructure. This results in 38% higher strain and one order of magnitude lower remanent polarization. In contrast, Bi-deficiency leads to a ceramic with a grain size six times larger than in the Bi-excess material and homogeneous distribution of compounds. Thus, the work sheds light on the rich opportunities that A-site stoichiometry offers to tailor NBT-based materials microstructure, transport properties, and electromechanical properties.T. F., A. A., and K. G. W. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft under WE 4972/2 and FR 3718/1-1. T. F. thanks Dr Edvinas Navickas for his help with the ToF-SIMS measurements. M. A. acknowledges the support of the Feodor Lynen Research Fellowship Program of the Alexander von Humboldt Foundation. M. D. and L. M.-L. acknowledge financial support from the Hessen State Ministry of Higher Education, Research and the Arts via LOEWE RESPONSE. L. M.-L. acknowledges financial support from DFG Grant MO 3010/3-1

Mechanisms of Enhancement in Lead-Free Piezoceramic Composites

The aim of this work is to investigate lead-free ferroelectric ceramic/ceramic composites, with the ultimate goal of elucidating the mechanisms of their enhanced electromechanical response. Previous work has shown that a composite comprised of a highly disordered nonpolar ferroelectric matrix material and an ordered polar seed material results in an increased electromechanical response under specific circumstances. The mechanisms used to explain this enhancement have been based on the electrical and mechanical interactions between the seed and matrix during application of an electric field. However, the interactions between the seed and matrix during processing also play a significant role in the enhancement observed in lead-free ferroelectric composite systems. The fabrication of ceramic/ceramic composites requires high-temperature sintering of the seed and matrix for formation of densified pellets. Fundamental laws of kinetics dictate that diffusion between the two constituents should occur at the high temperatures required for ceramics processing. In addition, a difference in the sintering trajectories will result in a nonzero stress state during sintering, which is well established to effect the microstructure. The structure-property relationships in composites can provide new insight into these mechanisms, but there have been significant challenges in investigating structure at the microscale. To that end, model systems of 2-2 composites were prepared and utilized to investigate these phenomena. In light of the influences of diffusion and internal stress on electromechanical behavior, the electromechanical response of several 0-3 and 2-2 composite systems are investigated. The influence of co-sintering interactions on the electromechanical behavior of electroceramics plays an important role in the improvement of lead-free material systems for the replacement of current commercially dominant lead-based systems

Phase transformation induced by electric field and mechanical stress in Mn-doped (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 ceramics

Electric-field- and stress-induced phase transformations were investigated in polycrystalline 0.5 mol. % Mn-doped (1�x)(Bi1/2Na1/2)TiO3-x(Bi1/2K1/2)TiO3 (x ¼ 0.1, 0.2). To characterize the effect of electric field and stress on the stability of the ferroelectric and relaxor states, polarizationand current density-electric field curves, as well as the stress-strain response as a function of temperature were characterized. Analogous to the observed electrical behavior, the macroscopic mechanical constitutive behavior showed a closed hysteresis at elevated temperatures, indicating a reversible stress-induced relaxor-to-ferroelectric transformation. The electrical and mechanical measurements were used to construct electric field–temperature and stress-temperature phase diagrams, which show similar characteristics. These data show that a mechanical compressive stress,similarly to an electric field, can induce long-range ferroelectric order in a relaxor ferroelectric

Enhancing Electromechanical Properties of Lead-Free Ferroelectrics With Bilayer Ceramic/Ceramic Composites

The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at room temperature. One layer consisted of a nonergodic relaxor,(Bi1/2 1 Na /2)TiO3–7 , BaTiO3 with an electric-field-induced longrange ferroelectric order, whereas the other is understood to be an ergodic relaxor [(Bi N 1/2 1 a ) /2 TiO3–25SrTiO3] that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of a bilayer with low diffusion between the two components is also demonstrated. By taking advantage of the different macroscopic strain– and polarization–electric-field responses of the two constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of the volume fractions and macroscopic properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics

The fate of aluminium in (Na,Bi)TiO3-based ionicconductors

The formation of associated defects (e.g.[AlTi–VO]c) upon acceptor doping is commonly seen as a reasonfor trapping of mobile vacancies in perovskite ionic conductors and electromechanical hardening inpiezoelectric perovskites. In order to clarify the presence of associated defects in Al-doped (Na1/2,Bi1/2)TiO3(NBT–Al) and Al-substituted ((Na,K)1/2Bi1/2)TiO3–BiAlO3(NKBT–BA), we employ a combination ofimpedance spectroscopy,27Al NMR spectroscopy, and electronic structure calculations. Our resultsindicate that associated defects between Al0Tiand oxygen vacancies can only be found in case of lowacceptor doping concentrations. This suggests a decreased driving force for defect association at highdoping concentrations as the reason for the non-linear dependence between acceptor concentrationand oxygen ionic conductivity for NBT-based ceramics. Furthermore, the combination of experimentaland theoretical techniques provides clear evidence for the successive occupation of the B-site, the A-site, andfinally the formation of a secondary phase with increasing Al3+content. Altogether, these resultscall for a new evaluation of the interaction between aliovalent dopants and O2�vacancies in acceptor-doped functional oxides, with implications for the design of ionic conductors as well as ferroelectricmaterials

Multilayer lead-free piezoceramic composites: Influence of co-firing on microstructure and electromechanical behavior

In this study lead-free 2-2 and 0-3 ceramic/ceramic composites comprised of the non-ergodic relaxor 0.93(Bi1/2Na1/2)TiO3–0.07BaTiO3 and ergodic relaxor 0.94Bi0.5(Na0.75K0.25)0.5TiO3–0.06BiAlO3 were investigated. The macroscopic electromechanical behavior was characterized as a function of continuent content,revealing an enhancement in the unipolar strain from the multilayer composite structure. Systematic evaluation of the effects of co-sintering on microstructural properties, such as grain size and porosity, revealed potential mechanisms by which the increase in unipolar strain was achieved. In addition, interdiffusion between the constituents was observed, providing evidence for the formation of a functionally graded ceramic by co-sintering. These data are contrasted with highresolution energy dispersive X-ray microanalysis for measurement of chemical composition across the interface of 2-2 ceramics. These findings provide insight into how synthesis routes can be optimized for tailoring the enhancement of electromechanical properties of lead-free electroceramic composite systems

Relaxor-ferroelectric crossover in (Bi1/2K1/2)TiO3: Origin of the spontaneous phase transition and the effect of an applied external field

The temperature evolution of polar order in an A-site complex perovskite (Bi1/2K1/2)TiO3 (BKT) has been investigated by measurements of dielectric permittivity, depolarization current, and stress-stain curves at elevated temperatures. Upon cooling from high temperatures, BKT first enters a relaxor state and then spontaneously transforms into a ferroelectric state. The analyses of temperature and frequency dependence of permittivity have revealed that polar nanoregions of the relaxor phase appear at temperatures higher than 560 ◦C, and also that their freezing at 296 ◦C triggers the spontaneous relaxor-ferroelectric transition.We discuss the key factors determining the development of long-range polar order in A-site complex perovskites through a comparison with the relaxor (Bi1/2Na1/2)TiO3. We also show that application of biasing electric fields and compressive stresses to BKT favors its ferroelectric phase, resulting in a significant shift of the relaxor-ferroelectric transition temperature towards higher temperatures. Based on the obtained results, electric field-temperature and stress-temperature phase diagrams are firstly determined for BKT

Designing properties of (Na1/2Bix)TiO3-based materials through A-site non-stoichiometry

Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on the impact of cation vacancies on electroceramics, especially in (Na1/2Bi1/2)TiO3 (NBT)-based materials. Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and functional properties in the representative ferroelectric (Na1/2Bi1/2)TiO3–SrTiO3 (NBT–ST). Experiments on NBT/ST bilayers and NBT–ST with Bi non-stoichiometry reveal that Sr2+-diffusion is enhanced by up to six orders of magnitude along the grain boundaries in Bi-deficient material as compared to Bi-excess material with values of grain boundary diffusion ∼10−8 cm2 s−1 and ∼10−13 cm2 s−1 in the bulk. This also means a nine orders of magnitude higher diffusion coefficient compared to reports from other Sr-diffusion coefficients in ceramics. Bi-excess leads to the formation of a material with a core–shell microstructure. This results in 38% higher strain and one order of magnitude lower remanent polarization. In contrast, Bi-deficiency leads to a ceramic with a grain size six times larger than in the Bi-excess material and homogeneous distribution of compounds. Thus, the work sheds light on the rich opportunities that A-site stoichiometry offers to tailor NBT-based materials microstructure, transport properties, and electromechanical propertie