Resolving structural contributions to the electric-field-induced strain in lead-free (1 - X)Ba(Zr<inf>0.2</inf>Ti<inf>0.8</inf>)O<inf>3</inf> - x(Ba <inf>0.7</inf>Ca<inf>0.3</inf>)TiO<inf>3</inf> piezoceramics

The large signal macroscopic strain response during an applied bipolar electric field is calculated from field-dependent in situ XRD data using expressions for, first, a lattice strain contribution and, second, a ferroelastic strain contribution. The lattice strain contribution is estimated using a weighted average of the lattice strains for the observed reflections along the field direction. The ferroelastic strain contribution is calculated by integrating the lattice parameter changes weighted with the ferroelastic domain distribution over all orientations relative to the direction of the applied field. Structural parameters are determined by means of both single peak fitting and Rietveld refinements. A large ferroelastic contribution is found for tetragonal (1 - x)Ba(Zr0.2Ti0.8)O3 - x(Ba 0.7Ca0.3)TiO3 materials that appears to be the dominant origin for the large signal macroscopic strain. The strong changes in lattice parameters and the decrease in tetragonality as a function of orientation and electric field also indicate a large influence of microstructure constraints on the macroscopic strain response. The total strain calculated from X-ray diffraction using both methods is in good agreement with macroscopic strain measurements. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Enhanced extrinsic domain switching strain in core-shell structured BaTiO<inf>3</inf>-KNbO<inf>3</inf> ceramics

Abstract Large electric-field-induced strain in piezoelectric ceramics is a primary requirement for their actuator applications. This macroscopic strain is generated from both intrinsic lattice strain and extrinsic domain switching and/or phase transformations. Among these contributions, non-180°ferroelectric domain switching can generate a large electric-field-induced strain due to the change in orientation of the coupled spontaneous strain. However, the large fraction of non-180°ferroelectric domain switching is a one-time effect during electrical poling. Here, we show that electric-field-induced non-180°ferroelectric domain switching in the microstructurally engineered material BaTiO3-KNbO3 (BT-KN) is largely reversible. In situ high energy X-ray diffraction showed approximately 95% reversibility in the switched fraction of non-180°ferroelectric domains during unipolar cycling. This reversibility is hypothesised to be due to the unique grain boundary structure of this material, where ferroelectric domain walls do not interact strongly with grain boundary defects. The domain switching behaviour of core-shell BT-KN has been contrasted with that of polycrystalline BaTiO3 and commercial lead zirconate titanate Pb(Zr,Ti)O3. The large and reversible non-180°ferroelectric domain switching of core-shell BT-KN offers a distinctive strain response. The results indicate a unique family of large strain lead-free materials based on enhanced reversible non-180°ferroelectric domain switching can be developed for future actuator applications

The effect of inter-granular constraints on the response of polycrystalline piezoelectric ceramics at the surface and in the bulk

The electro-mechanical coupling mechanisms in polycrystalline ferroelectric materials, including a soft PbZrxTi1-xO3 (PZT) and lead-free 0.9375(Bi1/2Na1/2)TiO3-0.0625BaTiO3 (BNT-6.25BT), have been studied using a surface sensitive low-energy (12.4 keV) and bulk sensitive high-energy (73 keV) synchrotron X-ray diffraction with in situ electric fields. The results show that for tetragonal PZT at a maximum electric field of 2.8 kV/mm, the electric-field-induced lattice strain (ϵ111) is 20% higher at the surface than in the bulk, and non-180° ferroelectric domain texture (as indicated by the intensity ratio I002/I200) is 16% higher at the surface. In the case of BNT-6.25BT, which is pseudo-cubic up to fields of 2 kV/mm, lattice strains, ϵ111 and ϵ200, are 15% and 20% higher at the surface, while in the mixed tetragonal and rhombohedral phases at 5 kV/mm, the domain texture indicated by the intensity ratio, I 111 / I 11 1 and I002/I200, are 12% and 10% higher at the surface than in the bulk, respectively. The observed difference in the strain contributions between the surface and bulk is suggested to result from the fact that surface grains are not constrained in three dimensions, and consequently, domain reorientation and lattice expansion in surface grains are promoted. It is suggested that the magnitude of property difference between the surface and bulk is higher for the PZT than for BNT-6.25BT due to the level of anisotropy in the strain mechanism. The comparison of the results from different methods demonstrates that the intergranular constraints have a significant influence on the electric-field-induced electro-mechanical responses in polycrystalline ferroelectrics. These results have implications for the design of higher performance polycrystalline piezoelectrics

Review of the mechanical and fracture behavior of perovskite lead-free ferroelectrics for actuator applications

There has been considerable progress in the development of large strain lead-free perovskite ferroelectrics over the past decade. Under certain conditions, the electromechanical properties of some compositions now match or even surpass commercially available lead-containing materials over a wide temperature range, making them potentially attractive for non-resonant displacement applications. However, the phenomena responsible for the large unipolar strains and piezoelectric responses can be markedly different to classical ferroelectrics such as Pb(Zr,Ti)O3 and BaTiO3. Despite the promising electromechanical properties, there is little understanding of the mechanical properties and fracture behavior, which is crucial for their implementation into applications where they will be exposed to large electrical, mechanical, and thermal fields. This work discusses and reviews the current understanding of the mechanical behavior of large-strain perovskite lead-free ferroelectrics for use in actuators and provides recommendations for further work in this important field

Composition dependence of electric-field-induced structure of Bi<inf>1/2</inf>(Na<inf>1-</inf><inf>x</inf>K<inf>x</inf>)<inf>1/2</inf>TiO<inf>3</inf> lead-free piezoelectric ceramics

Microscopic origins of the electric-field-induced strain for three compositions of Bi1/2(Na1-xKx)1/2TiO3 (x = 0.14, 0.18, and 0.22) (BNKT100x) ceramics have been compared using in situ high-energy (87.12 keV) X-ray diffraction. In the as-processed state, average crystallographic structure of BNKT14 and BNKT18 were found to be of rhombohedral symmetry, while BNKT22 was tetragonal. Diffraction data collected under electric field showed that both the BNKT14 and BNKT18 exhibit induced lattice strain and non-180° ferroelectric domain switching without any apparent phase transformation. The BNKT22 composition, in addition to the lattice strain and domain switching, showed an electric-field-induced transformation from a tetragonal to mixed tetragonal-rhombohedral state. Despite the difference in the origin of microscopic strain responses in these compositions, the measured macroscopic poling strains of 0.46% (BNKT14), 0.43% (BNKT18), and 0.44% (BNKT22) are similar. In addition, the application of a second poling field of opposite polarity to the first increased the magnitude of non-180° ferroelectric domain texture. This was suggested to be related to the existence of an asymmetric internal bias field

Electric-Field-Induced Domain Switching and Domain Texture Relaxations in Bulk Bismuth Ferrite

Bismuth ferrite, BiFeO3, is an important multiferroic material that has attracted remarkable attention for potential applications in functional devices. While thin films of BiFeO3 are attractive for applications in nanoelectronics, bulk polycrystalline BiFeO3 has great potential as a lead-free and/or high-temperature actuator material. However, the actuation mechanisms in bulk BiFeO3 are still to be resolved. Here we report the microscopic origin of electric-field-induced strain in bulk BiFeO3 ceramic by means of in situ high-energy X-ray diffraction. Quantification of intrinsic lattice strain and extrinsic domain switching strain from diffraction data showed that the strain response in rhombohedral bulk BiFeO3 is primarily due to non-180 ferroelectric domain switching, with no observable change in the phase symmetry, up to the maximum field used in the study. The origin of strain thus differs from the strain mechanism previously shown in thin film BiFeO3, which gives a similar strain/field ratio as rhombohedral bulk BiFeO3. A strong post-poling relaxation of switched non-180 ferroelectric domains has been observed and hypothesized to be due to intergranular residual stresses with a possible contribution from the conductive nature of the domain walls in BiFeO3 ceramics

A sample cell for in situ electric-field-dependent structural characterization and macroscopic strain measurements

When studying electro-mechanical materials, observing the structural changes during the actuation process is necessary for gaining a complete picture of the structure-property relationship as certain mechanisms may be meta-stable during actuation. In situ diffraction methods offer a powerful and direct means of quantifying the structural contributions to the macroscopic strain of these materials. Here, a sample cell is demonstrated capable of measuring the structural variations of electro-mechanical materials under applied electric potentials up to 10?kV. The cell is designed for use with X-ray scattering techniques in reflection geometry, while simultaneously collecting macroscopic strain data using a linear displacement sensor. The results show that the macroscopic strain measured using the cell can be directly correlated with the microscopic response of the material obtained from diffraction data. The capabilities of the cell have been successfully demonstrated at the Powder Diffraction beamline of the Australian Synchrotron and the potential implementation of this cell with laboratory X-ray diffraction instrumentation is also discussed.A sample cell for in situ electric-field-dependent structural characterization and macroscopic strain measurements is demonstrated. The results show that the macroscopic strain measured using the cell can be directly correlated with the microscopic response of the material obtained from diffraction data