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© 2017 The American Ceramic Society Surface chemistry and topo-physical properties determine the interactions of biomaterials with their physiological environment. Ferroelectrics hold great promise as the next generation of scaffolds for tissue repair since they feature tunable surface electrical charges, piezoelectricity, and sensing capabilities. We investigate the topography, wettability, chemical stability, and cytotoxicity in salient ferroelectric systems such as (1−x) (Na1/2Bi1/2)TiO3–xBaTiO3, (1−x)Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3, and Pb(Zr,Ti)O3 to test their suitability as biomaterials. The lead-free ferroelectrics promote in vitro cell viability and proliferation to a considerably high extent. 0.94 mol % (Na1/2Bi1/2)TiO3–0.06 mol% BaTiO3 showed the greatest potential leading to a cell viability of (149 ± 30)% and DNA synthesis of (299 ± 85)% in comparison to the reference. Lead leaching from Pb(Zr,Ti)O3 negatively affected the cultured cells. Wettability and chemical stability are key factors that determine the cytotoxicity of ferroelectrics. These variables have to be considered in the design of novel electroactive scaffolds based on ferroelectric ceramics.

In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, ΔTdirect = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase.

The influence of B-site disorder on the dielectric, microstructural, and structural characteristics of unpoled, lead-free (Bi1/2Na1/2)TiO3-0.06Ba(ZrxTi1-x)O3 piezoelectric ceramics with x = 0.02, 0.10, and 0.15 was investigated. The low and medium doping level introduced a stabilization of polar nanoregions reflected in the shift of the dispersive permittivity anomalies to higher temperatures and the development of lamellar rhombohedral domains embedded in the prevalent tetragonal nanodomain matrix. For higher Zr level, the regions of lamellar domains remain, but the dielectric characteristics indicate a reduction in the previous stabilization effect. This behavior is rationalized by a reduction in the correlation length due to the increasing amount of nonpolar sample volume with increasing Zr addition.

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 core–shell ceramics may gain similar interest. Generalities of core–shell functional ceramics features are addressed in this work. A model system, Bi1/2Na1/2TiO3–SrTiO3, 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.

The dielectric and electromechanical properties of 0.75Bi1/2Na1/2TiO3–0.25 SrTiO3 (25ST) as a function of temperature and frequency were studied. It is shown that the 25ST is a relaxor ferroelectric as evidenced by the temperature-dependent dielectric relaxations with an incipient piezoelectricity featured by the presence of a reversible electric-field-induced phase transformation at room temperature. The transition occurs on a broad electric field strength range depending on field amplitude and frequency. It is also accompanied by a huge strain that is attributed to repetitive poling and depoling originating due to the reversibility of the phase transition. The 25ST makes an attractive lead-free candidate for stack actuators as it presents a high normalized d33* of ~600 pm/V at a low electric field of 4 kV/mm for frequencies ranging from 0.1 up to 100 Hz.

This article details the influence of zirconium doping on the piezoelectric properties and relaxor characteristics of 94(Bi1/2Na1/2)TiO3–6Ba(ZrxTi1−x)O3 (BNT–6BZT) bulk ceramics. Neutron diffraction measurements of BNT–6BZT doped with 0%–15% Zr revealed an electric-field-induced transition of the average crystal structure from pseudo-cubic to rhombohedral/tetragonal symmetries across the entire compositional range. The addition of Zr up to 10% stabilizes this transition, resulting in saturated polarization hysteresis loops with a maximum polarization of 40 μC/cm2 at 5.5 kV/mm, while corresponding strain hysteresis measurements yield a maximum strain of 0.3%. With further Zr addition, the ferroelectric order is progressively destabilized and typical relaxor characteristics such as double peaks in the current density loops are observed. In the strain hysteresis, this destabilization leads to an increase of the maximum strain by 0.05%. These changes to the physical behavior caused by Zr addition are consistent with a reduction of the transition temperature TF-R, above which the field-induced transformation from the relaxor to ferroelectric state becomes reversible.