Matias Acosta

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(ZrxTi1-x)O3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO3 and (Bi1/2Na1/2)TiO3-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi1/2Na1/2)TiO3-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.

Strong electrocaloric effect in lead-free 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 ceramics obtained by direct measurements

Solid solutions of (1 − x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of ΔTEC = 0.33 K at ∼65 °C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about ∼50%. The discrepancy is attributed to the relaxor character of this material.

Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ferroelectrics

Analysis of polarization switching dynamics by means of the inhomogeneous field mechanism model allows insight into the microscopic mechanism of reversed polarization domain nucleation. For all chemical compositions studied, two distinct field regions of nucleation are established. In the high-field region, the activation energy barrier is found to be inversely proportional to the local field according to the Merz law. In contrast, the barriers in the low-field region exhibit a linear field dependence with a minimum in the compositional region of phase instability, which can explain the corresponding peak ferroelectric properties.

BaTiO3-based piezoelectrics: Fundamentals, current status, and perspectives

We present a critical review that encompasses the fundamentals and state-of-the-art knowledge of barium titanate-based piezoelectrics. First, the essential crystallography, thermodynamic relations, and concepts necessary to understand piezoelectricity and ferroelectricity in barium titanate are discussed. Strategies to optimize piezoelectric properties through microstructure control and chemical modification are also introduced. Thereafter, we systematically review the synthesis, microstructure, and phase diagrams of barium titanate-based piezoelectrics and provide a detailed compilation of their functional and mechanical properties. The most salient materials treated include the (Ba,Ca)(Zr,Ti)O3, (Ba,Ca)(Sn,Ti)O3, and (Ba,Ca)(Hf,Ti)O3 solid solution systems. The technological relevance of barium titanate-based piezoelectrics is also discussed and some potential market indicators are outlined. Finally, perspectives on productive lines of future research and promising areas for the applications of these ma...

Mechanical constitutive behavior and exceptional blocking force of lead-free BZT-xBCT piezoceramics

The free strain during unipolar electrical activation and the blocking stress are important figures of merit for actuator applications. The lead-free (1 − x)Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) system has been shown to display exceptional unipolar strain at room temperature, making it very attractive as an electroactive material for large displacement, low frequency actuation systems. In this work, the temperature- and composition-dependent blocking stress is measured with the proportional loading method. It was found that BZT-xBCT outperformed Pb(Zr1− x Ti x )O3 and Bi1/2 Na 1/2TiO3–based ceramics for electric fields up to 2 kV/mm.

In situ electric field induced domain evolution in Ba(Zr0.2Ti0.8)O3-0.3(Ba0.7Ca0.3)TiO3 ferroelectrics

In this work, the lead-free Ba(Zr0.2Ti0.8)O3-0.3(Ba0.7Ca0.3)TiO3 piezoelectric ceramic was investigated in situ under an applied electric field by transmission electron microscopy. Significant changes in domain morphology of the studied material have been observed under an applied electric field. During the poling process, the domain configurations disappeared, forming a single-domain state. This multi- to single-domain state transition occurred with the formation of an intermediate nanodomain state. After removing the electric field, domain configurations reappeared. Selected area electron diffraction during electrical poling gave no indication of any structural changes as for example reflection splitting. Rather, a contribution of the extrinsic effect to the piezoelectric response of the Ba(Zr0.2Ti0.8)O3-0.3(Ba0.7Ca0.3)TiO3 was found to be dominant.

Temperature dependent polarization reversal mechanism in 0.94(Bi1/2Na1/2)TiO3-0.06Ba(Zr0.02Ti0.98)O3 relaxor ceramics

The temperature at which the electric field induced long-range ordered ferroelectric state undergoes transition into the short-range ordered relaxor state, TF-R, is commonly defined by the onset of strong dispersion of the dielectric permittivity. However, this combined macroscopic property and structural investigation of the polarization reversal process in the prototypical lead-free relaxor 0.94(Bi1/2Na1/2)TiO3-0.06Ba(Zr0.02Ti0.98)O3 reveals that an applied electric field can trigger depolarization and onset of relaxor-like behavior well below TF-R. The polarization reversal process can as such be described as a combination of (1) ferroelectric domain switching and (2) a reversible phase transition between two polar ferroelectric states mediated by a non-polar relaxor state. Furthermore, the threshold fields of the second, mediated polarization reversal mechanism depend strongly on temperature. These results are concomitant with a continuous ferroelectric to relaxor transition occurring over a broad tem...

Mechanisms of electromechanical response in (1 − x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

Contributions to the piezoelectric response of (1 − x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics are quantified by small signal measurements made as functions of bias field and temperature. The highest fraction of intrinsic contributions is observed far from phase boundaries, of extrinsic contributions around phase boundaries, and of irreversible switching in the orthorhombic phase. The largest piezoelectric response, d33 = 475 ± 85 pC/N, is found near the orthorhombic to tetragonal phase boundary due to both reversible and irreversible switching. A peak in reversible switching above the Curie temperature for all compositions suggests a line of critical points associated with first order phase transitions, indicating that concurrence of triple and tricritical points in the zero-field phase diagram is not the responsible mechanism of enhanced piezoelectricity.

Tailoring ergodicity through selective A-site doping in the Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3 system

The morphotropic phase boundary composition Bi1/2Na1/2TiO3-20 mol. % Bi1/2K1/2TiO3 was chosen as initial material to do selective A-site aliovalent doping replacing Na and K by 1 at. % La, respectively. The materials were studied macroscopically by measuring dielectric and electromechanical properties. The Na-replaced material has a lower freezing temperature Tfr, lower remanent polarization and remanent strain, and thus a higher degree of ergodicity than the K-replaced material. These results are contrasted with local poling experiments and hysteresis loops obtained from piezoresponse force microscopy. The faster relaxation of the tip-induced local polarization and the lower remanent state in bias-on and -off loops confirm the higher degree of ergodicity of the Na-replaced material. The difference in functional properties is attributed to small variations in chemical pressure achieved through selective doping. Raman results support this working hypothesis.

Polarization dynamics variation across the temperature- and composition-driven phase transitions in the lead-free Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 ferroelectrics

The method of thermally stimulated depolarization currents (TSDC) and polarization switching experiments over a large field, time, and temperature regime are used to refine the controversial phase diagram of Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 and comprehend its relation to ferroelectric and piezoelectric properties. TSDC results suggest the existence of three ferroelectric phases for the composition range of 0.30 ≤ x ≤ 0.60, which can be assigned to the rhombohedral (R), presumably orthorhombic (O), and tetragonal (T) symmetries. Spontaneous polarization is maximal all over the entire intermediate phase region, where the activation barrier for polarization switching is small, not just at R-O or O-T boundaries as might be deduced from previous observations.