Hana Uršič

Influence of the critical point on the electrocaloric response of relaxor ferroelectrics

The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model.

Bulk relaxor ferroelectric ceramics as a working body for an electrocaloric cooling device

The electrocaloric effect (ECE), i.e., the conversion of the electric into the thermal energy has recently become of great importance for development of a new generation of cooling technologies. Here, we explore utilization of [Pb(Mg1∕3Nb2∕3)O3]0.9[PbTiO3]0.1 (PMN-10PT) relaxor ceramics as active elements of the heat regenerator in an ECE cooling device. We show that the PMN-10PT relaxor ceramic exhibits a relatively large electrocaloric change of temperature ΔTEC > 1 K at room temperature. The experimental testing of the cooling device demonstrates the efficient regeneration and establishment of the temperature span between the hot and the cold sides of the regenerator, exceeding several times the ΔTEC within a single PMN-10PT ceramic plate.

Dielectric, ferroelectric, piezoelectric, and electrostrictive properties of K0.5Na0.5NbO3 single crystals

The dielectric, ferroelectric, piezoelectric, and electrostrictive properties of K0.5Na0.5NbO3 single crystals (KNN s.c.) prepared by solid-state crystal growth are reported. The dielectric constant (e), dielectric losses (tan δ), remanent polarization (Pr), and coercive field (Ec) for KNN s.c. in the [13¯1] direction at room temperature are 1015, 1%, 17 μC/cm2, and 24 kV/cm, respectively. The influence of 180° domains to the linear piezoelectric response and quadratic electrostrictive response of KNN s.c. is discussed. The piezoelectric coefficient d33 and the electrostrictive coefficient M33 of KNN s.c. measured using atomic force microscopy at 2 Hz was 80 pm/V and 2.59×10−14 m2/V2, respectively. The extremely high M33 value can be explained by the extrinsic strain from the domain-wall motion. The properties of the surrounding polycrystalline KNN ceramic are added for comparison.

Domain-wall conduction in ferroelectric BiFeO3 controlled by accumulation of charged defects

Mobile charged defects, accumulated in the domain-wall region to screen polarization charges, have been proposed as the origin of the electrical conductivity at domain walls in ferroelectric materials. Despite theoretical and experimental efforts, this scenario has not been directly confirmed, leaving a gap in the understanding of the intriguing electrical properties of domain walls. Here, we provide atomic-scale chemical and structural analyses showing the accumulation of charged defects at domain walls in BiFeO3. The defects were identified as Fe4+ cations and bismuth vacancies, revealing p-type hopping conduction at domain walls caused by the presence of electron holes associated with Fe4+. In agreement with the p-type behaviour, we further show that the local domain-wall conductivity can be tailored by controlling the atmosphere during high-temperature annealing. This work has possible implications for engineering local conductivity in ferroelectrics and for devices based on domain walls.

Direct Measurements of the Giant Electrocaloric Effect in Soft and Solid Ferroelectric Materials

The giant electrocaloric (EC) effect is of great importance for application in cooling or heating devices of new generation, which would be friendlier for environment. Recent predictions of the existence of the giant EC effect in polymeric and inorganic ferroelectric relaxors are based solely on the indirect measurements of the electric polarization and not on a direct measurement of the EC effect itself. Here a method and analysis of direct measurements of the giant EC effect in various soft and solid ferroelectric materials in the form of thick and thin films is presented. The field dependence of the EC effect is shown for PbMg1/3Nb2/3O3 (PMN) ceramics and P(VDF-TrFE) (68/32 mol%) copolymers.

Direct Measurements of the Electrocaloric Effect in Bulk PbMg1/3Nb2/3O3 (PMN) Ceramics

Electrocaloric effect (ECE), i.e., a reversibly change in the temperature of an electrocaloric material due to the adiabatically applied electric field, is very attractive phenomenon due to its applications, for example in electric refrigeration. The first observations of the giant ECE were based on indirect measurements of the electric polarization and not on direct measurements of the ECE itself. By using direct measurements of ECE we show existence of the giant ECE in PMN ceramics near the electric field-induced ferroelectric phase transition.

Direct Measurements of the Electrocaloric Effect In Substrate-Free PMN-0.35pt Thick Films on a Platinum Layer

The electrocaloric (EC) effect attracted very recently considerable attention due to recent findings of the existence of a giant EC in polymeric and inorganic ferroelectric materials and because of its great potential in development of cooling or heating devices of new generation, which would be more environmentally friendly. A method and analysis of direct measurements of the large EC effect in self-standing [Pb(Mg1/3Nb2/3)O3]1-x[PbTiO3]x (PMN-xPT) with x = 0.35 thick ceramic films on a platinum layer are presented. Besides very large electrostrictive response already reported in these self-standing films, the experimental results confirm the existence of the large EC effect of similar magnitude as found in bulk PMN-0.35PT ceramics.

Dual strain mechanisms in a lead-free morphotropic phase boundary ferroelectric

Electromechanical properties such as d33 and strain are significantly enhanced at morphotropic phase boundaries (MPBs) between two or more different crystal structures. Many actuators, sensors and MEMS devices are therefore systems with MPBs, usually between polar phases in lead (Pb)-based ferroelectric ceramics. In the search for Pb-free alternatives, systems with MPBs between polar and non-polar phases have recently been theorized as having great promise. While such an MPB was identified in rare-earth (RE) modified bismuth ferrite (BFO) thin films, synthesis challenges have prevented its realization in ceramics. Overcoming these, we demonstrate a comparable electromechanical response to Pb-based materials at the polar-to-non-polar MPB in Sm modified BFO. This arises from ‘dual’ strain mechanisms: ferroelectric/ferroelastic switching and a previously unreported electric-field induced transition of an anti-polar intermediate phase. We show that intermediate phases play an important role in the macroscopic strain response, and may have potential to enhance electromechanical properties at polar-to-non-polar MPBs.

Temperature dependent piezoelectric response and strain–electric-field hysteresis of rare-earth modified bismuth ferrite ceramics

The rare-earth (RE)-modified bismuth ferrite (BiFeO3 or BFO) family of ferroelectrics have uncomplicated lead-free chemistries and simple perovskite structures. Due to the high Curie transition temperature of the parent BiFeO3 perovskite (∼830 °C), they are promising piezoelectric materials for use at elevated temperatures. However, the influence of the specific RE species on the electromechanical behavior at high temperatures and above the coercive electric-field is not widely reported. Here, structural analysis over multiple length scales using X-ray diffraction, transmission electron microscopy and piezoresponse force microscopy is coupled with a high electric-field cycling study and in situ converse d33 measurements up to 325 °C for three RE–BFO ceramic compositions, Bi0.86Sm0.14FeO3, Bi0.88Gd0.12FeO3 and Bi0.91Dy0.09FeO3. The ceramics exhibit different phase assemblages with varying amounts of polar rhombohedral R3c and intermediate antipolar orthorhombic Pbam phases as a function of the RE species. During electric-field cycling at electric-fields with amplitudes of 160 kV cm−1, peak-to-peak strains of 0.23–0.27% are reached for all three compositions. However, there are qualitative differences in the field-induced strain and electric current behavior as a function of electric-field cycling and the materials exhibit an electrical-history dependent behavior. Bi0.91Dy0.09FeO3 possesses an improved d33 stability as a function of temperature relative to the parent BFO perovskite and the highest depolarization temperature among the three RE–BFO compositions, with a stable d33 of ∼22 pC N−1 up to 325 °C.

Piezoelectric response of BiFeO3 ceramics at elevated temperatures

The high Curie temperature (TC ∼ 825 °C) of BiFeO3 has made this material potentially attractive for the development of high-TC piezoelectric ceramics. Despite significant advances in the search of new BiFeO3-based compositions, the piezoelectric behavior of the parent BiFeO3 at elevated temperatures remains unexplored. We present here a systematic analysis of the converse, longitudinal piezoelectric response of BiFeO3 measured in situ as a function of temperature (25–260 °C), driving-field frequency, and amplitude. Earlier studies performed at room temperature revealed that the frequency and field dependence of the longitudinal response of BiFeO3 is dominated by linear and nonlinear piezoelectric Maxwell-Wagner mechanisms, originating from the presence of local conductive paths along domain walls and grain boundaries within the polycrystalline matrix. This study shows that the same mechanisms are responsible for the distinct temperature dependence of the piezoelectric coefficient and phase angle and thus...