Direct measurement of electrocaloric effect in lead-free Ba(SnxTi1-x)O3 ceramics

In this study, we report on investigation of the electrocaloric (EC) effect in lead-free Ba(SnxTi1-x)O3 (BSnT) ceramics with compositions in the range of 0.08 ≤ x ≤ 0.15 by the direct measurement method using a differential scanning calorimeter. The maximum EC temperature change, ΔTEC-max = 0.63 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.11 at ∼44 °C around the multiphase coexistence region. We observed that the EC effect also peaks at transitions between ferroelectric phases of different symmetries. Comparison with the results of indirect EC measurements from our previous work shows that the indirect approach provides reasonable estimations of the magnitude of the largest EC temperature changes and EC strength. However, it fails to describe correctly temperature dependences of the EC effect for the compositions showing relaxor-like behaviour (x = 0.14 and 0.15) because of their non-ergodic nature. Our study provides strong evidence supporting that looking for multiphase ferroelectric materials can be very useful to optimize EC performance

The electrocaloric effect in BaTiO3_3 at all three ferroelectric transitions: anisotropy and inverse caloric effects

We study the electrocaloric (EC) effect in bulk BaTiO$_3$ (BTO) using molecular dynamics simulations of a first principles-based effective Hamiltonian, combined with direct measurements of the adiabatic EC temperature change in BTO single crystals. We examine in particular the dependence of the EC effect on the direction of the applied electric field at all three ferroelectric transitions, and we show that the EC response is strongly anisotropic. Most strikingly, an inverse caloric effect, i.e., a temperature increase under field removal, can be observed at both ferroelectric-ferroelectric transitions for certain orientations of the applied field. Using the generalized Clausius-Clapeyron equation, we show that the inverse effect occurs exactly for those cases where the field orientation favors the higher temperature/higher entropy phase. Our simulations show that temperature changes of around 1 K can in principle be obtained at the tetragonal-orthorhombic transition close to room temperature, even for small applied fields, provided that the applied field is strong enough to drive the system across the first order transition line. Our direct EC measurements for BTO single crystals at the cubic-tetragonal and at the tetragonal-orthorhombic transitions are in good qualitative agreement with our theoretical predictions, and in particular confirm the occurrence of an inverse EC effect at the tetragonal-orthorhombic transition for electric fields applied along the [001] pseudo-cubic direction.Comment: 12 pages, 11 figure

Fixed Volume Effect on Polar Properties and Phase Diagrams of Ferroelectric Semi-ellipsoidal Nanoparticles

For advanced applications in modern industry it is very important to reduce the volume of ferroelectric nanoparticles without serious deterioration of their polar properties. In many practically important cases fixed volume (rather than fixed size) corresponds to realistic technological conditions of nanoparticles fabrication. The letter is focused on the theoretical study of the behavior of ferroelectric polarization, paramagnetoelectric coefficient and phase diagrams of semi-ellipsoidal nanoparticles with fixed volume V. Our approach combines the Landau-Ginzburg-Devonshire phenomenology, classical electrostatics and elasticity theory. Our results show that the size effects of the phase diagrams and polarization of semi-ellipsoidal BiFeO3 nanoparticles nontrivially depends on V. These findings provide a path to optimize the polar properties of nanoparticles by controlling their phase diagrams at a fixed volume.Comment: 15 pages, 5 figures, we added the section IV. Paramagnetoelectric (PME) coefficient at fixed volume in this version and changed title and abstract accordingl

State transition and electrocaloric effect of BaZrx_{x}Ti1−x_{1-x}O3_3: simulation and experiment

The electrocaloric effect (ECE) of BaZr$_{x}$Ti$_{1-x}$O$_3$ (BZT) is closely related to the relaxor state transition of the materials. This work presents a systematic study on the ECE and the state transition of the BZT, using a combined canonical and microcanonical Monte Carlo simulations based a lattice-based on a Ginzburg-Landau-type Hamiltonian. For comparison and verification, experimental measurements have been carried on BTO and BZT ($x=0.12$ and $0.2$) samples, including the ECE at various temperatures, domain patterns by Piezoresponse Force Microscopy at room temperature, and the P-E loops at various temperatures. Results show that the dependency of BZT behavior of the Zr-concentration can be classified into three different stages. In the composition range of $0 \leq x \leq 0.2$, ferroelectric domains are visible, but ECE peak drops with increasing Zr-concentration harshly. In the range of $0.3 \leq x \leq 0.7$, relaxor features become prominent, and the decrease of ECE with Zr-concentration is moderate. In the high concentration range of $x \geq 0.8$, the material is almost nonpolar, and there is no ECE peak visible. Results suggest that BZT with certain low range of Zr-concentration around $x=0.12 \sim 0.3$ can be a good candidate with relatively high ECE and simutaneously wide temperature application range at rather low temperature

Ergodicity reflected in macroscopic and microscopic field-dependent behavior of BNT-based relaxors

The effect of heterovalent B-site doping on ergodicity of relaxor ferroelectrics is studied using (1 - y)(0.81Bi(1/2)Na(1/2)TiO(3)-0.19Bi(1/2)K(1/2)TiO(3))-yBiZn(1/2)Ti(1/2)O(3) (BNT-BKT-BZT) with y - {0.02;0.03;0.04} as a model system. Both the large- and small-signal parameters are studied as a function of electric field. The crystal structure is assessed by means of neutron diffraction in the initial state and after exposure to a high electric field. In order to measure ferroelastic domain textures, diffraction patterns of the poled samples are collected as a function of sample rotation angle. Piezoresponse force microscopy (PFM) is employed to probe the microstructure for polar regions at a nanoscopic scale. For low electric fields E < 2 kV.mm(-1), large- and small-signal constitutive behavior do not change with composition. At high electric fields, however, drastic differences are observed due to a field-induced phase transition into a long-range ordered state. It is hypothesized that increasing BZT content decreases the degree of non-ergodicity; thus, the formation of long-range order is impeded. It is suggested that frozen and dynamic polar nano regions exist to a different degree, depending on the BZT content. This image is supported by PFM measurements. Moreover, PFM measurements suggest that the relaxation mechanism after removal of the bias field is influenced by surface chargesopen2

Weak ferromagnetism and short range polar order in NaMnF 3 thin films

The orthorhombically distorted perovskite NaMnF3 has been predicted to become ferroelectric if an a = c distortion of the bulk Pnma structure is imposed. In order to test this prediction, NaMnF3 thin films were grown on SrTiO3 (001) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the Néel temperature TN = 66 K. For the dielectric studies, NaMnF3 films were grown on a 30 nm SrRuO3 (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics

A piezoresponse force microscopy study of CaxBa1-xNb2O6 single crystals

Polar structures of CaxBa1-xNb2O6 (CBN100x) single crystals were investigated using piezoresponse force microscopy. Increasing Ca content results in decreasing domain size and enhancement of the polar disorder. For the composition with x = 0.32 the characteristic domain size is similar to that reported for relaxor Sr0.61Ba0.39Nb2O6 (SBN61). However, decay of an artificial macroscopic domain in CBN32 takes place below the macroscopic transition temperature, contrary to SBN61, where random fields stabilize it above the transition temperature. We can conclude that CBN with 0.26 ≤ x ≤ 0.32 does not display classical relaxor behavior and might be considered as a disordered ferroelectric

Spontaneous and induced ferroelectricity in the BiFe1−xScxO3 perovskite ceramics

High-pressure synthesis method allows obtaining single-phase perovskite BiFe1-xScxO3 ceramics in the entire concentration range. As-prepared compositions with x from 0.30 to 0.55 have the antipolar orthorhombic Pnma structure but can be irreversible converted into the polar rhombohedral R3c or the polar orthorhombic Ima2 phase via annealing at ambient pressure. Microstructure defects and large conductivity of the high-pressure-synthesized ceramics make it difficult to study and even verify their ferroelectric properties. These obstacles can be overcome using piezoresponse force microscopy (PFM) addressing ferroelectric behavior inside single grains. Herein, the PFM study of the BiFe1-xScxO3 ceramics (0.30 ≤ x ≤ 0.50) is reported. The annealed samples show a strong PFM contrast. Switching of domain polarity by an electric field confirms the ferroelectric nature of these samples. The as-prepared BiFe0.5Sc0.5O3 ceramics demonstrate no piezoresponse in accordance with the antipolar character of the Pnma phase. However, application of a strong enough electric field induces irreversible transition to the ferroelectric state. The as-prepared BiFe0.7Sc0.3O3 ceramics show coexistence of ferroelectric and antiferroelectric grains without poling. It is assumed that mechanical stress caused by the sample polishing can be also a driving force of phase transformation in these materials alongside temperature and external electric field.publishe

Weakly-coupled barium titanate stannate-based relaxors as energy storage materials

Lead-free relaxor ferroelectrics are promising materials for energy storage applications. To find new material systems for electrostatic capacitors, many researches have dedicated their work to formulate new ecologically safe compositions. In our study, we have prepared (1−x)Ba(Ti0.93Sn0.07)O3−xBiYO3 (BTS7-BY) solid solutions via the solid-state method. The parent composition, Ba(Ti0.93Sn0.07)O3 is a ferroelectric-relaxor crossover material. It reduces its remanent polarization while still maintaining a high maximum polarization. Moderate substitution with BiYO3 further lowers the remanent polarization, resulting in slimmer polarization loops and increasing the energy storage efficiency to 90 %. Samples with x ≥ 0.06 show a true relaxor behavior with linear-like polarization loops with character of weakly-coupled relaxors. Breakdown strength measurements reveal a reduced breakdown strength with increasing BY content. Further investigations of the electrostriction, the band gap, and the electrical microstructure link the homogeneity and grain size of the microstructure to the resulting breakdown strength. This highlights the importance of a homogenous and small-grained microstructure for enhancing the energy storage properties. Samples with x = 0.06 exhibit the best energy storage properties with a recoverable energy density of 1.11 J/cm3 at 189 kV/cm with an energy storage efficiency of 74%.publishe