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

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

On the Influence of Ferroelectric Polarization States on the Magneto-electric Coupling in Two-phase Composites

Of particular attention in a variety of novel technical applications is the coupling between magnetic and electric field quantities. Materials that show magneto-electric (ME) coupling could enable new smart devices in the area of electric-field-controlled magnetic-data storage or highly sensitive magnetic-field sensors. In general, ME materials exhibit both a spontaneous magnetization and a spontaneous polarization. In this respect, they feature two ferroic states at the same time and are thus termed magneto-electric multiferroics. However, all natural and most of the synthesized ME multiferroics do not show an interaction between magnetization and electric polarization in the technically relevant temperature range. Thus, there is need for alternative realizations for ME coupling materials. A promising idea lies in the design and manufacturing of ME composites. These materials consist of a magnetostrictive and a piezoelectric phase and generate the ME coupling as a strain-induced product property. Since there exists a wealth of stable magnetostrictive and piezoelectric materials at ambient temperature, such composites yield the desired ME coupling also in a technically useful temperature range. In any case, the effective ME coupling is driven by microscopic interactions between the individual phases and thus highly depends on the microstructure of the composite. This calls for powerful homogenization methods that are able to predict the effective coupling for arbitrary microstructural morphologies. Motivated by that, we apply a two-scale computational homogenization framework for magneto-electro-mechanically coupled boundary value problems, which allows us to analyze the ME composite structures and calculate the effective ME-coefficient. Furthermore, by using a non-linear ferroelectric material model on the micro-level, we are able to simulate the polarization process of the ferroelectric phase. We show that this has a significant impact on the obtainable ME-coefficient

Sequential piezoresponse force microscopy and the 'small-data' problem

The term big-data in the context of materials science not only stands for the volume, but also for the heterogeneous nature of the characterization data-sets. This is a common problem in combinatorial searches in materials science, as well as chemistry. However, these data-sets may well be 'small' in terms of limited step-size of the measurement variables. Due to this limitation, application of higher-order statistics is not effective, and the choice of a suitable unsupervised learning method is restricted to those utilizing lower-order statistics. As an interesting case study, we present here variable magnetic-field Piezoresponse Force Microscopy (PFM) study of composite multiferroics, where due to experimental limitations the magnetic field dependence of piezoresponse is registered with a coarse step-size. An efficient extraction of this dependence, which corresponds to the local magnetoelectric effect, forms the central problem of this work. We evaluate the performance of Principal Component Analysis (PCA) as a simple unsupervised learning technique, by pre-labeling possible patterns in the data using Density Based Clustering (DBSCAN). Based on this combinational analysis, we highlight how PCA using non-central second-moment can be useful in such cases for extracting information about the local material response and the corresponding spatial distribution

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

Simultaneous direct measurement of the electrocaloric and dielectric dynamics of ferroelectrics with microsecond temporal resolution

A contactless technique for direct time-resolved measurements of the full dynamics of the adiabatic temperature change in electrocaloric materials is introduced. The infrared radiation emitted by the electrocaloric sample is sensitively detected with $\mu$s time resolution and mK temperature resolution. We present time-resolved measurements of the electrocaloric effect up to kHz frequencies of the driving electric field and down to small field strengths. The simultaneous recording of transients for applied electric field and induced polarization gives a comprehensive view on the correlation of electrocaloric and ferroelectric properties. The technique can further be applied to the continuous measurement of fatigue for $> 10^6$ electric field cycles.Comment: 12 pages, 11 figure

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

Temperature Effect on the Stability of the Polarized State Created by Local Electric Fields in Strontium Barium Niobate Single Crystals

The stability of ferroelectric domain patterns at the nanoscale has been a topic of much interest for many years. We investigated the relaxation of the polarized state created by application of a local electric field using a conductive tip of a scanning probe microscope for the model uniaxial relaxor system SrxBa1−xNb2O6 (SBN) in its pure and Ce-doped form. The temporal relaxation of the induced PFM contrast was measured at various temperatures. The average value of the induced contrast decreases during heating for all investigated crystals. Below the freezing temperature the induced state remains stable after an initial relaxation. Above the freezing temperature the induced state is unstable and gradually decays with time. The stability of the induced state is strongly affected by the measuring conditions, so continuous scanning results in a faster decay of the poled domain. The obtained effects are attributed to a decrease of the induced polarization and backswitching of the polarized area under the action of the depolarization field