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The impact of the differing phase change characteristics on the electrocaloric effect (ECE) and thermophysical properties of Ba(ZrxTi1-x)O3 (BZT) were investigated. It is demonstrated that the ECE reduces strongly with increasing Zr concentration, from 1.25 K to 0.10 K. The strong reduction of ECE is associated with the vanishing of the latent heat as the nature of paraelectric to ferroelectric phase transition changes. Despite the second order nature of the phase transition, an enhanced electrocaloric responsivity, 0.225 × 10−6 Km V-1, occurs at the triple point composition ( x = 0.13), where the stability fields of ferroelectric phases converge. The decreasing ECE and thermophysical properties with increasing Zr concentration result in a more than one order of magnitude reduction in a caloric figure of merit, from ∼3500 J m−1s−1/2 to ∼200 J m−1s−1/2. The caloric figure of merit was used to compare the performance of other representative electrocaloric, magnetocaloric and mechanocaloric materials.

The waste thermal/heat energy harvesting can contribute to more sustainable and efficient energy systems. The high energy harvesting and storage are indispensable with considering ferroelectric/antiferroelectric materials. In this work, Pb0.99Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98O3 (PNZST) is studied for high energy density (harvesting and storage). The Olsen cycle is performed on the materials for direct thermal/waste heat to electrical energy conversion. A large energy harvesting density of 1.0 MJ/m3 per cycle was obtained at temperatures of 303 and 403 K, cycling the electric field between 1 and 9 kV/mm. An estimated high energy harvesting is a result of the polarization change due to the ferroelectric (rhombohedral) to antiferroelectric (tetragonal) phase transition. The high energy storage of 0.9 J/m3 and energy efficiency of 81% were obtained at 403 K under an electric field of 9 kV/mm. The results will enrich our understanding of PNZST materials that offer high-performance energy harvesting and storage-based applications. This work can also be helpful in improving energy harvesting density via phase transition behaviors.

In this study, non-stoichiometric solid solution, Sr1–1.5(x+y)CexPryTiO3 abbreviated as (Sr, Ce, Pr)TiO3 (with 0.05 ≤ x ≤ 0.35 and y = 0.05, sintered in N2/H2 with 99% N2 and 1% H2) were prepared via a conventional solid-state method. By introducing (Ce0.25Pr0.05)3+/4+ on the A-site, the structure symmetry of the ceramic could be reduced, confirming cubic to tetragonal phase transition. Using Rietveld refinement and electron diffraction (TEM), the tetragonal phase was assigned to P4/mmm space group which posses a center of symmetry (centrosymmetric tetragonal unit cell). A small opening of hysteresis loop at higher Ce doping is assumed to be a caused by defect dipoles and conductivity. The dielectric anomalies were observed in all compositions, therefore, we assume that they are not related to structural phase transition but rather to some conductivity processes.

Induced relaxor-like behavior is reported by addition of a sintering additive to the (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 solid solution. The effect of Bi2O3 sinter additive on microstructure is determined. The phase transition behavior is highlighted by dielectric permittivity measurements. The electrocaloric temperature change is directly measured and comparison with literature data is provided on basis of the material related cooling power. Addition of Bi2O3 drastically increases the temperature stability and an ultra-wide temperature range of over 100 K is achieved. The findings path a way to tune electrocaloric materials for optimization of properties for solid-state coolers based on the electrocaloric effect.

Solid-state cooling technology based on the electrocaloric effect is attracting increasing attention as an important alternative to traditional cooling systems in microelectronic and integrated electronics. Lead-free bulk ceramics are considered as one of the promising candidates for middle and large-scale electrocaloric cooling because of their environment-friendliness and large heat absorption capacity. However, the room temperature adiabatic temperature change (ΔT) in lead-free bulk ceramics has long been limited by their relatively low dielectric breakdown strength (Eb < 60 kV cm−1), hindering their practical applications. In this work, we propose to use the tape casting technique as a practicable strategy to enhance the densification and decrease the porosity of lead-free bulk ceramics for achieving a high Eb and a large room temperature ΔT. An ultrahigh room temperature ΔT (1.6 K) was realized in a (Ba0.95Ca0.05)(Ti0.94Sn0.06)O3 (BCTS) bulk ceramic prepared by the tape casting technique, which is 4 times larger than that of the lead-free bulk ceramics with a similar composition prepared by the conventional ceramic preparing approach. More significantly, unlike the other previously reported results, which only show high ΔT in an extremely narrow temperature range, ΔT of the BCTS bulk ceramic increases from 1.6 K to 2.0 K in the temperature range from 300 K to 345 K, which is comparable with that found in the lead-based bulk counterpart. Most importantly, this work opens up a new avenue to explore lead-free bulk ceramics with a large room temperature ΔT for solid-state refrigeration.

The piezoelectric coefficient is a measure to quantify the potential use of a material in energy harvesting and sensor applications. High concentration of free charge carriers in piezoelectric materials can significantly impede the use of generated piezoelectric charge. In this study, undoped semiconducting ZnO single crystals with both Ohmic and Schottky contacts were prepared to quantify the effective piezoelectric response at temperatures from 20 °C to −140 °C and frequencies of mechanical loading from 0.5 Hz to 160 Hz. It was demonstrated that the formation of an electrostatic potential barrier at the metal-semiconductor interface increases the overall resistance, which provides access to unbiased piezoelectric coefficients of ZnO single crystals even at room temperature. These findings were verified using semiconducting ZnO for energy harvesting at room temperature and relatively low loading frequency.

ZnO is considered as one of the most promising semiconductor materials for future applications based on the piezotronic effect. Intense studies on ZnO nanowires had been carried out to understand the modulation of the Schottky barrier height at the metal ZnO interface via piezoelectricity. However, an experimental investigation on bulk ZnO single crystals and a fundamental comparison of the modification of the barrier height determined experimentally and theoretically are still missing. Therefore, an adjustment of the electrostatic potential barrier height at metal-ZnO single crystal interfaces due to stress induced piezoelectric charges was conducted, using both O- and Zn-terminated surfaces. In-situ stress dependent impedance and current-voltage measurements were utilized to extract the electrical properties of the potential barrier and to determine the reduction of the barrier height. The decrease of the interface resistance and increase of the capacitance reveal the presence of stress induced piezoelectric charges. The experimentally evaluated reduction of the barrier height reveals a moderate change of about 9 meV at 70 MPa and supports prior work on metal-ZnO nanowires. This change was found to be in good agreement with theoretical calculations based on the imperfect screening model if a thickness of the interface layer is assumed to be ~2 ��.

Relaxor ferroelectrics have drawn attention for possible applications in solid-state cooling and thermal energy harvesting, owing to their electrothermal energy conversion properties. Here, we have synthesized and characterized the structure-property correlations of a new Sn- and Nb-doped (Ba,Ca)TiO3 relaxor ferroelectric with large pyroelectric and electrocaloric effects over a broad temperature range. We observed two peaks for the temperature-dependent pyroelectric coefficient: (i) -(partial derivative P/partial derivative T) similar to 563 mu C/(m(2) K) at T similar to 270 K and (ii) -(partial derivative P/partial derivative T) similar to 1021 mu C/(m(2) K) at T similar to 320 K. In addition, a broad peak for electrocaloric temperature change is observed near 320 K with a relative cooling power of similar to 17 J/kg. These properties could be correlated to structural changes observed using X-ray diffraction at two different temperature ranges in the material. Analysis of high-energy X-ray scattering and specific heat capacity data revealed a transition from the cubic to tetragonal phase near T-m similar to 320 K, whereas an additional increase in the tetragonality (c/a) of the polar phase is observed below T-s similar to 270 K. CityUCity University of Hong Kong [7200514, 7004967, 9610377]; Danish National Research FoundationDanmarks Grundforskningsfond [DNRF93]; Danish Research Council for Nature and Universe (Danscatt); European UnionEuropean Union (EU) [778072]; Slovenian Research AgencySlovenian Research Agency - Slovenia [P1-0125]; DOE Office of ScienceUnited States Department of Energy (DOE) [DE-AC02-06CH11357] A.P. gratefully acknowledges funding support from CityU (Project Nos. 7200514, 7004967, and 9610377). M.R.V.J. is grateful for the support by the Danish National Research Foundation (DNRF93), and the Danish Research Council for Nature and Universe (Danscatt). N.N. acknowledges the European Union's Horizon 2020 research and innovation program under the Marie Skodowska-Curie grant agreement No 778072 and the Slovenian Research Agency under program P1-0125. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. A.P. gratefully acknowledges technical assistance from Mr. Daniel Yau.

International audience; The effects of 2% Zr introduction in Ba0.8Ca0.2TiO3 (BCT) system on its electrical and electrocaloric properties was investigated. BCT and Ba0.8Ca0.2Zr0.02Ti0.98O3 (BCZT) ceramics synthesized by solid-state processing were crystallized in a pure perovskite phase with a group space P4mm. After Zr insertion, the enhanced dielectric constant was obtained around the Curie temperature (Tc) in BCZT ceramic (εr = 6330 at Tc = 388 K) compared to BCT ceramic (εr = 5080 at Tc = 388.6 K). Moreover, the large-signal piezoelectric coefficient (d∗33) was improved from 270 to 310 pm/V in BCT and BCZT ceramics, respectively, under a moderate electric field of 25 kV/cm. The electrocaloric effect was determined via indirect and direct methods. In the indirect approach, the electrocaloric temperature change (ΔT) was calculated via Maxwell relation, and the measured ferroelectric polarization P (E, T) extracted from the P–E curves recorded at 24 kV/cm. The maximum values of ΔT = 0.68 K and the electrocaloric responsivity ζ = 0.283 K mm/kV obtained at 385 K in BCZT ceramic were found to be higher than those observed in BCT ceramic (ΔT = 0.37 K and ζ = 0.154 K mm/kV at 387 K). In the direct approach, ΔT was measured utilizing a modified high-resolution calorimeter at 14 kV/cm. As the direct method is more sensitive to the latent heat, it provided larger values for smaller applied field, i.e., ΔT = 0.474 and 0.668 K for BCT and BCZT ceramics, respectively. A significant ζ of 0.477 K mm/kV was obtained in BCZT at 385 K and 14 kV/cm that matches the values found in lead-based materials. These results suggest that BCZT lead-free ceramics could have an excellent potential to be used in solid-state refrigeration applications.

The sensitivity toward mechanical stress of barium titanate-based positive temperature coefficient resistor material was investigated by determining the resistance change with application of uniaxial stress from room temperature to 200 °C, which is well above the Curie temperature TC. Using the Landau–Ginsburg–Devonshire theory the resistance increases in the paraelectric state, the negligible impact of stress close to TC and the observed increase in TC with increasing stress could be rationalized. For the ferroelectric state, the stress-related resistance increase was attributed to ferroelasticity, a change in bulk permittivity and interfacial stress inducing a piezoelectric potential. The obtained results are also discussed with respect to recent endeavors to tune properties of potential barriers in piezoelectric semiconductors by mechanical stress.

The impact of charged point defects on the electrocaloric response of morphotropic Pb(Zr,Ti)O3 (PZT) ceramics is investigated using direct electrocaloric and polarization hysteresis measurements. The electrocaloric response determined for undoped, hard‐, and soft‐doped PZT is rationalized by considering the dipolar entropy change over polarization change and hysteresis losses. The highest electrocaloric effect is observed in poled hard PZT with the field applied along the poling direction, which is related to the reduced hysteresis losses caused by internal electric fields associated with defect complexes. The weak hysteretic dependency of polarization as function of the electric field in hard PZT also allows inducing an inverse electrocaloric effect. The experimental results are compared with model calculations that provide a physical interpretation. The results reveal how the electrocaloric response of ferroelectrics can be optimized by defect engineering.

Many ferroelectric materials that can serve as refrigerants in electrocaloric cooling devices have been identified. To compare them, material measures of cooling power are useful. The electrocaloric performance characteristics of ferroelectric materials are determined to a large degree by the nature of the paraelectric to ferroelectric phase change, which influences the magnitude of the electrocaloric effect and the variation with temperature of the thermal conductivity and heat capacity. Here, those properties are compared for three promising ferroelectric materials. The materials chosen for study have differing phase change characteristics ranging from first-order to diffuse/relaxor-like behavior, and include barium titanate and lead magnesium niobate—lead titanate single crystals and polycrystalline barium zirconate-titanate ceramics. Using measured thermophysical property data, and a simple model based on the Newtonian cooling of a thin plate, the dependences of the material cooling power on the dimensionless temperature, characteristic distance, and holding time for heat transfer are determined. Relationships existing between material cooling power and the nature of the phase transition, thermophysical properties, and the electrocaloric behavior are illustrated. Although device-level factors, such as thermal resistances at electrode interfaces are ignored, the results suggest a simple basis of comparison for electrocaloric materials.

The electric field–temperature phase diagrams of three bismuth sodium titanate-based relaxor ferroelectrics are reported, namely 0.94(Na1/2Bi1/2TiO3)–0.06(BaTiO3), 0.80(Na1/2Bi1/2TiO3)–0.20(K1/2Bi1/2TiO3) and 0.75(Na1/2Bi1/2TiO3)–0.25(SrTiO3). Relaxor behavior is demonstrated by temperature-dependent dielectric permittivity measurements in the unpoled and poled states, as well as by the field-induced phase transition into a ferroelectric phase from the relaxor phase. From temperature-dependent thermometry measurements, we identified the threshold electric field to induce the ferroelectric phase and obtained the released latent heat of the phase transition. We determined the nonergodic and ergodic relaxor phase temperature range based on the absence or presence of reversibility of the relaxor to ferroelectric transition. For all three compositions, the electric field–temperature phase diagram was constructed and a critical point was identified. The constructed electric field–temperature phase diagrams are useful to find optimum operational ranges of ferroelectrics and relaxors for electromechanical and electrocaloric applications.

The stability of the electrocaloric effect under electric field cycling is an important consideration in the development of solid-state cooling devices. Here we report measurements carried out on Ba(Zr0.2Ti0.8)O3 ceramics which reveal that the adiabatic temperature change, polarization-electric field hysteresis loops and dielectric permittivity/loss show stable behavior up to 105 cycles. We further demonstrate that the loss in electrocaloric response observed after 105 cycles is associated with the migration of oxygen vacancies. As a result, the electrical properties of the material are changed leading to an increase in leakage current and Joule heating. Reversing the polarity of the electric field after every 105 cycles changes the migration direction of oxygen vacancies, thereby preventing charge accumulation at grain boundaries and electrodes. By doing so, the electrocaloric stability is improved and the adiabatic temperature remains constant even after 106 cycles, much higher than achieved in commercially available barium titanate ceramics.

A study was made of the electrocaloric (EC) effect’s stability in relaxor Pb(Mg1/3Nb2/3)O3 multilayer elements. The sample was subjected to 106 unipolar cycles at an electric field amplitude of 110 kV cm−1. The dielectric and ferroelectric properties of the material change only slightly, while the microstructure does not reveal any detrimental evidence of the cycling. The initially measured EC temperature change of 1.45 K decreases by only 0.01 K upon cycling, exhibiting a fatigue-less behavior. The results justify the choice of relaxor multilayers as the working bodies in EC cooling devices, where the material should withstand numerous electric field cycles with high amplitudes, sometimes exceeding 100 kV cm−1.

When employed appropriately, ferroelectric materials present themselves as one of the most efficient means of waste (thermal/mechanical) energy scavenging. A large conversion potential can be obtained when appropriate materials are combined with high-field actuation (Ericsson cycle). However, waste energy rarely presents itself in an isolated form (heat or vibration). There is also a distinct lack of systems capable of simultaneous thermomechanical energy conversion, especially in the low-frequency range. In this regard a systematic approach to the concept of combined energy harvesting and storage potential of a singular material system is presented. Polarization versus electric field hysteresis loops were gathered as a function of temperature, uniaxial compressive stress, and electric field. Thereafter, a theoretical assessment was made to the effect of the biased and unbiased energy conversion potential of 0.50Ba(Zr0.2Ti0.8)O3– 0.50(Ba0.7Ca0.3)TiO3 bulk lead-free ferroelectric material. Maximum energy conversion potentials of 150 and 210 kJm@3 were obtained for thermal (5 MPa, 24–96 8C) and mechanical cycles (24 8C, 5–160 MPa), respectively. A slightly improved performance of 220 kJm@3 was obtained under simultaneous depolarization, despite performance degradation through individual biasing. However, the energy-storage density improved by 100% (80 kJm@3) and 50% (60 kJm@3), respectively, when operated under elevated stress (60 MPa) and temperature (90 8C). Results are indicative of a singular material system that could be used for combined thermomechanical energy conversion and on-board storage capacity.

The pyroelectric performance of Pb0.99Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98O3 ceramics was quantified by dielectric, polarization, and specific heat capacity measurements. The tunability of pyroelectric properties upon electric field application was studied as a function of temperature. The large pyroelectric coefficient of 0.28 Cm^-2K^-1 was associated with the ferroelectric (FE)–antiferroelectric (AFE) phase transition. The pyroelectric coefficient can be tuned over a broad temperature range of almost 40 K above the zero electric field phase transition. Electrical and thermal measurements were used to estimate various pyroelectric figures of merit. Pyroelectric figures of merit are almost three order of magnitude higher than those previously reported in FE materials. The large pyroelectric properties indicate that the investigated material is a promising candidate for many pyroelectric applications.

The fracture toughness in the lead-free relaxor ferroelectric (1-x)(Na1/2Bi1/2)TiO3-xBaTiO3 was investigated utilizing the surface crack in flexure method. To allow a comprehensive assessment, unpoled and poled samples from the rhombohedral, the tetragonal, and the morphotropic phase regime were considered. It was found that the fracture toughness is up to 23% higher for the poled state. In order to cover the transition from ferroelectric to relaxor phase, the temperature dependence of 0.97(Na1/2Bi1/2)TiO3-0.03BaTiO3 was studied as well. Fracture toughness values of up to 2 MPam1/2 were determined, which are considerably above data for lead zirconate titanate materials. The results are rationalized using a simple transformation toughening-type model in conjunction with investigations into the ferroelastic behavior. The presented model can be applied without fitting parameters but utilizes measurements of the coercive stress and remanent strain as well as the elastic modulus. This article is protected by copyright. All rights reserved.

Electrocaloric device structures have been developed as multilayer ceramics (MLCs) based on fundamental research carried out on PMN-8PT bulk ceramics. Two different MLC structures were prepared with nine layers each and layer thicknesses of 86 μm and 39 μm. The influence of the device design on its properties has been characterized by microstructural, dielectric, ferroelectric, and direct electrocaloric measurement. For direct characterization two different methods, ie temperature reading (thermistor and thermocouple) and heat flow measurement (differential scanning calorimetry), were used. A comparison of results revealed a highly satisfactory agreement between the different methods. This study confirms that MLCs are promising candidates for implementation into energy-efficient electrocaloric cooling systems providing large refrigerant volume and high electrocaloric effect. Due to their micron-sized active layers, they allow for the application of high electric fields under low operation voltages. We measured a maximum electrocaloric temperature change of ΔT=2.67 K under application/withdrawal of an electric field of ΔE=16 kV mm−1, which corresponds to operation voltages below 1.5 kV.

Antiferroelectric materials exhibit electric field-induced phase transitions between antiferroelectric and ferroelectric states, which enable their use in energy storage and other applications. However, the mechanisms of these transitions are insufficiently understood. Here, we considered the electric field-induced phase transition in the lead-free antiferroelectric NaNbO$_3$. Macroscopic measurements of polarization and longitudinal, transverse, and volumetric strain were complemented with simultaneous structural investigations using high-energy x-ray radiation, yielding crystallographic strain and unit cell volume changes. The field-induced behavior can be divided into the structural antiferroelectric–ferroelectric phase transition at about 8 kV/mm and the clearly decoupled polarization switching process at about 12 kV/mm, which is associated with a large increase in polarization and strain. Decoupling of the field-induced phase transition and polarization switching is related to the randomly oriented grains and mechanical stress present at the phase boundary.

Lead-free Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) piezoceramics with Bi2O3 additive were synthesized using solid-state ceramic processing. Various amounts of Bi2O3 (0.05, 0.1, 0.5, and 1.0 mol%) were added after calcination, milled, compacted, and sintered with no compensation at A- or B-sites. Addition of up to 0.5 mol% Bi2O3 was found to greatly enhance the densification and increase the piezoelectric properties, while higher amounts decreased the grain size and induced relaxor-like electrical behavior, obeying the Vogel-Fulcher model. The highest properties were obtained for the BCZT with 0.1 mol% Bi2O3 sintered at 1350 °C: d33 = 325 pC/m, d33* = 509 pm/V (at 3 kV/mm), kp = 0.42, and Pr = 10.4 μC/cm2. The microstructure, phase composition, and local structure were investigated by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The appearance of the A1g vibration mode in the Raman spectra and the shift of diffraction peaks to lower 2θ values indicate the incorporation of Bi3+ into the B-site of the perovskite BCZT structure.

Compositional engineering with a focus on structural phase transitions has been considered as the most important approach for enhancement of the functional properties of ferroelectric materials due to the critical fluctuation of physical properties. Of special interest are electric-field-induced phase transitions, which can terminate in a liquid–vapor-type critical point with a strong enhancement of functional properties. Whereas the critical point in liquid–vapor space considers changes in temperature and pressure, the critical point in this study is placed in electric field–temperature diagrams. In single crystals, temperature and electric field of a critical point are sharply defined and therefore not appealing for practical applications. However, in ceramics, it is demonstrated that the orientational dependence of the critical point leads to a broadened temperature and electric field range. The presence of a diffuse critical point in ceramics provides a conceptually novel approach for the enhancement of functional properties, such as piezoelectric and electrocaloric (EC) responses, as validated here on the example of the 0.75Bi1/2Na1/2TiO3-0.25SrTiO3 lead-free relaxor ferroelectric ceramics. The realization of a broad criticality range will further facilitate the development of the piezoelectric and EC materials and provide an alternative concept to manipulate the functional properties by application of an electric field.

The correlation between dielectric permittivity and electrocaloric (EC) temperature change (ΔTEC) has been investigated in (1 − x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN–100xPT, with x = 0, 0.05, and 0.10) relaxor ferroelectric ceramics. At a given electric field, both peak temperatures, including the temperature of the permittivity peak (Tm) and the temperature of the maximum of the ΔTEC (TEC max) increase with increasing PT content. The peak of the dielectric permittivity is, regardless of the applied electric field, always at a higher temperature than is the TEC max, and the temperature gap between both maxima progressively increases with increasing applied DC bias. This is particularly true above the threshold field, which induces the long-range ordered ferroelectric state. The results, which are explained in terms of the electric field–temperature phase diagram of relaxor systems, thus reveal that Tm can only roughly mark the temperature of the upper boundary of the temperature–electric field window, where the EC responsivity (ΔTEC/ΔE) is the highest.

Electrostatic potential barriers at doped ZnO‐ZnO interfaces can be modified bystress‐induced polarization charges. This concept was enhanced by preparingZnO‐based single crystal‐polycrystal‐single crystal structures by diffusion bond-ing. Increasing time for epitaxial solid‐state transformation results in structureswith a decreasing thickness of residual polycrystalline material in between twowell‐oriented single crystals. Microstructural and electrical analysis quantifies theinfluence of high‐temperature treatment during epitaxial growth on the stress sen-sitivity of the prepared structures. The orientation of the single crystals is definedto maximize the interaction between stress‐induced polarization charges and thepotential barriers at doped ZnO‐ZnO interfaces. With decreasing thickness ofresidual polycrystalline material, the percentage of grain boundaries with favor-ably aligned polarization vectors is increased resulting in a higher stress sensitiv-ity. This effect is compensated by an adverse effect of the high‐temperaturetreatment on the initial potential barrier height. Hence, a maximum in stress sensi-tivity can be observed for intermediate times of epitaxial growth. The preparedstructures close the gap between the varistor piezotronics based on bulk ceramicswith random orientation of the polarization vector and the bicrystal piezotronicswith perfect orientation of the polarization vector, demonstrating the capability ofmicrostructural engineering for varistor‐based piezotronic devices.

The electrocaloric effect in ferroics is considered a powerful solid-state cooling technology. Its potential is enhanced by correlation to the inverse electrocaloric effect and leads into mechanisms of decreasing or increasing dipolar entropy under applied electric field. Nevertheless, the mechanism underlying the increase of the dipolar entropy with applied electric field remains unclear and controversial. This study investigates the electrocaloric response of the antiferroelectric Pb0.99Nb0.02[(Zr0.58Sn0.43)0.92 Ti0.08]0.98O3 in which the critical electric field is low enough to induce the ferroelectric phase over a broad temperature range. Utilizing temperature- and electric-field-dependent dielectric measurements, direct electrocaloric measurements, and in situ transmission electron microscopy, a crossover from conventional to inverse electrocaloric response is demonstrated. The origin of the inverse electrocaloric effect is rationalized by investigating the field-induced phase transition between antiferroelectric and ferroelectric phases. The disappearance of the latent heat at field-induced transition coincides with the crossover of the electrocaloric effect and demonstrates that the overall electrocaloric response is an interplay of different entropy contributions. This opens new opportunities for highly efficient, environmentally friendly cooling devices based on ferroic materials.

The formation of polar nanoregions through solid-solution additions is known to enhance significantly the functional properties of ferroelectric materials. Despite considerable progress in characterizing the microscopic behavior of polar nanoregions (PNR), understanding their real-space atomic structure and dynamics of their formation remains a considerable challenge. Here, using the method of dynamic pair distribution function, we provide direct insights into the role of solid-solution additions towards the stabilization of polar nanoregions in the Pb-free ferroelectric of Ba(Zr,Ti)O3. It is shown that for an optimum level of substitution of Ti by larger Zr ions, the dynamics of atomic displacements for ferroelectric polarization are slowed sufficiently below THz frequencies, which leads to increased local correlation among dipoles within PNRs. The dynamic pair distribution function technique demonstrates a unique capability to obtain insights into locally correlated atomic dynamics in disordered materials, including new Pb-free ferroelectrics, which is necessary to understand and control their functional properties. (Less)

The temperature evolution of polar order in an A-site complex perovskite (Bi1/2K1/2)TiO3 (BKT) has been investigated by measurements of dielectric permittivity, depolarization current, and stress-stain curves at elevated temperatures. Upon cooling from high temperatures, BKT first enters a relaxor state and then spontaneously transforms into a ferroelectric state. The analyses of temperature and frequency dependence of permittivity have revealed that polar nanoregions of the relaxor phase appear at temperatures higher than 560 ◦C, and also that their freezing at 296 ◦C triggers the spontaneous relaxor-ferroelectric transition.We discuss the key factors determining the development of long-range polar order in A-site complex perovskites through a comparison with the relaxor (Bi1/2Na1/2)TiO3. We also show that application of biasing electric fields and compressive stresses to BKT favors its ferroelectric phase, resulting in a significant shift of the relaxor-ferroelectric transition temperature towards higher temperatures. Based on the obtained results, electric field-temperature and stress-temperature phase diagrams are firstly determined for BKT.

The formation and temperature evolution of polar nanoregions (PNRs) in relaxor ferroelectrics is an intriguing issue that is still under debate. Therefore, we present an approach to estimate the volume fraction of PNRs by the example of the relaxor ferroelectric, $(1\ensuremath{-}x)(\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{N}{\mathrm{a}}_{0.5})\mathrm{Ti}{\mathrm{O}}_{3}\ensuremath{-}x\mathrm{BaTi}{\mathrm{O}}_{3}$ (BNT-$x\mathrm{BT}$). A detailed analysis of the Young's modulus, which is highly sensitive to small structural distortions, at temperatures $25{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}l\phantom{\rule{4pt}{0ex}}Tl800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for both poled and unpoled samples, is correlated to the temperature evolution of PNRs by utilizing a composite model. The extracted volume fraction of the PNRs and the increasing Young's modulus above the formerly suggested Burns temperature indicate that the formation of the PNRs does not occur at a defined temperature but rather in a broad temperature range starting around $\ensuremath{\sim}720{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$.

Electrocaloric (EC) materials are prominent candidates for new generations of scalable and green refrigeration devices. While most often the research on EC materials has been focused on achieving high magnitudes of the EC temperature change, little is known about electrical losses and self-heating effects, despite playing a critical role in the cooling performance of these materials. Here, we analyzed the behavior of a set of ceramic materials under EC-device-like electric-field-driving conditions. The EC temperature response was studied focusing on the contributions to the self-heating in three different compositions: relaxor Pb(Mg1/3Nb2/3)O3 and two different (undoped and Nb-doped) rhombohedral ferroelectric Pb(Zr,Ti)O3 compositions. The specific relaxor and ferroelectric nature of the analyzed materials enabled us to separate the different contributions, such as domain switching and electrical conductivity, to their EC responses. We show that besides having a large EC temperature change, low electrical losses, leading to reduced self-heating effects, are another key parameter to be considered in the engineering of materials for future EC cooling devices.

An improved thermodynamic cycle is validated in ferroelectric single crystals, where the cooling effect of an electrocaloric refrigerant is enhanced by applying a reversed electric field. In contrast to the conventional adiabatic heating or cooling by on-off cycles of the external electric field, applying a reversed field is significantly improving the cooling efficiency, since the variation in configurational entropy is increased. By comparing results from computer simulations using Monte Carlo algorithms and experiments using direct electrocaloric measurements, we show that the electrocaloric cooling efficiency can be enhanced by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like $\mathrm{Pb}({\mathrm{Mg}}_{1/3}$/${\mathrm{Nb}}_{2/3}$)${}_{0.71}{\mathrm{Ti}}_{0.29}{\mathrm{O}}_{3}$.

In modern electronic systems and energy conversion, efficient capacitors with large energy densities are needed. Relaxor ferroelectrics show the potential to achieve those requirements. The lead-free relaxor ferroelectric 0.852(Na1/2Bi1/2TiO3)–0.028(BaTiO3)–0.12(K1/2Bi1/2TiO3) is investigated exhibiting the behavior of a wide range of Na1/2Bi1/2TiO3-based relaxor systems. The criticality of this system is analyzed and a full electric field–temperature phase diagram is constructed. The energy storage performance is determined from polarization measurements in a wide temperature range. It is found that energy density and especially the efficiency is largely increased in the vicinity of the critical end point of the relaxor system. The concept of criticality is widely applicable to lead-based and lead-free relaxor ferroelectrics and therefore an important approach to increase energy storage performance in those systems.

Mechanical confinement or restricted degrees of freedom have been explored for its potential to enhance the performance of ferroelectric devices. It presents an easy and reversible method to tune the response for specific applications. However, such studies have been mainly limited to uni- or bi-axial stress. This study investigates the effect of hydrostatic pressure on the ferroelectric behavior of bulk polycrystalline Pb0.99Nb0.02(Zr0.95Ti0.05)0.98O3. Polarization versus electric field hysteresis plots were generated as a function of hydrostatic pressure for a range of operating temperatures (298–398 K). The application of hydrostatic pressure was observed to induce anti-ferroelectric like double hysteresis loops. This in turn enhances the piezoelectric, energy storage, energy harvesting, and electrocaloric effects. The hydrostatic piezoelectric coefficient (dh) was increased from 50 pCN−1 (0 MPa) to ∼900 pC N−1 (265 MPa) and ∼3200 pCN−1 (330 MPa) at 298 K. Energy storage density was observed to improve...

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...

The piezotronic effect gained considerable interest during the last decade. It utilizes the interaction of stress-induced piezoelectric charges and the electronic band structure in piezoelectric semiconductors. This could lead to new applications like strain-triggered transistors or transparent strain/stress sensors. Apart from single Schottky barriers, double Schottky barriers in varistor boundaries in ZnO can be modified extensively by the application of stress. The gauge factors obtained by this method far exceed values for commercial strain sensors. The determination of the underlying physical mechanisms is therefore of utmost importance for applications in strain sensing. In this work, the experimental results of the influence of mechanical stress on the current–voltage characteristics of ZnO-based varistor ceramics are contrasted to simulations. It is verified that a recently introduced simplified physical model based on parallel conductive regions is capable of explaining the stress sensitivity of varistor ceramics and the evolution of the gauge factor as a function of mechanical stress. At electric fields below the breakdown voltage, small percolating pathways through the microstructure determine the current response of the ceramic. This current localization effect becomes even more pronounced when the material is mechanically stressed with an externally applied uniaxial compressive load. Considering application in stress sensors, current localization in the material could possibly lead to local heating effects and degradation of the material.

Application of a negative field on a positively poled ferroelectric sample can enhance the electrocaloric cooling and appears as a promising method to optimize the electrocaloric cycle. Experimental measurements show that the maximal cooling does not appear at the zero-polarization point, but around the shoulder of the P-E loop. This phenomenon cannot be explained by the theory based on the constant total entropy assumption under adiabatic condition. In fact, adiabatic condition does not imply constant total entropy when irreversibility is involved. A direct entropy analysis approach based on work loss is proposed in this work, which takes the entropy contribution of the irreversible process into account. The optimal reversed field determined by this approach agrees with the experimental observations. This study signifies the importance of considering the irreversible process in the electrocaloric cycles.

Electric-field- and stress-induced phase transformations were investigated in polycrystalline 0.5 mol. % Mn-doped (1−x)(Bi1/2Na1/2)TiO3-x(Bi1/2K1/2)TiO3 (x = 0.1, 0.2). To characterize the effect of electric field and stress on the stability of the ferroelectric and relaxor states, polarization- and current density-electric field curves, as well as the stress-strain response as a function of temperature were characterized. Analogous to the observed electrical behavior, the macroscopic mechanical constitutive behavior showed a closed hysteresis at elevated temperatures, indicating a reversible stress-induced relaxor-to-ferroelectric transformation. The electrical and mechanical measurements were used to construct electric field–temperature and stress-temperature phase diagrams, which show similar characteristics. These data show that a mechanical compressive stress, similarly to an electric field, can induce long-range ferroelectric order in a relaxor ferroelectric.