Polarization-electric Field Hysteresis Loops Research Articles

Enhanced energy storage properties in relaxor Pb(Mg1/3Nb2/3)O3-PbTiO3 thin-film capacitors by incorporating buffer layers

Recently, relaxor ferroelectric thin-film capacitors have attracted considerable attention for energy storage applications since their slim-type polarization–electric field hysteresis loops can yield large recoverable energy density (Wrec) and high efficiency (η). In this work, we study the effects of buffer layers on energy storage properties of 0.93Pb(Mg1/3Nb2/3)O3-0.07PbTiO3 (PMN-PT) thin-film capacitors with a 5 nm-thick SrTiO3 (STO) and LaAlO3 (LAO) films. The energy storage properties of Pt/PMN-PT/SrRuO3 (SRO) capacitors are found to be significantly changed by incorporating the STO or LAO buffer layer at the top Pt/PMN-PT interface, while inserting the buffer layer at bottom PMN-PT/SRO interface shows negligible effects on the electrical properties. Specifically, with the STO buffering, the breakdown field is dramatically increased in the Pt/STO/PMN-PT/SRO capacitor due to the existence of an internal field in the STO, which prevents the growth of electrical trees from the bottom SRO to the top Pt electrode, and a large Wrec of ∼48.91 J/cm3, more than three times of that of the PMN-PT capacitor, is achieved. However, buffered by the LAO, the Pt/LAO/PMN-PT/SRO capacitor exhibits a reduced relaxor character, which may be ascribed to a pinning effect of nanodomains associated with the charged LAO/PMN-PT interface. As a result, both Wrec and η are significantly lowered, compared to the non-buffered PMN-PT capacitor. These results provide physical insights into the modulation of relaxor and dielectric behaviors by designing the characteristics of buffer layers, demonstrating a way for enhancing energy storage properties in thin-film capacitors.

High-Temperature Ferroelectric Behavior of Al0.7Sc0.3N.

Currently, there is a lack of nonvolatile memory (NVM) technology that can operate continuously at temperatures > 200 °C. While ferroelectric NVM has previously demonstrated long polarization retention and >1013 read/write cycles at room temperature, the largest hurdle comes at higher temperatures for conventional perovskite ferroelectrics. Here, we demonstrate how AlScN can enable high-temperature (>200 °C) nonvolatile memory. The c-axis textured thin films were prepared via reactive radiofrequency magnetron sputtering onto a highly textured Pt (111) surface. Photolithographically defined Pt top electrodes completed the capacitor stack, which was tested in a high temperature vacuum probe station up to 400 °C. Polarization–electric field hysteresis loops between 23 and 400 °C reveal minimal changes in the remanent polarization values, while the coercive field decreased from 4.3 MV/cm to 2.6 MV/cm. Even at 400 °C, the polarization retention exhibited negligible loss for up to 1000 s, demonstrating promise for potential nonvolatile memory capable of high−temperature operation. Fatigue behavior also showed a moderate dependence on operating temperature, but the mechanisms of degradation require additional study.

Ultrahigh energy storage density and superior discharge power density in a novel antiferroelectric lead hafnate

Dielectric capacitors have been widely applied to pulse charge-discharge systems with medium energy density and high power density. In this work, (Pb1-3x/2Lax)Hf0.96Ti0·04O3 (PLHT) antiferroelectric (AFE) ceramics were synthesized by a solid-state solution. The field-induced AFE to ferroelectric transitions with double polarization-electric field hysteresis loops were observed for all studied components. The energy storage efficiency of PLHT ceramics was significantly improved by doping La3+. The sample of x = 0.05 (PLHT-0.05) exhibits excellent energy storage properties with a record-high recoverable energy storage density of 11.2 J/cm3, and a high energy efficiency of 88.9% achieved at an electric field of 360 kV/cm. The ceramic exhibits superior frequency and thermal stability against external environmental change. Besides, a record high discharge energy density (8.51 J/cm3) with an ultrafast discharge rate (t0.9<211 ns) was obtained under 360 kV/cm at room temperature in PLHT-0.05 ceramic. Underdamped discharge test shows the sample achieves an ultrahigh current density of 2098 A/cm2 and a giant power density of 378 MW/cm3, suggesting a significant advantage compared with the previously reported ceramic systems. The significant improvement implies the PLHT AFE ceramics can provide a realistic solution for high-power energy storage capacitors.

Bimodal polymorphic nanodomains in ferroelectric films for giant energy storage

Enhanced functional responses of ferroelectrics have often been achieved by complex compositional design of a morphotropic or polymorphic phase boundary. Here, we observe a pseudo-linear, ultra-slim polarization-electric field (P-E) hysteresis loop in simple composition, lead-free Ba(Zr,Ti)O3 of up to μm-thick. It features a low remnant polarization and a high maximum polarization after a delayed saturation process with a field-insensitive, thickness-scalable high energy efficiency of ∼90%. This giant energy storage performance is attributed to the self-assembled, bimodal polymorphic nanodomains consisting of two sets of coherent polymorphic nanodomains. The first set of domains has the best-matched, low index {110} interface. They manifest themselves as “entangled nanophases” and dominate in thinner or annealed films. The 2nd set of domains with a high-index {114} interface and an in-plane anisotropy leads to the largest reduction in bulk elastic energy. These domains show as “segregated nanophases” and prevail in as-grown thick films. The presence of multi-polar states in both sets of nanodomains helps reduce the remnant polarization and delay the polarization saturation. Furthermore, the segregated tetragonal phase in-plane nanodomains lead to a larger maximum polarization under a high poling field. Strain engineering of such nanodomain structures provides a promising alternative to chemical compositional design, for the optimization of dielectric thin films used in capacitive energy storage applications.

Ferroelectric Lithography in Single‐Component Organic Enantiomorphic Ferroelectrics

AbstractOrganic ferroelectrics are flexible, lightweight, and bio‐friendly, promising for bio‐harmonized electronic devices, while their ferroelectric lithography remains relatively unexplored. Here, by introducing homochirality and ZE photoisomerization, we obtained a pair of organic enantiomorphic ferroelectrics, di(benzylamino)‐substituted derivatives of muconic acids, the first ferroelectrics in the muconic family. Their ferroelectric and chiral features were confirmed by the polarization–electric field hysteresis loops and circular dichroism spectra, respectively. Piezoresponse force microscopy measurements demonstrate that the desired domain structure can be precisely achieved by applying a local electric field on a predefined pattern in their thin films. Moreover, thermogravimetric analyses reveal that their ferroelectricity can persist up to above 550 K. The precise pattern lithography and excellent thermal stability make them competitive candidates for ferroelectric lithography.

Ferroelectric Lithography in Single-Component Organic Enantiomorphic Ferroelectrics.

Organic ferroelectrics are flexible, lightweight, and bio-friendly, promising for bio-harmonized electronic devices, while their ferroelectric lithography remains relatively unexplored. Here, by introducing homochirality and ZE photoisomerization, we obtained a pair of organic enantiomorphic ferroelectrics, di(benzylamino)-substituted derivatives of muconic acids, the first ferroelectrics in the muconic family. Their ferroelectric and chiral features were confirmed by the polarization-electric field hysteresis loops and circular dichroism spectra, respectively. Piezoresponse force microscopy measurements demonstrate that the desired domain structure can be precisely achieved by applying a local electric field on a predefined pattern in their thin films. Moreover, thermogravimetric analyses reveal that their ferroelectricity can persist up to above 550 K. The precise pattern lithography and excellent thermal stability make them competitive candidates for ferroelectric lithography.

Deferred Polarization Saturation Boosting Superior Energy-Storage Efficiency and Density Simultaneously under Moderate Electric Field in Relaxor Ferroelectrics

High-temperature dielectric Bi0.5Na0.5TiO3 (BNT)-based relaxors near a morphotropic phase boundary are developed with excellent energy storage performance. Random distribution of polar nanoregions induced by composition modulation would disrupt the ferroelectric long-range dipolar alignment and weaken the coupling between the ferroelectric domains, yielding slender and deferred polarization–electric field hysteresis loops with relatively high saturation polarization. The reversible nano-domain orientation and growth in relaxors under a delayed electric field result in negligible remnant polarization and advantageous energy storage properties. Simultaneously, superior recoverable energy storage density and efficiency are gained, significantly surpassing the state-of-the-art dielectric energy storage materials under similar moderate electric fields. Vacancies, defect dipole behavior, and structural evolution that relied on an electric field and temperature are discussed to disclose the underlying mechanism associated with phase transition. Even thermal stability and large electrostrictive strain with low hysteresis are achieved in elevated temperatures. These features demonstrate the promising candidates for dielectric energy-storage application and provide a strategy in designing relaxors.

Ferroelectric properties of ZrO2 films deposited on ITO-coated glass

In this work, the ferroelectric characteristics of ZrO2 thin films grown on ITO-coated glass have been investigated. The ferroelectric nature of the ZrO2 films has been studied by polarization-electric field (P-E) hysteresis loops and found to be optimum for the films processed by rapid thermal annealing at 600 °C. The increase in the annealing temperature improves the ferroelectric properties through the increase of the in-plane strain that causes the formation of the ferroelectric orthorhombic phase. The formation of the orthorhombic phase was confirmed through high-resolution transmission electron microscopy. The effect of the electric field on the polarization switching kinetics of ZrO2 films has been investigated revealing that the switching kinetics follows the nucleation limited switching (NLS) model. The activation fields estimated from the peak values of the polarization currents (im) and the time (tm) at which im occurs are in good agreement with the values obtained from the switching characteristic time of the NLS model. This work paves the way towards the integration of (pseudo)-binary oxide thin films on cheap substrates like glass for the next-generation of non-volatile memories.

Ferroelectric and Dielectric Properties of Strontium Titanate Doped with Barium

Ferroelectric samples Sr1−xBaxTiO3 (BST), where x = 0, 0.2, 0.4, 0.6, 0.8 and 1, were prepared using the tartrate precursor method and annealed at 1200 °C for 2 h. X-ray diffraction, “XRD”, pattern analysis verified the structure phase. The crystallite size of the SrTiO3 phase was calculated to be 83.6 nm, and for the TiO2 phase it was 72.25 nm. The TEM images showed that the crystallites were agglomerated, due to their nanosize nature. The AC resistivity was measured as temperature dependence with different frequencies 1 kHz and 10 kHz. The resistivity was decreased by raising the frequency. The dielectric properties were measured as the temperature dependence at two frequencies, 1 kHz and 10 kHz. The maximum amount of dielectric constant corresponded to the Curie temperature and the transformation from ferroelectric to paraelectric at 1 kHz was sharp at 10 kHz. Polarization–electric field hysteresis loops for BST samples were measured using a Sawer–Tawer modified circuit. It was shown that the polarization decreased with increasing temperature for all samples.

Ferroelectric and magnetic properties in ε-Fe2O3 epitaxial film

The phase stability, ferroelectricity, and magnetism of highly crystalline epitaxial ε-Fe2O3 films deposited on SrTiO3(111) substrates are reported. Temperature-dependent x-ray diffraction measurements revealed that α-Fe2O3 appeared as a secondary phase after samples were annealed up to 1000 °C. A clear saturated and opened polarization–electric field hysteresis loop with the remnant polarization ∼2.6 μC cm−2 and coercive electric field ∼100 kV cm−1 was obtained at room temperature using a conventional ferroelectric measurement technique. The magnetic phase transition was observed at 155 K. Magnetization–magnetic field measurements revealed that magnetic softening via the phase transition accompanied a reduction in the coercive field from 10 to 6.8 kOe, which is smaller than the coercive field reduction previously observed for ε-Fe2O3 nanoparticles.

Wake-up Free Ferroelectric Rhombohedral Phase in Epitaxially Strained ZrO2 Thin Films.

Zirconia- and hafnia-based thin films have attracted tremendous attention in the past decade because of their unexpected ferroelectric behavior at the nanoscale, which enables the downscaling of ferroelectric devices. The present work reports an unprecedented ferroelectric rhombohedral phase of ZrO2 that can be achieved in thin films grown directly on (111)-Nb:SrTiO3 substrates by ion-beam sputtering. Structural and ferroelectric characterizations reveal (111)-oriented ZrO2 films under epitaxial compressive strain exhibiting switchable ferroelectric polarization of about 20.2 μC/cm2 with a coercive field of 1.5 MV/cm. Moreover, the time-dependent polarization reversal characteristics of Nb:SrTiO3/ZrO2/Au film capacitors exhibit typical bell-shaped curve features associated with the ferroelectric domain reversal and agree well with the nucleation limited switching (NLS) model. The polarization-electric field hysteresis loops point to an activation field comparable to the coercive field. Interestingly, the studied films show ferroelectric behavior per se, without the need to apply the wake-up cycle found in the orthorhombic phase of ZrO2. Overall, the rhombohedral ferroelectric ZrO2 films present technological advantages over the previously studied zirconia- and hafnia-based thin films and may be attractive for nanoscale ferroelectric devices.

A Metal-Free Molecular Antiferroelectric Material Showing High Phase Transition Temperatures and Large Electrocaloric Effects.

Antiferroelectric (AFE) materials, featuring an antiparallel alignment of electric dipoles in the adjacent sublattices, are keeping a great promise toward solid-state refrigeration applications on account of their electrocaloric (EC) effects. Although extensive studies have been performed on inorganic oxide counterparts (e.g., PbZrO3 and AgNbO3), metal-free molecular AFE alternatives with the above-room-temperature EC activities are quite scarce but urgently demanded in terms of environmental issues. Herein, we present a new metal-free molecular AFE, cyclohexylmethylammonium bromide (CMB), which exhibits the unusual antiferroelectric-ferroelectric-paraelectric phase transitions around 364 and 368 K upon heating. The phase transition temperatures are much higher than the majority of known molecular AFE materials. The practical utilization level of electric polarization (∼6 μC/cm2) is clearly evidenced by the typical double polarization-electric field hysteresis loops. Strikingly, large positive and negative EC responses with the temperature changes (ΔT) of 4.2 and -3 K are achieved under an electric field of 20 kV/cm. The origin of its antiferroelectricity and EC properties is elucidated by the antipolar reorientation of cations along with displacement of bromine anions, being distinct from the known mechanism of inorganic oxides. Such intriguing AFE behaviors, including large polarization and EC effects, reveal great potentials of CMB for the solid-state refrigeration. This study sheds light on further exploration of new AFE candidates toward environmentally friendly solid-state cooling devices.

Investigation of the wake-up process and time-dependent imprint of Hf0.5Zr0.5O2 film through the direct piezoelectric response

Ferroelectric HfO2-based thin films, which have been attracting a great deal attention because of their potential use in various applications, are known for their unique properties, such as a large time-dependent imprint and wake-up effect, which differentiate them from conventional ferroelectric materials. In this study, direct piezoelectric measurement was employed to investigate the state of polarization during the retention and wake-up process without applying an electric field. The polarization-electric field hysteresis loop of a sputtered Hf0.5Zr0.5O2 (HZO) film with a thickness of 10 nm showed a time-dependent imprint at room temperature during polarization retention, and the internal electric field that generated the imprint gradually increased from 0.05 to 0.6 MV/cm. While a space charge density of more than 1 μC/cm2 is required to form such an internal electric field, it was found that the magnitude of the direct piezoelectric response did not change at all during polarization retention. On the other hand, both the remanent polarization and direct piezoelectric response increased during the wake-up process. Based on the difference in the variation over time of these two characteristics, we concluded that the non-ferroelectric layer exists at the interface between the HZO film and TaN electrode and gradually transitions to ferroelectric phases through the electric field cycle.

Enhanced electrical properties of new lead-free 0.995Bi0.5(Na0.80K0.20)0.5TiO3-0.005LiNbO3 ceramics by (Ba0.98Nd0.02)TiO3 doping

ABSTRACT It is well known that the Bi0.5(Na0.80K0.20)0.5TiO3 or BNKT ceramic possesses quite high piezoelectric coefficient (i.e., d33 ~ 151 pC/N). This research attempted to prepare (1-y)[0.995Bi0.5(Na0.80K0.20)0.5TiO3-0.005LiNbO3]-y(Ba0.98Nd0.02)TiO3 or (1-y)[0.995BNKT-0.005LN]-yBNdT (where y = 0–0.05 mol fraction) lead- free ceramics by conventional sintering method. The optimum sintering condition was found to be 1100°C with a 2 h dwell time with a heating/cooling rate of 5°C/min. The XRD patterns of all samples showed pure perovskite phase. The optimized electrical properties were found in the sample having the composition of y = 0.03 mol fraction. Its dielectric constant was ~1980 with the dielectric loss of 0.0508 at room temperature and ~5987 with dielectric loss of 0.0540 at 268°C. The maximum S max value of 0.25% with d*33 = 491 pm/V, and the piezoelectric coefficient of 258 pC/N were also obtained. The polarization-electric field hysteresis loops of pure and lightly-doped samples indicated a normal ferroelectric behavior. The transition to anti-ferroelectric behavior was observed for the samples with high BNdT content. This showed that the electrical properties of BNKT-LN ceramics could be tuned by small addition of BNdT with the optimum FoM values of 3.80 pm2/N. These lead-free compounds may be suitable for energy harvesting applications.

Enhanced electrocaloric effect at room temperature in Mn2+ doped lead-free (BaSr)TiO3 ceramics via a direct measurement

(Ba1−xSrx)(MnyTi1−y)O3 (BSMT) ceramics with x = 35, 40 mol% and y = 0, 0.1, 0.2, 0.3, 0.4, 0.5 mol% were prepared using a conventional solid-state reaction approach. The dielectric and ferroelectric properties were characterized using impedance analysis and polarization-electric field (P–E) hysteresis loop measurements, respectively. The adiabatic temperature drop was directly measured using a thermocouple when the applied electric field was removed. The results indicate that high permittivity and low dielectric losses were obtained by doping 0.1–0.4 mol% of manganese ions in (BaSr)TiO3 (BST) specimens. A maximum electrocaloric effect (ECE) of 2.75 K in temperature change with electrocaloric strength of 0.55 K·(MV/m)−1 was directly obtained at ∼21 °C and 50 kV/cm in Ba0.6Sr0.4Mn0.001Ti0.999O3 sample, offering a promising ECE material for practical refrigeration devices working at room temperature.

Effect of uniaxial stress on energy harvesting, storage and electrocaloric performance of BZT ceramics

In this work, a systematic approach of waste (thermal/mechanical) energy harvesting and storage potential is studied in Ba0.85Zr0.15TiO3 (BZT) ceramics. The effect of stress on energy storage density (harvesting/storage) and electrocaloric performance is also studied. For this purpose, polarization–electric field hysteresis loops were recorded at various temperatures and uniaxial compressive stress. The Olsen cycle and electro-mechanical cycle are used for direct waste heat or mechanical energy to electrical energy conversion. A thermal energy-harvesting density of 42 kJ/m3 per cycle was obtained when the Olsen cycle was operated between 296–343 K and 0.25–1.5 MV/m. The electro-mechanical cycle-based energy harvesting is estimated as 78 kJ/m3 under the applied stress of 5–160 MPa and the electric field of 0.25–1.5 MV/m. The energy storage density is found as 39 kJ/m3 at zero stress field and 343 K, which increases to 53 kJ/m3 under the biased stress of 80 MPa in a wide operating temperature range of 296–328 K. It is observed that the high energy storage is a result of the reduction of the hysteresis loss. The electrocaloric temperature is found as 0.16 K and 0.18 K under the 0 and 80 MPa stress fields, respectively. Overall, the reported findings will enrich our understanding of the stress effect on BZT materials, which offers high performance for energy harvesting and storage-based applications. Moreover, this work can be also helpful in improving the energy storage density and electrocaloric effect via stress confinement.

Effect of ferroelectric Ba0.8Sr0.2TiO3 on the charge carrier separation of BiOBr at different temperature

The internal electric field formed by spontaneous polarization of ferroelectrics, as a potential driving force to improve the separation efficiency of photo-generated charge carriers, has become one of the research hotspots in photocatalytic field. Here, a series of Ba1-xSrxTiO3 (x = 0.1, 0.15, 0.2, 0.25) nanoparticles with different Curie temperature (TC) synthesized by hydrothermal method were combined with BiOBr to prepare the (1-y)BiOBr/yBa0.8Sr0.2TiO3 composite photocatalyts. The photocatalytic performance of ferroelectric Ba1-xSrxTiO3 (≤TC) is superior to that of paraelectric Ba1-xSrxTiO3 (≥TC). Polarization-electric field hysteresis loops (P-E loops), piezoresponse force microscopy (PFM), electron paramagnetic resonance (EPR) results and photocatalytic degradation experiments at different temperature indicate that the internal electric field of ferroelectric Ba0.8Sr0.2TiO3 can effectively boost charge carrier separation and significantly enhance the photocatalytic performance of BiOBr/Ba0.8Sr0.2TiO3. The photocatalytic degradation efficiency over ferroelectric 0.8BiOBr/0.2Ba0.8Sr0.2TiO3 can be reached to be more than 97% at 35 °C, which is almost 1.5 times and 1.3 times higher than that of individual Ba0.8Sr0.2TiO3 and BiOBr at 55 °C, respectively. This work provides an experimental evidence and a theoretical guidance for the enhanced photocatalytic performance of ferroelectrics and new ideas for the research and development of high efficiency ferroelectric photocatalytic materials.

Enhanced low electric-field energy-storage performance in Ba0.9Sr0.1TiO3-Bi(Mg1/2Ti1/2)O3 ceramics

Perovskite (1-x)Ba0.9Sr0.1TiO3-xBi(Mg1/2Ti1/2)O3 (BST-BiMT-x) ceramics were prepared by sintering the corresponding powders synthesized by combining of solid state reaction method with citrate sol-gel and selfcombustion techniques. Submicron grains morphology, high density and large resistivity were obtained in the BST-BiMT-x ceramics. In addition, the BST-BiMT-0.1 and BST-BiMT-0.075 ceramics exhibit ferroelectric hysteresis loops with slim shape which lead to enhanced energy-storage properties. The energy-storage density of these two ceramics increases almost linearly with increasing the applied electric filed. The energy-storage density and efficiency at 25 kV/cm of the BST-BiMT-0.1 and BST-BiMT-0.075 ceramics sintered at 1200?C are 141.2mJ/cm3 and 79.3%, and 158.1mJ/cm3 and 76.7%, respectively, surpassing many recently reported values for ferroelectric/antiferroelectric ceramics. The enhanced energy-storage density and efficiency under low electric field can be attributed to the slim polarization-electric field hysteresis loops, high density accompanied by submicron grains morphology and pure perovskite structure.

Preferences of polarity and chirality in triglycine sulfate crystals by alanine ghost

Triglycine sulfate (NH2CH2COOH)3⋅H2SO4, (TGS) has attracted much attention because it exhibits excellent ferroelectricity below the Curie temperature. Although TGS is composed of glycine and sulfuric acid, which are achiral molecules, it forms chiral crystals below the Curie temperature. Ferroelectric domain observations and polarization-electric field hysteresis loop measurements showed that TGS polarity become preferred after a small amount of doping with enantiomeric alanine. It has been confirmed that the preferred polarity appears when the internal bias electric field, caused by alanine doping, is larger than the coercive field. Furthermore, X-ray crystal structure analysis has revealed that l-alanine-doped TGS (LATGS) and d-alanine-doped TGS (DATGS) with the nominal amount of alanine above 50 mol% exhibited great preferences for either chirality, although the alanine content in the crystal was too low to be detected; alanine was a “ghost”. Correlation between the polarity and chirality, thus the “absolute polarity”, of LATGS and DATGS were identified, and the preferences induced by the alanine ghost were noted. We have found that the chirality of TGS grown from achiral molecules can be preferred using an extremely simple method; doping with a tiny amount of alanine.

Direct and indirect measurement of large electrocaloric effect in B2O3-ZnO glass modified Ba0.65Sr0.35TiO3 bulk ceramics

Barium strontium titanate (Ba0.65Sr0.35TiO3-BST) added with a 0.5 wt% of B2O3-ZnO glass phase was prepared via a solid-state reaction approach. The lattice structures and morphologies of the ceramics were analyzed using X-ray diffraction and scanning electron microscopy technologies. The dielectric and ferroelectric properties of the glass added BST ceramics were characterized using a precision impedance analyzer and a ferroelectric polarization-electric field (P-E) hysteresis loop tester, respectively. The electrocaloric effect (ECE) was directly measured by a computer controlled thermocouple and high-voltage power supply, and also indirectly calculated using the Maxwell relation and the polarization extracted from the P-E loops as a function of temperature and electric field. The results indicate that when added with 0.5 wt% of B2O3-ZnO glass phase, the breakdown electric field of BST ceramic can be increased, and the adiabatic temperature change ΔT was enhanced from 1.70 K to 2.29 K.