High performance relaxor ferroelectric textured ceramics for electrocaloric refrigeration

Relaxor ferroelectric ceramics have emerged as promising candidates for electrocaloric cooling systems due to their relatively higher heating and cooling capacities. However, simultaneously achieving high temperature changes (ΔT) and a wide operating temperature range remains a significant challenge, limiting their practical applications. This work proposes a synergistic strategy that involves precise compositional tuning of the BaTiO3-xKNbO3 system to customize the rhombohedral-to-cubic phase boundary around room temperature, coupled with engineering grain orientation of the ceramics. Based on this approach, a maximum ΔT of 3.9 K is achieved in <111>c-texture BaTiO3-KNbO3 ceramics, outperforming most environmentally friendly ceramics. Notably, the ΔT variation remains within ±10% across a temperature range of 30 °C to 80 °C, demonstrating a promising material for the design and application of electrocaloric cooling devices. This work provides new insights for the design of ceramics with optimized electrocaloric properties, offering significant potential for improving the efficiency and functionality of next-generation cooling technologies and devices.

Large low-field-driven electrocaloric effect in organic-inorganic hybrid TMCM-CdCl3

Due to environmental-friendliness and high-efficiency, electrocaloric effect (ECE) is widely regarded as a refrigeration technology for tomorrow. Herein, utilizing organic-inorganic hybridization strategy, we achieve the largest low-field-driven ECE and highest directly-measured electrocaloric strength (ECS) via packing sphere-like organic cation (CH3)3NCH2Cl+ (TMCM+) into inorganic one-dimension (1-D) CdCl3 chain framework. Single-crystal X-ray (SC-XRD) diffraction combined with Raman Spectra reveals that the simultaneous order-disorder transition of organic cations and dramatic structure change of inorganic framework are responsible for the large ECE. Moreover, the measured P-E loops and density function theory (DFT) calculations convey that the distinctive electric-field-induced metastable phase and consequential two-step meta-electric transition could lower the transition energy barrier and account for the low driving field. This work shows that the low-symmetry interaction between inorganic framework and organic cations plays a key role in achieving large ECE under low-field, which provides a method for designing high-performance electrocaloric materials via organic-inorganic hybridization.

Self-Restoration of a Wrinkled Hf0.5Zr0.5O2 Ferroelectric Membrane

Ferroelectric oxides are generally prone to brittle deformation, which impedes their applicability in flexible devices. Using a damage-free peel-off process, we successfully synthesized wrinkled 10 nm thick membranes of zirconium-doped hafnium oxide Hf0.5Zr0.5O2 (HZO). We studied their self-restoration dynamics via in situ scanning probe microscopy. Substantial deformations were induced as the tip descended by applying and sustaining a predefined static force at the crest of the wrinkled membrane. The membrane was fully restored to its original wrinkled state within a specific force range, with no observed damage after force removal. The membrane demonstrated self-restoration even after forces exceeding 100 nN, which completely collapsed the wrinkles, highlighting the exceptional flexibility of these freestanding HZO membranes─an uncommon property among functional oxides. Combining phase-field simulations, we observed the emergence of a region exhibiting continuous variation in polarization intensity within the strained area. The formation of this specific domain structure plays a pivotal role in the self-restoration behavior of the freestanding ferroelectric membranes. This self-restoration capability is essential for the long-term stability of flexible electronic devices, such as sensors, energy harvesters, and electronic skins.

Temperature-Insensitive Ferroelectric Polymer Film for Electrocaloric Refrigeration

Electrocaloric (EC) polymer materials are among the most promising candidates used in solid-state refrigeration with potential applications in electronics and human body thermoregulation. Although current EC polymers exhibit notable EC effects, their poor thermal stability, particularly around the die junction temperature (Tj ∼ 100 °C), poses a major barrier to large-scale applications. Here, we developed a P(VDF-HFP)-based polymer composite film by blending a small amount of P(VDF-TrFE-CFE) and dioctyl phthalate (DOP), followed by uniaxial stretching and fixed-end annealing. These processes, transforming the crystalline structure from spherulites to a fiber-like configuration, further increase the crystallinity and reduce the steric hindrance during dipole orientation, thereby enhancing the EC effect and forming a continuous ferroelectric (FE)-paraelectric (PE) phase transition over a broader temperature range. Direct measurements using an infrared camera show that the adiabatic temperature change remains stable (ΔT ∼ 3 K) from 30 to 100 °C. Moreover, the blend film demonstrates exceptional thermal stability, sustaining over 1400 cycles at 70 and 100 °C. In contrast, the pure P(VDF-TrFE-CFE) film fails after just 16 cycles at 70 °C and suffers dielectric breakdown immediately at 100 °C. Accordingly, these findings address critical challenges in the practical application of low-cost EC polymer films for thermoregulation and offer a pathway toward the commercial development of next-generation cooling devices.

Giant electrocaloric effect in high-polar-entropy perovskite oxides

Materials with a high electrocaloric effect (ECE)1,2 tend to favour a disordered yet easily tunable polar structure. Perovskite ferroelectrics3 stand out as ideal candidates owing to their high dielectric responses and reasonable thermal conductivity. The introduction of multielement atomic distortions induces a high-polar-entropy state4 that notably increases the ECE by effectively overcoming the constraints imposed by highly ordered, polar-correlated perovskite structures. Here we developed a lead-free relaxor ferroelectric with strong polar disorder through targeted multielement substitution at both the A and B sites of the perovskite, effectively distorting the lattice structure and inducing a variety of nanoscale polar configurations, polymorphic polar variants and non-polar regions. A combination of these multielement-induced features led to an increased density of interfaces, significantly enhancing the polar entropy. Remarkably, a high ECE for an entropy change of about 15 J kg-1 K-1 under a 10 MV m-1 field is observed for the material across a broad temperature range exceeding 60 °C. The formation of ultrafine, dispersed, multiphase lattice configurations leads to high-polar-entropy ferroelectric oxides with a high ECE and a long lifetime of over 1 million cycles that are suitable for manufacturing multilayer ceramic capacitors for practical electrocaloric refrigeration applications.

Zipper-Like Dynamic Switching of Coordination Bonds Gives a Polar Bimetallic Halide Toward Self-Driven X-Ray Detection

Polar molecular crystals hold a promise for controlling bulk physical properties originated in their unique switchable polarity via structural transformation. However, the mechanisms for switching polarization are mainly limited to displacive and disorder-order phase transitions, which rarely involve the reconstruction of chemical bonds. Here, we have switched and tuned electric polarization in a bimetallic halide, (Neopentylammonium)4AgBiBr8 (1), as verified by light-excited pyroelectric effect. Most notably, its Ag-Br coordination bonds show a zipper-like dynamic switching behavior from the 'locked' to 'unlocked' state, namely, reconstruction of chemical bonds. Coupling with the dynamic ordering of organic cations, this bond-switching transition makes a contribution to switchable polarization of 1. As expected, its polarity creates pyroelectric effect for self-driven X-ray detection with high sensitivity (3.8×103 μC Gy-1 cm-2) and low limit of detection (4.8 nGy s-1). This work on the bond-switching mechanism provides an avenue to design polar molecular candidate for smart optoelectronic devices.

Low field electrocaloric effect at isotropic-ferroelectric nematic phase transition

Electrocaloric effects (ECE) in solid state materials, such as ferroelectric ceramics and ferroelectric polymers, have a great impact in developing cooling systems. Herein, we describe the ECE of a newly synthesized ferroelectric nematic liquid crystal compound at the isotropic-ferroelectric nematic (I-NF) phase transition. While the Joule heat completely suppressed the ECE in a DC field, in an AC field with E < 1.2 V μm-1 and f ≥ 40 Hz, an increase in optical transmittance was observed, which in comparison with a zero-field transmittance versus temperature plot indicated a shift in the transition temperature. These findings implied that one can induce the desired phase transition using an electric field via ECE with an EC responsivity of ∼1.7 × 10-6 km V-1. Notably, the required electric field was two orders of magnitude smaller than the typical fields for other EC materials. EC effects observed under such low fields is a unique property of ferroelectric nematic liquid crystals. Furthermore, the specific EC energy could be increased considerably by reducing the ionic content, thus suppressing the Joule heat.

Enhanced Electrocaloric Effect of PVDF-based Polymer Composite with Surface Modified AlN

Poly(vinylidenefluoride-trifluoroethylene-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)) relaxor ferroelectric polymer exhibits a modest electrocaloric effect (ECE) at a low electric field near room temperature and a low thermal conductivity. The low thermal conductivity causes poor heat transfer when the terpolymer is used as a cooling device, even when the ECE of the polymer is substantial. By incorporating aluminum nitride (AlN) nanoparticles, which possess high thermal conductivity and good electrical insulation properties, into polymer matrices, we can enhance both the thermal conductivity and the ECE. However, weak bonding at the AlN-polymer interface can lead to a reduced breakdown electric field. Therefore, surface modification of AlN nanoparticles is performed to strengthen the chemical bonding between AlN and the terpolymer, thereby increasing the breakdown electric field. Following the addition of surface-modified nanoparticles, the breakdown electric field of the composite films remained around 240 MV/m, which is comparable to that of the pristine polymer film. Furthermore, the ECE and thermal conductivity were improved by 44% and 55%, respectively. We found three factors influencing the ECE, including the interface effect, the change in ferroelectric-paraelectric phase transition behavior, and the Joule heating effect. To assess the interfacial effect on the ECE, composite films with AlN particles of four different sizes were prepared. It was found that as the size of the nanoparticles increased, which resulted in a decreased interface area, the ECE decreased correspondingly. This finding further confirms the significant impact of the interface area in the composites on the ECE.

Thermally Stable Capacitive Energy-Density and Colossal Electrocaloric and Pyroelectric Effects of Sm-Doped Pb(Mg1/3Nb2/3)O3-PbTiO3 Thin Films

Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Sm-PMN-PT) bulk materials have revealed outstanding ferroelectric and piezoelectric properties due to enhanced local structural heterogeneity. In this study, we further explore the potential of Sm-PMN-PT by fabricating epitaxial thin films by pulsed laser deposition, revealing that Sm doping significantly improves the capacitive energy-storage, piezoelectric, electrocaloric, and pyroelectric properties of PMN-PT thin films. These Sm-PMN-PT thin films exhibit fatigue-free performance up to 109 charge-discharge cycles and maintain thermal stability across a wide temperature range from -40 to 200 °C. Notably, the films demonstrate a colossal electrocaloric effect with a temperature change of 59.4 K and a remarkable pyroelectric energy density reaching 40 J cm-3. By using scanning transmission electron microscopy and phase-field modeling, we revealed that these exceptional properties arise from the increased local structural heterogeneity and strong local electric fields along spontaneous polarization directions, facilitating the nucleation of polymorphic nanodomains characterized by a slush-like polar structure. These findings highlight the enormous potential of Sm-PMN-PT films in capacitive energy storage and solid-state electrothermal energy interconversion. Furthermore, this approach holds broad potential for other relaxor ferroelectrics by enabling the manipulation of nanodomain structures, paving the way for developing robust multifunctional materials.

Domain Dynamics Response to Polarization Switching in Relaxor Ferroelectrics

Nanoscale polar regions, or nanodomains (NDs), are crucial for understanding the domain structure and high susceptibility of relaxors. However, unveiling the evolution and function of NDs during polarization switching at the microscopic level is of great challenge. The experimental in situ characterization of NDs under electric-field perturbations, and computational accurate prediction of the dipole switching within a sufficiently large supercell, are notoriously tricky and tedious. These difficulties hinder a full understanding of the link between micro domain dynamics and macro polarization switching. Herein, the real-time evolution of NDs at the nanoscale is observed and visualized during polarization switching in an exemplary relaxor system of Bi5- xLaxMg0.5Ti3.5O15. Two fundamentally different domain switching pathways and dynamic characteristics are revealed: one steep, bipolar-like switching between two degenerate polarization states; and another flat, multi-step switching process with a thermodynamically stable non-polar mesophase mediating the degenerate polarization states. The two are determined by the distinct Landau energy landscapes that are strongly dependent on the intrinsic domain configurations and interdomain interactions. This work bridges the gap between micro domain dynamics and macro polarization switching, providing a guiding principle for the strategic design and optimization of relaxors.

A self-regenerative heat pump based on a dual-functional relaxor ferroelectric polymer

Electrocaloric (EC) cooling presents a promising approach to efficient and compact solid-state heat pumps. However, reported EC coolers have complex architectures and limited cooling temperature lift. In this work, we introduce a self-regenerative heat pump (SRHP) using a cascade of EC polymer film stacks, which have electrostrictive actuations in response to an electric field that are directed to realize efficient heat transfer, eliminating the need for additional transportive or regenerative mechanisms. The SRHP demonstrates a cooling of 8.8 kelvin below ambient temperature in 30 seconds and delivers a maximum specific cooling power of 1.52 watts per gram. The temperature lift of the SRHP is 14.2 kelvin. These results underscore the potential of the compact solid-state cooling mechanism to address the increasing need for localized thermal management.

Large Room-Temperature Electrocaloric Effect in Lead-Free Relaxor Ferroelectric Ceramics with Wide Operation Temperature Range

In order to obtain large room-temperature electrocaloric effect (ECE) and wide operation temperature range simultaneously in lead-free ceramics, we proposed designing a relaxor ferroelectric with a Tm (the temperature at which the maximum dielectric permittivity is achieved) near-room temperature and glass addition. Based on this strategy, we designed and fabricated lead-free 0.76NaNbO3-0.24BaTiO3 (NN-24BT) ceramics with 1wt.% BaO-B2O3-SiO2 glass addition, which showed distinct relaxor ferroelectric characteristics with strongly diffused phase transition and a Tm near-room temperature. Based on a direct measurement method, a large ΔT (adiabatic temperature change) of 1.3 K was obtained at room temperature under a high field of 11.0 kV mm-1. Additionally, large ECE can be maintained (>0.6 K@6.1 kV mm-1) over a broad temperature range from 23 °C to 69 °C. Moreover, the ECE displayed excellent cyclic stability with a variation in ΔT below ±7% within 100 test cycles. The comprehensive ECE performance is significantly better than other lead-free ceramics. Our work provides a general and effective approach to designing lead-free, high-performance ECE ceramics, and the approach possesses the potential to be utilized to improve the ECE performance of other lead-free ferroelectric ceramic systems.

Anomalous electrocaloric behaviors in (anti)ferroelectrics: a mini-review

Solid-state cooling, represented by the electrocaloric effect (ECE) in (anti)ferroelectric materials, has emerged as an alternative green refrigeration technology by virtue of its high efficiency and miniaturization and is expected to substitute conventional vapor-compression. Significant progress has been made in developing high-performance EC materials since its revival. However, anomalous EC behaviors are frequently observed, including asymmetric and negative EC profiles, and the physical mechanism behind this is still under debate. Its rationalization is of great importance since full utilization of anomalous EC behaviors could enhance EC strength and/or cooling capacity. This mini-review gives a brief overview of research advances in EC anomalies in (anti)ferroelectrics with the hope of provoking thought on the design of reconstructed refrigeration cycles and superior EC materials for application in solid-state cooling devices.

Excellent Barocaloric Effect by Modulating Geometrical Frustrations in Mn3Pt

Barocaloric materials hold great promise for next-generation solid-state cooling devices because of their green and efficient cooling performance. The insights into low-pressure-driven barocaloric materials are expected to pave the way for the widespread application of barocaloric refrigeration technology. Here, we reveal the low-pressure-driven large barocaloric effect (BCE) modulated by geometrical frustrations in Mn3Pt. The highest sensitivity to pressure of Mn3Pt in metal BCE materials results in an excellent temperature-change strength of 9.77 K 100-1 MPa-1. Neutron powder diffraction and first-principles calculations point out the dual effect of geometrical frustration on modulating the unusual BCE, which not only induces giant volume expansion by inspiring strong spin fluctuation and magnetic moment but also enhances the sensitivity of magnetic phase transition. The model of the dual effect of geometrical frustration in magnets with geometrical frustration is established, which will promote the research progress of barocaloric refrigeration devices.

Reversible Giant Barocaloric Effect with Broad Working Temperature Range in an Amorphous Ethylene Propylene Diene Monomer

The barocaloric effect of a solid material is an intense research topic due to its potential application in solid-state refrigeration. Among the proposed candidates, elastic polymers are distinctive because their barocaloric responses are independent from a pressure-induced phase transition which makes it possible to realize a broad working temperature range in principle. However, the barocaloric performance of most elastic polymers diminishes significantly as temperature decreases. In this work, giant and reversible barocaloric effects were observed in a broad working temperature range from 252 to 345 K in an amorphous polymer of ethylene propylene diene monomer, which are much higher than the investigated crystalline and partially crystallized ones. It is demonstrated that the degree of crystallinity can be a key factor responsible for the mobility of polymer chains and the corresponding barocaloric performance at low temperatures. The reversible giant barocaloric effects, broad working temperature regions, low cost, and absence of pressure-transmitting fluid make the ethylene propylene diene monomer attractive for solid-state barocaloric refrigeration.

Engineering Multiphase Phase Transitions for Exceptional Electrocaloric Performance and Ultraweak Electrostrictive Response in Ferroelectrics

In the pursuit of eco-friendly alternatives for refrigeration technology, electrocaloric materials have emerged as promising candidates for efficient solid-state refrigeration due to their high efficiency and integrability. However, current advancements in electrocaloric effects (ECEs) are often constrained by high temperatures and elevated electric fields (E-field), limiting practical applicability. Informed by phase-field simulation, this study introduces a (1-x)Pb(Yb1/2Nb1/2)O3-xPb(Mg1/3Nb2/3)O3 system, strategically engineered to incorporate highly ordered YN and disordered MN mixtures. The synergistic interplay between E-field/temperature-induced polarization reorientation and cation shift initiates multiple ferroelectric-antiferroelectric-paraelectric phase transitions. Our results demonstrate that under a moderate E-field of 50 kV cm-1, the x = 0.22 composition achieves remarkable performance with a giant temperature change (ΔT) of 3.48 K, a robust ECE strength (ΔT/ΔE) of 0.095 K cm kV-1, and a wide temperature span (Tspan) of 38 °C. Notably, the disrupted lattice structure contributes to ultralow electrostrains below 0.008%, with an average electrostrictive coefficient Q33 of 0.007 m4 C-2. The significantly weakened electrostrictive activity favors enhancing the performance stability of subsequent devices. This work introduces an innovative strategy for developing robust electrocaloric materials, offering substantial ΔT and low electrostrains, presenting promising advancements in ECE applications with an extended lifetime.

Synergy of Oxygen Octahedra Distortion and Polar Nanodomains Induced Emergent Electrocaloric Effect in NaNbO3-Based Ceramics

High-performance electrocaloric materials are essential for the development of solid-state cooling technologies; however, the contradiction of the electrocaloric effect (ECE) and temperature span in ferroelectrics frustrates practical applications. In this work, through modulating oxygen octahedra distortion and short-range polar nanodomains with moderate coupling strength, an EC value of ΔT ∼ 0.30 K with an ultrawide temperature span of 85 K is obtained in the x = 0.04 composition [(0.88 - x)NaNbO3-0.12BaTiO3-xLiSbO3 (x = 0-0.06)]. The LiSbO3 dopant induces a P4bm-to-R3cH phase transition and intensifies the oxygen octahedra distortion degree, accompanied by the ferroelectric domain smashing into polar nanodomains. Also, LiSbO3 addition enhances the relaxation degree with a downshift of Tfd (ferroelectric-to-diffuse phase transition temperature) and TJ (temperature of the maximal current density value), and Tfd is shifted to near room temperature with an absence of TJ in x = 0.04. Local energy barriers induced by oxygen octahedra distortion inhibit the phase transition in conjunction with activation of short-range polar order switching under thermal stimuli, which is the underlying mechanism for an excellent EC performance for x = 0.04. This work not only clarifies that ferroelectrics with oxygen octahedra distortion and short-range polar order are expected to achieve remarkable EC performances but also provides a design strategy to seek emergent EC behaviors in complex oxygen-octahedra-distortion materials.

Modeling Impact of Regiodefects on the Electrocaloric Effect in Poly(VDF-co-TrFE) Copolymers

Impact of regiodefects in the ferroelectric poly(vinylidene difluoride-co-trifluoroethylene) copolymer [poly(VDF-co-TrFE)] on the electrocaloric effect was studied with use of a developed analytical model and molecular dynamics (MD) simulations. It was shown earlier that the electrocaloric effect in these polymers is caused by the first-order phase transition from the ferroelectric β phase to the paraelectric conformationally disordered (condis) phase. MD simulations performed in the current work show that the presence of regiodefects in polymer chains makes this phase transition more gradual, the second order. The proposed analytical model is based on the Landau phenomenological theory of phase transitions, modified to ensure the correct behavior of polymer polarization at low temperatures. The model calculates the polymer polarization, P, and temperature change under adiabatic electric field variation, ΔT, as functions of temperature, applied electric field, and regiodefect concentration. Parameters of the free energy functional are calibrated through MD simulations of the poly(VDF-co-TrFE) crystal. The obtained results show that presence of regiodefects substantially changes the form of dependencies P(T) and ΔT(T), which significantly shifts them closer to experimental data.

Dependence of Magnetic Properties of As-Prepared Nanocrystalline Ni2MnGa Glass-Coated Microwires on the Geometrical Aspect Ratio

We have prepared NiMnGa glass-coated microwires with different geometrical aspect ratios, ρ = dmetal/Dtotal (dmetal-diameter of metallic nucleus, and Dtotal-total diameter). The structure and magnetic properties are investigated in a wide range of temperatures and magnetic fields. The XRD analysis illustrates stable microstructure in the range of ρ from 0.25 to 0.60. The estimations of average grain size and crystalline phase content evidence a remarkable variation as the ρ-ratio sweeps from 0.25 to 0.60. Thus, the microwires with the lowest aspect ratio, i.e., ρ = 0.25, show the smallest average grain size and the highest crystalline phase content. This change in the microstructural properties correlates with dramatic changes in the magnetic properties. Hence, the sample with the lowest ρ-ratio exhibits an extremely high value of the coercivity, Hc, compared to the value for the sample with the largest ρ-ratio (2989 Oe and 10 Oe, respectively, i.e., almost 300 times higher). In addition, a similar trend is observed for the spontaneous exchange bias phenomena, with an exchange bias field, Hex, of 120 Oe for the sample with ρ = 0.25 compared to a Hex = 12.5 Oe for the sample with ρ = 0.60. However, the thermomagnetic curves (field-cooled-FC and field-heating-FH) show similar magnetic behavior for all the samples. Meanwhile, FC and FH curves measured at low magnetic fields show negative values for ρ = 0.25, whereas positive values are found for the other samples. The obtained results illustrate the substantial effect of the internal stresses on microstructure and magnetic properties, which leads to magnetic hardening of samples with low aspect ratio.

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Self-oscillating polymeric refrigerator with high energy efficiency

Electrocaloric1,2 and electrostrictive3,4 effects concurrently exist in dielectric materials. Combining these two effects could achieve the lightweight, compact localized thermal management that is promised by electrocaloric refrigeration5. Despite a handful of numerical models and schematic presentations6,7, current electrocaloric refrigerators still rely on external accessories to drive the working bodies8-10 and hence result in a low device-level cooling power density and coefficient of performance (COP). Here we report an electrocaloric thin-film device that uses the electro-thermomechanical synergy provided by polymeric ferroelectrics. Under one-time a.c. electric stimulation, the device is thermally and mechanically cycled by the working body itself, resulting in an external-driver-free, self-cycling, soft refrigerator. The prototype offers a directly measured cooling power density of 6.5 W g-1 and a peak COP exceeding 58 under a zero temperature span. Being merely a 30-µm-thick polymer film, the device achieved a COP close to 24 under a 4 K temperature span in an open ambient environment (32% thermodynamic efficiency). Compared with passive cooling, the thin-film refrigerator could immediately induce an additional 17.5 K temperature drop against an electronic chip. The soft, polymeric refrigerator can sense, actuate and pump heat to provide automatic localized thermal management.

Field-Induced Antiferroelectric-Ferroelectric Transformation in Organometallic Perovskite Displaying Giant Negative Electrocaloric Effect

Antiferroelectric materials with an electrocaloric effect (ECE) have been developed as promising candidates for solid-state refrigeration. Despite the great advances in positive ECE, reports on negative ECE remain quite scarce because of its elusive physical mechanism. Here, a giant negative ECE (maximum ΔS ∼ -33.3 J kg-1 K-1 with ΔT ∼ -11.7 K) is demonstrated near room temperature in organometallic perovskite, iBA2EA2Pb3I10 (1, where iBA = isobutylammonium and EA = ethylammonium), which is comparable to the greatest ECE effects reported so far. Moreover, the ECE efficiency ΔS/ΔE (∼1.85 J cm kg-1 K-1 kV-1) and ΔT/ΔE (∼0.65 K cm kV-1) are almost 2 orders of magnitude higher than those of classical inorganic ceramic ferroelectrics and organic polymers, such as BaTiO3, SrBi2Ta2O9, Hf1/2Zr1/2O2, and P(VDF-TrFE). As far as we know, this is the first report on negative ECE in organometallic hybrid perovskite ferroelectric. Our experimental measurement combined with the first-principles calculations reveals that electric field-induced antipolar to polar structural transformation results in a large change in dipolar ordering (from 6.5 to 45 μC/cm2 under the ΔE of 18 kV/cm) that is closely related to the entropy change, which plays a key role in generating such giant negative ECE. This discovery of field-induced negative ECE is unprecedented in organometallic perovskite, which sheds light on the exploration of next-generation refrigeration devices with high cooling efficiency.

High-Curie-Temperature Elastic Polymer Ferroelectric by Carbene Cross-Linking

With the emergence of wearable electronics, ferroelectrics are poised to serve as key components for numerous potential applications. Currently, intrinsically elastic ferroelectrics featuring a network structure through a precise "slight cross-linking" approach have been realized. The resulting elastic ferroelectrics demonstrate a combination of stable ferroelectric properties and remarkable resilience under various strains. However, challenges arose as the cross-linking temperature was too high when integrating ferroelectrics with other functional materials, and the Curie temperature of this elastic ferroelectric was comparatively low. Addressing these challenges, we strategically chose a poly(vinylidene fluoride)-based copolymer with high vinylidene fluoride content to obtain a high Curie temperature while synthesizing a cross-linker with carbene intermediate for high reactivity to reduce the cross-linking temperature. At a relatively low temperature, we successfully fabricated elastic ferroelectrics through carbene cross-linking. The resulting elastic polymer ferroelectrics exhibit a higher Curie temperature and show a stable ferroelectric response under strains up to 50%. These materials hold significant potential for integration into wearable electronics.

Tuning ferroelectric phase transition temperature by enantiomer fraction

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S-3AMP)x(R-3AMP)1-xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 - 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

Low-k nano-dielectrics facilitate electric-field induced phase transition in high-k ferroelectric polymers for sustainable electrocaloric refrigeration

Ferroelectric polymer-based electrocaloric effect may lead to sustainable heat pumps and refrigeration owing to the large electrocaloric-induced entropy changes, flexible, lightweight and zero-global warming potential. Herein, low-k nanodiamonds are served as extrinsic dielectric fillers to fabricate polymeric nanocomposites for electrocaloric refrigeration. As low-k nanofillers are naturally polar-inactive, hence they have been widely applied for consolidate electrical stability in dielectrics. Interestingly, we observe that the nanodiamonds markedly enhances the electrocaloric effect in relaxor ferroelectrics. Compared with their high-k counterparts that have been extensively studied in the field of electrocaloric nanocomposites, the nanodiamonds introduces the highest volumetric electrocaloric enhancement (~23%/vol%). The resulting polymeric nanocomposite exhibits concurrently improved electrocaloric effect (160%), thermal conductivity (175%) and electrical stability (125%), which allow a fluid-solid coupling-based electrocaloric refrigerator to exhibit an improved coefficient of performance from 0.8 to 5.3 (660%) while maintaining high cooling power (over 240 W) at a temperature span of 10 K.

Multifunctional Flexible Ferroelectric Thin Films with Large Electrocaloric Effect and High Energy Storage Performance

Flexible ferroelectric films with high polarization hold great promise for energy storage and electrocaloric (EC) refrigeration. Herein, we fabricate a lead-free Mn-modified 0.75 Bi(Mg0.5Ti0.5)O3-0.25 BaTiO3 (BMT-BTO) thin film based on a flexible mica substrate. Excellent EC performance with maximum adiabatic temperature change (ΔT ∼23.5 K) and isothermal entropy change (ΔS ∼33.1 J K-1 kg-1) is achieved in the flexible BMT-BTO film, which is attributed to the local structural transition and lattice disorder near 90 °C. Meanwhile, a good energy storage density of ∼70.6 J cm-3 and a quite high efficiency of ∼82% are realized in the same ferroelectric film, accompanied by excellent stability of frequency and electric fatigue (500-10 kHz and 108 cycles). Furthermore, there is no apparent variation in performance under different bending strains. These prominent properties indicate that the multifunctional BMT-BTO ferroelectric film is a promising candidate for applications of flexible energy storage and EC refrigeration.

Twistocaloric Modeling of Elastomer Fibers and Experimental Validation

The twistocaloric effect is attributed to the change in entropy of the material driven by torsional stress. It is responsible for the torsional refrigeration of fiber materials that has been widely exploited as one of the solid-state cooling techniques with high efficiency and low volume change rate. The lack of theories and mathematical models of twistocaloric effect, however, limits broad applications of torsional refrigeration. In this work, a twistocaloric model is established to capture the relationship between twist density and temperature variation of natural rubber fibers and thermoplastic elastomer yarns. An experimental setup consisting torsion actuator and torque sensor coupled with a temperature measurement system is built to validate the model. Using the Maxwell relationship, twistocaloric coefficient is measured by quantifying the thermal effect induced by torsion under shear strain. The experimental characterization of the twistocaloric effect in natural rubber fiber and thermoplastic elastomer yarn are consistent with the theoretical predictions.

Colossal electrocaloric effect in an interface-augmented ferroelectric polymer

The electrocaloric effect demands the maximized degree of freedom (DOF) of polar domains and the lowest energy barrier to facilitate the transition of polarization. However, optimization of the DOF and energy barrier-including domain size, crystallinity, multiconformation coexistence, polar correlation, and other factors in bulk ferroelectrics-has reached a limit. We used organic crystal dimethylhexynediol (DMHD) as a three-dimensional sacrificial master to assemble polar conformations at the heterogeneous interface in poly(vinylidene fluoride)-based terpolymer. DMHD was evaporated, and the epitaxy-like process induced an ultrafinely distributed, multiconformation-coexisting polar interface exhibiting a giant conformational entropy. Under a low electric field, the interface-augmented terpolymer had a high entropy change of 100 J/(kg·K). This interface polarization strategy is generally applicable to dielectric capacitors, supercapacitors, and other related applications.

High cooling performance in a double-loop electrocaloric heat pump

Cooling through solid-state electrocaloric materials is an attractive replacement for vapor compression. Despite recent efforts, devices that are potentially commercially competitive have not been developed. We present an electrocaloric cooler with a maximum temperature span of 20.9 kelvin and a maximum cooling power of 4.2 watts under the moderate applied electric field of 10 volts per micrometer without any observed breakdown. Moreover, the maximum coefficient of performance, even taking into account energy expended on fluid pumping, reaches 64% of Carnot's efficiency as long as energy is properly recovered. We believe that this demonstration shows electrocaloric cooling to be a very promising alternative to vapor compression cooling.

A highly efficient solid-state heat pump

The high efficiency of a newly developed electrocaloric device brings theory closer to reality.

Pressure-freezing of dodecane: exploring the crystal structures, formation kinetics and phase diagrams for colossal barocaloric effects in n-alkanes

Barocaloric (BC) materials provide cheaper and more energy efficient alternatives to traditional refrigerants. Some liquid alkanes were recently shown to exhibit a colossal BC effect, matching the entropy changes in commercial vapour-liquid refrigerants. Dodecane was predicted to have the largest entropy change among the studied alkanes. Using synchrotron powder and single-crystal X-ray diffraction, Raman spectroscopy, and lattice energy calculations, we investigated the BC effects of n-dodecane at high pressures and room temperature. Remarkably, a colossal entropy change |ΔS| of 778 J kg-1 K-1 at 0.15(3) GPa and 295 K was observed. Spectroscopic studies revealed that this entropy change correlates closely with the conformational transition from mixed gauche to all-trans forms during pressure-induced crystallization. Additionally, the usage of a diamond anvil cell allowed the determination of the crystal structures of in situ crystallized n-un- and dodecane, as well as evaluation of the pressure-dependent crystal growth kinetics. Furthermore, our research suggests that the entropy change (per kilogram) upon compression should be similar for all n-alkanes within the range of 9-18 carbon atoms in the molecule, based on their lattice energies. Even-numbered alkanes are predicted to exhibit superior BC properties compared to their odd-numbered counterparts due to the more symmetric crystal structures and lower propensity to form plastic phases with lower transition entropy.

Molecular Ferroelectric Crystals with Superior Pyroelectricity, Plasticity, and Recyclability

The pyroelectric effect is used in a wide range of applications such as infrared (IR) detection and thermal energy harvesting, which require the pyroelectric materials to simultaneously have a high pyroelectric coefficient and a low dielectric constant for high figures of merit. However, in conventional proper ferroelectrics, the positive correlation between the pyroelectric coefficient and the dielectric constant imposes an insurmountable challenge in upgrading the figures of merit. Here, we explored superior pyroelectricity in [(CH3)4N][FeCl4] (TMA-FC) and [(CH3)4N][FeCl3Br] (TMA-FCB) molecular ferroelectric plastic crystals, which could decouple this positive correlation due to the nature of improper polarization behavior. Therefore, TMA-FC and TMA-FCB derive a high pyroelectric coefficient and a low dielectric constant simultaneously, yielding record-high figures of merit around room temperature. Furthermore, the favorable plasticity enables ferroelectric crystals to attach surfaces with different shapes for device design and integration. More interestingly, the molecular ferroelectrics could be softened and reshaped at elevated temperatures without decay in pyroelectricity, making them recyclable for cost savings and e-waste reduction. Combined with the facile fabrication process, the findings of this work would open avenues for employing molecular ferroelectric plastic crystals in the manufacture of high-performance pyroelectric devices.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Influence of Zirconium (Zr4+) Substitution on the Crystal Structure and Optical and Dielectric Properties of Sr0.8Mg0.2(Sn1-xZrx)O3 Ceramics

In this work, new compositions of Sr0.8Mg0.2(Sn1-xZrx)O3 0.00 ≤ x ≤ 0.06 ceramics are designed and synthesized by the conventional solid-state route. The influence of Zr doping on the phase, microstructural, optical, and dielectric properties is thoroughly investigated. The peaks (0 0 4) and (1 1 0) are observed to shift toward lower 2θ values, due to the variation of the ionic radius between Zr4+ and Sn4+. X-ray diffraction patterns reveal the orthorhombic structure with the space group Pbnm. Scanning electron microscopy images reveal the presence of pores and particles with a high degree of agglomeration. The functional groups and modes of vibration are determined by Fourier transform infrared spectroscopy of the prepared metal oxide samples. The existence of green emission of all the synthesized samples around 554.91 nm is identified by photoluminescence spectroscopy. The dielectric properties of the fabricated samples are measured by using an impedance analyzer. The values of the tangent loss and relative permittivity are found to decrease with increasing frequency.

Direct Electrocaloric Characterization of Ceramic Films

Reliable and accurate characterization of the electrocaloric effect is necessary to understand the intrinsic properties of materials. To date, several methods are developed to directly measure the electrocaloric effect. However, each of them has some limitations, making them less suitable for characterizing ceramic films, which rely almost exclusively on less accurate indirect methods. Here, a new approach is proposed to address the process of rapid heat dissipation in ceramic films and to detect the electrically induced temperature change before it thermally bonds with the surrounding elements. By using a polymer substrate that slows heat dissipation to the substrate and fast infrared imaging, a substantial part of the adiabatic electrocaloric effect in Pb(Mg1/3 Nb2/3 )O3 -based ceramic films is captured. Infrared imaging provides a robust technique to reduce the ratio between the adiabatic and the measured electrocaloric temperature change in micrometer-sized ceramic films to a single-digit number, ≈3.5. The obtained results are validated with another direct thermometric method and compared with the results obtained with an indirect approach. Despite different measurement principles, the results obtained with the two direct methods agree well. The proposed approach is timely and can open a door to verify the predicted giant electrocaloric effects in ceramic films.

Toughening of Ni-Mn-Based Polycrystalline Ferromagnetic Shape Memory Alloys

Solid-state refrigeration technology is expected to replace conventional gas compression refrigeration technology because it is environmentally friendly and highly efficient. Among various solid-state magnetocaloric materials, Ni-Mn-based ferromagnetic shape memory alloys (SMAs) have attracted widespread attention due to their multifunctional properties, such as their magnetocaloric effect, elastocaloric effect, barocaloric effect, magnetoresistance, magnetic field-induced strain, etc. Recently, a series of in-depth studies on the thermal effects of Ni-Mn-based magnetic SMAs have been carried out, and numerous research results have been obtained. It has been found that poor toughness and cyclic stability greatly limit the practical application of magnetic SMAs in solid-state refrigeration. In this review, the influences of element doping, microstructure design, and the size effect on the strength and toughness of Ni-Mn-based ferromagnetic SMAs and their underlying mechanisms are systematically summarized. The pros and cons of different methods in enhancing the toughness of Ni-Mn-based SMAs are compared, and the unresolved issues are analyzed. The main research directions of Ni-Mn-based ferromagnetic SMAs are proposed and discussed, which are of scientific and technological significance and could promote the application of Ni-Mn-based ferromagnetic SMAs in various fields.

Intrinsically elastic polymer ferroelectric by precise slight cross-linking

Ferroelectrics are an integral component of the modern world and are of importance in electrics, electronics, and biomedicine. However, their usage in emerging wearable electronics is limited by inelastic deformation. We developed intrinsically elastic ferroelectrics by combining ferroelectric response and elastic resilience into one material by slight cross-linking of plastic ferroelectric polymers. The precise slight cross-linking can realize the complex balance between crystallinity and resilience. Thus, we obtained an elastic ferroelectric with a stable ferroelectric response under mechanical deformation up to 70% strain. This elastic ferroelectric exerts potentials in applications related to wearable electronics, such as elastic ferroelectric sensors, information storage, and energy transduction.

Colossal room-temperature electrocaloric strength aided by hydrostatic pressure in lead-free multiferroic solid solutions

Solid-state cooling applications based on electrocaloric (EC) effects are particularly promising from a technological point of view due to their downsize scalability and natural implementation in circuitry. However, EC effects typically involve materials that contain toxic substances and require relatively large electric fields (∼100-1000 kV cm^-1) that cause fateful leakage current and dielectric loss problems. Here, we propose a possible solution to these practical issues that consists of concertedly applying hydrostatic pressure and electric fields on lead-free multiferroic materials. We theoretically demonstrate this strategy by performing first-principles simulations on supertetragonal BiFe_1-xCo_xO₃ solid solutions (BFCO). It is shown that hydrostatic pressure, besides adjusting the occurrence of EC effects to near room temperature, can reduce enormously the intensity of driving electric fields. For pressurized BFCO, we estimate a colossal room-temperature EC strength, defined as the ratio of the adiabatic EC temperature change by an applied electric field, of ∼1 K cm kV^-1, a value that is several orders of magnitude larger than those routinely measured in uncompressed ferroelectrics.

Microstructure and mechanical properties of BT/PVTC composite ferroelectric thin films

CONTEXT

Ferroelectric ceramic polymer composites have become the preferred electrocaloric materials due to their light weight and high polarization strength. But the mechanical properties were desired to be improved. In this study, the polyvinylidene fluoride trifluoro ethylene chloride (PVTC) and barium titanate (BT) composites were prepared, and the microstructure and mechanical properties were investigated by molecular dynamics simulations and experiments. It was found that with the increase of BT ceramic content in the composites, the yield stress is significantly reduced, which can be reduced by 16.07%. By comparing with the experimental data, the agglomeration and stress mechanism of the composites were proposed.

METHOD

The microstructure of the composite was analyzed using radial distribution function, self-diffusion coefficient, and glass transition temperature. The agglomeration mechanism of the composite was revealed from the microscopic point of view, and the rationality of the agglomeration behavior was verified by experiments. The calculations were performed by Material Studio 2019 software and the COMPASS force field was adopted.

Fluoropolymer ferroelectrics: Multifunctional platform for polar-structured energy conversion

Ferroelectric materials are currently some of the most widely applied material systems and are constantly generating improved functions with higher efficiencies. Advancements in poly(vinylidene fluoride) (PVDF)-based polymer ferroelectrics provide flexural, coupling-efficient, and multifunctional material platforms for applications that demand portable, lightweight, wearable, and durable features. We highlight the recent advances in fluoropolymer ferroelectrics, their energetic cross-coupling effects, and emerging technologies, including wearable, highly efficient electromechanical actuators and sensors, electrocaloric refrigeration, and dielectric devices. These developments reveal that the molecular and nanostructure manipulations of the polarization-field interactions, through facile defect biasing, could introduce enhancements in the physical effects that would enable the realization of multisensory and multifunctional wearables for the emerging immersive virtual world and smart systems for a sustainable future.

The First Demonstration of Strain-Controlled Periodic Ferroelectric Domains with Superior Piezoelectric Response in Molecular Materials

Achieving a periodic domain structure in ferroelectric materials to tailor the macroscopic properties or realize new functions has always been a hot topic. However, methods to construct periodic domain structures, such as epitaxial growth, direct writing by scanning tips, and the patterned electrode method, are difficult or inefficient to implement in emerging molecular ferroelectrics, which have the advantages of lightweight, flexibility, biocompatibility, etc. An efficient method for constructing and controlling periodic domain structures is urgently needed to facilitate the development of molecular ferroelectrics in nanoelectronic devices. In this work, it is demonstrated that large-area, periodic and controllable needle-like domain structures can be achieved in thin films of the molecular ferroelectric trimethylchloromethyl ammonium trichlorocadmium (TMCM-CdCl₃ ) upon the application of tensile strain. The domain evolution under various tensile strains can be clearly observed, and such processes are accordingly identified. Furthermore, the domain wall exhibits a superior piezoelectric response, with up to fivefold enhancement compared to that of the pristine samples. Such large-area tunable periodic domain structure and abnormally strong piezoresponse are not only of great interests in fundamental studies, but also highly important in the future applications in functional molecular materials.

Substantially Enhanced Electrocaloric Effect in Ba(Zr0.2Ti0.8)O3 Lead-Free Ferroelectric Ceramics via Lattice Stress Engineering

As an alternative to conventional vapor-compression refrigeration, cooling devices based on electrocaloric (EC) materials are environmentally friendly and highly efficient, which are promising in realizing solid-state cooling. Lead-free ferroelectric ceramics with competitive EC performance are urgently desirable for EC cooling devices. In the past few decades, constructing phase coexistence and high polarizability have been two crucial factors in optimizing the EC performance. Different from the external stress generated through heavy equipment and inner interface stress caused by complex interface structures, the internal lattice stress induced by ion substitution engineering is a relatively simple and efficient means to tune the phase structure and polarizability. In this work, we introduce low-radius Li⁺ into BaZr_0.2Ti_0.8O₃ (BZT) to form a particular A-site substituted cell structure, leading to a change of the internal lattice stress. With the increase of lattice stress, the fraction of the rhombohedral phase in the rhombohedral-cubic (R-C) coexisting system and ferroelectricity are all pronouncedly enhanced for the Li₂CO₃-doped sample, resulting in the significant enhancement of saturated polarization (P_s) as well as EC performance [e.g., adiabatic temperature change (ΔT) and isothermal entropy change (ΔS)]. Under the same conditions (i.e., 333 K and 70 kV cm^-1), the ΔT of 5.7 mol % Li₂CO₃-doped BZT is 1.37 K, which is larger than that of the pure BZT ceramics (0.61 K). Consequently, in cooperation with the great improvement of electric field breakdown strength (E_b) from 70 to 150 kV cm^-1, 5.7 mol % Li₂CO₃-doped BZT achieved a large ΔT of 2.26 K at a temperature of 333 K, which is a competitive performance in the field of electrocaloric effect (ECE). This work provides a simple but effective approach to designing high-performance electrocaloric materials for next-generation refrigeration.

An Active Pixel-Matrix Electrocaloric Device for Targeted and Differential Thermal Management

More than 55% of electronic failures are caused by damage from localized overheating. Up to now, there is still no efficient method for targeted temperature control against localized overheating. Although some existing thermal management devices handle this issue by full coverage cooling, it generates a lot of useless energy consumption. Here, a highly efficient pixel-matrix electrocaloric (EC) cooling device is reported, which can realize a targeted and differential thermal management. The modified poly(vinylidene fluoride-tertrifluoroethylene-chlorofluoroethylene) reaches a large adiabatic temperature change of 7.8 K and is more suitable for thermal transfer and electrostatic actuation at high frequencies. All active pixels in the EC cooling device exhibit a stable temperature span of 4.6 K and a heat flux of 62 mW cm^-2 , which is more than twice that of the one-layer EC device. Each refrigeration pixel can be independently controlled and effectively cooled down the localized overheating site(s) in situ. The surface temperature of the simulated central processing unit decreases by 33.2 K at 120 s after applying this EC device. Such a compact, embeddable, low cost, and active solid-state pixel-matrix cooling device has great potential for localized overheating protection in microelectronics.

Giant electrocaloric effect in a molecular ceramic

The electrocaloric effect (ECE) is an efficient and environmentally friendly method for solid-state refrigeration driven by an electric field. However, disregarding the ECE performance, the mass of materials also limits the amount of energy transferred in the cooling process. While molecular ECE materials have been attracting intensive attention with their excellent ECE properties, most reported molecular compounds can only be utilized in the form of thin films or single crystals. Unlike inorganic ceramics, molecular thin films and single crystals are very difficult to prepare in a large amount, which greatly restrains the future application of those materials. In this work, we report an excellent molecular ECE material in the form of polycrystalline molecular ceramics. Such molecular ceramics are composed of plastic molecular ferroelectrics, and can fulfil the requirement of large mass, easy processing, excellent performance and low energy consumption. Our molecular ceramic of HQReO₄ (HQ: protonated quinuclidine) demonstrates an isothermal entropy change of 5.8 J K^-1 kg^-1 and an adiabatic temperature change of 3.1 K. Notably, by a simple low-temperature pressing process without added adhesives (about 373 K), an HQReO₄ molecular ceramic block can be obtained, and its ECE performance is observed to be comparable to that of single crystals, for the first time. This work proposes a new application form for molecular electrocaloric materials, which opens up new ideas for solid-state refrigeration.

A colossal barocaloric effect induced by the creation of a high-pressure phase

As a promising environment-friendly alternative to current vapor-compression refrigeration, solid-state refrigeration based on the barocaloric effect has been attracting worldwide attention. Generally, both phases in which a barocaloric effect occurs are present at ambient pressure. Here, instead, we demonstrate that KPF₆ exhibits a colossal barocaloric effect due to the creation of a high-pressure rhombohedral phase. The phase diagram is constructed based on pressure-dependent calorimetric, Raman scattering, and neutron diffraction measurements. The present study is expected to provide an alternative routine to colossal barocaloric effects through the creation of a high-pressure phase.

Liquid crystal based active electrocaloric regenerator

The active electrocaloric (EC) regenerator exploiting electric conversion into thermal energy has recently become important for developing a new generation of heat-management devices. We analyze an active EC regenerator numerically. We establish a temperature span across the regenerator Δ T by commuting a liquid crystalline (LC) unit between regions with and without an external electric field E. In modelling, we use Landau-de Gennes mesoscopic approach, focusing on the temperature regime where isotropic (paranematic) and nematic phase order compete. We determined conditions enabling a large enough value of Δ T suitable for potential applications. In particular, (i) the vicinity of the paranematic-nematic (P-N) phase transition, (ii) large enough latent heat of the transition, (iii) strong enough applied external field (exceeding the critical field E _c at which the P-N transition becomes gradual), and (iv) relatively short contact times between LC unit and heat sink and heat source reservoirs are advantageous. Our analysis reveals that Δ T ≫ 1 K could be achieved using appropriate LC material.

Bio-Based Poly(hydroxy urethane)s for Efficient Organic High-Power Energy Storage

Fast, low-cost, and efficient energy storage technologies are urgently needed to balance the intermittence of sustainable energy sources. High-power capacitors using organic polymers offer a green and scalable answer. They require dielectrics with high permittivity (ε_r) and breakdown strength (E_B), which bio-based poly(hydroxy urethane)s (PHUs) can provide. PHUs combine high concentrations of hydroxyl and carbamate groups, thus enhancing their ε_r, and a highly tunable glass transition (T_g), which dictates the regions of low dielectric losses. By reacting erythritol dicarbonate with bio-based diamines, fully bio-based PHUs were synthesized with T_g ∼ 50 °C, ε_r > 8, E_B > 400 MV·m^-1, and low losses (tan δ < 0.03). This results in energy storage performance comparable with the flagship petrochemical materials (discharge energy density, U_e > 6 J·cm^-3) combined with a remarkably high discharge efficiency, with η = 85% at E_B and up to 91% at 0.5 E_B. These bio-based PHUs thus represent a highly promising route to green and sustainable energy storage.

Enhanced electrocaloric effect, energy storage density and pyroelectric response from a domain-engineered lead-free BaTi0.91Sn0.08Zr0.01O3 ferroelectric ceramic

A BaTi0.91Sn0.08Zr0.01O3 (BTSZ) ceramic was prepared by a conventional solid-state reaction method. Its structural, dielectric, ferroelectric, and pyroelectric properties were carefully studied. The Rietveld refinement was used to characterize the structural proprieties of the synthesized ceramic. The microstructure was observed by scanning electron microscopy. Phase transitions observed in the temperature dependent dielectric permittivity (ε r-T and tan δ-T) showed a transition close to room temperature, resulting in improved piezoelectric, pyroelectric and electrocaloric performance. In addition, it was found that an electric field poling process changed the character of ε r-T and tan δ-T plots. Resonance modes in the polarized state, where maximum power transmission was achieved, were observed in the impedance spectrum. The extra-slim hysteresis loops revealed a relatively low coercive field and hysteresis loss related to the diffuse phase transition, which can significantly improve energy storage efficiency up to 75% at 100 °C. To characterize the electrocaloric effect (ECE), indirect and direct methods based on the thermodynamic approach were used. Both methods results showed good consistency and revealed a large ECE peak evolving along the phase diagram. Furthermore, pyroelectric figures of merit (FOMs) for voltage responsivity (F v), current responsivity (F i), energy harvesting (F E), new energy harvesting and detectivity (F d) were calculated. Finally, thermal energy harvesting (N D) was determined by using the Olsen cycle. The obtained maximum N D was 233.7 kJ m-3 when the Olsen cycle operated at 25-100 °C and 0-30 kV cm-1. This study introduces not only a technique to produce a high performance ceramic for refrigeration devices, but also broadens the range of applications for BT-based lead-free ferroelectrics beyond actuators, sensors, and energy harvesting to solid-state cooling.