Abstract In contrast to the comprehensive understanding of novel ferroelectric [i.e., relaxor ferroelectric (RFE) and antiferroelectric] behavior in ceramics, RFE and double-hysteresis-loop (DHL) behavior in crystalline ferroelectric polymers have only been studied in the past fifteen years. A number of applications such as electrostriction, electric energy storage, and electrocaloric cooling have been realized by utilizing these novel ferroelectric properties. Nonetheless, fundamental understanding behind these novel ferroelectric behaviors is still missing for polymers. In this feature article, we intend to unravel the basic physics via systematic studies of poly(vinylidene fluoride- co -trifluoroethylene) [P(VDF-TrFE)]-based terpolymers, electron-beam (e-beam) irradiated P(VDF-TrFE) copolymers, and PVDF graft copolymers. It is found that both the crystal internal structure and the crystal–amorphous interaction are important for achieving the RFE and DHL behaviors. For the crystal internal structure effect, dipole switching with reduced friction and nanodomain formation by pinning the polymer chains are essential, and they can be achieved through crystal repeating-unit isomorphism (i.e., defect modification). Physical pinning [e.g., in P(VDF-TrFE)-based terpolymers] induces a reversible, electric field-induced RFE↔FE phase transition and thus the DHL behavior, whereas chemical pinning [e.g., in e-beam irradiated P(VDF-TrFE)] results in the RFE behavior. Finally, the crystal–amorphous interaction (or the nanoconfinement effect) results in a competition between the polarization and depolarization local fields. When the depolarization field becomes stronger than the polarization field, a DHL behavior is observed. Obviously, the physics for ferroelectric polymers is different from that for ceramics/liquid crystals and can be largely attributed to the long-chain nature of semicrystalline polymers. This understanding will help us to design new ferroelectric polymers with improved properties and/or better applications.

Novel Polymer Ferroelectric Behavior via Crystal Isomorphism and Nanoconfinement Effect

Magnetically driven thermal changes in magnetocaloric materials have, for several decades, been exploited to pump heat near room temperature. By contrast, their electrocaloric and mechanocaloric counterparts have only been intensively studied and exploited for little more than a decade. These different caloric strands have recently been unified to yield a single field of research that could help combat climate change by generating better heat pumps for both cooling and heating. Here we outline the timeliness of the present activity and discuss recent advances in caloric measurements, materials, and prototypes.

https://science.sciencemag.org/content/sci/370/6518/797.full.pdf

Caloric materials for cooling and heating

Calls to minimize greenhouse gas emissions and demands for higher energy efficiency continue to drive research into alternative cooling and refrigeration technologies. The caloric effect is the reversible change in temperature and entropic states of a solid material subjected to one or more fields and can be exploited to achieve cooling. The field of caloric cooling has undergone a series of transformations over the past 50 years, bolstered by the advent of new materials and devices, and these developments have contributed to the emergence of multicalorics in the past decade. Multicaloric materials display one or more types of ferroic order that can give rise to multiple field-induced phase transitions that can enhance various aspects of caloric effects. These materials could open up new avenues for extracting heat and spearhead hitherto unknown technological applications. In this Review, we survey the emerging field of multicaloric cooling and explore state-of-the-art caloric materials and systems (devices) that are responsive to multiple fields. We present our vision of the future applications of multicaloric and caloric cooling and examine key factors that govern the overall system efficiency of the cooling devices. Multicaloric cooling promises environmentally friendly and high-efficiency refrigeration. In this Review, the authors discuss emerging multicaloric materials and their physics involving coexisting ferroic order parameters, examine key factors that govern the overall system efficiency of potential multicaloric devices and envision future applications. 

Materials, physics and systems for multicaloric cooling

Dielectric composites boost the family of energy storage and conversion materials as they can take full advantage of both the matrix and filler. This review aims at summarizing the recent progress in developing high-performance polymer- and ceramic-based dielectric composites, and emphases are placed on capacitive energy storage and harvesting, solid-state cooling, temperature stability, electromechanical energy interconversion, and high-power applications. Emerging fabrication techniques of dielectric composites such as 3D printing, electrospinning, and cold sintering are addressed, following by highlighted challenges and future research opportunities. The advantages and limitations of the typical theoretical calculation methods, such as finite-element, phase-field model, and machine learning methods, for designing high-performance dielectric composites are discussed. This review is concluded by providing a brief perspective on the future development of composite dielectrics toward energy and electronic devices. 

Polymer‐/Ceramic‐based Dielectric Composites for Energy Storage and Conversion

Electrocaloric materials are promising working bodies for caloric-based technologies, suggested as an efficient alternative to the vapor compression systems. However, their materials efficiency defined as the ratio of the exchangeable electrocaloric heat to the work needed to trigger this heat remains unknown. Here, we show by direct measurements of heat and electrical work that a highly ordered bulk lead scandium tantalate can exchange more than a hundred times more electrocaloric heat than the work needed to trigger it. Besides, our material exhibits a maximum adiabatic temperature change of 3.7 K at an electric field of 40 kV cm−1. These features are strong assets in favor of electrocaloric materials for future cooling devices. The intrinsic efficiency of electrocaloric materials has been largely overlooked. Here, the authors use the parameter materials efficiency as the figure of merit to rank caloric materials, reporting on the materials efficiency of highly ordered bulk lead scandium tantalate PST.

/pdf/giant-electrocaloric-materials-energy-efficiency-in-highly-1y5rmqshpz.pdf

Giant electrocaloric materials energy efficiency in highly ordered lead scandium tantalate.

Cooling devices based on caloric materials have emerged as promising candidates to become the next generation of coolers. Several electrocaloric (EC) heat exchangers have been proposed that use different mechanisms and working principles. However, a prototype that demonstrates a competitive temperature span has been missing. We developed a parallel-plate active EC regenerator based on lead scandium tantalate multilayer capacitors. After optimizing the structural design by using finite element modeling for guidance and to considerably improve insulation, we measured a maximum temperature span of 13.0 kelvin. This temperature span breaks a crucial barrier and confirms that EC materials are promising candidates for cooling applications.

https://science.sciencemag.org/content/sci/370/6512/125.full.pdf

Giant temperature span in electrocaloric regenerator.

Coming up with sustainable sources of electricity is one of the grand challenges of this century. The research field of materials for energy harvesting stems from this motivation, including thermoelectrics1, photovoltaics2 and thermophotovoltaics3. Pyroelectric materials, converting temperature periodic variations in electricity, have been considered as sensors4 and energy harvesters5-7, although we lack materials and devices able to harvest in the joule range. Here we develop a macroscopic thermal energy harvester made of 42 g of lead scandium tantalate in the form of multilayer capacitors that produces 11.2 J of electricity per thermodynamic cycle. Each pyroelectric module can generate up to 4.43 J cm-3 of electric energy density per cycle. We also show that two of these modules weighing 0.3 g are sufficient to sustainably supply an autonomous energy harvester embedding microcontrollers and temperature sensors. Finally, we show that for a 10 K temperature span these multilayer capacitors can reach 40% of Carnot efficiency. These performances stem from (1) a ferroelectric phase transition enabling large efficiency, (2) low leakage current preventing losses and (3) high breakdown voltage. These macroscopic, scalable and highly efficient pyroelectric energy harvesters enable the reconsideration of the production of electricity from heat. 

/pdf/large-harvested-energy-with-non-linear-pyroelectric-modules-2hr8j2x9.pdf

Large harvested energy with non-linear pyroelectric modules

Actuation efficiency of polyvinylidene fluoride-based co- and ter-polymers

Using an infrared camera, we measured the latent heat of the first order phase transition in lead scandium tantalate at different applied electric fields. The entropy change value of 3.4 J kg−1 K−1 is consistent with differential scanning calorimetry measurements. The advantage of such an approach stems from the possibility to obtain both adiabatic temperature change and latent heat of the phase transition material only with an infrared camera or a thermocouple. This may prove useful for a systemic characterization of first order electrocaloric materials.

Pierre Lheritier

Papers

Giant temperature span in electrocaloric regenerator.

Giant electrocaloric materials energy efficiency in highly ordered lead scandium tantalate.

Large harvested energy with non-linear pyroelectric modules

Actuation efficiency of polyvinylidene fluoride-based co- and ter-polymers

Measuring lead scandium tantalate phase transition entropy by infrared camera