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International audience; 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 ($\epsilon _r$) 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 $\epsilon _r$, and a highly tuneable 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 synthesised with $T_g$ ~ 50 °C, $\epsilon _r$ > 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.

VDF-based polymers, such as poly(vinylidene fluoride) (PVDF) and its copolymers, are well-known ferroelectrics of interest for numerous applications, from energy storage to electrocaloric refrigeration. However, their often complex thermal phase behavior that typically leads to a low phase-stability can drastically affect the long-term dielectric properties of this materials family. Here, we demonstrate on the example of the terpolymer P(VDF

International audience; Active thermal control will be a major challenge of the twenty-first century, which has emphasized the need for the development of energy-efficient refrigeration techniques such as electrocaloric (EC) cooling. Highly polar semi-crystalline VDF-based polymers are promising organic EC materials, however, their cooling performance, which is highly structurally-dependent, needs further improvement to become competitive. Here, we report a simple method to increase the crystalline coherence of P(VDF-ter-TrFE-ter-CFE) ter-polymer in the plane including the polar direction. This is achieved by blending P(VDF-ter-TrFE-ter-CFE) with minute amounts of P(VDF-co-TrFE) co-polymer with similar VDF/TrFE unit content. This similarity allows for a cocrystallization of the co-polymer chains in the ter-polymer crystalline lamellae, preferentially extending the lateral coherence without lamellar thickening, as validated with a wide range of structural characterisation. This results in a significant dielectric and electrocaloric enhancement, with a remarkable electrocaloric effect, ΔTEC = 5.2 K, confirmed by direct measurements for a moderate electric field of 90 MV∙m-1 in a blend with 1 wt% of co-polymer.

Multilayer ceramic capacitors (MLCCs) of lead-free NBT-based ceramics are produced and their electrocaloric effect (ECE) is characterised for the first time. Dense MLCCs with 97μm-thick active layers are successfully produced by tape casting. Dielectric permittivity measurements reveal the MLCCs to have properties similar with that reported for the corresponding bulk ceramics, including Td∼50°C and TS∼100°C. Direct ECE measurements also reveal agreement and confirm the previously reported tendency of the high-field ECE peak to shift towards TS. The highest value of ECE, ΔTmax∼1.7K is measured at 90°C under 90kV/cm. A low breakdown strength of 93kV/cm needs to be solved to realise stronger electric fields and achieve commercially viable ECE values.

© 2018 Elsevier Ltd The influence of different substitutional mechanisms on the electrocaloric effect of a lead-free SrBi2(Nb0.2Ta0.8)2O9Aurivillius phase was studied for application in electrocaloric cooling systems. The A-site substitution with barium efficiently reduced the temperature of maximum permittivity from about 300 °C to 100 °C. The A-site substitution induced phenomena that are typical of strong relaxor ferroelectrics such as significant broadening of the permittivity peak and an increase in its frequency dispersion and with a depolarization temperature below room temperature. These features directly influenced the electrocaloric effect. A direct measurement system, based on a modified-differential scanning calorimeter, was used to analyze the EC effect of the dense (Sr0.5Ba0.5)Bi2(Nb0.2Ta0.8)2O9ceramics. In accordance with the relaxor characteristics, the EC effect was found to increase continuously over a broad temperature range above the room temperature. This was attributed to the alignment of field induced polar nanodomains. Directions for optimization towards a high-performing EC ceramic were identified.

International audience; Electrocaloric refrigeration is one of the most promising, environmentallyfriendly technology to replace current cooling platforms—if a notable electrocaloric effect (ECE) is realized around room temperature where the highest need is. Here, a straight-forward, one-pot chemical modification of P(VDF-ter-TrFE-ter-CTFE) is reported through the controlled introduction of small fractions of double bonds within the backbone that, very uniquely, decreases the lamellar crystalline thickness while, simultaneously, enlarging the crystalline coherence along the a-b plane. This increases the polarizability and polarization without affecting the degree of crystallinity or amending the crystal unit cell—undesirable effects observed with other approaches. Specifically, the permittivity increases by >35%, from 52 to 71 at 1 kHz, and ECE improves by >60% at moderate electric fields. At 40 °C, an adiabatic temperature change >2 K is realized at 60 MV m−1 (>5.5 K at 192 MV m−1), compared to ≈1.3 K for pristine P(VDF-ter-rFE-ter-CTFE), highlighting the promise of a simple, versatile approach that allows direct film deposition without requiring any post-treatment such as mechanical stretching or high-temperature annealing for achieving the desired performance.

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

A lead-free Aurivillius phase based on SrBi2(Nb0.2Ta0.8)2O9 was studied for potential application in electrocaloric (EC) cooling systems. Doping with praseodymium proved to be effective in reducing the temperature of maximum permittivity from about 300°C to 80°C. The praseodymium doping also led to a broadening of the permittivity peak and an increase in its frequency dispersion, which are typical features of strong relaxor ferroelectrics. A direct measurement system, based on a modified-differential scanning calorimeter, was used to analyze the EC effect of dense SrBi1.85Pr0.15(Nb0.2Ta0.8)2O9 ceramics. Unlike the trend observed in classic ferroelectrics, the EC effect in this relaxor ferroelectric was found to increase significantly over a broad temperature range well above the temperature of depolarization. This was attributed to the contribution of the polar nanodomain alignment that is induced by the external electric field. Directions for further optimization of this lead-free relaxor towards a high-performing EC material were identified. A comparison between the direct and indirect EC measurements showed significant discrepancies, not only in a magnitude but also in the trend. These discrepancies were attributed to the non-ergodic relaxor state of the Aurivillius phase, which cannot be described with Maxwell equations. For materials displaying such characteristics indirect methods to predict EC effect may be unreliable requiring that the EC effect be measured by a direct EC characterization technique.

We have analyzed the correlations between the electrocaloric (EC) temperature change and the EC coefficient, both obtained with experimental characterization of the materials in different forms: as thin films, thick films, single crystals and ceramics. We showed that higher EC temperature change has been obtained for the materials with lower EC coefficients as a result of very high applied electric fields that these materials can withstand due to their high dielectric strength. Based on that the dielectric strength has been shown to be one of the key parameters for the high-performance EC materials. The materials microstructural characteristics, required for the high dielectric strength, have been identified. We propose a number of future research and development directions towards the new EC materials with the high dielectric strength and high EC coefficient, which could result in the breakthrough in the EC refrigeration technology.

The electrocaloric effects (ECEs) of the morphotropic phase boundary (MPB) composition 0.82(Na0.5Bi0.5)TiO3-0.18(K0.5Bi0.5)TiO3 (NBT-18KBT) are studied by direct measurements. The maximum ECE ΔTmax = 0.73 K is measured at 160 °C under 22 kV/cm. This corresponds to an ECE responsivity (ΔT/ΔE) of 0.33 × 10−6 K m/V, which is comparable with the best reported values for lead-free ceramics. A comparison between the direct and indirect ECE measurements shows significant discrepancies. The direct measurement of both positive and negative electrocaloric effect confirms the presence of numerous polar phases near the MPB of NBT-based materials and highlights their potential for solid-state cooling based on high field-induced entropy changes.

This chapter describes lead-free electrocaloric materials and will mostly concentrate on bulk form. The materials will be described by their reported electrocaloric (EC) values or other essential factors associated with the electrocaloric effect such as ferroelectric phase transition, polarization, dipole entropy or heat capacity evaluations.

The electrocaloric effect (ECE) of two compositions (x = 0.06 and 0.07) of (1 − x)(Na0.5Bi0.5)TiO3-xKNbO3 in the vicinity of the morphotropic phase boundary is studied by direct measurements. ΔTmax = 1.5 K is measured at 125 °C under 70 kV/cm for NBT-6KN while ΔTmax = 0.8 K is measured at 75 °C under 55 kV/cm for NBT-7KN. We show that the “shoulder,” TS, in the dielectric permittivity, marks the upper limit of the ECE peak under high applied electric fields. These results imply that the range of temperature with high ECE can be quickly identified for a given composition, which will significantly speed up the process of materials selection for ECE cooling.

The electrocaloric effect (ECE) of SrxBa(1−x)Nb2O6 (SBN100x) single crystals, with a tetragonal tungsten bronze structure and a high ECE near room temperature, is studied by direct measurements. It is shown that although the onset of the ECE peak is closer to room temperature in SBN80 than in SBN75, the effect of the increase of the strontium content is very detrimental to the ECE performances with a decrease to ΔTEC = 0.23 K for SBN80 from the reported value of ΔTEC = 0.42 K under 10 kV/cm for SBN75 [F. Le Goupil et al., “Anisotropy of the electrocaloric effect in lead-free relaxor ferroelectrics,” Adv. Energy Mater. (published online)]. However, when 1.40% of cerium is introduced in SBN61, the temperature of depolarisation is shifted below 30 °C, while an ECE above 0.6 K is maintained over more than 70 K for a low electric field of 28 kV/cm. The maximum ECE ΔTEC = 0.85 K is measured at 61 °C. In addition to having an ECE peak close to room temperature, the ECE measured in Ce-doped SBN61 is comparable wi...

Dr. F. Le Goupil, Dr. A.-K. Axelsson, Prof. L. J. Dunne, Dr. A. Berenov, Prof. N. McN. Alford Department of Materials Imperial College London London SW7 2AZ , UK E-mail: f.le-goupil09@imperial.ac.uk Dr. A.-K. Axelsson, Prof. L. J. Dunne Department of Engineering Systems London South Bank University London SE1 0AA , UK Prof. M. Valant Materials Research Laboratory University of Nova Gorica Nova Gorica 5000 , Slovenia Dr. G. Manos Department of Chemical Engineering University College London London WC1E 7JE , UK Prof. T. Lukasiewicz Institute of Electronic Materials Technology 133 Wolczynska Str., 01–919 Warsaw , Poland Prof. J. Dec Institute of Materials Science University of Silesia 12 Bankowa Str., PL 40–007 Katowice , Poland

A direct electrocaloric effect (ECE) measurement system, based on a modified-differential scanning calorimeter (DSC), allowing the acquisition of both thermal (ECE, heat capacity) and electrical (P-E loops, leakage current) information simultaneously, was used to analyze 〈001〉-oriented PbMg1/3Nb2/3O3-30PbTiO3 single crystals. Different electric-field-induced phase transitions were identified on direct ECE measurements and confirmed by dielectric measurements. The strongest ECE (ΔTEC = 0.65 K) was measured for an applied electric field E = 10 kV/cm just above the temperature of depolarization. The direct ECE measurements were compared with indirect measurements obtained from dielectric polarization measurements versus electric field and temperature and a very good agreement was found. A region with negative ΔTEC was identified by both direct and indirect measurements. This phenomenon was attributed to the formation of a reversible field-induced phase transition towards a state with a different polar direction.