Pyroelectric and electrocaloric characterization has been determined for 0.75Pb(Mg1∕3Nb2∕3)O3–0.25PbTiO3 relaxor based single crystal and ceramic. Differential scanning calorimetry was used for measuring the electrocaloric response for different electric fields in the vicinity of the Curie temperature. For both ceramic and crystals the maximum activity is found to be around the transition temperature. On the other hand hysteresis loops for different temperatures were used to predict the electrocaloric effect with very good qualitative agreements with direct measurements. Pyroelectric coefficient is found to be much larger for ⟨111⟩ single crystals reaching 1300×10−6Cm−2K−1 whereas the ceramic reaches only 750×10−6Cm−2K−1. Higher pyroelectric coefficient and lower dielectric permittivity lead to outstanding figures of merits for sensors and energy harvesting, with a gain of 260% for voltage responsivity and more than 500% for energy harvesting. Although having a much larger pyroelectric activity, the electrocaloric effect is about the same for crystals and ceramics—around 0.40J∕g for 2.5kV∕mm electric field step. This result is interpreted by the decrease of the pyroelectric coefficient for high electric field. The electrocaloric activity is in fact limited by the saturation polarization and difference between Curie transition temperature and the working temperature. Those two parameters are very similar for crystals and ceramics. Single crystals are consequently very interesting materials in the framework of energy harvesting and sensor applications whereas no real improvement of performances can be expected for electrocaloric refrigeration devices.

International audienceThe electrostrictive properties of a polyether-based polyurethane elastomer and its corresponding composites filled with conductive carbon black (CB) were studied by measuring the thickness strain SZ induced by external electric fields E. For films with thicknesses of approximately 50 μm, the apparent electrostrictive coefficient M was measured at low electric fields, E ⩽ 4 V/μm, and different CB contents (up to a volume fraction of 2%). Dielectric measurements in AC mode were performed in order to determine the percolation threshold fc, which was 1.25 v%. This optimal volume fraction yielded a remarkable threefold increase in M, associated with an increase of the dielectric constant by a factor 7, in comparison with pure PU. This enhancement of the electric field-induced strain and apparent electrostriction was mainly triggered by an increase of the dielectric constant, even if the intrinsic electrostriction coefficient Q was decreased. The nanocomposites thus seem to be very attractive for low-frequency electromechanical applications. Above fc, their conductivity was raised and their electrostrictive activity lost. Finally, there is a good agreement between the experimentally determined dependence on the CB content of the M coefficient and the theoretical estimation calculated from dielectric and mechanical measurements

This study deals with the improvement of electric field-induced thickness strain of polyurethane (PU) elastomer films by carbon black (C) nanopowder incorporation in the polymer matrix. Different carbon volume concentrations—0.5, 0.7, 1 and 1.5%—have been tested. Weak-field dielectric and resistivity measurements revealed that a percolative effect is not induced by carbon filling up to 1.5 vol%. Thickness strain measurements showed that both pure PU and C/PU composite films exhibit similar strain variations which are not governed only by electrostatic forces (Maxwell stress) and/or electrostriction forces. The highest strain amplitude value observed was obtained for 1% C composite thin film (Sa = −7.4% at E = 17.8 kV mm−1). In comparison the highest Sa for pure PU thin film was −6.7% at E = 37.5 kV mm−1). In the case of thick samples, the thickness strain was not enhanced by C loading, which strongly suggests interfacial space charge effects in pure PU film, confirmed by the frequency dependence of strain level.

Polyurethane-based nanocomposite films were prepared by incorporating carbon-coated SiC nanowires (SiC@C) into the polymer matrix. Electric field-induced strain measurements revealed that a loading of 0.5 wt% SiC@C increased the strain level by a factor of 1.7 at a moderate field strength (6.5 V µm−1). Current–electric field characteristics and the film thickness dependence of strain demonstrated that the improvement of the electromechanical response was linked to a more pronounced space charge effect in the nanocomposite than in the polymer host. DSC measurements revealed that the level of phase mixing in the PU matrix remained unchanged after SiC@C filling; hence, the nano-objects themselves acted as charge traps.

This paper presents electrocaloric measurements on 0.75(PbMg1/3Nb2/3O3)–0.25(PbTiO3) ceramics. Reversible heat exchanged up to 0.15 J g−1 with an applied field of 1.35 kV mm−1 was obtained. The interpretation of this observation is based on direct polarization measurements. Starting from the integration along the electric field of the derivative of the polarization versus temperature, it was possible to predict the heat upon a decrease in electric field for values up to 3 kV mm−1. However the simulations differ from the experiments and the discrepancy is believed to be due to hysteresis in ferroelectric materials. Finally a practical limit of the use of ferroelectric 0.75(PbMg1/3Nb2/3O3)–0.25(PbTiO3) ceramics is evidenced through electric conductivity appearance when the electrothermal conversion is very high.

Electrocaloric properties of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer were determined by two methods. A modified differential scanning calorimeter measures the entropy variation when applying an electric field under isothermal conditions. Alternative technique consists of an infrared imaging camera that gives direct information on temperature variation in pseudo-adiabatic condition. Both techniques give similar results with a heat capacity of 1500 J/(kg K). For an electric field of 80 V/μm, entropy variation was measured at 15.1 J/(kg K) and a temperature variation of 2.75 K. High frequency measurement is possible using infrared imaging, and a strong frequency dependence of the electrocaloric effect was observed.

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International audienceNonpercolated composites based on polyurethane (PU) filled with low concentrations copper (Cu) powders of varying sizes were studied as electrostrictive materials for mechanical energy harvesting. The dispersion of the fillers within the polymeric matrix was investigated by scanning electron microscopy, and results showed a relatively homogeneous dispersion for the microsized fillers and the existence of agglomerates for their nanosized counterparts. Differential scanning calorimetry measurements displayed that there occurred no interaction between the polymeric matrix and the microsized fillers whereas the nanosized fillers slightly enhanced the glass transition of the soft segments of PU and significantly affected the recrystallization temperature. The dependence of the dielectric properties of the composites as a function of the filler volume fraction and filler size was investigated over a broad range of frequencies, showing an increase in the permittivity when fillers were used. This increase was more pronounced for the composites containing nanosized fillers. The measurement of the harvested current and of the harvested power also demonstrated an enhancement of the energy harvesting capability when nanofillers were employed. From the experimental data, it appeared that the electrostrictive coefficient Q was not proportional to the inverse ratio of the permittivity and the Young modulus for the studied composites. Finally, analytical modeling of the harvested current and of the harvested energy offered an accurate description of the experimental data