Benoit Guiffard

Electrocaloric and pyroelectric properties of 0.75Pb(Mg1∕3Nb2∕3)O3–0.25PbTiO3 single crystals

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 elect...

Enhanced electric field-induced strain in non-percolative carbon nanopowder/polyurethane composites

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.

Ferroelectric electrocaloric conversion in 0.75(PbMg1/3Nb2/3O3)?0.25(PbTiO3) ceramics

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.

Enhanced electroactive properties of polyurethane films loaded with carbon-coated SiC nanowires

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??m?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.

Low temperature synthesis of stoichiometric and homogeneous lead zirconate titanate powder by oxalate and hydroxide coprecipitation

A wet procedure to prepare stoichiometric and homogeneous PZT powder (Pb(Zr0.52Ti0.48)O3) is described. Starting reagents are tetra-n-butyl zirconate, titanate, and lead acetate. The synthesis is based upon the coprecipitation of metal hydroxides Pb(OH)2, ZrO(OH)2, TiO(OH)2 and lead oxalate PbC2O4 in the pH range 9–10. Monophasic and sub-micrometer-sized PZT powder was obtained by calcination of the mixture of hydroxides and oxalate at 700°C. The study shows that highly tetragonal PZT phase appeared after heating at 500°C only. No intermediate phase such as PbTiO3 was detected. The presence of two different lead precursors in the precipitate—Pb(OH)2 and PbC2O4—involved this low-temperature PZT formation without intermediate mixed oxide. Dense and compositionally homogenous PZT ceramics were obtained by sintering the calcined powder at 1200°C. These samples exhibited good dielectric and piezoelectric properties.

Modeling and experimentation on an electrostrictive polymer composite for energy harvesting

The harvesting of energy from ambient environments is an emerging technology with potential for numerous applications, including portable electronic devices for renewable energy. Most of the current research activities refer to classical piezoelectric ceramic materials, but more recently the development of electrostrictive polymers has generated novel opportunities for high-strain actuators. At present, the investigation of using electrostrictive polymers for energy harvesting (a conversion of mechanical to electrical energy) is beginning to show potential for this application. This paper discusses the development of a model that is able to predict the energy harvesting capabilities of an electrostrictive polymer composite (EPC). An equivalent electrical scheme has been developed by using the model of current that was recently developed by our group. After the validation of the model on a macroscopic level, an empirical relationship was established to predict the value of power from the electrostriction coefficient, the dielectric permittivity, and the compliance of the material. Finally, results indicated that the dielectric permittivity was the crucial parameter for energy harvesting.

Electrostrictive energy conversion in polyurethane nanocomposites

Electrostrictive polymers have demonstrated an ability to convert mechanical energy into electrical energy and vice versa. This energy conversion has been exploited in an extensive range of applications, including sensors and actuators. Recently, electrostrictive polymers have been investigated as electroactive materials for energy harvesting. The present work aims at establishing an analytical modeling based on electrostrictive equations for predicting a current that can be obtained from the first flexural mode of a beam which was attached by the electrostrictive polymers. The study was carried out on polyurethane films, either without filler or filled with nanosized SiC or a carbon nanopowder. Experimental measurements of the harvested current have been compared to the theoretical behavior predicted by the proposed model. A good agreement was observed between the two sets of data, which consequently validated that the modeling can be used to optimize the choice of materials. It was also shown that the i...

Effects of copper filler sizes on the dielectric properties and the energy harvesting capability of nonpercolated polyurethane composites

Nonpercolated 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.

Interpretation of the softening effect in PZT ceramics near the morphotropic phase boundary

In the last two decades, Eyraud tried to interpret the soft characteristics in lead zirconate titanate ceramics (PZT) by an electron transfer between constitutive ions and/or dopants such as Ti3+/4+ or Fe2+/3+. These electron transfers are supposed to minimize the space charges (ρ p ) existing at the domain walls interface due to polarization divergences (ρ p = −div ), making the domain wall movement easier (soft PZTs). Recently, the Eyrauds model has been refined and the softening mechanism is now explained by electron exchange between oxygen and lead vacancies owing to the presence of these defects in every doped PZT. Based on the analogy with BaTiO3, the proposed correction of the model is justified using the energy levels of barium and oxygen vacancies given by Daniels and Härdtl. It is supposed that the material will exhibit soft properties if acceptor and donor sites are associated with neighbouring energy levels allowing direct electron tranfer between each other, without contributing to electron transfers from the valence band or to the conduction band. So, the present paper briefly recalls the previous results about the Eyrauds model and secondly presents the new interpretation of the softening effect. Every basic doped PZT family will be considered, that is to say, PZTs doped with an acceptor, PZTs doped with a donor and the particular (Mg, F) and (Mn, F) co-doped PZTs developed in our Laboratory.

Analysis of AC-DC conversion for energy harvesting using an electrostrictive polymer P(VDF-TrFE-CFE)

Harvesting systems capable of transforming unused environmental energy into useful electrical energy have been extensively studied for the last two decades. The recent development of electrostrictive polymers has generated new opportunities for harvesting energy. The contribution of this study lies in the design and validation of electrostrictive polymer- based harvesters able to deliver dc output voltage to the load terminal, making the practical application of such material for self-powered devices much more realistic. Theoretical analysis supported by experimental investigations showed that an energy harvesting module with ac-to-dc conversion allows scavenging power up to 7 μW using a bias electric field of 10 V/μm and a transverse strain of 0.2%. This represents a power density of 280 μW/cm3 at 100 Hz, which is much higher than the corresponding values of most piezo-based harvesters.