This work deals with energy harvesting from temperature variations. It is shown here that direct pyroelectric energy harvesting (connecting an adapted resistance, for example) is not effective, whereas Ericsson-based cycles give energy 100 times higher. The principle and experimental validation of the Ericsson cycle are shown with the example of 0.90Pb(Mg 1/3 Nb 2/3 )O 3 -0.10PbTiO 3 ceramic. Harvested energy reached 186 mJ cm −3 for 50 • C temperature variation and electric field cycle of 3.5 kV mm −1 . A correlation between the electrocaloric effect and pyroelectric energy harvesting is then shown. Harvested electric energy with Ericsson cycles can be simply expressed as electrocaloric heat multiplied by Carnot efficiency. Several examples are then given from materials with the highest known electrocaloric effect. This leads to energies of hundreds of mJ cm −3 for a limited 10 • C temperature variation. Compared to Carnot's efficiency, this is much higher than the best thermoelectric materials based on the Seebeck effect.

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 audienceA new way to synthesize epoxy networks was reported in this work using phosphonium based ionic liquids combined with phosphinate, carboxylate, and phosphate counter anions. The influence of the chemical nature of the anions was investigated on the polymerization kinetics of epoxy systems as well as the thermal and mechanical properties of epoxy IL networks. In all cases, ILs displayed a high reactivity toward epoxy prepolymer and led to the formation of poly epoxy networks with high epoxy group conversion, i.e., up to 90%. In addition, epoxy IL networks have high hydrophobicity and an excellent thermal stability (above 350 degrees C) under N-2 with the glass transition temperatures (T-g) ranging from 90 to 140 degrees C depending on the chemical structure of ILs. For the first time, the mechanical properties of epoxy IL networks were also evaluated in terms of flexural properties and fracture toughness (K-Ic)