Brahim Dkhil

The interaction of domain structure and defects in ferroelectric thin films has been studied for decades. However, the role of dislocations and thermal stability of microstructures is still poorly studied. By combining transmission electron microscopy, x-ray diffraction experiments, and phase-field simulations, we show that dislocation pairs induced by post-annealing above 550 °C provide a stress field stabilizing a domains in 30 nm thick tetragonal PbTiO3 films on SrTiO3 substrate, initially exhibiting pure c domains. Based on phase-field simulations, we further discuss the effects of single dislocations and dislocation pairs on the nucleation of a-domains and the occurrence of non-ferroelastic 180° domains. Dislocations, and the possibility to tune them using an appropriate thermal annealing process, offer a path for modulating the domains and domain wall states and, thus, the physical properties of ferroelectric films.

A figure of merit for assessing the cooling efficiency of electrocaloric (EC) materials is defined, where the caloric properties are taken into account jointly with the material's losses. Using a specifically developed measurement setup, based on flexible thermistances, the caloric effect and the losses are directly measured on P(VDF-TrFE-CFE) electrocaloric polymer films. The data are used, jointly with our figure of merit, to extrapolate the cooling efficiency to be expected, under actual working conditions, from the studied EC material. Dielectric losses emerge as a major limiting factor for achieving the needed cooling performance. This finding shows that, beside the research for huge caloric response, material loss reduction has to be considered a key objective for researching an optimal EC refrigerant for cooling applications. Eventually, some strategies toward loss reduction are outlined.