Title: Size-Induced High Electrocaloric Response of Dense Ferroelectric Nanocomposites

Abstract: Analytical results obtained within Landau-Ginzburg-Devonshire approach and effective media models, predict that the synergy of size effects and Vegard stresses can significantly enhance the electrocaloric cooling (up to 7 times) of the BaTiO3 nanoparticles in comparison with a bulk BaTiO3. To compare with the considered effective media models, we measured the capacitance-voltage and current-voltage characteristics of the dense nanocomposites consisting of (28-35) vol.% BaTiO3 nanoparticles incorporated in organic polymers and determined experimentally the effective dielectric permittivity and losses of the composites. Generalizing obtained analytical results, various ferroelectric nanoparticles spontaneously stressed by elastic defects, such as oxygen vacancies or any other elastic dipoles, which create a strong chemical pressure, can cause the giant electrocaloric response of dense ferroelectric nanocomposites. We have shown that the advantages of the studied lead-free dense nanocomposites are the good tunability of electrocaloric cooling temperature due to the size effects in ferroelectric nanoparticles and the easy control of the high electrocaloric cooling by electric fields. This makes the dense ferroelectric nanocomposites promising for cooling of conventional and innovative electronic elements, such as FETs with high-temperature superconductor channels.