An All-Scale Hierarchical Architecture Induces Colossal Room-Temperature Electrocaloric Effect at Ultralow Electric Field in Polymer Nanocomposites.

Composed of electrocaloric (EC) ceramics and polymers, polymer composites with high EC performances are considered as promising candidates for next-generation all-solid-state cooling devices. Their mass application is limited by the low EC strength, which requires very high operational voltage to induce appreciable temperature change. Here, an all-scale hierarchical architecture is proposed and demonstrated to achieve high EC strength in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)-based nanocomposites. On the atomic scale, highly polarizable hierarchical interfaces are induced by incorporating BiFeO3 (BFO) nanoparticles in Ba(Zr0.21 Ti0.79 )O3 (BZT) nanofibers (BFO@BZT_nfs); on the microscopic scale, percolation of the interfaces further raises the polarization of the composite nanofibers; on the mesoscopic scale, orthotropic orientation of BFO@BZT_nfs leads to much enhanced breakdown strength of the nanocomposites. As a result, an ultrahigh EC strength of ≈0.22 K m MV-1 is obtained at an ultralow electric field of 75 MV m-1 in nanocomposites filled with the orthotropic composite nanofibers, which is by far the highest value achieved in polymer nanocomposites at a moderate electric field. Results of high-angle annular dark-field scanning transmission electron microscopy, in situ scanning Kelvin probe microscopy characterization, and phase-field simulations all indicate that the much enhanced EC performances can be attributed to the all-scale hierarchical structures of the nanocomposite.