세미나

In the first topic, we demonstrate the flexoelectric-boosted electromechanical properties of piezoelectric nanoparticles using an induced built-in strain gradient in heterogeneous core-shell nanostructure. The composition-graded core-shell structure of BaTiO₃@SrTiO₃ nanoparticles enables a significant increment of the effective piezoelectric charge coefficient via the chemical heterogeneities-induced lattice strain gradient. Through the combinations of ab-initio calculation and multiphysics simulations, the origin of the strain distribution over nanoparticles is theoretically interpreted with accompanying phase balance and diffusion criteria. In addition, our designed core-shell nanoparticles-based energy harvesting devices generate highly efficient and flexoelectric-boosted piezoelectric output signals. Individual core-shell nanoparticles and related elastomeric nanocomposites reported in this work represent state-of-the-art electromechanical properties compared to previously reported piezoelectric nanoparticles and composites.

In the second topic, the concept for the morphotropic phase boundary (MPB), which has been exclusive in the field of high-performance piezoelectric ceramics, has been surprisingly confirmed in P(VDF-TrFE) piezoelectric copolymers by the groups. This study demonstrates the exceptional behaviors reminiscent of MPB and relaxor ferroelectrics in the feature of widely utilized electrospun P(VDF-TrFE) nanofibers. Consequently, an energy harvesting device that exceeds the performance limitation of the existing P(VDF-TrFE) materials is developed. Even the unpoled MPB-based P(VDF-TrFE) nanofibers show higher output than the electrically poled normal P(VDF-TrFE) nanofibers. This study is the first step toward the manufacture of a new generation of piezoelectric polymers with practical applications.

In the third topic, electrospun silk nanofiber mat was soaked into ethanol (EtOH) for 2 h. Through the EtOH treatment, a phase transition from α-helix phase to β-sheet phase is observed. The molecular phase transition from α-helix to β-sheet induced the reorganization of silk fibroin’s phase structure, which influence the dipole moment of molecular hydrogen bonds. The electrospun- silk biodegradable piezoelectric generators were fabricated with as-spun and EtOH-immersed silk mat respectively. The output of EtOH treated device was 10 times higher (up to ~7 V and 150 nA) than the device fabricated with as-spun silk mat. To specify the relationship between piezoresponse property and molecular hydrogen bonding, electrical poling was applied to both as-spun and EtOH treated piezoelectric generators. Contrary to the general case of ferroelectric polymer, electrical output of both E-SBPGs were decreased when the high voltage was applied. We defined this result as a ‘quasi-piezoelectricity’. Due to the breaking of hydrogen bonds, dipole moment of hydrogen bonds is deteriorated, and it leads to decline of piezoelectric output. Finally, the EtOH-immersed E-SBPG attached to the different body parts to utilized as a self-powered motion detecting sensor.