Electrostatically induced β-phase alignment in the PVDF/thermally annealed CoFe₂O₄/Ni magnetoelectric laminates

Polymer-based magnetoelectric (ME) composites are garnering increasing attention for developing functional devices in various electronic applications. In terms of polymer-based ME laminates, which comprise piezoelectric (PE) polymer films and magnetostrictive (MS) substrates, the contact interface between the phases significantly affects the ME response owing to a mechanical and electrical interaction. In this study, poly(vinylidene fluoride) (PVDF)-based ME laminates composed of annealed cobalt ferrite (a-CFO)/nickel (Ni) (a-CFO/Ni) MS substrate were designed to trigger electrostatically aligned electroactive β-phase crystalline within PE films. The Ni substrates were treated through sol-gel coating of CFO precursor solution followed by thermal annealing under various temperatures. Next, the PVDF solution was cast on the multiphase MS substrate to build PVDF/a-CFO/Ni laminates with varying casting heights to investigate the effective distance for β-phase formation and alignment. Finally, for the ME laminates prepared with the lowest casting height, the highest ME coefficient of 1.24 mV/cm·Oe was observed, accompanied by a β-phase fraction of 91.7% and a crystallinity degree of 56.3% in the PE phase. It is particularly noteworthy that the prepared laminates exhibited ME voltage without an electrical poling process. This result was assumed to be because the effective alignment of β-phase crystallites was promoted by active electrostatic interaction between the phases owing to the negatively charged surface of a-CFO/Ni substrates. In summary, it has been demonstrated that surface-charged MS substrates have considerable potential to improve energy-conversion efficiency by raising the electroactive phase fraction and making the process more cost-effective without post-polarization treatment.

Poly(vinylidene fluoride) (PVDF)/annealed cobalt ferrite/nickel (PVDF/a-CFO/Ni) magnetoelectric (ME) laminates are designed to induce β-phase alignment. Multiphase annealed cobalt ferrite/nickel (a-CFO/Ni) magnetostrictive (MS) substrates exhibit a negatively charged surface. The highest value is 91.7% for β-phase fraction induced by surface-modified MS substrates in PVDF/a-CFO/Ni laminates. Investigation of the maximal ME response with self-bias effect in PVDF/a-CFO/Ni laminates.