Polymer conformation-dependent piezoelectric properties and self-bias magnetoelectric responses in adhesive-free laminate composites of Ni-foam/PVDF/Ni-foam

In the past decade, polymer-based magnetoelectric (ME) laminate composites have been developed for feasible applications such as flexible energy harvesters and wearable sensors. The 2-2-type ME laminate composites have been typically prepared by epoxy adhesion of piezoelectric and magnetostrictive layers, which is a drawback that disrupts strain transfer from the magnetostrictive phase to the piezoelectric phase due to its strong hardness after epoxy curing.In this study, to overcome the disadvantage of insufficient strain transfer due to the adhesive layers, we designed adhesive-free laminate composites consisting of porous magnetostrictive nickel foam (NF) and soft poly (vinylidene fluoride) (PVDF) films, which exhibited good physical coupling via the sandwich hot-pressing of NF/PVDF/NF. In addition, the self-bias ME effect could be expected from NF, since it exhibits butterfly-shaped magnetostrictive hysteresis loops. Therefore, NF/annealed PVDF(AP)/NF and NF/hot-pressed PVDF(HP)/NF were prepared for investigating the polymer conformation during the fabrication process of ME laminates. Polymer conformation-dependent piezoelectric properties were investigated after annealing and hot-pressing, respectively. The NF/AP/NF laminates (mM/mP = 0.88) were found to exhibit a β/α phase ratio of 2.07, degree of crystallinity (χc) of 46.5%, piezoelectric charge constant (d33) of 17.7 pC/N, and self-bias ME response of 1.88 mV/cm Oe (94% of the maximal ME response at Hbias = 20 Oe). The NF/HP/NF laminates (mM/mP = 1.26) were found to exhibit a β/α phase ratio of 1.40, χc of 32.2%, d33 of 12.4 pC/N, and self-bias ME response of 2.27 mV/cm Oe (92% of the maximal ME response at Hbias = 10 Oe). The high β/α phase ratio were carried out sufficient stretching effect during hot-press laminating process. The optimal piezoelectric properties of β/α phase ratio and χc were induced by predominant stretching of PVDF chains rather than recrystallization during the hot-press. Further, reliable self-bias ME responses were obtained in the adhesive-free laminate composites by enhancement of coupling effect between magnetostrictive and piezoelectric phases.As a result, the sandwich laminate composites of NF/PVDF/NF were successfully prepared via hot-pressing and reliable ME responses were obtained without requiring the electroding process. This demonstrates the feasibility of ME laminate composites for use in wearable energy harvesters or sensors through removal of the hard adhesive layer.