Vertically Printed Ferroelectret Microstructures

Using fused filament fabrication, ferroelectrets (i.e. polymer films with internal air-filled voids) can be manufactured from a variety of materials and with custom designed structures for optimum mechanical compliance and piezoelectric activity. In this study, we investigated a vertically oriented “double-wave” design, which results in structures with very low mechanical stiffness, and hence high piezoelectric $d_{33}$ coefficients. The structures were designed in an open-source parametric modeler and 3D-printed from polylactic acid (PLA) and polypropylene (PP). The total thickness is approx. $300 \mu\mathrm{m}$ and channel void heights are around $100 \mu \mathrm{m}$ . When charged in direct contact, the ferroelectret samples exhibited a remanent polarization of $0.25 \text{mC}/\mathrm{m}^{2}$ (PP) and $0.12 \text{mC}/\mathrm{m}^{2}$ (PP), respectively. Piezoelectric $d_{33}$ coefficients of up to $800 \text{pC}/\mathrm{N}$ were observed, but showed a strong decrease when static bias forces in excess of 1 N were applied to the 16 mm diameter electrodes. This behavior is likely caused by a stiffening of the double-wave structure as more compressive stress is applied. FEM calculations showed a very low effective Young's modulus of 125 kPa.