  Ferroelectrets are piezoelectrically-active polymer foams that can convert
externally applied loads into electric charge. Existing processing routes used
to create pores of the desired geometry and degree of alignment appropriate for
ferroelectrets are based on complex mechanical stretching and chemical
dissolution steps. As a simple, cost effective and environmentally friendly
approach, freeze casting is able to produce aligned pores with almost all types
of the materials, including polymers. In this work, we present the first
demonstration of freeze casting to create polymeric ferroelectrets. The pore
morphology, phase analysis, relative permittivity and direct piezoelectric
charge coefficient (d33) of porous poly(vinylidene fluoride (PVDF)
ferroelectrets with porosity volume fractions ranging from 24% to 78% were
analysed. The long-range alignment of pore channels produced during directional
freezing was shown to be beneficial in forming a highly polarised structure
after breakdown of air in the pore channels via corona poling. This new method
opens a way to create tailored pores and voids in ferroelectret materials for
transducer applications related to sensors and vibration energy harvesting.
