  Energy harvesting is an important developing technology for a new generation
of self-powered sensor networks. This paper demonstrates the significant
improvement in the piezoelectric energy harvesting performance of barium
titanate by forming highly aligned porosity using freeze casting. Firstly, a
finite element model demonstrating the effect of pore morphology and angle with
respect to poling field on the poling behaviour of porous ferroelectrics was
developed. A second model was then developed to understand the influence of
microstructure-property relationships on the poling behaviour of porous freeze
cast ferroelectric materials and their resultant piezoelectric and energy
harvesting properties. To compare with model predictions, porous barium
titanate was fabricated using freeze casting to form highly aligned
microstructures with excellent longitudinal piezoelectric strain coefficients,
d 33. Both model and experimental data indicated that introducing porosity
provides a large reduction in the permittivity () of barium titanate, which
leads to a substantial increase in energy harvesting figure of merit, , with a
maximum of 3.79 pm2 N-1 for barium titanate with 45 vol.% porosity, compared to
only 1.40 pm2 N-1 for dense barium titanate. Dense and porous barium titanate
materials were then used to harvest energy from a mechanical excitation by
rectification and storage of the piezoelectric charge on a capacitor. The
porous barium titanate charged the capacitor to a voltage of 234 mV compared to
96 mV for the dense material, indicating a 2.4-fold increase that was similar
to that predicted by the energy harvesting figures of merit.
