Orienting anisometric pores in ferroelectrics: Piezoelectric property engineering through local electric field distributions

Ferroelectrics are a technologically important class of materials that are used in actuators, sensors, transducers, and memory devices. Introducing porosity into these materials offers a method of tuning functional properties for certain applications, such as piezo- and pyroelectric sensors and energy harvesters. However, the effect of porosity on the polarization switching behavior of ferroelectrics, which is the fundamental physical process determining their functional properties, remains poorly understood. In part, this is due to the complex effects of porous structure on the local electric field distributions within these materials. To this end, freeze-cast porous lead zirconate titanate (PZT) ceramics were fabricated with highly oriented, anisometric pores and an overall porosity of 34 vol.%. Samples were sectioned at different angles relative to the freezing direction, and the effect of pore angle on the switching behavior was tracked by measuring simultaneously the temporal polarization and strain responses of the materials to high-voltage pulses. Finite-element modeling was used to assess the effect of the pore structure on the local electric field distributions within the material, providing insight into the experimental observations. It is shown that increasing the pore angle relative to the applied electric field direction decreases the local electric field, resulting in a reduced domain-wall dynamic and a broadening of the distribution of switching times. Excellent longitudinal piezoelectric (d33=630pm/V) and strain responses (Sbip=0.25% and Sneg=0.13%, respectively), comparable to the dense material (d33=648pm/V, Sbip=0.31%, and Sneg=0.16%), were found in the PZT with anisometric pores aligned with the poling axis. Orienting the pores perpendicular to the poling axis resulted in the largest reductions in the effective permittivity (ɛ33σ=200 compared to ɛ33σ=4100 for the dense PZT at 1 kHz), yielding the highest piezoelectric voltage coefficient (g33=216×10-3Vm/N) and energy-harvesting figure of merit (d33g33=73×10-12m2/N). These results demonstrate that a wide range of application-specific properties can be achieved by careful control of the porous microstructure. This work provides an understanding of the interplay between the local electric field distribution and polarization reversal in porous ferroelectrics, which is an important step towards further improving the properties of this promising class of materials for sensing, energy harvesting, and low-force actuators.