Freeze-cast porous textured BaTiO₃-polymer composites for energy harvesting applications

Porous piezoelectric ceramics exhibit a unique combination of high piezoelectric charge coefficients (d _ij) and low permittivity compared to their dense counterparts, which is desirable for achieving high piezosensing and energy harvesting performance. A further enhancement in performance can be achieved by inducing crystallographic texturing within the porous lead-free piezoceramic matrix while maintaining the aligned porous structure. Here, we report a process demonstrating the use of directional freeze-casting of BaTiO ₃platelets to fabricate lead-free porous textured BaTiO ₃ceramics with highly aligned porosity. A high degree of alignment of the piezoelectric BaTiO ₃platelets in the freezing direction was confirmed by using scanning electron microscopy. The degree of texturing was quantified by X-ray diffraction, yielding a Lotgering factor (LF) of ∼0.37. To enhance the mechanical strength and strain to failure for sensing and harvesting applications, the porous textured BaTiO ₃ceramics (∼60 vol % porosity, sintered at 1150 °C for 4 h) were infiltrated with polymers (epoxy and polydimethylsiloxane) of contrasting elastic properties. The BaTiO ₃–epoxy composite structure demonstrated a strain (%) to failure of 0.93 ± 0.005 at a high failure stress of 71.6 ± 3.05 MPa, with Young’s modulus of 7.6 ± 0.02 GPa. In contrast, the BaTiO ₃–PDMS composite had a flexible nature and exhibited a lower Young’s modulus of 0.015 ± 0.0012 GPa and a higher strain (%) to failure (>22 ± 1.5). The dielectric properties, polarization–electric field loops, and piezoelectric properties were examined in detail, and the poled BaTiO ₃–epoxy composite was used to fabricate a cantilever structure to demonstrate its energy harvesting and sensing performance. This work has shown that directional freeze-casting can produce an aligned porous and textured ferroelectric microstructure for sensing or energy harvesting applications.