Tailoring porous piezoelectric properties for selected modes of energy harvesting

The piezoelectric charge coefficient and permittivity, of a piezoelectric material are the governing properties in the widely accepted figures of merit (FoM) for stress-driven longitudinal and transverse modes of mechanical energy harvesting. The introduction of porosity into a range of piezo-ceramics has been developed as an effective method for reducing the permittivity and increasing the FoM. However, the FoM suggests that maintaining a high piezoelectric charge coefficient should dominate the final performance, and that porous materials are only suitable for certain harvesting modes. This work reconsiders this aspect, assessing the influence of introducing porosity, its connectivity and orientation relative to the poling direction in aligned structures formed by freeze casting. This was conducted for materials with a range of bulk FoM values, lead zirconate titanate, barium titanate and potassium sodium niobate, to assess its effectiveness compared to mechanical property trade-offs.