Piezoelectrics are an important class of materials for mechanical energy harvesting technologies. In this paper we evaluate the piezoelectric harvesting process and define the key material properties that should be considered for effective material design and selection. Porous piezoceramics have been shown previously to display improved harvesting properties compared to their dense counterparts due to the reduction in permittivity associated with the introduction of porosity. We further this concept by considering the effect of the increased mechanical compliance of porous piezoceramics on the energy conversion efficiency and output electrical power. Finite element modelling is used to investigate the effect of porosity on relevant energy harvesting figures of merit. The increase in compliance due to porosity is shown to increase both the amount of mechanical energy transmitted into the system under stress-driven conditions, and the stress-driven figure of merit, FoM X 33 , despite a reduction in the electromechanical coupling coefficient. We show the importance of understanding whether a piezoelectric energy harvester is stress-or strain-driven, and demonstrate how porosity can be used to tailor the electrical and mechanical properties of piezoceramic harvesters. Finally, we derive two new figures of merit based on the consideration of each stage in the piezoelectric harvesting process and whether the system is stress-(F X ij), or strain-driven (F x ij).

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The use of time constant related parameter fmax to obtain the activation energy for bulk electrical conduction (and any discussion of the possible conduction mechanisms) in ferroelectric materials should be used with caution.

A new micromechanical model is proposed to analyse the piezoelectric properties of freeze-cast porous composite materials based on a ferroelectric lead zirconate titanate-type (PZT) ceramics. The important influence of the composite microgeometry and the porous ceramic matrix on the piezoelectric coefficients * 3 j d and * 3 j g and the piezoelectric anisotropy factor * 33 d / | * 31 d | in the porosity range of m p = 0.2-0.6 is evaluated and discussed. The resulting piezoelectric parameters of parallel-connected freeze-cast composites with highly aligned pore channels are then compared to those of PZT-based porous materials with randomly distributed porosity. Due to the relatively large piezoelectric coefficients * 33

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