where for PZT ceramics d33 ≈ 2 |d31| and g33 ≈ 2|g31|. From this equation, the figure of
merit for dense PZT tends to zero. On the other hand in the porous piezoceramics a partial
decoupling of the piezoelectric response between the longitudinal and transversal direction
occurs. As a consequence, the more the porosity increases the more the |d31|value decreases
in respect to the d33 one. In this way, the electrical response is maximized along the most
useful direction (i.e. in the thickness, 33) and, at the same time, minimized in the other ones.
The figure of merit is in this way improved by more than three orders of magnitude,
decreasing the effect in the 31 direction while keeping constant the one in 33.