Notice that the relative permittivity of the cold sintered film is still much lower than the typical permittivity of PZT-5A [55]. The coercive field is ~2 times as high as Nb-doped PZT thin film. These results indicate the existence of residual porosity and possibly lead oxide-rich grain boundaries. In Fig. 2(d): closed pores can be seen in the sintered PZT and the grain boundaries are not clear. Consequently, after the cold sintered PZT film was poled at 3Ec (300 kV/cm), the measured e31,f was only 2.6±0.01 C/m2 (95% confidence interval), a significantly lower value than is typical of dense PZT-5A (16 C/m2) [55,56]. The sintered sample without CSP, however, returned zero e31,f.