*2.3.2. Formation of cracks*

on several other factors, such as the composition and stoichiometry of the material, tempera‐ ture, geometry of sample and electrodes, electrical loading characteristics, including the electric field amplitude and frequency. Hence a succinct classification of the entire phenom‐

Fig. 4 shows the typical hysteresis loops observed after different types of fatigue-like electrical loading. Fig. 4a is relative to the effects of fatigue under unipolar AC electrical loading after

P-E and S-E curves of PZT 4D ceramics previously poled under a DC poling field of ± 2.5kV/mm applied at T = 125 °C for 5 minutes.[17] It can be seen that the sign of the DC poling field significantly affects the shape of the P-E and S-E loops. In particular, samples pre-poled under positive DC field show P-E loops shifted towards left and S-E loops with a suppression of the left wing; opposite effects can be noticed in samples poled under negative DC field. However, it is worth recalling that to correctly establish the sign and value of polarization and strain in samples subjected to previously electrical loading, polarization and strain must be monitored for the entire electrical history. During DC poling of PZT 4D, polarization and strain were not monitored, therefore, the sign and values in Fig. 4c have relative validity; only the polarization and strain amplitudes are meaningful. Fig. 4d shows the P-E and S-E curves of BaTiO3 ceramics sintered at different temperatures after one loading-unloading cycle up to approximately 3 kV/mm field amplitude and 10 Hz frequency. It can be concluded that the deformation of the hysteresis loops is caused by the presence of an internal bias field which

**Figure 4.** Fatigue-like effects on P-E and S-E hysteresis loops: (a) after unipolar electrical fatigue cycles in PIC 151 ce‐ ramics, courtesy of N. Balke, after [15]; (b) after unipolar electrical fatigue cycles in 0.94BNT-0.06BT ceramics, courtesy of Z. Luo, after [16]; (c) PZT 4D after DC poling of 2.5kV/mm applied for 5 minutes at 125°C [17]; (d) after one AC

 cycles on the bipolar P-E and S-E curves of Pb0.99[Zr0.45Ti0.47(Ni0.33Sb0.67)0.08]O3 (PIC 151) ceramics.[15] Fig. 4b shows the P-E and S-E bipolar loops of 0.94Bi1/2Na1/2TiO3–0.06BaTiO3

electric field bipolar cycles.[16] Fig. 4c displays the bipolar

enology becomes challenging.

212 Ferroelectric Materials – Synthesis and Characterization

(0.94BNT-0.06BT) after 1, 104

109

*2.3.1. Hysteresis loops after different types of fatigue-like loading*

and 107

forces the systems into preferred polarization and strain states.

poling cycle in BaTiO3 ceramics sintered at different temperatures [17].

Fatigued specimens often show the presence of microcracks, although it is hard to establish whether microcracking is a cause or a consequence of fatigue. It has been conveyed that microcracking can occur in samples with low density, in compositions with large grain size, with large unit cell distortion, and in compositions near the phase boundaries, where electric field-induced transitions can be activated during electrical loading [18]. In addition, macro‐ scopic cracks can develop in the interface region between sample and electrode. In particular, two types of cracks were observed in Pb0.99[Zr0.45Ti0.47(Ni0.33Sb0.67)0.08]O3 ceramics: i) *edge cracks* (propagating obliquely from the electrode inside the material) and ii) *delamination cracks* (forming beneath the electrode and propagating parallel to the electrode during bipolar cycling. The latter appear at a later fatigue stage than the former due to a reduced amount of switchable domains during fatigue.[19]
