5.4.3 Poling of Piezoelectric Materials

As obvious from the previous sections (5.1 and 5.2), there are some piezoelectrics such as quartz which are not spontaneously polarized but get polarized upon application of stress, while ferroelectric which are anyway piezoelectric in nature are spontaneously polarized and show a change of polarization upon application of stress.

The values of piezoelectric coefficient of some materials are given below:

So, you can observe from this table that the level of strain generated is not so massive but is still important because of preciseness and reversibility of the effect.

Most ferroelectrics have to be poled to be useful as a piezoelectric. In the unpoled virgin state of the material, the ferroelectric domains of single polarization direction are randomly distributed across the material and in such a situation the net polarization would be zero. Application of stress to such a material would not achieve any change in the net polarization, thus making it useless as a piezoelectric.
Poling i.e. application of a large electric field near T_c (just below T_c)  orients the domains along the field and when the field is removed, the domain structure does not get back to the original condition giving rise to a net polarization along a certain direction. Now, when stress is applied to such a crystal, a noticeable change in the polarization can be observed.

Figure 5.21 Poling of ferroelectrics and application of stress on poled material