If a polycrystalline ceramic is ferroelectric, then it can be given artificial anisotropy, and thus piezoelectricity, on a macroscopic level. The anisotropy results from a treatment called poling, i.e., the application of a large electric field.
In practice, poling usually involves subsequent heating above the Curie point, application of the electric field, cooling below the Curie point, and finally removal of the electric field. Upon heating the material above the Curie point, the crystal structure becomes centrosymmetric, and all electric dipoles vanish. When the material is cooled in the presence of a sufficiently large electric field, the dipoles tend to align with the applied field, all together giving rise to a nonzero net polarisation.
After cooling and removal of the electric field, not all dipoles can return to their original direction. The result is a remanent polarisation throughout the ceramic as well as a permanent deformation. The polycrystalline ceramic now does exhibit an artificial anisotropy, enabling piezoelectric behaviour. The piezoelectricity is maintained as long as the material is not depoled, due to for example a temperature above the Curie point, or extreme electric or mechanical conditions.
The relationship between the polarisation in ferroelectrics and an applied electric field is shown below. For an unpoled, “fresh” ferroelectric the initial polarisation can be seen to be zero. Application of an electric field causes the individual dipoles to tend to align to this field, macroscopically observed as domain-wall motion. This process, causing a nonlinear increase of the overall polarisation, may continue up to a certain point at which maximum dipole alignment has been obtained. An even further increase of the polarisation then only stems from the increase of all individual dipole moments, known to vary linear with an increase of the electric field.
Removal of the electric field will leave a net remnant polarisation: the ferroelectric is said to be ‘poled’. A poled single crystal is referred to as ‘single-domain’. Apart from the remanent polarisation, Pr, of interest to us is also the spontaneous polarisation Ps obtained by extrapolating the polarisation at high electric field back to zero along a tangent. Ps is somewhat higher than Pr in ceramics, but is virtually equal to Pr in crystals. The difference can be explained by intergranular stresses present in ceramics, which cause part of the dipoles to revert to their original directions once the electric field has been removed. From the figure we observe that exposure to subsequent highly positive and highly negative electric fields causes polarisation reversal, leading to the typical dielectric hysteresis loop shown by ferroelectrics.