Why is piezo material polarized?
A strong electric field of several kV/mm is applied to create an asymmetry in the previously unorganized ceramic compound. The electric field causes a reorientation of the spontaneous polarization. At the same time, domains with a favorable orientation to the polarity field direction grow and those with an unfavorable orientation shrink. After polarization, most of the reorientations are preserved even without the application of an electric field. However, a small number of the domain walls are shifted back to their original position, e.g., due to internal mechanical stresses.