M4 Piezoelectricity and Ferroelectricity

Topic

Below a certain temperature, the unit cell of this ceramic loses its inversion symmetry: the constituent positive and negative ions no longer distribute themselves in a way that results in exact cancellation upon inversion about the cell's center, and this imbalance leaves a net electric dipole within each cell. The loss of inversion symmetry in the unit cell enables the electromechanical coupling that defines this material.

Because the unit cell has lost its inversion symmetry, the ceramic exhibits the piezoelectric effect in both its direct and converse forms: in the direct form, mechanical stress applied to the ceramic deforms the cell's electric dipoles, thereby generating a measurable electric charge on its surface; in the converse form, an electric field applied to the ceramic acts upon those same dipoles, resulting in mechanical deformation of the material. Since both forms of the piezoelectric effect share the same origin—the breaking of the unit cell's inversion symmetry—they are, in essence, the same electromechanical conversion process occurring in opposite directions.

This electromechanical conversion—in either direction—is never perfectly efficient; the electromechanical coupling factor quantifies how much of the applied mechanical energy is effectively converted into electrical energy, or how much of the applied electrical energy is effectively converted into mechanical energy. A higher electromechanical coupling factor for this ceramic results in more efficient conversion, whereas a lower factor means a larger fraction of the applied energy is lost without being converted into the other form of energy. However, the inversion symmetry breaking that characterizes the initial state of this model does not persist at all temperatures: as the ceramic's temperature approaches its Curie temperature, this symmetry breaking weakens until it vanishes completely at the Curie temperature, where the unit cell regains its inversion symmetry. This tendency of the unit cell to recover such symmetry causes the ceramic's dielectric permittivity to diverge near the Curie temperature—as described by the Curie-Weiss law—whereby the closer the ceramic's temperature gets to the Curie temperature, the higher the dielectric permittivity becomes, ultimately increasing without bound exactly at the Curie temperature point.

Consequently, this model—defined by the unit cell's initial inversion symmetry breaking—differs from the carrier transport mechanism described for semiconductors by other models in this same category; the latter mechanism involves the movement of charge carriers through the lattice rather than any inversion symmetry breaking. Conversely, the fact that this same inversion symmetry breaking underpins the ceramic's piezoelectric effect means that this model informs two other models within the group: one describing ferroelectric fatigue—where the aging of the established piezoelectric response leads to its eventual degradation—and another describing the ceramic's electrical characterization, where the hysteresis loop between polarization and the electric field serves to experimentally quantify this very ferroelectric response.

ID:4393