Ferroelectricity Explained: Classification, Characteristics, and Applications

Ferroelectrics are a class of insulators, known as dielectrics. Dielectrics are categorized based on their electrical properties:

These categories are hierarchical. Ferroelectrics are a subclass of pyroelectrics, and pyroelectrics are a subclass of piezoelectrics. All of these fall under the broader category of dielectrics. The diagram below illustrates this relationship.

When voltage is applied to a dielectric, current does not flow; instead, positive and negative charges within the material are displaced relative to each other. This phenomenon is called polarization. In paraelectric materials, polarization vanishes once the voltage is removed. In contrast, ferroelectric materials retain their polarization even after the voltage is reduced to zero.

Piezoelectric materials generate polarization when mechanical stress is applied (direct piezoelectric effect) and deform when subjected to an electric field (inverse piezoelectric effect).

Pyroelectric materials generate an electrical potential in response to temperature changes. Under steady-state conditions, surface charges are typically neutralized by ions in the air. However, when the temperature changes, the internal polarization changes, disrupting this balance and generating an observable voltage. Like ferroelectrics, pyroelectrics possess spontaneous polarization, but unlike ferroelectrics, the direction of this polarization cannot be reversed by an external electric field.

Since ferroelectrics are a subset of these materials, they exhibit both piezoelectric and pyroelectric properties. This means ferroelectrics respond mechanically to voltage and electrically to temperature changes and stress.

Historically, the prefix "ferro-" was adopted because these materials exhibit hysteresis loops similar to ferromagnetism, despite having no relation to iron.

Potassium sodium tartrate (Rochelle salt) was one of the earliest known ferroelectrics. Today, materials such as potassium dihydrogen phosphate (KDP) and barium titanate (BaTiO₃) are widely used.

Ferroelectric materials undergo a phase transition to a paraelectric phase as the temperature rises. Based on the mechanism of this transition, they are classified into two main types: "displacive" and "order-disorder."

The defining characteristic of ferroelectrics is spontaneous polarization that can be reversed by an external electric field. This polarization arises from the distortion of the crystal lattice, creating an electrical imbalance. Applying a voltage can shift the ions, reversing the direction of polarization.

Ferroelectrics also exhibit a hysteresis loop in their polarization-electric field (P-E) relationship, similar to the B-H loop found in ferromagnetic materials.