Origin of the Piezo Effect
The word “Piezoelectricity” comes from the Greek words meaning “pressure electricity”. It was discovered in 1880 by Pierre and Jacques Curie. They were investigating the well-known phenomena of generating a spark by striking certain materials such as quartz. The concept has been further refined to define piezoelectricity as the electric polarization produced by mechanical strain. The polarization is proportional to the strain and changes sign with it. The direct piezoelectric effect is the generation of electrical charge (polarization) from a mechanical stress. The converse effect is the generation of mechanical motion by the application of an electrical field.
Certain classes of crystals exhibit piezoelectricity because they lack a center of symmetry within the unit cells of atoms that make up their structure. In fact, about 2/3 of all materials can exhibit this effect. When you squeeze such a crystal structure it distorts in such a way that there is a net separation of the charged ions in the structure. This leads to a “positive” and “negative” direction in the unit cells. This is known as “Dielectric Displacement” and is manifested as a charge on the surface of the crystals.
A ceramic material, however, consists of millions of tiny crystallites all oriented at random. This random orientation causes the overall ceramic to appear completely non-polarized and thus to not exhibit a piezoelectric effect when squeezed. If the charge of each tiny crystallite can be aligned in the same direction then the overall ceramic can behave much like a single crystal. The alignment of the dielectric displacements is known as “Polarizing” or “Poling” for short.