Piezoelectric Effect

When certain dielectrics deform under external force applied along a specific direction, polarization occurs inside the materials, and opposite positive and negative charges emerge on their two opposite surfaces. Once the external force is removed, the materials return to an uncharged state. This phenomenon is known as the direct piezoelectric effect. If the direction of the applied force changes, the polarity of the charges changes accordingly.

Conversely, when an electric field is applied to the dielectric along its polarization direction, the dielectric deforms. The deformation disappears after the electric field is withdrawn, which is defined as the inverse piezoelectric effect. Sensors developed based on the piezoelectric effect of dielectrics are called piezoelectric sensors.

Piezoelectric Materials

Piezoelectric materials are crystalline materials that generate a voltage across their two end faces under pressure.

In 1880, French physicists Pierre Curie and Jacques Curie discovered that placing weights on quartz crystals would produce electric charges on certain crystal surfaces, with the quantity of electric charge proportional to the applied pressure. This phenomenon was named the piezoelectric effect. Shortly afterward, the Curie brothers also discovered the inverse piezoelectric effect—piezoelectric crystals deform under an external electric field.

Mechanism of the Piezoelectric Effect

Piezoelectric crystals feature low symmetry. When deformed by external force, the relative displacement of positive and negative ions within unit cells separates the centers of positive and negative charges, leading to macroscopic polarization of the crystal. The surface charge density of the crystal equals the projection of polarization intensity along the normal direction of the surface. For this reason, opposite charges appear on the two end faces of piezoelectric materials when they deform under pressure. On the contrary, when piezoelectric materials are polarized in an electric field, displacement of charge centers causes material deformation.

Applications

These properties of piezoelectric materials enable mutual conversion between mechanical vibration (sound waves) and alternating current. Therefore, piezoelectric materials are widely used in sensor components, such as seismic sensors, measuring units for force, velocity and acceleration, and electroacoustic sensors.

A common daily example is the spark produced by a lighter, which relies on this technology.