Piezoelectricity is the accumulation of electric charge in some solid materials such as crystals, ceramics, biological materials like bones, proteins DNA etc. The electrical charge accumulates when these materials are subjected to mechanical stress or heat. The word Piezo electricity was derived from Greek terms Piezein meaning squeeze or press and electric. The piezoelectricity is generated as a result of piezoelectric effect. The Piezoelectric effect is the linear electromechanical interaction between the electrical and mechanical states in the crystalline structure of the material.
The Piezoelectric Effect
The piezoelectric effect was discovered in 1880’s by Pierre and Jacques Curie. They found that when pressure is applied to certain crystals like quartz or ceramic, an electric voltage develops across the material. The piezoelectric effect is due to the asymmetry in the crystalline structure. This allows the ions to move along one axis than the others. When a mechanical stress is applied, each side of the crystal acquires opposite charges leading to a voltage drop across the crystal. This effect is linear and the voltage disappears when the mechanical or heat stress is removed.
Direct and Indirect Piezoelectric effect
The piezoelectric effect may be direct piezoelectric effect in which the electric charge develops as a result of the mechanical stress or reverse or indirect piezoelectric effect (Converse piezoelectric effect) in which a mechanical force such as vibration develops due to the application of an electric field. A typical example of direct piezoelectric effect is the generation of measurable amount of piezoelectricity when the Lead Zirconate Titanate crystals are deformed by mechanical or heat stress. The Lead Zirconate Titanate crystals also shows indirect piezoelectric effect by showing vibration when an electric potential is applied. This is the principle of sound generation in Piezo buzzer, Piezoelectric mic etc. The piezoelectric effect has wide applications in sound sensors, electronic frequency generation, Scanning Probe Microscopy such as STM, AFM etc. The simple application of piezoelectricity is the generation of Arc in Cigarette and Gas lighters.
How the piezoelectric effect develops?
The piezoelectric is closely related to the Electric dipole moments in solids. This may be induced for ions on the crystal lattice or may directly carried by the molecular groups. Dipoles tend to align in regions called Weiss domains in the crystals. These domains are randomly oriented in the crystals. The randomly oriented domains can be aligned properly through the process called Poling. The poling occurs when a strong electric field is applied at high temperature. All the piezomaterials do not show the poling phenomenon. The piezoelectricity depends on
1. Orientation of polarization (P) with in the crystal
2. Symmetry of crystals
3. Strength of applied mechanical or heat stress
Converse piezoelectric effect is the mechanical deformation of the crystal lattice due to a strong electric field.
CIRCUITS EXPLOITING PIEZOELECTRICITY