Piezoelectricity is the charge that accumulates in certain solid materials (notably quartz crystals, certain ceramics and biological matters such as human bone, DNA and various proteins) in response to applied mechanical strain (deformation) or stress. Some materials have the ability to produce electricity when subjected to mechanical stress. This is called "Piezoelectric effect". This stress can be caused by hitting or twisting the material just enough to deform its crystal lattice without fracturing it. The word piezoelectricity means electricity resulting from pressure. It is derived from the Greek word “piezo” or “piezein”, which means “to squeeze or press”, and “electric” or electron, which stands for “amber”, an ancient source of electric charge.
Piezoelectric effect also works in the opposite way, with the material deforming slightly when a small electric current is applied to it, encouraging their use in instruments for which great degrees of mechanical control are necessary. This is called “converse piezoelectricity”.
The first demonstration of the direct piezoelectric effect was in 1880 by the brothers Pierre Curie and Jacques Curie. They combined their knowledge of “Pyroelectricity” (the current created by heating or cooling the crystal) with their understanding of the underlying crystal structures that gave rise to piezoelectricity to predict crystal behavior, and demonstrated the effects using crystals of tourmaline, quartz, topaz, cane sugar and Rochelle salt (sodium potassium tartrate tetra hydrate) in which quartz and Rochelle salt exhibited the most piezoelectricity. The Curies, however, did not predict the converse piezoelectric effect. For the next few decades, piezoelectricity remained something of a laboratory curiosity. The first practical application for piezoelectric devices was “sonar”, first developed during World War I. During World War II, independent research groups in the United States, Russia and Japan discovered a new class of man-made materials, called “ferroelectrics”, which exhibited piezoelectric constants many times higher than natural materials.
Piezoelectric crystal consists of multiple interlocking domains which have positive and negative charges. These domains are symmetrical within the crystal, causing the crystal as a whole to be electrically neutral. The piezoelectric effect occurs when the charge balance within the crystal lattice of a material is disturbed. When there is no applied stress on the material, the positive and negative charges are evenly distributed and so there is no potential difference. When the lattice is slightly altered (when stress is applied on the crystal), the charge imbalance creates a potential difference, often as high as several thousand volts. Even a tiny bit of piezoelectric crystal can generate voltages in the thousands. However, the current is extremely small and only causes a small electric shock. The converse piezoelectric effect occurs when the electrostatic field created by electric current causes the atoms in the material to move slightly.