The piezoelectric effect in ceramic materials

The piezoelectric effect was discovered by Jacques and Pierre Curie in 1880. They found that if certain crystals were subjected to mechanical strain, they became electrically polarized and the degree of polarizationjwas proportional to the applied strain. The Curies also discovered that these same materials deformed when they were exposed to an electric field. This has become known as the inverse piezoelectric effect.

The piezoelectric effect is exhibited by a number of naturally-occurring crystals, for instance quartz, tourmaline and sodium potassium tartrate, and these have been used for many years as electromechanical transducer. For a crystal to exhibit the piezoelectric effect, its structure should have no centre of symmetry. A stress (tensile or compressive) applied to such a crystal will alter the separation between the positive and negative charge sites ineach elementary cell leading to a net polarization at the crystal surface. The effect is practically linear, i.e. the polarization varies directly with the applied stress, and direction-dependent, so that compressive  and tensile stresses will generate  electric fields and hence voltage of opposite  polarity. It's also reciprocal, so that if the piezoelectric ceramic crystal is exposed to an electric field, it will experience an elastic strain causing its length to increase or dicrease according to the field polarity.