The piezoelectric effect was discovered by Jacques and Pierre Curie in 1880. The initial observation was the appearance of a dielectric charge on a crystal proportional to an applied mechanical stress. Soon thereafter, the converse effect i.e. the geometrical strain of a crystal proportional to an applied electrical field was discovered.
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Basics on piezoelectric material
Piezoelectricity is the property of some materials to develop electric charge on their surface when mechanical stress is exerted on them. An applied electrical field produces a linearly proportional strain in these materials. The electrical response to mechanical stimulation is called the direct piezoelectric effect, and the mechanical response to electrical simulation is called the converse piezoelectric effect.
Different piezoelectric materials
Piezoelectricity is the property of nearly all materials that have a non-centrosymetric crystal structure. Some naturally occurring crystalline materials possessing these properties are quartz and tourmaline. Some artificially produced piezoelectric crystals are Rochelle salt, ammonium dihydrogen phosphate and lithium sulphate. Another class of materials possessing these properties is polarized piezoelectric ceramic.
In contrast to the naturally occurring piezoelectric crystals, piezoelectric ceramics are of a “polycrystalline” structure. The most commonly produced piezoelectric ceramics are lead zirconate titanate (PZT), barium titanate and lead titanate. Ceramic materials have several advantages over single crystal, especially the ease of fabrication into a variety of shapes and sizes. In contrast, single crystals must be cut along certain crystallographic directions, limiting the possible geometric shapes, but offer superior piezoelectric properties.
PZT crystal structure
PZT have crystal structures belonging to the perovskite family with the general formula AB03. In the following figure, the ideal, cubic perovskite structure is shown. PZT crystallites are centro-symmetric cubic (isotropic) above the Curie temperature and exhibit tetragonal symmetry (anisotropic structure) below the Curie temperature.
Before poling, a piezoelectric ceramic material consists of small grains (crystallites), each containing domains in which the polar direction of the unit cells is aligned. These grains and domains are randomly oriented; hence, the net polarization of the material is zero, i.e. the ceramic does not exhibit piezoelectric properties. The application of a sufficiently high DC electrical field (called poling process) will orient the domains in the field direction and lead to a remanent polarization of the material.
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