Physical Mechanisms Of Piezoelectric Properties Of Piezoelectric Ceramics

Piezoelectric ceramics are a kind of polycrystal, its piezoelectricity can be explained by the piezoelectric properties of the crystal, the crystal under the mechanical force, the total electric dipole moment (polarization) changes, thus presenting a piezoelectric phenomenon, so piezoelectric properties and polarization, deformation, etc. are closely related.
1. Microscopic mechanism of polarization
The polarized state is a state in which the electric field has a temporary balance and unity of mutual attraction between the force and charge that produce relative displacement of the charged mass point of the dielectric. There are three main polarization mechanisms.
(1) Electron displacement polarization - under the action of electric field force, the positively charged nucleus of the dielectric nucleus does not coincide with the negative charge center of the shell electrons.
(2) Ion shift polarization - the positive and negative ions of the dielectric undergo relative displacement under the action of electric field force, resulting in an electric dipole moment.
(3) Orientation polarization - the polar molecules that make up the dielectric, have a certain intrinsic (intrinsic) electric moment, due to thermal motion, the orientation is disordered, the total electric moment is zero, when an electric field is applied, the electric dipole moment is arranged along the direction of the electric field, and a macroscopic electric dipole moment appears.
For anisotropic crystals, the polarization intensity has the following relationship with the electric field wps60.jpg m,n=1,2,3
The key word in the picture is polarizability, or written as wps62.jpg with electric displacement
2. Piezoelectric effect
(1) Positive piezoelectric effect
When the piezoelectric crystal deforms under the action of external force, the center of positive and negative charges is displaced, and a different charge is generated on some corresponding surfaces, and the polarization intensity appears. This phenomenon of polarization due to deformation without electric field action is called the positive piezoelectric effect.
For anisotropic crystals, stress WPS63 is applied to the crystal.jpg; (corresponding strain wps64.jpg), the crystal will have a polarization intensity proportional to wps65 .jpg in the three directions of X, Y, Z, that is: wps66.jpg wps67.jpg in the formula wps68.jpg wps68 and wps69.jpg respectively called piezoelectric stress constant and piezoelectric strain constant.
(2) Inverse piezoelectric effect
When an electric field is applied to a crystal, not only polarization but also deformation occurs, and this phenomenon of deformation caused by the electric field is called the inverse piezoelectric effect. This is due to the fact that when the crystal is subjected to an electric field, stresses (piezoelectric stress) are generated inside the crystal, and piezoelectric strain is generated through stress.