Polarization of piezoelectric ceramic dielectric

Views: 16 Author: Site Editor Publish Time: 2018-09-15 Origin: Site

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Piezoelectric ceramics crystals are both dielectric and anisotropic dielectrics, so the dielectric properties of piezoelectric crystals are different from those of isotropic dielectrics.The dielectric is polarized under the action of an electric field, and the polarization state is a state in which the electric field exerts a relative displacement force on the charge point of the dielectric and a temporary balance of mutual attraction between the charges. The electric field is the external cause of polarization. The internal cause of polarization lies in the interior of the medium. With the microscopic processes inside the medium, there are three main mechanisms of polarization.

(1) An atom or ion that constitutes a dielectric. Under the action of an electric field, a positively charged nucleus does not coincide with the negative center of its shell electron, thereby generating an electric dipole moment. This polarization is called electron displacement polarization.

(2) The positive and negative ions that make up the dielectrics undergo relative displacement under the action of an electric field, resulting in an electric dipole moment called ion displacement polarization.

(3) The molecules that make up the dielectric are polar molecules with a certain intrinsic electric moment, but due to thermal motion, the orientation is disordered, and the total electric moment of the entire dielectric is zero. When an external electric field acts, these electric dipole moments will be aligned along the outer field,ultrasound piezoelectric crystal is producing a macroscopic electric dipole moment in the dielectric, which is called on orientation polarization.

Displacement polarization of an infinite molecule

When the electrodeless dielectric is in an external electric field under the action of the electric field force, the positive and negative charge centers of the molecule will produce relative displacements to form an electric dipole, and their equivalent electric dipole moments P are oriented along the direction of the electric field. For a dielectric piezoelectric as a whole, since each molecule in the dielectric forms electric dipoles, they are arranged in the dielectric. The positive and negative charges of adjacent electric dipoles in the dielectric are close to each other. If the dielectric is uniform, it remains electrically neutral throughout it, but on the surface of the dielectric that is perpendicular to the external electric field strength E0. There will be positive and negative charges, respectively, which cannot leave the dielectric and cannot move freely in the dielectric, This phenomenon of polarized charges in the dielectric under the action of an external electric field is called polarization of the dielectric. The stronger external electric field, the larger the relative displacement between the positive and negative charge centers of each molecule, the larger is the electric dipole moment of the molecule, the more polarized charges appear on both surfaces of the dielectric, and the more polarized high. When the external electric field of resonance frequency piezoelectric transducer is removed, the centers of the positive and negative charges are again coincident (P = 0), so this type of molecule can be regarded as an elastic electric dipole whose elastic force is connected by two equivalent equivalent electric charges. The magnitude of the electric dipole moment P is proportional to the field strength. Since the polarization of the infinite molecule lies in the relative displacement of the center of the positive and negative charges, it is often called a bit.

Oriented polarization of polar molecules

As for the polar molecular dielectric, the center of the positive and negative charges in the molecule is equivalent to an electric dipole. Under the action of the external electric field, it will be subjected to a moment, so that the electric dipole moment P of the molecule is turned to the direction of the electric field. Because of the interference of molecular thermal motion, this steering is tiny, and it is impossible to align the electric dipole moments of all molecules along the direction of the electric field. The stronger external electric field of piezoelectric electrode piezoelectric ceramic, the more tidy is the steering order of the electric dipole moment of the molecule. At the macroscopic level, the more polarized charges appear on both surfaces perpendicular to the dielectric and the external electric field, the higher degree of polarization. When the external electric field is removed, the direction of the electric dipole moment of the molecule becomes an irregular arrangement due to the thermal motion of the molecules, and the dielectric is still neutral. Polarization of polar molecules lies in the direction in which the equivalent electric dipole turns to the external electric field, so it is called orientation polarization. In general, while the molecules are polarized at the same time, there is also displacement polarization. Although the microscopic processes of polarization of two types of dielectrics, polar is different, but the macroscopic effects are the same. Polarized charges of different numbers piezoelectric plate sensors appear on the two opposite surfaces of the dielectric, and the external electric field increases. the more polarized charges appear. Therefore, when the polarization phenomenon of the dielectric is described macroscopically below, it is not necessary to divide into two kinds of dielectrics for discussion.