The basic principle of piezoelectric ceramics material

Views: 7 Author: Site Editor Publish Time: 2018-09-10 Origin: Site

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1.Domain:

Generally, the directions of spontaneous polarization of ferroelectrics are different, but in a small area, the spontaneous polarization directions of the respective unit cells are the same, and this small area is called a ferroelectric domain. The boundary wall between the two domains is called domain wall, and can be divided into a 90°domain wall, a 180°domain wall, and the like according to the spontaneous polarization directions of the two domains. The domain wall is usually located near the piezoelectric ceramics crystal defect, and the domain wall is not easily moved because of the internal stress in the defect region. The orientation of adjacent domains is generally "end-to-tail", and a special form of "head-to-head, tail-to-tail" will appear under the action of the stress field to facilitate the reduction of energy. The observation domain can be a chemical etching method, a polarizing microscope method or an X-ray topography method.

2.Ferroelectricity concept:

Some piezo crystals exhibit spontaneous polarization properties. There is an inherent spontaneous polarization electric moment in ferroelectrics;in ferroelectric crystals, the domain structure is usually accompanied, and the spontaneous polarization electric moments in the same domain are in the same direction; when the ultrasonic polarized piezo transducer crystal is large enough, different domains of electric piezo can cancel each .due to different orientations, so that the macroscopic polarization is not revealed. The spontaneous polarization of electric moment can change direction under the action of external electric field; under the action of alternating external electric field E, the relationship between the macroscopic polarization intensity p of the ferroelectric body and E appears on the return line. These properties of ferroelectrics are very similar to ferromagnetism, so they are called on ferroelectricity. In the ferroelectric body, the macroscopic polarization intensity p=0 is generally due to the domain structure. When the external electric field E is very small, p has a linear relationship with E. When E is large enough, a curve in which p lags behind E is called a hysteresis loop. After a repeated alternating polarization of a strong alternating electric field of a fixed amplitude, the hysteresis loop has a substantially stable shape.

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Ps is the spontaneous polarization of a single domain without an electric field; Pr is the residual polarization; Ec is a coercive electric field. When the external electric field starts to act on the unpolarized sample, the residual polarization Pr is generated on the sample. To reduce the residual polarization to zero, the coercive electric field Ec must be applied in the opposite direction. The electric field in the opposite direction increases the polarization in the opposite direction, thus forming the entire hysteresis loop. It changes along this curve each time it is polarized. Different piezoelectric transducer piezoelectric ceramics have different hysteresis loops. The polarization process is a very complicated process. When is polarizing,it is not only a higher electric field is required, but different thicknesses require different times, and the optimal polarization effect can be achieved at a higher temperature. The polarized piezoelectric ceramic material loses the polarization effect at a certain high temperature, and different piezoelectric materials have different failure temperatures. This should be noted when selecting piezoelectric ceramic materials.The polarization properties of piezoelectric ceramics are the knowledge that piezoelectric geophone designers must master. The difference in performance between piezoelectric pre-polarization and post-polarization of piezoelectric ceramic materials is large.

3. Piezoelectric effect:

Piezoelectric effect can be divided into positive piezoelectric effect and inverse piezoelectric effect .A bound charge occurs at both ends of the polarized piezoelectric ceramic sheet, so a layer of free charge from the outside is adsorbed on the surface of the electrode. When an external pressure F is applied to the ceramic sheet, the PZT materials piezo ceramic deforms under the action of an external force, and a polarization phenomenon occurs inside the piezoelectric ceramic element, and positive and negative opposite charges appear on the opposite surfaces of the ceramic sheet. A discharge will occur. On the contrary, pulling will cause charging. When the external force is removed, it will return to the uncharged state. The phenomenon that this mechanical effect is converted into an electrical effect belongs to the positive piezoelectric effect. In addition, piezoelectric ceramics have the property of spontaneous polarization, and spontaneous polarization can be transformed by the action of an external electric field. Therefore, when an external electric field is applied to a piezoelectric dielectric, a change as shown in the figure occurs, and the piezoelectric ceramic is deformed. However, piezoelectric ceramics are deformed because when the same external electric field as the spontaneous polarization is added, the polarization is enhanced. The increase in the polarization intensity causes the piezoelectric ceramic sheet to elongate in the polarization direction. On the contrary, if the reverse electric field is applied, the piezo ceramics are shortened in the polarization direction. This phenomenon of turning into a mechanical effect due to an electrical effect is an inverse piezoelectric effect.