Piezoelectric effect of piezoelectric ceramics

When the piezoelectric ceramic is polarized, there is a strong residual field polarization inside the ceramic material. When the ceramic material is subjected to an external force, the boundary of the domain moves is causing a change in the polarization intensity and resulting a piezoelectric effect.

 The polarized piezoelectric ceramic has a very high piezoelectric coefficient, which is about several hundred times that of a quartz crystal, but the mechanical strength is inferior to that of a quartz crystal.

Piezoceramic plate transducer are substances such as ferroelectrics, and are artificially fabricated polycrystalline piezoelectric materials having a domain structure similar to the magnetic domain structure of ferromagnetic materials. A domain is a region in which a molecule spontaneously forms. It has a certain polarization direction, so that a certain electric field exists. In the absence of an external electric field, the individual domains are randomly distributed on the crystal, and their polarization effects are canceled each other, so the original piezoelectric ceramic has zero internal polarization.

However, when the voltmeter was connected to the two electrodes of the ceramic sheet for measurement, the polarization existing inside the ceramic sheet could not be measured. This is because the polarization in the piezoceramics sheet is always expressed in the form of an electric dipole moment, that is, a positively bound charge occurs at one end of the ceramic and a negatively bound charge occurs at the other end. Due to the action of the bound charge, a charge from the outside is adsorbed on the electrode surface of the ceramic sheet. These free charges are equal to the bound charge sign in the ceramic sheet, which acts to shield and counteract the effects of polarization within the ceramic sheet on the outside world. Therefore, the voltmeter cannot measure the degree of polarization in the ceramic sheet.

If a pressure F parallel to the direction of polarization is applied to the ceramic sheet, as shown in the figure, the ceramic sheet will undergo compression deformation (dashed line in the figure), the distance between the positive and negative bound charges in the sheet becomes smaller, and the polarization intensity also changes. Therefore, a part of the free charge originally adsorbed on the electrode is released, and a charge phenomenon occurs. When the pressure is removed, the ceramic piece returns to its original state (this is an expansion process), the distance between the positive and negative charges in the film becomes larger, and the polarization intensity also becomes larger, so that some free charges are adsorbed on the electrode and charging phenomenon occurs.This phenomenon of changing from mechanical to electrical, or from mechanical to electrical, it is the positive piezoelectric effect.

Similarly, if an electric field of same polarization direction of high power piezo ceramic is applied to the ceramic sheet, as shown in the figure, since the direction of the electric field is the same as the direction of the polarization, the action of the electric field increases the polarization. At this time, the distance between the positive and negative bound charges in the ceramic sheet is also increased, that is, the ceramic sheet is elongated in the polarization direction (broken line in the figure). Similarly, if the direction of the applied electric field is opposite to the direction of polarization, the ceramic sheet is shortened in the direction of polarization. This phenomenon of turning into a mechanical effect due to an electrical effect or a transition from electrical energy to mechanical energy is an inverse piezoelectric effect.

It can be seen that the piezoelectric ceramic has a piezoelectric effect due to the spontaneous polarization inside the ceramic. After the spontaneous polarization is subjected to the polarization process and forced to be aligned, the residual polarization is present in the ceramic. If the external action (such as the action of pressure or electric field) can change the polarization,piezo ceramics will have a piezoelectric effect. In addition, it can be seen that the polarized charges in the ceramic are bound charges, it does not free charges, and these bound charges cannot move freely. Therefore, the discharge or charging phenomenon generated in the ceramic is caused by the change of the internal polarization of the ceramic, which is causing the release or replenishment of the free charge on the electrode surface.

When the piezoelectric ceramic is subjected to a force Fz in the polarization direction (Z direction) on the polarization plane (ie, the force is perpendicular to the polarization plane), then on the two silver plated (or gold) polarization planes, respectively positive and negative charges appear, the charge amount Qzz is proportional to the force Fz, ie:

Piezoelectric material:

Piezoelectric crystals, such as quartz;
Piezoelectric ceramics, such as barium titanate, lead zirconate titanate, etc.;
Piezoelectric semiconductors such as zinc sulfide, cadmium telluride, and the like.
        
Requirements for piezoelectric material properties:

The outstanding advantages of piezoelectric ceramic strips quartz crystals are very stable for performance, high mechanical strength and excellent insulation properties. However, quartz materials are expensive and have a much lower piezoelectric coefficient than piezoelectric ceramics. Therefore, it is generally only used in the standard instruments or sensors that require higher requirements. Because quartz is an anisotropic crystal, the physical properties (such as elasticity, piezoelectric effect, temperature characteristics, etc.) of wafers cut in different directions vary greatly. In order to design the quartz sensor, the cut shape of the quartz is correctly selected according to different application requirements.