In the production of any kind of piezoelectric components, we hope to choose high-quality piezoelectric materials that meet the requirements of the components. What is the basis for identifying the quality of the material? It is usually to select some parameters that can reflect the performance of the material, and the size of these parameters is the quality of the material performance. In addition to dielectric and elastic properties, piezoelectric materials also have piezoelectric properties. Therefore, there are many parameters reflecting the properties of piezoelectric materials, for example;
In terms of dielectric properties: dielectric constant and dielectric loss.
The elastic properties include: elastic constant and mechanical quality factor. Piezoelectric properties include: piezoelectric constant and mechanical coupling coefficient.
Piezoelectric ceramic sheet
In addition, there are Curie temperature, frequency constant, density, and parameters related to aging performance and temperature performance. Now they are introduced as follows:
1. Dielectric constant
The dielectric constant reflects the dielectric properties of the material, or the polarization properties of the material, and ε is usually used to express the dielectric constant. Piezoelectric components for different purposes have different requirements for the dielectric constant of the material. For example, ceramic speakers, microphones, etc. require materials with a higher dielectric constant; high-frequency piezoelectric components require materials with a lower dielectric constant. The dielectric constant of piezoelectric ceramics varies greatly with the formulation and process conditions.
2. Dielectric loss
The piezoelectric element heats up after working for a period of time under the action of alternating voltage. This suggests that when piezoelectric elements work, some of the electrical energy must be converted into heat. Usually under the action of ac voltage, the electric energy lost due to heating in unit time is called dielectric loss. Dielectric loss is one of the important quality indexes of dielectric. For example, a high-power transducer requires a very small dielectric loss, otherwise the transducer will be damaged during operation due to intense heat.
There are many reasons for dielectric loss. In piezoelectric ceramics, the main reasons are:
(1) When the applied voltage changes, the polarization state inside the ceramic will also change accordingly. When the polarization state of ceramic can't keep up with the change of applied voltage, the hysteresis phenomenon will occur, which will cause the dielectric loss.
(2) Dielectric loss due to leakage current in ceramics.
(3) Dielectric loss caused by uneven ceramic structure due to imperfect process.
3. Elastic constant
Any object undergoes different degrees of elastic deformation under the action of external force, and the elastic constant is a parameter that reflects the elastic properties of the material. The most frequently used elastic constant in piezoelectric materials is the elastic compliance constant, which is often expressed as s.
4. Mechanical quality factor
Mechanical quality factor Qm represents the mechanical loss of ceramic materials at resonance and is another important parameter to measure the performance of piezoelectric materials. The cause of mechanical loss is the internal friction of the material. When piezoelectric elements vibrate, friction is overcome and energy is consumed. Qm is inversely proportional to mechanical loss; Qm indicates that the mechanical loss of material is smaller than Qm; small Qm means large mechanical loss of material. Qm of general ceramics varies greatly with different formulations and process conditions. For example, Qm for lead zirconate titanate ceramics can be between 50 and 3000.
5. Piezoelectric constant
Piezoelectric constant is a parameter unique to piezoelectric materials, which reflects the coupling effect between the "pressure" and "electricity" of the material. Therefore, the piezoelectric constant is not only related to mechanical boundary conditions, but also related to electrical boundary conditions; in other words, not only related to stress and strain, but also related to electric field strength and electric displacement.
6. Electromechanical coupling coefficient
Mechanical and electrical coupling coefficient K is a parameter that comprehensively reflects the performance of piezoelectric materials. It represents the coupling effect of mechanical energy and electrical energy of piezoelectric materials, and is the most commonly used parameter in production. Because the mechanical energy of piezoelectric element is related to its shape and vibration mode, the electromechanical coupling coefficient corresponding to different shape and vibration mode is also different. The electromechanical coupling coefficient has no unit.
7. Frequency constant
The product of the resonance frequency of the piezoelectric element and the length along the vibration direction is a constant, called the frequency constant N (kH·m). It can be seen that the frequency constant of the piezoelectric element is only related to the nature of the material, and has nothing to do with the external dimensions of the element. If the frequency constant of the material is known, the external dimensions of the component can be designed according to the required frequency. In addition, the Young's modulus of the material can also be calculated by the above formula.