Characteristics of Ultrasonic Piezoelectric Ceramics
Ultrasonic piezoelectric ceramics are a class of electronic ceramic materials with piezoelectric properties. The main difference from typical piezoelectric quartz crystals that do not contain ferroelectric components is that the crystal phases that make up their main components are all ferroelectric grains Since ceramics are polycrystalline aggregates with randomly oriented grains, the spontaneous polarization vector of each ferroelectric grain is also chaotically oriented. In order for ceramics to exhibit macroscopic piezoelectric properties, it must be fired in piezoelectric ceramics. After being formed and combined with the composite electrode on the end face, it is placed under a strong DC electric field for polarization treatment, so that the respective polarization vectors of the original disorderly orientation are preferentially oriented along the direction of the electric field. The piezoelectric ceramics after polarization treatment, in After the electric field is cancelled, a certain macroscopic remanent polarization will be retained, so that the ceramic has certain piezoelectric properties.
Dielectric and elastic properties:
The dielectric property of piezoelectric ceramics reflects the degree of response of the ceramic material to an external electric field, which is usually represented by the dielectric constant ε0. When the external electric field is not too large, a linear relationship can be used for the response of the dielectric to the electric field:
For piezoelectric ceramics, P is the polarization strength, ε0 is the vacuum permittivity, E is the electric susceptibility, and E is the applied electric field. Different uses of piezoelectric ceramic components have different requirements for the dielectric constant of piezoelectric ceramics. For example, audio components such as piezoelectric ceramic speakers require a large dielectric constant of the ceramic, while high-frequency piezoelectric ceramic components require a small dielectric constant of the material.
The elastic coefficient of piezoelectric ceramics is a parameter that reflects the relationship between the deformation of the ceramics and the applied force. Like other elastomers, piezoelectric ceramic materials follow Hooke's law: Xmn=cmnpqxmnpq, where cmnpq is called the elastic hardness constant of the elastomer, X is the stress, and x is the strain. For piezoelectric bodies, due to the piezoelectricity, the value of the elastic coefficient is related to the electrical boundary conditions.
Piezoelectricity of Piezoelectric Ceramics:
The biggest characteristic of piezoelectric ceramics is piezoelectricity, including positive piezoelectricity and inverse piezoelectricity. Positive piezoelectricity refers to the relative displacement of the positive and negative charge centers in some dielectrics under the action of mechanical external force, which causes polarization, which leads to the appearance of bound charges with opposite signs on the surfaces of the dielectrics. In the case where the external force is not too large, its charge density is proportional to the external force, following the formula:
where δ is the surface charge density, d is the piezoelectric strain constant, and T is the tensile stress. Conversely, when an external electric field is applied to a piezoelectric dielectric, the positive and negative charge centers inside the dielectric undergo relative displacement and are polarized, and the displacement causes the dielectric to deform. This effect is called inverse piezoelectricity. When the electric field is not very strong, the deformation has a linear relationship with the external electric field, following the formula:
dt is the inverse piezoelectric strain constant, that is, the transposed matrix of d, E is the applied electric field, and x is the strain. The strength of the piezoelectric effect reflects the degree of coupling between the elastic properties and dielectric properties of the crystal, which is represented by the electromechanical coupling coefficient K, which follows the formula:
where u12 is piezoelectric energy, u1 is elastic energy, and u2 is dielectric energy.
Physical Mechanisms of Piezoelectric Properties:
The two ends of the polarized piezoelectric ceramic sheet will have bound charges, so a layer of free charges from the outside world is adsorbed on the electrode surface. When an external pressure F is applied to the ceramic sheet, discharge occurs at both ends of the sheet. On the contrary, if it is pulled, the charging phenomenon will occur. The phenomenon in which this mechanical effect is transformed into an electrical effect belongs to the positive piezoelectric effect.
In addition, piezoelectric ceramics have the property of spontaneous polarization, and the spontaneous polarization can be transformed under the action of an external electric field. Therefore, when an external electric field is applied to a piezoelectric dielectric, the change as shown in the figure will occur, and the piezoelectric ceramic will be deformed. However, the reason why piezoelectric ceramics deform is because when the same external electric field as spontaneous polarization is applied, it is equivalent to enhancing the polarization strength. The increase of the polarization strength makes the piezoelectric ceramic sheet elongate in the polarization direction. On the contrary, if the reverse electric field is applied, the ceramic sheet shortens along the polarization direction. This phenomenon, which is converted into a mechanical effect due to an electrical effect, is the inverse piezoelectric effect.
Piezoelectric ceramics have sensitive characteristics and can convert extremely weak mechanical vibrations into electrical signals, which can be used in sonar systems, weather detection, telemetry environmental protection, household appliances, etc. The sensitivity of piezoelectric ceramics to external forces makes it even possible to sense the disturbance of the air caused by flying insects flapping their wings more than ten meters away. Using it to make piezoelectric seismometers can accurately measure the intensity of earthquakes and indicate the azimuth and distance of earthquakes. This has to be said to be a great feat of piezoelectric ceramics.
The deformation of piezoelectric ceramics under the action of the electric field is very small, at most no more than one ten-millionth of its own size. Don't underestimate this small change. Control of precision instruments and machinery, microelectronics technology, bioengineering and other fields are a great boon.
Frequency control devices such as resonators and filters are key components that determine the performance of communication equipment. Piezoelectric ceramics have obvious advantages in this regard. It has good frequency stability, high precision, wide applicable frequency range, small size, no moisture absorption, and long life. Especially in multi-channel communication equipment, it can improve the anti-interference performance, which makes the previous electromagnetic equipment unable to look back and faced with the problem of being overwhelmed. Alternative destiny.
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